bob long part 2

15 The seismic design of ambienttemperature storage tanks

This Chapter describes the most commonly used method ofdesigning verticalcylindricalliquidstorage tanks for seismic loadings- This method is taken from API 650 Appendix E. Someindications of the origins of the various methods of calculation and the formulae used areorovided.

The equivalentdesign sections of BS 2654 and the forthcoming Euronorm are a lso described.

Contents:

15.1 lntroduction

15.2 The API 650 _approach'15.2.'1 The basic seismic data

15.2.2 The behaviour of the pmduct liquid'15.2.3 The overtuming moment

15.2.4 Resistance to overluming'15.2.5 Shell comDression

15.2.5.1 Unanchored tanks15.2.5.2 Anchored tanks

15.2.6 Ailowable longitudinal compressive stresses15.2-7 Slosh height and freeboard considerations

15.2.8 Other conditions arjsing from seismic loadings

15.3 The BS 2694 approach

15.4 The prEN 14015 approach

15.5 References

STORAGE TANKS & EQUIPMENT 263

15 The seismic design of ambient temperature stoftge tanks

15.1 lntroductionThe design of liquid storage tanks to resist seismic loadings is asubject which is taken very seriously these days. Despite thecollapse of a large steel water tank during the Long Beachearthquake of 1933, nothing was done to provide a frameworkof rules for the seismic design of liquid containing tanks until af-ter the Alaska earthquake of 1964. This seismic event causedwidespread damage and subsequent fire to a large number ofpetrochemical fuel tanks. The site contained a numberoftanksof different sizes, different proportions and filled to different lev-els with products with different specific gravities at the time thatthe earthquake struck. The study of the levels of damage sus-tained by the various tanks allowed the various theories relatingto seismic design to be tested. The Alaska event is described inReferences 15.1 and 15.2.

Two further seismic events involving damage to storage tanksalso contributed evidence to the discussions. These were the'1971 San Fernando and the 1974 Lima Peru eafthquakes de-scribed in References 15.3 and 15.4

The credit for the production of a workable and "user friendly"set of rules for the safe design of liquid storage tanks to enablethem to resist seismic loads owes a lot to the document com-monly known as TID 7024 (Reference t5.5) and to the work ofWozniakand Mitchell (Reference 15.6). The proposalsgiven in

this paper by Wozniak and Mitchell were adopted with minorchanges as Appendix E ofAPl 650 (Refe rence 15.7). AppendixL of API 620 (Reference 75.8) uses the same design methodBS 2654 (Reference 75.9) has "borrowed" the same rules, al-though not the seismic zones from the UBC, (The AmericanUniform Building Code).

This Chapter devotes most of iis efforts to the seismic design ofground based ve(ical cylindrical tanks as these form the largemajority of ambient tanks. TID 7024 provides some guidance

on the design of rectangular and elevated tanks.

The seismic design of liquid containing storage tanks has beena popular subject for study over the years and there are hun-dreds of articles and papers covering work which has been car-ried out. lt is proposed to include in this Chapter only the mostimmediately relevant references. lf more are required, then ref-

erence to chapter 6 of Philip Myers' book ( Refercnce 15.10)willprovide a list of 61 papers and reference to these in turn shou ld

Figure 15.1 Selsmic zone map ofthe lJnited States

From the Unifam Building Code

264 STORAGE TANKS & EQUIPMENT

produce sufficient material for even the most enthusiastic stu-dent of this subject.

15.2 The API 650 approach

15.2.1 The basic seismic data

Before the task ofdesigning a particular storage tank for a seis-mic event can start, it is necessary to have some idea of the ap-propriate set of seismic criteria which are to be applied. Thedefinition ofthe appropfiate seismic design datia for a particularsite location and geology is a complex business. This can beseen in Chapter 26 which deals with low temperature tank de-sign where the subject of seismic design is considered in agreater level of detail.

For ambient tanks it is usual to adopt a simpler approach andthis is reflected in the rules provided in API 650 Appendix E.

This Code allows tvvo aporoaches to the selection of the seis-mic design criteria:

Firstly it takes as its starting point the American Uniform Build-ing Code (UBC) (Reference 15.11).fhis document divides theUnited States of America into seismic zones ranging from zone0 (no seismic event to be considered)to zone 4 (the most oner-ous seismic event). This is illustrated as Figure 15.1.ltalso pro-

vides guidance regarding the appropriate seismic zones for lo-

cations outside the USA. This tabulation is shown in Figure15.2. The UBC approach is to express an earthquake event asa horizontal acceleration which can be applied to the structurein any direction.

The second approach has more in common with the practices

for low temperature tanks. By agreement between the pur-

chaser and the manufacturer, the horizontal accelerations maybe determined from site specific response spectra produced bythe purchaser. The accelerations should not be less than thosederived from the use ofthe UBC. Forthe impulsive componentof the liquid and the tank and roof self-weighb, the valuesshould be based on 2% damping, and for the convective com-ponent ofthe liquid, should be based on 0.5% damping. Unlessthe tanks are in some way special, this more exacting approachis rarely adopted.

-\l''r-Tt

1 5 The seismic design of ambbnt temperature sto@ge tanks

t.oc6lionAFRICA

AlgeriaAlge.Orar

Bdtin

G8boroneB'rrundi

Bujumbt'ra

Douala

Cap€ Verde

Cantral African RcpublicBatrgui

ChadNdjrmcna

CongoBrazavillcDjibouti

EgvPl

Cair0Pod Said

Equatorial CuineaMalabo

Etbiopia

GabonLibreville

GatubiaBanjul

Ghana

CuiDeaBissauConaky

Ivory CoaslAbidjan

KenysNairobi

L€sotho

Uberia

LibyaTripoliWi€elirs AFB

Malagisy RcpublhThna!arive

BlantyreLilongw€Zonba

Seismic Zone LocatonMali

BanakoMaudtaDia

NornkchottMaurillus

Pon lruis

Port LyautcyP.abqt

TangrerMozambique

MaputoNigcr

NiameyNig€ria

Iba&nKadunallgos

R€public of RwandaKiCali

scrcdDakar

Seychetles

SonaliaMogadishu

South AfricaCap€ TownDulban

Naral

TanzaniaDar es SalaamZanzibtl

Togo

TunisiaTuris

Ugadarial|Ipal!

' UppervoltaOuSado!gou

z,j,ir.BrkavuKi$hasatubumbashi

7ar$iaLukasa

ZimbrbweHardrc (srlisbury)

ASTAAfghribran

&bd

3

0

0

0

3

00

0

0

0

03

2A2A2A

o

33

0

0

t0

0

2A

2A

I

2A2A

0

3

S€l8nlc Zone


0

0

0

I2A

3

2A

0

0

0

0

3

2A1

2A

2A

2A2A

I

t32A

0

30

2A

ZA

3

Figure 15.2 Tabulation of tho seismic zones woddwide - pagg tFromthe Uniform Buildino Code

15 The seismic design of ambientlewenfiJre slorage fanks

LocttlonBib|3ir

MBnamr

Bangladcsh

BIuneiBandu Sci Begiunn

B!rmtMandalayRangooD

ChinaB6ijirgChctrgdu

' GuangzhouNenjingaiogdtoShaoShiSbenB|{t|t'tbiwm

AtlTihrva

xisrggangcypro6

Nicosr:a

IndiaBor|bryCalqnaMadr3lNcw Dclhi

IndorcshBendungJakrnaMedatrSursbrya

lmnIslbiarShirazTabrizTehmn

I'!qaaghdadBisra

lsraelllaifa

FukuokaIlrzi(e AFBMis8v,,a AIBNaha, Okinewaft*a/KobeSapporoTokyo

Jord!tr

KorcaKimhrcKwanAi!

KuNeilKuwiii

Figure 15.2 Tabulalion of the seismic zone6 worldwide - pags 2From the Unltom Bulldlng Code

266 STORAGE TANKS & EQUIPiIENT

Lao60 VllirtidD

Irbdiron3 Bdifill

Maley6is

I IOah LumFrrN.P3l

K'btnanduotlla!

\ Pddsranftlth.badKsncnihiorc

QatrrDoha

Srudi AnbiaAl BalhDhr$rJiild.!Khnis Mwl|arfRiysdh

Sing.poi!All

Soulh YdncnAdr[ Cily

Sri btrk.Colodbo

Syri.AloppoDimrs€rs

ThailandBrIgko*ChiuB irriSonglllla

Tn!ftcy

lsnirIsi'rbulKal"nluJsel

Unil€d Arab En ntesAbu DhabiDubai

Ho Chi Minh (Saigor)Y€Detr Aran Republic

Sanal

AT'I,ANTIC OCEAN AREA

AtlBcrtnudt

AICARTBBEAN SEA

hlruna IslsodsAI

CubrAI

Domiricar nepublicSanlo DoninSo

Frcndr Wbsl lndi€sMa iniquc

GrensdaSli Geolgcs

Sa|amlozone Locellon SstEmlc Zdao

1

.3

1

2A

I2A

.{

0

II

2AI0

I

3

0

33

I2A

0I

2A2A

4

3

00

0

I

33

'32A2A

3

2A2A

32AI

44

1

3l

3l3

343

3

7

II0

I

1

3

3

02A

0I

2A0I

2A2A

3

34

2A

3

0

3033

2A33

2A33

24

1

2A

0

ZA

ZAII

3

3

00

2L2A

02A2A

I

3

3

3

2A

2A

3

3

2A330

2-4,

03003

32B

3

1 5 The seismic design of ambient tempenture stgnge tanks

LocatlonHaiti

Kngsionlreward Islalds

AitTriridad & Tobago

A[SENTRAL AIVGRICA

B€lizoB€lmopar

Caral Zon!AI

Co$a RiaaSan Jo6c

El SakadorSan Salvador

GuatemalaGualrr||ah

HonduiasTcg|rcfgrlpa

MexicoCiudad JtlarczcudalajffaHentosillo

MrzatlanMcridaMcrica City

rijuNnaMcar.gra

Manrgua

CoiorGalclr

EUROPEAlbania

Tiran,

Salzbor8

Bclgiun

Blllss.lsB06oia-Hcccgovina

B.lgadeBulgaria

sofi3Croaria

TagnbCzecb06lovekia

BlalislavaPrague

Copenh"gcnFnlard

Heliloti

Bordca$rLyonMalseilleNic!

Ssls|nlczone Loca{onParis

3 stralbouB

Sabtnlc Zon€0

24

3

ZAZA

I2A

2A

3

3

2A1

I

I

2LI33

Ocrnrny, Fcdelsl RepublicBcdinBoBrcm.dDusseldorfFnnlfurlHalnburyMunicllsNuganVaihigcn

KrvrllaMakdRhodcsSruds BtyThessaloniki

H!ngaryBudapa{

IcebrdKcfla!ickRcytiavik

IrelardDubli'l

l|alyAvia@ AFBBrirdisi

MiIanNaplas

Sicily

Tudn

Malta

Nelherla sAlt

NorwayOslo

Polmd

PodugalLiSonOpporto

Russia

St. PelrlsbulgSpoin

BarceloraBilbaoMadridRottSeville

Colebo{gStoclhols

Figure 1 5,2 Tabulatlon of the selsmlc zones woddwide - pagg 3Ftufi fie Unitom Building Code

STORAGE.TANKS & EQUIPMENT 267

34

4

0

I

0

"AII

3

3

0

II

IIII

II

I

I

I

II

II

It

I

t

I

15 The seismic de'ign of ambient.bnpenture storage tanks

LdcellanSwi@crlald

Bir!

ZurichUkAin.

tricvUnilid (ingdom

B€|lastEdiiburgnEdzelClassow^cnfrewHaftiltonUvqpoolI.oddon

Thu$o

NORTI{ AMEIICAGrEcnland

AIICrradr

Agcnda NASCal$!y, AlbCturcni4 ManCold l,ah., AlbBdmortoq AlbE. ltffrnoo AIBForr W liams, Onlfrobirh€r N.W. Tei

HrlifaxMonbeal, Qucbc.O0awe,onlSl, Joh!'sNfdTbronb, Ont

Wimcpcg, Man

SOUTII AMERICA

Boliviala PazSanra Cruz

BlazilB€ldD

Brasilia

RecifeFio {bJanciroSalvadorSao Paulo

ChiltSa iago

ColombiaBogora

EcradorGuataquilQuito

Parrguay

Sdanrlczona kcalloo SeEfilC bn€

UrnPiu'"

Urugday

Carecas

PACIFIC OCSAN AREA

BisbaoeCantatra

P.nhSydmy

Carolin blrrdsKolsr, Psh! ls.

FrjiSuva

JohrEon IslandAll

l,{adann IstardsGuamSalpa!Tirisd

Mrlsbrll IslaidsAll

Nar Z.3latrd

llbliEtonPapau New Guincr

Porl r'rorubyPliIlFh€ Islrndo

BaSuoC€bul,Liila

S:motAll

\lblc khndAll

The above compilation is a parial ltsriry of s€ilmi@ zonnr forcities aod countries outside of the Unitcd Stares. Il has beetr Dr}vid€d in this cod€ prinanly as a source of inlormario4 and iraynot, i'l all cases, reflcct local ordtlances or cunent sic ifc in-

rffhen an authority having jlrisdiction requir€-! seisoic dasignforc.s tlal lre higler tha! wolld be indicsted by the above zorcs,lhe local rcquircments shall govem. When an aulhority havingir-risdiction requires seisrfc desjgn forces lha! arc lower lharwduld b. indjcated by the abovE z$es, and these farces havebcer develop€d with cotsider8tiotr of regjooat tectoiics ard urFto-dale geologic and seismologic fufomation, th6local rcquir€-

rl,hen no local s€iuic d$ign rcqldrcrlgnb cris!, prsperly dc-termircd iDformarion on sitc-specifc grourd tuotiotr! olay b€used to jlstify a lower seisBric zone. Such sile-spacifc grollldmotiotrs siall hav€ bsetr dlvelop€d with p4per comidcnlion ofregiooal tectonias and local geologlc and seisnologic inforlDs-tion, rnd ihll have no Elor€ &atr a 10 pGrc€r{ chrnce of bobg e,r.ceeded iB a SGyear period.

4

0

2A0

I

ZA10II

2A00I1

32A

3I3t

0

1I

00000000I

4

Flgura 15.2 Tabulallon of the seismic zonss worldwide - page 4From the Uniform Builditlg Code

268 STORAGE TANKS & EOUIPMENT

15 The seismic desjgn of anctea: :a-aa.a:--=::: =:: :: -. !

s.i\nrc FacLo. Seismr Zone FlcLor(lmm F,guE E- I o. oder souroes ) (horzon'3t acceierario! l

l

1A

2B

3

0.075

0.r5

020

0.30

040

Figure 15.3 Seismic zone factorFron APl650, Appendix E, table E-2

The API 650 Appendix E design acceleration for the impulsivecomponent of the liquid togetherwith the self-wejght ofthe tankshell and roof is the product of three variables. ihese are:. The zone factor (Z) from the UBC which is given in Figure

15.3.

. The importance factor (l) which is normally 1.0 and whichshould not exceed 1.25. This higher value should onty beapplied to tanks which are required to provide posfearth-quake service or tanks which store toxic or explosive sub_stances in areas where an accidental release of theircontents would be dangerous to the safety of the generalpublic.

. The lateral force coefiicient C1 which shall be 0.60.The corresponding acceleration for the convective componentof the liquid is again the product of three variables which are:. Z as above

. I as above

. The lateralforce coefficient C2 which is a function ofthe nat_ural period ofthe first mode ofsloshing (T) and the site coef_ficient (S).

T is determined from:

r = k(D05) equ 15.1

A soil ponle wirh ejrher a) a !o*-tii. oareiichemderizd by a she2J walc velocin EaE0E 760 r/s (2500 ftis) o. by ohd;!a!kneds of clAlindion or b) srjtr or d.!k eitconditions wb*e lhe $il deprh is le$ da Sln (200 fi).

A soil prcnb wir! stiff or derse soit condfuons*b€E lire soil depLh exc*ds 60 h (200 hl.

A soil profile l2 ft {4q n) or moE ir .teprh con-taining norc than 6 m (m n) of sofr ro mediunstiffclnybutmoFrhd t2n({Oft)of sonclay.

A soil profiic coffai ng moE ihe 12 n (40 f0

i.:

I i

Nor: The sne fs.ror shal be esrablsbed ftom properly [email protected] dar!. ID localions wher dE soil pDPcniei aE Dor tnopnin s!frcie de6il ro dercmtue th. soil Plofiie rypq lon pro6l. S.lshal bc us.d. Soi] pofte .ta ned not be a$Med unless the builiinEofbcial deremines rhsr soilproEte Sa nay b€ pllgt ar rhe e or ,;ihe ev€nl ttar loit pofitc 54 $ eslablished by gmrectuij@t dau

Figure 15.5 Site coefficients SFrcn API 650, Appendix E. table E-3

15.2.2 The behaviour of the product liquid

The way in which the liquid in a vertical cylindrical container be_haves when subjected to an earthquake was clarified bv one ofthe "giants" of seismic engineering. G.W. Housner. This is de_scribed in Refere,ce 75.72. This theory which is used to thisday, divides the liquid withjn the tank into two comDonents.These are termed:

. The impulsive component

. The convective component

The impulsive component is that part of the liquid in the lowerpaft of the tank which moves with the tank as thouqh it were asolid. lt experiences the same accelerations ani displace_ments as the tank and the subgrade upon which the tank isfounded. The tank is presumed to be rigid. This is not exacflvtrue, but for ambient tanks it is normalto make this presumotionand it provides answers ofsufficjent accuracv. The influence oftank flexibility. especially for steel tanks, is discussed further inChapter 26 on the seismic design of low temperature tanks.The natural period of vibration associated with this componentis a function ofthe size and the stiffness ofthe tank itseliand isusuallyto be found in the 0.i secto 0.4 sec range. Atypicalre-sponse spectrum for a seismic event is as shown in Fioure15.6. Cleady the impulsive component with its natural fre_

'lype

52

53

Note: This equation requires the tank diameter to be in feet. lfthe diamete_r is in metres, then the equation becomesT=1.811k(Do5)

Where k is taken from flgure E-4 shown in Figure 15.4

't.0

/, o8

0.6

0.54,0 7.0

Figure 15.4 Factor kFron API 650, Appendix E, figure E-4

or can be calculated from the following equation:

t,0

0.578

o

I6?

EbFor values of T less than or equal to 4.S sec:

^ 0.753.T

For values of T greater than 4.5 sec:

^ 3.3755t-

S is taken from Table E-3 (Figure .15.5)

equ 15.2

equ 15.3

PEFTOO (SECONDS)

Figure 15.6 Design response specAaFron Uniform Building Code

. . (s.azLfI/H

equ 15.4


15 The seismic design of ambient tempemture storage tanks

Figurc '15.7 lt4odel ol the impuls've and conveclive componenls

Figure l 5. S Model of the im pulsive and convective componenls, with theself-weights ofthe tank shelland roof added

quency will be subject to accelerations which are close to themaximum values shown.

The convective component is that part of the liquid in the upperpart ofthe iank which is free to form waves or to slosh. This partof the liquid has a much longer natural response time than theimpulsive portion and is usually to be found in the 5 sec to 10sec range, again depending upon the tank size. Referenceagain to Figure 15.5 shows that this portion of the liquid will besubjected to much lower accelerations.

The way in which the tank contents are modelled is shown inFigure 15.7. The impulsive component is rigidly linked to thetank wallswhilstthe convective component is attached by weaksprings. The complete model including the self-weighb of thetank shell and roof is shown in Figure 15.8.

The proportion of the product liquid which falls into the impul-sive and the convective portions is a function ofthe tank shape,and the calculation methods are different for short tanks withDiH greater than 1.333, (the majority of tanks fall into this re-gion) and for tall tanks with D/H less than 1.333.

API 650 Appendix E ignores tall tanks and gives the effectivemasses of the two components and the respective heights totheir centres of gravity in graphicalform, shown in Figures 15.9and 15.10.

The equations forming the basis ofthese graphs for short tanksare:

Ta Short

Tank diameier (m)Liquld n I height (m)

lmpulslve mass ftorne)Convectlve mass af onne)

27011 l6t 2%)13212132 8a )

42 OA

2A A752.0018.83

16443142.7o/o)22071157.3%)

1,0

{* o.u

s 0.4

*o.2

7.01.O 4.0

DIH

Figure 15.9 Etrective massesFrcn API 650, Appendix E,ligurc E-2

Fiqure 15 10 A comoarison belween the rmoulsve and convective masses fo'a 40 000m3 tank of iiifferenr orooorlions

t?471coshl 11 l- 1.0x, nn lDHl

H -'" 367 - /3.6?rD,',H lDiH'

where:

equ 15.8

equ 15.9

equ 15.10

Wr = weight of the impulsive liquid component

W2 = weight ofthe convective liquid component

Wr = total weight of the tank liquid contents

Xr = height from the tank bottom to the centroid ofthe impulsive liquid

Xz = height from the tank bottom to the centroid oJ

the convective liquid

H = maximum liquid filling height

Note: As long as the units used are consistent, then these for-mulae work in both Sl and US customary unib.

lf the tank is tall (i.e. D/H less than 1 .333), then it is suggestedthat W1 and X1 are modified to:

IL=r.o o.zra9WTH

! =o.soo o.os+ IHH

tanh 0.866 9vvr - H

& 0.866qequ 15.5

equ 15.6

equ 15.7

To see howthis works out in practice for a short tank (the major-ity of the larger ambient tanks fall into the short category) theexample ofa tank of40,000 m3 has been adopted. Two differenttank proportions have been chosen and the values of W1, W2,

Hj and H2 calculated.

The results are shown in Figure 15.11. Clearly the taller tankshows a higher proportion of its contents to be impulsive thanthe shorter tank where a higher proportion is convective.

15.2.3 The overturning moment

The overturning moment due to seismic forces to be applied tothe bottom ofthe tank shell shall be determined from the follow-ing equation:

& = o.zgo D

,unnl 3 67

Iw. H IDH'

! = o.szsrl


F

s

t'I

II

w"tw, _--r-X

I

1.0

0 1.0 2.0 3.0 4.0 5.0 6-0 7.0 8.0

DIH

Figure 15.11 Centroids of seismic forcesFron API 650, Appendix E, figute E-s

M = Zt(ClWsXs +C1WHr +C1W\ rCrWrXr) equ.15.11

Where the hitherto undefined variables are:

Ws = total weight of the tank shell

W, = total weight of the tank roof (fixed or floating)together with a portion of the snow load asspecified by the purchaser

Xs = height from the bottom of the tank shell tothecentre of gravity of the tank shell

Ht = total height ofthe tank shell

C1 = lateral force coefiicient for the impulsive com_ponent and is taken as 0.60

Cz = lateral force coefficient for the convective com-ponent

This moment is the moment due to the liquid acting on the tankshell only plus the moment due to the self-weighiof ihe shelland roof. lt is sometimes known as MEBp or moment excludjngbase pressure.

There is a second moment whjch is useful for the design ofstorage tanks known as MrBp or moment including base pres_sure. This moment, as its name suggests, also includes the in_fluence ofthe liquid tank contents upon the tank bottom. This isused for the tank foundation design and is calculated from thesame formula as equation 15..11, but with X1 being replaced byX., andXrby Xr. These new moment arms are calculated fromthe following:

For short tanks (i.e. D/H greater that 1.333):

0.866 qn

tanh 0.866 9H

equ 15.12

For tall tanks (i.e. D/H tess than 1.333):

x_nI =0.500+0.060:HHFor all tanks:

15 The seismic design of ambient temperaturc storage tanks

15.2.4 Resistance to overturning

Resistance to the overturning moment calculated in equation15.11 which applies at the bottom of the tank shell mav be oro-vided for anchored tanks by the weightof the tank shelliogeiherwith any portion ofthe tank roofwhich is supported by the tankshell and by the tank anchorage. For unanchored tanks, the re_sistance may be provided by the weight of the tank shell to_gether with any portion of the tank roof which is supported bythe tank shell and the weight of a portion of the tank contentsadjacent to the tank shell acting on the outer part of the tankbottom.

The evaluation of this liquid holding down component is as fol-lows:

In US customary units:

wL = 7.gtb\EGH

0.8

ti o.t

r 0.4

{0.2

+=0.,,1,,., ...[

This is subject to a maximum value of:

wL = 1 25GHD

this is subject to a maximum of:

wL = 196GHD

This maximum value of the liquid holding down effort is basedon a maximum permitted radial dimension of the uplifted por-tion of the tank bottom equivalent to about 7% of the iankradtus.

where:

wL = maximum weight ofthe tank contents that maybe used to resist the shell overturning momentin lb/ft of the shell circumference

tb = thickness ofthe bottom plate immediately be.neath the tank shell (inches)

Foy = minimum specified yield strength ofthe bottomptate immediately beneath the tank shell(lb/in'?)

G = design specific gravity of the product to besloreo

H = maximum design liquid tevel (feet)

D = tank diameter (feet)

In Sl units:

wL = ggtb1EycHequ 15.17

equ'15.18

Where:

to

Fby

H

D

equ 15.15

equ 15.16

,,]]

Y=,0

is in N/m

ts In mm

is in MPa or N/mm2

tstnm

isinm

equ 15.13

equ'15.14

For the detailed design of tank foundations, it is often useful toseparate the moment applied to the tank base area only andthis figure can be obtained bythe subtraction of MEBp frornM Bp.The actual distribution ofthis loading on the tank bottom js dl-scribed later.

15,2.5 Shell compression

15.2.5.1 Unanchored tanks

The API method divides the means of calculating the maximumcompressive force in the tank shell into four methods deoend_ent upon the value of:

rvy'[o'1w, + w,;]

wnere:


15 The seismic design of ambient tempercture storage tanks

= weight of the tank shell and that portion of thetank roof supported by the tank shell in N/m orlb/ft of shell circumference

Wnen rr,y'[o'?(w, + w.)] is tess than 0.785, there in no uplift of

the tank shell and consequently the compressive loading in theshell is distributed in a linear fashion across the tank diameteras in simpie bending. In this instance the neutralaxis ofthe tankremains in a central position.

Thus

. 1.273M'D' equ'15.19

where:

b = maximum longitudinal compressive force atthe bottom of the tank shell in N/m or lb/ft ofshell circumference

wnen ([o'?(w, + w.)] ls greater than 0.785 but less than or

equal to 1.5:

b can be calculated from the term b + wL

the value ofwhich iswt+wL

found from Figure E-5 (Figure 15.12) using the calculated. .. ,lf ^t , ._l

varue or rv/ LU ( wr + wL r l.

The bottom of the tank is lifted for a part of the circumferenceand a liquid holding down effort is created. The neutral axis ofthe tank is moved progressively away from the tank centrelineand the shell compression is concentrated in a decreasing por-

tion of the shell circumference. The explanation for this mecha-nism and the description ofthe derivation of the equations usedis best left to the paper by Wozniak and Mitchell (Reference15.6).

When the value oflvy'fo'(w, + w.)] is greater than 1 .5 but less

than or equal to 1.57 then b can be calculated from the follow-ing:

D+WL _r -ro 5

l. 0.637M

t DP(', . vv,)]

equ 15.20

stress, see Section 15.2.6), then the tank is structurally

unstable. Note that .: is the calculated longitudinal shellcom-121

pressive stress in US customary units, as is F,, i.e. lb/in2.

In this case the API Code makes the following suggestions:

. Increase the thickness of the bottom plate under the shell.This will increase the liquid holding down efrort.

. Increase the shell thickness. The way in which the calcula-tions are carried out is that the lower shell thickness arisingfrom the basic hydrostatic conditions is checked to see if it isstable. lf it needs to be increased in thickness to meet thetwo criteria given above, then all ofthe upper shell coursesshould be increased in thickness by the same proportion,unless a more sophisticated analysis is carried out to deter-mine the actual compressive stress at the bottom of eachshell course in turn.

. Change the proportions ofthe tank to increase the diameterand reduce the liquid filling height.

. Anchor the tank in accordance with Section 15.2.5.2.

15.2.5.2 Anchored tanks

Anchoring the tank shell causes the tank's neutral axis to re-main at the central oosition and as for the first of the unan-chored tank cases described above, the maximum longitudinalcompressive force is given by:

1.273MD=wr +

D2

When tanks are anchored, it is clear that no liquid holding downcan be utilised to reduce the uplifting loads as it requires theshell to lift to mobilise the term wL.

The anchorage system shall be designed to provide the follow-ing minimum uplift resistance in N/m or lb/ft of shell circumfer-ence:

1.273M* w, equ15.21D'

plus any uplift, again in either N/m or lb/ft of shell circumfer-ence, due to internal pressure. Uplifr due to wind loadings onthe tank shell and roof do not need to be considered in combi-nation with seismic loadings.

Anchorage is normally by means of bolts or straps. The points

of attachment of the anchors to the tank shell must be madewith due consideration to the local stress concentrationscaused. This part of the tank shell is already highly stressed in

hoop tension and local vertical bending. An acceptable designprocedure is given in Reference15.13.

The design of the anchorage should consider the following:

. The strength of the attachments to the tank shell shall begreaterthan the specified minimum yield ofthe anchors sothat the anchors will yield before the athchments fail.

. The spacing of the anchors around the tank shell shall notexceed 3.0 m exceptthatfortanks of less than 15 m in diam-eter the spacing shall not exceed 1.8 m.

. Anchor bolts shall have a minimum diameter of 25 mm, ex-cluding any corrosion allowance.

. The maximum allowable stresses shall be:

For the anchors an allowable tensile stress equal to 0.8times the minimum specified yield stress (this is 0.60times 1.33)

For other parts, 1.33 times the "normal" allowablestresses taken from section 3.10.3 of the Code.

't.490

When MlD'7(wr w.)l is greater than 1.57 or *n"n .,!, is

greater than F" (the allowable longitudinal compressive shell

t

{

0.8 1.0 1.2 1,1 1.6

M lID"(wt+ |9L)l

Not6: This ligure may b€ used 10 compL.te b when M / {d( pr + kJlis gGal6r than 0.785 bd bs rhan or 6qB1 lo 1 .5 (s€e E.5.1 ).

Figure 15.12 Compressive fofce bFrom API 650, Appendix E, figure E-5


The maximum allowabledesign stress in the shellattheanchor atiachment shall not exceed 170 N/lpa (25.000lbiinr) with no increase atlowed for seismic loading.

. The embedment of the anchor into the foundation shall beof sufficient strength to develop the specified yield strengthof the anchor.

. The purchaser shall specify any corrosion allowance to beapplied to the anchors. The uncorroded anchors shall beused to determine the design loads forthe attachments andthe embedments.

. When specified by the purchaser, the anchors shall be de-signed to allow for thermal expansion of the shell arisingfrom temperatures greater than 90 'C (200 'F).

15.2.6 Allowable longitudinal compressive stress

The maximum longitudinal compressivestress in the tankshell($ in US customary units or .

^b ^. in St units) shall not exceed12t 1000tthe maximum allowable compressive stress Fu which is calcu-lated from the equations given below These equations takeaccount of the reduction in the compressive buckling stresscaused by deviations from the perfect cylindrical shape due tothe fabrication and erection processes and the stabiljsing ef-fects of internal pressure due to the product liquid.

It must be borne in mind that the worst case seismic desions areall based on a tank filled to the maximum operating fill h;ight. Agood presentation of the influences of these vaiables is qivenin Reference 15.14.

In US customary units:

GHD,,When --" is greater than or equal to '106.

1061equ 15.22

15 The seismic design of ambient temperature storcge tanks

through the numerous papers which are listed in References15.16 and 15.16.

The classical buckling strength ofa perfect cylinder is given by:

s =0.6rR

equ 15.26

The value of given by equation 15.22 above (which representsmost storage tanks of normal proportions) is one third of thatgiven by equation 15.26.

15.2.7 Slosh height and freeboard considerations

API 650 Appendix E does not provide any specific rules for liq-uid sloshing. lt does give a general warning that "the purchasershallspecify anyfreeboard desired to minimise or prevent over-flow and damage to the roofand upper shellthat may be causedby sloshing of the liquid contents". lt seems curious to place thisresponsibility on the tank purchaser. The tank designer is, orshould be in a much better place to make decisions relating tothe freeboard to be allowed for seismic sloshing in anyparticular circumstances.

Forfixed rooftanks it is usualto arrange a freeboard sufficientto prevent the liquid sloshing wave plus any associated run upof product liquid up the tank shell from impacting upon the tankroof itself. Refererce 75.75does indicate means of calculatingpressures on the underside of the tank roof olates in caseswhere insufficient freeboard has been allowed. This documentis, to the author's mind, a quite excellent publication and essen-tial reading for those interested in the seismic design of storage€NKS.

For floating roof tanks it is usual to allow sufflcient freeboard toensure that the roof seals remain within the heioht of the tankshell. The presence of the floating roof is not;onsidered tomodify or inhibit the sloshing behaviour of the product liqujd.

For fixed roof tanks with intefnal floating roofs, it is usual to al-low sufficient freeboard to ensure that the internal roof and thetank roof, or its supporting structure, do not come into directconlact.

To enable these decisions to be made, it is clear that the heiohtof the sloshing wave must be calculated for any particular t;kgeometry and site location. ln the absence of any means tomake this calculation in API 650, it is not uncommon practice toborrow the following formula for the height of the first sloshingmode from Appendix L of API 620:

d = 1.124ztc.rzrann fa.rzl I l"l- L \Dr lequ15.27

where:

d = height ofthe sloshing wave in feet. lt is recom-mended that an allowance for liquid run uD thetank shell of 1 foot is added to this height

15.2,8 Other considerations arising from seismicloadings

There are a number of other areas of tank desiqn for seismicloadings which occasionally arise for ambient tanks. These arebriefly described below Some ofthese are revisited in Chapter26 (Seismic design of low temperature storage tanks) whereseismic design is considered in more detail.

. The columns ofcolumn supported roof type tanks must bedesigned to resist the lateralforces imposed on them by thecontained product liquid during the design seismic event.The work of Wozniak and Mitchell (Reference 15 6) gives asuitable and well-tried procedure for this.

,DlaHn2

When + is less than 106:

1n6rF"=r_+600JGH

In Sl units;

GHD,.wnen t2 - rs greater than or equal to 44:

equ 15.23

equ 15.24

equ 15.25

_ 83t.DcHn2

When :+ is tess than 44:

R?TFa =ffi+7.5JcH

However in all cases Fa shall not exceed 0.5 Fb/

where:

Fv = minimum specified yield strength ofthe bottomshell course in the appropriate units (Mpa,N/mm2 or lb/in2)

t = thickness ofthe bottom shell thickness exclud-ing any corrosion allowance.

The buckling of vertical cylindrical shells has been the subjectof a great deal of theoretical and test work over the vears.Those interested in studying this subject in more detail could doworse than to look atthe work of Wozniak and Rotter. orto oick


15 The seismic design of ambient temperaturc storcge tanks

. lt is sometimes necessary to calculate the local pressuresimposed on parts of the tank shell and bottom during theseismic design event. This could be for detailed design ofinternal fittings or for an assessment of the applied hoopstresses on the complete tank shell. Chapter 26 providesmeans of performing these calculations.

. In extreme seismic events, there may be a tendencyfor thetank to slide off its foundation. This is an interesting subject,about which there are differing views held by those knowl-edgeable within the industry This is again considered in de-tail in Chapter 26.

15.3 The BS 2654 approachAppendix G of BS 2654 admits in its introductory note to beingbased on Appendix E ofAPl 650. "Based on" is something ofanunderstatement. What is presented is Appendix E ofAPl 650 inmetric units!

There are a number of minor changes:

. In line with the philosophy of BS 2654 in designing all tanksfor the maximum anticipated product specific gravity, allseismic calculations are based on an assumed productgravity of 1.00.

o In place of the UBC zone coefficients which at one timewere only available for mainland USAand a few other loca-tions, the laterallorce coefficients are based on the ratio ofthe horizontal acceleration to gravity. This seems quite dan-gerous as the Code gives little guidance as to exactly whatthis acceleration should be. Should it for example be thepeak ground acceleration (PGA) which is the accelerationat time zero, or the acceleration appropriate to the naturalfrequency of the impulsive portion of the tank contents?These are ouite different numbers and there is often confu-sion as to what should be used. API 650 and the UBC usedtogether represent a coherent design system and there isless room for confusion and error. The more recentversionsof the UBC provide guidance for zone coefiicients for manylocations worldwide and it may be considered wise to makereference to this data.

. The Code introduces the concept of the Operating BasisEarthquake (OBE) and the Safe Shutdown Earthquake(SSE). For the OBE it suggests design seismic loads with a'10% probability of being exceeded in the structure's life-time. ln this event the allowable stresses should not be ex-ceeded. Forthe SSE it suggests a seismic design load witha 1% probability of being exceeded in the structure's life-time. In this event the ultimate strength should not be ex-ceeded. This all seems a little loose. The lifetime of thestructure is controversial and the means of determining andapplying the allowable stresses is not made very clear

15.4 The prEN 14015 approachThe new Euronorm for ambient tanks (Reference 15.17\ is cutrently in draft form. lt is anticipated that the industry commentswill be incorporated into this document during 2004.

Annex G of this document is entitled Recomrrendations forseismic provisions for storage tanks.

It is almost identical to the Aooendix G of BS 2654.

As for BS 2654 the requirement is for a specific gravity of thetank contents to be taken as 1.00 for the seismic calculations.This may well be an oversight as one of the differences be-tween this document and BS 2654 is that the requirement for aminimum product liquid specific gravity of 1 .00 to be used in allcases for the tank shell design has been removed. lt wouldseem inconsistent to design the tank shell course thicknesses


for the actual specific gravity of the stored liquid which couldwell be 0.8 or lower, whilst performing the seismic calculationsfor the same tank using the higher value of 1.00.

15.5 References15.1 Oil Storage Tanks, Alaska EaLthquake of 1964, The

Prince William Sound Alaska Eafthquake of 1964, volume ll-A, US department of Commerce, Coast andGeodetic Survey, 1 967, J.E.Rinnie.

15.2 Behaviour of Liquid Storage Tanks, The Great AlaskaEafthquake of 1964, R.D.Hanson, Engineering, Na-tional Academy of Sciences, Washington, 1973.

15.3 Damage of Storage Tanks, Engineering Features oftheSan Fernando Eafthquake, February gth, 1971,PC.Jennings, Earthquake Engineering Research Lab-oratory Report 71-02 Cal. Tech. June 1971.

15.4 The Lima Eafthquake of October 3rd, 1975. Damagedistributlon, R.Huisid, A.F.Espinosa and J.de las Casas,Bulletin of the Seismological Society of America, Vol-ume 67, no. 5, pp 1441-1472, October 1977.

15.5 Nuclear Reactors and Eafthquakes, by Lockheed Air-craft Corporation and Holmes and Narver lnc.,Chapter6 and Appendix E ERDA, TID 7024 August 1963.

1 5.6 Basisof Deslgn Provisions for Welded Steel Oil StorageIanks, by R.S.Wozniakand W.W..Mitchell, presented atthe Session on Advances in Storage Tank Design, APIRefining,43rd midyear meeting, Toronto, May 1978.

15.7 API STANDARD 650: Welded Steel Tanks for Oil Stor-age, The American Petroleum Institute, Tenth Edition,November 1998 plus Addendum 1, March 2000.

15.8 API STANDARD 620: Design and Construction ofLarge, Welded, Low-Pressure Storage lanks, TheAmerican Petroleum Institute. Tenth Edition, Februarv2002.

15.9 BS 2654:1989: British Standard for the Manufacture ofverlical steel welded non-refrigerated storage tankswith butt-welded shells for the petroleum industry, BSILOnOOn.

15.10 Above Ground Storage larks, Philip E.lvlyers,N,4ccraw-Hill, ISBN 0 07 044272 X.

15.11 2000 Uniforn Building Code, the International Confer-ence of Building Officials, Whittier, California, ISBN1 884590 94 2.

15.12 Earthquake Pressures on Fluid Containers, byG.W.Housner, A Report on Research Conducted underContract with the Office of Naval Research, CaliforniaInstitute of Technology, Pasedena, Earthquake Re-search Laboratory August 1954.

15.13 AlSl E-1, Volume ll, Patl Vll, Anchor bolt chairs.

15.14 Royal Aeronautical Society Structural Data Sheet, No04.01.01, (latet published by the Engineering ServicesData Unit).

15.15 Seismic desbn of storage tanks, Recommendations ofa Study Group of the New Zealand National Society forEarthquake Engineering, December 1 986.

15.'16 Guide linesforthe seismic design ofoitand gas pipelinesysferng Committee on gas, liquid fuel lifelines, ASCENovember 1984, ISBN O 87 262424 5.

15.17 prEN 14015 - 1:2000, specification for the design andmanufacture of site built veftical, cylindrical, flat-bot-tomed, above ground, welded metallictanksforthe stor-age of liquids at ambienttemperatures and above - Parl1 : Steel tanks.

16 Operation of ambient temperaturetanks

This Chapter provides some outline guidance on the usage, operation and maintenance ofabove ground vertical cylindrical storage ianks operating at ambient temperature.

This guidance has been taken from number ot sources, the references oJ which are included toenable the reader to obtain more detailed information on the various toDics discussed.

16.1 Tank type16.1.1 Fixed roof tanks

16.1.1.1 Fixed roof tanks with internalfloating mvers16.1.2 Floating roof tanks

16.2 Product identification

16.3 Operation of tanks16.3.1 Filling rates

16.3.2 Prevention of overfilling'16.3.2.1 Procedures'16.3.2.2 Communication

16.3.2.3 Tank gauging and sampling

16.3.2.4 Internal floating covers

16.3.2.5 Mixing of products

16-3.2.6 Sloos tanksI 6.3.2.7 Rundown temoeratures

16.4 The operation of fixed roof tanks16.4.1 Fixed roof ianks with intemal floatino covers'16.4.2 Tank conosion'16.4.3 Hazardous atmosDheres

16.5 The operation of floating roof tanks'16.5.1 Roof type

16.5.2 Pontoons

16.5.3 Tilting roof16.5.4 Mtxers'16.5.5 Access to floating roof'16.5.6 Venting

16.5.7 Managing leg supports'l 6.5.8 Static electricity control

16.5.9 Foam dams

,16.5.10 Floating roof seals

16.5.10.1 Vapour saving'16.5.10.2 VaDour loss

'16.5.1 1 Effects of roof type on drainage'16.5.12 Overflow drains

16.5.13 Collection sump details

16.5.14 Roof drain plug

16.6 Static electriclty16.6.1 Precautions to minimise or avoid static charges'16.6.2 Earthing and bonding

16.7 Heated storage

16.8 Tank and bund drainage16.8.1 Tank drainage

16.8.2 Bund drainage

16.9 Tank maintenance'l 6.9. 1 Permit-to-work systems

STORAOE TANKS & EQUIPMENT 275

16 Operation of ambient temperature tanks

16.9.2 Notice of issue of a permit

16.9.3 Working in tanks

16.9.4 Work on equipment in operation

16.10 Personnel and equipment requirements

16.1 1 Maintenance16.'11.1 lsolation

16.'l '1.2 Entry to tanks16.'1 '1.3 Gas-freeing

'l6.12 Tank cleaning16.12.1 Tanks which contain, or have contained leaded products

16.13 Tank inspection

16.14 Operational malfunctions

16.15 Further guidance

16.16 References


16.1 Tank typeThe type and nature ofthe product to be stored are the most im-portant criteria in selecting the type oftank to use i.e. fixed roof,floating roof or fixed roof with an internal floating coverFor hydrocarbon liquids, two lnstitute of petroleum oublica-tions, Refererces 16.1 and 16.2 and NFPA 30, Reference76.3, use systems of classification based on the closed flashpoint of the individual products to determine appropriate re-quirements. These classificatjon systems are different and it isimportant to define which applies when considering, for exam-ple, a Class l, ll or lll product.

16.1.1 Fixed roof tanks

Fixed roof tanks are generally used in refineries and storageterminals where the product stored does not readilV vaporise atthe ambient or stored temperature conditions. Thus ihey areused for Class lll and unclassified products, commonlv forClass ll (1) and rarely for Class I and ll (2). The size of theiankand the flash point of the product will atso influence the choiceof tank. These tanks are operated with a vapour space abovethe liquid.

Depending on the products to be stored, flxed rooftanks can bedesigned for storage at atmospheric pressure in which casethey are equipped with open vents. Alternatively, for Class I andll products, they can be designed for pressures up to a maxi-mum of56 mbarto tank Code BS 2654. Higher pressure tanksare permitted underdesign rules to tank Codes Apl 650 Appen-dix F, API 620 and European Code prEN 14015-1.

These pressurised tanks are vented bythe use ofroof-mountedpressure and vacuum valves. (See Chapter 8.)

Weak (orfrangible), shell-to-roof joints can, under certain con-ditions be incorporated to give structural protection to the tankin the event of an unexpected excessive build up of internalpressure. (See Chapter 3, Section 3.8).

Fixed roof tanks which are built stricfly to the requirements ofBS 2654 and API 650 are considered to be capable of with-standing an internal vacuum of 6 mbar and 2y2 mbat rcspec-tively, without the need to prove this by design. prEN .14015 willallow a vacuum rating of up to 20 mbar for the category 'veryhigh pressure tanks" (up to 500 mbar pressure) but there will bea need to ensure by design that the shell, roof and floor are ca-pable of withstanding the imposed loads due to this hjghervacuum.

16.1.1.1 Fixed roof tanks with internal floating coversSuch tanks are used, for example, where:

. Snow loading on a floating roof may be a problem.

. Contamination by rainwaterof a product stored in a floatingroof tank is unacceDtable.

. There is an environmental or vapour loss problem with fixedroof tanks.

. Contact of the stored product with air should be avoided.

Venting of these tanks is provided by means of large openingsaround the periphery of the roof and a centre open vent. Theperipheral openings are fitted with weather cowls and birdscreens with a mesh not lessthan 6mm square. The large ven!ing area so provided assists in reducing the vapour concentra_tion in the space between the fixed roofand the internalfloatinqcover to below the lower flammabitity limit.

16.1,2 Floating roof tanks

Floating roof tanks are generally used for Class I and Class llproducts to minimise product loss due to evaporation and for

16 Operation of ambient temperature tanks

safety and environmental reasons. There is a preference forfloating roof over fixed roof tanks as the size of the tank in-creases, as the vapour pressure of the stored product in-creases and when the flash point is below the storagetemperature.

The roofconsists of an arrangement of buoyancy pontoons andfloats on the stored product. lt is sealed against the shell ofthetank by a specially designed seal arrangement, (see Chapter 6,Section 6.5.3). The roof is provided with support legs which canbe adjusted to hold it in either of two positions. The upper posj-tion should be high enough to allow access for tank cleaningand maintenance personnel and equipment. The loweroperat-ing position should keep the roofjust above the inlet and outletnozzles, the drain lines, heatingcoils, side entry mixers and anyother accessories located near the tank bottom.

The tank shell must be provided with an adequate earthing sys-tem and the roof and all fittings, such as the rolling access lad-der must be electrically bonded to the shell as a protectionagainst lightning and static electricity. Also a internal fittingssuch as gauge floats, cables and mixers must be earthed toprevent the accumulation of static electricitv as djscussed inSection '16.6 and Chapter 6, Section 6.5.2.1 .

16.2 Product identificationlvlany of the products stored in tanks are highly inflammable,others may be corrosive or hazardous to health. These prod-ucts could cause pollution of the ground, ground water, sea,rivers or the atmosphere if they are allowed to escape. Therecould be a threat to the health of the employees working in thearea of the tanks and to the general public and also a seriousrisk of explosion or fire.

For these reasons, tanks should be clearly marked by a meanswhich meets the requiremenis of NFpA 704, Reference 16.4, oran equally equivalent system. The marking should not be ap_plied directly to the tank but should be located where it can bereadily seen, such as on the shoulderofan access-way or walk-way to the tank or tanks or on the piping outside the bundedarea. lf more than one tank is involved, the markings should beIocated so that the product of each individual tank can be easilvidentified.

16.3 Operation of tanks

16.3.1 Filling rates

Tankfilling ratesshould be limited to minimise the aeneration ofstatic electricity in the product. Free water dropleis or anothersecond phasewillhavean effect by electrostatic charging oftheproduct. These materials may be introduced either with the in_going product, or by disturbing tank bottoms during filling. Thus,it is recommended that anyfree water be regularly drained fromthe tanks. When product is pumped into a tank which may havea flammable atmosphere, a residence time of at least 30 min_utes should be allowed after the pumping has stopped beforemanually dipping or sampling.

Splash filling of hydrocarbons may result in the production of aflammable atmosphere (vapour or mist) inside the tank. Tominimise the risk ofelectrical discharge, the filling velocity in thepipeline should be restricted to 1 m/sec. (even with a dry prod_uct) until the outlet of the fill line in the tank is covered to a mini_mum depth of 0.5 m for fixed roof tanks or until any floating roofor internal cover is floating when the rate mav be increased.The maximum rate, which is governed by the fiiction losses inthe pipework, is in the region of 7 m/sec.

Electrostatic charges can be generated when a svnthetic taoeor cord (which may be used during dipping or sampling) is al-


;is

16 Operction of ambient tempeature tanks

lowed to run rapidlythrough an operator's gloved hand. In viewofthis, only naturalfibre tapes and cords should be used. Tanks

should only be manually dipped while receiving product if theproduct is of high conductivity i.e.above 50 picosiemens/metre(ps/m). Forfurther information regarding static electricity, referto Section 16.6.

16.3.2 Prevention of overlilling

16.3.2.1 Procedures

Clear formal written procedures should be established for thereceipt of product into a tank installation. These will vary in ac-cordance with the method of receipt employed i.e. cross coun-try pipeline, marine, rail or road. These will also depend uponthe quantities and grades of productto be delivered, the rates ofdelivery the numbers and capacities of the tanks to which deliv-eries are to be made, and the method of controlling the opera-tion ofthe inlet valves to the tanks. The procedures for change-over oftank and product grade, in addition to avoiding the risk ofoverfilling, should ensure segregation of grades and avoid riskof contamination.

1 6.3.2.2 Communication

There should be an efficient system of communication estab-lished between all personnel concerned in the operations, in or-der that the procedures referred to above are properly carriedout, and so that immediate action can be taken in the event ofan emergency.

16.3.2.3 Tank gauging and sampling

A reference depth should be clearly marked near dip hatcheswhich are used for gauging.

Dip hatchesfor manualgauging oftanks storing Class lor llpe-troleum products should be opened as infrequently as possible,

consistentwith obtaining tank gaugings for control of inventoryand tank filling.

Dip hatches should be properly closed when not in use lfatankis fitted with more than one dip hatch, only one should be

opened at a time.

lManual gauging should not be carried out when atmosphericconditions are liable to cause static or other hazard to person-

nel engaged in operations, e.g. an electric storm, hail, flying

sand.

No manual gauging or sampling should take place while tankfilling operations are proceeding, or for 30 minutes afierstopprng.

lf any object is accidentally dropped into a tank it should be re-ported immediately.

Floating roof tanks should be gauged from a gauging well, the

hatch of which is at the top ofthe access stairway, thus avoidingthe necessity of descending on to the roof.

Automatic gauging equipment should be checked against man-

ual dips at periodic intervals.

16.3.2.4 Internal floating covers

Increasing emphasis is being placed on reducing evaporationlosses byinstalling internal floating covers forlight hydrocarbonproducts stored in fixed roof tanks. Polyurethane (which is oflow conductivity) is often included as the principal material ofconstruction for some types of covers. lt is essential in thesecases that all metal attachments fitted to the cover are electri-

cally-bonded to the tank shell by a flexible bond to avoid thepossibility of a discharge from the cover to the earthed tank

shell. To prevent the build up of a charge on the polyurethane

cove( the resistance to earth at any point on the cover shouldnot exceed 103 ohms.


16.3.2.5 Mixing of products

Sudden mixing of products ofdifferent vapour pressures at dif-ferent temperatures can cause the rapid development ofvapour, or foaming in the tank. This can occur when:

. Stratified layers ofthese products are disturbed bythe useof a heating coil of the breakdown of an emulsion.

e When hot product is added to a tank containing a highvapour pressure prooucl.

. When a high vapour pressure product enters a hot tank.

16.3.2.6 sloos tanks

Heating coils in operation in slops tanks should always be com-pletely covered by the product. (Refer to Section 16.7.) Addi-tionally, water should be regularly drained from slops tanks.

Where product is discharged into a slops tank from a process

vessel under gas pressure, precautions should be taken to en-surethat, in an emergency, gas cannotbe released tothe atmo-sphere in large quantities via the tank.

16.3.2.7 Rundown temperatures

Rundown temperatures should be controlled to ensure thatproducts are delivered to tanks in a condition which will notcause a hazard due to the development of vapour or afroth-over. In the case offloating rooftanks and flxed rooftankswith internal floating covers, a check should be made to con-firm that the roof seal can withstand the rundown temperature.

16.4 The operation of fixed roof tanksThere are not many moving parts on a fixed rooftank butthe foflowing items should be periodically checked for serviceability :

. Pressure and vacuum valves

The weight pallets should be examined for corrosion andthat they move freely within the valve. For spring loadedvalves the action of the spring should be checked.

Check that the mesh screen is clean and not blocked withdebris.

. Free vents

Check that the mesh screen is clean and not blocked withdebris.

. Flame arrestors

Check that the tube bank is clear and ensure that there areno blocked passages.

. DiP hatches

Check that the hinge and the screwdown closure (when flt-ted) operate freely.

Check that the seal (when fitted) is not damaged.

. Emergency vents

Check that the cover opens easily and that the seal and theseating is not damaged.

. Float type level indicators

Check via the roof inspection cover thatthe float guide wires

are intact and that there are no kinks in the gauge operatingtape.

Check that the gauge tape is operating correctly by actuat-ing the float "lift and drop" mechanism on the gauge head

Check that the gauge reading window is not misted over

Check for corrosion or damage to the tape pipework'

sheaves and housings.

. Foam boxes

Check for corrosion and ensure that the bursting disc is in-

tact.

16.4.1 Fixed root tanks with internal floating covers

Additionally for these tanks, the internal cover (of the contacttype) when in the top position, should be visually inspected forleaks across its surface from a suitable manhole or inspectionhatch in the fixed roof.

Also it should be ensured that the anti-static cables and/orshunts on the seal are intact.

In day-to-day operations the lowering ofthe floating cover on toits supports should be avoided. lfthis does become necessarythen the filling rate should be reduced until the cover hasrefloated.

16.4.2 Tank corrosion

The tank itself should be checked for corrosion especially at thejoint between the shell and floor and the floor outstand beyondthe shell. Also the roof-to-shelljoint is another vulnerable area.Bracketed connections to the shell and roof and access stair-case connections also attract corrosion.

Thermally insulated tanks should receive special attention andsections ofthe cladding and insulation should be removed peri-odically to allow inspection of the underlying steel especially atdiscontinuities in the cladding i.e joints between claddingsheets, roof-to-shell joints, closures at wind girders, closuresaround nozzles and manholes and atthe base ofthe tank. lnsu-lation should ideally be stopped short of the floor plating by atleast 200 mm to prevent moisture being drawn up into theinsulation material by wick action.

1 6.4.3 Hazardous atmospheres

Access to the roofofa tank should be restdcted ifa toxic risk ex-ists (e.9. HrS, benzene), in which strict safety prccautions ap-propriate to the hazard, including the use of breathing appara-tus, should be adopted and warning notices posted at theaccess points to the tank.

16.5 The operation of floating roof tanksFloating roofs should be examined frequenfly to ensure thatthey are functioning effectively. lnadequate drainage of rainwa-ter, malfunctioning of roof ladders, sludge accumulation, iceformation, snow or perforated pontoons, can all result in can!ing ofthe roof. This can lead to jamming orsinking ofthe roof,with the possible generation of sparks.

Operational and maintenance procedures should cover thesepotential hazards. Instructions set out in the following Sectionswill assist in the operation of these orocedures.

16.5.1 Roof type

The general form of the floating roof will be either the doubledeck design, or the pontoon deck type.

Instructions may vary between the two roof designs and anypoints of difference will be highlighted.

16.5.2 Pontoons

Regardless ofthe roof type, the outer annulus will be divided ra-dially to form a ring of separate compartments. Each of thesepontoon compartments is fitted with an access hatch and cover.Ensure the covers are in place when access is not required.

Check each pontoon atfirst "float-off' after construction and af-

16 Aperatton at a-a e : ::- :+'2 .':. -: .

ter any outage for repairs. The compartments should be c_-i

Periodic examination of the pontoons for leakage is recc--menoeo.

16.5.3 Tilting roof

lftilting ofthe floating roof is noted, it may indicate one or moreflooded pontoons. Perform a visual check and if flooding isfound,landing ofthe roofmust be donewith greatcare, and theleakage drained ofi before landing.

Problems with the rolling ladder could be another cause of theroot tilting.

Another cause of a tilting roof, particularly at low levels near thelanding position, may be the result of accumulations of waxydeposits on the tank bottom. These sludges may build into sev-eral peaks and could cause damage to the roof if landed. lf ma-terials are stored, which could give rise to such deposits, regu-lar checks, should be made to determine the extent anddisposition of deposits. The roof support leg sleeves are oftenused as access points to dip for such deposits.

16.5.4 Mixers

Side entrymixers can cause severe vibrations in a floating roof.As a general rule, operators should avoid the use of mixers ifthe roof is within 4 m of the roof lever.

16.5.5 Access to the floating roof

Open top tanks normally have a means ofaccessto the floatingroof for a variety of purposes. The rolling ladder, with level ad-justing treads or simple rungs is the most usual means of ac-cess. These may be affected by high winds and need to bechecked periodically. Alternatively, noaccess may be provided,except by a short, fixed ladder for use when the roof is in itshighest position. Tanks without rolling ladders present a prob-lem when the roof is out of service for maintenance. A specialthrough-deck access way can sometimes solve this problem.

Rolling ladders with self-levelling treads require the treadmechanisms to be lubricated occasionally. The wheels shouldof course be seen to properly engage in the tracks and anysigns of wear on the flanges or track edges may point to offsetloads or alignment problems, Track and ladder length arematched to maximum and minimum roof height. Any attempttoalter levels, say by reducing the roof support legs length, maycause the ladder to jam if it is too near the vertical.

Where no rolling ladder isfitted, there may be a full height verti-cal ladder extending through a well in the floating roof. ln suchcases a fabric seal is usually the best that can be done to re-duce vapour losses at this source. Check the condition of thefabric and renew as required.

16.5.6 Venting

Floating roofs have to be protected against accidental damagewhen on their supports. Pumping out could pull a vacuum andfilling from empty may introduce pressure. In both cases, reliefis automatically provided by a bleeder vent, whose simple con-struction shown in Figure 16.'1.

Wjth the fitting of more effective sealing systems on the roofs, itis even more important that these vents function properly. Thevent has a gasketto prevent vapour loss when the roofis afloat.Check the gasket and replace if necessary ensuring suitabilityfor the product.

The bleeder deck vent is actuated by a leg, which contacts thetank floorjust in advance of the roof landing. Roof support legs


16 Operction of ambient tempercture tanks

Roof on supportstank being filled

Roof Iloating Roof on supportstank being emptied

Figure 16.1 Bleedet vents

usually have a low operating position and a high maintenanceposition and it is imperative that the leg and vent actuatorlengths must be checked for a match, for protection to be as-

sured.

The capacity of the vent valves is high, often around 10'000 bblper hour; but it maybewiseto check specific capacities against

current pumping rates, especially when any change of use isplanned.

A few roofs may not be fitted with emergency deck valves as

above, but carry P & V vents instead. Here the question of ca-pacity is even more pertinent and maintenance must be

assureo.

The only other venting requirement likelyto be found on a float-ing roofconcernsthe rim ventfor mechanicalseals This vent is

to guard against an unusual circumsiance; but one, which can

happen. lf a large quantity of air or gas enters the tank, say dur-

ing pigging, the valve prevents the metalshoeplates from beingpushed against the shell, jamming the roof by pressure. lt has

happened! Only mechanical seals need such a protective de-

vice.Thereshould beonevalveonsmalltanks, perhapstwoon

larger tanks.

16.5.7 Managing leg suPports

There are many different lengths of roof support on any glven

floating roof. This is becausethe floor may have been coned up

or down at the time of designing. The object however is to ob-

tain a level deck (or the desired slope in some cases) at twopossible levels - operating (low) and maintenance (high). The

iegsare simplypinned in one orotherposition (roughly760 mm

difference)

lf support legs are set in the maintenance position when the

roof is in service, the consequence may be that the roof lands

more frequently than is desirable and vapour conservation is

sacrificed. optimum operation will be achieved with the legs in

the operating position, except during tank maintenance lt is im-

portant therefore to remember to check that the bleeder deck

vent is in the corresponding pin position.

Remember also that the support legs are not designed to carry

the roof plus a water or product load Pontoons should be

checked before landing to ensure that they are dry

16.5.8 Static electricity control

Rim seals normally have a major role in safely conducting static

electricity to the shell and to earth. The increasing number of


shells with anti-corrosion coatings however means that an al-ternative method has to be used. This is more difficult than itseems for open top tanks due to wind action affecting any ca-

bling system such as is commonplace with internal decks lfshell contiact at the seal is not possible, cabled systems can be

arranged to partly follow the rolling ladder, taking precautions

against snagging on the projections offered by roof legs etc.

Check the condition and contact of rim seal shunts. Bend into

contact if required. The pitching of rim seal shunts is typically 3

m; but sometimes less, bY request.

16.5,9 Foam dams

Since fire hazards tend to be concentrated in the rim area'fire-fighting measures are similarly concentrated here. To re-

duce the amount offire-fighting foam and consequently speed

the extinction, it ls customary to limit the spread of foam by use

of afoam dam, as discussed in Chapter7' Section 7.10 Thisissimply a barrier, usually of steel running circumferentially at

around one metre from the tank shell and traditionally 12" high(300 mm). lt should be noted that there is a case for extending

ihe h"ight of the foam dam, to allow submergence of the tip ofany secondary sealfitted, by 50 to 75 mm. The foam may be de-

livered to the rim zone from overhead, via fixed foam pourers orfoam cannons from an external source

It is also oossible to use an on-deck foam soufce, which willgenerate foam on a signal and deliver it to nozzles located be-

tween the primary and secondary seals. This system obviously

has a limited capacity; howeverit has the advantages ofspeedyresponse and lowwastage since it is delivered to the seat of theproDlem.

Foam dams, which are integralwith the rim seal assembly, are

available and these are generally of a height compatible with a

secondary sealing system.

Whatever the foam dam in use, there will be mouseholes fordrainage at the bottom. These tend to fillwith debris and should

be clejned out as required. lf not keptclear, rainwatercan build

up behind the foam dam, leading to paint breakdown, corrosion

and damage, particularly to the seal mounting zone.

16.5.10 Floating roof seals

16.5.10.1 Vapour saving

Floating roof seals must of course be kept in good order The

demands of looking after the seals are not severe and the in-

vestment of a few hours say twice yearly will pay dividends

I

Specifically look at:

Mechanical seals have a polymer fabric joining the sheetmetal shoeplate to the floating roof rim. This material has to beslack to permit roof movement and therefore it tends to collectwater, corrosion products, wax etc in the loop. lf allowed to ac-cumulate, the weight of debris can pull the shoeplate off thewall, allowing vapour loss.

. With the passage of time, the rubber may deteriorate. Lookfot cnzing. The joints should be checked for tightness.Take action as necessary

. Examine the top ofthe metal shoeplates. lfcorrosion is evi-dent, the lower shoe may be corroded. Insert a piece ofwood behind the plate and look down, next to the shell.

. The mechanical seal will not be tight against the shell at allpoints; but if a location is noted where a large gap occurs, itmay indicate a problem with a pantograph hanger. Thiswould be unusual however.

. Aweathershield is a series ofoverlapped metal plates cov-ering the primary seal at an angle of about 50 degrees. Asthe name implies, it sheds off some ofthe rainwaterand cor-rosion products, protecting the primary seal. This will im-prove seal life, regardless of the primary seal type.

. A secondary seal does all that a weathershield does andmore. There are significant savings in vapour and the pri-mary seal well protected. lndeed, in the EU, tanks in motorspirit service are legally obliged to be fitted with both pri-mary and secondary seals.

Liquid-filled seals have a polymer bag containing a fluid, nor-mally kerosene. These primary seals are usually found in ser-vice with volatile products, such as gasoline, naphtha etc. Thefilling fluid may be contained within a separate tube or be held inthe scuff band itself. These seals should have weathershieldsor seconoary seats.

. Checkthe scuffband, which rubs againstthe shell, lookforsigns of deterioration. crazing etc.

. Check that the bag is containing fluid. lt should be pushingagainst the shell. lf the bag is limp, it has lost its fluid thestored liquid will be visible. In such a case, something haspunctured the bag or it has deteriorated structurally. Loosebolts can be trapped against the shell; but by far the mostfrequent destroyer of liquid-filled seals is the neglectedweathershield, which was put there to protect the seal. Thereason is that the weathershields are allowed to deteriorateto such an extent that they can be broken off when the roofis at a high level in windy conditions. The broken piece fre-quently falls into the rim space where it grinds away at therubber band until a puncture occurs and the fluid is lost.Weathershields must be kept in working order.

. To recover a punctured seal bag, provided the polymer isnot breaking down, obviously first remove the cause of thepuncture then replace the existing punctured tube, or if notube was fitted originally fit a tube in the envelope. This al-lows the repair to be made without removing the roof guidepole (this can be a problem with some liquidjilled seals,which are made as continuous rings). Refill with the fillingfluid. lt may be necessary to repair the scuff band if the hofang is extensive.

Foam-filled seals may suffer scuffing against the tank shell,especially where the material may be creased. Creasing ishowever inevitable since seals have to be made to allow forvarying rim gaps (usually plus or minus 100 mm).

. Lookforworn, ortorn, envelope material. Dependingon theenvelope material, patching may be possible, with a localwrap-around to protect the patch.

1 6 Aperaton of amtreq rcraE-a:-j:: . :

. Sagging may indicate the envelope has been penetralecand liquid has saturated the foam fllling. Complete repiac€-ment may be required.

. Look after any weathershields. They can damage the seal ifallowed to deteriorate.

Danger: With all liquid and foam filled seals think very carefullybefore doing any hot work on the tank or floating roof. lt isvery easy to forget even after cleaning and gas freeing thata large quantity of kerosene or vapour-impregnated foammay be prcsent in the tank. To avoid the risk of accidenialfire, these seals should removed before commencing anyhot work.

Warning: Take care when changing the product stored in atank, that the seal materials are compatible with the newproduct. Be especially wary of high aromatic liquids.

Secondary seals will be mainly of the compression plate type.Really there is very little to go wrong with then. There have ofcourse been a number of situations where compression plateshave turned down where there is a large increase in rim gap.e.g. at an outwards bulge in the tank shell. This problem is nowrecognised and can be safeguarded.

. Do not be concerned if the secondarysealtip is not makingcontact with the shell over the entire circumference. Majorgaps should not occur; but secondary seals act by interfer-ing with the wind action and small gaps do not negate thebenefits.

. Look out for any signs of loosening of the rim attachmentsand correct aS necessary

. Seal tip wear is unlikely to be found.

. EU legislation requires that secondary seals be fitted tofloating roofs with motor spirit contents; but says nothingabout operating the roof at a maximum level which retainsthe secondary seal within the tank shell top. Routinely tak-ing the secondary seal above the tank top is not recom-mended. lfthis is the case, it is not performing its function.

. Ensure that electrostatic grounding strips are in good condi-tion and contacting the shell. If not contacting, bend untilcontact is renewed.

Double seals are much the same as the secondary seals justdescribed. The lower, primary seal is visible ifa piece ofwood isinserted behind the secondary seal.

16.5.10.2 Vapour loss

API 2517 gives guidance on vapour losses from the roof legsleeves and the slotted guidepole etc. Support leg losses areindividually small; however there are many of them and the po-tential losses look significant afterthe major step offitting a sec-ondary seal has been taken. Leg covering socks are availableto deal with leg losses and are fitted during routine mainte-nance.

Slotted guide poles lose unexpectedly high amounts ofvapourand a number of solutions are available. However, because ofihe need to allow free movement of the roof around the guidepole and not to interfere with any internal liquid level floats, theshrouding of the guide pole presents a number of difficult prob-lems.

Geodesic dome roofs presentan opportunityto resolve manyofthe problems associated with open top floating roofs. The allaluminium roof can be fitted to existing open tanks with a mini-mum of site disruption. lsolated from the elements, the floaiingroof will no longer present painting problems, the seal losseswill reduce by up to 90% and water ingress to the productwilt beeliminated.


16 Opention of ambient tempercture tanks

16.5.11 Effects of roof type on drainage

The single deck, pontoon roof has a single layer of plating at itscentre and this plating will be below the normal liquid level ofthestored product when the roof is afloat. For any type of articu-lated pipe or hose attached to a sump in a roof of this type thenatural consequence of a leak or rupture in the conduit will bethat product will enter the conduit and flood the deck plating(particularly if the shell outlet valve is closed). Obviously, this isundesirable and as a consequence, the upper end of the drain-age conduit in pontoon roofsumps must be fitted with a suitablenon-return valve. Check this valve occasionally to see that it isfunctioning. The sump top screen should be kept clear of de-bns.

Double deck roofs have the drainage sump located below theupper deck, but above the product liquid level, and thereforethey do not require having non-return valves fitted in the sump.

When primary roof drains are closed in winter, measuresshould be taken to prevent the freezing of rainwater.

16.5.12 Overflow drains

Double deck floating roofs can permit waterto enter the storedproduct ifthe roof primary drainage system is not effective or isnot opened to remove rainwater before the water level on theroof exceeds the small upstand on the roofand spills down intothe product. These overflow drains are incorporated on doubledecks which have not been designed to carry the full designcondition waterload.

16.5.13 Collection sump details

Before rainwater is disposed of from the roof, it has to be collected at one or more points on the roof. Mainly the locus is theroofcentre. At this point there should be a sump, extending be-lowthe deck plating leveland with a screening device and coverto exclude large solids. Large roofs may have more than onesump.

Pontoon type roofs bytheir nature are flexible, and are intendedto be so. This has consequential effects on drainage, on paintcondition and indeed on the life ofthe roof. Care must be takenin the construction ofthese roofs to ensure the centre deck is asflat and even as possible; however despite balanced weldingtechniques etc, inevitably such decks will be distorted to somedegree and ponding will occur. Where distortion is severe,ponding tends to take place regularlyatthe same lowspots andpaint condition will deteriorate at these locations. Some roofsare fitted with under-deck stiffening rings in an attempt to con-trol deck distortion; though this is often not successful. Drain-age to the central sump of pontoon type roofs is not perfect andsome ponding is to be expected.

Efforts to overcome ponding have been made. Auxiliary pipinghas been connected from low spots to the central sump. Thishas to be done carefully however, since it could lead to hardspots where roof flexibility is impaired, rendering the roof un-able to cope with say tank floor settlement on landing of theroof. The interconnecting piping could also present problems.

Double deck roofs do not sufferfrom this problem to any degreesince their structure allows one or more oositive slopes to facili-tate drainage to central collection sumps.

Syphon drains have to be correctly designed and maintained.Their operation must be controlled thoughtfully, with a properunderstanding oftheir design and limitations. Changes of prod-uct, from say gasoline storage to a high density liquid, or onewhich could be affected adversely by water, e.g. MTBE, couldhave consequences which must be considered in advance ifsyphon drains are present.


Faults noted with these drains include decks flooding after aprolonged spell of dry weather. The pans may dry out and theessential leg ofwaterseal lost. The same result can occurfor adifferent reason; corrosion may perforate the water pan, allow-ing the product to flood onto the decK.

lf syphon drains are fitted, be aware oftheir limitations and per-haps consider using stainless steel units. At any rate at leastensure they are primed with water after any extended dryspeIs.

Syphon drains should be drained of water and plugged whenthe product temperature is belowfreezing, see Chapter 6, Sec-tion 6.5.8.

Rapid product movement could sweep out the waterseal and acover is often fitted to prevent this.

16.5.14 Roof drain plug

Pontoon type floating roofs carry a drain plug, nearthe roof cen-tre. This should be removed when the roofis outofservice. Anyarticulated roof drain which may be present will operate at itsminimum capacity when the roof is at low level. Opening thedrain plug ensures that the roof legs will be protected againstdamage by excessive water load whilethe roofis standing on itssuooorts.

Rememberto replace the plug before re-filling commences. At-taching a prominent streamer will trigger this action.

1 6.6 Static electricityThe generation ofstatic electricity is a surface phenomenon as-sociated with the contact and separation of dissimilar surfaces.With hydrocarbon products, the degree of charge, generationand decay is also a function of the type and concentration ofcertain trace compounds such as asphaltenes, oxidation prod-ucts, naphthenic and sulphonic acids.

The unit of conductivity normally used is picosiemens/metre,(ps/m). The siemens was formally known as the mho.

Hydrocarbon distillates with a low conductivity in the range of0.1 to 10 ps/m are strong electrostatic accumulators. Residuesand crudeoils have a much higher conductivity in the range 10"to 105 ps/m and any electrostatic charge generated is rapidlydissipated. Distilled water has a conductivity of about 103 pS im. Unless otherwise stated the hydrocarbon products referredto in this Section will be of the first group, i.e. those of low con-ductivity which are capable of accumulating electrostaticcharge.

The relaxation time ofa hydrocarbon product is that taken for itscharge to relax to 1ie of its original value and is inversely pro-portionalto its conductivity. For the above mentioned hydrocar-bon distillates the relaxation time would be in the ranqe 180 to1.8 seconds.

The generation of static electricity in itself does not present ahazard unless an electricfield is produced in a flammable atmo-sphere and that field can be discharged resulting in a spark ofsufflcient energy to cause ignition. The amount of energy re-quired depends upon the composition of the flammableatmosphere.

For further information on this subject, see Reference 16.5,which is to be superseded by a new Standard, see Reference76.6. Another most useful exposition on electrostatics is givenin Reference 16.7.

16.6.1 Precautions to minimise or avoid staticcharges

To minimise the build up of static charges when filling a tank

with Class I petroleum, or Class ll or lll petroleum static accu-mulator products, under conditions which may create a flam-mable atmosphere in the iank ullage space by vaporisation orformation of mist, refer to Section 16.3.1.

16.6.2 Earthing and bonding

Electrical continuity must be provided on a tank to ensure thatany electrostiatic charge, or lightning strike can be dissipated toearth. The following are examples of areas where electricalbonding is necessary:

. Between the tank fittings and valves, to the tank.

. Between the floating roof and the shell of the bnk.

. Between the rolling ladder, the floating roof and the shell ofthe tank.

. Between the floating roof seal and the shell of the bnk.

. Between an internalfloating cover and the hnk.

. Between the flanged joinb of the tank service pipework.

. The tank shell must also be adequately earthed to earthingrods or an earthing mat in or around the bund area.

Cautionary note: Where a tank has a cathodic protection sys-tem in place, then the method by which the tank is bonded andearthed must take account of the type of such a system.

16.7 Heated storageTanks storing heavyorviscous products and crudeoil maycon-tain heating coils or unit heaters through which steam, hotwateror oilflows. Heating is applied to keep the viscosity ofthe prod-uct low enough for pumping and sometimes to prevent the for-mation of wax. Suction heaters may be used if the productneeds to be heated when actually being withdrawn from thetank.

Electrical tank heating has a limited application for example, tosmall bitumen tanks. Coking ofthe bayonet type elements canbe minimised by ensuring adequate thermostatic control butcan still pose problems. The Institute of Petroleum BitumenSafety Code (Reference /6.8) should be consulted for furtheradvice on this subject.

Heating coils should be of all-welded construction. Due allow-ance must be made for expansion within the tank and for ade-quate support and location ofthe coils. The coils are usually de-signed in sections which can be isolated individually fromoutside the tank.

Consideration should be given to providing facilities such as alow levelalarm to ensurethatthe heating coilorelectric heatingelements are always covered by the stored product. This is toprevent overheating causing possible coking, froth-over orproduct deterioraljon, or at the worst, spontaneous ignition oc-curing in the vapour space.

Storage temperatures should be thermostatically controlled toensure lhat the stored product is held at a temperature whichwill not cause a hazard from vapour evolution or spontaneousignition.

lf there is a possibility of water being present in the tank, thetemperature of the tank contenb should be kept either

(a) below the boiling point of water, so that water bottomswill not flash to steam with a consequent violent eruDtionwithin the tank, or

(b) sufficiently high at alltimes to ensurewater bottoms can-not accumulate.

lfa tank has to be operated in a temperature range which fluctu-ates around the boiling point of water, measures should be

16 Operction of ambient temFeai-= =-.t

taken to exclude water from the bnk

Regular inspections should be carried out to ensure that thevents on heated tanks do not become blocked by polymerisa-tion, sublimation or condensed ofthe product or by icing, withthe possible consequent overpressurc or collapse of the tankunder vacuum.

16.8 Tank and bund drainage

16.8.1 Tank drainage

Water which is introduced into storage tanks containing hydro-carbon products which have a s.g. less than that of water, willaccumulate on the tank floor, and if this is not drained off it willcause corrosion ofthe tank floor plates and to the bottom part ofthe shell plating. lt is recommended that a water bottom shouldnot be mainiained in a storage tank as general policy; the onlyexception being in the case where a tank bottom is leaking, or issuspected of leaking. Water may then be introduced on the ba-sis that the seepage of water into the ground is preferable,rather than the stored Droduct.

Watermaybe introduced into atankduring a discharge of prod-uct from a ship, or pipeline which has been cleared by a waterplug, a practice which is not recommended. This water shouldbe removed as soon as Dossible after the tank contents havesettled. To ensure thatthe tank drain is closed off as soon as allthe water has been removed, a operator should be on stand-byat the drain point, unless an automatic waterdraw-off device isfitted to the drain line. On completion ofthe operation, the waterdraw-ofi valve should be closed and locked and the waterdraw-off connection blanked or otheMise secured.

Water drawn from tanks should be passed through an intercep-tor before passing to any external drainage system.

16.8.2 Bund drainage

The probability of a major leak from a well designed and main-tained storage terminal is low' particulady if the tanks areequipped with an overspill protection system. However, theconsequences of a spillage of flammable liquid are potentiallycatastrophic. Therefore measures to contiain spillages fromstorage tanks are essential.

Bunding is the method used to contain a liquid which has spilledor leaked from a tank, lt is recommended that bunding is pro-vided for all flammable liquids with a flashpoint of 55'C or be-low and for products which are stored at temperatures abovetheir flashpoints. Bunding for stored products with a flashpointof 32'C and below is required by HSE regulations, (see Refer-ence 16.9).

The purpose of bunding is to:

. Prevent the flammable liquid or vapour from reaching igni-tion sources.

. Prevent the liquid entering the drainage or water systemswhere it may spread to uncontrolled ignition sources.

. Allow the controlled recovery or treatment of the spilledprooucl.

. Minimise the surface area of the product and so reduce thesize of any fire that may occur,

. Preventthe spread of burning products which could presenta hazard to other plant or personnel both on and off the tanksite.

. Prevent contamination of land and water courses.

Rainwater, and water drained from the tanks which accumu-lates in the tank bund area may be drained from the bund by

-t.

STORAGE TANKS & EOUIPMENT 283

16 Operction of ambient temperature tanks

normal gravity drainage. In this case the area within the bundshould be isolated from any outside drainage system by an ex-ternally-sited valve, kept closed unless the bund area is beingdrained of water under controlled conditions.

Alternatively, the bund may be drained by means of a manu-ally-controlled pump, or by a syphon drain, which passes overthe top ofthe bund wall, and is primed by means ofa small man-ually-controlled pump.

Water from tank bunds should pass through a oil interceptorsystem before passing to any outside drainage system or wa-tercourse. Provision may be made for a valved by-pass roundthe interceptor, which would allow controlled flow ofuncontami-nated water in exceptional storm conditions orforthe release offire-fighting water.

16.9 Tank maintenanceHealth and safety law requires that plant and equipment ismaintained in a safe condition. Storage tanks and all associ-ated equipment, including walls and fences, should be properlymaintained. Only personnel who are suitably qualified andauthorised, and who fully understand the hazards, should carryout inspection and maintenance.

It is good practice to list the component parts of the installationon a preventive maintenance schedule, containing details ofthe scope and frequency of planned inspection and mainte-nance work. Attention should also be paid to periodic inspectionof electrical equipment and operation of isolation valves. Thereshould be regular inspection and cleaning of interceptors,bunds, vents, slop tanks, loading and unloading facilities, andany buildings where flammable vapour may be present.Fire-fighting equipment should be regularly maintained and,where appropriate, tested.

A competent person should carry out examination of tanks,pipe work and fittings. This could be a specialist inspection en-gineer employed by an insurance company or an employeewith the appropriate qualifications and experience. A writtenscheme of examination should be agreed between the user andthe competent person, to include the scope and frequency ofthorough examination. Intervals between internal examinationsshould bedetermined usinga risk assessment approach basedon tank service, maintenance history and known corrosionrates. Intermediate external examinations should also be car-ried out on above ground tanks. Records should be kept of allexaminations, tests, modifications and major maintenance.Schemes ofexamination should be in writing and should be re-viewed regularly. Hoses normally need to be examined andpressure-tested at least annually, and visually inspected onevery day they are used.

1 6.9.1 Permit-to-work systems

Many accidents have occurred while storage installations werebeing maintained, modified or demolished. The main cause isthe introduction ofa source of ignition, such as a cutting torch oran unprotected light, to pieces of equipment where flammablevapours remarn.

It is essential that any work carried out on equipment, whichmay contain a flammable liquid, or vapour is covered by a per-mit-work or similar system of authorisation. Permit proceduresare described fully in Reference 16.10.

A typical permit will specify:

. the area to which the permit applies.

. the work to be done and the method to be used.

. the time limjt on the permit.

. the precautions to ensure that allflammable materials have


been removed and cannot be accidentally reintroduced.

Care should be taken to ensure that contractors and subcon-tractors are also covered bythe permit or authorisation system.

16.9.2 Notice of issue of a permit

When repairs or alterations necessitate the dismanfling on siteof important items of equipment, such as valves, pumps orpipelines, or entry into tanks or vessels, specific notice shouldbe conveyed to all concerned and due acknowledgementrecetved.

16.9.3 Working in tanks

Hot work or other hazardous work should not commence insidea tank or vessel which has stored petroleum until it has beenemptied, isolated, cleaned and gas-freed. Also, it should notcommence until it is confirmed that there is no oxygen defi-ciency and the local areas have been cleaned so that there willbe no emission of product vapour on application of heat.

lf repair work involving hot work from the inside of the tank isnecessary on welded seams or plates ofabove ground verticaltanks, holes should be carefully drilled, under cold work condi-tions and gas tests should be carried out. This will ensure thatproduct or gas is not trapped between the tank plates and tanksurround or foundation, before the hot work is allowed toproceed.

16.9.4 Work on equipment in operation

Repairs or alterations to plant and equipment in operationshould not be permitted except for non-hazardous cold workcarried out under carefully controlled conditions, e.g. repairs oralterations to floating suctions, pressure and vacuum vents,float gauges, etc. Repairs or alterations should not be under-taken when tanks or vessels are being filled or emptied.

16.10 Personnel and equipment require-ments

Persons who are to carry outwork of maintenance or construc-tion in installations or depots, which are in operation, or whichare storing petroleum products, should be fully acquainted withall relevant safety regulations.

Work of inspection, maintenance or extensions shouldbe-planned and progressed by experienced and responsiblestaff, who should ensure that all persons engaged in the workobserve all relevant precautions.

When maintenance or extensions are being undertaken, con-tractors'or casual labour is frequently employed. These per-sons may not befamiliarwith the normal precautions adopted inpremises storing petroleum, and the necessary precautions tobe taken should be confirmed before commencement of thework. When such labour is employed there should be strict su-pervision to ensure that all relevant precautions are observed.

When mobile equipment, which is to be used for carrying outwork of maintenance or extensions, is temporarily stationed in ahazardous area, it should be ofsuch construction that it is not li-able to be a source of ignition and to cause a fire.

Contractors' equipment should not be allowed to be broughtinto use without written authority of the installation or depotmanager or his authorized representative.

16.11 Maintenance

16.11.1 lsolation

It ls essential, before any work is undertaken in a tank, that it isisolated. All lines and connections to the tank should be discon-nected or blinded off and a blinds list should be made out. ltshould be ensured that the tank is structurallv and mechanicallvsafe for the proposed work and that it is isolated from ailsources of motive power and electrical supply.

16.11.2 Entry to tanks

Tanks, which have not previously been gas-freed or tested forsufficient oxygen should not normally be entered for evennon-hazardous work or inspection even when breathing appa-ratus is worn. When entry under such conditions has to bemade for exceptional reasons, special authorization should begiven and the authorizing permit should set down the safe-guards to be taken, which include the wearing of breathing ap-paratus, safety harness and lifeline and the posting of compe-tent persons outside the tank to ensure that rescue can beundertaken if necessary

It must be recognized that in these circumsiances the vapourconcentration within the tank may fall to within the flammablelimits giving a further hazardous situation.

16.11.3 Gas-ff,eeing

The gas-freeing of a tank, which has contained volatile hvdro-carbons, has two main ourposes:

1) to eliminate conditions which might lead to a fire or explo-ston.

2) to eliminate possible toxic ef{ects and asphyxiation of per-sonnel entering the tank.

In all cases appropriate tests should be carried out and workprogressed under the direct supervision of a competent per-son. Gas-freeing and cleaning of tanks are interrelated and nowork of either gas-freeing or cleaning should start without anoverall plan.

Precautions must be taken to protect personnelfrom asphyxia-tion and the effects oftoxic materials. After removalofthe prod-uct, the tank must be isolated and then ventilated.

Naturalventilation is slow and purging oflarge tanks bythe useof steam is not practicable because of the large condensingsurface afforded by the tank shell and hence the difficulty ofraising the temperature in the tank space sufficienfly forthe ef-fective removal of vapour. Furthermore, with steaminq there isalso a risk of static accumulation on anv insulated co;ductorsthat may be in the tank.

Ventilation by an air or flameproof electric motor-driven fan oran air or steam-operated eductor sited at a suitable manhole isrecommended for reducing the vapour concentration within atank to a low figure relatively quickly (see R eference 16.11). Allequipment used for the venting of a tank must be correcflybonded and earthed.

During the gas-freeing of a tank which has contained volatileproducts, escaping flammable vapours may cause a hazatd-ous atmosphere outside the tank. In still atmosDheric condi-tions these vapours may travel beyond the limits of the usualsafety distances. For this reason, rapid dispersal of vapour isdesirable.

With ventilation by an air orflameproof electric motor, combus-tible gas indicator tests show that its atmosphere contains aminimum of hydrocarbon vapour but, in any case, below25% ofthe lower flammable limit. The tank may then be entered for in-

16 Operation ot ambient temperatute tanks

spection by personnel, wearing breathing apparatus, followedby de-sludging, cleaning or similar operations. There should beexternalsurveillance to ensure thatrescue can be undertaken ifnecessary

At this stage any activity inside the tank should not be capableof providing a source of ignition. Ventilation of the tank and gastesting of the almosphere inside should be continued whilstde-sludging cleaning, etc. are in prcgress, since evolution ofgas vapours can be expected from the oil-wetted surfaces ofthe tank and from the disturbance of sludge and scale. lf gastests give a reading above 25% of the lower flammable limit,work should be suspended. lt should not be resumed until ven-tilation has sufficiently reduced the vapour concentration tomake the tank safe for re-entry Lighting equipment should bewithout cables (battery operated or compressed air driven tur-bine generator type) as approved for Zone I areas or for tankcleaning purposes by the national authority responsible forsuch certiflcation.

Ventilation should be continued until the tank is gas free. Atankis considered gas free when, afterthe removalofall sludge andloose scale, combustible gas indicator readings taken at fiveminute intervals overa 30 minute period from eductor exhaust,tank manholes, roofgauging hatches and from several internallocations, particularly areas liable to vapour concentrations(e.9. sumps, pipeline entries, pontoons, around{ube seals, tu-bular legs, water draw-ofi facilities) are consisten y less than1% of the lower flammable limit.

16.12 Tank cleaningCleaning oftanks which are not completely gas free should becarried out only under the direct supervision of a comDetentperson. Such tanks should be kept as well ventjlated as possi-ble and all personnel entering should be equipped with suitablebreathing apparatus and protective clothing appropriate to thenature of the product and condition of the bnk.These factors will also determine the time for which personnelmay be permitted to remain inside, but it is recommended thatin no case should this exceed 1.5 hours with a half-hour breakbefore re-entry The time in the tank must not exceed the safelimit for the breathing apparatus being used. While work is inprogress there should be a competent person stationed outsidethe tank to ensure that rescue can be undertaken ifnecessaryln the cleaning of floating roof tanks, it is important to ensurethat pockets of flammable material are not trapDed in the tubu-lar roof support legs owing to blockage ofdrain holes by scale orsruoge.

For Class lll petroleum tanks, special precautions are not nec-essary provided that they are suitably isolated and adequatelyventilated during cleaning operations, and that personnelwearsuitable protective clothing. However, ifhotwork is to be under-taken, appropriate precautions should be observed.

During subsequentwork, tanks, which have been cleaned anddeclared free of gas, should be checked frequenfly (not lessthan twice daily) by an accurately calibrated combustible gasindicator. Effective ventilation should be maintained throuqhoutthe tank whilst work is in progress.

16.12.1 Tanks which contain, or have contained leadedproducts

Tanks, which have at any time contained leaded products, re_quire special precautions. and the recommendations of themanufacturers ofthe lead compound regarding procedure andsafety regulations should be rigidly ooserved.

The accumulation ofrust, scaleand sludgefrom a cleaning op-eration should be handled only in a wet state both in the tankand after removal. Disposal should be in accordance with therecommendations of the manufacturers of the lead comoound.


6ROUP sEivtcE@rDmofls

IIISPECT('{ FREQUETCY

EXIERIIALROUNilEVISUAL

lmootFl

EXTERIIAIo.t lhd vlrull

lmludlnE ulh&nlclhiclmaar

.to! .nd ?ool

NTEN A!O.ldl.d vl3u.l

Inclrdhg ulraaoructhhtt|..

bolb|n |rd.irI

CULATE OOIE fr€e bdor'E c a

Sbpr, cdfldte or agEssslv€cn6fi-rcab, ias !vd€r. bfi6(not iolernaly p|ol€d€d)

3 I

Sl|L ai Gllrp I axcaplslr.r! oaidt prct€dod arh Aro.rdh C.5.3

7

R€fig€tglld Sto.aga S6e App*dr F

Crud€ Ol 5 7 8 8 10

Fid oil, gr dl t !o oil,dl€ld ol. ..u.8 ra(b. ln€rloa rF|l.alE|tr*a chcnhal!,alrimrn lhuil

3 5 a to 12 1A m

Jsl Al fiilly i{.maly!|eded)

l0 't0 15 t5 5 I

Liolt prodocb. t€.osim.gsolkE, c..d(.d rtdllh..,lrbdod $/atar (nol i srEly

'loleded)

5 7 t0 12

7 H€aled and i|3{latad LrJG,

l'loi!: Ext mal UT maaclr+me.|b or ysrorrd b.do ol.h.x d et aalscad locdbtarourd rool d6rlot!6.y.

3 3 6

Climat cod63:A ' Wann ..d humid, o.g rqical rnd subtlphsl.lel8g = Temo€talo dlmale wftn f€quenl taln and |Nlnd

C ' Watrn and &v. e-s. d€6€n kEalixls

Commenta'Th€ insD€dbn freauencies indicated above ar€ for guidan@ only. After eadl detailed

extomal or intemql ingpedioo, tho Tenk Inlegrity Aeeeesor (nA) should detennlrE lhedate for the n€xi hspedion. This datr should ensure hat the rsjeclion limit€ EtatEd

ekewt|elt in thb publicalion arc nol sxceed€d.

lf lhe insDec{ion resultts indk:ale a more €pid deteriofation due to corosion or settement,other sinilar tanks may need to be inspeded earliet. On the other hand, if the inspeclionrssults arg tavourabl€. an extension of the lr6peclion InteNal may be conBldered'

Figure 16.2 Inspection frequencies

Fram EEMUA publication 159

and preferably either by means ofincineration under controlled Reference should be made to the two principal Codes which

conditions or by chemical treatment. deal with this subject and are given in References 16.12 and16.13

Both Codes give clear advice and recommendations for the in-spection and maintenance of storage tanks and guidance on

the frequency for inspections is also given. Figure 16.2 gives

details of inspection frequencies in EEMUA publication 159.

16 ODe@tion of ambient temDerature tanks

Care should be taken to avoid:

. skin contiact with products containing lead alkyls.

. inhalation of vapour from products containing lead alkyls.

. ingestion through contactofhands and finger nails with leadsruoge.

Any tank which contains or has contained leaded products, in-

cluding leaded slops, should have notices permanently fixed

adjacent to all manholes reading:.THIS TANK CONTAINS OR HAS CONTAINED LEADEDPRODUCT. IT MUST NOT BE ENTERED WITHOUTCOMPLYING WITH THE PRESCRIBED REGULATIONS'

This notice should only be removed when the appropriate de-

contamination procedures, as required by the lead alkyl manu-

facturers, have been implemented.

Total segregation of leaded slops is also recommended

16.13 Tank inspectionEvery tank installation should have an inspection regime in

place to ensure that the integrity of the tanks is maintainedTanks suffer from internaland externalcorrosion and it is impor-

tant to monitor this to ensure that a maintenance programme

can be put in place to rectify any serious corrosion problems


Figure 16.3 Intemalexplosion due lo hot work being performed

Figu re 'l 6.5 Effects of over-pressurisation

1 6.14 Operational malfunctionsFigures 16.3 to 16,6 demonstrate whatcan happen when a tankis mistreated.

Figure 16.3 showsthe result ofan explosion inside a tank due tohot work being performed on the tankwithoutthe correct safetvprecautions being observed.

Severe corrosion of the roof plates, found after the cladding andthermal insulation was removed from the roof of a tank. isshown in Figure 16.4.

Figure 16.5 shows the result of a severe over-Dressurisation.the roof-to-shell joint has ruptured and a roofstructure brackethas punctured the shell.

The result ofan internalvacuum condition, is illustrated in Fig-ure 16.6. This 29 m diameter tank was beinq steam cleaned.The operatives had covered up the roof vendto keep the heatin the tank and when they left the tank for a tea-break, thevclosed the shell manhole - again, to keep the heat in the tank,

Of course, during the tea-break, the iank atmosDhere cooleddown, the residualsteam condensed and the resultino vacuumsucked the tank in!

16.15 Further guidanceThe following publications provide useful guidance on iank op-eralton.

16 uperation of ambient temperature tanks

Figu re l 6.6 Resull of intehal vacuum condition

The Storage of Flammable Liquids in lanks, The Health &Safety Executive, HSE 176.

Tank maintenance, The Health & Safety Executive, HSE 176,page 18, Section 9.

Health and Safety at Work etc. Act 1974, Ch Z7 HMSO 1974lsBN 010 543774 3.

Aguide to the Health and Safety at Work etc. Act 1974, L'l HSEBooks 1992 ISBN 071 760441 l.The Health and Safety (Enforcing Authoity) Regulations 1989,st '1989/1903, HMSO 1989, |SBN 011 097903 6.

The Provision and Use of Work Equipment Regulations 1992,st 1992t2932, HMSO 1992, |SBN 01.1 025849 5.

16.16 References16.1 Refining Safety Code Paft 3,lnstitute of petroleum.

16.2 European Model Code of Safe practice in the Handlinaof Petroleum Products - Paft : Design. Layout anidConstruction, Institute of Petroleum.

16.3 Flammable and Combustible Liqui?s Code. NationalFire Protection Association. NFpA 30.

16.4 Sfandard System for the ldentification of Hazards ofMaterial for Emergency Response, National Fire pro-tection Association, NFPA 704.

16.5 Code of Practice on the Control of Undesirabte StaticElectricity, The British Standards Institution, BS 5958.

16.6 Code of Practice on the Control of tJndesirable StaticElectricv The British Standards Institution No. pDcLC/TR 50404.

'16.7 The lnternational Safety Guide for Oil Tankers and Ter-m/nals, (ISGOTT), www.seamanship.co.uk.

16.8 Bitumen Safety Code, Institute of petroleum

16.9 The Highly Flammabte Liquidsand Liquefied petroleumcasses Regu/ations 7922, HSE booklet HS(G) 51.

'16.10 Guidance on permit-to-work systems in the petroleumrndustry, HSE Books 1997, |SBN 07.1 761281 3.

16.11 Marketing Safety Code, Patt 2, lnstituteof petroleum.

16.12 Users guide to the lnspection, Maintenance and Relairof Aboueground Veftical Cylindricat Sfee/ SlorageIanks, The Engineering Equipment and Nlaterials U!-ers Association (EEMUA), No. 159: 2003.

16.13 Tank lnspection, Repair, Alteration, and Reconstruc-flons, API 653 3rd Edition, December 2O0l .

Figure 16.4 Roof plate conoson


17 Low temperature storage tanks

This chapter ranges widely overthe subject ofthe storage of industriargases in riquid fonn. Thegases stored in this way are risted and the significant properties described. pressurisedsystems for the storage of these gases which can be liquefied bv the imposition of oressurealone are described and their advantages, disadvantages and'economics are discusseo.Semi-refrigerated storage is an intermediate means of itoring certain types of gases failingbetween fully pressurised ambient temperature systems and fu[y refrigeriied storige at orius-tabove atmospheric pressure. Lasfly the most popular system of fuliy refrigerated'storag'e isdescribed.

The history ofthe development of lowtemperature storage systems is interesting. ltcombinesatangled web of factors and incidents involving the increasing understanding of the lowtemperature behaviour of various metals, increasing demands for improved safdty of storagefacilities, the developments in the design codes and otherregulatory requirements, the reactionto various accidents and incidents and, particurarry for LNG and to a re;ser extent for LpG andethylene, a dramatic increase in the overall capacity of terminals and the correspondingincrease in unit tank capacities. The various factors influencing the capacities oftanks leads to abrjef review of what is currently common practice.

The development ofthe design codes, especia[y in Europe, has red to the cateqorisation of rowtemperature storage lanks into single, double and full containment. The devel6pment of thesecategories emanated from increasing awareness of safety considerations both ior the terminalsites themselves and forthe surrounding areas. These three main categories relate to aboveground vedical cyiindrical storage tanks. The various codes provide quite specific definitions ofwhat is.required for each category. The membrane type oftank which was developed in France0y technrglz and which has its origjns in the marine transport of products such as LNG isdescribed. This type of tank is applicable to above ground and in_ground storage systems.spherical designs of tanks for rand-based, fu||y-refrigerated storage of products such as LNGhave been proposed for many years but have not tet been utiriied d;spite the attraction oftactory-based construction and transport to the job site in one piece. These arso have theirte_chnicalroots again in the marine transport of LNG. A smafl numberof rowtemperature tanksofabove ground, vertical cylindricalform have been constructed with both inner and outertanksconstructed from prestressed concrete. These designs were developed by the preloadCompany in the USA,(now.kading as Cryocrete lnc.). Th;se use a nu mbei of interesting designteatures, which are described.

ln-ground ljquefied gas storage systems divide themselves into two groups. The first group isthe vertical,cylindricar tanks, usuafly incorporating a membrane typiof riner, a rigid iniuraiionsystem and a concrete caisson wall are expensive, when compared with their a-bove groundequivalents, but in certain circumstances provide attractive advantages in that their incieasedperceived safety arrows croser tank spacing, which in turn makes bet'ier use of the area of randavailable This is especiarry important for areas where expensive recraimed rand in or crose tourban areas is involved. This type of tank is usuafly confined to the storage of LNG at marineterminals.

The second type of in-ground storage ls the cavern type. unlined caverns are constructed inrock at depths where the static waterpressure from a carefully controlled ground watersystemslightlyexceedsthe pressure required to maintain the stored p;oduct (usuaiiy LpG) in riquid format ambient pressure. A number of novel liquid gas storage systems involving floaiing or gravitybased structures most usually directed at LNG are alsodescribed.

Contents:

17.1 The low temperature gases

17.2 General

17.3 Historical background

17.4 Tank sizing considerations

17,5 Storage systems and containment categories

17.6 Single containment systems

17.7 Double containment systems

17.8 Full containment systems

17.9 Membrane tanks17.9. 1 Development history'17.9.2 Detailed description of land-based membrane system


17 Low temperaturc storcge tanks

17.9.2.1 The metallic membrane

17.9.2.2 The insulation system

17.9.2.3 The outer tank

17.9.3 Comparison of above ground membrane tanks and conventional tanks

17 .9.4 fhe lined mined rock cavern initiative for future LNG storage

17.10 Spherical tanks

17.11 Concrete/concrete tanks17.11.1 History of cryogenic concrete tanks

17.'11.2 Details of concrete/concrete tanks

17.1 1.3 Arguments for and against concrete/concrete tanks

17.12 In-ground tanks17.12.1 In-ground membrane tanks

17.12.2 Cavern slorage systems

17.12.3 Ftozen ground systems

17.13 Novel solutions

17.14 References


17.1 The low temperature gasesThe gases commonly stored in liquefied form are listed in Fig-ure '17.1 together with their significant properties.

The commonly used term Liquefied Petroleum cas (LPG) re-fers to a mixture of normal butane, jso butane and propane inany proportion. In some ofthe American Codes (i.e. NFpA 58)LPG is referred to as LP-Gas. This isjust a quirk ofthese Ameri-can Codes and does not have any other significance.

Similarly the term Liquefied Natural Gas (LNG) refers to a mix-ture of methane with smaller proportions of ethane and smalleragain proportions of LPG gases. Small quantities of othergases are also found in LNG, in particular nitrogen. Themakeup of an LNG is not constant, but varies within relativelyclose limits from a single geographic source and varies morewidely between diferent geographic sources.

An example is given in Figure 17.2 which compares the differ-ent compositions of LNG from sources in Trinidad, Algeria andOman.

17.2 GeneralThe various low temperature products listed above can bestored in a number of different ways.

Products such as buiane, butadiene, propane, propylene andammonia can be mainiained in liquid form bythe application ofpressure alone. Consequently in the early days of the petro-chemical and chemical industry when these products were re-quired in relatively modest quantities, the favoured means ofstorage was by using pressure vessels. As the quanfities be-came larger, the pressure vessels became larger and becauseof the pressures required (i.e. for propane storage a designpressure of around 15 bar is required) more expensive. A typi-cal LPG storage facility using spherical vessels is shown in Fig-ure 17.3. Spherical pressure vessels commonly used fortheseproducts began to become problematic at around 5000 m3 ca-pacity. A spherical vessel for this capacity would be some 22 min diameter and for propane seryice would have a shell thick-ness of around 50 mm. This is an expensive construction andclearly other storage systems were required.

17 Low temperaturc storage tanks

Figure 17.2 Typical LNG composiiions

For a while semi-refrigerated systems were considered andmany were constructed. These eased the problems by reduc-ing the design pressures but were an unsatisfactory half wayhouse for a variety of reasons.

Eventually, partly driven by cost, safety and increasing storageunit capacity requirements, the standard solution for the stor-age of large quantities of these products was as fully refriger-ated liquids in vertical cylindrical hnks.

Initiallythese tanks followed the oil industry practices, being es-sentially oiltanks with insulation. This is perhaps a litfle simplis-tic. These new tanks had to contend with the low temperaturemetallurgical requirements together with new problems ofavoidingfrost heave in the underlying ground and coping withahigher design pressures (usually from 70 mbar upwards) thanhad been the case for ambient tanks. The first tanks were ofsin-gle containment type. As time progressed and safety require-ments increased, there was a move to superior forms of con-tainment and this subject will be discussed in later Chapters.

It would be untrue to say that pressurised storage ofthese prod-ucts has ended. In certain quarters spherjcal vessels for thestorage of modest volumes of LPG are still being constructed.However increased safety standards togetherwith the memoryof some spectacular and very public accidents make this formof storage unpopular and in many instances a prohibited prac-Itce.

To accommodate the requirement for the storage of modestquantities ofthese products, an alternative arranoement known

Figurc 17.1 Gases commonly stored in liquefied formFron prEN 14620 - 1,Iabl6 A.1

I'lame ChemlBllormula

MoI.iryeight

Bollingpoint

Latentheat otl;quid

Liquiddenslty

Gasd6nsity

Vol, ofgas

liberatedbyl msot

liquld(exP. to15"Crt1 bar)

g/mol kcarkg kdm' kdm'N-Butane CrHro 58.123 - 0,50 92.100 601.40 2.7@ 239

lso Butane CrHro 58.123 - 11.70 87.600 593.40 2.420 236

Ammonia NHg 17.030 - 33.35 327.1o..c 682.00 0.905 910

Butadiene CrHo 54.091 - 4.50 99.800 650.40 2.550 279

Propane CsHa ,14.096 - 42,05 101.750 502.00 2.423 3llPropylene QHa 42.080 . 47.72 104.600 613.90 2.365 388

Ethane qH6 30.069 - 88.6€ 116-74n 546.r19 2.054 432Efrylene czHr 28,0t{ - to3.72 115.330 567.92 2.08s 442Methan6 16.043 - |61.52 121.860 422-62 1.819 630Oxygen o, 31.SS9 - 1A2.97 ' 50.869 1,141.00 4,475 854Nitrogen N2 28.013 - 19s.80 47.459 808.61 4.614 691

Argon 39.94a - 185.86 38.409 'I,392.80 5.8s3 835

NOTE 1 Uquld petroleum gas€s (LPG) is a general tem for commercjal butiane, pFpane and thekmlxtJre in any proposition.

NOTE 2 Commercial butano is a mixture in N-Bulan6 and isobuiane with smallcontenl of propano andDenlane.

NOTE 3 Commercial pFpan€ is propane whh small content of ethane and bltane.


17 Low tempercture storage tanks

Figure 17.3 Atypical fully-pressu sed LPG storage facility

as "mounded storage" was developed. This consists essen-tially of horizontal pressure vessels installed on a bed of sandand backfllled with sand. An example ofsuch a facility ls shownin Figure 17.4. This system seems to meet with approval fromeven the most safety conscious operators.

Figurc 17.4 l,lounded storage

Coutlesv of Whessoe

A further development, most usually applied to the storage ofpropane, is "cavern storage". In this system a series of unlinedhorizontial tunnels or caverns are constructed at a depth whichdepends to some extent on the level of the local water tableThe principle of the system is that the vapour pressure of thepropane is slightly over balanced bythe ground water pressure.

Hence no propane leaks out ofthe system and modest quanti-

ties of water leak into the caverns. This water is continuouslypumped out and used to prime a series of header pipes whichensure the continuous and constant ground water pressure.

This system is again the subject of later Chapters.

Products such as ethane and ethylene cannot be maintained in

liquid form bythe application ofpressure alone, but can be suc-cessfully subjected to a combination of pressure and low tem-Derature. There are some examples of this form ofstorage butin generalthese products are stored in fully refrigerated form.An example ofethylene stored in both the fully refrigerated form

and the semi-refrigerated form is shown in Figure 17.5. The

semi refrigerated storage is in the spherical vessel which is

double walled, i.e. an aluminium alloy sphere suspended withinan outercarbon steelsphere. This solution posed some seriousdesign and construction problems.

The remaining products (LNG oxygen, nitrogen and argon) are

always stored as liquids in the fully refrigerated form.


Figure 17.5 An example ofa semi-refrigerated and fully-refrigeraled ethyleneslorage

Courtesy of whessoe

1 7.3 Historical background

The first thoughts regarding the liquefaction and transport ofgases go some way back in time. In 1915 Godfrey Cabot pro-posed the shipment of LNG by means of river barges. In 1937SirAlfred Egerton proposed thatthe British gas industry whichwas at that time based on the carbonisation ofcoal, should ex-tract a fraction ofthe methane contentfrom the coal gas, liquefyit and store itforsubsequent evaporation for peak shaving pur-

ooses.

The firstattempt to store liquefied naturalgas in bulkon a com-mercial scale was not promising.

The facility at Cleveland, ohio, was the first LNG peak shavingplant ever built. It was constructed to provide an economicaland reliable energy source during the winter months for the lo-cal industry Much of this local industry was related to WorldWar ll munitions manufacture. The LNG storage part ofthe fa-cility consisted of one vertical cylindrical tank of 4,100 m3 ca-pacity and three 2,000 m3 sphericaltanks and had been in ser-vice for some four years with no apparentoperating problems.

The tanks had been filled to their full capacity in readiness forthe iorthcoming winter when, on the afternoon of October 20'i1944, the cylindricaltank suddenlyfailed releasing all of its con-tents into the nearby streets and sewers of Cleveland. The re-sultant gas cloud ignited immediately and a fire ensued whichengulfed the nearby tanks, residences and commercial estab-lishments. After about 20 minutes, with the initial fire havingnearly died down, the spherical tank closest to the cylind cal

tank toppled over and released its contents. The 2,000m3 ofLNG immediately evaporated and ignited. In all 130 people

were killed and 225 injured. The area directly involved wasabout 2 square kilometres of which an area ofaround 120 hect-ares was completely devasbted.

Although sabotage was first suspected, a thorough investiga-tion showed that the accident was due to the low temperatureembrittlement of the 3.5% nickel steel inner tank shell. 3.5%nickel steel is now known to be susceptible to brittle fracture atLNG temperatures (around -160 "C). In addition the tianks were

situated close to a heavily used railroad and a bomb bodystamping plant. ltwas considered that excessivevibration ema-nating from the railroad engines and the stamping plant proba-

blyaccelerated the crack propagation from some smalloriginaldefect in the innertank shell. The outercarbon steeltankwouldhave cracked on contact with the emerging LNG

lll

f]

The accident was aggravated by the lack of adequate dikingaround the tanks, the ability of the LNG to enter the local sew-age system and the proximity ofthe plant to nearby residentialareas. The reason for the second release of liquid from thespherical vessel was found to be due to the fact that the sup-porting legs of the sphere were not fitted with fire protection.The prolonged exposure to the fire weakened the legs of thesphere to the pointwhen they buckled causing it to topple over.

This event cast a blight overthe storage of bulk refrigerated liq-uids and of LNG in particular, until '1958 when the bulk storageof LNG was re-examined as a part of the beginning of theworld's LNG trade. Over the intervening years a great deal ofresearch work was carried out on the low temperature behav-iour of various metals and the lessons of this sad event gaverase to serious considerations ofplant layout, fire protection andother safety related sub.iecb.

The next step in the development ofthe LNG storywas the pio-neering scheme to transport gas in liquid form from LakeCharles, Louisiana, USA, to Canvey lsland in the United King-dom. A liquefaction plant built for an eadier barge transport in-vestigation wasiaken to Lake Charleswherea 2000tonne LNGtank was constructed. An ex-liberty ship, The Normafti, wasprocured, converted into an LNG carrier of 5000m3 capacityand renamed The Methane Pioneer. Al Canvey lsland two1000 tonne capacity perlite insulated aluminjum alloy LNGtanks were constructed. On 20th February 1959 The MethanePloneerdelivered a cargo of 2020 tonnes of LNG to Canvey ls-land after a voyage from Lake Charles of22 days and 18 hoursduration.

As Dennis Rooke, whowas a crew member on that originalvoy-age and who rose to become Chairman of British Gas, re-marked, "thus was the LNG industry born". The Methane pio-neerwas to make a further seven voyages carrying a technicalteam between Lake Charles and Canvey lsland, and a greatdeal of data and experience was gained which was pivotal tothe further development of this industry This part of the historyis described in considerable detail in Refe rences 17.1 and 17.2.

Following thls pioneering work, the first commercial LNGscheme was set up. A liquefactjon plant and loading terminalwere constructed at the port of Azew in Algeria. British Gas or-dercd, The Methane Princess from Vickers Armstrong (Ship-building) Ltd at Barrow-in-Furness and The Methane progressfrom Hadand and Wolff in Belfast. The carriers were 27,000m3in capacity designed to deliver 12,000 tonnes of LNG per vov-age.

So successful was The Methane Pioneer that only modestchanges in deiail were required to be made for the larger newships. The Methane Pflncess, built in 1964, remained in serviceuntil 1997, delivering cargoes of LNG (and two of Lpc) fromArzew to Canvey lsland. Le Havre (the French equivalent ofCanvey lsland), Barcelona, Cartagena, Huelva, Fos sur lvlerand Staten lsland. The old ship sadly ended its days being cutup for scrap on an Indian beach.

The original contract was for 700,000 tonnes of LNG Der vearfor 15 years. At the receiving end of the chain, five new 4000tonne perlite insulated tanks were built to add to the two 1000tonne tanks constructed earlier. The intention was that gas inliquid form would be transported by road to regional distributioncentres where it would be regasified and delivered to consum-ers.

One such distribution centre was built at Ambergate, inDerbyshire. Unfortunately for the new LNG industry in the UK,the discovery of large amounts of oil and gas in the North Seameant that the development was not continued and directtransmission of the newly discovered gas into the NationalTransmission System at Bacton and St Fergus took its place.Despite this, theAzeW/ Canvey lsland project pioneered many


of the technical advances and served as a model for the LNGimporuexDort terminals to follow.

Two further significant accidents played their part in determin-ing the nature of future low lemperature storage facilities.

The first was the explosion in the LNG tank for Texas Eastern atStaten lsland in the IJSA. This tank was of a novel design. Thenew design evolved from original work carried out in 1958 and1959. A new research and test programme was implementedbetween 1960 and 1966. A test tank of 6.1 m in diameter and8.5 m in height was constructed and successfully tested. MostLNG tanks up to that time were of the single containment typewith aluminium alloy or 9% nickel steel inner tanks within aperlite insulated carbon steelouter tank. The Staten lsland tankhad a aluminised mylar layer in contact with the product liquid,supported by polyurethane foam thermal insulation which wasin turn supported by a thick concrete wall mounded with graveland earth. The details are shown in Figure 17.6.

The tank, which entered service in March 1970 and which wasalso for storing LNG from Algeria, was found to be sufferingfrom leakage of LNG through the mylar liner into the supportingPUF, concrete wall and the supporting earth beam. Despitethis, the tank remained in service until January 1972. It wasthen decommissioned and there followed a protractedgas-purging period. The tank was under repair when the acci-dent happened. lt is thoughtthat residual heavy ends (propane)had accumulated behind the liner either within the PUF or be-hind the concrete wall and that some of this liquid suddenlymade its way back into the tank and was ignited by variousnon-explosion proof tools being used for the repair work. Theexplosion caused the concrete roof to fall killing the 40 menworking insidethetank. This event brought to a haltalldevelop-

Figure 17.6 The Texas Eastern LNG tank at Staten tstand

?

t3

v



ments using this form of liquid containment and this is still thecase looay.

Secondly an accident occurred at the Qatar Lpc terminal. Onthe 3 April 1977 an explosion and subsequent fire totally de-stroyed the $43 million plant for the processing and storage ofliquefied propane, butane and pentane at Umm Said in Qatar.Seven people were killed and 13 injured. Apart from an emo-tive report entilled "Frozen Fire" (Reference /23), litfle hasbeen published about this event, which is strange given the tre-mendous effect that it had on the future development of lowtemperature storage systems. The protracted legal processwhich followed the accident also did little to help identifl/ thecentral cause for the failure, only that a sudden and cata-strophic failure of the shell of the propane tank occurred whichin turn led to the failure of the other tanks on the site.

Amongst the main suspects were:

BS 4741:1971(Reference 17.4) The British Standard forthe storage of liquids down to -50'C, which was in place atthat time. This Standard included a new and complexmethod of material selection together wjth a partial ratherthan full height hydrostatic test requirement.

Incorrect operation possibly involving over filling or overpressurising of the iank.

- Poor fabrication and inspection Standards

- Sabotage

Despite the failure to isolate the prime cause of the failure, anumberof major operating companies led by Shell (References17.5 and 17.6) began to give further thought to designs whichwould reduce the possibility of brittle fracture and reduce theconsequences ofa failure if it did occur. The earth berm remotefrom the Qatar propane tank had singularly failed to serve itspurpose ofcontaining the spilled liquid following the failure. TheGas Research Institute sponsored a programme of researchinto materials better able to resist initiation or propagation ofbrittle failure.

Much of this work was presented at the Brugges Conference(Reference 17.7) in 1984 and at the TWI Seminar in Newcastle(Reference ?7.8) in 1986. BS 4741 was amended shortly afrerthe Qatar accident to enhance the fracture toughness require-ments of the steel materials and to institute a full heioht hvdro-static test for the tanks.

The desire of the industry to move away from single contain-ment systems towards double and full containment was ham-pered by the lack of a suitable Standard. The existing Americanand European Standards were all based of the philosophy ofsingle containment for both LPG and the colder products suchas LNG. Consequentlythe Engineering Equipment and Materi-als Users Association (EEMUA) in London was approached toform a committee to consider rules for the full range of coniain-ment options and in 1986 oublished EEMUA Publication 147(Reference tZ9). After a suitable period of time this documentwas passed to the British Standards Institution, BSl, who in1993 published BS 777711993 (Reference 77.70). This is morefully discussed in Section 17.5.

17.4 T ank sizing considerationsForthe gaseswhich can be maintained in liquid form bythe im-position of pressure alone, the maximum size of refrigeratedtank is established by a combination of the following:

The practicalities of pressure vessel design

The relative costs of pressure versus iiquefied storage

Safety considerations

Local and international regulations


There are a few mounded LPG storage facilities with total ca-pacities ranging up to 20,000 m3 (involving a numberofsmallerpressure vessels) and some spherical vessels with individualcapacities up to 4,000-5,000 m3, but these are comparativelyunusual. Fully refrigerated storage tanks for these productscome into their own (depending upon local circumstances)around 5000 m3 and become increasinglythe onlyviable optionas the volume to be stored increases. The maximum size of re-frigerated tank for these products is established by a combina-tion of the following:

The maximum lower shell course thickness Dermitted bvthe design Codes

Seismic design considerations

The relative economics of larger storage units

- Operating flexibility consideration (i.e. two smaller tanksmay be preferred to a single larger tank for reasons con-nected with malfunction or majntenance etc.)

Safety considerations

Local and international regulations

Current practice suggests that 80,000 m3 is large for an LPGtank with 50,000 m3 being a more usual size. For ammonia60,000 m3 is a big tank with 30,000 m3 being a more normalchotce.

For the gases which cannot be maintained in liquid form by theapplication of pressure alone, the field divides into threegroups:

- Oxygen, nitrogen and argon

- Ethane and ethylene

_ LNG

Oxygen, nitrogen and argon are not required by industry andother users in the same quantities as the other liquefied gasproducts. An air separation plant will commonly have refriger-ated tanks with capacities of 500-2,000 m3 for liquid oxygen(LOX) and liquid nitrogen (LlN) storage and smallerfor argon ifit is involved. Atthe consumer end ofthe supplychain, factory-manufactured vertical cylindrical double-walled and vacuum-insulated vessels of a few hundred cubic metres are commonlvinstalled.

For ethane and ethylene a big tank would be 50,000 m3 with20,000-30,000 m3 being a more common size.

LNG is in a class of its own when tank capacity is concerned.Because of the volumes of LNG traded around the world andthe size of carriers involved, (several currently being con-structed of around 145,000m3 capacity), the trend has been tobuild biggerand bigger storage tanks. The unitstorage costperm3 of LNG tends to decrease as the tank capacity increasesand this is illustrated by Figure 17.7, the resultofa studyforsin-gle and full containment type tanks. The basis of this compari-son is given in Reference 17.11.

Currently, the biggest above ground free-standing LNG tank isin Senboku in Japan. This is of 180,000m3 capacity and is de-scribed in detail in Reference 17.12. In-ground membrane typetanks have also been constructed at 200,000m3 in Korea andJapan and one of these at the Ohgishima LNG Terminal of To-kyo Gas in Yokohama is the subject oI Reference 17.13. Asmaller number of large above ground tanks tend to makebetter use of the available site area when compared with alarger number of smaller tanks. Comparisons using the tankspacing rules from NFPA 59A willfeature elsewhere in StorageTanks & Equipment, to makethis point. Factors influencing themaximum size of the free standing, above ground type of LNGtanks are:

E

9 0.60

P o.qo

50 rm 150 2@ 250

Capacity x 1000 cuM

Fjgure 17.7 Unitstorage cosl per m3 of LNG

- The maximum inner tank lower shell plate thickness oer-mitted by the design Codes

Foundation considerations

Seismic design considerations

- Economic considerations

- Operating flexibility constrainb

The reference list of LNG tanks designed and constructed byWhessoe, given in Figure 17.8 clearly shows the changes oftank capacity with the passage of time. lt is also interesting as itshows the changes in the materials used and in the storagecontainment categories adopted. This subject is further dis-cussed in Sections 17.6, 17 .7 and 17 .8.

17.5 Storage systems and containmentcategoriesThis Section relates to vertical cylindrical tanks of the conven-tionaltype (i-e. those with a metallic self-supporting innertank).

As has been described earlier, the Qatar incident gave the in-dustry cause to reconsider its practices regarding the use ofsingle containment systems for the storage of lowtemperatureproducts. The design Codes in force at the time ofthis incidentwe re AP | 620 (Refe rence 1 7 .1 4) (Append ix R for prod ucts withstorage temperatures down to -60 oF and Appendix e for prod-ucts down to -260 oF) in the USA, and 854741 for products

Figure 17.8 List of LNG ianks designed and built by WhessoeCouftesy of Whessoe

17 Low temperature storcge (anks

down to -50 "C and 8S5387 (Reference tZt5) for productsdown to -196 "C in the UK.

These Codes only consjdered single containment systems.There were no Codes or regulatory guides which provided aframework for the design of other containment systems. Theformation of the EEMUA committee, largely at the instigation ofShelland chaired byJohn deWitof Shell, was aimed at provid-ing a document which filled this void.

ln 1986 the EEMUA recommendations were published and atlastthe industryhad some rulesforthe design and constructionofotherforms ofconiainment. As has been mentioned earlier.the EEMUA recommendations were passed to BSI after a suifable period of time during which the opinions ofthe industry re-garding the suitability of the document were sought and foundto be generally favourable. BSI converted the EEMUA recom-mendations into BS 7777, a Code which closely followed thespirit of the EEMUA document.

The definitions of the various categories of containment sys-tems for conventional vertical cylindrical tanks are given belowand are those developed by EENIUAand repeated in BS 7777.EN1473 (Reference tZ 76) also made an attemptto define thevarious storage systems and followed the spirit ofthe EEMUAdefinitions but added definitions for systems not considered byEEMUA. i.e. membrane and concrete/concrete tanks.

The early designs of double and full containment systems forboth LPG and LNG considered a sudden or unzipping failure ofthe inner liquid containing tank. The loadings on the inner sur-faces of the outer iank were considerable and were evaluatedas a result of test work caried out by N.J. Cuperus, again ofShell (Reference 17.1n. fhe apprcximately six-fold increasein the static liquid pressures caused by the sudden failure oftheprimary container was an onerous design condition and as a re-sult, the early full containment tanks had outertanks which con-sisted of reinforced concrete walls whjch were supported by anexternal earthen embankment. This arranqement is illustratedin Figure 17.9.

Whjlst this system provided admirable protection for the tankarising from external incidents such as missiles and fire scenar-ios, it was expensive. lt required a Iarge site area to accommo-date the slope of the embankment and gave rise to complica-tions with the base heatjng system which had to be extended tocover the tank walls as well as the bnk base.

Research and testing programmes coordinated as descrjbedearlier by the Gas Research Institute were undertaken to look atthe abilities ofvarious steels in their prevention offracture initia-

r.60

l zO

0.20

0.00

lr{B)

OUIERcol{cR€tE

EflELLUAU|D

OUTLEI

1

1

2

1

2390

9000

":*12000

50000 N

N

ALUMNIJM

ALUMNUM

ALUIIII{IUM

9% NEKEL

9% N'CKEL

9% NTCKET

9% NEKET

CARAONSTEE!

CARBO'.ISTEEL

CARBON STEEI

CARBON'sTEEI

CAREONSTEEL

CARBON SIEEL

CARBONSTEEL

CARBONSTEEL

CARBONSTEEL

N

N

N

N

N

Y

NPII

N

N

N

N

N

N

N

N

N

N

SINGIE

SNGTE

SNGIE

SINGLE

StIGLE

DOUALE

DOUBIE

DOUBLE

DOUBI.E

CANvEYSLAND

AMBERGATE

PARTIIGION 1

PARTNGTON 2 50000

50000AVONMOUTH 1

EIE OF GRAIN 1 & 2

N

50@0

50000

65000

N

N

N

AVONMOUTH 3

DEPA

IRI{DAD

DABHOL

2 PRESTRESSED CONCRE'E

PRESTRESSEO CONCRETE

PRESTRESS€D CONCRETE

PRESTRESSEO CONCRETE

2

1

1020@ N

105000

160000

9% NPK€I

9% NICKEL3 N


II

1 7 Low temperaturc storage tanks

Figure 17.9 Typical early full containment tank

courtesy of Whessoe

tion and fracture arrest aspec6. The result was that suddenfailure ofthe inner liquid containerswas considered a non-cred-ible event and the difiicult design condition was removed. Thismeant that the outer tank could become a prestressed concretestructure without the embankment, with consequent mst andother savings.

Acontributoryfactor to this reduction in the design requirementwasworkcarried out on the damping influence ofthe perlite andglassfibre blanket matedalswhich made up the shellinsulationof most LNG storage systems and some other lowtemperaturesystems. This work is described in References 17.18 andt7.t9. The story of the gradual evolution of the low tempera-ture storage systems has been told a number of times and is

Sne boundary

Single conlalnmenl lank

covered in much deiail in References 17.20. 17.21, 17.22and17.23.

An essential element of the reasoning which led up to the intro-duction ofdouble and full containment systems was the consid-eration oflire scenarios which could occurfollowinq the failureof the primary liquid containment.

Presuming that the low remote bund wall served its purpose incontaining the leaked liquid, a single coniainment systems as il-Iustrated in Figure 17.10 would give dse to a large diametershallowpoolof litiuid at localgrade levelin the event offailure ofthe primary container. The subsequent, almost inevitable, poolfire would be large in area, low in elevation and heat from thelarge area ofground within the remote bund wallwould ensurerapid evaporation of the liquid product, an altogether undesir-able event.

ln the case ofa double coniainment system again as illustratedin Figure '17.10, the outer tank would contain the leaked liquid.Again the probability is that the pool of liquid would catch fire,but in this casethe poolfire is smallin diameter, is elevated wellabove local grade and the liquid evaporation rate is slower, amuch less dangerous sltuation is that pertaining to single con-tiainment.

Forfull containment systems as illustrated in Figure 17.10 theleakage of the primary contiainment resulG in the liquid beingcontained by the outertank and the vapour being contained anddisposed of to a safe location (i.e. a flare stack). This eventcan

Full conlanrnenl l.nl

Fgure 17.11 Postfailure vapour dispersion scenarios for single. double andfullcontainmenl tanks

Fullconlalnmsnl lank

Figure 17.10 Postfailure fire scenarios for single, double and fullcontainmentianks


DolbL co.hint|.rn bnk

not result in a poolfife. Atworsta relief valve tailpipefire couldbe considered. Fire scenarios of this type have a considerableinfluence on other equipment on the site (for example neigh-bouri-ng storage tanks, vaporising equipment etc.) and on thelayout ofthe site itselfto ensure that risks from fire exposure topeople and property outside the site boundaries is kept to aminimum.

Aseparate but related subject which plays a part in safety andplant layoutdiscussions, is vapour dilution. lt is clearly sensibleto arrange for the mixture of product vapour and air whichcrosses the site boundary to be at a concentration below thelower flammable limit of the parlicular mixture. In this way thevapour cloud can not be ignited as it passes across the siteboundary into the uncontrolled area outside. The influencesthatthe differenttypes of containment have on this matter are il-lustrated in Figure 17.11.

These topics are the subject of various regulatory documentsand Codes from such bodies as The Institute of Petroleum (lP),The National Fire Protection Association (NFPA), The US De-

1 7 Low temperature storage tanks

partment of Transport (DOT) and the European Committee forStandardisation (CEN). These will be discussed in some detailin later Sections of Sforage Tanks & Equipment.

17.6 Single containment systemsThe deflnition taken from BS 7777 is

Single containment tank

Eithera singletank or a tank comprising an innertiank andan outer container designed and constructed so that onlythe inneriank is required to meet the low temperature duc-tility requirements for storage of the product.

The outer contiainer (if any) of a single containment stor-age tank is primarilyfor the retention and protection of in-sulation and to constrain the vapour purge gas pressure,but is not designed to contain refrigerated liquid in theevent of leakage from the inner tank.

Asingle containment tank is normally surrounded by a lowbund wall (see below) to conhin any leakage.

Figure 17.12 is a reproduction offigure 1, taken from BS 7777.

The parallel deflnition from EN 1473 (Refe rence 17 -21), whichitshould be remembered is only relevant to the storage ofLNG is:

Single containment tank

Asingle primary container and generally an outer shell de-signed and constructed so that only the primary containeris required to meet the low temperature ductility require-ments for storage of the product.

The outer shell (if any) of a single containment storagetank is primarily for the retention and protection of insula-tion and to contain the purge gas pressure, but is not de-signed to contain refrigerated liquid in the eventofleakagefrom the primary container.

An above ground single containment tank shall be sur-rounded by a bund wall to conbin any leakage.

Figure 17.13 A single coniainmeni tankFrom EN 1473,figure Hl

d)

Figure 1 7.1 2 A single containment lank

Frcm 857777, figure 1


1 7 Low tempercturc storage tanks

Figu€ 17.14 Typical single coniainment LNG tanks

Courtesy of Pift4es Moines, lnc

Figure 17.1 3 is a reproduction of figure H l taken from EN 1473.It is clearthatthe two documents are in close aqreement on thismatter.

Figure 17.14 shows typical single conbinment LNG tanks.

It is interesting that EN 1473 makes ita mandatory requirementfor all categories of containment that there shall be no penetra-tions ofthe primary and secondary (where provided) containerwalls or bottoms. This makes the use of in-tank DumDs a man-datory requirement for these tanks.

The American storage tank design Codes have never tried todefine the containment categories in the same way as the Brit-ish or European Codes. API 620 in both its low temperatureAppendices R and Q deals with what are essentially single con-tainment systems. The rules from the API Code are commonlyused to design the metallic components ofstorage tanks whichfall into other containment categories

Similarlythe American Codeswhich coverthe land-based stor-ageand handling of LPG and LNG do not consider containmentin the same way.

Single containment storage systems are the cheapest from ofstorage for refrigerated liquids. In addition to the potentially di-sastrous consequences of a failure of the primary liquid con-tainment, they are also vulnerableto damage from a numberofother loadings including:

- Radiation from adjacent tanks or equipment fires

- Inplant generated missiles

- Explant generated missiles

.oirEnslon Xshal oqlal o. qc€€d h€ qlmof drnensloo yplrg trgeqtval€nt tFad In LNG ol lh€ prcsg|r€ h dr6 y{or space abovgtho fouH.ea@l: Whdl tl'€ holgfn ot tto.{k or htrDltrtV wrtt b aqrftl to,or gleeler lha4 tl?e fir&nrm frqdd btol, X nay ltsw arv r&E

. ohpnsion X b hs dshnco ft!.n f|e hnsr wall ot fio contaln€r b t|gdo6€t froe ol he dko or lnpoundk|g wall

. otrcrFbn l.b t|e &tmco tom tls mdlnnm teld lsv€l h heconhlnsr b th6 lop ot thg dlka oa lrpoundrg vrarl.

Figure 17.15 Bund/distance relationship

ftom NFPA 59 and 59A


- Blast loadings

Provisions required to protect single containment tanks fromthese loadings such as active flre protection, increased tankspacing, structural modification and enhanced pressure reliefvalve system capacity must be taken into accountwhen consid-ering the overall costs of single containment systems.

The most commonly accepted rules concerning the capacity,height and distance of bunding systems are given in NFPA 59(Reference 17.24) for LPG and NFPA 59A (Refere nce 17.25)for LNG storage systems.

Figure 17.15 shows the bund heighvdistance relationship fromReferences 17.24 and 7225. This criteria is also usually fol-lowed for double and full coniainment svstems as described inSections 17.7 and 17.8.

17.7 Double containment systemsThe definition taken from BS 7777 is

Double containment tankAdouble coniainmenttank is designed and constructed sothat both the inner self-supporting primary container andthe secondary container are capable of independenflycontaining the refrigerated liquid stored. To minimise thepoolof escaping liquid, the outertank orwall is located at adistance not exceeding 6m from the inner tank.

The inner tank contains the refrigerated liquid under nor-mal operating conditions. The outer tank or wall is in-tended to contain the refrigerated liquid product leakage

Figure 17.16 Double containment tank

From BS 7777 :1993, figurc 2

Nob$

liq'rd

(r8ulat€d)

from the inner iank, but it is not intended to contain anyvapour resulting from product leakagefrom the innertank.

Figure '17.16 is a reproduction offigure 2 taken from BS 7777.

The'parallel definition from EN 1473 (LNG only) is;

Double containment tank

Adouble containmenttiank is designed and constructed sothat both the inner self-supporting primary coniainer andthe secondary container are capable of independentlycontaining the refrigerated liquid stored. To minimise thepoolofescaping liquid, the secondarycontainer should belocated at a distance not exceeding 6m from the primaryconGtrner.

17 Loq tengate gd4E 7E

The primary container coniains the reftilecated lilil l'}-der normal operating conditions. The seconday cs>tainer is intended to contain any leakage of therefrigerated liquid, but it is not intended to contain yvapour resulting from this leakage.

Note: Examples of double containmenttanks are given in fig-ure H3.

Note: Figure H3 does not imply that the secondary contain€ris necessarily as high as the primary conbiner.

Figure 17.17 is a reproduction offigure H3 taken from EN 1473-

Again it is clear that the two documents are in close accord onthis matter. lt is interesting that the Codes have decided thatthe point where single containment becomes double contain-mentwhen the remote bund wall moves towards the liquid con-tainer and becomes taller in accordance with the NFPA59 and59A rules, is reached when the spacing between the bund andliquid containing tank is 6m. This seems a sensible choice andthe tank shown in Figure 17.18, which is one of a series of50,000 m3 capacity LNG ianks builtfor British Gasas partoftheUK peak shaving system, uses 6 m for this spacing. Figure17.19 shows two LPG tanks with prestressed concrete outertanks. The spacing between the steeland concretetanks is lessthat 6 m in this case.

The design of the bund wall, which for the single containmenttypes oftanks was a low structure made from earth, reinforcedearth or reinforced concrete, has now become a more demand-ing task. As has been mentioned in Reference 77.5, it is nownot usualto require the bundwallto be designed forthe suddenfailureofthe liquid container. Despite this, the gradualiilling ofthe bund resulting from a slow leakfrom the innertankwhich isconsidered fullto capacity at the time ofthe incident is still a de-manding criteria.

Some bund walls are of metallic construction. The majority ofbund walls are of prestressed concrete. The "Preload" exter-nally wire-wound type of wallwas a popular choice for this mm-ponent for many years. The circumferential prestressing wasapplied to the concrete wall by an externalwinding of a singlestrand whilstthe vertical prestressing was applied by macalloybars cast into the wall. Figure 17.20 shows the wire winding inprogress for such a wall.

The alternative arrangement for bund walls uses embeddedtendons for the circumferential prestressing. lt is not usual toapply a vapour linerto the innersurfaces ofthis concretewallsothat in the event ofthe inner tank failure, the permeability of theconcrete wall would allow small volumes of product vapour toescape. This is not viewed as posing a problem. The construc-tion of thejoint between the base slab (usually reinforced con-

Figure 17.'19 Two 50,000 m'double containment tank for LPG

Courtesv of Antwen Gas Terminal Nv

Figure 17.17 Double containment tank

From EN 1473, tlgue H3

Figuro 17.18 A 50.000 m3 capacity double conbinment type LNG tank for Brit-isi cas at the lsleof Grain

Courtesy of Advantica


1 7 Low tempemture stomge tanks

FiguE 17.20 Wire winding of a concrete bund

Courtesy of Preload lnc.

crete) and the bund wall has been the subject of debate andcontrove rsy for a numberofyears. The prime contenders beingthe siiding, pinned and fixed joints. This is the subject of moredeiailed comment in Chapter 18.

The space between the bund walland the liquid containing tank(Figure 17.1 7) or the insulation protection outer tiank can eitherbe lefr open to the atmosphere or be closed off by the applica-tion ofsome type ofweathersealing roof. The open topped wallalternative must be fiUed with a suitable means ofremoving therainwater or the flre water which could accumulate in thisinterspace and which could cause problems of corrosion orfloatiation. This water removal system should not prejudice theability of the outerwallto perform its product liquid containmentduties in the event of an inner tank leak or failure.

Consequentlythe tank illustrated in Figure 17.18 had a systemwhich mllected the interspace rainwater into a substiantialsump and pumped it out over the 19m high bund wallto a suit-able external drain. The closed top alternative must be fittedwith a suitable wealherproof and long lasting roof whilst stillhaving provision for removal of any water accumulation in theinterspace. This solution has been used most frequenflyfor liq-uid ammonia storage and the problems associated with theprovision of a suitable roof are discussed in Chapter 21.

lf double contiainment systems are to be used forthe storage ofproducb whosevapour is heavierthan air at ambient tempera-tures (i.e. LPG), careful consideration must be given to themonitoring of the space between the tank and the outerwallforthe potentially dangerous accumulatons of product vapoul:

Double containment systems are quite unusual these days.This may wellbe due to the relative costs ofdouble vs full con-tainment systems. lf the two tanks shown in Figure 17.21 arecomDareo:

- The inner 9% nickel steel tanks are the same

- The insulation systems are the same

The concrete walls are similar

- The base slabs/base heating are similar

- The outershell in 17.21a is deleted

- The wall and base liner in 17.21b is added

- The in-tank pumping system/fittings/pipework are similar

- The roof/suspended decks are similar

The overall result is that both solutions willcost aboutthe sameand iake similartimes to construct. Consequently itwould seemthe obvious choice to choose the full containment oDtion as

3OO STORAGE TANKS & EQUIPMENT

Figti.e 17.2'la Double containment type tank

Figure 17.21b Full containment lype tank

better value for money, i.e- better containment at the sameonce.

17.8 Full containment systemsThe definition taken from BS 7777 is:

Full containment tank

A double tank designed and constructed so that both theinnertank and the outertank are capable of independenflycontainingthe refrigeratedliquidstored. Theoutertankofwall should be 1m to 2m dishnce from the inner iank.

The inner tank contains the refrigerated liquid under nor-mal operating conditions. The outer roof is supported bythe outer tank. The outer tank is intended to be caDableboth of coniaining the reffigerated liquid and of conholledventing ofthe vapour resulting from product leakage aftera credible event.

Examples of fufl containment tanks are given in Figure ,17.22,

which is a reproduction of flgure 3frcmBS 7777.

The parallel definition taken from EN 1473 is:

Full containment tank

A tank designed and constructed so that both self-sup-porting primarycontiainerand the secondarycontainerarecapable of independentlycontaining the refrigerated liguidstored and forone ofthem its vapour Thesecondarycon-tainer can be lm to 2m distiance from the Drimarv con-tainer.

Figufe 17.22 Examples offull containment tanksFron BS 7777 : 1993, figure 3

17 LgLt "= "t ::: ---= :-'

The primary container contains the retrigeraie.3 :, : ,--der normal operating condilions. The outer.3c' s s-:_ported by the secondary container. The seca-::-.container shall be capable both of contarning re .e-::j-ated liquid and ofcontrolled venting ofthe vapou..es_: -.:from product leakage after a credible event.

Figure 17.23 is a reproduction of figure H4 from EN 1473.

Full containment represents the final step in the transfomato-ofthe bund wall. Initially a low and distant bund of earth or reii-forced concrete impounding a shallow pool of product liquicj cilarge d iameter open to the atmosphere (single contarn merr . :has moved inwards to become a tall bund wall constructecclose (i.e. < 6m) to the primary liquid contajnment, and ir-pounding a much deeper pool of product liquid of much smallerdiameter, again essentially open to the atmosphere (doubiecontainment), and now it has further moved inwards to becomethe outer tank itself, spaced between .1 and 2 m from the pri-mary liquid containment, and impounding a deep pool of prod-uct liquid of small diameter which is no longer open to the atmo-sphere (full contiainment). As has been mentioned eadier thischange brings about significant improvements in safety. Thepost leakage tank fire is no longer considered a credible event.Most storage facilities or terminals house a number of storagetanks and the adjacent tank fire has always been one ofthe ma-Jor tank spacing criteria. This is explici y recognised by EN'1473 which in Tables 1 and 5, shown in Figure 17.24, indicatesthe maximum radiation exposure figures and failure scenariosfor difierent types of storage tanks. lt should be noted that toget into the full containment category this Code will require thetank to have a concrete roof.

It is generally agreed that the elimination of Denetrationsthrough the inner and outer tank walls and bottoms is a orereo-uisite of full containment systems. These connections wouldrepresent points ofweakness in the containment from the pointof view of mechanical and structural design, as well as provid-ing the undesirable possibility of bypassing the containmentsystem via local damage to liquid connections outside the outercontainment envelope and before the first isolation valve in theconnected pipeline. This in turn requires the use of roof pene_trations for all connections to the tank and the use of in.tankpumps for liquid removal. These pumps and their associatedequipment are discussed further in Chapter 19.

For LNG storage in particular, where the outercontainer is con-structed from concrete, two further safety enhancing elementshave become very much de rigueur.

The first is the secondary bottom. This is laid beneath the in-ner containment bottom, eitherabove orwithin the thickness ofthe base insulation. lts function is to prevent possible leakagefrom the inner tank bottom from penetrating the base thermilinsulation and causing a cold spot on the reinforced concretebase slab, be this ofthe on-ground or elevated tvpe. A localisedcold spot in an otherwise ambient temperature base slab of thistype brings the possibility of through-thickness crackinq andconsequent product leakage. This is discussed further in ahap_ter 18.

The second is the bottom corner protection system. Leak_age of product from the prlmary container may well result in awarm outertank base slab and a cold outerwall. This combina-tion of events, partjcularly for tanks of the fixed bottom cornerdesign (the majority of tanks currenfly in service or under con_struction), will result in shear stresses in the lower part of theconcrete outer wall which cannot be adequatelv catered for inthe design without the creation of through thickness cracking.The application of a Section of thermal insulation to the lower3to 5m of the inner face of the outer wall, impermeable to theleaking product liquid, will convert some of ihe shear to localbending and provide a designable situation. Thjs is also dis_cussed in Chapter'18.

Figure 17.23 Examples of full containmenl ianksFron EN 1743, tigurc H4


Eqolpmc tDrtdcbourdory UirluoDtharodrrdhtlontlur (tw/E2)

Concrete out€r surface of adiacentstorage tar {9: unpnrtectedl) 3) orbehind thermal protectionz)

32

Metat out€r surface of a4iacent storagetanlG: unprptectedu, or b€hind thennalprotectironz), (see P3)

15

The outer sudaces of a4iacent pr€ssuesiorage vess€ls and process facilities(see P.3)

C,onhol room, Maint€nance workshops,laboratories, warchous€s, etc. (see P.2)

Adninistative buildings (see P.2)'r For prestressed concrete tanks, maximum radianon nuxesmay be determined by the requirements given in ]{.6.2.2) Such facilitjes are protFctFd by means of walcr sprays, fireproofing, radiation screens or similar sysiems.3l Prolaction is pmvided by spacinA alone.

1 7 Low temperature storcge tanks

lypc of tlrt All metdllc oronly wlthmetdllc roof

PreatrGs3ed

(lnclurtlnScorcrcte roo0

Single contalment l)

Double containn€nt

F\ll contaiuneni 3)

Membrane

Clyogenic conqete 2)

Sphedcal r)

Ingtomd 2) 3)

Scensaio€ to be conddercdr) ln ctse of collrpse of rh€ t r* pdnEry contaimr, fue poolsize con€spon& to dre lrnpounding area.2) In crs€ of cotlq€e of the tank rcot, the 6re pool sizeconcsponds to the secondsy c tain€tr3) No colap6e lB consider€d for thee tank b?es.

Figure 17.24 Exposufe figures and failure scenariosfor differcntlypes of

From EN 1473. tables 1 and 5

17.9 Membrane tanks

The membrane solution for low temperature liquid storage iswidelyused in the fields of marine transport, above ground landbased systems and in-ground land based systems. lt also hasexcellent prospects ofapplications in floating storage and grav-ity based storage systems, a number of which are being ac-tively considered at this time.

A further potent'al area of application is in below ground linedmined rock caverns. A pilot scheme is currently underway inSouth Korea and this is described;n Section 17.9.5.

The essential difference between the membrane system andthe conventional systems for liquid storage lie in the separationof the structural support function for the product liquid and theliquid tightness function. This separation and how it is achievedis discussed in 17.9.3 below. The relative merits ofthe conven-tional and the membrane systems are discussed in '17.9.5

below

lvlembrane tanks forabove ground use are notwithin the scopeof codes such as API 620 ot BS 7777. NFPA 59A includesmembrane containers in its definition section but provides littlemore information on the subject. EN 1473 recognises mem-


Figure'17.25 Examples of membrane tanks

From EN 1473, figure H5

brane tanks as a separate category and gives the followingdefinition:

"Amembranetank should be designed and constructed sothat the primary container, constituted by a membrane, iscapable of containing both the liquefied gas and its vapourunder normal operating conditions and the concrete sec-ondary container, which supports the primary contiainer,should be capable of containing all the liquefied gas storedin the primary container and of controlled venting of thevapour resulting from product leakage ofthe innertank. Thevapour of the primary container is coniained by a steel roofliner, which forms with the membrane an integral gas tightcontainment. The action of the liquefied gas acting on theprimary container (the metal membrane) is transferred di-rectly to the pre-stressed concrete secondary containerthrough the load bearing insulation.'

Examples of membrane tanks from EN 1473, figure H5 areshown in Figure 17.25.

The Japanese RPIS Code considers membrane typetanks butis only applicable within Japan.

The forthcoming Eurocode on low temperature tanks (prEN14620) which will replace BS 7777 has also chosen to recog-nise membrane tanks and provides detailed rules forthe designand construction of such tanks.

It is interesting just how similar the developments of the marinetransport (i.e. liquid gas carriers) and the land based storagesystems are. In gas carrier design there are two mainphilosophies:

The use of rigid tanks supported within the structure ofthe ship.Spheres of the Moss Rosenburg type or prismatic tanks suchas those designed and manufactured by lHl. A tanker of thespherical liquid contiainertype is illustrated in Figure 17.26. Theprotrusion ofthe spherical tanks through the deck of the ship isclear to see. The spherical tanks require support at or close to

Figufe 17.26 A carrier ofthe sphedcal iank type

Figute 17.27 A carrier oflhe membrane tank type

Courtesy of Universal Shipbuilding Corporation

their equator and are relatively inefficient in making use of theavailable space within the ship's hull.

The use of membrane technology supported bythe structure ofthe ship. This solution is much more efficient in terms of spaceutilisation. The liquid containing tanks can be tailored to theshape of the ship's hull. The distribution of loads from the tanksto the (usually double hulled) ship's structure is more efficientthan is the case for spherical or prismatic rigid tanks. Atanker ofthe membrane type is illustrated in Figurc 17 .27 .

These two philosophies are very much reflected in the landbased storage systems where the conventional self supportedtanks and the membrane solutions compete for territory Usu-ally both are confined to the vertical cylindrical form for struc-tural reasons. The ability ofthe membrane system to take othershapes will bring possible advantages in the floating and GBSareas of future activity.

17.9.1 Development history

The early work on membrane systems was directed towardsmarine gas carriers, and in particular to LNG carriers. The firstliquid gas carriers had self supporting tanks and it was consid-ered that this did not represent an ootimal solution in terms oJspace utilisation.

Starting from a Norwegian patent, the French company SNTechnigaz developed a stainless steel membrane system. Thismembrane had two sets ofcorrugations running at right anglesto one another. These corrugations acted like bellows allowingthe membrane to accept the thermal contraction stresses,which would cause tensile failure in a flat membrane. The struc-


tural support for the liquid could cleady not be supplied by theflexible membrane itself, but came from the structure ofthe shipitself by way of the load bearing insulation system. This insula-tion was in the form of panels consisting of two layers of ply-wood sandwiching a layer of a suitable insulation material. Theinitial patents for this liquid storage system were filed in 1964.

During the same period of time, a second French company wasbusy developing a different membrane concept. The companywas Gaz Transport and its solution to the problem was to use amembrane made from a 36% nickel-64% iron alloy which wasoriginally patented by lmphy in 1896 and is more commonlyknown as Invar. As well as being physically strong and capableof being welded, this material has the added merit of possess-ing an almost zero coefficient of thermal contraction across awide range oftemperatures. This makes it idealfor ihis purposein thatalmost no thermal contraction stresseswould be presentand consequently no corrugations would be required. This lineragain needed structural support from the ship's structure viathe load bearing insulation. The original patents for this systemwere filed in 1963 and 1965.

The technology for both of these liner concepts was licensed toa number of shipbuilding companies around the world. Up todate the LNG tanker fleet consists of 141 shiDS. 68 of the twomembrane types, 71 using the Moss spheres and two using adifferent rigid self-supporting tank concept. There are 54 shipsunder construction of which 38 are of the membrane type and16 ofthe Moss sphere type. Options existfor a further 25 shipsof which 20 are ofthe membrane type and 5 ofthe moss spheretype.

The first move to adopt the membrane system for land basedstorage systems was taken by SN Technigaz in the early '1970s

with two above ground liquid ethylene tanks of relatively modestcapacity being completed in France in 1972. Technigaz contin-ued to develop its system and completed two 120,000 m3above ground LNG tanks for Gaz de France at Montoir deBretagne in 1980 and ten 100,000 m3 above ground LNG tanksat Pyeong Taek in South Korea from 1986 to 1998.

The move from marine to land-based storage systems for theInvartype of membrane did not come about and the reasons forthis failure to move into whatwould seem to be attractive alter-native markets is something ofa mystery There appears to beno technical reason whythis transition was not made. Perhapsit comes down to no one being interested in pursuing the mat-ter The French Code for lowtemperature above or semi-buriedtanks (Reference 77.26) clearly suggests that the Invar mem-brane system is suitable for this purpose and indeed providesspecific rules and requirements for the design and constructionof this type of membrane. The Technigaz membrane systemwas the subject of a number of improvements around 1990.These improvements addressed amongst other things variousconcerns of its customers and costs of the system. These aredescribed in Section 17.9.3.

The next move in the historical develooment of the membranesystem was its adoption for use in in-ground storage tanks.Again the system chosen was the Technigaz membrane. Thetechnologywas licensed to various Japanese and Korean com-panies. One of the Japanese licensees (NKK) has completed20 in-ground membrane tanks up to the end of2002. Other Jap-anese companies (l\y'H1, lH I and KHI) have developed their ownmembrane systems for land-based in,ground use.

Currently the biggest LNG tanks in the world are of thein-ground membrane type at Negishi and Ohgishima terminalsin Japan. In-ground membrane tanks are described in moredetail in Section 17.12.

STOR,AGE TANKS & EQUIPMENT 303


17.9.2 Detailed description of the land-based mem-brane system

17.9.2.1 The metallic membrane

The material used forthe stainless steelcorrugated membraneis generally 1.2 mm thick ASTM A 240 type 304. The corruga-tions are of two different sizes and are created at right angles toone another. The manufacturing techniques to produce thiscomplex component are cold forming or stamping. The coldforming method is considered to have certain advantages interms of residual stress and plate thinning.

Figure'17.28 shows a typicalabove ground tank wall panel aftermanufacture and indicated the sizes ofthe large and small cor-rugations. A wall panel would be approximately 3280 mm x1150 mm in size. The seriously clever element ofthe syslem isthe detailed design and configuration ofthe area where the cor-rugations cross. This is termed "the knof'. The pitch betweenthe largersized corrugations is 560 mm. These run verticallyonthe tank shell and radially on the tank bottom. For the smallersize of corrugations the pitch is 650 mm. These run circumfer-entially on both the tank shell and bottom. The design method-ology for the membranes of above ground tanks will appearpublically in a code ior the first time in the forthcomingEuronorm for low temperature tanks.

For in-ground tanks, most of which are either located in Japan,Korea and Taiwan and have been designed and constructed byJapanese contractors, the details ofthe system is slighfly modi-fied. The membrane is 2.0 mm thick at the behest ofthe Japa-nese utility companies and the national regulaiory authorities.The corrugationsare of similarform butare biggerand pitchedat 1360 mm in eitherdirection and areformed using thestamp-ing method instead of the cold forming technique.

For marlne service the membrane is again 1.2 mm thick, butdue to the more onerous fatigue environment caused by cyclicelongation due to the ships hull deforming, the corrugations aremore closely pitched at 340 mm x 340 mm.

Quite recently the Technigaz membrane has undergone someimprovements as mentioned above. The main change relatingspecifically to the metallic membrane is that the corrugationsare now pitched at 680 mm in both directions and advantagehas been taken of the stainless steel manufacturers ability toproduce largersheets. Fora large tankthis latterchange willre-duce the number of sheets in the tank wall by some 23% andthe lengths of welded seam by 8%. The bottom of the tankwhere the layout of the corrugations was previously radial andcircumferential, has now been modified such that the standardsquare pitch panels can be used for the majority of the basewith transition pieces derived from marine practice used at thebottom corner. For a large tank this will reduce the length ofwelded seam by some 39%.

The method of securing the membrane sheets to the support-ing thermal insulation js by way of stainless steel inserts whichare anchored jnto the plwood inner facing of the insulationpanels. The membrane is then welded to these inserts throughholes in the membrane itself. This is illustrated in Figure 17.29.

The wall-to-base detail of the membrane system involves theuse of special panel components and the top of the wall issealed to the concrete wallor its liner by a suitable detail. Fig-ures 17.30 and 1 7.31 show typical details ofthe bottom and topcorner of an LNG tank. The membrane system has a limitedability to cater for local loads such as may be caused by theneed to support or guide pump columns or stilling wells. Thisnecessitates special details such as that illustrated in Figure17.32.

Figure 17.33 shows a viewinside a large LNG tankof the mem-brane type and Figure 17.34 shows details ofthe corrugationsand "the knot'. The joining of the membrane sheets is by lap


Figure 1/.28 Typic€l above ground tank wail panel, indicating lhe targe andsmaI corrugatons

Coutesy of SN Technigaz

Figure 17.29 N,{ethod ofsecuring the membrane sheets lo supporting thermal

Courtesy of SN Technigaz

welding. The development ofthe automated welding system forthe installation of membrane sheets is an essential part of theoverall system. lt is based on a TIG method with no added fillermetal. The welding system is supported by ra'ls which attach tothe membrane liner surface at the knots using a specially de-signed clamping device. Post weld inspection is carried out af-ter installation of the complete membrane by conducting an

KNOT OEIAIIs

3ru@fuc^no'|'Mu

17 Low tempenturc storage tanks

F

Figure 17.32 Special €quirements forsupportng tocal toadsCourtesy of SN Technigaz

Figure 17.30 The wall-to-base detail of lhe membrane system

Coulesy of SN Technigaz

Figu.e 17.31 Containment sysiem bottom-to-wa junction


ammonia test. This is a technique commonly used in the nu-clear industry and consists of introducing a mixture of 20% am-monia vapour and 80% nitrogen into the insulation space. Anyleakage is detected on the inner surface of the weld seams bythe application of a sensitive paint which changes colour fromyellowto blue in the presence of ammonia vapour. Helium leaktesting is also used as an overall leakage test for the Invar typeLNG carriers and for repair testing on the stainless steelmembrane type of LNG carriers.

Unlike the free sianding types of primary containers, the mem-brane which serves as the primary tightness barrierfor the liq-uid but not as its structural support, is not subject to a hydro-static test. A hydrosbtic test of the prestressed concrete outertank is required and this can be to a water equivalent to 1.25times the weight ofthe innertank levelcontents up to the full liq-uid height in addition to a pneumatic test of 1.2S tifles the de-sign pressure. This test is obviously carried out prior to the in-stallation of the insulation and of the membrane ibelf.

F--l

Figure 17.33 Aview inside a large LNG membrane type tankCourlesy of JFE Engineeing Corpontion

Figure 17.34 Details ofthe corrugations and 'the knot',

Coulesy of JFE Engineeing Corporation


17 Low tempenturc storcge tanks

During service the insulation space behind the membrane is

filledwith nitrogen gas. This insulation space is constantly mon-itored for the lifetime of the tank for traces of the stored productvaoourwhichwould indicate a leak in the membrane liner. In theevent that such an indication were found, the nitrogen flow rate,which in normal operation is very low, would be increased tosweep the insulation space to attempt to ensure that the gas

concentration is maintained below30% oJthe lowerflammablelimit for the oroduct in question.

17.9.2.2 The insulation system

Forthe tank base and the tank wallthe basic insulation materi-als are either polyvinyl chloride foam (PVC) or polyurethanefoam (PUF). The densities of the materials used vary depend-ing upon the liquid and vapour loading in the different parts ofthe tank. For PVC foam densities of between 65 and 90 kg/mJ

are typical whilst for PUF densities of between 65 and'120 kg/m3 are used. This insulation is supplied to the site in fac-tory made panels with the insulating material sandwiched be-tween layers of plywood. The liquid side face plywood is gener-

ally 12 mm thick and the concrete side may be of the samethickness or thinner (typically I mm). A typical wall insulationpanel would be 1930 mm x 540 mm. The thickness of the insu-lation componentwould depend on the service and the requiredheat leak, but a typicalvalues using PVC foam for LNG servicewould be 210 mm to 350 mm depending on the heat leak re-ouirements.

The wall insulation panels are attached to the inner surface ofthe concrete by two means. Firstly 10 mm stainless steel studsare fitted into the concrete priortothe panelerection. There areusuallytvvo studs per panel. Secondly a bonding mastic is used.The holes where the studs penetrate the panels are filled withplugs of PUF The gaps between the adjacent panels are filledwith PUF or compressed glass wool. To ensure that the con-crete wall is resistant to the ingress of moisture from the atmo-sphere, the internalsurface is coated with a suitable primerandmoisture barrier prior to the hydrostatic test. Atypical arrange-ment ofthe wallinsulation erection sequence is shown in Figure17 .34.

Figure 17.35 A typical arrangement of the wall inslllallon erection sequence

Couftesy of SN Technigaz


The roofinsulation is glassfibre laid on and supported by a sus-pended deck of either the flat plate type or the corrugated sheetand structural section type.

For a 100,000m3 LNG tank, a typical design heat leak for amembrane type tank intended for an import terminal would be

between 0.08% and 0.1% ofthe full tank contents per day. Foran exportterminal heat leaks ofbetvveen 0.05% and 0.08% arepossible.

As is the case for the membrane itself various other improve-ments have been made to the system:

. The vaDour barrieron the innersurface ofthe concrete tankwas improved in its crack bridging abilities bythe addition ofa glass cloth layerand in its applicability by an improvementin formulation.

. The insulation panelsize hasbeen increased in linewiththeincrease in the membrane sheet size. The standard wallpanel is now 2020 mm x 1340 mm instead of 1930 mm x540 mm.

. The ability of the membrane system to resist the effects ofliquid leakage better into the insulation space have beenmade. The possibility of through thickness cracking at thebase ofthe ore-stressed concrete wallwhere built in bottomcorners are used has been discussed in Section 17.8. To

avoid the harmful effects of the low product temperatureson the inner surface of the concrete tank, the insulation ofthe base and lower 5 m of the wall is modified. A liquid-tightmaterial is provided on the insulation paneljoints and on theinner plywood face. This liquid{ight barrier is very muchequivalent in function to the secondary bottom and thelowershell protection used for the free standing types offullcontainment tanks described above.

17.9.2.3 The outer tank

The outer tank base slab is constructed of reinforced concrete,most usually ground-based, but occasionally elevated on piles

or stub piles. The ground-based alternative would incorporatebase heating ofthe electrical or other types. The outertankwallis ofthe post tensioned type and the joint between the wallandthe base slab is of the fixed or encastre type. The verticalpre-stress is applied by tendons in U-shaped ducts. The hori-zontal pre-stress is applied by tendons in ducts within the wallrunning between a number (usually four) of stressing but-tresses. Traditionaltyconcretes ofthe 40 - 50 MPa compressivestrength grades are used.

The roof is most usuallyofthe reinforced concrete type. Asteelsheeting supported by a structural framework is erected at thefulltank height, or on the tank base slab and airor mechanicallylifted into place. With the assistance ofinternal air pressure, this

supports the newly placed concrete (either in one or two pours)

until it becomes self-supporting.

lmprovements have been made in this area too. These aremainly in the use of stronger concretes giving in addition to in-creased strength, advantages in improved permeability andfaster curing times.

One of the membrane tanks at the Pyeong Taek LNG terminal,is shown in Figure 17.36, with an outer bund, which would nowbe considered redundant.

17.9.3 Comparison of above ground membranetanks and conventional tanks

As mentioned previously, this discussion has been ongoing fora considerable period of time. lt would not seem helpful to ex-press a view on this complex and multifaceted argurnent, butrather to indicate where the differing views can be found ex-oressed in full in various references.

17 Low temperature storcge tanks

Figure 17.36 Membrane tank with redundant bund at the Peyong Taek LNG

Cowlesy of SN Technigaz

The argumentsfor parity of containment betweenthe two typesof tanks can be found in References 17.27 and 17.28.

The French AGT Code (Reference 17.26) appears to express aview in one of its tigures that above ground membrane tanksconstitute single containment whereas in-ground membranetanks are full containment. This document has rather been leftbythewayside jn recentyearsand is notas infl uential as itoncewas.

References 17.10 and 7Z 76 require the primary and second-ary containment storage components for both double and fullcontainment ianks to be "capable of independently containingthe product liquid". The membrane system's separation of liq-uid tightness function from the structural support function isperceived by the "inequality" camp as not fulfilling this "inde-pendent' requirement.

One difference which should be aired in this discussion ofequivalence is the fact that the outer concrete tank forthe mem-brane type is hydrostiatically tested and that for the 9% nickelsteel/preslressed concrete type is not. This test can be ex-pected to reveal any major construction defects in the outertank.

Notwithstanding this long, and occasionally acrimonious dis-cussion, it would seem that most users of these two types ofstorage tank are coming to accept their equivalence.

17.9.4 The lined mined rock cavern initiative tor fu-ture LNG storage

The possibility of storing LNG in below ground cavems hasbeen discussed for many years. The early attempls to use un-lined vertical cylindrical holes in the ground were not a success,and all but one have now been decommissioned and filled in.The main problem lay in the extensive fissuring ofthesunound-ing ground giving rise to excessive heat leaks and an ever-ex-tending area of frozen ground around the bnks.

The use of a suitable impervious barrier would solve some ofthese problems, but the behaviour of the water, ice and rocksurrounding a mined cavern was alwaysan area of u ncertajnty.

Recently, a group of companies has joined together to designand construct a pilotfacilityto investigate the possible problemareas. G6ostock Co., Saipem.Technigaz of France and S.K.Engineering & Conshuction Ltd of South Korea, have come to-gether to pool their considerable skills in in-ground and mem-brane storage systems. They are constructing a I IO m3 pilottank at Deajon in South Korea.

This will be a mined cavern some 20 m belowground. The rockfaces will be faced with reinforced concrete and the Technigazmembrane and insulation syslem will be applied to this lining.

Figure 17.38 Shows the overall facilitCounesy of LNG Joumal

Figure 17.39 Asimple P & | D of the facility

Couftesy of LNG Journal

Provision will be made to drain the surrounding rock and the fa-cility will be heavily supplied with inskumentation. The tank wibe cooled with liquid nitrogen at -196 'C.

Figure 17.37 shows a view inside the test tank, Figure 17.38shows the overall facility and Figure 17.39 is a simple piping &illustration drawing, (P & | D) of the facitity. (See Reference17.17.)

Figure '17.37 A view inside the test tank



17 Low temperaturc storage tanks

17.10 Spherical tanksIn addition to the spherical tanks discussed above being usedfor the storage ofthose products which can be maintained in liq-uid form at ambient temperature by the imposition of pressurealone, or at semi-refrigerated temperatures bya combination oflow temperature and pressure, there have been proposals tostore various lowtemperature liquids in spherical vessels underfully-refrigerated conditions- These schemes have been almostexclusively aimed at the storage of LNG

Some 50% of the world's LNG carriers are of the skirt-sup-ported spherical vessel type, so it is no surprise that theseschemes to use land-based spheres come from the world ofship building.

one such scheme is the subject of Refere nce 17.29.J|1is papelproposed spherical vessels of80,000 m3 capacity, which givesa diameter of around 54 m. The spherical vessel was to be con-structed from either 9% nickel steel to ASTIV1 A 553 or of alu-minium alloy to ASTM B 209 type 5083-0. The design codeswere a mixtufe of the ship design IMO Codes and the estab-lished pressure vessel Code ASIVE section Vlll. Briefly the de-sign conditions were:

. Design temperature -163 "C

. lnternal design pressure 0.25 bar

. Vacuum design pressure -0.10 bar

. Product density 450 kg/m3

. Boil off 0.054% of the full contents/day

. Hydrotest - partial to 66% liquid fill capacity

The vesselwas to be supported at its equatoras is the case forconventional ships'tanks and to have a thermal insulation sys-tem consisting of polystyrene foam of 500 mm thickness. To re-duce the heat in leakage at the skirt support, a stainless steelplate some 2500 mm in height was to be placed between thestiffened supporton the vesselitselfand the concrete supports.Thus the heat leak at this pointwas reduced to 10% ofthe total.

The only connections to the sphere were to be in the top capwhere liquid inlet and vapour outlet connections would be lo-

cated. Liquid removal was to be by the use of in{ank pumps

which would also penetrate the vesselthrough the top cap. Theouterconcrete protection was notto be designed to provide liq-uid containment in the event of an inner tank failure as this wasdeclared impossible due to good understanding of the designand behaviour of the spherical vessel, much better by implica-tion than would be the case for a vertical cylindrical bnk.

This argument would attract little support today. The evidencefor the need to design this concrete structure for the effects ofadjacent tank fires was, by modern viewpoints, similarly sus-pect. As presented the scheme represented single contain-ment by today's standards. A cross-section of the proposed

scheme is shown in Figure 17.40.

Three alternative construction proposalswere suggested in thepaper:

. Prefabrication ofsections ofthe sphericalvessel in a ship-yard for normal assembly on the job site into the concreteouter vessel.

. The spheres and their supporting structure to be assem-bled inthe shipyard and skidded onto transport to site (prob-ably suitable barges) and skidding off to be united with thepart constructed outer concrete vessel.

. The comolete structure is fabricated and assembled at a

ship or offshore yard suitable for deliveryto site as self-floa!ing units or by heavy lifr ships (Figure 17.41).

The orooosed construction schedule of 30 months or betterwas arguably a little better than for equivalent conventional


Figure 17.40 A cross-section oflhe proposed scheme for sphericalskirt-suDooded tanks

Figure 17.41 Examples of self-delivering anangements

tanks ofthe same capacity, and thiswas its main attraction. Thefinal costs would be expected to be similarto or greaterthan itscomparable conventional rivals. Despite the volume of effortput intothisscheme, itdid notgetoffthe ground and none havebeen built to date.

A variation on this proposal was to use the facilities which had

been set up around the world to manufacture the sphericaltanks used in the LNG carriers. These are highly sophisticatedsemi-automated factories which were a necessary part of thetanker building industry. The obvious advantage was that theycould construct spherical vessels quickly, cheaply and to highstandards. The disadvantage was that the spherical vesselsused in the carrierswere ofa smaller unitsize than the 100,000m3 plus which would normally be required on land-based stor-age facilities.

A "normal" ship's tank would be some 35,000 m3 in useable ca-pacity. Stretching the production facilities to their maximum sizelimit and adding an equator parallel course could perhaps in-

crease this to 45,000 m3. In recent years, for LNG facilities inparticular, construction time is ofren equally, and on occasions,more imDortantthan finished cost. The extra revenue, which an

early plant commissioning can earn during the time saved, maywell outweigh additional costs of such novel storage solutions

Self-delivering arrangements such as integrated barges(placed into the final position by a dry docking procedure) andattention to superior containment systems would seem to pres-

ent an attractive option for the industry

Summarising the advantages and disadvantages;

Advanbges:

. Factory production ensures fast delivery. cheapness andhigh quality

. Site civil work and vessel production are parallel activities

. System is pre-designed

. Units could be moved to another site if reouired

. Units could be pre-ordered and stockpiled to further savefurther time

Disadvantages:

. Unit size too small

. More sets of pumps and instrumentation required

Despite the apparent attractions, this scheme also failed tobear fruit. The reasons why are not immediately apparent.

17.1 1 Concrete/concrete tanksPrestressed concrete has become the most commonly usedmaterialforthe outercontainers of full containment lowtemper-ature tanks, both ofthe conventional 9% nickel steel innertanktype and the membrane type. lt is no surprise that the questionwas asked "Why not use pre-stressed concrete for the innertanks?" What is surprising is just how long ago this questionwas asked and how early the flrst development and test workaimed at determining the lowtemperature properties of this ma-terial were carried oul.

17.11.1 History of cryogenic concrete tanks

For many years, especially in the USA, water storage tankshave been constructed using reinforced concrete. The reinforc-ing to resistthe dominanthoop loadings was frequently appliedby the wire winding method. This consists of winding a largenumber of high tensile steel wires around the outside of a con-crete cylindrical tank. The wires pass through a die which in-creases the tensile load in the wire whilst increasing its strengthdue to the cold forming which takes place. The wires are fre-quently protected from corrosion and othersources ofpotentialdamage by the application of a sprayed concrete of the"Shotcrete" type.

Any vertical prestressing is applied using internal tendons lo-cated in the centre of the concrete wall.

ln 1950, the Linde division of the Union Carbide Corporationbegan an investigation into the possible use of pre-stressedconcrete for the storage of lowtemperature liquids such as liq-uid oxygen. By January 1951 test reports were available whichshowed thatthe thermaland mechanical properties of wire rein-forced concrete improved down to -196 'C.Based on this data, Preload Inc., a company who specialised inpre-stressed concrete structures, desi gned and constructed forLinde, a double-walled tank of 2,650 m3 capacity, for the stor-age of liquid oxygen. This tank remained in service until 1985when it was decommissioned and demolished. ln the late'1960s the American Gas Association initiated a testprogramme. This work was conducted jointly by the Institute ofGas Technology and the Portland CementAssociation and wasreported in Reference 77.30.

Esso Research and Engineering produced technical specifica-tionsiortwo40,000 m3 double wallconcrete LNG tanksfor GAzNatural (now Enagas). These tanks were constructed at theLNG import terminal in Barcelona and went into service in1968. These tanks have been in continuous successful servicesince that date. ln 1981 a similarLNG tank of80.000 m3caoac-itywas constructed on the same site and has given similarly un-troubled service.

In 1974 the Philadelphia Gas Works in the USAcommissionedtwo peakshaving LNG ianks each of92,500 m3 capacity. Thesetoo have been in uninterrupted trouble-free service. Figure17.42 shows these tanks during construction. Despite the un-


Figure 17.42 Construction of a peak shaving LNG tank

Coutesy of Preload Inc.

blemished record of service, this type of tank has not enjoyedthe success that it deserved.

Some years ago MW Kellogg, who in the eyes of many in theLNG field, is a major league arbiter of what is technically ac-ceptiable, gave the concrete/concrete concept its seal of ap-proval. The technology for this concept was owned by preloadInc. in the USAand is now in the hands ofa newcompany calledCryocrete. Since 1998, Whessoe, (formerly Whessoe Interna-tional Skanska), has had a license agreement with Cryocreteand a number of changes to the basic design have been pro-posed which will perhaps see a change in the fortunes for thisparticular type of tank.

17.1 1.2 Details of concrete/concrete tanks

The Preload type ofvertical cylindricalwall has been describedin some detail in Section 17.8 where it has been used as liquidcontaining high bunds for double containment systems of theconventional type. This type of wall is used for both the innerand outer tanks.

The base insulation in the central area ofthe tank bottom is usu-ally PVC foam. Under the inner wall the insulation consists ofprefabricated blocks made from plywood and balsa wood,which are subsequently sealed into a fibreglass shell. Theseblocks are designed for the compressive loads arising fromself-weight and seismic conditions.

To allowthe innertank to contract radially durlng cooldown andto accommodate shear loadings from seismic events, shearkeys are fitted to the underside of the annular plate and to thetopside ofthe secondary bottom. This arrangement is shown inFigure 17.43.

The insulation between the two walls is site-exoanded Derlite. Aconventional steel plate and section roof is provided whichcould form the outer roof in its own right, or provides the sup-porting formwork for a reinforced concrete roof. The inner roofis a suspended deck of one ofthe usual designs, which willsup-port the glass fibre insulation. The outer wall has a carbon steelvapour barrier which is installed close to the outer face andcompressed vertically by the vertical internal pre-stressing ten-dons and circumferentially by the wire winding.

This arrangement is protected from the weather and other ex-temalevents bya shotcrete layer. The slidingjoint between theouterwblland the base.slab precludes the need for the bottomcorner thermal protection measures required to control theshear stresses in the case of a built in corner detail describedabove.

The annularand bottom plates are made from g% nickelsteel.


1 7 Low tempetature storage tanks

Figure 17.43 Arangemenl for radial contraclion ofinnertank cooldown, using

17.11,3 Arguments for and against concrete/concrete tanks

As was the case for the similar discussions relating to conven-tionaland membrane tanks, a greatdeal has been written aboutthe subject, some of it objective and some less so emanatingfrom entrenched or self-interested view points. Ratherthan en-ter into this ongoing fray, it is proposed to list the subject areaswith hopefully simple and non-partisan comments:

For:

o Cost

. Enables largertanks to be constructed. Current designs areavailable uo to 250,000m3.

. lnner wall is stable under seismic axial compressive load-ings.

. Concrete and concrete placing skills may provide advan-tages in certain geographic areas over 9% nickel steel andthe metalworking skills required. lt mayforexample serve tomaximise the "in country" content work content

. The inner wall is better able to resist the external loadingsfrom the thermal insulation and may not require a resilientblanket.

Against:

. Unfamiliar technology despite track record of satisfactoryservice.

. Vulnerability of outer tank wire windlng to damage from ex-ternal missiles, fire and corrosion.

. The space required for the wire winding machine meansthat a wide interspace is required if simulianeous construc-tion of the tvvo tanks is to be achieved, otherwise the inner

tank must be built and pre-stressed before the outertank is

constructed. This clearly has programme implications.

. Concerns relating to the ability of the shear keys to with-stand high seismic loadings

. Concerns relating to the time required to decommission the

tank for internal insoection, should this be needed.

17.12 In-ground tanksThe three categories of in-ground tanks described in this Sec-

tion are what could be described as "true" in-ground systems.

There are a few examples of tanks, which at first glance would

seem to be candidates for this category, buton closerexamina-tion prove not to be suitable.

The LNG storage tanks at both the Zeebrugge and theRevithoussa import terminals are constructed with their bot-


toms well below the local grade levels, in the former case byaround one half of the shell height and in the latter case by al-most the full shell height. In both cases the reason behind thisunusual and very expensive departure from normal practice

was because of planning restrictions on the elevations of thetank profiles. Consequently what was constructed were con-ventional full containment 9% nickel steel/pre-stressed con-crete above ground tanks located in a purpose-built pit. The ad-ditional containment and protection provided by the pit was a

bonus and not the maln objective of the exercise. This type oftank is frequently described as the "in pit" type. Aphoto ofthe "inpit" tanks at Revithoussa is shown in Figure 17.44.

Figure 17.44 The "in pil" LNG tanks at Revilhoussa lsland, Greece

Coutesy of Whessoe

17.12.1 In-ground membrane tanks

ln-ground membrane tanks are closely related to the aboveground membrane tanks described in Section 17 9- There are a

number oftanks ofthis design to be found in Japan, Korea and

Taiwan. The first in-ground LNG tank was built in Japan at the

Negeshi Terminal in Yokohama using the 1-2 mm thick stain-

less steel membrane technology licensed from SN Technigaz.Later NKK had to develop a 2.0 mm thick membrane to complywith new Japanese regulations.

A number ofother Japanese contractors (MHl, lHl and KHI) de-

veloped theirown membrane designs, which were similarto the

original SN Technigaz design. These companies have also de-

signed and constructed in-ground LNG tanks. The world's big-g;st tank at 200,000 m3 capacity has been constructed in Ja-pan and is the subject of Reference t7 73 These tanks are

both expensive and time consuming to construct. In-groundstorage onlybecomes aviable option wheresome combination

of the following circumstances apply

. Reclaimed or very expensive land requiring optimisation ofeffective use of the area available

. Abnormally high safety standards perhaps due to conges-tion, adjacent industrial plant or high risk locations such as

Dort areas

. Highly seismic areas

Apart from the changes to the membrane thickness and the

size and spacing ofthe corrugations, the metiallic membrane is

as described in Section 17.9-

The civil engineering works associated with this design are in-

teresting and to the mind of the author quite remarkable. These

tanks are frequently built in areas of reclaimed land where soil

conditions are poor and ground water table levels are at orclose to grade.

To constructthe tank it is necessaryto de-waterthe area. To ac-

complish this, a slurry wall is built of a thickness (some 1.2 m)

sufficientto resistthe external pressures from the ground waterand to a depth where competent rock is to be found (up to 100

PRESTR:SSED COXCREI€

Figure 17 45 Asection lhrough a lypical in,ground tank

Cauiesy af SN Technigaz

m). When this slurry wall is completed, the excavation work cancommence and a reinforced concrete wall is built from the topdown on the innerface ofthe slurry wall. This can be up to 2.0 min thickness for a large tank. The base slab of reinforced con-crete must be of sufficient strength and thickness to resjst theground water pressure and can be up to 7.0 m in thickness.Within this inner wall and base, the conventional insulationpanel and metallic membrane is installed.

To avoid the problems associated with soilfreezing, this type oftank will require heating for both the base slab and the cylindri-calwall. The roof will be of the plate and section type and rnaybe sub-sequentially covered with reinforced concrete. lt isusual to employ a suspended ceiling supporting glass flbfe in-sulation in these tanks, although recent developments have in-volved the elimination ofthe suspended ceiling and the attach-ment of the roof insulation directly to the underside of the roofstructure itself. A section through a typical tank of this type isshown in Figure 17.45.

Reference 77.37 provides guidance for the design and con-struction of such tanks, applicable in Japan.

17.12.2 Cavern storage systems

The storage of LPG in unlined mined rock caverns has beenavailable for a number ofyears. The LPG is stored in liquid format a pressure appropriate to the temperature of the rock at thedepth of the cavern. The pressure from the local groundwaterexceeds the pressure of the stored LPG such that water leaksinto the cavern rather than LPG leaking out. This water collectsin a sump in the bottom ofthe cavern and is pumped out and fedinto a water curtain arrangement which maintains the groundwater pressure at a constant level in the area of the caverns. ltis clearly necessary to have suitable rock for mining of the cav-erns at a suitable depth appropriate to the product pressure.Thus propane would be stored in caverns located at a greaterdepth than would be necessary for the storage of bubne.

Cavern storage systems depend upon the appropriate subsoilconditions being available at the chosen location. lt is morecommon to find the necessary conditions at greater depths andaS a consequence Some facilities store propane in cavernS andbutane in above ground conventional tanks.

The caverns themselves are of a constant cross-seciion, thesize of which is dependent upon the local rock characteristics,and are often ofconsiderable length (some 100s of metres). lt iscommon to have a series of parallel caverns with total storagecapacities well in excess of 200.000 m3.


Fig!re 17.46 A simplifed sectlon through a cavefn

Pipework connections into the caverns are via vertical tunnelscarrying liquid inlet, liquid outlet using deep-well or submergedpumps, water removal pumps and the necessary instrumenta-tion. Asimplified section through such a cavern is shown in Fig-ure 17.46.

The advantages of cavern storage over conventional aboveground tanks from a safety point of view are self-evident. Fur-ther advantages come from the area of land used. lt is possibleto locate caverns beneath othet surface plant and in some cir-cumstances, beneath the sea for a large part of the system.

'17.12.3 Frczen grou nd systems

The development ofthe frozen ground tank was an effort to pro-vide cheap and safe storage for low temperature liquids. Sadlyitwas not a great success. There is only one iank ofthis type stillin service. This is locaied in Algeria at the Arzew liquefactionand export site.

These tanks bear some similarities to ihe in-ground membranetype in that they consist of a vertical cylindrical excavation withthe roof at local grade level. To enable the excavation to takeplace, the ground is frozen to allow the walls of the excavationto be self-supporting. This is done by drilling a series of holes ina circle to the full depth of the excavation. lnto these holes dou-ble concentric pipes are inserted into which liquid nitrogen or re-frigerated brine is circulated to freeze a vertical cylinder ofground. This circulation must be maintained untilthe tanks arecommissioned. There is no lining or thermal insulation pro-vided. Based on small-scale tests, the assumption was that thefrozen ground would give sufficient liquid tightness and supplythe required thermal insulation. At the local grade level a rein-forced concrete ring wall was provided around the top of the ex-cavation.

At Arzew the 38,000 m3 LNG tank had a carbon steel roofframework which was not plated over in the conventjonal man-ner and supported an aluminium suspended ceiling. This ceil-ing in turn supported the thermal insulation and a series of con-crete weights whose function was to counterbalance theinternal pressure. This tank is described in a paper given atLNG 4 in 1974 (Reference 17.32) and is said to have been inseryice for some B years at that time and to have taken two


17 Low tempercturc storcge tanks

years to construct, which makes the date ofthe originalconceptand design around 1964. The four 50,000 m3 LN G tanks of thistype built at Canvey lsland were similar except that the roofframework was plated over

The failure of these tanks to perform in a satisfactory mannerwas due to a number of problem areas:

. Thefrozen soilwas notastighta barriertothe liquid andthevapour as had been anticipated.

. The soil fissured and cracked allowing the area of frozensoil to extend much further from the tanks than was antici-pated. This was a particular problem at Canvey lslandwhere the region of frozen soil threatened to pass beyondthe site boundary

. Frost heave gave rise to significant ground movementsmaking the connection between the frozen soil and the con-crete ring beam and between the ring beam and the roofstructure difficult to seal against vapour escape.

. The frozen ground did not provide the thermal insulationthat had been hoped for, resulting in a higher than antici-pated heat in leak with the consequent practical and eco-nomic Problems.

The tanks at Canvey lsland were decommissioned. This wasno easy task. The integrity of the excavation had to be main-tained. and heat had to be supplied to warm upthe bynowvastvolume of frozen soil. Safety had to be maintained in the face ofLNG and in particular the warmerfractions leaking from the soilover a large area. The filling in of these pits with warm purged

sand is an interesting tale in its own right.

17.13 Novel systemsThere have been a number of novel systems proposed whichhave a large element of low temperature liquid storage, usuallyfor use with LNG.

Many of these schemes are aimed at LNG import terminals andinvolve floating or gravity-based systems (GBS). The advan-tage ofthese over conventional land-based terminals is that theowner is freed from the need to find a suitably protected anddeep harbour close to the market for the product being im-ported. This is often a major stumbling block for a terminal pro-ject.

There are also advantagesto be had from moving the construc-tion from the job site, as is the case with conventional terminals,to a ship or offshore rig yard where productivity rates may be

higher.

It had been hoped to describe some of these interestingschemes in some detail, but at this moment in time, the ownersand designers of the various schemes are concerned with theconfidentiality of their proposals and have not given their per-

mission to discuss the details and merits.

Atrawl through the proceedings of the LNG and Gastech con-ferences and the LNG Journals will reveal outlines of some ofthese schemes and hopefully in the near future some facilitieswill be constructed and the technical dehils published

17.14 References

17.1 J.A.Ward and R.S.HildreW LNG 1 Chicago 1968.

17 .2 LNG tndustry-A retrospecfive, Sir Dennis Rooke, LNG9. Nice 1989.

17.3 Frozen Fire - Where willit happen next?, Friends of theearth, San Francisco, 1979, ISBN 0-913890-30-8.

17.4 BS 4741 : 1971 VerticalCylindicalWelded SteelTanksfor Low Temperature Service: Single Wa Tanks for

312 STORAGE TANKS & EQUIPMENT ^

Temperatures down to -50 'C, British Standards lnstitu-tion, London, (Now superseded by BS 7777:1993).

17.5 Cryogenic Storage Facilities for LNG and NGL, N.J.Cuperus (Shell), 1oth World Petroleum Congress, Bu-charest 1979.

17.6 Developmentsin Cryogenic Storage lanks,6th Interna-tional Conference on LNG, Kyoto, 1980.

17 .7 Transport and Storage of LNG and LPG, Royal FlemishSociety of Engineers' International Conference, BruggeMay 1984.

17.8 Fracture Safe Deslgrs for Large Storage lanks, TheWelding Institute International Symposium, Newcas-tle-upon-Tyne, April 1986.

17.9 EEMUA Publication No. 147, Recommendations fortheDesign and Construction of Refrigerated Liquefied GasStorage Tanks, Engineering Equipment and MaterialsUsers Association. London. 1986.

17.10 BS 7777 : 1993 Flat Bottomed Veftical Storage Tanksfor Low Temperature Servlce, British Standards Institu-tion, London.

17.11 Bigger and Cheaper LNG lanks, Bob Long, LNG 12Perth, Australia, May 1988.

17.12 Development of above ground Storage Tank Designs inJapan, HitoshiHiose - Toyo Kanetsu K.K, LNG JournalNovember/December 1998.

17.13 Construction of an underground Storage Tank,Yanagiya and Ogawa - Kaiima Corporation, LNG Jour-nal November/December 1999.

17.14 API 620 Tenth edition February 2002, Design and con-struction of large, welded, low-pressure storage tanks,American Petroleum Institute

'17.15 BS 5387 : 1976 Veftical cylindrical welded storage tanksfor low-temperature Service: double walltanks for tem'peratures down to -196 'C, British Standards lnstitution.

17.16 BS EN 1473: 1997 lnsta ation of equipmentfor lique'fied naturalgas -design of On-shore instal/aflons, Euro-pean Committee for Standardisation and BritishStandards Institution.

17.17 Developments in cryogenic storage fanks, N J.Cuperus - SIPM, LNG 6 Session ll, paper 13, Kyoto'Aoril 1980.

17.18 Dynamic load attenuation for double wal/ tarks, R AVater - Pittsburgh-Des-Moines Corporation, Gastech84. Amsterdam. November 1984.

17.19 Experimental dynamic compaction of perlite insulation,T. Kauos - CBI Industries Inc., Gastech 84, Amsterdam,November 1984.

17.20 tntroduction of the EEMUA recommendations for thedesign & construction of liquefied gas storage tanks,John de Wit - SIPM, Chairman EEMUA Tank Commit-tee, Chairman BSI Tank Committee, API/BSI Confer-ence, San Diego, May 1986.

17 .21 LNG storage tanks : Developments & key elements, C.B. van Liere - SIPM, LNG owners' seminar, Session lV :

1988.

17.22 Developments in the standardisation of single, doubleand fult containment tanks for the storage of refrigeratedliquef,ed gases, D. Dickie - Motherwell Bridge ProjectsLtd, R. Long - Whessoe Pfojects Ltd, Gas Engineering &l\.4anagement, Vol 24, September 1989.

17 .23 Cryogenic storage of liquefied gases (Pafts 1' 2 & 3)'Fritz Papmahl - Noell LGA, Hydrocarbon Asia, April,Mav. June 1996.

17 .24 NFPA 59, Utility LP-Gas Plant Code, 2001 Edition, Na-tional Fire Protection Association, Quincy, Massachu-setts.

17.25 NFPA 59A, Standard for the production, sforage &handling of liquefied natural gas (LNG), 2001 Edition,Nationalfire ProtectionAssociation, Quincy, l\4assachu-sefis.

17.26 French LT tank Code above-ground or semi-buriedtanks for low pressure liquefied gases, guidelines fordesign and construction, Association technique deI'industrie du gaz en France (AGT), Publication date un-known.

17 .27 Comparative SafetyAssessment of Large LNG StorageIanks, R.Giribone (Bureau Veritas) and J.Claude (SNTechnigaz), LNG 11, Birmingham, July 1995.

17.28 Quantifaction and Comparison ofthe Risks ofLNG Stor-age Concepts- Membrane and Full Containment,

te mqe ntu rc storage tan ks

PGenoud (SN Technigaz) and N.Ketchell, R.G.A.Rob-inson (AEA Technology), LNG 12, Perth, May 1998.

17.29 Spherical skitt suppofted tanks for onshore LNG stor-age, lEinstabland (Selmer), E.H.Hektoen (KvaernerBrug), R.Schrader, (l\.4oss Rosenburg Verft), LNG 7 Ja-karta May 1983.

17.30 Preslressed concrete at cryogenic temperatures,Eighth Congress of the Federation lnternational de laPrecontrainte, London 1 978.

17.31 Recommended practice for LNG in-ground storage,Japanese Gas Association, First published March1979.

17.32 Huit ans d'activite d'un stockage souterrain de GNL,A.Benadi Chef du Service Camel Algeria, LNG 4, Al-giers Juin 1974.


314 STORAGE TANKS & EAUIPMEN+

18 The design of low temperaturetanks

A considerable part ofthe design procedure for low temperature tanks is based on the practicesused for storage tanks for the containment offluids at ambient temperatures. These practiceshave been described in earlier Chapters. Where the different containment arrangements, lowertemperatures and higher pressures cause these procedures to require modification, thesechanges are discussed in this Chapter.

The Chapter is restricted to consideration of the following design Codes:

API 620 Aopendix R

API 620 Aooendix Q

BS 7777

prEN'14620

Contents:

18.1 General

18.2 Tank capacity

18.3 Shell design18.3.1 The API 620 Appendix R approach

18.3.1.1 Hoop tension - liquid containing tanks18.3.1.2 Nonliquid containing tanks

18.3.1.3 Axial comoression

18.3.1.4 Wind and vacuum stiffening

18.3.1.5 Shell stiffening for external insulation loadings18.3.2 The API 620 Appendix Q approach

18.3.2.1 Hoop tension - liquid containing tanks18.3.2.2 NonJiquid containing tanks18.3.2.3 Axial compression


18.3.2.5 Shell stiffening for external insulation loadings'18.3.3 The BS 7777 aooroach

18.3.3.'l Hoop tension - liquid containing tanks1 8.3.3.2 Non-liquid containing metallic tanks18.3.3.3 Axial compression

18.3.3.4 Wind and vacuum stiffening18.3.3.5 Shell stiffening for external insulation loadings18.3.3,6 Addendum to BS 7777 on partial height hydrostatic testing

18.3.4 The prEN 14620 approach

18.3.4.1 Hoop tension - liquid containing metallic tanks1 8.3.4.2 NonJiquid containing tanks18.3.4.3 Wind and vacuum stiffening

18.3.4.4 Shell stiffening for external insulation loadings

18,4 Bottom and annular design18.4.1 The API 620 Appendix R approach

18.4.'1.1 Liquid containing metallic tanks18.4.'1.2 Nonliquid containing metallictanks

'18.4.2 The API 620 Appendix Q approach18.4.2.1 Liquid containing metallic tanks'l 8.4.2.2 Nonliquid containing metallic tanks

'18.4.3 The BS 7777 aDDroach

18.4.3.1 Liquid containing metallic tanks1 8.4.3.2 Nonliquid containing tanks

18.4.4 The prEN '14620 approach


i

18 The design of low tempeftture tanks


18.5 Compression areas . ,18.5.1Thd API 620 appRiddi18.5.2 The BS 7777 approach


18.6 Roof sheeting18.6.1 The API approach (Appendices R and Q)18.6.2 The BS 7777 approach


18.7 Roof franreworks18.7.1 The API approach (Appendiees R and Q)18.7.afhe BS 7777 approach'18.7.3 The prEN '14620 approach

18.8 Tank anchorage18.8.1 The requirements ofAPl 620 Appendix R

18.8.1. 1 Liquid containing tankiq8.8.1.2 Non liquid containing lanks

18.8.2 The requirements ofAPl 620 Appendix Q18.8.2.1 Liquid containing tanks18.8.2.2 Nonliquid containing tanks

18.8.3 The BS 7777 reouirements


18.9 Tank fittings18.9.1 The requirements ofAPl 620

18.9.1.1 General reouirements of API 620 section 518.9.1.2 The particular requirements of API 620 Appendix R18.9.1.3 The particular requirerhents of API 620 Appendix Q18.9.1.4 The design of heat breaks

18.9.2 The reouireiTents of BS 777718.9.2.1 Outer container mountings

18.9.2.2 Inner tank and outer liqriid containing tank mounlings18.9.2.3 Connecting pipework between inner and outer tank connections


18.10 Suspended decks18.'10.1 The requirements ofAPl 620

18.10.2 The requirements of BS 7777


18.11 Secondary bottoms

18.12 Bottom corner protection systems

18.13 Outer tank concrete wall and bottom liners

I 8.14 Connected pipework

18.15 Access arrangeinents

18d6 Spillage collection systems

'18.17 Reinforced and prestrbsseu concrete Component design'18.17.1 General

18.'17.2 Tank bases

18.'17.3 Tank walls18.17.3.1 Above ground tanks

18.17.3.2 In{round tanks18.17.4 Bottoni comor details'18.17.5 The toD comer details

18.17.6 Tank roofs

18.18 References

18.1 GeneralThe design of low temperature tanks has evolved from the de-sign of tanks for ambient temperature service. In many areasthe design methods are the same or very similar to the ambientpractices. In these cases reference will be made to the earlierChapters concerning the origins ofthe design methods and thederivations of the formulae used for ambient tanks. Where thelow temperature practice differs from the ambient designmethods, this will be described in ful

There are a number of different codes covering the design oflow temperature tanks in force around the world. The followingdocuments will be considered:

. API 620 Appendix R (for products down to -60 'F)

. API 620 Appendix Q (for products down to -260'F)

. B57777 (fot products down to -196'C)

r prEN 14620 (for products down to -165 "C)

There are Codes from other European countries (DlN andAFNOR for example) but for reasons of simplicity and becausethese documents will shortly be replaced by the new EuropeanCode, they have not been discussed in dehil here.

There are also Codes which are no longer curreni, but havebeen important in the development process which has givenrise to the existing regulatory documents. BS 4741, BS 5387and EEN/UA 147 allfall into this category and will be mentionedwhere appropriate.

18.2 Tank capacityBefore the real calculations start, it is necessary to determinethe main dimensions ofthe tank. The initial estimate ofthe tankgeometry for a particular storage facility, is arrived at by consid-ering a large numberofdifferentvariables. Amongst these are:

. The operating volume required.

. Any planning constrainb on the tank height.

. Any limitations related to the maximum discharge pressureof the pumps of ships discharging into the tank.

A - 600 Tm min n"Jn I provded by rhe choser rr-ranh purp sLpprelB=400mm )

C =Coresponds lo the lullopeEting volume

(Volume between low€sl levelatfulllow pump down and maximum nomalopeEting level)

D = Conespondsto X minutes flN at maimlm liquid import rate

E = corespondsto Y mnltesflowat maxinum liqlid import €teF = CoffespondsioZ mnutesflowat maxirnum iquid inpoir raleG = min mum ireeboad (commony500 mm)

D+E+F+G = height requ red to contain product wave due to €arlhquake

1B The design of low temperature tanks

. Allowable subgrade loadings.

. Seigmic design criteria dictating tank proportions and sloshheight.

. Site space constraints.

. The tank contractor's views on the most economical tankaspect ratio.

. The performance limitations of the chosen in-tank pump (ifuseo).

. The elevation ofthe suction outlet connection (if used).

. Operational considerations concerning pumping rates andrequired response time intervals related to the various levelalarms.

In order to help in sorting out this multifaceted problem, it is of-ten helpful to produce a sketch illustrating the various signifi-cant liquid levels. Such a sketch for the inner tank of a full con-tainment LNG tank is illustrated in Figure 18.1.

When the tank diameter is chosen, it is necessary to make al-lowances for the thermal contraction of the liquid containingtank. For metallic tanks the data provided in Figure 18.2 shouldprove helpful.

For double-walled tanks, the diameter and height of the outertank will be based on the chosen dimensions of the inner tankand consideration of the following:

. The interspace width required. This willin turn be related to:

- The wall insulation thickness required.

- Access for personnel to work in this area.

Access for resilient blanket and wall liner installationfrom suspended cradles.

. The insulation hopper volume where loose fill insulants likeperlite are used.

. Any requirements for the total impoundment volu me arisingfrom regulatory Standards.

. Physicalspace requirements in the top corner ofthe tankforroof insulation, internal runway beam access, etc.

Flgure 18.1 Miscellaneous innertank levels


1B The design of low temperaturc tanks

l,"1

I

)

Flgure 18.2 The thermal contraclion of lhe liquid conlaining lank

18.3 Shell designThis Section confines its attention to the design of the metallicshells of vertical cylindrical tanks. The material selection crite-ria are mentioned in passing in this Chapter, but are discussedmore fully in Chapter 21. The design of concrete tanks of thisform are dealt with in a later part ofthis Chapter As discussed in

Chapter 16, these steel tanks may be single-skinned tanks orthe inner and outer shells of double-walled tanks. These shellsare the subject of combinations of the following loadings:

. Hoop tension caused by the maximum head of product liq-uid together with any associated internal operating pres-su re.

. Hoop tension caused by the maximum test water head to-gether with any associated internal test pressure.

. Axial compressive loadings caused by combinations ofself-weight, internal vacuum, external (i.e. interspace) pres-sure, wind loadings, snow loadings and insulation loadings.

. Axial tension loads caused by combinations of internal tankpressure and wind loadings (this is usually of little conse-ouence).

. Shell buckling loadings caused by wind loadings.

. Inner tank buckling loadings caused by external loadingsarising from loose fill insulation systems, occasionally incombination with internal vacuum and external (i.e.

interspace) pressures.

. Various loadings arising from seismic events and their im-pact on the tank structure. These are dealt with separatelyin Chapter 26.

18.3.1 The API 620 Appendix R approach

When this Appendix was originally written, refrigerated tanksfor the storage of products down to -60'F were only ofthe singlecontainment category These would be either single wall tanksin contact with the product fluid or double-walled tanks wherethe inner tank would contain the low temperature product (andin the case of flxed roof inner tanks, the vapour pressure aswell). The outer tank would contain or supp@rt the insulationand contain the vapour pressure (or in the case of a fixed roof


inner tank, the interspace purge gas pressure).

With the passage of time, tanks of double or full containmentcategories came to be required bythe industry Despitethe factthatthe API Codes do not considerthese forms of containmentwithin theirscope, the rules ofthe lowtemperature appendicesare commonly used to design the metallic tanks ofdouble orfullcontainment systems.

API 620 Appendix R divides the various tank components intothree different categories. The Code goes into some detail toensure that the various components are categorised correctly.The categories are:

. Primary components. In general primary components in-clude those components whose failure would result in leak-age of the liquid being stored, those exposed to therefrigerated temperature, and those subject to thermalshock. The primary components shall include, but not belimited to, the following parts of a single walltank orthe innertank of a double wall tank: shell plates, bottom plates,knuckle plates, compression rings, shell manholes andnozzles including reinforcement, shell anchors, piping, tub-ing, forgings and bolting. Roof nozzles in contact with therefrigerated liquid shall be considered primary components.Primary components shall also include those parts of a sin-gle wallor an innertank that are not in contact with the refrig-erated liquid but are subjectto the refrigerated temperature.Such components include roof plates, roof manways andnozzles with their reinforcement, roof supporting structuralmembers and shell stiffeners when the combined tensileand primary bending stresses in those components underdesign conditions are greater than 6000 lb/in2.

. Secondary componenls. Secondary components arethose whose failure would not result in leakage of the liquidbeing stored. Secondary components also include thosecomponents which are not in touch with the refrigerated liq-uid but are subiect to the refrigerated temperature vapoursand have a combined tensile and primary bending stressunder the design conditions which does not exceed 6000lb/in2. Secondary components which could be designedwithin this reduced stress are roof plates, including roofmanways and nozzles with their reinforcement, roof sup-porting structural members and shell stiffeners.

alnararin^ ^.ea.D

to =

- lgSwoHo + Po l+ ca

ZUDO

Test case:

1, = L 196*, 11, * o, 1' 20sr 'where:

. Basic components. Basic comoonents are those that con-tain the vaporised liquefied gas from the stored refrigeratedgas but primarily operate at atmospheric temperature be-cause of insulation system design and natural ambientheating. These components shall comply with the basicrules ofthis Standard (API 620). Examples ofsuch compo-nents are the outer wall and roof of double wall tanks androof components above an internally insulated suspendeddecl(.

l\.4uch of the distinction between the various types of compo-nents is concerned with the material selection and the impactiest requirements. This subject is covered in detail in Chapter22.

18.3.1.1 Hoop tension - liquid containing metallic tanks

The following applies to liquid containing tanks, i.e. sin-gle-walled tanks, the inner tank of double-walled tanks wherethe outer tank is non-liquid containing and the inner and outertanks of double-walled tanks where both the inner and outertanks are designed to contain the product liquid.

The basic formulae used are derived in the same way as hasbeen described in Chapter 4, Section 4.7 for ambient tanks. lt isconvenient to express them in the same form. Hence in metricunits they become:

18 The design of low tempercturc tanks

suggested that the Table 3-2 footnote 2 rules are fol-lowed, i.e. the lesser of 30% of the specified minimumultimate tensile strength or 60% of the specified mini-mum yield point using the 0.92 quality factor where ap-propflate.

6. The allowabletest stress limits can be taken from Ta-ble Q-3 or be based on the lesser of 85% ofthe speci-fied minimum yield skength or 55% of the minimumspecified tensile strength of the material.

The design point is at the bottom edge ofthe course under con-sideration and not 0.3m (or one foot) abovethis levelas permit-ted by BS 2654, API 650, BS 7777 and the new EN.

The minimum thickness requirements are as usuala function ofthe tank diameter and are reproduced in Figure 18.5.

The radiographic inspection requirements given in paragraphR.7.6.1 for '! 00% radiography of all shell plate joints where theactual operating stress across the welded joint is greater than0.1 times the specifled minimum tensile strength of the platematerial (i.e. all vertical seams of liquid containing tanks),means that a joint factor of unity allowed by Table 5-2 ofAPl 620(Figure 18.6) will always be applied.

Forthe inner shells ofdouble-walled tanks, the addition ofa cor-rosion allowance is quite unusual. The combination of the lowtemperature and products which are benign from a corrosionDoint of view make the inclusion of additional metal unneces-sary. Ammonia tanks where stress corrosion cracking is aproven hazard are a possible exception. The addition of mate-rial to the minimum calculated thickness to allow for futuredressing out of surface cracking may be seen as money wellspent. This is discussed further in Chapter 20.

For single-walled tanks or the outer shells of double-walledtanks the addition of a corrosion allowance to cater for the pos-sibility of external corrosion is not unusual. Un-insulated outershells are particularly vulnerable atthe shell-to-bottom junctionwhere rain water rnay pond and in the vicinity of external shellstiffeners. Good housekeeping in terms of regular external in-spection and the maintenance of protective paint systems isself evidently a sensible precaution.

18.3.1.2 Non-liquid containing tanks

Paragraph R.5.3.2 allows single lap-welded or single-sidedbutt-welded shells wherethe thicknessdoes notexceed %" and

double-sided buttwelds not having complete penetration orfu-sion, at any thickness for tanks not in contact with the vaporisedliquefied gas. Clearlythe single-sided lap and butt welds shouldbe made from the outside surface for reasons ofcorrosion pre-vention. Such tanks having fixed inner roofs are quite unusualthese days. Paragraph R.5.3.1 requires a minimum shellthick-ness of %6" in this case. This is appropriate for very small tanks,

but may lead to axial stability problems as the tank size in-creases. lt would seem wise to apply the methods described inSection 18.3.1.3 to ensure that a safe structure is specified.

Where the tank shell is in contact with the vaoorised liouefiedgas, Paragraph R.5.3.3 requiresthe rules ofthe body (i.e. sec-tion 5) ofthe Code to be used. This means that the shell must bechecked for hoop tension caused by vapour pressure and pos-sibly any internal pressure due the loose fill insulation. The for-mulae given in Section 18.3.1.1 can be used and the allowablestresses shown in Figure 18.3. The minimum thickness re-quirements are again as Figure 18.5.

Bear in mind that for outer tanks the level of radiography is nolongerdictated by Appendix R and can be such as to require theuse of a joint factor of less than unity. The relationship betweenjoint factor and level of inspection is shown Figure 18.6.

'l 8.3.1.3 Axial compression

The behaviour of thin cylinders whilst subject to compressiveloadings is an interesting subject in its own right. Agreat deal of

equ 18.1

ntct

Il

)e

)e

3rS,

)-te

s.'t-

)-3.

rd

al

e

toto

''l-

k-

ede

t.S

is

0d)f)-

equ 18.2

to = shell thickness due to operating case (mm)

tj = shellthickness due to test case (mm)

D = tank diameter (m)

Ho = height from the bottom of the course underconsideration to the highesi product liquid level(m)

Hr = height from the bottom of the course underconsideration to the highest test water level(m) lnote 1]

wo = maximum anticipated SG of product liquid butnot less than 0.577 (equivalent to 36 lb/ff)

wt = SG of test water [note 2]

po = maximum vapour pressure above the productliquid (mbar) lnote 3]

pi = test pressure (mbar) lnote 4]

ca = corrosion allowance (mbar)

So = allowable stress for the operating case(N/mm'z) [note 5]

q = allowable stress in the test case (Nmm'?)

lnote 6l

Notes: 1. The maximum test water level is required by Para-graph R.8.3.3 of the Appendix to be equal to the maxi-mum product liquid level.

2. Usually 1 .000 but in unusual cases could be sea wa-ter with an SG of up to 1.025.

3. In the case of open-topped inner tanks this is zero.

4. Required by Paragraph R.8.4.1 to be 1.25 x po.

5. Allowable operating stress limits illustrated in Figure18.3 or in Figure 18.4. For non-APl listed materials it is


18 The design of low temperatute tanks

Spcci6cation(Scc Notc l) Gradc Notcs

Tcnsilc Strcnqth(lbf/in'21-

Spcciliod Minimum Maximum AllowablcTcnsilc Sucss for

Tcnsion. S,.(lbf/in.2, scc Notci2 and 3t

Yicld Polnl(lbf/in.z1

ASTM A 36ASTM A I3IASTM A I3IASTM A13IASTM A 283ASTM A 283ASTM A 285

ASTM A 516ASTM A 516ASTM A 516ASTM A 516

ASTM A 537ASTM A 537ASTM A 573ASTM A 573ASIM A s73ASTM A 633

ASTIYI A 662ASIlr,l A 662ASTMA 678ASTM A 678ASTM A 73?ASTMA E4I

rso 630tso 630

Cla$ IClass 25865mCrdD

BcAEBClas! I

44,5and64

4and54,5and6

:

4ud84ard?

Platcs

s8,m058.00058,m058,m055,0@@,(m55000

55,00060,0m65,000?0.0@

?0.00080,00058r0065m?0,0@?0,000

65.0@70J@70,0@E0,0@?0J@70J00

59Jm65Jm6530069J@61,[email protected]@

36,m034,0m34.00034,00030,00033,0m30,m0

30,00031000350m38,000

50,0@@,0@32,00035,0004a00050,0@

40,0043.0@50,0@60,m50,0m50.m

37.?0043,50050,80050,t0037,00048,J@

r6,m0r5.200r 6,000r6,000| 5,200| 5,2@| 6,500

16,5001E,000t9J002r,000

'2t,m24,000r6,0@1E,000r9,30019J00

r9.50021,0@l9J@22,[email protected]

16,40018,m018,0m19200l7,l@19,600

BcscDc

55@6570

CSA G4021-M 2Ow .nd 2@WrCSA C40.21-M 300w lnd 300wTcsAG4o.2r-M 350WcsA G40.2t-M 350WT

Ee75 Qurlfuy q D8355 Quality C, D

7

4444snd7

444444

ctrnlcS!API SpocASTr,l AASTM AASTM AASTM AASTI{ AASTM AASTM AASTM A

Elccric-FusionWcklcd

ASTM A I34ASTM A I]4ASTM A I39ASTM A 67IASTM A 67IASTM A 6? I

ASTM A 67IASTM A 67IASTM A 67IASTM A 6? I

ASTM A (t7 |

Figure 18.3 Maximum allowable stress values for simple tension - page 1

Frcn API 620, table 5-1


5LB138106 B106 c333 l333 3333 6524 I521 tl

4,5and95and99999

7and97and9

PtF

@,m060000@,0070,m055J0065.0m

@,&055,000

[email protected][email protected],J65.ffX)'lo.(m70.fix180.(xxl55.(XXl({1.{xxl

35m35,00035,00040,0m30,m35,000

t5,00030.m

30,0m30.00035.000rc.(m:12.0mt5.000t8.fix)50.fin$.u.x)l{).00(ll:.txxl

l&[email protected],000r&0m21,0@16,5m19J00

t8,m0r6.500

r2.lmr1.20014.,fi)t:r.200| 4.400l5.({x)r6.8(X)l6.ltult9.!(xl|.i.:(xll-l..llxl

A 281 Cr"dc CA 285 Gradc C

cA55cc60cc65cc?0cD?ocD8()cE:5CE({,

18 The design of low temperature tanks

a

'd 3) Spccifrcd Minimum Maximum Allowable

Tcnsilc Strcss forTcnsion. S,.

(lbf/in.2, sc€ Nok;2 and t)Spc6ifrcatiort(Scc Noe l) Orade Notcs

T.nsilc Srens|h(lbflin.2)-

Vcld Pojnl(lbf/in.z1

ASTM A IO5ASTM A I8IASTM A I8IASTM A 350ASTM A 350ASTM A 350

Forgiogs

60,0m60,0m?0,0@60,m70,m70,0@

10,m0r0,00036,00030,00036,m040,0m

I

LFILF2LF3

I E.000t8.0002 r.000r&0m2 r,0m2!,0m

ASru A 27ASTM A 36ASruA 193ASTM A 30?

ASTM A 307

ASTM A 320

60.30For alcho. boltingB7B for frngcs8rd pflsturr parlsB for lruc{l.rlporB s|d rnctorboltingvt

Crstings lrd bolthg

60,00058,000

125,0@55,m0

s5,000

l25pm

lollllll lrd 12

ll

ll

30,00036,000

r05j00

r05,0m

14,4{nt5,3m,w

8,400

r5.000

24,(m

Suqc-ln"l sh.p6 Rcsisting lnt (rlal Prersur€

ASTM A 36ASTM A I3IASTM A 633ASr r A 992csA c40.21-Mcs^ c40.21-McsA G40.2I-M

260W rnd 26OWT3mw |rd 300WT350W lnd 300wT

4rrrd 64 ,',A6

4.nd64lnd64{rd64and6

58,0m58,0@63,m65,m59J006530069,600

36,00034,(m42.(m50,0m37Jm43,5@50,800

t5Jml5:ml?,4001520015200r52mr5200

Notcs:

l. All pcnircnt rnodifcrtioos rtd limiradoru of rpocifioti,os tlquild by 42. tlFugh 4,6 shdl bc complicd with.2. Erc.p( for drcac 'rscs wlrt! ldditiond fdors or litlittrias rc |pplid 16 indicdld by trfcrcnclr io Notcs 4, 6, l0 |nd 12, rhc .llowsblct!tl3ilc![!3s \|.lu(r gitttr in lhi! trblc for matlrid3 o{trr tllln boltiog rtcl l[c tlE lcasrr of (.) 30% of rhc rpccificd ninhum ulrimlc acnrilcstslngth f6 thc msr.{ial or O) 60& of $c spccifi.d minimultr yicld loiol3._Ercspt whctr . joiot cfficic{tcy &crot is alrcady lrdc$d in 0|c.pccifcd allowlbh sallr vsluc. r' idicatcd by thE lrfctlnccs ro Nolc lO, or{llcrr d|c vrlur ofrvdcEdfncd in racord'r|c€ with 5.5.33. i! lcss rhln rlrc .pplicsblc joinr cffciqrcy giv.n in'ibblc 5.2 (ud 0rcnforc cficcbr grtx.r.€duclion in rllowrblc rfrts th.n seuld lhc F&&

'in rfficicrrctfldq, ifrpplicd), rhc ipccifica *ts rnluas for w€tds in rslslon

shrll bc multipllt by dlc rpplicrbk joinr cfficicrry lrctor, E, gircn in Trblc 5.21

4. Swss valucs for strucrurtl qudity stc.ls inch& ! quality frdor ofo.92.5. Platcs.M pip. shrll not bc rsrd in $iqkncss grcarcr thln l/. in.6. St.q(r valucs arc limir.d ro rhosc for srccl thal hrs.n lltin|at. rcnrilc snrngth of only 55.000 lbf/in.l.7. lrss than or cqual ro 2ll2 in. thickncss.

8, Lrss than o. cqllll to I l/2 in. $ickncss,9. Stcsr valucs for fusbn-s'cldld pipc includc a wcldcd-joifl cf,icicncy fsdor of O.m (rcc 5,23.3). Only strrighr"srrm pipc shall hc oscdi thcusc of spiral.scam pifrc is prohibicd.

10. Strc$ vllucr for cr"slings irrlude a qualiry f|cror of0.80.ll. Sc( 5.6.6

f2.Affowahlcsttd$shascdonSectionvllloftltcASMEBoifu..1PtI.tturryrrrclCrrdcInullinhcdhythc.atioof0rdc\rfnstrc\\tiKr(rrsn!this rlandld rnd Secraon Vl | | ot rhc ASME (i'dc. niJrnlctv 0 lt!{).ti

: !ure 18.3 lvaximum allowable stress values for simpte tension - page 2-..m APl620, table 5-1


1 I The design of low temperaturc tanks

Spccid Minimun Allov$L Stcsr

ASTM sFcifa.|bns TcoiiksrEngltt Yi.:ldstittlth DrsiSo

PlatcandSEua all Mat art

lm.mo t5,mA 353

A 553.Typc I

A 645

A 240, TyF 304

A24o,qpc 304L

I 209, Alby m34E aD,Alloy 5052{

B 209,Altoy 5m'0B.Ulg,Allot 506&0

E 209, Alby 5154.0B 209,ltby 5456-0

I 22l,Alby 3(xl3{I22l,Alby 5052 o

B 22l,Alby 563-0B 221, Alhy 5086'l)

E 22l.llby 515{4

E 221, Nhy 5456{B 221, Nloyr 6(}61-T,l.dT6

t00,0m95S0

?5,m700m

r4,0q,t5S4),m335,m0

30,m

'12@3

l4,m25,000

85,m65.m

o$o25,000

500095m

r8.$0rt4,06

,0m19,0s

rJmr0@

3t,?@

22,s18,750

3,7507,t00

t3J00r10J00

8r50r4pmr

3,?f,1?r00

12@t0,s)83Cl

13.fl08Jm

8@

423d

NMn-w

4J(D8550

r6rtr12,6m

9,9@l?.rG

4Jm9,m

l4/@r16009,9d)

r?.1@l0,m

ro@

16.m0140srtrsl9@

39,000

35.0m30,0@

4Lm24.m

UNd)6l-T4 rodlt

A33f,Gndc tA 33l,Grdc 8

A 213. ond. TP, Bpc 304

A 2 t3, Cndc TP,lYF 3O4L

A 312" C,t!d. TP, DT. l04cA 3la GndcaP,BF 304Lc

A 35& G(!d. 3O4. c1|cr I

B 210. Alloy 3303.0

B 2 10, Alby ml-H I 12

B 210. Alloy 5052{

MO. Alloy :i086{)I 2 | 0. Alloy 5154{}

0 Xl.Alloy 5052-0

tl :.1l. Alk'y 5{|[|1'0

Figure '18.4 Msximum allowable st€ss values - page ?

From API 620, table @3


PlFinS |nd lbbhtl00nmt00,m

75,(m70000

75s?0,m

75.0(I)r4.0(D

14.(m25.(tro

35,000!or(I)

25,m39.0m

75,m?5,0$

30.0m25.000

1!@25p00

[email protected]

5rq)togn

14p0or11,000

10,0001q00o

22Jmt&7$

nfir&7x)

tffi3.750

3.?50?_aD

o-lh8,250

7500u,000

27S022Jm

2?.mzlj00

2?IID4.500

4.500etm

r]Irn9p00

9,000140oo


Strcss vatuc (tbflin.r)

SF.ial Minimum Allowablc Stress

ASTM Spccificatioos Tccrilc Strenglh Yicld Srcngh Dcsig|l

B 24l,AUoy $86{

B 24l,Alloy 5454{)B 241, AUoy 5456-0

B ,1,14 (UNS-1.I06625), GrJdc IB ,144 CJNSnm6625), Grdc 2I 619 (UNS-N10276), Ct si lcB 622 (t NS-NlOtr6)

35,000

31,000

41.0@

120,000

100,000

rm,mo100,000

r4.000

1e000t9.m0

60,000

400004to@41,000

r0,500

9.000r3,650

12,600

t0,80017.r00

54,0@r36,m0136,90d36,900f

40,mof30,mof30J5d30J5d

Forging:

A5nA 182, C,rad! F,'IYpc 3&A 182, &r& F, lYF 304L

B ZT,Aloy 3003-Hl12B 247, Aloy 5083-Hl 12 Modd

lmpm?5,000

65,000

1,1,000

3qm

75,000

30,000

25,0@

5,000

16,000

3,75012,0m

2?O0022fi

4J00t4,400

22,fi18,750

Bolting.

B 2l l, Alloy 606l-T6

A 320 (sfi..io+arddtcd cn& 88, BgC, B8M.od B81)

s3h'.f,r>%-1in.>l-l[4i&> lt/4 - lrzir.

A 320 (rolutiotr{r.lFd rrd !frin+{d.d gradcsr,}ao c&ldcd)

Grrdcs 88, BtM, rd BAFdI riz6

42,m

r25,mlt5.m105,@0

r00,@0

75,(m

35!O

lm,od)80,(m65,000

50,(m

30,000

lojm

30,m026tro2r,m016.000

r5,m0

Nob8:

rThc alloq[blc rEErsls for thcsc tDfirdrb rtr bssad oo thc lowcr yiald rnd Ensilc lErhgth of thc wEld firral or baJc mct l, s3 drl.Fldncd by Q.6.1, .rd *l. d!6ign rulcs in Q.3J2. Thr Einimum mc.surcd tcoiilc s@o8lh rhalt be 95,m0 lbf n.2 and mhimum mcr-lu|ld yickt ilrryth shdt bc 52,500 lbflir2. The maximrm p.rnidcd vrluca !o bc uscd fo. dcr.rminin8 rhc illow.btc ar.ss arc100,0m bffurz. for r.nsilc slr.ngor rnd 5&OOO lbflin.z for yicld itrrnglh,DBascd oo tha yicb and Gtt$iL atEnglh of lhc wrld nEtrl, rs dctlnnin€d by Q.6.1. Thc oinimum rncssu.cd r.arsile rtcng$ shall be95.0@ Fi nrd dr odnirun mc-aru.ld yi.ld rltngrh shall bc 52J00 lbf/rn.l.qior u/clding pipint or tubing. a joi .ffici.rra of 0.80 shall bc spplicd lo tlrc .llorJablc suts.€s for longiMind Fir{s in accordanccwith 5.23.3.(rnE

dcaignalion Mod rcquitts thar lhc maiimum tmsilc and yicld srcngrh ard lhc rnioirnum clong.rion of lhc marcrial conform |o rhcli.ntu of B 209, Alloy 50814.

gThcs. allowablc $rc$ vrluer arc for ma(crials ihickness up ro end irKkrding | -5 ir. hr rhicknBr ovcr L5 in.. atlow$lc srrcrs vrh|!\ arclo k eit blistEd pcr Q..1..r.1 usiog ASTM dara of lcnsilc (ul(matc) and yic'd srftngrh Jor rhcsc Crad!:t.tNor ro bc rrscd fo. op.ning rqinforccmcnr whcn uI!{ with A:151. A jsl. nnd A 64i.

Figure 18.4 Maximum allowable stress values - page 2Frcn APl620, table Q-3


18 The design of low tempercture tanks

(fr)Noninal Plate Thickness

(in.)

< 25 3116

>25-60 ta

o0- 100 5ro

,00 r,r

Figure 18.5 Tank radius versus nominal plaie thickness

Frcm API62A,bble 5-6

theoreticaland experimental work has been done in this area

The ability of a thin cylinder to resist axial loadings has been

shown to be related to numerous variables, amongstwhich are:

. The cylinder diameter

. The cylinder length

. The cylinder wall thickness

. The material of construction

. The degree of imperfection in the cylindrical shape

. Any co-existent internal or external pressures to which the

cylinder is subjected

The relationship between these variables is complex. One at-

tempt to summarise this situation is shown in Figure 18.7.

A design Code has to reduce this substantial body of work to a

relatively simple set of easy to use rules and this is what API

620 has done.

Axial compression loadings on cylinders take a number offorms. One is a uniformly distributed loading around the periph-

ery of the cylinder due to such loadings as self-weight, roof

loadings and insulation loadings. Asecond is non uniform load-

ing, usually distributed as a bending load due to wind or seismF

cally induced loadings (note again that the seismic loading is

not discussed here in detail but is dealt with in Chapter 24) Athird loading is a point load applied at a specific point located on

the periphery ofthe cylinder. The tank design Codes are not ex-

actly forthcoming regarding this last loading and the way in

whi;h a point load is fed intothe structure is leftvery much to the

designer. Fortunately such loadings, caused perhaps by local

attaChments for stair\/vays or pipe supports, are usually small

when compared with the other loadings applied to these tanks

and this fact should be reassuring for the designer'

In general the various loadings are considered to be applied to

emptytanks asthis is the worstcase For those tanks which can

be subject to liquid loadings, the internal pressure is found to

stabilise the tank shell and increase its resistance to compres-

sive buckling. The exception to this is the seismic design case

for full tanks where the axial loading is a function of the liquid

level within the tank and this is dealt with in Chapter 26

Although not entirely in accordance with the letter of Paragraph

3.5.4 ofAPl 620, it has been common practice to use the allow-

able compressive stress from Paragraph 3.5.4.2 which as-

sumes that there is no co-existent hoop tensile or compressive

stress. This is sensible because for outertanks the tensile hoop

stress due to internal pressure is modestand known to stabilise

the tank. For inner open-topped tanks' there is no co-existent

hoop tension and compressive stresses due to the loose fill in-

sul;tion system are also low. As it is very unusual for a tank

shell to have a ft ca) / R of more than 0.00667, then the allow-

able axial comdressive stress is given by:

This converts almost exactly to the following in metric unib:

S"" = 12.5(t ca)/R equ184

where:

S* = allowable compressive stress (N/mm2)

t = sheltthickness (mm)

ca = corrosion allowance (mm)

R = radius of tank wall (m)

When the local axial stress is primarily due to a moment in the

shell, then the allowable compressive stress may be increased

by 20%. Where the bending ofan empty orfulltank is caused by

wind loads or the bending of an empty tank is due to seismic

loadings, then in addition to this 20% increase, the allowable

compressive stress may be increased by an additional one

third.

For butt-welded shells, the joint factor used in compressive

stress calculations shall be taken as unity irrespective of the ex-

tent of inspection of that joint. In the now unusual case oflap-welded shells, the joint factors to be used in compressive

stress calculations shall be tiaken from Table 5-2 of API 620

An anecdote from the author's experience may be of interest at

this point. A 50,000 m3 capacity LNG tank was being con-

structed in the south ofthe United Kingdom some 25 years ago'A somewhat unusual erection technique was being used The

outer tank was being erected by the conventional jack building

method. However, the open-topped inner tank was suspended

from the outer tank roof and constructed from the top down-

wards. Bythis means a single set ofjacks could be used forthebuilding of both tanks.

During the finaljacking operation the entire weight of the inner

tank was being supported by the outer tank shell. lt was noticed

at this ooint that the two minimum thickness upper outer shell

courses had developed the pre-buckling lozenge pattern'

Needless to say this was recognised as being towards the"hairy end" of the scale and the load was rapidly jacked down

onto supports. Later unpicking of the loads and stresses in-

volved for these tvvo courses revealed that the applied com-pressive stress was close to the limiting value ofequation 18 3'

Agreat deal ofthe research work in the area of buckling of cylin-

ders has been preformed on small scale specimens, but here

was a full-size specimen complete with real construction de-

fects such as locked-in stresses and deviations from the true

theoretical shape. An increasingly dim memory suggests that

the outer tank shape was not good. Nevertheless, this experi-

ence serves to reinforce the beliefthat the limiting stress given

by equation 18.3 should be taken seriously

18.3.1.4 wind and vacuum stiffening

Forthe wind and vacuum stiffening of single-walled tanks or the

outer wall of a double-walled tank, reference ls made to Para-

graph 5.10.6 ofAPl 620. This provides rules for the number and

aize ofthe wind stiffening required for a vertical cylindrical shell'

These rules are essentially the same as those for ambient

tanks described in ChaPter 4.

The maximum unstiffened height of the tank sidewall is given

by;

ca

R=corrosion allowance (in)

radius if tank wall (in)

Hr = 6(100t)

where

n1 maximum unstiffened height of thetank shell (ft)

s"" = 1,800,oooKt ca) /R]

where:

s""

t

allowable compressive stress (lb/in'?)

shell thickness (in)

loor)3-l


equ 18.3 equ 18.5

8.4

Basic

Maxim0mJoiflt

Efficiency

Typc of JoinlJoid Radiographcd (%: sce

Limirations Emcicncy (%) (Sce Notc I ) Notc 2)

me;edby

nic0te)ne

ive3X-

ofiveL

)n-lo.henged

he

nunPin$. auaincaother mcans appov.d by lhe porchascr,lhar roofs abovc liquid lcvcl. Full (sce 100willobtain th. quality ofd.positcd trcld melal Nore 3)on thc insid€ and oubid. wcld surfaccs lftaragrces with $c rcquircmcnls of PaDgraphUW-15 in Scction VIII of thc ASME C.rd.:x/clds using mctal backing sldps Uat rcmainin plec! arc crcltrdcd.

Singl.-wddcd boujoint with backing srrip orcouiwlcnt o{hcr than thosc includ€d !bo\,L

Singlc-c,cldcd bun joint witfi outbacking rt ip,Dolble full-fillct lapjoint (sccNo& 4).

Roofs abovc liquid lev.l,

Longitudinal or nEridional cilclrmlcrcmr or latitudinali)i s bctwcar plater not rnorr than I r/4

in.lhick nozzlc attachmcnr *EtdiDg withoutthiclnass limitatioo.

Roofs abow liquid lcvcl.

Nozzla 4tachfircnt wcLfing. 10

LnShudinal or tn ridionrljoinls &d.quivak 70(sc. Notc 5) cirflrnfcdlial or ladEdind iobts*twcco ptatcs nor rnorc ard 3,/8 h hickj;inrs ofthis tyF shall not 6c uscd for longibdintl ormcridionaljoints Ihat {rc p(ovisioru of5.12.2icqui.c to bc butt-r,6ldcd

OtlEr citcurnfcfiotial oa latildinal iointsbct*Ecn platcs not morr thro 5/8 in-thiclc 6s

lr$giMinal or m.ridionelilints ltld ci.cumfcr- 35cflial or lariurdi&ljoiots bctw€an phtas rblnorc drao ,8 in lhickFinB of this tyF sh.ll norbc nscd for lodgiudinal or Dcridiood joinrs tluithc Fovisions of 5.12, rcquir wh.r! rhc rhirutc{plalcjoincd cxc.Eds r/4 irl"

For 8n*hlnanl ofh€adJ conv€x to pcssurc nol 35morc lhsD 5/8 in rcquid lhickncss, o{ y wirh useof lh. fillc{ wcld on rh! i$idc of tlF 0o2dc.

AlrchrEnt wclding for nozzlcs and thch rcin-forccmcnts,

7o spo,Full (!ccNor! 3)

?5 SpotFUU (r.cNotc 3)

?0 "_-. 70JPwr

Full(sc! ;iNor4 3)

(lscludcd inthc sftnghfactors io3.16.8.3)

't085100

7585

70

70

65

35leredell'n.

ne

n-

L

n-ree-te

1-tn

tea-ldit.

nt

In

5

Singlc full-fllct lapjoint (scc Nolc 4).

Sioglc flu-filkt lrp joints for hcrd-ro-noz.lcJOmts

Nozztc-anachn€dt fi llct w.lds

Plt g rrcldr (scc 5.245)

: jJre 18.6 lL4aximum allowable efiiciencies fof arc-wetded joints:'.n APl620, table 5-2

Attachmcnt wclding for nozzlc rdnfo.t mcnB 80 - 80(scc Notc 6).

| . Scc 5.26 ard ?. | 5 for d.mimdofl rcodrcnrcnts.2. Rc8rtdla$ of any vducs givcn in ftis colunul. thc cfiicicncy for htrwcldcdjoinls bcrwcco plar.s wilh surhc.s of dorbtc cunarur. thar have a com-pfli$vc iE-css acro6.r llrc jolnl lrom a ncgativc taluc of P. or otlEr cxtcmal l@dinr nrav bc rakc$ as unitvl such comorcssirc vrass shalt nol €rcccd700 lbfto r, Fot 3ll othrt ltpwcldcd joint!. tllc joid cmcicrrcy facror musr bc appliid o rrr atlr,rattc coriprcssira sriss. \,. Thc cflicrcncy for furl.pcn tmllon D|rn'wclddt romtr. khich arc in comprccsion adDrs !h. cntirc thickEss of rhc conE rcd plar.s. may bc t i€n as unit vI All ruin butl.wcldcd joints t scc 5.26.1.2) dbll bc complcrcly radiogr.phcd as sFcifi.d in ?. 1 5.1 fid .ozzlc and ninforccmcnr hchmcnt wckl.

'nE $arr bc c,(amrncd by rh€ magftdc.paniclc mcrhod rr srfciftd in ?.15.2.

4. Thictocss hmrrarions dr not appty ro llat borlomr supfDncd unifomty ona fo dation.J Fo. thc poro$s affh lable.3 circumfcrcn[al or laritudii.l ioins shallb. considercd suhiccr ro rhe samc rcouir.nEnrs and limiullionl !s arc k,n.gitudtnolor m$idionalroint\ shcn $rch a circumt rcorial or lainudinaljoinr is locarcd(ar

';a sttuncal. lori slhcricalor Lllips{ndrl .hrltL o n I'r

ofc. slrlic. of doublc curvlrur. lbr ar fc iuncrion hctwcrn l Looicat o; dishcd rmf &v thrron irnd c.rlindricai sidcw l s. t|s ;tnsidcnd in j. [ .1 ( rItt it r rimilariuncruru at citbLr cnd of I i;nsiion sccrioo or r€ducc. ls sh(sn in Figuft 5.96''.|'.d0icDnc!l]tctGis|towt|ir|i||c|wd|ds!ndph'!lwt|dsi'.n(i|lC)t|p


i flgt re :l

0.6

0.5

04

0'3

K

',ii-

l

i-lili'i

.f...:.::::'ny'R\.E\TI

_; *:: -l-* ;

-'r' 'ir:i. ,?0-: : .

j::-:

't0

.'|':

l:+Ili-:.:,;ll;

':

i,':

:,-if.:

ttt

.rll:iiitt

:l,i

'::

: , l.T:i--:i l i.-r !r i.i:li$|++

t-i i ;.l.<-r;t.ilt;

':i I i+ ii-,. rlii !i_ I.jil:r:

:t L:l t i :lI

i

'l_l : i:i.1- l.i:

0.1

j-r,I.i

;iiijl-:: I';:li ri:i

-l illl ill I i:1 |tili t-l- i,- '.tOl,-:il:iriiill;l

i.l.;i i* i:li.i.;-1rlr:.-.* ir'i:.t.

]+:;-:-l:li:l1i;lili i: I .l.i il

rir..l.l

i,-il

:i'10 r: iU,

.:l 'i:1. ili:irlf,6i,: i:ltt:i ,i.i iii

r;i;t;::l Ti:-:l:,

l.ll r i-',riil'-'.-

i:i,ili;i

18 The desian of low lemperaturc tanks

Efecttoe\

0.10

o0l

Figurc 2

A=M.an lln.

I, B= Ltdltt for E0 Pol c€nlof t.rdtt

C, C=ll|nltr lor .ll t.tultr

04.01.01-NorE.EXTERNAL PRESSURE

ie. E\: ) <o.

B'rcklinS mey occur uDder- ex_

t ma,l Dresaure aloDe at roweroressurei tban indicated bY tlisbata Sheet io tle long wavelengthmode cov€red bY Data sbeet04.09.01. Tberefole in externaloressufe ca!c6 the str€D8tb -ol tleivlinder rhould elao b€ cneckeo\riitrr Date Sheet 0l 09 ol'

Daceitbet 1963t00 Rt

Figure 18.7 Buckling stress coefflclents for tirn-walled Llnstiffened circular cylinders under combined axial pressure - page 1

From Royal Aeronautical Society Structunl Data Sheet no 04 01 41


04.0t.01BUCKLING STRESS COEFFICIENTS FOR THIN.WALLEDUNSTIFFENED CIRCULAR CYLINDERS UNDER COMBINED

AXIAL COMPRESSION AND INTERNAL PRESSURB

-, (s.cona ke6 ld! t*)

l-le.gth of cylild.l (i!.)R = radius of c/iider (in.),= tlid.ncss of cylind.r warl (iaJo=llaximun inw&d initial iff€sula.ity (in.)

-E= Youos.'s nbdurus 0b./in.),- iitcmd ple3urc 0b,/i!.')t'=critiel codpr$sivb sttB (lb./inj){-buck$ng strss co.frci€nr dd!.d by i=rr (r/R)

The cuns on Fislr. r 6ay be Bed to dcrcdine rle brcklinE sr.6co.f6ciet5 for thin-eau.d un.tj-ff€nd ci@lar cytinder3 uader @obin;d djalcomprBioa and irt.rnal pcsurc. The buaklins 3rlB @effidst K i3plotted.sainst 6/r ror var;ous value or thc palme@r (lrlE) (Rl)r.

The dclrh 3 of initial nr.glladriB i! one of rhe D.inciDal faclorc a{erinE rbebuckling str6s of circular cyliddeE. Dispcitiod ofiuch i.rcsalerideJ ba!ben panly tAkcn into a@unr by thep.rticular d.6nidod 6dopted tor 6, i,., tnemaxidun inward irregula.iry of the surfa@ of rhe cylinde. from arv slraicbtedg. placed in an axial phnc. Appreciabte vanatioi in buckri'q si;ess dirsrstiu bc expered for cyij^deB haling tnc aame 3/r rclio but wi{h di8s€nllyshaped ed difcrcndy l@ted inidal i.r.Autari!$.

h h.lways dificuh to irreprer actulnsur@ots of i(egularities, Somegu;ddce or the vatues of 6 fdr uee wh€o etjnaLine tte bickti@ strese ofqylioders f rch Fignrc r ;s given in Flgurc :. Tle dr;es or Figw ; bave lenderiv.d Iron experimental r6ulrs, etr@rive vatues of 5/r b.ios obrained b!wo.ki.g back lrcm Ficur. r. The rers on which tle cu.i6 ar.-bar.d @vcrcfa wide raoge of date.iah abd cylinder 3izcs and. saner b.nd is show!;cuNc A i3 the de linc dnwn rbrough rle elperimenral poinls, [ns B, B,liow thc lidirs for eigbry per cenrol rhe r€ults and lines C, C, show the limirsfor all the tuult!. lt i5 beliq.d rl.( by u6ios t!$e cun6 a rc*onabtr6rimaie of rhc ndufactu.ing inaeuraci* wjlr 6e obrained for use iD d.ttr-hidng rle buckling srr6s of cyund.E under .xial @hprgsion aud irrer!.1pre$ure rrom ! !gur. r,

on Fislre , thc cude ror (t/E)(Ri 'f =o is {or $e es€ of toadids u.der

a:ial @mpe3ion alone. Tb€ efect of incrnal pre$urc iE !o redua rhl inir'61iiregularitie in rhe crlinde. and hence to lncrease (h€ huckline srre$coefhciert.

"Nts*iv! walurs of.(?/E) la/4r giv€ rheefr*t or cxte..d tr6sure.

lhe orde. o! th.se elfecrs is:ndi@red by ihe dotred cuwes.Tn€ curu6 are appli@ble obly to cyl,ndec whcre tllR): )' :{ lrr F), bur rhd

may be used to deremin. rh€ buckling srras of dng{risened cyiinaqs if i jsrakeD 4 $e distance btrw.en two rinss and dre above ondjrron k fu$ued.For slDrter c/Undss .ne bucklirg srr-s will be hgher Lhan tlat given by

Cox and P{r!RAx. Th€ Ei€menrs of th. Bucklins of Cufred plat6.loudal olrhe Rotdt A.o@stn:at Sociar, S€ptdber rta.

Cox. Unplblisbed rork,Lo, CurE and ScEwdrz. Bucjkline of Thin-Watled Cvtinder undir Aaial

Compd3io6 ud lrrerDal Pcsulc. N.A.C,A. Rclorr !ozi, tqst.FunG and S3a!R. Buc&tina or Tii!-walr.d Ci@td CvnndcF uoder

A,xiar Compcsion .nd Iarernat Prusurc. Jddd of r,,. A.@tu;tknr Sci.@es,

H^nRB m ^1.

Thc stabiliry of Thin-waLd UNdfiocd Cir.utar Cvtind*ubde. Axiel Compesio! I.cludirg rhe Efct3 of tDt mal prssurc. jor@tol t^6 Aero@uficat

'.toq.r, Augu3t 1957,

To find tbe buc&ling 6r€s of a cylinder under @mbided axial oopcsion

R=18 itr., t-0.036 in,, l-t8 jr., E-rox!o. tb./in.,snd ,=ro.o lb./in,'(l/x),=r.o a.!d ,5 {t/R)=o.os; lhcrcfore (t/.R}'> .s (r/R).R/t=5@ ed, f6h cwe A of Flgure ,, 3/r-o.15.(r/E) (R/r)r=o.,s dd, kom Fisurc t, x-o.ao.

Thc€forc lb=8,@ lb./in.rA cylinder of muc! l* rld averaao *snda'd of manufacrure in l.fts of

lhe magritud. of its cylirdriel inpcrt@liols, misht bav. ar cfi@tive t/t ofo.95 (upp.. C li* on Fig!rc ,). Suc! a cyrindt would have . huch t;werbuckliqg srcs rh6 rhat €titut d abovc j ia woutd bc <,4oo lb. /io.' Ukewis€a crlinder of much geare. cyli.dri€l p.rfetiod misht;;hie. an .f,etive 6/ror orly o.oo9 0d.i C ltn on FiEw ,) .!d beyc a buckliE !rG! &_rr.,@.1b./io.r The po$ibiury ol3!ch variariols i. bucklilg dtr;s must b.


t = thickness ofthe top course ofthe tank wall (in)

[note 1]

D = tank diameter (ft)

Notes: 1. The Code states that the thickness used shall be the"as ordered" thickness unless otherwise stated. Thismeans that for tanks where a corrosion allowance hasbeen soecified. it is the uncorroded thickness that shallbe used. Clearly if the designer, owner or other inter-ested parties feel that the added security of having astiffening system based on the corroded shellwould bebeneficial, then there is provision to allow this, but theonus is on these parties to demand this change.

2. This formula is based on the following:

The design wind velocity (V) is 10q mph which imposesa dynamic pressure of 25.6 lblf(. The velocity is in-creased by 10% for either height above the ground orgust factor. The pressure^is thus increased to 31.0lb/fr". An additional 5.0 lb/f is added to this for internalvacuum (5.0 lb/f is equivalent to a vacuum of approxi-mately 1.0" water gauge). lf the design is to cater foranotherwind speed specified by the customer, then tl'!evalue of H1 can be adjusted by multiplying by (100iv)' .

Theformula can be similarly adjusted to cope with windvelocities which are expressed in terms of wind pres-sure rather than velocity and for other levels ofvacuum.The background to this is based on the work ofMccralh ( Refere nce 1 8. 1 \

To take account of the differing thicknesses of the tank shellcourses, the shellmust be transposed to an equivalent shell, allof the minimum or upper shell course thickness using the fol-

:;-.e 18.7 Buckling stress coefficienis for thin-walled unstiffen€d circular cyl--::rs !ndercombined axialpressure - page 2

-'.- Royal Aeronautical SocieE Sttuctural Data Sheet no. 04.01.01

equ 18.6

tunrom = thickness ofthe top sidewall course (in), as or-dered, unless otherwise specifled lsee Note 1

abovel

t"d,"r = thickness of the sidewall course (in)forwhichthe transposed width is being calculated, as or-dered, unless otherwise specified

W = actual course width (ft)

Wr = transposed course widh (fr)

The sum of the transposed widths gives the total height of thetransposed shell. lf this height is greater than the value of Hjcalculated from equation 18.5, then one or more intermediatewind girders must be added to stabilise the shell.

If H1 > I W, then no intermediate wind stiffeners are re-qurred

lf H1 < >W, <2Hr then one intermediate wind stiffener isreourred

lf 2H1 < I Wr <3H1 then two intermediate wind stiffenersare reoutreo

Etc....

It is generallyagreed that the spacing between the wind stiffen-ers should be equally pitched on the transformed shell as farasis oossible.

This is efficientdesign. lt provides the maximum shellstability inthat all of the unstiffened parts of the shell are equally stable.The stiffeners may be attached to either the inner or outer sur-faces of the shell and must not be located within 6 inches of ahorizontal welded seam.

The required minimum section modulus of the intermediate

lowing relationship:r--''-I r l-

wt. = w.i rlrlrq I

I L 'acruar I

where:


18 The desian of low tempercture tanks

5i9ru"13"tJ[:,33""F"tins area or the shellto be included in the wind siirr-

wind stiffener is given by the following equatron:

Z = O.O0O1D'H, equ 18.7

where:

Z = minimum section modutus of the stitfener (in3)

and the Participating shell

The participating shellwhich may be included in the calculation

of the sectlon modulus on the stiffener' both above and below

the point of attiachment is given by:

quired for the stairway, then the opening shall have the same

minimum section modulus as the stiffener itself This is de-

scribed further in ChaPter 4.

The stiffening ring itself can be a structural section or be fabri-

cated from plate. ln this case the joint between web and flange

may be an intermittent weld on alternate sides'

One rathole of minimum radius0.75" shallbe provided ateach

ioint between stiffener sections and at each point where the

itiffener crosses vertical shelljoints This detailis shown in Fig-

ure 18.9.

All fillet welds shall consist of a minimum of two passes The

ends of the fillet welds shall be 2" fromtheedgeof therathole

as illustrated, and these welds shall be deposited by starting 2"

from the rat hole and welding away from the rat hole An accept-

able alternative to stopping the fillet welds 2" short of the rat

hole is to weld continuously through the rat hole from one side

of the stiffener to the opposite side All craters in the flllet weld-

ing shall be filled bY back welding.

Anv ioints between sections of stiffener shall be made such that

tne minimum required moment of inertia of the combined

shell/stiffener shall be maintained

Welded joints between adjacent sections of stifiener shall be

made with full thickness complete penetration butt welds The

use of backing strips is permitted.


For double-walled tanks where the thermal insulation system

consists of a loose fill material (usually perlite) in association

with a resilient blanket (usually glass fibre)' the insulation sys-

tem imposes an external pressure on the outerface ofthe inner

tank. fhis mechanism is described further in Chapter'19 The

external pressure is at its maximum value when the tank is

emDtv and warmed up to ambient temperature following a pe-

r.iod in lo* t".p"tutuie service The evaluation of the magni-

tude of the insulation component of the loading is also dis-

iuiied in Chapter 19. ln the case of inner tanks with fixed roofs'

the insulation loading can be increased by the mostpessimistic

combinations of interspace pressure and inner ianl( vacuum'

The Code does not give any guidance for the design of a suit-

able shell stiffening system.

Adesiqn methodology which has wide acceptance in the indus-

try is b:ased on the work of L.P Zick of Chicago Bridge (Refer-

eice 78.2). This work has been republished with minor modifi-

cations in a publication by the American lron and Steel lnstitute

most recentiy revisedin 1gg2 (Reference 18 3) and it upon this

Tank shell

I

iIIII

to = 1 .a7[Dt1',

where:

lp = length of the participating shell (in) (see Figure

18.8)

Where tanks arefitted with spiralstairways and thewind stiffen-

ing is to be fitted to the outside of the tank shell, stiffeners ex-

tending up to 6" from the outer surface of the shell plate with

stairwiys of at least 24" nominal width are permitted wlthout

modification. For stiffeners which are widerthan 6", a minimum

unobstructedwidth of stain/ayof 18" mustbe maintained lf the

stiffener is of such a width that an opening in the stiffener is re-

1 i" I r l r" rr-i,.. r,rt. "

Fgure 18.9 APl620 - Shell slrffenet Jornt deta'ls


equ 18.8

Con nu@sfr|e|EE (s*a 3 s)

.ne

te-

rchlne

9n-rge

-he

oe

:.: :re following is based. This document gives design meth-- : -:'or three d;fferent types ofvessels and loadings. lt is Type C- :'r is most appropriate for storage tanks ("Storage Tanks of

-:'ie Diameter Subject to Radial Loads Only, ot Small Vacu-, -: Wherc the Axial Load is Negligible").

--. external load applied bythe insulation system on a vertical:. -dricaltankhas been shown by experimental and theoreti-:: :iudies to be nearly uniform overthe tank height. This has.::. assumed in this design procedure.

-: :3count for the fact that real tank shells are constructed from: ::ries of courses, usually of different thicknesses, and the

-ry in Reference 78.3 is based on a shell of constant thick--::s. it is first necessary to construct a shell of constant thick--:-is equivalent to the real shell. This is for convenience based: - :re minimum shell thickness used in the real shell and is: - -structed as in Section 18.3.1.4 by repeating the calculation:':quivalent course width on a course-by-course basis using:: -ation 18.6.

_:fe:

-1. = total height of equivalent tank shell (in)

.termediate shell stiffeners-^: David Taylor lvlodel Basin Formula is used to decide upon.- = citching of the stiffeners on the tank shell. This is taken from: ,vork of Windenburg and Trilling (Refe rence 18.4), some of

, - ch is based on test work carried out on behalf of the US Navy:: 'ar back as 1929.

--? fofmula is:


half of the distance from the centre of the stiff-ener to the centre of the next stiffener on theother side (or to the top or bottom of the shell)(in). This is the part of the shell supported bythe stiffener and this is illustrated in Figure18. 10

D = tank diameter (in)

t = thickness ofthe thinnest (i.e. top) shell course(in)

E = Young's modulus ofthe tank shell material(lb/in2) usually taken as 29.0E06 for steelmaterials and 10.6E06 for aluminium alloys

= factor of safety with respect to predicted failure

- suggested minimum value = 2.0

p = external pressure (lb/in2)

p = Poisson's ratio - for steels 0.30

This formula is valid for values of L" greater than O.916i.

For tanks of conventional proportions, it is most unusual forshell stiffeners to be this close together.

The number of stiffeners required to stabilise the shell is givenby:

equ 18.9

equ 18.10

half of the distance from the centre of the stiff-ener to the centre of the next stiffener on oneside (or to the top or bottom of the shell) plus

|2")pt-

rae)ro-

iat)ed

be-he

emronys-ner-he

ce-lni-

)fs,sticTt,

uit-

!, < D

, _ere:

L,

(rounded up to the nearest whole number)

Note: lt is usualto arrange for shellstiffeners to be a minimumdistance above or below the circumferential shellseams (measured on the real shell). This can be theminimum spacing permitted by the tank design Codesor a greater distance to suitthe method oftank erection.

It is next necessary to calculate the number ofwaves into whichthe stiffening ring will buckle, which is taken as the same num-ber of waves into which the complete unstiffened shell willbuckle:

N.,n =t 1

Nz - -:: > 100H lt^..

Di] D

where:

equ 18.11

equ 18.12

equ 18.13

US-

iift-utents

N = number of waves rounded up to the nearestwhole number - Note that the maximum per-mitted value is 10

H = total height ofthe real shell (in)

t", = average shell thickness (in)

Next is the calculation for the required moment of inertia of thestiffener and the participating area of the shell:

' FPL'DI' sE(N'z-1)

where:

l" = moment of inertia of the shell stiffener and theparticipating area of the tank shell (in4)

L" = actual or real height ofthe tank shell associ-ated with the particular stiffener (in)

The participating area ofthe shell is the lesser ofl .1t"vbi (equalto the participating shell width on either side of the stiffener of

t-i078-/r aiven in eouation 18.15).\2 -

This is computed by reversing the move from the real shell to

2.42 E (t Df f0.45 + _4 |

Fp(l-u')''' l

: :Lr€ 18.10 The podion of the lank shell supported by a stiffener (1")


1 8 The design of low tempenture tanks

Figure I 8.1 1 Example of transition from the equlvalent to the real shell

the equivalent shell in equation 18.9 above. An example ofthiscalculation is given in Figure 18.11.

To preventyielding ofthe stifener, the following minimum area

Diametor 72mHeight 35mFillheight 34mProduct LNGsG 0,48

Course Width (m) Thickness (mm) Equivatent ht (m) Realheight (m) .€quival€nt height (m)(per course) (cumulativ€) (clmulative)

1-bottom 3.5 35.4 0.134 35.000 13-913

2 3.5 31.7 0.177 31.500 ',t3779

3 3.5 2a-1 0.235 28.000 13.602

4 3.5 24.5 0.336 24.500 13.364

5 3"5 20.8 0.507 2',1.o00 't3.o27

6 3.5 17.2 0.815 17.500 12,521

7 3.5 ',13.6 1.465 14.000 11JO68 3.5 9.9 3.241 10.500 10.241

I 3.5 9.6 3.500 7.000 7.000'lGtop 3.5 9.6 3.500 3.500 3 500

requirement shall also be met:

. PL.D

wnere:

c{T

equ 18.14

Stitrener s2e6

Top 9lo x 305 x 76 x8 mmA" = composite area ofthe stiffener and the partici-pating shell (in'?). The participating shell width

' to be included on either side of the stiffener it-self shall be:

0.78 equ 18.15

Note: This is more generousthan permitted byAPl620 Para-graph 5.10.6.8 for wind stlfiening (see equation 18.8)

which in the same units works out to be060{; .

To ensure that the stifiener is a sensible minimum size, the areaofthe stifieneralone shall be not less than one halfofthe valueof A" calculated.

F" = Allowable compressive shess (lb/irP)and shallbe taken as '15,0001b/in'? for steel.

This procedure results in a stiffener anangement with the shellstiffeners congregating towards the top of the tank shell and in-creasing in size from the top of the tank to the boftom. A typicalarrangement is illustrated in Figure 18.12.

The conservative assumption, built onto equations 18.13 and18.14 is that allofthe load on the tank shell is taken by the inter-mediate stiffeners.

The proportions ofthe plate stiffeners shallconform to the AISCrules for compression members (Reference 78.q, or the de-signer can use the simpler 16t rule.

End (i.e, top and bottom) stiffeners

Forthe design ofthese stifieners, it is assumed that one halfofthe total radial load on the tank shell is transferred to the endstiffeners. In this case where the external load is uniform, theend stiffeners are equallyloaded, each receiving one quarter ofthe total load on the tank shell.

The required moment ofinertiaforan end stiffener is given by:

FpHD3

'|

10

!1

12

13

't4

15

152x9 mm

159xe

165x8

171x8

184x8

197x8

203x8

&x 9.6

wnere:

H = total actualtank height (in)


equ 18.16

Figlre 18.12 A tpical shell stiffenlng arrangement

Eileclive area wilhoul annular stilfener

lzr"lL"l

-

Effective area with annut& sliffening

: l-:e 18.13 Boitom comer arrangement where additional area is required

::r open topped tanks N should be taken as 2 for the top stiff-

:lr a tank with a fixed roof with a radial rafter framework, N

=:uals the number of radial rafters subject to the maximum,: ue of '10.

::. a flat bottom or a self-supporting roof (i.e. no framework) N:^all be calculated using equatlon 18.12.-"e required cross-sectional area ofthe end stiffeners is calcu-=:ed from:


outer tanks.

API Appendix Q divides the component parts of the tank intotwo categories:

Primary components. lt is worth quoting paragraph e. i.4.1 infull as there is frequently some confusion in this area:"ln general, primary components include those componentsthat may be stressed to a significant level, those whose failurewould permit leakage ofthe liquid being stored, those exposedto a refrigerated temperature between -60'F and -27O.F, andthose that are subject to thermal shock. The primary compo-nents shall include, but will not be limited to, the following partsof a single wall tank or of the inner tank in a double wall tank:shell plates, bottom plates, roof plates, knuckle plates, com-pression rings, shell stiffeners, manwaysand nozzles includingreinforcement, shell anchors, pipe, tubing, forgings and bolt-ing".

Note: When roof plates, knuckle plates, compression rings,manways and nozzles including reinforcement are pri-marily subjected to atmospheric temperature they arereclassifi ed as secondary componenb.

Secondary components. ln general, secondary componentsinclude those components that will not be stressed to a siqnifi-cant level by the refrigerated liquid, those whose failure wiit notresult in leakage of the refrigerated liquid being stored, thoseexposed to product vapours, and those that have a designmetal temperature higher than -60"F

Much ofthis categorisation is concerned with material selectionand impacttesting. This is discussed more fully in Chapter 21 .

18.3.2.1 Hoop tension - liquid containing tanksThe two formulae from Section 18.3.1.1, (equations 18.1 and18.2) are used again here with the following exceptions:

. Ht is no longer required to be the same as Ho. The exactwording of the Code on this subject is:

Q.8.1.1 states; "Except as limited by foundation or stressconditions, the test shall consist offilling the tank with waterto the design liquid level and applying an overload air pres-sure of 1 .25 times the pressure for which the vapour sDaceis designed. Where foundations or stress conditions do notpermit a testwith waterto the design liquid level, the heightof the water shall be limited as stated in e.8.1.2 andQ.8.1.3.',

Q.8.1 .2 states: "The load on the supporting foundation shatlpreferably not exceed the established allowable bearinavalue for the tank site. Where a thorouqh evaluation of lh;foundation justifies a temporary increaie, the establishedallowable bearing may be increased for the test condition.but the increase shall not be more than 25%.',

Q.8.1.3 states: "The maximum fill shall not produce a stressin any part of the tank greater than 85% (may be gO% forstainless steel oraluminium materials) of the specified min j-mum yield strength of the material or 55% of the speciflecminimum tensile strength of the material.

What this boils down to is:

- The minimum test water fill level is given by:

Hr = 1.25Howo

equ 18.17

' t is available, (i.e. a top stiffener located below the top of the

=rk shell), a participating width ofshell on each side ofthe stiff-:1ef, as given in equation 18.15, shall be included in the area.:lr the bottom end stiffener, the participating section without--^e addition of extra material is the annular plate outstand. the:rrtion of the annular plate beneath the shell and the effective,1 dth of annular plate (given above) together with the participat--9 shell area from equation '18.15. This is usually sufficient to'- fll the moment of inertia requirements ofequation 18.16, but-.3metimes falls short of the minimum area required by equa-: cn 18.1 7. An economical means of providing the extfa area is:: use a small annular plate stiffener to mobilise additional an--Jlar plate. This is illustrated in Figure 19.13.-te maximum value for L" is usually taken lrcm Reference

18.3.2 The API 620 Appendix e approach

is has been described in Section 19.3.1, when this Standardr.as written it only considered single containment tanks for the::orage of products at temperatures down to -270.F. This is-ade clear in the scope in Paragraph e.1.1 of the Code. The

=nks considered are a single-walled insulated tank (most un--sual these days) ora double-walled tank consistinq of an inner:nk for storing the refrigerated liquid and an outer tank enclos--g an insulation space around the innertank. This outer tank is-ot designed to contain the product liquid.

,','ith the passage of time and the increasingly frequent appear-:''rce of double and full containment tanks, the rules of this AD-::ndrx have also been used to design liquid containing metaliic

equ 18. i 6

- The shell th ickness will normally be governed by the ope-ating condition but a check must be made to ensure tia:the stress limits given in Q.8.1.3 are not exceeded.

There are occasions when the test water level is reouirecto be higher than that given by equation 18.18. These c:-cumstances could be associated with local requlations o.customer preference. So a test water height up to H co.t obe required, and in this case the shell thickness wiil proba-

nHn"4F


1 8 The design of low temperature tanl$

srdric shell b2sidr to AFt 620 Ap

-

Tank diohel€aHeight of fork shellFill height - op€'.olir'g

Fill heighi - lastP.oduct S€Test wster 56Operuting pre3sure - fixed rrof tonk only

Tesi pr6s|tFe - fixed roof tonks only

,rloteridl sel€cted :

9% Nlckd: AST A553 ?

2O.& |n

ntdrgmborg

lrlinimun thickie9scorrosion ollowonce

Weld nefol properties -

Density of steel

Ullimole Tensile Sif engih

Yi€ld Sir€ng{h

li i: 9,600 hn6,a=[ffilrnn

uts.--ZEiE-ru/rnrn'yS = 399.91 t{,hm,

p= 8O0O.0O k9lm3

-

Fron APr 620 : Q.3.3.2 (desqn) d Q.8.1.3 (tesi)

Allowoble strqte"s - opc.stirE; 1/3 x UTsor 2/3 xYS

trst : 0.55 x UTs

or 0.85 x YS

229,8 N/hn'?266.6 1.1166?

379.2 N/mnf339.9 Mmm'z

229.8 N/mtn'g

339,9 N/nrn'?Dasign slressTest stress

-

The colculofion of the shell fhiclap$ cohat fron API 620 | 3.10.3.2.

t= Tt +"oSoE

where T, = lditrjdindl unil fofce (cirdrhferehtiol unii forcas)sB. marii rm ollowobl€ stf€55 for simple iension

E= efficiency of joint

Totofl,eioht: 952.55 Ie

where;

Hd = Height of liquid h€ld under oPer{tlng codifions oi level being conslde.ed (n)

Hr = Heighl of liquid head under ta# conditio[' dt level being contidered (|n)

td = DesiEn thickr€s of 3hell (nm)

fd + cq : Dedgn thicknesg + corrosion ollowonce (mm)

ir: Tert thlckness of shell (nu'n)

l" = selccted shell thickness (nm)

Coufse wdih(tn)

Ha

(m),+(rn) (nn) (run)

td seleclea(|n|n)

lr(wn) (lt'n)

Wei9hi(Te)

I

3

78

lo

3

34

30.50

273023.fi20.00

16.W13.00

9.506.00

".50

20.,{()

16.90

13.,tO

6.402904.60-4.10

-7.60-11.10

25,O8

19.91

17.33

14.75

12.17

9.59

7.O1

4.431.84

25.O8

22.tu19.91

t7.3314.7511 17

9.59

7014.43

1.84

[email protected]

17.33

12l79.60

9.609.60

9.60

17.56

10.29

9.609.60

9.60

9.609.609.60

22.4?0,o17,414.4l?..29.69.69.69.6

154.91

142.50

126.67

110.20

77.2760.EO

60-90

60.80

60.80

'3

Figure 18.14 Example calculatlon showing how the test water heighl influenced the shsll thickness - page t


Smt Stntl Oesron to fpt fZO fo ,rldteriol selected :

sxnl"k"'-llllr1ssl ETonl dion terHeight of tdr* lheilFillheight - opcmthgFill height - tcstProduci SG

Tesr wdter SgOp.i.afi4t pr$$? - fixed foof lqaks mlyTesl prErure - ftx.d noof iohk only

r'ffi*34.00 rhw

wffi"uo"glW|rnbqrg

i,r = 9.600 rnn61=W$nrnr

UTS = 689.50 Wm|nzYS = 39.91 N/mr:

p= e000.00 kglnri

l'linihqn lhiclaEasCoirrosion qllowonce

Weld metol properiiat -

D.nsityof st el

Uhitndte TeBile St|.engf h

Yield Strength

F.on APr 620 r q3.3.2 (desqn) & Q.8.1.3 (tusr)

Allowqble stre3ses - opa.atlrB =or

tst:or

Desi$r stressTe6tStr€93

n

V3 x UTs2/3 xYS0.55 x UTS0.85 { YS

229.8 wtrtnz266.6 N/hn,379.2 f.t/61614

339.9 Nh|n,

e29.8 N/mn,339,9 Nlnrmr

llle co,qulqii$ of the rhell thick'|ess cones fron Apf 620 I 3,f0,3.2.

t= T" +aa whereS*E

Tr: ldtitlrdinql unit force (clrrunfer€diol unit fofc€s)g, = mxinw$ ollowoble strEis for simple terlsiolt

E 3 efficiency of joinf

rornl weiol* = -TiSd7l-.re

Hd = Heigl* of llqrdd hsad under olerlting corditig.s af level b€ing congidersd (ttl)Hr = Height of liquld hend qldef lert conditjons at levcl bsing consider€d (n)td = Design thickness of shell (nrn)

td + ca. Detign thickn€€S + corrociqn dbwree (nUn)

tr : Terf thicknssr of shell ftnn)tr : s.lecr€d shell thickne6s (mn)

Cou|le Wtdllt(|n)

H6

(rn)ll'(n)

id(run)

ld+co(|n|l|)

id selected(hii) (nn) (mm)

Weighi(re)

1

4

o

34305027.@

eo.00

16.50

13.00

9.ffi6.@

e50

34.00

27ffie35020.00

16.50

13.00

' 6.!02.&

25.08

19.91

,7.33t4:751Zt79.597.at4.43!34

25.08

22.50

r9.91

17.33

14J5rzv9.59

4.43'| a4

25.08

19.91

17.33

14.75

[email protected]

9.609.60

353431.69

28.0524.42

4.7817.L4

9.879.60930

36.431.728.121.520.8t7.z13.6

9.99.69-6

224.20

m.77177.97

155.17

131J4108.94

E6l362JO60.8060.80

1 8 The design of low temperaturc tanks

l|r1 Ernple calculation showing how the test water height Influenc€d the shell lhlckness- page 2


1B The design of low tempenture tanks

Nominal Cylindcr Diamctcr(f() Nominal Plate Thickncsr (in.l

Slainlcss s{.cl ed nickel stael

<60 r/rr,

dO- l4O ttl> 140-220 5tt6

>22O r/s

Aluminum

<20 3h6

2O-Im ttt> 120-2m 5h6

>M l/s

Figure 18.15 Nominal thickness of inner iank cylindrical sidewall plaies

Fran API 624, hble Q-5

bly be dictated by the test condition.

- An example of this is shown in Figure '18.'14. In this casethedesign ofthefoundation and the supporting base insu-lation material must be checked to ensure that they aresuitable for the higher test case loadings.

. Minimum values for the design specific gravity of the vari-ous products (Wo) are given. These are:

- Methane 29.30 lb/ft3

- Ethane 34.21 lblft3

- Ethylene 35.50 lb/ft3

(i.e. an SG = 0.470)

(i.e. an SG = 0.546)

(i.e. an SG = 0.569)

. Values for the allowable stress for the design condition (So)

and the allowable stress for the test condition (Sr) for com-monly used ASTM materials are given in Table Q-3 (Figure18.4). The values ofSo given in this Table for plate materi-als are based on the lesser of:

(a) one third of the specified minimum ultimate tensilestrength of the material or

(b) hi,/o thirds oJ the specified minimum yield strength [75%in the cases of stainless steel, aluminium alloys and nickelalloys - which does not include 9% and 5% nickel steelsl.The values of S, are derived as stated above.

For materials where the strength of the weld metal under-matches the strength of the plate material such as 9%nickel steel, footnote a of the Table allows a range of weldmetal propertieson whichthe allowable stresses foroperat-ing and test conditions can be based. Paragraph Q.6.1 pro-vides means of determining the weld metal strengths.

. The minimum shell plate thicknesses are given in Figure18.1 5. lt is interesting to note that the values given for stain-less steels and nickel steels are different from those givenfor aluminium alloys. They are also different from the val-ues given forAPl 620 Appendix R tanks (see Figure 18.5).

It is interesting to note that the properties ofthe parent plate orweld metal, upon which the allowable stresses are to be based,are determined at room temperature. The actual properties atthe design temperature will be higher, and in some cases con-siderably higher, than the room temperature properties. Thus inoperation an additional factor of safety exisb.

18.3.2.2 Nonliquid containing tanks

Appendix Q of API is interestingly non-committal regarding thedesign of the non-liquid containing tanks. A minimum shellthickness of %6" is specified which is appropriate for smalltanks

but should be checked for largertanks as suggested in Section18.3.1 .2 . In general it is usual practice to turn to the body oftheCode (i.e. section 5) for suitable rules for the design of outernon-liquid containing tanks.

18.3.2.3 Axial compression

For both inner and outer liquid containing tanks and for outer


non-liquid containing tanks, the rules from section 5.4 of theCode are used as described in Section 18.3.'1 .3. For aluminiumalloysthe allowable compressive stress shallbe reduced bytheratio of the modulus of compressive elasticity to 29,000 (note

US customary units of lb/inz) for values of [- c) less than

0.0175 and by the ratio of the minimum yield strJgth of the alu-ft -c)miniumalloyto30,000forvaluesof ' _' equaltoorgreater

than 0.0175.


For single-walled tanks (most unusual to Appendix Q), andouter liquid and non-liquid containing tanks, the rules outlined inSection 18.3.1 .4 are used. The only difference is that AppendixQ allows the use of stainless steels and aluminium alloys. Forthese materials the requirement for a minimum of two pass filletwelds is not mandatory


The Code gives no guidance here and the methods describedin Section 18.3.1.5 above are commonly used. The only differ-ences are in the values of Poission's ratio and the allowablecompressive stress to be adopted when aluminium alloys are toDe useo.

In eouation 18.10 a value of u of 0.33 should be substituted andin equations 18.14 and 18.17 the allowable compressive stressof 5483 lb/in'? (based on 15,000 lb/in'z times the ratio of theYoung's modulii of aluminium alloy to steel) should be used.

18.3.3 The BS 7777 approach

As has been discussed in earlier Chapters, BS 7777 is basedon the work of the EEMUAcommittee and its recommendationscontained in the EEMUA 147 publication. EEMUAand its pre-decessor, OCMA, worked for several years to pfoduce a set ofrecommendations which included double and full containmenttanks which were increasingly being required by clients inter-ested in increased safety for storage facilities for the variouslow temperature gasses.

BS 7777 does not use the primary secondary and basic cate-gories forthe component parts ofthe storage tanks, rather pre-ferring to place the various parts of the structures in the cold(i.e. design temperature based on the temperature ofthe storedliquid)and warm (i.e. design temperatures based on minimumambient temperatures) categories.

In Part 3 of this Standard, rules are provided for the design ofconcrete parts of low temperature tianks.

18.3.3.1 Hoop tension - liquid containing metallictanks

The basic design formulae are:

Operating case:

t^- D l98w^rH^-0.3t-o^I 'ca" ,)nQ ( ' ")

Test case;

t. = D {gew, -i, o.gt*0, }' 20s, '

equ 18.19

equ 18.20

The variables are as defined in Section 18.3.1.1withthefollow-ing exceptions:

. No minimum product density is given.

. As originally published, the test water fill height was to bethe same as the maximum productfill height. Alateramend-mentwas published changing this requirement which is dis-cussed in Section 18.3.3.6.

. For outer liquid containing tanks, the design product fill

lhn} dismeter , Mlnlmum shell thlctness

D <3030<D<5050 s,

8

l0

NOm. This thickness may inciude any corrosion allowanc€provided that the shell is shown by calculation to b€ safe inth€ corroded condition and in accordanc€ wjth therequirement of 7.2.i1.3.

)f then ium,ythe(note

than

) alu-

-'ater

: gure 18.16 IVinimum shell plate thickness

"on BS 7777 : Paft 2, table 12


Carbon manganese lU'IS,2.35 or YS/L5steels

Improved toughness ltns/2.35 or YS/1.5caxbon marganese

l,ow nickel steels I UTS/2.35 or

10.2 % Ps/1.5

I % nickel steel I tlTS/2.35 oi

10.2 % Ps/1.5

Austenitic stainless IUTS/2.5 or I % PS/1.5

Figurc 18.18 Determination ofthe maximum allowable stress desgn

Fron BS 7777 : Part2,table 11

Nornnrl cont lrs dLmeterD

Noblnal sh€U thrch€r)

,<10l0=D<3030<r<6060<r<75

5

6

8

l012.5

', The r€quirenenl to. minimm nomiMl thickn€ss is ne€dedfor onshuction pu.pces, and nay include any colruDnauowe.e, prdid€d that the shell b shom by cdculariotr robe sfe in the cormded condition and to b€ in accordancewith ?.1.4.2 and ?.1.43.

anoed in)ndtx. Forfillet

gs-ibed

ilfer-/aotere to

tano:IESS

i the-.d.

Material type l,hximum shell thickness'/

lype It?es II and IIIt?es lV and V

1}?e vl

30 mm

25 rnm

30 mm2)

25 nm'r When material thicknesses are required in excess of thes€values, additional requirements to maintain the same level ofsafety are lo b€ ageed between purchaser and manufacturer

'z) see footnore to tabte 2.

: lJre 18.17 l\,4aximum shell plate lhickness: .n BS 7777 : Paft2, table 4

height shall be the level achieved when the full inner tankcontents are released into the outer tank.

-1ese formulae applyto tanks ofthe single, double and full con-

= .rment categories.

--e design point is taken as being 0.3m above the bottom edge:':re course under consideration.

r.y additional height to accommodate seismic sloshing need-:: be included in the product design level (Ho).

--e minimum thickness requirements are given in Figure' a 16. These minimum thicknesses are not the same as those-i:Jired for non-liquid containing tanks. The footnote makes it: =ar that the minimum thickness can include the corrosion al-:,,, ance providing the shell is thick enough for the internal pres-

i --e according to equations 18. 19 and 18.20. Corrosion allow-: -:es are not commonly applied to low temperature tanks as-=-i already been mentioned.

--. r'naximum shell plate thicknesses for the various materials:'= jiven in Figure 18.17. lt is interesting to see that this Table

= ,:s a let out in Footnote 1) for circumstances where the sheil

:::e thicknesses exceed the values given in the Table. This" : - I normally be in the form of additional or more onerous- -:-cy V-notch impact testing which would be agreed between:-= : Jrchaser and the manufacturer. Footnote 2) states that for-'.3 service and double or full containment categodes, "ordi--= r' 9% nickel steel (type lV) should be replaced by "im-: -: ..d- 9% nickel steel (type V) or by austenitic stainless steel-.:e Vl)when the shellthickness is between 30mm and 40mm.

-- = 3 jlowable stresses for the operating case are given in Fig-

--- - 3.18 subject to a maximum value of 260 N/mm2.

-- -. s the same maximum as is imposed by BS 2654 for ambi-:- - :: rks. lt is there for similar reasons of avoiding problems of:;--::Jfal instability and excessive anchor rotations of con--?::=: pipework (despite the fact that the Code seeks to dis-: : - -=3e the use of shell pipework connections).--: : owable stresses for the test case are 85% of the mini----_, eld or proofstrength ofthe parent plate orthe weld metal"- :-ever is the lower) subject to a maximum value of 340

--: - :.nate tensile stress (UTS), proof stress (PS) and the: : ::-,ess (YS) are all to be determined at ambient tempera-

--= ,i nere the weld metal under matches the strenoth of the

Figure 18.19 Conlainer nominal shell plate thickness

Fron BS 7777 : Pad 2. tabl-e Iparent plate, the weld metal properties used in the determina-tion of the allowable stresses shall be demonstrated to havebeen achieved by the use of a strain-gauged, cross joint tensiletest which is defined in the Standard.

18.3.3.2 Non-liquid containing metallic tanks

Only steel tanks are considered.

The shell plates shall be checked for internal pressures usingthe following formula:

tseo:ions

et ofnentrter-ious

,PDt^ =--+ca" 20s

nof

(s

1.19

cotd)rednum

rbernd-

drs-

equ 18.21

Where the variables are as defined in Section 18.3.'1 .1 withtheexception of:

p = internal pressure as a combination of internalgas pressure and insulation pressure (mbar)

S = design stress being the lesser of 260 or twothirds of the material minimum vield strenoth(N/mm2)

BS 7777 does not permit the use of lap-welded outertanks.

It is most unlikely that equation 18.21 will dictate the shellthickness.

The minjmum shell plate thickness shall be taken from Fig-ure 18.19. Again the footnote allows the inclusion oJthe cor-rosion allowance.

The maximum shell plate thickness shall not exceed 35 mm.

18.3.3.3 Axial compression

The thickness of the shells of non-liquid containing tanks areusually less than is the case for liquid containing tanks. For thisreason it is necessary to check the stability of the shell underthe influence of axial compressive loadings. The basic equationto determine the allowable compressive stress is given by:

/ t,^\S^ = 12.s ]j--ilc equ18.22

where:

S" = allowable compressive stress (N/mm2)

t = shell plate thickness at the point under consid-

J.20

R



eration (mm)

corrosion allowance (mm)

radius of the tank shell (m)

A factor for different loading combinations:

= 1.00 for deadweight above the point underconsideration plus insulation load plus 50% ofpipe load plus superimposed load

= 1.25 for deadweight above the point underconsideraLion plus insulation load plus pipeload plus wind load plus 50% of superimposedtoao

= 1.33 for deadweight above the point underconsideration plus insulatjon load plus seismicload plus 50% of the superimposed load

BS 7777 makes this design requirement specific to the outercontainer (i.e. by definition non-liquid containing) shells. Forsingle-walled tanks it would seem sensible to carry out thesame checkforthe uppercourses of tanks, particularly in caseswhere unusually high roof loadings must be accommodated(for example a Canadian West Coast snow loading which canbe uo to 5.0 KN/m'?).

Equation 18.22 is exactly the same as equation 18.4. Clearlythis has been "borrowed" from API 620. The onty difference isthat the load combinations and associated factors are different.


The section on the design of the wind and vacuum stiffening forexposed shells giveninBS7777 Patl2is exactlythe same asthat for secondary stiffening given in BS 2654 and described inearlier Chapters.


BS 7777 gives some limited guidance on the design of shellstiffening for external loadings. This is confined to a brief com-mentary on the factors contributing to the loose fill insulationcomponent of the design load, the application of a factor ofsafety of 2 on both stiffener pitching and sizing, the use of thewind and vacuum loading methodology for determining thenumber and positioning of the stiffeners and the statement thateach stiffener shall be designed for the loading on the panel ofshell associated with it, including an unspecified portion of thetank shell plate adjacent to the stiffener.

Whilst this is helpful, it falls some way short of the detailed de-sign procedure outlined in Section 18.3.1.5. The designer is leftwith the option ofusing this method and borrowing rulesfor stiff-ener area, moment of inertia and contributing shell area fromanothersource (BS 5500, Reference 18.15, wouldseemtobea suitable candidate) or adopting the method described in Sec-tion 18.3.1.5.

Cturce l/lldfi Thidoess Tr'icki€s! thii<n€{m) (.'yn) {ffin} ttrm)

1 3.500 25.1 35.0 Z2.O2 3W 22.5 3t.!t 19.73 15@ 2(}.O 27.A 17 44 3.:,00 17.4 24-1 15-25 3.5@ UA A.5 1296 3_500 122 16.9 1207 3.5{rO 9-6 132 120a 3.500 9.6 120 1209 3.5@ 9.6 12.O 12.0

9526 1297.7 9323

CaseA Design to APt620 - partiat hydrcte.tCase B Design ro Bs 772 - Fu[ hydrotestCase C Design to BS 2777 - Partiat hydroresl

rigu e 18.20 Companson o' unk shetts desiqred lo APt 620 Aooerd r e. BS7777 {fLll t'e ghr lesr) and BS /277 tparlial neigt^t lesl)

18.3.3.6 Addendum to BS 7777 on partial height hydro-static testing

The decision to require all low temperature tanks to be hydro-statically tested to the full design liquid level was made byEEIVIUA 147 as a reaction to the events surrounding the QatarLPG tank facility failure which have been described in Chapter'17, Section 17.3. This was carried over to BS 7777.

For large LNG tanks where the difierence between the productdesign specific Aravity (usually around 0.48) and the test waterspecific gravity (1.00) is large, designing for a full hydrostatictest fill height gives a test overstress to all the shell courses ofaround 2.08. This is considered excessive. For LPG tanks witha design specific gravity of around 0.6, the overstress is of theorder of 1.67 and for ammonia with a design specific gravity of0.68, the overstress is 1.47. These are considered more rea-sonable.

Figure 18.20 shows a large LNG tank where the shell is de-signed to API 620 Appendjx Q (partial hydrostatic test height),BS 7777 (full height hydrostatic test) and BS 7777 modified tothe API permitted test water fill height. lt as clear that the fullheight test requirement results in an uneconomic design oftankand this meantthatthe industryavoided specifying BS 7777 asthe design Code for this type oftank

With this in mind, Technical Committee PVE/15, the committeeresponsible for BS 7777, decided that something needed to bedone and in 2000 published PD 7777 .20OO (Reference 18.6\.PD stands for published document. The reasons for publishingthese new requirements as a PD ratherthan as an amendment

72.0035.0034.0034,00

20,400,481,00

Figure 18.21 Longiludinal Charpy V-notch impaci test requiremenls

Frcm PD 7777 : 2000, table 2


Tegr.nerdt ol !l^r.rq ldt t.mD.rttu. tor l2O J'"c

Te.t .r..8y ot reld n.trl

C.Mn fTMCP) 27Jat 50 'c -A1d -20 50 J at 50'CLow Ni (TMCP or Q & T) 2?J at -80 rc -^ts -50 50 J ar -80'C9% Ni sl€els 100 J at - 196 'Ce Not rcquired 75J at - 196'C

' Ener$/ izile is th€ minihon av€mg€ of three sp€cime.i with only oh€ sin8le elue ls rhd rne ulue sp€in.d &d viur no sindeElur lN lhs ?5 96 of $e value speified-b For steltnj.lor* ls ria 1l m, lOmh x 6hh sul$ize spennes shoutd be u*d, ed shdld demoGF.!€ ?O9i ot rhe tdu6spe.l6d in lhis Lble. For I Ni stel5, ihe vzlue lor l0 M x 6 m $b-sia speineB should be m 96 ol the \arue sD€cjn€d in rhis

c Irnpact ia$ing shqld be cdi.d o on erh llaie 10 demoBtnre rhe lequirEd inlac{ }€lue. th addition, r€sir8 ar a nequenc}, ofone r6t ol uE sp€imN per40 toM6 ba!.I shoutd be cMied @t 10 d€noNFate t20J.i rhe r€mDer*urc spei6ed. tte de6niiionof plab aid tEtch re giren in DN 10026.

d Relerere should be nEde to md A ol BS ??7-2.

'For horimt veld5 in 9 96 Ni srels, lne wel<l m€Ll rEquifthent ne€d or y be 60 J at 196rc.

'oro-

latarlpter

: . --: 18.22 Required n |ductility lfansition lemperatures and r.inimum tesi temperaiures for no brcak resutts

18 The design cf 1a,, ie-aa-aa,-. aa-. :

,< 10

l0<D< 30

30<D< 60

60<o

5

6

8

10

NOTE The cqui.€menl for nininum rhickness sneeded fo. @nsltuction purposes, and may irctudeany cotrosDn arrowance provided lhar he sheu sshoM by €ldlallon to be safe in the @rcded

' - ?D 7777 :2000, table 3

': 3S 7777 was associated with the CEN rules which state that: ,' 'rg the period when a new European Standard is being pre-:: -:C, all national standards must be subject io standstill. This-:: rs that the national standards cannot be amended (unless. -: lnlficant error is discovered), and the use of a PD was in ef-::: a device to modify the Standard within the EU rules.--= criginal British Standard based the material selection on- : :: on resistance to brittle fracture and included the full test-; ---i to provide added safety in this respect. The new pD::-::s rts material selection on initiation resistance and on the

",::: capability of fracture arrest for fractures which may ex-:-: acfoss a horizontal weld seam or be associated with a lo-r: :- itle region of a vertical weld seam. This is explained in, : -: detail in the introduction to this PD.-: :-sure that the fracture arrest criteria are met, the PD re-r - ':: enhanced plate and weld metal Charpy V-notch impact::- - 3 as Figure 18.21 and nil-ductility transition temperature': - ) determinafion in accordancewith ASTN/I E 208 (Refer-

' :: -8.4. The NDTT requirements are as Figure 18.22. The: - : .essomeadditionaldetail relatingtospecimenselection,: -::.d welding methods etc.

: ': : 'rg that the material has been selected and tested in ac-: ::-:ewiththe requirementsof the PD, then thefollowing is

, : - --ary ofthe hydrostatic testing requirements:

. : - single containment tanks manufactured from steels.'-.. lhan 9a nickel steel or austenitic stainless steel, for, r: at temperatures warmer than -105'C, full height hydro-:=:: testing is required.

. :: - s ngle containment tanks manufactured from 9% nickel!::: or austenltic stainless steel, it is not necessaryto carry: ,: : full hejght hydrostatic test.

. :r-:re n ner tanks of double or fu ll containment tanks, a full-: :-i hydrostatic test is not necessary

. :,-::eouter(metallic)tanksof doubleandfull containment=

-. : hydrostatic testing is unnecessary. -= -e steels as specified and tested to the pD are used for-: :.yef courses only of outer tanks, there Should be no-:=::o carryouta hydrosiatic test. Additional surface crack

-::::: on ls requtred.

- : " - : ^ .,d rostatic test referred to above is one where the test.: :- .,: is sufficient to impose a stress in the bottom cou rse-: : -:s the design stress arising from the product loading.

I : j The prEN 14620 approach

. ' : : ::=r mentioned earlier, this Code (Reference 1g.g\has-: :oJ'se of being wriflen for some years now lts cur-: :-, _: s that the docu ment has been completed, issued for: :-"'rent and the comments are at Dresent (December:: -: consolidated into the final version which will then

be issued as a full Euronorm Standard. What follows in th sSection and similar Sections of this Chapter is taken from thedraft or provisional version of the document. lt is not thoughtthat there will be major changes resulting from the commentconsolidation process, but itwould be wiseforthose using Stor-age Tanks & Equipmentlo refer to the final version when thisbecomes available.

The normal evolutionary process which occurs when new Stan-dards are written is that they are based on what is perceived asthe best of earlier Standards added to by any new practiceswithin the particular subject area.

This is very much the case with this document, but one signifi-cant difference from earlier Standards is the introduction ofthedesign of steel components by limit state methods. lt was ini-tially proposed that in compliance with the demands of oiherEurocodes, the limit state methods would replace the allowablestress methods. However, in recognition of the long hjstory ofsteel component design for storage tanks by allowable stressmethods, the familiarity ofthe industry and those active within itwith these methods and the possibility of old dogs proving un-able to learn new tricks, the allowable stress methods were re-tained and sit in the prEN alongside the limit state methods.

This is in many ways a parallel with what happened in the UK inthe area of the design of structural steelwork. The new Stan-dard, BS 5950 (Reference 78.9) introduced limit state designmethods and it was intended that the old Standard, BS 449(Reference 78. /0) with its allowable stress methods would co-exist for a brief period and then be replaced. What actually hap-pened was thatthe old Standard refused to fade away and bothnow exist in parallel. The intention ofthis new Standard was thatthe various factors would be adjusted so that both design meth-ods would provide the same end result and consequenflv thesame levels of safety.

The scope of this Standard is to provide rules for the design,fabrication, erection, testing and commissioning for of single,double and full containment, plus for the first time, membrane

d uct,aler

tatic)s ofwttnf thety ofaea-

)d to) fulliank

rileeooe

h lngnent

Figure 18.23 fulinlmum shell plate thickness

Fran p.EN 14620-2:2003, table 6

'c

NU-ductility tldsition

'c

aemler.tu.lor tFo

'cButare -10 -45hopane,{Propylene -50 90

Ethane,/Ethylene -105 - 145 -140Met]'ane (ING) -165 N/Ab 196 c

'This temp@t@ od the eiated NDTT 6 b€ @d for mtenal *lection Drovided tlar rhe spe.i6ed dsiSF remp€mt@ js notmore rhd 5'C colder thd tlis assMed

'Elft.b T6tirg below -196 rc is coNidered unpEcricd. Two nonrak 6drs at -196.C eertain a NDTT ol ar lea* z0l qc d4 Filh CAT = ND1T + 400 rc 16l, 16l, giv6 a minimMCAT of -16l'C. W'rih the boilin8 point of ING belng o!1y0.6'C lower (-161.6'C), ed the coBMtim olOe CAT = NDTT + 40"crelntio4 lllie !5 cNiderEd a $6c!ent requiremoi io sp4iry @ck resl


Type of steel Allowable stress in sewice Allowable stress duringhydrostatic test

Tlpes l, ll, lll the lesser of:

O 43 f, or 0.67 fy or 260 N/mm'?

the lesser of:

0.60 f" or 0.85 fy or 260 N/mm'z

Tlp€ lV the lesser of:

0.43 f"at 0.67 fv

the lesser oi:

0.60 f, or 0.85 lv

Tlpe V lhe lesser of:

A .40 f, ot A .67 f"

NOTES

1) f" ls minimum ull mate tensile slrenglh in N/mm'?and t s minimurn yield strength Ln N/mm:

2) For iow nckeland 9 o/o Nistees fyisequalto0.2%otprcolslress

1 8 The design of low temperature tanks

Figure 18.24 Determination ofthe maximum allowable desrgn slrcss

From prEN 14624-2:2443, bble 4

Figure 18.25 Partial load and mater al factors for types l, ll, lll and lv steel

Fron ptEN 1 460-2 : 2403, table 5

storage tanks for products stored at between -5 'C and -165'CIt was envisaged that the Standard would have its scope ex-tended to cover products down to -196'C by the addition ofanAppendix or extra part similar to BS 7777 : Part 4. Sadly, thislooks unlikely to happen, due largely to a lack of interest fromthat part of the industry dealing with the storage of liquidoxygen, nitrogen and argon.

18.3.4.'l Hoop tension - liquid containing metallic tanks

The formulae used for calculating the thickness ofthe tank shellunderoperating and test conditions are exactlythe same as aregiven in BS 7777 (equations 18.19 and 18.20). The minimumthickness requirements which apply to primary and secondaryliquid containers are slightly different to those in BS 7777 andare given in Figure 18.23. The maximum shell plate thick-nesses are as follows:

Types l, ll and lll steels 40 mm

Type lV steel 50 mm

Type V steel no limit given

A let out clause, similar to that included in BS 7777 is added al-lowing these thickness limits to be exceeded when additionalmaterialtesting is carried outto demonstratethat the same lev-els of resistance to brittle fracture have been achieved

For the allowable stress design, the allowable stresses are sim-ilar to those given in BS 7777 and are shown in Figure 18 24.

Note: There has been considerable debate about this and it, would be wise to consult the "real" EN Code when it is

published.

The partial load and material factors for the limit state designmethod are given in Figure 18.25.

All vertical and horizontal welds must be butt welds with fullpenetration and fusion. The distance between verticaljoints in

adjacent courses must not be less than 300 mm. lt would seempreferable to use a larger spacing of say one third of a plate

length here to avoid possible distortion problems


The axial stability criteria are exactly the same as are used in

BS 7777 (equation 18.22). The minimum shell thicknesses are


the same as are given for liquid containing tanks in Figure'18.16. The maximum thicknesses are as stated above.

18.3.4.3 wind and vacuum stiffening

The method of calculating the number and size of shell stiffen-ing is exactly the same as is given in BS 7777 (see Section18.3.3.4) which is in turn taken originally from BS 2654.


The only guidance is a reference to a calculation method given

in a Japanese Gas Association document (Reference 18.11)This leaves the choice of method to the designer and the abovereference or the method described in Section 18.3.1.5 wouldseem sensible candidates.

18.4 Bottom and annular designThe Code rules for bottom plates of metallictanks are quite sim-ple and straightfoMard. The plate thickness and the means ofjoining are specified. The Codes do not give guidance regard-ing the plate layoutsto be adopted. This is lefttothe contractorswho have their own particular favourite plating arrangementsbased on experience and suitability to fit in with the erectionmethods and, at the end ofthe day, to provide a flat bottom. Thefinished profile oftank bottoms can be flat, coned up to the tankcentre or coned down to the tank centre. This latter arrange-ment is often associated with a tank centre bottom outlet con-nection.

Annular plates and theirjunction with the lowestshell plate is an

area in a liquid containing metallictank where a complex stresssituation is found. Yielding and shakedown are known to hap-pen during the hydrostatic test. A great deal of research has

been carried out in determining and predicting stresses in thiscomponent. This work has been reduced to a slmple set of ruleswhich experience has shown provide a satisfactory service his-

tory if followed by bnk designers.

18.4.1 The API 620 Appendix R approach

'18.4.1.1 Liquid containing metallic tanks

Bottom plates for liquid containing tanks shall be a minimum

Operating conditions Test conditions

1.36 d > 1.57

1.10

c! < 1.57

1.7210

1.06 o.> 1.42

1.11

s < 1.42

1.57|d.

NOTE o is the tensile to yield strength ratio fJfy

'fr is the partialfactor for actrons;

f" is ultimate tensile strength of steel;lM is the factor for material shongth;

fyis lhe yleld strength of the steol

18 The design of low tempetature tanks

ofFirsr Shcllcou6c (an.)

Dcsign Sresb in Fnsr ShcllCou6c(Psi)

s 20.0m 24,0m

<0.75

> 0.75 - t.00

> 1.00 - L25

> 1.25 - 1.50

'Thc thicrncsscs sd widdr_ (sc. R.3.4. I ) u bas.d on rE folndarion providinS a uniforn suppon und€r rhc fuflwro(n or rhc enular ptac. Untess ftc found.lion is propcrty cr,npacrcd, Fnicutarty at thc iciac of a concrcannSwar. scnkmenr wrl ptroducr addidonat sElsscs i, rhc annul& otalc.b'Ilc sucss shall bc calcllar€d ushg ftc fomuta (2.6Dxffdyr. phcrc D = nominal dianc&. of thc r.nl, in ft; I/=jnai$tum nr rrng hergN ot fte tlnk for dcsign, in Iq C: dcsign lp.cifc graviryi snd , = d€sign rhickrcss of rhen6r snc coursqcxctudrng conosion aloeoc!. in.

F g ure 1 8.28 Welded joint req utremenls

Fron APl620, figure 5-3

true circle at the outer periphery and a scalloped effect atthe in_ner profile. This arrangement is economical in terms of use ofmaterial and fabrication work.

The radialjoints between annular plates shall be butfweldedwith complete penetration and fusion. The Code does not 0re-clude the use oI backing strips for these welds and for annulafplates this is often considered convenient in avoidinq the com_plications ol lifting the annular to gain access to weld from ir-eunderside.

Butt welds in annular plates shall not be closer than 12,,(305 mm) from any vertical weld in the tank shell.

The Code requires 25% ofthe the outerend of radialjoints to beradiographed.

Note; For welded joints where a backing strip is to remain inplace, the welded seam must be examined bv mao-netic particte (Mpt)or by tiquid penetrant lLpt; inspei_tion methods after the first two weld beads have beendeDosited.

For the welded joint between the lower course of the tank shelland the annular plate, the Code allows two options. The firsi isthe use of a fillet wetd laid on each side ofthe shell pla1e. Thesewelds shall conform to the following:

. The size of the fillet weld shall not be greater than %,,(12.7 mm), not be less than the thinner of the two Dlaiesjoined. and not less than the values given in Figure 1g.22.

. When the specified minimum yield strength of the shetlplate rs greater than 30.000 lbiin, (207 N/mm2), each filtet

- rgure

tiffen-gction

1gs

given8.11).rbover'r'ou to

: ture 18.26 Table R-6 from Apt 620

:rickness of %,, (6.35mm).

-f less otherwise specified, the bottom plates shall be:c-welded together with a minimum lap of i,, (25mm).Three: ate lap joints shallnot be closerthan 12,,(305mm)from each::"er and from the butt welds ln the annular plate. There are::"npanies who specify butFwelded bottoms for reasons asso-: eted with a perception thatthis reduces the oossibilitvof leak_::e. lt is usual to allow the use of backing strips for buti-welded: f,itoms and provision must be made to accommodate these::.king strips in the top surface of the base insulation.: iquid containing metallic tanks must have butt-welded annu_=- plates. The wjdth of the annular plate shall provide a mini_- - m distance of 24,' (61omm) between the jnside of the tank: -: I and any lap-weldedjoint in the remainder ofthe bottom to_

=1er with a minjmum projection beyond the shell of 2',

: - rm). Agreaterwidth of annular plate may be required when-=:Jired by the following formula:

-_"=esJHG

equ 18.23

-::e:

= radial width of the annular plate (in)

:. = nominal thickness of the annular plate (in)

"j = maximum tiquid design height (ft)

3 = design specific gravity ofthe liquid to be stored--: :hickness of the annular plate shall not be less than the- :.1esses given in Table R-6 (Figure 18.26).-- - - ng of annular plates shall have a circular outside circum_:-=-:e but may have a polygonal inner profile, with the number: : res being equalto the number of an nular plates adopted.

: : lnvenient to make the number of annu lar plates the same: . :-: number of shell plates, or if this proves to be wasteful of- =::la1, then twicethe n umber of shell plates. lt is not uncom-*: - 'cr the innerand outeredges ofannularplates to be cut to-: ,.:me radius. This radius is that of the outer edge giving a

-. stm-tns of

rctorsnen$)ctton

) tanklnge-con-

I nasr this

-' his-

Maximum Thickncssof Shcll Plalr (in.)

Minimum Sizc ofFillct Weld (in.)

0.1875

> 0.1875 - 0.75

> 0.75 t.25

> 1.25 - 1.50

3h6

27 Sidewall,lo-bottom fitlel wetd forflat botlom cylind cattanksa2A. hble 5-4


1 8 The design of low temperature tanks

Egur6 18.29 Fitting and welding of lapjolnts

From API 620, tlgute ,2weld shall be made with a minimum of two passes

. For annular plates thicker than %" (12.7 mm), the joint shallcomply with the requiremenb of Figure 18.28.

. The welded joint shall be examined as follows. The initialweld passes on either side of the shell shall have all slagand non-mebllic debris removed and be examinedl/isually.Aftercompletion of thelillet or partial penetration welds, thespace between the welds shall be pressurjsed to 15 lb/in2(1 mbar) and examined for leakage using soap solution.Steps shallbe taken to ensurethatthis pressure reaches all

parts of theweld peripherybyensuring a blockage in the an-nular passage and ananging the air supply feed to belocated at one side of this obstruction, and the pressuregauge at the other side.

The second option is to utilise a full penetration joint, whichdoes not require the pressure testing described above.

For a large tank subject to significant seismic loadings, the an-nular width and thickness requirements given above are oftenexceeded, sometimes by signilicant margins as the tank de-signer attempts to arrive at a stable design. This is done by in-creasing the holding down ofthe annularplate bythe containedliquid to the limib permitted by the Code. Annular plates of30mm thickness and 2.5m width are not unusual in these cir-cumstances. Wide and thick annular plates bring theirown con-struction problems.

18.4.1.2 Nonliquld containing metallic tanka

The minimum nominalthickness ofthe bottom plating shall be

not less than %6" (4.8 mm).

The Code does not insist on the use ofa ring of butt-welded an-nular plates. So, lap-welded bottom plates to the outer periph-ery are permitted. Lap joinb immediately beneath the shellshall be fitted and weldedto provide a smooth surface as shownin Figure 18.29. Where the bottom plates are thicker than /s"(9.6 mm), the joinb beneath the shell shall be butt-welded.

D.sigr S!!$r in Firsr StEll Cou'*. (lbt/in.))

I t9,000

s 0t5

> o.?5 - t.00

> 1.00 - 1.25

> 125- 150

eaz

tVtt

1/r5

ea|

llal

tkz

rA

tA

9/t2

tA

tA

thl

1/rc

t74lz

t+,2

t142

2l^.2

Not r:rr! |hicb.!..3 trd vldda (r€. Q3.4.1) ln dd! ribL |1! h.sdoo tE foudrlin FlvidiiS rttnlfodn r4porltrd.r dE fu wt&hof dE tlmu-

Ir pL!c. Udc!. rb. fotndd.o b F!0.d, comF.r.4 pdq{rdy ! lIt ind& of r.onc$rc dIttw.ll, r.dlaEn stll p.ldlce.ttdiddd,ftr!.r h dlc.na!||' ptrte. fhc &lcbr.s ofdE.!outr. bo.roo phr.r |l.cat t.t ..d tlE tiichEt d6c tur thcll oo|!|c Thc ftinimumthbhs!.: fo. uuhl hotbll9t|l!. es! (bivldb$.d dt |ftdguc otclo lifc of lomcJd.s fordunimo Lrt .

rThc r!!$ rhdt tccrtsrLr.d llliig tE fo(n'rh t(r6D) x (rfC)yr, vrtrc! D c mminrl di.rn r.r ofd'c trlh In f},t= m|Iitnutn filittg lEi8ltt

ot &. r.d fo(d!rlg[, h & C = dc!i8! rpc.ift Snviry; rd r = dclEr 6icbr.ss of0r f.|r rlEll c6oric,.rdodint co'ltltion dhvmcc, h ir

FigLrre 18.30 Minimum thickness for the annularbottom plate: steeltanks

Fron API 620, table Q4A

ofFinlSh.llD6itD St$r ln F6( SlEll Co{tlc (lb0i! 2)

r5.06

<0.50

>050-0,75

>0.75 - lr0

> l!0 - 125

> t25 - t50

> lsn - r.75

> t.75-20

ea,

r3A2

ter1'

lA

aaz

lt46

llat

,ttln

rh6

rk

lA

t\t

ts,l.z

ti

1A

I

^6h

rv|6

tw

lta

trrar

ti6

,rb

N/a

rh

rt6

t.a

thr

l5a1

rha

3ho

lsA2

1116

Not 3:

Th. rhicte$i ..d {i.trh3 (i.! Q-1.4. | ) in ihis rltlc s b$.do. In. fond.rio. Fovidin! ! milom ,ut,Pon !.d.r !h. flll {idtn of d!unltd pl!|!. Untc$ rr|. fd,idrk t i3 p.qc.ty @p.cr.d. ptnidldry n |h. i i& of | cNErc dnlwdl. t { 6n will Fdle .ddi_

riond .uc.s i. tn. |![st . ot r.. ft. 6i.tFi, of rtrc |Nl! borron pl.r6 ftcd .or .rc..!t rlE tlricttB of dE 6r$ ih.ll cdre TrE

ni.iMo lhicrEi* td hnuhr borrom da6 *c &.iwd b$cd on . Otiguc .yct. lif. of 1000 .y.l* for tllminuo ldts.Ith. $6r rhrll t oldl!&d urang fi. adltt l(2 64t) , (rO)l ,' whsc D; mhintl diamrd of ltE tet in ft tr = mriBuh lill'tr8

, h.igh ol $. |.nt rq dc.i8n. in fr a; = d.iign ipc.itic Irrvry: .nd r = d*itn rhktn.$ ol IIE liBr .hrll .@Ge. .rclldin! co(osid tlk'w!rec. h in

Figure 18.3'l Minimum thicknoss for the annular bottom plate : aluminium tanks

Ftun API 620, table +48


te an-io DeSSUTE

which

oftenK Ce-Dy In-3inedes of

rcon-

rll be

rnph-shell

'' /aoe0.

Backing strips %" (3.2 mm) thick shall be used, or the weldsshall be made from both sides. These butt welds shall havecomplete fusion through the thickness of the plate and extendinwards at least 24" (6'10 mm) from the tank sidewall.

For larger tanks of this type, it is often considered convenienttoit a butt-welded annular plaie.

18.4.2 The API 620 Appendix Q approach

'18.4.2.1 Liquid containing metallic tanks

3ottom plates for liquid containing tanks constructed from 5%rickel, 9% nickelor stainless steels shall have a minimum thick-.ess of %6" (4.8 mm). Curiously this part of the Code does notspecify a minimum thickness for aluminium alloys.

Jnless otherwise specified, the bottom plates shall beap-welded together. The lap welds shall consist of a minimumrf two passes (for materials excepting aluminium alloy).-hree plate laps shall not be closer than 12,,(305 mm) from:ach other or from butt welds of the annular plates.

3utt-welded annular plates are mandatory and shall fulfil the24" (610 mm) inside of shell to any lap-welded joint and the 2,'50mm) outstand requirements gjven in Section 18.4.1.1, un-:ss a greater width is required by the following::cr steels:

390trt_. - GG as given in equation 18.23.

1g,The design of low temperature tanks

,''.z:z€Section Z-Z

Figure 18.33 Typ cal sketch of plale joint under shett ptates for tanks of container withoui ann!lar plates

Frcm BS 7777 : Part2, figurc 3

Bottom plates shall be lap or butt-welded together

Lap welds shall consist of at least two passes. Lap joints shallhave a minimum lap of 5 times the bottom plate thickness.

The use of butlwelded annular plates is mandatory

The minimum width (i.e. the total width from the outer edge tothe inner edge beneath the bottom plating) shall be 650mm.

The minimum thjckness ofthe annular plates is given in Figure18.32.

Butt welds in bottom of annular plates may be welded from bothsides or from one side using backing strips.

The joint betur'een the lowest shell plate and the annular platernay be double fillet- or full penetration welded. For double filletwelding, each weld shall be of at least two passes and be of aleg length equal to the annular plate thickness. The inter weldpressure test is recommended but not made mandatory Forthe full peneiration option, a warning about possible annularplate distortion is included.

18.4.3.2 Non-liquid containing metallic tanksThe minimum thickness of bottom plates shall be 6 mm.

The bottom plates shall be lap-welded with a minimum lap of btimes the thickness of the plate. The minimum length of astraight sketch (i.e. a cut to fit part plate) shall be 500 mm. Thisavoids the use of very small plates.

The minimum distance bet\,veen three Dlate laDS shall be300 mm.

Tanks, where the bottom shell course is greater than .lO mm,shall have a rjng of butt-welded annular plates with a minimum(total) width of 500 mm and a thickness of 8 mm. For thebutt-welded radialjoint a backing strip of minimum thickness 5mm shall be used. The minimum bottom olate laD onto the an-nular plates shall be 60 mm.

Tanks, wherethe bottom shellcourse is notgreaterthan 10 mmmay have a ring of annular plates or be constructed withlap-welded bottom plates to the perimeter. Where this option ischosen, the deiails shall be as Figure 18.33.

The attachment between the bottom edge of the lower shellplate and the bottom plate or annular plate shall be fillet-weldedcontinuouslyfrom both sides ofthe shell plate. The leg ,ength ofthe fillet weld shall be equal to the thickness of the shell plate,the bottom plate or the annular plate, whichever is the least.


This draft Code only allows steel bottoms with annular plates.The rules are the same for the primary liquid containe( the sec-ondary liquid container and tanks for vapour containment onlv

:tr1. aluminium alloys:

equ 18.24

--e minimum thicknesses of annular plates shaJl be as Figure- a.30 for steels and Figure 18.31 for atuminium alloys.

l-:t welds in annular plates shall not be closer than 12": -:5 mm) to any vertical shell weld.

--e joint between the annular plate and the lowest shell course. :ermitted to be double fillelwelded or of the full penetration-.:e as described in Section 18.3.1.1. For the double fil::-,velded option the same interweld pressu re test is required.' 3.4.2.2 Non-liquid containing metallic tanks-- e outertank bottom shall have a minimum nominal thickness-':._. (4 8 mm).

-. :bove, the Code does not insist on a ring of butGwelded an-- - ?. plates and fillefwelded plating to the periphery is permi!.:: Again for large tanks, the use of butt-welded annular plates

' : ,en iound to be convenient.

' :-e outer tank bottom is exposed to the vaporised gas as is- : -it usually the case, the fillet welds must have a minimum of

'3.4.3 The BS 7777 approach

': .1.3.1 Liquid containing metaltic tanks--: .J'rinimum nominal thickness of bottom plates shall be

. 32 1,/ir imlm tt^tchnF"s o. anlLtar ptare

' :- :: i7 /7 : Pan 2, hble 10

The lesser of 8 mm or tlIO

12.5


18 The design of low tempenture tanks

tt6r (rnar)-/

I Wl|.! $in3 0. .lr.m.t dxrt Fnb. (rh. @f Dtrc udd dE..qFsrio. b& $ rho*n n Cr.||it !t l. rlEFrrcn ..r .hdld .dridlr or o* of cruki.f ro .{rq4 rlE dn,dF .f ,rrnfr ,. llE &.r .t rtE fi|Lt ktdI D'mnnoi A i. hr.ik h .id

' rlrrtd .ot .rc..d diftBd ,{

I Sc T.bk 5.2 td linir.ttoc cm.min8 kE.rb wlEc vres rypc, .t ql<t€d pr.k nr! b. u,..t.

Figure 18.34 Figure 56 from APl620

The minimum thickness of boftom plates, excluding conosionallowance shall be 5 mm. The remaining requirements for bot-tom plates are the same as stated above for BS 7777.

The annular plate shall have a minimum thickngss, excludingconosion allowance of e€ given by:

18.5 Compression areasThe basic logic and derivation of the formulae used to dssigncompression areas is described in Chapter 3 on ambient iankdesign. The various Codes use whal are essentially the samerules, the only differences being in the debil_

18.5.1 The API 620 approach (Appendices R and Q)

API 620 allows two different designs for ihe cylindricalshett-to-roof ju nction.

These are the knuckle and the plate types.

The knuckle type is quite unusual these days. The fabricationcosts ofthe hot-pressed knuckle plates, the needforveryaccu-rate fit up in thefield, the difiiculties otjoining the radial ioof sup-port members to the shell and the lack ofany significanfmate-rialsaving tended to push the industrytowards the plate type ofdesion.

e" =(3.0 rej/3), but nottessthan I mm

where:

14, = 20,1,

, urt not lessthan 600 mm- .ipH

equ 18.25

el = thickness ofthe bottom shellcourse (mm)

The minimum width of the annular plate, W" is given by:

equ 18.26

wnere:

H = maximum design liquid level (m)

p = design product specific gravity

The remaining requiremenb are as stated above forBS 7777.


PENM|SSIBLE

1

{OT PERI'ISSELE

The basic rules forthe design of a knuckle type of compressionarea given are limited to geometry limitations and direction to-wards the design requirements as discussed in API 650, seeChaDter 5. Section 5.10.

It is stated that the radius of curvature of the knuckle in the me-ridional plane shall be not less than 6% and preferably not lessthan 12% of the diameter of the tank sidewalls. A radius of 6%will frequently require an excessively thick knuckle plate.

In the eventthata knuckle is not provided, the participating por-tions of the roof and upper shell course plating shall resist thecompressive forces generated by the internal pressure. lt isusually the case that the plating in this area is not sufficient forthe task and local plate thickening or additional area is required.Permissible and non-permissible arrangements in this area areillustrated in Figure 18.34.

The relevant formulae are:

wh =o.61Fdq-

18 The design of low temperaturc tanks

Rool ol lank

w" -0.6ff" cJ

equ 18.27

equ 18.28

equ 18.29

equ 18.30

Q = Trwn + Tr"w" - tR"sin cr

A" = Q/15,000 or Q/S"E

,vhere:

= The width of the roof plate considered to par-ticipate in resisting the circumferential forceacting on the compression ring region (in),see Figure 18.35

= corresponding width of the participating side-wall (in)

= thickness of the roof plate at and near to theroof to sidewall junction including any corrcsion allowance (in)

= corresponding thjckness of the sidewall at andnear to the same junction (in)

= roof spherical radius or Rc /cos cr (in)

= sidewall radius (in)

= meridional (radial) unit force in the roof plating= 0.5PsR, (lb/in)

= correspondinglatitudinal(circumferential)unitforce in the roof plating = PqRs (lb/in)

= circumferential force in the sidewall (lb/in)

= angle between the slope of the roof and a ver-tical line (see Figure 18.35)

= total circumferential force acting on the com-pression area (lb)

= net area exclusive of corrosion allowances required for the compression area (in2)

= maximum allowable stress for simple tensjonas given by Figure 18.3 (lbiinr)

= efficiency of meridional joints in the compres-sion area in the event that Q should have apositive value (see Figure '18.6)

= design vapour pressure (lb/in2)

- s almost always negative and in this case the design com-:'=ssive stress of 15,000 lb/in2 is used for steeltanks. ln the un-

. : . case that an aluminium alloy tank requires a compression: -:: a compressive stress of 15,000lb/in, factored bythe rela-: ,:

= values of the materials would be appropriate.

L

R2

a

Tr

T2

Tz.

ct

w. = 0.6 /FJ[--tit

L_

Cyindturl sldewall ol tank

Table 18.35 Compression ring region

Fron API 620, table 5-5

Thickness of lhe Thickcr ot lhcTwo Patu Joincd(in.)

Minimum Siz. ofFillcr Wcld (in.)

< tt4 ltrc

> tll -314

>\4 - ltt4 5h6

> ltt| lE

Figufe 18.36 l\,4inimum size ofilllet weld

Fron API 620. table 5-8

lf Q is in iact positive, then the allowable tensile stress is basedon the allowable tensile stress from Table 5-'1 and thejointfac-tor from Table 5-2.

The requirement that the composite centroid of the compres-sion area provided shall not lie above or below the horizontalline through thejoint between the participating shell and roof ar-eas by more that 1.5 times the average thickness of the twomembers joining at the corner is made. This is for reasons oflimiting the secondary stresses as described in Chapter 4.

There are a number ofother requirements forthe compressionarea details which are outlined briefly:

. The horizontal projection of the compression area shallhave a width in the radial direction of not less than 0.015times the horizontal radius of the tank shell. This onlv aD-plies if Q is negative indicating compression. which is themajority of cases.

. Where the minimum sidewall and roof plate thicknesses donot provide the area required by equation 18.28, then theadditional area shall be provided by (a) locally thickeningthe sidewall plate, the roof plate or both to comply with therequirements or (b) adding an angle, a rectangular bar or ahorizontally disposed ring girder at the juncture of the side-wall to roof plates or (c) using a combination of these.

. The horizontal projection of the added angle, bar or ringgirder shall not be less than 0.015 times the horizontal ra-dius ofthe tank shell. However, when the added area is lessthan one half of the required total, this width requirementmay be disregarded ifthe horizontal projection ofthe partici-pating roof area is equal to or greater than 0.015 times theshell radius; or when the angle, bar or ring girder js locatedon the outside ofthe tank, the sum of the horizontal projec-tion of and the horizontal width of the added material isequal to or greater than 0.015 times the horizontal radius ofthe tank.

. The projecting part of the roof compression area, an added

r)

gnnkne

on)u-rp-te-of

,,i}-;r,


1 8 The design of low tempemturc tanks

Figure 18.37 Alternative details forjoint between thickened shell compressionar€a and uDoer course of lank shell

angle or ring girderwithout an outer vertical stiffening flangeshall not exceed 16 times its thickness. OtheMise the hori-zontial or nearhorizontal parts of the compression area shallbe braced at intervals around the periphery ofthe iank. lt isusual to choose the compression area proportions such asto avoid the necessity for lhese bracings, but if they are re-quired, then rules are provided to allowthe number and sizeof such bracings to be calculated.

. The minimum size of fillet welds between the various com-pression area componenb shall not be less than indicated

Figure 18.38 Shell-roof compression a€as

From BS 7777 : Pan 2, frgurc 6


in Figure '18.36.

Where a thickened shell compression area plate is to bewelded to the thinner minimum thickness shell plate, thenthe joint shall be as indicated in Figure 18.37.

The Code is notexplicitin its requiremenb for the maximumthickness of compression area.parts. A thickness of 2Z "would not seem unreasonabb as long as the material se-lection requiremenb are met.


The requirements of BS 7777 for compression areas are basedon those of BS 2654which are described in Chapter3, Section3.7. Theformula for the minimum area required and the deriva-tion of this is also mvered in thisChapter. The formula only con-sidersthe radialforce in the roofsheeting and ignoresthe othertwo terms which are of opposite sign and cover the circumfer-ential loadings in the participating roof and shell areas whichare included in equation 18.29.

The allowable compressive stress of 120 N/mm2 (17,400lb/in'z)is higher than the 15,000 lb/in'? permifted by APl.

The general layout and minimum angle sizes are given in Fig-ure 18-38.

Two other differences between the 857777 and the BS 2654practices are that BS 7777 adds the API requirements for thehorizontal projection ofthe effective compresslon area to be notless than 0.0'l5timesthe horizontal radius ofthe tank shell, andthe rule limiting the vertical position of the composite centroid.


Forthe plate type ofcompression area, the rules are exactly asgiven for BS 7777 and described above.

t-ff \ |*f-

t-tl-\ /*n

Correldor d top ccb tngle recdon ard lhdrdr[et€r

D

,<loLO<D<m8<Ds3686<D3488<D

60x 60x 6

eox 60x 880x 80x l0l00xlmxu160x160xl0

NqIE. Whq r!d!.ltt , tn€ inov€ 6lt{t|M dr€t ol ioD cub.rd. dbuld b. !d!H.

trr - radlu! of curljgqrr€ of rDof (in n)- (for contcrl mo& - X/sin d);

ahcll lnd tjoof

- ladtos of sttel (tn m);

trea.

t - thida|€ss of itrell (in rutr);t - tNcfgresr of sngle rtifleder (ln nun);

6 - thtclate$ of mol date al aoltrl,rldsiorrdrg (in nur)i

f. - nu$mum wldth of drcn pbttotgcotddered to mr.L up the cornlale8;

l/" - luJdtnum wldth of rDof Pbtingcorddercd to mrke uP the corn


Single filleiweld JF = 0 35be

nen

rum

:it "

se-

Knuckle type compression areas are permitted but no rules fortheir design are given. Reference is made to the Japan Gas As-sociation Standards.

18.6 RoofsheetingThe roof sheeting design is very similar to the ambient tankpractices from which it has evolved. One significant differenceis that the design internal pressures are higherfor low tempera-ture tanks than is usually the case for tanks operating at ambi-ent temperatures. BS 2654 has categories for design pres-sures of 7.5, 20 and 56 mbar, whereas BS 7777 allows up to140 mbar, and with the agreement of all parties, higher figuresare permitted.

For tanks where the metallic roof sheeting (usually steel) is ex-posed to the atmosphere, there are a number of different possi-c lities.

The roof sheeting may be supported by, but not joined to thesupporting framework. ln this case the possibilities for joining:he sheets are topside lap welding, double-sided lap weldingand butt welding with or without backing straps. For this unat-:ached sheeting case, the roof sheeting alone must resist therternal tank pressures.

-he roofsheeting may bejoined to and act compositelywith the:upporting framework. ln this case it is usual to use the roof:upport structure as backing straps for the welding of the roofsheets. Whilst in practice the roof framework may contribute to:1e tanks ability to resist internal pressures, its influence in this-espect is usually ignored in the calculations.:cr tanks with reinforced concrete roofs, the roofsheeting and:s supporting framework act as erection shuttering. During ser-. ce the roof sheeting is exposed to the full tank design pres-sJre (which with current designs may be as high as 300 mbar):Jt the structural resistance to this loading is the task of the re-^'orced concrete. The design basis for the roofsheeting is now:ased on the internal and external loadinos which will occur:Jring the construction process.

:1er the roof has been erected in its final position, and this may- ,i olve air raising, jacking or piece small erection at the fulltank-:ight, it will be subjected to loads during the placing ofthe rein-':.cing and the subsequent placing of the concrete. lt is the: acing ofthe wet concrete which usually gives rise to the worst:ading case. For a large LNG tank the concrete roof will be-.:me 450 mm thick. lf this thickness of wet concrete were: 3ced on the roof. lt would represent a loading of around 11.0'\1m2.

-- s is significantly more than the usual roof loading of around' 2 KN/m2,and it would require a substantial roof framework=-C thickened roof sheeting to support this loading unaided.::f this reason, it is usual to seal the tank envelope and apply:- nternalair pressure to balance the wet concrete loadingsfor:-: period of concrete placement and curing.

::. if the full450 mm ofconcrete were placed in a single pour, a:: ancing pressure ofaround 110 mbarwould be required and. :ie roof were poured in two equal thickness pours, then this: : a ncing pressure would be arou nd 55 mbar. These pressures:-:1 become the design pressures for the tank roof plating.

18.6.1 The API 620 approach (Appendices R and Q)

:-:'n the body of the Standard, equation (7) from Paragraph: ' 1.2.5 gives the following for dome roofs:

I

/- Double fillet weld JF - 0 65

\,--'.-----------------_._______\___.

Backing stnp bultweld JF = 0 75

Eutt weld JF = 0 5.0 65 or 1.0{depending on teler ar inspBcrion)\rc=

sed:ionva-on-herfer-rch

in,)

-rg-

i54Inenotrndi.

. oR.

tu-nequ 18.34


( Backing stnp butt weld uslng rafter JF = 0 75

-->r r _-lr---; --1*1f--

Figufe 18.39 Various poss bililies for joining roof plates

Tr = tSr.E

Then:

PR

2S'F

equ 18.32

equ 18.33

where:

t = calculated roof plate thickness excluding anycorrosion allowance (in)

R" = R] = 6615p1r"rical radius (in)

The remaining variables are as defined above.

The minimum thickness ofthe roof plating (exclusive ofany cor-rosion allowance) is 216" (4.8 mm).

As for the tank shell, the allowable stresses are shown in Figure'18.3 and the joint factors shown in Figure 18.6.

The various possibilities for joining the tank roof plates areshown in Figure 18.39.


BS 7777 allows both cone and dome shaped roofs.

It imposes the following shape requiremenb:

Cone roofs shall have a slope of 1:5.

Dome roofs shall have a spherical radius of between 0.8 and1.5 times the tank diameter.

The design requirements are very similar to those given in BS2654 for ambient tanks.

For internal pressures for conical roofs:

Slrapped flletweld JF = 0 65

equ 18.31


For internal pressures for domed roofs:

. PR,' 2os4

For external pressures for both cone and dome roofs where nosupporting structure is present:

| 1nP t"-L = 40R.1 :rs

I'tFl equ 18.36

where:

t, = calculated thickness of the roof sheeting exclu-sive of any corrosion allowance (mm)

Rr = for dome roofs the spherical radius (m)for cone roofs = Rr/sino

0 = slope of the roof at the roof-to-shell junction(degrees)

S = design stress taken as two thirds ofthe mate-rial minimum yield strength subject to a maxi-mum of 260 (N/mmr)

I = joint factor (single-sided lap welds 0.35,double-sided lap welds 0.65 andbutt wetds 1.00)

P" = external loading (KN/mr)

E = Young's modulus of the roof plate material(N/mmr)

The minamum thickness of roof plates shall be 5.0 mm.

Roofs without supporting structures shall be butt-welded ordouble lap-welded.

Lapped roof plates shall be continuously fillet-welded on theoutside with a minimum lap of 25 mm.

For roof plates which are lapped, it is recommended that theloweredge ofthe uppermost plate should be beneath the upperedge of the lower plate to minimise the possibility of condensa-tion entering the joint.

Seams in the roof plating that are included as part of the com-pression area shall be butt-welded.

With the exception of the small diameter tanks, often for thestorage of oxygen, nitrogen and argon, conical roofs are quiteunusual for low temperature bnks.


The minimum thickness of 5 mm and theformulae forcalculat-ing the roofsheet thickness are the same as are given above forBS 7777.

A joint factor of 0.70 is given for backing strip butt welds, other-wise the rules and guidance are as BS 7777.

18.7 Roof frameworksFot the cases where the roof sheeting is exposed to the atmo-sphere, the roofframework design loadings are based on com-binations of the following:

. The self-weight of the f€mework

. The self-weight of the roof sheeting

The self-weight of any supported insulation

The self-weight of a suspended deck (if fitted)

External roof loadings (i.e. snow live etc)

lnternal tank negative pressure loadings (i.e. the designvacuum)

equ'18.35

. Local loadings from fittings and other similar roof-mountedequipment

. Differential pressure loadings on the suspended deck (iffit-ted)

. Wind loadings

. Seismic loadings

. Blast loadings

. lmpact loadings

. Erection loadings

Forcases where the roofsheeting and supporting structure areto be used to form shuttering to enable the concrete roof to beerected, the roofframework design loadings are based on com-binations of the following:

. The self-weight of the framework

. The self-weight ofthe roofsheeting priorto concrete place-ment and thereafter if not tied to the concrete roofwhen thisbecomes self supporting

. The self-weight of the suspended deck and any supportedinsulation

. Loading from reinforcing placement including any pointloads from accumulations of material

. Any loadings during concrete placementwhich are not bal-anced by ajr pressure

. Differential pressure loadings on the suspended deck

. Seismic loadings

. lmpact loadings which may be transmitted to the framework

. Erection loadings

In this last case the roof framework is largely redundant, apartfrom forming a convenient means ofsupporting the suspendeddeck, from the point in the erection processwhen the reinforcedconcrete roof becomes self-supporting. Sadly it is impractica-ble to remove and reuse it.

One wayofgaining some advantage from the redundantframe-work and the sheeting which has been reduced to pressuresealing steel wallpaper, is to place the roof framework abovethe roofsheeting and arrange good connection to the concreteby the use of shear connectors. By this means the roofframe-work and sheeting can replace some or all of the lower part ofthe reinforcing required for the concrete.

The design of supporting frameworks for tank roofs is an areawhere the design codes are less prescriptive than is the casefor other areas oftank design such as tank shells and bottoms.Consequently different companies have developed their owndesig ns. For the larger dome roof tanks which form the majorityof low temperature tanks being built, the following roof frame-work types are commonly used:

. The simple internal polar rib arrangement where the polarribs all run from the roof periphery to the centre ring, fittedwith circumferential bracing as required, having either unat-tached or attached roof sheeting.

. The internal polar rib arrangement where intermediate cir-cumferential rings are used such that the numberof ribs re-duces as the tank centre is approached. Again havingcircumferential bracing as required and either unattachedor attached roof sheeting.

The externallyframed versions ofthe two arrangements de-scribed above with the roof sheeting being attached to theunderside of the framework.

A geodesic arrangement similarto that used for some oftheproprietary roof designs for retrofitting to ambient tanks as



unleo

(iffit-

e are(o oelom-

valu€ for Mcmbcrs NotSubj.c( (o

Pt surclmposcd t adsValuc for M.mb.ff Subjccrro P..$uc-!mpo-scd Loads

( lbflin..)

ace-r this

)lIed

)oint

oaF

Pins dd $mcd bolc in rcmcd or drillcd holes

wcbs of b.ans !d platc sirdcs whcE At is normoF lha. 60, o. wh@ wcb is adcqlably srif-.n d, d gross stid ot *bWc!6 of bcds dd platc 6n&ls wh.E scb is rcr.d.quat ly sriffcmd sd /I/l is mor! tto 60, on

Fillcl wcl.li wh.rc load h pcrFndiculd to trlcnglh of ecu on tlE s.ction 0lough thc thrcar(se 5. | 6J3. il.m b)filbl eddt wl|cr! lold i! F.rllcl to 0E l.rg$ ofw!14 oo dE s.dion 0uooeh ttlc tlm6t (s5,15,&3. icm b)Pbg wclds or clor vclds, on ctr@rivc fayirS-suFfr.t lrci of *cld (s.. 5.24.5 md Tlblc 5-2)Burt sdA! on Lrst ('oss-r.ctional arca, in or atdsc of wcld (s.€ 5.16.8.3, illh a)

Pins od !|nt.d bohs in rlocd or drillcd hoLst .d qplicd !o hoh .r oou one sidc of lllc mcm,

lrad disEibltsl uodoEnly,.pproximarcly, lcrossthi.kEss of ih. m.mbcr corctcd

t !d +plicd io bolt at only orc sid. of ihc mcm-

t rd dittibut d uaifo.nly, alproxioatcly, acru!thickn*of th. m.mbd connct d

ll,50010.000

12.000

r8.0001t + t rt?0041

12,6m

9,0m

| 1,700

14,400

Bc.ring

u,ffi

30,000

16,0m

20,0m

r2,0008.0m

?^ Gnsion val!. rrom Tablc 5- I

(tcmiq uluc fton Tablc 5- I )

n + Q?n?Nt Jl

?0% l.nsion valuc ftDm Tablc 5- I

50% tctrrion ulrc ftom Tabt 5' I

65% t nsion !a!u. froh Tablc 5- |

80% tcnsion valuc frcm T.blc 5" I

1.33 xt nsion vatuc fron TabL 5- I

1,67 x Ensibn wloc frcm T6b'c 5- l

0,{B xt Bion vallc fred TlbL J-|

L I x &.sion wlu. iion Tablc 5' I

l. Thc v$irblcs io lhc comFEssirc $r.rs .qu|tioN e d.6n d $ follows:, = onb..c.d l4gth of thc 6luma in in.i . = con spoding hat r.dius ot &ddrion ol rhc colurn in ioi r = OickrEsr of rhc rlbule column,in ic: /= unity (lO) for v.lo6 of /R c$sl to of gElt r hr! O0l5ty =(2,.j)Ilm,ltr)l l2<2/])llm)/r)ll ftr tih.s of rA lcss rlun 0.15.

2. Thc vdiab,er in lhc bcnding strcs .qulrions src dcfincd 5s folld.s: , : uruuppo.Ed t..8rh of !h. m.Dbcr; fd . cerjlcvd b.m nor iu ltyshy.d at ns outcr cnd agaiist Fe.lador o. rctadon, / stEil bc rrkln ss rwic. ri. tcn$h of ll)c @opE$ion ndg., in in.; d = dcprh ot ihcn nb.r, in i.,r , = widnh oa irs @nprc$ion flegq in b.i I = thickBs of ils conpcssion fleg., in io.

l. Thc vuiablcs in thc shc&ing stE* cquarions e .kfincd as follNs: I = cl@ dii.irc. b.rw. wb Adgc! in in.: I = urkrncrj of dE w.b./ork

partoedced

Tte-ure3Ve

etene-tof

featsens.wnrity1e-

)ir-'e-

e-

'e

'rg

: :-'e 18.40 Maximum aliowable slress values for structu€ members

' ':'1 API 620, kble 5-3

allowed by API 650 Appendix G for aluminium roofs. Thiswould normally be with athched roof sheets.

::aching the roof sheeting to the iramework brings significant-:'eases in the vertical, and particularlythe horizontal stiffness:' :re composite structure. This is of particular benefit in cases,-ere high seismic criteria have to be accommodated in the

: =: gn. lt also allows the radial ribs to be used as backing straps'- ch is convenient when higherjoint factors for the roofsheet-- I are required to allow for hagh design pressures. lf the roofr-:ets are laid in a radial petal plate arrangement, then onlythe: -:Jmferential seams will require separate backing straps.--: different roof framework arrangements come with various. : . antages and disadvantages. The simple polar rib to the tank::^:re type is easy to erect requiring only a central king post to:-:lo the centre ring, but is wasteful of material. The more:: -plex types are more efficient in terms of material usage, but-:-e complex to erect. lt is difficult to be specific as to which- :: ls best as the merits of the different types tend to change:::ending on the tank size, the local circumstances and the de-.'- and erection method experiences of the contractors-.: ved.

'3.7.1 The API 620 approach (Appendices R and Q)

--= rrajority of roof frameworks are subjected to ambient or-:.' ambient temperatures. Consequently the body of the-::: (section 5) provides some basic design rules based on- -: -,can siructural steel standards for these frameworks. The--,

= are shown in Figure 18.40 and make reference to Figure

--: ^ominal thickness, including any corrosion allowance, of

any part of any internal frarfework shall not be less than 0.17"(4.3 mm).


B57777 : Pan 2 ptovides the following guidance, similar to andbased on the requirements of BS 2654:

The steel for construction of roof members shall have a mini-mum nominal thickness of not less than 5 mm.

The roof-supporting structure shall be designed in accordancewith BS 449 : Part 2 :1969 (Reference 78.9) or with BS 5950(Reference 18.10\.

The spacing of roof purlins for cone roofs shall be such that thespan between them does not exceed 1.7 m. Where one edge issupported by the top curb ofthe shell, the maximum span shallpermitted shall be 2.0 m.

Where the roof plate is not welded to the supporting framework,for all roofs exceeding '15 m in diameter, cross-bracing in theplane ofthe roof surface shall be provided in at least two pairs ofadjacent bays. These braced bays shall be equally spacedaround the tank circumference. This stiffenino is known as windbracrng.

For coned roofs with roof plate thicknesses greater than theminimum thickness, and for domed roofs, the purlin spacingpermitted may be increased by agreement between the pur-chaser and the manufacturer. A sensible peripheral pitch for alarge radial rib dome roof would be around 2.5 m.


18 The design of low tempenture tdnks

trualn tast stsrtt$!(Im $5I sYlr&r afrn tut Do$t

I

T

ilIt

lltl

Figure 1 8.41 A tank anchorage system penelrcting both the secondary bottom plate and the outer concrete tank liner


1 8 The design of law temperature tanks

sgcloit E-E. EfiEC]EOI CASI136x di Cnfln ffn€NCE




This provides little in the way of design rules or guidance. Ref-erence is made to ENV '1993-1-1, the applicable Eurocode forthe design ofsteel stucturcs (Reference 18.12).

18.8 Tank anchorageThe designs ofthe anchorage systems for single-walled metal-lic tanks and for the inner and outer walls of double-walled me-tallictanks are based on the methods described earlierforam-bient temperature tanks in Slorage Tanks & Equipment.

The task is made more difficult by the following:

. The tank design pressures, and consequently the upliftforces are greater

. The liquid-coniaining tank shells are subject to thermalmovements which must be accommodated in the design

. The anchors for the inner tanks of double-walled tanks willhave to penetrate the outer tank envelope

The total design loading on the tank anchorage is made up ofcombinations of the following loadings:

. Self-weight of the metallic tank parts (in the corroded, i.e.the lightest condition)

. Self-weightof the participating insulation when appropriate

. The internal pressure

. Wind induced uplift

. Seismically induced uplift (it is usual not to have to considerwind and seismic evenb occurring simultaneously)

The form ofanchorage chosen is again based on ambienttankpractices. The boltand chairis less frequently used for low tem-perature tanks and the welded-strap type is generally pre-ferred. This is for two reasons. The flexibility of a thin strapmeans that loads caused by thermal movements of the tankcan be more readily accommodated. Alsothe attachmenttothetank shell ofthe conventional bolt chair gives rise to higher mo-ments in the tank shell than does the strap attachment. This isimportant in an area ofthe tank shell already being subjected tohigh hoop and vertical bending loads.

It is usual to consider the liquid-containing tanksto be emptyatthe time that the uplifting loads are applied, with the exceptionof the seismic load case described below

Open-topped inner tanks are only subject to uplift loadings aris-ing from seismic loadings. ln this case it is usualto resist the up-lift by mobilising a part of the contained product liquid to assistthe tank self-weight to hold the tank down. This is described inmore detail in Chapter 26.

There are however situations where the mobilising of the con-tained liquid cannot be used, or where it is impractical. Thesecould be wherethe seismic upliftcomponent is simplytoo big tobe resisted _by self-weight and liquid hold-down. This could inturn be in situations where the site geometry does not allowthenecessary adjustments to the tank proportions to enable ade-quate liquid hold-down to be made. ln these cases hold-ing-down anchors must be provided which attach to the innerliquid-containing shell and must penetrate the outer metallicshell, or the liner of the concrete outer shell, and on occasionsthe secondary bottom as well. These penetrations must not al-low leakage of the product vapour during service. Bearing inmind the various thermal movements to which these compo-nents are subjected, and the high anchorage loadings in thestrap, this is a complex design pfoblem.

An example of such an arrangement is shown in Figure 18.41 .

This particular case is about as difficult as innertank anchoragesystems get. In addition to the design problems associated with


the penetration of the secondary bottom as mentioned above,the design process was made more complex by the fact thatthis tank was built by the spiraljacking system. This means thatthe shell seams between adjacent courses are not horizontal,butfollowa spiralpath. This in turn meansthatthe length oftheholding-down straps vary to avoid the strap shell attachmentsfalling on this seam.

It is interesting to note the use ofthe tube set into the concretebase slab to artificially lower the lower fixed point and the re-quirement for accurate erection presetting in the radial direc-tion. The former allowed strains due to differential thermalmovements in the vertical direction to be accommodated, andthe lattertook account ofthe thermal horizontal displacement ofpoint B where the strap passed through and is attached to thesecondary bottom.

The various codes give allowable tensile stresses for tank an-chors under the differing loading regimes. The actual load inanchors where a thermal movement is involved is a combina-tion of tension (due to the uplift actions) and bending (due tothermal movements). As has been mentioned already, the useof an anchor where the proportions are deliberately chosen tohave a low stiffness in the radial direction minimises the bend-ing component of the stress, leaving more available effectivestrength for the pure tension component. This generally pro-vides a more efficient solution for the design of the strap itselfand limits the bending loads transferred to the tank shell. Thereis however a limit to this freedom to adjust the stiffener strapproportions based on a sensible strap width and minimumthickness. Strap widths up to say,200 mm, would seem asensible maximum unless special circumstances dictateotherwise.

Sorting out the various loading components and load cases fortank anchors is a tedious business and is an area of designwell-suited tothe use ofbespoke computer programs or spreadsheets.

18.8.1 The requirements of API 620 Appendix R

18.8.1.1 Liquid containing metallic tanks

For single-wall tanks, or for the inner shell of a double-walledtank, the following requiremenb apply:

. The anchorage shall accommodate movement ofthe tankwall and bottom caused by thermal changes.

. Stainless steel should be considered for tank anchors, orprovision should be made for corrosion wherecarbon steelsare used. lvlaterials for tank anchorage shall meet the re-quirements given for primary materials (see Chapter 22).

. When the too shell course is the minimum thickness asgiven in Figure 18.5 and the top corner arrangement is as

So rca ofUplift Prcrsurc

AllowlbL TdrsionSEarC (psi)

Tanl dcsign Fcssurc

Tank &sign prcsrurc pluswrnd or €$hquakc

Tank tcst prcssurc

Allowablc dcsign ircss. S,,

{scc Tablc J- I )

Smalle' of 1.33 t,or 80% orlhc sp€cificd minimum yicld

Smallcr of l.:11t,or 80% ofrh€ sp.cili€d tninimurn yicld

I'Thc rlksab[ tcnsion srrc\{ d]".t! irnrn{d xl ttE Ini nMnocrsc.t(rn or trnsr|t strc\s cr dt rhc rn.hor

Figure 18.42 Allowable tension stresses for uplift pressure conditions

From API 620, table ,7

above,rct thatns thatzontat,r oftherments

ncretetne re-0rrec-

'rermal

o, anonentofto the

nk an-oad innbrna-0ue to're usesen tobend-'ectivey pro-) itselfTherestrap

rimum)em aictate

es forlesignpread

t

valled

) tanK

re re-22).

is as

rs, orileets

Figure 18.34 details a-e, h and l, the minimum anchorageshall be designed for normal loads ("normal loads" meansthe upward loadings less the downward loadings with noadditional factors applied). In this case the allowablestresses for carbon steels, shown in Figure 18.42. For alu-minium alloys and stainless steels the following shallapply:

- The maximum allowable tensile stresses for designloadings combined with wind or earthquake loadingsshall not exceed 90% of the minimum soecified vieldstrength for the material.

For the test condition where the tank is filled with waterto the maximum product design level and an over pres-sure of 1.25 times the design pressure is applied (whereapplicable), the tensile stress shall not exceed 90% ofthe minimum specified yield strength of the material or55% of the minimum specified tensile strength of thematerial.

Allowable tensile stresses shall be taken from Fioure18.4.

r When the top shell course is thickened as in Figure 18.34details f and g, or where a knuckle is used, the anchorageshall be designed for three times the internal design pres-sure. The allowable stress for this loading is 90% ofthe min-lmum specifled yield strength ofthe anchorage material. Asan alternative, the purchaser may specify a combination ofnormalanchorage and emergency venting (it is usualto fol-ow this latter course of action, the addition of emergencyventing systems being generally considered a cheaper op-tion than the consequences of increased anchorage loadsand the foundation provisions that will occur).

-'./ ng decided on the anchor loads to be appljed, it is clearly-::essary to provide means of resisting these loads. The Code., es thefollowing guidancefortankfoundations to resist uplift:

. For tanks with an internal design pressure less than 1 lb/in2,rhe uplift shall be taken as the smaller of the maximum upliftcalculated in the following conditions:

The internal design pressure times 1.5, plus the designwind load on the shell and roof.

The internal design pressure plus 0.25 lb/in2 plus thedesign wind load on the shell and roof.

. For tanks with an internal design pressure of 1 lb/in2 andtrver, ihe uplift shall be calculated considering 1.25 timesine internal design pressure plus the design wind load on:ne shell and the roof.

. ,Vhere the anchorage is designed for the three times the:esign pressure as described above, the foundation shall:e designed to resist the uplift that results from three times:'re design pressure with the tank full to the design liquid:veJ.

:: - 3 r\i of the above it is permissible to utilise friction between-: soil and the vertical face of the ringwall (if used) and all of

-: :ffective liquid weight.--

= Code does not make it clearjust what constjtutes the effec-: quid weight. The author's view is that this should be calcu-

:-=: n the same way as WL in the seismic design case, (see, - =r:ef 26).

' : 3.1.2 NonJiquid containing metallic tanks:: ' :'rese non-liquid containing tanks the requirements of sec-

: - a of the Code apply.-- ,i 3 rlows tanks with cou nterbalance ( j.e. where a counterbal-:-: -g structure such as a ring wallor a slab type foundation as-: ,.:-. :ne tank self-weight in resisting the uplift) and tanks with-: -: :cunterbalancing weights. In this latter case, no anchors:': :'ovided and the uplift loads are resisted by a stiffened bot-

18 The design of low temperatute tanks

tom. This is most unusual for these tvoes of tanks and will notbe considered further.

The tank anchorage shall be designed based on the following:

. The design stresses shall be in accordance with Figures18.3 and 18.42.

. When corrosion is specified fortheanchors, thickness shallbe added to the anchors and the attachments. lf bolts areused for anchors, the nominal diameter shall be not lessthan 1 in plus a corrosion allowance of %" on the bolt diame-ter

Attachments of anchors to the shell shall be designed usinggood engineering practices. This means giving appropriate aftention to the minimising ofany moments and stress concentra-tions aDDlied to the tank shell.

As above, the counterbalancing structure shall be designed toresist the uplift based on 1.25 times the internal design pres-sure plus any wind load on the shell and roof. lf seismic loadsare specified, uplifr shall be calculated using the internal designpressure plus the seismic loads. Wind and seismic loads neednot be combined.

18.8.2 The requirements of API 620 Appendix Q

18.8.2.1 Liquid containing tanks

The requirements are very similar to those spelt out in Section18.8.'1.1. A couple of extra cond;tions are imposed:

. Allowabletensile stresses shallbe taken from Figure '18.5.

. 9a/" or Sok nickel steels, stainless steel oraluminium alloymay be used for anchorage and carbon steel may be usedwhen a corrosion allowance is provided and the tempera-ture regime is suitable for this material. Aluminium alloy an-chorage shall not be embedded in reinforced concreteunless it is suitably protected against corrosion.


The requirements of Section 18.8.1.2 apply.

18.8.3 The BS 7777 requirements

BS 7777 does not differentiate between the requirements forthe design of anchors for inner and outer tanks. Although laidout in a different wayfrom APl620, the actual requirements areessentially very similar.

The design loadings are set out as follows:

. Inner tank service loads The uplift produced by roof de-sign vapour pressure with seismic loads, counteracted bythe effective weight of the shell, roof, roofstructure, roofin-sulation and any permanently atbched insulation.

. Inner tank test loads The uplift produced by roof testvapour pressure counteracted by the effective weightoftheshell and roof structure.

. Outertankservice loads The upliftproduced bythe annu-lar space design pressure with either the wind uplift andoverturning, or, the seismic loads (but not acting simulta-neously), counteracted by the effective weight of the shelt,roof, roofstructure, any associated structure attached to theroof or shell and any permanently athched insulation.

. Outer tank test loads The uplift produced by the annularspace test pressure, plus 60% of the wind uplift and over-turning, counteracted by the effective weight of the shell,roof, roofstructure and anyassociated structure attached tothe roof or shell.

The allowable stresses are given by:


1B The design of low temperctute tanks

. Service case: not to exceed 50% of the (minimum sDeci-fied) yield shength of the material of construction.

. Test case: not to exceed 85% of the yield strength of thematerial of construction.

Various other items of advice are offered in a fairly haphazardfashion and these are listed as follows:

. The internal and external tanks or containers (the BS 7777terminology for non-liquid containing tanks) shall be re-garded as independent structures where neither contrib-utes to the other in resisting uplif

. Asuspended deck shall be considered as an integral part ofthe outer tank.

. Where insulation of the loose fill type is used, it shall notcontribute to the resistance to uplift of either tank.

. Wind uplift and overturning loadings shall be based on BSCP 3 : Chapter V : Paft2: 1972 (Reference 18.13).

. Anchorages shall not be attached directly to shells or bot-tom plates, but shall be attached to pads or brackets (lt isunclear if this means that anchors can be attached to bot-tom plates, which is considered undesirable).

. The design temperature for anchorage and anchorage at-tachments shall be either the design metal temperature ofthe tank of container, or a temperature agreed between thepurchaser and the designer.

. Where the top shell course is thickened to provide addi-tional compression area, the anchorage should be de-signed for 3 times the design vapour pressure. Theallowable stress for this loading may be increased to 90% ofthe minimum specified yield strength ofthe anchorage ma-terial. The Code then goes on to explain the thinking behindthis requirement, which is helpful. The reason for this is that,with a thickened top shell course, the anchorage is under-designed relative to the shell-to-roof connection. Designingthe anchorage for 3 times the design pressure load, en-sures that the anchorage (strength) is in I'ne with the roof-to-shell connection (strength) in the event of an extremeover-pressure. This ensures that the weakest point in anover-pressure situation is not the bottom{o-shell junction.

. Where the top course of the shell is thickened, but normalanchorage is retained, the use ofemergency pressure reliefvalves should be considered.

. lnsulation firmly attached to the inner or outer tank may beregarded as resisting uplift on either tank. Insulation is notnormally applied until after tank testing.

. Anchorage design should allow for adjustment due to set-tlement during commissioning. lt is common to attachwelded-strap type anchors to the tank shell pads during thehydrotest of liquid containing tanks to avoid the need for ad-justment, which is inconvenient to incorporate into the de-sign.

. All anchorages should be embedded into the tank founda-tion.

. On no account should inner tank anchorages be embeddedin the base insulation for the purpose of resisting uplift.

. Tank (and anchorage) design should accommodate move-ments due to temperature change to minimise inducedbending stress in the shell. Any additional stress induced inthe shell by the anchorage attachment should be checkedto ensure that the allowable stress level of the shell is notexceeded forthe conditions ofanchorage load considered.

. Heat breaks may be required at the anchorage of innertanks to prevent chilling of the outer tank and foundations.Heat transfer to the colder parts of the tank structure should


be limited, to avoid failures of the anchoraqe due to ice for-mation or water condensation.

. A corrosion allowance of 1 mm should be aoolied to all sur-faces of anchorage parts.

. Any initialtension in the anchorage members resulting frombolting loads or loads due to transient or long term thermalmovements, should be considered in the anchor loadings.

. No initial tension should be applied to the anchorage. ltshould only become effective when an uplift force developsin the shell of the tank or container.

. Steps should be taken before the tank goes into service toensure that anchorage bolts cannot work loose or becomeineffective over a long period.

. Any anchor bar, bolt or strap shall have a minimum crosssectional area of 500 mm2.

. Anchorage points should be spaced at a minimum of 1 mand at a maximum of 3 m and should, as far as possible, bespaced evenly around the circumference of the tank.

When reading through these requirements, it should be bornein mind that the language is used in a specific way as requiredby BS 0 (The British Standard for Producing British Standards).Hence "shall" indicates a mandatory requirement whereas"should" and "may" indicate a recommendation of goodpraclce.


The guidance given for tank anchorage is essentiallythe sameas is included in BS 7777.

Stress limits for tension are given as:

Normal operation 0.50 f"

Test 0.85 fy

oBE 0.67 fv

ssE 1 .00 fv

The point is made that shell attachments shall be designed for aload corresponding to the full yield capacity of the un-corrodedanchor bolts or straps.

18.9 Tank fittingsThis Section deals with fittings which penetrate both the liquidcontaining and the non-liquid containing tanks. Other items ofequipment which are some times included under thls headingsuch as in tank pumps, stilling wells, fill columns, etc. are dis-uur-ru rIv,,dP(v, zv.

As is the case for ambient tanks, there are a variety of types offittings for low temperature tanks. Roof fittings are the mostcommon, being used for liquid, vapour, instruments and ser-vices. Shellfittings, mainlyfor liquid outlet connections, are lesscommon. Fittings penetrating the tank bottoms are uncommonthese days and are confined to liquid outlet connections.

For fittings in double-walled tanks where the inner tank is de-signed for the product temperature and the outer tank or con-tainer is designed for temperatures based on ambient tempera-tures, there is a need to protect the outer tank materials fromthe temperature ofthe cold product in the emerging line. This isachieved by the use of heat breaks. The thermal insulation ofthese heat breaks is discussed in Chapter 19.

Current oractice is to preclude the use of shell and bottom con-nections. These are seen as weak points in the storage system.The stress concentrations and discontinuities associated withfittings are potential sources of leakage from the primary con-tainment and damage to the externally connected pipe work

o ice for-

o a SUr-

:rng fromthermal

)adtngs.

)rage. ttevelops

)rvice to)ecome

n cross

of 1mible, bek.

) borne)quireddards).hereas'good

:truld lead the tank to dump its conients. Consequen v all con_- 3ctions to the tank are required to be through the tink root,="rd this involves the use of in-tank pumps. These pumps and:-e associated equipment are described in some detail inlrapter 20.

-iere are two areas where this practice is not adopied. The::3rage of liquid ammonia traditjonally has not used in_tank: -mps as thesewere not availablefor usewiththis product until:-'re rec-ently. This subject is discussed at some length in-'aDter 2l-e storage of liquid oxygen, nitrogen and argon has tradition_: ., been in double-walled single containment tanks with liouid

: -: et connections in the lower shell or tank bottom. lt is aroued: . the part of the industry that deals with these productJthat:-:y pose a lesser risk to personnel and the environment that::-er products, being non-flammable and being stored in rela_

, aly small quantities. The use of internal shut_off valves is:: 1]mon practice and goes some way to answer the contents: - -'rping question. The liquid ouflet connections for such tanks:-: usually small, rarely greater than 6" in diameter. For con_-.:tions of this size, effective internal shut_off valves are'-:Jily available. For other prcducts, the sizes ifthe liquid ouflet-:-necllons are much larger, making the use of internal.--i-offvalves a less viable option.:::TAPI 620 and BS 7777 permit and give rules for designing

lfora'ooed

!rq!loTrs ofrdrng) drs-

es ofr"lostser-iessmon

:on-era-ronis isnoi

pn-3m.t iih

ork

Suspended deck vapoLrr veni

Fgure 18.44 Typ catsuspended deck penetrations

tank shell connections. Apl 620 does this without comment asto the desirability of such connections, which js no surprise asits origins go back to tjmes which predated the use oi in tankpumps for land-based storage and concepts other than sinolecontainment type tanks. BS 7777 expresses a rather nril;tyworded disapprovalofthe use ofsuch connections as follows:"All pipe connections should preferably be made via the roof ofthe.tank.-This is based on the phllosophy that the risk of ser ousreaKage lrom the innertank is thereby reduced to a minimum. lnthis way. the possibility ofsurroundings flooded by teaking pfod_uct, with the risk of fire and explosion, is minimised.',Some of the roof fittings are quite large. For large LNG tanks.in-tank pump columns of 42,, in diameter are not uncommon.with liquid inlets and vapour ou ets of 32', in diameter. Bearingin mind that these fittings will need to be fitted with heat breaks:then thediameterc of the actual penetrations through the outertank roof will be even bigger. For such large fitting;, their verysize gives rise to structural problems. ThJ compjnenis whjchgo to make up the roof loadings are:. The self-weight of the fitting, the heat break and ihe at_

tached. internal and external pipework together with t-eInermat tnsulatron

Any roof mounted isolation valves \a typical24" balt la Elor tow temperature service can wejgh around 2700 kE,

Any internal attachments (i.e. in-tank pump columns).Loadings from externally connected pipework wntcr :s -s __ally considered to be anchored at the roof conneci o.Seismic loadings acting on the roof, the fitting and the cotr-nected pipework.

Tanks where the structural part of the roof is made of reinforcedconcrete are generally better suited to catering for these load_ings than those made from steel.

Pener€fions throuih suspended deck

Thertulins!alon Vapourfl4

' : - : - 8 4 3 Typical roof iitting arrangements


1 8 The design of low tempercture tanks

Figuro 1€.45 Typlcal roof frttng arangements for tanks with fixed inner and

Otherlarge rooffittings are the roof manways wh ich, forperson-nelaccess can be 36" in dianieter, andthose usedfor materialsaccess during the tank construction period are ofren up to 60" indiameter A number of difierent tank roof fitting anangementsare illustrated in Figure 18.43.

For tanks with suspended roofs, connected internal pipe workmust pass thrcugh lhe deck. There are also fiftings in the deckitself for personnel access and for product vapourflow acrossthe deck. Typical suspended roof fiftings are illustrated in Fig-ure 18.44.

Nowadays, for the relatively rare case, where the tank is pro-vided with fixed innerand outer roofs, the need to cater for thedifferential thermal movemenb between the tianks is an addeddifficulty. Some arrangemenb which have been used in thepast are illustrated in Figure 18.45-

354 STORAGE TANKS & EQUIPiIENT

For shell connections which Denstrate both tho inner and theouter walls, a similar probtem exlsb. This is usually overcomeby the use of bellows or pipe work running in the interspace.Some typical arrangerrents are shown in Figure 18.46.

Tenk bottom connections are unusual but not unknown. Somepass straight down through the base insulation andlhe support-ing base slab, and som6 turn through a aight angle and run outthrough the base insulalion, which may be thick enough to ac-commodate tho liquid ouflet line, or may require to be locallythickened to avoid localchilling of the outertank bottom wherethis is not provided in a lowtemperature steel. Some altemative

Inr.nEl plp*ork lpe

Flgure 1 8.46 Some typical double-walled tank sh€ll conn€ction amangements

Inr€n.l b€{m .fugsdt

Plpe$qrk belw€€n roof3 io provld€ n€xiblliv

Figure 18.47 Typicaltank bofiom @nn€ction arangements

1B The design of low tempercture tanks

and theercomerspace.

aqckng srrp, il used, may be ren eed an€f w€tding

l! d t? ncl l€s than 0.71_ d%ncn

r. Someupport-run outh to ac-locally

rwherernative

Ptml g Plmr n

: ; -

_: 1 8.48 Acceptable types of nozzles and olher conneclions - page I: -- lPl 620, figure 5-o

:-=-gements are shown in Figure 18.47.

'3.9.1 The requirements of Apl 620

18.9.1.1 General requirements of API 620 section 5

These requirements cover the maximum sizes of nozzleswhich do not require any additional reinforcing, reinforcing ofnozzles which do require additional reinforcinq material to besupplied. the details of suitable nozzle and reiriforcing plate at-tachments to the tank shell, stress relieving requirements and anumber ofother points. These rules are applicable to nozzles innon-liquid containing tanks and form the basis ofthe desion ofnozzles for liquid containing tanks.

Single openings in tanks whjch do not requjre reinforcementother than that inherently available within the provided nozzleneck and the tank wall thicknesses are:

. 3" pipe size welded into bnk walls of %,, thickness or less

. 2" pipe size welded into tank walls of thickness qreaterthan%.

. Threaded connections where the opening in the tank wall isnot greater than 2" pipe size

The minimum area or reinforcement required is based on the


--: : ready mentioned, tank fittings can penetrate the roofs,:-: s orfloors ofboth innerand outertanks. Forthe innertank: ; - -ection the fitting is desjgned to either Appendix R or e de-:i -: rg on the temperature of the product liquid. For metallic:- : containing outer tanks the fittings are similarly designed

.: ::cendix R or Q. For metallic non-liquid containing tanks the--:-ls are designed to the API basic section of the Code (i.e.

-:-- cn 5).

-- > situation is made more complex by the two Appendices:,: -cwing" parts of section 5 and then imposing modifications: '-:se rules. What follows is an attempt to simplify and sum--:- se these rules. For those involved in the detailed desion of=

-. "tt,ngs. it is probably necessary to read the full text oi the

"-:JS sections to ensure a full and accurate understandino of-: : erhora of deiailed requirements and advice providedl


where:

d

Paisl P Plnel q

F gure 18.48 Acceptable lypes of nozzles and othef connections page 2

Fron API 620, figure 5-B

100% replacement of area rule. The basic formula is:

,\ = (d + 2c)(t c)E' equ 18 37

It must lie within a distance measured circumferentially orvertically along the tank shell from the axis ofthe opening ofthe greater of:

Adistance equalto the diameter opening after corrosionot

A distance equal to the radius of the opening plus thethickness of the nozzle wall plus the thickness of thetank wall, all in the corroded condition

It must lie within a radial distance measured along the fittingitself from the inner or outer surface of the tank wall of thelesser of:

- Adistance equal to 2.5 times the thickness of the tan k

wall less corrosion allowance or

- Adistance equalto 2.5 times the thickness ofthe nozzlewall less its corrosion allowance plus the thickness ofany additional reinforcement inside the tank wall if avail-able less its corrosion allowance(s)

= area of reinforcement to be provided (in'?)

= the clear inside dimension across the opening(usually considered as perpendicular to the di-rection of principal applied stress) (in)

= corrosion allowance for the part in question (in)

= joint factor for the part of the tank wall inques-tion

The reinforcement shall be provided in the tank wall, an addedreinforcing plate or in the nozzle body within the limits outlinedbelow. The tankwallmay be arbitrarily thickened to make avail-able more reinforcing area.

The limitations on the effective reinforcing area are:


l::

c

E'

ly orrg of

s|on

thethe

ttngthe

ank

2)eiofail-

leThedesign otlov@te*

Enh€. d8ttr.d ol .tta.lwd k larid.dory

F!o.l F P.n lF2.

\-l 3 9le6 slz€, nrr

ll-l. t*trEl5\pPf,.-A-3.rssrr

NOTES:

,- : nornmal thickners of the irnl wall, in in.,including cormsion allowEnce,

r. = loninal ninimum thickness of rhe nozzle neck,in in.in., includina @rrosion allowance,

r, = nominal rhickress of rhe rcinlorcing pa4 ir in.,includirg co.rosion allowanc€ if thc pad iser(posed io coFosion,

. = conosion allowanc.. in rn..

tmin = fte smaller of 3/4 in. or $e thickn.ss lcss fieco.rosion allowanceof eirherof rbc pans ioinedbya filler weld or Foove weld.

I or/: = a value not less than the small€rotl/4 in. oro.7rn,Di the sum t + irshallnor be tesr hanl.?5rmi,,.

rr = rhe rmrllcr ol l/arn,ar0./(r,,-.' {lnsrd.(or-nerselds nlav h€ Iunher limired h! ! le$er

Fgure 18.48 Acceptabte types of nozztes and other connections - page 3F@rn API 820,ligute 5-8

liletal in the tankwall in excess of the thickness required forthe$uctural requirements for 100% joint effciency may be mn-sltered as available reinforcing material if the fitting is locatedertirely within the plate material (i.e. does not pass through ar€lded seam). This is useful for roof penetrations.

f b not usualto cany out site stress relief of storage tanks fol-bvting erection for reasons to dowith the size, weightand needb.localsupport during the operation. There are however localsfress relieving requirements to minimise local residual stressconcentrations. The necessity to carry out such operations,Gually in the fabricator's works, afrer the fiftings have beenrelded into the tank shellplate, are based on the shelland noz-Ae body thicknesses. Hence stress relief is required when:

. Tank sections which have a wall thickness greater than1.25" (this can be increasedto 1.5,,forcertain matedals pro-viding a minimum preheat temperature of 200.F has beenmaintained during the welding process).

. Welded attachmenb and nozzle bodies where the thick-

length of projcctior of d|! nozzle watl b€yondrhc imide face of dle iant wall.)

,4 = a r"luc not lcss than ojrnnF

t5 = a value nor less than o.7rnnn,

t, = norninal h.rd tlliclmcls, h in.

l.TtE wcld dimcc'oN indicatcd i! Ihir 6gua e pcdiqcd on 0lcasuhprion thal no .drosio. tu .nticiprt d or th. oubirlc of firl,nk. lt oul\idc cotuion is.rpdl!d. dc oukirL w.ld dihcnsio$shall b. ircra\ed a@ordingly,

2. Erpos.d cdgs sh(M as rudcd hay b. 6nish.d by lighl Eind-ing b ar tast r 14 in- 6dius or chdf.Ed ar 45 degRs ro ar lcan a

IFd!n FF r'a &.1 smallcr. s cxempoons h 5.16.9. L

ness at any welded joint, in inches, is greater lhan(D+50y120.

where:

fitting nominal diameter (in)

For fittings less that 20" diameter, D shall be taken as 20f.

This requirementfor stress relief is not applicable to tank com-pression areas where the thicknesses of plates used fequentyexceed these thicknesses. This is fonunate as the sib s-tressrelief of a completed compression area wouh be impradicaland the restriction imposed on the designerof timiting the platethicknesses to 1.25" or 1.5" would in effecd limit the maximumdesign pressures for large bnks.

Figure 5-8 from section 5 ofAPl620 provides guidance regard-ing acceptable details for welded nozzles and other @nnec-tions. These are shown in Figure 18.48.

Singb-mH bp sEtd lor haads c.@r b plgsr.



18.9.1.2 The particular requirements ofAPl620 Appendix R

Appendix R contains a number of detailed additional require-ments relating to the types of connections which are permittedin primaryand secondary components, the stress reliefrequire-ments and aspects concerning spacing and inspection.

At fear of repetition, the number of detiailed requirements in thisarea makes it necessary that the designer studies the rulescarefully and is hopefully experienced in this type ofwork. Themain Doints raised are:

. All openings in primary components shall have completepenetration and fusion. Acceptable details are shown in Fig-ure 18.43 details a,b,c,g,h,m and o.

. In primarycomponents, shop stress-relief of the welded as-sembly is requires unless:

- The stress in the plate is less than 10% ofthe minimumtensile stress ofthe plate materialand the opening rein-forced for this low stress

The impact requirements for the plate and the weldingfulfil the requirements for primary componenb and thethickness ofthe shell plate is less than %" for any diame-

ter of connection or less than 1 .25" for connections thathave a diameter of less than 12". The thickness of thenozzle neck without stress relief shall be limited to(D+50)/120 as above.

. The opening is reinforced with a forging as shown in Figure18.48 details o-1 to o-4

. The butt weld around the periphery of a thickened insertplate or the fillet weld around a reinforcing plate shall be atleast the greater of 10 times the shell thickness or 12" fromany butt-welded seams, except where the completed pe-ripheryweld has been stress-relieved. In this case the spac-ing shall be at least 6" from any vertical seam and 3" fromany circumferential seam subject in both cases to a mini-mum of 3 times the shell thickness. These rules shall alsoapply to the shell-to-bottom joint. lt is allowable to extendthe thickened insert plate or the rejnforcing plate to theshell-to-bottom joint and not require stress reliefof the weldto the bottom or annular Dlate.

. All welds in opening connections that have not been com-pletely radiographed shall be inspected by magnetic parti-cle (MPl) or liquid penetrant (LPl) inspection methods. Thisshall include nozzle and manhole neck welds and neck toflange welds. The root pass and each additional %" of de-posited weld mebl shall be similarly inspected.

. Butt welds around the periphery of thickened insert platesshall be completely radiographed

It is interesting to see that this section of the Code (and theequivalent section of Appendix Q) allows the use of slip onflanges with the agreement of the purchaser.

18.9.1.3 The particular requirements of API 620Appendix Q

The detailed requirements of Appendix Q are essentially thesame as for ADoendix R. Stress relief of stainless steel or alu-minium alloys is not required. For 5% and 9% nickel steels,stress relief is a function ofsurface strain and as such can usu-ally be avoided.

18.9.1.4 The design of heat breaks

A has been mentioned, many tank fittings have heat breaks as-sociated with them. These heat breaks are frequently sub-jected to high axial and bending loads from the attachedpipework and associated fittings, particularly in the largersizesof connections and where seismic loadings are involved. TheCode does give some guidance which can be used for the de-


tailed design of these items, but this is limited and it is not un-usualfor additional guidance to be soughtfrom pressure vesselcodes such as ASI\.4EVlll (Reference 18.74) or a similar Code.This area of design is tedious and repetitive. Well-designedcomputer programs or spreadsheeb are a boon forthis type ofworK.

18.9.2 The requirements ol 85 7777

BS 7777 seeks to separate the area of tank fitting design intothe following categories:

. The design ofoutercontainer (i.e. non-liquid containing me-tallic ianks) mountings

. The design of inner tank and liquid containing outer metallictank mountings

. The design of connections between the openings in innerand outer ianks

The detailed reouirements and recommendations are summa-rised in the following Sections.

1 8.9.2.1 Outer container mountings

The rules follow almost exactly the ambient tank practices laidout in BS 2654 described in Chapter 3.

ln brief. the main Doints are:

. No reinforcing required for nozzles oflessthan 80mm diam-eter

. Reinforcing to be 75% of the removed area

. Reinforcing to be provided either

By a thickened shell insert plate

By a thickened nozzle body

By an added reinforcing plate

. As an alternative the thickened nozzle barrel protruding onboth sides ofthe tank wall such that the j factor does not ex-ceed 2.0 in accordance with the calculation method given

There are some minor modiflcations to the oermitted weld iointdetails.

18.9.2.2 Inner tank and outer liquid containing tank mount-Ings

The rules again follow the BS 2654 practices for shell connec-tions. Table 13 (Figure 18.49) makes slightly different require-ments for the minimum nozzle body thicknesses.

The Code makes a number of comments and recommenda-tions (i.e. non-mandatory requirements), some of which areworth repeating in full:

. All pipe connections should preferably be made via the roofof the tank. This is based on the philosophy that the risk ofserious leakage from the inner tank is thereby reduced to aminimum. In this way, the possibility of surroundings beingflooded by leaking product, with the risk of fire and explo-sions, is minimised.

Minimtrm manhole ddnozzlc body thicknes

d"< 50

50 <d^< 70

70<dn<100100<dn<200200<4=300300 < d-

5.0

5.06.0

8.010.0

t2.5

Figure 18.49 Manhole and nozzle body thickness

From BS 7777 : Paft 2, table 13

i not un-e vesselrr Code.esrgned; type of

ign lnto

ing me-

netallic

n inner

Jmma-

es laid

dram-

ng onf,t ex-)jven

ljoint

runt-

. Where side or bottom entry is specified, the design shouldtake account of nozzle leakage, and its consequences,from the following causes:

- Differentialmovement

- Thermal stress

- Stressintensification

Pipe loads

- Difiiculty of inspection/maintenance

- Foundationheatingdiscontinuities

Vulnerability of pipework to damage

' Where possible, shell-mounted nozzles should be shop-fabricated into shell plates

. Where used, shell-mounted inlet and ouflet connectionsshould be provided with internal shut-of valves

-^ s section ofthe Code also gives rules and advice regardjng:-€ design of bottom connections. The one rule (mandatory) is:€t connections to inner tank bottoms shall be Drovided withf .jt off valves for internal emergency shut off conditions. The1:.r mandatory comments and recommendations are also-:eresting and useful:

. Connections to the inner tank bottom should be avoided.

. Bottom connections for emptying the tank totally prior towarm up and inspection are normallyof smalldiameter lt isnot advisable to operate the tank through the bottom con-nections, unless the design allows some liquid to remain inthe tank (this is presumably to avoid accidental tank warmup during service).

. Where openings (i.e. bottom openings) are unavoidable,the following design procedure should be adopted:

- Entry to the tank is as close to the shell as possible, butnot less than 650mm (to keep the opening close to theshell but not in the annular plate - this could be a prob-lem where the annular plate is made wider for reasonsof seismic design. The fitting should always be inboardof the inner edge of the annular plate).

The nozzle is positioned in an annular or sketch plate,enlarged if necessary for this purpose.

- The nozzle opening is reinforced on a replacement ofarea basis by a doubler plate, or thickened annular orsketch Dlate.

- The design should impose negligible bending momenton the inner tank bottom under all conditions of oDera-tion, particularly with reference to the difierential con-traction ofthe innertank relative to the outertank. (Oneway to do this is to support the horizontal portion of thepipe from the underside ofthe innertank shell-to-bottomjunction bya strap welded to the underside ofthe annu-lar plate and the top of the pipe).

- The pipe should always be full of product in service.

- A high standard of construction and inspection is sDeci-fied. (lt is advised that the nozzle assembly is prefabri-cated into the thickened bottom plate and subiect toinspection and testing prior to laying on the base insula-tion).

- The unsupported area underthe nozzle is keptto a mini-mum and the surrounding insulation is designed for thehigher load imposed on it. (lt should also be capable ofwithstanding damage at the edges of the hole).

- The space surrounding the nozzle and pipe is filled witha suitable insulating material.


Pqtked ,,ih insulonon

Figure 18.50 Possible connection option

From BS 7777 : Paft2, figure 25

nec-uire-

Figure 18.51 Further shell conneciion option

From BS 7777 : Patt 2, figure 26

- Anti-vortex provisions are considered.

- Additional localised base heating is considered.

Much of these mandatoryand advisory requirements werewrit-ten following the events surrounding the bottom ou et connec-tions installed on the original Das lsland LNG tanks. poor con-struction details, leakage and poor thermal insulation led tothese tanks having to be decommissioned and taken out ofser-vice and eventually being demolished and replaced.

18.9.2.3 Connecting pipework between inner and outertank connections

The Code provides two sketches of possible shell connectionarrangements. The first uses bellows to provide the reouiredflexibility (Figure 18.50), whilst the second uses a pipe loopwithin the interspace for the same purpose, (Figure.lg.5.l).

The Code provides a number of mandatory reouirements andadvisory guidance, all of which is useful.

The mandatory requirements are:

. Connections shall have a detailed design. (Meaning thateach connection shall be fully designed in its own ight andnot be a tepeat or copy of an eadier similar connection.)

. A heat break shall be fitted to connections between innerand outer tanks. (ln this case the detailed design ofthe heatbreak could follow the requirements of BS 5500. (Refer-ence 18.15).

. Connections between openings in the inner and outer tankshells shall be designed to accommodate the differentialmovement between the shells. (lt is importiant to considernot only the longitudinal thermal differential movemenG,but also the translation caused by thermal contraction andthe rotations caused by product loadings.)

nda-are

roofik oftoa-jingplo-



. Flanged joints shall not be located within the intersoace be-tween the inner and outer shells.

. Where welded pipe js used, the welded seams shall beradiographed for the full length of the weld.

. Connections between openings in the inner and outer tankroofs shall be designed to accommodate the differentialmovements between the roofs.

. For the suspended deck, connections shall be desioned asan extension to the nozzle in the outer tank roof.

The guidance is:

. Particular attention should be paid to the design ofthe inter-connecting mountings. This is to ensure that no mainte-nance or inspection is necessary during the operational lifeof the tank, since access to the interspace between the in-ner and outer tank is normallv imDossible.

Thermal and hydrostatic forces cause relative movementbetween the inner and outer tank, thus jncreasing or de-creasing the interspace gap. Strain absorbing connectionsare necessary to ensure that the relative movement doesnot cause unacceptable local stressing of the inner and/orouter tank. (Guidance on local loads on cylindrical vesselscan be obtained from Reference 18.15.)

Heat breaks should be designed to prevent over-cooling ofthe outer shell.

Connections should be made from seamless oioe.

lvlovements between the inner and outer tank roofs arisesfrom either differential thermal expansion or contraction, ordiffering internal or external loads.

. For suspended decks the connection should be able tomove freely through the suspended deck, thus eliminatingadditional loads on either the outer roof or the suspendeddeck. (l\ileans of preventing roof insulation from fallingthrough any gaps and entering the stored product shouldalso be made.)

l\y' uch ofthis is self-evident and repetitive, but is nevertheless allsensible advice.


For membrane tanks all liquid inlet and outlet connections areto be routed via the tank roof. For othertanks connections to theprimary container and other parts of the secondary containerare discouraged but reluctantly allowed. In cases where bottom(and possibly lower shell, the wording is unclear in its intention)inlets and outlets are used, either a remote operated internalshutoff valve must be installed or the bottom (and shell?) con-nection shall be designed as part of the primary container withthe first valve being a remote operated type which must bewelded to the bottom (or shell?) connection.

The guidance provided is similar to that in BS 7777 and refer-ence is made to the appropriate sections ofthe prEN on ambi-ent tanks in Chapter 3.

18.10 Suspended decksSuspended decks or ceilings are simple covers over the top ofthe liquid containing tanks. They can be fitted into single-walledtanks or above the inner liquid containing tank of double-walledtanks. They are suspended from the tank roof framework andtheir function is to support the roof insulation. They do not re-quire to be product vapour tight, in fact very much the reverseas the product vapour should be free to flow across the sus-pended deck with little resistance and consequenfly causing liftle differential pressure.


Figure f8.52 Aflat plate type suspended deck

The use of a suspended deck means that the roof insulation isnow within the tank envelope and is protected from the windand any chemically aggressive atmospheres. As a conse-quence, cheap materials such as glass fibre of a very basjcgrade (i.e. household loft insulation type), mineralwoolor looseor bagged perlite can be used ratherthan more expensive insu-lation systems such as PUF protected by vapour tight metalliccladding. lt is not only the cost ofthe roof insutation that is sionif-icant. placing it in a protected environment gives longevity-ad-vantages as well. Further advantages relating to the need to nolonger require a low temperature grade of steel for the roofstructure and sheeting also exist.

It is usualto arrange a generallyflat profile ofdeck flush with thetop of the inner tank for double-walled tanks, and located at asuitable elevation to avoid thermal stress problems for sin-gle-walled tanks.

The design Codes have little to say about the design of sus-pended decks, so the designer is lefr very much to his or herown devices. A number of different arranqements are in com-mon use:

. A flat plate system stiffened by circumferential mem.bers The flat deck is usually of lapped and fillet-welded con-struction. The inner part of the deck is stiffened bycircumferential flats attached to the upDer surface of thedeck and serving as the attachment points for the hangers.The outer edge of the deck has a more substantial periph-eral stiffener fitted.

The hangers are usually of steel cable or rod material se-lected to be suitable for the loading and temperature. lt isnot unusual for the hangers to be made from two different

I't

,

Figur€ 18.53 A struclural frame and profiled sheeting type suspended deck

ation is€ wtndconse-y basic|tloosee insu-netallicrsignif-,ty ad-dtonore roof

fith the,data)r sin-

rf sus-or hercom-

mem-I con-dbvf the€ers.)nph-

.ltiserent

;

tItI

=g rte 18.54 A plywood paneltype of suspended deck

materials. The lowerpart being suitableforthe lowtempera-ture whilst the upper part is of a cheaper material suitablefor the local ambient temperature. An example of such adeck is shown in Figure 18.52.

The design ofsuch decks ofren involves the use offlat platetype bridge deck formulae from The Steel Design Manualproduced by the United States Steel Corporation (Refer-ence 18.16\. This type of deck is usually made of carbonsteel with suitable low temperature properties or of alu-minium alloy. The plate thickness chosen for the deckplates isfrequently5 mm. This type is quite robust and suit-able for foot traffic and for use as a working platform foranyerection tasks in this area of the tank. The deck is heavierthan the structure and profiled sheeting type and is gener-ally more expensive on an area-for-area basis. lts use isusually confined to smaller diameter tanks.

Aframework of structural sections supported from themain roof members by cables or rods This framework inturn supporb a deck of profiled sheeting. lt is usualfor thestructural members and the sheeting to be made from alu-minium alloy. lt is usualforthe design Code minimum thick-ness requirements of %6" or 5 mm to be ignored for thissheeting which can be obtained in thicknesses down to 0.9mm. To simpliry the supporting arangements, it is usualforthe structural framework to follow the layout of the roofframework which is a normally radial rafter type.

This type of deck is light and economical and is often usedfor the bigger LNG tanks. The profiled sheeting, especiallyin the thinner thicknesses, is susceptible to damage by foottraffic and erection activities. lt is frequently necessary touse walking boards or similar devices to protect it at thisstage. Atypicaldeck ofthis type is shown in Figure 18.53.

Individual deck panels supporled at their corners bysuspended plates Atypicalanangement using plywood inshown in Figure 18.54. This is an economical and easilyerected system. One unforeseen disadvantage with the useof plywood is its water contdnt. lt is normally delivered wilhsome 10% moisture by weight. This has been the cause ofdifiiculties in obtaining a suiiably low dew point during thetank purging phase.

The first two types described are well-suited to a tank roofwhich6to be airlifted. The suspended deckcan be erected at ground.$/el, attached to the main roof framework and air-lifted into itstu€l position. The latter type is ideal for erection at the full iankr€ht.


Figure 18.55 A suspeid€d deck vent

18.10.1 The requirements of API 620

Essentially none.

18.10.2 The requirements ot BS 7777

BS 7777 has more to say about suspended decks. The manda-tory and advisory requiremenb are summarised as follows:

The suspended deck support structure shall be designedforthe lowest temperature encountered by any part in prac-flce

The design shall ensure that the outer roofshall always beat ambient temperature where the hangers are attached to

The structure shall be designed foranyone hanger becom-ing ineffective

Materials for the suspended deck shall be agreed betweenthe purchaser and the manufacturer and may include:

- Carbon manganese steel

- Aluminium alloy

9% nickel steel

- Stainless steel

- Timber

The suspended deck shall be insulated such that the outerroof does not cool below its design metal temperature, thatboiloffofthe product is limited and thatexcessive roofload-ing due to ice build up is avoided

The design shall preventthe passage ofinsulation materialinto the Droduct

The design shallallow product vapourto breath through thedeck to limit anydifferential pressures to less than 2.4 mbar.Suitable vents shall be provided which allow this breathingand prevent cold vapour impingement on and chilling oftheroof structure. Such a vent in shown in Figure 18.55.

Fittings which pass through the roof space above the deckand which could cause cooling of the vapour above thedeck shall be suitiably insulated

The deck shall be at a level not less than 0.5 m above theproduct design level

In areas of high seismicity the deck should be at a level toavoid sloshing liquid impingement and be designed for anylateral loadinos


18 The design of low tempamture tanks


This Code has little to sayon the subject ofsuspended deck de-sign - apart from requiring the roofand its supporting structureto be designed for the minimum design temperature, to be de-signed forthe failure of any one hanger and to have ventilationopenings such thatthe pressure difierence across the deckwillnot exceed the weight of the supported roof insulation.

18.11 Secondary bottomsSecondary bottoms are a comparatively recent addition to thelist of component parts of low temperature storage tanks. Forfull containment LNG tanks with concrete outer tanks, second-ary bottoms are now more or less much a standard part. Forother types oftanks for other products, they are an occasionalrequrrement.

The function ofthe secondarybottom isto protectthetank baseslab, either of the on-ground or elevated types, from damagecaused by an inner tank bottom leak. A leak in the inner tankbottom will make its way through the sandwich of liquid perme-able materials which together make up the tank base insulationand locallycoolthe base slab, which is usually madefrom rein-forced concrete. This local cooling can cause through{hick-ness cracking and consequent liquid leakage either to theground or to atmosphere depending on the type of base slabadopted. There is also a possibility that damage to the baseheating system and anotherpossible route ofliquid leakage viathe heating element conduits may occur.

The secondary bottom concept came about in orderto preventthe possibility of liquid leakage to the local environment. Themetallic form of this member consists of another tank bottomlocated either within or above the base insulation sandwich.The construction details ofthis secondary bottom are similar tothe inner tank bottom, i.e. a lap-welded membl-ane of minimumthickness. There is a possibility that the thickness could be lessthan the Code minimum requirements of %6" or 5 mm, but themost commonly used material (9% nickel steel) is not currentlycommercially available at thicknesses less than 5 mm.

There are possibilities of secondary bottom systems usingnon-metallic materials and one manufacturer, has developed aproprietary design using polyurethane foam in conjunctionwitha glass reinforced plastic covering which has been used in anumber of low temperature tanks. The most common system ishowever the metallic secondary bottom.

The secondary bottom is usually used in conjunction with a bot-tom corner protection system which is the subject of the nextsecnon.

API 620 and BS 7777 have nothing to say regarding secondarybottoms.

The design and practical requirements include:

. The thermal contraction must be considered, particularlywhere the bottom is attiached to.a bottom corner systemwhich may provide a peripheral constraint. The bottomwhich is embedded within the base insulation willclearly notcontract as much as a bottom which is located on too ofthebase insulation.

. Differential thermal movement between the various materi-als must be free to occur without restraint.

. Local cooling of the embedded type of secondary bottomfrom some intermediate temoerature in normal service tothe product liquid temperature in the leakage situation mustbe accommodated.

. Constructing thin bottoms to tight local flatness tolerancesis difficult. Provision for practical undulations must be de-signed into the system.


. The inspection regime must be similar to that of the innertank bottom with of course the exception of the hvdrostatictest.

18.12 Bottom corner protection systemsAgain, like the secondary bottom, this is a comparatively newphenomenon in the design of low temperature tanks. API 620and BS 7777 have nothing to say about this part ofthe storagetank system.

The function of this component is to avoid the unsustainablestresses which can occur in the bottom corner of concrete outertanks ofthe fixed or encastr6 design. The fixed bottom corner,as opposed to the pinned and sliding designs, which are dis-cussed in Section 18.16.4, is currently the most commonlyused detail for this part of the concrete outer tank.

In the innertank leak or failure case where the product liquid iscontained by the concrete outer tank, a situation is createdwhere the bottom slab is warm (protected by the base insula-tion) and the outer wall is cold. This gives dse to shear stressesat the wall-to-base junction which cannot be easily designedfor. Without some modification in this region, there exists a sig-nificant possibility of through-cracking ofthe concrete wallandleakage ofthe product liquidto the environment. This is particu-larly the case for LNG tanks. Their large size and low designtemperature exacerbate the oroblems.

For LPG tanks, the less onerous design temperatures oftenmakes this problem manageable without resort to bottom cor-ner insulation. To make this bottom corner designable, thermalinsulation is applied to the lower portion of the outer wall for a

\1Figure 18.56 A 36% nickel / 64% iron alloy bottom corner protection arrange-

Figure 18.57 The installalion ofa 36% nlckel/ 64% iron alloy bottom cornerarangement

qdal-I-rdeT!Iff-'IF'If[r-

q

the innerdrosktic

emsr'ely newAPt 620storage

tainable)teouterr corner,are dts-mmonly

liquid iscreatedr insula-tresses)signeds a srg-vallandparticu-design

s often)m cor-nermalrll for a

-erght which is usually between 3 and 5 m, although there are:esigns wherethis insulation is applied for the full height of the:,-ier tank wall. This insulation prevents the lower portion of the:,-:er wall from cooling to the product temperature in this acci--iElrt situation. ltchangesthe stress regime inthe bottom cornertea from one of almost total shear stress to one which is a- xture of shear and bending, and is more readily accommo-

=:ed in the design process.-: ensure that this insulation performs its purpose, it is impor-E:: that it is protected from the leaked product liquid in ther:erspace. This liquid may be at the full equilibdum level andl-E static head will tend to force liquid into the insulation and-:- Jer it ineffective unless suiiable steps are taken to avoid this:,:ssibility.-=ditionallythere have been two approaches to the solution of: s problem. The non-metallic solution involves the type of ap-:r:ach to the secondary bottom described in Section 18.11,r'€h is continued around the corner and uo the inside of the


outer tank wall for a limited distance, or to the full tank height.

The metallic solution has divided itself into two different ap-proaches. The essential design problem for this component isthe conflicting need for the liner material to thermally contractand at the same time to be able to sustain the head of leakedproduct liquid. Without the structural support of the insulationmaterial, the liner on the inside of the wallwill have to resist thefull product head. For a large LNG tank this could result in a 9%nickel steel liner of some 25 mm in thickness. This is both ex-pensive and causes design difficulties at the point where thisliner is attached to the tank wall due to the large local forces tobe accommodated.

One solution to this problem is to use a protective liner madefrom the 36% nickel / 64% iron alloy which is marketed undertrade names such as Invar or Pernifer. This alloy has a coeffi-cient of thermal contraction over a wide range oftemperatureswhich is close to zero. Consequently the thermal contractionproblems largelydisappear. This is illustrated in Figure 18.56.

The gap at the point where the alloy bottom protection linermeets the outer edge of the secondary bottom is deliberatelycreated to allow the secondary bottom to contract and pull theliner into contact with the outer edge ofthe base insulation ringwalland obtain the necessary structural support. This detailhasbeen supplied for the LNG tanks installed in Greece and Trini-dad. The alloy is expensive at some 4 to 5 times the rate/tonneas 9% nickel steel. lt is available at thicknesses which are con-siderably less (down to 0.7 mm) than the minimum availablethickness of 9% nickel steel (around 5 mm), so this offsets thebasic difference in the material costs. The alloy is easily fabri-cated and welded providing the correct equipment and proce-dures are adopted. A photograph of such a liner during installa-tion as shown in Figure 18.57. The liner thickness in this casewas 1 .2 mm.

The second approach is to use a 9% nickel steel in the configu-raiion shown in Figure 18.58. This does not incorporate the ver-tical leg at the outside of the base insulation of the previous so-lution. The liner is constructed from steel of 5 mm to 12 mmthickness and usually incorporates a swept corner. The accom-modation of the conflicting needs of thermal contraction andstructural support is achieved by the flexibility of the liner. Thevalidation ofthis design requires some sophisticated finite ele-ment analysis togetherwith the use ofsignificantly higher allow-able stresses than are used in the other metallic componentDarts of the tank.

Guidance is usually taken from the various pressure vesselcodes regarding the allowable stresses. A photograph of a linerof this type during installation is shown in Figure 18.59. The ap-parent step atthe top ofthe sweptcorner is because the insula-tion has not yet been applied to that part of the wall prior toerecting the wall portion of the mebllic liner itself.

The thermal insulation used on the inside ofthe concrete wall isusually constructed from cellular glass. This is most usually at-tached to the wall carbon steel liner with a suitable adhesiveand of a thickness around 100 mm. The welding of the liner, beit of either material, is carried out from one side by necessityand cellular glass is tolerant of the high local heat inputs in-volved.

18.13 Outer tank concrete wall and bottomlinersThe concrete outer tanks offull containment systems are gen-erally agreed to require to be made vapour tight. The preven-tion of productvapours from passing outwards through the con-crete structure and the avoidance of atmospheric water vaDourfrom passing into the insulation (particularly of the loose filltype) are seen to be desirable, if not essential attributes of thesystem. There are those who suggest that modern concretes

:l= 18.58 A 9% nickel sleel boltom comer protection arrangemenl

'"i-: ': ag The installalion of a 9% nickel steet bottom corner ananoement


18 The design of low tenperaturc tanks

correctly placed have a sufficienfly low permeability and do notrequire any additional vapour proofing and that the volumes ofproduct or water vapour passing through the wall is insignifi_cant. This view is however not in the ascendant in the industrvat this time.

API 620 does not venture into this area, not beinq interested infull containment systems. BS 7777 does not s;ecificallv de-mand the use of a liner but points quite strongly in that dire;tion.The intention of words like "Liners or membranes are aDoliedprimarily as a vapour barrier. Full containment concrete tankswill normally be required to inhibit both the passage of bothproduct vapour and water vapour. Unless specified by the pur-chaser it is not normally required to provjde ljners or mem-branes to double containment walls, base slabs orinterspaces." and "Liners and coatings are commonly includedin the design of concrete components for refrigerated storagesystems for the following reasons: a) to make the componentgas andior liquid tight b) to prevent the ingress of water vapourinto the tank." are difficult to misinterpret. Customers or theirengineers often have strong views on the necessity for, andtypes of liner systems to be used on, their facilities.

Two different approaches exist:

The non-metallic route involves the direct aoDlication of suchmaterials as sprayed GRP to the faces of the concrete floorand walls. These are usually proprietary systems, the exactdetails of which are fiercely protected by their owners. Onepossible disadvantage of these types of liners is that theirapplication is generally incompatible with other constructionactivities taking place within the tank at the same time. Thisgives a programme problem. The extension of overalltimescale and the inconvenience of having to stand downthe mechanical erection team are significant diffjculties.Building two or more tanks on the same site, suitably out ofphase, may help with the latter difficulty. Developmentworkis currently afoot to flnd ways of addressing these difficul-TIES.

The metallic route is the more commonly adopted solution.The material selected is usually a carbon steel without lowtemperature properties. This means that the liner is not ex-pected to survive a major inner tank leak incident. lt is ar-gued that such an event will result in tank decommissioningand possible demolition and problems of a failed vapourbarrier are well down the list of concerns. For the tank bo!tom the liner is fillet-welded and can be laid like a conven-tional bottom, or can be attached to casfin inserts for bettercontrol of local flatness. The liner thicknesses often usedare less than the minimum values specified by the Codes.3 mm is a practical thickness for handling and welding.

For the tank walls, it is usual to arrange for inserts of carbonsteel to be cast into the inner surfaces of the concrete outerwall. These inserts may be vertically or horizontally disposed.The wall liner sheets are then fillet-welded to these inserts. Thedesign of the wall liner must take account of a number of inputssuch as changes in the concrete wall dimensions due to cir-cumferentialand vertical pre-stressing, creep and shrinkage ofthe concrete, thermal movements, insulation loadings, vacuumloadings and welding shrinkage of the liner plates.

Atypicalwall liner system is shown in Figure 18.60. lt is impor-tant that the liner conforms reasonably closely to the concretewall and that water is not allowed to accumulate behind the linerduring the construction period.

The practice of supplying a liner that is not expected or de-signed to survive an inner tank leak or failure is somewhat atvariancewith the expressedwishes ofBS 7777: part 3 which inParagraph 6.7.2 states "Where contact with a product occursas a result of spillage or leakage, the materialof liners or mem-branes should be selected to withsiand the product temDera-ture".


cast tnto inn.r faceot concrete wall

Figure 1 8.60 A typical wall liner system

Note: lncidentally, note the use of "should" throuohout part3ot BS 7777 . This is because this section ;f the Cooeprovides a set of recommendations (non-mandatorvlas opposed to Part 2 which provides a set of specifici-tions (mandatory).

To select a materialfor the colder products which will be suit-able for the low temperature, it will be necessary to use g%nickel steel or stainless steel. However, anchoring the un-insu-lated linerto the concrete wallwillcause stresses in the liner inthe accident case (where at least for the flrst period of the inci-dent, the liner is cold and the wall is warm)which can almostguarantee to give rise to linerleakage orfailure. So, it is simplynot possible to design for liner survival for an un-insulated svs-tem: and if insulation is used, then why use an expensive lbwtemperature liner material?

The possibility of liner corrosion on the concrete side is some-times a concern. The view most commonly adopted is that thealkalinityofthe local environment in this region is such as to in-hibit corrosion.

1 8.14 Connected pipeworkThe design and construction of pipework systems is a subjectthat in general belongs elsewhere. lt is howeverworth highligh!ing a few points where the pipes connecting to, and often sup-ported by the tank, impact upon the tank structure.

. Pipe connections are often anchors in the piping systemand as such can be subject to high loadjngs and momentswhich a steel tank shell, bottom or roof is ill-equipped tocater for. Care in the piping layout and positioning of sup-ports can minimise such loads and momenb.

. lvlany companies are set up so that the piping engineer de-signs the pipe work and passes the loadings to the struc-tural engineerwho designs the structure to support or guidethe pipe workand in turn passes the loadings to the tank en-gineer who then has to design the tank to cope with theseloadings. This compartmentalisation results in each indivjd-ualsolving his orherown private puzzle without consideringthe effects on their downstream colleagues. lt is importantthat these people talk to one anotherto minimise any prob-lems, or indeed as often used to be the case, when a singleperson designed the complete system and had no interestin making his own life any more difficultthan it needed to be!The other problem with the separate discipline approach, isthat factors of safety get compounded to produce an ineffi-cient and expensive final Droduct

. Insulation for low temperature pipes is expensive and time-consuming to fit. The use of prefabricated sections of piping

-)

t

ta

.

.:l

I Part 33 Codedatory)ecifica-

)e suit-se 9%f-insu-liner inle Incl-elmostsrmplyd sys-r'e low

rome-at theito ln-

rde-lruc-uideien-€seivid-nngtant'ob-

Eterestbe!l, is..fi-

rbjectrlight-sup-

stemrents)d tosup-

ne-Ing

=q--e 18.61 An unusual access/pipe insulation arrangement

:.:'-.tesy of MB Engineeing Services Ltd

systemswhich can befabricated and pre-insulated in a suit-able factory environment is ofren well worth considering.

. For LOX and LIN tanks in paftcular, as much of the pipeworkas possible is run within the tank interspace so thatthetank insulation provides the pipe work insulation as faras ispossible.

. The combined access tubular spiral stairway/cold box ar-rangement shown in Figure 18.61 would seem to offer sav_ings in certain circumstances. The tubular soiral stairwavwas factory-fabricated in sections. The main pipe work run-ning to the tank rcof passes within this structure which pro-vides the thermal insulation to the vertical legs ofthe pipesby virtue of being filled with site-expanded perlite.

. Bespoke low temperature insulated pipe supports are veryexpensive and their use should be minimised.

18.15 Access arrangements:.cess arrangements are a necessary part of all tanks. The

":Jipment required for low temperature tanks is similar to that

-scribed earlier in Chapter 7 for ambient temperature tanks.-:w temperature tanks are more complex and have more tank-ounted equipment and instrumentiation. For this reason the'rcfs and operating platforms are more frequenfly visited byx€rating personnel.

3enerally the requiremenb are:

. Means of access to the inside of tanks and intersDaces.This is usually by shell manways. or where the type of con-tainment precludes these, then by way of openings in thetank roof and internal ladders.

Figure 18.62 A typical.roof platfom fof a large LNG iank

. Accessfrom localgrade to the tank roof. This can be via spi-ral staiMaysattiached to the outer tank shell, by access tow-ers (which may also support the pipework and cablingrunning to the tank roof) or in the case ofsecondary escapefacilities, by vertical caged ladders with suitable side-step-ping platforms. In recent times it has become common prac-tice to fit large tanks with hoists giving access to the roof.

. Roof walkways and platforms giving access to roof-mounted equlpment. For items such as in-tank pumps, theroof platforms can become substantial. ExamDles of theseare given in Chapter 20 which deals with ancillary equip-ment.

. Walkways from tank-to{ank, tank{o-bund or to other itemsof plant.

. Runway beams. These can be located within the tank forerection and shell inspection purposes or around the outerperiphery local to the top corner for inspection and sitepainting purposes.

. Peripheral hand railing providing protection to those whomay have to work on the tank roof away from the existingplatform and walkways.

Such equipment is designed and constructed in accordancewith local codes relevant to the items concerned. Some furtherguidance on this subject is to be found in the tank design andother related codes. Many of the large process plant contrac-tors and plant owners have their own standards reoardino theextent of access equipment to be provided for vario-us diff6renttypes of tanks.

During the design ofsuch equipment the requirements for pro-tection from the radiation caused by local and adjacent tankfires and the need to avoid damage by direct liquia impinge-ment from roof liquid spillage shall be considered.

Atypicalroof platform for a large LNG tank with in tank pumps isshown in Figure 18.62.

18,16 Spillage collection systemsThe spillage of lowtemperature liquids anywhere on a storagefacility is a source of potential risk. problems associated withflammability, personnel contact with cold Iiquids, toxicity andasphyxia make the quick and efficient collection and orooerstorage or disposal of any spilled liquid a desirable event.For low temperature tanks fltted with roof-mounted liquid inletand outlet conneclions, the main concern is one of collectinoany leakage at the tank roof level and conveying this to loca]grade and thence to a safe impounding basin.

The first step is to decide how much liquid is to be collected.For LNG the design spill is given jn Tabte 2.2.3.5 of NFpA59A.




For full containment tanks, which currenfly constitute the major_ity of LNG tanks, the requirement of ,,the largest flow from inysingle line that could be pumped into the impounding area witirthe containerwithdrawalpump(s) considered to be d;livering atthe full rated capacity" has been the subject of much coniro-versy and different jnterpretations over recent years.

Does for example, the liquid inlet from an unloading tanker,where rates of up to 12,000 m3/hr are common, form the basisofthe 10 minute spill? lf this is to be collected and controlled.then this is an onerous design problem. The confusion maystem from the fact that the US codes generally do not conslderfull containment and the rules consequenfly do not fit this typeofstorage tank. This is something which it is understood wilibeaddressed by the appropriate committee in the near future.It is generally considered that the source of leaks at tank rooflevel is the pipe work and fittings such as valves. Flanged jointsare obviously suspect in this regard and allwelded systems areto be preferred.

The consequences of a liquid spill are obviously related to thetank roof material. A roofconstructed of a non-low temDeraturesteel is clearly more susceptible to damage than one madefrom reinforced concrete. Steps can and have been taken to aD_ply local thermal insulation to protect the materials which wouldotherwise be damaged by conhct with the cold liquid.Perceptions as to what constitutes a sensible spillage collec-tion system vary widely. At the minimalist end of the scale thesimple stainless steel troughs following the route of the roof-mounted liquid containing pipe work is a cheap option which re_lies for its success on the view that the leaking liquid will obedi-

entlyfall from the damaged or defective pipe work and fittingsinto the troughs for collection and disposal. A photograph ofsuch a system is shown in Figure 18.63.

At the other end of this scale there is the view that the leakinoliquid will be projected as far as the leak scenario and the liouidpressure will allow in the most unfavourable foreseen circum_stances. In this case a large part ofthe tank roof, or indeed thewhole tank roofwillform the collection system. Such a system isillustrated in Figure '18.64.

Figure 18.64 Atotal roofspittage co ectjon system


Th€h€]ghtolthep|inihswllhi.lh6sp a9€ 6rca are sovefted by rhe

Figure 18.63 Atypical minimalist lrough type roofspi age co ection system

rnd fittingstograph of

he leakingI the liquid)n circum-ndeed thesystem is

18.'17 Reinforced and prestressed con.crete component design

18.17.1 ceneral

--e design of the concrete parts of liquid storage systems is."ry much an area for the specialist exponents ofthls region of: . I engineering. Some of the loading scenados give cause for:-e use of siate-of-the-art sophisticated analysis tools, which is:r:bably better left to those familiarwith and experienced in this-,:e of work.

--e design Codes provide some general guidelines in this arear.l these will be discussed in this Section. lt is probably fair to-_. that in the design ofthe concrete components, the designer,< 1 many ways less constralned than is the case for the de-:,;rer of the metallic components of a liquid storage system.

:.:.crete parts of lowtempe€ture liquid storage systems occur:--rughout the various types of bnks:. For all above ground tanks the base slab (which may be di-

rectlyon the ground or elevated)orthe foundation ringwall.

. For single containment above ground tanks, the low bundwalls remote from the tank (these may also be constructedof other materials).

. For double containment above ground tanks, the bund wallsclose to the tank.

. For full containment, conventional (i.e. the free standingmetallic inner tank type) above ground tanks, the outer tankJyalls, and in some cases the tank roofs.

. The outer tank walls, and in some cases the roofs of mem-Srane tanks.

. The inner and outer walls, and in some cases the roofs ofmncrete/concrete tanks.

. The caisson walls, base slabs and in some cases the roofsof in-ground tanks.

. Various parts offloating and gravity based systems.: ;:eat deal has been written about concrete tanks. Two excelp,-: books on the subject are by Harry Turner (Reference', 14 and Professor Bruggeling (Reference 18.78). Both arel:.trd starting off points in the study of this area of design. Other,-ful information is contained in Refere nces 16.27 and 16.31.,', -at follows is a very brief tour through a subject which could:;sily merit its own book.

--e design Codes are variable in the extent of their specific re-: - 'ements and advice offered in this area. API 620 has noth ino': say. NFPA 59A, which it shoutd be remembered is specific t6-'.G tanks, provides some general advice. The design of con-::.:e containers is to complywith ACI 318 (Reference 1A.19\.- 3ddition it provides guidance on the allowable stresses in re--'rrcing bars and prestressing strands. BS 7777:pan 3 pto'.,:es quite a lot of detailed and general guidance. BS 9110

"ference 18.20)isreferenced as the main civildesign Code.

18-17.2 Tank bases

rr:st above ground low temperature tanks have base slabs. A:A are constructed with the ringwalland compacted filltype of--ndation where the ground conditions allow. lt is difficult to-f,arate any discussion on base slabs from considerations of:-e complete tank foundations which are the subiect of Chaoter

:; $in"t""t base slab arrangements are illust;ated in Figure

--e function of the base slab is to support the loadings which=-se from the dead weight ofthe tank structure and its contents

18 The design of low tempe@ture tanks

on gbund 3tab- slppon.d eithd on @mpete soiuockor pil.d/imptuved r@d b€a.ing soil

lltl

Elevaled 3tab - Suproited bv stub @1umn; onro agEund &sed stab or on exlonded oites

Figure 18.65 Base slab affangemenls

during the hydrostatic test phase and service life, togetherwithany imposed loadings from events such as snow wind andearthquake. Whefe above ground full containment is the cho-sen type of storage, it is usual for a common base slab to sup-port both the inner and outer (steel or concrete) tanks. In thecase of above ground double containment systems, the baseslab is usually required to support the inner tank and the outerconcrete or steel bund wall, although there are a few caseswhere the outer wall is supported on a separate ring wall foun-dation. The base slab must be strong enough to span betweenpoints of local support such as piles or stub columns. lt mustalso accommodate the base heating system where this is fitted.

There are two distinct types of base slab.

On ground type. This is directly supported by the groundwhich may require to have its load bearing abilities en-hanced by the use of piles, stone columns, replacementmaterial or other means as discussed in Chapter 24. Innearly all cases, the use ofthis type of tank support will re-qujre the installation of a base heating system. Detailed de-scriptions ofthe various forms of base heating are includedin Chapter 20.

Elevated type. ln this case the base slab is elevated abovethe local grade by the use of pile extensions or by stub col-umns or similar supports from an on ground slab or direcflyfrom the ground where the local conditions permit (i.e. atank constructed on competent rock). Forthis type of base,the heat input to prevent the formation of ice lenses comesdirectly from the air circulating beneath the base slab andno additional heating is required.

It is usual to arrange for the height ofthe elevated base slabto be such that access for personnel is possible beneath theslab for inspection purposes. Aspace of 2.0 m makes the in-spection tasks comparatively pleasant, 1.5 m is a recipe foran inspector's bad back and 1.0 m is a thoroughly unpleas-ant place to spend protracted periods of time! Clearly, ahigherspace belowthe base slab costs more and may giverise to other problems associated with the elevatinq of thecentre of gravity of the tiank and its contents during ieismicevents and the raising ofthe finished tank profile where thisis a contentious planning issue.

The means of supporting the upper slab where a groundbase slab is used can be either by the use ofshort stub col-umns at suitable centres, or bythe use of a system offadial



walls. The arrangement shown in Figure 'l8.66 allows for acircumferential layout of supports around the tank perime-ter to cater for the high line loads from self-weight and seis-mic loadings, and an orthogonal arrangement in the centralarea more suited to the uniform loading from the tank prod-uct.

Tank base slabs are usually constructed from reinforced con-crete. There are some designs where some circumferentialprestressing of the concrete base has been adopted, but theseare comparatively rare.

Where in-ground tanks are concerned, the base slab is subjectto different loading conditions. The ground water pressure be-neath the base is pushing upwards with considerable force andthe base may be tied into the wall. Usually some form of groundwater collection and removal is adopted to control these pres-sures. A typical tank base for such a tank is shown in Figure'18.67, where it is some 7 m thick and contributes to theanti-buoyancy system by virtue of its dead weight. The baseslab for the 200,000 m3 LNG tank built at the Ohgishima LNGTerminal in Japan and described in Reference 18.21 is evenbigger. At around 77 m in diameter and 9.8 m thick, this requireda staggering 11,000 tonnes of concrete.

An unusual tank base was that on the island of Revithoussa,Greece, designed and constructed for LNG tanks. The basewasofthe elevated slab type (in this casefitted with seismic iso-lators), constructed within a pit. The rock forming the bottom ofthe excavation was found to have some unexpected disconti-nuities which gave rise to the need for a 2.5 m thick heavily rein-forced pit bottom. This is shown in Figures 18.68 and 18.69

18.'17.3 Tank walls

18"17.3.1 Above ground tanks

Concrete walls of above ground low temperature tanks fall intoa number of categories, each of which is described briefly

The design of these walls is complex, having numerous me-chanical and thermal load cases. Reference to Harry Turnerand Professor Bruggeling's books (References 18.17 and78.78)would be a good place to start for those intent upon mas-tering this area of expertise. Both books are now rather datedso it will be necessary to study some more recent publicationsas well.

One basic input to the design process is to decide what the in-ternal liquid loading will be for the various types of concrete

ililaI11'::-

ilEE

::i

*l

F gure 1 8.66 A lypica I support ayolt for an elevaied base sla b

Figure 18.67 The pit base arrangementfor an in-ground LNG tank

Caulesy af LNG Journal

Figure 18.68 A section through the Revithoussa lsland LNG tank base

Couiesy of Whessoe


)fane-

traltd-

: ;!re 18 69 The Revllhoussa LNG tank base stab during construction

:.tnesy of Whessoe

:3nks. The innertanks ofthe double concrete type will require to:e designed for the product liquid loading. lt is unusualforthese

=nks to be subjected to a hydrosta c test as is always the case'lr steel tanks. For the outer walls of double or full containment:anks, the liquid loading to be considered depends on the type'inner tank failure that is envisaged. In the early days of full

rcntainment tanks where the spectre ofthe eatar accident and:Jdden unzipping tank failure was paramount, the overridinqjes gn condition was the toading resulting from such a lailure.-'re first work to determine what this loadinq should be came.. J'r'r experimental work carned out by Cup;rus for the Shelllompany. These are reported in Reference 18.22.

lJperus built scale models of double-walled tanks where the- ner tank wall could be unzipped vertically for its full height. He:^en measured the resulting pressure profile on the inner sur_'?ce of the outer tank wall. In simple terms, the peak pressures-easured were around six times the hydrostatic pressure. This

18 The design of law tempercture tanks

work was carried out with an empty interspace between the twowalls. The amplification factor of six is an onerous design condi-tion and thought was given to ihe possible ameliorating effectsofthe perlite and glass fibre insulation which is normallv presentin the interspace. especially for targe LNG and tiquid eihylenetanks. (This work is reported in References 18.23 and 18.24.\The conclusion ofthese theoretical and test studies was that anamplification factor of six is valid for an empty interspace, butthe presence ofthe insulation will reduce this factor to betweentwo and three.

The development of metallic materials less susceptible to sud-den brittle failure and better understanding of the mechanismsinvolved in such failures together with means of predicting leakbefore failure scenarios, meant that sudden failure became nolonger a design requirement. This was firsi voiced in EEN/IUA149 where the sudden failure of the metallic inner tank was nota design requirement for the outer tank, unless specificallv re-quested by the client.

Particularly for LNG tanks, where double or full containmentsystems together with large unit tank sizes were the norm, thisresulted is significant cost savings. lt is currenfly normal prac-tice to design the outer concrete tank for a non-dvnamic liouidloading. This presumes a gradual filling of the out;r rank Lo theequilibrium liquid level, frequently specified in terms of a time tofill or as the physical dimensions of the design leak.

An extreme case of outer tank wall design Jor exceptional load-ings are the 144,000 m3 LNG tanks built for Distrioas at Statenlsland. New York. Because of their tocation, lhe t;ks were re-quired to be capable ofabsorbing the horizontal loadings from afully-laden Boeing 747 lJavelling at 2OO knots without damaqeto the liqlrid conta;ni19 i11er tank. This was ach,eved by a wi I

constructed from a combination of prestressed and mass con-crete to a total thickness of around 3 m as shown in Fiqure1B.70.

Prestressed concrete wall - internal tendon typeThis type of construction is the rnost commonly adopted for theoutef concrete walls of full containment stofaqe svstems_ Theteloons fo. tl-e 1or zonrar prest.ess ng io. peiha[s rrore co.-'ectly post-tensioning ) o' the talk wal, a.e LSudl y of t1e .nLltstrand type running in ducts close to the mjddle ofthe wallandanchored at stressing buttresses equally spaced around iheoutside of ihe wall. Figure 18.71 shows a typicat buttress.There are usually four such buttresses. These accommodatethe anchorages which anchor the individual strands and pro-vide suitable details for the attachment ofthe prestressing jackswhich impart the required tension to the strands. Followino thestressing ofthe tendons. they can be grouted (i.e. of the bo;dedtype) or remain non-bonded. These days the popular choice isthe bonded type. This has the advantage that the failure of astrand or anchorage does not have the same detrimental ef_fects as jn the non-bonded type where the total orestressinoload of that strand is lost. the grout feeding the load from th;failed strand into the adjacent strands. lt is important that thegrouting system adopted is proved to be satisfactory, i.e. thatthe grout reaches all parts of the tendon within the duct andbonds to it in a satisfactory fashion.

It is a commonly adopted practice to carrv out a full_scale orouttest on site to demonstrate thar the maierials and instaltitionprocedures are suitable. This will jnvolve the sectioninq of thetest ducvtendon sample to demonstrate Lhat the desiqnersaims have been achieved under site conditions. For a laraeLNG tank d is common to use medium-sized tendons from rhecommercially manufactured range available. These would tvDj_cally use 19 strands of 0.6 drameter wire jn each duct. iireducts in this case would be 100 mm in diameter and most usu_ally made from a corrugated piastic or steel of suitable robust_ness to survive the site conditions and handling. Larger ten_dons are commercially available, but their use is restricted by

lral)se

ectle-rndrndes-Jfe:hetse\G'en'ed

rseto-roflit-'in-)

te-rerndis-edIS

n-;1-a

ll!01! !!q!q!g- 1

j6

. t-re 18.70 Details of the LNG tanks at Staten tstand. New york


18 The design of low temperature tanks'

Figure 18.71 Prestressing buttresses for a full containmeni type LNG lank

practical spacing considerations. The steels that the tendonstrands are made from have a very high tensile strength (typically in accordance with prEN 10138-1 1991 with a minimumguaranteed UTS of 1860 N/mm2). An example ofsuch a tendonand its prestressing anchorage is shown in Figure 18.72.

The vertical prestressing can be of the single bar (MacAlloy)type, which is usually reserved for smaller tanks, or ofthe samemulti-strand tendon type as is used for the circumferentialstressing. There are a number of current wall designs wherevertical prestressing is deemed not to be required. These de-signs are at this time the sub.iect of an active, and occasionallyacrimonious debate bet\,veen the interested parties which willdoubtless be resolved in the near future. Clearly there are sig-


Figwe 1A.72 A typical multi-strand prestressing tendon and anchofage

nificant commercial and programme advantages to the con-tractorwho can produce a design avoiding the need for verticalprestress.

Where access permits, the vertical tendons can be prestressedfrom the bottom end, i.e. where an elevated base slab designhas been adopted. An example ofa tendon being stressed fromthe bottom end is shown in Figure 18.73. When an on groundbase is adopted, this is not possible and either single tendonsfitted with a blind anchor at the bottom end as shown in Figure18.74 are used, or the 'U'shaped tendon is used. In both casesthe prestressing must be carried out from the top corner of thetank.

It is usualfor the thickness of walls ofthis type to be ofthe orderof 600 mm. There is a practical minimum thickness required toaccommodate the circumferential and vertical tendons and toprovide cover for the reinforcing steel needed for local crackcontrol. lncreasing concrete thickness means more reinforcingsteel, but may provide increasing blast and missile protection.Clearly there is a construction cost versus the quality of acci-dent condition protection argumenvquestion which is commonin this area of design and construction.

Figure 18.73 Vertical prestressing from beneath the elevated base slab

t-th

rru-

t''I

a

t!!.|

a

II

:a

I

con-rtical

:- g!re 18.74 A blind prestressing tendon anchorage

The vertical prestressing is usually applied first to give the wallthe capacity to accommodate the vertical bending induced bythe circumferential prestressing.

BS 7777 gives some guidance in this area which is summarisedas follows:

. Transfer and residual stresses should be taken into ac-count. The amount ofprestress should be calculated takjnqinto account all losses.

. The total loss of prestress is the sum ofthe individual lossesarising from transfer, creep, shrinkage, friction, curvatureand relaxation. Losses should be calculafed in accordancewith BS 81'10 : Part 1 : 1985 :Annex C.

. lt js necessary to use concrete of a high quallty level, with astrength of 40 N/mm3 or more.

. The totaltensile load capacjty of the tendons, jncluding anynon-stressed reinforcement, should be greater than thetensile load capacity ofthe concrete tank wall. This is to en-sure that in the event of overloading, sudden failure orbursting of the tank in a "brittle" manner js not a credibleevent.

. The layout ofthe tendons, especially adjacent to the stress-ing buttresses, should be such practicable spacings areachieved and the placing and vibration ofthe concrete is notimpaired.

. Under maximum design loading conditions, including theliquid and the temperature loading due to inner tank leak_age, the minimum residual average compressive stress of1.0 N/mm2should be provided inthe principal direction(s) ofprestress. This is normallytaken as the hoop direction only.

Prestressed concrete wall - wire wound type1is type ofwalldesign has been menfioned briefly in Chapter

' 7. lt is often known as the "preload" type of wall, as the designaas patented by the American company preload Inc.

: 1as been used widely in the USAfor water storaqe tanks and- :s some history of use in the UK as the chosen d;sign for the: -ter concrete walls of double coniainment tanks. The Datent is-:,v owned by another American company called Cryocrete ofn-om Whessoe is currenfly a licensee.-^e double concrete design (i.e. a prestressed concrete inner=-l concrete outer tank of the full containment type) uses this-::nnology. The vertical prestressing is supplied by internal:?'s or tendons which run in the centre of the wall. The cir-: -'r'rferential prestressing is applied in the form of a continuous4 'e strand wound around the outside surface of the concrete4:L

Figure 18.75 A view of the outer surface of a banded wire wound concrete

Earlier designs had the wire distributed evenly over the wholeouter surface ofthe wall. More recent designs have the strandscollected into bands which may each contain some hundreds ofindividual strands, and distrjbuted vertically as required for thedesign loadings up the wall. This can clearlv be seen in Fioure18.7 5.

The application of the strands is accomplished bv the use of aspecially designed carriage which runs around ihe outside ofthe wall, usually supported by rubber tyres running on the top ofthe wall. This carriage is driven circumferentiallvvia a Revnoldschain placed around the concrete wall. lt also his the caoabilitvro raise and lower providing coverage of tne complete outeisurface of the wall. The stressing strand, which is initially some'10 mm in diameter, is passed through a die mounted on the car_riage, as it revolves around the tank wall. This has the com-bined eftect of work hardening the wire strand and applying therequired tension to it. As the wire is placed, a suitably stronggrout is applied the wall to provide both bonding and weatherprotection for the strands.

The positioning of the circumferential prestressing wires closeto the outer surface of the wall has on occasions oiven rise toconcerns of possible loss of prestress caused by-external fireevents and of possible damage to the wires caused by missileimpact.

There have been a small number of cases where this orout hasnot properly performed its weatherprooflng duties. This wastraced to bad application procedures and material selectjon,and should now not be a problem. lt did however pose an inter-esting technicalproblem for the owners ofthe affected facilities.Clearly some corrosion of the stressing wires had taken place.It was self-evidently between 100% and 0%, varying over theheight and circumference of the wall. To add some iraction ofthe original stressing to the wall (i.e. on top of the existing)WOUIO;

. Crush the concrete if most ofthe original prestressing werepresent

. Leave the wall understressed if most of the orioinalprestressing were damaged

An interesting quandary!

Considerable effort was put into trying to devise methods ofnon-destructive testing to determine the extent of theprestressing loss. This was eventually unsuccessful and the


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only solution was to physically remove the existing prestressingsystem entirely and replace it. The author witnessed one suchrepair. The thought was that the defective grout/shotcretewould fail if a band of highly stressed wires were cut through.Thus a complete band of the circumferential stressing materialcould be removed in a single operation.

l\,4uch worryingwas done concerning the energy locked into theprestressing bands and the potential problems if this energywere to be suddenly released. Would the band let go ofthe con-crete wall in an explosive manner? Finally a suitably protectedoperative (in a substantial metal cage) was allowed to tenta-tively apply a disc grinder to the selected band of wire winding.He eventually cut through all of the wires in the band and noth-ing happened. The ability of the grout to hold the wires in placegreatly exceeded the influence of the tension in the wires toseparate the band of wires from the concrete tank shell. Havingcut through the complete band of prestressing wires, itwas thennecessary to remove the whole band manually, which proved tobe a very difficult and time consuming task.

Forthe innertankofa double concretetankit is usualto providea metallic barrier. This is usually made from carbon steel with-out low temperature properties and is applied to the outside ofthe concrete wall before the vertical prestressing and the cir-cumferential wire winding is undertaken. The argument used tojustify the lack of low tempefature properties for a componentwhich will clearly see the full liquid design temperature in ser-vice, is that it will be in both vertical and circumferential com-pression due to the prestressing of the concrete throughout itslife.

Some of the wire wound concrete tank walls were constructedfrom prefabficated sections, something which may be worth re-visiting in times where the finished cost of a project is often ofless significance that the overall construction programme time.

Reinforced concrete wall with earth embankment

Following the Qatar incident, which has been described inChapter 17,the first full containment tanks designed had outerwalls of this type. For various reasons this design has fallen outof fashion. An example of such tanks is shown in Figure 18.76.The majority ofthese tanks werefitted with steelouter roofs, al-though there is no reason why concrete roofs should not be fit-ted.

One interesting diffefence between this type of tank and theprestressed types is in the need to provide heating for not onlythe tank base, but also for the tank wall. The design ofthis heat-ing system to allowfor maintenance and replacement of heaterelements and to cater for base/shell movements during con-struction and service was no easy task.

The uncertainty which surrounded the inward loading from theearth or rock embankment (the eouivalent of the circumfer-ential prestressing) meant that this loading had to be viewed atits minimum to provide the inward load to resist the liquid load-ing from the inner tank failure condition, and at its maximum todes;gn the reinforced concrete wall to resist crushing duringservice. The accommodation of these two opposites could giverise to an expensive wall design. The early versions ofthis typeoftank were designed for ihe full sudden unzipping failure of theinnertank. This gave rise to a robust and expensive walldesign.

A section through such a tank, taken from BS 7777 is shown inFigure 18.77.

The reasons fof the current unpopularity of this type of tank arebased on cost, construction programme time and the site areasrequired to accommodate the embankments. The slope of theembankment is usually around 1.0:1.5 (vertical to horizontal).So. for a 100.000 m3 tank with an inner diameter of 70 m andheight of 28.5 m, a circular footprint of some 156 m in diameterwill be required. The embankment does provide excellent resis-tance against external missile and blast loadings.


Fioure 18.76 LNG ianks wth reinforced concrete earlh erabankmenl walls

Figure 18.77 Example ofa reinforced concrete wallwlth earth embankr.ent

Fron BS 7777 : Pan2, figure 4

18.1 7.3.2 In-ground tanks

The in-ground membrane type of tank as shown in Chapter 17,Figure 17.42 will require a concfete wall for a number of rea-sons. During the construction sequence it allows the excava-tion of the pit and during service holds back the surrounding soiland prevents the ingress of ground water.

The 200,000 mr LNG tank at the Ohgishima LNG Terminal ofTokyo Gas in Yokohama, Japan, is described in Reference18.21 . lt ts built entirely underground, that is to say, even thetank roof is beneath the finished ground level as indicated inFigure18.78. lt has a caisson wall of 77.1 mdiameterx68.5mdeep with a wallthickness of 1 .5 m. This was constructed by theslurry wall technique. This enabled the excavation to be com-pleted and inner structural wall consisting ofa 0.15 m thick se-ries of Drecast Danels and a cast in situ 2.2 m thick concrete ele-ment. The total thickness of the wall (i.e. the slurry wall, thestructural concrete and the precast panels) was 3.85 m.

The design for the external pressures from soil and ground wa-ter is usually quite straight forward, requiring only vertical pre-StTESS,

't 8.17.4 Bottom corner details

The issue ofthe type of bottom corner detailto adopt is mainly aconcern of the above ground tank fraternity. Much has beenwritten on the relative merits of the different types of bottom cor-ner details by those in the industry who have frequently devel-

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do.ts qi.horad ot buttreses

V..indl dnd.i..!0feretidtp.*tres3 rorpl.ted b.fde

\rall .ointorsent i5 pfovided -In oddition foverti.ul pr6h.ss

0tuuf,terentiol pr$hett

vertkol pRrh.*

Figure 18.78 200,000 m3 in-ground LNG tank in yokohama. JaDanCoutesy of LNG Jounal

oped strong partisan opinions on the subject.

The three designs are:

. The sliding joint. In this case the wall is supported by thebase slab and is free to move horizontallv underprestressing and service loadings. To ensure thai the wallcannot move laterally due to wjnd or seismic loadings, ra_dial guides should be provided to ensure lhat all move_ments are concentic with the base slab. A flexlble seal.frequently in the form of a stainless steel strip is provided toprevent leakage of liquid or gas at the joint.

. The pinned joint. In this case the wall is supported by thebase slab and during prestressing is free to move horizon-tally. After the prestressing operatjon is complete, the wallispinned in position. The joint between wall and base slab isdetailed such that it can transmit shear loads from wall tobase slab, but not bending moments. Again a flexible sealwill be required to prevent local leakage.

. The fixed joint. This is where the walland base slab are ofmonolithicconstruction. prestressing, servjceand accidentloadings must be transferred from wallto base slab withoutthrough-cracking. The provision of local thermal insulationto allow the accident condition loadings to be accommo_dated is described in Section 18.i.1.

The three types ofjoint are illustrated in Figure 19.79. The cir_cumferential prestressing system can be either by internalten_dons or bywire winding in each case. The figure does not makethis clear. The relative merits are briefly summadsed as shownin.F_igure 18,80. There is scepticism regarding the ability ofthesliding jointtype to accommodatethe high lateral loadings usu_ally associated with seismic events. The radial guides required

Figure 18.79 Typicatjoints for prestress wa ,to_base iunciionFrom BS 7777 : Paft 2, figure 3

forthese high loadings appearto run into practicalspace limita_!ons.

The tendency within the industry to specify ever-increasingtank design pressures gives rise to another problem. Undeinormal circumstances, the self-weight of the outer concretetank walland roofwould be more than sufficient to balance theuplifi due to internal operating and test pressures. With internalpressures rising to 300 mbar, this cannot be relied on and occa_sions arise where it becomes necessaryto anchor the tank wallto the iank base slab, and indeed possibly to require the an_chorage of the base slab. The anchorage of the wall to thebase, whilst allowing for radialmovement, gives rjse to some in_teresting design problems fortanks with the sliding and pinnedjoint details.

The current favourite within the industry seems to be the fixedjoint.

18.17.5 The top corner details

For full containment type tanks, a suitable joint is required be_tween the wall and the roof of the tank for concrete walls withboth concrete and steel roofs. BS 7777 : part 3 gives someguidance in the design of this detail. The loadings ihould mn-sider the following:

. Gas tightness

. Radialand vertical loadings applied to the wall, both uDward


Slresrs rre D..dicr.d rvith goorl

Second.ry slreses are relrtiaclr

Dependent on adequaa_ ofjoint seal

Sone un.crtrinry ove. de$ee of sli.ling

Pfrst.css is tr{rii(l.ri \vith good

Ilaxinrun nlrm$l ..(u6 rn wallarv,y from lh{:.ioinl-s. rt level when'cnd effects from ver'li.rl t.n.l,Ds,rc hrklv srnollcd oul

Subsequenr selond.ry srr6ses a.. lcss

l,alle shers and hiru la.ge mon.nls

it(n,usl f.r N of lonshucti.n PFdi.lnn otslrc$cs is un.dlxnr

LlEe rrnnents rn(l slrea':

!.{r1r.sse(l {rll! ne.essitrt,. lhe lsc ol(dnu)us presllBse{l basr 1d s!*itrlrhennal Uote.rnnr ar tlF l(,inl nrtry beinlri'(\rccd to rc(lu( rLl(litioral srrlssesduc to llrcmd niranrg

]h\im!m nromenr o.rnF rt the ion{

Figute 18.80 Summary of the advantages and disadvanlages of lojnls in ihe wal-to-base lunct on


Fron BS 7777 : Pan 3. bble 4

and downward vertical loadings

. Radial thermal loadings

. Abnormal loadings such as blast, fire or seismic

To this list should be added:

. Intefnal pressure loadings

. Constructionloadings

The top corner is usually of the fixed type where a reinforcedconcrete roof is used. This is clearly a congested area withplenty of both vertical and circumferential stressing membersand reinforcing to be accommodated. Where a steel roof is tobe connected to a concrete wall, BS 7777 suggests the use of asho( vertical steel shell section. The thinking being that this el-ement will allow flexibility to accommodate ihe various differen-tial movements between roof and wall. There are a number ofcases where this cylindrical steel element has not been in-cluded.

18.17.6 Tank roofs

It is usualfor concrete roofs io be constructed from reinforcedconcrete. A finished thickness of 300 mm to 500 mm is usualwith reinforcing mats close to the upper and lower surfaces. Asteel roof framework supporting the roof sheeting is usually uti-lised to act as forrnwork for the construction of the concreteroof. As mentioned in Section '18.6, it is possible to utilise thestructurally redundant roofframe and sheeting to replace someof the reinforcing steel.

The main design criteria for the tank roof are the internal pres-sure, the self-weig ht of the roof platforms and any missile load-ings. For the LNG tank at Revithoussa lsland in Greece, twomissile loadings were specified. The first was a soft missile,which was a helicopter from the local army-training base, andthesecondwasa hardmissilefiredatthetankwithmaliciousin-tent. Both were defined in terms of their rnass, velocity, angle ofapproach and area of application.

18.18 References18.1 Stability of API Standard Tank She/is, R.V.McGrath,

Proceedings of the American Petroleum Institute, Sec-tion lll, Refining APl, New York, 1963, Volume 43 pp458-469.

1a.2 The design of lanks fo Reslsl External Prcssures,L. PZick, Document no. 7'1l33508 dated N,4 ay 1971, Pte-sented to the British Standards lnstitution TechnicalCommittee PE/12.


18.3 Steel plate Engineering Data - Volume ll- Paft lll- Ex-ternal pressures on cylinders. Published by The Ameri-can lron and Steel Institute in cooperation with the SteelPlate Fabricators Association Inc., Washington DC1992.

18.4 Collapse by lnstability of Thin Cylindrical Shells underExternal Pressure, Dwight F. Windenberg and CharlesTrilling, ASME proceedings Vol 56 No 11, November1934.

18.5 Table 8.5.1 ofthe American lnstitute of Steel Construc-tion - Manualof Steel Construction (Allowable slress de-slgn), Ninth Edition, 1989, ISBN 1 56424 000 2.

18.6 PD 7777 : 2000, Alternative steel selection and its effecton design and testing of tanks to BS ZZZZ, The BritishStandards Institution, London

18.7 ASTM E208: Standard method for conducting drop-weight test to determine nil-ductility tnnsition tempera-ture of ferritic steel : 1995.

18-8 prEN 14620 : March 2003- Design and manufacture ofsite built, veftical, cylindrical, flat bottomed steel tanksforthe storage of refrigerated, liquefied gases with oper-ating temperatures between -5 'C and -165 "C: Pafts 1

to 5 : CEN.

18.9 85 449 : Paft 2 : 1969, The use of structural steel inbuilding ,fhe British Standards Institution, London.

18.10 BS 5950 i Pad 1 : 1990, Structural use of steelwork inbuilding Part 1:Code of practice for design in simple andcontinuous construction: hot rolled sectiors, The BritishStandards Institution, London.

18.11 Recommended practice for LNG above ground stor-age:1981, fhe Japanese Gas Association

18.12 ENV 1993-1-1 Eurocode 3: Design of steel structures -Paft 1-1 General rules and rules for buildings.

18.13 BS CP 3 i Chapter V: Paft 2 : 1972 Code of basic dataforthe design of buildings- Chapter v Loading- Paft 2Wind loads, The British Standards Institution, London.

18.14 ASME Section Vlll Divisions 1 and 2, rules for construc-tion of pressure vesseis, ASME, New York 2002.

18.15 PD 550A :2000 specification for unfired fusion weldedpressure vesse/s, BSI London, ISBN 0580 33080 X.

18.16 Steei Deslgn Manual, Unlted States Steel Corporation,Section 8.2.2.

18.17 Concrete and Cryogenics,F.H.f utner, AViewpoint pub-lication, The Cement and Concrete Association, ISBNo 7210 1124 1.

- Ex-nefl-ite€lDC

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18.18 Prestressed co ncrete for the storage of liquefied gases,Professor Dr lr. A.S.eBruggeling, AViewpoiht publica-tion, The Cement and Concrete Association, ISBN 07210 1187 X.

1Al9 ACI 318, Ameican Concrete lnstitate Building CodeRequiremenb for Reinforced Conctete 1999.

18.20 BS 8110: Patt 1 : 1985, Structuraluse of concrete Codeof practice for design and consttuction,fhe British S'tan-dards Institution, London.

18-21 Construction of an underground storage tank, KenjiYanagiya and Tomoyuki Ogawa, Kajima Corporation,Japan LNG Journal, Nov/Dec 1999.


18.22 Developments in cryogenic storage tanks, N.J.Cuperus, Shell Intemationale Petroleum, Maabshappij8.V., The Hague, LNG 6 Conference.

18.22 Damping effect of pedite/fibreglass insulation on outertank dynamic loads in double-walled cryogenic tanks,A.S.Adorjan, bo(on Production Research, Houston;D.B. CraMord, M.WKellogg, Houston; S.E.Handman,M.W.Kellogg, Houston; LNG 7 Conference.

18.24 Dynamic load attenuation for double wall tanks, R.A.Vater, Pitbburgh des Moines Corporation, PittsburghGastech 1984 Conference.


19 Insulation systems for lowtemperature tanks

Insulation systems for low temperature tanks form an essential part of the liquid mntainmentsystem. The rlaterials used and installation techniques adopted are many and varied. ThisChapter is of necessity a brief review of the thermal insulation for the different parts ofthe tiank,the guidance given by the various Codes and the calculation methods to be adopted to designan efiective thermal insulation system.

Contents:

19.1 General19.1. 1 Basic requiremenb of the insulation system

1 9.1.2 Insulation categories

1 9.1.3 Installation considerations

19.1.4 Basic design and material requirements

19.1.5 Design Code requirements

19.2 Base insulation'19.2.1 General

19.2.2 The central area

19.2.3 The peripheral area

19.2.4 Design methods

19-2.4.1 lnner arca19-2.4.2 Periohe'al arca

'19.2.5 Detailed design Code requirements

1 9.2.5.1 EEMUA reouirements

19-2.5.2 BS 7777 rcouirements

19.2.5.3 Draft of new Euronorm DrEN '14620

'19.2.6 Base insulation materials - central area

19.2.6.1 Cellular glass

19.2.6.2 PVC foam

1 9.2.6.3 Polyurethane foam

1 9.2.6.4 Lightweight concrete

19-2.6.5 Composite systems

19.2.6.6 Blast furnace slag

19.2.7 Base insulation materials - peripheral area

19.3 Wall insulation19.3.1 General'19.3.2 General reouirements

19.3.2.1 Insulation for the walls of single-walled metallic tanks19.3.2.2 Rigid insulation for the walls of double-walled tanks'19.3.2.3 Loose till insulation systems


19.3.4 Wall insulation materials

19.3.4. 1 Polyurethane foam'19.3.4.2 PVC foam19.3.4.3 Other Dlastic foam materials


19.3.4.5 Mineralwool19.3.4.6 Perlite loose fill insulation systems

19.4 Roof insulation19.4.1 General'19.4.2 Externai roof insulation'19.4.3 Internal suspended deck insulation

19.5 Insulation of heat breaks for fittings

STORAGE TANK$ & EOUIPIIENT 377

19 lnsulation systems for low tempemture tanks

19.5.1 General'19.5.2 Heat breaks for roof connections

19.5.3 Heat breaks for tank sidewall connections

19.5.4 Heat breaks for tank bottom connections

19.6 Internal pipework insulation

19.7 External pipework insulation

19.8 Heat leak calculations19.8.1 Basic calculation methods

1 9.8.2 Thermal conductivity values

19.8.3 The influence of different interstitial gases

19.8.4 Calculation of the hot face temperature

19.8.5 Overall heat leak

19.9 Heat leak testing

19.10 The use of the infra red camera

19.11 lnsulation problems from the past and their lessons'19.1'1.1 Base insulation failure

19.11.2 External vaPour sealing

19.1 '1.3 Bottom corners

19.1 1.4 Perlite settlement

19.12 References


19.1 GeneralSystems for the thermal insulation oftanks storing lowtempera-ture liquids are areas ofinterestwhich could easilyform the ba-sis of a book in their own right. What follows is something of ar'r'histle stop tour and is of necessity limited in the amount of in-fofmation and detail provided. The subject has to be included,even in an abridged form as it forms an essential part ofstoragesystems which must, in many respects, be considered as com-tosite systems rather than a series of independent compo-rents.

The following is based on the assumption that the tanks inquestion are vertical and cylindrical in form. lvluch ofthis chap-:er is relevant to other types of storage tanks, but to include allrfthe possible variatjons and exceptions would tend to obscure:he various ooints made.

rhe insulation systems associated with the membrane type of:anks, both above and in-ground, are quite particular and are:iscussed in the Sections 17.9 and 17.12 in Chapter 17.

19.1.1 Basic requirements of the insulation system

-he essential functions of the thermal insulation system are:

. To limit the product boil off due to heat in leakage from theatmosDhere to below the specified limib.

. To protect the non low temperature parts of the tank systemfrom the temperatures of the stored product.

. To provide structural support where required (i.e. to the tankoase).

. To limitthe cooldown ofthe tankfoundations to ensure thatcooling of the sub soil and possible damage as a result of"frost heave" is avoided.

. To prevent or at least minimise condensation and icing ofthe outer surfaces of the tank.

1 9.1.2 Insulation categories

:or simplicity and to give this section some structure, the insu-:ted parts of the tank will be discussed separately under the': lowing headings:

. Base insulation

. Wall insulation

. Roof insulation

1 9.1.3 Installation considerations

-^ere are various installation related considerations which are::ecific to particular insulation systems and materials. In addi-: :n there are some points which are relevant to the installation:'insulation in general and these should be considered in all

. Installation within a closed space requires careful attentionto the following:

Ventilation. l\y'anyof the materials used are problematicwhen used in confined spaces. For example many ad-hesives, sealants and mastics give off harmful toxjcand/or inflammable vapours.

- Certain works such as the sanding offoamed insulationmaterials can produce fine particles which are suscepti-ble to flash fires.

The provision of adequate access for personnel andmaterials.

19 Insulation systems far law tempercIurc tan|s

Protection from moisture. l\rany insulation materials willreadily absorb water. This usually has a detrimental efiecton their thermal conductivity. Consequently care must betaken to ensure that materials are maintained in a dry stateduring their transport to the construction site, during anystorage on the site, during jnstallation and up to the pointwhen the tank is placed in its intended service. There arenumerous examples ol this happening, which have provedto be both expensive and time consuming mistakes, as wetinsulation materials are often impossible to subsequentlydry out in situ.

Storage tanks are often large in both diameter and heightand are frequently constructed on coastal sites. The combi-nation of size and severe weathercan cause installation dif-ficulties for external insulation systems. High winds canremove all but the most robust weather protection and amodest amount of rain, blown to one side ofthe tank can re-sult in a virtual waterfall on the down wind side of the tank.Strong and well thought out protection systems are a neces-sity.

Some insulation materials are susceptible to mechanicaldamage (i.e. cellularglass). Appropriate care must be takenduring transport, storage and installation to ensure thatbreakages are kept to a minimum.

19.1.4 Basic design and material requirements

The following considerations must be taken into account whendesigning a thermal insulation system:

Thermal conductivity of the materials used at the designtempefature. Any anticipated changes of thermal conduc-tivity with time should be taken into account.

Any long term structural loadtngs to be sustained by the in-su ation. Any aniicipated changes in strength with time andany creep effects should be taken into account.

The chemical orooerties of the materials used should besuch that if they are in contact with the product vapour dur-ing service, theywillbe unaffected by long term contact withit. Intheeventthattheinsulationmaterialwillcomeintocon-tactwith the product liquid during an accident scenario, thenthe structural propertles in particular shall not be adverselyaffected.

The materials shall be suitable for the anticipated operatingand accident condition temperatures.

The insulation system must not give rise to corrosion ofthesteel or other tank components. Some insulation materialscan give rise to acidic conditions when subjected to waterpenetration.

The insulation system, where exposed to the atmosphere,must resist the ingress of moisture. For cold insulation sys-tems, the most harmful "aggressor" is the continuous ten-dency forwatervapourto invade the insulation and make itsway to the cold face. For many insulation materials, smallchanges on moisture content can bring about majorchanges in their thermal properties.

Problems arlsing from any envisaged fire scenario.

Problems arising from installation difficulties.

Considerations caused by commissioning and decommis-sioning requirements.

l\y'echanical damage by humans and birds.


Cellular slass

loos€ ttu msulants;eleanded perlite orvetmiculite

U*d h ohbinstion $trt to@ fi insutdt3 nomeuy for tls mecharncal pbp.ni6.

19 lnsulation systems for low temperaturc :6ln(s

Flgure 19.1 insutation maiertals for refdgerated storageFrom BS 7777 : Patl 3. tabte 2


Traditionally, the tank design Codes had litfle to say on the sub_ject of thermal insulation. The American Codes siill adoDt thisapproach, whilstthe British and European Codes have devotedmore space to this subject with each new document issued.BS 7777 has a section on thermal insulation in part 3. Table 2,Figure 19.1, provides a useful list of commonly used insulationmaterials and identifies for which component of the tank thesematerials are normally utilised. The forthcoming Eurocode,(prEN 14620), on low temperature storage will have a full sec-tion devoted to tank insulation and will dealwith the subject inconsiderably more detail. lt makes the important point th;t thatthermal insulation system for low temperature tanks differsfrom ambient temperature tanks where it is considered a pe_ripheral component. For low temperature tanks the thermal in_sulation is an essential part of the overall storage arrangement,such that without a correcfly designed, installed and maintainedinsulation system the storage tank will not operate correc vThere are also numerous Codes which deal with the productionand testing of individual insulation materials. This is a specialistarea into which the authors of Slorage Ia nks & Equipmenthavechosen not to venture.

The specific requirements ofthe tank Codes to the desiqn ofthevarious pa rts of the tank insulation systems are discuss6d in thesections which follow

19.2 Base insulation

19.2.1 ceneral

Although generally self-evident, it is worth repeating that thebase insulation systems for low temperature tanks perform twobasic duties:

. Structural support for the low temperature tank. The materi_als should have suitable long term load-bearing prope(iesto ensure that the tank is supported throughout its intendedservice life and is not subject to unacceptable setflementsresulting from uneven compression of the base insulation


as the loadings from the tank and its contents are trans_ferred to the foundation.

. Thermal insulation ofthe base of the storage tank. The sys_tem chosen should limit the heat leak into the tank. Dreventthe formation of ice on the underside of elevated base slabsand prevent ice lens formation for on ground type founda_tlonS.

Despite these modest aims, it is interesting just how few materi_als are available with the appropriate combination of lonq termcompressive strength. suitably low thermal conductiviW, theability to sustain the low temperatures and at an appropriatecost. This limitation of choice has not been helped bv the es_sentially conservative nature of the liquid storage industry andthe unwillingness of its participants to be ,,guinea pigs" foi newsystems and materials.

The base insulation for storage tanks divides into two areas.These are the central area and the peripheralarea. These aredescribed in Sections 19.2.2 and '19.2.3.

19.2.2 The central area

For a single-walled tank it is quite clearwhatthe centralarea ofthe base insulation comprises. For cedain double-walled tanks.especially those with the base insulation placed beneath theouter tank bottom, there is also an area of the base insulationbeneath the interspace between the two tank shells as illus_trated in Figure 19.2. This part of the base insulation is sub_jected to loadings which are similar to those applied to the cen_tral area, particularly in the post leakage scenario. To avoidover complication, only the central area of a single_walled tankis considered. This is subject to the following loadings:

. A uniformly distributed load arising from the product liquidhead and in certain circumstances, the product vaDourpressure.

. A uniformly distributed load arising from the watertest headand in certain circumstances, the air test pressure.

:';

;ys-/ent

roa-

;er -

trmlheaIe

: gure 19.2 D agram of bottora corner ofdouble-walled tank

. A uniformly distributed loading arising from the self-weightof the tank bottom plates and the insulation system itself(usually insignificant).

. A non-uniform loading arising from the seismic loadings towhich the tank is subjected (usually SSE and OBE). Theseismic loadings are made up from combinations of:

Vertical loadings arising from the combined overturningmoments from the action ofthe horizontal design eventon the impulsive, convective and self-weight compo-nents of the tank and jts contents. This loading, in a sim-ple analysis may be considered as triangular indistribution, but in a more complex analysis may well beof a different form.

Vertical loadings arising from the action of the verticalevent on the self-weight on the bnk contents.

The various combinations ofthe two horizontal componentsand the one vertical component of the loadings to be con-sidered are described in Chapter 26 . A not uncommonlyused rule for this combination is:

100%Fx +/- 30%Fy +l- 31o/a Fz ol

3Q'/oFx +l- 1004/oFy +l- 3ookFz ol

30%FX +l 30%f!/ +l- 100okFz

On occasions the 30% comoonent is increased to 40%.

. Shear loadings resulting from horizontal loadings from windor earthquake.

. Point loads arising from internal fittings which rest directlyon the tank bottom, such as stilling wells, inlank pumpwells, fill columns etc.

19.2.3 The peripheral area

-ihe peripheral area for a single-walled tank is clearly that partrf the base insulation which is beneath the tank shell. For dou-!le-walled tanks there may be two areas of the base insulation,'/hich are loaded in a similar fashion, i.e. the part beneath therner shell and the part beneath the outer shell.-hese areas are subjected to line loadings from the variousshells which are made up of combinations of the following:

. Self-weightof the tank shelland attached parts including in-sulation as appropriate (i.e. during operation but not per-haps during hydrotest)

. Roof loadings including snow load, live load, internal vac-u um.

. Wind overturning loadings

. Seismic ovefturning loadings for botr OB= a^: S S = :, = - =

19.2.4 Design methods

19.2.4.1 Inner area

For the inner area of the base insulation systern i'e.:-:-,<loadings described in Section 19-2.2, are surnmaT se: - :-:most pessimistic combinations which may occu'. as

j: :.,.:

. Normal operation

. Hydrostatic testing

. OBE seismic event

. SSE seismic event

These loading combinations are then assessed for acceo:e:itytaking into account the compressive strength (and in the ca:cases with seismic components, the shear strength) of the var -

ous materials making up the base insulation system togethe.with the factors of safety described in Section 19.2.5.

19.2.4.2 Peripheral area

As in Section 19.2.4.1, the various loadings are summarisecunder the same headings. lt is common to assume that the llneload spreads out intothe base insulation at45degrees as llus-trated in Figure 19.3. Based on this assumption it is usual towork downwards through the insulation system to assess theacceptability of each layer taking account of the materalstrengths and factors of safety again taken from Section 1 L2.5.A worked example of this procedure in given in Figure '19.4.

This includes the mechanical loadings on both the central andthe peripheralareas otthe base insulation. lfthe simple triangu-lar load distribution is considered inadequate, then a more e e-gant analysis may be undertaken, but the weight of past experi-ence suggests that the simple method has served the industrywell and there may be a relucbnce to depart from it.

As is the case for the inner area of the base insulation, consid-eration must be given to the shear strength of the materials orcomposite system where seismic loadings are involved.

es-andtew

;as.

!rs-

JOli.rr

rofks.iheion

3n-odlnk

Loading on section A - A

\Worst case for rinqwall

Flgure 19.3 Lineload in the base insulaton ng wall

STORAGE TANKS & EQUIPMENT :.:'

I

I

i

I

II

I

III

I

19 lnsulation systems for low tempenture tanks

Figure 19.4 Examplo ofa calculation ofthe loadings in an insuladon ringwall- pag6 tCourtesy of Whessoe


olE 55EL!r. l,rtt Srnr srn'c4Esr. Rdc40 ll?,5 /ktt 607C.€tt. 6!.d C2O 322.! t.6a 2,?9Pa.rlt dstt. Sq{r' 3 Q2.t tJZ l.Btlalit &'tcti.6o0l9/o3 !o2a.t o.'o

2M

20.loJ

to,3

C'Fg0Lcorir.t!c{, &tqaqBnlc @irar" czo roiP.llt..iasbr0di9/# ll.OFdif en6.r!t00olg,/# a5Lrrii!oorwr.8€t9/# 5p?!,lit.c.isd. rck/ii 3A

FoiBrd HJt6o0 rlP..i'9lu}fgia{P ltf..indqrt|.llzoo t2Foi9brH3t000 rOPl'liL 4r.rri. sftldi3 q,Pqlli.!olEvt700rq/n3 .aOll|{F. c.E!t! 60ltlgir3 a2

l*4l6ldnl Jrrdr .f bdr hlElttd 1

I&rf!n.h.ll.drs tl *n r,t5|'.{ }! bsrhn rtld rirrV.rtlcd A!.d'.tbn (oE|, q,ldlc.l ,^cd...it.|| (sse, o,

(t cda&fid. .snr. ! L'bad ?Fd.0l) c.r$rorjtdrt|qrfi h€.iiurdong.idt|*

ftidi*F .'d C.hF-tw Snqdb

Fd.lit @i.Er.000tq/ilNtlt|!.!Er|! 6a0k/ri3

20n

t0,3

t{ot: I r .idlls anptdtt/t rtrr.. l6.ors.d. h.. b.ar iEdtfird h o..odo*, $?h BS!ll0.

-

i) fi@dffn long 'diml

dqrdiw tcB. b

D varid d@t rarld or d! at =*!!4.J3- =txD

i) $do.rofic hrld (ot+ri'r)rt ?r.q.. GF{tlrg)irD r,i/'icdcfic t..d (i€r)

D Tdorg'rdt dri|{&rqt h.d (O8Ol) v..fl.d ..c.l.r!tid ot pnder (q8qili) Trh'AddV drrdhtvn b{d (65qb) Y.rrk l cc..Lrrnon of pru4Er (Sse)

OBEI a0?.4o tnyi559 | 64€.40 tfvn

OBE | 3,t6 ri^Vh95E r 5.27 $Vft

0166 l{,hifOD29 tVni9,?3n |\yn#0036 |yir#

t

F-rgure 19.4 Example of a calculation ofthe loadings in an insulation fingwalt- page 2Coulesy of Whessoe

1 9 lnsulation systems lv tow tawane axs

Th" lin" Ioods frcm the 3h.llonto fh" innsr tank ringb€en ar€ colorlar€d bdowr

i) Concr€i. scrc€d C4o

lidxinlm conprcssivc si€n9th :,qctudl conpf€ssive str€55 (OBE) =lcnjql compr€$ivc slross (SSE) :

il) Concr€|. scr€ed C20

lltoxinun conprc!3iv€ slr€ngith :lctual conpr€lsiv. stre$ (OBE) :Actudl coirprelrivc stre$ (sSE) :

iii) P"rlitc concrcle 800k9/n3f{dxinlm cohprcssive !t cnglh =

iciuql compr€esive rrresr (OBq :icnialco|nprlstivc sfr.ss (SSE) :

iv) P"rlir. con$et. 600k9/it3

^lldxirnun conpr*live sircngth :

Actualc.nprcasivc airess (OgE) =Aciual .ornprar.6ive slr.$ (55E) :

-

FOS: 4.80

+ FoS: 3..1O

+ FOs: 6.2t:+ Fos: 4.at

:::::> FOS: 3.77

+ Fos: 2.7f

+ FOS = 3.18

-

FOS: 2.'06

2O62 N/mm?4.30 N/mm'6,07 Vmm?

1O.31 N/rnm?

1.66 Mnrnz2.29 N/mn?

5.OO N/nrra?

1.32 N/nrm?1.81 Nhrn2

2.2O N/nrnrt0,69 N/nvn?

0.90 N/mnr?

ft€ loqds on rhe inncr bdle insuhiion dre cdbukdcd below;

Inhcr bo3c injuhtion us€d in design :

}{v.rog€ c.rnpr.ssiva str€ngth =

Fcclor of sdfcty fo. op€raling cond;lioE :Fqcror of sdfely fof hydroi.st condilions :Focror of sdf.ty for OBE scisnic

"v€nt :

Faclor of Jdfery f6r SSE sebmic cvcni :Notcr POS vdlu€s idken from forthconing EN 265

i) Op.roting loods

Op.ratirq pr.6slrc :lnsu htion, scrc.d, Plqte eic :

TOTAL

Fo€mglas H.3 800

N/nm'z

-os =

4.00

iii)

iD

Insuhtion, Scn .d, Platc ctc :

szismic load (oB€)Op"r6liB bo4t :Verlicol o...l2ruiion of prodqcf :Triorgulort disrribuf cd load

Ssilnic lo{d (SSE)

Operdtirts loods :Vrrlicol dccEl.rdiion of prcduct :T"idrg'rhrv dirtribut.d locd

O236 N/nrrf0.036 N/nrrn?

O0O6 N/nnzrornl@uz^nr €os: 2.87

0.200 N/mmz0.O08 N/mm'?

0.043 N/mharoreL--T75E-p7amz ===+os. 3.18

0.20O N/nrna0.013 N/mma0.05,r N/mn,

roleL--EE-p7- , :s r 3.01 ok

iv)

o_t65

o.o29

o.oo50.200


19 lnsulation systems for low temperature tanks

19.2.5 Detailed design Code requirements

19.2.5.1 EEMUA 147 requirements

The EEMUA recommendations, first published in 1986. werethe first to include specific requirements for safety factors to beused for the mechanical design of insulation systems. Thesewere, not surprisingly, confined to the base insulation andaimed largely at cellular glass materials, at that time and almostcertainly stillto this day, the most popular choice for this compo-nent. The requirements state:

"lnsulants placed beneath a tank bottom shall be able towithstand the loads imposed upon them. For service andtest conditions, the safety factor for cellular glass should beat least 3.0 based on the average compressive strengthproperties of the grade selected, or 2.5 based on the mini-mum compressive strength guaranteed. Other materialsshould be viewed in the same manner, and it is preferable touse mrntmum guaranteed properties rather than averagevalues."

19-2.5.2 BS 7777 requirements

BS 7777 was first published in1993 and was based upon theearlier EEMUA 147 recommendations. ln the case of thequoted factors of safety for base insulation it has adoptedslightly different values. The Code states:

"lnsulation material located beneaih a tank bottom shouldbe able to withstand the load imposed upon it. The allow-able compressive stress for service and test conditions forcellular glass should be either 0.33 times the guaranteedaverage compressive strength or 0.5 times the guaranteedmin jmum strength ofthe grade selected, whichever is less."

It also suggests that other materials can be used in the samemanner (meaning presumably using the same factors ofsafety)and suggests that guaranteed minimum strengths should beused rather than average values.

19.2.5.3 Drafl of new Euronorm prEN 14620

This Standard is at the stage of final editing having been issuedfor public comment. Although not in its final form, part 4 of thisdocument probably represents the latest thinking on the sub-ject.

The structural design of the base insulation system may bebased on the allowable stress method or by using the limit staterneory

The insulation materials are divided into two categories:

. Brittle materials, i.e. those not subject to creep under sus-tained compressive loadings. This category would includecellular glass and light weight concrete.

. Materials subject to creep. This category would includeplastic foams such as polyurethane foam (PUF) and potyvi-nyl chloride (PVC) foams.

Forthe brittle materials methods are given to allowthe nominalcompressive strength to be determined by test. For allowablestress based designs, minimum factors of safety are applied tothis nominal compressive strength as follows:

. Normal operation 3.00

. Hydrosiatic test 2.25

. Earthquake (OBE) 3.00

. Earthquake (SSE) 1.50

For the materials subject to creep, a complex method of deter-mining the nominal compressive strength is given based oncompressive creep testing, some tests being required to have a10,000 hour duration. Safety factors are then given, to be ap-plied to the nominal compressive strength to arrive at designvalues forthe various loadings forthe allowable stress designs


An appendix is provided which gives the basis of the Iimit statedesign methodsforthe twocategories of materials. This is verycomplex and may have to be simplified before it is let loose onthe industry.

19.2.6 Base insulation materials * central area


Cellular glass is formed when powdered glass is heated to-gether with other carefully selected chemicals which ernitgases at the appropriate point in the manufacturing process tocreate a glass foam. The foam material is cooled down carefullyresulting in a series of near spherical small glass cells contain-ing an insulating gas. The material is 100% closed cell. vervstable at ambient and low temperatures and with zero perme-ability. lt can be made in various grades to have a range of dif-fering thermal conductivities, densities and compressivestrengths.

As discussed, cellular glass is the most commonly used mate-rial for the bases of low temperature tanks. The manufacturingprocess is complex and must be subjected to a high level ofquality control if the end product is to have consistent proper-ties. Pittsburgh Corning Corporation (PC) was the first to de-vote the necessary resources in terms of research and devel-opment into the understanding of the material properties andinfluence on these properties of the manufacturing process.Thus for many years this company was, quite righfly, the almostexclusive source of cellular glass blocks for tank bases and itsproducts were a specific requirement of many tank specifica-tions and invitations to tender

The materials manufactured by PC are marketed under theproprietary trade name of Foamglas@. lt is an indication of thiscompany's dominance in this market over the years thatFoamglas@ is often taken incorectly as the generic name for allforms of cellu lar glass irrespective of the manufacturer. pC has.over the years, also devoted considerable efforts to the desionand installation methods for systems utilising their materials fo.which they should be given credit.

For tank base insulation the appropriate grades of Foamglas@are the High Load Bearing (HLB)grades made and marketed in

DENSITY AND COMPRESSIVE STRENGTH

Gr.deof Aver.Ce Den y

k9/mr

os@ .(*r4 b asrM @o

HLB 300 r35

HLB 1000 140 0.6s

HLB 1200 145 1.20 0.32

HLB 14OO

HLB 16(x) 165

THERMAL CONDUCTIVIry

HLA 1@O 0.053

HLBl2OO

0,056

Figure 19.5 Basic propedies ofthe HLB grades of Foamglas@

Couftesy of Pittsburgh Caming Corporation

fie

ry)n

t>lritblyr}ry

n€

F.19

6r-

*ds-sa

tsa-

Eb*I

nT

Ih


Thermal conduouvlty (lf l|nlq

EeFqECSSq'PT€tnperdrre (.Celsius,

Erre 1 9.6 Thermal conductivity vs temp€rature p'ot for the HLB gmdes of FoamglasrD

Fgrrs 19.7 Basic properties of various gEdes of Cell-U-Foam

htttesy of Ce -U-Foan Cwonlion

lve grades of increasing density and compressiv€ strengthtsn HLB 800 to HLB 1000. Asixth stronger grade (HLB 2000)b curently underdeveloprnent. There is the usual tmde-ofi be-xeen increasing density/strength and decrgasing thermal effi-cbncy. The basic physical properties are shown in Figure 19.5and the all important thermal conductivity-mean tempeftlturecrrves are shown in Figure 19.6. The significant reduction inlErmal conductivity with temperature is clear. An excellent

book has been published by PC covering the manufacture ofFoamglas@, its properties and uses, together with a fund ofother useful informalion (Reference 19.1).

Overthe years a number of mmpanies from different countrieshave aftempted to gain i foothold as manufacturers of cellularglass products ofsuitable quality for the insulation of low tem-perature trank bases. Until recently none succeeded in produc-ing a product suitable for this purpose. Some five years ago a

7.

/z

'/a./.,

./.,'/

Property

CompesgivE at]englh, avetrge 6t brsaldng polntpti (kPal (ASTM C 165, c 24o, C 552)

Den6lty, average, |l]sr''ts lkgltrql (A9TM C 93)Flerural strength, block average, psi (kPa) (A'snt C tu3, C z4o)Llnear coefficlent ot thermal expanslon,p.t'F lpet'K) 6sru E 22a)

Specific hoat, Btu/brF (Jftg."K)Thermal conductivity, Blu-in/hr-ft2-"F (W/mrK)(ASTMCt77,C51e)

Thermal diffusivity, tt2lhr (m 2/sec)

Ult e-CuF"to:ll Ultra-CUF 929gsn€ralEervlca cryog€nlcservlcs

1m (689)

I (128)

80 (552)

4.8x10s(8,6x1f)

0.18 (830)

0.30 @ 50"F0.31 @ 75"F

(0.043 @ 10 "c)(0.04s @ 24 "c)

0.019 (4.9 x 10-i)

87 (6m)

7.5 O2O)

64 (441)

5,0 t 10{ (9.0 x 10{)

0.18 (83O)

0.28 @ 50"Fo.29 @ 75"F

(0.038 @ 0 .c)(0.040 @ 10 "c)

0-019 (4.9 x 10-1

Compressive slrength, ASTM mlnimumpsi lkglcm2l (Lowet speciticaton linitwith AQL = lo/" Det ISO 3951)

Comprcssive st€ngth, ASTM avohgepsi {kg/cm2)

Nominal average densily. lb/fl3 (kg/m3)

Thermal conductivity, maximum aveaageBtu-irthr-lta"F @ 75"F {W/m-"K @ ro"C)

Thermal co.duclivity. highest single valueBtu-idhr-fl."F @ 75'F (W/m.oK @ rooc)

Superior compressive strength for tank bases

Propony $-gr:l|t ujla-culg urrra.cuF rT4 uj:j::1379.8 (5.61) 100.1 (7.04) 118.9 (8.36) 139.2 (9.79)

116 (8.16) 14s (10.19) 174 112.23) 203 (14.27J

9.2 (135.04) e.5 (140) LS (144.97) 10.2 {14S.e3)

0.340 (0-048) 0.345 (0.050) 0.370 (0.052) 0.380 (0.054)

0.368 (0.0s1) 0.382 (0.053) 0.396 (0.0ss) 0.410 (0.057)

STORAGE TANKS & ESUIPMENT 385

10i-//

I I r

.dIv7

'{929

-t lI Themalconduciivity

I vercus remporature(Brirish units)

---r-------r--- r T

19 Insulation systems for low temperature tanks

Figure 19.8 Thermal conductivity vs temperature plot for various grades ofCell-U-Foam

Coutlesy of Cell-U-Faam Caeoration

company which was a part of ACS Industries in the USAandtrading underthe name ofCell-U-Foam made serious and suc-cessful efforts to break into this area. They have now becomeestablished as suitable suppliers of these materials by most, ifnot all ofthe world's tank specification writers and builders andproduce a range offour enhanced compressive strength mate-rials marketed as Ultra-CUF in grades from 116 to 203. The ba-sic physical data and low temperature thermal conductivitycurves are shown in Figures 19.7 and'19.8.

It should be mentioned here that cellular glass is an unusualmaterial. Without venturing too deeply into this subject whichagain could be the subject of a book in its own right, a few wordsof explanation should be given.

The intrinsic compressive strength of cellular glass materials isquite high. The problem is in developing this strength (i.e. ingetting the load ffom the tank bottom intothe insulation materialand out again into the foundation). The manufacturing processfor cellular glass ensures that the finished surface is irregularwhen examined in close up, consisting ofcutthrough part cells.

When two blocks are placed together, the actual area ofcontactis a minute proportion of the total area ofthe blocks, consistingin the main of glass cell walls crossing each other on the twofaces at various angles. When any load is applied to this systemlocal breakdown occurs atthese highlystressed contact points.This is clearly not a satisfactory situation. To enable a higherproportion of the intrinsic strength of the material to be devel-oped, it is necessary to interpose other materials between theblocks.

This effect is clearly indicated in Figure 19.9 which comparesthe compressive strength of cellular glass (in this instanceFoamglas@ from Pittsburgh Corning Corporation)with no inter-leaving and with different interleaving materials. These materi-als must be capable of better distributing the load into the cutcells of the surfaces of the adjacent blocks. lvlaterials com-monly used for this purpose are:

. Hot melt bitumen. This material is applied in liquid form,being dispensed onto the surface of the lower blocks whilstthe upper blocks are placed into the liquid bitumen. This isprobablythe mostcommonly used method oflaying cellular


STFIAIN %

: ff '"n"":'#::; l::,"..,.,".4 Aare, ho cappins

NOTE: lhErepresenlttypicalb.haviorand should nol b. used rd

EigLre '9 9 Tte 11Le,rce of rrlerleaving materalon i1e compresrve slrength

glass tank bases, but suffers from a number of disadvan-tages. To ensure that the cells on the lowerside ofthe upperblocks are correctly filled with the bitumen, to make certainof the correct load transmission, is difficult to achieve andimpossible to check in a non-destructive manner A greatdeal ofthe success ofthis method deoends on the skills ofthe installation operatives and their supervision. Anotherdisadvantage ofthis method is that hot melt bitumen is car-cinogenic and its use, particularly in confined spaces whichmay occur in partially constructed double-walled tanks, isincreasingly unacceptable.

. Damp-proof course material. These are proprietary ma-terials, made for the building industry to be used as damp-proof courses between bricks or other building blocks. Thetypes most suitable for this purpose are usually about 3 mmthick consisting of a bitumen-impregnated hessian type offelt. Providing that they are used correctly and consider-ation is given to points such as: carefully butting the edgesof adjacent rolls with no overlapping, the temperaturedependancies of the hardness/soft ness of the material andthe capping of the roll ends to prevent edge damage ontransit and handling, then this method provides a satisfac-tory solution without some of the disadvantages associatedwith the hot melt bitumen method.

. A base insulation free of organic materials, which it isoccasionally necessary to construct, (for example, for usebeneath liquid oxygen tanks). The materials discussedabove are clearly not acceptable in this case and substi-tutes must be found. One commonly adopted solution inthese circumstances is to use an inorganic powdersuch asKieselguhr powder. This is applied dry and it is clearly eas-iertofillthe uopercellsofthe lowerblocksthan it is to filltheunderside of the upper blocks. Despite this practical diffi-culty, the method has been used successfully for manyyears. lt is perhaps fortunate that despite the relatively highspecific gravity of liquid oxygen, the product is normallystored in smaller tanks than the other lowtemDerature liouidgases and consequentlythe loadings on the base insulationare lower Hence a lower load transfer is not so important inthis case. Another solution to this particular problem is touse a glass fibre interleaving material.

- ensure that the compressive strength data provided by the-3terial manufacturers are on the same basis and that inter-:aving effects are equalised, the test methods are specified in::nsiderable detail in various ASTM. Euronorm and ISO stan-:3rds.

>'!rilarly the measurement oi the thermal conductivity of the-aterial, particularly when measured at low temperatures, is: .|cult and should only be undertaken by laboratories and test- lL.rses with the specialised knowledge, experience and equip--ent.:.:turning to Reference 79.7, there is an interesting chapter on:^e cellular glass base insulation of the Ambergate LNG tank.- ris tank was designed and constructed in 1969 for the East! dlands Gas Board at Ambergate, Derbyshire in the Unitedr. igdom, as a strategic inland storage facility and extension of:-e Canvey lsland LNG importterminal. The subsequent devel-: lment ofthe North Sea gas flelds made this facility redundant:rd it was decommissioned and dismantled by British Gas in'986.

-re British Gas Engineering Research Station at Killingworth.1as given a budget to use this tank as a research tool and car-- ed out a number of tests, eventually leading to the collapse of:-e inner tank under internal vacuum loading. These tests were':ported at LNG I (Reference 19.2).lt is interesting to specu-:ie if this opportunity to use a full-scale test piece would be::(en and funded today. As a part ofthis investigation, samples:'the cellular glass base insulation material were taken and:ent to the laboratories at Liege University in Belgium. The ma-::rial was Foamglas@ S3S grade (which has subsequently:een replaced by one of the HLB range of grades). The impor-:ant properties of thermal conductivity and compressive-r:rength were measured and found to be unchanged by the 17, ?ars in low temperature service.

19.2.6.2 PVC foam:VC foam is now a generally acceptable material for base insu-allon. lt has a history of being used successfully for membrane

:., pe tanks in the prefabricated insulation panels and beneath:everal concrete/concrete type LNG tanks. lt is now being pro--oted for use with the more usual types of low temperature:3nks. lt claims certain advantages over cellular glass:

. The material is supplied in larger sized sheets, up to 2.4m x'1.2m. This will reduce handling and installation costs andalso installation times.

. No interleaving materials are required again reducing in-stallation costs and time scales.

. The materialis considered to have bettertolerance of expo-sure to moisture during installation.

. The material is more elastic (conversely less britfle) thancellular glass and thus perhaps better able to resist damageresulting from seismic loadings and locally imposed vibra-tions. There is some history of cellular glass bases beingdamaged by nearby pile-driving activity and proximity to ro-tating machinery

{t the end of the day the choice of material will probably be3ased on economic considerations.

19.2.6.3 Polyurethane foamrhis material has a considerable history of use in membrane:)/pe tanks. Most of what is mentioned in Section 19.2.6.2 isagain relevant here.

1 9.2.6.4 Lightweight concretethere are a number of lightweight concrete materials available.These obtain their light weight and consequent low thermal:onductivity either by the use of lightweightaggregates such as,erlite, Lytag or blast furnace slag, or by the addition of a foam-ng agent to the concrete mix which provides ajr or gas entrain-

19 lnsulation systems for low temperaturc tanks

ment. The lightaggregate concretesare usuallybespoke mate-rials and are consequently expensive to produce. Their use ismore normal in the more highly-loaded peripheral areas of thebase insulation. Air entrained concrete materials are producedon a much larger scale for the building industry and are mar-keted under such trade names as Thermalite@, UltraLiterM andSiporexrM. These do not have the same thermal efficiency asthe materials described above, but are cheaper Clearlyan eco-nomic assessment is reouired here.

19.2.6.5 Composite systems

A system using a combination of perlite concrete tubes and freeperlite has been used on a number of occasions. The perliteconcrete tubes are laid on end in contact with the adjacenttubes and the spaces filled with free site-expanded perlite parti-cles. The perlite concrete tubes provide the compressivestrength whilst the thermal efficiency comes from the perlite.Some suitable capping for this system is required before the in-ner tank bottom is laid. This system is illustrated in Figure19.10.

19.2.6-6 Blast furnace slag

Afew tanks have been built on bases consisting of blast furnaceslag. When suitably graded, dried and compacted this materialhas a suitable compressive strength and thermal properties inthe same region as the lightweight concretes. The advantage isthe almost free raw material.

19.2.7 Base insulation materials - peripheral area

The higher loadings experienced by the peripheral areas of thebase insulation means that frequently the materiais describedin the preceding Section are unsuitable by virtue of their limitedcompressive strength and need to be replacedwith otherstron-ger materials, or be used in different combinations.

Using the 45 degree load-spreading rules described in Section19.2.4.2, the compressive loading decreases as the point inquestion moves downwards from the point of contact of theshell line load. 1t is important to maintain the best possible ther-mal efliciency whilst catering for the compressive loadings with

Figure 19.10 View ofcomposite perttte concreie tube/toose periite base insula


1 I lnsulation systems for low tempenturc tanks

accepiable factors of safety required bythe design Codes. Thisis necessaryto minimise the overall heat leakfor reasons ofop-erating costs, and to reduce the problems associated with baseheating systems where large differences in the heat flux be-tween the central area and the periphery can prove complex orexpensive to design for. Commonly used solutions to this prob-lem are:

. The use ofa composite system using layers ofcellularglassof different grades, frequently capped with reinforced con-crete with suitable low temperature properties to allow forthe high loading occurring immediately beneath the tankshell.

. Similarv the use of layers of different grades of foamedPVC or polyurethane foam, again frequently capped withreinforced concrete.

. The use of purpose-designed and factory-manufacturedblocks incorporating layers of reinforced concrete and dif-fering densities of perlite concrete. The design and manu-facture of such blocks should be entrusted to comoanieswith the necessary experience and expertise in this area.

. The use of purpose-designed and factory-manufacturedblocks using wood. Balsa and pine have been used in thisway. The compressive strength and thermal conductivity ofwood materials are very dependenton the orientation ofthegrain with respect to the direction of loading. Again the de-sign and manufacture of such blocks should be lefr to suifably expert and experienced suppliers.

19.3 Wall insulation

19.3.1 General

Wall insulation systems for low temperature tanks falls intothree calegories:

. Insulation for the walls of single-walled metallic tanks fixedto the outer surface of the tank shell.

. Rigid insulation systems for the walls of double-walledtanks. For metallic outertanks this can be fixed to the outeror the inner surfaces of the outer wall. There are a few ex-amoleswherethe insulation is attached to the outersurfaceof the innerwall. Fortankswith Dre-stressed concrete outertanks this insulation is most usually attiached to the inside ofthe outertank. Again there are a few exampleswhere the in-sulation is attached to the ouler surface of the inner wall.

e Loose fill insulation systems for installation between thesteel or pre-stressed concrete outer tank and the steel andless commonly pre-stressed concrete inner tank,

1 9.3.2 General requirements

19.3.2.1 Insulation for the walls of single-walled metallictanks

In addition to having suitable long term thermal properties, thesystems used in this category must give due consideration tothe following:

. The provision of a suitable vapour barrier with the abilityover the design life of the system to prevent the ingress ofatmospheric moisture into the insulation material driven bythe thermal gradient.

. The vapour barrier must also act as a weather barrierto pre-vent the ingress of rain water and to sustain wind loadings.Storage tanks are tall structures, frequently constructed inexposed coastal areas where weather conditions are ex-tremely onerous. Rain falling on a large tank roof is fre-


quently concentrated at the tank wall to water fall like pro-portions

. The insulation and its vaDour barrier/weathercover must besuitably attached to the tank shellorbe freesianding in theirown right.

. The system must cater for the worst stresses caused bythermal and mechanical changes in the tank shape.

. The system must caterforany loadings caused by externalice build up.

. Depending on local circumstances and fire protection sys-tems, the insulation system must have the necessary resis-tance to anticiDated heat flux levels.

. Suitable resistance to loadings caused by fire water im-pingement.

19.3.2.2 Rigid insulation for the walls of double-walledtanks

Applied to the outer surface of the outer wall

The requirements ofSection 19.3.2.1 apply in this case.

Applied to the inner surface of the outer wall

The attraction of this method of providing the wall insulation isthat the outerwallitself provides the vapour barrier and weathercover In normal operation the outerwall is at ambient tempera-ture. A number of new considerations need to be taken into ac-count:

. The vapourtightness ofthe insulation system during normaloperation when the insulation is exposed to the productvapour(i.e. an open-topped suspended decktype oftank).

. The liquid tightness of the insulation system in the event ofinner tank leakage or failure.

. The chemical resistance ofthe various components oftheinsulation system to contact with the product vapour or liq-uto.

. The abilityto gas free and decommission the tankwithin theprescribed period.

Applied to the outer surface of the inner wall

This is quite an unusual arrangement. In this instance the fol-lowing considerations must be hken into account:

. The inner tank leak case will result in the comolete loss ofthe insulation system. This will result in a very high heatleak, theevolution of largevolumes of productvapourwhichmust be handled safely by the relief valve system to avoidthe possibility of outer tank damage due to over pressuris-ation, and the possibility of substantial ice build-up on theouter tank shell, possibly giving rise to structural stabilityproDrems.

. Careful detail design at the junction between the wall androof insulation.

. lfthe insulation materialchosen is permeable, provisions toorevent convection within the insulation itself.

19.3.2.3 Loose fill insulation systems

This usually means a system using pedite and someform ofre-silient blanket.

Conceming the perlite loose fill insulation material, allowancemust be made for the following poinb:

. Settlement of the perlite during service. This is usually pro-vided for bythe provision of a suitable hopper volume in thetop corner ofthe tank above the product liquid level and bythe use of vibration equipment during the initial installationofthe material. The sizing ofthe hoppervolume is a functionofthe geometry and service ofthe tank in addition to the ex-

DTO.

ih,

:ma

]S S-

lled

?a-

t-c:

perience of the tank designer and the insulation contrac-tor/designer

N,'luch of this information is of a proprietary nature, some-trmes based on test work and is not discussed further otherthan in general principles. The techniques and equipmentnecessary for the vibration of perlite during installation areagain of a proprietary nature, closely guarded by their own-ers and not for general discussion. Suffice it to say ihat arunway beam above the insulation interspace to supportthevibration equipment is necessary that means for position-ing the vibrating elements accurately are provided andmeans of determining the extent of vibration compaction bylocal level measurement are provided.

. Provision for topping up the perlite during the service life ofthe tank in the event that this is found to be necessary Thisis usually accommodated by the provision of topping upnozzles located around the periphery of the tank above theinsulation interspace. A sensible size for these connectionsis 6 inches in diameter


--e design Codes give little gujdance on the detailed design of-^e various forms of wall insulation systems. The general points: scussed above are included.

' 9.3.4 Wall insulation materials

--e following is of necessity a brief review of the more com--lnly used materials and some points regarding their correct::p ication. As has been mentioned earlier, there is no substi-:,:e for experienced designers and installation contractors in:^ s area. lvluch of the installation equipment and methodolo-I es are of a proprietary nature.

1 9.3.4.1 Polyurethane foam: -obably the most commonly used material for the wall insula-: :n of tanks discussed in Section 19.3.2.1.: trlyurethane foam (PUF) can be applied to the tank walls ln a-

"mber of different ways:

. lt can be supplied as factory-made slabs and attached tothe tank wall by the Llse of a suitable adhesive. The fac-tory-manufacture ensures a good quality of foam with con-sistent properties. The adhesion to the tank wall is moreproblematic, but good site quality control and attention todetails such as the correct primer in a suitable condition onthe steel wall should ensure success.

For insulation thicker than 50 mm, it may be wise to con-sider using two layers of PUF with staggered joints. Clearlya vapour barrier/weather cover will be required where theinsulation is exposed to the atmosphere. This could be ametallic sheeting of galvanised steel, aluzinc sheeting, alu-minium sheeting or stainless steel sheeting, in all cases ofeither flat, corrugated or profiled section.

Attention to detail in the design and installation of the jointsbetween the sheets and the attachment of this barrier to thetank are essential to ensure adequate long term vapourtightness and wind and weather resistance. An alternativevapour barrier/weather cover may be of the sprayednon-metallic type. This could be of a number ofdifferent ma-terials depending on the local conditions prevailing.

. ltcan bespray-applieddirectlytothetankshell. Theoverallthickness being made up of a number of layers, each usu-ally about 'l2mm thick. This requires considerable experi-ence and skilled operatives if it is to be successful. The finalquality ofsprayed PUF is notoriously susceptible to temper-ature and humidity. For this reason one contractor went to


F gufe 19.11 Weaiher protection system for the insulation of an LPG tank

Courtesv of AntwerD Gas Terninal NV

the trouble and expense of providing an inflated dome roofattached to the top periphery of the outer concrete bundwall, air conditioning equipmeni and a suitable air lock forthe entry of personnel and materials, as shown in Figure19.11 to ensure control ofthese variables and to produce asuitable finished product applied to the outer surface of thesteel inner tank. In this type of PUF applicaiion, the vapourbarrier/weather cover must also be of the spray appliedtype.

. It can be foamed in situ between the tank shell and thevapour barrier/weather shield. This method is only relevantto external shell insulation. The metallic vapour barrier iserected on all or part of the tank shell using suitable spac-ers, usually made from PUF slab rnaterial, to ensure thecorreci finished thickness of the foam. The tlvo constituentswhich react to fofm the foam are mixed and injected into thespace between the tank shell and the cladding, eitherthrough holes pre-dril ed in the sheeting or poured over thetop of the partially erected cladding. The reaction betweenthe components produces a foam ofthe appropriate qualitywh ch fllls the space and adheres to both the cladding andihe tank shel . This sounds straightfoMard, but there aremany possib e piifalls which can only be overcome by atten-tion to deiail and expefience.

19.3.4.2 PVC foam

PVC foam is a factory-made material and as such can only beapplied to a tank shell in ihe first ofthe three methods describedabove for PUF.

19.3.4.3 Other plastic foam materiats

The tabulation in BS 7777 : Part 3, (see Figure 19.2), indicatesphenolic foam and polystyrene foam as possible materials forthis purpose. The use ofthese materials is outside the author'sexperience and is suspected to be an uncommon practice.


Cellular glass materials have been used successfully as the ex-ternal wall insulation in numerous cases. The cellular glass caneither be attached directly to the metallic iank shell bythe use ofa suitable adhesive, or be installed as a free-standing wall. Forthicknesses up to about 70 mm it is possible to use a singlelayer of cellular glass. For greater thicknesses it would be wiseto consider the use of two layers, installed with theirjoints stag-gered. As always a suitable weathercover/vapour seal must beused.

The producers of cellular glass products have their own de-tailed designs and installation methods for such insulation sys-tems. Typical details are shown in Figure 19.12.

Cellular glass is also used extensively as the bottom corner in-sulation in numerous double-walled tanks with pre-stressedconcrete outer tanks. This insulation is necessary to limit the

STORAGE TANKS & EQUTPMENT 389

19 lnsulation systems fot low temperaturc tanks

2 FOAMGLASo@llu|3rgless

4 PITTCOTEo 404 (irspecitied)5 PCo FABRIC 79P {rf specilied)

I Tankwall2 Tank root? FOAMGLAS@cetlutarglass4 PC9AA ADHFSIVE5 Resilienl insulaiing matenal6 PITTSEAL@,144 sealei7 PITTCOTEO 404 + PC@ FABRIC 79P

Figure 19.12 Wall nsulation using Foamgtas@

Couftesy of Piltsburgh Corning Corporatian

shear stresses at the junction of the wall and the base resukirEfrom the innertank leakdesign scenario. This is particularlythecase with the fixed orencastr6 bottom cornerdesign. The insl-]-lation layer is applied to the bottom of the interspace and to rnelower part of the tank wall, normally to an elevation of 3m to Smfrom the base slab.

A typical arrangement for a large LNG tank is shown in Figure19.13. Most pre-stressed concrete tanks have a metallic lineron their inner wall and base surfaces. The cellular glass mustbe suitably attached to this liner and protected from the productliquid in the inner tank overfill or leakage design case. The casewhere the full inner tank contents are contained by the outertank is the most onerous for the designer The most frequenflyused metallic barrier employed for this purpose is faced with rnefollowing two interesting and conflicting requiremenb:

. To be of an economic thickness, the protective layer mustrely on mechanical support from the underlying thermal in-sulation material to resist the hoop stresses which the fulleaked product head will impose.

. The temperature change brought about by coniact with theproduct liquid willcausethe protective layerthe contract in-wards and loose contact with the supporting insulation

The various ways of solving this apparent conundrum is dis-cussed elsewhere in Sforage Tanks & Equipment.

Non-metallic solutions to this problematic area will require vali-dation by suitable test work.

One of the attractions of the use of cellular glass materials inthis area is its ability to resist the heat generated by installationwelding activities.

19.3.4.5 Mineralwoo

For externally-applied wall insulation there are a few caseswhere mineralwool products have been used. This system willrequire even more careful attention to the weather cover/vapour seal, as mineral wool has a low intrinsic resistance tothe flow of air with in itself. For this reason the orevention of con-vection loopsbeingsetupwithinthe mineralwoolinsulationwillalso have to be considered.

19.3.4.6 Perlite loose fill insulation systems

Aloose fillinsulation system installed in the interspace betweentwo concentric vertical cvlindrical tanks consists of two comDo-nents. These are:

qL49S EIBBE 8AlNrcL. 350 IXPANDED PERLITE

{UNCOMPRESSED) ICOMPACIEO)

9% Ni ]NNER TANK EOTTOIl

50 mm SCREED

5 mm 9% Ni SECONDARY BOTTOM

50 mm SCREED

4x t00mmTHIC(FOAMGLAS (HLBSOO)

INTERLEAVEDWTHPLUVEX

l00mmSCREED

3 mmc STEELVAPOUR BARRIER

CONCRETE OUTER TANK

Figure 19.13 Boltom corner proteclion system

390 STOR,AGE TANKS & EQUIPMENT

CRYOGENIC CONCTE]E250 rim THICK

iNNERTANKSHELL

9% NiLINER(VARY NGTHICKNESS)5000 mm l\,llN. UP VERTICALOUTERTANK

3 mnTTFICKCARBONSTEEL LiNER

1 X75 mm THICKFOAMGLAS (ILB 8OO)

SAND

. CONCRETE OUTER TANK

s0 mm RESILIENTBLANKET

50mn RES L ENIELANKET

2r 100 mm Tl-llCK FOAMGLAS {HLB 1400)INIERLEAVED WITH PLUVEX

. The loose fill insulation

. The resilient blanket.

The loose fill insulalion:or loose fill insulation systems pedite is reallythe only choice.)erlite is a naturally occurring volcanic glass rock. lt is a boundj.uminium silicate which contains some 3% water of:rystallisation. When the material is ground to a suiiable size:lrd subjected to rapid heating to around 1000 'C, the glass:articles melt and the crystallised water evaporates and ex-:ands the particles to between '10 and 40 times their original. clume forming rigid foam cells. The resulting material is white- colour and has excellent thermal properties. In small quanti---es the material can be produced in a suitable factory and:ansported to site in bags.

-nis approach is a possibilityfor small tanks such as the small

=rd of the LOX, LIN and argon storage tanks. For larger tanks:-rs rapidly becomes impractical in view of the quantities in-, trlved. For a large multi-tank LNG facility, thousands oftonnes:' expanded perlite will be required. At 60 kg/m3 density, 1000::1nes of perlite requires a volume approaching '17000m3. The':ctory route is clearly not a sensible proposition in this case.-'e solution is the use of portable expansion plants. These can:e set up on the construction site and the unexpanded perlite:-e can be more sensiblytransported to site in 1 tonne big bags:. in containers.

--e perlite is produced by continuously feeding the graded ore-:o a purpose-designed furnace. The expansion is almost im--ediate and the product is exhausted from the furnace in the-at gas stream which is filtered to separate the expanded:€dite. This is then pneumatically conveyed to the tank where it

-ttles out of the air stream and fills the inter-tank void. Special?re must be taken to ensure that this conveying process does-:t damage the perlite as attrition maytake place. To avoid this:-e conveying velocity must be kept as low as possible, thei.gths of hose should be kept as short as possible and any:€nds as few as possible and of the maximum practical radius.I re diagram showing this process is shown as Figure 19.14.

I :/pical example of the vibration equipment required to avoidi,cessive settlement of the pedite as discussed in Section': 3.2.3 is shown in Figure 19.15.

The resilient blanket

--e guroose of the resilient blanket is to control the external:-3ssures which the loose fill insulation material exerts on the--er liquid container These external pressures change in- agn itude du ring the life cycle of the storage tank, but can gen-:?lly be assumed to have their maximum value when the tank: lecommissioned following a period in service. In simplistic:."ns the wayin which this maximum design pressure is arrived:: s illustrated in Figure 19.'16. Perlite in its expanded form is: -3sumed to be incompressible at the low levelofloadings pre-,, ing in this situation (higher loadings will cause expanded:.'ite to break down in an irreversible fashion) and to be able to':,v freely into any voids which may be created.

--e problem was first given serious attention during the design-" ase of the LNG tanks at the Canvey lsland lmport Terminal.--e client was the East Midlands Gas Board (EMGB) and the

='rtractor was Whessoe Ltd. Ajointlyfunded research and de-,E cpment programme was undertaken and a series of box:s:s weredevised which simulated the intersoace between the--er and the outertanks, the insulation materials which were to:: installed in this space and the thermally-induced move--e'rts of the inner iank as it cooled to LNG temperature and. -rsequently warmed up again. The test facility had the equip--elt necessary to measure the pressures on the outer sur--3:es of the simulated inner tank at various stages during the:.s: cycte.

19 lnsulation systems for low temperaturc tanks

Figure 19.14 Line diagram showing a site-based perlite expansion plant

Flgure 19.15 A perlLte vibraiion sysiem

I

The solution chosen to solve this problem was to suspend aglass fibre blanket from the top of the outer surface ofthe innertank shell. This blanket acted like a spring and controlled theexternal loadings on the inner tank, especially when the innertankwarmed upfollowing a period in service. Atthis point in thetank's life cycle it is empty and most at risk from possible dam-age caused by external pressures. This test work enabled thedesign pressures to be predicted with confidence and the innertank stiffening system to be designed. The design of the innertank stifiening is discussed in detail in Chapter 18. The result ofthis work was that the resilient blanket system was the subjectofajointly held patent, and the technologywas licensed to othercontractors until the patent cover expired in the late 1970s.

As low temperature tanks, and LNG tanks in particular, becamebigger, the suspended method of blanket installation became aproblem. The glass fibre manufacturers could only producesuiiably-faced materials in limited lengths and the vertical load-ings on the blanket materials themselves became a limitation.Providing a second support ring at a point halfway up the tankshell and using two drops of blanket was one solution. Anotherwas to impale the blanket material on pins which were attachedto the outside ofthe innertank with suitable adhesives. Afurthersolution was to arrange forthe inner tank stiffening to be on theouter surface ofthe innertank (these stiffeners are most usuallyto be found on the inner su rfaces of the inner tanks) and for thestiffeners themselves to be at a constant Ditch. The resilientblanket material was then installed between these stiffenersand held in place with wire laced between the outer edges ofthestiffeners.

STOR,AGE TANKS & EQUIPMENT 39,I

1 9 lnsulation systems for low tempenture tanks

Outerwall 'Perlite

Over the years furthertest work has been carried out and Brit-ish Gas (of whom El\,lGB was formerly a part) continued its in-terest in the subject and allowed some of its 50,000m3 LNGtanks to be fitted with pressure and blanket thickness measur-ing equipment. Thus a full-scale test facility, withoutthe uncer-iainties brought about by specimen end effects, which were al-waysa problemwith the box tests, was made available. Mostofthe data collected was analysed by the late Dr lan Leadley ofWhessoe.

The relationship between the perlite, the resilient blanket andthe resultant pressures on the inner tank is complex and in-volves a bewildering numberofvariables. Amongst these are:

. The physical dimensions ofthe hnks and the interspace

. The product and ambient temperatures

. The materials of the inner and of the outertanks

. The density ofthe perlite

The thickness of the resilient blanket

The compressive properties of the resilient blanket

The angle of internal friction of the perlite (this varies withtime)

The coefficient offriction at the various interfaces

The number of full and partial thermal and liquid filling cy-cles for the inner tank

. The numberof pressure cycles for the outertank (applies tosteel outer ianks only)

All of the tank designers have their own ways of solving thiscomplex problem. These are usually closely guarded secrets.The whole subject is shrouded in uncertainty because ofthe dif-ficulties with the perlite/blankevpressure relationship, the largefactors of safety used in the stiffenersize and pitch calculationsand the small number of lowtemperature tanks which are sub-jected to the full loading cycle and then decommissioned andexamined.

19.4 Roof insulation

19.4.1 General

The most commonly found forms of roof insulation fall into twocategones:

. For single-walled metallic tanks the insulation system is at-tached to the external surfaces of the roof sheeting.

. For some single-skinned tanks and for the majority of dou-ble-walled tanks, the insulalion system is supported on asuspended deck supported from the tank roof frameworkand located within the tank.

19.4.2 External roof insulation

This is usually similar in form tothe externalshell insulation sys-tems described above. The insulation materials used and theneed for an effective weather cover/vapour banier are thesame. lf anything, the local environment on the tank roof isworse than for the tank shell in terms of exposure to severewinds, rain and chemical aggression. For this reason it is notuncommon for single-walled tanks to be constructed with inter-nal suspended decks. In addition to the advantages of remov-ing the insulation from this difficult environment, there are otherpotential savings associated with the design temperature forthe outer roof sheeting and its supporting framework beingbased on atmospheric temperatures and noton the lowerprod-uct temperature. The insulation material may be cheaper i.e.

Blanket

- inlorspace filled wih Pedite

'"1"'.1*:"*o" \

Cold - inn6r tank conlracts

Inn€r tank warms up and oxpands

Pe ite / bl.nket intBrface remains in same position as c) ,boveOhis gives maximim exlamal prossure on inner tank)

Non-f n6ar €lastic prop€rli6s of reslient bbnket

Figure l9.'16 Resilient blanket pressure history


iJ r-el

:o;o'

rit-in- l

!G

i:c liIt'- ::

:r density glass fibre rather than polyurethane foam. Ther, eather cover/vapour barrierwill no longer be required. These::vings may well offset the cost of providing the suspended:eck and its supports and the longevity ofthe system is almost::rtain to be jmproved.

19.4.3 Internal suspended deck insulation

-.e most commonly used insulation materials for this purpose:-e low density glass fibre ofthe type often found in domestic at-: :s or lofts, (usually around 0.75k9/m3 in density), mineralwool,=!ain at the lower end of the range of densities available, and:€dite. The pedite is most usually site-expanded and installed:.3se or in bags. For suspended deck insulation, the following::nsiderations must be taken into account:

. Steps should be taken to prevent the roof insulation fromcontaminating the product liquid. For plate type suspendeddecks this is only a problem around the periphery of thedeckand atfittings penetrations. A simple fabric shroud willnormally suffice. Fordecks constructed from structural sec-iions and troughed sheeting where there are gaps betweenthe sheets, this is more ofa problem and frequently requiresa layerofsuitable material between the deck and the insula-tion to covef such gaps. A commonly used material for thispurpose is a glass reinforced paper. Perlite in particular is aproblem. lt has a tendency to flow through small gaps, andfor an apparently delicate material, can cause problemswith abrasion of the moving parts of certain downstreamprocess equipment, for example pumps.

. Where fibrous materials are used for deck insulation andperlite for the wall insulation, the fabric shroud or the perliteretaining wall must prevent the perlite from getting onto thedeck in addition to its other duty of retaining a sufficient hop-per volume of perlite above the interspace.

. Steps must be taken to prevent the deck insulation from be-ing "rearranged" by vapour movements above the deck orfrom getting into the product via the deck vents.

. Steps should be taken to ensure that the final, installed deckinsulation thickness is what has been specified and used inthe thermal insulation calculations. A commonly used de-vice is to put adhesive tape markers on the suspended decksupporting rods or wires to indicate the required finishedthickness. By this means any settlement during installation0ecomes oDVtous.

. For tanks where there is a possibility of condensation of theproduct liquid on the underside ofthe outer roof (a problemfor butane tanks where the ambient temperature can occa-sionally be lower than the storage temperature, and in ar-eas ofthe world subject to very low temperatures, a similarproblem for propane tanks), provision must be made to en-sure that the condensed liquid can make jts waythrough thesuspended deck. For double-walled tanks it is necessary toensure that the condensation will also not enter the wall in-sulation interspace.

19.5 Insulation of heat breaks and fittings

19.5.1 General

-he majority of fittings in conventional vertical, cylindrical low:emperature storage tanks pass through the tank roof. The lacklftank wall penetrations is a prerequisite ofthe fullcontainment

=tegory of storage and is strongly preferred for the single and:ouble containment categories.

-here are a fewcircumstances where side walland bottom con-rections are used. One of these is for the storage of liquid am-

19 lnsulation systefis for low tempercturc tanks

monia where the supply of suitable in{ank pumps is problem-atic. This is discussed in Chapter 21. Oxygen and nitrogentanks traditionally have side wallor bottom connections ratherthan in-tank pumps. Tanks which predated the general moveaway from single containment to the higher containment cate-gories were also often fitted with side wall or bottom connec-tions, mainly for liquid import and export.

The function of the fittings heat breaks are:

. To prevent excessive local heat gain

. To provide a mechanical load transfer mechanism betweenthe attached pipework and the tank roof, wall of bottom

. To protect Darts of the tank which are constructed from ma-terials which are not suitable for contact with the orod uct Iio-uid design temperature

. To avoid condensation orlocalice build uD around thefittino

19.5.2 Heat breaks for roof connections

Figure 19.17 shows a typical roof heat break arrangementfor alull containment LNG tank. Roof fittings for product liquid orvapour can become quite large. The in-tank pump columns fora typical LNG export terminal tank may well be up to 42" in di-

Fqure 19.1/ -ypical rool heal break arrange-lent for LNC se-vrce


1 I lnsulation systems for low temperature tanks

ameter and it is not unusual for the liouid import line and thevapour return line for these tanks to be 32" in diameter

ln this case the tank roof is constructed from reinforced con-crete with a non low temperature steel liner, i.e. the roof sheet-ing. With the product storage temperature at about -162 'C anefficient thermal heat break is required. The mechanical loadsto be transferred across the heat break arising from theself-weight ofthe internal and external pipework and its insula-tion, the piping system anchor loads and any local valves andfittings will be substantial, particularly when the tank is to beconstructed in an area subjectto a severe seismic environmentand significant amplification of these static loadings can takeplace. Consequently a robust connection is required.

Cleady the need for thermal efficiency and the need for me-chanical strength are in conflict. This makes the design ofthesefittings interesting. A lot of experience in the use and behaviourofthe various proprietary designs is useful here. lt is sometimeswritten in specifications that the heat breaks shall prevent iceformation orcondensation on the tank rooflocalto thefitting un-der allatmospheric conditions. This is a quite unreasonable re-quirement which is impossible to comply with. There willalwaysbe some measure of cooling ofthe roof or the warm side com-ponents ofthe heat break adjacent to the fitting and under unfa-vourable atmospheric conditions, some condensation or icebuild up is inevitable. lt would be more reasonable to requirethat the heat break design will seek to minimise this phenome-non.

For double-walled tanks where the inner tank has a fixed roof,the situation is more complex as a means must be provided toaccommodate the differentialthermal movements which will bepresent. These types oftanks are quite uncommon these daysalthough there are some companies in the Far East who still re-main wedded to this arrangement. lt is most usual to include a

bellows in the penetration. This can be located between the tworoofs or external to the outer tank roof. Atypical arrangement is

illustrated in Figure 19.18.

For the heat break arrangement shown in Figure 19.17, experi-ence has shown that the most effective form of insulation isPUF. This can be installed in the fitting prior to its erection intothe tank roof. The most convenient way of achieving this is tofoam the PUF in situ into the inverted fitting using one ofthe pro-prietary foam kits available on the market. The use ofglass fibreor mineral wool packed into the insulation space has not beengood for the lowertemperature end of the product range. Prob-lems of internal convection have frequently led to the appear-ance of external ice spots.

For warmer products such as butane. ammonia or propane, a

simpler arrangement is frequently adopted. One such is illus-trated in Figure 19.19. One of the significant savings resultsfrom the elimination of the expensive stainless steel conepiece. Forthese products the use of glass fibre or mineralwoolinsulation may be appropriate.

19.5.3 Heat breaks for tank sidewall connections

For single-walled tanks, there is no need fora heat break, sincethe wall, fitting and the connected pipework all fallwithin the in-

sulation envelope.

For the now unusual, double-walled single containment type oftanks, a typical arrangement is shown in Figure 19.20. The de-

sign of such flttings was not easy, the accommodation of thethermal and mechanical movements of the inner shell and thereduction ofthe external loadings from the connected pipework

to acceptable limits had to be considered. External spring sup-ports and pipe loops were frequently required to resolve thesepro0rems.


Figure 19.18 Atypical roof fitting for a double roof type of tank

Gl.ss flbre, miner5l wool

Figure 19.19 Atypical rcoffilting fot LPG or ammonia seruice

Forthe much smallertanks for the storage ofoxygen, nitrogenand argon, the use of sidewall connections is common. Theseare frequently incorporated into a cold box built into the shell ofthe outer carbon steel tank. Atypical arrangement is indicatedin Figure 19.21.

19.5.4 Heat breaks for tank bottom connections

Despite the apparent attractions oftank bottom connections interms of maximising the use of the tank capacity and of en-abling the operators to drain the full tank contents for rapid de-commissioning, bottom connections are uncommon.

For single-walled tanks there is no need for any form of heatoreaK.

For double-walled tanks with small-sized iittings, the penetra-tion of the inner tank bottom is connected by pipework runningin the base insulation and exiting through the outer shell. Here aheat break arrangement similar to that required for Section'19.4.3 is necessary

Figure 19.22 shows a detail which was used for a double con-tainment liquid butane tank constructed at l\Iobil Oil's CoMonrefinery This was a double-walled metallic tank where in;or-mal service the inner pipe is cold and the outer pipe is warm.Under accident conditions both inner and outer pipes are cold.This was a fitting which was difficult to construct, but which has

1 e t,s,Eji9!:EE!:!!!9y!!29EyE !9!

Figure 19.22 The bottom connection at Mobil Oit's Coryton refinery

worked successfully for some 15 years. ln this case the heatbreak is the bellows.

19.6 Internal pipework insulationThe only pipework to be found within the tank envelooe is nor-mallythatwhich runs from the cold liquid orvapour roofconnec-tion down to the suspended deck. The insulation is simDle usu-ally consisting of multiple layers of a suitably-backed glassfibre, wrapped around the pipe and held in place with wire orplastic ties, or of preformed pipe insulation sections usuallymade from glass fibre or mineral wool.

19.7 External pipework insulationThe insulation of lowtemperature pipework is a signifi cantsub-ject jn its own right and more properly belongs in a publicatjondealing with pjpework systems.

Regularvisits to sites where lowtemperature liquids are storedand processed will rapidly revealto the observerthat the maior-ity of problems of ice build-up and condensation occur in ihepipework. In addition to causing expense by virtue of excessiveheat gain, the ice build-up can also be the source of safetv Drob-lems by inhibiting thermal movements at bellows and pipe sup-oorts.

Good low temperature pipework insulation is the result of cor-rect system design, specification, material selection and proba_bly most importantly, the careful installation and site supervisionby experienced specialists.

Traditionally,pipework insulation has been fitted followino erec-tion ofthe piping system. In recent years there has been Jmoveaway from site installation and it is now common for orefabri-cated pipe spools to be sent to an insulation specialist to re-ceive the thermal insulation prior to erection in the field. Thisbrings the benefits of controlled factory environments for thisactivity, leaving onlythejoints between spools to be insulated atsite. Clearly good planning and dimensional control are a Dre-requisite ofthe successful implementation ofthis method of in_stallation.

The most commonly used materials for the insulation are pUFand cellularglass. Both ofthese materials are available in fac-tory-formed pipe profiles. lvlaterials commonly used for thevapour seal/weather protection are galvanised or Aluzinc steelsheeting, aluminium sheeting, stainless steel sheeting, poly-ethylene and vadous non-mebllic mastics.

For manyyears an argument has raged concerning the relativemerits of the materials and combinations of materials in resist_ing heat radiation loadings from various accident scenarios.

---i-*-t,.-l',+ atr-j.r.r,t fr ,././/)l !I - -

tiKU .''

ll'1 "",""i

Fjgure 19.20 A typicat sidewall connection fora double-wa ed sjngle contain-

Figure 19.21 A typical sidewatt connection for a LOX or LtN tank


19 lnsulation systems fot low tempercture tanks

This still continues, with allsides frequently claiming victory. lt isnot unusual for composite systems using PUF for the innerlayers and cellular glass for the outer layer to be specified.

An area where particularcare is needed is in the fitting of insula-tion to valves and other fittings.

Infrared thermography discussed in Section 19.10 is a usefultool for inspecting low temperature pipe insulation systems.

The identification of the causes of failure or lack of longevity ofsuch pipe insulation systems is not always straightforward. Arefinery in the UK had problems of early breakdown of its care-fully installed cellular, glass-based insulation. The cause ofthisproblem was eventuallyhaced to the fact that the workforce fre-quently used the larger insulated pipes as walkways to themore inaccessible parts ofthe site. The brittle cellularglass wasno match for the workers' boots!

19.8 Heat leak calculationsThe basis of the heat leak calculations is quite straightforward.Itisonlyin the detail that the subject becomes a little more inter-esting. The tank insulation system is divided up into the areaswhere similar materials or combinations of materials have beenused. For a typical full containment tank these would be:

. The central area ofthe tank base

. The oerioheral area ofthe tank base

. The lower tank wall where thermal orotection has been in-stalled

. The upper tank wall where no thermal protection has beeninstalled

. The tank roof

19.8.1 Basic calculation methods

The basic equation to calculate the heatflux through a particu-lar component to be adopted where a slngle insulation materialis used is:

l\4ost thermal insulation materials have thermal conductivitieswhich change with temperature. This is illustrated for cellularglass by Figure 19.6. Hence the mean temperature of a layer ofinsulation material within the multi-layersystem must be knownbefore the thermal resistance of that layer can be calculated,and the thermal resistance is required to establish the meanremperarure.

The way out of this apparent impasse is to assume a tempera-ture profile for the various interfaces within the system, use thisto calculate the mean temperatures of the individual layers andthis in turn to obtain the k values of the various materials to per-form the initial calculation. The results of this calculation allowthe interface temperatures to be recalculated and the k valuesto be revised. Asecond calculation is carried out and the resultsofthis allow a further revision of the k values. A couple offurtheriterations should show temperature values at the interfacesconverging and this should be sufficient. Figure 19.23 shows a

numerical example of this Drocess. This is a tedious calculationto carryoutbyhand and is ideallysuited to Excel spreadsheets.

19.8.2 Thermal conductivity values

Initially, the source of the thermal conductivity values (or K val-ues) to be used in the basic calculations is the manufacture'stechnical literature. Mosi low temperature tank designers willproduce their own detailed technical specifications for the sup-ply and installation of the different parts of the insulation sys-tem. Within such specifications it would be unusual if there werenot some means of confirming the K values of the materials.This could take the form of regular samples being taken fromthe place of production, be this a factory for materials such asslab stock PU F, PVC foam, cellular glass, glass fibre or mineralwool, or the construction site for such materials as perlite or siteexpanded PUF.

These samples would be sent to an agreed laboratory wherethe K values would be verified at the appropriate temperature(or range of tem peratures). lt is usual for the eventual owners ofthe tank to witness this testing, either themselves or via theirhired engineering or inspection companies. As has been men-tioned earlier, the measurement of low temperature insulationproperties is noteasyand should be leftto those skilled and ex-perienced in this work. Note that this testing is usually con-ducted in air.

For porous materials such as perlite, glass fibre or mineralwool, the vapour within which the insulation material is operafing wiil have a significant effect on lts effective K value. Thus thetest results which are based on air as the interstitial gas willhave to be adjusted to account for the presence of a differentgas. This is discussed in Section 19.8.3.

Where the insulation material has been penetrated by itemsmade from different materials, such as is the case for resilientblankets supported by being impaled on pins, the effective Kvalueto be used in the calculations must be adjusted to take ac-count of the short circuiting effect ofthe pins. Asuitable calcula-tion method for making this adjustment is given in section ,q3 ofReference 19.3.

Certain insulation materials have thermal properties whichchange with time. PUF used as external insulation may displayan increase in its K value as the original foaming gas within thecells is progressively replaced byair.ltis importantthatin thesecircumstances a suitably aged property is used in the calcula-tion.

19.8.3 The influence of different interstitial gases

The various equations which allow the K values of the vapoursofthe various low temDerature oroducts to be calculated at dif-

equ 19.1

where:

H = heat flux through component (W)

k = thermal conductivity of the insulation material(W/m'K)

A = area of component (m2)

AT = hot to cold face temperature range ('K)

L = thickness of component (m)

Where more than one material is used the following methodtaken from section A3 oI Reference 79.3 is used:

H = k xAxAT i L

U = 1/ (R1+ R2 + R3+.....+Rn)

wnere:

R=L/K

U = thermal transmittance (Wm'? 'K)

R1 & thermal resistance ofthe various insulationR2 etc = components (m2'K iV) - calculated from

equation '19.3 below

equ 19.2

equ 19.3

Hence the equation to calculate the heat flux through a multi-layer component is:

H = U xAxAT equ 19.4

It is not normal to include surface resistance in these calcula-tions.


Installed perlle densitylnstallsd glass fibre d€n8itySection through wa[:

Matedal Thickne6s(mm)Outside Conc€te 600

Sleel linet 5 lgnor€ h thermalcalculationPsdite 960Glasslibre 240

Inside 90/6 ni inner tank 10 lgnore in thermalcahulalion

Assumed thermalgradienl ('C) CK)

A€Bumptions for the exercise:To Outer surface tamperatureTi lnner sutfsoa ternperatureIntef,stltialgas

Air/concrelgConc|Eie,/perliteP€rlite/gla8s tibreGlass fibr€/product

TRIAL No 1' Calculate K values:ConcretePerlite step l

60 kglm324 relm3

40 31330 3{X}-125 148-'165 108

Taken as 1.60 Wm'K tiroughout calqiationcalculate K value of hterstitial gas (f€1)ATp 155 'KKS1 0.0243158 w,lm'Kcalculate K value of petlib (KP)

R 8.252384i|y 0.137422Kp 0.0401584 Wm'Kcalculate K value of interstitial gas (lQ2)ATfg 40'Kl{gz 0.0133521 dm"Kfuctor ftom Figure 19.25f 1.455calculale K value of glass fibr€Kg 0-0194273

Thidmess (mm) K value Themalresistance(rn2 "K iv) AT600 1.6 0.3750 2.0985960 0.0401584 23.9054 133.771407240 0.0194273 12.3538 6S.1301

+40'c-165 'C

Methane

Total 36.6341 m5.0000

19 lnsulatjon systems for low temperature tanks

step 2

Gla6s fibre step 1

step 2

step 3

Total h€at f,ux (tdal 1). Matedal

ConaGtePerliteGlass fib|e

Heat FlI( (dmz) 5.5959

3'13.0000 313.0000 313,0000310.C015 310.256 310.7349117.1301 174,7136 174.5*1108.0000 108,0000 108-0000

Figure 1 9.23 An sxample of a multilayer ln6ulation @mponent cslcllation - page t

STORAGE TANKS & EQUIFII'ENT 397

19 lnsulation systems for low temperdturc tanks

TRIAL No 2Calcxrlate K values:ConcretsPedite step 1

st€p 2

Glass fib|€ step .l

step 2

step 3

Heat Ftux (dmz)

TRIAL No 3Calculale K values:ConqeiePedite step 1

step 2

Glass fibr€ step 1

step 2

step 3

Total heat flux (biat 1 )Material Thlckness (mm) Kvalue Thermal resistance(mr.XLn) atConcret€ 600 1.8 0.3750 2,2744Perlfte 960 0.042808 22-4287 136.012036cfass fibr6 240 O.O21B|B7 10.999g 66.7,13€

Taken as 1.60 Wm'K thrcughout calculationcalculate K value of intersfital gas (lg1)ATp 133.7714'KKgl 0.0266994 w/m'Kcalculale K value of pertite (Kp)R 7.87Tt517y 0.1374?2.Kp o.o428oa w/m'Kcalculale K value of inteGfitial gas (Kg2)Arfu 69.1901 'KKg2 0.01499s6 w/m'Kfactor from Figure 1 9.2St 1.455calculate K value of glass fibreKfs 0.0218187

Toiat $.9005 208.0000

6.0650

Taken as 1.60 w/m'K throughout c€lculalioncalculate K value ot interstitiat gas (Kg1)ATp 136.0i20 'KlQl 0.0285485 w/m'Kcalculate K value of pedite (Kp)R 7.9020321y 0.137422.Kp 0.0426242 wtm'Kcalculale K value of interstitial gas (Kg2)ATfs 66.7136 'KKg2 0.0148s87 dm'Kfactor from Figure 1 9.25

1.455calculate K value of glass jibreKfg 0.02i6194

33.9986 205.0000

Total heat flux (trlat 1)Matedal Thickness (mm) Kvalue The.malresistance(flf "tgu/) ATConcrete 600 1.6 0.3750 2.2611pedite 960 0.0426242 Z2.Sn4 135.8027i18Glass fibre 240 0.0216194 11.'1011 66.936.1

Total

H€at Flux (dm2) 6.0297

Figure 19.23 An example ofa multi-tayer insulation component calcutation - page 2


Fomula for themal conductivity In wm"K

o oorou * u'"tito'Lt'Jli

o.oos6r r 5 9l7 t 1o' lT'l''

o.oorot . uuttilo'[t.ll'

I 258'10 7 .-,e6rr,nT t' lr

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-- ^T

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'efent temperatures are given in Figure '19.24. The source oftis data is Reference 19.4.

: g u re 1 L24 Formulae for calculating the K values of various gases at d ifferent

Forcalculating the Kvalue ofperlite with the air replaced byonecf these gases at a particular set oftemperature conditions, the'ollowino formula can be used:

equ 19.5

Kp = thermal conductivity of perlite in the replace'ment interstitial gas (Wm 'K)

Ks = thermal conductivity of the interstitial gasattheappropriate temPerature (Wm "K)

R" = 0.114i Ks+ 3.608

y = 3.9x10'3xp087

p = installed perlite density (kgim3)

For calculating the K value of glass fibre or mineral wool, thegraph shown in Figure 19.25 can be used.

(Both equation 19.5 and Figure '19.25 are attributed to the lateDr lan Leadley of Whessoe.)

1 I lnsulation systems for low temperaturc tanks

19.8.4 Galculation of the hot face temperature

The design specification will require a certain maximum heatleak into the tank contents as described in Section 19.7 Ofrenthe only data given is the maximum design ambient tempera-ture, which is a shade temperature at the tank's geographic lo-

ca on.

The actual maximum temperatures to which the tank roof andwalls will be subjected influenced by the local solar radiationmaximum levels, the attitude ofthe surface in question, the pre-

vailing weatherconditions (clear orcloudy skies)and the natureof the external surfaces. Data and calculation methods allowingthe actual maximum temperatures to be calculated for any setof circumstances are given in section AG ot Reference 19 3

For the tank base. two possibilities exist:

. For tanks built on the ground, the hot face temperature isbased on the operational settings of the base heating con-trol system. lt is clearly unwise to have the base heatingsystem maintaining the base temperature at too high a

level. Purchasing expensive energy, be it electrical, steamor heated brine to boil off more product than is necessaryincurring further costs in terms of re-liquefaction or product

loss to atmosphere is clearly a nonsense. Consequently a

design hot face value as low as possible is used. -5 'C is a

not unusualvalue to use in the calculations in these circum-stances.

. For tanks built on elevated foundations, a hot face designtemperature equal to the maximum shade temperaturewould seem to be a sensible choice. There is perhaps a

case for using a lower temperature. Experience suggeststhat the space beneath the base slab of such tanks is a coldplace to be, even on hot days.

19.8.5 Overall heat leak

Acommon wayforthe tank maximum heat leakto be specifledto the tank and insulation system designer is to express it interms of the escape to atmosphere of a percentage of the fulltank contents perday. Hence for a large LNG tank we may see:

"The maximum heat leak shall not exceed 0.05% of the fulltank contents per day on the assumption that the tank con-tents are considered to be pure methane."

The latter requirement to consider the tank contents as a pureproduct is to avoid the complication of working out the latentheat ofthe LNG which may have a range ofcompositions and to

xo=Kn(-v)+ /^,+o-1fvhere:

t

lhermal conduclivity glass fibre= factor x average conductivity of gas

Figurc 19.25 K value ofglass fibre of mineralwool


19 Insulation systems for low temperature tanks

avoid subsequent contentious arguments. lt is normal to makethe same form of wording for any tank containing a mixed prod-uct, expressing the permitted heat leak in terms ofa percentageofthe major constituent. The following points are worth bearingin mind:

. lt is importantto use the correct density in the calculation ofthe permitted heat leak. For LNG a latent heat of 507.0kJ/kg should be used with the pure methane density of0.422. Using the design density of LNG (frequently given as0.48) will give too high a value of the permitted heat leak.

. lt is often presumed that the worst conditions pertaining atany point on the tank outer surface at any time during thewhole day will persist for the full 24 hours. This has occa-sionally become a point of dispute beh,veen the owner andthe designer, with the tank designer claiming that it repre-sents an unnecessarily conservative interpretation.

. The full tank contents is usually taken to mean just that, i.e.with no deduction for in-tank pump NPSH etc.

. Whilst the calculations seek to cover all of the sources ofpossible heat leakage from tank to atmosphere, there willprobably be some which have been ignored or overlookedsuch as the smaller connected pipe connections. To coverfor these uncertainties, it is usual for the designer to aim fora calculated heat leak lower than the full target value. A notunusual starting point would be to aim for 85% of the fullvalue in the first instance.

A well set out heat leak calculation for a large full containmenttype LNG tank is shown in Figure 19.26. This makes use of aseries of linked Excel soreadsheets.

19.9 Heat leak testingWith the customer or his engineer setting a heat leakage re-quirement for the tank and the distinct possibility that at leastsome of the process equipment will be designed based on thisfigure, it would seem sensible to test the finished storage sys-tem to see that it fullllls this performance criteria. This is not assimple as it would appear for a variety of reasons:

. lvleasuring the heat leak will require either a significantchange in the tank liquid level to occur, which may takesome days depending upon the accuracy ofthe level mea-suring equipment provided, or will require the accuratemeasurement of the vapour flow through the vapour outletline, something difficult and expensive to achieve. Vapourflow measurement is not a normal part of the tank instru-mentation. For a large LNG tank with a specified boil off rateof less than 0.05% (a typical figure for such tanks) the levelchange will be of the order of 15 mm/day. To get a sensiblemeasurement which will be sufiicient to negate any uncer-tainty caused by tolerances on gauging accuracy, it is clearthat the test duration must run into several days.

. The tank must be fullor close to full at the time of the test toavoid contentious arguments revolving around the extrapo-lation ofthe heat leakfrom a lowerliquid levelto a fulltank.

. The tank must not be subject to any liquid movements dur-ing the test period.

. The test must be carried out at a time when barometricpressure is anticipated to remain relatively constant.

. Ambient temperature must be monitored throughout thetest period.

. For LNG LPG and other mixed products, it is necessary todetermine the composition of the liquid in the tank. This willrequire sampling as the iank is filled.


. lt may be necessaryto run the in-tank pumps during the testperiod to ensure proper mixing. Allowance for the energy in-out from this source must be made.

. Boil off is known not to occur at a uniform rate, but rather asa series of irregular "burps". This is another reason why thetest must be conducted over a protracted period.

. Arrangements must be made to record and take account ofthe effects of wind and solar radiation.

These difficulties combine to make a physical heat leak testtime consuming, expensive and inconclusive. To avoid thisproblem area, the following procedure is often adopted:

. The tank designer must prepare detailed heat leak calcula-tions together with the appropriate certification (and possi-bly QA records if these are available at the time) todemonstrate that the materials used havethe required ther-mal properties.

. These calculations and the associated documentation willbe submitted to an expert third party, previously agreed byboth the owner and the tank contractor, who would reviewthe calculations and whose findings would be binding onboth parties.

One of the added advantages of this procedure is that in theevent of a shortfall being found in the thermal insulation pro-vided, then this can be made good prior to the tank entering ser-vice. This could perhaps be by a simple addition to the thick-ness ofthe insulation on the suspended deck, an action with noknock-on effects. With the physicaltest route, this pre-commis-sioning adjustment is not possible.

19.10 The use of the infrared cameraAn infrared camera will produce images which will identify ar-eas where the heat leak is abnormal or merelydifferentfrom thesurrounding areas of insulation. lt is a useful tool both at thetime of tank commissioning and as an occasional maintenancedevice to locate any changes in the thermal insulation systemand its performance, perhaps due to such time dependent phe-nomena as insulation material degradation or perlite settle-ment. The equipment is nowadays quite cheap to purchase, orthere are companies who will come and perform this service.

19.1 1 Insulation problems from the pastand their lessons

19.11.'l Base insulation failure

Two LNG tanks belonging to GAZ Metropolitan in lMontreal,

Canada, had been in continuous satisfactory service until July1990, when instrumentation in the tank base ofone of the tanksbegan to show evidence of cold spots. After double checkingand adding new thermocouples, the problem persisted and itbecame obvious that the tank required to be taken out ofservice.

Following decommissioning it was found that the cellular glass

base insulation was the subject of massive cracking and me-chanical breakdown.

This damage eventually necessitated the lifting ofthe innertankandthe complete replacement of the cellularglass base insula-tion, an expensive and time consuming process.

An investigation into the cause of this base insulation failurewas carried out and this is reported in Reference 19.5.

The investigation revealed that in July 1990, blasting work had

been carried out within 200 m ofthe two tanks. The tank nearestto the blasting was full of Iiquid at the time and undamaged. Thetankfurthest awayfrom the blasting location was fllled to 20% of


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19 lnsulation systems for low tempenturc tanks

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1 9 lnsulation systems for low tempenture tanks

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*t

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tt'EgE5EdE;

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f

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Figure 1 9.26 A typical heat leak calculation for a large LNG tank - page 7

I

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sa{i6lF

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et9:iE;_EF-d6

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E* r:*a

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19 lnsulation systems for low tempercture tanks

Figufe 19.26 A typ cal heat leak calculation for a Lafge LNG tank - page I


:s capacity and was damaged as described. The investigation:ound that the partiallyfilled tank had a higher natural frequency:han the fuller tank and was thus more susceptible to damageiom the blasfinduced ground motions.-lhe lesson from this incident is clearly that any blasting on thesame, or adjacentsites should be avoided, and ifthis is not pos-sible, then detailed investigations should be carried out to de-iermine the susceptibility of tanks with brittle base insulationnaierials to damage.

19.11.2 External vapour sealing

A number of low temoerature tanks with external thermal insu-ation on their shells, and in some cases also on their roofs,rave come to grief over the years. The reasons for these fail-Jres are usually associated with poor performance of their ex-:ernal vapour sealing arrangements. This has allowed mois-iure-laden air to invade the insulation material and form ice/vithin the insulation or on the tank shell beneath the insulation.The damage can manifest itself immediately following the tankcool down or following several years in service.

Higher than anticipated heat in leak and consequent productSoiloff, the appearance of external condensation orofice spotsor (in at least one case) sudden failure and collapse ofallor partof the shell insulation are the usual signs.

The lesson here is that the external vapour seal and itsong-term abilityto keep atmospheric moisture out ofthe insula-tion material is viialto the survival ofthe insulation system. Cor-r.ect material selection, sDecification and installation are all ac-livities, which will help to ensure that the required performanceand service life are obtained.

19,11.3 Bottom corners

The bottom corner of tanks where the wall insulation is on theouter surface ofthe shell and the base insulation is beneath theiank bottom, have on occasions given rise to problems. Again,the cause is moisture ingress and the reason is poorwaterandvapour sealing materials and details. This is a difficult area ofthe insulation system to design for, for a number of reasons:

. Large radial thermal movements caused by tank contrac-tion.

. High shell line loadings, requiring materials with good, me-chanical and thermal DroDerties.

19 Insulation systems for low temperaturc tanks

. Exposed to frequent waterfall events, due to concentratedrainfall from the roof and shell.

. Complications to detailed design caused by holding-downanchors.

. The correct selection of materials, detailed design andcareful installation together with regular inspection all havetheir part to play in this area.

19.1 1.4 Perlite settlement

Double walled tianks using perlite insulation have on occasionshad a history of poor performance. Aring of condensation oriceat. or close to the top ofthe outer shell, is an indication of exces-sive perlite settlement. The reasons for this can be:

. Lack of, or inadequate, or ineffective vibration ofthe perliteduring its site expansion and insiallation.

. The provision of insufficient hoppervolume atthe top cornerof the tank.

. On at least one occasion, the location of a large diesel-pow-ered generatoradjacentto the tank following perlite installa-tion.

The use of experienced perlite installation companies using ap-propriate methods and equipment will help to avoid this prob-lem and its solution, expensive in service topping up of theperlite.

19.12 References

19.1 Foamglas@ lndustrial lnsulation Handbook, PittsburghCorning NV Waterloo, Belgium.

19.2 Research into the structural integrity of LNG tanks,D. Neville and G. White, British Gas Engineering Re-search Station. LNG 9. October 1989.

19.3 The lnternational Heating and Ventilating Guide, fheChartered Institution of Building Services.

19.4 The Handbook of Cryogenic Engineering, J.G.Weisland ll, Taylor & Francis, London, 1998.

19.5 Damage to base ofLNG tanks from blast loadings - Acase study, R. Tinawi, A. Filiatrank, C Dor6, Journal ofPerformance of Constructed Facilities. Vol 7. No 3. Au-qust 1993.


410 STORAGE TANKS & EQUTPMENT

20 Ancillary equipment for lowtemperature tanks

This Chapter provides a brief review of the ancillary equipment which goes to make up a lowtemperature liquid storage system.

Contents:

20.1 General

20.2 In-tank pgmps and their handling equipment20.2.1 In-tank pumps

20.2.2 In-tank pump removal systems20.2.3 Pumo columns

20.3 Filling columns

20.4 Base heating systems

20,5 Tank cooldown systems

20.6 Internal shut-off valves

20.7 Venting systems

20.8 Fire protectlon aystems20.8.1 Detection systems

20.8.2 Safety systems20.8.2.1 Fire water system

20.8.2.2 Foam systems20.8.2.3 Dry polvder systems

20.8.2.4 Local proteclion of vulnerabb equiprner

20.9 Instrumentation20.9.1 Level measurement20,9.2 Pressure measurement

20.9.3 Temperature measurement

20.9.4 Level/temperature/density (LTD) measurement20.9.5 Leak detection

20.9.6 Internal cameras

20.10 Civil monitoring systems

20.11 Refurences


20 Ancillary eguipment for low temperature tanks

20.1 GeneralThe ancillaryequipmentassociatedwith lowtemperalure tanksfalls into two categories.

. Equipmentwhich is generally uniqueto the lowtemperaturetiank category such as in-iank pumps, base heating sys-tems, civil monitoring systems and some items of inslru-mentation.

. Equipment which is similar to that normally provided fortanks for ambient temperature service, but in certain waysdifferent, perhaps by being required to be more accurate orto be made to higher safety standards. Certainly suchequipment is generally more expensive than the equivalentitems for ambient temperature service. Into this categorywould fall level, pressure and temperature measuring in-strumentation, venting and fire protection equipment.

20.2 In-tank pumps and their handlingequipment

20.2.1 ln-tank pumps

As has been discussed in earlier Chapters, in-tank pumps havebecome very much a standard item of equipment for low tem-perature tanks, especially for double and full containment sys-tems.

The pumps and their driving motors are fitted into pump wellswhich are immersed in the product liquid. The pump wells runverticallyfrom the bottom ofthe liquid-containing tank upwards,through the suspended deck where this is fitted and exitthrough the pressure-containing tank roof. Such a pump is illus-trated in Figure 20.1. The pump motor is cooled directly by thepassage ofthe product liquid for all of the low temperature liq-uids with the exception of ammonia. The reasons why liquidammonia is the odd one out are described in Chapter 2l .

The modern land-based in-tank pumps owe much to the shipping industry where similar pumps have been used for manyyears and from whence they were developed. These pumpsare required to lifr the product liquid out of the storage tank, ei-therto deliver it to external pumps which willimpart the requiredpressure to the product, or to impart this pressure themselveswithout the assistance of external pumps.

As would be expected, the biggest pumps are associated withLNG tanks which are in general larger in capacitythan the tanksfor other products and require greaterthroughput. lt is generallyaccepted that in order to avoid excessive demurrage charges,a large LNG tanker should be loaded at 12,000 m3/hr. Presum-ing thatthis tankeris being loadedfrom two land-based storagetanks each fitted with three in-tank pumps, then pumps with adischarge rate of some 2000 m3/hr will be required. A typicalin-tank pump assembly is shown in FigUIe 2o.2.

Pump manufacturers are currently developing pumps with unitcapacities of 3000 m3/hr. Awide range of pressuremow combi-nations are available and these are illustrated in Figures 20.3and 20.4.

Clearlythe capacityofan in-tiank pump willdepend on the situa-tion and function ofthe storage tank. For a liquid exportterminalthe requirement may be for a large capacity to speed the load-ing of shipping, but a comparatively modest delivery pressure,sufficientto liftthe liquid from the tank and deliver it through theconnecting pipework and loading arms to the ship. For an im-port terminal, the requirements in terms ofthroughput are moremodest being related to the needs ofassociated processes, butthe requirements in terms of delivery pressure may be signifl-cant. Where external pumps are notto used in the system, it isnot unusualforthe in-tank pumps to be required to raisethe liq-uid pressure to that of the final user, who may require a linepressure of 40 bar or greater.

Pumps for high throughput but lowpressures tend to be large indiameter with a single stage. High-pressure pumps with low

Operalionsl condilion

Figure 20.1 Alypical section lhrough an inlank pump

Court$y of Nikkjso Cryo Eurcpe


Figure 20.2 A complete in-tank pump assembly

Cou esy of Mkkjso Cryo Europe

-a_..

HEAD(m)

60Hz zoc{

- lure 20.4 Pump pressure/flow raie curues

:lroughput tend to have many stages and be smaller in diame-.:r

lne area of concern for ihe tank designer is the ability of the: Jmp to pump down to a low liquid level without losing suction:td the minimum liquid level at which the pump may be re-:iarted. These characteristics combine to dictate the unusable:pace at the boiiom of the storage tank, which economics re-ruires to be minimised. What was at one time expressed as a:lngle NPSH figure has in recent times become more complex.-he variables are;

. The minimum head at which the pump willcontinue to pumpat its full-rated capacity is of interest from overall processreasons.

. The minimum head ai which the pump willcontinue to pumpat reduced capacity is of interest from tank decommission-ing reasons. The minimum level of residual liquid left in ihetank after the pump has ceased to function will have to beremoved by other means and will have an influence on theovefall decommissioning period and consequent costs.

. The minimum liquid level at which the pump can be re-started will have process consequences. The pump is both

2A Ancillary equipment fot law tempercture tanks

100 200

FLOW(m3/h)

cooled and lubricated by the product liquld and restarting atlow levels may have pump wearand frequency of major ser-vjcing effects.

The unravelling of these is usually decided by discussion be-tween the owner, the pfocess engineers, the tank designersand the pump suppliers.

The electrical power supply to the pumps is usually made viacables which penetrate the pump column head plate and passdown ihe pump column to the pump itself. They are supporiedfrom ihe lifting cable. There is another less ffequently adoptedsystem which uses power cables that run inside a tubular sup-port, which is also used to lift the pump.

For service and maintenance reasons the in-tank pump ar-rangements must be such that the pump can be safely removedfrom the tank without any form of interruption to the tank opera-tion. For this reason it is usual to fit one spare pump and its as-sociated equipment. The most commonly used purnp removalsystem is described in Section 20.2.2.

To ensurethatthe pump is notdamaged bythe ingestion ofanyconstruction debris which may have been left in the tank or in

HEAD(m)

50HZ 2ooo

1500

500

5

: lure 20.3 Pump pressure/flow rate curves

r500

r000

7m

500

4C0

300

200

150

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20 Ancillary equipment for low temperature tanks

the connected pipework, it has become usual practice to installa coarse filter around the bottom of the pump column.

The pumps should not be installed prior to the tank hydrostatictest.

The European code for the design of onshore LNG installationsis EN 1473 (Reference 20.1). This document requires thatthere shall be no penetrations of the primary and secondarycontainer walls or bases of storage tanks. This requirementmakes the use of in-tank pumps mandatory for all classifica-tions of containment (i.e. single, double, full, membrane, dou-ble concrete or spherical above ground types).

20.2.2 ln-tank pump removal system

The system described here is the most commonly adoptedwithin the industry. The pump column is fifted with a head piatewhich is bolted to the top flange of the pump column. This headplate is fitted with a lifting device which can slide up and downfor a limited distance through a suitable gland arrangement.The pump body is attached to this lifting device by the pump

support cable. There is a second cable attached to the pumpwhich also runs up the pump column to the head plate and thesurplus of this longer lifting cable is stored beneath the headolate. The head olate also contains the electrical connectionswhich pass through suitable pressure tight glands to an exter-naljunction box. Atypicalhead plate arrangement is shown in

Figure 20.5.

The bottom of the oumD column is fitted with a foot valve. This isa spring-loaded valve which is held shut by the spring loadingand the product liquid pressure until it is opened by the imposi-tion of the dead weight of the pump which has been lowereddown the column onto this valve. Atypicalfoot valve is shown inFigure 20.6.

The function ofthe foot valve is to allow the pump column to be

emptied of product liquid prior to removing the in-tank pump.

Before removing the head plate, the pump weight is lifted offthefoot valve allowing it to close. This is done using the pump sup-port cable. The pump column is then pressurised with nitrogen

Figure 20.5 Atypicalpump column head plate affangement


gas which forces the liquid out of the pump column through thefoot valve. This then allows the removal ofthe head plate from anitrogen gas purged column, a much safer arrangement thanwould be the case if product liquid and vapour were present in

the column during this operation.

The head plate is lifted sufficiently to allowfor the pump weightto be suDoorted whilstthe lift cable is attached to the drum ofthepump lifting arrangement. With the head plate removed, thepump can be lifted and the power cables, which are attached tothe support cable, can be wound onto a suitable drum for safestorage. This sequence of events is illustrated in Figure 20.7.The replacement of a pump is simply a reversal of this proce-d ure.

Hoists for lifting the pumps, which can weigh as much as4000 kg, are either of the gantry U beam or pedestal type. Thelifting arrangements must clearly cover all of the pump col-umns, any other associated equipment such as cable storagedrums, and be capable of lowering pumps and other itemsdown to the local grade. lt is occasionally felt necessaryto pro-

vide a canister at roof levelforthe pump to be placed in immedi-ately following removal to permit it to warm up and safelyrelease any product vapours.

There is an alternative in-tank pump support and removal sys-tem which uses a system of connecting stainless steel tubes.The use of this system is relatively uncommon.

These operations require considerable space at the tank rooflevel and result in large platforms, an example of which isshown in Figure 20.8.

20.2.3 Pump columns

Pump columns are usually sized by the pump supplier A Iargepump column would today be around 42" in diameter' The ma-jority ofthe column length is a comfortable clearance fit aroundthe pump. The lower portion, foraboutthe heightofthe pump, isa closer fit and serves to locate the pump accurately onto thefootvalve and contains a means of preventing pump rotation. ltis usual for the tank builder to provide a dummy pump of thesame physical dimensions as the real pump. This is used todemonstrate that the pump column will allow the pump to pass

without obstruction. The column is designed to pressure vesselCodes and Standafds for the pump delivery pressure and usu-ally for full vacuum or maximum externalliquid pressure ifthis ishappensto be greater. lt is usualto blank off and carry out an in

situ pressure test of the pump column.

The most commonly adopted material for pump columns is

stainless steel. This is due to the ready availability of suitablelarge diameter tubes in this material. There is no reason why

FOOT VALVE OPEN

Figure 20.6 A typical foot valve

FOOT VALVE CLOSED

20 Ancillary equipment for low tempemture tanks

C Headpr.G 'e@r

INIIIA! UF'

Figure 20.7 The pump lifting sequence

other materials such as 9% nickel steel should not be used ifthese could be shown to be economical.

Pump columns may be supported from the base of the tank orsuspended from the tank roof. The former arrangement wouldreouire some device such as a bellows to accommodate differ-ential thermal movements between the tank and the pump col-umn and a suitably strong base insulation material to allow forthe local loadings. lf the suspended route is adopted, guidesmust be fitted to restrain the bottom of the column. For both so-lutions it is not uncommon to provide guides or restraints be-tween the column and the tank shell at higher levels, especiallywhen seismic sloshing loadings are to be allowed for. To evalu-

Wozniak unless more sophisticated methods are readily avail-aote.

It is wise to position the liquid outlet connection on the side ofthe column some way down from the head plate. This allows avapour cushion to exist betvveen the maximum liquid level andthe head plate, especially at start-up when the product liquidrapidly ascends the column. Early experiences where this con-nection was placed close to the top resulted in the liquid con-tacting the head plate and flange which promptly contractedwhilst the bolting remained at ambient temperature. This fre-quently caused a transient leakage situation on start-up whichchallenged the operators fleetness of foot.

20.3 Filling columnsThese have become a common feature, especially for thelarger tanks with high liquid inlet rates. Theirfunction is to sepa-rate the liquid and the flashing vapour as they enter the tank.They are fitted immediately beneath the main liquid bottom fillline and are suDoorted from the tank base.

The most commonly found detail for this purpose is the propri-etary design owned by MW Kellogg Ltd. A typical detail is indi-cated in Figure 20.9. These fittings can be substantial in size.The columns forthe DabholLNG tanks were 2.2 m in diameter.Such large and heavyfittings must be accommodated in termsof base insulation loading and must be located sufficiently farfrom the tank shell not to inhibit the uplifting of the tank shell inunanchored tank seismic design.

Doubts have been exDressed as to the usefulness ofthese ex-pensive fittings, however a large number of tanks have been fit-ted with these details, and there is no evidence thatthey do notperform their intended function.

20.4 Base heating systemsLow temperature tanks require base heating, and in somecases wall heating, to avoid the possibility ofthe occurrence of

Figure 20.8 Atypical roof platform for pump handlingCourtesy of Chicago Bidge & kon Conpany (CB & l)

ate seismlc sloshing loadings, it is usual to use the work of


Figure 20.9 Deiails of a lypical bottom llllconnectionCaunesy of M W Kellagg Ltd

a phenomenon known as frost heave. This is wherethe zero'Cisotherm can penetrate the subsoil and cause the formation ofan ice lens, most usually beneath the tank base. This ice lenscontinues to grow and will eventually lift the tank bottom giving

rise to damage and possible leakage. Different types oi subsoilare more susceptible than others to the formation of ice lensesand the location ofthe localwatertable is clearly important Anearly paper on this problem is given as Reference 20.2.

Tanks with elevated bases do not require base heating Aboveground tanks, with bases located directly on the subsoil, are al-most alwaysfitted with base heating systems. There are a fewexamples ofground-based tanks for the warmer products suchas normaland iso butane where no base heating is provided. In

ground and earth-bunded tanks will also require heating of thetank walls.

The most common heating systems are electrical These usecables running in conduits such that any individual heater unitcan be removed and replaced. This is simple for the aboveground tanks, but requires more care in the detailed designphase to ensure that a heater unit traversing both tank wallsand base is readily removable and replaceable.

The less commonly adopted system uses warm water or brinecirculating through pipework.

BS 7777:Part 3 gives some guidance relating to base heating

systems. In brief these are:

The temperature of the supporting soil or concrete duringnormal operation should not be less than 4'C.

When the soil is sensitive to drying and shrinkage, a maximum temoerature should be defined.

The heating system should be designed to minimise exces-sive temperatures, which could lead to high boil-off rates.There is clearly little merit in purchasing expensive energyto boiloff more expensive productthan is absolutely neces-sary and then purchase more energy to reliquefy it.

The arrangement should take account of differences in thethermal characteristics of the base insulation. For instancethe difierence between the central area ofthe base and thepeflpnery

The system should allowfor the replacement of any heatingelement or temoerature sensor

The design ofthe system should be such asto allowthe reg-ular monitoring of its performance.

. The thermal design ofthe system should be based on theheat leak calculations. Two sets of boundary conditionsshould be considered, the design temperature of the prod-

uct and a soiltemperature at a depth belowthe tank. Forthislatter flg u re a value of 7'C at a depth of20m is suggested.

. A factor of safety of 1.3 is suggested for the heat input.

. The heating circuit output should be contlolled bytempera-ture sensors. These should be strategically located throughthe system.

. All temperature controllers should have spare sensorsavailable and permanently installed to allow for rapidchangeover in the event of sensor failure.

. Electrical heating should consist of a number of independ-ent parallel circuits designed such thatthefailure ofanyonecircuit does not decrease the power supply to the remainingcircuits.

. Heaters should be located such that the deficiency causedby the failure of any one heating circuit is spread evenlythroughout the heated area.

. Electrical heating should be designed sothat in the event ofthe failure of a main supply cable or a power transformef:

- Sufficient time is available to repair the equipment beforedamage occurs due to excessive cooling.

- Provision is made for connecting to a stand by heatingpower source.

The general flow diagram for a brine-based heating system is

shown in Figure 20.10.

For LNG tanks, EN 1473 requires 100% redundancy to be pro-

vided in the design of base heating systems.

ttHealing coils intank base slab

Brine slorage

Figure 20.10 A flow diagtam lor a brine-based heating syslem


SECIION A-A VAPO1IR OIJTLFI

,t')a -'t...\/r/..,.)..

20.5 Tank cool-down arrangementsThe cooling down of lowtemperature tanks is an important partof the commissioning procedure. The rate of cooling and thepermitted differential temperatures between various parts ofthe tank structure should be determined in advance of the cooldown operation. lt is usualto cool the tanks with the product liq-uid to be stored. The one exception to this general rule is thecase of a number of recently-constructed LNG tanks whichhave been cooled with liquid nitrogen rather than with LNG

The means of cooling is to spfay the cooling liquid into the tankthrough a roof-mounted connection located close to the centreline of the tank. Clearly for tanks with suspended decks, thespray connection should be beneath the deck. The spray ar-rangements should create finely dispersed liquid dropletswhich will result in a symmetrical convection loop which willhopefully cool the tank in a suitably uniform fashion. lt is impor-tant that the arrangement chosen produces a fine spray of liq-uid droplets over the full range of cool-down liquid supply flowrates and pressures. A device which simply rains liquid downonto the tank bottom immediately beneath the spray locationwhen the supply of cool-down liquid is throttled back is of nouse. A number of different arrangements are in common use:

Figure 20.11 A typical propr etary spray connecUon

. Asingle spray nozzle. The nozzle is a proprietary design se-lected to provide the required dispersion over the full rangeof anticipated supply conditions. A typical spray nozzle isshown in Figure 20.11. This arrangement is prone to block-age and should be used in conjunction with a suitable filter.

. An internal ring pipe with a series on drilled holes. Unlesscarefully designed these can turn into watering cans whichis not good.

. An internalring with multiple proprietaryspray connections.

. A removable spray head. This arrangement allows for thesingle spray nozzleto be removed from the tank without anydecommissioning. The spray nozzle can be cleaned or re-placed if the original is not found to be performing correctly.A sketch of this arrangement is shown as Figure 20.12.

Having decided on the intended cool-down rate (usually be-tween 1'C and s'C/hour) it is possible to calculate the roughrange of liquid flow rates required. Precise control of the flowusing a suitable valveand flow measuring device is necessary

A number of temperature measuring points are located withinthe tank - around a dozen distributed over the tank floor andwall with one located immediately below the spray arrange-ment. An assessment of what constitutes an acceotable tem-perature difference between adjacent thermocouples and whatthe maximum temperature difference between any twothermocouples should be made prior to commencement ofcool-down. The coofdown operation follows tank purging sothat the tank will be full of nitrogen gas. Spraying a cold liquidinto a nitrogen atmosphere can produce sub-cooling belowthetemperature of the incoming liquid. Those carrying out thecooldown should be aware of this Dossibilitv and take the

Figurc 20.12 A removable spray nozzle arrangement

necessary steps to ensure that damage due to overcoolingdoes not occur.

In addition to the primary function of allowing a controlled tankcool-down, the cool-down system may be used during iank op-eration as a means of controlling tank pressure.

20.6 Internal shut-off valvesThe use of internal shut-off valves is uncommon for low temper-ature tanks with the exception of those storing liquid oxygen, ni-trogen orargon. BS 7777 : Pat14, which is specifically applica-ble to tanks storing these products, makes lheir use mandatoryfor all shell and bottom conneciions of 50 mm or laroer in diam-eter.

Figure 20.13 A typical internal shuloff valve and iis ope€ior



20 Ancittary equipmenl [ot low temperature lanks

The most commonly found valves of this type are simple flap

valves which are held open during service by cables running to

tank roof connections. In the event of a mishap being detected

by the instrumentation provided for that purpose, operators lo-

cated on the tank roof release the cables and shut the valves lt

is normal to reset (i.e. reopen) these valves manually from thetank roof. Atypical valve and operator is shown in Figure 20 13.

20.7 Venting systemsAs is the case for ambienttanks, the function ofthe venting sys-

tem is to protect the tankfrom excessive levels of internal posF

tive pressure and negative pressure (i e. vacuum).

The design and description of venting systems for ambient

tanks has been described earlier in Chapter 8. The design and

types of equipment used for low temperature storage systems

is essentiallythe same with a few differences to allowfor the low

temperature and higheroperating pressures commonly assocF

ated with low temperature storage and in some cases to cater

for the very high throughputs required.

For the larger, more sophisticated tanks, it is usual to provide

two pressure relief systems. The first of these is connected to a

system of pipes which will deliver the relieved vapours to a re-

mote location, usually a flare stack. where the vapours may be

safely disposed of. The second system relieves directly to at-

mosphere. lt is usualto set the relieving pressure ofthe system

to flare at a lower setting than the relieving system to atmo-

sDhere. This ensures that the safer option istriedfirst and onlyifthis fails or is of insufficient capacity for the particular process

circumstance causing the upset, does the less desirable option

of relieving direct to atmosphere become operational

A typical series of pressure and vacuum settings for a large

LNG tank are shown in Figure 20.14.

The basis of the design of pressure and vacuum relief valve

systems is API 2O0O (Reference 20.3). Fot low temperature

tjnks, the section on refrigerated tanks is to be applied This

document is, unusually, also quoted in BS 7777 : Part 1 lt is

quite unusual for American Standards to be referenced in Brit-

ish Standards.

The design of product vapour (for pressure relief) and air (for

vacuum relief)flow capacities are made up ofthe most onerous

combination of the following:

a) Vapour displaced during filling at maximum rate

b) Maximum rate of withdrawal of liquid product

c) Possible escape of product under emergency conditions

d) Suction capacity of comPressor

e) Heat leakage to the tank from atmosphere

f) Barometric pressure variation

g) Fire exposure

h) Any other special circumstances (e.g roll over)

The tankdesign Codes have contributionsto make in thisarea

API 620 requires that:

. Automatic pressure relieving devices shall be fitted to pre-

ventthe pressure atthe top ofthe tankfrom rising more than

1O7o above the maximum allowable working pressure (usu-

ally taken as the tank design pressure) except as provided

DCIOw'

. Where an additional hazard can be created bythe exposure

of the tank to accidental fire or another unexpected source

of heat external to the tank, supplementary pressure rellev-

ing devices shall be installed. These devices shall be capa-

bl; of preventing the pressure from rising more than 20%

above the maximum allowable working pressure. A singlepressure relieving device may be used if it satisfies the re-quirements of this Paragraph

Note: lt is normal practice to test low temperature tanks to25% overthe design pressure The 20% maximum ex-ternal fire induced excursion is then less than the pres-

sure to which the structure has been tested.

. Vacuum relieving devices shallbe installed to permitthe en-

try of air (or another gas or vapour if so designed) to avoid

the collapse ofthe tank wall ifthis could occur under natural

operating conditions. These devices shall be located on the

tank so that they will never be sealed off by the tank con-

tents. Their size and pressure (or vacuum) setting shall be

such that the partial vacuum developed in the tank at the

maximum specified rate of air (or gas) inflowwill not exceed

the partial vacuum for which the tank is designed.

. The system shall be designed in accordance with the re-

quirements of API 2000.

Pressure relieving devices shall be installed such that they

are readily accessible for inspection and removable for re-

pairs. The practices suggested in API RP 520 (Reference

20.4) shall generally aPPlY.

lf the relieving devices are not located on the tank roof (a

most unusual situation), they shall be installed on the piping

connected to the vapour space as close to the tank as ispracticable. lf the relieving devices are vented to atmo-

sphere, they shall be at a sufflcient heightto prevent chance

ignition.

The opening from the tank leading to the relieving device

shall have a diameter at least equalto the inlet size ofthe re-

lieving device.

When a discharge pipe is fitted to the outlet side of the re-

lieving device, itshallbeofan area at leastequaltothe area

ofthtoutlet ofthe relieving device, or if a single plpe is pro-

vided for the discharge of several relieving devices, it shall

have an area not less than the aggregate areas ofthe multi-

ple valves. The discharge pipe shall have an open drain to

orevent wateror other liquids from accumulation on the dis-

charge side of the relieving devices.

Discharge pipes shall be installed such that no undue stress

is placed on the valve body. open discharges shall be

placed and orientated such thatthe oumowis directed away

irom thetankand willnot cause a hazard to walkways, stair-

ways or operating Platforms.

MRP, RVs oischa.gi.q to al,nospne6 319

sP, RYs Oischarging lo Almosphere 290

MRP, Rvs D6chrrglng lo FlaE 245

sP, RVs D schaE ng Io Fla€

255

Maxidum Nodal OPe6ling P(essuE

Minimum Nofral Op€€ling Pessure 100

30

15

0

vaclum R€lEf FlllFloe

laEcGlossaryi MRP =

RV=PABH =PAH .-PA! =

Manmum Relieving Pessuc

HEh Pressure Tdp {PressuE Alam High HEh)High Pressure Peal.mLN Pessure PE-alam

Figure 20.14 Atypical LNG tank pressurc setting tabulation

courlesy of l\4 W Keilogg Ltd


. A vacuum relieving device shall have as direct an inflow tothe tankas is possible, and shallhave no pipeworkahead ofthe inlet apart from a weather cover.

. Stop valves, if used between the relieving devices and thetank to allow seryicing of these devices, shall be locked orsealed open, and an authorised person shall be present ifthis situation is changed. lf the tank is in use, the authorisedperson shall remain there until the locked or sealed openstate of the relieving devices is restored.

It is interesting that API 620 does not make mandatory the pro-vision ofa spare valve in any set and the use of stop or isolationVAIVES.

BS 7777 : Part 1 make similar demands. but with some minordifferences:

. For "normal" pressure relief (i.e. as determined from a)to e)listed above), the internal pressure at the top of the tankshall not exceed the design internal pressure by more than10v..

. For vacuum relief, the internal negative pressure shall notexceed the internal negative design pressure by more than2.5 mbar.

. For "emergency" pressure relief (i.e. as determined from f)and g) listed above), supplementary pressure relief shouldbe installed. These pressure relieving devices should notallow the tank internal pressure from exceeding the designinternal pressure by more than 20%.

. Where a single valve will satisfy the emergency venting re-quirement, a duplicate valve should be installed to facilitiateinspection and maintenance.

. Where multiple pressure relief valves are required for theventing duty, they should allbe ofthe same capacityand atleast one additional valve should be fitted of the same ca-pacity as a standby for inspection, maintenance and possi-ble replacement purposes.

. The use of isolating valves installed between the pressurerelief valve and the tank should be agreed between the pur-chaser and the contractor, i.e. is optional. Appendix C ofthispart of the Code suggests rather more strongly that thisshould be the case and normal industry practice usually de-mands this facilityfor all pressure and vacuum reliefvalves.

. Appendix C repeats some of these requirements and pro-vides additional guidance on this subject.

The newprovisional Euronormfor low temperature tanks, prEN14620, is strangely reticent on the subject of pressure and vac-uum relief bearing in mind the amount of detail that its ambienttank counterpart (prEN 14015) has chosen to go into. Annex Lof this document provides comprehensive requirements forthedesign and installation ofventing systems, which for once is notbased upon API 2000.

Summarising the requiremenb of prEN 14620:

20 Ancillary equipment for low temperaturc tanks

Normally the pressure and vacuum relief valves are sepa-rated from each other However a combination may beused. This is taken to mean that valves with a combinedpressure and vacuum relieffacility may be used. There aresuch valves on the market and the minimising of the roofconnections and isolating facilities makes this an attractivechoice. However, care must be taken to ensure that full sys-tem capacity is maintained at all times and the distribution ofvapours beneath the tank roof is carefully thought out.

For a full containment tank, the pressure relief system shallbe designed such that it can accommodate the vapoursgenerated from an innertankfailure. For this purpose a holein the flrst course ofthe tank shell of 20mm in diameter maybe assumed. The origin ofthis notional leak is a mystery notleast to the authorwho served on the committee which cre-aied these rules! Clearly a concentration failure!

The number of pressure relief valves required shall be cal-culated based on the total product vapour outflow specified.ln addition, one spare valve shall be installed for mainte-nance purposes. The inlet piping shall penetrate the sus-pended roof, where such a roof is fitted, thus preventingcold vapour from entering the warm space between theouter roof and the suspended roof under relieving condi-tions.

The number of vacuum relief valves shall be calculatedbased on the total air intlow specified. In addition one sparevalve shall befitted for maintenance purposes. The vacuumvalves shall allow air to enter the vapour space located di-rectly under the roof.

Because the design pressures of low temperature tanks aregenerally higher than is the case for ambient tanks, the use ofdead weight valves for pressure relief is unusual, and in thelarger valve sizes, becomes impractical because of the physi-cal size of the weight required. lt is normal practice to use pilotoperated valves which have advantages of not leaking at pres-sures close to their set point, having a reduced blow down andbeing more suited to operation with pipe-work on the inlet andouilet sides. This is discussed in some detail in ADDendix C ofB57777 : Pat11. Atypical pilofoperated pressure reliefvalve isshown in Fioure 20.15.

Venting to atmosphere shall be excluded from tanks de-signed to contain toxic product.

For tanks designed to store non-toxic products, sufficientmargin shall be provided between the operating pressure(i.e. the reliefvalve set pressure) and the design pressure ofthe tank to avoid unnecessary (i.e. too frequent) venting.

The relief valve capacity (pressure and vacuum) shall bedesigned based on normal operation and abnormalopera-tion scenarios. Failures at interconnecting facilities, i.e. pro-cess plants, vent or flare systems etc., shall also beconsidered. Note however that the responsibility for speci-fying the flow rates forthe relief valve system is given byAn-nex C, Part 1 of this document to the tank Durchaser.

l3 I r+ I'T/-_l -T --lf,T*;r I FT]"r Ill-ul ll'.:>|

"....:r+,..,.':FFigure 20.15 Atypical pilot-operated pressure retief valveFrom BS 7777 : Patl 1, figurc C.3


20 Ancillary equipmentfor low temperaturc tanks

e!'

2x3t'x4' ft4x6' wo

6xaAxlO' !,10\17 t,l2'x 16 .q

A{nedyGs.e!5.Bodyxg|b.|qb.

NOIE vImN isan eunpl€ ol a suluble produd a\aihnF.onner.rxlvTlis infomauv. s sven rof ihe eneeni€hce ot LsE or rhis Pan of BS 77?7

Figure 20.16 A typical dead-weighi vacuum relief valve

From BS 7777 :Paft 1, figure c.4

Vacuum relief valves are commonly of the dead-weight type

and an example is shown in Figure 20 16 Proprietary designs

for pilot-operated vacuum valves and valves which combine

the pressure and vacuum relieving elements within the same

valve are available. These provide the possibility of reducing

the number of roof connections and consequent cost savings'

For tanks of the suspended deck type using the conventional

system of separate valves for pressure and vacuum relief, it is

usualto locate the pressure inlet to the valves beneath the sus-

pended deck and arrange for the vacuum valves to discharge

directly into the roof space above the deck The use of com-

binedvalves clearly does not permitthis arrangementand deci-

sions have to be made aboutthe configuration beneath the tank

roof. Adjustments to the suspended deck venting system may

be fequired if these combined relief valves are used

It is usualto mount pressure reliefvalves, vacuum reliefvalvesor combined pressure and vacuum valves on individual rooffittings. The fittings for pressure and combined valves in non low

temperature roofs willrequire heat-breaks to be fitted Vacuum

valves, drawing in air at atmospheric temperature will have no

need of heat-breaks. Similarly pressure and combined valves

will require to be manufactured from materials with suitable low

temperature properties whilst vacuum valves have no such re-

quirement. There are occasions where two valves or more are

mounted on a single roof penetration

For pressure relief valves, the higher tank pressures for low

temperature tanks means higher flow velocities and conse-

quently larger pressure losses in the connected pipework on

both the inlet and outlet sides of the valve. These losses must

be calculated and subtracted from the total pressure difference

between the tank and atmosphere to give the pressure drop

across the valve itself which is used to calculate the flow rate

through the valve. Some relief valve manufacturers provide

software to assist the tank designer with this task

Reliefvalves are available in a range of sizes depending on the

vapour throughput required. An example of a range of pilolop-

erated pressure reliefvalves is shown in Figure 20.17'

The orecise evaluation of the vapourflow rates resulting from a

tank rollover event is difficult to calculate. In recent times the


Figure 20.17 An example of a range of pilofoperated pressure relief valves

Coulesy of Tyca Valves & Conttols

need to design for a rollover has been largely removed by the

orovision of level/temperature/density (LTD) measuring equip-

ment together with provisions for top filling, bottom filling and

product mixing via the in-tank pumps. These provisions gave

ihe tank ooerator an indication ofsituations within the tank con-

tents which were prone to initiate a rollover and the means to

take the appropriate preventative action.

ln earlier times the rollover had to be considered, and one com-

pany designed and manufactured a large capacity relief-valve

ior this puipose. tnis valve was made in large sizes (24" and

36" diameter) and had correspondingly large throughput of

vapour. The basis of operation was a frangible link within the

vaive which held the valve closed in normal service and would

fail at a predetermined over pressure to allow the valve to fully

open. The valvewasfitted with a spring which closed the valve'

albeit at a much lower pressure than the opening pressure'

These valves were also used when sudden inner tank failure

was a design requirement. An example ofsuch a valve, known

as the reserve capacity relief valve, is given in Figure 20 18

For LNG tanks, EN 1473 suggests the use of rupture discs for

rollover relief and references the French AGT Code forthe ba-

sis oftheirdesign. The use ofrupture discs has not been a pop-

ular choice for this purpose, possibly due to the proportionate

uncertainties in the actual failure pressures of bursting discs at

the comparatively modest pressures associated with low tem-

perature storage tanl(s.

Where pressure relief valves are fitted in a system where multi-

ple relief valves are manifolded into to a system of closed

oipe-work (i.e. to a flare) it is necessary to provide isolation

valves on both the inletand outletside ofthe relief valveto allow

for valve removal from the system

Fire orotection for relief valve tail pipes is covered in Section

20.8.

20.8 Fire protection systemsFire protection systems for above ground, low temperature

tanks are generally a part of the overall facility fire protection

svstem. The development of such a system, sultable for ad-

diessing all ofthe equipment on the site and all of the perceived

ohn.ioN,l.che.lmnl\6lE C D

lbllsl

:igure 20.18 A reserue capacity relief valveaauttesy of Anderson creenwood

'rsks is a majortask, which is best lefl to companies and jndivid-Jals who possess the necessary expertise. Codes such asNFPA58, NFPA59, NFPA59Aand EN 1473 give some generalguidance on the subject, but this falls a long way short ofwhatthe system designer will require. The design of a facility's fireprotection system is intimately connected to the riskand hazardassessment process.

What is included in this Section is a brief look at some of the as-pects of fire protection systems as they relate to the storagetanks themselves.

20.8.1 Detection systems

EN 1473 (which it should be remembered is applicable onlytoLNG facilities) lists the various detection systems as follows:

. Gas detection

. Cold detection

. Smoke detection

. Fire detection

It is clearly an essential part ofthe protection system to have agood detection system which will trigger the appropriate re-sponse from the fire fighting equipment provided.

20.8.2 Safety systems

These are the various items of equipment provided to addressproblems arisjng from gas leakage, cold liquid leakage, poolfires and adjacent tank or plant fires.

Fire protection systems are eitherclassified as being active (i.e.water spray systems, deluge or sprinkler systems) or passive(i.e. fireprooflng coatings or shielding systems).

20.8.2.1 Fire water systems

The application of water to items of plant and equipment hasmore to dowith keeping them cool, when theyare the subjectofheat radiation from fires in adjacent areas, than its contributionto the efforts to extinguish fires. Indeed, in certain circum-stances, the application ofwaterto liquid poolflres may aggra-

20 Ancillary equipment for low tempercturc tanks

vate the situation, increasing the vapour formation rate whichmay increase the burning rate.

The fire water system for the storage tanks is fed from the sitefire watersystem. This may be based on the use of fresh water,sea water or a combination of both. A large diameter fire mainruns around the site passing the various items of equipment,adjacent to which suitably sized off-takes are situated. lt is im-portant that this fire main is itself suitably protected from fire.This is usually accomplished by burying it. Thefire main is in itsstandby state filled with fresh water which is maintained at thedesired operating pressure (commonly around 9 bars) byjockey pumps. Two fire water pumps must be provided with in-dependent powersources, such that if one system becomes in-operative, the second willfulfil the full capacity required.

Tanks with steel outer shells and roofs are clearly more suscep-tible to damage from adjacent tank fires than those with con-crete outer shells and roofs. Not withstanding this fact, it is notunusualfor cjrcumstances to conspire to require the use ofde-luge systems for concrete tanks.

The tank deluge system used to be a simple matter of a waterpipe to the crown of the tank roof, which delivered the desiredquantity of water to a distribution crown arrangement. Thiscaused the fire water to flow to the periphery of the roof andthence down the iank shell to local grade. The coverage of theroof and shell provided by this arrangement was extremely ap-proximate and was adversely affected by roof mounted ob-structions and wind. Where steel tanks had external stiffeningand outstanding compression areas, rings of deflector plateswere needed to ensure that the fire water ran down the tankshelland did notmerely run clear ofthe shell from the stiffeners.

lvlore recently, where tankdeluging is a requirement, the Codesand facility owners require that a system ofdirect impingementoffire water on both the tank roof and shell be used to ensure abetter distribution of the fire water This requires a series of fifewater pipes running over the tank roof and shell to distribute thewater to suitable spray nozzles. One such arrangement is indi-cated for an LNG tank on Figure 20.'19. The water flow rates fora tank deluge system arise either from detailed calculations orfrom flre protection Codes. A commonly adopted figure is 10.2litres/min/m2 which is derived from NFPA 1 5 (Reference 20.S).

It is common to arrange the layout ofthe deluge system to suitthe local situation. For example, where two storage tanks arelocated adjacent to one another, the adjacent tank fire risk foreach tank is associated with only one half of the shell and partof, or perhaps all of the roof. For this reason it may be thoughtwise or economic to concentrate the fire protection on the ex-posed parts of the tanks, or to arrange a zoning system whichwill allow different parts ofthe target tanks to be selectively pro-tected.

Fjgure 20.19 An LNG tank rcof water drench system

Fdl trirt @q Fl3| l4| trsl

lsl tsiol tE4 tr164 t4t wl


20 Ancillary equipmentfor low temperctute tanKs

lf insufficient fresh wateris availableon the site, itwillbe neces-

saryto feed the fire water system with sea water in the event ofan emergency. In the interests of corrosion protection following

exposure of all or part of the system to salt water, it will be nec-

essary to arrange suitable facilities for flushing the appropriateparts of system with fresh water.

It is usual to require the system to be tested on at regular inter-

vals. This is normallycarried outwith fresh waterand limitations

in the available quantities of fresh water may necessitate the

subdivision of the system such that it can be tested in smallersections. The system illustrated. which is for roof deluging only.

is for this reason divided into six sections.

The equipment used to set the deluge system into action ls of-

ten large, sophisticated and expensive Roof-mounted deluge

valves will themselves require a high level of fire protection

In addition to the deluge system, it is common to add a number

of fire monitors. A monitor is a means of providing a spray or

stream ofwaterfrom a fixed station to a locaiion where it is re-

quired for fire fighting or equipment cooling. The control of the

waterspray and its direction can be achieved by either manual

or remote operation.

20.8.2.2 Foam systems

For areas where liquid may accumulate such as tank bunds or

spillage impounding basins, it is usualto installa system of high

expansion foam generators These will allow remotely-con-

troiled blanketing ofthe spilled liquid which will either douse the

fire or reduce the flame size and consequently the radiation

rate. Suitable systems, designed and tailored for the specific

circumstances, are supplied by fire protection companies who

specialise in this type of work. Asystem would consist ofthe fol-

lowing elements:

. High expansion foam generators

. Stoo valves

. Foam concentrate storage tanks

. Foam inductors

It is usual to test the system at least once per yeat

20.8.2.3 Dry powder systems

Fires in pressure relief valve tailpipes are not unknown Forthis

reason it is common practice to fit a dry powder extinguishing

system. This will inject into the relief valve tailpipes a mixtureof

carbon dioxide and fire extinguishing powder in the event of a

tailpipe fire. The system is fitted localto the reliefvalves on the

tank ioof and should be capable of local or remote operation lt

is usualto allowsufficient storage of powderand propellantgas

to allow for two attempts to extinguish the fire. These systems

are the product of specialist companies and are often supplied

skid mounted.

20.8.2.4 Local protection of vulnerable equipment

It is imoortant that certain equipment associated with low tem-

peratuie storage tanks continue to perform their intended func-

iions when the tank is exposed to heat radiation, perhaps aris-

ing from an adjacent tankfire. ltems which fall into this category

arL relief valves, deluge valves and certain parts of the struc-

tural steel supporting critical equipment.

These are roof-mounted and may require specific fire protec-

tion. This is usually passive fire protection and can take the

form of proprietary intumescent paints, cementious coatings orpurpose designed shielding.

The principal value of this fireproofing is realised during the

earlystages of a fire when efforts are mainly directed at setting

in motion the various fire suppression equipment and prevent-

ing exacerbation by way of the addition of further fuel to the

evlnt. lf the fire is intense and prolonged' then passive fire-

proofing may prove ineffectual in preventing damage


20.9 lnstrumentationThe level of instrumentation which is supplied with a low tem-

perature storage tank is usually specifled by the owner or his

engineer in the tank specification documentation The tank de-

sign Codes such as API 620, BS 7777 and prEN 14620 give lit-

tl; or no guidance as to the extent of instrumentation which

would be appropriate to supply with a low temperature tank to

ensure its correct and safe operation

Codes and regulatory documents which have a wider scope

dealing with the overall storage and handling facility such as

NFPA 58, NFPA 59, NFPA 59A and EN 1473 are more forth-

coming on the subject. The latter Code outlines what is now a

pretty huch agreed industry standard level of instrumentation

for LNG tanks.

20.9.1 Level measurement

It is clearly necessaryto measure the levelofthe product liquid

within theiankfor reasons such as inventory controland avoid-

ance of overfilling. The normal form of level measuring equip-

ment used for these tanks consists of a roof-mounted servo

gauging unit connected via a suitable roof connection to a float

iitnin iperforateo stilling well inside the tank itself. Atypicalar-rangement of this type is illustrated in Figure 20.20. The level

gauging unit will be located where it can be readily accessed

f-ronithe roof-mounted platform and may have a roofreading fa-

cility.

It is usual to have a remote transmitter associated with the

oauoinq unit (proprietary equipment on the market often has

6uiliin iransmission equipment within the gauging unit) to send

the various level signals to the terminal control room lt is usual

to provide each tank with two separate level measuring sys-

tems, indeed EN 1473 insists upon this (with the exception ofpeak shaving tanks where the level changes are slow and pre-

dictable). The level gauging equipment is almost always fitted

with a set of levelalarms. These would normally be low low level(LLA), low level (LL), high level (HL) and high high level (HHL)

It is also normal to install another instrument which is specifi-

cally to detect the HHL only The setting up of this system

Figure 20.20 A typicallow lemperalure iank level gauge serup

should be such that a safety shut-down of the pumping equip-ment(often the delivering ship's pumps) is triggered bythe vari-ous high level alarms.

lfthe tank level measuring equipment is to be used to measurethe exact capacity ofthe tank, or the amount of liquid product in-troduced to or abstracted from the tank for commercial or cus-toms purposes, it is important that the equipment is as accurateas Dossible and that the tank has been calibrated. Calibration isthe precise measuring of the finished or as built primary liquidcontaining element of the tank. When the measurements havebeen made, and appropriate corrections have been made forthermal contraction and mechanicalexDansion. a set ofcalibra-tion tables are produced which relate the measured liqujd levelto the liquid capacity. This activity is the preserve of specialistcompanies who carryout this service. lt used to be the case thatihe various measurements were made by mechanical strap-ping of the structure, but nowadays there are clever electronicsurveying instruments which can gather the necessary infor-mation from a single site within the tank.

20.9.2 Pressure measurement

It is clearly important to know what the pressure ofthe productvapour is within the tank. Too high a pressure would give rise tounnecessary and expensive venting, and in the extreme, en-dangerthe tank structure, and too low a pressure would createa vacuum and cause air to be introduced into the tank, which isa minor disaster from an operating point of view

It is usualto provide equipment to monitorthe following param-eters:

. Absolute tank pressure

. Gauge tank pressure

. Tank pressure alarms (high and low) and local pressure in-dication

As with level measurement, it is usualfor provisions for the con-tinuous pressure data to be transmitted to the control room tobe incorporated into the system together with the triggering ofsuitable safety related activities (i.e. stopping the boil-off com-pressor when low pressures are detected).

It is usualto mount pressure measuring equipment on tank roofpenetrations.

For double-walled tanks where the inner tank has a fixed roof, itis particularly important to measure and control the pressure inthe interspace around the inner tank to ensure that no exces-sive external loads are applied to the inner bnk shell or roof.

20.9.3 Temperature measurement

There are three areas where temperature measurement is im-portant. These are:

. During tankcooldownto controlthe operation and avoid ex-cessive temperature gradients

. During normal operation

. To monitor the base heating system

The normal temperature measuring elements are three wireplatinum resistance temperature detectors (RTD). The numberrequired for the various functions are either for the owner tospecify, or to be based on the tank contractor's experience. Itshould be remembered that the temDerature elements cannotbe replaced with the tank in service. Any redundancy requiredmust be in the form of additional RTDS.

It is usual to arrange for a special fitting on the tank roof withsuitable glands to allowthe RTDS to penetrate the pressure en-velope of the iank.


20.9.4 Level temperature density (LTD) measure-ment

As has been mentioned elsewhere, the need to have knowl-edge ofthe temperature and densitydistribution within the bodyofthe product liquid is important to avoid situations which couldresultin a rolloverevent, with the possible vapour release whichmay be difficult or impossible to accommodate within the nor-mal tank operating parameters. For this reason, and especiallyon tanks containing mixed products like LPG and LNG, the LTDinstrument has been developed. EN 1473 makes the supply ofthese mandatory for LNG tanks.

The instrument is in essence a very sophisticated servo drivenlevel gauge. Like the levelgauges it is roof-mounted and has asensor head which can track up and down inside the tank withina stilling well. lt can measure the product temperature and den-sity at each location and convey this information to the controlroom where suitable software will produce a level/tempera-ture/density plotfor the tank contents. Armed with this informa-tion and perhaps detailed knowledge of incoming shipments,the experienced tank operator should be able to spot potentialroll over situations and take the appropriate actions. These ac-tions could be to top fill, bottom fillor stir up the tank contents byrunning the in-tiank pumps on recycle or by other means.

The relationship between the many variables associated withroll over and the various remedial actions are ouite soDhisti-cated and not alloperators are conversantwith what representsa dangerous situation and what actions should be taken. Gazde France has developed some proprietary software into whichthe LTD parameterscan befed and which willprovide advice onthe appropriate acfions.

Both the measuring instruments and the operator's softwareare expensrve.

20.9.5 Leak detection

The provision of leak detection systems is usually confined todouble-walled tanks. The function ofthe leak detection systemisto identifyant leakagefrom the innertank. This could befromthe innertankshell, bottom or (foropen-topped innertanks)theresult of overfilling or seismic sloshing.

The most commonly adopted system consists of a number ofRTDS installed on the inside of the outer tank at the bottom ofthe intersoace and at various levels close to the bottom of theinterspace. These record a temperature change in the event ofa liquid leak which will be relayed to the control room and raisethe alarm. lt is normal to install a number of RTDS at each level,say four equally spaced around the tank.

Another system involves the installation of an optic fibre withinthe lower interspace to detect leakage.

20.9.6 lnternal cameras

These are thought to be an expensive luxury by manywithin theindustry They are normally only to be found in large LNG tanks.The camera can either be located within the tank and subjecttothe low temperature, or be external with suitable optics to beable to view within the tank. In addition to the camera, lightingand camera manipulation and focussing systems will be re-quired.

Some tanks have been fitted with cameras. Films taken withinLNG tanks show that the cameras have remarkable resolutionand can identify small marks on the innertank bottoms. LNG isa very clear liquid. Quite what the realfunction ofsuch a systemis, and whether it justifies the price tag (some $'1.00 million) isan interesting question. Technically it is an impressive feat.


20 Ancillaty equipment for low temperature tanks

20.10 Givil monitoring systemsIt is not uncommon, especially in the case of large LNG andLPG tanks, for instrumentation to be required to monitor theperformance of the tank foundauon-

For tanks which sit on ground-based slabs or slabs which areelevaled above local grade, the measurementof the global set-tlementand the tilt at the peripheryof the slabs are quite simple.Allthatis requjred isthe esiablishingofa site datum and a num-ber of equally spaced points around the base slab plus somebasic surveying skills. The behaviour ofthe slab away from itsedge, particularly for ground-based slabs, is more difiicult. Thecasting into the base slab of two inclinometer tubes, set at rightangles to one another, and the use ofsuitable inclinometers willprovide the dab required on anychanges ofshape. lt is usualtotake readings after construction, during and afier the hydro-static test and at intervals during service.

The LNG tanks on Revithoussa lsland in Greece, were built inan area of high seismicity. The elevated base slabs were fittedwith a seismic isolation system. To monitorthe performance ofthese isolators, a number of accelerometers were fifted to theiank, some below the level of the isolators and some above.Under normal operating conditions these accelerometerswould not be remrding, but in the event of a seismic eventabove a certain th reshold, the system was triggered to record at

its full speed- Thus following an earthquake, there should besuffcient realdata to pmvide adetailed description of the eventitselfand ofthe modification ofthis event caused by the seismicisolation system. This will allowthe tank and the isolator design-ers to check the anticipated behaviouragainstthe recorded be-haviour. At least one seismic event of sufiicient magnitude totrigger the system has occurred. No doubt when the data be-comes public, learned papers will appear and theories will bemnfirmed or will have to be revised.

20.11 References2O.1 EN1473:1997, lnstallation & eqaipment for liquefied

natural gas, Design of onshore installafions, CEN

20.2 Problems in connection with the foundation of tankscontaining a lower temperature media, E. Zellere\Linde AG, Munchen, Paper ftom LNG 1.

20.3 Venting atmosphertc and low pressure storage tanks,non-refigerated and refrigerated,'fhe American pefo-leum Institute, API 2000, Fifth edition, April 1998.

2o.4 Sizing, selection and installation of pressure relievingdevices in refineies, API RP 520, Part 1 , sizing & selec-tion, The American Petroleum Institute.

20.5 NFPA 15 : watet spray, fixed system


21 Ammonia storage - a special case

Ammonia is a much used chemical in a number of industries and as such it is in need ofsystemsfor its safe and economic bulk storage.

Ammonia is different from the other liquefied gases discussed in chapter 17

These differences in turn give rise to a variety of challenges in the search for suitable storagesystems. In particular the toxicity ofthe gas, the ability ofthe liquid to conduct electricity and thesusceptibility of carbon steels in contact with either the gaseous or the liquid phase to stresscorrosion cracking, give rise to problems. The problems related to toxicity speak forthemselves.The truly lethal nature of the gas means that special measures are required for the design,construction and especially the operation of storage facilities.

The significance of the electrical conductivity of the liquid is less obviously a problem area, butthis has, or at least has untilvery recently, prevented the development of in-tank pumps, and asa consequence, offull containment storage systems for this product.

The stress corrosion cracking of carbon steels in contact with ammonia caused problems for theearly ambient temperature pressurised storage systems and more latterly has been found tooccur in refrigerated storage systems as well. This subiect has been researched extensively,and would indeed probablyfurnish sufficient materialfor a book in its own right. The problem andthe main findings are discussed and references provided for further study if required.

The commonly adopted forms of refrigerated storage are described as well as an interestingalternative system. The requirements for periodic inspection and repair of liquid ammoniastorage systems are also described.

Finally, a dramatic incident involving a liquid ammonia tank in Lithuania and the lessons to belearnt are discussed .

Contents:

21,1 General

21.2What makes ammonia storage special?21.2.1 Flammabiliiy

21 .2.2 f oxicity

21 .2.3 Latent heat

21.2.4 Eleclrical conductivity

2'1.2.5 Stress corrosion cracking (SCC)

21.3 Refrigerated storage of liquid ammonia21.3. 1 Conventional systems

21.3 2 An alternative storage system

2'1.3.3 Chemical Industries Association guidance

21.3.4 Recent developments

21.3.5 Insulation systems

21.4 Inspection and repair of liquid ammonia storage systems

21.5 Incidents involving liquid ammonia tanks

21.6 References


21 Ammonia storcge - a special case

21.1 GeneralAmmonia is manufactured in large quantities and has numer-ous uses in the chemical industry Amongst these are:

- The manufacture of nitrogenous fertilisers

- The manufacture of explosives

- The manufacture of dyes

The manufacture of man made fibres

As a chemical reagent in the forming ofamines and ammo-nium compounds

- As a refrigerant

Fof these reasons safe and economic bulk storaoe ofthis mate-rial is clearly necessary

Ammonia can be liquefied bythe application ofpressure alone.At a maximum design temperature of 38 'C, the comparativelymodest pressure of 14.7 bar is required to maintain the gas inliquid form. For many years, smaller quantities of ammonia (saybetween 500 and 3000 tonnes) were stored in cylindrical orspherical pressure vessels. For reasons which will be ex-plained in Section 21.2.5, this practice has virtually ceased.

Liquid ammonia has also been stored in semi-refrigerated facil-ities. At a storage temperature of 0 'C the pressure required tomaintain the liquid state is only 3.0 bar. Semi- refrigerated stor-age is usually in spherical vessels, again in the 500 to 3000tonnes range of unit capacity. This form of storage is also nowquite unusual. Fully pressurised and semi-refrigerated storagesystems are the subject of a Chemical Industries Association(ClA) Code of Practice (Reference 21.1). This document is nowno longer published, perhaps an indication of the falling out offavour of the pressurised and semi-refrigerated methods ofstorage.

The majority of liquid ammonia storage facilities are now of thefully-refrigerated type in which the liquid is stored at its atmo-spheric pressure boiling point of minus 33 'C. Lowtemperaturetanks with capacities of up to 60,000 m3 are not uncommon.The fully-refrigerated storage systems are also the subject ofaCIA Code of Practice (Reference 21.2).

It is estimated that there are currently around 1000 fully-refrig-erated liquid ammonia tanks in ooeration worldwide of whichsome 50 are located in Europe.

21.2What makes ammonia storagespecial?Liquid ammonia differs from the other liquid gas products listedin Chapter 17, Figure 17.1, in a number ofways.

21.2.1 Flammability

ln common with allof the gases listed in Figure 17.1 with the ex-ception of oxygen, nitrogen and argon, ammonia is flammablewith flammable limits of between 16% and 25%. lt has the rela-tively high auto ignition temperature of 651 'C and for this rea-son liquid ammonia storage installations are not regarded asrepresenting significant fire hazards in the same way as is thecase for the bulk storage of LPG, ethane, ethylene and LNG.

21.2.2 Toxicily

Ammonia is also highly toxic and it is this property that requiresparticularcare to be taken with the design and operation of bulkstorage systems for this product. The health hazards are sum-marised in Figure 21.1 taken from References 21.1 and 21 .2mentioned above. lt is to some extent fortunate that ammoniacan be detected by the average person at the low concentra-tions of around 50 ppm, well below the 500 - 1000 ppm levelswhich are considered dangerous. lt is important that due atten-tion is given to operator training and that the necessary sitesafety facilities are provided. Reference 27.2 provides guid-ance !n this regard.

One example of the special provisions required is that ammo-nia storage facilities must have one or more wind socks fitted inhigh and prominent positions and that these must be illumi-nated at night to allow personnelto choose the correct escaperoute in the event of a leakage incident.

Vapourconcentration(ppm v/v) Generrl effect Expmure period

100

Odour detectable by mostpersons.

No adverse effect foraverage worker.

lmmediate nose andthroat irritation.

Immediete eye irritation.

Convulsive cougbingsevere eye, nose andthrort irritation

Convulsive coughingsevere eye, nose andthroal irritation

Respiratory spasm. Rapidasphyxit,

Thr€shold Limit ValueMaximum for I hourworking period.

Deliberate exposure forlong periods not permitted.

1/2 - t hour erposure ceusesno serious effect.

12 - t hour exposure causes

ro serious effect.

Could b€ fat'al after l,2 hour

Could be fatal afler 1/4 hour

Fatal within minutes.

700

1,?00

2,000 - s,000

5,000 - 10,000

Figure 21.1 Vapour concenlrauon health hazards


21.2,3 Latent heat

Liquid ammonia is also unusual in having a high latent heat(327.10 kcal/kg as opposed to the next highest listed gas whichis methane at 121.86 kcal/kg). This makes it relatively easy toachieve low atmospheric boil ofifigures, usually expressed as a70 of the full tank contents per day, for liquid storage systems.The commonly used insulation systems for liquid ammoniatanks are discussed in Section 21.3.5.

21.2,4 Electrical conductivity

Liquid ammonia in its pureform hasa high dielectric constant. ltdoes however have a high affinity for water In addjtion, for rea-sons associated with the propensity of carbon steels to sufferfrom stress corrosion when in contact with ammonia as de-scribed in Section 21.2.5, water is deliberately added to storedliquid ammonia. This generally gives a water content of be-iween 1000 and 2000 ppm and at this level the liquid will con-duct electricity. The significance of this is that until recenflV itwasnot possible to develop an in-tank pump for liquid ammoniaserytce.

Conventional in-tank pumps as described in Chapter20forusewith the other low temperature gases rely on the product liquidbeing pumped to both lubricate and cool the pump motor byflowjng directly through the motoritself. This is clearly not possi-ble in the case of liquid ammonia. The influence of the lack ofavailability of suitable in-tank pumping systems on the contain-ment systems is discussed in Section 21.3.

21.2.5 Stress corrosion cracking (SCC)

Stress corrosion cracking (SCC) has been known for manyyears to be a problem for the storage of liquid ammonia in car-bon steel vessels at or close to ambient temoeratures. paoerswere published on this phenomenon as early as 1956 (Refer-ences 21.3and 21.4). Although the potentialfor SCC to occur incarbon and low alloy steels in ammonia service was recoo-nised, it was not until the 1970s that inspection technology ha-ddeveloped to the point where the problem could be identifiedand the effects quantified. This led to the discovery of wide-spread SCC in liquid ammonia storage spheres. For this rea-son, many of the facilities which stored ammonia in thefully-pressurised or semi-refrigerated form were decommis-sioned and were replaced by fully-refrigerated storage sys-tems.

To provide more data on the problem of SCC in liquid ammoniastorage systems using carbon steel containment vessels. acorporale research programme was Set up at the Institute forEnergy T€chnology in Oslo, Norway. This was sponsored join yby BASF, DSN/, Kemira O! Norsk Hydro AJs, tcl, E I Dupont deNemours Company Inc. and the UK Health and Safetv Execu-tive. The work was all carried out by Lunde and Nyborg and theearly work was published in the proceedings of various confer-ences and in papers listed as References 21.7 to 21.11.

The general terms of reference for this work were:

a) To investigate the effect of operating parameters (espe-cially water and oxygen concentrations and temperature)on stress corrosion cracking.

b) To determine s€fe/unsafe operating conditions for ammo-nra slorage spheres.

c) To investigate the influence of material composition andmechanical properties on susceptibilityto stress corrosioncracking for both parent material and weld mebl.

d) To investigate possible means of preventing stress corro_sion cracking in ammonia environmenb.

21 Ammonia storage _ a special case

Although the main thrust ofthis effort was aimed at the storaoeof ammoniaasa liquid in spherical vessels at ambienttempeL-tures, the findings are of interest and relevant in part to refriger-ated ammonia storage systems. The main conclusions were:a) SCC initiation is influenced by the water and oxygen con-

tent of the ammonia as shown by Figure 21.2.

b) Sufficient water addition to avoid cracking in liquid ammo-nia may not always prevent its occurrence in the vapourphase, in the event ofcondensation, due to adverse oarti-tion of oxygen and water

c) Under conditions typical of those known to cause SCC(3 ppm oxygen and 50 ppm water) crack growth ratesfound in the studies were similar to those found in service,i.e. 2-6 mm/year dependent on stress intensity.

d) Crack grolvth rates decreased markedly with time.

e) Lowerstrength steels showed generally lower susceptibil-ity to SCC for both parent materiat and weld metal.

f) The initiation of SCC is more djfficult and its propagationslower at -33 'C than at 18 'C, and is less affected by theoxygen content at the lower temperature.

Both References 21.1 and 21.2include the following text:"ln order to minimise the risk of stress corrosion crackinothe welding consumables should overmatch the tensiliproperties ofthe plates by the smallest practicable amountand carbon molybdenum electrodes shall not be used inany circumstances. Furthermore, the tensile strenoth oftheplates shall not be allowed to exceed the maximum-detailedin the plate soecifications."

The original versions of these guides to good practice werepublished before the studies mentioned above had been car-ried out.

It is clear from the volume of work published during the 1970sand 1980s that SCC, particularly in the ambient temperaturepressure storage area was seriously under the microscope.Work published by Cracknell in 1982 (Reference 21.5\ andTowers in 1984 (Reference 21.6) lutlnet explored the problemand served again to confirm the importance of variables suchas oxygen content, water content, steel strenqth and stress re_lief. Both suggest that refrigerated storage is l;ss tikely to sufferfrom this phenomenon than ambient temperature pressurestorage, but in the light of more recent findings, were wise nottohave been too adamant that SCC will not occur at all in refrioer-

1 10 100 t0o0

orys.n in trquid ph.$ ippn w/w)

A Itrsp€.t rt normlt fftquency,

B Insp.ct rt le8l rwice mrnrt frequeng_

C Donolop€fut in lhisrrer.rrr b bdng op.nring .ondi.iois iib zom A or B_

Figure21.2 S-C-C, susceptibilily of C,lVn steels wilh diffefenl oxygen and watercontents at 18 'C

t! 1000

e3

ig

! 100

a

AI

7

B c


21 Ammonia storcge a specialcase

ated storage systems. Alan Cracknell ends his paper with thefollowing sensible suggestion:

"ltwillbe appreciated that if SCC does turn out to be a prob-lem in refrigerated storage, it is likely to affect all companiesusing as-welded equipment. Proving thatit is absentortak-ing precautions against it can prove expensive. lt is sug-gested therefore thatthe companies involved should set outto share information on their findings in much the same wayas companies involved in the bulk (ambient temperaturepressurised) storage of ammonia. Hopefullythe informationderived will benefit not only the refrigerated storage indus-try, but will also give clues to the solution of the generalproblem of avoiding SCC in ammonia storage."

The earlyworkdone by Lund and Nyborg suggested that stresscorrosion cracking was also a possibility at the temperature ofthe low temperature storage systems, i.e. -33 'C. This wassomething altogether new for the industry which had up to thistime believed that refrigerated storage of liquid ammonia wasnot susceotible to this oroblem.

The inspection of the 12,000 tonnes liquid ammonia tankowned by BASF at the Seal Sands site in the UKwas somethingof a turning point. This tank was designed and constructed byWhessoe to BS 4741 and the CIA guidelines and entered ser-vice in 1978.

The tank was previously owned by Monsanto who at that timewere devotees ofacoustic emission (AE) methods of non-intru-sive inspection. The owners decided to override the CIA guide-lines for the first internal inspection at six years after enteringservice, by carrying out an AE examination in 1984 and a fur-ther examination in 1985. These test procedures had the ad-vantage that the expensive de-commissioning, internal exami-nation and re-commissioning could be avoided. No defectswere found during these tlvo examinations.

ln 1985 the ownership of the site transferred to BASF. Thiscompany did not favour the use of AE testing and arranged foran internal insDection to take Dlace in 1987. This examinationfound a large number of internal stress corrosion cracks, manyassociated with original hydrogen cracks. The defects were inthe main associated with welded seams and areas of construc-tion attachments (such as blank nuts and erection brackets) inthe lower shell where the stronger steel (minimum yieldstrength 355 N/mm2) had been used. The upper courses wheresteels of lower strengths had been used (minimum yieldstrengths 280 and 245 N/mm2) were found to be almosi com-pletely free of signs of SCC.

This tank insDection is reDorted in considerable detail in Refer-ences 21.12 and 21.73. The defects were dressed out by localgrinding and the tank was re-commissioned wlth suitable care.Some years following this inspection the tank was again de-commissioned and subject to an internal inspection. By thistime the SCC was such that it was decided to remove and re-place the tank bottom and annular plating and the lower five(higher strength steel) shell courses. This was an expensiveand time-consuming modification and suggests that if SCC canbe avoided by the use ofweakerand less economical materials(in the short term), by correct selection of welding proceduresand consumables and by careful commissioning, de-commis-sioning and operating procedures, then this is money wellspent.

BASF also owned and operated two refrigerated ammoniatanks at Ludwigshafen Germany. These were each of 25,000tonnes capacity, constructed in 1969 and 1981 by Ktockner.Alerted by the Seal Sands experience, BASF decided to in-spect these tanks internally. Both were found to have indica-tions of SCC. This is reported in Reference 21 .14.lnteteslingly,the older tankwas less badly affected than its newer partner (27reported defects as compared to 214). The report is at a loss toexplain this difference. The tank bottom was of butt-welded


construction using backing straps. lMany of the cracks werefound in the tank bottoms and were repaired by fitting localcover strips, a procedure which the authorwould not endorse. ltwould be interesting to find out how these tanks have fared fol-lowing their subsequent inspections.

The industrywas atfirst slowto hold its hand up to the existenceof this problem. Indeed, it is probably unfair to blame those in-volved in the refrigerated storage of liquid ammonia of an os-trich-like disingenuous self-interest for their failure to immedi-ately acknowledge the difficulty. ldentifying stress corrosion onthe internal surfaces of carbon steel liouid ammonia tanks wasnot easy until detection techniques became more sophisti-cated, largely due to workdone in the UK by NationalVulcan.

Separating the evidence of SCC from original construction de-fects and from hydrogen cracking is not easy. Since the publica-tion ofthe reports of the BASF ammonia tank problems and theavailability of the means of detecting and identifying this phe-nomenon, most of the liquid ammonia storage tanks in the UKand Europe have been inspected. Some have been found toexhibitthis problem and some have been found free of any signofthe complaint. The reasons forthese apparent differences inbehaviour between storage tanks (all builtfrom carbon manga-nese steels) is not clearly understood.

The signiflcant variables would seem to be the same as thoseidentified for ambient temperature pressurised storage, i.e.:

- Stress in the parts ofthe tank exposed to the product liquid

oxygen content within the tank during its early life (ie dur-ing commissioning)

- Water content ofthe stored product

- Stress relief

Welding techniques related to heat input and local hard-ness

- The selection of a weld metal which closely matches thestrength of the parent plate

It is uncertain if this Iist includes allof the important variables. ltis also uncertain which individual variable, or indeed combina-tion of variables is the most important. There is however con-siderable circumstantial evidence to link these to the problem. lthas become common practice to use a low strength steelfortheinner tank in contact with the product liquid and vapour (i.e. a275 N/mm2 yield strength grade steel rather than a 355 N/mm2grade whichwould otherwise provide a more economic storagetank) and to pay particular attention to the othervariables listedabove.

Later work, again by Lunde and Nyborg of the Norwegian En-ergy Institute and sponsored by the ammonia storage industrywas presented to the A.l.Chem.E. Ammonia Safety Sympo-sium held in Vancouver in October '1994 (References 21.15and 2t. t6). These papers are well worth reading for those witha special interest in this problem area and contain a large num-ber of useful references for further study. The figure comparingthe stress corrosion susceptibility of carbon steel as a functionof oxygen and water content at temperatures of 18 'C and-33 'C is interesting and is shown in Figure 2'1.3.

21.3 Refrigerated storage of liquidammonia

21.3.1 Conventional systems

The early liquid ammonia tanks were of the single containmenttype with remote low bunds as illustrated in Figure 21.4. Assafety standards increased, the tank type most commonlyadopted by the industry became the double containment type

E

c

E

E

8^lyl

A

u aY-B 8!5

{lrl .! 83


as illustrated in Figure 21 .5. The final move through the types ofcontainmentfrom double to full, which was made in the cases ofmany of the other low temperature gases, was not followed in

the case of ammonia. The reason for this is associated with thelast of the properties listed in Section 21.2.4, i.e. the ability ofthe liquid ammonia to conduct electricity. For many years thisprecluded the industry's ability to develop an in-tank pump foruse with liquid ammonia, a central requisite for the eliminationof bottom or lower shell liquid outlets required for full contain-ment systems.

As has been stated in Section 21.2.4, in-tank pumps for theother low temperature products are directly cooled and lubri-cated by the pumped liquid. The first of these is clearly not pos-sible with Iiquid ammonia. This problem was not aided by a fur-ther unhelpful property of liquid ammonja - its affinity forattacking copper bearing alloys. Recent developments whichwill hopefully overcome this problem are described in Section21.3.4.

The liquid containing metaltanks weredesigned in accordancewith API 620 appendix R or to BS 4741 and more latterly to itsreolacement Code BS 7777.

The outer wall may be constructed from low temperature car-bon steel or prestressed concrete designed to contain the fullliquid contents of the inner tank without leakage. l\ilany of theconcrete wallswere prestressed using the "Preload" wirewind-Ing system.

The tanks were most usually supported on elevated unheatedreinforced concrete base slabs supported on pile extensions orby other suitable arrangements. The connection between theconcrete wall and the base slab was usually ofthe sliding or the

0.1 1 10 100

Oxygen ppm

:igure 21.3 Comparison ofihe susceptibilily of cabon steelio SCC as a lunc-ron ofoxygen and waler content at temperatures of 18'C and -33 'C

On ground

: gure 21.4 Single containment storage arrangements

e.<

-.+'

: qire 21.5 Double conlainmenl storage arfangements



Thin dudntum $cdon b Pdvidowaalh.rp@nno M tll(t, vt !@l &hof2.nbl @6trEt of bnk b bto cla6

Figure 21.6 Interspace roofdetailof lcl North Tees ammonia tank No.2

pinned type. The insulation systems were quite unsophisti-cated as described in Section 21.3 5

It is important to prevent rainwaterfrom entering the interspacebetween the inner insulated steel shell and the outer steel orDrestressed concrete wall. The undetected accumulation ofrainwaterand possible condensation within this interspace hasled to a number of tanks of this configuration having to be de-commissioned and be the recipients of expensive and timeconsuming remedial work. Clearly an effective roof coveringthe interspace is a necessity.

The design of an efficient roof with a sensible lifespan wouldseem straightfoMard, but for the reasons listed is not easilyachieved and requires carefuldesign and careful construction:

- The roof is some 30 m above local grade and conse-quently subject to strong winds.

Ammonia tanks are mmmonly built at coastal locationsand are subject to adverse corrosion regimes.

- The roof sDans between the steel inner tank which is sub-ject to thermal movemenb and the concrete outer tankwhich is not subject to thermal movements (at least not tothe same extent).

- The roof is difficult to inspect and repair.

The arrangement indicated in Figure 21.6 was adopted by lclfor the reolacement roof for the No. 2 ammonia tank at NorthTeesWorks. This has served its purposewelland seems to an-swer the various problems posed.

It is also important to inspectthe interspace regulailyto ensurean early indication is obtained of anywateraccumulation and tohave a suitable drainage system together with site operatingDrocedures to remove such water before it can damage thetank base insulation.


Prsbrn d ctc6d 6ll polytrm€

21.3.2 An alternative storage system

The storage arrangement shown in Figure 21.7was developedbetween the plant owner and the tank contractor for a22,000 m3 ammoniatankat a site in the UK and seemsto getasclose to full containmentas the lack ofsuitable in-tank pumps atthat time would allow. The outertank is designed to contiain thefull liquid contents ofthe innertankwhich are assumed to reach

O InBrsh.rl @@ oui*3h€n @@ shlbtrv.N. @@ Bsh.lt.lloi @

Figure 21.7 An alternative storage anangement

the equilibrium level in the outer tank within 5 minutes of theleak commencing, i.e. a fast but non-zip type of failure.

The outer tank shell and bottom are cold in service whichmeans that there will be no thermal shock in the event that theinner tank leaks its contents into the outer tank. The cold outertank also means that there will be no significant evolution ofvapourinthe inner tank failure case. The vulnerable lowershellliquid outlets are protected by shut-off valves within theinterspace between the inner and outer tank shells as indicatedin Figure 21.8.

21 Ammonia storcge - a special case

Figure 21.9 Bottom corner details forthe altemative arrangement

sons associated with the stress corrosion cracking problems.The ability to get as much product liquid out of the tank asquickly as possible leaving the minimum volume to be removedvia the atmospheric heatleak route willclearly be useful in mini-mising the decommission/inspecvre-commission period andconsequent costs associated with unavailability of the tank. Asmall drain connection also protected by two interspace valveswith a suitable external pump arrangement maywell be a goodinvestment. A photo of this facility is shown in Figure 21 .10.

Figure 21.'10 An ammonia storage tank ofthe alternative lype

21.3.3 Chemical Industries Association guidance

Much ofwhat is discussed in Sections 21.3.1 and 21 .3.2 is alsocovered by the latest version of Reference 27-2. This dividesthe types of tanks to be used for the storage of refrigerated liq-uefied ammonia into the nowfamiliar three categories. The ex-act definitions vary from those given in Chapter 17, Section'17.6 onwards which were derived from BS 7777 and EN 1473,and for this reason are ouoted in full:

. Single containment

This is a tank designed and constructed so that only thecontaining element in contact with the refrigerated ammo-nia is required to meet the lowtemperature ductility require-ments for storage of the ammonia. Any outerwall ofa singlecontainment storage system is primarily for the retentionand protection of insulation and is not designed to containliquid in the event of ammonia leakage from the innertank.

@@

@

Figure 21.8 Liquid oullet details for the allernative arangement

The first valve (i.e. thefirstvalve that the exiting liquid meets) isa manually-operated valve whose purpose is to allow the sec-ond valve to be serviced. This second valve is open during nor-mal service, but is pneumatically-closed in the event of liquidleakage being detected by instrumentation located close to theliquid outlet connection. The purpose of this arrangement is toprotect the plant and its surroundings from an incident involvingthe liouid outlet externalto the outertank and before the first ex-ternal shut-off valve in the outlet pipework. Without this ar-rangement, this incident would cause the tank to dump its entirecontents through the liquid outlet connection.

With this equipment in place, the amount of liquid escaping tothe environment would be that which would flow out ofthe tankduring the period that the detection equipment took to identifythe problem and the second pneumatically-operated shut-offvalve took to close. This is bad enough with a toxic product likeammonia but much preferable to leakage of the full tank con-tentS.

The apparently curious arrangementwhere the inner and outertanks share a common roof is important. This allows theinterspace between the two tank shells to be filled with air. Thisair space is within the insulation envelope and consequentiallyat the product temperature of -33 "C. Cold and inhospitablethough this interspace is, it allows the valves in the liquid outletlineto be accessed and serviced which is vitaltothe overallvia-bility of the system. This arrangement does require the innertank to be fitted with holding down bolts or straps as the internalpressure is predominantly applied to the part of the single roofattached to the inner tank shell. The design of the anchoragesystem which must penetrate the outer tank bottom can proveinteresting. The bottom cornerdetail for this tank is illustrated inFigure 21.9.

As a side issue, when designing such a system it is importanttoremember that ammonia tanks are likely to have to be decom-missioned, probably more than once during their careerfor rea-


21 Ammonia storage a special case

A single containment tank is traditionally surrounded bysecondary containment in the form of a low bund wall tocontain any leakage.

Afootnote elsewhere in the document shtes:

"storage tanks, for example tanks containing petroleumproducts, are frequently surrounded with low earthern orconcrete bunds which will contain the liquid in a large openDool should failure ofthe main tank occur. lfthis method wasadopted with an ammonia tank, evaporation from the pool

of ammonia would cause toxic concentrations considerabledistances downwind and is not considered desirable, nor

acceotable in the UK."

We also find:

"lvlany tanks were originally built as single containment type

but have been retrofitted to become double containmenttype".

These are quite strong indicators that single containment is not

currently preferred.

. Double containment

This refers to an inner tank designed and constructed with

secondary containment in the form of a wall or outer tank

Both the inner tank and the wall or outer tank shall be capa-

ble of containing the refrigerated liquid ammonia. To mini-

mise the pool of escaping liquid, the wall or outer tankshould be located at a distance not exceeding 1.5 m from

the inner tank. The innertank shall store the refrigerated liq-

uid under normal operating conditions. The wall or outertank shall be able to contain the refrigerated liquid ammonia

leakage from the inner tank.

The outer tank is not designed to contain vapour released

due to ammonia leakage from the inner tank

. Full containment

This refers to a tankdesigned and constructed with second-

ary containment in the form of a wall or outer tank. Both the

inner tank and the wall or outer tank shall be capable of con-

taining the refrigerated liquid ammonia

The innertank shall store the refrigerated liquid and vapourammonia under normal operating conditions.

The wall or outer tank shall be capable of containing both

the refrigerated liquid and vapour ammonia resulting from

ammonia leakage from the inner tank

It is a shame that the authors of this document have sought to

modify the definitions which were developed in EEN'4UA 147

and repeated in BS 7777. The wording of the full containment

definition suggests a fixed inner tank roofto contain the product

vapourduring normaloperation, whereas the figure (taken from

BS 7777) clearly indicates that a suspended ceiling is accept-

able.

The outer containment of double and full containment systems

is oermitted to be of steel or reinforced or postlensioned con-

crete. For steel components the rules of BS 7777 are required

to be adopted.

As has been mentioned, when this guide was written (1997),

in-iank pumps for ammonia service were not available. Conse-quently, liquid outlet connections which penetrated the inner

and outer tank walls could not be avoided. The guide gives thelollowing advice regarding liquid outlet connections:

1 . The liquid outlet pipe (or pipes) should be taken from theside oithe innertank, as close to the base as permissible'

2. Each outlet pipe shall incorporate a valve for remote clo-

sure. Where internal valves are the preferred type of re-mote closing valve, two outlet pipes are recommende0'

3. When a remote closing valve is fitted externally to the in-

ner tank, it should be sited in the annular space between


the inner tank and wall. An isolation valve is required to be

fitted betvveen the remote closing valve and the tank andshould be located as closetothetank as possible Asingleoutlet is permissible.

4. ln the event of power failure, the remote closing valve(s)should close automatically. The consequences to down-stream operations should be evaluated

5. The outlet pipe (or pipes) shall be anchored into the baseslab (or wall if this is preferred in a structure which is notoost-tensioned) and due allowance made for the move-ments which will occur when the bnk cools down

6. The tank should preferably be fitted with two liquid outlets.Both shall be fitted with internal valves capable of remoteclosure.

Items 1 to 5 apply to double containment systems with concreteouter walls. ltems 1 to 6 apply to double containment systemswith steel outer tanks and to full containment systems with both

steel and concrete outer tanks/walls.

The document makes it mandatory for steel shell plates with

mountings (penetrating fittings) to be stress-relieved. A sensi-

ble SCC precaution.

The document has a section on commissioning and de-com-

missioning. Again SCC raises its head and the following advice

is offered:

"Recognition should be given to the possibility of stress corro-

sion cracking (SCC) occurring in ammonia storage tanks. Ap-propriate design and construction techniques wlll minimise thisrisk but it should also be noted that research work has shown

the following:

. SCC does not occur without the presence of oxygen

. The oresence of water may inhibit Scc

Therefore the purging ofthe tank with inert gas priorto the addi-

tion of ammonia, and the maintenance of a water content in the

ammonia of 0.15% to 0.20% should be considered The inertgas purging ofa tank priorto the addltion of ammonia is recom-

mended as a standard industry practice."

21.3.4 Recent developments

Quite recently in-tank pumps suitable for use with liquid ammo-

nia have been developed. The pump is described in detail in

Reference 21 17.fhe design of this pump incorporates the fol-

lowing features to deal with the problems of liquid ammonia's

electrical conductivity and its highly corrosive action on copper

and copper bearing alloYS:

. Electric motor housed in a liquid and gas tight enclosure

. Magnetic coupling to connect motor to pump

. Motor cooled indirectly by the product liquid

. Nitrogen-purged motor enclosure and electrical contain-

ment system

. Power cable assembly enclosed in a flexible bellows type

hose assemblY

. Filtered product liquid bearing and coupling lubrication sys-

tem

The Dumps can be installed with the usualtype offootvalve, op-

erated by the pump self-weight and allowing the pump column

to be purged and freed from ammonia Iiquid and vapour priorto

removal or replacement, especially important for personnel

safety with this particular product.

Pumps of this type have been specified and installed at theplant operated by Shanghai Golden Conti Petrochemicals in

China. They have been in service for some four years and other

eoe

than problems relating to the product cleanliness, have oper-ated successfully.

For land-based storage systems, this development opens theway to true full containment for liquid ammonia.

These new pumos could also be a useful addition to the marinetransport of liquid ammonia where external motors and longdrive shafts have traditionally been used.

21.3.5 Insulation systems

Because of the high latent heat of this gas, the tank insulationsystems are usually quite straighforward.

For the tank base, two layers of cellular glass with a load bear-ing ringwall dependent upon the seismic environment normallysuffices. For the tank walls, polyurethane foam, either foamedn-situ behind metallic cladding or spray applied with a mastic

weatherproof coating are commonly used on the outside sur-faces. There are a few double-walled tanks which use perlite in-sulation.

The tank roof could have a similar arrangementto the tankwallsorfor reasons associated with weather protection, befitted withan internal suspended deck supporting a glass fibre or mineralwool insulation.

21.4 Inspection and repair of liquidammonia storage systemsReference 27.2 also provides guidance regarding inspectionand maintenance of refrigerated liquid ammonia storage tanks.Clearlythe possibilityof SCC is the reason why rules specific toammonia storage are required. The Reference suggests:

"All tanks should be thoroughly inspected, both internallyand externally, not more than 6 years from the date of inltialcommissioning. Thereafter, the interval between major in-spections should be determined by the tank owner, depend-ing on past experience. For example, if SCC was found atthe first inspection, then it may be necessary to carry outsubsequent inspection at an interval of less than 12 years,and if it (the extent of SCC) was sufficient to warrant sub-stantial repair or rebuilding, then it is recommended that thenext major inspection should be carried out within a further6 years."'

For such an important issue in terms of public safety, theseguidelines are remarkably non-specific and non-mandatoryThe Reference goes on to say:

"For guidance, inspection intervals in the region of 12 yearsare considered an appropriate balance between the need tomonitor the tank and the risk incurred during de-commis-sioning and subsequent re-commissioning."

And:

"lf, outside the recommendations of this guidance, a singlecontainment design of tank is operated, then the inspectionperiod shall (mandatory) not exceed 12 yearc."

It recommends that the first inspection should include:

. Magnetic particle flaw detection (MPl) of all tee welds infloor plates, for a length of 230 mm along each arm of theweto.

. l\y'agnetic particle flaw detection in accordance with BS6072 of lojo/a of first course vertical and horizontal welds(including the shell-to-floor weld), plus at least 50% of thetee welds in the remaining shell plates, for a length of230 mm along each arm of the weld, plus all internal attach-ment welds below the first to second course circumferentialweld. This shall include all areas of temporarv attachmentwetos.


. Careful visual inspection of:

All other welds in floor and shell plates

All other internal brackets and attachments

Although it is not expressly stated, it is presumed that this in-spection will be confined to the internal surfaces ofthe primaryliquid container. The outer surfaces ofthe primary container areoften inaccessible due to the presence of the thermal insula-tion.

Some guidance is given regarding the methodology of MPI tobe used, but this is in outline only and would not impress thoseorganisations skilled in the techniques necessarylor the detec-tion of SCC. The sensible observation that "Those carrying outthe tests should be experienced with the techniques and of theinterpretation of the results obtained." is included.

External inspection is confined to looking for insulation coldspots and ammonia leakage around fittings and pipework. Forthis the tank should be three-quarters full oi liquid ammonia.Four holding down bolts and their boxes or holding down strapsshould also be inspected.

An interesting more recent development is the "Recommenda-tions for safe and reliable inspection of atmospheric, refriger-ated ammonia storage tanks" published in October 2002 by theEuropean Fertiliser ManufacturersAssociation (EFMA) (Refer-ence 21.18). This document brings attention to the fact thatthere are major differences between the national regulationsand/or Codes of Practice from different EuroDean countries re-garding the frequency of inspection of liquid ammonia tanks.This is illustrated in Figure21.11.

These regulations and Codes of Practice also do not discfimi-nate between tanks of diflerent lypes of construction, differentoperating practices and posing different risks to the surround-ing envifonment. This new guidance is based on Risk Based In-spection (RBl). and seeks to evaluate the pfobability and con-sequences of failure of each lndividuaL tank. This process is inturn intended to optimise the lnspection frequency to obtainknowledge about the tank and its cond tion and the negative ef-fects of opening the tank for intefnal lnspection which could in-crease the potential for the occufrence of SCC

An inspection procedure is suggesied (Figure 2'1 .'12). A proce-dure is given for determining the maximum tolerable defect size

Figu re 21 . 1 1 Nalional rules or recognised standa rds for th e frequ ency of inspeclion of atmospheric (lquld) ammonia storage tanks n Europe

Fram EFMA document. Abjendix 1

Maximum inspcctioD iotcrvrl for 2tnrosph(flc.

20 years, but a notijled body can de(jde shoner

None (VAWS, weter protection act, state 5, 1 0 or I 5

depending on inspecuon r€sults. according to CLA +

C!€eicrl lldst y Ass@iatiod Cuidd@ for &c lege scale sromge offtly.rny.LM ql@olia in ihc UK Jue 1997.


SteD I Step 2 Step 3

I00%100%

B;ftom plates 50% r00%

SheLl Dlat6, T-w€lds ir co@e I &d 2 t00%Sh€ll plales, honzontal atrd verhcsrwelds itrcowse I ed2

t0% ICD%

Shell Dlats. T-welds in @us€ 3 !o top to% 50% ILD'/O

Shell plat6, horizontal md vertic.lwelds in couie 3 to tot

to% tatJ%

Marhols, pip€ cotne.tio&s, purnp sinkdd other sDeaial details

1rfro/o

clamp marks or t@pomry fabrieiion l0/o 100%

89qq!@!e poeeadon weldsAr.a! s biect to pr€vious repaiB

100%100./"

21 Ammonia storage - a specialcase

Figure 21 12 Inlernal nsoeclion Iecommendatol

Frcm EFMA docunent, Aq?endix I

tank of carbon steel to retiain and protect the perlite thermal in-

sulation. The tank was supported on an elevated base slab andwas surrounded by a concrete wall supported at local grade

level and described as reinforced (as opposed to prestressed).

The inner tank was 30.3 m in diameter and 21 3 m high.

Itwould seem thatan operating erroralloweda small quantityofwarm ammonia liquid to enter the tank and this caused a largequantity of vapour which over-pressurised the tank. Failure ofthe shell-to-bottom joint and of the holding down anchors then

occurred and the failure sequence is illustrated in Figure 21.14

which is taken from the SUPRA report.

In much the same way as the Qatar LPG tank failure' the tank

was moved sideways by the reaction to the exiting liquid and

smashed its waythrough the concrete bund wall, tlnally landingsome 25 m from the originalfoundation. The tank bottom was

left on the elevated foundation slab.

The liquid ammonia vaporised and caught fire, in turn setting

fire to an adjacent 15,OOO tonnes store of NPKwhich continuedto burn/decompose for 3 days. The official number offatalitieswas 7, a surprisingly low figure for the magnitude of the event

and the number of employees. Jonava was evacuated for a

short period of time.

It is a pity that there is so little published information relating tothis incident. The lessons which can be learnt would seem to

be:

The necessity for careful process control, especially dur-

ing unusual operating conditions such as start up orre-commissioning activities, to avoid the unforeseen im-port of warm liquid.

- The design of a pressure relief valve system adequate to

cater for all uPSet conditions.

- ln an extreme overpressure incident, the tank failure

modes must be reviewed and provisions made to ensure

that failure cannot occur in elements which will cause thetank to release the contained liquid, i e failure in the roof

sheeting or the shell{o-roof compression area ratherthanthe shell, the shell-to-bottom junction or the holding down

arrangements.

The reinforced concrete secondary liquid containing wall

must bedesignedto containthe productliquid in anycredi-ble inner tank leakageifailure scenario.

- The location ofthe storage tank relative to othervulnerableeouiDment or materials on the site

21 .6 References

21 .1 Code of Practice for the Storage on Anhydrous Ammo-nia Under Pressure in fhe UK The Chemical IndustriesAssociation Ltd - January 1980, (nowno longer avail-able).

21.2 Code of Practice for the Large Scale Storcge of FullyRefrigerated Anhydrous Ammonia in the UK, TheChemical lndustries Association Ltd - 1997.

21.3 Behavior of welded pressure vessels in agriculturalam-monia service, T.J. Dawson, The welding Journal Vol

35, pp 568-574, 1956.

2'1.4 Sfress corrosion cracking of steeis ln agricultural am-monia, A.W. Loginow and E.H. Phelps, Corrosion 18(8), 1962.

21.5 Stress Corrosion cracking of Steels in Ammonia,A.Cracknell, lcl The lnstitute of Refrigeration, Paperpresented 6 MaY, '1982.

21.6 SCC in welded ammonia vesse/s, O.L Towers, MetalConstruction, August 1984

E6

0

10yrs

12 24 3.0 4.8 60 72

Fallure Probabllity number

Figure 21.13 Inspection frequency diagram

From EFMA dacunent

based on BS 7910 (Reference 21.19). An RBI evaluation

method is provided which results in arithmetic scores forfailureprobability and failure consequence. These scores are entered

in the inspection frequency diagram (Figure 21.13)and indicate

an inspection frequencyforthe particulartank in question rang-

ing from greater than 20 years to a minimum of 3 years This

seems a sensible and disciplined approach in an area where

there is a confusing amount of non-specific advice.

21.5 Incidents involving liquid ammoniatanksThe history of liquid ammonia storage has been comparatively

free of incident. A number of minor incidents involving vapourreleases to the atmosphere, non-performance ofvacuum relief

valves and foundation problems usually associated with pooror

inadequate base heating systems have happened over theyears.

The one serious incident involved a liquid ammonia storage

tankin Lithuaniain 1989. Little information is available concern-

ing this incident due to the site's military links, but despite this

the site was visited some two months afterthe event by a group

from the Swedish National Rescue Group (SUPRA) and a re-

port was written by a member of that group (Reference 21'20)

This is interesting reading and the dearth (thankfully) of serious

incidents in this area makes it importantthatthose that do occur

are recorded and published aswidelyas possible so that all the

lessons are learnt and any modifications to industrial practices

and regulations are made and implemented as soon as practi-

caole.

The accident took place at the "Azotas" fertilizer plant'12 km

from Jonava in Lithuania. Jonava has a population of around

40.000 and the plant around 5OOo employees. The ammonia

tank was of 10,000 tonnes capacity and at the time of the inci-

dent held some 70OO tonnes of ammonia The tankwasofJap-anese design and was constructed by Soviet personnel in

1978. The innertank was of lowtemperature steel and the outer


f-rF

Position before accidEnt

21 Ammonia stonge - a specia/ case

Failure of anchors

Base break-away

Agitation of tank begins

Position of tank afier accident

Figure 2'1.'14 Lithuanian ammonla lank faitu€ sequonc€


21 Ammonia storage - a speclal case

21.7 Stress corrosion Uacking of some metallic maErtab inammonia at ambient and low temperatures, L. Lund andR. Nyborg, UK Corrosion 1985, Hanogate UK.

21.8 The Etrec't of Orygen and Water on Stress corrosionCracking of Mild steel in Liquid and Vapotous AmmoniaP/ant L. Lunde and R. Nyborg, Operations ProgressVol 6, No 1, Januayl987.

21.9 Sfress Corroslon Cracking in Different Steels in Liquidand vaporous Ammonia, L. Lundeand R. Nyborg, cor-rosion '87, Paper no 17, San Francisco, USA.

21.10 Sf/'ess Corrosion Crack Growth Rate of Carbon Manga-nese Sfee/s, L. Lunde and R. Nyborg, Corrosion Pre-vention in the Process Industries Conference, 1988 -Amsterdam, The Netherlands.

21.11 Effect of Welding Electrode Composition and StorcgeTemperature on Slrcss Corroslon Cracking of Carbonsteels in Liquid ammofla, L. Lunde and R. Nyborg,NACE Corrosion Conference 1991, Cincinnatti, USA.

21.12 Sfress corroslo n in a 12,000 tonne fully-refigerated am-monia storage tank, J.R. Byrne and F.E. Moir (NationalVulcan Engineering Group Ltd) and R.D.Williams(BASF Chemicals Ltd), 1988 Ammonia Symposium -Safety in Ammonia plants and related facilities, AICE,Denver Colorado, August 1988.

21 -13 gructural lntegrity of a 1 2,000 tonne Refrigerated Am-monia gorage Tank in the presence of Slress ConosionCracks, R.A. Selva (NationalVulcan Engineering GroupLtd) and A.H. Heuser (BASF AG).

21 .14 New Cases of Stress Co/'rosrbn Cra cking in Large Am'monia gorage Tanks, M. Appl, K. Fabler, D. Fromm, H.Gebhard and H.Portl, (BASF AG).

21.15 Measures for Reducing Sfress Conosion Cracking inAnhydrous Ammonla Storage lanks, Rolf Nyborg andLiv Lunde, lnstituftfor Energiteknikk Norway, presentedto the A.l.Chem. E. Ammonia Safety Symposium, Van-couver, Canada, October 1994.

21.16 Sfress Corro sion Cracking in Low Temperature Ammo-n,a Slorage lanks, Rolf Nyborg and Liv Lunde - InstituttforEnergiteknikk Norway, presented to the A.l.Chem. E.Ammonia Safety Symposium, Vancouver, Canada, Oc-tober'1994.

21.17 Advanced design for submerged liquid ammoniapumps, D- Cullen and H. Kimme!, Ebara InternationalCorporation, Nevada, USA, International Journal of Hy'drocarbon Engineering, April 1998.

21 .'18 Recommendationsfor safe and reliable inspection of at'mosphefic, reftigerated storcge tanks, P repared by theEuropean Fertiliser Manufacturers Association,(EFMA), October 2002.

21.19 BS 7910 : 1999, Auide on method for assessing the ac-ceptability of flaws in metallic structures, British Stan-dards lnstitution, London.

21.20 March 20 1989 accident in Lithuanian feftilizet plant'B.O. Andersson, Swedish National Rescue Board (SU-PRA). A.l. Chem. E. Technical Manual, Vol 31, 1991.


Am-r, H.

gtnanonted'lan-

vno-Etutt1. E.Oc-

,NE)nalHy-

fat-lheon,

ac-En-

ant,su-1.

22 Material selection criteria for lowtemperature tanks

The means of selecting appropriate metallic materials for the various component parts, whichmake up the structure of a low temperature tank, are described in this Chapter and thedifferences between the American, the British and the draft European Codes are discussed.An interesting historical material selection procedure is also described.

Contents:

22,'l General

222 The requirements of API 62022.2.1 API 620 Appendix R

22.2-1.1 Materials lor parts subjected to ambient temperatures22.2-1.2 Maletials tor parts subjected to low temperatures

22.2.2 API 620 Appendix Q22.2.2.1 Matenals tor parts subjected to ambient temperatures22.2.2.2 Mal€tials lor parts subjected to low temperatures

22.3 The requirements of BS 7777 | Paft222.3.1 Materials for parts subjected to ambient temperatures22.3.2 Materials for parts subjected to low temperatures

22,4 The requirements of 857777':Patl422.4.1 Parts subjected to ambient temperatures22.4.2 Parts subjected to low temperatures

22.5 The requirements of PD 7777 l2OOO

22.6 The requirements of prEN 1462022.6.1 Materials for parts subject to ambient temp€ratures22.6-2 Matenals for parts subject to low temperatures

22.7 An example of a material selection method ftom the past

22.8 References

STORAGE TANKS & EQUIPMENT

22 Mateial selection criteia for low temperaturc tanks

22.1 GeneralThis Chapter is devoted to the rules covering the selection ofmaterials suitable for use in low temoerature tanks of metallicconstruction. lt limits its interest to the selection of plate materi-als for the main parts ofthe tank structure, i.e. the bottom, shelland roof plating. The various Codes give guidance for the se-lection of materials such as structural sections, pipe, forgings,bolting, etc. Those interested in this sort of detail should makereference to the appropriate parts of these Codes.

To venture deeply into the philosophywhich underpins the ba-sis of the metallic material selection for these tanks is beyondthe scope of Slorage Tanks & EquipmenL and certainly beyondthe abilities and knowledge of the author. The detailed discus-sions surrounding such subjects as avoidance of brittle frac-ture, fracture arrest, critical defect sizes and the like must befound elsewhere.

What will be discussed are the relatively simple rules for mate-rial selection which have been developed from a great deal ofdetailed study and test work, carried out over a considerablenumber of years, resulting in the requirements of the variousdesign Codes.

The material selection falls into tlvo separate areas:

. Materials for primary and secondary liquid-containing parts.

- These are the inner tanks of single containment tanksand the outer metallic tanks of double and full contain-ment tanks. These parts are subjected to both the mini-mum product liquid temperature and the full hydrostatichead.

. Materials for the outer metallic tanks of single containmenttanks.

- These are subjected to the minimum ambient tempera-tures and comparatively modest stress levels arisingfrom internal vapour pressures and self-weight, windand seismic loadings. For these components the mate-rialselection rules followthe ambient temperature prac-

tices.

The material selected for the various parts ofthe structure mustpossess the necessary strength, the ability to be fabricated intothe required forms, weldability and the necessary toughness atthe design temperature to avoid the possibilityof brittle fracture.Ensuring that the materials possess the necessary propertaes

to demonstrate their suitability for the first two of these consid-erations is relatively straightfoMard. The usual tensile testingrequired by the material specifications to determine yield

strength, ultimate tensile strength and elongation to failure arenormally sufficient. These properties are usually measured atroom temperature.

As most materials are stronger at the lower service tempera-tures, this gives rise to a factor of safety additional to that im-posed by the Codes in the determination of the allowable ten-sile stresses. For certain materials this can be substantial. lnthe case of 9% nickelsteels used at LNG temperatures, recentdata suggests that this "hidden" factor can be as high as 50%for both the plate material and the weld metal. lt should be re-membered that for LNG service, it is the latter property whichcontrols the selected plate thickness. lt is the last of the re-quired properties that tends to cause the most concern. Ferriticsteels at warm temperatures, are malleable and can be plastic-ally deformed without risk of fracture provided the materialselongation has not been exhausted. When the service temper-ature is reduced, the material may become brittle and failurecan occurat low stresses with little or no plastic deformation.

The temperature at which the transition from ductile to brittlebehaviour occurs is dependent upon the steel chemistry the


heat treatment, the applied stress and the number, size andshape of any defects in the finished assembly.

There are a number of test methods which are used to deter-mine the material toughness at a particular temperature.Amongst these are:

. Wide plate tests

. Pellini-type drop weight tests

. crack opening displacement (coD) tests

Allofthese tests are difficult, expensive and time consuming toperform, but arguably get somewhere close to determining theintrinsic toughness of the steel.

A test which is suitably quick and cheap, and with which thesteel producers are familiar and comfortable with is CharpyV-notch impact testing. Unfortunately the industry is less confi-dent ofthe ability ofthis test to revealthe true nature ofthe prop-erty of interest for the materials in question. So, the unsatisfac-tory situation exists of tests which provide an answer which canbe believed with some confidence, but which are too expensiveand slow to be of use as a production quality control tool, and atest which is quick and cheap, but which produces answerswhich are contentious.

Fortunately much work has been done to accommodate thisapparent dilemma. lmproved production methods haveresulted in generally tougher steels and the design codes havein some cases become more conservative in the materialselection area. As a consequence, for ferritic materials theCharpy V-notch impact test is used as the toughness criteriondespite its limitations.

Aluminium alloys and austenitic stainless steels are not sus-ceptible to temperature dependent tough/brittle transitions in

the same wayas ferritic steels and consequentlytheir base ma-terials are excluded from impact testing requirements.

22.2The requirements of API 620The requirements of API 620 regarding material selection re-quirements are based in the main on the use of ASTM stan-dards. Some Canadian Standards Association (CSA) and In-ternational Standards Association (lSO) steels are alsoincluded in the listings. The useofsteels manufactured to othernational and international standards requires careful study ofthe steel specification and the code rules to ensure that all ofthe necessary requirements have been fulfilled. It is frequentlynecessary to have to resort to additional testing of such steelsto be able to demonstrate equivalence. The code contains alarge number of rules, specific exceptions and footnotes relat-ing to material selection. For those involved in the task of mate-rial selection, there is little choice but to become immersed in

the fine print of the Code and consequently there is no need toquote "chapter and verse" in Storage Tanks & Equipment. Abrief overview is thus offered with some ofthe significant pointshighllghted.

With the exception of plates which have their thickness se-lected based on minimum thickness requirements, plates arepermitted to be a maximum of0.01" thinnerthan the calculatedthicknesses required, based on edge thickness measure-ments.

22.2.1 API620 Appendix R

22.2.1.1 Materials for parts subjected to ambienttempera-tures

The Code describes these parts as basic orsecondary compo-nents.

22 Mateial seledion aiteia for low temoenture tdnks

no

er-ae.

Pcrmi$ribh Spacifi crtio0sDerign Mctdlbmpcnum(icc 4.2,1)

Plnc Thickn6sIrEludirS Co.rosion

Allounmc (iD.) Spcci[c.rion Grr&Spocid Roquilrrncnts(in ddirion to ,1.2.3)

65'F rrd or€{

25'F rnd o\rcr

Any listql io 2-2.3ASTM A 36CSA C,|{}.21-MAny lhtld in 2"23ASTM A 36 Mod 2ASTM A I3ICSA C'lll2l -M

ASTM A I3ICSA G4O2I.M

ASTtr{A 13lASruA516ASIl,lA 573ASruA662ASruAn?ASTI'iA talcsAeo2t-M

ASTI| A 516ASIUA 53?ASTlr,lA t3ASTMA d}3ASTtr{A 662

^SII{A 6?t

ASruA73?ASruAglcsA c(t2t{lISO 630

Astlt A t3tASTM A 516ASruAtJ?ASTM A

']ASTI| A 633ASTM A 662ASTI{A 678ASTMA 737ASruAglcs^ G402t-Mtso 630

ASTM A IlIASTM A 516ASTM A 5I?ASTM A 5?3ASTM A 6!tASTM A 66?ASTM A 6?8ASTM A ?I?ASTM A 8,1|csA Glo.] -Mrso 6.1(l

1o*,mr|/,l5o*

B26|}w3(x)w350w

B260fl,3SW,35{tW

cs55,6(},65.705& 65,mBrdCBClsl26OW 3dtu 350W

NoocNoocNonc I

NorNcNoncNoocNoncNotc I

NotrcNonc

NUlcNoc INd. INelNaleNqENotc 2

hepvIIF)Ptc-anreta:{s

Eveveialhem

e-tl-[}x)Ef(tdrylsa

rt-e-itbAas

Itohe

rE

d

- 5"F jd ov€r

<l>l

3.tksl

>lsth

>th

>l

'150630 E Zt5. 8355 Qudiry D NccE l.rd2

1S-

hE-

55,60,65, mCl|s|.r I rdz5& 65, ?0CqdDB rdCA rdEB('|ttt60w'3r[1v.:]fntE 275, E355 Qutiry D

cs55,60,65,70at-..Gl I .nd 25tC|rdDBrndCA|trlBBCl|sr I2601V.3mw' 3t0wEin5, E355 |nlQudhyD

cs55.60,65, mCh*.cs I .||d 25tCrndDEaMCArndBClrss IBl6{rwT. .r(x}wT. r50 wTE2?5, El55Qlalily D

l,lqFNcteNoorl{o.tlooeI\toEI\bEt\hocNoc 2NGtrd2

l{ficNoac 3NqrcNorc 3NqrcNoac 3NoneNoo!NoocNdrs 2 rnd 3Norcs 2 |ld 3

Noac 4Notc$ 1 d4Norc 4Nolcs I rnd 4Nqc 4Nqcl| 3 rrd 4Norc 4N(xc.lN(|rtcNdcs 1..1. irnd 4N(*:\ :- J. and J

<l

I All pht6 ovd lla l. ttkl d.ll b. mr'Idlad2 Tb! *.cl lrtl lhdl L U[.a lrd nrd. rta! th. grdr prrcd.e3 Tlc plrt . |rl[ bc .omdh.d o. qrcrcr r.nFrid (..c {.24r)a Er.b pLae |irll D. lDp.ct ae.!.d tr rccsrd co wXt {2J

tgurs 22.1 Minimum requkemenb for pl6te sF€cifcaflons to be us€d for design metal temperatureBo.n API 620, Awendlx R, table +1

STORAGE TANKS A EQUIPMENT 439

22 Matedal selection cdteia for low tempercturc tanks

D.dgn M.rd TcmFnfiE ofsccoid.rJ Compon rt-6lPF to Bclow -20PF -20'F lo +40'F

Pip.

M ..Lb 19 llsr.d in Tlblc R-4

ASTM A 106

Mr|ri* r! lilt dinnbb R4

As lis&d in 2J

na. or pipc .! li$.d lbovc

Suucnlrrl lhat€i ar liir.d io 2.6 or a.r li$cd ud.r E- 6fF to - m'F llmpcr.|un h€rding

SEudonl ntnbcrs Plra or pipc !s list d .borcASTM A 36 Mod | lluctunl dupca (i.c 2.6)

ASTM A l3lOr?&CSCSA C4021-M C6d.s 26OW..100W. rnd lsOW (s.. Mt!)

Forlrngs ASTMA 105

Eolu ASTM A l9l Gnd{ a?

ASTM A 120 Cad. L?

Nor(: Thc \lcclthxll h.lolly hillcd m'j madc |oli*-g.n ptulr.c

Figure 22.2 IVaterials for secondary componenls

Fron API 620, Appendix R, table R-3

Minimum D6ign Mctal Tcmpcmturc, oF

Plarc Thickncss lncluding CoFosion Allowsrlcc. in,

GroupSpccific8don

Numbct G'adc th6-3ls >3!E-th >l/2-l > t-lr/2

| (scmikillcd)

ll (fuUy ldll.d)

Itr (fully killcd utdhigh 3ulNrglh)

A 573A l3lA 516A 516ISO 630cs^ c40.2r-M

A 573

A 516A 516A 537A662A 633A 6?8AWISO 630csA c40.2t-McsA G40.2r-M

58'cs55 rr|d 6055 and 60P

E 2?5 Qudity Db26oWD

65 dd ?0

65 .rd 7065 rrd ?0 Mod lrl.rdzBandCCtndDA udE

E 35sQurliry DD3mwo2{0wD

A 36 Mod 2.A I3I BcsAG4o.2r-M 260W150630 E 2?5 Qualhy Co

-m-m

0-m

-30-60-30-40-30-,t0

-40-60-40-60-@-60-30-io-30

-t0-t0+10-10

-J.'-N-ln-n-30

-N

-m-30-50-30-50-50-50-m-30-10

+5+J

+zt+)

-10-35-t0

-10-15

_10

-t0_15

-15_15-35-35-35-10-15+J

+)

+5

+50

-m0

-m-20-m+5

0+m

-m0

00

NoEi:Wh€rt Dorndizi4 mrtcrirl$ in 6is r.blc |!ry bc urcd at tcmpqafils 20"F b€lo$, drc6! shown (dccpt fgr A t3l Gndc CS, A 537 Cl.!sc! |

rnd e A 633 Grld.s C ard D, A 6?8 Crad.! A rnd B, And A 737 Gradc B). lf lmpaca tclas 'l!

Equircd for thc metrrid! listld itr thit tablc,

0l.y rh.ll bc in occordrtE! wfth Tablc f,-5.rscc 4,2.3 for r cottrpLtr dclcaiption of ddr hrtrrial.blftc n cl ihall bc frllly killcd aDd na& witb 6nc-8rain Frclicc, vithout lonnrlizin& fo( thickEoslg of 116 in. ttugu8h I lD in.cThc mragmcac cootcd shall bc in thc nDg. from 0,&5% ro ll0% by bdlc amlysi!.

Figure 22.3 lvlinimum pemissible design metal temperature for plates used as secondary components without impacl testing

From API 620, Appendix R, tabla R-4

Basic components are those that contain the vapodsed lique-fied gas from the stored refrigerated liquid, but primarily oper-ate at atmospheric temperature because of insulation systemdesign and naturalambient heating. Examples ofsuch compo-nents are the outerwalland roofs ofdouble walltanks and roofcomponenb above the internally insulated suspended deck.

Secondary components are those whose failure would not re-sult in leakage of the liquid being stored. Secondary compo-nents also include those components that are not in contactwith the refrigerated liquid but are subject to the refrigeratedtemperature vapours and have a combined tensile and primarybending stress under design conditions that does not exceed


6000 lb/in'z. Secondary components which could be designedwithin this reduced stress are roof plates, including roofmanways and nozzleswith their reinforcement, roof supportingstructural members and shell stifieners.

These components are furtherdivided into thosewhich containthe vaporised gas and those which do not. The materials fortheformer may (a curious choice of word, a BS or CEN standardwould have used "shall" here) conform to one ofthe following:

. Table4-1 (Figure 22.1 )fordesign metal temperatures downto -35'F (i.e. a lowestone day mean ambient temperature of-35'F) without impact testing unless these are required byTable 4-1 or by the purchaser.

22 Material selection citeia for low temperaturc tanks

Plate Impacr valueb (ft-lb) W.ld Innact Value (fllb)Speai6cation

NumberRangc in

Thickness (in.) lndividual lndividualGrade

ASTM A I3I

ASTM A 516

ASTM A 516

ASTM A 516

ASTM A 516

ASTM A 84I

ASTM A 53?

ASTM A 53?

ASTM A 662

ASTM A 678

ASTM A 678

ASTM A 73?

ASTM A 84I

Csc

55 and 60

65 ard'tO Mod td

65 and ?0 Mod 2d

I

I

2

BandC

B.

I

,16 - llh11j -23t6-23tft -23tft -z3116 -zJlt6 - 2

3116-2

31rc-2

3h6 - tt t2

3l16-2th6 -23116 -2

25

25

25

25

25

25

30

25

25

30

25

20

20

20

20

20

20

20

25

20

m25

20

20

20

20

20

20

20

20

20

25

20

m25

20

20

t5

t5

l5

l5

ml5

l5

20

l5l5

l5

ISO 630 E 355 Qualiry D..ac ,rc-2 25 mcsA G40.21-M 260WT.& 3t6-2 25 ?!CSA G4O.2I-M 3oovrFn ,\6-2 25 mcsA G40.21-M 350s/tc'd' tlrc-2 25 20

m20

mm

Notcs:.SrcR.2.l.2.bFor design mc(al tcmpcratures of- 40oF ,nd lowcr {tc p(ate impact values shall bc nis.d 5 fl-lhcThc tiequcocics of tcsting for ncrharical and chemical prcpcnies shall be at l.ast e4ual ro $osc of ASTM A 20.dscc 4.23 for a complctc dascripiion of rhis matctial.Thc sEel shall bc fully kill€d aM madc with fine-grain Factice.

r-rgure 22.4 IVinimum Charpy V-noich impact requlremenls for primary components plate specimens (transverse) and weld spec mens incJuding ihe heal affected

Fron API 620 Appendix R. table R 2

. Table R-3 (Figure 22.2) for design metal temperature downto -60"F without impact tests unless they are requifed by Ta-ble R-4 (Figure 22.3) or by the purchaser

. lfapproved bythe purchaser, the material may be seiecledby the requirements of paragraph 4.2.2 of API 620. Thisprovides three oossibilities for the reduction in the stfin-gency of the material selection criteria where the actualstress under design conditions in the component in ques-tion does not exceed one third of the allowable tensilestress.

For the parts of an outer tank which do noi contain the vapor-sed gas (i.e. in the case of a design using a fixed roof innertank), the material may conform to any of the materials listed inTable 4-1 . Consideration ofthe design metal temperature is notrequired if the actual stress in the component of the outer tankn question does not exceed one halfofthe allowabletensile de-sign stress for the material.

22.2.1 -2 Materials tor parts subjected to low temperatures

These parts are described as primary components which are inturn defined as components whose failure would result in leak-age of the liquid being stored, components which are exposedto the refrigerated temperature and those subject to thermalsnocK.

These primary components shall include, but not be limited to,the following parts ofa single walltank or the innerwallof a dou-ble walltank: shell plates, bottom plates, knuckle plates, com-pression rings, shell manways and nozzles including reinforce-ment, shell anchors, piping, tubing, forgings and bolting. Roofnozzles in contactwith the refrigerated liquid shall also be con-sidered as pfimary components.

Also included are those parts of a single wall or an inner tankwhich is not in contactwith the refrigerated liquid butare subjectto the refrigerated temperature. Such components include roofplates, roofmanways and nozzles with their reinforcement, roof

supporting structufal members and shell stiffeners where thecombined tensile and primary bending stress under designcond tions exceeds 6000 lb/in2.

For components falling into this category, Charpy V-notch im-pact testing at or below the minimum design metal temperatureshall be carried out to achieve the energy values given in tableR-2 (Figute 22.4). f he impact specimens shall be taken trans-verse to the difection of final plate rolling. This is different to therequirements of BS 7777 which requires the impact test speci-mens to be taken parallel to the direction of final plate rolling(i.e. longitudinal). Some steel specifications quote impact en-ergy levels for both longitudinal and transverse specimens, inwhich case the steel seleciion is quite straightforward. Otherspecifications only quote longitudjnal Charpy V-notch require-ments, in which case additional testing must be undertaken toensure that the required properties are provided.

It is not wise to estimate the transverse plate properties basedon the longitudinal properties determined at the same tempera-tures. The relationship between the two properties is depend-ent upon the directionality of the plate which is in turn a funciionof the amount of cross rolling in the production process. As aguide, the transverse energy levels can be between 50% and90% of the equivalent longitudinal values.

Note: Table R-2 also gives energy values for the weld metaland the heat-affected zone (HAZ)for use in the weldingOrocedure Oualifications.

22.2.2 API620 Appendix Q

22.2.2.1 Malerials lor parts subjected to ambient tempera-IUres

The Code describes these as secondarv comDonents definedas follows:


22 Mateial selection c teia fot low temperature tanks

Secondary components include those components thatwill notbe stressed to a significant level by the refrigerated liquid, thosewhose failure will not result in leakage ofthe liquid being stored,those exposed to product vapours, and those having a designmetal temperature of -60'F or higher. When roof plates,knuckle plates, compression rings, and manways and nozzlesincluding reinforcing are primarily subjected to atmospherictemperatures, these shall also be classified as secondarycomponents.

The selection of materials for these oarts is as described inSection 22.2.1 .1.

22.2.2.2 Materials tor parts sublected to low temperatures

These are described as primary components, and again API620 launches into a lengthy definition of which parts fall into thiscaregory:

Primary components include those components that may bestressed to a significant level, those whose failure would permitleakage of the liquid being stored, those exposed to a refriger-ated temperature between -60"F and -270'F and those that aresubject to thermal shock.

The primary components shall include, but will not be limited to,the following parts of a single walltank (most unusual to Appen-dix Q), or ofthe innertank in a double walltank: shell plates, bot-tom plates, roofplates, knuckleplates, compression rings, shellstiffeners, manways and nozzles including reinforcing, shellanchors, pipe, tubing, forgings and bolting.

This part ofthe code allows the use ofthe following materials forpnmary components:

. 9% nickel steel

. 5% nickel steel

. Austenitic stainless steel

. Aluminium alloy

. Nickel alloys (i.e. Invar, Hastelloy etc)

9% nickel steel is the most commonly used material for LNG,ethylene and ethane service. lts strength, weldability, availabil-ity and cost make this the obvious choice.

5% nickel steel is less widely available and its use is normallyrestricted to ethylene and ethane service.

Austenitic stainless steel is uneconomic for large low tempera-ture tanks, but is commonly used for the smaller tanks for thestorage of liquid oxygen, nitrogen and argon.

Aluminium alloy was once wideiy used for the construction oflarge low temperature tanks. The manufacture of aluminium al-loys is an energy intensive process and the significant in-creases in energy costs which took place in the early 1970scaused the pendulum to swing towards 9% nickel steel as theeconomic choice, and this remains the case today.

The use of nickel alloys is most unusual and will not be furtherconsidered.

The commonly used AST[,4 materials are listed in table Q-1(Figure 22.5).

PlaGs ar|d Sruclu.alMcmbcrs

Pipingand Tubing Forgings BoltinB

A 153 (Scc not. l)

A 553, TyF I (scc not. l)

A 645

A 24O, Typc 304A 240, Tnc 3ML

B 209, Alloy 30034 (scc notc 5B 209, Alloy 5052{ (s.t notc 5)B 209, Alloy 50E3{ (!.c nolc 5)B 209, Alloy 508tu (scc notc 5)B 209, Alloy 51544 (!cc notc 5)

B 209, Alloy 545@ (scc notc 5)B 22t,Alloy 6061-T4 udT6B 3O8, Alloy 6061-T2 ard T6

A 333, C.adc 8 (scc not.2)B 444 (UNS-N06625), Cr. I

A 334, Gradc 8 (scc notc 2)B 444 (UNS-N06625). Gr. 2

B 619 (UNS-N l0276xscc norc 3)B 622 (UNS-N102?6)

A 213, Gr.de TP 3O4

A 2 I 3, Gradc TP 304 LA 312, Crrdc TP 304 (scc notc 3)A 313, GrNde TP 304L (s.c norc 3)

A 358. Gnde 304, Class I(!cc Dote 4)

B 210, Alloy 3003-0B 2 10, Alloy 3003-H I 12B 2lq Alloy 5052{B 210, Alloy 508tuB 2l0,Alloy 5154{

B 24l,Alloy 50524B 241, Alloy 5083-0B Zl, Alloy 5066{B 241, Alloy 5454{B 241, Alloy 5456{)

A 182, Gradc F 304A | 82, Cradc F 304L

B 247, Alloy 3m3-HI12E 24?, Alloy 5083-Hlt2Mod

A 320, CradcsBE, B8C, B8Mand B8T

82l I, Alloy 606l -T6

Notas:

l. Whc{r Fcssurc para d! rndc ofASTM A 353 orA 553 rnat dd or nickcl rlloy, pipe nangc! or pipc may bc rustcnitic stainlcss sl.cl ofr rr?c that cannot bc hardcdcd by h€at EEaEncnL Pipc nanSca or pipc may bc wcldcd to nozzlc nccks of dE prcssurc part mat tial if thcbutr wcld b locltrd rno(c than I dist$cc cqual !o thc 4 mcrsurcd from thc facc ofthc rcinforccmcnl whcrr r= insidc ridius of $c nozzlcncclq in iL rnd r s thick|ess of thc mzzlc ncck in in. Thc d.sigr of thc nozzlc ncck rhall bc bas.d on thc allo*€blc stcss veluc ofwcrkcr mrlc.ial.2. Scrmlcss piping rnd rubing only.

3. Wcldcd piF sha bc wcld.d ftom rhc ootsidc only by thc tungstcn-arc inscn gas-shieldcd CfC) prcccss nithout $c addition of 6llcrmctrl and Ehall bc hydroslaticdly testcd.

4. Impacl lcst of wcld! shall bc madc for thc w.lding proccdurc whcn rcquncd by Q.6,3.

5. ASTM B 221 structural scctions dE also Dcrmittcd.

Figure 22.5 ASTM materials fof primary components

Fron API 620.Iable A-1


22 Mate al selection criteria for low temperature tanks

V.lu. R.qoicd for Mhimln V.l@ Wi$ootAeco(li.C Rcquiriq R.r.!P

(rr.lb) (ftlb)

vrlcRcqli!dd MiniooD hlc Wirh&lA...ptd R.qrili.t Ra6tr

(ftnb) (finb)

l0 x 10.00

l0 x ?J0

l0 x667

l0 x5.0

r0x333

t0x2-!).NG,abil-

rally

N'ld:rA$dg. of dr!. rFdeaBlonly G .pcdm of. .c!

: gure 22.6 Charpy V-notch impact value

+om API 620, table Q-2

mpact testing is not required for primary components madeiom austenitic stainless steel, nickel alloys or aluminiumaUoys.

mpact testing is required for primary components made from3% and 5'/o nickel steels.)laie matedals shall be imDact tested in accordance with the'bllowing:

. lmpact test soecimens shall be taken transverse to the di-rection of final plate rolling.

. Charpy V-notch specimens shall be cooled to a tempera-ture of -320"F for impact testing (conveniently the tempera-ture of liquid nitrogen).

. For 5% nickel steel only, the test temperature may be raisedIo -22O'F.

. The transverse Charpy V-notch impact energy values shallconform to table Q-2 (Figute 22.6).

. Each test shallconsist of three specimens, and each speci-men shall have a lateral expansion opposite the notch of notless than 0.015".

The Charpy V-notch impact energy levels quoted in Table Q-2afe low by comparison with the levels usually associated with5% and 9% nickelsteel olate materials ofrecent manufacture.-ihe code also gives rules for the impact testing of structuralshapes and forgings in these materials. The manufacture andrse of these is most uncommon.

22.3 The requirements ol BS 7777 i Paft 2'.1inimum supplied plate thicknesses for tanks designed to thisrode fall into two categories:

. For shellplates where thethickness is based on axialstabil-ity or internalvapour pressure considerations, for roofthick-nesses based on minimum requirements, bottom orannular plates, The minimum thickness based on edgemeasurement shall not be less than the specified thicknessless one halfof the total thickness tolerance specified in EN10025:1990.

. For shell plates where the thickness is based on fluid pres-sure considerations or roof Dlates where the thickness isbased on a calculated thickness. the minimum thicknessbased on edge measurements of the plates supplied shallnot be less than the calculated values.

:or 9% nickel steels where the required thickness is based on:re properties ofthe weld metal ratherthan the properties oftherlate, local thinning remote from the edge of the plate, due to'olled in scattered scale on the plate surface, is acceptable pro-/ided that the measured thickness is not less than g0% of the:alculated thickness of the Dlate.

25

t9

t7

a

6

2A

ll

t0

1

5

Mlntnrmddlgn metd .

Thtclness Mste d2)

T>O ts15

15<t<36

BS EN 10025 : 1990Fb 430 B orFe 610 B

BS EN 10026 : 1990Fe 430 C orFe 510 D1

T> -20 ,<16

15<ts35

BS EN 10026 : 1990Fe 430 B orFe 610 C

BS EN 10026 : 1990Fe 430 Dl orFe 510 DD1

'/ See 6.2.2.

'?) Other matelials may be used pmvided th€y ar€ equivalent

to rhos€ sp€cilied, and pfolided rhe purchas€r andmanufacturer aSree [o such a subsriturion.

at5

t3

l0

7

5

l6

t2

t0

8

5

the

1of

in-70sthe

ner

F gure 22 7 Stee s fof outer tank compresson area, roof and roofstr!ct!rewhere the minim!m desgn tempe€ture is based on ambient lempe€ture

Frcn BS 77/7 : Pai2, table 6

22.3.1 Materials for parts subjected to ambienttemperatures

For steel outer tank parts where the minimum design tempera-ture is based on ambient temperatures, the materials shall beselected from Table 6 (Figure 22.7). These parts would nor-mally be the compression area, the roof plating and the foofstructure. lt is recommended that for the uK the design metaltemperature should be taken as -10'C. For areas outside theUKthedesign metaltemperature should betaken as the lowestdaily mean temperature (i.e one half of the sum of the dailymaximum and minimum temperatures).

For the outer tanks of double-walled single containment tanks(i.e. those for containment of gas only), the materials shall beselected from Table 7 (Figure 22.8).

Note: The steel designations given in these Tables are thosefrom the earlier version of BS EN 10025 (Reference22.1). fhese have now changed and the appropriatenew designation should be used taken from the latestversion of this Code (Reference 22.2).

22.3.2 Materials for parts subjected to low tempera-tures

The Code focuses its attention on the use ofsteels for inner andouter tanks designed to contain refrigerated liquid.


22 Material selection citeria for low temperature tanks

Figure 22.8 Steelfof outer conlainers forsingle containmeni ianks

BS 7777 : Patl2, table 7

The use of aluminium alloys is subject to agreement betweenthe purchaser and the manufacturer of the tank in question. Ashas been mentioned earlier, the use of aluminium alloysfortheconstruction of low temperature tanks these days is very un-usual.

Guidanceforthe use of aluminium alloys is given in AnnexC ofB57777 : Patt2- This Annex requires the use of plate materialsto BS 1471 (Reference 22.3) and structural materials to BS'1474 (Reference 22.4). No doubt these rules could be ex-tended to cover materials of US origin to ASTM standards byagreement. The maximum shell plate thickness is restricted to55 mm.

For the use ofsteel materials, the code has defined six materialtypes:

. Type I steel: Normalised carbon manganese steel con-forming to BS EN 10028-3 (Reference 22.5) gtades P275NLI or NL2 or grade P355 grades NL1 or NL2.

. Type ll steel: lmproved toughness carbon manganesesteels.

. Type lll steel: Low nickel steels (normallythese are propri-etary steels with nickelcontents of between 1 .5 and 3.5%).

. Type lV steel:9% nickelsteels conforming to BS 1501: Part2 | 1988 (Reference 22.6) type 510 (this code has subse-


Fjgure22.9 l\4aterial iypesfortankshell and bottom

BS 7777 : Part 2, table 2

quently been replaced by BS EN 10028-4 (Reference22.7)).

. Type V steel: lmproved 9% nickel steels conforming to BS15O1 : Par12: 1988 type 510 improved.

. Type Vl steel: Austenitic stainless steel conforming to BS1501: Part 3 : 1990 (Reference 22.8).

It is interesting to note that the BS Code does not quote or allowthe use of a 5% nickel steel which API 620 indicates as beingsuitable for ethane and ethylene storage tanks. A reason putforward for this was that the use of 5% nickelsteelwas quite un-usual and the infrequent manufacture ofthis steelwas inconve-nient to the mills which charged a premium for its supply. This,together with the difierence in strength between 5% and 9%nickel steel, combined to make gyo nickel steel a more eco-nomic proposition and consequently the 5% nickel steel be-came irrelevant. A recent re-examination of this supplycosUstrength issue suggests that this perception is no longertrue.

The use of the six types of steel for the various products to bestored is described in table2of 857777 :Paftz, (Figurc 22.9).For single containment systems, the material chosen is gener-ally one type higher in quality than is the case for double and fullcontainment systems. Although experience has demonstratedthat the risk of failure of a single containment system, correctlydesigned and fabricated in accordance with British Standards,is very low, an enhanced material quality for this type oftank ismade in the attemptto offset the more serious consequences offailure. This table gives no rules for materials to be used for thestorage of liquid oxygen, nitrogen and argon. These productsare covered by BS 7777 :Paft4 (Reference 22.9) ,the rcquie-ments of which are described in Section 22.4 below.

The Charpy V-notch impact testing requirements for the platematerials is given in Table 3 ofthe Code (Figure 22.10). Notethat the impact testing is specified as longitudinal which is dif-ferent from the API requirements.

The CharpyV-notch impact testing forthe weld metal is given inTable 5 of the Code (Figure 22.11).

The philosophy behind the materialselection and testing ofthetypes ll and lll steels needs some words of clarification. The ba-sic requirement is to provlde a toughness of at least 27J at thedesign metal temperature in the heat-affected zone (HAZ) ofthe production welds of tanks for double or full containmenttypes, and 27J at 25"C +/- 5"C lowerthan the design metaltem-perature for single containment tanks. The steels used in thesecategories are well known to be susceptible to degradation ofthe weld metal impact properties in the HAZ.

The normal practices of allowing the tank manufacturer tospecify and purchase the steel, the welding methods and

]Iinimum design

7'

Thickness Materiatl)

T > r l0 I =:10 BS EN 10O25 :

1990Fe 360 B,Fe 430 B orFe 510 B

+10>7>0 t s25

2'o <t <35

BS EN 100251990Fe 360 B,Fe 430 B orFe 510 B

BS EN TOO251990Fe 360 C,Fe 430 C orFe 510 Dl

0 > 7 > -20 t < 12.5

12.5<r<20

20<r<35

BS EN 10025 :

1990Fe 360 B,!'e 430 B orFe 510 B

BS EN TOO25 :

r990Fe 360 C,Fe 430 C orFe 510 C

BS EN 10025 :

1990Fe 360 Dl,Fe 430 Dl orFe 510 DDI

rrSee 6.2.2.z) Other ma.tedals may be used provided they are equivalentto those specified, and provided the purchaser andmanufacturer agree to such a substitution,

Product SinBlecontrlnment

Double orfullcont$lnment

Typicalproductetordgetempetatute

Butane

AmmoniaPropane/propylene

Ethare/ethyleneLNG

Iype IITlpe IITYpe iII

Tlpe W

D?e V orVI

Tlpe IType tr

lVpe IV

I}pe IVr)

*10.c-35 0C

*50.c

- 105 0c

- 165 0C

'r For thicknesses geater than 30 rnrn and less than or equalto 40 mm, 'lvpe V or VI is necessary,

BS

. gLre 22.10 Longitudinal

33 7777 : Pat12, table 3

Weld metrl tor desiglnrtrdsteel tyle

Charpy V-not h lnpact te8tenefgy,

For matedals used atambient temperatures

For lype I steels

For $pe tr steeis

For TYpe III steels

For l}pe [V steels

For lYpe VFor $pe VI

27 J at -L0 6C

60 J at -30 'Cz)60 J at - 60 ocz)

50 J at -80 ocz)

35 J at - 196 oCa)

35 J at - 196 oC3)

Not r€quircd

', Enerty velue ls the minihum average of three sp€cim€ns,with otrly one sirui]e !'alue leis than the value specili€d andMth no single value less than 76 % of the vatue specified.u ) Th€ intent of this speciJication is to ensurc that theFoduction \ileld6 in the tank meet ihe 20 J minimum at thete8t temperoiue gi\,€n above. In ordet to achieve thjs, theprocedur€ t€st of weld m€tal for \?es I, II and III steels isrequir€d to demoistmte a higher Charpy V-notch energ/value to compensate for the scatter oi rcsults inherent inChaey V testiru of weld mat€dals.Employing the forcgoing welding prcc€dure, prcduclion testplates are nolmally unn€cessaq/, and ai€ not a feaiure of thjsstandard. However, if Uoduction test plates arc called for t'ythe pllldns€! minimum averaS€ energy values of 2? J forthree speeimem with no single mlue less than 20 J arerequLed at the te8t temperature given.3) 'Ihe minimum average impact enemr for the weld metal for'Ihes lvand V is based on the high nicket, austenitic weldmetal. In the event of weld met€l b€ing offered incomposition matching that of I % ntckel plate, additiona.lspeciallst advice ehould be obtelned.

NmD. Where austenitic weld metal is us€d for weldin!T}?e I or'IVpe m st€€ls, Charpy V-nokh inpact rcsring ofthe tlcld metal is not r€quk€d.

't-,e 22.11 Weld metat C harpy V-notch impact test energy:'.n BS 7777 : Patt 2, tabte 5

r:'rsumables, and to then demonstrate that the 27J had been::rieved in the HAz by means of production weld testinq on: ::. carries with it the possibility offailure to attain the req;ire--ent. This would have a devastating effect on the cost and:'rgress of the project in question as by this time the steel, : JId have been purchased, delivered, fabricated, erected and:;'iially welded. To avoid the possibility of encounters with this:-barrassing and expensive scenario, it was decided to devjse: -ethod of pre-qualification for the steels commonlV used for:-:se products.

-- s pre-qualification procedure was based around the deter-- -atron of the AT shift for the material jn question. This is the-:asurement by testing of the actual decrease in the touoh_- .:s ofthe parent plate and the HAZ of the steel. Annex Aof

-BS

---7 : Patt 2 gives the details of the methods of determinino

22 Material selection citeria for tow temperature tanks

|- :loo t lo -

Hut weld

I rJf-_-lE ut_l_

NqlE l A mininun oi 15 pllre specinero are taken ar mid rhickne$ ioh rhe 3de side

Longitudinal dis of sD€cinens ii !o be p€rpendicutd to weld uis.NOlIj 2. A ninjhum of 15.pecimens.F taken ar mjd rhickne$, wiih rhe noLh tdated inlhe tlAz, and e etched io denon6i6ie that rhe norch is hoi in veld melat o. par€nt

F gute 22 12 Lacal a. af Charpy V,noich impact energy test spec mensFron BS 7777 : Pan 2. figure A3

this lT. involving a standardised weld specimen as illustrated jnfigure A.1, A.2 and A.3 (F)gwe 22.12).

Th js allowed a minimum of 15 Charpy V-notch specimens to betaken from both the parent plate and the HAZ, tested at differenttemperatures to allow the 27J transjtion graph indicated in Fig-ure 22.13lo be produced. With the AT determjned, the platematerial could be tested at a temperature of AT below the de_sign metal temperature to ensure that the required 27J will beachieved in the HAZ at the design metal temperature.

To ensure that the material does not exhibit a flat CharpyV-notch transition, a batch test (one test per 40 tonnes) is re-quired to show a longitudinal impact of .120J at either _20"C(type ll) or -50'C (type lll).

This then pre-qualifies the particu lar steel. lf any ofthe followingchange, then the test must be repeatedl

. Steelmaker

. Steelmaking process

. Deoxidation process

. Desulphurisationprocess

. Casting practice

. Heat treatment

. Changes in specified chemical composition including mi_cro-alloy additions.

This materialselection method has given rise to a long-runningand occasionally acrimonious argument. The UK a;d Euro_pean-based steel producers were opposed to the proposal andrefused to carry out the test work necessary to pre-qualify theirsteels. When BS 7777 was presented as a possible draft for in_corporation into the new European lowtemperature tank Code,

tsS

rgl..li

.S,

).:

)?-

ie:

n?e ITYpe tr

\?eIIl'I!"e IVI}pe vTJpe VI

Nomalized carbon-manganese

Irnproved toughness caxbon-manganese

Low nickel steel

I % nickel steel

Improved I % nickelAustenitic stairiless steel

27 J at -50 'C27 J at -50 oC - LIa)27 J

^t -80 oC - L?a)

36 J at 196 oC

100 J at -196 .C

No impact testing requircd

Not required

-20 ac

-50 oc

Not required

Not requircd

'' Ener$ lelue i5 the minirnufr 8k€ge oI itRe sleimens, wi$ only one siqle r?rue bs than the i?tue specified and with nostnele Elue les than ?6 * ol the lalue soeified.a, For matedrl lh'cknes ls rhm 11 nn, 10 nn x 6 m sub,si@ specihens are ro be u*d, rnd demonstate ?O % of thevalues speilied lh thri ia.ble_ Ibr 1}?e V sieet, rhe lulue js ro be 50 % of the Btu€ spejfl€d in tnis t5bte,3r lrnp&i t$rhg i5.arn€d our on esch Dtare ro demorotrate the required inpacr rrlue. In ad<tftion, rGring rr s frcquency of onet4t p€r40 t batch i! to be clrded out to demomtrate the 120 J requirem€trr (4e anner A). The deti iions of pl&te.nd iaichaE given in BS EN 10025.

', RefeFnce should be hdde to urex A,

Charpy V-notch impact lesling


22 Mateial selection citeia for low temperature tanks

Figure 22.13 Temperature shift (AT)at the 27 J toughness tevel

Fram BS 7777 : Parl2, figure A.4

this issue was atthe heart ofan ill-tempered series of meetingswhich delayed the production of the new standard for years, areaction which seemed to be disproportionate to the impor-tance of the issue. The end result is that the new EN will nothave this selection method for the materials to use for butane,ammonaa, propane and propylene tanks. The pre-qualificationroute always seemed sensibleto the author, and it is hoped thatnottoo many tank builders fall into the holes described above.

Type lV steel is now something of a historical irrelevance thesedays. lmprovements in production methods and chemistryhave meant that nearly all9% nickelsteels produced are nowofthe improved quality. lt is not uncommon for the tank specifica-tion writers to require restrictions on sulphur and phosphoroustevets.

22.4 The requirements of BS 7777 : Part 4As has been mentioned earlier. this Dart ofthe Code is for tanksfor the storage of liquid oxygen, nitrogen and argon. lt was wri!ten at the request of the UK paft of the industry which had formany years specialised in the production, storage and distribu-tion of these products. This group argued that their intrinsicallysafer products did not require double or full containment sys-tems which form much of the thrust of the remainder of BS7777.

They were of the opinion that single containment was appropri-ate to their industry and persuaded the committee to let themwrite BS 7777: Part 4 which gives the rutes which they consid-ered that they needed.

22.4.1 Parts subjected to ambient temperatures

The materials for these parts are carbon steels selected in ac-cordance with the appropriate parts of BS 7777: Pan 2 GeeSection 22.3.1).

22,4,2 Parts subjected to low temperatures

TypeV orVlsteel in accordance with BS 7777: Part 2 (see Sec-tion 22.3.2) or aluminium alloy in accordance with Annex C ofBS 7777: Pan2 shall be used.

The most usual choice is austenitic stainless steel.

22.5 The requirements ot PD 7777 : 2000This published document (Reference 22.10) has been dis-cussed in Chapter'18. lt is concerned with the use of 9% nickelor of austenitic stainless steels which can be demonstrated topossess crack arrest properties and the influence ofthis fact onthe hydrosiatic testing requirements.


At the time when this PD was published, there were no nationalor international standards for ferritic steels in which toughnessrequirements are specified for crack arrest capability. Anumberof proprietiary steels have been developed for which some testdatia have been provided, and these are summarised in Table 1

(Figure 22.14)with their related references.

It isthe responsibility of the purchaserand the manufacturertosatisfy themselves that the steel suppliers possess the neces-sary expertise to produce suitable steels and provide the nec-essary evidence that these properties can be achieved in pro-duction. Where possible such evidence should be based onproduction data, but, because of the stage of development ofthese steels, the evidence may have to be based upon trial pro-d uction run data. In this case, the PD recommends that the ma-terial property data should be based on a minimum of two castsof steel of at least 250 tonnes each.

The PD goes on to make a number of points relating to the steelmaking process and the chemical composition of the steel. Inbrief these are:

. All steels should be produced by a basic oxygen processand vacuum degassed.

. Steels should be made to fine grain practice.

. It is likely, but not mandatory that these steels will be pro-duced by either a thermo-mechanical controlled process(TN/CP), or by quenching and tempeing (Q&T).

. The tank manufacturer should obtain from the steelmakerthe composition limits of the elements specified in EN10028-4 (Reference 22.7)together with those elements, asagreed between the purchaser, the manufacturer and thesteelmaker, which are likely to afiect the final properties ofthe steel.

. The chemical composition as measured by the ladle analy-sis should be reported.

. Requirements for product analysis and for carbon equiva-lent should be agreed between the purchaser, the manufac-turer and the steelmaker.

Next, detailed Charpy V-notch testing requirements for theplate and drop-weight testing to determine the nil ductility testtemperatures (NDTT) of the plate and the HAZ, are given inconsiderable detail.

Two points are worth making at this stage:

. The various parties required to come to the above agree-ments will need to have knowledge in depth of thesteelmaking process and the influence of the variables onthe steel orooerties.

. This process makes the steel very much a non-standardproduct. Bearing in mind the arguments surrounding the ATproposals which took place in the CEN committee, much ofwhich was lead by European steel makers who were reluc-tant to manufacture non-standard or special steels, it willbeinteresting to see if this proposal gains in popularity. Per-haps the "carrot" ofthe steel weight-saving associated withthe partial hydrostatic test may prevailand force the issue.

22.6 The requirements of prEN 14620The material selectjon requirements of this draft Code are ingeneral similar to those of BS 7777 : Part 2, described in Sec-tion 22.3. The AT method of pre-qualif,/ing steels oftypes ll andlll has been excluded for the reasons given. At risk of tediousrepetition, it should be remembered that this is a draft or provi-sional document and the formally adopted Euronorm may con-tain changes.

22 Material selection criteria for low temperature tanks

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Bibliographytll SJ. Carwood and C S, wi€Fl er., lan tetupetuhttein tute reqiru,nenls drtd' qflah atrest @ aeptsforit;roge tar*s, Proc Conf. "Bulk Storage lbnlis', Paper 4, IBC Guif Confelenceq Abu Dhabi, 1996'

[2] C.S. wie6net and B. Hayesi A rari4u oJ L'rt/* araest t2sta, nadds unl' applica'i'7t5' Ptoc Conf 'Crackiirest Concepts for Failure hevention and Life b<tension", Pap€r 3, Abington PubXshing, 1996'

[3) C.S. Wiesner, SJ, Ga$rood and J.B, Denham; Thz s"ecili/ntiorl oi crock arrest Pvperti$ Jor stornse tanks'b;ci$dnd

d.nd tqownadati.otls. ASTM SI? 1337, Symposia on Design Cdteda to assue Stuc'h|ral Integdry,

New Odea$, May 1997.

[4] c.P Smedley; Pr"dtcrira and. specifiaatiotu oJ crad anest poperti.4s oJ stcd plalr. Intemational Jounal forPr€ssure vessels and Piping, 40 (f989), pp 279 - 302

[5] C.S. Ui€net B. Hqyes and .!LA Mlowhb$ Ctudk d|'tzst in nadern steels ond, theb udhnetts --Cowarison, behtwn snat! and large scdle tzst fta ,s. Int4rtBtional Joumal for Pr€s$rc V€ssel md Ptping,

53 (1993), 100.

[6] C.S. Wiesnef Predtcaing sttltotrEal, cftek qnest bdtsviour 6i/ng s1t81Ls@14 ,nilefial chdlv.Eti&liorti€srs. Int€rnational Journal for hessule vessel ard Piping' 69 (1996) pp185 - 196.

lfl Y. fawa€uchi and A-A- Willoughby; Ctu.& orsst tpst and t].eir uae i,n etxhaliw nraterialytpefli8,'fheW;lding Institute Sympooiuft\ Newcasde upon Tne' Apiil f986.

[8] B€6syo, Ozaw4 lbwaguchi and Nalonishi n@cerrt dadrpnett of 6beLplat"s for lan lqtuperfr1re sbt!4e;aalis. 'Ihe Sumitomo Se;ah No. 2q November l9B4 Sumitomo Metal llrdusbies Ltd, 'IblvoI9l K Bessyq lMcPsred plorpJo lau tzrhperal'un stotuge ta*s, ?m Int€nrational hEss|ne VessdConference, Druseldo4 September l9g2

l10l hirate corununication' British Steel Corporatioq June 1984

Illl Doucet, hessouyre, Bourges, Blondeau and C?diou Raenl yrogress in 9% Ni stzelfor LNG 6ppli.olions:'un'

WCS gron" "bel.

E;oya n;mish Society of Engineers Lt€rnational Confelence, Brugge, MAy 1984.

llzj $4P Benter, alrd WJ M!trplry, Erpldsion bulge and d'tq ueight bsnkt4 oJ QT I oA Ni stql A&IEPeholeun MedEnical EngineeF Confercnce, September 1967.

: !ure 22.14 Suiiable steels:'.n PD 7777 :2000,table 1


22 Mateial selection criteria for low temperature tanks

22.6.1 Materials for parts subject to ambient tem-peratures

The steelfor the vapour containing outer tank shall be selectedin accordance with Table 3 Figure 22.15. alternative types ofsteel are permitted providing equivalent properties can be dem-onstrated.

22.6.2 Materials for parts subject to low tempera.tures

The plate materials are again classified into a numberoftypesin the same way as 857777 :Patt2.The normalor unimproved9% nickel steel is no longer included for reasons already dis-cussed. The steeltypes, products to be stored and containmenttypes are the subject ofTable 1, Figure 22.16. The general re-quirements of the five steel types are as follows:

a) Type I steel:

A Type I steel is a fine grained, low carbon steel which shall bespecified for pressure purposes attemperatures downto minus35 'C. The steel shall meet the following requiremenb:1) The steelshallbe specified to meetthe requirements ofan

established European Standard (e.9. EN 10028-3). Steelswith a minimum yield skength greater than 355 N/mm,shall not be used.

2) The steel shall be in the normalized condition or producedby the Thermo-Mechanical Control Process (Tl\4CP).

3) The carbon content shall be less than 0.20 %. The carbonequivalent Ceo shall be equal to or less than 0.43 with

C",r = C + Mn/6 + (Cr + Mo + V)i5 + (Ni+ Cu) 115

b) Type ll steel:

AType ll steel is a Jine grained low carbon steelwhich shall bespecified for pressure purposes at temperatures down to minus50 'C. The steel shall meet the following requiremenb:'l ) The steel shall be specified to meet the requirements of an

established European Standard (e.9. EN 10028-3). Steelswith a minimum yield strength greater than 355 N/mm2shall not be used.

'c

Marenalsrade (EN 1002sr1993)

S 235 JRG2orS;75 JR orS 3551R

S 235 J0 or S 275 J0 or S 355 J0

S 235 J0 or S 275 J0 or S 355 J0

S 235J2G3 orS 275 J2G3 ors 355 J2G3

s ?35 J2G3 0r S 275 J2G3 0r S 355 J?G3

s 235 J2G3 or S 275 r2G3 or S 355 J2G4

Fo'desiQn meral lempeEtures below 20 "C 6nd/or for rhicknessesabove 40 n6. lhe charovv notch value shll be 27.1 rono rud,nar

NOT€ For desiqn neiar Gmp€ra|lr6 be]ow 0'c, rhe rouqhness or l}E Ne d

Figure 22.15 Sieel for vapour container/outer tank

Fron prEN 1 4620-2 : 2003, table 3

2) The steel shall be in the normalized condition or producedby the Thermo-Mechanical Control Process (Tl\y'CP).

3) The carbon content shall be less than 0.20 %. The carbonequivalent Ceq shall be equal to or less than 0.43 with

C"o = C + lVn/6 + (Cr + N.4o + \4i5 + (Ni + Cu) 115

c) Type lll steel:

AType lll steel is a fine grained low nickel alloy steel which shallbe specified for pressure purposes at temperatures down to mi-nus 80 'C. The steel shall meet the following requiremenb:1) The steelshallbe specified to meetthe requirements ofan

established European Standard (e.9. EN 10028-4);

2) The steelshallhave been heattreated to obtain a fine, uni-form grain size or produced by The Thermo-l\.4echanicalControl Process (TlVlCP).

d) Type lV steel:

A Type lV steel is a I %-nickel steelwhich shall be specified forpressure purposes at temperatures down to minus 196'C. Thesteel shall meet the following requiremenb:

1 ) The steel shall be specified to meet the req uirements of anestablished European Standard (e.9. EN 10028-4);

2) The steel shall be quenched and tempered

e) Type V steel:

A Type V steel is an austenitic stainless steel type which shallmeet the following requiremenb:

1) The steel shall be of 18 Cr 9 Ni type with low carbon con-IENI,

2) Flat plates shall be obtained from rolled coil, cold finished.

3) The steel shall be delivered in the solution annealed con-dition.

Plate thickness tolerances for Types I to lV shall be in accor-dance with EN 10029 : 1991 , Table 1, classB, Reference 22.12.

Plate thickness tolerances for Type V shall be in accordancewith EN 10082-2, Reference 22.12.

Charpy V-notch impact test values for base material; HAZ andweld metal shall be in accordance with Table 2, Figve 22.17 .

Note the change lrom longitudinal to transverse specimen ori-entation.

The document goes on to make a number of more generalpoints amongst which are:

. For material thickness less than 11 mm, the largest practicalsub-size specimen shall be used. The minimum CharpyV-notch impact test values shall be in direct proportions tothe value sDecified for full size soecimen.

. lmpact testing shall be carried out for each inner tank shellplate and annular plate. For other components impact testsshall be carried out per heavcast of the material.

. The degradation effect due to welding shall be taken intoaccount.

Low remperature ca6on-

lmoroved 9 % n,ckelsleel

NOIE For nidrer base weLd nerars ior Type rv sGsrs th€ rmp8cr roughn€s 6n6Qy tu w6ld m5r6land heal €fiecred 20n6 shan be 55J

Figure 22.17 lrinimum Charpy V-notch impact lesl enercy

Fran prEN 14620-2:2043, bble 2

Figure 22.16 Product and steelclass

Fron prEN 1 4620-2 : 2003, table I


:;gure 22.18 l\/linimum Charpy V-notch inrpact requifemenls for ptates and sections-.an BS 4741, figure 6

22 Material selection citeria fot low temperature tanks

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22 Material selection criteria for low tempercture tanks

. For certain materials, higher Charpy V-notch impact testvalues of lower test temperatures may be needed for the

base materials to meet the requirements in the HAz

The latter two requirements perhaps represent an olive branch

to the losers of the pre-qualification arrangemenb.

22.7 An example of a material selectionmethod from the pastTravelling back in time, the British Standards for low tempera-ture lanks were published as two separate documents. BS

474'l (Reference 22.73) dealt with tanks for service down totemperatures of -50"C and BS 5387 (Reference 22.14) deallwith tanks for service down to temperatures of -196'C.

As was, and indeed still is the case for API low temperature

codes, these British Standards contained rules for single con-

tainment designs only.

The material selection criteria for BS 5387 were unremarkable.For BS 4741, which confined its attention to carbon manganese

steels, the material selection methodswere more adventurous

Based on test work on a series of notched and welded Wells

Wide Plate Tests (WWP tests) carried out by the Welding Insti-

tute (reported in the main in The British Welding Journal ir''1964), a new selection process was devised The tesl work

showed that the resistance to brittle fracture ofcarbon and car-

bon manganese steels at low temperatures is dependent upon:

. The notch toughness ofthe material.

. The plate thickness.

. The extent ofcrackiike defects present (The introduction to

the Code states: "lt is believed that if such defects are ab-

sent, brittle fracture will not be a problem at normal rates ofloading").

. Degree of local embrittlement at the tip of pre-existing de-

fects. (The introduction to the Code goes on to explain that:

for carbon and carbon manganese steels, post-weld heat

treatment in the stress-relieving range temperature is effec-

tive in removing severe embrittlement arising as a result ofwelding or flame cutting during fabrication and construc-

tion.)

. The influence of mechanical over-stressing during the hy-

drostatic test.

In an attempt to relate all of these variables to the material se-

lection process, figure 6 was produced (see Figure 22.18) This

involved the use ofscales L. M and N and was quite difficultforthe designer to use. The result of using this figure often meant

that a single tank shell could have four or five different steels

Sadly the early version of this code was held to be a contribu-

tory cause to the Qatar LPG tank failure, and the rules were

changed. In particular the elegant figure 6 was removed and re-

placed by a simpler API type material selection method, and a

full height hydrostatic test requirement was imposed

So it was discredited and deleted, but nevertheless to the au-

thor's mind, itwas an heroicattemptto relate a large numberof

different variables to a material selection method on a single

sheet of Dapef. lt also reflected a time when the material cost

savings in a structure were seen as being a major factor in the

final price. The increase in the cost of the labour element ofsuch work in relation to the material costs in the interveningyears, has made this rather fussy procedure seem less rele-

vant.

22.8 References

22.1 BS EN 10025: 1990 specification fot hot rolled productsof non-alloy structural steels and their technical deliveryconditions.

22.2 EN 10025:1993 plus Amendment number 1, dated 15

June 1995, Hot rolled products of non-alloyed steels -Tech n i c al de I ive ry co nd iti on s.

22.3 Bs 1471:1987 Specification for wrcught aluminium al-loys for general engineering purposes: plate, sheet andstriP.

22.4 85 1474: 1987 Specification for wrought aluminium al-loys for general engineeing purposes: bars, extrudedround tubes and sections.

22.5 BS EN 10028-3:1993 Specification for flat ptoductsmade of steels for pressure vesse/purposes - Weldablefine grain steels, normalised.

22.6 Bs 1501:Paft 2 : 1988 Specification for alloy steels -ptates.

22.7 BS EN 10028-4 :1995 Specification for Flat productsmade of steels for pressure purposes - pa ft 4 : Nickelal-loy steels with specified low temperature propeiies

22.8 BS 1501 :Patl3:1990 Specification for corrosion andheal resisfing sfee/s: plates, sheet and strip

22.9 BS 7777 : Paft 4 : 1993 FIat bottomed, vertical, cylindri-cal storage tanks for low temperature seruice: PatT 4

Specification for the design and construction of singlecontainment tanks for the storage of liquid oxygen' liq-uid nitrogen or Iiquid argon.

22j0 PD 7777 : 20OO Altemative steel selection and its effecton design and testing of tanks to BS 7777

22j1 EN 10029 : 1991 Specifications for tolerances on di-mensions, shape and mass for hot tolled steel plates 3

mm thick or above.

22.12 EN 1OOB2-2 Staintess steels - Paft 2 : Technical deliv-ery conditions for sheet/plate and strip for general pur-poses.

22.13 BS 4741:1971 Specification for vedical cylindricalwelded steelstorage tanksfor low temperature serutce:singte walltanks for temperatures down to -50'C (sub-jeci to major amendments in January 1980).

22.14 BS 5387:1976 Specification for veftical cylindricalwelded storage tanks forlowtemperature lanks servlcedouble watl tanks for temperaturcs down to -196'c'


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23 Erection considerations for lowtemperature tanks

Efficient erection techniques have always been important to trank constructors for reasons of'finished costs". Nowadays it is frequently the case that a short construction timescale is asimportant as the cost. Driven by the need for fast, cheap and high quality site construction andfurther driven by tha often-onerous financial time penalties for non-performance, numerousnovel construction techniques have been adopted by the industry.

Whilstseeming to lag behind industries such as shipbuilding and offshore rig building in terms ofthe scale of modularisation and prefabrication methods adopted, tank builders have mademodest steps in the same general direction and some of these are described in brief in thisChaoter.

Contents:

23,1 General

23.2 Air raising of tank roofs

23.3 Tank jacking (or jack building)

23.4 A fast track ethylene tank

23.5 A fast track liquid orygen tank

23.6 Spiral jacking

23.7 The construction of tanks with reinforced conclete roofs

23.8 Concrete wall construction

23.9 Wall and base linels

23.10 Modular construction and p.efabrication techniques

23.11 Automated welding methods

23.12 Large inground LNG tanks

23.13 References

STORAGE TANKS & EQUIPITIENT 451

23 Ercction conaiderctions lor low tempercture tanks

23.1 General

Large low temperature tanks are often the critical componentsof a terminal or storage facility in terms of time. This fact tendsto place a great deal of emphasis on the tank construction pe-riod. Time is often equally, or on occasions, more importantthan cost to the facility owner. Early completion equals earlyrevenue, and forfacilities such as LNG terminals, the revenuesare enormous and so speak loudly in this equation.

As a consequence, the tank contractors and their subcontraetors are always under pressure in their efforts to obtain con-tracts by promising tighttimescales and furtherdriven in this di-reclion, once successful, by the financial penalties associatedwith not performing to these anticipated schedules. Over theyears, these pressures, together with those aimed at'Tinishedcosts", have given rise to some interesting and ingenious con-struction methods. lt would be thought that novel and advanta-geous construction methodswould remain the closely guardedsecrets oftheir originators, but forvarious reasons the geneElmethods tend to leak out into the Dublic domain and becomecommon knowledge quite quickly, whilstthe details and equip-ment used tend to remain the property of individual companies.

Some of these methods and sequences are described in thefollowing Sections.

The basic methods oftank erection, particularly relating to thesteelcomponents, are based on the methods used for ambienttanks and these are described in ChaDter 12.

23.2 Air raising of tank roofsThis is a commonly adopted practice for large tanks of the fullcontainment type, although there is no reason why the tech-nioue should not be used in other circumsbnces.

Where a pre-stressed concrete outertank is adopted, there aretwo options for the construction ofthe steeltank roofframeworkand roof sheeting which will be required for both steel or rein-forced concrete roof types:

It can be built at the fulltank height afterthe outertank shellis completed.

It can be constructed within the outer tank whilst the con-crete shell is being built and air lifted into its final positionwhen both are complete.

The former route requires the outerconcrete tank shelland theroof framework construction operations to be in series, whilstthe latter puts these operations substantially in parallel. Thetime required to design, procure the materials, carryout the fab-rication and erect the steel roof is often comparable with thetime to construct the concrete wall, which is convenientfrom aprogramming point of view.

Figure 23.1 Atypical airlifted roof conslruction sequence


theient

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The roof framework is erected within the concrete outer tankduring the construction period forthe latter method, starting assoon as materials become available and the concrete workswillallow lt is usual to provide a central king post and a peripheralring ofsupports to construct the roofframework. For certain de-signs of dome roof structures further intermediate supportsmay be required.

The roofsheeting can be constructed to be a loose membraneonly supported by this framework, or may be welded to theiramework to provide a generally more robust structure. lt isusual to construct the suspended deck and arrange for it to belifted with the roof. The sealaround the Derimeter ofthe roof canbe a specially designed segmental rubber type or a simple ar-rangement of polythene film and wire mesh. Other aspects lobe considered are:

. Suitable details must be provided to ensure that during thelifting operation the roof remains level, circularand concen-tric within the outer shell.

. When the roof arrives at the top of the shell, suitable ar-rangements are made to secure it from its upper surface.

. Trial lifts are helpful to trim and balance the roof before thefinal lift.

. The roof and the concrete shell must be surveyed prior tolifting to determine that no unacceptable out ofshape prob-lems exist.

: 9(]re 23.2 Air lifiing a roof up the inside of the inner shell of a double-watted

: gure 23,3 Large dome roof framework under construction:curlesy of Whessoe

23 Erection considerations for low temperature tanks

Figure 23.4 Large dome roof under construction, showing king post in positionand slart of roof platingCautesy af Whessoe

Figure 23.5 The dome roofemerglnq towards the end of a successiutair-tiftCouftesy af Whessoe

. Arrangements must be made to collect and dispose of rain-water which falls during roof construciion period, particu-larly if other works are being carried out beneath the roofprior to air-lifting.

Many of these details are of a company-confidential or propri-etary nature. Figure 23.'1 shows a typical construction se-quence involving an air-lifted roof.

Alternative arrangements involving air lifting are possible andFigure 23.2 shows diagrammatically one such, where the roofwas air-lifted up the inside of an inner steel tank.

Cantibwr s€ction of roofTempo€ry extension to


Figures 23.3 and 23.4 show a large dome roofframework (77 m

in diameter) being erected within the outer pre-stresseo con-

crete wall. The two people to be seen walking around the perim-

eter support in Figure 23.3 gives an indication of the scaie of

this particular structure. Figure 23.5 shows a dome roof emerg-

ing into view towards the end of a successful air-lifting opera-

lon.An alternative, but generally similar erection technique involves

the roof structure being erected at a low level. Instead of the

roof being lifted into place by air pressure, a mechanical lifting

method using multiple strand type jacks is used

23.3 Tank jacking (or jack buitding)Aswith ambient tanks, jack building hascertain attractions For

a single-walled tank or the outer metallic wall of a double-walled

tank, the usual construction sequence is:

. Erect and weld the top two shell courses on temporary sup-

ports.

. Erect the compression area

. Erect the tank roof.

. Install the jacking system.

. Jack up the tank and install the remaining courses one at a

time and weld.

. Installthe annular plate.

. Jack down the tank and weld the shell-to-bottom joint

Jack building can have certain advantages lt clearly depends

upon local circumstances but these can be:

. The tank is protected from the possibility ofwind damage as

shown in Figure 23.6 |n this example, the outer tank of a

double-walled LNG tank was being built by conventional

methods. lt blew down just before the compression area

and tank roof could be installed, which would have stabi-

lised the structure. The temporarywind stiffening used was

clearly inadequate. With the jacking method, the roof is in-

Figure 23.6 Wind damage to an outertank being buili by conveniional meth-


Suspended deck erected

Figure 23.8 A possible variation on jack building

stalled at a low leveland the permanentwind stiffening is in-

stalled progressively as the shell increases in height,

consequently the structure is saieguarded from possible

wind damage at all stages of the erection procedure

. Roof erection is carried out at a low level.

. Shell erection, welding and inspection is carried out at a low

level and this technique can be combined with automated

welding techniques.

. "staging out" ofthe tank shelland subsequent clean down ls

avoided.

The disadvantages can be the cost of the jacking equipment

and a lack oi flexibility if anything goes wrong

Fioure 23.7 shows a tank beingjack built. Figure 23 8 illustrates

a Sossible variation which has been used where the outer tank

is jacked and the inner tank is hoisted up with it such that.both

tanks are builtfrom the top down and allerection and welding is

conducted at low levels.

23.4 A fast track ethylene tankThis tankwas a double containmenttype consisting ofan inner

tank of 9% nickel steel, an outer tank of carbon steel wlthin a

hiqh Dre-stressed concrete wall ofthe Preload wire wound type

tni base was ofthe elevated type built on a poor riverside allu-

vial site which required heavy piling. The tank was of modest

orooortions, the innertank being 23 0 m in diameterand 23 5m

in height giving a useful capacity of 9730m3 A set of unusual

lomm"ercial colndition" relating to the price and availability of

liouid ethvlene pertained at this time ('1977) which dictated the

necessity of a very fast construction programme 1o allow the

owner to take advantage of this transient economic situation'

t

Figure 23.7 Atank beingiack built

I

:=

1) - Sta.i pemanenttoundalion prling

- Erect temporary ioundaton

2) - Consrrucl pehanenr elevared slab

- Remove tedporary f oundaton

5l E.e6t nne.9% nickelste€r sherl- Erdl outer pr6lr6sed conc.ete shell

23 Etection considerctions fot low temperature tanks

The tank owner was lCl at North Tees Works, situated in thenorth east ofthe United Kingdom, who at that time had its ownengineering and construction staff. The tank design, fabricationand erection contractor was Whessoe Heavy Engineering whohad its head offices and fabrication facility nearby in Darlington.

The erection sequence proposed involved the placing in paral-lel of the following activities:

. The construction ofthe piled base slab and the erection onan adjacent temporary foundation ofthe outercarbon steeltanK.

. The construction of the base insulation and the inner 9%nickel steeltank and the outer ore-stressed concrete wall.

The first of these parallel activities required the moving of thecompleted outer steel tank from its temporary foundation ontothe permanent elevated base slab. This was accomplished bythe hovercraft technique of attaching a specially designed skirtaround the bottom of the tank shell. The tank was to be lifted bythis means and towed onto its new permanent base slab. Thistechnique has been around for many years and has been com-monly used to move mainly ambient temperature tanks, usuallyto other locations on the same sites. ln this instance the tankwas quite heavy and calculations showed that the pressure re-quired to lift this tank could damage the shell-to-bottom detaildue to high radial loadings caused by the pressure beneath thebottom plates.

To avoid this possibility of damage, air was bled into the tank tobalance the pressure beneath the tank bottom. During this op-eration, an interesting and amusing event took place. The skirtwas fitted, the air blowers installed and the operation com-menced. The air pressure beneath the tank bottorn was moni-tored and rose slowly to the pressufe calculated to equate tothe tank weight. Nothing happened. The pressure continued torise to around iwice the calculated lift pressure and still nothinghappened. Greai consiernaiion. much head scratching and thebeginn ngs of rurnblings amongst ihe numefous onJookers of:"lt was a silly scheme anwvay. , lt will never work." and "lt willhave to be cut down and erected conventionally." took hold. Allof a sudden, there was a loud bang, the tank leapt some 3 feetinto the air and lurched alarmingly, sending the accumulatedonlookers scuttling away from the tank in panic. The tank thenbehaved perfectly and was duly towed to and installed on itsnew base.

nternal plate handling arEigemenl

6)- Complete nnerlank, hydrc tesl- Codpl.16 ouler concGle wll, prslrds

Complete fLfiings ] a E, elc

Figure 23.10 Simultaneous conslruction of the carbon shell ouler tank and theelevated perma nent fou ndation

3) - Aj. I comp eled outd tank ahdmo!€ onto comdeted elevaled

::-re 23.9 The fasl track elhylene tank erection sequence


23 Erectian cansicleratians far low tenperctLtre lanks

Fg!re23.1lTre nf atec sk rt used to fl lh-..!1-.. isrrk

.t

F.-F3"rb.!+*FF=?.=_

Fgure 23 13 The neary compleied strLrcture

23.12) and a view on the roof showing the nearly compleiedtank (Figure 23.13).

The existing good relationship between the owner and the tankcontractor, the r geographical proximity, the existence of a sub-stantia site work force who could be quickly mobilised to sortout problems and obstacles, and a generous penalty/bonusscherne all conspired to get this project compleied some threemonths ahead of the programrne, to the satisfaction of all of thepart es involved.

23.5 A fast track liquid oxygen tankThis tank was built at the British Gas Research Centre atWestfield in Scotland. lt was a part of a development project toupdate the gas from coal technology and for reasons whichhave now van shed into the past, was fequlred to be built in ashod period of time.

The tank was supported by a ground-based, electrically-heatedslab which was supported by piles. The tank itself was a typicalliquid oxygen tank consisting of a stalnless steel lnnef tank wiiha fixed dome roofsituated withln a carbon steel outer tank with a

radial rafter-supported cone roof.

The construction/erect on sequence adopted involved:

. Start construction of the permanent tank foundation andsiart construction of two temporary foundations immedi-ately adjacent to the permanent foundation.

. Start construciion of the lnner stainless steel tank on onetempofary foundatlon and the outer carbon steeltank on thesecond temporary foundation.

. Lift the outer tank onto the permanent foundation as soonas both are comp eted.

. InstalL the cellular glass base insulaiion within the outertank.

. Lift off the removable roof of the outer tank.

. Lift the now completed inner tank into the outer tank to besupported by the base insulation.

. Replace the removable roof of the outer tank.

. Complete the tank insulation and fittings.

F g!re 23.12 Wife w nd ng afrangernents

The problem was tfaced to the temporary foundation being fin-ished with a sand /bjtumen layer. The welding of the iank bot-iom had effeciively glued the tank down, and refused to allowair beneath to I ft it, untiL such a point where it dec ded to sud-denly unstick itselfl

The second of the paralel activities required the steel outertank to be fitted with lnternal liftjng arrangernents to handle theinner tank insulation and I % nickel steel plate materials to-gether with a sultably waterproofed foof material accessarrangement.

This construct on sequence s I ustrated in Figure 23.9.

Photographs show the simu taneous construction of the outercafbon steel tank and the elevated base slab type foundat on(Figure 23.10), the beginning of the air rnoving operation (Fig-ure 23.11 ), the cjrcumferential wire w nd ng in progress (F gure


1) slanphqror pemaneft base srab

23 Erection considerctions fot low temperature tanks

This sequence put various operations in parallel and produceda very shorl constructlon period. The erectlon sequence is illus-tfated n Figure 23.14.

What made it possible was the relatively small size and weightsofthe two tanks lnvolved. The inner tank was 12.0 m in diameterand 12.35 m high whilst the outer tank was 16.0 m in diameterand 16.9 m in height. The useable capacity was 1326 m3 whichis not unusual for a liquid oxygen tank. With careful attention tothe detailed design, it was possible to arrange for both tanks tobe lifted with a reasonable sized crane.

Figures 23. 15 and 23.16 show the removal of the outer tankroof and the installation ofthe innertank within the outertank.

23.6 Spiral jackingSpiraljacking is an erection technique which has been aroundfor a number of years and has been used for both ambient andlow temperature tanks. The basics of the technique are illus-trated in Figure 23.17.

The top and bottom course are made from tapered plates andthis seis up the spiral. The remaining plates are of constantwidth and are reciangular, as for conventional tank erection.Thetop and bottom coursesare set up and their vertical seamswelded. The two parts of the tank shell rest one upon the other,supported and separated by a system of rollers, and can bepushed around in a circumferential direction to produce a slotinto which one of the rectang u lar plates can be fitted. lt is usualto join a numberofthe rectangular plates togetherand feed thisstrip into the slot formed by rotating the upper part of the shellon the lower part.

.::

I!

i=

TWTWV,Z - Z-2)-comprere peJma^0dbasesrab

5) Lfl ,enovabreroorottoder taik aid

6)- Lift 6nerlankhlo oute,bnk. R€p ace oulerbnk,ool'comdere nsurdon r& E. rttnss pipework etc

Figure 23.15 Remov ng lhe roofofthe outer carbon sieeltank

Eq 2 16I1-q -l .<SSee ra e.(dr. orreol.p tda-

: ;-re 23.14 The fast track lquid oxygen tank ereclion sequence


23 Erection considerctions for low temperature tanks

SlKlniionu'rv "r""t

ti,e bottom course of tankStep 2 -coivenLrona ly erect the top course of tank completew h toD strfiener directly on top of bottom course.lnstatl iackrnq equrpmeni and drect suooort steework forplate cbrl and'wdldlng machines (rnside and outside oftank)

\,.-N----l\De ivered plates

prepared at mil

Step 3 -Stari plate coild outside of the tank.Tack and roofweld vertical ioints.

i-t\N\\fl\ _Delivered platesprepared at mill

MJJe p ate coil forward and acdnext plate We d veriical jo nt rnsrde.

wprepared at mill

Step 5 -Contrnue assernbIng plates Completewelding vertical joints.

Delivered p ates

Step 6 -Spiia top course round 1 plate length, move coil n and make vertical

conrnJe sorralino lanl shelland wed Lpp€'l'o'iTortar€:ms end no sltreneB nsrdeLan^ hsb;ctweloeo sedn5

Figure 23.17 The spiraljacking erection technique


-re equipment used io allow the shell parts to slide on each

-:her and be pushed circumferentially is of a proprietary natureird the patents are owned by the Swedish companyiodoverken. lf spiraljacking is the chosen erection method, it is

-sualto hire the necessary equipment and the site supervision'lr its operation from this company.- re advantage of this erection method is that all of the plating,,,e ding, inspection and repair activities take place in fixed ar-

?s which can be arranged to have more sophisticated equip--ent that might normally be the case. lt is particularly suited to

-echanised plate handling, welding and inspection methods A: g LNG tank of 75 m in diameter and 32.5 m shell height will-?ve some 242 identically sized (apart of course from the plate

:-lckness) rectangular plates to erect. This volume of repetitive., ork is suited to the setting up of a factory-like environment and':r continuous shift operation to reduce costs and programme: Tles. No scaffolding and subsequent cleaning down of the:lmpleted tank shell will be involved

-re disadvantages are thai the equipment is expensive to hire: rd if anything goes wrong, perhaps with the plate material: Jpply chain or with the equipment used, the method leaves lit-: : flexibility for alternative methods to be considered, whereas:cnventional erection would provide a numberofways of possi-: y circumventing the problems.

-1e design of tanks for erection by spiral jacking is interesting.

'',/ith the exception of the upper courses of minimum thicknessjs required by the design Codes, all of the other shell plates,'.nere the thicknesses are derived from hydrostatically-in-:Jced loadings, can in theory be of different thicknesses. The:'oblems of manufacturing all of these shell plates at margin-. ly different thicknesses and of maintaining their identification:r avoid getting them mixed up during ihe subsequent fabrica-: on, transport and erection processes are well known. To min!-

-;se these potential problems, it is usual to arrange ior the: ate thickness to change only for each full or half revolution of:re tank circumference. This small sacriflce n overall shel,,eight is usually considered as money well spent.

-'re disposition of the shell stiffening is also lnteresting. lt LS

-sual to arrange these members as a series of fings set paralle:l the ground. This however does not suit the erection and', elding mechanisms for which an arrangement with the stiffen-:'s running parallel to the spiral is more convenient. A tech-- a ue which has been used is to allow the stiffeners to follow the:eifal for the majority of their circumference and to have a spe-: al section which crosses the main tank spiral seam ioining the:,vo ends of the ring of stiffening. The detailed design of such an:rrangement to cater for wind, vacuum and insulation loadingss quiet demanding.-he full containment 105,000 m3 tank built at the Cartagena ter--.lnal in Spain had its inner 9% nickel steel tank built using this

= gure 23.18 The 105,000m3 Cartagena terminal LNG tank:.unesy of Enagas

23 Erection cansiderations fat law tetlperctute tanks

erection method. The outer pre-stressed concrete tank wasbuilt whilst the outer steel portions of the roof were being con-structed within it and air lifted to its final position when it wascompleted. The inner tank was built by spiraljacking with the"tail" of shell plates, joined by having their vertical seamswelded, protruding out of the concrete shell via a suitable ac-cess opening. This tank in its finished form is shown in Figure23.18.

23.7 The construction of tanks with rein-forced concrete roofsFull containment tanks often have reinforced concrete roofs forreasons associaled with providrng protection From externalmissiles, fire or blasi loadings. Such tanks will also require a

steel roof sheeting to provide a product vapour and mols-ture-tight lining fof the tank roof.

To erect such a roof sheeting clearly requires a supporting roofframework, as does the placing of the concrete feinforcing sys-tem and the uncured concrete. lt is this latter loading whichcauses the problems. A 500 mm thick layer of concrete at 25

kN/m 'willgive a roof loading some 10 times the 1 .2 kN/m' usedto design a "normal" roof framework. To design for supportingthe full 500mm ofwet concrete will give rise to a massive frame-work and the costs associated with such a structure. Thisseems a very wasteful concept, made the more so by thethoughtthat the roofstructure is only required to fulfll its full loadbearing function for a brief period during the placement and ini-

tial curing of the concrete. Indeed, if it were practicable to re-

move ihe roof framework once it had done its job, this would be

done providing rneans were made to support the roof sheetingfrom the now load-beafing reinforced concrete stlucture.

Even if the con cfete ls placed n two or rnore layers ltlsstillex-penslve to provide a foof framewofkwhich w i dlrectly supportsuch a loadlng. So. as concrete toofs becarne more common,the ndustry cast about fof means of avoid ng these costs. Mostof the tank contfactors came to the sarne conclusion which wasto use internal air pressure to balance the weight of the uncuredconcrete and to maintain this pressure unti such a time that theconcfete roof could be considered self-supporting.

Points which require consideration when designing such an

erect,on syster wrll include the tollowing:

. lt is normal to provide an erection opening in the shell ofconcrete outer tanks and this opening will be required afterthe roof concrete-placing operation. Conseq uentially a suit-able air tight door is needed to provide a temporary seal forthis opening. lt is usual to incorporate connections for theincoming air and control and measuring equipment as apart of this door. A typical arrangement is illustrated in Fig-

Figure 23.19 Atempofary closure lor the concrete outer tank wall

Couftesy of Whessae an.l DEPA


23 Ercction considerctions fot low tempetuture tanks

ure 23.19. Such a closure may also be useful during the in-ner steel roof air lifting operation if this is the chosenerection method.

lf a concrete roof can be designed which is self-supportingwith only a part of the cured concrete present (typically onehalfor one third ofthe totalfinished thickness), then this canprovide advantages. For large diameter tank roofs, the airpressure required to balance the whole weight of the con-crete placed in a single thickness pour may give rise to in-

creases in the plating thickness above the Code minimumvalues of 5 mm ot 3/16".

lvlultiple layer concrete placement may lead to costly clean-ing ofthe upper parts ofthe reinforcement which may havebeen effected by earlier concreting operations.

The air oressure must be maintained until such a time thatthe newly placed concrete can be considered self-support-ing. This usually takes a few days depending upon the de-tails and the concrete mix chosen. Arrangements must be

made to ensure that the air supply is secure for this period.Duplication of blowers and power supply to drivers willavoidany unpleasant surprises. The thought of watching the roofslowly succumb to the full weight of the wet concrete anddisappear from view, to take its piace on the tank bottomwith the carefully constructed roof frame crumpled beneathseveral hundred tonnes of heavily reinforced concrete, cur-ing rapidly in the hot sun, is one which should concentratethe mind on this matterl

The slope of the roof at the periphery may be such that thenewlyplaced concrete tends to slide off the roof. lfthis is thecase then shuttering may be required to contain the un-cured concrete for some distance radially inwards to thepoint where there is no longer a tendency to slide. Amock-up of this oortion ofthe roof can be useful in assess-ing the need for shuttering.

The loading on the roof during concrete placement shouldbe maintained as circumferentially uniform as possible

Placing the concrete in a series of single or multistart cir-

cumferential pours is the usual choice. The radial width ofthese pours should be related to the speed of placement to

ensure that no problems related to the joints between adja-cent poureo nngs occurs.

23.8 Goncrete wall constructionPre-stressed concrete walls for double orfull containment stor-age systems generally divide into the wire wound or Preloadtypes and the internal multi strand tendon types. Both have thevertical pre-stress (where required) applied by internal tendons

located within the walls, the difference is in the manner of the

application ofthe circumferential stressing. Both types are sub-ject to similar construction methods.

The two principal contending methods are slip forming andjump forming;

The first involves the setting up of a complete inner and

outer circumferential form and the continuous casting ofthe wall from bottom to top. This requires the continuousinstallation ofthe reinforcing bars, the post-tensioning ten-

don ducts and the placement of the concrete

The second uses a small number of inner and outer formswhich are in an area where the rebar and ducting are al-

ready installed and remain in place until the concrete is

placed and partially cured. when they will be relocated cir-

cumferentially untilthe complete lift is finished. Atthis point

the next lift of the wall is started

The relative merits of these methods have amongst them thefollowing;


. Slip forming is fast

. Slip forming systems must run continuously once started toavoid expensive cold joint work Th's means shift working.

. Afullset of slip formworkfor a big tank is expensive. This willmilitate against the simultaneous construction of more thanone tank at a time on a particular site.

. Cleaning ofjump formwork is labour intensive. Cheap la-

bour favours this aspect.

. Slip forming requires the whole supply and installation sys-tem to work close to perfectly. Jump forming is more accom-modating of delays and interruptions.

. Different areas of the world have their own traditions andskills. An area where slip forming is common will have com-panies, workpeople and supply infrastructures who areused to this technique. Introducing it into other areas maypresent problems.

Which ever construction method is adopted, there are a lot ofdifferent activities going on in a confined area and good organi-sation and supervision are essential to good performance.

Some indication of the congestion may be gleaned from Fi9-ures 23.20 and 23.21 . Figure 23.22 shows a jump form at theDabhol site in India.

Figure 23.21 Rebariltting above the iomworkCauttesy of WhessoelTaylor Waodrow Canstructian

Figufe 23.20 Tank wallformworkCounesv of whessaelTaylor Woodrow Construction

to

9.

an

/s-

nd

trea),

E9-I'e

tftl

JJi

it8

p'


:gure 23.22 Example ofajump form:cunesy of Whessoe

23.9 Wall and base linersAs has been discussed in earlier Chapters, the provision of aner, impervious to a degree specified in the tank specification

:c the product vapour and to watervapouris a common compo-

"ent, particularly for the inner surfaces of the outer3re-stressed concrete tanks of full containment systems

\on-metiallic systems involving the spray application of propri-alary coatings, such as are marketed and installed by Recinco.f Belgium, have been successfully used, either in isolation or:ombined with an insulation layer.

-he more commonly adopted system is to use a metallic car-tron steel barrier for both the wall and the base of the bnk.

, he base liner is usually lap-welded from 3 or 5 mm thick steel:lates. For reasons to do with the finished flatness of such aner, it has sometimes been found convenient to weld this liner

:J cast-in inserts in the tank base slab.

-te wall liner is also usually made from 3 or 5 mm thick steel:lates and the most common means of installation is to weld--'rese plates to inserts cast into the concrete wall. Vertical in-

-.erts at between 1.5 and 2.0 m centres (usually dictated by the.vailable width of carbon steel strip mill plate) seem to be the:Jrrent favourite.

inother method which has been utilised is to make use of the-ner steel liner as the inner shutter for the construction of the:oncrete wall. This involves the earlier construction ofthe liner,: ther as a complete tank shell or course by course in advance:! the construction of the concrete wall. The liner/shutter must:e strong enough to resist the external loadings from the con-:'ete as it is placed. This will require an increase in the thick--ess of the plating or temporary/permanent stiffening being:dded.

23.10 Modular construction and prefabri-cation techniques-f're tank construction industry has, to the mind of the author,agged behind the shipbuilding and offshore industries in the

-se of modular construction techniques. The building of low::mperature tanks is no exception. The building of ships fre-:Jently involves subsiantial component parts being made in

r'fferent locations, commonly in different countries, being:'ought together and assembled in a single location. Similarly:^e offshore industry has equipped itself and become familiar.,,.th the lifting and moving over considerable distances, of^:odules in excess of 5000 tonnes in weight.

?nk builders are new boys on this particular block, indeed in

:.rms of modular construction techniques, may even be mov-

23 Erection considerations for low temperaturc tanks

Figufe 23 23 A modulaf stairlower

ing backwards. This is a shame as there seems considerablemerit in pursuing these methods. As has been discovered bythe shipbuilders and the offshore rig-builders, productivity andquality can both be improved by operating as far as is possible

in a factory environment. The examples described in Sections23.4 and 23.5 show a tentative step in this direction, althoughboth of these examples are now somewhat dated. Furthersteps in this direction involve:

. Prefabrication of steel shell plates intodoublewidth panelsThis can save some 50% of the onsite circumferentialwelded seam length.

. Prefabrication of roof frame sections away from the tankand lifting these into place

. Prefabrication of concrete tank parts for wire-wound typetanks

. Modularising the roof platforms, especially the in-tankpump platforms. These could be factory-builtand lifted ontothe tank roof as a single module.

. Modularising the tower stairway. Figure 23.23 shows a stairtowerwhich was factory-built in three sections and fitted outwith the stairway, the lighting and electrics priorto being de-livered and erected in a single day. The intention was to in-clude all of the pipework (including the thermal insulation)running to the tank roof in these modules, but circum-stances prevented this.

. For membrane type tanks there is considerable prefabrica-tion in the production of the stainless steel panels and theinsulation panels, both of which are factory made.

Clearly there is a lot of work to be done in this area.

23.11 Automated welding methodsThe low temperature tank builders have followed the practicesof the ambient tank builders in this respect. There seems con-siderable room for automated welding methods, perhaps ac-companied by automated inspection methods, to be borrowedfrom other industries and used in the tank building area. Forconventional tanks, the focus of automated welding has beenthe tank shell, and in the past particularly the circumferentialseams. In more recent times, the vertical seams have alsobeen welded by automated methods. As has been mentionedearller, automated welding techniques are particularly suited tothe spiral jacking erection methods.

One area where this apparent Luddite approach to automationis not the case is in the construction of membrane tanks. Auto-matic welding of the seams between the membrane sheets isan essential part ofthe construction of these tanks. The smaller

23 Ercction considerations for low temperaturc tanks

Figure 23.24 Atotally buried 200,000 m3 tankCouftesy of LNG Journal

'.._.-.-\

t'-

Figure 23.25 A200,000 m3 tank buried up 1o the-shell-to-roofjunctionCoulesy af LNG Journal

size of the fabficated stainless steel panels means that thelengths of welded seam in these tanks are much greaterthan isthe case for a conventional tanks.

23.12 Large in-ground LNG tanksThe majority of in-ground membrane LNG tanks are to be foundin Japan, with a few in both Taiwan and South Korea. Their ad-vantages over the above ground tanks are numerous:

. Being buried provides protection from adjacent tank firesand external missiles and blast loadings. Some like the200,000 m3 tank at the Ohgishima Terminal of Tokyo Gasare totally buried (illustrated in Figure 23.24) and some likethe tank of the same capacity at the Chita l\ilidorihama Ter-

minalofTohoGas, are buried up to the shell-to-roofjunction(shown in Figure 23.25), so the roof is exposed and must be

suitably protected.

. lt is argued that in-ground tanks can be designed and con-structed at greater capacities that the above ground tanks

. Because of the protection provided by burial, in-groundtanks do not need such large separation distances fromeach other or from site boundaries. This means that moreproduce can be stored in a given area. This is important


Figure 23.26 construcuon sequence for a 200,000 m3 in-gtound LNG tank in

Couftesy of LNG Jaurnal

Excavalion

Bottom SIab and Side Wall

@

when land is expensive, perhaps because it has been re-claimed from the sea as at Inchon in South Korea, or is justexpensive or in short supply.

. The partial or full burial is a useful plus in planning discus-sions, and may allow in-ground tanks to be built in locationswhere the visual impact of above ground tanks would pro-hibit their use.

The disadvaniages lie in the increased construction costs,f,/hich may be offset bythe better use of land or planning issuesTentioned above, and in the increased construction time. Foraoig LNG tank the approximate construction time would be 54Tonths which comDares with around 36 months for an abovelround iank.

The construction of thesetanks, currentlyata maximum capac-ty of 200,000 m3, is quite remarkable. There are a number ofDapers and articles which describe the construction processrublished in the LNG Journal (References 23.1, 23.2and 23.3)rr presented at the LNG conferences.

The 200,000 m3 LNG tank, described in Reference 23.3 andil-ustrated in Figure 23.25, at the Chita Midorihama Terminal is

rypical. The construction sequence is shown in simplified formn Figure 23.26. The following lists some ofthe construction as-rects of this project:

. The liquid containing membrane is 74.0 m in diameter and46.6 m deep.

. The site has been reclaimed from the sea and was of suchpoor load bearing capability that a major soil improvementprogramme had to be undertaken before construction vehi-cles could access the site.

. The nature of the subsoil was such that a slurry wall 100 mdeepand'1.4 m thick hadto be constructed to reach downtothe impermeable rock layer The excavation ofthis retiainingwall, the balancing ofthe soil pressures with heavy benton-ite muds, the insertion ofthe 100 m deep reinforcing cages

23 Erection considentions for low tempetaturc tanks

and the placement of the concrete from the bottom up-wards, so as to displace the bentonite mud, is a remarkableachievement in its own right. 35,300 m3 of concrete and2,300 tonnes of rebar were used.

This retaining wall allowed the inner excavation to takeplace. Around 284,000 m3 of material were removed.

When the excavation was completed, a gravel layer wasplaced in the bottom of the hole and a heavily reinforcedbase slab 9.0 m thick was cast involving 42,200 m3 of con-crete and 7,600 tonnesofrebar. Thethickness and strengthof this base slab is a function of the high groundWater upliftoressures.

The concrete side wallwas 2.5 m thick and required 33,200m3 of concrete and 5,100 tonnes of rebar

The steel roof was assembled on the bottom ofthe tank and€ised to its permanent position by air lifring orjacking.

Both the walland the base werefitted with a heating systemto protect from frost heave.

The fitting of the thermal insulation and the membrane fol-lowed the conventional membrane type bnk methods.

23.13 References

23.1 Development and construction of large LNG in-groundstorage tanks, Junji Umemura and Sasao Goto, TokyoGas, LNG Journal November/December 1996.

23.2 Construction of an underground storage tank, KenjiYanagiya and Tomoyuki Ogawa, Kajima Corporation,LNG Journal November/December 1999.

23.3 Construction of a new LNG receiving terminalin centralJapan, Naoshi Furukawa, Toho Gas, LNG JournalMarch/Apil .1997.


24 Foundations for low temperaturetanks

This Chapter provides a brief review of the basic requirements for the foundations of lowtemperature tanks. The guidance provided by the tank design Codes on the subject is ou inedand a few examples of past practice and some of the problems encountered are discussed.

Contents:

24.1 General

24.2 Code requirements and guidance24.2.1 API62024.2.2 BS 7777

24.2.3 DIEN 14620

24.3 Some examples and problem areas

24.4 References


24 Foundations for low temperature tanks

24.1 GeneralThe foundations for above ground vertical cylindrical tanks forlow temperature service are generally similar to those de-scribed in Chapter 13 for ambient temperature tanks. The addi-tion of base heating systems and the use of base insulationconstructed from brittle materials such as cellular glass meansthat in general low temperature tanks have less tolerance tosettlement and foundation movement durino service that theirambient temperature cousins.

For these reasons the foundations tend to be of a higher quality.Reinforced concrete ring wallswith graded infillare uncommonfor LPG tanks and almost unknown for LNG tanks. The mostcommonly adopted foundation types for low temperature tanksare the ground-based and the elevated reinforced base slabtypes.

The finished shape of the foundation, especially around the pe-

riphery is important. Agood levelfoundation in this area meansthat the steel tank erector has a better chance of producing agood shaped tank. A poor foundation means that he willalwaysbe struggling to achieve the desired tank shell tolerances.

ln-pit tanks tend io have the same types of foundations asabove ground tanks with the added complications broughtabout by considerations of possible floatation and ground, rainor flre water removal.

In-ground tanks are a specialist subject area, which is not dis-cusseo nere-

24.2 Code requirements and guidance

24.2.1 API620

Appendix C of API 620 provides some general guidance onfoundations, which is intended for tanks operating at ambienttemperatures. This Appendix is invoked by both of the low tem-perature Appendices (Q and R), which both go on to give spe-cific instruction regarding the design loadings for downward orbearing loading and resistance to uplift from anchorage wherethis is fitted. lt will be remembered that an essential differencebetween these Appendices is in the test water levels required.Appendix Q allows a partial test filling height whilst Appendix R

demands a test fill to the maximum operating liquid level.

One requirement oJ Appendix C is worth highlighting. For con-crete ringwallor slab foundations, the leveltolerances are given

as+%" in any 30 feet of tank circumference and t %" around

the total circumference. As it is common for the foundation con-tractor and the tank contractor to be difierent companies, thesetolerances often become the subject of heated discussion atthe point of handover.

Specific limits for foundation settlement are not provided, al-though the section dealing with tank hydrostatic testing does re-quire level readings to be taken at the start of the test and at"reasonable" intervals during the test. These readings are to beplotted promptly in a suitable form to see if undue or unevensettlement is occurring. The results ofthese observations shallbe reported to the tank erector and the purchaser or his engi-neering representative.

lf any excessive rate or amount of settlement is found, the fillingshall stop until a decision is reached as to what, if any, correc-tive measures are needed. lf a minor amount of settlement is

found to occur during filling and this continues after the tank is

at the full test water height, the water level in the tank shall notbe lowered until the settlement has substantially ceased, or adecision is reached which suggests that it is unsafe to maintainthe full test water level.

All of this is sensible advice, but for various reasons, non-spe-


cific and consequently of little help in resolving contractualdisputes between the parties involved in a particular project.

24.2.2 BS 7777

Part 3 of BS 7777 has quite a lot to say about foundations ofver-tical cylindrical above ground storage bnks.

It should be remembered that this part ofthe Code represents a

set of recommendations, and hence its contents are not man-datory

The foundation design should be in accordance with BS 8004(Reference 24.1).

Prior to the design and construction of a foundation, a numberof site specific investigations are suggested;

. Soil investigation

. Ground water investigation

. Seismicinvestigation

These should be carried out by individuals or companies suit-ably experienced in this work.

The following types of sjtes should be avoided:

. Sites where part of the tank is on rock or other firm undis-turbed ground, and part is on fill. Sites where the depth offlLlis variable. Sites where the ground under part of the tank ispre-consolidated.

. Sites on swamps, or where layers of highly compressiblematerial lie beneath the surface.

. Sites where the stability of the ground is questionable forreasons which may be associated with proximity to deepwater courses, mining operations, excavation or steeplysloping hillsides, karst topography or gypsiferous mater;alswhich could include weak lenses subject to dilution.

. Sites where tanks may be subject to flood waters.

. Sites near active faults or susceptible to liquefaction duringseismic excitation

The Code addresses the subject of permissible settlement duf-ing the life ofthe tank. The limits are to be agreed between thefoundation designer and the tank designer and between thepurchaser and the contractor(s) and take account ofthe designof the tank, the size of the tank and the local subsoil conditions.The values given in Figure 24.1 are to be used for guidance.

Advice is given on the means of monitoring foundation move-ments during the construction, test and operating phases ofthetank's life. These include conduits within the foundation to ac-commodate suitable instrumentation.

Where the subsoil on the chosen site is found to be incapable ofcarrying the loadings without sufiering excessive settlement, a

number of methods of improvement are suggested:

. Removal and replacement of unsatisfactory material by asuitably compacted granular fill

Diffcrcntial setllement llEil

Tilt oI the tank Ir500Thnk floor srttlcnlenialong a mdial line fromthe pcriphery to the

1r300

Settlement aroun.l theperipherl of thc tank

1 : 500 but notexce€ding thc maxim'rm

calculated for tilr ol thc

2

.:

-

Flgure 24.1 Differential settlement limiis

Frcn BS 7777 : Pat13:1993,table 3

ntractualproJect.

1S Of Ver-

esents a)ot man-

35 8004

number

es suit-

undis-th oftilltank is

-sssible

ble for) deepiteeplyterials

luring

rlour-ln tneIn theesigntlons.nce.

love-lf the

)le ofInt. a

Dya

. lmprovement by vibration or dynamic compaction

. Pre-loading with a temporary overburden

. Enhanced sub soil draining with pre-loading

. Stabilisation by chemical or grout injection

. Piling

idvice is also offered on the avoidance of frost heave and local:?lnage.:our different types of foundation are suggested and a brief:ommentary is provided on each type. These are:

. The ring beam type

. The surface rafr type

. The pile supported base type

. The elevated base slab type-he commentary includes much sensible advice. Amongst,{hich is the suggestion that the minimum space beneath an el-?vated base slab is 1.5 m and that gas detection equipments'rould be insialled within this space.

-eveltolerances forthe as-constructed base slab are given as6 mm in any 10 m of tank circumference and :t 12 mm be-

://een any two points around the circumference. These arelouble the values allowed by API 620.

24.2.3 prEN 14620

Part 3 ofthis provisional Standard provides a limited amount oflformation regarding tank foundations which are classified asshallow foundations and piled foundations. The shallow typeare further divided into Iaft (or ground-supported slab) and ringr€am types.=he rafr type is appropriate for soils which are shown to be ca-trable of supporting the designated loadings and may require:rickened sections in areas ofadditional loads such as beneath:re tank shell or concrete wall.-he rjng beam type is the cheapest foundation option and is:rearly dependent upon the ability of the subsoil to bear the im-ssed loadings. The ring beam must be designed for the shell're loadingsand any uplift loadsfrom tankanchors. Differential

:ettlement between the ring beam and the soil or iniill must be:onsidered for its possible effects on the tank bottom, the base:sulation and the base heating system.-he piled type includes both on ground and elevated bases.

Part 2 of the provisional Standard provides tolerances for the3s-built tank base. The peripheral tolerances, which include an:rea 300 mm inside and 300 mm outside the line of the tanksnell, aret 3 mm in any 10 m of circumference and t 6 mm be-:,,veen any two points on the complete circumference. Theseare interestingly the same as API 620 and one half of the BS-777 tolerances.

-ne Code also provides a localflatness tolerancewhich can be'nportantforthe correct insiallation of base insulation materialssJch as cellular glass. This is: Any deviation, measured with ai m long template, shall not exceed 15 mm.

\o guidance regarding permissible settlement is provided.

24.3 Some examples and problem areas-he choice between a ground-based slab and an elevated'cundation and the reasoning behind this choice is interesting.

lleady a ground-based slab will require some form of base-eating which is itself expensive and has ongoing costs in:erms of power and maintenance. The elevated slab does not-ave any ongoing costs in the same sense, but may be more


expensive to construct in the first olace.

For a site which has been piled, the elevated slab may be con-structed on extended piles which will make the cost differencebetween on-ground/elevated solutions perhaps less thanwhere a ground-based slab, columns and an upper slab arereoutred.

Where seismic isolation is required, an elevated slab is a matterof necessity. lsolator inspection, maintenance, removal and re-placement may have an impact on the design.

Worries about product vapour accumulation beneath the ele-vated slab, particularly for LPG, may have an influence on thechoice. A butane iank at the CoMon refinerywas designed andconstructed with an elevated base slab. Worries about safetycaused this to be subsequently back-fllled with sand and fittedwith a base heating system to give the arrangement shown inFloute 24.2.

*\- -

Figure 24.2 The CoMon butane tank foundation

The owners of an liquid ethylene tank built on a Teesside sitewere concerned about the vulnerability oftheir elevated slab todeliberate sabotage. Their solution was to fit a substantial gril-lage around the perimeter of the elevated slab to prevent ac-cess to the space beneath the slab.

Tankswhich have bottom liquidllttings, which are quite unusualthese days, give rise to problems for theirfoundations. The firstLNG tanks on Das lsland were eventually demolished and re-placed for reasons closely associated with problems to theirfoundations and bottom connections

Clearly the decision as to the choice of the base slab type re-quires all ot these considerations to be taken into account on ajob-specific basis.

Most elevated slabs are supported by a number of columns ofsquare or circular section. The number and layout ofthese columns may be dictated bythe shength ofthe elevated slab orthepile layout. An alternative scheme, which has been used be-neath the liquid ammonia tank built at Ravenna in ltaly, uses aseries of radialwallsto separate the ground bearing and the el-evated slabs. Some of these walls run almost to the centre ofthe slab and some stop short. lt had been questioned as to



Figure 24.3 The Reviihoussa lsland LNG tank arrangement

Coulesy of Whessoe

Flgure 24.4 The Trinidad LNG piling anangement

Cowlesy of Whessoe

whetherthis arrangementwould allowa suitablyfree circulationof air beneath the slab, but experience has shown this not tohave been a problem.For the LNG tanks on Revithoussa lsland in Greece, the ele-vated slab located within a pitwith an interspace covershown in

Figure 24.3 presented problems of limited air circulation to pro-

vide heat to both the tank bottom and the tank walls. This waseventually overcome by the installation of a quite sophisticated(and expensive) system of forced ventilation.The LNG tanks at Point Fortin in Tdnidadwere constructed on a

site with very poor load-bearing ability. The eventual solutionchosen was to use a ground-based slab supported by some'1200 steel pipe piles, each of 600mm diameter and around 30m long, beneath each tank base. This is shown in Figure 24 4.Returning to the LNQ tanks in Greece, the excavation ofthe pit

Figure 24.5 Concreling oflhe reinforced base slab at Reviihoussa lsland

Couiesy of Whessoe

revealed a widecrack in the limestone, running acrossthe base

ofone ofthe tanks. Aprotracted discussion took place to decideif this was a harmless defect, a seismic fault or an ancient de-bris-filled sea cave. lt was eventually sentenced to be the latterfor a mixture of reasons, a decision which allowed the project toproceed. The technical solution adopted was to construct a

massive 2.5 m thick, heavily reinforced slab. This shown underconstruction in Figure 24.5.

24.4 References24.1 BS 8004:1986 Code of practice for foundations


cide

?tte-qIc

rde-

25 Regulations governing the layoutof refrigerated liquid gas tanks

There are a host of regulatory documents, which seek to dictate the layout on a particular site ofrefrigerated liquid gas tanks and associated equipment.

The more commonly used ofthese regulatory documents are examined in this Chapter and theirdifferences highlighted.

There are some interesting differences in approach, especially between the European and theUSA's approach to the various problems which arise.

Contents:

25.1 Introduction

25.2 Regulations governing LPG storage tacilities25.2.1 NFPA 58

25.2.1.1 Pressurised LP-Gas storage

25.2.1.2 Rettigetaled LP-Gas storage

25.2.2 NFPA 59

25.2.2.'1 Pressurised LP-cas storage

25.2.2.2 Reftige.aled LP-cas storage25.2.3 fhe Institute of Petroleum rules

25.2.3.1 Genetal25.2.3.2 LPG pressure storage (Volume 1, Chapter 2)25.2.3.3 Refrigerated LPG storage (Volume 2, Chapter 3)25.2.3.4 Storage tank spacing

25.2.3.5 Vapour travel requirements

25.2.3.6 Bunding requirements

25.2.4 APt 251025.2.4.1 Pressurised LPG storage

25.2.4.2 Re'friget ated storage

25.3 Regulations governing LNG storage facilities25.3. 1 DOT.CFR rules

25.3.2 NFPA 59 A rules

25.3.2.1 Origin and development of NFPA 59A25.3.2.2 lmpoundment

25.3.2.3 The design spill

25.3.2.4 Thermal radiation

25.3.2.5 Vapourdilution considerations

25.3.2.6 Minimum spacing requirements25.3.3 EN 1473:1997 rules

25.3.3.1 Scope

25.3.3.2 Scenarios to be considered25.3.3.3 Design spill


25.3.3.5 Vapourdilution25.3.3.6 Minimum spacing requirements

25.4 References


25 Regulations governing the layout of refrigerated liquid gas tanks

25.1 lntroductionThis Chapter is devoted to the codes and regulatory guidelineswhich govern the layout on site of vertical cylindrical fu lly refrig-erated tanks for the storage of the various products describedin Chapter 17. For completeness and for purposes of compari-son some of the regulations relating to pressurised storage ofthese products are also included in this chapter.

The various codes and standards are soecificallv addressed tothe storage of LPG and LNG.

The discussion ofthese regulations has been confined to thosemost widely used. The National Fire Protection Association(NFPA) standards are legal requirements in the USA and arecommonly employed elsewhere in the wodd. The US Depart-ment of Transport Code of Federal Regulations(USA.DOICFR) is mostly confined to use in the USA andmakes many references to the NFPA regulations over which itinterestingly takes precedence. The American Petroleu m Insti-tute (APl) publish guidelines for LPG installations which fall out-side the NFPA area of regulation. The Euronorm Regulations(EN) are more recent documents which reflect European prac-tices taking into account the various categories of containmentdiscussed in Chapter 17. The Instltute of Petroleum (lP) repre-sents UK Dractices for LPG and it is oresumed that the lP ruleswill eventually be replaced by EN rules.

There are a multitude of other regulatory guidelines, somecountry specific or even more locally based where certain localcircumstances may necessitate their own requirements. No at-tempt has been made to address these other documenb.

25.2 Regulations governing LPG storagefacilities

25.2.1 NFPA 58 - Reference 25.1

The areas ofapplicabilityofthis documentare not entirely clear.It appears that it is; "The design, construction, installation andoperation of marine terminals whose primary purpose is the re-ceipt of LP-Gas for delivery to transporters, distributors or us-ers". This is then conditioned by two exceptions:

Exception I - Marine terminals associated with refineries, pe!rochemicals and gas plants.

Exception 2 - lvlarine terminals whose purpose is the deliveryof LP-Gas to marine vessels.

This seems to leave LP-Gas marine import terminals not asso-ciated with refineries, petrochemicals and gas plants - not avery wide area of application.

The Code also excludesa numberofother applications includ-ing frozen ground containers and underground storage in cav-erns.

Note LP-Gas is defined as "any material having a vapourpressure not exceeding that allowed for commercialpropane that is composed predominantly of the follow-ing hydrocarbons, either bythemselves or as a mixture;propane, propylene, butane (normal butane or isobu-tane) and butylene". This is similar to but not exactlythe same as LPG as defined in Chaoter 17. Section17.1.

25.2.1.1 Pressurised LP-Gas storage

The Code provides a bewildering number of spacing rules andexceptions to these rules. Much of this is devoted to small ca-pacity storage units (cylinders) and is of little interest to storagetank designers. Figure 25.1 giving the separation distances be-tween containers, important buildings and other properties is

interesting particularly because of the apparent advantages ofmounded or underground containers over above ground con-tainers in terms of spacing requiremenb.

25-2-1.2 Relrigercted LP-Gas storage

lmpoundment

The main rules governing impoundment for refrigeratedLP-Gas tanks are given briefly below:

- lmpoundment shall have a volumetric holding capacityequal to total volume of liquid in the container assumingthat the container is full.

- If an outside container wall is used as a spill containmentthe materialto be used shall be suitable for exposure to thetemperature of the refrigerated LP-Gas liquid.

- lmpoundment structures, and any penetrations thereof,shall be designed to withstand the full hydrostatic head ofimpounded LP-Gas and the effect of rapid cooling to thetemperatures of the liquids to be confined.

Provisions shall be made to clear the impounding area ofrain or other water.

Minimum spacing requirements

For refrigerated tanks designed to operate at below 15 psig, theminimum spacing from occupied buildings, storage containers

Midrnum Dltt ncca

rllhi.. Crpdtyplr Cont incr

Moundcd o.Und.igmurd rt5ov.!tound

Conr.lncri'

s.t

$n

'S.c !.2.2: Exc.Ptiotr No. ,rsbc !,2.t.:(g).s..3.2,t:(0.ds.. !.2.?.2(r).'scc !.?.2.2(b), (.), sd (d).

Figure 25.1 Sepafation disiances betlveen conlainers, imporlani buildings and olher propedies

Fron NFPA 58, table 3.2.2.2


<129d

125-250

25t-60050r-2000

200r-a0,00030,001-?0,000

?0,001-90,000

90,001-120,000

I m,001-200,000

200,001-1,000,000

>1,000,000

<o.5d

0.5-1.01.0r-1.9l.9r-?,67.6+l 14

11{r-265265F:XlWlt-454SL-157

751+-3185>3?85

lol0l0l050

50

50

50

50

50

50

0l0l025

50

t00125

?00

300

400

0"3

37.6

l523

30

38

6t9l

00

3

3

5

00

II

diamctc$ ofaqjaccnt con-

25 Regulations governing the layout of reftigerated tiquid gas tanks

i asso--nota

nctuo-'t cav-

/apour'ercialbllow-xture:Sobu-xactlyeclion

s andl ca-

es is

con-

r' .E

):te

\thicr CryacityPcl coIrtaber

AbovcgrourdCodain ll

cd

Walcr C.pa.ity

s.r (-)<70,000

>?0,000

965>265

: -:Lrre 25.2 IVinimum disiances

:'on NFPA 58, table 9.5.2

'rr flammable or combustible liquids and lines of adjoining:ropertywhich can be built upon shall be in accordance with Ta_:le 9.5.2 as shown in Figure 25.2.-he edgeofa dike, impoundment or drainage system that is in-:ended for a refrjgerated LP-Gas container shall be 100ft orrore from a property line that can be built upon, a public way, or: navigable waterway.-he minimum djstance between above ground refrigerated-P-Gas contiainers shall be one half of the diameier of thearger container.

Fire exposure-he reference to fire exposure is concerned only with the nec-3ssary provision for pressure relief related to fire exposure forvhich detailed rules are provided.

Vapour dilution requirements\FPA 58 does not have vapour dilution at site boundary re_:uirements.

25.2.2 NFPA 59 - lReference 25.21

-he scope of this Code states that it shall apply to the design,:onstruction, location, installation, operation and maintenancerf refrigerated and non-refrigerated utility gas plants. Installa-:ons having a capacity less than 15.14m3 are referred to NFpAtd.

25.2.2.1 Pressurised LP-cas storage>ressur-ised storage is generally referred to in this Standard as.on-refrigerated storage. For above ground coniainers the-r|nrmum spaqng requjrements are given by Figure 25.3 whichs the same in most respects as the NFPA 58 Table 3.2.2.2 butloes not consider mounded or underground systems.-he following rather ominous requirement for above groundoressurised systems is made:

"Containers shall be orientated so that their lonqitudinalaxes do not point towards other containers, abovJgroundLNG tanks, and flammable liquid storage tanks on th5 sameor adjoining property".

Up lo 70,000 (265) 15

70,00r to9o,0,00 (265.0 3{r} 100

90,m1 to l1O,O00 (!41 to4t{) 7

tm,oor to mO,00O (454 to 757) 20O

200,001 ro 1,0O0,00O (757 to 3?85) 300oi,€r 1,00o,mo (3785) 400

'l)100

,:c'

Figure 25.4 Refrigerated conlainer insta ations minimum distances

From NFPA 59. table 3.5.1

25.2.2.2 Retrigercled LP-cas storagelmpoundment

These requirements are the same as those required bv NFPA58 (see Section 25.2.1.2).


Spacing of refrigerated propane containers (and presumablycontainers of other LP-gases) from important buildings, stor_age for flammable or combustible liquids and lines of adjacentproperty that can be built upon shall be in accordance with Ta_ble 3.5.1 as shown in Figure 25.4.

The remaining spacing requirements are the same as requiredby NFPA 58 (see Section 25.2.1.2).

Fire exposure

These requirements are the same as are required for NFpA 5g(see Section 25.2.1.2).

Vapour dilution requirements

NFPA 59 does not have vapour dilution at siie boundary re_quirements.

25.2.3 The Institute of Petroleum rules -(Reference 25.31

This Code is Part g of The Institute of pet roieum Modet Code ofSafe Practice in the Petroleum Industry.fhe latest edition waspublished in February 1987 replacing the earlier edition pub_lished in 1967. lt represents a series of recommendations forsafe practice ratherthan a set of rigid rules. lt interestingly sug-gests that this more flexible approach should more easilv allowthe use of new methods, techniques. materials, etc., whi;h mavbe developed in the future and which meet the requirements forsafe practice given in the Code.

The Code is arranged as two volumes with the followino con_tents:

(m)

lrrrter .rf-tq ofEad Coa.atrer B€tw.e! Conbirct'

trlulcont ber ao Ned€'i l|bDortratBuildlag or Cloop of Bolldlob Not

AsEo.tat d *16 ine udtt.y c.3 Phoi"or r rit€ of Adjorohg pip€rty Tha!

Cian Bc BuIt UDod

20Ol ro E0,000

3O,O0l to 70,000

70,001 to 90,000

9O,0Ot b 120,000

12o,O0l to 200,000200,o0l ro I,OO0,0O0

I ,0OO,0OI or more

7.6 nt ll4ll4 to 2d5

265 b 341

A4l @ 4U451to 757

747.ogiA5ovcr 3785

r/i of surt of diaoet r! ofadjec€orcoIttainerl

23

30

E8

6l9ltn

50

100

125

200

300

{00Note Thc.pacinE of@ntdD4 &om butlditra! at5_l2r.d sith u.iug, ts pLnt3.h.lt b. F.r ned b b. rcduccd @ 50 pcrccDr of the dilqncci inTe.blc 2.{.t.2, eirh e minlmluE epndon of 50 n (l5 m). -

:igure 25.3 Above ground container instaltation minimum djstances

=rcn NFPA 59, table 2.4.1.2


25 Regulations governing the layout of rcfigented liquid gas tanks

Volume I. Chapter 1 - General information applicable to LPG

. Chapter 2 - Pressure storage at reflneries, bulk distributionplants and also industrial consumer premises, where suchstorage is large

. Chapter3-RefrigeratedLPG

. Relevant appendices

Volume 2

. Chapter 1 - Pressure storage at industrial, commercial anddomestic premises

. Chapter 2 - Plant for filling, handling and storage of cylin-oers

. Chapter 3 - Transport by road and rail

. Relevant aDoendices

25.2.3.1 General

This document contains a great deal of sensible and practicaladvice and guidance, only a little of which will be described in

this Section. Three baslc points relating to safety of LPG stor-age systems in general are worth repeating;

. LPG at ambient temperature and normal atmospheric pres-sure is normally heavier than air. Commercial butanevapour and commercial propane vapour are approximately2.0 and 1.5times as heavyas air respectively. LPG vapourswill therefore sink to the lowest levels of the surroundingsand flow along the ground or through drains or similar pas-

sages. Under still air conditions the natural dissipation ofaccumulated vaPour may be slow

. LPG has a low viscosity. Hence it is more likelyto find a leak-age path than water or most other petroleum products.

. Although LPG in its liquid and vapour phases is colourless,the evaporation which occurs when liquid leaks results in

water condensation or water freezing which appears as awhite mist or cloud.

25.2.3.2 LPG pressure storage (Volume 1, Chapter 2)

The scope of this chapter covers above ground, mounded orbelowground storage of LPG involving vessels ofindividualca-pacity greater than 135m3 or group storage greater than 450m3

Storage in frozen ground or underground caverns are not in-

cluded.

The rules governing equipment layout are numerous and in-cruoe:

LPG storage vessels shall be located to ensure that theminimum distances to fixed sources of ignition are:22.5m for storage vessels not exceeding 337m330.0m for storage vessels exceeding 337m3.

The radiation flux levels given in Table 1 of Appendix'1 (Fig-ure 25.5) shall be complied with. Calculation methods fordetermining the radiation flux levels are given in Appendix2.

The rate ofspillage and its duration used to calculate the ra-diation flux levels shall be based on identified potentialleaksources in the system. The identification and quantitativeassessment of such leak sources requires a systematicevaluation of the design and operating procedures takinginto accountfailure modes and the likelihood oftheir occur-rence. Examoles of potential leak sources and indicationsof leakage rates from them under specific conditions usingsimplified typical equations are given in Appendix 3.

Provisions shall be made to minimise the prcbability of aflammable cloud resulting from a spill as defined abovefrom reaching the site boundary Typical provisions may in-clude such measures as spacing, limitation of spill poolarea, screening and vapour dispersion equipment. Appen-dix 3 gives examples of leak sources and Appendix 4 pro-vides means of calculating hazard distances for thesereleases.

The permitted radiation level on thermally protected adja-cent LPG storage vessels is based on the protection oftheadjacent vessel bythe application of cooling water at a rateof 7 litres/(minute,m3).

There shallbe a minimum spacing between adjacentaboveground vessels of 1.5 m or 0.25 times the sum oi the adja-cent vessel diameters.

For below ground-mounded vessels, the spacing betweenadjacent vessels shall be determined by the site conditionsand the requirements for safe operation/ removalofthe ves-sels in addition to inspection, testing and malntenance re-qutrements.

The maximum number of vessels in any group shall be 6.

Prcesurc siongc

Mesimum radialiott lux hrcbrwd BTU(hd)

BquipdcrtThcoutcrrurfrcasof adjaccdtprsuEtaftge Y?rtcb(l)

TrAnfltlb proi.crcdQ)Unpro&cr.d (3)

44 13,7502,ffi

f h! outcrrurf acas ohdjaccitoragc kr&rco rinin&flamttabLproducts (4) mdproccss tacilitics

Th.nnallt protcd.d (21

UnFot€dcd (3)10,00025@8

F liog/dirdErgc poitrts 2,51X)

Pc.Eonncl iosidc boundaryProca$ .tc! (5)Protr.lcd wolk rrca (6)Wo.t lrcr(7)Critical src.a (8)

2,5m2,5m1,5m

500

885

Plant boundaryRomorc rroa (9)Urban lrcr (10)Critical arcr (8)

4,0001,500

50051.5

Figurc 25.5 Radiation flux levels for pressure storage

From The lnstitute of Petroleum Code of Safe Practice, Appendix 1 , table 1


SJ

2a

'e

]a'.a

5t

2l

ano ln-

rat ihe

1(Fig-rds forcendlx

:ne ra-alleak:rtativeematictakingcccur-ationsusing

:lofa:oove'ay in-

pooicpen-.1 pro_

. Any one group shall be separated from any other group by15m.

. In any group vessels shall be in a single line, i.e. shell toshell and not shell to end or end io end.

. The layout and spacing of above ground vessels should re-ceive careful consideration to ensure accessibility for firefighting and to avoid spillage from one vessel from flowingunder any other vessel or other vulnefable equipment.

. The provision of bunds around above ground pfessurisedLPG storage is not normally required.

. Separation cufbs may be required to direct spillage awayfrom storage vessels and other vulnerable equipment.

-he Code gives wide spread advice on good practice in pres-:urised LPG storage facilities under ihe following headings;

. Pressure storage (i.e. the design of the pressure vesselsthemselves)

. Piping, valves and tlttings

. Foundations and supports for vessels and piping

. Pumps, compressors and meters

. Road and rail loading and unloading facilities

. Electrical, static electricity, lightning protection

. Reouirements for fire orotection

. Operations

. Inspection of pressure storage

25.2.3.3 Refrigerated LPG storage (Volume 2, Chapter 3)

The scope ofthis chapter covers refrigerated LPG tanks, above;round, fully in-ground and partly in-ground. lt does noi coverstorage in frozen earth pits, in underground caverns or partially'efrigerated storage.

-he three categories of liquid containmeni are discussed to-:ether wlth guidance for provisions fof spil age containnrent:nd handllng, the avoidance of leakage, the minirnising of,'apour formation following a liquid leak and reducing the con-sequences of a fire following a liquid leak.

25.2.3.4 Storage tank spacing

Ihe rules for location and spacing of refrigerated storage tanksare summarised as follows;

. Refrigeraied LPG storage tanks and their containment sys-tems shall be located and soaced so that the minimum dis-tance to any fixed source of ignition is 30m, irrespective ofradiation flux levels.

. Containment systems, i.e. tanks and the associated bundsand impounding basins, shall be located and spaced so thatin the event of a fire, eg a tank fire or fire resulting ffom theignition of spillage of flammable products, thermal radiationflux levels shall not exceed the maximum levels given in Ta-ble 2 ofAppendix 1 (see Figure 25.6)

. The thermal radiation flux levels in the above requirementshall be based on the ignition of flammable product either ina tank or from spillage. In the case of spillage, the poolformed will be dictated by the spillage rate, the evaporationrate and the duraiion ofthe spill (see below) and the topog-raphy/locatlon of the site and facilitjes associated with it.

. Reference should be made to Appendix 2 for guidance onihe calculation of thermal radiation levels with respect toLPG facilities. Appendix 2 includes comprehensive guid-ance, references and a number of excellent worked exam-ples on this subject.

. The rate of leakage of flammable product and its duration

25 Regulafions goveming the layout of rcfigercted liquid gas tanks

used above shall be based on ideniified potential leaksources in the facilities which need to be considered. Theidentification and quantitative assessment of leak sourcesshould be based on a systematic evaluation of ihe designand operating procedures of such facilities, taking into ac-countfailure modes and the likelihood oftheif occurring. Ex-amoles of ootential leak sources from LPG facilities andindications of leakage rates from them under specific condi-tions are quoted in Appendix 3.

25.2.3,5 Vapour travel requirements

Provision shall be made to minimise ihe possibility of a flamma-ble vapour cloud of LPG from a spill as defined above fromreaching the site boundary. Such provisions may include suchmeasures as spacing, limitations of spill area, insulation of thebund or impounding areas, screening or vapour dispersionequipment. Appendix 4 contains references relating to variousmathematical models for calculating hazard disiances arisingout of spillages of LPG and the resultant vapour cloud forma-tton.

25.2.3.6 Bunding requirements

The various requirements relating to the need for bunding, thebund capacity and the bund design include the following:

. No refrigerated tank shallbe located within the bund enclos-ing any other storage tank.

. Each refrigerated storage tank shall be completely sur-rounded by a bund unless the topography of the area issuch, either naturally or by construction, that spills will be di-rected safely by gravity drainage and diversion walls (if re-quired) away from adjaceni tanks, equipment and sensitiveareas to an impound!ng basin suitably located within the siteoounoary

. Fu I and double containment systems by def nition fulfil theabove requirements but low bunds may be required aroundthe tank main connections to contain leaks ffom externalpiping. valves and fittings.

. Forsystemswhichdonotconformtodoubleorful contain-ment, the bund/impoundlng basin sha I be capable of re-tain ng the tota content above ground level or of the largesttank connected to any shared impounding basin.

r Where bunds are provided around tank connections theyshould be of sufficient capacity to contain the anticipatedspill volume.

. Bunds and impounding basins shall be provided with waterremoval systems designed to prevent LPG spiilages escap-ing into any system outside the area of the bund/impound-in9 basin.

. The capacity and reliability of water removal systems forrain and fire waier where aDolicable shall be sufficient toprevent the accumulation of such quantities of water aswould cause damage to the tank foundations, bund wall orwould lead to tank flotation.

. Consideration should be given to the monitoring of bundedareas for LPG leakage.

This section of the Code also gives sensible advice on a num-ber of other related subjects.

25.2.4 API2510 - (Reference 25.41

The scope of this Siandard includes the design, constructionand location of both pressurised and refrigerated LPG storagevessels, loading systems, unloading systems, piping and re-lated equipment for installations at marine and pipeline termi-nals, reiineries, petrochemical plants and tankfarms. Excludedfrom the scope of this Standard are:

adJa-trf ther rate

coveadla-

I een:rons/es-I re-


25 Regulations goveming the layout of refigerated liquid gas tanks

Rc(rigc.atcd storsgc

Maximam rtennat radiationfur kv.l\kwm2 B11r(h ftr)

EquipmcntThc outcr surfaccs of adj accotI cf r4c rutc d s to n Ee la n ks

Thznw lproucad(z)Unprotc.tcd (3)

10,0002,5m

328

Thc outcrsurfaccs of adiacent itorugcunkr @n|,'ininglfa,''|tnatLproducts (4)

The,rnaIy prokd.n (2)Unprot..lcd (3)

10,m02,500E

Thc outcr surfaccs of adj aocnt LPGprcrsufa ttorrga v.Jsels and proccssfadlitics (U) 2,vn

Pclsontrcl i8idc boundaryProccss arca (5)P.otc€tcd$,ork.r.a (6)Work.!ca O)Clitical arca (8)

8651.5

2,5002,ffi1,500

500

Plart boutrdaryRcrdotcarca (9)Urbno arca (10)Gitical area (8)

Not€s(1) The distance from all LPG presrare storsgc v.ssel to a refrigeruted storage rark is detcrmincd by thc

re{uircmcnt of Chapter 3 (Eee 3.3.1) ard Tsble 2 of this appcndix.(2) Such facilitieJareas arg protcctod by means ofwater sprays, iosulation, mdiation screens or similar systems.(3) Protecion is providcd by spacing alooe.(4) Spc<ial co[sidcration Ehould bc givcn to the location of floating roof tan&s containing high vapour pressurc

produds si[cc cffcctivc watcr cooling of thcir roof structures is impraciicablo.(5) A normalty unocorpied arca occasionally Dattncd by traincd and suitably clothcd pcrsons familiar both with

cscapc rout6 and opportuoitics for tcmporary shcltcr afforded by tbc proccss plant,(6) A pcrmancnt building whcrc pcrsonn€l insidc arc shicldcd and/or bavc shicldcd mcans ofcscapc.(7) An open arca or small (c,g. temporary) building without shicldcd mcaus of cscapc.(8) This is cithcr an uoshiclded 8(ca of critical imponaoce where pcoplc without protcctivc clothing may bc

rcquircd it all timcs iocludiog during cmcrgencies or a place diffiorlt or dangerous to cvacuatc rt shortloticc (e.g, a spors stadium).

(9) An arca only infre4uently occupied by small numbcrs of persons, c.g. moorland, farmland, dqscrt.(10) A.u arca which is noither a remote area oor a Gitical arca.(11) Thc allowablc thermal ndiation flux levcl is rcstrictcd for thcr facilitics h vicw of the potcntially longer

duratioo of crposure resulting from a rc/z'gc rafud t4nkl bund fiIe.

1,0001Joo

5001.5

TigLre )5.6 The'mal'ad alion irux levels forefrge'aled slotage

From The lnstitute of Petroleum Code af Safe Practice, Appendix 1, table 2

. Frozen earth pits

. Underground storage caverns

. Underground or mounded storage tanks

. Above ground concrete tanks

. Tanks covered by NFPA 58 and NFPA 59

. Tanks with capacities less than 2000 US gallons

It is not entirely clearjust howthe decision is made, presumablyby the local regulatory authorities, as to which facilities fallwithin the scope ofthis Standard, of NFPA58 and of NFPA59.

25.2.4.1 Pressurised LPG storage

The Standard provides spacing and impoundment rules forstoragevessels and other equipment which are briefly summa-rised in the following Sections.


. The minimum horizontal distance between the shell of apressurised LPG tank and the line of adjoining property thatmay be developed shall be as shown in Figure 25.7.


. Where residences, public buildings, places of assembly, orindustrial sites are located on adjacent property, greaterdistances or other supplemental protection must be pro-vided.

. The minimum distances betvveen the shells of pressurisedLPG tanks or between the shell of a pressurised LPG tankand the shell of any other pressurised hazardous or flam-mable storage tank shall be as follows:

- Betvveen two spheres, between two vertical vessels, orbetween a sphere and a verticalvessel, 5 ft or halfofthediameterof the largervessel, whicheveris the greater.

- Between two horizontal vessels, or betlveen a horizon-tal vessel and a sphere or vertical vessel, 5 ft or threequarters ofthe diameter ofthe largervessel, whicheveris the larger.

. The minimum horizontal distance between the shell of apressurised LPG tank and the shell of any other non-pres-surised hazardous or flammable storage tank shall be thelargest of the following subject to a maximum of 200 ft:

Water Capaciry of EachTnnk (gallons)

MinimumDistance (feet)

2,000-30,000 5030,001-?0,000 7570.001-90.000 10090.001-120.000 125

120,00t or grcater 200

:rqJ'e 25.7 ll"e mrnrmum d stance between lhe -nel' ola p'essur sed LoG:ana drd t1e rine ol adjoirilq propeny thal Fay be developed

non APl251A, bble 1

- lf the other storage is refrigerated, three quarters of thegreater diameter.

lfthe storage is in atmospheric tanks and is designed tocontain a material with a flash point of 1000 F or less,one diameter of the larger bnk.

- lf the other storage is in atmospheric tanks designed tocontain materials wiih a flash point greater than 100" F,

half of the diameter of the larger tank.

- 100 ft.

. The minimum horizontal distance betlveen the shell of anLPG tank and a regularly occupied building shall be as fol-lows:

lfthe building is used for the controlofthe storage facil-ity, 50 ft.

lf the building is used solely for other purposes (unre-lated to the control of the storage facility), 100 ft.

- Both of the above requirements may be replaced bycomDliance with API 752.

. The minimum horizontal distance between the shell of anLPG tank and facilities or equipment not covered aboveshall be as follows:

- For orocess vessels 50 ft.

For flares or other equipment containing exposedflames, 100 ft.

For other fired equipment, including process furnacesand utility boilers, 50 ft.

For rotating equipment, 50 ft, except for pumps takingsuction from the LPG tanks, 10 ft.

- For overhead powertransmission lines and electric sub-stations, 50 ft. In addition siting shall be such that abreak in the overhead line shall not cause the exposedends to fall on any vessel or equipment.

For loading and unloading facilities for trucks andrailcars, 50 ft.

. The minimum horizontal distance betlveen the shell of anLPG tankand the edge ofa spill containment area for flam-mable or combustible Iiquid storage tanks shall be 100 ft.

. Pressurised LPG tanks shallnot be locatedwithin buildings,within the spill containment area of flammable or combusti-ble liquid storage tanks as defined in NFPA 30, or within thespill containment area for refrigerated storage bnks.

. Compressors and pumps taking suction from the LPG tanksshould not be located within the spill containment area ofany storage facility unless provisions are made to protectthe storage vessel from the potential fire exposure. Exam-ples of such include (a) a submerged motor direct-coupledpump with no rotating equipment outside the pump contain-ment vessel or (b) a submersible pump within the LPG tank.

. Horizonial LPG tanks with capacities of 12,000 US gallonsor greater shall not be formed into groups of more than six

25 Regulations governing the layout of rcfigercted liquid gas tanks

tanks each. Where multiple groups of horizontal LPG ves-sels are to be provided, each group shall be separated fromadjacent groups by a minimum horizontal shell-to-shell dis-tance of 50 ft.

And again - the ominous Note: Horizontal vessels used tostore LPG should be orientated so that their longitudinal axesdo not point towards other facilities (such as containers, pro-cess equipment, control rooms, loading or unloading facilities,or flammable or combustible liquid storage facilities or offsitefacilities located in the facility of the horizontal vessel). The vi-sion of a ruptured LPG vessel "rocketing" through a controlroom is difficult to suDoress!

SDill containment

Spill containment may be achieved by either remote impound-ment or by the provision of a dike around the tank. The mainpoints to consider are:

. For remote impoundment systems:

- The remote impoundmentarea shall be located at least50 ft from the vessels draining into it and from any hy-drocarbon piping or other equipment

The holdupofthe remote impoundmentarea shallbe atleast 25% of the volume of the largest vessel draininginto it. lf the material stored in the vessel has a vaoourpressure that is less than 100 psia at 100'F, the holdupforthe remote impoundment shall be at least 50% ofthevolume of the largest vessel draining into it

. For diked systems:

lf an LPG sphere is diked, each sphere shall be pro-vided with its own diked area. lf LPG stored in horizontalvesels, a single diked area may serve a group oftanks

- The holdup ofthe diked area shallbe at least25% ofthevolume of the largest vessel within it. lf the materialstored in the vessel has a vapour pressure that of lessthan 100 psig at 100 'F, the holdup for the diked areashall be at least 50% of the volurr]e of the larqest vesselwithln it.

25.2-4.2 Retrigetale d storage

Again a number of requirements are included in the Standard.


The following requirements apply:

. The minimum horizontal distance between the shell of a re-frigerated LPG tank and the line of adjoining property thatmay be developed shall be 200 ft. Where residences, publicbuildings, places oi assembly, or industrial sites are locatedon adjacent property, greater distances or other supple-mental Drotection shall be orovided.

. The minimum distance between the shells of adjacent re-frigerated LPG tanks shall be halfthe diameterofthe largertaNK,

. The minimum distance between the shell of a refrigeratedLPG tank and the shell of another non refrigerated hydro-carbon storage facility shall be the largest of the followingsubject to a maximum distance of 200 ft:

lf the other storage is pressurised, three quarters ofthelarger tank diameter

lf the other storage is in atmospheric tanks and is de-signed to contain materialwith a flash point of 100'F orless, one diameter of the larger tank.

- lf the other storage is in atmospheric tanks and is de-signed to contain material with a flash point greaterthan100 'F, half the diameter of the larger tank.

- 100 ft.


25 Regulations governing the layout of rcfrigeated liquid gas tanks

Soill containment

As is the case with pressurised storage, spitl containment maybe achieved by the provision of remote impoundment or bydiking.

Remote impoundment

The following requiremenb apply:

. The remote impoundment area shall be located at least 50 ftfrom the vessels draining into it and from any piping or othereouroment.

. The holdup of the remote impoundment area shall be atleast 100% of the volume of the largest vessel drain ing intoit.

Diking

The following requirements apply:

. Each refrigerated LPG tank shall be provided with its owndiked area. The holdup of the diked area shall be at least100% of the volume of the tank.

EXCEPTION: More than one tank may be enclosed withinthe same diked area provided provisions are made to pre-vent lowtemperature exposure resulting from leakage fromany one tank from causing subsequent leakage from anyother tank.

. When dikes are used as part of the spill containment sys-tem, the minimum height shall be 1.5 ft measured from theinside of the diked area. When dikes must be higher than6 ft, provisions shall be made for normal and emergency ac-cess into and out of the diked enclosure. Where dikes mustbe higher than 12 ft or where ventilation is restricted by thedike, provision shall be made for normal operation ofvalvesand access to the top of the tank or tanks without the needfor personnel to enter into the area of the diked enclosurethat is below the top of the dike wall. All earthen dikes shallhave a flat top section at least 2 ft wide.

25.3 Regulations governing LNG storagefacilities

25.3.1 DOT.CFR rules

The appropriate section of these documents from the tJS De-oartment of Transport is:

DOT.CFR Title 49

porated documents are listed with the appropriate edition re-lated to each DOT.CFR paragraph.

In brief, the salient points from this document are:

. ln the event of conflict between DOICFR and NFPA 59A.the DOT.CFR prevails.

. This Standard applies to any new LNG facilities placed inservice after l\4arch 31, 2000.

. lf an existing LNG facility is replaced, relocated or signifi-cantly altered after March 31 , 2000, it must comply with thesalient part of these regulations (with certain exceptions).

. Each container must have a thermal exclusion zone in ac-cordance with NFPA 59A 2-2.3.1 (see Section 25.3.'1).

. Each LNG container must have a dispersion exclusion zonein accordance wjth NFPA59A2-2.3.2 (see Section 25.3.2).

. Average gas concentration at boundaries = 2.5%.

. The design spil! shall be determined in accordance withNFPA 59A 2-2.3.3.

. An outerwallofa component served by an impounding sys-tem may not be used as a dike unless the outerwall is con-structed of concrete.

. A covered impounding system is prohibited except for con-cfete wall desjgned tanks where the concrete wall is anouter wall serving as a dike.

Note: These last two points suggest that this Agency has be-gun to accept the concept offull containment tanks andthe advantages which accompany this category of con-tainment.

. Each impounding system serving an LNG storage tankmust have a minimum volumetric liouid imDoundment ca-pacity of 1 10% of the LNG tanks maximum liquid capacity(for an impoundment serving a single tank).

. Aflammable non-metallic membrane liner may not be usedas an inner container in a storage tank.

Note: A throwback to the Staten lsland LNG Tank incident,see Chapter 17, Section 17.3.

25.3.2 NFPA 59A rules

25.3.2.1 Origin and Development of NFPA 59A

Acommittee of theAmerican GasAssociation beganworkon aStandard for liquefied natural gas circa 1960. Intheautumnof'1964 a drafr was submitted to NFPAwith the request that it beconsidered asthe basisforan NFPAStandard. The first officialedition was adopted at the 1967 NFPA annual meeting.

By early 1969 it had become apparentthatthe use of LNG wasexpanding and the API suggested that its Standard PUBL2510A (Design and Construction of Liquefied and PetroleumGas (LPG) Installations) be used to help develop a Standardhaving a broader scope. The committee on Liquefied NaturalGas was established and in 1971 a new edition encomDassinothe broader scoDe was adoDted.

Subsequent editions were adopted in 1972,1975, '1979, 1985,1990, '1994 and 1996. The 2001 edition was developed in ajoint effort between NFPAand the Canadian Standards Associ-ation LNG committees to harmonise the requirements of NFPA59A and CSA Z 276.

25.3.2.2 lmpoundment

The rules governing impoundment for LNG tanks are givenbriefly below. lt should be noted that for an LNG storage andhandling facility, areas in addition to the storage tanks such asprocess areas, vaporisation areas and transfer areas will alsoreouire their own imooundment.

Transportation

Other regulations relating totransportation (continued)

Research and special pro-grams administration

Department of Transporta-tion (continued)

Subchapter D Pipeline safety

Part 193 Liquefied natural gas facjli-ties

Federal safety Standards

This is lengthy and so is normally referred to as DOT.CFR.49.Part '193 (latest version dated October 2000).

This document has little to say in its own right, but makes refer-ences on most subjects to other Standards (mainly NFPA 59Awhich is discussed in Section 25.3.2) which are described as"N,4aterials Approved for Incorporation by Reference". Interesfingly and rather confusingly, the DOT document makes refer-ence to different earlier versions of NFPA 59A in different sec-tions and not the most recent publication as one might expect.

Fortunately the document is well indexed and the various incor-


Subtitle B

Chapter 1

25 Regulations governing the layout of rcfigehted tiquid gas tanks

,59A,

red in

g n ifl-n tne)ns).

't ac-

zone2\

,,y'tth

SYS-

iaa

ce-anctc l-

. Flammable liquid and flammable refrigerant storage tanksshall not be located within an LNG container impoundment.

' The impounding area shall have a minimum volumetricholdlng capacity equal to the total volume of liquid in thecontainer assuming the container is full. Thisisforanim-poundment for a single LNG tank and allowance shall bemade for any displacement due to snow accumulation orother eouiDment.

. Dikes and impounding walls for LNG containment shall beconstructed of compacted earth, concrete, metal or othermaterials. They shall be permitted to be independent of thecontainer, or they shall be permitted to be mounded integralto or constructed against the container. They,andanypen-etrations thereof, shall be designed to withstand the full hy-drostatic head of impounded LNG, the effeci of rapid coolingto the temperature of the liquid to be confined, any antici-pated fire exposure and natural forces such as earth-quakes, wind and rain.

. Where the outer shell of a double wall tank complies withthese requirements, it shall be permitted to be consideredas the impounding area for the purposes of tank siting dis-tances etc. lf the containment integrity ofthe outer shellcanbe affected by an inner tank failure, mode, then an addi-tional impounding area shall be provided.

. The relationship between dike and impounding wall heightsand their distance from the primary container shall be asFigure 25.8.

. Provision shall be made to clear rain or other waterfrom theimpounding area. Clearly such provisions shall not allowthe escape of LNG.

. The impounding system shall, as a minimum, be designedto withstand an SSE event whilst empty and an OBE whilstcontajning the maximum volume of LNG as defined above.After an OBE or SSE, there shall be no loss of containmentcapability.

25.3.2.3 The design spillFigure 25.9 attempts to define the design spill, forthree typesof storage tank:

. Containers with penetrations below the liquid level withoutinternal shuloff valves.

. Containers with penetrations below the liquid level wiih in-ternal shufoff valves in accordance with certain desion re-

ContaineBwith A3pil thougha$ Us€ rhe fomrulap€n€tfations assumedoDeDinsbelo{ &e liquid aq ara eq;a iD 4 a r-il-:Tli9::-- ffea to, tha! pe!€- q = aa-4nrnremalsnuron Eation b€loi r}re until rhe differen-

IiI: l:"lto*.o tial head aadng onP-q T 9I gryt the openins b otlow rom alr rnntiauy full containex'*'"''

Us€ the coolzin€rni& the largeJi flowif more itlao on€contair€r ia theiErgouddi-og erea-

Coltaioenwith Th€ largerr0ow The largestdowover-drc-top 6ll, tom anyslagte ftoma{ringle !!ewltn oo Peft$a- lile llra! could b€ rlEtcould beF.99.*r:. Y tn. puEp€d i.oro rlrc puop€d hro itle[qurorever' impoundtagarea i-por*a;"g..*

with ihe cotrtaiqer lvith tbe cootais€rwiihdiav-al withdrrl|?lpulrpG)puep(s) coDsid- d€li\€rirg fie tujlered to be deliv€r- raoed capacirFilrg the full rared (l)ForloEitruG!c.rpacity. ifsun€illalrce atrd

shurdovrx b d€la-oBtrated atrdapProved by theaurhodry harhgjuridictioo.(2)For the tilDe[€eded to ernpt/ efull cotrtaileirvhere rurveillanceand 3hutdor{tr ir0or app(red,

Codtaitrerswidr Thendw*|rowhao Us€ the formulaPeleo-atioos assuEedoP€ohgaqoeloav urc xqrxq ?.od equ"l io area !o, 4aE11,LT-T.1."_5.':_ tlar p€oetndoD q = iz-4 4

fl"J:A'#* F\*,n"r,q*g forrhour.with6.s.3.3. *1l"'.-lto

regrlt n tnela g€s!flow fto(o a! ilitialyfirfl coDtainer.

ContaiaerD€stp Sp[I

D€sigtr SpllDuraiio!

2n4

LDpoundingarea Th€ oow bom any For l0 mioures or*rvt(Ig only sirgle accidenral for ? shod€r tjroe!?Ponzauon,.., leakagesource. ba5edoodemon-proceJs, or LNU sFable surveiltrarBfer areas

shuadol{n provi-siors acceptable tothe aufioriry hav-irgjuridicrion.

Nore gbdteflosBre lftmin (m3,hin)l o{tquld dirdrcdianFcr [i!.(mm)l of tank Fr.trariod bdos rhc &uid t€'icl. Ll! dl. h.qttt tft (m) lofliquld abo!€ p.n duion ln dE contetne. *hcn fic @nt2lncr t fdt

Fgure 25.9 Design sp llfor three types of siorage lank

From NFPA 594, tabte 2.2.3.5

quirements.

. Containers with over the top fill, with no penetrations belowthe liquid level (also presumed to be equipped with in-tankpumps for unloading).

For the first two tank types ihe calculation of the design spill isquite straightforward. The formula given is:

4 ^-o = d'Jh'3

where

q = flow rate (ft3/min)

d = diameter of the tank penetration (ins)

h = height of liquid above the penetration when thetank is full (ft)

The spill duration is also given in Figure 25.9.

Noiac. oimensbn Xshall €q]a, or €xce€d the sum of fmendoo yDlus t|esqival€nt head ln LNG of he prgssu.o In tt€ vapor spaco abovot|s huld.Excf,pfKn: WtBr tto h€,lght ol he dl@ or hrvwNing *aI ls equdt Aor gealer tlgn, tE nptdmrm qtfr le'/el, X ntsl hd!.e any velue-

. Do|enslon Xb h€ dshnco ipm fie hn€r wall of fis cootaln€r b tl6closest hce of |hs dik6 or [npounding walt

. DinFnslon yb fls dstance ftom t|€ marmun lilrld lgvel h hoconbln€r b tho iop ol th€ dlks or trpoundng wafl.

Figure 25.8 Dlke or impoundment wallproximtty ro conlarners

Fran NFPA 59A, figute 2.2.2.6

equ 25.1



For the third tank type the picture is less clear. The industrypractice has been to take the maximum export rate (i.e. thecombined flow of all of the installed in-tank pumps assumingthese to be manifold together) as the 10 minute duration spill(presuming a suitable shutdown system is provided). However,the design spill wording "The largest flow from any single linethat could be pumped into the impounding area with the con-tainer withdrawal pump(s) considered to be delivering the fullrated capacity,", leaves room for doubl.

Fora typical LNG import terminal, the liquid pumpout rate froma large tank may be ofthe order of 2000 m3/hr whereas the liq-uid import rate (i.e. the carrier's maximum unloading rate) maybe as high as 12000 m3ihr The 10 minute spillvolume in onecase is 2000 x 10/60 = 333 m3 and in the othercase is 12000 x10/60 = 2000 m3.

The logic of postulating a possible failure in the case ofthe liq-uid export line whilst not doing the same in the case ofthe liquidimport line seems contentious. The differences between thetwo figures in terms of the cost and difficulty of containing thespill at the tank roof level, the conveying ofthe spilled liquid tolocalgrade and the further guiding of liquid to the site impound-ing basin are substantial.


NFPA 59A requires provisions to be made (almost always byadjusting the site layout)to preventthe thermal radiation from atire from exceeding the limits listed belowundera defined setofatmosphe c conditions (zero windspeed, 70oE 50% relativehumidity):

o 1600 Btu/hr/fi3 (5000 w/mr) at a property line that can bebuilt upon for ignition of a design spill (see Section25.3.2.3).

. 1600 Btu/hr/ft, (5000w/mz) atthe nearest point located out-side the owners property line that, at the time of plant siting,is used for outdoor assembly by groups of 50 or more per-sons for a fire over an impounding area containing the fulldesign volume (see Section 25.3.2.2).

3000 Btu/hr/ftz (9000 Mm2) at the nearest point ofthe build-ing orstructure oulside the owners property line that is in ex-istence at the time of plant siting and used for occupanciesclassified by NFPA 101 (Life Safety Code) as assembly, ed-ucational, health care, detention and correction or residen-tial for a fire over an impounding area containing the fulldesign volume.

',OOOO t,urn7ftz (30000 wim2) at a property line which canbe built upon for a fire over an impounding area containingthe full design volume.

The calculations of radiation distances shall be calculated bythe following method:

Gas Research lnstitute Reoort GRl0176 "LNG Fire:Ather-mal radiation model for LNG fires"

lfthe ratio ofthe majorto minor dimensions ofthe impound-ment does not exceed 2 then:

d =F./a

where

equ25.2

d = distance from the edge ofthe impounded LNG(ft)

A = surface area of the impounded LNG (ft )

F = flux correction factor as follows: 3.0 for 1600Btu/hr/ft2 2.0 for 3000 Btu/hr/ft, 0.8 for 10000Btu/hr/ft'z

An example using the simpleformula for tanks for the storage of160,000 m3 of LNG of the single and full containment types


shows the large differences in the various spacing require-ments and the savings in site area arising from the use of thedifferent types ofcontainmentcategory The summary ofthe re-sults of this exercise are given in Figure 25.10.

One additional requirement relating to thermal radiation is thatthe LNG container impounding areas shall be located so thatthe heat flux from a fire over the impounding area shall notcause major structural damage to any LNG marine carrier thatcould Drevent its movement.

25.3.2.5 Vapour dilution considerations

NFPA 59A has the following requiremenb:

. The spacing of an LNG tank impoundment to the propertyline which can be built upon shall be such that, in the eventofthe design spill(Section 25.3.2.3), an average concentra-tion of methane in air of 50% on the lower flammability limit(LFL) does not extend beyond the property line that can bebuilt upon. Three different calculation methods are given:

- GRI 0242 "LNG vapour dispersion prediction with theDEGADIS dense gas dispersion model".

- GRI-96/0396.5 "Evaluation of mitigation models for ac-cidental LNG releases-Volume 5; using FEM3Aforac-

Figure 25. 1 0 Comparison of site layouts with differenl types of liquid contain-

z,1

lr

2T

]=(

requtre-se of theof the re-

In is thati so that;hall notrier that

rropertye eventcentra-lity limitcan begiven:

/iih the

for ac-lor ac-

cidental LNG accident consequent analysis".

Another model subject to stated guidelines.

. Provisions shall be made to minimise the possibility of aflammable mixture of vapours from the design spill fromreaching the property line that can be built upon and thatwould result in a distinct hazard. Flammable mixture disper-sion distances shall be calculated in accordance with GRI0242 subject to certain stated conditions.

25.3.2.6 Minimum spacing requirementsn addition tothe spacing requirements arisingfrom the require-Tents of impoundment (Section 25.3.2.2) thermal radiationSection 25,3.2.4) and vapour dilution (Section 25.3.2.5) con-

siderations, the following minimum spacing requirements ap-lly:. The minimum distance from the edge of the impoundment

to buildings or property lines shall not be less than 0.7timesthe container diameter or 100 ft whichever is the greater.

. The minimum distance between LNG storage containersshall not be less than 1/4 ofthe sum ofthe diameter of adja-cent containers or sfr whichever is the greater.

Hote: Both ofthe above applyto LNG tanks with capacities inexcess of 265 mJ. Forverysmall tanks refer to Table2.2.4.1 of NFPA 59A.

. In no case shall the distance from the nearest edge of im-pounded liquid to a property line which can be built upon, orthe near edge of a navigable water way as defined by fed-eral regulations be less than 50fr.

xoie: Clearly for tanks with capacities >265 m3, the first con-dition above willgovern for disiances to property lines.

25.3.3 EN1473 : 1997 rules

25.3.3.1 Scope

-r;s European Standard gives guidelines for the design, con-3:!ction and operation of all onshore stationary LNG installa-:ons including those for the liquefaction, storage, vaporisation,

-nsfer and handling of LNG The Standard is valid for the fol-c$iing plant types:

. Export terminals between the desjgnated gas inlet bound-ary limit and the ships manifold.

. Receiving terminals, between the ships manifold and thedesignated gas outlet boundary limit.

. Peak-shaving plants, between designated gas inlet andoutlet boundary limits.

. LNG satellite plants with a totial storage capacity above 2OOtonnes, including the loading station up to the designatedgas outlet boundary limited.

tlote: Satellite planb with a total storage capacity less than200 tonnes are excluded from the scoDe of this Stan-dard.

-^is Standard is very difierent in philosophy and content from1-€ USADOT.CFR and NFPAStandards. lt is based on hazardrd consequent risk assessment methods and is generallyless

=ecific in its requirements than the American Codes. The fol-or/ing Sections are an attempt to abstract the salient points:r',en jn this EN Standard astheyrelateto LNG storagetanks.25.3.3.2 Scenarios to be considered-'ese are summarised for the full range oftank types in Figure:5 11 . lt is interesting to note the distinction made between full

=.ltainment tanks without concrete roofs where the pool fire;ze is based on the secondary contiainer and full containmentE-KS with concrete roofs where no collapse is considered andr., implication no poolfire is to be considered.

25 Regulations govening the layout of rcftigerated liquid gas tanks

e Part 6.1 ofthe Standard states "Tanks can be Dlaced on theground.....

.....The raft of the tank can be supported by raised piles.

. Part 6.2.3 states "there shall be no penetrations of the pri-mary and secondary container walls or base.

.....The absence of wall or base penetrations requires theuse of submerged pumps." Le. tanks ofall types must be fit-ted with in{ank pumps, This is very different from NFPA59A.

25.3.3.3 Design spillThe Standard mentions "loss ofcontainment of LNG and of nat-ural gas" in Section 4.4.3.1, "Evaporation of spilled LNG" in4.4.5.1, "Atmospheric dispersion of LNG vapour'' in Section3.3.4 and "Provisions for control of leaks or spillage" in Section4.5.2. lt does not provide means of evaluating the flow rate orduration ofthe design spill. The Standard provides guidance onhazard assessment using probabilistic and deterministic ap-proaches, the use of hazard and operability studies (HAZOP),failure mode effect analysis (FMEA), eventtree method (ETM)and fault tree method (FTM) together with a listing of possibtehazards of both internal (i.e. within the site) and external origin(i.e. from outside the site).

There is also some briefguidance on the estimation of probabil-ities (Section 4.4.4) and the estimation of consequences (Sec-tion 4.4.5 and Annex F). Whilst allof this guidance is admirable,it leaves the deflnition ofthis important design event a matter ofconjecture and debate which will require a lengthy and expen-sive site specific studyto resolve. The much simplerapproachto this issue taken by NFPA 59A, albeit not entirely clear in all ofits aspects, may appealto terminal designers and contractorsand to the various regulatory authorities in view of ib clarity.


Figure 25.12 gives recommended maximum incident radiationvalues for equipment within the site boundary including tankswith outer surfaces constructed of concrete or steel. oressurevessels, process facilities, control rooms, workshops and ad-ministrative buildings arising from an LNGtankpoolfire. Theseare maximum values to be used unless deflned otheMise in lo-cal regulations. The Standard then moves on to suggest thatthe maximum radiation flux levels for each main structure shallbe calculated and provides some guidance on how this may beachieved.

For LNG storage tanks, the permissible radiation flux shall bedetermined taking into account the following factors as a minimum:

. lf no deluge system is installed, watercooling is deemed toapplyafterthe time required to provide firewater in sufficient

SingIE conteinnent l)

Double co0unn$eltt 2)

Fltl contairunent 2) s)

Mernbrane 2) 3)

Ctyogmic cons€te 2l 3)

Slraical l)

fngrttrd 4 3)

S@Ei6 to b€ @Eid.rE l:r) Ir .3le of @Uar6e of tlE Lrik lrlnrry .onr.h€r, !n poolrlr. conldpd& !o the inpdtndng.ie..4 h ce ol @0qe o( dE tsn* rcoq tlE !E loot tdr!@nt{d& b dE €lDdlry cont lEr) No @U!9e i! @Etder.d for tl€se t !* h,!..,

Figure25.11 Scenarioslobeconsidered inlhehazad assessmentasfunctionoftank types

Frcn EN 1 473, table 5



Fig ure 25.1 2 Allowable thefmal radiation flux exclud ing sola r tadiation inside lheDounoary

FFrn EN 1473. table I

ouantities from external sources

. Loss of strength of container

. Pressure build up within the container

. The temperature ofthe safetyvalve shallnot reach the autoignition temperature of the flammable substance in thetank.

. Surface emissive powers

Figure 25.13 similarly provides maximum recommended inci-dent radiation flux levels arising from an LNG poolfire within theboundary on different types ofareas outside the site boundaryThe maximum recommended incident radlation levels arisingfrom the flare are given in Figure 25.14 for areas within theboundary and in Figure 25.15 for areas outside the boundaryThese Tables give two levels of radiation fluxfor normal and ac-cidental operations.

25.3.3.5 Vapourdilution

EN1473 gives no specific rules for dilution levels at site bound-aries. lt does give some guidance on the calculation methodsto determine the furthest distance to the lower flammable limitand of what factors to take into account in this calculation, butno specific requirements.

25.3.3.6 Minimum spacing requirements

Again EN1473 gives little guidance on this subject. lt doesstater "The spacing between two adjacent tanks for which no

collapse is considered (i.e. full containment, membrane, cryo-genic concrete or in-ground tanks, in each case with a concreteroof), shall be (as a) minimum equal to a half diameter of thesecondary container of the larger tan k". Th is is essentially sim-ilar to the NFPA 59Aone quarter times the sum ofthe diametersof the adjacent containers requirement.

25.4 References

25.1 NFPA 58 Liquefied Petroleum Gas Code 2001 Edition,National Fire Protection Association, Quincy, Massa-chusetts.

25.2 NFPA 59 Utility LP-Gas Plant Code 2001 Edition.

25.3 lnstitute of Petroleum Code of Safe Practice in the Pe'

Eqqlpn..r luld. bouduy

Concrete outer 6urf9ae of adiacmtstorage tanls: unprotectedi) 3) orbehind thennal proteciionz,

32

M€tal outer surface of adja4ent storagetanl€r unplotected3) or behind thermalgot€ctionz), (see n3)The outer surfaces of aqjacent pr€ss&estorage v€ss€ls alrd plocess facilities(s€e ng)C4ntrol room, Maintenaice workshops,laboratoriG, werehous€s, etc. (see P.2)

8

Administrativ€ buildin$ (see P.2) 5rJ For prest@d concrelt t2.k, naximum ddiation nuesmy be detlnnined bt fie requiremenG given in 4,5.2.:)such facih es are prot crld by m.es or watf,r srrEys, fircDmiing, rad8tlon *reens o.simil& systms3' Proteciion is Drcvid€d by sDa.inr .lone.

(lwD'l)Remote areal) t3

Urban atea 6

Critical area2)

t) An area only infEqu4tly o.cupied by smal nmb€B ofpeMF, e8. moorlmd, fanntand, de&D'ftis is ettle-r !n u$hhlded e. ofailical lnlortrnce eh@Deoole sit rout prctecdre .lothi.8 ce b€ iequjred at 3I tirolnctudinS duing energendes or B plae difrodt o! dangercus toeEurte !t dblt nodce (eg sports stadrum, play groud,

Figure 25.13 Allowable themal radiauon flux excluding solar rad ation outside

Frcm EN 1473, table 2

Eq{lDn.rt luide bouar.ry

Flow rtte e d€nled lD 11.6

Base of the flai€ in a t€sbctedal€a wher€ well trainedoperators wil be present onlyfor maintenarce. (s€€ P.3)

5 I

Roads and open ar€as 3 6

'Ibiks erd proce$ equipments 5

Contsol room, mahten nceworkshops, Izboratones,

1,5 5

Adminjstrative buildings 5

Figufe 25.14 Allowable themal radiation flux excluding solar radiation insidethe boundary

Fron EN 1473, table 3

Flow rrte a delined ir 11.6

radistion nux (kwn'?)Nomal I Ac.ident'l

Remote arear.) 3 5

Urban area 3

Criticrl area2) 1,5

1r An @a only infrequ€ntly @cupi€d by sall nmbe6 ofpcen' e,g, m@rle4 fmland, des€rr,) This is eirler d NNelded ea ol cdtical importa@ ttErepsple witlDur pmtectiv€ clothing cd be required at all tinesinctuding duinS emergencies or e pl&e drfficuli o. dege.os toevacuare at short nolice (e,9. sports stadjw, pl4v groud,

Fjgure 25.15 Allowable thermal radlation flux excluding solar radiaiion outsidethe boundary

Frcm EN 1473, table 4

troleum lndustry - Part I Volumes 1 and 2 - LiquefiedPetroleum Gas February 1987.

25.4 API 2510 Eighth edition, May 2001 Design and Con-struction of LPG lnstallations.

25.5 United Sfates of America Deparlment of TranspotlRules.- Title 49 - Subtitle B - Chapter 1 - Subchapter D

- Part 193 - October 2000.

25.6 NFPA 59A, Standard for the production, storage & han-dling of liquefied natural gas (LNG), 2001 Edition,National Fire Protection Association, Quincy, Massa-chusetts.

25.7 BS EN 1473 : 1997 lnsta ation of equipment for lique-fied natural gas - design ofOn-shore installaflo,s, Euro-pean Committee for Standardisation and BritishStandards Institution.


fed

bn-

26 Seismic design of low temperaturetanks

This Chapter represents a brief visit to what is a large and complex area. Much has beenresearched and wriften on the subject and it is difficult to do it justice in so few words. lt hasbecomethe practice to approachthe seismic design of low temperatu re tanks at a higher levelofsophistication than has been the case of tanks storing products at ambient temperatures.

The Chapter is restricted to the design of metallic, liquid-coniaining, vertical cylindrical tanks.Those looking for detailed information on the seismic design of low temperature tanks ofdifferent geometries and construction materials will have to seek the information they requireelsewhere.

The importantareas of seismic design have been reviewed and illustrated by means ofa workedexample, using an LNG tank of modest proportions.

Contents:

26.1 General

26,2 The basic seismic design data

26.3 Damping

25.4 Directional combinations

26.5 The behaviour of the product llquid

26.6 Natural frequencies26.6.1 The horizontal convective frequency26.6-2 The horizontal impulsive frequency26.6.3 The vertical barelling Aequency

26.7 Ductility

26.8 Calculation of the design accel€rations

26.9 Product liquid pressures acting on tank shells

26,10 Tank stability under seismic loadings

26.11 Tank sliding

26.12 Liquid sloshing

26.13 Seismic isolation

26.14 The design Codes

26.15 Gonclusion

26.16 References

nrtYD

fl>'ot

,aa-

IFto-istr

STORAGE TANKS & EQUIPMENT 48,I

26 Seismic design of low temperaturc tanks

26.1 GeneralChapter 15 describes the relatively straightforward rules given

in the various ambient storage tank design codes to cover thedesign of such tanks for seismic loadings. These are based on

the work of Wozniak and Mitchell and use as their input the ac-celerations given for the various zones given in the AmericanUniform Building Code or similar sources. This approach ig-

nores the influence of the vertical components of the designseismic events. The recent earthquakes, particularly those inKobe in Japan and in Taiwan have provided evidence that thiscomponentis particularly destructive and has traditionally beenunderestimated.

For low temperature tanks where the risks associated with Iiq-

uid containment failure are generally considered to be more se-rious and certainly the costs of the facility are generally greater,

the seismic design is taken more seriously as has been men-

tioned in Chapter 15. Whether the risk or the cost arguments in-

deed justify this approach is perhaps contentious. Large singlecontainment refined product tanks are themselves dangerousbeasts, the consequences ofwhose failure in refineries are per-

haps the equal in terms of risk and cost of similar events inmany low temperature facilities.

Passing over this point, it is the case thai low temperature tankshave traditionally warranted this enhanced level of attentionPerhaps the apparent recent trend of placing large LNG export

and import terminals in areas of high seismic activity has had

something to do with this. The LNG tanks located at the Bay ofMarmara in Turkey, Revithoussa lsland in Greece and PointFortin in Trinidad are all examples of this trend.

The amount of theoretical and test work carried out in this area

is awesome. Not only have the giants of the subject like

Housner, Veletsos, Yang, Haroun, Manos, Wozniak, Newmarkand Hall been publishing learned works for many years. but

they are joined by a host of other authors working in the same

area. A fook at the list of publications in References 26.1, 26.2

and 26.3 will illustrate this point.

In addition to the volume of work existing in this area, there are

also a variety of different approaches to the seismic design ofthe various components which go to make up the difierent types

of low temperature liquid storage systems described in Chapter

17. For example the approach of the designer of the steel inner

liquid container is often very different to that of the designer ofthe outer pre-stressed concrete tank.

It is clearly not possible to describe the whole breadth of thisarea of activity in a single chapter, indeed it would be difficult to

envisage doing itjustice in a single book. Forthis reason the fol-lowing Sections confine their interest to the seismic design ofvertical, cylindrical, flat-bottomed steel primary liquid contain-

ment tanks. These are one of the most commonly found com-

oonents of low temperature liquid conbinment systems

The level of sophistication of the analysis is that which the au-

thor has found to be sufficient to satisfy the owners of newstor-age facilities togetherwith their engineers and technical consul-

tants. To those seeking information on the seismic design of

-500

F gure 26.1 A typical real 1me history ground ac.elerai on rccord


concrete tanks, membrane tanks, roof-mounted equipment,in-ground tanks and tanks of shapes other than vertical and cy-lindrical, apologies are given. However, a brief search of thesubject area should produce the necessary information.

The various national and international Standards and regula-tory organisations also have views on the subject ofthe seismicdesign of liquid storage systems. These are reviewed in the ap-propriate Sections of this Chapter and in Section 26.13.

26.2 The basic seismic design dataA real or a design seismic event can be expressed in a numbero{ different ways. A real time history is a plot of the actual re-corded ground motions (acceleration, velocity and displace-ment) against the elapsed time of the event. lt is accelerationwhich interests the designer most, and an example of such a

plot is shown in Figure 26.1. There are techniques which allowthe design process to be based upon such realtime histories,butthese are outside the chosen scope ofthis Chapter. In suchcases it is usual to use a number of real time histories of eventsfrom a similar geological setting and of a similar magnitude, orscaled to the same magnitude.

A more useful starting point for the designer is the responsespectrum. The response spectrum repfesents the peak re-sponse ofa single degree offreedom damped system to a baseexcitation. Such a single degree of freedom system is illus-trated in Figu re 26.2. The precise means of getting from a real

time history considered relevant for the particular site to an

agreed design response spectrum are complex and are bestleft to those with the appropriate expertise in these matters

Figure 26.2 Asingle degree offreedom damped system

The provision ofdesign spectra to the tank designer is usuallyatask placed in the hands of specialist geo-technical consul-

tants. These are companies with wide and respected experi-ence in the study of seismic events. The days when a single nu-

merical reference to an anticipated seismic event using suchdevices as the Richter Scale was considered sufiicient input forthe tank designer are long past. The RichterScale isstillused in

the media to describe the magnitude of seismic events, and forinterest the scale and a subjective set ofeffects is shown in Fig-

ure 26.3.

The usual outputfrom such specialists is a site-specific seismic

I

I

L______'

SUPPORT I.IOTION:

. '-.:

:=

:J,

It,MASS f10Ti0N:v, v, v

<Q

il5

-oquipment,caland cy,arch of thertion.

nd regula-he seismicJ in the ap-

'.13.

a numberactual re_displace-)elerationof such arich allowhistories,r. ln suchlfeventsritude, or

esponse)eak re-cabaseis illus-

m a realetoanrre Destrlters.

tatry a)NSUF(pel.-e nu-suc-ut fo-

d fo.

EM


Modified MercaliIntensity Scale Acceleration

RichterMagnitude Scale

Mletected only by very

rE srDve rn$rumenis

'-:tt by "

b* persols at

Lspended objecls swing

I F"[ l"d"-"jut""id=cognised as an earthquake:;TA

Indoot€ bv a fevruolor cars rock nbliceably

, Feitby most. Som€..ndows break. Little damage

;"- b*r r"r*:. mneys. Damage-smatl

t-rn "t*" -ar*rependrng upon construction

l1l Fall of watts. cht'nnevs-ionuments. A fair amoutit

x B!ildinqs 6hifted otf=fi dation-s. Much damage

;;""*'""*"" d"",,**3rcund cracks, Landslide's

a**u|"" "

or"r*f,iiges destroied

"-"."t*","t W**s€en on oround [email protected],tiecls thrown in sr

,005

.01

.05

0.1

0,5

1.0

u3

M4

u5

u0

SZ

UC

:€ure 26.3 The RichterScale and set of subjeclive effecis

-azard assessment (SHA). This is a substantial document:ank designers have been known to unkindly suggest that this:ocument is sold by weight alone). lt takes account of the fol-owing in arriving at ib conclusions:

. The detailed geology and tectonics ofthe selected site

. The seismic history of the area

. The details of the matedals and structures which will lie be-tween the bedrock and the base of the structure (this mav

26 Seismic design of low tempercture tanks

necessitate a site investigation if the information is not al-ready available)

. The nature ofthe structure, its contents and the possible in-fluence ofthese on the people and property adjacent to thesite

The sections of the SHA whjch the tank designer is most inneed of are the design response spectra. lt is usualforthese re-sponse spectra to be presented for both the horizonial and thevertical seismic events. Until quite recently only the horizontalspectra was provided and a factorwas given for scaling this tobe used as the vertical seismic design criteria. This factor hasgrown with time from 0.5 to 0.6 and 0.67 as the significance ofthis part of the seismic event became apparent. Following therecent events in Japan and Taiwan it could well be the case thatthis factor will increase and approach unity. lt is now usual topresentseparate vertical design spectra, whichtake account ofthe magnitude ofthis part ofthe event and anydifferences in theapplicable spectral shape.

The usual practice is to considertwo levels of seismic event forthe design of a particular structure. The allowable stress andlevels of permitted damage criteria which apply to each level ofevent are described below There have been a varietv of differ-ent definitions for these two event levels used over ihe vears.The most commonly used in the past have been the operatingbasis earthquake (OBE) and the safe shutdown earthquake(SSE). These were events with return periods of 475 and10,000 years respectively. ln recent time things have becomemore complex. NFPA 59 A which is a significant design docu-ment for most LNG tanks has chosen to introduce the maxi-mum considered earthquake (MCE) which is the event having a2% probability of exceedance within a 50 yearperiod (mean re-currence interval 2475 yearcl. subject to a number of definedexceptions. The liquid container and the impounding systemare then designed for an OBE and SSE which are deiined asfollows:

The OBE shall be represented by a ground motion responsespectrum in which the spectral acceleration at any period Tshall be equal to 2/3 of the spectral acceleration of the NICEground moiion defined. The OBE ground motion need not ex-ceed the motion represented by a 5% damped acceleration re-sponse having a 10% probability of exceedance within a S0

Typical Horizontal Response Spectra

0.500

0.450

0_400

0.350

P€riod T , secs

:igure 26.4A Atypical horizonlat seismic design response spectra in graphicatform for 5% damping

OBE - 5% DamplngI o.3oo

E 0.250

69 0.200

0.150

0.100

0_050

0.000

0 0.5 11.52 2.5 3 3.5 4 4.5 5 5.5 6 6.5 77.58 8.5 9 s.5 10

1

j

tl

\


0.100

0.050

0.000

0.5 1.5

Typical Vertical Response Sp€ctra

4.5 5 5.5

Period T , secs

Figure 26.48 A typical vertical seismic design response spectra in graphicalfom for 5% damping

year penoo.

The SSE ground motion shall be the motion represented by a5% damped acceleration response spectrum having a 1%probability of exceedance within a 50 year period (mean returninterval 4975 years). Howeverthe spectral acceleration of theSSE resDonse spectrum shall not exceed twice the corre-sponding OBE spectral acceleration. This seems to be a recipefor confusion, but doubtless there are good reasons for thechangeswhich willbe apparenttothose skilled in the prediction

of seismic design criteria.

It is most common to present the seismic design spectra ingraphicalform and atypicalexample is shown in Figures 26.4A& B. These are given for 5% damping which is also usual andfor both the OBE and the SSE events. Plots, which include thedisplacement and velocity data in addition to the accelerationdata, are frequently used and these can includethe data atvari-ous different levels of critical damping. These tend to get con-gested and are sometimes difilcultto usewiththe required level

Figure 26.5 A typical t.ipa.tite acceleration, veloci9 and displacement plot


-OBE - 5% Damping

-ssE - 5%

of accuracy when extracting design data. An example is given

in Figure 26.5.

Damping levels arediscussed further in Section 26.3. On occa-sions the seismic design spectra are presented in the form ofaseries of equations. An example ofthis type of presentiation, to-getherwith the graphs which can be derived from these equa-tions is shown in Figures 26.6 A & B. lt is usual for the time-scaleto run from zero upto, orjustbeyond'10 seconds. Thiswillcover the sloshing or convective periods of the biggest tankscurrently envisaged,

Where a site specific SHAis not carried out, the seismic designdata can be based upon artificial response spectra. Onesuch is

Desisn Spectlm {5%)Hodzontal

eadhd@ke. acceler,alion (a)

0,+1.0

OBE oos+

ssE o-+

c 0.200

E o.tso

Figure 26,6AAn example of s€ismic design data presented as a sedes ofequalDns and also In grapnrc€r rcrm

-9

lE

nsf,rrtxilc('r€(

"-'ia3@ ?t\r) or+

0,52.05.07,0

10.0

l Cr

1.0i.0i.0t.0

3_54

2.612.27t.90

5.954.253 ri2_12

2,14

-1.202.t02.051,88l.?0

rlv nnum sohd displacnenr r ekcn proporriont io nuxinrosound accelsalion, dd is 36 in. for Brornd accelsationol l.0 gravjry.

:Acelchrto. rid disFlacenrem xmplifiolioi raao* rs bk.n f'orlreonhond"roir siver rnEfergncc l

Amprili.ation F .rou ro' conror Poina

0.52.05.01.O

10.0

t.0t.01,01.0i.0

4.963.542.612.27t.90

t 6rl4.052.942.592.17

2_t3).671.311.25r.l3

: trre 26.68 An example ofselsmjc design data presenled as a series of::.a1ions and also in gfaphical form

',-at given in the USAEC Regulatory cuide 1.60 (Referencer5.4). The spectral shape is expressed in a tabularform cover--g a fange of levels ofdamping (Figure 26 7). When plotted oui--is is as shown in Figure 26.8. The point where the accelera-: rn intercepts the zero time axis is known as the peak ground::celeration (PGA) and in these plots it has a value of unity. lt is-ecessary to know the PGAappropriate to the chosen site to be

26 Seismic design of low tempercturc tanks

able to use this guide meaningfully. The use of an artificial re-sponse spectra is a cheaper but very much inferior means ofproviding a seismic design basis than that provided by a sitespecific SHA study.

26.3 DampingIt is important to select the appropriate level of damping for thedjfferent component parts of a storage system for the (usuallytwo) levels of seismic design event being considered. Guid-ance can be found in Iable 4.2 of Reference 26. / which is reoro-duced in Figure 26.9. This reference uses the ierms "probabledesign earthquake" (PDE) and "contingency design earth-quake" (CDE) in place of the more familiar OBE and SSE.

Where the seismic design data provided is tied to a singledamping level (usually 5%), it is necessary to have a means ofconverting to other levels of damping as indicated in Figure26.9. The accepted means of doing this is to use the Newmarkand Hall amplification factors as shown in the table (Figure26.10). The three columns headed A(B), V(p)and D(B)apply tothe constant acceleration, constant velocity and the constantdisplacement parts ofthe response spectrum. This is illustratedin Figure 26.11. T2 is usually around 3 seconds.

So, by way of an example:

The natural frequency of the impulsive portion of the tank liquidcontents falls between To and T1 and is 0.609 taken from theOBE 5% critical damping horizontal seismic response spec-trum. lt is required to determine the acceleration appropriate to2o/o ctilical damping as required for OBE design as shown inFigure 26.9.

The acceleration is then

Nt3\(2V"\nA^ '"- -" 4 B ttsv" )

Using the data from Figure 26.10 this becomes:

0.60 y::: = 0.81o' 2.71

26.4 Directional combinationsAny seismic event is made up of a combination of motions in thetwo horizontal (X and Y) and the vertical (Z) directions. Clearly itis unreasonable to assume that the three orthogonal compo-nents will be simultaneously at their maximum values. The in-dustry has decided that the following represent sensiblecombinations:

. 100% X +l- 30o Y +l- 30%Z

. 30'/oX +/- 1004 Y +l- 3O%Z

. 3Oo/oX +/- 30a Y +l- l}OYaZ

It is convenient to combine the two horizonlal components.

So for the first two combinations listed above a facior oft, :- --

J(10' 0.3') - 1044can beuseoandforthethirdcombination

a factor of = 0424 is appropriate.

There have been occasions when a 100% +l- 40% +l- 40okcombination has been required, bui this is less usual.

26.5 The behaviour of the product liquidThe means of calculating the effective masses ofthe impulsiveand the convective portions ofthe stored liquid and the heightsabove the tank bottom of their centres of gravity are exactly thesame as are used for ambient temperature tanks. This is de-scribed in Chapterl5, Section 15.2.2.

'q ue-

is 9-c-s

'Maxjmum grolnd displaetent is r.l.n troportionit to nariDungrou.d a.eletation and is 36 i.. aor eround acehrarion of I 0 gnvily.

! Acceleration anllticarion facros aor fie vertical dcxrsn r$pon*sp.ctra aE eqkl 16 ihos ior horizonlal deslgn rcspoise 3p.cttr ar r sivcnrr.qu.ncy, lhereas di$lacamenl aBpuliBIion lacroB aIo 2/3 nb$for lrori.anlal destgn lesponse specta. Itles rdios b€lveen llre amptitierion tuctoufor the r{o desiri

'espone spectarretn agre.ftenr pnh lhosg tecomneidcd

in retercnce I .

!Th6e valu.s were changed lo ruke rhis tablc consistem Nilh rhe dncusion dl veniel.onponenls in Sectiod B ol tirguide.

Figure 26.7 Tables I and ll from ihe IJSAEC Regulatory cuide 1.60

03+ 03'z)


26 Seismic design of low tempenture tanks

Figure 26.8 The USAEC date plotted out

26.6 Natural frequenciesIn orderthatthe designer can extract the appropriate accelera-tions from the design seismic response spectra provided, it isnecessary to determine the naturalfrequencies associated withthe significant loading componenb. These are the horizontalimpulsive frequency of the liquid tank system, the horizontalconvective frequency, and the vertical "breathing" mode fre-quency. There is a considerable volume of information on thissubject and the influences of tiank flexibility and foundation stiff-ness. In the interests of avoiding over complication, the meth-ods considered in this ChaDterare limited to those discussed inReferences 26.3, 26.5 and 26.6.

26.6.1 Horizontal convective frequency

From APf 620 (Reference 26.q

This is the same method as is used for ambient temperaturetanks (see API 650 Appendix E ) and is described in detail inChapter 15. For convenience this is repeated here:

The period of the first liquid sloshing mode is given by:

r = t(D)o u


where:

T = first liquid sloshing period (sec)

K = a factor taken from figure L-4 (Figure 26.12)


H = liquid fill height {ft)

Note: lf the tank diameter is given in metres, this equation be-

comes T = 1.81 1k(D)' '. (Rememberto use consistent

units for D and H in Figure 26.12.)

This calculation method is probablythe most commonly used todetermine this period.

From References 26.1, 26.3 and 26.5

The same equation forthe frequency ofthejth sloshing mode isoiven in each of these references and is:

E

i

!

ll

.1'2n .F'*"6H

where:

R

n

tank radius (m)

liquid fill height (m)equ 26.1

equ26.2

t!

;I

)'obdble De)lgn biplig, v"ssels, 5!a.ts, eqJio- to ?:.rt\qudre (P0F)

flusr r4.in elasric


constanr cssbnr comt.nrI 'c.6r€rarion I v€r.dry I dr!pr.c.m.d

+--+--,- _,__+

Fig ure 26. 1 1 lllustraling the porlions of the response spectru m associated wilhconstanl acceleration, conslant velocity and constant displacement

1.0

4.O

DIH

7.0

product liquid fill height (m)

period coetficient from figure C2.29 (Figure26.15)

liquid density (kg/m3)

Young's modulus of the tank shell material(N/m,)

lelded sleel, p.est.essedconcreter rcll relnforcedconcfete (only sli9hr c.acking)

Reinforced .o^.rete Hithconside.able c.acklns

Bolted andlo. .iveted sreel,{ood st.uctu.er rith nailed

eral Tanir - .onvecrlve {slosh)- inpulsive

c.itical D;npjne'

Pltinq, v4iele, sract5, equlp-ne.t, and ce.laln sttuclures that

telded steel, Orert..ssedconc.ete (rjthout coopleteI oss or P.€strets)i Presr.ess.n

prethess lef!

Bolred and/or rivered steel, roodstfuclu.es rlrh bolred jojnts

$ri nal red joln.s

Ileial ranks - convective (slosh)- rnp! r5lve

0.8

0.6

0.580

Soil-str0ctu.. iiteraction effectr nor in.1!qe!

:gufe 26.9 Damping levels appropriate for different componenls-.ken fram Table 4.2 from Relercnce 26.1

In rhi$ obr. rhe f!.rots ^

(P), v (P) and D (p) ryty ro ie conea r..l.mnD. !,n$d \ dsi!. rdr.nianr dBprrancm p.niotu. esperildy. ofrtrc tl.ssn sp.(nnn.

:igure 26.10 Newma* and Halldamping amptificalion factors

g = acceleration due to gravity (m/s2)

li = 1841

using real numbers (D= 65 m and H= 25 m), equation 26.1gives a period of8.76 seconds, whilstequation 26.3 gives 8.95seconds which is reassuringly close. The small difference is al-most certainly due to the manual interpolation of Figure 26.12.Equation '15.2 in Chapter 15, gives means of calculating K ex-actly.

26.6.2 The horizontal impulsive frequency

The method given in References 26.1 and 26.5

The formulae given in these References are essentially thesame and take account of the flexibility of the tank itself. Thenatural frequency ofthe tank{iquid system, using the terminol-ogy given in Reference 26.5, is given by:

Figure 26.12 Figure L-4 from API 620 Appendix L

Young's modulus of tank shell material (N/m2)

density of the tank shell material (kg/m3)

A dimensionless coefficient which depends onthe tank proportions (H/R and VR in consistentunits), the Poisson's ratio of the tank shell ma-terial and the relative densities of the oroductliquid and the shell material (p /p.)

nence:

;-1

P'

where:

pw = density ofiresh water (1000k9/m3)

pr = density of the product liquid (kg/m3)

pn = density ofthe tank shell material (kg/m3)

C" = factor taken from Figure 26.13 (also plotted asFigure 26.14)

It as suggested that for a shell of non-uniform thickness, the av-erage thickness is used. The data in Figures 26.13 and 26.14assumes a Poisson's ratio of 0.3 for the tank shell material.

The method given in Reference 26.3

The period ofvibration ofthefirst impulsive tankiiquid horizon-tal mode is given by:

_ 5.61rH n;rr = -./=-Kn ll tr9

equ26.4

where:

TI

kh

\tsE

It is again suggested that for a shell of non-uniform thickness,the average thickness is used and that the data in Figure 26.15

E

pm

Cr

.fF{ _ "1 iL

z^n ! pm

wnere:

fo = natural frequency (Hz)

H = liquid fitl heisht (m)

equ 26.3



tlR=00fd5 I JR = 0.N1

Frequen t @6cient, C-, in expressio! for fthdmertalMtdal frequelcy, to, of tels fll wirh wateri v=03,

AIP =0'127

HIR vzt@ ol c.

o28

0.20

.9

5I0 064 1

0.065 00.065 1

0.50.60.7O,E

0.91.0

0 050 600538005640 058 ?0.060 50.062 0

o.t1r90.97630.079 90.0€2 90.065 40.08'/ 5

0.090 30.09150.091?

Figure 26.13 Table 1 from Reference 26.5

Figure 26.14 Data from Figure 26.13 extrapolated and plotted

is based on a tank shell material with a Poisson's ratio of 0.3.

Using real numbers again (D = 65 m, H = 25 m, product liquidSG = 0.48, shell material density = 8000 kg/m3 and E= 210 x 10e

N/m2, average shellthickness = 20 mm) both methods give aperiod of0.35 seconds, which is again reassuring and suggeststhat they emanate from the same source.

All of these References (26.1, 26.3 and 26.5) provide workedexamples which are helpful, particularly in sorting out the ap-orooriate units.

26,6.3 The vertical barrelling frequency

The method given in References 26.1 and 26-5

The natural frequency of the axisymmetric breathing mode isgrven by:

{,YLl:z,rn \l pm

equ 26.5

Where the variables are as given in Section 26.6.2 with the ex-ception of:

C" = factor taken from table 7 .5 of Reference 26.1(Figure 26.16) or table 4 ot Reference 26.5(Figure 26.17).

Note: This table is for materials with a Poisson's ratio of 0.3


Height !o radius ratio, H/Rm

Figure 26.15 Figure C2.29frcm Relerence 26.3

andaql ol0.127 (i.e. a steeltank containing water).

These tables give a very limited amount of data when com-pared with Figure 26.13 and for steel tanks, is confined to a VRratio of 0.001 . This makes it quite hard to use in practice. It alsorequires adjustment for different product specific gravities inthe same way as C1 in Section 26.6.2.1.

Thus the value of this factor to be used in equation 26.5 in theplace of Cu is:

The method given in Reference 26.3

The period ofvibration ofthe flrst tank-liquid vertical (breathing)mode is given by:

2e

ti(

equ 26.6

Where the variables are as given in Section 26.6.2 with the ex-ceDtion of:

ku = factor taken from figure c2.30 (Figure 26.18)

When the same real numbers given above are used, these twomethods give values for this period of 0.32 and 0.30 secondsrespectively.

26.7 DuctilityThis is a complex subject about which much has been written.In this level of analysis it is dealt with in a very simple manner.Wozniak and Mitchell in their paper (Reference 26.7) support-ing their proposal for a seismic Appendix to API 650 suggestthe following:

"The increased hoop tension due to earthquake ground motionshould be added to the hoop tension due to hydrostatic pres-sure. The hydrodynamic portion (i.e. that part due to the earth-quake) should be divided by a ductility factor of2.0 for applica-tion in the design at the normal allowable stresses."

This method ofanalysis treats ductility in a different way. FortheoBE condition the structure must remain elastic and a ductilityfactor on 1 .0 is used. For the SSE condition the structure mustremain nearly elastic and ductilityfactors of 1 .2 to 1.3 are usual.

A factor is calculated using the formulal

u = ductility faclor (i.e. 25"k is given as 1 .25)

-ris factor is then applied directly to the design accelerations:s indicated in the following section.

26.8 Calculation of the design accelera-tions-aving calculated the naturalfrequencies ofthe Iiquid-tank sys-:em for the horizontal convective, horizontal impulsive and the.ertical barrelling modes, the basic accelerations for each

'rode for both the OBE and the SSE cases can be found from:re appropriate response spectra.

arom the datia given for the horizontal and vertical seismic

2

26 Seismic design of low temperature tanks

events at 5% damping in Figure 26.6 and using the periods cal-culated above (i.e. horizontal convective = 8.95 sec, horizontalimpulsive = 0.35 sec and vertical barrelling = 0.30 sec) the ba-sic accelerations can be taken from the graphs, or more accu-rately calculated from the formulae given.

These accelerations must now be adjusted for the corlect levelof damping, the inclusion of the second horizontal componentand the influence of ductility.

Figure 26.6 does include a means of adjusting the dampingfrom the 5% figure provided to othervalues, but in this instancethe more commonly used Newmark and Hall relationships willbe adopted (see Figure 26.10).

The damping values suggested in Figure 26.9 for the OBE andthe SSE for various actions have been adopted. These adiust-ments are illustrated in Figure 26.19.

26.9 Product liquid pressures acting ontank shellsThe tank shell is normally designed for the hoop tension result-ing from either the maximum operating liquid fill height orfor thehydrostatic test condition as described in Chapter 18. Wherethe tank is subjected to seismic loadings, the hoop stresses areincreased.

The total hoop tension is made up ofthe following components:

The stress due to the normal operating condition (P")

The stress due to the horizontal impulsive action (Pi)

The stress due to the horizontal convective action (P")

The stress due to the vertical barrelling action (P")

These are all the maximum values calculated when the tank is

full of product liquid. From Section 26.4, it can be seen thatthese actions do not act simulianeously at their maximum cal-culated values. but give rise to the following two combinations:

. Ps +P +P.+0.3P,

. Ps + 0.3(Pi + Pc) + P"

The means of calculating the actual values of these variouscomponents are taken from Reference 26.7 This refefence

uses US units.

R = 2.6YDG

wnere:

equ 26.8

P" = hoop tension per inch of shell heightduetonormal product filling (lb/in)

Y = distance from the point on the tank shell underconsideration to the liquid surface (ft)


6 = product liquid specific gravity

For tanks where D/H is greater than 1 333:

equ 26.9rY .v''l , .^^DtP .4.5A.GDH| ' -0.51 -l I tanh 0.5oo' JH tt-lrl

C0EFFICIENT Cv IN EXPRISSIoN FoR FUNDAT4ENTAL FRIQIIENCY 0F

AXISYI'1I,IETR1C, SREATHIN6 I,IODT OF VIBRATI(]N OF TULL TANKS

Sol utionBendingSol uti on

Bending

!.?00.30

0.50

0,75

1.0

1,5

?.0

3.0

5.0

0.0467

0,056/

0.0705

0.0808

0.0868

0.0925

0.0949

0.0969

0.0979

0.052I0.06 L0

0.0738

0.0834

0,0889

0.0950

0.0964

0,0982

0.0992

0.0787

0.09/ 3

0. t230

0.1423

0.1533

0.1641

0.r688

D,TI24

0.1/44

0.1135

0,1250

0,1414

0.1564

0.1650

a -r7 26

0. i/560. l7 70

0.1775

* h/R = 0.001, erlp = 0.127, ! = 0.30

t h/R = 0,01, o!/o = 0.40, ! ' 0,17

:.ure 26-16 Table 7.5 from Reference 26.1

Co€frcienl C, in exPr€ssionfor tundalaental ftequency ofaisymmetric. breatbing nodeol vibration of full lanh ' =0.3, tlR = 0'$1, PtlP =0127

050751015

. aJrc 26.11 Table 4lrcm Reference 26 5

_,1'- t^ ,

-J zlt - |

0.ff70 50.080 80-086 8o.w5

equ 26.7

€0rwhere:

Pr

TI

hoop tension per inch of shell height due to thehorizontal impulsive action (lb/in)

horizontal impulsive design acceleration (g)see Section 26.8 above

liquid filling height (ft)

For tanks where D/H is less than 1.333:

Y< 0.75D:Figure 26.18 Figure C2.30ftom Reference 26.3



Figure 26.19 Adjustments to the basic acceleraions

equ 26.10

equ26.11

equ26.12

P" = hoop tension per inch of shell heightduetothehorizontal convective acceleration (lbi in)

A" = horizontal convective acceleration (g)see Section 26.8 above

oosignaerSmlcEV6nl

lIod6 P€riod Batrc g€cond

tonzonlElCrilicaldafiprn0

s6lgclgd(%)

Dqmpingadjul|medl€clor

Ductilityfactor

0uctilltyadjultm!faclar

O..lonEdjustod

(c)faalor A'B'CD

Horizont€l conv6ctiv6HorizonlEl inpLrlsiveVertlcel barrsling

Horizontel conv€cliveHorizont€l impuliiveVarlical ba €ling

8S5035030

0.35030

0.00280.1€750.1153

0.01120.75000.4610

1.U41.U41.000

1.044LU41.000

0.5

2.O

0.5

5.0

1.512

1.35i

1.512'1.000

1.m0

1.001.001.00

1.251.251.00

1.0001.0001.000

0,8160.8161.000

0,00440.2645

0.01440.63E90.4610

P -277AGD,[ Y o.sl Y l'z

L 0.75D r 0.75D'

Y> 0.75D:

R = 1 .384AiGD'z

/HY)cosh 3.68 ^'P^ 0.975A-cD/ u

/ H\cosh 3.68 I I

T D,j

wnere:

I rr-r-v'lP" = AP,cos ,r * |

L \ zn 1)

where:

equ 26.13

P" = hoop tension per inch of shell heightduetothevertical barrelling acceleration (lb/in)

A" = vertical barrelling acceleration (g)see Section 26.8 above

It is normalto calculatethe hoootension in the tank shell result-ing from the operating static loadings combined with the mostonerous combinations of the OBE and SSE loadings.

The allowable tensile stress in the tank shell for seismic OBEand SSE loadings are given in the various tank design Codes.Briefly these are:

. APt 620

Appendices Q and R suggest that 1.33 times the allowablestress for "normal" operation may be used. As API 620 doesnot consider the seismic events at two different levels, thisallowable stress is usually taken to apply to the OBE case.Q.3.3.6 does apply the additional criteria of not exceeding90% of yield stress for tanks of stainless steel and alu-minium alloy only. The criteria for the "normal" allowablestress is as described in Chapter 18. As API 620 gives nocriteria for the SSE case, the rule in NFPA 59A is frequentlyborrowed. This limits the tensile stress to the minimumspecified yield strength of the plate, or of the weld metal ifthis under-matches the plate material.

. BS 7777

In Annex B of part '1, the tensile stress in the OBE case islimited to 85% of the plate or weld metal minimum yieldstrength, and for the SSE case to the yield strength of theplate or weld metal.


o prEN 14620

This allows '1.33 times the allowable stress for normaloper-ation (as Chapter'18) for the OBE case and minimum yieldof the olate or weld metal for the SSE case.

The required thickness of the tank shell to comply with thesecriteriaforthe OBE and SSE casescan becalculated and com-pared with that derived from the static normal operation rules.As the seismic loadings can be the governing criteria for shellthickness at different levels, it is necessary to calculate thethicknesses required for both the OBE and the SSE cases foreach course of the tank shell. This is lengthy and tedious work,best carried out by bespoke computer programs or by the useof spreadsheets.

It is customary to use the fixed fraction combinations of thethree components for the OBE thickness calculations and thesouare root ofthe sum ofthe souares forthe SSE thickness cal-culations. Using the 65 m diametertank adopted as an exampleearlier, and the accelerations from Figure 26.19, Figure 26.20shows these calculations. lt can be seen that to accommodatethe OBE. courses 4.5 and 6 need to be increased in thicknesswhilst the SSE courses 2,3,4,5 and 6 need to be increased in

thickness.

The distribution of Pi, P", and Pu over the height of the tank areshown in Figure 26.21. fhe impulsive and vertical barrellingcomoonents are at their maximum value at the bottom of thetank whilst the convective comoonent is at its maximum valueat the liquid surface. There is a school ofthought that Pu shouldbe distributed in a Iinear fashion over the tank height. Figure26.21 shows the more onerous ootions.

The distribution of the impulsive and convective componentsaround the periphery of the tank shell is a function ofthe cosineof the angle to the point under consideration whilst the verticalbarrelling pressure is equally distributed around the tankcircumference.

That partofthe seismic moment which acts onthe basearea ofthe tank may be required to be evaluated for reasons offounda-tion design or base insulation compressive loadings. As hasbeen described in Chapter 15, this moment is the difference be-tween l\illBP and MEBP The distribution of this moment interms of base pressures across the tank bottom is given in Ref-erence 26.3.

26.10 Tank stability under seismicloadingsThe basic calculation methods used are the same as are usedfor ambient tanks and described in Chapter 15. These comefrom API 620 AppendixL (Reference 26.6) which is based onthe workofWozniak and Mitchell (Reference 26.4. Appendix L

of API 620 is essentially the same as Appendix E of API 650.

The methods of calculating the impulsive and convective


TANK SEISMIC DESIGN .SUMMARY OF HOOP LOADINGSFixed fraction load combinationSquare root of lhe sum of the squares combinalion

OAE LOAD CASE

Design code API 620

)e1

:--

-9:_ -:-:-

:_::_:

:'a-=

:-

:_:_--:

Ph*, = The maximum hoop tension as a rcsult ol the combined masses in lhe horizontat;

P",". = The maximum hoop t€nsion as a resuttoflhe cdombined masses tn the vedjcal;

P","" - The squarc rcol ot the sum of the squares

Ts: Thickness ofsnettdue to hyd.ostatic toadings onty

Th = Thickness otshelldue to maximum hoopiension as a resutt of ho zontalmorion;

T,= Thickness ofshellduelo maximum hoop lension as a resutr of verricai motionj

T*" = Thickn€ss ofshelldue to maxrmum hoop tension irom squarc rcot otthe sums sqlaredi

TANK SEISMIC DESIGN .SUMMARY OF HOOP LOADINGSFixed fiaclion load cambinatlonSquarc rcol ofthe sum of the sqlares combination

SSE LOAD CASE

Design code API 620

Ph,* = The maximum hoop tension as a resu[ o he combined masses in the hodzontat]

P*,"= The maximum hoop tension as a rcsutt ofthe cdombined masses in the vedicat;

P"* = The square rootoithe sum ofth€ squares

Ts: Thickness ofshelldue to hydroslatic toadings only

Th = Thickness of shell due to maximum hoop le.sion as a resutr of ho zontat motion;

T" = Thickness ofshelldue to naximum hooptension as a resutt of vedica{motion;

T"* = Thickness ofshsttdue to maimum hoop tension frcn square root ofrhe sums squaredl

: gure 26.20 Spreadsheets showing the calculaiions for siabjtity and axiat compressive toadings

phnax=ps+pi+pc+0.3pv

Ps/ss=Ps+!@-t[7;j

phnat=ps+pi+pc+0.3pv

Ps+ Pv + A.j ei + Pc)

csns=es+1/jF +a +Ej

H, Tank ht:A i =l 0.2645 Iac =[ ooo+l ]& =fo.r55tl

Kl= 9774,Q28K2= 42.944

D/H = 2.600

A{rowabre srress =L9q!!9L25.000

P' P. Pa,* P",.. P".., Ts

23

567II10

2.700 22.3AA2.700 13.6002.704 16.9002.704 14.2402_704 11.5002_7AA 8.8002_7AA 6.'1002.744 3.400

1947117114147561239910041764453262969

33523207297A264722491786'1272

721

48304659437439753r'62283520941239

43454A51

5662fa7990

25350227402006917219142341111774724503

24245217321905316254133461033972474085

25351227702044317175141701103577774404

14.8313.03

9.448.008.008.008.00

14.52 13.91 14.52'13.04 12.44 13.0411.49 10.91 11.489.86 9.31 S.84815 7U 8.116 37 5.92 6.324.51 4.15 4.452 5a 2.U 2.52

G. Product SG =

A r =l 0-63E9 |

Ac =-o.oiA"=T;36fi--l

K1 = 23609.174K2 = 140.556

D/H = 2 600

Allowable stress =l 400.000

25.000

23

56

8I10

2.700 22.3002.700 19.6002.700 16.9002.700 14.2002.700 11.5002.700 8.8002.700 6.1002.700 3.400

'19471

17114'14756

1239910041768453262969

99199489878878346655528537632133442

11667'11254

1056596024362684850582993

3425631362281132451A2054416321117426861

3478531835284992479220730163361 16356653

14.8313.0311.249.448.008.008.008.00

142147156168'184

204229259

3293230023267602316319260150841067560781338

15.00 14.42 15.2313.73 13.15 13.9412_31 11.72 12.4814.74 10.14 10.869.01 8.43 9.087.15 6.61 7.155.14 4.68 5.103.00 2.66 2.91



o =P

0.8

p.g

t

o 0.4-

0.2 0.4 0.6

Pressure (1.0 = maximum value)

1.0

Figure 26.21 The distribution ofthe ho zontal impulsive, horizontal convective and the vedical barrelling pressures on lhe tank shell

masses and their moment arms, the resistance to overturningprovided by the liquid action on the annular plate, the axialcom-pressive stresses in the tank shell and the overall stability areexactly as described in Chapter 15.

One difference is that the calculations are carried out for boththe OBE and the SSE seismic events and for a numberofload-ing casesfor each event involving the directions [horizontal (H)and vertical (VI and coexistent proportions (usually 100% and30%). lt is usualto look at the following for both OBE and SSE:

. 100%H + 30%v

. 100%H - 30%v

. 30%H + 100%V

. 30%H - 100%v

This represents quite a lot of work and is best achieved by theuse of spreadsheets. The means of including the vertical com-ponent into the calculation is to increase or decrease the den-sity of the product liquid and the effective weight of the tankshell. These calculations have been carried out for the 65m di-ametertankand are shown as Figures 26.22 to 26.26. Notallofthe load cases have been included as it is clearthattheydo notall play any part in the selection of the tank scantlings.

It is clear from these soreadsheeb that the various OBE loadcases do not change anything other than the annular platewidth, which is increased from the API 620 minimum require-ments. The dominant load case is the SSE 100% horizontalplus 30% vertical combination. To achieve a workable solutionfor an unanchored tank, the annular plate must be thickenedand made wider, and the lower shell courses be made thicker.The axial compressive stress criteria adopted in this calculationhas been theAPI 620Appendix Lvalue increased bya factorof1.33.

As has been mentioned in Chapter 15, the API allowable axial


compressive stress is one third ofthe classical buckling stressfor a perfect cylinder When the influence of the internal liquidpressure is considered, there is clearly room to increase thisstress, especially for extreme load cases like the SSE. NFPA59A, which is often one ofthe significant design codesfor LNGtanks, requires the design limit for the SSE case to be "critical"for buckling.

l\.4uch has been written about allowable buckling stresses invertical cylindricalvessels. Before choosing a value for the SS E

condition it would be wise to look at in Figure 18.5, in Chapter18, (The Royal Aeronautical Society Structural Data sheet).This takes account of fabrication and erection imperfectionsand the influence of internal pressure on axialbuckling strengthof vertical cylindrical vessels.

A similar relationship between these variables is illustrated in

Figure 26.27 taken ftom Reference 26.3. Also the elastic plasticcollapse (elephant's foot buckling) criteria given in this Refer-ence could usefully be explored. Figure 26.28 shows a tankwhich has suffered an eleDhant's foot buckle in the first course.It may be that in the light ofthese investigations, a factor of 1.33is found to be conservative figure for SSE buckling. Indeed, BS7777 in Annex B to part 1 suggests that the same value for al-lowable compressive stress as is permitted byAPl 620 is used

for the oBE and a vatue oto.+lEllis used for the sSE. This is\R/two thirds ofthe "critical" value and is the same as using a factorof2.0 rather than 1.33.

lf it isdecided thatan anchored design can be adopted, then thesituation becomes easier in certain respects. The axial com-oressive stresses in the lowercourses ofthe tank shellare con-siderably reduced, as are the local high line loadings being fedinto the base insulation and thence into the foundation.

As has been discussed in Chapter 15, this is due to the bending

N\

\

\ ,a\ \

\.

\ Pi

Pv \,,

\

\

\

-e!:'f,.ae,-:C

:lr'

uidhisIPA

\iG

tnSEter

lm

lnbc

'tk

'JSF

d

tf

3

l

-eutral axis of the cylinder being maintained in a central posi-:on, ratherthan being displaced to one side and effectively con-:entrating the axial compressive portion of the bending loadrto a small Dortion of the shell circumference. In certain cir-:umstances, perhaps where the site is of restricted area and:re spacing rules diciate a tank of proportions unhelpful to thes€ismic design process (i.e. tall and of a small diameter), thenanchorage is inevitable unless expensive options such as seis--ric isolation are adopted. Where either design is a possibility,:ren a number of other factors come into play:

. Where the tank can withstand the seismic loadings withoutincreasing the tank shellthickness, the only increased costofthe unanchored alternative being an increase in annularplate width and thickness, then it is usualto choose the un-anchored alternative.

. When significant increases in shell thicknesses are re-quired to make the unanchored alternative viable, then aneconomic assessment will help to make the decision, un-tess:

- The owner, or his engineer have strong views regardingthe use of anchors. These usually relate to the non-de-sirability of attaching anchors to an already highly-stressed part ofthe tank shell. The anchors may require

Seishic Design of Storooe Tonlc

!!q 65m dio x 27ln high LNr€ tonk

Led @s. OB€ 100% horizodol+ 30% vedi@l


to be substantial and will need to be attached to the low-est shell course where the hoop and vertical bendingstresses are high during the defining seismic event. i.e.when the tank is full of product liquid. The addition oflarge localstresses into this area is seen by manyas en-hancing the risk of tank failure under seismic loadingconditions.

26.11 Tank slidingAs has been mentioned earlierin Chapter 15, there are differingviews held by those with seismic design wisdom on the need toprevent tanks from moving horizontally during seismic events.One school ofthought, which probably represents the majorityview and certainly has determined the Code requirements onthe subject, requires there to be no movement during the mostonerous seismic design event. The second school of thoughtconsiders the horizontal seismic desagn event to be a sefies ofrapid reversals as is illustrated in Figure 26.1 with the input en-ergy equally distributed around the zero acceleration axis. Con-sequently the tank shuffles around on the spot and any lateraldisplacemenb are small. This shuffling about is considered auseful and harmless way for the system to increase dampingand dissipate the input energy.

Seismic Desiqn of Storoqe Torks

Mg 65n did x27n high LN6 tonk

Ldd .ose OBE 100% horizoni.l - 30% ve.ticol

*zon c.efticientEss€ntidl Fociliti.s fodor

weighi of l@k shell. insulotion

Roof l@d c.frled by 5h€ll

Height oJ tdnr shell'fork Dionete.

Produ.t specif ic 6.dvityliloxihuh liquid level

rhickne3 of firsr course

T1lkkness of 6d!ld.s pldte3

Yield sfEngth of onNldrJ

l in yieid of 1st.ourseForc€ @efficient

Sate Atnplif i@tloh f oclof

W.i9hl of tdnk .orterrs

z.fl--o.+loo -lr.[--iE--l

Hr= 88-58 ftD= 213-25 fr6.fi---5024 -lH= 42,02 ft

0,657 in

0.1rc9 in

'Zo^z c.ztticient€i<entidl Focilitier f octor

Weighl of rank shell+ insuldtion

Roof led cd.ied by shell

Height of tonk shell

Tdnk DiohetefP.odlct specif i. Gravit

Moxilnln liquid level

Thi.kn€$ of first colrteThicknels of onnsldrs Plotes

Yield strengih of dnnulo.s

l ii yield of 15i @u.se

Force coeffi.i.htSite Anplif i.otion f octor

W€ight of tonk coni€nts

W,= 1021577 l')w.= O lbHJ. 88.58 frD: 213.25 lt6=f-nE6-lH. A2.OZ ttt = 0.657 in'rb' 0.409 ir

T?rnT-rlt ,?.@-ll-6l-00 I

frjoo I

fto.?o lfrcro l

Tsoa-r-nrl|- ?r.oo If6;_oo I

frj.oo ITl-.io IT-ioE-]

D/H = z 6ot)k : 0,613

'r= kool = 8.958

c?= 3-375 3/12. 0.0166

Wtlwt- 0:4344 wl= 39914438.12

Wzlwt. 0.5309 Wz= 44742749-44x/H: 0.3750 Xr. 30.76xz/H- 0.5693 Xz- 46.69

iLn.nr (EBP) lb-fr : 334rt99504.9

P.oduct weight wL = 6053.70 lblfiMin ridth of onnu,or = 4925 ftsloshing mve height d = 0.656 lt

r. koo5). 8.958c2.3.375 S/t2 - 00166

Wr/W'= O 4344 W1= 36355188.86

W?/q. 0.5309 Wz- 44432695.35

X1/H. O.37aO X1 = 30.76xz/H - 0.5693 x, . 46.69

l enent (E8P) lb-ft = 322887096.9

Productweight WL= 5777.49

W,. 37961434.83 k9

D/H = 2.600k. 0613

- 4.28d. 0.656

Wi. 1524.85

. 0.972b: 10388.93

't316.76

GHD,/1,shelllii€ lood

M/ID1WJ+ W!)iEX loi9 @lnp force

longiiudiml cohp str€ss

. 4335010

: I.0O9

b- l$47.O1

l4al.29

Mii widih.f dnrulorsloshihg wove heighi

GHD,/t,5helllihe lood

M/tD1%* w,)nBx bng .ohP rorce

longitudiml comp stress

tb/tt

fr

oll.wable conp shs F.. 3083.11

che.k stresses : Ok

check stobjliiy : Ok

l.!ote: ZlCi A\and ZIC2. A.,{nnriL. plate thick|l6i fron,lPI620 rabl. Q-4A

allowoble cohp stfess F". 3083.11

ch.ck dr6se5 : Okcheck stdbilily ' ol

Note: ZTcFAno d ZIcz=A.

^n.!lo. ploi€ thi.kh.$ from API620 Tabl€ Q 4A

Figure 26.22 Spreadsheet calculations for stability and axial compressive|oaorngs

Figure 26.23 Spreadsheet calculations forstability and axial compressivercaorngs


26 Seismic design of low tempenture tanks

Scienic Dcsion of Slorooc lanks

!!t 65n dio x 27n high !|€ ronk

l4s!L!s$ sse rco% ho.itontdl * 30U wrt 6l

s.ishic Desion of Storooe Tqnk

!S9- 65n dio x 27|n nigh LNG tonlL.od c.t sSE I0o% horieitdl- 3o%v.tti6l

API 620 oppendix L

rPl650 oppcndix €

'Zoe .oetficiqtB34tiol F4ilti* fdri4

W.ight of td|( 3h.ll. i@krionRoof l@d 6ried by shell

H.jght of t@k sh.llldk Dio|n.i4

Prod@t Sp4'fi. 6mtttMqiNn liquid l.vel

Thi.lB of fhst .dEThi.l6*s of @nuhB pldtd

Yi.ld siretdn of omhrsl in yi.ld of 13t @urs.

FoE e.fficidtsir. I nplifi..rion fodd

W.irhl ot ronk @nr.ii3

0. ?13.25 ft6 =[-i.-6d.ll1= 4232 ft

r,,.f-J5@-*ltuzin'rn = l--ffi--lturin'"'=|"-_-ls.l 1.0o I

'ZotE ..ettiei'n€.!€rtiol F.cilhi.s f dctor

Weight of tonl sh.ll+ in3rhiionRoof l@d .drri.d by shcll

Height of tdnk lhellTonl Diomeler

Produ.r s?€cific orovitl^.rihuln liquid l.vel

_rhickn $ of fa.sr cou.s.

ThicloBs of .irEh.s pldta

Yield srreryth of orrul@s

l^in yield of lsi cours?

For.e coefficientsite Arnplif icotion fdctor

Weight of tonk cont€nis

z=[---iG-lr

= f---i-oo--l

t1: 88.58 tD = zB.e5 fta

= [-o.-ii:?-lH: 42.02 ft

Fq.f--ffi-lturin'r,".|--ffi-.]tuuin'c' =f-o;-ls:l r.@ I

wl: 3€112963719

t ,5,@ l"'f ezoo l*T ro.oo l.'"

lJSOrffi.rO ap|.""O"n""t calculations for stability and axial compressive

The calculations are simple and are shown in Figure 26.29 forthe 65 m diametertank. The direction ofthe verticalcomponentis taken to give the minimum effective self-weight and conse-quently the minimum horizontal restraint.

The maximum value of the required coefficient of friction be-tween the tank and the underlying base is calculated, for theSSE case with the verticalcomponent in the unhelpfuldirection,as 0.3478 in this case. For mnventional LNG tanks, the innertank bottom will be constructed of9% nickelsteeland the baseinsulation willmost usuallybe capped with a concrete screed orsimilar lt is this interface that predominantly concerns the de-signer, although it is worth considering the possibility of therebeing another interface within the base insulation "sandwich"

which may be the more likely slip plane. This is not usually thecase. Assuming the 9% nickel steeliconcrete screed situationin this case, the efiective ooefficient of friction can vary widelydepending on the surfacefinishes ofthe steeland the concrete.The steel can be supplied untreated, shot-blasted or shot-blasted and primed. The concrete screed can be supplied witha wide variation of compositions and finishes. Proprietary mate-rialssuch as are often used onfactoryfloors and concretewalk-ways can be used to increase the coefficient of friction.

The best source offriction datia is derived from laboratorytests


6/H= ?.600

I : 0.613

T. k(Dot) = 8.956c2 :3.375 s/.'2. 0.0135

wy'W.= 0.4344 Wt. 32459497.b?wrl4. 0.5309 W,: {01503?0.8

X,/H : 0.3750 X, = 30.7b

X/H: 0.5693 X, = 46.69

iLn r$ G8P) lb-fr : 6984?51Y.4P.oduci r€i9hi wt= 9o42.al lb/L

l in wadrh of .nelqr = 7.304 f+Sloshiis *6ve heighi d - 1.291 tr

6Hb'/{ . t36a295shelllihe lodd wl: 1378.39 lb/ft

l 4D'z(wi* wD = 1 475

|mx loig conp for@ b = 59472.33 lh/trlongiiudiml .onp s+f€ss 4662.34 lb/i,J

.lloebr. comp dr63 F,= 49W.6a lb/i^z

check sire$e. : Olchak ndbility : Ok

N.ie ZIc'=iiand 2TCr:,t" (wiues of Z 6nd C, adjusted 1o erit)lnruld. pldle thickrcss inc@!€d to dllo{ on uekhored solution

Sottoh colrse sh.ll plot. thickn€s. incM!.d fo dlloa oi un nchor.d sorution(e. Figta 26-24)

Figure 26.25 Spreadsheet calculations for stability and axial compressivetoaotngs

using the materials and finishes proposed. Small-scale testingmust be carried out with care by those experienced with thistype of work. The apparently simple small scale friction testscan be misleading. There is some published data available andone such source is Reference 26.8.

The various design Codes give some limited guidance in this

. API 620 Appendix L This suggests the use of a coefficientof friction of 0.40.

. BS 7777 This gives no advice regarding this matter.

o prEN 14620 This gives the following mandatory (i.e. shallrather than should) requiremenb

- Horizontal sliding ofthe tank shall not be allowed

- Friction factors shall be based on literature ortesting

- For the OBE case a factor of safety of 1.5 shall beap-olied

- Far the SSE case a factor of safety of 1.0 shall beap-plied

The resistance-to-sliding throughfriction between the tank bot-tom plating and whatever lies beneath will impose significant

0/H - 2.600, = 0.613

T.k(Dd5). a.95aC2: 3.375 s/T? : 0.0135

wl/Wi. 0.4344 wt= 44O7f444.59W,/W'. 0-5309 W,.53870759,14

Xt/ta= O3IAJ Xr = 30.76

x,/H: 0,5693 X2= 46.69

l.ndt (ESt) lb-fi = 937391231Product rejghr wL. 1212996 lb/ft

l{in width of omkr : 73U lt5l6hin9 ME h.ignr d = 1-?91 ft

6H6"F . 1331396

sh.ll liE load Wt . M46.72 lbltt

^.!/lo'(wi*wJ . 7.475

M lo'g .olnp fft. 6 : 797A2.A6 l6/ltr4itudjel @nP 5116 6254.5a lb/inz

otkMbl€ coinp iifd F,. 49A4.68 lb/i.z

chck !i..sg = Wornhg dlebl. conP.6iv. !tr4 sceed.dEnhohc.d olloftbL @hPte$iv. st,B O(

chal srohili'y - ol'

zIq+\atdzrc2=A. (mlu.s of z did c,.djGr.d r. suir)amslor lhte rhicktEs [email protected] to lllow d tukhor.d $iurrohEotros sh€ll couE. 6lso ind.@d to ollow oh uMchorsd $lutionOthc. low *ell cdr36 tjll rcquir. ch€cli'g fof 4Pr6ive str4 dd 5ow

fu'-s3lf!"0 IF ,r"oo Ifrr-06-1

T ,5'o 1

|z16-lf-63o-l

rc

tr

f

ta

Btn

nn

I

II

XE

ilr"-Jr"lJ'lJh

l''

ingN|s

no

lrs

nt

pSTORAGE TANKS & EQUIPMENT 495

| lflr 1

f-0.i-l| 27.00 |

f6r.oo I

Ta5.oo Irirzii-lT ,oJo I

D/H: 2.600k = 0 613

T= ktDo6) = a.958C2= 3.375 Sff2. O!:o41

WtIWr= 04344 Wt:440774aa.59WrlW1: 0,5309 Wr= $47O75t14

x/l: 0.3750 x= 30.76X?Al: 0-5693 Xz= 46,69

l 6at (EBF) lb-ti = 27361536!.3PFodu.r w.ighr WL= 1?12956 l6/tr

Min odth of qnnula : 7.304 fisl€hingMw h€ighr d: O.ll7 ll

a\bz/rz : 1831396

sh.ll litr. load Wr = t84A.72 lb/lrM/tD1W'tw.) . o43o

nd hn9 .ohP f.r.. b = 94f,7-9o lb/I1longitudind conp.rr.s 74537 lb/it'

.lloeble conp dr.# F. = 49t4.68 lb,li?chak.t..#ee = Okch.ckltability: Ok

ACiArad4cz=A\ {vdlue of zod c, adjBred to surrAnnuld Phterhi.kn # in+.c.d 16 i{ir rcO%H+30AVcG€ (i..li9 26.24)

=igure 26.26 Sp.eadsheet calculalions fof stabilily and axial compressiveo€drngs

loadingson the bottom as a membrane. The loadings which arethe cause ofthe sliding are acting on the tank walls and the re-sistance to this are acting on the iank bottom. Consequentlythere is a need to transfer the loading from the bottom to thewall. For reasons which are not entirely obvious, the industryhas largely turned a blind eye to this aspect ofthe seismic de-sign. lf this scenado is to be taken sedously, it could result in in-creases in the bottom plating joint factor (i.e. from single-sidedlap welds to double-sided lap welds or buttwelds), increases inthe bottom plate thickness or changes to both joint factor andthickness.

lfthe requirements for sliding resistance cannot be met and theargument that sliding does not matter are not accepted, thenthe designer has no option butto provide some form ofphysicalrestraint or to change the tank geometry The design of a sys-tem to provide physical horizontal restraint to a large tank isboth difficult and problematical.

An early design forthe 65,000 m3 LNG tanks at Revithoussa ls-land in Greece had a system of inclined seismic anchorsforthispurpose. These are illustrated in Figure 26.30. The anchorstraps were large (some 300mm x 25mm in section) and gaverise to serious congestion in the bottom corner ofthe tank. Theloads transferred to the tank shell were massive and the argu-ments which have been used agalnstthe use of simple holding-


Figure 26.27 Figure C4.5lrom Reference 26.3

Figure 26.28 Alank whjch has suffsred an elephant's fool buckle in the tirct

down systems were even more relevant. This option, despitebeing cheaperthan the eventually chosen seismic isolation ar-rangement, was rejected for reasons associated with the per-ceived increased risk of failure due to stress concentrations inthe lower tank shell. lf there are particular constraints whichprevent either of these courses of action being appropriate orpossible, then the next steps are relocation of the facility orseismic isolation.

26.12 Liquid sloshingThis is discussed in Chapte|l5, Section 15.2.7. The basic Aplformula for calculating the slosh height is given in equation15.27. There is a suspicion thatthisformula may not be correct.

Seisnic besion of Storooe Tar*s

EL 66h dia x 27n high L|.l6 tdnk

l,.d .@ SSE 301 hod:ontal + $0n varicdl

rzor..o.tfi.i!nt€rt€rtidl F@iliris f.dor

W.i+t of iq.k 3h.lllin{ldrionRoof lo.d.dried hy!h€ll

Holgnt ot tok4.llTokOl6etcf

Prodlcr Sp.cltic 6.dviyMdxinub liqlid l.!.1

Thickn.# of fir.t .oqrr.Thickn.s of d.ular pldr.r

Yalld rtrcngth of dnnuldH

fiin yi.ld of l.r couB.Forc. co.ffi.iqt

5it. Anplif icarioi f @r.r

W.igl'r of rdk.6ir.nr.

z=[-o.orE *l

r=[--i]ii-l&= 1238549 rb

D: 2F-45 fto.[-i]EEiilH: 42.02 fi

r',=l---5i6-o_lruzi"'Fo =f-166-d--lttri"'c,=f o.o--ls:l 1.00 |

2 6 s e i ]!f39:p!!! j!!!!9E!Ep!t

Figure 26.29 Sliding calculations - pag6 I

496. STORAGE TANKS & EQUIPMENT

Inc_rsd-di(a|a

Who|tl tt'" cclier 6ladrtilm5 thc 6lbr'1l|g p€sk gr€od dccetariicrs d[I b. ur.d r. d"rrfriEif tl'. i||'s td* is sbirct t6 taizurtol stdiB due ro s.irric fr.c.i

,tcc€bnrion of itqdd!. .o{pqsr,

/tcc€l€rlti)|| of 6aE in:coi?orE r'.

,t...lel!tia of raridl coi?aEr,

TIle dio4.€|n sl|ors d'. drsfim of dcftE fof.es;

o!!E

",=f.:,645-1q..r3ffi-l.,=r-T:iE-l

55E

f o.e:r, 1

rT:6m-l

f-6r6o I

@Cdr|lcti€{dcc

*E- lqdicft?cc

.@

hortzortrlslidiqIt'

s€lf |.dgkr

Wll'a folbaing l.3ds nrrst S€ oo.6id.rc4

o 5.F r"&||l of tu* dE[, niffas! drd 6rry aiidch.j in$hticr W :

b E tecti€ tr6 of tutt c.frbs rhich nlows iI l,|is.ri rith rE k C*ll is irqrli\*; Y,r:

c Ef6".ti!" tl6otftikdrdlnt3di.fi ne!€s in t E fst slo*iT hode i.c-.rn crirc %:

&srn'c td* s?F r.iglt Fc*.rr&ds tipubw liquid

Thr€e 3".isric eE|rs dbfi bc cordd:rc,*

I Horizlrdrl s€isnic "!'t|rt

ont2 lhrizontdl + O-3 V"rli.al s€ieric ?Etrt3 o'3 lhri:ontal + V.rtiool s.isfc ?srt

irors Dirr.tiofi df l.rlic.l ac.dadi|n is idl(.,' to educc €ff..tiE slidh9 rt*rdi.c

f:G6]r.fEzlr.fiiifiIlr.


m3.1 OOE das.

T6id .tf,lctiri sdf rdghr.

r+: oYrrytq . (w'q,

4: 4:n 5-OO T.

S'Wi YrW,rW?

5Ir: t69ta@ TG

R!+irdcocffid.nrof fti.t.\ i= jL . O-t?re,SI'

3,2 sSE CG

lhi.(rrhl frtr.G, t+= (y . yiq + (g?)q.

f+! rtg-17 L

Totd dnlciiF.dfndgftt, Sl{: t' + yr. y,

SAr: 3AIa.lD Tc

F.{itd *fftdarot frLrhr. t* = o-gfff

%aant./t.f OBE d@

lhdzrrrd ft.c., l*= (W*Ur)c * (Vtq.

+: ,r7 .q) Tc

Tordcft rliE.dt $rhr SW: tW. Vr + W.) - O.3atU + Ur . W.)-O.3 tdtid -{* er,

5Y: 37@a97 f.r.4*ld.ocffi.irrt.ffii.tbq rt : O.ta,3

fl

4.2 SSE Cca

Tstd .ff.clir! rdf r4tr- Oi tr.rfi.d *-*rr,

E S.!d .o.ffi.-riof 6icrbr.

!t: (v.woq + (w!)q

l|f: tt66-ry Jc

su. (W r g ' lyr - o,3o.(V + ry1, W,

Stit: 33535,64 T.

|lt= _!L : O-:ifrtfl

Frgure 26.29 Sliding calculations - page 2


26 Seismie design of low tempenturc tanks

Fgcrh5.1 OBE f@o3lr,rard fq!.. lt= or'[*'t1". ft;* ]

+: 143E8O TG

Totdcff*li*rdf righr, S[tr (V r lttr . vr) - * {rry,rv!)- V.rnid aidsckt

ff: 3?45{.54 TG

Rlqild.o.fftd.'t|ot ftidn , 4 : O-Oa3a

F

5.2 SsE Cot

o.3lci',id fd!.. rlr: *'fo-*,1.,. (r * ]llr = 3,1t9.34 T.

Taid.f6EiiEdf dgft!. slt: (ly. Wr + r, -.r (I+w|tlrt- trrlird ..*ric .{t|t

54.! 20976,80 T.

Fleftd @ctfi.i''taf ticfdt. 6! = }t . o-165t?5v

Figure 26.29 Sliding calculations - page 3


MNCRETgBASE 5LA8

PAO

ANCHORAGE

INTO BASE

SLAB --__\


It does seem to give similarslosh heights fortankswith verydif-ferent geometries. For this reason it is probably wise to checkthe value given by equation 15.27 using the following:

d=0.837xRx,\

wnere

equ26.'14

R = tank Edius

A = design convective acceleration (g)

This is taken from Reference 26.1.

It is generally accepted thatseismic calculations should be car-ried out based on the normal maximum tank filling level and notthe very maximum under excursion conditions, (i.e. HHLAlevel).

26.13 Seismic isolationBy placing the tank on seismic isolators, it is possible to reducethe horizontal seismic loadings which act on the tank and itscontents. These isolators do little or nothing about the verticalcomponent of the seismic loading which is passed to the tankundiminished.

The arguments for and against the use of seismic isolation in-volve a mixture of cost and safety considerations which arebriefly reviewed as follows:

For seismic isolation:

. The facility can be constructed on the desired site

. Large local loads on the tank shell due to holding-downstraos or lateral restraint are avoided

. Savings to the inner shell due to thinner shell and thin-ner/narrower annular olate are Dossible

. Loadings on the foundation, especially the high peripheralline loads, are reduced

. Sliding problems are reduced

Against seismic isolation:

. Costs ofthe second base slab are high

. Seismic isolator units are expensive

. Construction programme is extended

. Large horizontal movements may cause problems with thedesign of the connected pipework

There are a number ofdifferent types of seismic isolator available, illustrated diagrammatically in Figure 26.31. These in-clude the conventional steel/rubber sandwich type of isolatorsimilarto those used to isolate vibrating machinery the conven-tional steel/rubber sandwich type with an energy absorbinglead core and the friction pendulum type. The latter two are pro-prietary designs which are marketed by the companies whichown the various patent rights.

A photograph of a friction pendulum isolator is shown in Figure2632. fhe relative merits of the different types of isolator is acomplex and contentious area involving such subjecb as:

. Cost

. Efficiency in reducing the horizonbl seismic loadings

. Resistiance to property changes due to ageing

. Virgin cycle problems

. Vertical load bearing ability (dichtes the number required)

lf it is decided that the isolators willnot require reDlacementdur-ing their service life, the upper and lower base slabs may beconstructed with a small space between them. lf it is decidedthat the isolators should be capable of being inspected, and if

AASE

INSJLATId'l

Figlre 26.30 A proposed system ofseismic restraint using inclined anchors



Figure 26.32 Afriction pendulum lsolator

Couiesv af Eanhquake Protecliot Systems /nc

The calculations involved surrounding the influence of a partic-

ular type of isolation system on the design horizontal seismic

design event (i.e. the reduction caused by the isolators to the

site design criteria to provide the tank design criteria) are com-plex and should be left to specialists with the necessary

expenrse.

26.'14 The design CodesMuch of what the tank design Codes have to say on the subject

of the seismic design of low temperature tanks has been in-

cluded in the relevant Sections eadier in this Chapter.

For LNG storage facilities in particular, a number of the basicprinciples, in addition to the definition of the OBE and the SSE(see Section 26.2) come from NFPA 59 A and these are per-

haps worth repeating as they relate to the tanks:

. The LNG container and its impounding system shall be de-

signedto remain operable during and afreran OBE. The de-

sign shall provide for no loss of containment capabilityoftheprimary container, and it shall be possible to isolate and

maintain the LNG container during and after the SSE'

^____l

(M.s.)

2

:::

Figure 26.31 Different types ofselsmic isolation units

required, replaced during theirservice life, then the space must

be large enough to allow personnel to enter and room for the

various activities to take place. This will require a gap ot some

2.0 m.

Two of the better known examples of seismic isolation of large

low temperature tanks are to be found in the numerous conven-

tional and membrane type of LNG tanks of 100'000 m3 capacity

constructed at Inchon and Pyeong Taek for KOGAS in South

Korea and the two 65,OOO m3 LNG tanks constructed for DEPA

in Greece. The former used conventional steel/rubber sand-

wich isolators and the latter the friction pendulum type'

The Greekexample is perhapsthe most sophisticated, and cer-

tainly the most expensive example. Each tank required some

216isolation units, each costing around $10,000 each There

is more to be found regarding the design and construction of

this facility in References 26.9 and 26.10. The layout of the

seismic isolators is shown in Figure 26.33


Lead CoreSteel Load PlatesSteel Reinforcing PlatesInternal Rubber Layers

Flgure 26.33 Layout of seismic isolatorsfor a typical LNG tank

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' ihe impounding system shall, as a minimum, be designed:c withstand an SSE while empty and an OBE whilefull. Af-:eran OBE oran SSE there shallbe no loss ofcontainment:apability.

. An LNG contiainer shall be designed for an OBE, and astress-limit check shall be madeforthe SSE to ensure com-cliance with above. OBE and SSE analyses shall includeJre effect oi liquid pressure on buckling stability. Stressesior the OBE shall be in accordance with the rules given inAPI 620 and forthe SSE, tension shall not exceed the mate-rial minimum yield strength, and for compression the criticalbuckling stress (this is forthe metallic components, differentrules are Drovided for Dre-stressed and reinforced concreteoarts).

-rind these rules lie two simple but slightly contradictory con-

. The impounding system is normally empty and must ofcourse be designed to resist the SSE in this condition. TheSSE may cause the innertank to leak, filling the impoundingsystem. The aftershocks ofthe originalSSE are assumed tobe at the OBE magnitude and the impounding system mustresist these whilst full.

. The primary containment is designed to continue to operateas normal during and following an OBE and to maintain itsliquid containmentfunction during and following an SSE, al-though this event may cause the primary containment dam-age which may require its subsequent repair orreconstruction.

-:.le provisional European code prEN 14620 makes reference:tr other European codes as would be expected for the seismic:esign of low temperature storage ianks. These are ENV'998-1-1:1996 (Reference 26.11) and ENV 1998-4 :1999 (Ref-

=rence 26.12\

26.15 GonclusionThe foregoing outlines a calculation method of modest com-rlexityto justify the design ofa vertical cylindrical metallic liquid:ontaining inner tank. lt is clearly possible to carry out a much'nore rigorous and sophisticated analysis of this component.

\othing has been said about the seismic design of the outerusually pre-stressed concrete) outer tanks or impoundments.

This is again the province ofthose armed with the appropriate:ools and expertise. The design of roof-mounted equipment?nnot meaningfully commence until the outer tank designers


have evaluated the influence of the outer tank itself on the de-sign seismic criteria (usually a subsbntial enhancement).

26.16 References26.1 Guidelines forthe sebmic design of oil and gas pipeline

systems, Chapter 7 - Seismlc response and design ofliquid storage tanks. Committee on Gas, Liquid fuel life-lines, American Society of Civil Engineers, November1984, ISBN 087262428 s5.

26.2 Above Ground Storage larks by Philip E. Myers,Mccraw Hill, ISBN 007 044272 X.

26.3 Seismlc des/gn of storage tanks, Recommendations ofa Study Group of the New Zealand National Society forEarthquake Engineering, December 1 986.

26.4 USAEC Regulatory guide 1.60 - Revision 1, December1973, Design Response Spectra for Seismic Design ofNuclear Power Plants, the U.S. Atomic Energy Com-mtsston.

26.5 Se,'smlc Des,gn Rules for Flat Boftom CylindricalLiquidStorage Tanks, N.J.l.Adams. The International Journalof Pressure Vessels and Piping, lssue no. 49 (1992) pp6'1-95.

26.6 API Standard 620 - Design and Construction of Large,Welded, Low-Pressure Storage lanks, Appendix L,Tenth edition, February 2002.

26.7 Basls of selsmic design provisions for welded steel oilstorage tanks, R.S.Wozniak and W.W.Mitchell.

26.8 Nippon Kokran technical repod Overseas No. 42,1984.

26.9 Seismiciso/ation of LNG tanks,Bob Long, LNG Journal,Jan/Feb 1997.

26.10 Seismlc lso/ation of industrial tanks, Victor Zayas andStanley Low of Earthquake Protection Systems, pre-sented to the American Society of Civil Engineers, Pro-ceedings of the Structures conference, April 1996,Chicago.

26.11 ENV 1998-1-1: 1996. Eurocode I - Design provisionsfor eafthquake resistance of structures - Part 1-1: Gen-eral rules- Seismic action and generalrequirementsforslrucrures.

26.12 ENV 1998-4:1999. Eurocode 8- Design provisions forearthquake reslsfance of structures - Paft 4: Silos,tanks and pipelines.


27 Miscellaneous storage systems

Miscellaneous storage systems such as gasholders, silos and elevated tanks, althoughperipheral to the main thrust of Storage Tanks & Equipmenf, are of interest for a number ofreasons.

Gasholders have a long and interesting history and have been constructed in substantialcapacities for a surprisingly long time.

Silos face a variety of new problems arising from the properties of the products stored andelevated tanks are very much a part of our landscape.

Contents:

27.1 Gasholders27.1.1 Wet seal gasholders

27.1.2 Dry seal gasholders

27.2 Silos27.2.'l Materials of constuction

27.2.2 Silo shapes

27.2.3 Product removal

27.2.4 Silo design

27.2.5 Codes and design guidance

27.3 Elevated tanks

27.4 References



27.1 GasholdersGasholders are probably the most visible large tanks, espe-cially around the UK. Stemming in the main from the days whenmost towns ofany size possessed theirown gas works, the spi-ral-guided wet seal types are still a common site, often withjnthe built up areas of towns. The dry seal types of gasholder areoften associated with industrial plant such as steelworks andare more often sited in industrial areas, but are highly visibledue to their height.

Although Sforage Tanks & Equipmenf is directed mainly to-wards liquid storage systems, it is interesting to look atgasholders which in many instances led the way in the designand construction of large storage tanks.

Little seemsto have been written about gasholders bycompari-son with their liquid storing companions. What is available issomewhat dated, so that the brief descriptions which followmay not reflect current practices accurately, for which the au-thor apolog ises. lt is interesting that in the introduction to Refer-ence 27.1, the afihor l\,4r R.J.Milbourne observes "At the pres-ent time (1923), when the attention of the gas industry theInstitution of Gas Engineers, and the Board of Education is be-ing so wisely directed to the education and technical training ofyoung men forthe Gas Engineering profession, it is a matterforregret that the publications obtainable on Gasholder Designand Construction are neither numerous nor exhaustive.".

There do not appear to be any Standards or codes of practicecovering the design and construction of gasholders.

There is a document published by the lnstitution of cas Engi-neers (Reference 27.2) which covers the design of both wetand dry seal gasholders in a very general manner. The mainthrust of this document seems to be the safe operation andmaintenance ofthese structures. NeveTtheless, it does containa lot of useful information, some of which is repeated below

A quite detailed description of the design of spirally-guidedgasholders by M.A.Thompson has also come to light (reference 27.3), so ifthere is a resurgence ofinterest in this subject,then we will at least have something to refer to, albeit ratherdated.

Thanks must go to Mr Peter Hutchinson who has also provideda lot of what follows.

27.1.1 Wet seal gasholders

l\,4ost ofthis Section is abstracted from Reference 27 i .fhis pa-per is a compilation of a series of articles which appeared in1922, written by Mr R.J. lvlilbourne, managing director of C & WWalker Ltd, describing various aspects of the design and con-struction of a 12.5 million cubic feet capacity gasholders in Syd-ney, Australia.

Thjs type of gasholders seems to have been made in large ca-pacities for a long time. In 1880 the world's largest holder wasrecorded at 5.5 million cubic feet (155,700 m3). In 1887 twoholders of6.25 million cubicfeet (172,100 ms)were built in Bir-mingham and in 1888 a holder of 8.5 million cubic feet (249,700m3) capacity was constructed at East Greenwich. In 1892 asix-lift holder of 12 million cubic feet (339,800 m3) was built. Forsome years this was the world's b;ggest gasholder until it wasovertaken by two in the USA, each of 14 million cubic feet(396,400 m3) capacity.

Bearing in mind the fact that we currently consider a crude oiltank big at 100,000 m3 and large LNG tanks are being consid-ered at 200.000 m3, these are massive structures and one canonly admire the nerve of the designers and erectors of suchholders.

Three types of wet seal gasholders are described. The first twoarethe externally-framed typeswhere the various moving parts


Figure 27.1 Askeich ofan exiernallyJramed wei seal gasholder

Figure 27.2 Asketch of a spiral-guided wet seal gasholder

or lifts are supported against lateral loads and guided by a sub-stantialframework. An example is shown schematically in Fig-ure 27.1. The first is where the external guide framing is equalto the full height of the top of the top lift when the holder is fullyinflated. The second is where the guide framing is shorter al-lowing the top lift to protrude above itwhen fully inflated, and thethird type having no externalframing at all. This latter type is thespiral-guided gas holder which is most familiar to us today. Thespiraltype is shown schematically in Figure 27.2 and a photo-graph of such a holder is shown in Figure 27.3.

The reasons for such large capacities may lie in a combination

Flgure 27.3 A spiral-guided wet sealgasholder

27 Miscellaneous storage sysfems

g

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2,l6,2.,il

OE.TAILS OF CUF CARRIAGES

: !ure 27.4 Some detail ol ihe 12.5 million ft3 wet seal gasholder

PLAN Of. ]OP CARR1ACESOIJ}ER LIFT



of the economies of scale together with the presumption at thetime that to ensure economical working of the facility, the stor-age capacity provjded should equal the daily output of thegas-making plant.

The 12.5 million cubic foot capacity holder described in Refer-ence 27.1, was ordered in 1913 by the Australian Light GasCompany, of Sydney, NSW, from Messrs. C & W Walker Ltd,Nilidland lron Works of Donnington, near Newport, Shropshire,UK. All ofthe steelwork was fabricated in the Donnington workswith the first shipment leaving in September 1913 and the last inDecember 1915.

The excuse for this lengthy fabrication period is attributed toproblems associated with the enlistment of workmen, the diffi-culties of transport and the prior claims of the N4inistry of Niluni-tions and the Admiraltyfor the manufacture of munitions of war.In March '1917, the holder was inflated, tested and put into com-mission. lt apparently gave every "satisfaction in service". Thefixed lower shell was 300 feet (9'1 .44 m) in diameter, the guideframe is 188 feet (57.30 m) high and the crown ofthe roof is 202feet (61.57 m) above the internalwater level. There are four liflsand some indication ofthe complexity ofthe watersealing sys-tem and various details can be seen in Figure 27 .4.

The completed holder is shown in Figure 27.5.

The internal gas pressure varied as the holder filled due to thechanging weight supported, so that when:

. the inner lift only is inflated 5.6 ins watergauge

. the inner & second lifts are inflated 7.8 ins water gauge

. the inner, second & third lifts are inflated 9.8 ins watergauge

. allofthe lifts are raised 11 .6 ins watergauge

Distant memories of elderly relatives complaining that "the gaspressure is down" whilst cooking Sunday lunch, may not onlyhave been due to the peaking of gas consumption, but also thedecreasing pressure in the system as the local holder wasemptied.

In several ways these gasholders were easier to design thantheir liquid containing counterparts:

. The internal pressures were modest as described above

. There are no axial compressive loads applied to the liftsapart from the self-weight of each lift

. The roof framing is always supported by internal gas pres-sure when the inner lift is raised or by an internal woodenframing when it is lowered

The bulk of the vertical shells or lifts are carbon steel of either0.25" or No. 9 lmperial Standard Gauge (0.144") thickness.

The majority of the roof plating is No. 8 lmperial StandardGauge (0.160") thickness.

27,1.2 Dry seal gasholders

Dry seal gasholders are most usually associated with steel-works where their function is to capture and contain the gasesissuing from the blast furnaces for both economic (these gaseshave usefulflammable portions) and the more obvious environ-mentat reasons.

There are a number of proprietary designs for dry seaLgasholders which are licensed to a small number of fabricatorsand constructors. Amongst these are the M.A.N., Klonne.Wiggins and Hammond types. Little has been written about th stype of gasholder over the years.

An exception to this trend was an article in The Gas Joumal.dated August 1933, by lvlr W. Beswick, who was at that time themanaging director of Ashmore, Benson and Pease Ltd ofStockton-on-Tees, who were licensed builders of the Kl<innetype of dry seal gasholders. This article describes a holder of126' 0" (38.4 m) in diameter and 180' 6" (55.0 m) in heightto thecurb. This was of2.0 million cubic feet (56,600 m3) capacity andwas erected in the UK atYork. A generalelevation ofthis struc-ture is shown in Fioure 27.6.

Figure 27.6 A generalelevation of a 2.0 m llion ft3 capacity dry sealgasholder

The complex arrangement of guide rollers, dry seals and oiseals are shown in Figure 27. 7. The stability of the piston wasimproved by arranging a concrete weight as low as possible asshown in Figure 27.8. As in ship design, this gave a centre oibuoyancy above the centre of gravity. This could also be sizedto suit the desired gas storage pressure.

The shell was a true cylinder, (rather than the polygonal shapeadopted by some other gasholder types), and was of rivetedconstruction using counter sunk rivets to give a smooth internalsurface. The turret located at the centre of the dome roof con-tains an electrically-powered liftwhich gives access to the top ofthe piston. Such wasthe confidence in the efficiency ofthe seal-ing arrangements that operators entered this space and wefelowered onto the top surface of the piston for inspection andgreasing operations whilst the holder was in service.

A list of gasholders of this type in service at the time of writingthe article is shown in Figurc 27.9. The 6.0 million ft3(170,000 m3) gasholder for the Syracuse Lighting Company inthe USA is a monster by any standards and perhaps the world'sbiggest.

Dry seal gas holders of this type were frequently fitted with an

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Figure 27.5 The completed 12.5 million ft3 wei seal gasholder


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:igurc 27.7 The sealing arangemenls for a dry seal gasholder

:igure 27.8 The installation of concrete on the underside of the piston

external dial, operated by cables, which indicated the level of'Jre piston, and hence the volume of gas stored. The Kliinneholder illustrated in Figure 27.10 shows such a dial.

The N.4.A.N. gasholder also originated in Germany. This type ofholder uses a sealing arrangement using oil or tar which isstored in tanks on the holder roofand is allowed to run down theinside of the tank to form a liquid seal around the edge of thepiston. The liquid is contained in a trough sealed to the sides ofthe holder wall by a sailcloth membrane held in place by coun-terweighb. This arrangement is shown in Figure 27.11. Fromthis trough the liquid leaks and runs down to the bottom of thetank where it is collected and pumped back to the tanks on theholder roof.

The Wiggins gasholder uses an internal piston in combinationwith a synthetic rubbercoated fabricseal. This is a trulydry sys-tem and has been used to store gases as varied as ammonia,sulphurdloxide, LD convertergas, ethyleneand dimethyl ether.

27.2 SilosSilos, bins and hoppers are the terms most commonly used forstructures whose function is to store particulate solids. The ma-terials stored rangefrom dryfine powdered products through to

Figure 27.9 A lisl of'Kl6nne-type" dry seal gasholders instalted up to 1933

Figure 27.10 An externalview of a typical Kldnne holder

coarse granular products and damp or wet products. Typicalmaterials to be found in silos include flour, wheat, sand, coal,coke and cement. All possess very different properties whichhave an influence on the manner of their bulk storaoe and theconiainers used.

The bulk handling industry has an extensive historyof failures.These range from catastrophic collapses to shortfalls jn perfor-mance. Whilst the former events are infrequent, they attract theattention of the media. The latter are more common and resultin inefficient performancewith its consequentcosts and delavs.


_)

27 Miscellaneous stonge syslems

Weights !o hold rubbing

Dspth ottar measursdbydippin9

Steel rubbing

Figure 27.11 Atypicaltar sealarrangement for a M.A.N. type dry seal

27.2.1 Materials of construction

The most commonly encountered materials are carbon steeland reinforced concrete. Wood was a widely used material inthe earlydays ofthe storage ofsolids, butwould not be a sensi-ble or economic choice these days.

27.2.2 Silo shapes

The mostcommon shape is a vertical cylindrical shell fitted witha conical outlet. Rectangular section silos with symmetrical ornon-symmetrical, pyramidlike outlets, are also common.These are fequently supported at a suitable elevation so thattheycan discharge theircontents into a truckor railcarbygrav-ity alone. Silos are ofren built in groups, eitherwithin a buildingor comprising a single composite structure.

Atypicalsilo of the cylindricalshelland conicalbottom is shownin Figure 27.'12. This is one of a number of PTA silos made forlCl's Wilton Works. The materialwas stainless steelto ASTM A240 type 321and the silos were 11.5 m in diameter and 17.5 min cylindrical length. They were manufactured in an offshore rigyard on one side ofthe RiverTees, transported across the riverby heavy lift floating crane and taken toWilton Works by road onlow loaders.

Figure 27.'13 A typical fat-bottohed silo

Cowiesy of Whessoe

Flat-boftomed silos are not uncommon and relyon internalme-chanical handling equipment to ensure efiicient product re-moval. One such silo is shown in Figure 27.13. This is a 35,000tonne silo built some years ago, in Dubai for the storage ofAlumina.

27.2.3 Product removal

The reliable and consistent removal of Droduct ftom silos hasbedevilled the solids storage and handling industry for manyyears. The behaviour of particulate solids is complex and thevagaries and range of flow possibilities is illustrated by the fol-lowing possibilities for elevated conical outlet type silos:

No flow. This is when the material being stored remains inequilibrium under the action of gravity and the internalforces within the material itself. Commonly known as arch-ing, bridging or doming, it can result from poor design or co-hesion between the particles, often caused by dampness,surface tension or electroshtic effects.

Coreflow(also known as funnelflow). This is wherethere isno flow of that part of the product in contact, or close to thevesselwalls. This means that onlythe materialthe middle ofthe vessel is discharged so that the full capacity ofthe ves-sel in not realised.

Ratholing or piping. This is an extreme case of core flowwhere only a very smallfraction oi the vessel capacity is uti-lised.

Mass flow. This is what is needed for efficient oDeration.Arching, coring and ratholing are avoided and the wholevolume of the vessel is utilised.

Erratic flow This is where the flow rate and bulk density ofthe discharged product is highly variable. This is commonwith find powder materials. When a fine powderis placed ina silo, it de-aerates and compresses under the pressure ap-plicable at the particular location within the silo. When thispowder tries to flow out of the silo outlet, the reduction inpressure causes the powder to expand leaving voids thatmust be filled with gas, and this requires an inflow of airthrough the outlet. This inflow of air obstrucb the outflow ofproduct and this phenomenon is known as slurping, and re-duces the outflow to a small fraction of what was dntici-pated.

Uncontrolled flow. Ofren referred to as flushing or flooding,

Figure 27.12 A typical cylindrical silo with a conicaloutlet

Coulesy of lcl PlcMhessoe


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this occurs with fine particle matedals in core flow situa-tions. lt is the resultofa sudden instabilitywithin the productresulting in very rapid and uncontrolled outflow of material.

. Pulsating flow. Often referred to as silo quaking, this is mostcommon in poorly designed mass flow silos. The surface ofthe stored product has regular periods of movement andrest whilstthe materialcontinues to flowoutofthe silo. Thiscan result in high shock loadings on the shell and support-ing structures. This is the subject of a paperby Roberb andWiche in Reference 27 .4.

. Eccentric flow. Usually found in silos which have off centredischarge connections. This can give rise to high transientover pressures and variable material discharge rates.

Silos which do not perform as anticipated, in particular with re-gard to the rate and uniformity of product outflow offen bearthemarks ofthe oDerators' efforts to persuade them to conform toexpectations, These can be marks on the cone resulting frombeatingsor rodding holes drilled in the coneto overcome block-ages or inadequate flow rates.

To ensure that the outflow rates are suited to the plant produc-tion or the vehicle loading requirements, it is common to utilisedevices for this purpose. These take a number of forms involv-ing air injection, vibration and local modification of the dis-ciarge connection. Many propriebry devices and details arefound in this area of aclivtty. Reference 22.5 contains descrip-tions of a number of commonly adopted solutions with com-ment on their various applications and advantages.

Flat-bottomed silos require specialised equipment to removetheir contents. This usually takes the form of screw type dis-chargers, often fitted into suitable recesses in the silo base.

27.2.4 Silo design

The design of silos for the storage and handling of particulatesolids is more difficult than the design of liquid storage tanks.The determination of the required properties ofthe stored ma-terialsandthe loading applied bythese materialsto the silo un-derstatic and dynamic conditions are complex. One ofthe ma-jor differences between liquid storage ianks and silos is thatthesolid product gives rise to longitudinal or axial loadings on thevessel shell in addition to the internal pressure loadings. lt ishese loadings which tend to dictate the wall thickness of thesilo.

The early silos wereforthe storage ofwheatand were designedusing methods derived from the field of soil mechanics. Thework of Janssen (Reference 27.6) dating from 1895, is cited inMilo S. Ketchum's book, (Reference 27.n as the method of de-termining the radialand verticalforces in silowalls (in the chap-ter on Steel Grain Elevators). This remained the method of de-riving these forces until the 1970s when work of Jenike,Johanson and Carson were published ( References 27.8, 27.9and 2Z 70). These provided more sophisticated methodswhichtook account of certain additional oressures which occurredduring product movement conditions, especially in the cylindri-cal-to-conical jointarea. Atoraround thistime there seemed tobe an upsurge of interest in silo design and numerous paperswere published. Atrawlthrough the bibliographies ofthe listedreferences will provide the interested student with any amountof reading on the subject. A plot ofthe internal pressures com-puted using various different methods is included as Figure27.14 where the pressure spike at the cylinder/cone junction isquite apparent.

27.2,5 Codes and design guidance

A useful guide to silo design is found in Reference 27.11.

A second publication, again produced jointly, see Reference


Fisure 27.14 computed w", "J"i*ffi"J

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From Refercnce 27.9, figure ll-3

27.12, is anothet wide ranging document containing a greatdeal of useful intormation regarding steel silos.

When discussing silo design, it would be unfair not to mentionProfessor J.M.Rotter of the Universities of Sydney and Edin-burgh, who over the years has published an enormous volumeof material on this subject.

27.3 Elevated tanksMost elevated tanks are for the storage ofwater and also serveto maintain the waterpressure in the locality. Some arethe rect-angular Braithwaite types, supported by brick or steel struc-tures which are afeature ofairfields, factories and occasionallyurban settings. The Braithwaite type of tank is rectangular inform and made up of factory-fabricated panels, which arebolted together. The internal pressure from the contained liquidis resisted by a system of internal rods stressed in tension.

The cylindrical elevated watertianks, usually supported by rein-forced concrete structures are also common.

In the USA in particular, elevated water toweF made of steeland supported on either a steelframework orthe more elegantsingle central column are to be found. These are often made in

the form of a well-known local product or other interesting

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Figure 27.15 The steel "peach" water tower

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27 Miscellaneous stonge systems

Figure 27- 16 "The world" water tower

CowTesy af Chicaga Bidge & lron Conpany (CB & l)

Figure 27,17 An elevated watertoweron a concfete masonry column

Coulesy of Chicago Bidge & lron Conpany (CB & I)

shapes. Many of these water towers in the USA were con-structed by Chicago Bridge & lron Company, indeed the com-pany's in house magazine was called The Water Tower. Someexamples are shown in Figures 27.15, 27 .'16 and 27 l7 . fhedesign of such structures for seismic loadings is both interest-ing and challenging. Any failures would be very public evenb!

The design of steel water tanks in the USA is covered by theAmerican Water Works Association Code D 100, (Reference2Z 73). This is an interesting document in that it is not so tied tothe practices ofthe petrochemicalindustry as most ofthe otherliquid storage codes. lt is written in a refreshingly different wayand has its own view of what is appropriate. lt is well worthspending a bit of time with for those interested in storage tank

design. The seismic design section is especially good. Thereare companion Standards published by the same organisationcovering factory-coated bolted steel tanks (Reference 27.14),wire and strand wound circular prestressed concrete watertank lReference 27. t5) and circular prestressed concrete wa-ter tanks with circumferential tendons (Reference 27.16\.

27.4 References27.'l Design and construction of a 12.5 million ft3 gasholder,

R.J. l\.4ilbourne, Managing Director of C. & W. WalkerLtd, Donnington,Shropshire, first published as a seriesofarticles bythe Gas Journal ('1922) and lateras a com-pilation by Walter King Ltd (1923).

27.2 Low-pressure gasholders storing lighterthan air gases:Safety recommendations: IGE/SR/4 Edition 2 Commu-nication 1624, The lnstitution of Gas Engineers, Lon-don, June 1996

27.3 Designing spira y-guided gas holders, M.A.Thompson,reprintedfrom the Gas Times by Hallowsand Slaughter,Grove Road, Leighton Buzzard, 1 940.

27.4 Bulk 2000 : Bulk materials handling towards the year2000, International conference sponsored by the BulkMaterials Handling Committee of the Process Indus-tries Division of the L Mech.E., October 1991.

27.5 Solving problems in hopper and sl/o sysfems, from aseminar organised bythe Bulk Materials Handling Com-mittee of the Process Industries Division of thel.Mech.E., June 1996.

27.6 Versuche Aber cetreidedruck in Silozellen,H.A.Janssen, Zeitschrift des Vereins DeutscherIngenieure1895.

27.7 The Design of Walls, Binsand Eleyators, M.S.Ketchum,Mccraw Hill 1909

27.8 Bin loads - Paft 2: Concepts, A.W. Jenike, J.R.Johanson, J.W. Carson, ASME, Paper no. 72-lVlH-11972.

27.9 Bin loads - Paft 3: Mass Flow Bins. A.W. Jenike. J.R.Johanson, J.W. Carson, ASME, Paper no. 72-MH-21972.

27 .10 Bin loads * Pad 2: Funnel Flow Bins. A.W. Jenike. J.R.Johanson, J.W. Carson, ASME, Paper no.72-MH-31972.

27.11 The Draft Code of Practice forthe Deslgn ofs,'7og B,ns,Bunkers and Hoppers, The Institution of MechanicalEngineers and the British Materials Handling Board.

27.12 Useful lnformation on the Design of Stee/ B/ns and S/-/os, John R. Buzek, - The American lron and Steel Insti-tute and the Steel Plate Fabricators Association lnc..1989.

27.13 ANSI/AV|/WA D 10096: AVIWA Standard for WeldedSlee/ Slorage lanks for Water Storage , AW WA, Denve rColorado.

27.14 ANSUAWWA D 103-97: AWWA Standard for Fac-tory-coated Bolted Steel Tanks for Water Storage.AWWA, Denver. Colorado.

27.15 ANSI/AWWA D 110:95: AIUWA Standard for Wire andStrand Wound Circular Prestressed Concrete WaterIanks, AWWA, Denver. Colorado.

27.'16 ANSUAVVWA Standard for Circular Prestressed Con-crete Water Tanks with Circumferential Tendons.AWWA. Denver. Colorado.


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28 Glassification guide tomanufacturers and suppliers

The classification guide summarises ambient and low temperature liquid storage tank,classifying them according to tank type, maximum diameter or capacity, materials ofconstruction, products stored, materials of construction, containment type etc.

The guide has been categorised in a particular way to try to impose tight boundary limits onproduct types and classifications, with the specific aim of simplifying the choice of supplierfromthe useis point of view.

The guide also covers the essential components and ancillary equipment and servicesassociated with the industry. Trade names are comprehensively lisled as well.

These sections are preceded bythe names and addresses and contactdetails ofall companiesappearing in the guide. These are listed alphabetically, by country of origin.

The information in Section 28.3 is provided for guidance only. lt is strongly recommended thatdirect contact with all companies is made to ensure their details are clarified wherevernecessary-

Contents:

28.1 lntroduction

28,2 Names and addresses

28.3 Storage tanks

28.4 Ancillary equipment and services

28.5 Trade names


28 Classification guide to manufactuters and suppliers

28.1 IntroductionThe guide classifies liquid storage tanks into main groups:

. Ambient temperature

. Low temperature

The principal ambient liquid storage tank types are then classi-fied by:

. Vertical cylindrical above ground hnks

. Maximum diameter

. l\y'aterials of construction

. Other types of above ground tanks

. In or below ground lanks

Low temperature Iiquid storage bnk types are classified by:

. Tank type

. Capacity

. Products stored

. Containment type

. Materials for primary and secondary conbiners

The guide has been classified in this way to impose tight bound-ary limits on categories with the specific aim of simplifying thechoice of supplier from the user's point of view.

Although these are not always strictly logical, it should be obvi-ousto both userand manufacturer what is meantbya particulargroup or ancillary equipmenVservice.

The guide also covers the essential ancillary equipment andservices usually available for both ambient and low tempera-ture tanks. Trade names are comprehensively listed as well.

The storage tanks section (Section 28.3) is preceded by thenames and addresses and contact details of all comoanies ao-pearing in the guide.

Names and addresses - Section 28,2

This Section has been based on a questionnaire sent to manu-facturers and suppliers woddwide. Where possiblethe informa-tion supplied has been used. Full company and contact detailsare given. Companies are listed alphabetically, by country oforigin.

Storage tank types - Section 28.3.

The data presented in this Section is based on the same ques-tionnaire. Where possible all the information supplied has beenused. Discussions were held with many companies to ensurewherever possible, that their activities were correcfly inter-preted. There will however inevjtably be some overlapping dueto limitations of descriptions and space, and the informationgiven is for guidance only. Where there was doubt in interpret-ing the data, some of it has been omitted. lt is strongly recom-mended that direct contact with all companies be made to en-sure their details are clarified wherever necessary

Ancillary equipment and services - Section 28.4

Also based on the same questionnaire, this Section lists com-panies alphabetically underthe relevant productor service andwith their country of origin. Ambient temperaiure tank ancillar-ies are listed first, followed by those for lowtemperature tanks.

Trade names - Section 28.5

This Section has been compiled similarly. lt lists companies al-phabetically under the relevant trade name and with country ofoflqtn.

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28.2 Names and addresses

AUSTRALTA (AUS)

Angus Fire AustraliaA'1 Janine StreetScoresby Indust.ial ParkScoresbyVictoria 3179AustraliaTel: 03 9765 3800Fax: 03 9765 3801E-mail: [email protected]: www.kiddeuk.com

Baillie Tank Equipment Ltd10b/201 New Sooth Head RoadEdgecliffNew South Wales 2027Australiafelt 02 9327 5481Faxt 02 9327 5488E-mail: [email protected]: wwwbaillietank.com

Syltone Australia Pty LUUnit 1, 100 Station StreetNunawadingVictoria 3131AustraliaTel: 03 9874 2900Faxi 03 9873 2422E-mail: [email protected]: wwwsyltone.com

AUSTRTA (A)

NBN Elektronik cmbHRiesstrasse 146A-8010 GrazAustriaTel: 0316 402805Fax: 0316 40 25 06E-mail: [email protected]: www.nbn.at

Sattler AGSattlerctrasse 454-8041 GrazAustriareli 0316 41O 42A2Fax: 0316 410 4354E-mail: [email protected]: www.sattler-europe.com

Wolf Systembau cmbHFischerbrlhel 1

A-4644 ScharnsteinAust.iaTel: 07615 3000Fax: 07615 300313E-mail: [email protected] www.wolfsystem.at

BELGIUM (B)

Egemin NVBaarbeek 1

B-2070 ZwindrechtBelgiumrelt 03 641 12 12Fax: 03 64'1 '13 13

E-mail: [email protected]: wwwegemin.com

FCX Truflo Rona SAParc Industrial des Hauts Sarts8-4040 HerctalBelgiumTel: 04 240 6886Faxt 04 248 0246E.mail: [email protected]: wwwfcx{ruflo-rona.com

Fernand DesplentereOostendestraat 329B-8820 TorhoutBelgiumTel: 050 22 06 36Fa* 050 22 02 08E"mail: [email protected]: wwwdesplentere.be

G&G International NVMolenweg 109B-2830 WillebroekBelgiumTel: 05673 2121Fax: 056 73 4040E.mail: [email protected]: www.ggi.be

Geldof Metaalconstructie NVBroelstfaat 208-8530 HarelbekeBelgiumfelt 056 732121Fax: 056 73 4040E-mail: [email protected]: www.ge dof.be

Ortmans Inox SARue Fernand Houget 13B-4800 VerviersBelgiumTelt OB7 322811Fax: 087 31 59 98E-mail: [email protected]: wwwortmans.be

Pommee SA3idme Avenue15 Parc Industriel des Hauts-Sarts8-4040 HerstalBelgiumTel: 04 256 90 00Fax: 04 256 90 09E-mail: [email protected]: www.pommee.be

RecincoHoogveld 5B-9200 DendermondeBelgiumTelt 052220127Faxt 052 22 61 13E-mail: [email protected]

VBR Flir Systems ABUitbreidingstraat 60-62B-2600 BerchemBelgiumTel: 03 287 87 10Fax'. 03 287 87 29

28 Classification guide to nanufacturcrs and supplierc

E-mail: [email protected]: wwwflir.com

wLsSmederijstraat '16

B-2960 Brecht (St Job)BelgiumTel: 03 633 16 10Fax: 03 633 25 12E-mail: [email protected]: wwwwls.be

Zeppelin Belgium NVI\,4unsterenstraat I Genk-Zuid - Zone 78-3600 GenkBelgiumTel: 0Bg 62 94 00Fax: 0Bg 61 18 31

E-mail: [email protected]: wwwzeppelin.com

BRAZIL (BRA)

Syltone do Brasil LtdaAvenue Ricardo Bassol Cezati 1620Campinas, SPcEP 13050-080BrazilIeli 015 3269 4247Fax: 019 3269 5083E.mail: [email protected]: www-syltone.com

CANADA (CAN)

l\,lix Bros. Tank Services14707 - 17 Sireet NEEdmontonAlbe.taT5Y 6E2CanadaTel: 780 471 1386Fa* 780 474 0877E.mail: [email protected]: www. mixta n k- cjb. nei

Tanksate lnc208 3112 -lfth St NECalgaryAlbedar2E 7 J1

CanadaTel: 403 291 3937Fax'. 403 291 5125E-mail: dualtank@tanksafe_comWeb: wwwtanksafe.com

Oenso North America lnc90 lronside CrescentUnii 12TorontoOntarioM1X1M3CanadaTel: 416 291 3435Fax: 416 2910898Web: wwwdensona.com

UAS Canada lnc2656 Deacon StreetAbbotsford

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BYGGWIK (U.K.) LTD

TANK JACKING ANDHEAVY LIFTING

NEW CONSTRUCTION

Emair: [email protected]

r.r: + 44 {0)1$4 47s244

Fax: + 44 (0) 1534 37s243

BASE REPAIR

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29 Reference lndex

The refercnce jndex conlains a large n umber of key words used wiihin the industry. lt ists the page numbers on which the keywofds are used. The list ofcontents al the siad ofeach chaDteralso Dfovides a usefutouide.

Abovsground and in orbelowground stoEge systems

Above ground low temp€rature bnksAcoess arrangements

Air Eising ot tank roofs

Double-Walled lank

Pre-Shessed concete ouier tank

Aluminium alloys 438, 351,

Ambient temp€hture lanks

Ancillary equipmenl and nliings

Design

Design ol tank rools lixed

Design oftank roois - floating

Erectiorconsid-arations

Fabr calion considerations

Layoul ol tank installalions

Non-verlical cylindrcal tanks and other iypes

Nozle des gn and efiecl ofapplied oading

Materia se eclon crileria

Amedcan P.troleum hstitute (APl)

Ame can Unifom Building code (UBc)

American water works Association (AVVWA)

APt 520

As{onstructed foundation tolerances

APl650 requirements

BS 2654 EquiremenlsprEN 14015 requnernenis

ASItE 8.96.'l

asMEv t

Austenitic stainless stesl

Automaied welding method6

Axial compression loadings

B

6

367

365

307

247

452

453

452

361,442

't5

185

15

113

153

235

249

231

257

215

103

219

263

245

8, 13

264

14

291,394,507

418

234

250

25Q

251

251

220

216

13,438

99,434,442461

7,'t0022, 26, 319, 334, 335

324

E

E

E

E

E

E

tII

I

I

Inspection and repairof iqu d ammonlastorage systems

Ammonia storage - what mak* itspecial?

Elecl.i@l@nducl vty

Slress Coffosior c€cklng (SCC)

Ancillary equ ipment for low tsmperature tank6

aNsr/Awwa 0103-97

425

426

433

426

427

426

427

427

426

412

39,338

214

7,220

7

' 197,414

197

281

a, 13, 30, 117 , 220, 277 , 295, 302, 314,338, 34,t, 347, 358, 361, 418 438, 466, 44,t, 490

8, 19,21, 30,75,90,93, t04, 106,114, 154, 190,206,232, 251, 264, 277

252

Base heating sFtems

Baseinsulallonrnat€rlals centalarea

Base insu ation free oforganic maleriats

Base insuation materials- periphera area

B asl iurnace sag

composit€ sysloirs

Damp-proof course materia

Lightweight concrete

Poyurelhanefoam

Deia led design Code Equircments

BS 7777 requirements

EEMUA147 requirements

prEN 14620

B.low-ground cavems

Bitumen storage tanks

Bitumen.ba€ed paint system

Bolted cylindrical hnksBottom and annular d*ign

APl620 Appendix O apprcach

Liquid containing meta lic lanks

415

380,466

384

386

387

387

384

. 387

386

386

387

387

387

380

381

381

381

384

344

384

384

380

381

367,424,461

7

1,6201

101

181,279

218

338

341

341

aPt 12C

aPr 2000

aPt 2210

aPt 2517

APt650

APt653


Non LquLd conlaining meiallctanks

APl620 Append x R appDach

L quid @nlaining mela lic lanks

Non lqud conlan ng metal clanks

Llqu d conla n ng metallc tanks

Non- iqLid contalning metallc tanks

prEN 14620 approach

Bottom boom of the truss

Boltom oorner protection systems

Brine-based heating sYsten

British slandards Institution (BsD

BS 150'l

BS 2454

BS 2654

BS 5950

BS 6072

BS 7777

147

188

188

1AT

190

97,115

9, 13,294,428, 450

9, 't3, 34, 295, 450

341

338

338

340

341

341

341

131

362

4,2't6,509416

34

13

220

I

216

337

466

232

295

492

492

236

237

236

238

238

239

238

283

206

291

6,291,394,467

19,20.27, 90, 114, ',t54, 190, ',|92, 193,206, 221,222,225,232, 236, 246, 274,277

Civil monitoring syslems

column-supported roof s

Compression area forfixed roor tanks

API lmitatons lor the length ol the rooi compresslon area

CalcLlating lhe compresslon zone area

Compression zones

BS and API Code difier€nces oi allowab ecompressive svess

Compression zone area toAP Code

Compresson zone area lo BS Code

cosl-effecl ve design

Derivation ofthe requned conpresson zone aea

Effecl of rooi slope on cross-sectrona area

Economyoidesign

Rationa islng the calcu ation

Roof compresson area

Shell compression area

Effeci of nterna presslre

Eslab shing lhe compressiof area

M nlmuff c!rb ang!e requnemenls

Angle sizes for lixed rooilarks

Cases where minimum curb angLe requ renents

Effect ol inlernal pressure and iank d ameter onrequ red compressDn afea

Pos tiofi.g the centroid ol area

AP Code Appendix F

Praclical cons derations

Provlding the required compression area

For the APlCode

For lhe BS Code

AP 620 appDach (Appeidices R and a)BS 7777 appDach

prEN 14620 approach

Concrete raft foundations

Cavern storage syslems

Cenira, crown rinq design calculation

charpyv-notch impact testing

chicago Bridge & hon Company

Concrete wall construction

concrele/concr€te tanks

A€umenlslorand against

cryogen c concrele tanks

311

385

384,466

133

22 t,225, 337 , 4X8, 444

13

424

115

80

83

83

81

82

a2

81

88

8l

81

86

86

86

86

86

80

83

83

85

85

85

88

88

8a

88

83

a2

a2

a2

342

342

42

BS 2654 requirementsfor shell nozles

F lsh lype cear-oLt doofs

Nozzes 80 mm outslde d ameter and above

Nozles lesslhan 80 mm outside diameler

BS 449

BS 4741

BS 5387

BS 5500

433

9,30, 294, 298, 300, 302, 334, 344,345, 347 , 351 , 35A, 446, 490, 416, 444

BS 8004

BS EN 10028

BS EN 10029

8S4741

E ephanfs loot buck lng

Ereclng the she by lhe lradiliona method

Peaking and banding

Piate m sa ignment

Calculation olthe design accelerations 489

calculations for slabilily and axial compressive toadings 491

Carbon manganese steel

Carbon sleel contamination

cathodic protecrion system

220,361

232

19, 25, 35, 48, 6't, 232, 351

243

345,459,479

460

460

460

460

309

310

309

31,41,62

STORAGE TANKS & EOUIPIVENT 543

Construotion and erection of ambient temperature ranks

Joints ir wind girders

Other forms ot constructon

Anlifting a roof rlo position

Co umn-supporled rcois

Pejabrlcaled roof section

The roof shuclure

Construction oftanks with reinforced condete roofs

contenrs measudng sysl€ms

High accuracy .adar lank gauge

F gh accuracy seryo lankgauge

Automatic tank gauge

F oal, board and target syslem

Temperalue measuemenl

Coniingency Design Eanhquake (CDEI

cra6k Openins Displacement (COD) tests

Cryogenic stonge - history

D

David Taylor Model Basin Fomula

Dead-weighl vacuum rcli.f valve

Design and construction regulations

Amedcan slandards

Chicago Bridge Eng neerng Standards

Company Standards

EEMUASlardard

Euopean slardards

Euon basic P€cilcesolher Eurcpean nallonal Slanda.ds

standards for other prodLcls

Bs code 2654

Alternal ve lrformalion to be supp ied by ihe purchaser

Inlormafon to be igrced behveen ihe plrchaserand ihe manuiactu€r

Information ro bespecilied bythe purchaser

Eurcpean Code prEN 14015 -1 2000

Annex A (normative) Techn ca agreemenls

Oesign metal temperature

Ma{mum temperaiures

API 650

BS 2654


prEN 14015

'Vinimum temperatures

APt 650

BS 2654

p|EN 14015

D$isn ofa sing le{ecr floating roofDesign of ambied r6mp6rature storagelanks

Compression area ior nxed rooftanks

Fransible roof joinl, or weak roottcshell joint

Semi-buied ianks for the storage oi aviai on fuel

Tanks produced in slain ess steel malerials

'Variable desigr point method

Wind and vacuum stiffenlng

D6sign of fixed roofs for ambient tempeEture tanks

Differences behveen lixed and floaling rools

Design of low t€mperature tanks

Bottom and annu ar design

prEN 14620 approach

BS 7777 apprca.h

Diking

DIN

DtN 4119

Directional combinations

Double containm.nt tanks

BS 77TT

EN i473

Double stringer spiral stairca6e

DuctilityDye peneirant tssting

E

EEMUA

EEMUA147

Elaphant's foot buckllng

364

235

241

243

243

243

243

243

236

241

459

459

459

194

196

195

195

195

195

194

195

485

438

135

132

I

485

76,329

420

't83

3

13

13

13

I13

13

13

13

13

20

21

20

2A

21

20

21

21

221

222

222

222

222

221

221

221

222

157

15

80

20

89

100

94

36

99

26

56

43

113

317

u1317

334

317

131

476

507

't3

t3274

467

243

294, $2,454294

299

2A1

't99

423

179

507

,l88

247

269

9,13,29413,336,384

E

E

E

III

I

I

7

183

492

467

509

509

509

181

EN 10025

EN 10028

EN 10088

EN 10113

EN't0210

EN 13445

EN 1473

274

225,226

19'22699

226

226

216

295, 297 , 299,302, 414, 420

214

454

456

456

456

415

200,420

421

240

200

202

241

241

421

422

421

422

422

259

174,240

43,206

114

115

115

115

116

115

278

278

t54

154

154

154

220

36,43,142239

193

194

193

194

2TA

183

241

384,387

460

250

253

253

25X

236

236

9,13,294501

101

242

451

460

452

459

292,334,426

292,334, 426, 454, 467 , 507

F

Factory-manufaclured bnks made fromnon-metallic materials

Fasl t6ck erhylone tank

Wire winding atrangenents

Fast track liqui<I orygen tank

Fire protoction syslems

Deteciion sysiems

R msea loam poureB

Top loam poure6

Dry powder systems

Fne waler systens

Loca protecl on olvllneEble equ pffenl

Fire.fiqhting

Float type level indicators

Floating roof designs

Prncipal ollhe lloating roof

Types offloating roof

APl650 requ rcments

BS 2654 requiremenls

Elropean Code prEN 14015 requiremenls

Formwork low temerature tanks

Concr ele slab fo0ndation

Foundalron to erances

Engineering Equipment and lvlaterialsUsers Association (EElllUA)

ENV't998

Epoxy-based p6inl system

Erecting theshell by thejacking method

Condete wall construcl on

Tanks w th reiniorced concrete rools

European Code requiremenls for sh.llnoz2les

Euf opean Fertiliser lManufacturcrs Association (EFMA)

EUrop€an Pressure Equrpme urecrve te/ruJrE!)Amb enl temperalure

European Standard Committeo TC 265

European sieel Slandards

European tank desig. Codes

European Standard prEN 14015-1 : 2000

Annexes 10 ihe Slandard

Fixed and noatng roofdesgn

Primary and secondary w nd g.ders

Rool to sheilcompression zone

Temperature rating

German sioragetank Code DIN 4119

External f loating roof appurtenances

R n f re delect on

Exlernalfloating rooG

Floating roof design

Des sn of a sinsle-deck roof

Types oI erternal noal ng roof

Doube-decklype

Other types ofloallng rool

Snge deck pontoon type

External pipework insulation

Externally.framed dome roof

13

190

433

'|

13

13

I226

19

l919

19

t9

19

19

19

19

19

19

2A

2A

2A

174

1Ta

1Ta

154

156

156

155

155

155

155

155

395

140


BS 2654

prEN 14015

Foundations for ambietrt temperature ta.ks

Foundalions in a horzontalplane

Foundations of low temperature tanks

Code requnemenls and guidaice

APt620

BS 7777

prEN 14620

Examples and problem areas

Fransible roof Joint, of weak roof-to-shorl joint

APl650 Code - anchor EqliEments

Allowable stresses in anchoB

lMinimum bo t d amelef

Spacing olanchors

conflicl ol design inleresls

"Service' and "Emergency' desigr condilions

Difference between codes

Examples oilrangible and non+angib e rcoljoinls

Tali desqled Io an opp."tr1g p€5sure ofr0 rba'Tankdesgnedforanoperatingp.esslreof 7.5mbar

Fomua as exprcssed n APL650

Addiliona requiremenrs to APl650

Formula as expressed in BS 2654

Addit onal requiremenis lo BS 2654

Frangib e rooliointtheory

EE]\4UA

l',4arimum @mpression zone area a lowab e

Other iactors affecting lhe frangible roofconneclon

Sizeolwed atthe rooi pare to shel connection

Tank anchorage a means lo lrangibi ily

Delermining anchof age.equirements

EnsLring a frangible roofcon.ection using ancho€ge

Furtherdes gn check

Olher anchorage considerat ons

Worked example

Gas stomg. - history

Drysealgashodels

Wel sea gasholders

Geodesic dome roofs

GEvity.based systems (GBS)

H

Handrall construction

Hoat Afected Zone {HAZ)

Heat leak calculation for a large LNG tank

Heat leak calculations

Basic calculation methods

CalculaUon oI lhe hol face temperature

The influence of diffeEnl interstitial gases

Thermal 6nduct viiy values

Heated storage tanks

Hsavy or viscous products

Historical background lo stobge tanks

History of the design and construction regulations

American standards

Chicago Bridge Engineerrg Standards

Cornpany Slandards

EEMUASiandard

Eubpean Standards

Euon basic P€clL@s

Olher European nat onal Slandards

Slandards for other producls

236

236

236

250

255

250

250

250

465

466

466

466

467

467

89,94

93

u93

91

91

91

91

91

91

90

90

90

90

89

94

89

89

90

90

90

92

92

92

93

93

92

274

500

312,379,463

311

363,432,479

217

6

1,504

506

504

142

7

141,241

312

42,254,467

175

240

198

234

7

400

396

396

399

399

396

396

400

1,6,243283

30

3

7

8

13

13

13

I13

13

13

13

13

409

Friction pandulum isolatoF

Frozen ground sysiems

Full containment LNG tank

Full coniainment sysiems

Full containment hnksBottom comer Proieclion syslem

BS TTTT

EN 1473

Fully-pre$u sed LPG storage


Holding-down anohoF

Hydrocarbon pro.luels

I

tL58

26,47,81, 318, 319, 331,334,338

217

284

241

242

21, 30, 36, 39, 41, 56, 60

21, 18, 220, 252, t36.334, 367

244

226,228

476

206

400

217

300

301

300

300

301

292 In€round hodzontal cylindrical storags tanks

In.ground membrane tanks

lnspection and tesling the tank

Fixed roof plate tolrt lesllng

Floaing roof lesllng

Floor p ale joint testing

Hydroslatic tank testing

Radiographic inspectlon

BS 2654

prEN'14015

SheLf lo-boltom joint tesiing

Testing ol sheLl nozzLes and aperlures

Inspection f requoncies

Institute of P.tfoleum llP)

Leve neasurement

Level temperature density {LTD) rneasurcmenl

Pressure measurement

Temperalure measuremenl

Insulation mat. als for refrigeraled storage

lnsulatiod of hear broaks andfiltings

Heai breaks lor roof conneciions

Heat breaks iortank botom connections

Heat breaks tortank sidewallcofnecl ons

Base insulalion failure

Exlernal vapoLr sealing

Insulation syslems for tow temperature ianks

Basic design and maleral requirements

Basic requ rements oi the insLlaiion sysiem

Design Code f equirements

ExlernaL pipework insu ation

lnsla lalon cons derations

Insulation c€tegones

The inirared camera

Wealher proleciion syslem

368

294,310

310

310

310

246

247

247

246

244

246

246

246

246

247

244

286

13

422

423

423

422

423

423

423

380

393

393

395

394

400

400

409

409

409

377

379

375

380

395

379

379

400

389

ln-tant pumps and their handling equipment 400, 412, 427 , 432

Joinas in wind girders

K

L

Lafge in.ground LNG tanks

Laser measuring nethods

Layout of ambienr t.m perature bnk installations

Above ground tanks

LocaUon and ayoul

Separal on d slan@s ior grcups ol sma lanks

Separalion dislances lor large tanks

separation from olher dangerous subslances

layoul ol refrigeraled liquid gas bnks

Regulaiions governlng LNG storage faciitesReguiations govern ng LPG

Leak detection and pfevention ofground conbmination

EENIUA

Probabil iy oi botlom eakage

Liqueried Natuhl Gas (LNG)

Liqueried Peholeum Gas (LPG)

Liquid Natural Gas (LNG)

LNG

Loadinq on the foundations

Loadings in an insulation ringwall

LODMAT

Low temperatu re ranks

Anc llary equipment and littings

Ammon a slorage a sp-"c alcase

Double cortainment sysiems

Erectior considerallons

Ful conla nmenl syslems

Historica background

LNG tank at Slaten sland

Qatar LPG term na

295

241

396

241

48,101

462

't96

257

254

264

254

254

254

258

254

259

259

264

260

469

507

285

254

254

255

255

6, 29'1, 293, 30?, 310,317.36A,393, 400, 426, 462

7 , 216, 291, 292, 420.473, 426

467

I456

217

495

2al6,291,426

6, 216, 2S3, 303, 308, 412

412, 423, 463. 495

Pump refiova system

Iniernal f loating roofs

Types of inlemaL I oal ng rooG

Pontoon and skin roof

Internal pipework insulation

Interna! shuGoff valves

156

173

173

'173

395

417

454

242

250

342

222

289

411

425

294

435

300

291

292

293

294

377

STORAGE TANKS & EQUIPIIIENT 547

Layoui of refrlgerared lquid gas ranks

Low lemperalue gases

lllale al selection cribna

Single contairmeni systems

Storage of induslria gases in liquid lorm

Tank slzjns considerations

LOX and LIN

LPG

M

Magnetic Panicla Flaw Detection {MPl)

itaterial groups, 5lUni6Itaterial quality control

Material selection criteria for ambient tenperatufe

Requiremenis of the tank design Codes

APl650 requnements

BS 2654 requi€menls

prEN 14015 lequiremenls

Itaterial seclion crit€ria for low tempeEturetanks

Example of a material selection meihod

Reqlirements ofAPl 620

APl620 Appendix Q

l\4ateria s for parts subjected toambienttemperalues

Maierials lor parts subjecled toowlempe€tures

APl620 Appendix R

469

291

437

441

297

308

308

249

294

30

365, 395

7 , 216, 291, 426

476

291

438

483

57,63,76, 10't, 327

241

42

302,463

302

302

Malerials ior rhe ourer meraLlic ranks of s ng e @ntainmeni

Itaximum Considered Earthquake {iilCE)

Membrane in foundation

BS 7777

Comparison ot above ground membranetanks

l,laier]als for parts subjected lo low lenperatures

Requirements of BS 7777:Part 2

Matehalsfof oarls sub ected to ambient lemDeraiures 438

and convenlional lanks 306

DeveLopment history 303

EN 1473 302

Land-based membrane sysiem 304

lnsllation sysiem 306

lMeia lic membrane 304

Outeriank 306

The knot 304

Lined mined rock cavem iniliative fof tuturc LNG slorage 307

LNG cariiers 303

NFPA 59A 342

prEN 14620 342

lrreial arc w€lding 241

lrelhane 334

lrilf scale 234

Mineralwool 390

Minimum impact test r€quifemenb 224

Minimum spacing requirements 475

Miscellaneous storagesFtems 503

Morton 46.80

itodulafconstructionandprefabricationt€chniques 461

Roof frame seclions 461

Roof plaiforms 461

Sleelshel plates 461

Tower slaituays 461

Wire wound lype 461

Mounded storage 217,292

lvlouseholes 241

llulti-strand tendon typs 460

lllultl-layerinsulationcomponentcalculation 397

N

National Board ofFire Und€Mritere {NFBU)

Natio.al Fire Protection Association (NFPA)

Ho.izonlal convecUve lrequency

Horizontal impulsive lEquency

Veriical banelling irequency

NFBU 30

Nil Ductility lest Temperature {NDTr)

507

433

285

295

245

234

441

234

223

221

232

220

222

222

225

226

438

450

438

438

442

438

443

Malerials lor pa.ts sublecled to ambieniiemperatures 443

443

446

446

444

lv,laledals ior parts subjected to low tempeEtures

Requnements of BS 7777:Pan 4

Parts subjeci to ambient iemperatures

Parts subjected lo iow lempe€lores

Requ rements of PD 7777:2000

Requ remeris of prEN 14620

Malerials fof parls slbjecl 1o amb ent lemperalures

Materials for pads subjecl to low temperatures

7

7

486

486

447

488

489

7

42r

I291,421,470

302, 421, 471 , 476

421,500

277

442

446

351,442,454lvlaterlals ior primary ard secondary liquid-@ntaining pans 438


ODaraton and maintenance of ambient temDerature tanks 275

Nozzles and nozle design

Assessment of nozle oadings

APl650 approach

Determinatior ol a lowab e oads

Determ nation oi loads on lhe nozle

Loadirg on the nozze

Meihod of analysis example

Nozz e loading exampLe

She ldefeciion ard roiai on

oOBE

"One foot" method or rule

Operating Basis Earthquake (OBE)

Product identlicanon

Wilh lnternaL i oat ng covers

Floating rooi lanks

Op.raiion of fixed roof hnksF xed roof lanks with interna ioaiing coveB

Hazardous atmospheres

Operation of floaling roofhnksAccess lo the ioallng rool

Colecl on sump delaisEfiecls oi roof type on dra rage

lManaging eg suppots

static electricily @nirol

Operation ofbnks - general

Preveniion oi overfi ing

Operational malfunclions

Outer hnk concrete wall and bottom liners

P

Peak Ground Acceleration (PGA)

Pellini Dfop Weight Test 221,434

Penile 317, 387, 391, 393, 396,399,409

Personnel add equipnent requirem6nts 244

Petrochemical and other industrids 6

Petrolstation forecolntanks 6

Pilot-operated pressure reliefvalve 419

Piping loads 103

Plat6 fabrication 232

Pfate mills 233

Pfate thiokness tolerances 232

Polyurethane foam (PUF) 306

Polyvinyl ch lorid. foam (PVC) 306

Pontoon manholes 183

Poslwold Heat-Treatment {PWHT) 234

"Preload" wire winding system 4, 299, 371, 429, 454, 460

prEN140rs 9, 19,30, 99, 114, 154, 193, 194,207,251,277prEN14620 9, 302, 341, 346, 350, 446, 419, 490, 50',1

Pressu re and vacuum .elief valve 206, 277 , 414

Pressure Equipmenl Regulations I

Pressurevesse,s 1,215

Pre-stressed concror.lanks 309, 460, 461

Preshessing bultresses 370

Probablo Design Earlhquake (PDE) 445

Product identification 277

Producl liquid pressuresacting on tank shells 489

APt620 490

B3T77T 490

prEN 14620 490

Protessor A. S. Tooth 31 , 43, 56, 63, 'l04, 1'16

Propane 6,291,394

Propylene 291

R

Radial raiter type cone roof

Refrigerated liquefied gas storage - history

Refrigeraled stora9e of liquid ammonia

a ternative storage system

Chemical Induslies Associaion gu dance

Convenl onal syslems

Inspecllon and ma ntenance

Recenl developdents

Regulations governing LNG storage facilities

EN1473: 1997 rules

lMin m!m spac ng requiremefts

Scenaros to be considered

Themalradation

214

30

312

104

102,344

106

106

108

106

110

30,105

108

107

109

106

5

28, 60

2f4,443

2AT

277

2TT

277

277

277

274

279

279

279

279

279

242

242

2AA

2AA

284

279

242

279

242

240

279

279

277

2TT

278

247

206

363

234

1

485

'124

472

216

6

6

424

430

431

424

433

433

432

476

476

479

479

480

479

479

479

480

476


l4inimurn spacins equirements 479

Odgin and Development 476

Thermal radiation 47a

Vapourdilution considerations 478

Regulations goveming LPG storage facilities 470

hsltule ofPetroeum rules 471

Bunding requnemenls 473

GeneEl 472

LPG p€ssure siobge (Volume 1, Chapter 2) 472

Reirigeraied LPG sto€se (Vo ume 2, Chapler3) 473

9orage tank spacing 473

Vapour tEvel requirements 473

APl2510 473

Pessurised LPG storage 474

Reffigerated storage 475

NFPA58 47O

Pressudsed LP-Gas siorage 470

Relrlgerated LP Gas storage 4TO

NFPA5g 471

Pressurised LP-Gas storage 471

Refr seraled LP-Gas sioEge 471

Reinfofced and prestrcssed concrete component design 367

General 367

Tank bases 367

Tank rools 374

Tankwals 368

Above groLnd tanks 368

Bonom corner delails 372

In-groundtanks 372

Preslressedconcretewal-intemaltendonlype 369

Prestressed corcrete wall - wirc wound lype 371

Reiniorced conc.ete wal wilh earlh embankment 372

Top comerdetais 373

R€infored base slab 468

Reinrorced concrete 101

Reinfored concrete roofs 345, rt59

Refoase Prevention Barier(RPB) 254

Resetue capacily relief valve 420

Rim venb 174

Risk Based Inspection (RBl) 433

Rivetted and weld.d structures f

APl620 apprcach (Appendices R aid a)

prEN 14620appmach

Exiernal roof nsulation

Internal suspended deck insLrLalion

APl650 requiremenls

BS 2654 requi€ments

Eurcpean Code prEN 14015 €quiements

APl650 requlemenls

BS 2654 l€quircmeftsELropean Code prEN 14015 requiremenls

Roor plaforms ror plmp handling

Compresslon p ale type sea s

Seals incorporating foam dams

Liqu d flled fabric sea

Resi i€nl foam-rlled seal

APl620approach (Appendices R and O)

prEN 14620 approach

Roof water drench system

Roof5 wlth no supporting st.ucture

Co umn-supported oofs

cdumnseeclion

selisuppodns cone (of membEne oo0

Thickness of roof plating

Folded plate lype

Roofs wilh supponing strLclures

Cenlral crown ring

Externallyj€med lype

Radia €frertypo

Extemal y{ramed roois

Design calcu ation

Geodesic dome roois

Amoured nexible hose

Aniculaied piping syslem

DEin design Codes

BS Code

Helicalflexible hose


45,101,133

279

'182,232

174

179

180

179'179

180

180

' 180

180

179

3,16

347

347

350

392

392

393

't93

193

193

193

192,441

193'192

193

415

176'177

177

177

177

176

176'176

345

345

345

346

234

174

421

116

142

143

116

116

116

117

116

118

122

'122

122

122

122

127

123

123.128

136

139

136

136

..137

141

142

Trussed lrame type

Rool-lo'shell 60mpression zone

S

Saie Shutdown Earthquake {SSE)

She lslfien ng lor exlerna insLlation oadings

Wind and vacuum slfiening

AP 620 appendx R approach

End (ie lop and boltom) stifferers

Hoopiension lqud conlaining meialictanks

lntemediale shel stifieneE

Non'lqu d contain ng tanks

Shelslllening for enernal insr ation loadlngs

Wind and vacuLm slffenirg

Hoop tenson iq! d conlanlng metal c ia.ks

Addendumlo Bs 7777on parlialheight hydroslal c lesting

Nor Lqu d conlanng metalLlclanks

Shelstifeningforexle.nalinsllation loadirgs

Wind ard vacuum stffening

Hoop ters on lrquid 6ntaining mela lic lanks

Nonliquid @ntaining lanks

Sh€l stiffening ior external lnsu allon load ngs

Wind and vacLum stifien ng

Shellfiltings

APl650 fequ rements

BS 2654 reqlnements

European Code prEN 14015

Shell thickness formula

Shellwelding sequence

sherl-roof compression a.eas

Codes ard design guidance

tvlale als ot consir!cliof

127

82,92,24326, 50, 52, 53, 55, 89

3'18

319

330

319

329

319

324

324

336

335

335

336

336

337

338

333

338

338

352

192

192

192

192

103

23,27,3A

240

3,14

80. 89

40.342

l. 5!7

409

aa9

::?

:::zar 291 !31

!'.:

241

251

aa,

.t1

252

503

190

192

190

192

192

216, 302,308

274,477,443

Salt domes T

Saunde6 andwindenberg 45,101

sampte/dip hatch 183

sealing syste'ns 279

Secondary bottons 362

Secondaryseals 241

Seismicdesign of ambient temperature tanks 264

AP 650 approach 264

Alowabe longitLdna compressive slress 273

Basc sesmc data 264

Behav our oflhe product lqud 269

Othefconside€liors arising from seismic oadings 273

Overturning moment 270

Res siance lo overtlrning 271

Shel compression 271

Anchored lanks 2T2

Unanchored tanks 271

Slosh he ghl and lreeboard consideralons 273

BS 2654approach 274

lniroducllon 264

prEN 14015 approach 274

Seismic design of lowtemperature tanks 441

Basic seismic design dala 482

Behaviour olthe product liquid 485

Calculation ofihe des gn acceleraions 489

Danprg 485

Design Codes 500

Ducllily 488

General 442

Liquid sloshng 495

Natural lrequencies 446

Productliquid presslres acting on tankshels 489

seismc isolation 499

Tark siding 493

Tankslablily under seism c load ngs 490

Seismic Hazard Assessmenl (SHA) 251,442

Seismic isolalion 499

seismic loads 2a

Seismic zones worldwide 265

Self-supporting 6one roofs 1'16

Semi-bu.ied lanks forthe storage of aviation fuel 100

semi-refrigerat€dandfllly.refrigeratedethylenestorage 292

Separation distances forsmalllanks 259

S.trlement in seNice 252

Shellbuckling 318

shetldesign 318

APl620AppendixQ approach 331

Axlal compression 334

Hoop tenson - iquid cortaningianks 331

Non liquid @ntaining lanks 334

16, 44, 53,60, 334, 336, 336, 328

Single containnent tanks

BS TTTT

EN 1473

BS 2654

prEN 14015

Solids storage and h2.. aiSpacing of welds a..--i ::-,-e-,-s

AP 650'::, _:-: =

BS 26ar -.:, _: : =Eurcr.:_:r:- =: --: .F rs_::e ::: -: :::-

Spher€ssc'e1- =-rr

S-:F-r- -+$.r: 4 :QUIPMENT 551

Spillage collection systems

Spihlly-9uid6d gasholders

Staircases and slaiMays

Parliclpaling rcoi and shellplate area

Seciion size tof the secondary wind girder

Shellwird girder ca cuLalion

Shell-lGroof compression zone

wrd loading to APl650

Tank anchorage 6ystensTank and bund drainage

Tank bottom annular pate analysis

Tank capacity

Tank cool-down anangements

Stain ess steel naleria s

Surfa@ protectior iof plates and secllons

Tank appurtenar@s

p€N 14620approach

Requiremenis ofAPl 620

APl620 Appendix R

APl620AppendixQ

APl620 secUon 5

Design of heat breaks

ReqLirements of BS 7777

Innerlank and olterliquid @ntaining lank

Connecting pipework bendeen innef andoulerlank @nnect ons

Outer @niainer moLntings

Earihing and bonding

Precautons lo minirnise or avoid staiiccharges

Steel Tank lnstitute (STl)

Storage ol f lammable liquids

Storagesysteds and conhinment categories

Storage lank dab sheol

Stomge t nks - delinition

Stnp and bolt ancho.age

Surface proteotion fof plates and sections

prEN 14620 approach

Requ rcments oJAPl620

Requ remenis oi BS 7777

Tankaccess 198

Horizontal p adorms 199

Rada slancase 198

sp Elsla rcase 198

Vert€lladde6 199

Tankanchorage 350

BS 7777 requirehents ?51

p|EN 14620 app@ach 352

Requiremenls ol API 620 Appendix Q 351

Liquid conlaining tanks 351

Nor-Lquid @ntainng tanks 351

Requnenrenls ol API 620 Appendix R 350

Liqlid conlaning metalictanks 350

TankanchoEge furthef considerations 94

Anchorage attachmenl 94

Spacifg ofanchors 94

Wind loading and internalseryice pessurc 94

Worked example 94

Anchorage ca cu alion 97

Check for iranqib lity 99

Comp eiion oiiank design 95

Design oflhe ancho€ge 98

Alax mum urstifiened height otlhe shell 95

Ovefturning momenl due io wind action only 9T

Overturning moment due io wind acton while in sewice 97

552 STORAGE TANKS & EQUIPI\,IENT

274,

220, 232, 304,

475

283

457

457

459

504

417

477,443

351,361

234, 365

242

283

282

7

105, 109

260

295

22

93

100

244

234

360

362

361

361

180

96

96

97

95

95

95

99

348

283

234

49

317

245

248

417

421

231

234

232

234

234

360

355

358

358

355

358

358

American Code requnernenls

Annular plates >12.5 mm thick

F oorairangementlor ianks requiring optmum drainage

F oors lormed from lap welded plates only

Ldpppd loor pates. oraniJldrpa,a' 12.5 rm ifi.kShel tofloorplatewelds considebtionfor speciic maleria s

Tank foors wh ch requne specia @nsideralion

Brtish Code rcquirenrenls

Tanks above 12.5 m diamelef

Tanks up to and includng 12.5 m diameler

Env ronmenial @nsideralions

Floor p aie ar€ngements

Tank ja6king {or jack buildins)

Welding and inspeclior

Permii-lo-work syslems

Work on equipment n operation

246,

359

358

36

39

40

40

4A

36

37

36

42

36

424

433

454

454

454

454

284

244

244

Tank stability under seismic loadings

Prcssurc and vacuum (P &V)valves

Tanks produced in slainless steel materials

Technigd m€mbEne

Alowab e compressNe srress

Axial skess in the shell

Actual compress ve sress

Allowable compressive slresses for she I colrses

Axialstress due io wnd loading on ihe shel

Dervallon and assessmeni oI dialstress ln

a cylindrielshel 31

BS 2654 2T

Alowab e slee stresses 29

Ex@ption to"one iool' rnethod 2a

l,laximu.n afd minimum operating tempeEtures 30

Maximum and mlnimum sh€l thickness 29

Praclical appl calion ofihickness iormu a ,a

Pressure in lhe roofvapouf space 30

Princjpalfaclorsdeterminngshellthickness 2a

Specilicg€vily or relative densLtyolthe slored producl 30

Tank shelldesign ilustration 31

Design ofthe lank shel 26

Failure along the length oflhe cylinder 27

Fa lure around the crcumlerence ofihe cyinder 26

Thermal insulation 363,463

Timber 361

Top boom of the truss 130

Towns gas 1

Toric materiats 245

Type I steel 444,444

Type flsteel 444,444

Typ. fll steel 444,444

Type lV steel 444,444

Type V steel 444,444

Type Vl steel 444

USAEC Regulatory Guide

Vacuum bor testing method

"variable design point" method

Botlom shel course

Conparison of the thickness resulls

Delaibd "vadable des gn point method €lcllaionMethod deveLopmeni

Shel siifiening -wind gnde6

Comparison behveen Brilish and Americansecondary wind girder requn€merts

Primary wind g rders lo APl650

secondary wind gkdeE to APl650

Ventilation - maintenance

v.nting of ambient tanks

Deslgn code rcqu rements

APt 2000

lr.4eans oi veniing

Pressure limitaUons

Re iel valve nstalaiion

Venting requiremenls

BS 2654

prEN 14015

Venling eqlirements

R€liefvalve equiPment

very Larye Crude cariers {VLCCS)

Waltand base liners

Design Code requiremenls

Insulal on for the wals ofsingle-wa ed meiallic tanks

Loose fil insllation systens

Rigid lnsulaton lorlhe wals oidoub e-walled lanks

Wa i nsulaiion nater a s

Loose li I nsulallon

Olher plasiic loam matera s

Perlile loose lil nsulaUon systems

PoLyurelhane loam

Reslienlbankel

Water cooling systems

Specialcase - Floating rooflanks

244

147

493

495

496

490

196

197

197

196

197

99

304

241

242

242

26

35

31

34

34

34

485

246

24,206

306

19,56

5T

63

63

56

60

76

78

76

76

60

285

205

206

209

212

212

212

209

206

206

2AT

207

206

206

206

212

414

26

131

6

243

461

388

389

388

388

388

389

389

391

390

389

390

389

389

391

203

203

uUL 142

Umbrella type dome roof

UndeMrite6 Labohtories Inc (UL)

Unifomly Dist buted Loads (U.D.L.s)

United Kingdom Pressure Equipment Regulations

Unrestrained shell deflection and rotation atthE nozle centreline

7

'123

T

129

109

STORAGE TANKS & EQUIPII4ENT 553

Tank cooling methods

Fixed and lraileFmountod water cannons

Water sp€y and detuge sprinkt€r systems

Weld edge preparalion

Wells Wde Plat6 (wWP) tests

Equiv€lent shell method

Numbor of gideB rcquted

worked example

Vertical bending of the shell

Ro€non and stress analysis

Shell-to-bottom connection

lvind and vacuum stiftuning -low temperature lanks

Saiety measurcs against wind damage

Wlndenburg and Tdlling

wlnd girder *ctions

Wire wound .oncrele lank walls

Wlre wound concrele waGr tanks

z

203

204

203

233

29, 221, $4, $O

45

45

46

4a

4A

324, 334, 336, 338

239

240

329

318

30

372

1

57,63,328

Wind and vacuum stiffonlng - ambisnt temperature bnt(s

Ch@sing BS or APt shelt thickness design methods

Shelldesign slresses

Shell plate lhicknesses

Use of shell design fomuiae

Worked examples

P mary wind gideE

R€fining the dsign technique

Secondary wind sidels

507

43

51

53

51

51

53

53

56

43

43

43

45

554 STOMGE TANKS & EQUIPMENT

Acknowledgements

The publishers acknowledge ihe help and assistance ofthe following organisaiions in supplying dala,photogEphs, illusttations and where apprcp ate, permissionto reproduce matefialfrom their own publications.

Advant caAlibert Buckhorn UK Ltd

Amadeus P€ss LtdAnderson Greenwood

Angus FieAntuerp Gas Termina NV

API (Amedcan Pelrcleum Inslitute)BSI

Cell U Foam CoQoralionCEN (Eump€an Commifiee for Slandad zalion)

chi@so B dse& lrcn Company(CB&l)DEPA S,A.

DOT (US Depadmeni of Transportalion)Eadhquake Proleclion Syslems Inc

EEMUAEFIVA

EnagasEndrcss+Haus€r Systems & GauginS Ltd

Energy 'nstiluie

(formedy the Institute ofPetrcLeum)tci Plc

lnslitute of Petroleumlntemalional Code Council (lCC)

JFE Engineering Corpo€iionLNG JoumEl

lM W Kellogg LidIMB Engineering Se ices Lid

McTayMolhsrwel Conlrol Syslems Lld

N FPA (NaUonal Fire Poleclion Association)Nikkiso Cryo ELrrop€

Philips Pelrcleum CompanyPilt-Des Molnes lnc

Pittsburgh Corning CorpoalionPrsload lncRecinco NV

Roya A€ronautca Soc elyRoya Vopak

Taylor Woodrow ConstruciionThe Mot Otrce

Ty6 Va ves & ConlrolsUniversal Shipbuilding Corporation

WhessoeWoodside Petoleum Ltd

Aspeclal acknowl€dgement to Protussor A. S. Tooth

Bob Gamer is greaily indebted to the late PrcfessofAlwyn Tooth,Prcfessor of Mechan ical Engineering, at the lJnive|siiyof Srathclyde, Glasgoq

lor his huge contribulion to ihe undersbnding ofthe theory of siorage lank design.

Much ofthe explanation to the backgrcund ofthe theory included inthe ambient tank Section of this book is taken from work produced bv Professof Toolh.

STORAGE TANKS & EOUIPMEI{T 556

xviii

527

xxvi

x)o(t,5zl

xxviii

527

XXII

Inside Back Cover

XViii

xvi

xxviii

viii

XXXi

Inside Ffont Cover

xxiv

xiv

xxviii

527

vi

Outside Back Cover

I

lndex to advertisers

AConsult

Allibed Buckhom Uk Ltd

Bayham Ltd

Braby Ltd

Brimar Plastics Ltd

Bureau Veritas

ByggwikUK Ltd

Cookson & Zinn (PTL) Ltd

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Drayton Tank & Accesso es Lld

Ebara Intemalional Corporction

Emco Wheaton GmbH

Fod Vale Engineedng Ltd

Franklin Hodge lndustries

H[,1T Rubbaglas Ltd

HSB Inspection Quality

Lloyd's Regisler

I\,1W Kellogg Ltd

IvlB Engineedng Services Ltd

MC Inieg Ltd

I\,,1cTay

I\,lixing Solulions Ltd

I\,lotherwell Control Systems LdNikkiso Cryo Europe

Nomanby Induslries LH

Prccolor Saies Ltd

Seetru Ltd

Sinclair Slainless Fab cations Ltcl

SN Technigaz

Sui Generis Intemational Ltd

Taylor Woodrow Construction

TesTex NDT Ltd

Tmctebel Gas Engineering GmbH

&t*556 STORAGE TANKS & EQUIPMENT

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bob long part 2

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