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21/5/2016 New Constr uction Above Gr ound Storage Tanks - Matcor, Inc.

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ABOUT NEWS RESOURCES CONTACT A CORROSION EXPERTHome Resources Technical Papers New Construction

Above Ground Storage Tanks

New Construction Above Ground

Storage Tanks

OVERVIEW:

This technical bulletin addresses the design and installation

of cathodic protection systems fornew construction above

ground storage tanks (ASTs). Cathodic protection (CP) is

typicallyapplied to all above ground storage tanks built on a

ring wall foundation to protect the externaltank bottom incontact with the soil/sand foundation. Smaller tanks built on

concrete slabfoundations typically do not have cathodic

protection applied to them.

In many locations, cathodic protection is mandated by local

regulations for tanks storinghydrocarbons orhazardous

materials; however, even in theabsence of such mandates,

goodengineering practice would generally dictate cathodic

protection.

Summary/Conclusion:

MATCOR typically recommends the use of linear anodes in a

concentric ring configuration asthe most reliable system

design for new construction above ground storage tanks,

combiningeconomy of material requirements with ease of

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installation.

Galvanic vs. Impressed Current

Historically, various configurations of galvanic anodes,

including discreet anodes and ribbontype anodes, have beenused to protect AST bottoms. Experience has shown that

these systemsdo not provide the uniform current distribution

necessary over the entire CP system design lifeand result in

premature failure as the galvanic anodes consume. ASTs

require significantcurrent, which generally precludes the use

of galvanic anodes. Almost all AST CP systemstoday are

designed with impressed current systems to provide the

current required over a longperiod of time.

Design Information:

The following information is required to develop a CP design

for ASTs:

Tank Diameter

This is necessary to calculate the surface area to be

protected.

Tank Bottom Coating

Typically tank bottoms are bare plate steel, but in

some cases the plate steel may becoated on the bottom,

which reduces the current required for cathodic protection.

CP isstill recommended for tanks with coated bottoms.

Current Density Required

Typical design current density requirements of 1

mA/ft2 (10 mA/m2) are sufficient toachieve NACE criteria forcathodic protection (see discussion on operating

temperature).


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Depth of Anode

The separation distance between the anode and the

tank bottom affects current spread andanode spacing.

Sand/Soil Resistivity

This information is necessary to estimate overall

system resistance necessary to properlysize the rectifier

voltage. In many cases, it may make sense to install the

anode systemand test the actual circuit resistance using a

portable rectifier or car battery beforecommitting to a

specific rectifier size.

Tank Operating Temperature

Corrosion rates increase significantly with elevated

temperature, necessitating morecurrent. MATCOR uses the

following temperature correction formula for its CP

designsfor heated tank bottoms: for every 10 C above 30 C

the current requirement increases25%.

Area Classification

Rectifiers and junction boxes must be designed with

suitable enclosures for the areaclassification where they will

be installed. Often, these can be located in non-

classifiedareas with minimal additional cost of cable while

saving significantly on the cost of theenclosures and classifiedcomponents.

Secondary Containment Liners

If polyethylene or other such plastic liners are to be

placed underneath the tank, theseliners act as a barrier to the

flow of current and the anodes must be placed between

theliner and the tank bottom. If Claymax or other conductive

type liners are to be used, theanodes do not have to be

placed between the liner and the tank bottom.


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Design Life

Typical design life for new construction ASTs is 25 to

30 years. It is important to notethat the actual operating life

of ASTs often exceeds this value and depending on

thedesign of the tank, its location, and the selection of a

containment liner. Replacement ofthe CP system may be

difficult to impossible so some consideration should be given

tothe economic value of extending the design life. For

MATCORs SPL-FBR concentricring configuration, the

incremental anode cost to go from 30-year design life to 50-

yeardesign life is approximately 25% additional anode cost,

with no increase in installationcosts, making this a very

attractive alternative.

CP Configurations for New Construction ASTs

Shallow Distributed Anodes around Tank Periphery:

One common design approach to AST bottom CP is to installa shallow distributed pointanode system around the

periphery of the tank (Figure 1). These are typically augured

into depths of 5-10 feet. This design approach only works

when there is no electricallyisolating secondary containment

liner under the tank.

For these designs, the critical issue is assuring that sufficient

current reaches the center ofthe tank. Above ground storage

tank bottoms are large bare surfaces requiring a lot ofcurrent.

To assure that current distributes properly, the anode depth

and distance fromthe tank are critical. Shallow peripheral

anodes are not able to throw current to the centerof all but

the smallest of ASTs.

MATCOR generally does not recommend shallow


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peripherally distributed anodes forASTs with tank diameters

exceeding 20 ft (6 m) due to the quantity of anodes

requiredand the risk of poor current distribution to the center

of the tank.

Deep Well Anode SystemsThis approach is based on using one or more deep

well anode systems located well belowthe tank bottom to

provide current uniformly to the tank bottom. This approach

has somelimitations in heavily congested plant environments

where current can flow to otherburied structures. When

multiple deep wells are employed to protect more than one

tankin a cluster, care must be taken to assure proper current

distribution.

As with any deep well, there are concerns with drilling

(typically 150+ feet to bottom ofhole) including access issues

for a drill rig, environmental concerns, permitting

andhandling of drilling spoils. Even with a deep wellapproach, when dealing with newconstruction, reference

electrodes should be installed under each tank.


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Many operators prefer a close coupled dedicated CP system

for each tank rather than theblanket coverage afforded by a

deep well system.

Should the design for the new construction tank utilize a

containment liner that shieldscurrent (i.e. polyethylene liner),

the deep well anode system cannot be used.

Distributed Anodes Below Tank Bottom

While not as common today, many older designs utilized

individual anodes laidhorizontally along the tank bottom and

connected parallel to header cables exiting thering wall. The

economics of this design, both in terms of installation costs

and materialcosts, are not favorable and this design has been

dropped in favor of either a grid systemor linear anodes in a

parallel concentric ring arrangement.

The Grid System

Developed, patented and heavily promoted by Corrpro, this

proprietary system involveslaying out parallel titanium

conductor bars and then running mixed metal oxide

(MMO)ribbon anode perpendicular to the conductor bars.

The MMO ribbon anode is field spotwelded to the titanium

conductor bar to provide both mechanical andelectricalconnections. Wherever the titanium conductor bars

cross, they too must be field weldedtogether. Power feeds

(preassembled cables with a flat plate to connect to the

conductorbar) are secured to the titanium bar in multiple

locations and routed to the ring wallpenetration.

Patented in 1991, this system continues to be used by

Corrpro; however, it is a labor andQA/QC intensive

installation process requiring significant field welding and on-


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sitetesting to assure electrical continuity. The attachment of

the power feeds to the titaniumgrid is critical to the system

reliability. From a design perspective, the spacing of

theanodes and conductor bars must be sufficient to assure

even current distribution.

SPL-FBR Linear

Anode Systems

Unlike the Grid

system promoted

byCorrpro,

MATCORs SPL-

FBR linearanode

system for AST

tanks can be

factoryassembled

to eliminate the

need for anyfield

fabrication, which

greatly

simplifiesinstallation and reduces QA/QC issues,eliminating

field welds and power feedconnections that are relied upon

with theGrid system to assure electricalcontinuity andsystem integrity.

The principal advantage of the linearsystem is that everything


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under the tank isfactory assembled and tested prior

toinstallation and the only installation effortis to lay the

anode assemblies in accordancewith the design drawings and

installationinstructions. This provides for anexceptionally

simple installation whileassuring the highest system

reliability;installation costs are minimal. Please referto

MATCORs Installation, Odperation andMaintenance (IOM)

Manual for a detailedinstallation description with pictures.

There are two primary configurations forlinear anodes under

tank bottoms as shownin Figure 2. The parallel linear

anodearrangement has multiple parallel anodesegments,which are fed from each end ofthe anode. The anode

connections are fieldspliced to loop cables, which terminate

attwo anode junction boxes.

The use of concentric rings offers two keyadvantages over

parallel anode segments. The first advantage is that this

configurationdoes require junction boxes on both sides of

the tank, thus eliminating one of the anodejunction boxes

and reducing the cabling required to run from two anode

unction boxesback to the transformer/rectifier unit. The

second key advantage to this configuration isthat it

eliminates the need for two field splices for each anode

segment. Each ring can bemanufactured with the appropriate

length of header cable to run each end directly to thesingle

anode junction box. These field splices are weak links subject

to premature failureover the life of the anode system.

As with any system, spacing between anode segments is

another key design element.MATCORs experience with ring

configurations is extensive and we have determinedthrough

numerous installations and our own in-house testing that for


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tank bottomapplications with bare bottom plates and an

anode depth of 1 foot, concentric rings with aspacing of 10

feet provide thorough current distribution. When the anodes

can be placeddeeper than 1 foot, the anode spacing can be

extended. Based on the 1 foot depth, typicalambient

temperature tanks can be protected for 30+ years with 16

mA/ft linear anode,while a 50+ year design life is typically

achieved with a 25 mA/ft rated anode with amodest 25%

increase in the anode cost.

Provisions for Testing

With any CP system for tank bottoms, it is critical that

provisions for testing be installed with theanode system.

Once the tank is erected, making accurate potential

measurements at variouslocations along the tank can only be

accomplished if reference electrodes have been

installedbelow the tank. Typically, copper-copper sulfate (Cu-

CuSO4) reference electrodes are installedin strategic

locations underneath the tank. These reference electrodes

are often mistakenly calledpermanent reference

electrodes; however, they are not permanent as over timethe coppercoppersulfate solution becomes contaminated

and ceases to provide accurate information. Oncea baseline


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for performance is established over asufficient operating

period, maintaining theappropriate current output to achieve

NACEcriteria is all that is required.

In some cases,

clients may also

specify

dualreference

electrodes such

as both zinc and

coppercoppersulfate. While the zinc reference electrodesare

not as consistent, they provide a much longeroperating life

and can be calibrated against the copper-copper sulfate

electrodes.

In addition to the fixed reference electrodes, it isalso

common to provide a reference electrodetube/conduit

underneath the tank bottom to allowsliding of a calibrated

reference electrode throughthe monitoring tube to take

potential readings.These can also function as leak detection

tubes.

Maintenance/Inspection

MATCOR typically recommends annual testing/inspection of

the tank CP system by a qualifiedNACE CP level 1 or higher

technician familiar with testing CP systems for ASTs. As with

anyimpressed current system, monthly rectifier checks should


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be performed by plant maintenance toassure that the

rectifier is on and that the voltage and current outputs remain

stable.

Appendix A MATCORS DESIGN PROGRAM

System Design:

Once all of the design parameters have been established,

MATCORs design program uses thefollowing process to

create a completed design.

1. Basic Calculations

a. Tank Bottom Surface Area simple geometric calculation

of the area of acircle. If the tank bottom is coated MATCOR

assumes a 75% coatingefficiency (25% bare) as some coating

degradation can be expected at allof the weld seams. b.

Current Requirement the bare surface area times the

temperaturecorrected current density requirement provides

the total design currentrequired.

2. Anode Spacing MATCOR typically assumes that the

anode will be located aminimum of 1 foot below the tank

bottom. Based on 1 foot depth, MATCORuses 10 foot

spacing between anodes. This is not based on any

theoreticalmodeling but has been derived empirically by

testing performed by MATCORand validated on numerous

actual applications. MATCOR has researched theavailable

literature and has not found any published data or theoretical

modelingthat can be used to determine current distribution

for concentric rings beneath astorage tank bottom. For

depths greater than 1 foot, MATCOR increases theanode

ring spacing at a rate of 0.5 feet/additional foot of depth.


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Occasionally, clients have askedMATCOR to provide a

theoreticalbasis or some sort of calculation toconfirm the

spacing of the anodes.NACE, in its CP3 design

course,provides a geometric guideline of2*tan60 for spacing

of galvanic ribbonanodes under a tank bottom to

assuredistribution; however, we have foundno reasonable

scientific or

theoreticalexplanation as

to how this value

wasselected (Figure 4).

We have discussed thiswith NACE instructors

and Senior Corrosion

Engineersand they cannot

provide any justification for this calculation. It just makes for

anice geometric value.

MATCOR has tested numerous tanks with impressed current

CP systems using 10foot spacing between anodes with a

depth (d) of 1 foot. This equates to ageometric equation of

2d tan(78.7), which is not as elegant a number as 60. Butwe

know from our testing using reference electrodes that have

been placed inclose proximity to the tank bottom that

current distributes sufficiently to meetNACE criteria between

ring segments.

3. Current Capacity once the preliminary anode spacing is

determined, the totalnumber of rings can be calculated with

the rings spaced equally apart inconcentric circles. The total

anode length is then calculated. The current rating ofthe

anode is a function of the total current required divided by

the anode length.For ambient temperature tanks, using 10


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foot spacing, the typical design requiresMATCORs SPL-

FBR-16 mA/ft anode and yields a 30+ year design life.

Thesame design using MATCORs 25 mA/ft anode will

generally provide for a 50+year design life for only a modest

increase in material cost.

For elevated temperatures MATCOR calculates a

temperature correction factorbased on increasing the current

required 25% for every 10 C above 30 C. Toaccommodate

the higher current requirements at elevated temperatures,

either ahigher output anode is necessary or tighter spacing

with more anode length isrequired. MATCORs designprogram allows for the anode spacing to beadjusted to

optimize the anode length and anode rating as needed to

meet thedesired design life requirements.

4. Anode Resistance Calculations each anode ring is

connected parallel to oneanother through the anode junction

box to the rectifier. The resistance of eachanode ring is

calculated using Dwights equation for a ring of wire:

where:

R= resistance of each ring

? = soil resistivity

D = diameter of ring

d = diameter of anode

s = twice the depth of anode


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Current Density

Current Required amps 11.3

Total Number of

Rings

6

Anode Spacing ft 10

Anode Selection 16 mA/ftAnode Life (est) yrs 32

Total Resistance ohms 2.8

Rectifier Sizing voltage 40

amperage 15

Total Anode

Length

ft 1130.9

Total CableLength

ft 600

Ring

#

Diameter

ft

Linear

ft

Total

ft

Cable Tail Length

ft

1 10 31 31 150

2 30 94 126 1303 50 157 283 110

4 70 220 503 90

5 90 283 785 70

6 110 346 1131 50

Typical AST Tank Ring Cathodic Protection System

AST Cathodic Protection System Detail Drawing

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Ring Configuration
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