Report of the Committee on

Explosion Protection Systems

David C. Kirby, Chair Union Carbide Corp., WV [U]

1L F. Schwab, Vice Chair Allied-Signal Inc., NJ [U]

Luke S. Morrison, Secretarj Professional Loss Control Ltd, NB, Canada [I]

Joe IL Barton, Nat'l Starch & Chemical Co., IN [U] Rep. NFPA Industrial Fire Protection Section

John V. Birtwistle, Monsanto Co., MO [U] William J. Bradford, Brookfield, CT [SE] Relnhard E. Bruderer, Pred-Engr Inc., FL [U]

Rep. Ciba-Geigy Corp. Kenneth L. Cashdollar, Nat'l Inst. of Occupational Safety & Health,

PA [El Gary A. Chuhb, Peabody TecTank, Inc., KS [M] David G. Clark, The DuPont Co., DE [U] Robert L. DeGood, Fike Corp., MO [M] Henry L. Feho, Jr., Factory Mutual Researdi Corp., MA[I] Mark A. Fry, Mark A. Fry & Assoc., Inc., NJ [SE] Jsoseph P. Gillis, Westboro, MA [SE]

tanley S. Grossel, Process Safety & Design, Inc., NJ [SE] Edward J. Haas, Jr., Johnson & Higgins, NY [I] {~ mes G. Hansel, Air Products and Cbemicals, Inc., PA [M]

ichael D. Hard, Hard Fire Suppression Systems, Inc., OH [IM] Rep. Fire Suppression Systems Assn.

Robert L. Henley, Ashland, Inc., KY [U] Walter B. Howard, St. Louis, MO [SE] A! Lewis, Kemper Nat'i Insurance Cos., IL [1] Geror~e Lobay, Canada Dept. of Natural Resources, ON, Canada

[ a ~ j

IL A. Mancini, Amoco Engr and Construction Corp., TX [U] Rep. American Petroleum Inst.

Robert W. Nelson, Industrial Risk Insurers, CT [1] John A. Noronha, Eastman Kodak Co., NY [U]

l ohn Joseph Plunkett, U.S. Coast Guard, DC [E] oseph A. Senecal, Kidde-Fenwal, Inc., MA [M] effery W. Sutton, Liberty Mutual Group, IL [I] Rep. The Alliance of American Insurers

Robert G. Zalosh, Worcester Polytechnic Inst., MA [SE]

Alternates

Laurence G. Britton, Union Carbide Corp., WV [U] (Alt. to D. C. Kirby)

Krls Chatrathi, Fike Corp., MO [M] (Alt. to R. L. DeGood)

James D. Dick, Kemper Nat'l Insurance Cos., WA [I] (Alt. to A. Lewis)

Walter L. Frank, The DuPont Co., DE [U] (AlL to D. G. Clark)

Thomas A. Gray, Akzo Nobel Inc., IL [U] (Ait. to J. 1L Barton)

Dan A. Guarieci, Fenwal Safety Systems, MA [M| (Alt. toJ . A. Senecai)

Paul F. Hart, Induslrial Risk Insurers, IL [I] (A]t. to R. W. Nelson)

George A. Krabbe, Automatic Suppression Systems Inc., IL [IM] (Alt. to M. D. Hard)

Arnold L. Mundt, BS&B Safety Systems, OK [M] (Voting Alt. to BS&B Safety Systems Rep)

Samuel A. Rodgers, Allied Signal, VA [U] (Alt. to R. F. Schwab)

John Valiulis, Factory Mutual Research Corp., MA [I] (Alt. to H. L. Febo)

Nonvoting

Harry Verakis, U.S. Dept. of Labor, WV

Shaft Liaison: Martha H. Curtis

This list represents the membership at the time the Committee was balloted on the text of this edition. Since that time, changes in the membership may have occurred. A ke'j to classifications is found at the back of this document.

Committee Scope: This Committee shall have primary responsibility for documents on explosion protection systems for all types of equipment and for buildings, except pressure venting devices designedto protect against overpressure of vessels such as those containing flammable liquids, liquefied gases, and compressed gases under fire exposure conditions, as now covered in existing NFPA standards.

The Report of the Technical Committee on Explosion Protection Systems is presented for adoption.

TilLs Report was prepared by tile Technical Committee on Explosion Protection Systems and proposes for adoption_ a complete revision to NFPA 68-1994, Guide for Ventmg of Deflagrations. NFPA 68-1994 is published in Volume 10 of tile 1997 National Fire Codes and in separate pamphlet form.

This Report has been submitted to letter ballot of the Technical Committee on Explosion Protection Systems, which consists of 30 voting members. The results of tile balloting, after circulation of any negative votes, can be found in the report.

668

N F P A 6 8 - - A 9 8 R O P

(Log #1) 68- 1 - (Table 4-3): Reject SUBMITTER: Edward S. Naidus, Glen Rock, NJ RECOMMENDATION: O n line 3 of Table 4-3, replace ti~e first "G" value of 0.17 witll a

"C" value o f 0.10; replace tile second "C" value o f 0.045 witll a "C value of 0.026. SUBSTANTIATION: The basis for r e c o m m e n d e d change in "C" values in Table 4-3 for propane-l ike def lagrant mixtures is out l ined in the following: The re is widespread criticism of tile cu r ren t "C" value of 0.17 because it leads to vent ing ratios for many low- s t rength enclosures that are unrealistically high, e.g., for a cube- like enclosure the ratio approaches one sq.ft, of vent per one cu. ft. of enclosure. The cu r r en t value of "C" = 0.17 is based on data summar i zed by L. Britton ( 6 / 2 6 / 8 6 ) tha t accepted witllout er ror analysis every result publ i shed in the reports covered by the summary . In what s e e m e d like a conservative "worst-case" t reamlent , the Commi t tee calculated a "C" value by a data- enveloping p rocedure tilat has the effect of emphas iz ing the h ighes t P max values wi thout regard to tileir exper imenta l validity. A reexamina t ion of the Brit ton m e m o nsing convent ional error mlalysis revealed that many of the ltigh values listed and included by Britton are shown to be statistically invalid and no t useflll as "worst-case" boundar ies . (The l ump i ng of da ta arrays f rom different investigators all seeking "true" values for propane-air deflagrat ions is known as meta-analysis and, in principle, gives more significazlt values of the variable, P m a x , t han a single set of data - in any case, subject to tire same error analysis).

O f great concern is the protective value of vent ing of low-strength enclosures tlrat are filled with more than 25 percen t of a def lagrant mixture . A review of ven ted and u n r e n t e d incidents such as, (grain elevators, process line ruptures , s tarch handl ing) suggests tha t such real-world def lagrat ions would no t be successfully protected. Deflagrat ions timt occurred witll "low-fill" scenarios were adequate ly pro tec ted with unsoplt is t icated ratios o f one to thirty (Av:V) equiw.dent to a Swift-Epstein C value of 0.051. It has been sugges ted tha t useful risk .'Lssessments utilizing NFPA 68 shou ld inc lude cons idera t ions of tn rbu lence (self or induced) , as well the probabi l i ty of a "fill-ratio" less than 25 percent . COMMITTEE ACTION: Reject. COMMITTEE STATEMENT: Tile Commi t t ee disagrees with ti~e submit ter ' s statistical analysis of tl~e data s ince it has a bias for a less s:ffe approach. T he Commi t t ee is unab le to de t e rmine any errors in tile "C" valne da ta to sugges t tha t it shon ld be rejected, therefore the s u b m i t t e r ' s use of a statistical analysis to reject the cur ren t "C" values is no t an acceptable basis for the Commi t t ee to reject the cl,ata.

The Commi t t ee believes that there is confi rmat ion of the "C" vahles publ i shed in NFPA 68, 1994 edition, reference 23 (W.B. Howard, " Interpre ta t ion of a Building Explosion Accident") a n d reference 30 (W.B. Howard & A.H. Karabinis, "Tests of Explosion Vent ing of Buildings"). Bod~ exper imenta l da ta a n d industr ial exper ience have conf i rmed that the established "C" values are valid. NUMBER OF COMMITTEE MEMBERS ELIGIBLE TO VOTE: 30 VOTE ON COMMITTEE ACTION:

AFFIRMATIVE: 26 NEGATIVE: 1 ABSTENTION: 1 NOT RETURNED: 2 Birtwistle, Clark

EXPLANATION OF NEGATIVE: SENECAL: There is ,an entry error in Table [)-2. T he first

chemical n a m e shou ld be changed f rom 1,1-dichloroetbane to l,l-difluoroetllane. The error was mine on submiss ion o f tire

original table. COMMENT ON AFFIRMATIVE:

ZALOSH: Al though I agree widl the Commi t t ee Action Oil this item, I disagree with tile Commi t t ee S ta tement suppor t ing the nse of the cu r ren t "C" wdues. The "C" valnes need to be reviewed carefiflly in light of new data ob ta ined s ince 1986 as well as a more comprehens ive analysis of ti~e original &~ta compi led b X L. Britton. The Commi t tee S ta tement tha t "Boti1 exper imenta l data and industrial exper ience have conf i rmed tllat tile estal~lished "C" values are valid is incorrec t and provides tile reader of NFPA 68 witll a false sense of security and little motivation to heed all tire caveats and caut ions we have in Sections 3 and 4 of NFPA 68.

In part icular I suppor t fu r the r review and ,analysis of the data to aid tile "considerat ions of tu rbu lence (self or induced) , as well as the probability of a "fill-ratio" less t han 25 percent" as sugges ted in Ed Naidus ' proposal .

EXPLANATION OF ABSTENTION: HENLEY: Recently appo in ted to Technical Commi t tee on

Explosion Protect ion a n d have no t comple ted review of tile Report on Proposals (ROP).

(Log #2) 68- 2 - (5-4.2): Reject SUBMITTER: Scott L. Donn , Cv Safety Products /REMBE, Gm b H RECOMMENDATION: If it is necessary to locate enclosures that require def lagrat ion ven t ing inside buildings, ven t ducts should be used to direct ven ted material f rom tile enc losure to tl~e outdoors. Flames and pressure waves d i scharg ing f rom the enclosure du r ing vent ing represen t a dlreat to personne l and could damage otiler equ ipmen t . Newer deve lopments indicate tiaat f lame p.ropagation f rom explosion vented e q u i p m e n t u n d e r certain condit ions, can be ven ted within a bui lding by us ing approved diverters, e.g. mechanica l f lame barriers (f lame arrester, q u e n c h i n g devices witi1 dus t retainers) . SUBSTANTIATION: Techno logy now exists, tha t u n d e r certain condi t ions f lame propaga t ion f rom tile ven t ing of deflagrat ion can be s topped. This will facilitate control led discharge widfin a building. Devices for dtis purpose will facilitate efficiency in p lant des ign a n d p romote opt imal m a n u f a c t u r i n g processes.

Note: Suppor t ing material is available for review at NFPA Headqua r t e r s . COMMITTEE ACTION: Reject. COMMITTEE STATEMENT: The Commi t tee believes that the revised text ,as p roposed by tile submi t te r allows broad applicat ion of vent ing deflagrafions witifin bui ldings widlout suppor t ing test da ta to limit its use. The Commi t tee does not believe daat sufficient suppor t ing material has been made available to define file condi t ions of applicability of this technology. At tile presenta t ion made to die Commi t tee in October 1995, tire Commi t t ee reques ted fllat tile submi t te r provide addit ional test da ta so dlat it could be used to establisil appropr ia te l imitations on tile use of this technology. With no new data submit ted , die Commi t t ee c anno t suppor t tile submi t te r ' s r e c o m m e n d a t i o n . NUMBER OF COMMITTEE MEMBERS ELIGIBLE TO VOTE: 30 VOTE ON COMMITTEE ACTION:

AFFIRMATWE: 26 NEGATIVE: 1 ABSTENTION: 1 NOT RETURNED: 2 Birtwistle, Clark

EXPLANATION OF NEGATIVE: SENECAi= See my Explanat ion of Negative on Proposal 68-1

(Log #1). EXPLANATION OF ABSTENTION:

HENLEY: See my Explanat ion of Abstent ion on Proposal 68-1 (Log #1).

(Log #CP1) 68- 3 - (Entire Documen t ) : Accept SUBMITTER: Technical Commi t t ee on Explosion Protect ion Systems RECOMMENDATION:

I Completely revise tile 1994 edit ion of NFPA 68, Guide for Venting of Deflagrations to read as shown at the end of this report. SUBSTANTIATION: The Commi t t ee has completely revised the 1994 edit ion of NFPA 68 for tile following reasons:

1. The Commit tee upda t ed the te rminology used t h r o u g h o u t ti~e d o c u m e n t to be consis tent witi1 cu r ren t industrial practice and for clarification purposes .

2. The Commi t tee revised, reorganized and consol idated tile material p resen ted in the d o c u m e n t for ease of use. In addit ion, new material was added to upda te tile t echnology and to stay consis tent with o ther s tandards developing organizat ions.

3. Tile Commi t t ee added new informat ion on fl~e effects of vent ducts a n d tile met l lod of calculating d lese effects.

4. Tile Commi t tee added new informat ion on vent area calculations and tile effects of ven t discharge.

5. T h e Commi t tee revisions inc luded recognizing tile use of "weak roof- to-shel l" joint des ign as a means for vent ing silos a n d bins.

6. The Commi t t ee provided new informat ion on explosions in e longa ted vessels.

669

N F P A 6 8 ~ A 9 8 R O P

7. The Commi t tee revised die material p resen ted in dae d o c u m e n t to clarify tile provisions for restraint panel secur ing means and to clarify die Commit tee ' s in ten t that vent closure assemblies are to be instal led in accordance widi dleir design.

8. The Commi t tee revised die material p resen ted in die d o c u m e n t to clarify file provisions for inspect ion a n d ma in t enance of def lagrat ion vent closures. COMMITTEE ACTION: Accept. NUMBER OF COMMITTEE MEMBERS ELIGIBLE TO VOTE: 30 VOTE ON COMMITT E E ACTION:

AFFIRMATIVE: 27 ABSTENTION: 1 N O T RETURNED: 2 Birtwistle, Clark

C OMMENT ON AFFIRMATIVE: SUTTON: I am voting affirmative on this Proposal as I feel d ie re

is a lot of good new informat ion in dfis revision dlat needs to be got ten to the public. However, equat ion (17) in paragraph 6-2.3 sbou ld not be used as dlis equat ion is only based on a single da ta poin t and except for this one data point, d lere is no technical validity for the use of this formula. I feel dais fo rmula is only reaching f rom me ,ms to a ccoun t for die L / D (Leng th /D iame te r ) ratio gap presen ted by equat ion (16 and the 1994 edit ion of NFPA 68. A better app roach would be to state in dae d o c u m e n t d ia t flieir is no t sufficient test in format ion available to accurately calculate die vent a rea of h igh s t rength enclosures hand l ing f lammable gas mixtures with L / D ratios greater than 2. EXPLANATION O F ABSTENTION:

1-1.6 Tiffs guide does no t apply to pressure relief devices on e q u i p m e n t such as oil-insulated t ransformers . It also does not apply to pressure relief valves on tanks, pressure vessels, or domes t ic (residential) appl iances.

1-2 Purpose . The purpose of dfis guide is to provide die user wifla criteria for vent ing of deflagrations. It is impor tan t to note flint vent ing will no t prevent a deflagration; vent ing can, however, minimize dae destructive effects of a deflagration.

1-3 Applicability.

1-3.1 This d o c u m e n t applies where the need for deflagrat ion vent ing has been established. Noth ing in tiffs guide is m e a n t to require die installation of vents on any enclosure.

1-3.2 It is no t i n t ended flint fine provisions o f dais d o c u m e n t be appl ied to facilities, equ ipmen t , s t rnctures or installations with deflagrat ion vent ing which were existing or approved for construct ion or installation prior to die effective da te o f die documen t , except in those cases where it is de t e rmined flint the existing si tuation involves a distinct hazard to life or property.

1-3.3 Vents act as a system, in conjunc t ion wifll file s t rengdl of die pro tec ted enclosure. However, some l ightweight structures, such as damage- l imi t ing buildings, can be cons idered totally relieving a n d would require no specific vents.

HENLEY: See nay Explanat ion of Abstent ion on Proposal 68-1 ::.. (Log #1). 1-4 Definitions. F.o.g..'~~.,,. purpose of dais guide, d ie following terms

have die meanin~iv~*elow.

Approved. ' . :~cc" ' : '~e": '" ~ ' " to the author i ty having jurisdicti on. i

NFPA 68 B u r n i n ~ . . i ~ i ~ . ~ . T b ~ : ~ . q %t~.:.flame propagat ion relative to die veloci~#::i~f" t.ht":'finburned ~ i g ] l e a d of it. (See Fundamental Burning

Guide for Vent ing o f Def lagrat ions Velo~...) %::, .:.:...#"

1998 Edition C o m l ~ : ~ ' ] " % chemical process of oxidat ion that occurs at a rate fast L~.t..gh to p roduce hea t a n d usually light, in die form of

NOTICE: An asterisk (*) following the n u m b e r or letter "~ii~i~:!::::.. eidler a g ld~ '~ . : . .~mes. des ignat ing a paragraph indicates that explanatory materi,'fl on the ":.~!~..:%':~:-:: ..... "-:.:'~:S:" paragraph can be f o u n d in Appendix A. ":~!. " ~ . . L . ! . . . l ~ ' i h a g bui ld ings . Rigid-framed buildings with all walls

Informat inn on referonced rmblicationg can he f o u n d in ~i:,o.r.::#W'~!..~!{ind roof cons t ruc ted of l ightweight materials an d Cha'pt'erl"l'~-Nu-mber-sin--btTackVe-~fol-]owing te---xtrefer t o t h - e . ~ : : : : ":ii~igne~!:~o re l i evea t a m ! n i m u m pressure. Tile wa l l s and roof

. . . . . . . :~ A . . . . .4.... ~ .:.._.'~-'.: x:.::i:~::':~:: ~.l~lould De des igned to wttllstand expected winds torm torces. r e f e , ~ , , ~ ~, ,~FV~.,~,., , . . ":.:.:"" "?:-:':."~: "S:,. ".~- N u m b e r s in brackets following text refer to die list o.g...~.~.Fenc~:,,,. "~ ~ f l a t r r a t i o n Pronatration of a combus t ion zone at a velocity dmt

i " F .:~:.-...x:~:::::::.. 4:..:::::::::£:::::::-~ ":':: ~.. - o • r o- • • ~ ' n Appendtx . " "~"::-:i.'::::" ":':~j.:}::$~s less than the speed of s o u n d in d ie un reac ted med ium.

.,.,~.:."~!~i~iii!ii~.:]-!i::::, ":'~!~!::...~:. Deflagrat ion Index . IL (See K a and Kst. ) Chapter 1 General ..#" ":"~:...~i'-S.-, ~.'-';..'#

. . . . ~.:..'::, "::i!~iii~. f f Detonation. ProoalLmtion o f a combus t ion zone at a velocity fllat %Si:!::'::., :~::~., ~s greater than die speed of s o u n d m dae unreac ted m e d m m .

1-1.1. Tins g m d e a p p h e s t o die destgn, Iocat, on, ~ l a t a ~ Dust . Any finely divided solid, 420 nficrons or less in d iameter m m n t e n a n c e , a n d use oi aewces anct systems m a t wfl¢ tile d e ~ a ~ a f i o n ( tha t is, m.ater,ml capable of pass ing flarough a U.S. No. 40 combus t ion gases a n d pressures resul t ing f rom a widfin an enc losure so d~at s t ructural and mechanic~ '~damage is s t andard steve,t. minimized. The deflagrat ion can result f rom fine ignition of a f l ammable gas, mist, or combust ib le dust.

1-1.2 This g u i d e is considered as a c o m p a n i o n d o c u m e n t to NFPA 69, Standard on Explosion Prevention Systems , which covers explosion prevent ion measures and can be u s e d i n place of or in conjunct ion widl NFPA 68. T he choice o f die mos t effective and reliable m e a n s for explosion control shou l d be based on an evalttation dlat includes the specific condi t ions of the hazard and objectives of protection. Vent ing of deflagrat ions will only minimize file damage resul t ing f rom combust ion .

1-1.3 This guide does not apply to detonat ions , bulk autoigni t ion of gases, or u n c o n f i n e d deflagrations, such as open-air or vapor c loud explosions.

1-1.4 This guide does n o t apply to devices tha t are des igned to protect s torage vessels against excess internal pressure due to external fire exposure or to exposure to odler beat sources. (See NFPA 30, Flammable and Combustible Liquids Code.)

1-1.5 This guide does not apply to emergency vents for runaway exo the rmic react ions or se l f -decomposi t ion reactions.

Enclosure. A conf ined or partially conf ined volume, for example , a room, building, vessel, silo, bin, pipe, or duct.

Enclosure Strength (Per,). Tile m a x i m u m pressure dial can be widistood by die weakest s truculral e l emen t of the vented enclosure which is des igna ted not to fail.

Explosion. The burs t ing or rup tu re of an enclosure or a conta iner due to tile deve lopmen t of internal pressure f rom a deflagration.

Flame Speed. The speed of a f lame f ront relative to a f ixed reference point. Flame speed is d e p e n d e n t on turbulence , file e q u i p m e n t geometry, a n a t h e fundamen ta l bu rn ing velocity.

F lammable Limits. The m i n i m u m a n d m a x i m u m concent ra t ions of a combust ib le material, in a h o m o g e n e o u s mixture ~4th a gaseous oxidizer, tha t will propagate a flame.

F lammable Range. Tile range o f concent ra t ions between die lower and uppe r f lammable limits.

670

N F P A 6 8 - - A 9 8 R O P

Flash Point. T h e m i n i m u m tempera tu re at which a liquid gives o f fvapor in sufficient concent ra t ion to fo rm an ignitable mixture with air nea r die surface o f the liquid, as specified by test.

Fundamen ta l Burn ing Velocity. The bu rn i ng velocity of a laminar f lame u n d e r stated condi t ions of composi t ion, t empera tu re , a n d pressure of the u n b u r n e d gas.

Gas. T h e state of mat ter character ized by comple te molecular mobility a n d un l imi ted expansion. Used synonymously with the t e r l n "vapor."

Hybr id Mixture . A mix ture of a f l ammable gas with e i ther a combust ib le dus t or a combust ib le mist.

K G. The deflagrat ion index of a gas cloud as described in 2-2.3.

KSt. The def lagmt ion index o f a dus t c loud as descr ibed in 2-2.3.

Listed.* Equ ipment , materials, or services inc luded in a list publ i shed by an organizat ion acceptable to the anthor i ty having

irisdiction and conce rned witi~ evaluation o f products or services tha t main ta ins per iodic inspect ion of p roduc t ion o f listed e q u i p m e n t or materials or periodic evaluation of services a n d whose listing states ei ther tha t the equ ipment , material , or service meets identif ied s tandards or has been tested a n d f o u n d suitable for a sped f i ed purpose .

Lower F lammable Limit (LFL). T h e lower f lammable limit is the lowest concen t ra t ion o f mater ia l dmt will p ropagate a f lame f rom m~ ignition source t h rough a mix ture of f l ammable gas or combust ib le dus t d ispers ion wida a gaseous oxidizer. LFL is also known ,as m i n i m u m explosible concent ra t ion , MEC.

M a x i m u m Pressure (P ). T h e m a x i m u m pressure developed m a x

in a conta ined def lagrat ion for an o p t i m u m mixture .

Max imum Rate o f Pressure Rise . T h e m a l ~ ' m u ~ !

rate o f pressure rise is the s lope o f the s teepest part ofthe %~ recorded d u d n g deflag~.+o..n i ~ i ' ~ pressure-versus-t ime curve in

closed vessel. (See Appendix B.) ~ 1 ~ " ~

Minimum Ignition Energy (MIE). T he mi~*' . .~m amoug~" !~ energy released at a po in t in a combustibFi~: ~ r e tha t wl~-cause! "~' f lame propagat ion away f rom tha t point, u n d , ~ e d te.~: condit ions. T h e lowest value of t he m i n i m u m i~ is , ~ e !~gy f o u n d at a certain o p t i m u m mixture . It is this valu/~.~;.'~:.gl ; o p t i m u m mixture , tha t is usually quo ted as the m i n i n ~ ignit ion energy.

Mist. A dispersion of f ine liquid drople ts in a gaseous med ium.

O p t i m u m Mixture . A specific mix ture of fuel a n d oxidant that yields the mos t rapid combus t ion in t e rms of a specific measu red quant i ty or tha t has dae lowest value of the m i n i m u m ignit ion energy or dlat p roduces the m a x i m u m deflagrat ion pressure. The o p t i m u m mix ture is no t always the same for each combus t ion proper ty measured .

Oxidant . Any gaseous material tha t can react with a fuel (ei ther gas, dust, or mist) to p roduce combust ion . Oxygen in air is the most c o m m o n oxidant .

Rate o f Pregsure Rise . T im increase in pressure divided

by die t ime interval requi red for fl~at increase to occur.

Reduced P r e ~ u r e (Pred). T h e n~aximum pressure developed in a

vented enclosure du r ing a vented def lagrat ion.

Static Activation Pressure (Pstat). Pressure daat activates a vent

closure when the pressure is increased slowly (with a rate of pressure rise less than 0.1 bar / ra in . )

Stoichiometr ic Mixture. A mixture of a combust ib le material and an ox idan t in which the oxidant concent ra t ion is j u s t sufficient to completely oxidize tile fuel.

Ul t imate Strength. The pressure tha t results in the destructive failure of the weakest c o m p o n e n t of an enclosure.

U p p e r F lammable Limit (UFL). T h e u p p e r f lammable limit is the h ighes t concent ra t ion of a material tha t will p ropaga t e a f lame f rom an ignition source t h r o u g h a mix ture o f f l ammable gas or combust ib le dus t d ispers ion with a gaseous oxidizer.

Vapor. See Gas.

Vent. An open ing in the enclosure to relieve the developing pressure f rom a deflagration.

Vent Closure. A pressure-rel ieving cover placed over a vent.

1-5 Convers ion Factors. T h e conversion factors in Table 1-5 will be useful in u n d e r s t a n d i n g the data p resen ted in this guide.

T a b l e 1-5 C o n v e r s i o n F a c t o r s Length 1 m = 3.28

= 39.4 in. .:~iin. "", = 2.54 c m

d :'~$~,', = 30.5 c m ~]~ l " ~ 9 n -6

~: ( m r ~ e t e r ) = 1.00 x 10 m .-'..%':5:- x¢

,.& 1 m 2 = 10.8 2

. f0" s2 . -~ v

"::" ":::i:~:::#" .:Y::::@ 1 in. 2 ~-~ = 6.45 cm V o l ~ 1 liter 3

"~'-'.~. ~ fc 3

~ * , , . ~ . - ' 1 m i~ ~i~ ~ ::::.~"

1 ~ (IJ.S. / ~....~"

Pressure 1 a tmosphere

1 psi 2

1 N e w t o n / m l i a r

Enerlgy

KG and KSt Conversion

Concent ra t io n

1 ki lol~ram/cm 2

1 k i l o ~ a m / m 2

I J 1 Btu i j 1 ba r -me te r / sec

1 psi-f t /sec 3

1 oz. Avoir /f t

= 61 .0 in. = 7.48 U.S. gal

3 = 35.3 = 264 u.s. = 3.78 liters

3 = 231 in.

3 ffi 0.134 = 760 mil l imeters

Mercury ( ram Hg) = 101 kiloPascals (kPa) = 14.7 psi = 1.01 bars = 6.89 kPa = 1.00 Pascal

= 100 kPa = 14.5 psi = 0.987 a tmosphere = 14.2 psi

= 0 205 Ib/ft 2 . . (p = 1.00 Watt-seco~Sa~

1055,1 = 0.738 ft-lb = 47.6 ps i - f t / sec

= 0.21 b a r - m e t e r / s e c 3

= 1000 g / m

1-6 Symbols. For the purpose of this guide, the following symbols have the mean ings given below.

2 2 A - - A r e a , m or ft 2 or in.

A S - - I n t e r n a l surface area o f enclosure, ft 2 2

or m 2 2

A v - - V e n t area, m or ft C - - C o n s t a n t used in vent ing equat ions as def ined in each

specific use. d P / d t - - R a t e of pressure rise, b a r / s e c or ps i / sec K G - - D e f l a g r a d o n index for gases, ba r -m/ sec

671

N F P A 68 - - A98 R O P

K r I R e a c t i o n force constant, Ib KSt --Deflagration index for dusts, bar-m/sec L n ~ L i n e a r dimension of enclosure, m or ft (n=1,2,3) L x --Distance between adjacent vents L/D - -Leng th to diameter ratio, dimensionless LFL --Lower flammable limit, percent by volume for gases,

weight per volume for dusts MIE i M i n i m u m ignition energy, mJ p - -Per imeter of du ct cross section, m or ft P i p r e s s u r e , bar (ga) or psig P --Enclosure strength

e s

Pmax - - Maximum pre~ure developed in an unrented vessel, bar (gauge) or psig

Pred - - Reduced pressure (i.e., the maximum pressure actually developed during a vented deflagration), bar (gauge) or psig

Pstat --Static activation pressure, bar (gauge) or psig dP --Pressure differential, bar or psi S u IFUndamenta l burning velocity, cm/sec Sf - -Flame speed, cm/sec tf --Durat ion of pressure pulse, sec UFL - - U p p e r flammable limit, percent by volume

3 3 V IVOlume, m or ft

NOTE: All pressures are gauge pressure unless otherwise specified.

Chapter 2 Fundamentals of Deflagration

2-1 Scope. This chapter presents tile essential points pertaining to deflagrations in air which result in the rapid development of pressure in enclosures.

2-2 General.

2-2.1 Deflagration Requirements. Tiae following are necessa~.,,$~ initiate a deflagration: ....~.:?.~'~?.,

(a) Fuel concentration within flammable limits . . . . ~

Oxidant concentration sufficient to support c o m b u s t ~ . (b) ...~.:!::: ' ~ t ~

"#~ "~" ~ - " ~ (c) Presence of an ignition source .... ~.-':~;?.'~..%. ~ "

....,.-$~.,:. ~:~$:~.- 2-2.2 Deflagration P r e s s u r e . . . . . ~ i ~ . . ~ $ ! : : ~ : . ~:~:':~.,x.-.~ • .~ ~...-..:. ~:-:::,

~'~s 2-2.2.1 The deflagration pressure, P, in a c l o s e t " ~ e , l ~': related to the temperature, T, and molar quantity~-~.~r~'~e following ideal gas law: ~'.'.-"$'::"

4:

nRT P = ~ (1)

V

Where: R = universal gas constant

2-2.2.2 The maximum deflagration pressure and rate of pressure rise are determined by test, over a range of fuel concentrations. (See Appendix B.) The value of the Pmax for most ordinary fuels is

in dae range of 6 to 10 times flae absolute pressure at the time of ignition.

2-2.3 The maximumpressure generated and the maximum rate of pressure rise are key factors in the design of deflagration protection systems. The key characteristics of closed-vessel deflagrations are the maximum pressure attained, Pmax, and the maximum rate of pressure rise. The deflagration index, K, is computed from the maximum rate of pressure rise attained by combustion in a closed vessel with volume, V as follows:

(2) \ a t ) m a x

The value of will be a maximum for a particular fuel

t / l ax

concentration, referred to as die "optimum" concentration. (See examples in Figure 2-2.3.) The deflagration index is denoted as K G for gases and KSt for dusts.

Table 2-2.$ Hazard Classes of Dust Deflah~ratlons Hazard Class 3 3

KSt Pmax bar-m/sec bar [{au~e

St-1 < 200 ~ 10 St-2 201 - 300 < 10 St-3 > 300 < 12

N o t e s :

1 The application of the nomographs is limited to an upper KSt

value of 600. 2

See Appendix E (~.'..'i~,~amples of KSt values.

K and Pm,~ ~:".'.~..termined in approximately spherical ~t ...:~::" -::.:~::~::~ calibrated te~: i~ess~ ~ at least 20-L (5.3 gal) capactty as per ASTM E I ~ s . '-~!~,

.~!%..... ~, " ~ . ~ ~,

O~

el.

6

4

3

2

1

0 I I I I 1000 2 0 0 0 3 0 0 0 40O0

Dust concentration (g/rrP) 5000

F'~[ure 2-2.3 Variation of deflagration pressure and deflagration index with concentration for several dust~ (Adapted from

Ref. 109)

2-2.4 Burning Velocity and Flame Speed.

2-2.4.1 Tile burning velodty is file rate of flame propagation relative to file velocity of file unburned gas ahead of it. The fundamental burning velocity, Su, is the burning velocity for a laminar flame under stated conditions of composition, temperature and pressure of file unburned gas. Values of S u have been measured and published for many gases. (See Appendix C.)

2-2.4.2 Flame speed, Sfl is the speed of a flame front relative to a fixed reference point. Its minimum value is equal to the fundamental burning velocity times an expansion factor equal to file ratio of the density of the unburned gas to that of the burned gas.

672

N F P A 68 ~ A 9 8 R O P

2-2.5 Deflagrations occurring in enclosures flint are not s trong (bar) enough to accommodate the pressure could result in explosions, "~ damage to fl~e system, and injury to nearby personnel .

r l

2-2.6 Venting is one means o f limiting d~e pressure genera ted in an enclosure by a deflagration. By releasing expanding gases

t o darough an open ing engineered for tile purpose, it is possible to ~ 5 limit d~e pressure genera ted to a reduced maximum pressure, o_ Pred, that is below flint which would cause unacceptable damage E to flue enclosure. .~_

2-3 Fuel. ~:

2-3.1 General. Any material capable of reacting rapidly and exofl~ermically wid~ an oxidizing medium can be classified as a (bar/s) fi~el. A fuel can be in a gas, liquid, or solid phase. Liquid fuels to when dispersed in air as fine mists, solid fuels when dispersed in

\ ! vc os,

_ Baking flour Methyl

, , ~ - ~ ,,,,L/cellulose

~ + ~ " " , , -1% 0 )

~ 200 air as dusts, and Iwbrid mixtures pose similar deflagration risks as gaseous fuels•

2-3.2 Concentrat ion. The concentrat ion of a gaseous fuel in air is E=._~ 100 usually expressed in terms of volume percent , vol%, or mole percent , mol%. Tile concentrat ions of dispersed dusts and mists .~

[ - ' a , ~ J 1 m ~ - - vessel - - , " ~ [ high ignition energy

are usually expressed in units of mass per uni t volume such as ~ 0 • - , 3, ~ "[~.:.. 200 300

grams per cubic meter ( g / m ) . A::':":<::~: . . . . . . :.':K::'. Mealan value (mlcronsl

2-3.3 Flammable Gas. ~ : ~ . ~#iii "::~!~!i~::

2-3.3.1 There are concentrat ions of f lammable gases in air, below _ _ ~ . . t r t t _ ~ . . . . . . : . , ~ ~ : ~ ~ t . . . . . . . . . . . . . _ . _ and above which fl'~ey v~ll not burn. These concentrat ions are die A.':. ss{~r"e d r~.,,..~.~ rate o . . . . . • . . .::::~pre an m a m t ressure rise. t~l f lammable limits, which consist of due lower f lammable limit, LFL, ..:~--" ...::. ~ P ,and tile upper f lammable hmlt, UFL. Between dlese ..:$~$!:!:'::.. ~:~:~:t.:, ~" concentrai ibn limits, ignition and f a m e propagat ion are possible. <!::" "":~ii:'..::,<:~::;-:~i~:. lgnition of mixtures outside dlese composit ion limits fails because ~ . insufficient energy is given off to beat the adjacent unburned gases .. 600 ",'L.~.~.,:::.. ~ i i i to d~eir ignition temperature . Lower and upper f lammable limits %~:~::<. "%!!ii!:~i~f"::' are de te rmined by test and are test medaod dependent• "~'-'i""" i~i~i~i~iE~:~.-..:.. .....:..:..s:" Flammable limits for numerous fuels are oublisbed. See NFPA ".~:,<. " ~ ! :~.-.'~:" . . . . . ~::- d::'-:" ~ :~" $25, Proper t ies o f F l a m m a b l e L = q m d s a n d V o t a t d e So l id s . . . . . "::!::., i~'::" 501~ ::, [ 2-3.3.2 The mixture composit ions taut are observed to s u p p ~ ! ~ : "~ii~i ~. ".:~ / maximum pressure P and d~e maximum rate of pressl~e ~ t ~ ~ , I [ ' max' ,::~....~ ~:~::' , ~.#.," ~ /

( dP'~ ~':":::'~-....-::. ~ i ~ i ~" 400 / / ~ / , for a deflagradon are commonly on d~e fuel-r~'i$'.'|ide "~.~'~'. > / k. dt )== . ~ : ~ . . ~"~i!~i~i~. ...... ~ ! / of ti~e sto~clnometr~c mixture. Note flint ti~e c~4Rment r~ . . fo r t l ~ . $ +" c / maximum rate of pressure rise and tile con..gg,."~ation f o ~ : Pm~:" =* 300 can differ. " "::~.~::, !i-"::~! "= ~ / • ~ : : " ":': ':" " ' :~ '~:::::: 0

""$'::':.:::'.::.-, /~: "E ":::.-%'.:::::, :~::" o~

2-$.4 Combustible Dust. "::':-~i'-;~':. ..-'~-~' ""

2-3.4.1 Solid particulates smaller dmn 420 microns ( . ~ a b l e of ._~ passing d~rougb a U.S. No. 40 s tandard sieve) are cl~sified as .--q / dusts. The "fineness" of a particular dust is characterized by ~ particle size distribution. The maximum pressure and KSt increase 100 with a decrease in d~e dust particle size, as shown in Figure 2-$.4.1.

2-3.4.2 Particle Size.

2-3.4.2.1 Dust particle size can be reduced as a result of attrition or. size segregation dur ing material, handl ing and processin, g. This might lead to file g radua l reduc t ton of the average particle size of the material being bandied and can increase the deflagration hazard, of the dust.. Minimum. ignition . energy is strongly d e p e n d e n t on parucle size. [1] See Ftgure 2-3.4.2 for an illustration of dfis effect.

2-3.4.2.2 A combustible dust dispersed in a gaseous oxidizer and subjected to an ignition source does not ,always deflagrate. The ability of a mixture to propagate a deflagration depends on factors such ,as particle size, volatile con ten t of solid particles, and moisture content .

0 I I I I 0 4 0 80 120 160 200

Average particle size (microns)

a g F i • g ' u r e 2-3.4.2 Ef fec t o f average particle d i a m e t e r o f a typical cultural dus t on the m i n i m u m ignit ion energy . ( U n p u b l i s h e d

data c o u r t e s y o f U .S . M i n e Safety and Hea l th Adminis trat ion . )

2-3.4.4 There is a min imum dust cloud concentrat ion, commonly known as the lower f lammable limit, LFL, and also as tile min imum explosible concentrat ion, MEC, that will suppor t flame propagation. Tile LFL of a dust is d e p e n d e n t on its composit ion and particle size distribution. Large particles participate inetticientiy in die deflagration process.

2-3.4.5 Combustible dusts accumulated on surfaces in process areas can become airborne by sudden air movement or mechanical disturbance. Dusts can pass darough ruptured tilter elements. In these instances, a combustible concentrat ion of dispersed dust can become established wbere normally it would no t be present.

673

N F P A 6 8 - - A 9 8 R O P

2-3.4.6 Combustible dusts do not, for most practical purposes, exhibit upper flammable limits in air. This is a consequence of the flame propagation mechanism in dust clouds. Thus, deflagrations cannot usually be prevented by maintaining high dust cloud concentrations. 2-3.4.7 The combustion properties of a dust depend on its chemical and physical characteristics. The use of published dust flammability data can result in an inadequate vent design if the particular dust being processed has a smaller mean particle size or other property differences from the dust for which data are given. The particle shape is also a consideration in the deflagration properties of a dust. It is preferable to verify the flammability characteristics of a particular dust by test. (See B-5.)

2-3.5 Hybrid Mixture.

2-3.5.1 The presence of a flammable gas in a dust-air mixture reduces the apparent lower flammable limit and ignition energy. The effect can be considerable and can occur even though the gas is below its lower flammable limit and the dust is below its lower flammable limit. Careful evaluation of the ignition and deflagration characteristics of the specific mixtures is required.

(Figure 2-3.5.1 will be shown in the Report on Comments.)

2-3.5.2 It has been shown that the introduction of a flammable gas into a cloud of dust that would normally he a minimal deflagration hazard can result in a hybrid mixture with increased maximum

pressure, Pmax, and maximum rate of pressure rise, ( d P ) m a x .

Ati example of this phenomenon is the combustion of polyvinyl chloride dust in a gas mixture. (See Fig~zre 2-3.5.2.)

® [bar] / I

g ~ s ~ .,.:.,.~.~ . . . . #.$-:v~.-.~$!~: / -#" %!

o _ ~:::::::~::::-~: :::::::::::::::::::::::::::::::::::::::::::::::: ~:@~#

1 ,

[bar/s] ~ ~ , ~ : ~ e O ~ i

"=~ . t ~ 7 volo/o ",~'-~ z ;o ii ,.volo/o

"~ ,~ 1~)1% n" 0 ~ ~without methane

0 500 [g/m a] PVC dust concentration

~gure 2-3.5.2 Deflagration data for hybrid mixtures of polyvinyl chloride dust and methane gas in air. [4]

2-3.5.3 Situations where hybrid mixtures can occur in industrial processes include fluidized bed dryers drying solvent-wet combustible dusts, desorption of combustible solvent and monomer vapors from polymers, and coal processing operations.

2-3.6 Mist.

2-3.6.1 A mist of flammable or combustible liquids has deflagration characteristics analogous to dusts. The LFL for dispersed liquid mists varies with droplet size in a manner analogous to particle size for dusts. The determination of these deflagration characteristics are complicated by droplet dispersion, coalescence, and settling. A typical LFL for a fine hydrocarbon mist is 40 to 5 0 g / m 3, which is approximately equal to the LFL for combustible hydrocarbon gases in air at room temperature. Mists of combustible liquids can be ignited at initial temperatures well below the liquid's flash point.

2-3.6.2 Deflagration of Mists of Combustible Liquids. Combustible mists will ignite not only at temperatures above the flash point temperature of the liquid, but also at temperatures below the flash point temperature. [62, 63, 64, 65] The design of

deflagration venting for many combustible mists can be based on Eq. 16 in 6-2.2 using the propane K G of 100 bar-m/see.

2-4 Oxidant.

2-4.1 The oxidant for a deflagration is normally the oxygen in air. Oxygen concentrations greater than 21 percent tend to increase the fundamental burning velocity and increase d~e probability of transition to detonation. Conversely, concentrations less than 21 percent tend to decrease the rate of combustion. There is for most fuels a limiting oxygen concentration below which combustion will not occur. (See NFPA 69, Standard on Explosion Prevention Systems.)

2-4.2 Substances other than oxygen can act as oxidants. While it is recognized that deflagrations involving reaction of any of a wide variety of fuels and oxidizing agents (oxygen, chlorine, fluorine, oxides of nitrogen, and others) are possible, discussion of deflagration in dais guide is confined to those cases where the oxidizing medium is normal atmospheric air consisting of 21 vol% oxygen unless specifically noted otherwise.

2-5 Inert Material.

2-5.1 Inert Gases. concentration. inerting gases. S~stems.

2-5.2 Ine

2-5.~.~.~ J

d u s t ~ rise and

; can be used to reduce the oxidant carbon dioxide are commonly used Standard on Explosion Prevention

e'~-:powder can.gi~duce the combustibility of a dust by the i ~ . e a t . AddiSon of inert powder to a combustible ~'t- ~ t u r e will reduce die maximum rate of' pressure

|~.:increase tlletile m'minimum concentration of combustible ces~..9..~.-':ignition. See Figure 2-5.2 for an example of the f a d ~ inert powder. The amount of inert powder

• to~.~event a deflagration is large, with concentrations of ~.'~i~ing required. £..:-

"l~ome inert powders in small concentrations, such as silica, counterproductive because d~ey can increase the ibility of the combustible dust.

40 , , , , , , , , 00 t 20

13 i

t~

fit.

7 i

6

5

4

3

2

1 10

i i i m i i i

20 30 40 50 60 70 80 90 Limestone dust in mixture (%)

Figure 2-5.2 Effect of added inert dust on deflagratlon data for coal dust in air. [ l l01

2-5.3 Presence of Moisture (Water Content).

2-5.3.1 Increased moisture content of a dust can increase tile minimum required energy for ignition, ignition temperature, and

6 7 4

N F P A 68 - - A 9 8 R O P

f lammable limit. Increased moisture content o f a dust can decrease dae maximum rate of pressure rise. Moisture in a dust can inhibit accumulat ion of electrostatic charges.

2-5.3.2 Moisture in tim air (humidity) surrounding a dust particle laas no significant effect on a deflagration once ignition has occurred.

2-5.3.3 A moisture addit ion process should not be used as dae basis for reducing file size of deflagration vents. The quantity of moisture to prevent ignition, o f a dust. by most common sources, normally would result m fide dust being so damp dial a cloud would not readily form. Material containing dais much moisture will usually cause processing difficulties.

2-6 Ignition Source. Some types of ignition sources include electrical (arcs, sparks, and electrostatic discharges), mechanical (friction, grinding, and impact) , ho t surfaces (for example, overhea tedbear ings) , and flames (welding torches and so forfla).

2-6.1" One measure of fide ease of ignition of a gas, dust or hybrid mixture is its min imum ignition energy, MIE. The min imum iflg~lanition energy is typically less flaan 1 mJ for gases and often less

100 mJ for dnsts. Minimum ignition energies are repor ted for some gases and dust clouds. [7-17, 90, 92]

2-6.2 An ignition source such as a spark or a f lame can travel f rom one enclosure to anofller. A gr inding spark (hot glowing particle) can travel some distance and can ignite a f lammable mixture anywhere along die way. Similarly, s t ronger ignition sources, such as f i c h e j e t ignitions, deserve special consideration. A flame produced by an ignition source in one enclosure can become a much larger ignition source if it enters anoflmr enclosure. This increase in fide energy of tim ignition source can increase fide maximum rate of pressure rise developed during a deflagration.

40

3O

8 ,=

20 .= O=

E

10

I I t / 758 ° F 342 ° F 1 3 3 ° /

/ Po = 4.8 bars a b s ~ / / "

Po = 2.8 ba/m absolute J

bars absolute

1720c 55oc

Reciprocal of initial temperature 1 OKx 10 -2 2-6.3 The location of die ignition source widfin an enclosure can affect dae rate of pressure rise. For splaerical enclosures, ignition at the center o fdae enclosure will resuh in dae highest rate o f pressure rise. For e longated enclosures, ignition near the no~ vented end of an elongated enclosure results in a higher r a ~ pressure rise allan ignition in dae center of the enclosure..,x~ "~ '~

2-6.4 Simultaneous multiple ignition sources will

deflagration [daat is, greater

2-7 Effect o f Initial Temperature and Pre~su/#~ii~y chang'i.g':~in d ie initial absolute pressure of dae fue l /ox idan t m i x ~ , a t a gi.~in initial temperature will p roduce a propor t ionate c ~ e ¢ ~ P " f l l e maximum pressure developed by a deflagration of ttl~{.~xture in a closed vessel. Conversely, any change in d ie initial a~golute temperature at a given initial pressure will produce an inverse change in die maximum pressure attained. (See Figure 2-7.) Tiffs effect can be very substantial wida vapor explosions at cryogenic temperatures.

2-8 Effect o f Turbulence.

2-8.1 Turbulence causes flames to become stretched which increases dae ne t flame surface area exposed to u n b u r n e d materials leading to increased flame speed.

2-8.2 Initial turbulence in closed vessels results in h igher rates of pressures rise, and some increase in maximum pressure dlan would be obtained ff dae fue l /ox idan t mixture were at initially

~,~ iescent condit ions resulting in increased required vent area. dis is shown in Figure 2-8.2, including dae effect of fuel

concentrat ion.

2-8.3 Turbulence is also created dur ing deflagrafion as gases and dusts move by obstacles widlin dae enclosure. In e longated enclosures, such as ducts, turbulence genera t ion is enhanced, flame speeds can increase to very high values causing transition from deflagration to detonat ion. Venting, because of die flow of unbu r ned gases darough dae vent opening, can cause turbulence both inside and outside the enclosure.

o

¢¢

O .

E .E

I[

:Effect of initial temperature on the maximum essure of near-stoich]ometrlc mixtures of methane-

alr at three initial pressures, P0. [19]

2OOO Maximum pressure ( t u r b u l e ~ ~

6 1500 Q.

4 1000 ~¢

Maximum rate o_

2 ( t u r b u l e n t / ~ 500 n-~ ~ ®

/ Maximum rate '~ (nonturbulent)

o o 4 6 8 10 12 14

Methane (percent)

Figure 2-8.2 Effect of turbulence on the maximum pressure and rate of pressure rise for methane-air mixtures. (Adapted from

references 20 and 21.)

675

N F P A 6 8 1 A 9 8 R O P

Chapter 3 Fundamentals o f Venting o f Deflagrations

3-1 Basic Concepts.

3-1.1 A deflagration vent is an opening in an enclosure through which material expands and flows, thus relieving pressure. If no venting is provided, d ie maximum pressures developed during a deflagration of an opt imum fuel /a i r mixture will typically be between 6 and 10 times the initial absolute pressure. In many cases, it is impractical and economically prohibitive to construct an enclosure that will withstand or contain such pressures. In some cases, however, it is possible to design for conta inment of a deflagsation. (See NFPA 69, Standard on Explosion Prevention s~a~s. )

3-1.2 Nothing in this guide is meant to prohibi t tile use of an enclosure with relieving wails, or roof, as long as the potential dmnage and potential for injury are addressed.

3-1.3 Tile vent ,area can be reduced from that indicated in Chapters 4 through 7 if large-scale tests show that tile resulting damage is acceptable to tile user and anthorities having jur isdict ion.

3-1.4 Tile design of deflagration vents and vent closures requires consideration of many variables, only some of which have been investigated in depth. Tile calculated vent area will d e p e n d on several factors including the size and strength of tile enclosure, the characteristics of the fue l /ox idan t mixture, and tile design of the vent itself. Tile design techniques use one or more empirical factors that ,'allow simplified expressions for vent areal The design factors are tile result of analyses of numerous actual venting incidents and venting tests which have allowed certain correlations to be made. The user o f this guide is urged to give special at tention to all precautionary statements.

3-1.5 The rate of pressure rise is an important parameter used in the design of deflagration venting. A rapid rate of rise means that only a short per iod of time is available for successfiil venting. Conversely, a slower rate of rise permits tile venting to proce, ,,. more slowly, yet still be effective. In terms of required yen ~i~ tile more rapid the rate of rise, the greater the area need.~ ~ venting to be effective, all o ther factors being equal. ~:x~'~"':"'i ~ "

3-1.6 Vents are provided on an enclosure to limit l ~re deve lopment . . . . . . (Pred) to a level acceptable to th%:.~#~. ~t ~,~,~

an thon ty havmgjunsd tcuon . It can be at a I.¢.t~i w] • . g . - : . : . : .

to the enclosure ts hkely, or where some d ~ . . $ , of deformat ion is tolerable. ~: '::'~.~.::, %..'...-:..~ 3-1.7 Tile larger the vent area provided, the Iowe]~'~ ie reduced pressure for a given static activation press~ ~ e vent closure. ( )pen vents are more effective than cover~ ~ts. Vents with lightweight closures are more responsive than tl ~se with heavy closures.

3-2 Consequences of a Deflagration.

3-2.1 Damage can result should a deflagration occur in any enclosure that is too weak to withstand tile pressure f rom a deflagration. For example, an ordinary masonry wall [g-in. (20- cm) brick or concrete block, 10 ft (3 m) high] cannot withstand a pressure difference f rom one side to the o ther of much more than 0.5 psig (0.03 bar ga). Unless an enclosure is des igned to withstand file expected deflagration pressure, venting or a deflagration suppression system should be considered. (See NFPA 69, Standard on Explosion Prevention S3stems. )

3-2.2 Limited data are available on file reaction forces exper ienced by file structural e lements of an enclosure during a deflagration. Designs should be based on the specifics of each enclosure, its material of construction, its resistance to mechanical and thermal shock, and the effects of vents (including file magnitude and durat ion of consequential darust forces) should be considered. The enclosure design should be based on widlstanding file maximum pressure attained dur ing venting, P red of d ie deflagration.

3-2.3 Exposure from the Venting Process. Flames and pressure waves emerging from an enclosure dur ing the venting process can injure personnel , ignite o ther combustibles in die vicinity, result in ensuing fires or secondary explosions, and result in pressure

damage to adjacent buildings or equipment . For a given quantity of combustible mixture, tile amount that will be expelled from file vent, and file thermal and pressure damage that occurs outside of the enclosure, will d e p e n d on d~e volume of ti~e enclosure, the vent opening pressure, and file magnitude of Pred" For a given

enclosure and a given quantity of combustible mixture, a lower vent open ing pressure will result in more unburned material being discharged th rough file vent, resulting in a larger fireball outside tile enclosure. A higher vent opening pressure will result in more combustion taking place inside tile enclosure prior to the vent open ing and higher velocity through the vent. (See Section 7-7.)

3-2.4 Deflagration venting will generate pressures outside the vented enclosure. This pressure is caused by venting file primary deflagration inside tile enclosure and the secondary deflagration outside the enclosure.

3-2.5 Location of Deflagration Vents Relative to Air Intake~ Deflagration vents should no t be located in such positions that tile vented material can he picked up by air intakes.

3-3 Enclosure Strength.

3-3.1 The force e x e ~ time. Work by H ~ ' " ~ i enclosure is a s s ~ to pressure, P ~ - ~ s ' ~ l i e

on an enclosure by a deflagration varies in ~..d Karabinis [SO] indicates that file r~spond as if the peak deflagration :1 as a static load, provided some

~but not catastrophic failure) can be

3-3..~.~n d ~ i n g an en~i~sure to prevent catastrophic failure w t ~ . a . ~ . n g some inelastic deformation, tile normal dead and I ~ , ~ : . ~ ' ~ o u l d no t be relied on to provide restraint.

mbers be to Structur~i~i~.,.:~ ~ should des igned suppor t tile total load.

' "~ ,$ D e s i g n ' ~ ' s s u r e Selection Criteria. "~....:...: ..~.~: 37~$~ : ~"~ 'mon l y , design standards allow P r e d to he selected up f two-tfitrds tile enclosure s t rength for equipment , provided

..~'f.ogmation of tile equ ipment can be tolerated.

f ~ . 3 . 2 " The design pressure of a ductile high strength enclosure is mlected based on whether:

(a) Pe rmanen t deformation, bu t no t rupture, of the enclosure can be accepted

(b) Pe rmanen t deformation of the enclosure cannot be accepted

1 .SPre a For P,, - (3)

F.

15Pre d P a = ~

Fy Where:

P~ = Enclosure design pressure, psi (bar), to resist P~a

P ~ = Maximum pressure developed in a vented enclosure, psi (bar)

F u = Ratio of ultimate stress of the enclosure to the allowable stress of die enclosure per ASME code

~1 = Ratio of file yield stress of the enclosure to the owable stress of file materials of construction of file enclosure

per ASME

3-$.4 Ductile design considerations should be used. For materials subject to brittle failure, such as cast iron, special reinforcing should be considered, ff such reinforcing is not used, tile maximum allowable design stress should not exceed 25 percent of the ultimate strength.

5-4 Vent Variables.

34.1 Vent Location, Size, and Shape. The Pred developed in a vented enclosure decreases as tile available vent area increases, ff tile enclosure is small and relatively symmetrical, one large vent

676

N F P A 6 8 ~ A 9 8 R O P

can be jus t as effective as several small vents of equal combined area. For large enclosures, location of multiple vents to adtieve uniform coverage of the enclosure surface to die greatest extent practicable is recommended . Rectangular vents are as effective as square or circular vents of equal area.

3-4.2 Inertia o f Vent Closure. The free area of a vent does not become fi t ly effective in relieving pressure until file vent closure moves completely out of the way of the vent opening. Until dais occurs, tbe closure obstructs the combustion gases issuing from tile vent.

3-4.3 The greater the mass of the closure, file longer tile closure takes to completely clear d~e vent opening for a given vent opening pressure. Conversely, closures of low mass move away from the vent opening more quickly, and venting is more effective.

$-4.4 The addit ion of a vent discharge duct can substantially increase the pressure developed in a vented enclosure. (See Section 5-4.)

3-5 Vent Operation.

3-5.1 Vents should fimction dependably. Closures should not be h indered by deposits of snow, ice, paint, corrosion, or debris, or by buildup of deposits on their inside surfaces. Closures should not be bonded to d ie enclosure by accumulations of paint. Materials used should be chosen to minimize corrosion. Clear space should be mainta ined on both sides of file vent to enable operation without restriction and without impeding die free flow through tile vent.

3-5.2 Vent closures should be mainta ined in accordance with Chapter 10 and manufacturers ' recommendat ions .

3-6 Basic Considerat ions for Venting.

3-6.1 Chapters 4 through 8 provide guidance on design of vents. Chapter 4 addresses low strength enclosures capable of withstanding pressures of not more than 1.5J.apsig (0.1 bar : t ga ,, Higher s trength structures are addressed by file equations ~ i.".-:.:~.":~. Chapters 5, 6, and 7, with added conditions for elongate.d.?'gh ~!i~ and ducted vents in Chapter 8. ,,'#~iii~.

ict 3-6.2 Tbe equations in Chapters 4 through 7 do not exactiy'~ file required vent area for all enclosures under ~ t i o n s ~, Certain [44, 98, 991 indicate that the g a s . ~ ' ~ ' t i ~ c ~ data not indicate file sufficient venting in every ~ . Also, teg~:~.i..-~: ~-'.-'~ involving extreme levels of both congestidaC/-"~:i.nitial tur .~ 6'" demonstrate that pressures exceeding flmse i n ~ . . t . e d by d(~ ; equations can occur. [42, 87] For d*e present, Y/~.g..ver, ..~:i use o f the equations is r e c o m m e n d e d on the basis of s ' ~ . . . ~ l industrial experience. "~i.*'

4":"::" 3-6.3 Tile material discharged f rom an enclosure during the venting of a deflagration should be directed outside to a safe location. Property damage and injury t o p e r s o n n e l due to material ejection during venting can be m i n i m i z e d o r avoided by locating vented equ ipmen t outside buildings away from normally occupied areas. (See 3-2.3.)

3-6.4 Tile vent opening should be free and clear and not impeded, ff the vent discharges into a congested area, the pressure inside the vented enclosure will increase. T h e major blast pressure could be caused by ignition of unburned gases or dusts outside tile enclosure.

$-6.4.1 It will be necessary in some cases to provide restraining devices to keep vent panels or closures from becoming missile hazards.

3-6.4.2 Restraining devices should not impede tile operation of the vent or vent closure device. (See Chapter 9.)

3-6.4.3 An alternative means of protect ion is to provide a barrier.

3-6.5 Appropria te signs should be posted to provide warning as to the location of a vent.

"$-6.6 If vents are fi t ted with closure devices timt do not remain open after activation (i.e., self-closing), it should be recognized that a vacuum could be created when gases within the enclosure cool.

$-6.7 Interconnect ions between separate pieces of equ ipment present a special bazard. A typical case is that of two enclosures connec ted by a pipe. Ignition in one enclosure will cause two effects in the second enclosure. Pressure development in the first enclosure will force gas th rough the connec t i ngp i pe into the second enclosure, resulting in an increase in bodi pressure and turbulence. The flame f ront will also be forced through the pipe into die second enclosure, where it will become a very large ignition source. The overall effect will d epend on the relative sizes of the enclosures and file pipe, as well as on d~e length of the pipe. This has been investigated by Bartknecht, who found the effects can be large. Pressures developed in the pipel ine itself can also be quite high, especially if dae deflagration changes to detonation. Where such in terconnect ions are necessary, deflagration isolation devices should be considered, or the in terconnect ions should be vented. Without successful isolation or venting of tile interconnect ion, vent areas calculated on die design bases herein might be inadequate because of creation of high rates of pressure rise. (See References 58 and 66 and NFPA 69, Standard on Explosion Protection Systems.)

3-6.8 Reaction forces resulting from venting should also be considered in the design of file equ ipment and its supports. (See 5-2.9.)

:: . i

ICts used t # ' : ' : ~ $ v e n t e d gases from die vent to the outside ding s l ~ be of noncombust ib le construct ion and lou~l~ffb ~ t a n d the expected P _. Ducts should be as ~..-.-~--::.~..::~:. r ea p.~...j.~, and '~ .~ . . r ab ly should no t have any bends. (See 5-

3-6.9 Ducts u s e d of a building strong e n ou gx!~b

shor t as p ~ 2. 9.) ,::~:., ~-:i~!~:~.~=

3 - 6 r.~ii~ Sit U"~.~..o n s

all. It is ~ " ~ t e t

U~.~gns can oct,~i: in which it is not possible to provide ~ r a t i o n venting as descr ibed in Chapters 4 darough

~de." "~This is not just i t icat ion for providing no venting at ~ t e d flaat the "maximum practical" amount of

.venting be ~ d ~ d , since some venting should reduce file damage i{i~..O..tial. | n~ i i l i t i on , consideration should be given to taller t l ~ ~ E l prevent ion methods. (See NFPA 69, Standard on .J~{C~'.":"5~,raention Systems.) ~:~Y" ~.-.-'7::" %6.1J *:.:.:....:.:...._ Enclosure Wall Effects. Tile reduced pressure, Pred , in a v ~ e d gas deflagration can be significantly reduced in certain .'~tuations by lining the enclosure interior walls with an acoustically absorbing material, such as mineral wool or ceramic fiber blankets. These materials inhibit acoustic f a m e instabilities responsible for high flame speeds and amplified pressure oscillations in deflagrations of initially quiescent gas-air mixtures in unobst ructed enclosures.

3-6.12 It is not possible to successflllly vent a detonation.

3-6.13 The maximum pressure that will be reached dur ing venting, Pred, will always exceed the pressure at which the vent device

releases, Pstat; in some cases it will be much higher. This

maximum pressure is affected by a number of factors. These should be considered when designing the enclosure that will be protected.

3-6.14 Effects o f Higher Inertia Vent Closures. For a given vent area, a greater mass per uni t area (higher inertia) of a vent closure results in a h igher maximum pressure during venting. Similarly, h inged vent closures can increase maximum pressure during file venting process by reducing the rate at which tile available vent area opens with time.

3-6.14.1 A vent closure should have low mass to minimize inertia thereby reducing opening time. If the total mass of a closure divided by the area of the vent opening does not exceed 12.2

k g / m 2 (2.5 Ib/ft2), all vent area correlations presented later in this guide can be used without correction.

3-6.14.2 For gases having a K G value no greater than those for methane or ammonia (and where there are no internal turbulence inducers), file vent area correlations for low st rength enclosures presented later in this guide can be used without correction if the mass of the closure divided by the area of the vent opening does

not exceed 39 k g / m 2 (8 lb / f t2) .

3-6.14.3 Tile effects of higher vent closure inertia and hinged vent closures is de te rmined by testing, and is usually expressed as an

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N F P A 6 8 ~ A 9 8 R O P

efficiency factor (See Section 7-3. 9-3.4.1, and Reference 104.) Design changes can be made to compensate for these inefficiencies by increasing the venting area, by increasing dae enclosure strength, or both. A vent closure should have no counterweights; counterweights add more inertia.

3-6.15 A vent closure should withstand exposure to the materials and process condit ions within the enclosure being protected. It should also widastand ambien t condit ions on the nonprocess side.

3-6.16 A v e n t closure should release at its Pstat or within a

pressure range specified by the vent manufacturer.

3-6.17 A v e n t closure should reliablywitlastand pressure fluctuations dlat are below Pstat" It should also withstand

vibration or o ther mechanical forces to which it could be subjected.

3-6.18 A vent closure should be inspected and properly mainta ined in order to ensure dependable operation. In some cases, this could mean replacing a vent closure. (See Chapter 10.)

3-7 Correlating Parameters for Deflagration Venting.

K G values, in file quiescent state, that are similar to or less than

that of propane.

3-10.1.2 The susceptibility of deflagration to detonat ion transition in a turbulent system increases with increasing values of the fundamenta l burn ing velocity (see Appendix B). Inpart icular , compounds that have K G values close to that of hydrogen are

higldy susceptible to detonat ion when ignited under turbulent conditions.

$-10.2 Enclosure Appurtenances . Internal appur tenances within a vented enclosure can cause turbulence. [55, 102]

3-10.3 Dusts. The values of Pmax and KSt for dusts are

de te rmined from tests conducted on turbulent dust clouds. The equations given herein for calculating vent area for dust deflagrations use the values of Pmax and KSt so determined.

3-11 Deflagratlon o f Mists. The design of deflagration venting for mists can be based on the propane venting equation. For more detail on mists, see Chapter 2.

3-7.1 Tile technical literature reports extensive experimental work on venting of deflagrations in large enclosures. Equations have been developed that can be used for de termining the necessary vent areas for enclosures• [101]

3-7.2 Tile equations superseded techniques based on a linear relationship of vent area to enclosure volume. The area-to-volume techniques for vent sizing are no longer r ecommended in dais

3-12 Venting D e f l a ~ o n s of Flammable Gases Evolved from Solids. In certai~:~f~"~"~es, f lammable gases can evolve f rom solid materials. .~;i~!e solid is combustible and is dispersed in the gas /oxidant ~td~.ik..~.. might be the case in a fluidized bed dryer, a hybrid m.. ix~e res'f~i:~.(See Section 7-8.)

3-13 v . . . . ~ n : " ~ i ~ ! : D e f l ~ , ' ~ ' ~ n Ducts. Most deflagrations of d a b b l e $ ~ mixtures i ~ l e ducts occur at initial internal pr .~g.~. , o~::~arly aunos~i'heric. The venting of deflagrations in su~:h i t '~: . . t~:~l~ussed in Chapter 8. guide•

%i~i~ .... 3-7.3 Consideration of the L /D ratio of enclosures is important in ...:..: .... 3-14 H y b ' ~ i ~ . . . m r e s . Special considerations are given to hybrid die design of deflagration venting. For long pipes or process ducts %~i~S~..m.~tures in ~ l ~ S n 7-8. The propert ies of hybrid mixtures are or low-pressure enclosures wbose L /D ratio is 5 or greater, file ~ i ~ . g . . . v , ely d i~ussed in References 3 and 66. The effective K~t deflagration vent design should be based on the information given "!'~:" va i i f~ : ~ : : : ~ t combustible dusts is raised b the admixture o f a m Chapter 8 ... ~ Y • " " ....::...'~.':::',.:::, "~.'."-"~mbus~ie gas even if the gas concentraf ion is below the lower 3-8 Effects o f Vent Discha e Duets ,..-..~.".'::'~'~:""~i~!~: :"~smmable limit. An alterna(e approach is to conduct tests to

rg . .-::#" "~: ~ i n e the equivalent K q t using worst-case conditions and A~'~i~i~i~i~::.. ~.:...,.....,~ % :..'..-~.:">" . - - . . ,~ ly tile a ro mate dust ventm e uauon 3-8.1 If it is necessary to locate enclosures with deflagi~ti~.. ' .~.. ,~": ' :~.: ' : ' l~.~P PP P g q •

inside buildings, vent ducts should be used to direct venteff'%~f~: ":."-'~':':~:'::::" material f rom the enclosure to the outdoors. .:-::!:!!!~::-:.. %$~:.

~::~:: ...... ""~:.::::.~:, ~ : : " Chapter 4 Venting of Deflagrations in Low Strength Enclosures ..:+- -.,.::.,..,.:::~?:~ ~:.:.:::::~ - 3-8.2 Tile use of vent ducts will result in a n ~ r e a s e in P";~.::'~$. T I ~ :2" . . . .

. . . . . . .......:.: ~-. :--"~..."~::, ~ - '::" 4-t tntroOuction. vent tlucts snouia nave a cross-section at l~tst'~.....i~...eat as th$~of file ve~:d~el ~ 51~n~Clreet~thl?s P[e:~are2~ie t o e t l ~ i ~ v e n t : : ~ c t ~ a s 4-1.1 This chapter is applicable to tile design of deflagration vents • g g - ( ~ : i~ , g,...a~ for low st rength enclosures capable of wiflastanding pressures, P ~ , Figure 5-4(b) for dusts. The use of vent ducts of I ~ i ! ~ i - o s s of not more than 1 5 osi~ (0 1 bar if'anise} Ea (4) has been section than the vent might result in a smaller incre~:i:~="n the develooed from res'ul~ o'f tests and fl~ le~an'alysis of ' industrial pressure developed dur ing venting (Pred) than wbeti 'nsing vent accide~ats. Deflagratlon vents have been effective in mitigating the ducts of equivalent cross section [95] but this effect is difficult to consequences of many industrial building explosions. It should,

however, be no ted that flames and pressure waves from file with total length less than one hydraulic diameter, no correction is explosion might be hazardous, as described in 3-2.3 and 3-2.4 of

this document• Further, test work has demonst ra ted flint

quantify because of l imited test data. For vent ducts and nozzles

required.

3-8.3 Vent ducts should be as short and straight as possible• Any bends can cause dramatic and unpredictable increases in file pressure developed during venting.

3-9 Location of Deflagration Vents Relative to Air Intakes. Deflagration vents should not be located where tile vented material can be picked up by air intakes.

3-10 Effects of Initial Turbulence and Internal Appurtenances for Enclosures with Initial Pressure Near Atmospheric.

3-10.1 Gas.

3-10.1.1 In many industrial enclosures, the gas phase is present in a turbulent condition, ff the gas system is initially turbulent, the rate of deflagration is increased relative to tbat observed in initially quiescent conditions. [3, 35] In dais case, tile equations do not apply directly. It has been found that initially turbulent methane and propane exltibit K G values similar to that of initially quiescent

hydrogen. For this reason, tile K G value for hydrogen should be

used in tile venting equation for initially turbulent gases that have

deflagrations of f lammable gas mixtures in enclosures containing turbulence inducing objec ts (such as process equipment , pipework, cable trays, and so forth) can develop pressures significandy higher than predicted by Eq. (4)• It is therefore r ecommended that building vents should be used in addit ion to taking measures to minimize d ie potential for f lammable gas accumulations in enclosures. It is in tended that tltis chapter be used along wida dm information conta ined in file rest of this guide. In particular, Chapters 3, 9, and 10 should be reviewed before applying the information in this chapter.

4-1.2 Chapters 5, 6, and 7 of this guide provide recommendat ions for enclosures of higher strengths. Typically, fills chapter will apply to buildings, ovens, dust collectors and t i l l e r similar equipment .

4-1.3 No venting recommendat ions can presently be given for fast- burning gases, with fundamental burning velocities greater than 1.3 times that of propane, such as hydrogen. Tltis is because the r e c o m m e n d e d me thod allows for initial turbulence and turbulence-generat ing objects, ,and no venting data have been gdenerated to address such conditions for fast-burning gas

eflagrations. The user is caut ioned that fast-burning gas deflagrations can readily undergo transition to detonation.

6 7 8

N F P A 6 8 ~ A 9 8 R O P

Alternate measures are given in NFPA 69, Standard on Explosion Prevention Systems.

4-2 General.

4-2.1 Deflagration venting is provided for enclosures to minimize structural damage to the enclosure itself and to reduce the probability of damage to o ther structures. In the case of buildings, deflagration venting can prevent structural collapse. However, personnel widfin the building could be e x p o s e d t o the effects of flame, heat, or pressure.

4-2.2 The venting should be sufficient to prevent the maximum pressure developed within the enclosure, Pred ' f rom exceeding

enclosure strength, Pes-

4-2.3 Doors, windows, ducts, or o ther openings in walls in tended to be pressure resistant should also be des igned to withstand Pred"

4-2.4 Care should be taken to ensure that the weakest structural e lement is identified, as well as any equ ipment or o ther devices that might be suppor ted by structural elements. All structural e lements and supports should be considered. For example, floors and roofs are not usually des igned for loading from beneath. However, a lightweight roof might be cons idered sacrificial, as long as its movement can be tolerated and its movement will no t be h indered by ice or snow.

4-2.5 Reaction thrust forces for suppor ted enclosures are discussed in 5-2.9 and A-5-2.9. Examples of these types of enclosures include dust collectors and vapor collection ducts for incinerators.

4-2.6 The vent area should be distr ibuted as symmetrically and as evenly as possible.

4-2.7 For low st rength enclosures Pred should always exceed Pstat by at least 0.35 psi (0.02 bar). ..::::~

4-5 Calculating the Vent Area. ~!~ :?:i ~ _ _ ~

4-3.1. The r e c o m m e n d e d venting equation for low st~i~ng~.~.~.ff ~ structures is as follows: " - ~ ' - ' - ° x~:-~.

Av - 1/2 ered

where:

A = v

C

A = S

P m

red

2 2 vent area (ft or m )

venting equation constant (see Table 4-3.1)

internal surface area of enclosure (ft 2 or m 2)

The maximum pressure developed in a vented enclosure

dur ing a vented deflagration. P ~ , in this application, shall no t exceed Pes. (in psi or bars, not to exceed 1.5 psi or 0.1 bar).

Table 4-3 Fuel Characteristic Constant for Ventin uat ion Fuel ~dlglJ~ " Metric

Anh),drous ammonia Methane Gases with fundamental burn ing velocity less than 1.3 times that of p ropane* St-1 dusts St-2 dusts St-3 dusts

C(psi) C(bar) 0.05 0.013 0.14 0.037 0.17 0.045

0.10 0.026 0.12 0.030 0.20 0.051

*Note: Includes hydrocarbon mists and organic f lammable liquids.

4-3.2 The form of the venting equation is such that there are no dimensional constraints to the shape of the room provided the vent area is no t applied solely to one end of an elongated enclosure. (Other general vent considerations are given in Section 3-6.) For e longated enclosures, the vent area should be applied as evenly as possible with respect to the longest dimension, i f the available vent area is restricted to one end of an elongated enclosure, the ratio of length to diameter should not exceed 3. For cross sections o ther than circular or square, the effective

d iameter can be taken as the hydraulic diameter, given by ( A ] ,

wilere A is file cross-sectional area normal to the longitudinal axis o f the space, and p is the per imeter of the cross section. Therefore, for enclosures with venting restricted to one end, the venting equation is constrained as follows:

L 3 <

where

L 3

A

P ~9

many in~ end, then

(5)

s t ~ e n s i B h of the enclosure (ft or m)

ea (ft 2 or m 2) normal to the longest

~ e r of c ~ % o n (ft or m)

~ osure c ~ contain a highly turbulent gas mixture is restricted to one end, or if the enclosure has

obstructions and the vent area is restricted to one L[.~ of the enclosure should no t exceed 2, or:

( f t or m ) (0)

~ . 2 Where these dimensional constraints on the enclosure are ~:ot met, tile alternate methods as described in Chapters 6 through 8 should be cons idered for possible solutions.

4-3.$ Vent ing Equation Constant. Tile value of C in the venting equation in 4-3.1 dlaracterizes the fuel and clears the dimensional units. Table 4-3.1 gives some r e c o m m e n d e d values of C. These values of C pertain to air mixtures.

4-$.3.1 The values of C in Table 4-3.1 were de te rmined by enveloping data. If suitable large-scale tests are conduc ted for a specific application, an alternate value of C can be determined.

4-$.$.2 The data cited in References 28 and 30 through 45 are mostly for aliphatic gases. It is believed that liquid mists can be treated tile same as aliphatic ~as, es provided that the fundamental burn ing velocity of the vapor is less than 1.3 times that o f propane.

4-$.$.3 No recommendat ions can presently be given for hydrogen. Unusually high rates of combust ion ( including detonat ion) have been observed in actual practice dur ing turbulent hydrogen combustion. As condit ions become severe, combust ion rates approach those of detonat ion for o ther fast-burning fuels. In addition, as rates of pressure rise increase, the inertia o f vent closures becomes more critical. (See 4-7.2 and 9-3.4.1.) (Even if de tonat ion does no t occur, it might be impossible to successfully vent fast deflagrations in some cases.

4-4 Calculation o f Internal Surface Area.

44.1 The internal surface area, A , is the total area that constitutes s

the per imeter surfaces of the enclosure being protected. Noustructural internal partitions that cannot withstand the expected pressure are not considered to be part o f the enclosure surface area. The enclosure internal surface area, A , in the

s venting equation includes the roof or ceiling, walls, floor, and vent area and can be based on simple geometr ic figures. Surface corrugations are neglected, as well as minor deviations f rom the simplest shapes. Regular geometr ic deviations such as saw-toothed roofs can be "averaged" by adding the contr ibuted volume to that

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N F P A 6 8 ~ A 9 8 R O P

of d~e major strncture and calculating A for file basic geometry of $

die major structure. The internal surface of any adjoining rooms should be included. This includes adjoining rooms separated by a partition incapable of widlstanding die expected pressure.

4-4.2 The surface area of equ ipment and contained strnctures should be neglected.

4-5 Enclosure Strength. The user is referred to Sections 1-4, 3-3, and 4-2 for specific remarks relating to enclosure strengdL

4-6 Methods to Reduce Vent Areas. In some circumstances file vent area calculated by dae formula in 4-3.1 will exceed die area available for installation of vents. When such situations arise, it is r e c o m m e n d e d flint one of file techniques indicated in 4-6.1, 4-6.2, or 4-6.3 be used to obtain file needed protection.

4-6.1 The calculated vent area, A,, can be reduced by increasing file value of P~a. The value of Pr~ should not be increased above 1.5 psig (0.1 bar gauge) for design under dais chapter. If P ~ is increased above 1.5 psig (0.1 bar gauge), the medaods of Chapters 5, 6, and 7 should be followed.

4-6.2 The calculated vent area, A , can be reduced by installation v

of a pressure-resistant wall to confine tile deflagration hazard area to a geometr ic configuration widl a smaller internal surface area, A . The new wall would be des igned in accordance wifll Section

s 3-3.

4-6.3 The calculated vent area, A , can be reduced if applicable v

large-scale tests demonst ra te that tile f lammable material has a smaller constant, C, than indicated in Table 4-3.1. (See 4-3.3.1.)

4-6.4 The need for deflagration vents can be eliminated by d ie application of explosion prevention techniques described in NFPA 69, Standard on Explosion Prevention S3stems.

4-6.5 The vent area can be reduced for gas deflagrations i n ~ relatively unobst ructed enclosures by the installation of ....~:" ~ noncombust ible , acoustically absorbing wall linings p r o d dl .~.~.:

tes~" s ' ( ~ . ~ large-scale test data confirm die reduction. These -~": ~:::,x ~m~ conducted widl tiae highest anticipated turbulence levels ~nt~.'~'~l die proposed wall lining material and dtickness.~.:~?,~.~.-"~:~,~:.: ~ : " ~ i ~ .

~ ' % : : % , ... . .

4-7 Vent Design. (See also Section 3-4, Vent ,:~ables')a.:.,.,~, ::~ii~ "~i$ .(.::: .~.::~:~:..:..:::,

4-7.1 Where inc lement wearier , e n v i r o n m e n t ~ i i ~ t z m i n a ~ n , or loss of material is not a consideration, open ven~.:.~.~, t be u~ed and are r ecommended . In most cases, vents will be coF~' ~#........~'some type of vent closure. The closure should be designe:~ ~.:'.'~fbnstructed, installed, and mainta ined so flint it will release readil! r and move out of die padl of dae combustion gases. The closur, : should also no t become a hazard when it operates.

4-7.2 Weight o f Panel Closure Assembly. The total weight of die closure assembly includin~ any insulation or hardware should be as low as practical, to minimize fl~e inertia of dae closure. The

vent closure weight should not exceed 2.5 Ib / f t 2 (12.9 k g / m 2) when using Eq. (4) widaout consideration for vent closure efficiency. (See 3-6.14.2.)

4-7.3 The material of construction of dae closure should be compatible wida die envi ronment to which it will be exposed. (Refer to d~e National Association of Corrosion Engineers Handbook.) Some closures, upon activation, are blown away f rom dieir mount ing tints.. Brittle materials will fragment, producin, g missiles. EPacb mstallation should be evaluated to de termine d~e extent of the hazard to personnel from such missiles. Additionally, it should be recognized dlat d~e vented deflagration will discharge burning dusts or gases, posing a personnel hazard.

4-7.4 Deflagration vent closures should release at as low (P~t) as practical, yet remain in place when subjected to external wind forces producing negative pressures, to prevent vents from being

pulled off. In most cases, a (P~t~t) o f 20 lb / f t 9 (0.01 bar gauge) is acceptable. In areas subject to severe windstorms, release

9 pressures up to 30 lb / f t (0.015 bar gauge) are used. In any case, locating vents at building corners and eavelines should be avoided due to die higber uplift pressures in dlese areas. In hurricane areas, local building codes often require higher resistance to wind uplift. In these situations, die limitations of P~t in 4-2.6 should be recognized, and strengfl lened internal structural elements should be provided.

4-7.5 If an enclosure itself is subdivided into compar tments by walls, partitions, floors, or ceilings, d ien each compar tment flint contains a deflagration hazard should be provided witil its own vent closure(s).

4-7.6 The vent closure(s) sbould cover only die required vent area for die compar tment being protected.

4-7.7 Each closure should be des igned and installed to move freely widaout interference This ensures dlat ~# an obstructed cl .~.r ,

4-7.8 A vent su ii iniury c o u l ~ : . ~ l l t fi

ft, prevent:: t

4 - 7 ~ h e ~ r i a ~! b ~ k h ~ e a~ tak ~ t e c { : " d l e ice. " ,:~i~..~.~..

obstructions such as ductwork or piping. t~ of combustion gases is not i m p e d e d b y

open if personnel fall or lean on it. If s event, guarding should be provided to ~in¢:'against vent closures.

for dxof~esign of roof-mounted closures are as dlose for wall closures. Measures should be

closures against accumulations of snow and

{..~?1 C ~ g i d e r a 20 x 30 x 20 ft (6.1 x 9.2 x 6.1 m) (LWH) i~iispens~g room for Class 1 f lammable Anticipated liquids. ~ . m a b l e liquids have fundamental burn ing velocities less daan 1.~'~.~mes dlat of p ropane (see Table C-l). The room is located ,~ainst an outside wall and, in anticipation of deflagration venting requirements , die dlree inside walls are des igned to widistand P~a of 100 psf (0.69 psi). Design of d ie venting would p roceed as follows. For most f lammable liquids, Table 4-3.1 gives a vent ing equation constant, C, of 0.17.

Internal surface area of die room = 3200 fC (297 m 2)

017X ®) (61 m Vent area, A v , - 0.691/2 - 655 ft 2

This is more dlan is available in die outside wall, so some modification is necessary.

If die wall strength were increased to resist a Pr~ of 150 Ib / f t 2

(0.072 bar), a v e n t area of 533 ft ~ (50 m 2) would be required. This wall s t rengdi can usually be achieved, and is r e c o m m e n d e d over die common wall s t rength to resist P ~ of 100 lb / f t 2 (0.048 bar).

4-8.2 Consider die building illustrated in Figure 4-8.2 for which deflagration venting is required. The building is to be protected against a deflagration of a hydrocarbon vapor having die burning characteristics of propane. The maximum P ~ dlat dlis building can widlstand has been de te rmined by structural analysis to be 0.5 psi (3.45 kPa).

680

N F P A 68 ~ A 9 8 R O P

~ 1 7 0 ft (51.8 m)--~

I~- 50 ft "~ (15.25 m)

[ 3~ft (9.15 m)

lO tt -------- 3 13.05 m!_ l "q

6o.--4 j (18.3 m)

r

L

Figure 4-8.2 Building used in sample calculation (not to scale).

4-8.3 Divide die building into sensible geometric parts (Parts 1 and 2) shown in Figure 4-8.3.

~ 1 7 0 tt (51.8 m ) ~

60 ft (18.3 i i

5o. (15.25 m)

o tt ------1 4 1

13'i' " I.-- j

(18.3 m)

Figure 4-8.3 Building used in sample calculation

4-8.4 Calculate die total internal surface area of each part of die building.

Floor

Roof

Rear wall

Front wall

Side walls

(Rectangular part)

Side walls

(Triangular part)

Part 1 Surface Area

170 x 30 = 5100ft 2 (474m e)

170× 31.6= 5372 R~ (499m e)

170×202 3400f~(316 e)

(120 x 30) + (50 x 10) = 4100f~(381 m e )

2×30 x 2 0 = 1200fF ( l l l m e)

= 30x 10=300fff (28m e)

Total internal surface area o f Part 1:

Floor

Roof

Front wall

Side walls

As1 = 19,472 ft 2 (1809 m ~ )

Part 2 Surface Area

= 5 0 x 3 0 = 1 5 0 0 f t 2 (139m e)

= 5 0 x 3 0 = 1 5 0 0 f t ~ (139m e)

= 5 0 x 2 0 = 1 0 0 0 f t 2 (93m e)

= 2 x 3 0 x 2 0 = 1 2 0 0 f F ( l l l m e)

Total internal surface of Part 2:

A n = 5200 ft ~ (483 m e)

Thus, the total internal surface area for tile whole building, A , is s

given by: ~ . - % .

A = 19,472 + 5 ~ = 21~fi72 ft ~ (2292 m e) S

4-8.5 C a l c ~ : ! ~ t h e ~ '~! :yent area requirement using:

.-:.~#i~i~A~i~ i ' % ~ . ~ : ~ ,..::#:~:" GJ ::&:::::~ ~ . . ~ ,. (8)

F ;" (2292 m 2)

psi (3.45 bar)

l/e (psig) (0.045 bar) (from Table 4-3.1).

(0 672) (551m 2) A v = 0.5112 = 5932 ft 2

The total vent area requirements of 5932 ft 2 (551 m ~) should be divided evenly over die outer surface of tile building and should be apport ioned between tile parts in the same ratio as their surface a r e a . Thus, Part 1 -

= A v ( A s l ] = 5 9 3 2 ( 1 9 ' 4 7 2 " ~ = ( 4 3 5 m 2 ) ~ . A s j ~ 2 4 , 6 7 2 J 4682 ft2 A v l

Part II -

= A v ( A s 2 ] = 5 9 3 2 ( 5200 ~ = 1250 ( l 1 6 m 2 ) ~. As j ~ 2 4 , 6 7 2 J f12 A v 2

4-8.6 Check to determine whether sufficient external surface area on file building is available for venting.

In Part 1, file required vent area [4682 ft e (435 me)] can be obtained by using parts of file front, rear, and side walls or the building roof.

In Part 2, the required vent area [1250 f& (116 me)] can be obtained by using parts of tile front and side walls or tile building roof.

NOTE: Only tile outer "skin" of tile building can be used for vent locations; a deflagration cannot be vented into other parts of tile building.

681

N F P A 6 8 - - A 9 8 R O P

4-8.7 An irregularly shaped building can be squared off to approximate a building of regular geometry whose internal surface area can be easily calculated. This is particularly applicable to buUdings with "saw-toothed" roofs or o ther such architectural features.

4-8.8 Situations can arise in which the roof area or one or more of the wall areas cannot be used for vents, eRher because of the

~ lacement of equipment , or because of exposure to other uildings or to areas normally occupied by personnel . In such

cases it is necessary to s t rengthen file structural members of the compar tment so that the reduced vent area available is matched to the vent area required. The minimum pressure requi rement for the weakest structural member is obtained by substituting into the equation the available area, the internal surface area, and the applicable G value, and then calculating P~a, the maximum allowable overpressure. The vent area should still be distr ibuted ,as evenly as possible over the building's "skin."

4-8.9 ff the only available vent area is located in an end wall of an elongated building or structure, such as a silo, an evaluation should be made to de te rmine whether tbe equation can be validly applied. (See 4-3.2.) Chapter 5 Venting of Deflagrations in High Strength Enclosures -

G e n e r a l

5-1 Introduction. It is in tended that dais chapter be used along with the information contained in file rest of dfis guide. In particular, Chapters 3, 9, and 10 should be reviewed before applying the information in dais chapter.

5-1.1 This chapter and Chapters 6 and 7, apply to enclosures such ,as vessels or silos capable of withstanding P,~,t of more than 1.5 psig (0.1 bar gauge).

5-1.2 I)eflagration vent requirements are dependen t on many variables, only some of which have been fully investigated. The teclmology of calculating the required vent area in an enclosure subject to detlagration is based on a limited number of tests the analyses of actual explosion incidents. The testing a n d ~ e.:+,~, analyses conducted to date have allowed certain g e n e r a l i ~ ] o n s ~" be made; the r e c o m m e n d e d calculation methods p r e s ~ i . ~ , in t guide are based on these generalizations. The calcula~o~-":~,.::~:. ~:~:: met lmds should, therefore, be regarded as approximate Ol~!i! ~ he user of this guide is urged to give special attenu.'~.tii~.'.~i~:: + ~i::.. precautionary statements. --::!:+':::#::+" "::%:,~iii'~

y ven~:~:~ %iii: " 5+1.3 It is not possible to successfull, ~.,s..~.t°nati°n. . . . . . . . .

5-1.4 Tile maximum pressure that will be reacbe~ .~ . r ing ~ r i n g , Pred ' will always exceed tile pressure at which the + ~ c e .....

releases; in some cases it will be much higher. This.:~::~ximum pressure is affected by a number of factors, described in dais chapter and Chapters 6 and 7 which give guidelines for de te rmin ing dais maximum pressure.

5-2 Basic Principles. Certain basic principles are common to rite venting of deflagrations of gases, mists, and dusts. These include but are not limited to the following:

5-2.1 Enclosure strength. The user is referred to Sections 1-4 and 3-3 for specific comments relating to endosu re strength.

5-2.2 The vent should be designed to prevent the deflagration pressure inside the vented enclosure f rom exceeding two-thirds of the enclosure strength. Tiffs criterion anticipates that the enclosure could bulge or otherwise deform.

5-2.3 Vent closures should open dependably. Their proper operat ion should not be l t indered by deposits of snow, ice, paint, sticky materials, or l~olymers. Their operat ion should no t be prevented by corromon or by objects that obstruct the open ing of the vent closure, such as piping, air condit ioning ducts, or structural steel.

5-2.4 When a tatpture d iaphragm device, of low mass-to-area ratio, vents a deflagration, the vent closure ruptures in the prede termined pattern and provides an unrestr ic ted opening. On account of its low inertia, it i:ontributes little to the Pred which develops dur ing the venting. However, if a similar venting device has a substantial mass/area, that inertia can cause an increase in

Pred under certain conditions. Vent closures should therefore have a low mass per unit area to minimize inertia in order to reduce opening time. This has been conf i rmed by tests with f lammable gases and dusts. [50, 98, 99] An example is given in Table 9-3.4.2. See 3-6.14 for fur ther guidance for effects of higher inertia vent closures.

5-2.5 Vent cl6sures should withstand exposure to the materials and process condit ions within the enclosure being protected. They should also withstand ambient condit ions on the nonprocess side.

5-2.6 Vent closures should release at pressures reasonably close to their design release pressures.

5-2.7 Vent closures should reliably withstand fluctuating pressure differentials that are below the design release pressure. They should also withstand any vibration or o ther mechanical forces to which they could be subjected.

5-2.8 Vent closures should be inspected and properly maintained in order to ensure dependable operation. In some cases, dais could mean replacing the vent closure at appropriate t ime in te~a~. (See Chapter. 10.)

~$i'....'::+. ....,:.... 5-2.9* The suppg...~.~ng':'~cture for the enclosure should be strong enough to w i f f ~ : , , a n y reaction forces developed as a result of operation o f ~ v ~ : . T h e equation for these reaction forces has been e s t a b l ~ f r o ~ . . t results. [46] The following equation is

c ~ t e ~ i ~ o r e n ~ ' f ~ e ~ w i t h o u t vent ducts: only ap

(Pred )-:i::: (7)

= ~ i m u m reaction force resulting from combustion :x~" ~::+¢enting, lb f

2 = Vent area, in.

= Maximum pressure developed during venting, psig

A V

P red

gauge

Fr= 119 (Av) (Prod) (8)

Where:

F r = Maximum reaction force resulting from combustion venting, kN (kilo newtons)

2 = Vent area, m

= Maximum pressure developed dur ing venting, bar

5-2.9.1 The total thrust force can be considered equivalent to a force applied at the geometr ic center of the vent+ Ins ta l la t ion of vents of equal area on opposite sides of an enclosure cannot be depended upon to prevent thrust in one direction only. It i s possible for one vent to open before another . Such imbalance should be considered when designing enclosure restraints for resisting thrust forces.

5-2.9.2 Reference 46 contains a general equation that approximates tile durat ion of the thrust force of a dust deflagration. It only applies to enclosures without vent ducts. Knowing this durat ion can aid in the design of certain support structures for enclosures with deflagration vents. The durat ion calculated by the following equation will be quite conservative:

(9)

682

N F P A 6 8 - - A 9 8 R O P

C

t f

K s t

V P

red gauge

A V

Where (metric units):

- (O.0001) = durat ion of pressure pulse, sec

deflagration index for dust, bar-m/sec

3 vessel volume, m

= maximum pressure developed dur ing venting, bar

2 = area of vent (without vent duct), m

Where (English units):

-4 c = (4.42x 10 )

t = durat ion of pressure pulse, sec f

K = deflagration index for dust (bar-m/see) St

3 V = vessel volume, ft

P = maximum pressure developed during venting, psig red

A V

2 = area of vent (widmut vent duct), ft

Note: KSt measurements are always repor ted in metric units and d~at KSt should be used in bod~ equations.

5-2.9.3 The equivalent static force daat a structure support ing a vented enclosure, will exper ience dur ing deflagration venting i: given by file following equations: .d~

...:~.~.~*

F s = 0.62 (A v ) (Pred) " ~ i ~ " . ( •

Where: "::::::: ...::.:~:%~.~. "~ .:..~ - ~:.:.....:... ..:~, ".~:~;~-.:.,':~..

= Equivalent static force experienced..:.~ support$~,i:...

sm~cture, lbf <*~ "'N-i~i~.. ' ~ A = Vent area, in. ~ ~:~:'~4~.~ ~g':

P red = Maximum pressure developed d u r i n g v e n g

FS = 61.48 ( A v ) ( P r e d ) (11)

Where:

F = S

lilk = V

Pred =

Equivalent static force exper ienced by suppor t ing

structure, kN 2

Vent area, m

Maximum pressure developed dur ing venting, bar gauge

5-3 Parameters for Deflagration Venting.

5-3.1 Tile technical literature reports extensive experimental work

on venting of deflagrations in enclosures up to 250 m (8830 ft ) in voinme. [3, 49, 50, 51, 52] As a result, a series of equations l~as been developed for calculating dae necessary vent areas for enclosures.

5-3.2 For long pipes or process ducts whose L / D is ~{reater titan 5, d~e deflagration vent design should be based on die reformation given in Chapter 8.

5-3.$ The equations for deflagration venting of gases (Chapter 8) and for deflagration venting of dusts (Chapter 7) are based on experimental data. The equation for gases cannot be used for dusts, and vice-versa. A/so, a K c of a given value is no t equivalent

to a Kst of d~e same value, in terms of tileir respective significance for venting.

5-3.4 The venting equations are based on deflagrations in which die oxidant is air. They do no t apply to venting when some od~er gas is die oxidant.

5-4 Effects of Vent Discharge Ducts.

5-4.1 The deflagration vent area requi rement is greater when a vent discharge duct is used. When venting a defla[~ration d~rough a vent duct, secondary deflagrations can occur in tilts duct reducing tile differential pressure available across die vent. The sizing equations and graphs in Chapters 6 and 7 are based on vent ing deflagmtions to a tmosphere widiout vent ducts.

5-4.1.1 When using equations 16, 19, and 20 in Chapters 6 and 7 widi vent ducting, a lower value should be used in place of Prod' This value, P ' ~ , can be de te rmined for gases using Figure 5- 4.1.1 (a) and for dusts using Figure 5-4.1.1(b), or calculated with dm equations in 5-4.1.3 and 5-4.1.4. Keep in mind fllat Pr~ is still die maximum pressure developed in a vented deflagration. P'r,d is not an actual pressure.

2 .:$.x..

E k--.~

g

~.., o o 8 "":~":.E:~ . . . . .

%

' ~ ~g~i~ v:-:. "" " ~ - ~ : ~ , . -:" ~:.. ::~:~'~#" .

% . . 0 2

it~..:.:.:.~. ~ :.:'~v" 0 6 1 1.4 1 8 2 2 2 6 3 3 4 3 8 4 2 ~' "~ . , , .~ , , ,,~,'~ . . . . . . . . ~ l ~ P roe - - maximum vented pressure - - bar gauge with duct

!~:~.~.-.....::.~. .~..~.}.~. Figure 5-4.1.1(a) Maximum pressure developed during venting of

gas, with and without vent ducts. [101]

¢~ 1.8 o~ 1.6

i.4

0 . 8 ~

0.6

"~ 0.4

0.2

0.6 1 1.4 1.8 2.2 2.6 3 3.4 3.8 4.2 Prod i max imum vented pressure - - bar gauge with duct

Figure 5-4.1.1(b) Maximum pressure developed during venting of dusts, with and without vent ducts. [101]

5-4.1.2 Tes t ingbas been done with 3 m (10 ft) and 6 m (20 ft) duct lengdls. Unt i l more test data is available, duct lengths shorter than 3 m (10 ft) should be considered to be 3 m (10 ft) for calculation purposes. The effect of ducts longer allan 6 m (20 ft) has not been investigated. If longer ducts are required, P r~a should be de te rmined by appropria te tests.

5-4.1.$ The equations of din lines in Figure 5-4.1.1 (a) are as follows:

683

N F P A 68 1 A 9 8 R O P

(a) For vent ducts of less than 3 m (10 ft)

P'r~t = 0.779 (Pred) 1"161 (12)

where P ' r ed is tile resulting pressure wifll vent duct, bar gauge (psig)

(b) For vent ducts of length $ to 6 m (10 to 20 ft)

P'rcd = 0.172 (Pt~A)I"936 (1.$)

where P ' r ed is the resulting pressure with vent duct, bar gauge (psig)

5-4.1.4 The equations of the lines in Figure 5-4.1.1(b) are as follows:

(a) For vent ducts of length less d~an .$ m (10 it)

P'red = 0.394 (Prexi) L529 (14)

(b) For vent ducts of length .$ to 6 m (10 to 20 ft)

P'~ = 0 .100 (Prea)2.094 (15)

5-4.1.5 The vented material discharged f rom an enclosure during a deflagration should be directed to a safe outside location to avoid injury to personnel and to minimize property damage. (See 3-2.3.)

5-4.2 ff it is necessary to locate enclosures dlat require deflagration venting inside buildings, it is r e c o m m e n d e d flint vents not disclmrge wiflfin file building. Flames and pressure waves discharging f rom file enclosure dur ing venting represent a threat to personnel and could damage odler equipment . Therefore, vent ducts should be used to direct vented material f rom the enc|o,. ~ e to flae outdoors. .~.-'~ ~:~'."~ ~. ~.'-':~:~.~

I M..$ If a vented enclosure is located wldfin buildings d "% placed close to exterior walls so dmt tile vent ducts w i l ~ ~ as possible. "~..~ N

ic 5-4.4 The vent ducts should have a cross s ec f i .~ at ~ g r that of fl~e vent itself. The use of vent ducts ~.~'.. larger criY~]ie fl~an fl~e vent might result in a smaller m ~ ' . . . g 9 the p r e " ~ devetoped dudng venting (ere d) a,an d" uc.' of

• ".'~-~.. .#." equivalent cross section, [93] but dais effect is ditll~to..: .~Lrantify because of limited test data. There is a special r e q u ~ . ' ~ n t for vent duct cross-section when d~e vent closure device .i~$-a hinged panel. This is discussed in 7-5.

5-4.5 Vent ducts should be as straight as possible, l n g e n e r a l , any bends will cause increases in dm pressure developed dur ing venting. If bends are unavoidable, d3ey should be as shallow- angled as practical (dmt is, have as long a radius as practical).

5-4.6 Vent ducts that vent d~rough d~e roof of an enclosure should take into considerat ion climatic conditions. (See Section 3-5.)

2 Av, in m :

Chapter 6 Venting Deflagrations o f Gas Mixtures in High Strength Enclosures

6-1 General. This chapter applies to enclosures capable of widlstanding more titan 1.5 psig (0.1 bar gauge). It is in tended that fllis chapter be used along widl the information conta ined in die rest of this guide. In particular, Chapters 3, 5, 9, and 10 should be reviewed before applying d~e information in dxis chapter.

6-2 Venting of Enclosures of L/D of 2 or Less, by Low Inertia Vent Closures Such as Rupture Diaphragms.

6-2.1 The lengdl-to-diameter ratio, L/D, of the enclosure determines the equation(s) to be used for calculating d ie necessary vent area. For non-circular enclosures, tile value to be used for d iameter is file equivalent diameter given by

- , xl/2 t a x

w A . ° . . a . o m o \ ' * /

die longitudinal axis of file space.

6-2.2 For L/D' bottom of this [ calculating d ie i

where:

2 or less, the equation shown at tile Reference 101, is to be used for

':'~e n t a r e a ,

,,t gauge ~ ' ~ ~ gauge and at least 0.05 bar > Pstat

".:.<.-:~.-:b. _~.:~... S V, ei$:~$I~.t,~:volume, _~ 1000 m • ,.:...'-~..:.~...:..~: •

:'-':.~:~. 4 ~ ~: ~.'..M.....::i~ressure before ignition < 0.2 bar gauge

~ rom tests made under tile following conditions:

$ Volumes of test vessels: 2.4, 10, 25, and 250 m

L of test vessels approximately 1

D

Initial pressure: a tmospher ic

P~t: 0.1 to 0.5 bar gauge

Ignition energy: lOJ

stationary gas mixture at t ime of ignition

No turbulence inducers

(16)

684

N F P A 68 - - A98 R O P

L 6-2.3* For -- values f rom 2 to 5, and for P ~ no h igher daan 2 bar

D gauge, the vent area, Av, calculated f rom Eq. (16), in 6-2.2, is incre.~sed by add ing more ven t area, AA, calculated f rom Eq. (17) below. The equa t ion is as follows:

A r K G D -2

AA - (17) 750

Eq. (17) is subject to die l imi tadons stated in 6-2.2. For long

L pipes or process ducts where - - is grea ter than 5, use the

D guidel ines in Chapte r 8.

6-2.4 In addi t ion to calculation of vent a rea by Eqs. (16) and (17), the ven t a rea can be d e t e r m i n e d by use of the graphs in Figures 6- 2.4.1 (a) t h r o u g h 6-2.4.1 (g), which are based on die two above equations. T h e restrictions given for Eq. (16) apply equally to dae graphs. The graphs c,'m be used as a pr imary means for de t e rmin ing ven t area, or they can be used as a backup to check dae vent area calculated by file two equat ions. Similarly tile equat ions can be used to check the vent a rea de t e rmi ned by the graphs.

6-2.4.1 Following are direct ions for us ing Figures 6-2.4.1 (a) t h r o u g h 6-2.4.1(g), togedler with an example.

15 14 13

12 11

10 < 9

~ 6 ~ 5 ~ " - - ' -

4 ~ ~

3 2 1

0 50 100 150 200 250 300 350 400 450 500 550

KGvalue (gas)

6-2.~.~'~.) Vent~..._.__~_:.: sizing for gas. P,,= = 0.5 bar Figure

P r e ~ . - - - - " ~ bar - ._ . . - -

- - - - - " " " - - - - ~ 0.8 bar / 1.0 ba, r

i

1.5 bar 2.0 bar _

2 2~ 24 2~ 2 J ~ 3~ 34 36 ~B 4 42 44 4 0 40

U-

<

LL

1 4

1 3 - -

1 0 / _ ~ . . . , . . . ~ . . v I _ _

0.6 ba.,~( , . - - - . . ~ J " ~ . ~ _ - - - v I ..... / ~ ~ ~ I".0 bar ::-:~::-::::.:

-.:+:.:

'::: ii:%-, "::'::: '::!i~!~:: ::.:.::S "<

50 100 150 200 250 300 350 400 450 500 ~::"

K o vam~ (gas) ":':':::'

Figure 6-2.4.1(a) Vent sizing for gas. P,t= = 0.1 ba r

15 14 J 131 0 3 b a r --.'-'-"

12 11! ~ , ~ r ~ ~ ~

9 / ~ 0.6ba..~Lr ===-

7 i / ~ 1.0 bar

5 ! ~ - - " " - - 2 . 0 b a r ~

3 2 1 0

50 100 150 200 250 300 350

KGValue (gas)

400 450 500 55(

Figure 6-2.4.1 (b) Vent sizing for gas. P,t= = 0.2 bar

0.16

0 .14 O

0.12 o ,~ O.lO;

U. J

v u . l u j ~,

0.08 . /" f 0.06 /

0.04 f~ /

O.O2 /

o i 0 1 2

E l o n g a t e d v e s s e l v o l u m e c o r r e c t i o n . Factor B - for gas.

J ~ f

v

j ~

j r

J

3 4 5 6 7 8 9 Volume (cubic meters)

I

Figure 6-2.4.1(e) Volume correction. Factor C - for gas.

%.

10

685

NFPA 68 - - A98 ROP

1

0.9

0.8

0.7

0 0.6

0.5

u. 0.4

0.3 /

/F 0.2

0.1

J

I f

/

I f

I "

f ~

0 10 20 30 40 50 60 70 80 90 100

Volume (cubic meters)

6-2.4.2 Example Problem: Find the ven t size to protect an enclosure f rom a gas deflagration.

Where:

K G = 150 ba r -m/sec

P,t~t 0.2 bar gauge

P,,a = 0.4 bar gauge

Volume = 30 m s

L / D 4.4

Factor A = 8.65

Factor B = 2.15

Factor G = 0.45

Figure 6-2.4.1(0 Volume Correction. Factor C - for Gas.

5

4.5 J

J 4 I

j l 3.5 I -

O 3 i r 2.5 J '

u. 2 /

1.5

t , / /

0 ,. ~i l l . 100 200 300 400 500 600 700

Volume (cubic meters) ~.:::~

Figure 6-2.4.1 (g) Volume correct ion.~:¢~$gr C - ~...

Factor A: Select the graph with the appropri~i~at...: in _..t# heading. Plot a line f rom the K value at the bott~iig.o, t t ~ P r ed

G ":: : : : : : : : : : : . : : . : : :"

line and t hen across to the left to de te rmine Factor ~::iiii::"

L Factor B: If the vessel laas an - - greater than 2, a n d if Pred is less

D t han 2, de te rmine the value of Factor B. To do tiffs, use the "Elongated Vessel Correct ion" g raph Figure 6-2.4.1(d). Plot a line

L f rom the - - ratio up to die K G line and daen across to the left.

D

Note: If the length- to-diameter is 2 or less, Factor B is equal to

L 1.0. For h igher values of - - , use Chapter 8.

D

Factor C: Use one of the "Volume Correction Factor" graphs. Plot a line f rom the volume value up to the graph line and then across to the left.

With file three factors thus de t e rmi ned the vent size,

A v (m 2) = Factor A x Factor B x Factor C

Vent ,area = A v = Factor A x Factor B x Factor C = 8~65 x 2.15 x 0.45 = 8.57 m ~

6-2.4.$ The m o ~ c u r a i ~ ' v a l u e of K G is tha t d e t e r m i n e d directly by test, as oudl~e~i~ i :Appendix B. ff test ing canno t be done to d e t e r m i n e . K ~ o r a : ~ : . c . u l a r gas, the K G can be approx imated by rafioing.~g... '~!-~ K a d:~!~p:...~e (= 100 bar -m/sec) on the basis of the cg.~:~sE.on~ing f u n d ~ : ~ bu rn ing velocity (Appendix C) of pro.~:"" . . . . . . . . . e ('_Z:'~ cm/ sec ) a ~ that, of file part icular gas in quest ion. ( S W ~ ~ . . { . K G values.)

6-2.4.4 K'~i~..~.u .~...... T h e m a x i m u m rate of pressure rise can be :.....:::::::::__:,.::~..unalized ~!i"-..::...p~:e the K G value. (See Eq. 27 in Appendix B. ) It " ~ ..... ~:,~.owe~;'er, be no t ed tha t K G value is n o t constanl a n d will ~.: ::..,....:::::..:.::::.

i i . ~ ' d t [ 5 ~ . ~ i n g on test condit ions. In part icular increasing the : ~ ] u m e ~ the test enclosure a n d increasing the ignition energy can

" : " S " " C - - - - ~... u..l:.t In m reased K G values. Alflaough the K G value provides a : ~ s of compar ing the m a x i m u m rates of pressure rise of known" .:.-:~'d unknown gases, it shou ld be used only as a basis for

vent sizing if the tests for bofla materials are pe r fo rmed in enclosures of approximate ly the same shape, size, arid wida tile same kind of igniter having consis tent ignit ion energy. See Appendix D for sample calculations of K G values.

6-2.4.5 Some papers have p roposed calculation of vent areas for gases on the basis of f undamen ta l f lame and gas flow proper t ies and experimental ly d e t e r m i n e d constants. [26, 78, 79] These ca lcu la t ionprocedures have no t yet been fully tested a n d are no t r e c o m m e n d e d .

6-2.4.6 Use o f the Equat ion with Hydrogen. The user is caut ioned that hyd rogen / a i r deflagrat ions can readily unde rgo transi t ion to detonat ions . It is flaerefore r e c o m m e n d e d t h a t , before us ing Eq. (16) for hydrogen, considerat ion be given to the potential for a de tona t ion to occur. Tiffs could require test work. Alternate measures are given in NFPA 69, Standard on Explosion Prevention Systems.

6-5 Effects o f Initial Tu rbu lence and Internal Appur tenances for Enclosures with Initial P r e s s u r ~ N e a r Atmospheric .

6-3.1 In many industrial enclosures, the gas phase is p resen t in a tu rbu len t condit ion. An example is the con t inuous feed of a f l ammable g a s / o x i d a n t mixture to a catalytic partial oxidat ion reactor. Normally dais mix ture enters the reactor head as a high- velochy tu rbu len t flow th rougb a pipe. As the gas enters the reactor head, still more tu rbu lence develops due to the sudden en l a rgemen t of the flow cross section. Appur t enances within an enclosure e n h a n c e turbulence .

6-5.2 If the gas system is initially turbulent , the rate of deflagration is increased. [3, 35] In dais case, Eqs. (16) a n d (17) do not directly apply. It has been found that initially tu rbu len t m e t h a n e

686

~ T A 68 1 A 9 8 R O P

(d t) and propane exhibit high values. For dais reason, the

m a x

hydrogen K G (= 550) should be used for venting initially turbulent (dp) g ~ e s that have values, in file quiescent state, that are

max similar to or less than that of propane.

6-3.3 The susceptibility of a turbulent system to detonat ion

increases with increasing values of tile quiescent . In m a x

particular, the compounds that have values close to m a x

that of that of hydrogen are highly susceptible to detonat ion Mien ignited unde r turbulent condiUons. It should be no ted that venting will t end to inhibit transition f rom deflagrafion to detonat ion, but is no t an effective me thod of protect ing against the effects o f a detonat ion once the transition has occurred. Where the potential for detonat ion exists, al ternate solutions such as those in NFPA 69, Standard on Explosion Prevention Systems, should be considered.

6-4 Effect of High Ignition Energy.

6-4.1 Tile amoun t and type of ignition energy can affect die effective flame speed and the venting. The exact amoun t of ignidon energy dlat can occur in enclosures cannot normally be predicted. In many industrial cases, however, the ignition energy can be quite large.

6-4.2 Where two enclosures are connec ted by a pipe, ignition in one enclosure will cause two effects in the second enclosure. Pressure deve lopment in the first enclosure will force gas t] h the connect ing pipe into the second enclosure, resulting i~ ~

The flame ~ increase in both pressure and turbulence, fro..~ also be forced th rough the pipe into the second enclost~.~

The overa#:~ it will become a very large ignition source. ;~ de pe nd on the relative sizes of the enclosures and the ell as on file length of the pipe. This has been i n v e ~ y Bartknecht, who found the effects carl be larg¢~ ' -~7~ l~i.~it ~ developed in the pipeline itself can also be ~ i t e high ~ ~ file deflagration changes to detonat ion. ~ i ~ . t j c h o '~."" prevail in equ ipment design, refer to Refe~en~: .7 . , an !

.:.~.~.).-'.~:.... 6-5 Effects of Initial Elevated Pressure. ""~:"'~" #" 6-5.1 Eqs. (16) and (17) or the enclosed graphs car~:~i used directly to establish file required vent area for an enclosure containing a gas mixture at an initial pressure, before ignition, no higher than 0.2 bar gauge. If file initial pressure, before ignition, is be tween 0.2 and 3.0 bar gauge, the correlation in this section can be used. (See 6-5.3.)

6-5.2 For a given vent size, the maximum pressure developed dur ing file venting of a deflagration will vary as a function of file initial absolute pressure raised to an exponential power, ~¢. For this calculation, as shown in 6-5.3, file ratio of the absolute l)ressure . . . . of open ing of vent closure to d ie absolute pressure at t ime of lgnluon Is ,assumed to be constant. The r ecommended values of the exponen t vary inversely with the ratio of the vent area, Av, to the 2 /3 power of the enclosure volume. V; that is, ~" varies

A~ inversely with " ~ - ~ - . Tbis is shown in Figure 6-5.2. The solid

lines for p ropane ,and hydrogen were developed from the data in Reference 59. References 61 and 79 support the exponen t value of 1.5 for prop,me. Tile line for prop,me can be used for gases having K G values no higher than 1.3 times that for propane. The line for ethylene represents an untested interpolation. The dashed extension lines are extrapolations.

%%.

1.5 "'~

c -

o t~

1.1

" ' , . \ . \

Hydrogen'~ ~ ~ ",,

0.1 0.2 0.3

Adv~ ~,:

Figure 6-,5.2 ~ u e ~]t.~exponent, y as a function of

#.~.~*'#" ~'.'-~ ~'~" (Reference 59)

6-5.3 stens

the vet~t ignition,

h v

V2~ •

use the following

ae ratio o f the elevated absolute pressure at which ~pens to the elevated absolute pressure before aple, operating pressure. This ratio is ~to a Pstat in absolute pressure units that will be ions or the graphs after conversion to gauge

Ett~tblisb the available area of vent opening.

From the vent area and the enclosure volume, de termine

" ~ , and from that and Figure 6-5.2 de te rmine the value of the

exponent, 7.

(d) Using Eqs. (16) and (17) or the graphs, and file value of Pstat f rom (a) above, the vent open ing area, and the enclosure volume, de termine the Pred , in bar gauge, which becomes Pred,1 (in bar absolute).

(e) Calculate the maximum pressure developed during the venting f rom file initially elevated pressure by using file following equation:

= (p ,)(P2P1) '

where:

P = atmospheric pressure = 1.0 bar absolute

P2 = elevated initial pressure before ignition, bar absolute

P~a.~ = P ~ as de te rmined in (d) above, converted to bar absolute

P = actual maximum pressure, bar absolute, developed by the deflagration in a vented enclosure when the initial elevated pressure before ignition is P2, bar absolute.

Note 1: The value to be used for P2 should be carefully chosen to represent the likely maximum pressures at wltich a flammable gas mixture can exist at the t ime of ignition. It can be the normal operating pressure. On the other band, if there can he pressure excursions during operation, it can be the maximum pressure excursions dur ing operation, or the pressure at relief valve fully open position.

687

N F P A 6 8 ~ A 9 8 R O P

Note 2: Venting f rom enclosures at initially elevated pressures will result in severe discharge conditions. The user should consider locating dale enclosure to take into account dae blast wave associated wida dte venting process.

Example problem: Determine maximum pressure during venting for tile following conditions:

Enclosure volume, V = 2.0 m 3

Vent area, A = 0.45 m 2 V

A v 0.45

V 2/3 - 1.59 = 0.28

3', f rom Figure 6-5.2

Maximum operating pressure at time of igniuou

= 1.23

= 2.125 bar gauge

From Eqs. (16) and (17) or dae graphs, P,~t ~" 0.6 bar gauge

P = ~ = 1 + 0.6 = 1.6bar absol'ute

5. Perform dile calculation described in 6-5.3(e):

1 . 2 3

P = (0. f i+l~ (2.125 + l ) r e d , 2

1

= 6.5 bar absolute

= 5.5 bar gauge

6-5.4 As in any vent calculation procedure, any one of dae variables (e.g., vent area, Pstat, Pred) can be determined, provided dae

odilers are held constant. Thus, dile exact sequence of steps depends on d~e variable to be determined. The above procedure and example assume daat actual P ~ and dile vent area are fixed. However, dae medilod for accounting for elevated initial pressure can also be used if a different set of variables is fLxed, but file steps

Vent closure o enm ressure 2 75 bar u e p " g p = . , g a g would be oerformed:~, .a different sequence than is given here. - ~ • . .:.'.~ -q-,,

Material in enclosure ro a n e / m r • " ~" P P 6-6 Effect of l ~ . , T e m p e r a t u r e The effect of initial temperature ~ .x::::::::.*.:~

"" 1 ~ is discussed i . ~ . ~ t i ~ e in Chapter 2. In most cases, an increase r ropane t ~ = w in initial t e . . m ? ~ f a t u r ~ result in an increase in max imum rate of

. . . pressure ~ ..... a d e ~ .rlz..1. irr dte pressure generated by 1 Perform the calculauon described m 6 5 3(a) ~ ~ ~ _ ,., " " " : comb .u~'~'n ]:~a~a u n ~ ' . . i - ~ l ' c l o s u r e . Wlffle rates of pressure rise

. . . . . . . . . are oi~erve~..to increase a~4~qevated temperature, wlffch suL, gests 1 X n a t a n s o l u t e ..':.':::.~-.. ,~.~

• -- " d l ~ ' ; : : " ~ n c r ~ in vent a~ea would be required, research ["6'0] on 2 125 + 1 "*~ ":'~" :': . . . . • v e n t ' S + t e m p e r a t u r e showed dilat increased mltaal

t e m p e r ' 2 ~ d i d not result in increased values of P red- = 0.2 bar gauge "~':J~-".:~ ~i

~t.~. . . . . . . .~ . . . . . - p . . . . . . . . v - - tl m ~ . ~ , . Effects ~'{:~ombmattons of Variables. There are insufficient - -sh?t..w,.: "~ 7 '~q'" ' : ~' ~ £ ' : ' ~ i ~ . . ~ . ¢ t # n e precisely how combinations of variables affect

anct I1 / ) or rue grapns "% d l ~ t ~ n pressure developed during vendng (P-~-0.

2. Determine tlae ,area for venting [6-5.3(b)]: ~ " ~ ~ ~" . . . . . . . . . . . . ,:-'~'~" :~'.~:~:" ~ Meuatrration o t r o a m s o t t~omnustibte Liqmas roams or

.4#" $~ : : • . .x~-: .. . . . . " In dills example, dais is "qven as 0 45 m ~ ...:~:'~, .~" ~ ~ . u s f i b ~ e hqulds can burn. ff tile foam is produced by

• v ~' " " ~f i :~: : , ~ ~ l b b l i n g air darough d~e liquid, d~e bubbles will contain air for . . . . . . . . . " ~ : " ~ ' ~ # o u r n i n g . Combustion characteristics will deoend on a number of o . d e t e r m i n e d a b o v e , == l . z ; a ".'.'i:::i:i.~:~x .-s:-- . - : . . . . - .

~/ ~*:.-'.-'.x, ":~¢.?.'.:..:.::-.'~ properties such as d~e speofic hqmd, size of bubble, and duckness 4#~:~'~'~.:: "::'~:'..:,.,,+ of bubble film There is, however, a more bazardous case. If a ..::::.. ~.~.:::::::::~. ~....':~::~ . . . . . . .

4. Determine Pred as described in 6-5.3(..~" .~'.¢~::'~ .~-.~::" combustable hqmd ts saturated w~d~ mr under pressure, and ff dale ~$! ' . . . . . , ~'..~} ";:," liqutd phase ts flaen released f rom pressure wtda the formation of a

~ t a b ~;°h ~ . . . . . . . . . . . or" ' " ~ . . . . .~i~i! foam, dale gas phase in tile bubbles can be preferentially enriched ~ a u s t s o t t F r o J u s l n f f E u . l i B ) a n ( 1 ( 1 1 ) n i l e ' q t l L " : ~ D f i s t o t [ ~

. . . . w~:: .:.:~: in oxwen. Tiffs is because tile solubility of oxygen in combustible followin conditions: :~-i~:~.%. ..5.'? . . , o . . . o o g ,:::.-:::.~.-::. ..-~:- hqmds m hugher d~an dmt of mtrogen. The increased oxygen

"~)'-::'::" concentration will result in intensified combustion. Therefore, it Enclosure volume, V = 2.0 m $ ,:$i~::" is recommended fllat combustible foams be carefidly tested

relative to design for deflagration venting.

Vent area, A~ = 0.45 m 2 6-9 Venting Deflagrations of Flammable Gases Evolved from

Pstat = 0.2 bar gauge Solids. In certain processes, combustible gases can evolve f rom solid materials. If tile solid is itself combnstible and is dispersed in die gas /oxidant mixture, as might be file case in a fluidized bed dryer, a "hybrid" mixture results. ( S e e S e c t i o n 7 - 8 f o r m o r e d e t a i l . )

6-10 Effects of Vent Discharge Ducts. The effects of vent discharge ducts are discussed in Section 5-4.

6-11 Venting of Deflagrations in Conveying and Ventilating Ducts. Most deflagrations of combustible gas mixtures inside ducts occur at initial internal pressures of nearly atmospberic. Tile venting of deflagrations in such ducts is discussed in Chapter 8.

6-12 Pressures External to Vented Enclosures. A vented deflagration will develop pressures which can damage external structures. An example of external pressure is shown in Table 6- 12. [95, 101] In extreme cases, dlese pressures have been shown to be as high as Pred widlin 1 m of die vented enclosures, and will vary with distance from lille vent opening.

688

N F P A 68 - - A 9 8 R O P

Distance from Vent to External Obstruct ion meters (ft)

0,63 (2.1)

1.0 (3.3)

2.0 (6.6)

Table 6-12 Pressures External to a Vent (Reference 95) Measured Pred Pressure Measured at External Vent Pressure,

bar gauge (psig) 0.144 (2.09)

0.172 (2.49)

0.160 (2.32)

External Surface

bar gauge (Wig) 0.070 (1.02)

0.060 (0.87)

0.020 (0.29)

Percent of Pred

48

35

13

where:

3 Test volume = 2.6 m

2 Vent area = 0.55 m Pstat = 0.1 bar gauge Fuel = 5% propane in air

Chapter 7 Venting o f Deflagrations o f Dust Mixtures in High Strength Enclosures

7-1 Introduction. This cbapter applies to enclosures capable of withstanding pressures greater allan 1.5 psig (0.1 bar gauge). It is in tended that dais chapter be used along with dae information contained in die rest of dais guide. In particular, Chapters 3, 5, 9, and 10 should be reviewed before applying the information in dais chapter.

7-1.1 Some sections of Reference 104 are udlized in tiffs chapter. Additional technology is utilized f rom od~er sources as n o t e d in die text.

7-1.2 K is a measure of the deflagration severity of a dust and is establislSted by the test requirements of ASTM E1226, S t a ~ Method for Pressure and Rate of Pressure Rise for Combustib"..Ousts. The ~ t values established are sample specific. See C h ~ i ~ , 2 fm variables ,affecting test results and Appendix B-5 for d ~ t ~

7-1.3 K~t values of dusts of file same chemical with the physical propert ies such as size and sl and moisture content. K values from pub[ i~ therefore examples only, and represent tirt¢:~$~ (See Appendix B. )

7-2 Venting by Means o f Low Inertia Vent Closures ~ h as Rupture Diaphragms. ~:~

L 7-2.1 The lengdl-to-diameter ratio ~ of die enclosure

D determines die equation(s) to be used for calculating die necessary vent area. For non-circular enclosures due value to be used for d iameter is die equivalent d iameter given by

D = 2 , where A* is the cross-sectional area normal to

the longitudinal axis of tl~e space.

7-2.2 For

be used t~ equatio.,~

L dlan 2, tile following equation [104] is to

~ c e s s a r y vent area, A, in m 2. (This ~ i ~ . ~ t i o n s stated below.)

2 area, m

Initial ~ i ' e before ignition (operat ing pressure) < 0.2 bar

P ~ ' : - maximum pressure reached dur ing deflagration of an opt imum mixture of combustible dust and air in a closed vessel, bar gauge

(a) 5 bar gauge > Pmax< 10 bar gauge for

10 bar-m/s >_ KSt~ 300 bar-m/s

(b) 5 bar gauge _> Pmax _K 12 bar gauge for

300 bar-m/s > KSt < 800 bar -m/s

Pstat: 0.1 bar gauge < Pstat < 1 bar gauge

Pred: 0.1 bar gauge < Pred < 2 bar gauge

3 3 V:0.1 m < V < 1 0 , 0 0 0 m

Note: Static burst i[~ressures (Pst~t) below 0.1 bar gauge (1.5 psig) can be used, nowever, in Eq. (17) a min imum value of 0.1 bar gauge (1.5 psig) should be used.

A v = [3.264 x I0 "5 x Pmax x Ks t x Pred "°'569 + 0.27 (Pstat-0.1) x Pred -°'5 ] X V °'753 (19)

689

N F P A 68 - - A 9 8 R O P

L 7-2.3 For - - values equal to or grea ter than 2, but less than 6, and

D Pred smaller d~an 1.5 bar gauge (22 psig), dae vent area, Av, calculated f rom Eq. (19) in 7-2.2 is increased by adding more vent area, ~ , calculated f rom Eq. (20)_below [104] This equat ion is subject to tile l imitations in %2.4.

= - - (20) AA A v -4.305 x lOgl0 Pred + 0.758 × l ° g l 0 D

where:

The symbols are the same as for Eq. (19) in %2.2.

and:

L = enclosure height, or length

D = enclosure equivalent d iameter (See 7-2.1.)

It is noteworthy daat dais addit ional vent area calculated f rom Eq. (20) in 7-2.3 is sensitive to Pred. For low values of Pred, d~e addit ional area is relatively large. For Pred values of 1.5 bar gauge (22 psig) and above, use Eq. (19) in 7-2.2 alone. For long pipes or

L process ducts where - - is greater d~an 6, use d)e material in

D Chapter 8.

7-2.4 T h e limitations appl ied to Eq. (19) in 7-2.2 also applies to Eq. (20) in 7-2.3.

7-2.5 No test da ta are available for Pmax values above 12 bar gauge (174 psig) or for Kst h igher d~an 800. For such dusts, riffs guide is no t applicable and reference should be made to deflagrat ion prevent ion measures such as in NFPA 69, Standard on Explosion, s .

,..,.....~:,~ ~-:....,..~ Frevention Systems.

7-2.6 In addi t ion to calculation of vent area by die E q s . x ~ ) a n d ~ (20), d~e vent area can be de t e rmined by d~e use of ff~*'~.~.:.bs ~ i i below, which are based on d~e same two eouauons The -~,~:~ • ~ : ~ . . - ~

restrictions no ted for d)e equations apply equall~.:,~ P ~ / ~ The graphs can be used as a primary means fol::::~'~ ; vei ' t l~i area, or they can be used as a back-up to cb~l~ 'd~ calculated by Eqs. (19) and (20). F o l l o w i ~ . ~ i, ~for'~ .$~i~'~" using d~e graphs, toged3er wid~ an examp[i~. ~':~:':':':. ~.."-'.'..'.-"..'.-'~.-... -'.~- • %

15 14

13 12

11

10

8 7

5 ~

- - - --2.0 o a r - . .~ : ~ ~_-. ~ ~ ~ ~ I

I 450 500 550 600

#

3 2 1 v o ~ ~ = = =

50 100 150 200 250 300 350 400

Kst value (dust)

so Ioo i~0 2~i 25o 3oo 35o 40o 450 ~ 550 eoo

K=~IU J {a~l)

Figure 7-2.6(b) Vent sizing for dust. P . = = 6 bar , P u = 0.2 bar.

15 14

13 12

11 10

9 ~ • - -

4 - ' ~ " " ~

~Y" ~ - " "~-;:~:~:i . -~:~;~:

150 200 250 300 350 400 450 500 550 600

"~-".~::":~ Kst va lue (dust) ">.:.:~:_::~._.

Vent sizing for dust. P__ = 6 bar, Pa,~= 0.5 bar.

12

1, lo

< 9 /

1 0

5O 100 150 200 250 300 350 400 450 500 550 600

Kst va lue (dust)

Figure 7-2.6(d) Vent sizing for dust. P . ~ = 8 bar, P~== 0.1 bar.

Figure 7-2.6(a) Vent sizing for dust. P u = 6 bar, P== = 0.1 bar.

690

N F P A 68 - - A98 R O P

<

LL

15

11 ~ 0 . ~ -~ 11 ~o - ~o / ~ " " I 9 I / f

2 ~ - :3

0 0 50 100 150 200 250 300 350 400 450 500 550 600 50 100 150 200 250 300 350 400 450 500 550 600

Kstvalue (dust) Kstvalue (dust)

Figure 7-2.6(e) Vent sizing for dust. P,,u. = 8 bar, P,,,,= 0.2 bar. Figure 7-2.6(h) Vent sizing for dust. P,,~ = l0 bar, P=== 0.2 bar.

15 15

~;":"~: " I 14 14 ~.~ ...... %

11 ,0.6 ~ I 11 ""i!~i! I

o I . !i~::ii~:.-.i .... ....-...-.o.. .:,., ....~- 50 100 150 200 250 300 350 400 450 $00 0"~!~i:<~: 050 100 150 200 250 300 350 400 450 500 550 600

<

LL

Kstvalue (dust) ..... ~$~:~i";-:-:i:i ....... ~=, "'" j : : .......... %1%., ~i~::i~i~:~-~ -~:

Figure 7-2.6(0 Vent sizing for dust. P = . ~ bar, P,,~%~!~ bar.~ii i'i::" .~!!;-:. ;~:!~- ..-

11 11 ,o / ,o / / .

1 ~ r 0

50 100 150 200 250 300 350 400 450 500 550 600

Kst value (dust)

Figure 7-2.6(g) Vent sizing for dtt~t. P.,~ = lO bar, P=== O.1 bar.

Kst value (dust)

Figure 7-2.6(i) Vent sizing for dust. P , , , = 10 bar, P,~,t = 0.5 bar.

0 50 100 150 200 250 300 350 400 450 500 550 600

Kst value (dust)

Figure 7-2.6(j) Vent sizing for dust. P ~ = 12 bar, P.,,= 0.] bar.

6 9 1

N F P A 6 8 - - A 9 8 R O P

15 14 13 12 11 10

9 .< ~ 8 ~ 7

U. 6

5

/ / F i ' - '

3~ .~4 . , ~ f ~ . ~ ~ . ~ _ ~ .~ ~

0 50 100 150 200 250 300 350 400 450 500 550 600

gstvalue (dust)

Figure 7-2.6(k) g e n t sizing for dust. P , = = 12 bar, P , ~ = 0.2 bar.

15 14 13 12 11 10 9 ,<

/ / / ~ ' / ~ ' - I

6

4 ~ " ~ i ~ " ~

• ~.~:~

1 ~i~-':~ 0 ~: ::'~$~$"~ :: 50 100 150 200 250 300 350 400 450 500 "~.~

Kstvalue (dust) " "~''"::~:'~" ' i~

-S:" %~?.':~:.. F ~ 7-~.6(I) Vent , i~,g for d , . r..~i::'.'~ bar, r ~

3.8

3.6 ~:.,~ ~.~ ~-~ 3.4 3.2 ~ ! .~ $ ~

3 J ~ " - -

2.6 / 7 = ~- I I 2.4 / i ~ . . ~

1 . 8 ~ ~ " ~ I I 1.0 bar _ . . _

14 - - [ - V t 1.2 ~ " " " " ~ ~ ~ 1 P[~ '>- 1.5 bar

1 1.4 1.8 2.2 2.6 3 3.4 3.8 4.2 4~6 5

Height/diameter

0.2

0.18

0.16

0.14

0 0.12

o.1 u. 0.08

0.06

0.04

0.02 / 0

0

0.8

0.7

0 0.6

0<.~ ~ ~.~."

J I

. f

J J

J f

J J

f

1 2 3 4 5 6 7 8 9

Volume (cubic meters)

Figure 7-2.6(n) Volume correction. Factor C - for dusts .

10

1 f . , . I

o.g . ~ 1 1

: . , . " . % ~ : 0<.~

02f? -~0 20 30 40 50 60 70 80 90 1 O0

~:" Volume (cubic meters)

Figure 7-2.6(0) Volume correction. Factor C - for dusts .

6 I ~ 5 . 5 I ~

5 4.5

4 o 3.5

w 2.5

2

1.5 J r

/ 1

0.5 0

I

f I

.I .ff

.Jr f

f

I

m~ ~-m

100 200 300 400 500 600 700 800 900 1000 Volume (cubic meters)

Figure 7-2.6(p) Volume correction. Factor C - for dusts.

Figure 7-2.6(m) Elongated vessel vo lume correction. Factor B - for dusts .

692

N F P A 68 - - A98 R O P

35

30

25

0 2O

15 ,,,? 15

10

5

I " /

r

I

I I

I f

. . I I "

I

I

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10,000

Volume (cubic meters)

Figure 7-2.6(9 ) Volume correction. Factor C - for dusts.

Factor A: Select thegraph [Figures 7-2.6(a) to 7-2.6(q)] with the appropriate P,m and dust Pm~ in the heading. Plot a line from dae Ks, value at the bottom up to d~e P~a line across to dae left.

Factor B: Use the "Elongated Vessel Correction" graph. Plot a line

L from tile ~ ratio up to the P ~ line and then across to ti~e left.

D L

Note: If the I is less than 2, then Factor B is equal to 1.0. D

Factor C: Use one of the "Volume Correction" graphs. Plot a line from the Volume value up to the graph line and then across to the left.

2 Vent size, m , = Factor A x Factor B x Factor C

Example: Find the vent size to protect an enclosure fro~.-.~ ' ~ deflagration when the conditions are as follows: . ~

PK, = 10 bar gauge

K = $50 bar-m/sec ~ / ~ , . ~

P m, = 0.2 bar gauge A,'.:'-"~.,.~ ":~.~3".' ~

=O.0bargauge A

Volume = 25 m ~:Y

L/D = 3.O

Then vent size = Factor A x Factor B x Factor C 2

= 6.0 x 1.82 x 0.35 = 3.82 m

7-3 Effects of Higher Inertia Vent Closures.

7-3.1 Defiagration vents with hinged closures are less effective than open vents or vents with lightweight rupture diaphragms. The efficiency of a specific hinged dosure will be dependent on its design details, and can be measured experimentally. [104] In view of the reduced efficiency of hinged enclosures, lightweight rupture diaphragms are preferred. However, based on industrial experience, acceptable vent performance can be acltieved with hinged closures provided:

(a) There are no obstructions in the path of the closure preventing it from opening

(b) Operation of the closure is not restrained by mechanisms such as corrosion, sticky process materials, or paint

ff file weight of a hinged closure is greater than 1.2 lb/ft 2 (0.54

kg/m2), its venting efficiency should be determined by test. [107]

7-3.2 In general, a hinged vent closure results in a higher Pred than does a rupture diaphragm. The hinged vent closure with its geometric area A , mass, and static relief pressure Pstat is tested in position on an enclosure under suitable conditions of dust KSt , Pstat, and ignition. The Pred is determined experimentally under these conditions, and this is related to a corresponding vent area A 2 for an inertia-less vent closure such as a rupture diaphragm which relieves at the same Pstat and gives the same Pred. The venting efficiency, E, is given by the equation:

E = 100 = Percent Efficiency (21)

If a hinged vent closure is followed by a vent duet, special consideration should be given to the clearance between tile front edge of the closure panel and the duct wall throughout the course of the opening arc. This clearance should not hinder flow during the venting while tile vent closure is swinging open. The amount of clearance needed f ro~ the front edge of tile hinged closure, in the closed position, to t~] "~.Jl of tile vent duct is approximately half of

~."~osure from tile hinge to the front edge. the length of the .l~fig

3 It is ve~m%~ '~' ~ t that hinges on hinged vent closures he capable o f ~ ' ~ i g ~ p e c t e d forces. If hinges are weak, or they a r e ~ t t ~ c h e ~ if~$he door frame is weak, tile vent closur..~t~an, fear aw~y i n ~ o u r s e of venting a deflagratlon. They

7-3.3

cl osu .r...~"

after veri which in

~ gt~ction is strong, file vent closure can rapidly close "" This can result in . ~ a partial vacuum in file enclosure, t~J'~..~o~d result in inward deformation of the enclosure.

~.~uum br ~a.~l~ can be installed to prevent this provided they are lgiy enough to Pred during venting or the

~ W ) like rupture diaphragms to leave a clear opening.

t~.~.~ Figure 7-3.3.2 [104] shows tile vacuum relief vent area, as a ~ o n of enclosure size, to prevent the vacuum from exceeding vacuum resistance of the enclosure, in millibars.

T-4 Bins, Hoppers , and Silos.

7-4.1 Deflagration venting for bins, hoppers, and silos should be from the top or the upper side, above tile maximum level of the material contained, and should be directed to a safe outside location. (See Sections 5-5 and 7-7.) Deflagration venting can be through vent closures located in the roof or sidewall, or by making the entire enclosure top a vent. In all cases, the total volume of the enclosure should be assumed to contain a suspension of the combustible dust in question. No credit should be taken for the enclosure being partly full of settled material.

7-4.2 For deflagration venting accomplished by means of vent closures located in the sidewall of the enclosure, the closures should be distributed around the wall near the top. For a multiple application, the closures should be symmetrically placed to minimize the effects of potential reaction forces. (S~5-Z9.) Gare should be taken not to fill the enclosure above the bottoms of the vent panels, as large amounts of dust could be blown out into the atmosphere, be ignited, and form a Imge fireball. Dust piled above the bottoms of vent closures can ldnder venting.

7-4.3 Defiagration venting can be accomplished by means of vent closures located in the roof of the enclosure. The vent operation procedures outlined in Section 3-5 are to be followed.

693

N F P A 68 ~ A98 R O P

1.0

F 0,1

E

0.0i

0.001 10

IIII f i l l ~ " ' ' Ill I I I I -- i ~ ', ',',',

! # ~ /

.v .¢~ 2 " 1 I I ~ l I l l / / . r l I I I I I I I

/ / ~ I i k ' t ' I ~ ~ ' / / 3rfll ~,'Tlli " / 2/'1111 IIII " l i l t S / II

I ~ r l I I I I J r I I I I I I

. I I l l I l l i .V" I l l l Ill

/ Illl Ill

Illl III 100 1000

Volume of enclosure (cubic meters)

dust deflagration vent should be considered when locating vents and vent ducts.

7-7.1 In the case of dust deflagration venting, the distance, D, is expressed by Eql. (22). If the vented material exits f rom the vent horizontally, this would be an anticipated horizontal length of the fireball, the height of the fireball could be the same dimension, with laalf the height below the center of the vent and half the height above. It is extremely impor tant to note dlat the fireball can in fact ex tend downward as well as upward. (References 91 and 108) In some deflagrations, buoyancy effects can allow the fireball to rise to elevations well above these distances.

I/S D = 1 0 ( 3 / ) (22)

where:

D = Maximum flame distance f rom vent opening, ft or m V = Enclosure volume, ft s or m ~

7-7.2 External Pressure Effects.

7-7.2.1 When a dust deflagration is vented from an enclosure, pressure effects are created in the a tmosphere external to the enclosure. These are due to the effects of both the vented products and the fur ther deflagration of excess f lammable dust. There are two pressure maximums (peaks), one from the venting process and one from the d e f l a ~ . . . ~ ' o f the dus t /a i r mixture external to the enclosure. Only J ~ t " ~ l a t a are available for correlation to approximate value of dais maximum le

7-7.2.2

,unt of this latter pressure. The maximum ists at a distance of about 1/5 of the

ae fireball as calculated in %7.1.

by the foil! a cubic vessel, the value is indicated i equation [108]:

• ~.:.-....-.,s,:~.~ ~-..:~:. Av0 .1 V 0.18 P m a x f i : ~ "2 P~o,t x x (23)

F'~yure 7-3.3.2 Graph to Determine the Vacuum Relief Area for Vacuum Vents on Enclosures. [104]

7-4.4 The entire enclosure top can be made to vent deflag~ ~ i In such cases, design and operat ing conditions (internal ~ ~ . ~ external pressure, wind loads, and snow loads) can cat .~.@ of the roof to exceed that prescribed for deflagration ~'ffff~ Roof panels are to be as lightweight as possible and are no~ at tached to internal roof supports. APIO50, ~ ~ . ( r ~ ~ . . Oil Storage, offers guidelines for the design of . ~ . ~ ~ roof joint . Al though API 650, Welded Steel ~' iksfor . n v ~ ~:~Y"".:::"~?" frangible roof design is not in tended to se~'..'~.~.defl~ , , ra t i~ ~ g , there is exper ience to show that such roofg h ; i ~ c , msfulI~ e d - denagrations. The use of a trangihle root desi~# .~ x ~ recommended as tile inner roof on enclosures tlla'~.~ .~e.~" headhouse or penthouse. Equipment, piping, arid at tachments should no t be directly connected to the, ~ o f which could restrict its operat ion as a vent closure. The remaining port ions of the enclosure, including anchoring, should be designed to resist tile calculated Pr~ based on the vent area provided. (See Section 3-3.)

7-5 Effects o f Vent Discharge Ducts. The effects of vent discharge ducts are discussed in Section 5-4.

7-6 Venting of Enclosed Bag Dust Collectors. It is desirable to design bag filter vent closures in such a way as to minimize the potential for bags and cages to interfere with the venting process. The filter medium might not segregate the clean and dirty sides of the collector dur ing the deflagration. Therefore, the entire volume of each side s h o u l d b e used when calculating the vent area. In multi-section dust collectors, venting should be provided for each section. [41]

7-7 Flame Clouds from Dust Deflagratlous. Normally when dust deflagrations occur, there is far more dust present than there is oxidant to burn it completely. When venting takes place, large amounts of unburned dust are vented from the enclosure. Burning continues as the dus t mixes with additional air f rom the surrounding atmosphere . Consequently, a very large and long fireball of burning dust develops that can extend downward as well ,as upward. Personnel enveloped by such a fireball would likely no t su ture . The potentially large size of the fireball extending from the

~max, ~i -~'~ external pressure, bar gauge ) k...-:# ~:.:*" A v = vent area, m 2

K <200 St

3 V = enclosure volume, m

For larger distances, r (in m), the maximum external pressure, P m a x , r , is indicated approximately by Eq. (24):

Pmax, r = Pmax,a (0.20 D/r) (24)

where:

D = Maximum length of fireball, m

r = Distance from vent _> 0.2 D, m

Eqs. (22), (23), and (24) are valid for tile following conditions:

enclosure volume: 0.3 m ~ > V < 10,000 m 3

static activation pressure of the vent closure: Pstat >- 0.1 bar gauge

reduced pressure: P r e d -> 1 bar gauge

deflagration index: KSt < 200 bar-m/s

7-8 Hybrid Mixtures.

7-8.1 Hybrid mixtures of f lammable gases or combustible dusts could be ignitable even if both consti tuents are below their

C_~94

N F P A 6 8 ~ A 9 8 R O P

respective lower f a m m a b l e limits. The propert ies of hybrid mixtures are discussed in References 3 and 104. Certain dusts that do not form combnstible mixtures by themselves could do so if a f lammable gas is added, even if tile latter is at a concentrat ion below its lower f lammable limit. The min imum ignition energy of a hybrid mixture is typically less than dlat o f the dust alone. (See 2- 3.50

7-8.2 The effective K~t value of most combustible dusts is raised by tile admixture of a f lammable gas, even if d~e gas concentrat ion is below the lower f lammable limit. This, in turn, leads to an increase in d~e required vent area. For hybrid mixtures, use tests to de termine the equivalent K~ using worst-case condit ions and use the applicable dust equation. When test data are not available for hybrid mixtures wid~ gases having similar combustion characteristics to p ropane (f imdamental burning velocity < 1.3 times that of propane) and St-1 and St-2 dusts, use Eq. (19) in %2.2 wida Pmax = 10 bar gauge ,and KSt = 500 bar-m/sec.

%9 Deflagration Venting o f Enclosures Interconnected With Pipelines.

7-9.1 Eqs. (19) and (20) in 7-2.2 and 7-2.3 can give instffficient vent area if a dust deflagration propagates from one vessel to another d~rough a pipeline. [98] Increased turbulence, pressure piling, and broad flame je t ignition results in an increased deflagration violence. This results in an elevated deflagration pressure higher than timt used to calculate vent area in Eqs. (19) and (20) in 7-2.2 and 7-2.3.

7-9.2 For in terconnect ing pipelines of inside diameter no greater daan 1 ft (0.3 m) and length no greater than 20 ft (6 m), the following are r ecommended . [104] Interconnect ing pipelines having an inside d iameter greater than 1 ft (0.3 m), or longer than 20 ft ( tm) are not covered in this document .

Note: Alternate protect ion measures can be found in Chapter 8 of riffs docum e n t and in NFPA 69, Standard o n Explosion Prevention S3stems.

(a) The venting device for the enclosure should be des igned for a Pstat < 0.2 bar gauge. ..:..,..:..:.,:.,..,

(b) Enclosures of volumes within 10 percent of each ~ e . sho~"~'": be vented according to Eqs. (19) and (20) in 7-2.2 a n ~ ...%.ey~ "~ !

(c) If enclosures have volumes differing by m o r ~ . : ~ 10 i ~...~... t, design the vents for both enclosures as if P r e d ~ ~ , t . ~ t~., less. Design the enclosure with P es equal to,..~"'gar~:~--gau~., unu~,.~

':':"'"" ure in ~; (d) If it is no t possible to vent die smaller e ~ u r e accordance widl this document , allen this s m a l l ~ c l o s u ~ thould be des igned for the maximum deflagration pressut~...~l~i.~,~ and the vent area of d~e larger enclosure should be doubled.:.~

(e) Deflagration venting of smaller enclosures will no t be effective if the larger enclosure cannot be vented or otherwise protec ted as descr ibed in NFPA 69, Standard on Explosion Prevention @stems.

Chapter 8 Venting o f Deflagrations from Pipes and Ducts Operating at or Near Atmospheric Pressure.

8-1 Scope. This chapter applies to systems operat ing at pressures tip to 3 psig (0.2 bar gauge). This chapter does no t apply to vent discharge ducts. This cbapter applies to pipes, ducts and elongated vessels widl length-to-diameter ratios of 5 or greater.

8-2 General.

8-2.1 Several factors make the design of deflagration vents for pipes and ducts a different problem from the design of deflagration vents for ordinary vessels and enclosures. These include die following:

(a) Deflagrations in pipes and ducts with large length-to-diameter (L/D) ratios can transition to detonations. Flame speed acceleration increases and higher pressures are genera ted as L /D increases.

(b) Pipes and ducts frequently contain devices such .as valves, elbows, and fittings or obstacles. These devices cause turbulence

and flame stretching that will p romote flame acceleration and increase pressure.

(c) Deflagmtions originating in a vessel will precompress the combustible material in tlle pipe or duct and will provide a strong flame f ront ignition of the combustible material in tile pipe or duct. Bofla of dlese factors increase the severity of the deflagration and the possibility a detonat ion will occnr.

8-2.2 In comparison to venting of vessels, relatively little systematic test work is published on the design of deflagration venting for pipe~, and ducts. The guidelines in dfis chapter are based on information contained in References 3 and 68 through 75. Deviations from these tgluidelines should be in the direction of providing more vent area

ann r ecommended .

8-2.3 Wherever it is no t possible to provide vents as called for in this chapter, two alternative approaches can be employed:

(a) Provide explosion prevention measures as described in NFPA 69, Standard on Explosion Prevention Systems, or

(b) Design file piping or ducts to wifllstand de tonat ion pressures and provide isolation devices to protect in te rconnec ted vessels. Systems having a design pressure of 10 bar gauge will be acceptable for Class St-1 dusts.

8-2.4 The use of defl...agration venting on pipes or ducts cannot be relied on to ston f l a t l t ' ~ o n t propagat ion in the pipe Venting will

. . ~ - . ~ ' ~ - " ~ • • .

only prowde refi~f-d?~'~l~ressures genera ted d u n n g a deflagrauon.

8-S Design

8-3.1 T | ~ total c ~

T|~".~.~g~d~|i~!elines " ~ . ~ . ~ d on providing vent area equal to dae total #'s~s.e.cffonal area ~ : 1 ~ vent location. The required vent a r ~ be~complished:.~.~y using either one or more than one v ~ c l l i ~ a t i o n . Tile cross-sectional area is file maximum e f f e c t i ~ { a '~a at each vent location. For non-circular cross sections, '~. '-j~ydraulic d iameter is equal to 4 A /p , where A is the c r o s s - s e c t i o ~ and p is file per imeter o f the cross section. ~:..,. , ~ - .-.~,..~ . ~. . . ~;~. .~. . t . i .~ .~ vent locauons can be prowded along the length of : l ~ * d u c t to reduce the maximum pressure dur ing a- ~ i a ~ n . ~$.$., Deflagrations vents should be located close to possible ~ ' o n sources where these sources can be identified (for example. t#ffine blowers or rotating equipment) .

3-5.4 Pipes or ducts connec ted to a vessel in which a deflagration can occur will also require deflagration protect ion. This can be accompl ished by installing a vent with an area equal the cross- sectional area of the pipe or duct. It should be located on the pipe or duct no more than 2 diameters distant f rom the point of connect ion to the vessel.

8-3.5 For systems handl ing gases, vents should be provided on each side of turbulence-producing devices at a distance of no more than 3 diameters of die pipe or duct.

8-$.6 In order to use the correlations presented later in die document , die welght of deflagration vent closures should no t

exceed 2.5 Ib/f t 2 (12.2 k g / m 2 ) o f free vent area.

8-3.7 The static burst pressure of file vent closures should be as much below Pred as practical and consistent with operat ing pressures.

8-3.8 Deflagration vents should discharge to a location that will not endanger personnel .

8-3.9 Consideration should be given to reaction forces that develop during venting (see 5-2. 9).

8-4 Vent Placement to Prevent Run up to Detonation.

8-4.1 Vents can be placed on pipes and duct.s to prevent a deflagration from transitioning into a detonat ion.

8-4.2 From file ignition location, dae distance necessary for a deflagration to transition into a detonat ion is described as a length- to-diameter ratio ( L / D for detonat ion) . This L / D is d e p e n d e n t upon ignition source strength, combustible material, piping system

6 9 5

N F P A 68 ~ A 9 8 R O P

geometry, roughness of pipe walls and initial conditions wifltin the pipe.

8-4.3* The curves in Figure 8-4.3 should be used to determine the maximum allowable length of a smood~, straight pipe, duct, or vessel that is closed on one end and vented on die odter where no additional deflagration vents are provided, ff L/D ratios greater than d~ose shown in d~e figure are present, dtere is a risk that detonation can occur°

~20 I

100 [ \ ~ 80 ~ E

- , . . . \ .

20

0 0

I I I I I L = Distance between dellagration vents l

or Length o~ pipe or duct having one end open

I I • Dusts with Kst < 200

/ , Propane, dusts with Kst < 200 - -

2 3 Diameter (meters)

Figure 8-4.3 Maximum allowable distance, expressed as length-to- diameter ratio, for a smooth, straight pipe or duct.

8-5 Use of a Single Deflagration Vent on a Pipe or Duct.

8-5.1 If the lengd~ of a pipe or duct is greater d~an fl~e L/D indicated in Figure 8-4.3, d~en a single vent will not provide enough vent area. (See Section 8-6.) Figure 8-4.3 includes safety factors for typical long radius elbow systems. While very few conveying pipes are eidaer straight or smooda, Figure 8-4.3 can be used for most applications. It will not apply when conveying pipes have sharp elbows or orifice plates along d~eir lengd~.

8-5.2 System Flow Velocity 2 m/sec or Less - Flammable Ga ~.~.,--. The maximum pressure during deflagration venting ( P ~ ) ,~j ~ " ~ or duct conveying propane can be estimated from ~igur~.~! 2." "~ Figure 8-5.2 can also be used with gases [laving a f u n d C ~ t ~ burning velocity less dmn 60 cm/sec. Curves are pro~ied:-'~ .?::-~e~ different pipe diameters. For edger pipe diamete~:~.~,~a car. -~:,.:.~ determined by interpolation. The distance b e # ~ . ~ [ ~.-'. location and vent location is expressed as a .L~..J~ ratid."*.~: ./I~ ~?':~','~....~{~ rado is used in conjunction wid~ d~e a p p r ~ t e curves"~t ;tirq#{e d~e P~a. <'$:~ ~ '''-~ :':" :~' ,~ .-.~

~ D = 1.6 m ~ / ~ ' : -.<.<¢ 0.4 ~ , ~ I / ~:.~.~-"

/

0 0 20 40 60 80

Length-to-diameter ratio

Figure 8-5.2 Maximum pressure developed during deflagration of propane/air mixtures flowing at 2 m/sec or less in a smooth,

straight pipe closed at one end.

8-5.3 System Flow Velocity 2 m/sec or less - Dusts. The maximum pressure during deflagration venting (P,~.a) in a pipe or duct conveying dusts can be estimated from Figure 8-5.3. Curves are provided for three different KSt value. For dusts having other KSt, P ~ can be determined by interpolation.

,o I I /

2O0 ~ s t i

0 ~ "~"~ " ' - 0 20 40 60

Length-to-diameter ratio

I

I I 80 1 O0

Figure 8-5.3 Maximum pressure developed during deflagration of dust /a i r mixtures flowing at 2 m/sec or less in a smooth, straight

pipe closed at one end.

8-5..4 For system flow velocities, greater., than, 2 m/sec and for gases wldi fundamental burmng velocmes greater d~an 60 cm/sec, a single vent is not recommended. Additional vent area should be provided by using multiple vent locations.

8-5.5 Initial Velocity Greater than 20 m/sec, or Gases Having Burning Velocities More than 1.3 Times that of Propane, or Dusts With K=> 300. For dlese situations, vents should be placed no more than 1 to 2 m apart..<:--~>...

..::i.::2.::-'..'< ~i?~:, 8-5.6 Turbu len~ . rodu ' /~ng Devices. For ducts or pipes containing t u r b u l e n c e - p r ~ " ~ : devices as previously described, vents should be placed as~eciff~i![~. 8-3.5. Additional vents, as specified e l s e w h e r e ~ : : ~ t i o n ~;.-i~:an also be required.

8-6 , ~ " o ~ l t i p l e D e f l ~ i i o n Vents on a Pipe or Duct.

8-~!"l::~i~:.r:~'i':::~.l"" ~'" should be used to determine the maximum distanc'~ ~ee~ each vent i ~ . for a maximum pressure during d e f l a g r a t l ~ n t l p . g of 2.5 psig (0.17 bar gauge). The figure applies :~..o. system f l ~ c i t i e s up to 20 m/sec (66 ft/sec). It is applicable . . " ~ widl ii-~St less than or equal to 300 bar-m/sec and

15

0

. c

c ® 5

[ Propane and dusts with Kst _< 300 J initial velocity between I J 2 m/s and 20 m/s

\

O0 1 2

Diameter (meters)

Figure 8-6.1 Vent spacing required to keep P~d from exceeding 0.2 bar gauge for propane and dusts with a KSt less than 300 bar-m/sec.

8-6.2 For Other Gases. For gases odler than propane, die maximum pressure during deflagration and d~e distances between vents can be calculated using Eqs. 24 and 25. The equations are limited to fundamental burning velocities below 60 cm/sec (2 f t /sec).

696

N F P A 6 8 - - A 9 8 R O P

2

Pred,x = Pred,p Su, x

L× = L p Su, p

Where: Pred,x = maximum pressure predicted for gas, psig

Pred,p = 2.5 psig - maximum pressure for propane

L x = distance between vents for gas

Lp = distance between vents for propane

Su, x = Fundamental burning velocity of gas

Su, p = Fundamental burning velocity of propane

(24)

(25)

Additional venting is required for die 20-m (66 ft) section. The flow of 100 m S/min corresponds to a velocity of 6 m/sec (20 ft/sec). Hence, Figure 8-~(b) should be used. According to this figure, the vents should be placed at intervals no greater than 11 diameters, or approximately 6.5 m (21 ft) apart. The distance between vents C and F is 17.2 m (56 ft); therefore, 2 additional vents (D and E) at approximately equal spacing would meet file requirement.

Tile total vent area at each vent location should be at least equal to tile cross-sectional area of the duct. This will result in a value of 0.2 bar gauge (3 psig) for P~,d. According to 8-3.7, die vent release pressure should not exceed half P~a and therefore must not exceed 0.1 bar gauge (1.5 prig).

(Figure 8-7.4 will be shown in the Report on Comments.)

Chapter 9 Description of Deflagration Vents and Vent Closures

9-1 General.

9-1.1 Many types of deflagration vents and vent dosures are available. This chapter gives some basic information on vent design and performance.

8-7 Examples.

8-7.1 A dryer handling a dust whose Kst is 190 is 2 m (6.6 ft) in diameter and 20 m (65.6 ft) long and is designed with a single vent. What pressure can occur during a vented explosion?

(a) Check maximum allowable length: According to Figure 8-4.5, an L/D of about 25 is allowable. The dryer has an L/D of 10, so this is acceptable.

(b) Maximumpressure: According to Figure 8-5.3, a pressure of approximately 0 .Sbar gauge (7.3 psig) will b e developedin this equipment by file deflagration of dtis dust. Hence, the equipment should have a design pressure of at least this value.

9-1.2 Some vent typ~ and vent closure assemblies are commercially available and can ~ " "-~..p~]~.c.hased ready to install. Others can be cus tom-fabr ica te~n mt@~oy die user. The following descriotions can be used a s . . . . ~ for the selection or design of vent and vent closures ~I~ ":~4',

9-M*.~.he ~ t effective ~ a g r a t i o n vent is an unobstructed o ~ L ~ no closure. Open ven~ are an option whereve~ e q ~ r ~ * b m s do not need to be totally closed. However, dlere are~[~_.paratively few situations where ol~erations with an

~ ! ~ h e r e n t d ' ~ # o n h ~ r d ~ be conductea in o p e n equipment.

' ~ ~ ~ . ~ d Openings. Openings fitted with fixed louvers can 8-7.2 A flare stack is 0.4 m (1.3 ft) in diameter by 40 m (130 ft) .,.tall ~ b ~ " 6 d as open vents. However, the construction of die

~ d ~ ~:..~nt arvers~rtiallY obstructs the o p e n i n , t h u s c t reducin the net free and is equipped, with a water seal at i t s . base. What should i . . . . ~ " v ersea" Tile obstructton presentel~ by the louvers decreases t h e . g pressure be in order to protect it from file pressure deve

having properties s i m i l a r ~ s e o~,~.~,o... ~ a t e of gases passing dwough the vent and increases the ~~..~ ssure drop across the vent. Tills obstruction increases Pred and • "$~$~shoubhould be accounted for in die system design.. The pressure drop

ength: From Figurg. . . -~ , through file louvered vent should be determined by gas flow ,d. This stack h ~ : ' ~ ( . ~ l u a l t ~ - ' ' - ' ' : ~ # " calculations, and P red adjusted. teslgned to with~Land a d ~ t i o n ~ "

~:~:i¢:, " ~ e~. ~ other m e an~:~@:~., '~.'~.'}.- - ,.,- ~-~,..-!...-..~::. " ~ • . : : . ~ . - ' . ~ : - .

diameter a r i~ i~ ~ m ( 3 ~ ft) : ~ s , #"

i:~.'.'.'~'~ e or ..~tlagration

dr:signed

ignition of a fuel/air mixture having properties s imi lar propane?

Check maximum allowable length: From Fi maximum L/D of 28 is allowed. This stack 100. Therefore, it should be designed to wi should be protected by some

ft)

of

to open

8-7.3 A straight duct 1 m (3.3 ft) in long is to be protected by deflagration vents. It c hydrocarbon/air mixture having properties similar propane. What vent spacing is required to limit the pressure to 2.5 psig (0.17 bar gauge), the vents are at 0.05 bar gauge (0.73 psig)?

From Figure 8-6.1, file vents should be placed no more than 7.6 m (25 ft) apart. In order to meet this requirement, it is recommended that a vent be placed at eadi end and that 13 additional vents be evenly spaced along the duct.

8-7.4 Provide deflagration vents for the ducts in the system shown in $ Figure 8-7.4. The gas flow through the system is 100 m / m i n (3500

Zmin) ' and all ducts are 0.6 m (2 ft) diameter. The maximum allowable working pressure for the ducts and equlpment is 0.2 bar

u 3 " ga Ige ( pstg) and file maximum operat lngpressure in the system is 0 .05bar gauge (0.73 pslg). Tile system handles a Class St-2 dust. It is furdmr assumed dlat the dryer and dust collector are equipped with adequate deflagration vents.

As required by 8-3.4 and 8-3.5, A and B should be located, respectively, within 2 diameters of file dryer outlet and no more than 3 diameters upstream of the first elbow.

- C located 3 diameters distance.

- F located at a position approximately 2 diameters upstream of the dust collector inlet based on 8-3.3.

9-2.3 Hangar-type Doors. Large hangar-type or overhead doors can be installed in the walls of rooms or buildings that contain a deflagration hazard. The doors can be opened to provide sizeable unobstructed vents during operation of the process or equipment in which fllere is an inherent deflagration hazard. However, die opening is a vent only when the door is not in place. Interlocks with process systems creating a deflagration hazard should be provided to assure dlat the doors are opened when file process is in operation.

9-3 Normally Closed Vents.

9-3.1 It is the responsibility of die vent closure manufacturer or designer to document die value and tolerance of the Pstat of a vent closure when installed according to file manufacturer's recommendation in die intended application. If the vent is custom fabricated on site, the manufacturer or designer should provide the same documentation.

9-3.2 Testing should be carried out to establish the Pstat for any given closure release mechanism, with that mechanism installed on the vent closure and tested as a complete assembly. This applies to all types of closure mechanisms, including pull-through fasteners, shear bolts, spring magnetic or friction latches, and rupture diaphragms.

9-3.2.1 Large panel closures as installed on buildings or other large, low strength enclosures cannot be tested as a complete assembly. For these closures, the designer should document that the entire assembly will release at the P,t~ specified. This information should include the design P,~, Pint, enclosure surface area, closure area,

697

N F P A 68 - - A 9 8 R O P

Vent Closure

Mass

"C-factor" used in the design, types of fasteners, spacing, and quantity. T h e design records a n d installat ion drawings shou ld be ma in t a ined by die bui ld ing owner a n d operator .

9-3.2.2 X~qlen ven t closure mechan i sms or fas teners are used, daey shou ld be listed for tile application.

9-3.5 The ven t closure shou ld be des igned to release at as low a pressure as practical a n d shou ld be compat ib le with die service condi t ions to which it will be exposed. Vent closure shou ld be des igned for dleir expected t empera tu re range.

9-3.4 Vent Closure Identification.

9-3.4.1 The vent closure shou ld be identif ied as an deflagration pressure-relieving device and should be marked wad1 dae release pressure.

9-3.4.2 The vent closure should be des igned to funct ion as rapidly ,as is practical. Tile mass of the closure shou ld be as low as possible to reduce die effects of inertia. T h e total mass of die movable part of die ven t closure assembly shou ld no t exceed 2.5 lb / f t e (12.2 kg /m2) . Counterweights shou l d no t be u sed because they add to the inert ia of the closure. Insulat ion added to panels is to be inc luded in dfis mass. Table 9-3.4.2 demons t ra tes die effect of vent mass on P red.

Table 9-3.4.2 Reduced Pressure (P~a) Developed Dur ing Deflagrat ina Vent ing In f luenced by Mass of Vent Closure

5% propane in air, 2.6 m s enclosnre [95]

Static O p e n i n g Vent Closure R e d u c e d ( P ~ ) Pressure (Pint) Response T i m e

Ib/ft 2 millibar gauge mil l isec millibar gauge

0.073 103 14.5 156 0.68 96 31.0 199 2.29 100 42.6 235 4.26 100 54.0 314

L / D ~ i - ~ =2.3 Test series repor ted = #17, #1, #3 a n d #4. A = 64.8 ft ~ (6.0 m 2 )

v

9-3.5 The closure shou ld be des igned to widlstand natural such as wind or snow loads, opera t ing c o n d i d o ~ . ~ . , ~ , it pressure f luctuat ions and internal t e m p e r a t u r . ~ ' n d e ~ corros ion. ~:.:'~ "i~i~iii~i

9-4 Types of Building or Room Vent Closures? of vent closures are i n t ended for use with large, enclosures such as fllose covered by Chapte r 4.

~g types

9-4.1 Hinged Doors, Windows, and Panels Closures::Y/These closures are des igned to swing outward a n d have latches or similar lmrdware flint automatical ly release when inf luenced by slight internal pressure. Friction, spring-loaded, or magne t i c latches of die type used for doors on industrial ovens are the usual type of hardware. For personne l safety, d ie door o r p a n e l shou ld be des igned to remain intact and to stay a t tached. Materials tha t t end to f r agmen t a n d act ,as sh rapne l shou ld not be used.

9-4.2 Shear and Pull -Through Fasteners. Specially des igned fasteners tha t will fail u n d e r low mechanica l stress to release a ven t closure are commercial ly available a n d some have been tested by listing or approval agencies. Shear and pull-fl lrough fasteners can be used where the ven t des ign calls for very large ven t areas, such as die ent ire wall o f a room.

9-4.2.1 T h e shear-type fastener is des igned to break f ro m flae shear stress flint develops in die fas tener when file pressure f rom a deflagrat ion pushes laterally on tile vent closure.

9-4.2.2 Tile pul l -dl rough type of fastener uses a collapsible or deformable washer to ho ld tile closure panel in place. Tile force of the def lagrat ion on the closure panel causes d~e washer to be pul led th rough die m o u n t i n g hole and tile panel can d len be p u sh ed away f rom tile vent opening.

9-4,2.3 Vent closures mad relief devices dlat fail u n d e r tens ion or shear migh t require higiler forces for opera t ion u n d e r dynamic condi t ion daan u n d e r d~e static condi t ions at which they are usually tested. These h igher forces migh t no t be compat ible ~i tb file design requ i rements of tile vent system.

9-4.3 Friction-Held Closures.

9-4.3.1 Some commercial ly available ven t closure assemblies use a flexible d i aph ragm he ld a r o u n d its edges in a restraining frame. W h e n a def lagrat ion occurs, d ie pressure de fo rms die d iaphragm, pusl t ing it f r om i t s . ~ . [See Figures 9-4.3.1(a) and (b).] This type of vent closure ~i.'.mbly':'~ well sui ted for large s t ructures such as rooms, buildi....~:; '~:~veyor enclosures , silos, dus t collectors, and b a g h o u s e s . . ~ i ~ a l ~ i ] ~ c u l a r l Y sui ted to ductwork opera t ing at or close to a t m t ~ h e r i c X ~ u r e .

closure

frame/curb

Figure 9-4.3.1(a) Exploded view of manufactured vent closure.

698

N F P A 6 8 - - A 9 8 R O P

Roof-mount: Built-up roof

• ~'~]Ve'nts

Wall-mount: Building

Roof.mount: Metal roof Wall-mount:

Filter collector holder

Duct mount

Figure 9-4.3.1(b) Typical applications for manufactured vent closures.

9-4.3.2 At locations where personnel or equ ipment might be struck by flying vent closures, te ther ing of the vent closure or o ther s~.¢..:~ measures is r e commended . ~.:'_-'ii~::

9-4.4 "Weak" Roof or Wall Construction. All or a po~ or wall can be designed to fail under slight pressure. ' vent closure, suitable lightweight panels can be used.

9-4.5 Large Area Panels. Large area panel multiple layers (insulated sandwich panel). figures (Section 9-5) refer to tests carried..<.. [30] Alternate methods for o ther types of careful engineer ing design and, preferably, ,assembly.

9-5 Restraints for Large Panels. ...,.:.-

9-5.1 Where large, lightweight panels ,are used as vent closures, it is usually necessary to restrain the vent closures so they to the do no t become missile hazards. The restraining med lod sbown in Figure 9-5.1 illustrates one me thod that is particularly suited for conventional single-wall metal panels. The key features of die system included a 2 in. (5 cm) wide, 10-gauge bar washer. The length of the bar is equal to the panel width, less 2 in. (5 cm) and less any overlap between panels. The bar washer /vent panel assembly is secured to the building structural frame using at least three 3 /8 in. (10 nm0 diameter dirough-bolts.

9-5. I. 1 The restraining techniques shown are very specific to their application. They are meant to be examples only. Each situation will require its own design. Arty vent restraint design should be documen ted by file desiguer.

9-5.1.2 No restraint for any vent closure should result in restricting the vent area. It is possible that a closure te ther could become twisted and bind the vent to less than the full vent opening area.

9-5.1.3 Any hardware added to a vent closure is to be included in the mass determinat ion of the closure, subject to 9-3.4.

i,f Vent panel .(:- . . . .

Girt c \ , in. d i ~ . Building thru-bolt I girt

G i r t - - -

Bar washer L (10 gauge) G i r l - - -

Girt

+1- Lap

[ Vent I panel

ii I washer

Elevation showing vent panels and bar washer assemblies

Figure 9-5.1 An example o f a restraint system for single-wall metal vent panels.

9-5.2 When die vent closure panel is a double-wall type (such as insulated sandwich panels), die restraint system shown in Figure 9- 5.1 is not r ecommended . The stiffness of the double-wall panel is much greater d l a n ~ f a single,wall panel. The formation of the Diastichin~e ~ d l 1 . 4 ~ c u r ~ r e slowly, and rotat ion of the panel can de incomptete#~'=..~: factors will t end to delay or impede venting during a defl.a.~ad~.,.,~.

9-5.3 T ~ n t sys't~. for d 9 .~] e-wfi]]~" pan els. l imi . t~ to 3"$.~{t ~ (3.1 m ~) ;.

~.~l,.~wn in Figu re 9-5.3 is r e commended ~ccessful functioning, die panel area is :its mass to 2.5 lb/f t~(12.2"kg/m~).

Vent

Bar washer

• -:':;:~$.:::" I Sheetmetal subgirt (10 gauge)

Blind rivet t Girt

i Roof girder

-e

Z\x in. dia. forged eye bolt sorber (C\zn in. thick

r--fro' om to move through

~ " Z \ v in. dia., 4-11 long galv. n(" V tether wire rope /

~ / ~ - - :~ \x in. dia. bolts

9-z \x i~ ' shock absorber

-4in.-.l

Figure 9-5.3 An example o f a restraint system for double-wall insulated metal vent panels.

Tests employing fewer that 3 rope clips have in some instances resulted in slippage of the te ther flirough the rope clips, thus permit t ing d ie panel to become a free projectile.

Forged eyebolts are necessary. Alternatively, a "U" bolt carl be substituted for the forged eyebolt.

699

N F P A 6 8 ~ A 9 8 R O P

A "shock absorber" device with a fail-safe tether is provided. The shock absorber is a thick, Dshaped piece of steel plate to which the tether is attad~ed. During venting, the shock absorber will form a plastic hinge at the juncture in the "L" as the outstanding leg of the "L" rotates in an effort to follow the movement of the panel away from the structure. The rotation of dais leg provides additional distance and time over which the panel is decelerated while simultaneously dissipating some of the panel's kinetic energy.

9-6 Equipment Vent Closures.

9-6.1 Hinged Devices.

9-6.1.1 Hinged doors or covers can be designed to function as vent closures for many kinds of equipment. The hinge should be designed to offer minimum frictional resistance and to ensure that the closure device remains intact during venting. Closures held shut with spring, magnetic, or friction latches are most frequently used for tins form of protection.

9-6.1.2 Hinged devices can be used on totally enclosed mixers, blenders, dryers, and similar equipment. It is difficult to vent equipment of this type if the shell, drum, or enclosure revolves, ttfrns, or vibrates. Charging doors or inspection ports can be designed to serve tiffs purpose where their action does not endanger personnel. Special attention should be given to the regular maintenance of hinge and spring-loaded mechanisms to ensure proper operation.

9-6.2 Rupture Diaphragm Devices.

9-6.2.1 Rupture dlaplwagms can be designed in round, square, rectangular, or other shapes to effectively provide vent relief area to fit the available mounting space. (See Figure 9-6.2.1.)

Q

\ '\~ .,

e e o e o m dp o

\ \

/ ° u ~ u - /

Figure 9-6.9.1 Typical rupture diaphragm.

9-6.2.2 Opening Characteristics. Some materials used as rupture diaphragms can balloon, tear away from the mounting frame, or otherwise open randomly, leaving the vent openin~ partially blocked on initial rupture. Although such restrictions migtlt be momentary, delays of only a few milliseconds in relieving deflagrations of dusts or gases having high rates of pressure rise can cause extensive damage to equipment. For these reasons, only rupture diaphragms with controlled opening patterns that ensure full opening on initial rupture should be utilized.

able to function as ir closure is properlygg operated or b e e n ~ might hinder i ~

9-7 Flame-Arresting Vent System. Certain equipment items have been developed and tested to stop the dust flame as it exits from the enclosure. These devices consist of a vessel attached to the vent opening, and have an opening equal to or greater than that of the vent. Hot gases and toxic products of combustion will exit these devices upon operation. Therefore, they should be directed to a safe outside location. Only limited information is presently available on the design. The user is advised that installing these devices on existing equipment can result in significant increases in P~a, or require the vent area to be increased.

Chapter 10 Inspection and Maintenance

10-1 General.

10-1.1 This chapter covers the inspection and maintenance procedures necessary for proper function and operation of vent closures for venting deflagrations.

10-1.2 The occupant of the property in which the deflagration vent closures are located is responsible for inspecting and maintaining such devices.

10-2 Definitions. For the purpose of dais chapter, the following terms have the meanings shown.

Inspection. Visual verification that the vent closure is in place and as m~nded. This is done by ensuring that the vent l y ~ e d and identified (see 9-3.4), that it has not t,-':~m" p e ~ l with, and that there is no condition that

proper and remedial actions taken to ensure

l O - ~ i ~ p e ~ o n Frcquen~'~"and Procedures.

10-$.I ~ p t a ~ c e mspecUons and apphcable tests should be conduc L ~ . e r insinUation to establish that the vent closures have

IT" " ~ b r d i n g to been insta ~ manufacturer's specifications and i~ .try acc~i~:~l practices. They should be clearly marked as an ~ ~ ~f device. The relief path should be unobsu'ucted and

to areas wherepersonnel might be harmed by the relief ..'.~. essur~:~ld fireball. (See 3-4.2, Section 7-7, and 7-7.1.)

"~h .~ Vent closures should be inspected on a regular basis. The luency will depend on the environmental and service conditions

which the devices will be exposed. Process or occupancy changes that can introduce significant changes in condition, such as changes in the severity of corrosive conditions, increases in accumulation of deposits or debris, can require more frequent inspection.

10-3.3 Inspections should also be conducted following any activity that could adversely affect the operation and the relief path of a vent closure (for example, after process changes, hurricanes, snow accumulations, or following maintenance changes) or following maintenance turnarounds.

10-3.4 Inspection frequency and procedures should be carried out according to manufacturer's recommendations.

10-3.5 Inspection procedures and frequency should be in written form and should include provisions for periodic testing, where practical.

10-3.6 To facilitate inspection, access to and visibility of vent closures should not be obstructed.

10-3.7 Any seals or tamper indicators tiaat are found to be broken, any obvious physical damage or corrosion, and any other defects found during inspection should be corrected immediately.

10-$.8 Any structural changes or additions dmt could compromise the effectiveness of vent closures or create a hazard to personnel or equipment should be reported immediately and should receive corrective action.

10-4 Maintenance. Vent closures should receive appropriate preventive maintenance as recommended by fl~e manufacturer.

10-5 Recordkeeping. A record should be maintained showing the date and the results of each inspection, and the date and

700

N F P A 68 1 A 9 8 R O P

description of each maintenance activity. The records of at least tile last three inspections should be kept.

Chapter 1 i Referenced Publications

11-1 The following documents or portions thereof are referenced within this guide and should be considered as part of its recommendations. The edition indicated for each referenced document is die current edition as of the date of the NFPA issuance of this guide. Some of these documents might also be referenced in riffs guide for specific informational purposes and, therefore, are also listed in Appendix F.

11-1.1 NFPA Publication. National Fire Protection Association, 1 Batterymarch Park, P.O. Box 9101, Quincy, MA 02269-9101o

NFPA 69, Standard on Explosion Prevention Systems, 1997 edition.

11-1.2 ASTM Publication. American Society for Testing and Materials, 100 Ban- Harbor Drive, West Conshohocken, PA 19428- 2959.

ASTM E1226, Standard Test Method for Pressure and Rate of Pressure Rise for Combustible Dusts.

Appendix A Explanatory Material

This appendix is not a part of the recommendations of this NFPA document but is inchMed for informational purposes only.

A-I-4 Approved. The National Fire Protection Association does not approve, inspect, or certify any installations, procedures, equipment, or materials; nor does it approve or evaluate tesdng laboratories. In determining the acceptability of installations, procedures, equipment, or materials, the authority having jurisdiction may base acceptance on compliance with NFPA or -taller appropriate standards. In the absence of such standards, said audlority may require evidence of proper installation, procedure, or use. The authority having jurisdiction may also refer to the listings or labeling practices of an organization concerned with product evaluations that is in aposi t ion to determine compliance with appropriate standards for tile current production of listed items.

A-14 Listed. The means for identifying listed equipment m ~ . ~ ' ~ for each organization concerned widl product evaluation, .~..~_e':~ whida do not recognize equipment as ~isted unless it is . ~ label~' .

authority ha.ngjurisdiction should utilize the by tile listing organization to identify a listed product. "¢.'-a'~:.: "- " - : . ~

e is A-2-6.1.1 At present there is no ASTM s t a n d a r ~ ~ ~]:- ~ determining die minimum ignition energies .~-:'dnsts gases). Al~ough several teK methods for ~ h a v e ,ee developed by different companies and or~i]]~..~.ons, l?eet~resul ts might not be equivalent. Reference 92 p u b l i s h ~ . . , ~ew .o..$" ignition energy test methods for dusts and gases ~ ad I~:en developed. "~ ~ p ~ "

represent~ of a sU'ess- A-~-3.3.1 The following curve is a general on strain curve for low carbon steel. See Figure A-3-3.3.1.

P ultimate x \ c UllJma~ *~'-- '-- ' ' ' '~

Yield /

f f l

s (strain) Figure A-3-3.3.1 Stress-straln curve for low carbon steel.

A-3-6.11 Data in Reference 45 show tile effects of us ing 5-cm (2-in.) 3

thick glass wool linings for propane deflagrations in a 5.2-m (184-

It s) test vessel equipped with a 1-m 2 (10.8-ft 2) vent for which P =

star 24.5 kPa (3.6 psi). The value of PredWaS 34 kPa (4.9 psig) in the unlined vessel and 5.7 kPa (0.8 psig) (that is, a reduction of 83 percent) when the glass wool lining was installed on two of the vessel interior walls.

Data in Reference 37 illustrate the effects of a 7.6-crn (3 in.) thick mineral wool lining for natural gas deflagrations centrally ignited in a 22 m 3 (777 ft s) test vessel equipped with a 1.1 m ~ (11.8 ft e) vent for which Pstat = 8 kPa (1.2 psi). The measured values of Pred were

about 60 kPa (8.7 psig) in tile unlined vessel and about 8 kPa (1.2 psi) (that is, a reduction of 87 percent) when tile lining was placed on tile floor and three walls of the vessel.

Similar dramatic reductions in Pred have been obtained in propane deflagration tests in a 64-m s (2260-cu ft) enclosure using ceramic fiber blankets on dlree interior walls. [102, 103]

A detailed discussion of tile role of acoustic flame instabilities in vented gas deflagrations can be found in Reference 43. Acoustic flame instabilities and enclosure wall lininl~s are important factors in unobstructed, symmetrical enclosures wath ignition near the center of the enclosure. Other types of flame instabilities, such as those described in Reference 44, that are not influenced by enclosure wall linings can have a greater influence on Pred in other

.:::'::.. situations. ..:¢~'~:'.~.~

A-4-$.1 N u m e ~ e t h o d s have been proposed for calculating tile vent closure ,..~.~a. ~.~,.27] Some venting models use tile surface area of th~.e.i~o.., su re~ , . .bas i s for determining vent area. Analysis of a v a l l ~ [ S O - ~'~'~9:..~ that such methods overcome certai~eficiehcies of pr~...~s methods of calculating vent area.

A ~ " '--'-;: ,~ii!..~x. ~ of calculation of reaction force during venting, for the foffi"::": ~:" "~: " $ ~ . g conditions:

~'.,.~#- ~= 2 ~.,~A = ~ 1550 in.

bar gauge = 14.5 psig

F, ffi (1550)(14.5)(1.2) = 26,970 lb.

Example of calculation of duration of thrust force resulting from venting of a dust deflagration, for the following conditions:

K ffi 160 bar m/sec St

3 V = 20 m

P = 0,4 bar gauge red

2 A = l . 4 m

V

t F

t F

( , ^ - 4 sec2 ~ (160) (20 )

7J(o.4)(1.,) = 0.57 sec

A-6-2.3 This equation was developed based on tile following considerations:

(a) Flame speeds and values of Pr~ increase rapidly in elongated vessels with L/D greater than tile maximum value for which Eq. (19) is applicable

(b) Gases with higher values of K G are more prone to flame acceleration in elongated vessels

701

N F P A 6 8 - - A 9 8 R O P

(c) Limited data on f lame speeds a n d pressures are available in Sect ion 5.1 of Reference 101 fo r p r o p a n e def lagra t ions in an open- e n d e d vessel with L / D approx ima te ly = 5

A-8-4.3 T h e curve identif ied "Dusts with Kst < 200" in Figure 8-4.3 is based on file data in Refe rence 75 for gasol ine vapor Oetmgrations. Ti le curve ident i f ied "Propane , Dusts with Ks, > 200" in Figure 8-4.3 is ob ta ined by r educ ing Reference 75 (L/D)m~, da ta f o r g a s o l i n e vapor by 50 percent• Thus , the Commi t t ee has e x e r c i s e d eng inee r ing j u d g m e n t in adap t ing tile da ta fo r use with dusts as well as gases.

A p p e n d i x B G u i d e l i n e s f o r M e a s u r i n g D e f l a g r a t i o n I n d i c e s o f Du s t s an d G a s e s

This appendix, is. not a art .fo the recommendations, of this NFPA document but is included for informational purposes only.

B-I Genera l C o m m e n t s . This a p p e n d i x discusses h o w file test p r o c e d u r e relates to ven t ing of large enclosures, b u t tile test p r o c e d u r e is n o t descr ibed in detail. ASTM E1226, Standard Test Method for Pressure and Rate of Pressure Rise for Combustible Dusts, sets fo r th a m e t h o d for d e t e r m i n i n g m a x i m u m pressu re a n d rate o f p res su re rise of combus t ib le dusts. [96] Since gases are n o t

o f p res su re rise that was r eco rd ed w h e n each was def lagra ted in a l - m s closed test vessel. T h e m a x i m u m rate of p ressure rise f o u n d in dais 1- m s vessel was des igna ted "K ." K is n o t a f u n d a m e n t a l

St St mater ia l proper ty , bu t d e p e n d s o n the condi t ions of tile test. The classification work carr ied o u t in the 1- m s vessel provides the only direct link be tween small-scale closed vessel tests a n d the large-scale ven ted tests on which tile n o m o g r a p h s are based.

The K G index m i g h t similarly be d e t e r m i n e d in a 1- m s (35 ft s)

vessel, b u t pub l i shed K values c o r r e s p o n d to tests m a d e in smal ler G

vessels. K G is known to be v o l u m e - d e p e n d e n t a n d s h o u l d n o t be

cons ide red a constant . Its use is res t r ic ted to normal i z ing

" " ~ ' [ d_f_} data g a th e r ed u n d e r a f ixed set of test condi t ions. \ U L ] m a x

1}-3.2 S tandardiza t ion o f a Tes t Facility. Tile objective of s tandard iza t ion is to be able to c o m p a r e tile def lagra t ion behavior of a par t icular mater ia l with o the r s for which full-scale test data are available. Wi th o u t access to tile 1-m s (35-ft s) vessel in which file original K classifications were made , it is essential to s tandardize

addres sed in ASTM E1226, Standard Test Method for Pressure and Rate , St . . . . . . . . . s _r o . . . . . . D:.- r .- ,~---L..,:LL r~+ ,. +^0+ . . . . . a . . . . . . . a: . . . . . a :-- m e test contri t ions empioyect us ing samples testect e iu ler in u le l -m 71/+: . . . . . . .1-. Y V (35f t s) vessel or I n o n e s tandard ized against it. ASTM def ines tile u . a .tVpcuuLx. s tandard iza t ion req..:.:.:.~tfi[~ ents for dusts. T h e h o m o g r a p h s identify a

At p r e sen t there is no ASTM s tandard m e t h o d for determinincr die series of gas m i x t # g ~ . w e r e u sed in tile full-scale tests. In o r d e r . . . . . . . o to calibrate for ........ the actual K values are n o t critical. Tbis is m i n i m u m tgmt ion energaes of dusts (as there is for gases). . ~ : . C.

AJ though several test met i lods fo r dusts have been deve loped by . . . . . . . . ~'_t..':"~':~-;~ . . . . . t.~ - .- . .• . . . . . . . ¢ . . . . . . . . :.~ ^¢ c t t t t e r en t com a n l e s a n c t o r mzat lons, the test results ml n t n o t D e __~:_.t^. . . .~.*~Y . . . . . £"~;~ .k^ . ^C .t . . . . . • .~ . . . . :'.~_~+:t:.^~:_.I.~

• P g a . . . . . . g , a l U L U | ~ I L L . : ~ I ~ I ~ L U I C I ~ l : ~ k : K L I L I ~ L O J L I ~ I C ~ 1 ~ l l l l ~ k L U l l ~ I U I ~ I / I ~ I L I C U I l l t H E eqt, lvalent+ Reference 92 p u b h s n e d a resaew of , g m u o n energy test - .4:+:-'z:~:+:::-. "+. m e t h o d s fo r dusts a n d gases tha+ h a d been deve loped . . . . #:" "+ [ ~.].~+:~

n o ~ a p | % ~ If these [ ~ | values are all m e a s u r e d u n d e r ,oo . . . . . . . . . . . . . . . . : ~ +i::"::~,:..':.,.. ::~....',~.:. \ ~ t ) ~ .

I I I I I I I I I I I I I I I I " . . : : ~ : :~ : :~ .~ : : ,e~ ,~ : : : : : : . . + . ~ I I I I I I I I I I l l [ l t I - - ~ • . . . 2::..~.... • ~ . . . . . . illll~ t i tml~ ~ ~ i i i ill,,, , , I t m l ident lc~i~ . .~dl t lons In a vessel mee t i n g cer tmn cri teria (see Section B. I I I I I I I I I l l l l l l l ~ I I I l l l ] l l l I I l l l l l e . . .~ . . . . .

700 i i l l l i ' [ I I [ ; f l ; l I i ~ i " l l 4), the nOt*..'.~..'..~Zrap..[)s m i g h t be used by lnterpolataon. In o rde r to " - - I I I i i 1 1 1 1 I I I [ t i l l ~.~:.::..%:.. • " : : :~" "~ ~'~" . . . . . ~ L H ~ r O l ~ f l ~ 7"-" I , . . . . . . . . . . . . . . :~;~i.~:...¢....allbrate f o t ' ~ , winch c a n n o t be identafied by compos i t ion

, , , , , , - . . " , " ~ ,,,,,, - - , , , ,,,,,, , , , ~ w#.:."~m~+ it is + o ~ ' ; + ~ , , , , o b t m | n e~mnlac havin~ oe~l~lieh-ed IZ t~l,,+e *00 I I ~ I I I I l l l l I - - I I I I I l l l l I I , # ' I l l l l : : ~ * . . ~ . . ~ . ~ . ~ : . + ~ . ~ o ~ 7 m ~ ~ = + o ~ t a a [ , , a ~ o / ~ ~ o ~ . . o [ , " ~ t ~ . , ~ o IL 3 1 1 I I I I I I I H ! I I I I l l l l l I ~ I I l l l l ::-.%. ======================= +.m" ~ [

- - l , , , I I , , , , , , , , , , , , - - , , , , , , , , , ~ , , . : :m -:-::;:,~:::-.,:+~ ~" t , t , , , , i , t m u , - - , , , , , , u ~ , , i l l l l l $'-';: (sed.:'~e~';~::B-S).

I l l l l l l l l l l l l l l - - Z~ .+~ : . - - - -~ :++e . • ~ ~ --~-~ ~ i ~llll~ __~ ~ . ::;i:::~ > .<:+,

~- ~ e " ' ' " ' " ' ___~ ..~ , , , , .... ~:::+,T|~.~ !:.~ :~.$.$ D e t e r m m a t l o n o f t h e K a n d K Indices. I f the max~mum x ~ + ~ II I I l l l l l l _ _ #~ i i i i i i i i i +.:~i. • i I l l l l . ~ ~'~'~ ~% P ~ 1 ~

. . . . . . . . . . - - . . . . . . . . . . .:.+;.~;_,, I I . . . . . . + ~ * • • • o 4¢11 ~ , H fl , 11 H i l l ~ , I 11 ,,.I}Y.'-:-:-:t.~I I , 11~" .~*~¢'.% . ~ r ~ o f p r e s s u r e n s e i s m e a s u r e d i n a v e s s e l o f v o l u m e o t h e r t h a n 1

+ . . . . . . . , , , ..... ; , , ' 1 ~ "" ~ (35 ft ), the followintz re la t ionsh ip is u sed to normal ize the value 'l l~t',',', ', ', I I I I I ' , ] - - ~ "~'~'::@obtained to a 1-m s vessel. I I I I l l l I I I I I I I I I

" I | | t I I I l i i l l a ~ , .4-'.- 'I I I | rm.l~:-~44,~( I I | i t { ::~£~:"

+tin i ill01 i t.~quH _____ r. i i ~ t,lUt..x<,r..:.+.t i ~l~l $- I O r l [ 11"4'~ - - - i I I I l l l ~ 1 I l l l l l . ~ l +:~ I I I I l l i l l . : .~ - ; -~ i l l i k + + t +

. . . . . . . . - . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . / / I V / = K (29) ', ' , t ~ ~ A . ~ , , ~ ' - ' - - ~:~.~' , ' , ' , ' , ' , ' , ' , ' ,~ ; " , I I I I k d t , ~ x

' ~ l ~ ' " ' ~ + ~ < " q l lllll' , ~ ~ - ~ - J ' " - "

i¢;~.~-, i i . . . . . . . . . . . Ill

• v l . l ~ c v ' . . . . . . . . . _ ~ ~ ~ " i ".):.:¢.:c

~ 1 I I I I l l | i - - [ 1";~Sg-~ll l .':-:q I I I I I I I I I I I H I I i I I I I J i I I I ' : $ : t i ~ & : : : : " I I I I I I I I I [ l l l I I I I l l l l l - - i i I I i ' t ~ f " I I I I I I I

1 0 1 0 ~ 1 0 0 0 I O,0O0 ¢ ~ , 0 0 0 1 , 0 0 0 , 0 O 0

TO,A v l ~ . s ~ d v e ~ w n e ( 1 ~ )

Figure B-I E f f e c t o f t e s t v o l u m e on K G m e a s u r e d in spher ica l

vesse l s .

B-2 Purpose . T h e p u r p o s e o f d e f l a g r a t i o n index m e a s u r e m e n t s i, to p red ic t the effect of tile def lagrat ion of a par t icular mater ia l (dus t or gas) in a large enc losure w i thou t carrying ou t full-scale test work.

B-3 Basic Principles. T h e h o m o g r a p h s p r e s e n t e d in this gu ide a n d those in VDI 3673 [104] a re based on large-scale tests carr ied ou t in ven ted vessels u s ing a variety o f test mater ia ls a n d vessel sizes. [3, 47] For each test mater ia l a n d vessel volume, tile m a x i m u m reduced def lagra t ion p ressu re (Pred) was f o u n d for a series o f vents

with var ious areas (Av) m}d o p e n i n g pressures (Pstat) . Use of the

n o m o g r a p h s requ i res only tha t a single material classification (tile K or K index) be exper imenta l ly ob ta ined by the user. Knowing

G St t he vo lume and mechan ica l cons t ra in ts of file enc losure to be protected, the use r can then d e t e r m i n e the vent ing r e q u i r e m e n t s f r o m the n o m o g r a p h s .

B-$.I T h e K G and KSt Indices . T h e test dusts used d u r i n g the

large-scale test work were classified accord ing to tile m a x i m u m rate

W h e r e :

P = p ressu re (bar ) t = t ime (s)

3 V = vo lume ( m ) K = normal ized K o r K index (bar m / s e c )

G St

Tile m e a s u r e d m a x i m u m def lagra t ion pressure , P , is n o t m a x

scaled for vo lume a n d tile expe r imen ta l value can be u sed for des ign purposes . The m a x i m u m rate of p ressure rise is no rma l i zed to a vo lume of 1 m 3 (35 ft s) us ing tile above equat ion . If tile m a x i m u m rate of p ressure rise is given in uni t s of s ingular b a r / s e c

s and the test vo lume in uni ts of m , tile equa t ion def ines the K or G

K index for tile test material . St

Example: T h e vo lume of a spherical test vessel is 26 L (0.026 m s)

a n d the m a x i m u m rate o f p ressure rise, , f o u n d f r o m the

m a g

s lope o f tile p r e s s u r e / t i m e curve is 8300 p s i / s e c (572 b a r / s e c ) . Subst i tu t ing these values in t he equa t ion above, the no rma l i zed l/s index is equal to 572 (0.026) , or 169 ba r -m/ sec .

7 0 2

N F P A 6 8 ~ A 9 8 R O P

B-3.4 Effect o f Volume on K and K . In die case of many G S t

initially qu iescen t gases, die normal ized index K G is found no t to be

cons tan t but to increase widl vessel volume. Figure 13-1 shows die variation of K widl vessel vo lume for me thane , propane , and

G ent,ane as measu red in spher ical test vessels• [77] The increase of

is related to various f lame accelerat ion effects as descr ibed in G

References 44, 78, and 79. It is for dlis reason dlat K values G

m e a s u r e d in vessels of different sizes canno t be directly compared, even if all od ler factors affecting K G are he ld constant . Any K G

m e a s u r e m e n t shou ld be made in a spherical vessel at least 5 L (0.005 ft s) in vo lume a n d the values ob ta ined shou ld be used only to interpolate between die ven t ing requ i rements of gases identif ied in die h o m o g r a p h s (set Section B-4).

The effect of vessel vo lume a lone on k~ values ob ta ined for part icular dusts has no t been well established. Dusts c anno t be s u s p e n d e d in a qu iescen t m a n n e r and die initial tu rbu lence in t roduces a nonscaleable variable. However, it c anno t be a s sumed fllat KSt in file equa t ion in 13-3.3 is i n d e p e n d e n t of vessel volume. It

3 has been f o u n d [47] dlat K vahies obta ined in file original 1-m

S t 3 (35-f t) classifying vessel c anno t be r ep roduced in spherical vessels

$ of less allan 16 L (0.016 m ) vo lume nor in file cylindrical H a r t m a n n apparatus . All existing facilities dlat have s tandardized e q u i p m e n t use a spherical s test vessel of at least 20 L (0.02 ft ) vo lume or a squat

cylinder of larger vo lume (such as die 1-m classifying vessel itself). T h e principle of KSt s tandardizat ion in sucli vessels is to adjust test

condi t ions (particularly initial tu rbu lence) until it can be demons t r a t ed fllat a series o f dusts all yield K values in acceptable

St 3

a g r e e m e n t widl die values fllat [lave been establisiled in dae 1-m 3

vessel. [96] If vessels of vo lume d r i e r allan 1 m are used, Eq. (29) in 13-3.3 has to be used. This m i gh t lead to errors dlat are d e p e n d e n t on K t. Such errors si lould be cons idered when

applying test da ta to vent design. [77] #:~,,. ~ .~{ .~

B-3.5 Effect of Initial Pressure . T h e initial pressure f o g ~ a g r a ~ . i g n • . • # ". :-x " ~ .

t e s t i n g i s o n e s t a n d a l ' d a t l q f l o s p h e r e ( 1 4 . 7 p s l a , 7 6 0 m r r t i ~ ' ~ . ~ . . . 1 ¢ ~ bar). Alternatively, a s t andard pressure of 1 bar could be ~ t i ' d l " negligible error. If initial pressures are no t of s ~ . . ~ . v a l u e llave to be repor ted and correct ion m e t h o d s a ~ !' ' ~ :::::::: •

proport ional to initial test pressure a n d an~g .~e l en initial test pressure a n d one s t andard a t m ' d b t ~ tiplied by die def lagrat ion pressure rado (usually b~.~tw66~i~ and 1...~: in the

( _ . ~ . ] """~!b.. ~ d P ':*-::~*~::. --:-'-:" m e a s u r e d P vah,e. Measured values"~..~'ge affected

max max

to a smal ler degree. T h e effect of initial pressure is mos t impor tan t where tests are conduc t ed at ambien t pressure. Ambien t pressure can v , ~ f rom ext remes o f 12.9 to 15.6 psia (0.89 to 1.08 bar), even at sea level, a n d decreases wifll elevation. For example , at an elevation of 2 km (1.25 miles), the average pressure in lati tude 50°N is 11.5 psia (0.79 bar). It is readily seen tha t a P value measu red

m a x at sucil an elevation would be about 20 percen t lower than would be measu red at one s tandard a tmosphere , a s sumi ng a 10:1 deflagrat ion pressure ratio. It is always preferable to conduc t tests u n d e r suandard condi t ions raffler t l lan to correct file measu red values.

B-4 Gas Testing. The test vessel used for gas test ing shou ld be spherical wifli a volume of at least 5 L a n d preferably 20 L or greater. Since die only source of initial tu rbu lence is the ignition sonrce employed, an impor tan t considerat ion is dlat the f lame f ront no t be undu ly distorted by die ignition process. T he ignition source shou ld be centrally placed a n d shou ld approx imate a po in t source. A discrete capacitor d ischarge carrying no great excess of energy above dlat needed to ignite file mix ture is r e c o m m e n d e d . Fused- wire a n d cllemical igniters migh t cause mul t ipo in t ignit ion and shou ld no t be used for rout ine I¢~ m e a s u r e m e n t s in small vessels.

Stan&trdization gas mixtures, as identif ied in die nomographs , migh t be initially tested in file system. Each gas mixture has to be verified to be well mixed and qu iescen t immedia te ly pr ior to

ignit ion. The m a x i m u m rates of pressure rise are measu red systematically for several composi t ions close to file s to idf iometr ic mixture until file m a x i m u m K G value has been found. A table of K o values is fllen establ ished for die s tandardizat ion gases as m easu red in file test vessel. These values will no t necessarily be die same as die I ~ values given in tile gas n o m o g r a p h s (set B-3.4).

In order to subsequendy apply the nomograpias to a test gas, d ie m a x i m u m K G value for dae test gas has to first be foun d u n d e r identical condi t ions to dlose used for s tandardizat ion. The test material is compared with s tandardiza t ion gases having K o values above and below the test value as measu red in die test vessel an d die vent r equ i rements are d l en f o u n d by interpolat ion between die r equ i r emen t s for the s tandardizat ion gases.

A data base shou ld be establ ished for the test e q u i p m e n t in which K G values are given for a wide variety of gases tested u n d e r die s tandardized condit ions. K G values sl lould no t be repor ted unless this data base, or at a m i n i m u m file K o values f o u n d for fide s tandardizat ion gases, are also repor ted.

Most f l ammable gas mixtures at d ie o p t i m u m concent ra t ion migh t be convenient ly igni ted in small vessels us ing a capacitor spark of 100 mJ or less and dais m igh t be a no rma l ignit ion source for s tandardizat ion. However, dae igni t ion r equ i r emen t s for certain exceptional gas mixtures migh t s t e a d y exceed dlis figure. Before a gas mixture is des igna ted as noncombus t ib le , it shou ld be subjected to a s t rong ignit ion s...9+.t, rce (see Section B-6).

...~.~¢.'~..

~aiougia ale q . ~ o ~ s deal widl deflag~atious of g~es in ~r, it migh t be n e c e ~ i ~ : : p r e d i c t d ie effect of oilier oxidants such as chlorine. It ~ e c o ~ i ~ n d e d dlat file K c concep t no t be ex t en d ed to such s ' ~ c e p t ~1 expert ise can c~ . . ~ . e considerable be demons~$ ~ .~ t . f l le te i~ : ~ i ~ . Many gaseous mix tures will be

~ d l file bl, m a ~ ' ] of tile test vessel a n d mi l l any trace i u CO ..~...:~."'h ....... :"~"~" co n : . . . .~ !na~.wid l in it, i r k ~ d i n g traces of humidity. Expert b ~ . ~ l ': ":" ".b, sough t in applying such test da ta to tile

re enclosures . O

protec

I !

Figure B-4 Per fora ted r ing dispers ion system.

B-5 Dus t Testing. Dust samples having d ie same cllemical compos i t ion will no t necessari ly display similar K values or even

St similar def lagrat ion pressures (P ). The bu rn ing rate of a dus t

m a x d e p e n d s markedly on the particle size dis t r ibut ion and sllape, a n d on oil ier factors such as surface oxidat ion (aging) and mois ture content . The fo rm in which a dus t is tested shou ld bear a direct relation to the fo rm of that dus t in file enclosure to be protected• Owing to fide physical factors in f luenc ing fide deflagrat ion propert ies of dusts, file h o m o g r a p h s do no t identify file dusts involved in large- scale test ing except by flleir measu red KSt values. Ald lough

Appendix D of tiffs guide gives both K and dus t identit ies for St

3 samples tested in a 1-m vessel, it shou ld no t be a s sum ed dlat oilier samples of tile same dusts will yield die same K values. These data

St canno t be used for vessel s tandardizat ion, bu t are useful in de t e rmin ing wends. Tile test vessel to be used for rou t ine work sl lould be s tandardized us ing dus t samples whose K a n d P

St m a x

7 0 3

N F P A 68 ~ A 9 8 R O P

3 characteristics have been established in file standard l-m ($5-ft s) chamber [96].

B-5.1 Obtaining Samples for Standardization. Samples should be obtained having established K values in Dust Classes St-l, St-2, and

St St-3. At the time of the writing of thisguide, suitable standard samples (with file exception of lycopodimn dust) were not generally available. ASTM E1226, Standard Test Method for Pressure and Rate of Pressure Rise for Combustible Dusts, defines fl~e agreement required

3 with values generated in Ode standard 1-m (35-fC) chamber.

B-5.2 Effect of Dust Testing Variables. For a particular spherical test vessel [20 L (0.02 ft s) or greater] and a particular prepared dust sample, the following factors affect the measured K :

St

(a) tim mass of sample dispersed, or concentration

(b) the uniformity of dm dispersion

(c) tim turl)ulence at ignition

(d) the ignition strength

The concentration is not subject to standardization since tiffs should be varied for each sample tested until the maximum KSt has

been found. The maximum KSt usually corresponds to a

concentration several times greater than stoichiometric. ASTM E1226, Standard Test Mettwd for Pressure and Rate of Pressure Rise for Comtmstible Dusts. recommends a series of concentrations to test. A plot of measured K is made against concentration, and tests are

St continued until dm maximum bas been found. By testing progressively leaner mixtures, tile minimum explosive concentration (lean limit or LFL) might similarly be determined. Tiffs limit might be affected by ignition energy.

B-5.2.1 Obtaining a Uniform Dust Dispersion. The uniformity of dust dispersion is implied by die ability to achieve consistent and reproducible KSt values in acceptable agreement with tile .,-:~..:?-.~

establisbed vahles for the samples tested. Poor dispersiola.....s~i::% to low values of KSt mad P . ..~ii!:,

max ,::~,.:..-.::~::::...-...

A number of dust dispersion methods exist. For small v e ~ i most common types are the perforated ring and:..~,,~£ing~.-;.':',.'! bose." Tim perforated ring (Reference 96, AF~.~fl E 1 2 " ~ / a ~ ..'.'~ Test Method for eressur, and Rate of Pres sure .~ for Cornli ~ ¢¢~.w.. ..... Dusts, Appendix 17-2) fits around tim i n s i d ~ ' ~ e . o f the ~ t ve; I and is designed to disperse the dust in many ' ~ g n s . A ~ g of tiffs type is described in Reference 47 in relation "~:~,e du.~:

%%:,@ classifying work in Ode 1-m (35 ft s) vessel. However~:~iossible problem with this device is clogging in dae presence~ 'waxy materials, low-density materials, and materials that become highly electrically claarged during dispersion. To minimize these problems, the whipping hose [77] has been used. Tiffs is a short length of heavy-duty rubber tubing dmt "whips" during dust injection and disperses the dust. Comparison of these two metimds under otherwise identical conditions [77] indicates that tiaey might not be interchangeable and fl~e dispersion method should be subject to standardization.

B-5.2.2 Standardizing Turbulence at Ignition. During dust injection, the partially evacuated test vessel receives a pulse of air from d~e air bomb which brings d~e pressure to 1 atm. (absolute) and disperses dust placed below tile dispersion system. Some time after the end of injection, tide igniter is fired. The following test variables affect turbulence at ignition in tile test vessel:

(a) Air bomb volume

(b) Air bomb pressure

(c) Initial vessel pressure

(d) Injection time

(e) Ignition delay time

References 77 and 80 describe combinations of these variables that have yielded satisfactory results. For example, a 26 L test vessel [77] employed a 1 L air bomb at 300 psia (20.7 bar). Having established tim air bomb volume and pressure, the initial test vessel reduced pressure and injection time are set so that after dust injection the test vessel is at 1 aim absolute. It should be noted that the air bomb and test vessel pressures need not equalize during dust dispersion. Injection time and ignition delay time are set using solenoid valves operated by a timing circuit. For standardization, reproducibility of timing is essential and it might be found that file optimum ignition delay time is in the order of 10 milliseconds. Fast-acting valves and accurate timing devices should be employed.

Standardization using well-characterized samples (see B-5.1) is complete when samples in dust classes St-l, St-2, and St-3 have been slmwn to yield tim expected KSt (to within acceptable error) witia

no adjustment of the variables listed in this section. Also, the mode of ignition (see B-5.2.3) should not be changed for standardized testing.

B-5.2.$ Ignition Source. The ignition source might affect the K & values obtained even if all oilier variable are held constant. It has

3 been found [47] flaat in a 1-m vessel, capacitor discharge sources of between 40 mJ and 16 J gave comparable K and P data to

St max those obtained using.&10 kJ chemical igniter. In dm same vessel, a permanent spark ~ . . d e r r a t e d both K and P for a range of

..:-:-- -::-'?---. 5t max samples. In R ~ c e s 77 and 81, it is described how comparable K and P ~ i ~ # r e obtained in vessels of approximately 20 L

:st m ,'l~. ":-':~i~ii::.. . . using b e ~ p . ' ~ : a n d 6 " ~ ' ~ a l [ y placed electric match Jgmters each

V...#~..~.s ~ of electri~lly initiated chemical ignition source d ~ i c ~ ~ e n satisfactory during routine test work, tile most popu la~ : : :~g two 138J electric match igniters or two 5 kJ pyrotechrJ}~.~.~vi.c~s. These ignition sources are not interchangeable ~ d s t a n d a r ~ " o n should be based on a t'Lxed type of igniter. The ~ . ¢ ~ hav~.=..:.Efisuffident power to ignite ,all combustible dust m ~ : . ~ : " F o r dais reason, any dust appearing to be St-0 should ..~-retes'~.~ using two 5 kJ pyrotedmic igniters (see Section B.6). The ~.fiutine"~ise of tile pyrotechnic igniter as a standardized source

res a method of correction for its inherent pressure effects in ~ ' i vessels. [77] Therefore, neither source is ideal for all ~tplications.

!1-5.$ Dust Preparation for KSt Testing. A dust has to be tested in a

form that bears a direct relation to its form in any enclosure to be PtakrOtected (see Section B-5). Only standardization dusts and samples

en from such enclosures are normally tested in dae "as-received" state. The following factors affect fide KSt:

(a) Size distribution

(b) Particle shape

(c) Contaminants (gas or solid)

Although dusts might be produced in a coarse state, attrition can generate "fines." Fines might accumulate in cyclones and baghonses, on surfaces, and in file void space when filling larg.e enclosures. For routine testing, it is assumed fl~at such fines m,ght be represented by a sample screened to sub-200 mesh (75 mm). For comprehensive testing, cascade screening into narrow-size fractions of constant weight allows K to be found for a series of

St average diameters. Samples taken from file enclosure help in determining representative and "worst-ease" size fractions to be tested. If sufficient sample cannot be obtained as sub-2O0 mesh, it might be necessary to grind tide coarse material. This might possibly introduce an error by affecting file shape of the fines produced. The s ecific.surface. .°f a. sample, which, affects burning rate, depends on ~oth stze distribution and parUcle shape.

When considering fines accumulation, fide accumulation of additives also has to be considered. Many dust-handling processes can accumulate additives such as antioxidants timt are added as onl) a small fraction of the bulk. Such accumulation might affect K

St

704

N F P A 6 8 1 A 9 8 R O P

and, by reducing the ignition energy necessary to ignite the mixture, might increase die probability of a deflagration. [77]

Flammable gases might be present in admixtures with dusts (hybrid mixtures) and many accumulate with time owing to gas desorption from the solid phase. Where dfis possibility exists, both KSt and ignition energy might be affected. The effect of hybrid

mixtures can be synergistic to the deflagration and a gas present at only a fraction of its lower flammable limit has to be considered. [3] Testing of hybrid mixtures can be carried out by injecting the gas-plus-dust mixture into an identical gas mixture already present in file test vessel. Tile gas concentration (determined on the basis of partial pressure at the time of ignition) should be systematically varied to determine file range of hybrid KSt values that might apply

to file practical system.

Tile use of a whipping hose (see B-5.2.1) should avoid the necessity of using inert flow-enhancing additives to belp dust dispersion in most cases. Tile use of such additives in testing is not recommended.

B-6 Classification as "Noncombustible." A gas or dust mixture cannot be classed as noncombustible (for example, Dust Class St-0) unless it has been subjected repeatedly to a strong chemical ignition source of 10 kJ. I fa material fails to ignite over the range of concentrations tested using tile standard ignition source then, after tile equipment is checked using a mater ia lof known behavior, the test sequence is repeated using a 10-kJ chemical igniter. It has to be established that the strong ignition source will not yield a pressure history in the vessel dial might be confused with any deflagration produced by it.

It might not be possible in small vessels (for example, tile 20-L vessel) to unequivocally determine whether a dust is noncombustible. This is because strong igniters such as 10-kJ pyrotechnics tend to overdrive the flame system, in addition to producing marked pressure effects of their own. C, ashdoilar and Chatrathi (Reference 97) have demonstrated the overdriving effect when determining minimum explosible dust concentrations. Apparently-explosible mixtures in a 20 L vessel would not propagate

3 flame in a 1-m vessel at the same concentration. These anthors recommend use of 2.5 kJ igniter for lower flammable limit ~ measurements which gave similar results to tile 10-kJ igni..~.~ n a".~

3 . .,~,:,.'~ .,,'x":~x m vessel. The ideal solution is to use large (10 kJ) , g ~ *~nZ" ~

3 x ~ g larger ( 1 - m ) vessels. They further recommend tile .ignitio~ criterion of an absolute pressure ratio greater i t h ~ - ' ~ i ~ a "::::"

greater than 1.5 bar-m/sec. ..~# . . . . . ~...x:.! S~ ~g:~

An alternative to the use of the strong igniti,~..u..rce and' s associated pressure effects in small vessels is to '~.~.ner-si.~ fractions than tile routine sub-200 mesh. Dust igvu~::..e~. ~'rgy varies with approximately die cube of particle diameter [ 7 ~ : n c e , the

identifi* use of electric matches might be extended to ion of St-0 ~ t dusts. Similarly, the dust lean limit concentration might be subject to ignition energy effects, which decrease with decreasing particle size of the sample. Such effects largely disappear when sub-400 mesh samples are tested. In tile case of gases, a strong ignition source consisting of capacitance discharges in excess of l O j or filsed-wire sources of similar energy migbt be used. Such sources are routinely used for flammable limit determination.

B-7 Instrumentation Notes. Data can be gathered by analog or digital methods, but tile rate of data collection should be capable of resolving a signal of I kH z or higher frequency (for digital medlods, better than 1 data point per millisecond). For fast-burning dusts and gases, particularly in small vessels, faster rates of data logging

might be required to resolve Data logging systems n.12t X °

include oscilloscopes, oscillographs, microcomputers, and other digital recorders. An advantage of digital methods is that both the system operation and subsequent data reduction can be readily automated using computer methods. [77] A further advantage of digital methods is that expansion of tile time axis enables a more accurate measurement of slope of the pressure/t ime curve than can be obtained from an analog oscilloscope record. When using automated data reduction, it is essential to incorporate appropriate logic to obviate the effect of spurious electrical signals. Such signals might be reduced by judicious cable placement, grounding, and screening, but are difficult to avoid altogether. It is advantageous to

manually confirm automated values using tile i l l a x

pressure/t ime curve generated.

When making up gas mixtures by the method of partial pressures, it is important to incorporate a gas temperature measuring device, for example, a the1 ~ u p l e , to ensure this is done at constant temperature. C,a f i a l '~ , i s preferable where such facilities exist.

~... It has been ~ t piezoelectric pressure transducers are

satisfactov ~ . n pressure measurements in test systems ot this kim ~ o o ~ . ~ . ~ t i o n stability. The u~msducer should

and be flu.~ ~ t h e ' ~ wall of file vessel coated with s i l i ~ ~:er, thereby ~ i m i z i n g acoustic and thermal effects.

• ~ f $ : .

¢~ie~ ~ '~-system should be routinely maintained and subject~ !~::periodic tests using standard materials of known bebavio ~ . , . ~ e r initial standardization, it is advisable to

~¢ .pa re l a r g ~ a n t i t i e s of well-characterized dust samples (Classes ~ g t ~ ar~'~t-3) of a Woe not subiect to a~in~ or other effects.

tests.

C Fundamental Burning Velocities for Selected Flammable Gases in Air

This appendix is not a part of the recommendations of this NFPA document but is included for informational purpose on O.

The values of fundamental burning velocity given in Table C-1 are based on NACA Report 1300 (Reference 82). For the purpose of fltis guide, a reference value of 46 cm/sec for tile fundamental burning velocity of propane has been used. The compilation given in Perry's Chemical Engineer's Handbook [83] is based on the same data (NACA Report 1300), but uses a different reference value of 39 cm/sec for the fundamental burning velocity of propane. The reason for using the higher reference value (46 cm/sec) is to gain closer agreement with more recently published data.

705

N F P A 6 8 - - A 9 8 R O P

ii i v Gas

i Acetone Acetyiene Acrolein Acrylonitrite Allene (propadiene) Benzene

,n-butyl- ,ter~obutyt- ,1,2-dimethyl- 1,2,4-trimethyt-

1,2,-Butadiene (methylallene) 1 ,$-Butadiene

,2,3-dimethyl- ,2-methyl-

n-Butane ,2-cyclopropyl- ,2-2-dimeth~4- ,2,3-dimethyl- ,2-methyl- ,2,2,8-trimethyl-

Butanone 1-Butene

,2-cyclopropyi- ,2,3-dini~yl- ,2-eth~- ,2-methyl- ,S-methyl- ,2,5-dimethyi-2-butene

2-Buten l-yne (vinylacet34ene) l-Butyne

,3,5<limethyl- 2-Butyne Carbon disulfide Carbon monoxide Cyclobuume

,eth~ ,isopropyl- ,methyl- ,methylene

Cydohexane ,methyl-

Cydopentadiene Cyciopentane

,methyl- Cyclopropane

,cis-1,2-dimethyi- ,tram-l,2-dimethyl-

,l,l,2-trimethyi- trans-Decalin (decahydronaphthalene) n-Decane l-Decene Diethyi ether Dimethyl ether Ethane Ethane (ethylene)

a ~ m m t ~ Velocltlm of Selected Gases and Vai~om Table GI F Gm

Veloe~, era/me Ethyl acetate lee* 06 5O 87 48 S7 39 S7 39 68 64 52 55 45 47 42 43 43 42 42 51 50 46 46 46 49 44 89 68 56 61 5 8 46 07 5S 46 52 61- 46 44

Ethylene oxide E _thvlenimine n-tteptane - Hexadecane -- 1,5-Hexadlet3e n-Hexane 1-Hexene l-Hexyne 3-Hexyne Hydrogen lsopropyl alcohol lsopropylamine Methane

,diphenyl- Med~y[ alcohol 1,2-Pentadiene (ethylallene) cis-l,8-Pentadiene trans-l,8-Pentadiene (piperylene)

,2-methyi-(cos or tram) " 1,4-Pentadiene 2,3-Pentadiene n-Pentane

,2,2-dimeth ~ ,~S~Umeth~ ,~,4~Um~- ,2-methyl- ,~nethyl- ,2,2,4-trim

l-Pentene'~

c i s - 2 - P ~ n t 8 ~ l-Pentene b,

42

47 54 47 80*

Propylene oxide (1,2-epoxypropane) 1-Propyne S~ropenume

Toluene (meth~4beazene) Gasoline (100.octane) Jet fuel, gradeJP-I (average) Jet fuel, grade JP-4 (average)

htdameaUd Burning Veloci~ cm/sec

108 46 46 44 52 40 50 57 5~

312" 41 31 40*

56 61 55 54 46 55 60 46 41 43 42 43 43 41 50 47 48 51 03 53 61 54 46* 50 40 40 40

9

41 55 44 58 82 82 71 48 39 41 40 40 41

*Gases so marked have been critically examined in Refereaces 84 or 85 with regard to fundamental burning velocity. Table compares the selected values from these references with ttmee given in Table G:I.

C-2

706

N F P A 68 1 A 9 8 R O P

Table C-2 Comparison of Fundamental Burning Velocit ies for Selected Gases

i

Fundamental Burning Velocity, cm/sec

Gas Table Andrews and France and C.1 Bradley (84) Pritchard (85)

tin air) (in ox,/~n) 0u air) I140 Acetylene 166 158 Ethylene 80 79 - - 0 Hydrogen 312 310 1400 347 Methane 40 45 450 43 Propane 46 - - - - 46

Appendix D Deflagration Characteristics of Selected Flammable Gases

This appendix is not a part of the recommendations of this NFPA document but is included for informational purpose on~.

As stated in 6-2.4.4 and Appendix B, the K G value is not constant

and will vary depending on test conditions such as type and amount of ignition energy, volume of test vessel, and other test conditions. Thus, a single value of K for a particular set of test conditions is

G but a "snapshot" among a continuum of values that vary over tile range of test conditions.

Figure B-1 shows K G values for methane, propane, and pentane

over a range of vessel sizes. [77]

Table D-1 lists K values determined for several gases. The values G

were determined by tests in a 5-L sphere with ignition by an elecuic spark of approximately 10J energy. Where the fuels had sufficient vapor pressure, tile tests were done at room temperature. Where tile fuels did not have suffidendy high vapor pressure, the tests were done at elevated temperature, and the test results were then extrapolated to room temperature. The source of the test data is the laboratory of Dr. W. Bartknecht, Ciba Geigy Co., Basel, Switzerland (private communication).

A K G value for a flammable gas can be approximated f r o ~

known K G value for another flammable gas by the f o l l ~

equation:

( K G ) 2 = ( K G ) 1 (Su)2(Su) 1 ~~~) ~ Tile values for P for the two gases can be me ~ 'actual

max ge test under closely similar conditions, or they can bo t~ calculated

for adiabatic combustion conditions. However, one ? cannot max

be calculated and the other measured by test. "Optimum mixture" means a mixture of that composition that gives the highest maximum pressure during combustion. Usually this m not a stoichiometric mixture, but one that is sllghdy ridler in fuel gas than stoichiometric. This equation applies best wilere the two flammable gases have similar values of K G.

Table D-I Flammabillty Properties o f Gases (5 liter where, E =1 10 , Normal Conditions; Reference 101'

Flam~mable/~laterial ' t~ma x [bar] K G [bar-m/sec l I

Acetophenone a 7.6 109 Acet)4ene 10.6 1415 Ammonia b 5.4 10

i

c 4.4 36 B-Naphol Butane 8.0

8.1 92

115 78

Diethyl Ether a 8.4

Dimethyl Formamide a 7.3 112

DimethyI Sulfoxide a 7.8 106

Ethane Eth~! Alcohol 7.0 78

a 7.4 96 Ethyl Benzene

a 7.8 83 lsopropanol Methane 7.1 55

a 7.5 75 Methanol Meth]dene Clfloride 5.0 5 Methyl Nitrite . 11.4 111 Neopentane .,.~.):~. 7.8 60

a . . . . . . ~ 6.7 95 Octanol ~ .

"~a " " 8.0 116

Pen P r o ~ e q~.~ ..$ ~ 7.9 100 S q # ~ r i ~ r u d e Oil " 6.8 - 7.6 36 - 69 C a r b o ~ ] f i h ~ 6.4 105 H ~ l r o ~ e ~ d e . 7.4 45

~ . . a ~ # r 7.8 94 ~ e n e ? ~ r

H ~ ~ 6.8 550

tes: 7" ured at elevateti temperatur and ex apolated to ~(77°p at normal conditions. t . ~ E = 100-200J

C

2oooc (S92°F)

D-2 Using New K¢ Data. The method of developingK o values has not been standardized. As such, values determinedby a laboratory might deviate from those employed by W. Bartknedlt in developing tile correlation coefficients for tile vent area equation recommended for use with flammable gases. In order to maintain consistency in application of the vent area equations in Chapter 6, it is recommended that K o data be adjusted to put them on an equivalent basis with the Bartl~echt data. The procedure uses the Bartknecht K o values for methane (55) and propane (100) as points of reference. The following procedure is recommended:

(1) Develop K o values for propane and methane using tile same equipment and method as employed for obtaining data on other gases of interest;

(2) Compute tile linear adjustment coefficients A and B as follows:

B = [Ko (propane) - K o (methane)]wB [I~ (propane) - K¢ (methane) ] s ~

A I K¢ (propane) wB - B x K o (propane) N,~

Where the subscripts WB and New refer to Bartknecht data and New data, respectively.

(3) The adjusted value of lr~ determined by the new method is calculated as followm

Kot,dj=t~a) = A + B x K c N~*

707

N F P A 6 8 ~ A 9 8 R O P

Table D-2 Gas Explosibility Data as Measured and Adjusted to to Bartknecht Basis 1111]

As Pm~,, bar measured Adiusted

1,1-Difiuoroedlane 59 75 7 . 7 e gau~

Acetone 65 84 7.3 Dimethyl Ether 108 148 7.9 Ethane 78 103 7.4 Ethyl Alcohol 78 103 '7.0 Ethylene 171 243 8.0 Isobutane 67 87 7.4 Methane 46 55 6.7 Methyl Alcohol 94 127 7.2 Propane 76 100 7.3 Hydrol~en 638 * 6.5

*Not recommended due to excessive extrapolation. Adjusted K G = -14.0 + 1.50 K G (as measured)

2 5 0 :

225 ~

2oo E ~ 175

150 "10

125 == ~ ' 100 .<

75

50 2 5

y = 1.5× - 14

I 50 75 100 125 150 175

KG, as measured (bar-m/s)

Figure D-I Reported K a data. [ l l l ]

Appendix E Deflagration Characteristics of Selected Combustible Dusts

This appendix is not a part of the recommendations of this NFPA document but is included for informational purposes only.

E-I The following tables are based on information obtained from Forschungsbericht Staubexplosionen: Brenn- und Explosions- Kenngrossen yon Stauben, published by Hauptverband der gewerblichen Berufsgenosseuschaften e.V., Langwartweg 103, 5300 Bonn, 1, West Germany, 1980. [86]

For each dust, die tables show die mass median diameter of the material tested as well as the following test results obtained in a 1-

3 m vessel: minimum explosive concentration, maximum pressure developed by dae explosion, P , and the maximum rate of

m a x

pressure rise . Also shown is the KSt value, which is

m a x

equivalent to because of the size of the test vessel, and

m a x

the dust hazard class as used in the nomngraphs in Chapter '7 of dais guide.

E,-2 The user is c a . ~ . ~ that test data on flammability characteristics o~.-:-~ts a~:~ sample specific. Dusts of the same chemical iden~i. ' .~. . . r example, as chemical, or from nominally the same s o ~ s , s ~ . ~ , grain dust, can have wide differences in K value~.F./~.~.exanf~::yarious calcium stearate dusts have been

founck:¢~ h~ve'~nges of ~ S . ~ u e s that place the respective dusts in ..--~ % .~e

C ~ . ~ t - l ~ e u g h St-3.'~'For these reasons, care should be taken when t ~ t l ~ o m these tables.

~....::.~:. "N ~

708

N F P A 6 8 - - A 9 8 R O P

Table E-I A~rieultural Products

Material

Mass Minimum median f lammable Dust

diameter concentration P,~, bar K s Hazard microns g / m s ]~aul~e bar-m/sec Class

Cellulose 33 60 9.7 229 1 Cellulose 42 30 9.9 62 1

pulp Cork 42 30 9.6 202 2 Corn 28 60 9.4 75 1 Egg white 17 125 8.3 38 1 Milk, 83 60 5.8 28 1

powdered Milk, non- 60 8.8 125 1

fat, dry Soy f lour 20 200 9.2 110 1 Starch, 7 10.3 202 2

corn Starch, rice 18 60 9.2 101 1 Starch, 22 30 9.9 115 1

wheat Sugar 30 200 8.5 138 1 Sugar, milk 27 60 8.3 82 1 S u g ~ , beet 29 60 8.2 59 1 Tap ioca 22 125 9.4 62 1 Whey 41 125 9.8 140 1 W o o d f lour 29 - 10.5 ci~i.l~ 2

~. "% Table E-2 Carbonaceous DustsA~ 9: ~-~!ig~...

IJ~ ' I l.lj.lj.,.. Mass M i n i m u m ..:.%~, ~.,: '%'.::~.'.:::.. ~,~"~ ::~. ~.:~-~... .<..

m e d i a n f l a m m a b l e ..::~ " ' -x~ . . ~::~:.,- Dust d i ame te r concen t ra t ion P ~ 4 b ' a r ~.. K~ ":~?" H a z a r d

Material m i c rons ~r/mS ~ e % b a r - m / ~ " Class

Charcoal activated

Charcoal , wood

Coal, b i t uminous

Coke, pe t ro leum

Lampb lack I ~ g t f i t e

Peat, 22% H 2 0

Soot, pine

Material

~>" ":~.%'k\ ~

28 6o 7. ~.~)..::~::. ".:.$:~::~

14 60 ~:~i:-':'~'.:-'-~ - 9.0 " ~ . : ~

.... '-'-:,:~ ~ : - :.,:....-.-':"

.-.-~:~ ~- .:::::x.:::::. ~" < 10 ~..':'.::.-':<, 60:.~ .,~ ~'~:~" 8.4

s2 ~¢-::~'~.:~!~:,.~ ~ : ~ 10.0 -~.~. ~:~ ~ :.?~::.~'::

.<.<:~:~::':~:::~<.. ~.~.-.. -':::~::'::~:~ ~i'-':.$~x ~:-'.~ ;~ .: . .~ q-~ ",:q~::,%~

.~(..~ ] 0 "~[~$:: -'-~-~:~i~-"" 7.9

~'-<~;~.. ~ .

"~tiiiii.-'..'.-.,. ~ b l e E-3 Chemical Dus ts

M ~ ~'-':" M i n i m u m m e c l ' ~ ~ f lammable

d i a m e ~ < concentration Pm=, bar mnerons ~ / m s ~aul~e

14 1

10 1

129 1

47 1

121 1 151 1 67

26 1

Dust K~ Hazard

bar-m/sec Class

Adipic acid < 10 60 8.0 97 1 A n d w a q u i n o n e < 10 10.6 364 3 Ascorbic acid 39 60 9.0 111 1 C a l d u m acetate 92 500 5.2 9 1 Calcium acetate 85 250 6.5 21 1 Ca lc ium 12 30 9.1 132 1

steal'ate Carobxy- 24 125 9.2 136 1

medlyl- cellulose

Dextr in 41 60 8.8 106 1 Lactose 23 60 7.7 81 1 Lead s teamte 12 30 9.2 152 1 Medlyl- 75 60 9.5 134 1

cellulose Paraform- 23 60 9.9 178 1

a ldehyde S o d i u m 23 60 8.4 119 1

ascorbate Sod ium stearate 22 30 8.8 123 1 Sulfur 20 30 6.8 151 1

7 0 9

N F P A 68 ~ A 9 8 R O P

Table E-4 Metal Dusts Mass Minimum

median flammable diameter concentration PK, bar

Material microns g/m s ffau~e

Aluminum 29 30 12.4 Bronze 18 750 4.1 Iron carbonyl < 10 125 6.1 Magnesium 28 30 17.5 Zinc 10 250 6.7 Zinc < 10 125 7.3

Dust

bar_Imam/see Hazard Class

425 31

111 508 125 170

Table E-5 Plastic Dusts Mass Minimum

median flammable Dust diameter concentration Pm~ bar K~ Hazard

Material microns ~ /m s gauge bar-m/sec Class

(poly) Acrylamide 10 250 5.9 12 (poly) Acrylonitrile 25 8.5 121 (poly) Edwlene < 10 30 8.0 156

(low pressure process)

Epoxy Resin 26 30 7.9 129 Melamine Resin 18 125 .-:- Melamine, molded 15 60 7.5

(wood) flour and ~ . , mineral filled .~f~!~- x ~ : . phenol- ~ ::~: ~,..~,~ formaldehyde) ~.:.-;-:.,.:.. "%'.'.: ¢¢~.:. .

Melamine molded 12 60 ~ ~ *~" 12"q:~&~ ~ (phenol- . .*..:~:" ~ "~'~'#~ cellulose)

(poly) MedwI 21 30 - ----~.~.~.~.~.~.~ 269 acrylate ' ~ l ~ , . x ~ 202 (poly) MedwI 18 ,_,.~ . acrylate, emul- " ~ : ~ ~ ~ . sion polymer

Phenolic resin <10 1 129 (poly) Propylene 25 3 0 ~ ~.~8"~'~t 101 Terpene-phenol 10 ~ ' . ' ~ 15 . 1.7 143

10.2 formaldehyde/

cellulose, molded (poly) Vinyl .~,~.~,~,~2 ' ~ 30 8.6 119

acetate/ethyle.t~ T M " ~ - ~ . ,~.~:~-,~,,,:.~" ~opolymer ,~T ' ~ . 7

(only) Vinyl ~ . o l 26~ . ' 60 8.9 128 lpoly) VinylXb u ~ 65 ~ 30 8.9 147

(poly) Vinyl ~ . ~ . 107.~ 200 7.6 46 chloride ~ ¢ e

(poly) Vinyl 60 8.2 95 chloride/vinyl .~ acetylene emulsion copolymer

(poly) Vinyl 60 60 8.3 98 chloride/ edwlene/vinyi acetylene suspension copolymer

710

N F P A 68 1 A 9 8 R O P

Appendix F Referenced Publications

F-I The following documents or portions thereof are referenced within this guide for informational purposes only and are tiros not considered part of its recommendation. The edition indicated here for each reference is the current edition as of dae date of die NFPA issuance of this guide.

F-I.1 NFPA Publications. National Fire Protection Association, 1 Batterymarch Park, P.O. Box 9101, Quincy, MA, 02269-9101.

NFPA 30, Flammable and Combustible Liquids Cod~ 1996 edition.

NFPA 69, Standard on Explosion Prevention Systems, 1997 edition.

NFPA 325, Properties of Flammable Liquids and Volatile Solids, 1994 edition.

F-2 Other Publications.

1. Jacobson, M., Cooper A. 1L, and Nagy, J.; ExplosibUlty of Metal Powders, Report of Investigations 6516, US. Bureau of Mines, Pittsburgh, 1964.

2. Ball.d, D. IL and Lefebure, A. H., "Ignition and Flame Quenching of Quiescent Fuel Mists", Proceedings of the Royal Society, London, Vol 364, 1978, pp. 277-294.

3. Bartknecht, W., Explosions: Course, Prevention, Protection, Springer-Verlag, New York, 1981.

4. Ibid.; p. 51.

5. Ibid.; p. 50.

6. Field, P.; "Dust Explosions", Handbook of Powder Technology, Volume 4; Elsevier Scientific Publishing Co., New York, 1982, pp. 88-90.

7. Haase, H.; Electrostatic Hazards: Their Evaluation and Control, Verlag Chemie, New York, 1977.

8. Harmanny, A., Europex Newsletter, pp. 7-10 (April, 1993)

9. Harrison, A.J., and Eyre, J. A., Combustion 32 121-[:~/" % ~ (1987). . ~ £ ~

10. Jbertager, B.H., Fuhre, K., & Bjorkhaug, t~omt)us. ~e~.~-...~ Tech. 62,239-256 (1988). ....:.~.:~:,:..,..~ :,.. ~ : ~ .

11. C~lcote, H. F., Gregory, C. A.Jr., Barrg.~" C. M ~:..'"""':"!~Gilm~ ~ IL B., "Spark Ignition, Effect of Molecular ~ t u r e " , lnctt~lfial i

Engineering Chemistry, Vol. 44, p. 2659, 1~~.?..!::..

12. Jacobson, Cooper, and Nagy, op. cit. "%!~'~"':~i ~.~y- ~

13. Nagy, J., Dorsctt, H. G. Jr., and Cooper, A. P~.,.: ~'ploslbHity of Carbonaceous Dusts, Report of Investigations 6597, ~ .S. Bureau of Mines, Pittsburgil, 1965.

14. Dorsett, H. G. Jr. and Nagy, J., Dust Explosibility of Chemicals, Drugs, Dyes, and Pesticides, Report of Investigations 7132, U.S. Bureau of Mines, Pittsburgh, 1968.

15. International Bureau for Explosion Protection and Plant Safety (INBUREX), PGVENT, Version 1.2 (March 30, 1993)

16. Jacobson, M., Nagy, J., Cooper, A. IL, and Ball, F.J., Explosibility of Agricultural Dusts, Report of Investigations 5753, U.S. Bureau of Mines, Pittsburgh, 1961.

17. Jacobson, M., Nagy, J., and Cooper, A. IL, Explosibility of Dusts Used in flae Plastics Industry, Report of Investigations 5971, U.S. Bureau of Mines, Pittsburgh, 1962.

18. Eckhoff, IL, "Toward Absolute Minimum Ignition Energies for Dust Clouds?", Combustion and Flame, Vol. 24, Elsevier Scientific Publishing Co., New York, 1975, pp. 53-64.

19. Fenning, R. W., "Gaseous Combustion at Medium Pressures", Phil. Trans. Royal Society, London, Serial A, Vol. 225, 1926.

20. Nagy, J., Seiler, E. C., Corm, J. W., and Verakis, H.C., Explosion Development in Closed Vessels, Report of Investigations 7507, U.S. Bureau of Mines, Pittsburgh, 1971.-

21. Nagy, J. and Verakis, H. C., Development and Control of Dust Explosions, Marcel Dekker, New York, 1983.

22. Hartmann, I. and Nagy, J., Effect of Relief Vents on Reduction of Pressures Developed by Dust Explosions, Report of Investigations 3924, U.S. Bureau of Mines, Pittsburgh, 1946.

23. Howard, W. B., "Interpretation of a Building Explosion Accident", Loss Prevention - Volume 6; American Institute of Chemical Engineers, New York, 1972, pp. 68-73.

24. Runes, E., "Explosion Venting", ibid., pp. 63-67.

25. Rust, E. A., "Explosion Venting for Low-Pressure Equipment", Chemical Engineerin~ McGra~Hill Co., New York, Nov. 5, 1079, pp. 102-110.

26. Swift, I., '~¢enting Deflagratious - Theory and Practice", Plant/Opgcations Progress, VoL 3, No. 2, American Institute of Chemical Engineers, New York, April, 1984, pp. 89-93.

27. Yao, C., "Explosion Venting of Low-Strengdl Equipment and Structures", Loss Prevention-Volume 8; American Institute of Chemical Engineers,...~ew York, 1974, p. 109.

28. Swift:, I. an~.-~.pstel~- M., "Performance of Low Pressure Explosion V e n ~ : e r 84<1; 20th Annual Loss Prevention Symposium, ~CilE":~{~fqng National Meeting, New Orleans, LA,

29.~ Res .~~I

30. Venting Institute

H., British Shipbuilding Report

B. and Karabinis, A. H., "Tests of Explosion kgs"; Plant/Ol:~erations Progress, Vol. 1, American cal Engineers, New York, January 1982, pp. 51-68.

~ G., "Gas Explosion Tests in Room-Size Vented Loss Prevention -Volume 13, American Institute of ~:ineers, New York, 1980.

Forsok, Bromma 1957. Slutrapport, April 1958.

33. Tonkin, P. S. and Berlemont, C.F.J., "Dust Explosions in a Large-Scale Cyclone Plant", Fire Research Note No. 942, Fire Research Station, Borehamwood, UK, July 1972.

34. Sapko, M.J., Furno, A. L., and Kuchta, J. M., Flame and Pressure Development of Large-Scale CH4-Air-N 2 Explosions;

Report of Investigations 8176, U.S. Bureau of Mines, Pittsburgb, PA. 1976.

35. Harris, G. F. P. and Briscoe, P. G., "The Venting of Pentane Vapor-Air Explosions in a Large Vessel"; Combustion and Flame, Vol. 11, Aug. 1967, pp. 329-388.

36. Cubbage, P. A. and Simmonds, W. A., "An, Investigation of Explosion Reliefs for Industrial Drying Ovens, Gas Council Res. (London) Communications GC23, 1955.

37. Thorne, P. F., Rogowski, Z. W., and Field, P., "Performance ot 3

Low Inertia Explosion Reliefs Fitted to a 22 m Cubical Chamber," 4th Int. Symp. on Loss Prevention and Safety Promotion in the Process Industries, EFCE Publication Series, No. 33, Vol. 3, F1-F1O, Sept. 12-16, 1983.

38. van Wingerden, C.J .M. and Zeeuwen, J. P., '"tenting of Gas Explosions in Large Rooms," 4all Inc Symp. on Loss Prevehtion and Safety Promotion in die Process Industries, EFCE Publication Series, No. 33 Vol. 3, F38-F47, Sept. 12-16, 1983.

39. Yao, C., deRis, j., Bajpai, S. N., and Buckley, J. L., "Evaluation of Protection from Explosion Overpressure in AEC GIoveboxes," Factory Mutual Research Corporation, FMRC Report 16215.1, Dec. 1969.

711

N ~ A 68 1 ~ 8 R O P

40. Burgoyne, J. H. and Wilson, M.J .G. , "The Relief of Pentane Vapor-Air Explosions in Vessels," Symp. on Chemical Process Hagards, Inst. Chem. England (London), 1960, p. 25.

41. Lunn, G. A. and Cairns, F., "The Venting of Dust Explosions in a Dust Collector," J. Hazardous Materials, Vol. 12; 1985, pp. 85- 107.

42. Zalosh, R. G. and ColI, J. P., "Explosion Venting Test Program for Municipal Solid Waste Slaredders," presented at 1981 A n n u a l Meeting of Society of Fire Protection Engineers, S.F.P.E. Seminar on Engineering Fire Safe Conservation andResource Recovery, Dallas, Texas, May 18-21, 1981.

43. Cooper, M. G., Fairweafller, M., and Tite, J. P., "On dae Meclaanisms of Pressure Generation in Vented Explosions," Combustion and Flame, 65: 1-14, 1986.

44. Solberg, D. M., Pappas, J. A., and Skramstad, E., "Observations of Flame Instabilities in Large-Scale Vented Gas Explosions," 18dl Symposium (Int.) on Combustion, Combustion Institute, pp. 1607-1614, 1981.

45. van Wingerden, C.J .M. and Zeeuwen, J. P., "Flame Instabilities in Vented Gas Explosions," Combustion and Flame, 51, pp. 109-111, 1983.

46. Faber, M., Recoil Forces of Vented Explosions in Technical Installations, Europex Newsletter, Dec. 1991, logs. 2-3.

47. Donat, C., "Application of Explosion Pressure Relief as a Protective Measure for Industrial Plant Equipment", Loss Prevention-Volume 11, American Institute o f Chemical Engineers, New York, 1977.

48. Donat, C., "Release of dae Pressure of an Explosion With Rupture Discs and Explosion Valves", ACHEMA 73, Frankfurt am Main, Republic of Germany, 1973.

49. Donat, C., Staub-Reinhalmng der Luft, Vol. 31, No. 4, April, 1971, pp 154-160.

50. Bartknecht, W. and Knhner, G., Forschungsbericht 45, Burdesinstitut fur Arbeitsschutz, 1971. .,..-::i:-::~::.:,

51. Bartknecht, W., Explosions: Course, Prevention, P.~:tectiot~!~,. Springer-Verlag, New York, 1981. ...-'~i~. ,

Dangreaux, J ................... ":"~'."~. 52. Pineau, J., Gilt'tire, M., and "Etude .::(::~ :'-'m~..~i-?::..~ '

d'explosions de poussieres en recipients de 1,,./.~" et 100!-'.'~:~'..', N~ii~i:: No. 1005-83-76, Caltier de Notes l ) o c u m e n ~ . s , No. 8 3 ~ : ' ~ Trimestre, 1976. -:.:-"!::"'~!~%. ~ii

53. Simpson, L. L., "Equations for dae VDI an~'ii~:t.knec~ Nomograms", Pkmt/Operation Progress, Vol. 5, N~i~i~::~.'..-~.#herican Institute of Chemical Engineers, New York, Jan. 1 9 8 6 ' ~ . 49-51.

54. Siwek, R., and Skov, O., Verein Deutsche Ingenieure Berichte, No. 701, Vol. 2, pp 569-616 (Dfisseldorf; VDI-Verlag, 1988)

55. Lee, J. H. S. and Guirao, C. M.; "Pressure Development in Closed and Vented Vessels", Plant~Operations Progress, Vol. 1, No. 2, Ainerican Institute of Chemical Engineers, April, 1982, pp. 75-85.

56. Bartknecht, W., op. cit., pp. 18-23 and p. 124.

57. Ibid; pp. %26.

58. Ibid; p. 111.

59. Cousins, E. W. and Cotton, P. E., "The Protection of Closed Vessels Against Internal Explosions", American Society of Meclmnical Engineers, Paper No. 51-PRI-2, 1951.

60. Maisey, H. R., "Gaseous and Dust Explosion Venting", Chemical mad Process Engineering, Vol. 46, Oct., 1965.

61. Chippett, S., "An Investigation of Vented Explosions at Initially Elevated Pressures for Propane/Air Flames", Report of Research Project conducted for NFPA Committee on Explosion Protection Systems, Feb. 2, 1984.

• . ~ : -

62. Burgoyne, J. H. and Cohen, L., Proceedings of die Royal Society, Vol. 225, London, 1954, pp. 375-392.

63. Browning, J. A., Tyler, T. L., and Drall, W. G., "Effect of Particle Size on Combustion of Uniform Suspension", Industrial and Engineering Chemistry, Vol. 49, 1957, pp. 142-148.

64. Vincent, G. C., and Howard, W. B. , "Hydrocarbon Mist Explosions - Part I: Prevention by Explosion Suppression", Loss Prevention - Volume 10; American Institute of Chemical Engineers, NewYork, 1976, pp. 43-47.

65. Zabetakis, M., Flammability Characteristics of Combustible Gases and Vapors, Bulletin 627, U.S. Bureau of Mines, Pittsburgh, 1965, pp. 6-7.

66. VDI Ricbdinie 3673, Pressure Release of Dust Explosions; Verein Deutscher Ingenieure - Kommission Reinhaltung der Luft, Dfisseldorf, 1979 and 1983, VDI Verlag GmbH, Dfisseldorf.

67. Schwab, 1L F., private communication.

68. Rasbash, D.J. and Rogowski, Z. W., "Gaseous Explosions in Vented Ducts"; Combustion andFlame, Vol. 4, No. 4, Butterworth, London, Dec., 1960, pp. 301-312.

69. Rasbash, D. Propane/Air Mix1 Second Symposiu Reference to Plaa London, 1963:~# i

7 0 Plant", ~ Indus~$~s. ICfferr

71. London

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