Report of Committee on Explosion Protection Systems

A. Richard Albrecht, Chairman Dow Chemical Co.

(Rep. Chemical Manufacturers Assn.)

Gary C. Batz, Fike Metal Products Corp. William J. Bradford, Brookfield, CT Laurence G. Britton, Union Carbide Corp. George P. Garland, Crown Fire Protection

Rep. Fire Suppression Systems Assn. Joseph P. G i l l i s , Fenwal Inc. Stanley S. Grossel, Hoffmann-LaRoche Inc. Randal D. Hamilton, BS&B Safety Systems Walter B. Howard, St Louis, MO George A. Krabbe, Automatic Suppression Systems Inc. R. A. Mancini, Amoco Oil Co.

Rep. American Petroleum Inst. John Nagy, Library, PA Edward S. Naidus, APC Corporation

Rep. Architectural Aluminum Mfg. Assn. Robert W. Nelson, Industrial Risk Insurers John A. Noronha, Eastman Kodak Co. Anthony Santos, Factory Mutual Research Corp. R. F. Schwab, All ied Corp. C. E. Scott, Kemper Group - Southern Area

Rep. Alliance of American Insurers Harry Verakis, Mine Safety & Health Admin.

Alternates

John V. Bir twist le, Monsanto Co. (Alternate to Monsanto Co. Rep.)

Robert L. DeGood, Fike Metal Products Corp. (Alternate to G. C. Batz)

J. D. Dick, Kemper Group (Alternate to C. E. Scott)

David C. Kirby, Union Carbide Corp. (Alternate to L. G. Britton)

Edward D. Leedy, Industrial Risk Insurers (Alternate to R. W. Nelson)

Arnold L. Mundt BS&B Safety Systems (Alternate to R. O. Hamilton)

Gregory G. Noll American Petroleum Inst i tute (Alternate to R. A. Mancini)

Thomas K. Palmer Automatic Fire Protection Systems (Alternate to G. A. Krabbe)

Parker Peterson Fenwal Inc. (Alternate to J. P. G i l l i s )

Edwin Dale Weir Ciba-Geigy Corp. (Alternate to R. Albrecht)

Robert G. Zalosh Factory Mutual Research Corp. (Alternate to A. Santos)

This l i s t represents the membership at the time the Committee was balloted on the text of this edit ion. Since that time, changes in the membership may have occurred.

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

This Report was prepared by the Technical Committee on Explosion Protection Systems and proposes for adoption a Complete Revision to NFPA 68-1978, Guide for Venting of Deflagrations. NFPA 68-1978 is published in Volume 7 of the 1985 National Fire Codes and in separate pamphlet form.

This Report has been submitted to le t te r bal lot of the Technical Committee on Explosion Protection Systems which consists of 20 voting members; of whom 15 voted aff i rmatively, 3 negatively (Messrs. Bir twist le, Bradford, and Nelson), and 2 members abstained (Messrs. Nagy and Verakis).

Mr. Bi r twist le 's negative vote is based on his opposition to the reduction of the Runes equation constants.

Mr. Bradford's negative vote is based on his opinion that the revision requires considerable additional edi tor ia l work and because some conf l ic t exists between the explanation of the nomographs and their inherent degree of accuracy.

Mr. Nelson's negative vote is based on his opposition to the reduction of the Runes equation constants.

30

(Log #1) 68- I - (Entire Document): Accept in Principle SUBMITTER: Edward S. Naidus, APC Corporation RECOMMENDATION: ( I ) All required.explosion r e l i e f vents and associated components shall be of an approved type. Approval by the o f f i c i a l having ju r i sd ic t ion shall be based on acceptable performance tests and standards ( fo r the ent i re assembled device and not only unassembled components) by a qua l i f ied testing organization such as Fenwal Laboratories, Ashland, Massachusetts, Factory Mutual Laboratories, Norwood, Massachusetts, or others.

(2) Tests shall include both pressure and t ime measurements expressed as average maximum pressure (at the instant of release) in pounds/sq f t (Kg/sqm) and as average time (mil l iseconds) to release (from ign i t ion time to release time). The integrated product of "maximum pressure" mult ipl ied by "release time" is called "impulse" and should range below 10 percent of the estimated "impulse" load resistance of the building. (Example: most buildings of conventional steel and masonry block can res is t an "impulse" of lO0 Ib/sq f t for 0.5 seconds). The vents should open f u l l y at not over 30 Ib/sq f t in not over' 50 mill iseconds.

(3) Unit vents shall not be used For access and when in closed posit ion, shall not have f ixed openings ( to the outside) in the unit greater than I percent o f the vent area.

(4) Unit vents mounted on a roof shall have provisions for restraining a 200 Ib person from f a l l i n g through the vent in the nonactivated condition.

(5) Materials of construction shall be durable and functional (without frequent inspection and maintenance) in the expected envir(mment of weather, corrosion, temperature, ign i t ion sources and mechanical loading as well as any known special hazards. The releasable portions of the vent shall be tethered to minimize " f l y ing object" hazards and shall not give r ise to pro ject i les or shards that may cause in jury . SUBSTANTIATION: Standards for exp'l'osion venting have been expressed in rat ios of ~ent area to building volume in NFPA standards and various model code provisions. However, deta i ls as t,) explosion vent performance have not usually been :~tated and the approving o f f i c i a l has been forced t o r e l y on general pr inciples. In recent years, standards and test performance have been developed to the point where basic engineering information can now be furnished to architects, builders, engineers and code o f f i c i a l s . (See references.)

Ref: (1) Accidental Explosions - H. A. Strebiow

Nasa Cr 134779 - June, 1975 (2) Factory Mutual System - Loss Prevention Data 7-76

August, 1976) p. 16 (3) Dust Explosions and Fires - K. N. Palmer -

Chapman & Hall - London (1973) I t is now known that damage to buildings or large

structures by internal pressure waves depends on a character ist ic time response of the building. When the pressure wave moves more rapidly (most explosions) than the building can redis t r ibute the energy of the pressure wave (the bui lding is toe r ig id or not f l ex ib le enough) then a port ion or a l l of the bui lding collapses. The purpose of an explosion vent is to reduce the tota l energy on the building before the building reaches the force x time impulse tha t .w i l l cause damage. The character is t ic time response decreases with increasing r i g i d i t y but fo r conventional masonry, steel or timbered structures, the time constant wi l l range close to I /2 second.' Therefore, venting must occur well below I/2 second a f te r ign i t ion to be of value. Use of vents for uses other than primary emergency explosion r e l i e f may cause excessive wear, damage or other performance impairment.

Current safety objectives fo r construction, maintenance or emergency personnel are met by providing a safety structure around or over the vent.

For specialized structures, such as ducts over 6 f t in diameter of 4 f t x 4 f t in area, elevator legs, conveyor throughways, etc. , uni t vents should be placed on a l l ex ter ior faces and the vents should extend the f u l l width of the structure. The vents should have a smallest dimension of 3 f t (I meter) and a dimension ra t io of not over 2.

Structures with L/D greater than 3 require higher vent ratios than near-cubical buildings. (Example: Elevator legs should have vents no less than 20 f t on center.)

I'

COMMITTEE ACTION: Accept in Pr inciple. i! COMMITTEE COMMENT: The Committee believes that the Submitter's concerns are adequately addressed~by this complete revision of NFPA 68.

(Log #4) 68- 2 - (2-2.1.9): Accept in Pr inciple SUBMITTER: Robert D. Coffee, Eastman Kodak Company RECOMMENDATION: Revise as foll'ows s tar t ing with l ine 17:

"In order for moisture to prevent ign i t i on of a dust by common sources, the moisture content must be normally around 13-14 percent or greater. Moisture contents of this magnitude frequent ly render the dust so damp that dust clouds can not be formed." SUBSTANTIATION: Published data of U.S. Bureau of Mines (34) (Figure 2-2.1.9(b)) , Eckhoff (see comment on 2-2.1.5), and unpublished data of Eastman Kodak Co. show i n a b i l i t y to obtain dust explosions at moisture contents above 14 percent fo r many materials. ~OMMITTEE ACTION: Accept in Pr incip le. COMMITTEE COMMENT: Section 2-8 of the revised dra f t addresses this point. !

( Log #5) 68- 3 - (A-3): Reject SLIBMITTER: Thomas E. Frank, Tacoma, WA RECOMMENDATION: In paragraph beginning "The use of vent ducts can lead to substant ia l ly increased pressure." delete or revise the las t two sentences, beginning with "When duct lengths exceed about 10 f t

I I

SUBSTANTIATIQN: I believe these two sentences are misleading, when taken out of context. The 10 f t vent duct length may have produced the pressure increases in the cited example (Ref. 22), but surely th is doesn't apply to a l l cases ( i . e . , d i f f e ren t vent sizes). I believe th is e f fect is a function of L/D not just L, as well as numerous other variables (e .g. , pressure ra t io across the vent at the moment of release, densit ies and speci f ic heat values of gas/dust mixtures, e tc . ) . To state a "crltic~El length" of lO f t is a drast ic overs impl i f icat ion of the problem, and is i n er ror when taken out of context. The statement regarding I0 f t vent ducts should be put in proper context o~" deleted.

Also the statement concerning t rans i t ion to detonation in very long ducts is pr imar i ly a phenomenon of gaseous mixtures. To my knowledge, the only case of detonation observed for dusts was in small scale tubes using aluminum dust (Class ST-3) suspensions enriched with pure oxygen. This cer ta in ly does not represent most industr ia l conditions and the statement~.should perhaps be qual i f ied in regards to dust explosions. COMMITTEE ACTION: Reject. COMMITTEE COMMENT: Chapter 8 of th is d ra f t presents a l te rnat ive guidelines that the Committee feels provide a greater degree of safety.

(Log #6) 68- 4 - (A-5) : Reject SUBMITTER: Thomas E. Frank, Tacoma, WA RECOMMENDATION: Revise to read as f o l l o w s :

A-5 Venting of Gas or Dust Combustion Inside A i r Conveying Ducts. Most of the cases of flammable gas or dust mixtures inside ducts of the a i r ven t i l a t i on type occur at i n i t i a l internal pressure of nearl~ atmospheric. The venting of gas combustion in such ducts is discussed in Appendix C. Since the gas/a i r test mixtures used in Appendix C have'cdbic~law. constants (Ko) s imi lar to Class ST-2 dusts (Kst), the guidelines presented in Appencix C can b e considered applicable to most. Class ST-I. and ST-2 dusts. SUBSTANTIATION: No guidelines are suggested fo r combustible dusts in elongated vessels or ducts (L/D ~ 5). Many such s i tuat ions are encountered in industry (e.g., plywood and part ic leboard sander dust co l lect ion systems, f lash tube dryers, e tc . ) . The guidelines in Appendix C have been used fo r wood dust conveying systems with numerous explosions successfully vented.

31

From Figure 2-2.1.8 in th is standard, Methane/air under turbulent conditions generated a maximum rate-of-pressure r ise of 12,500 psi/sec in a ] c u f t closed vessel. This gives a cubic law constant of:

K G = 12,500 psi x (1) ~/3 = 12,500 psi - f t = 259 atm.-m sec sec sec

Compare wi th wood dust (Class S t -2 ) :

Kst = 230 atm.-m ( f rom Table A-3(b) in t h i s s tandard) . s e c

Appendix C should y i e l d s a t i s f a c t o r y ven t ing gu ide l i nes f o r Class ST-1 and ST-2 dusts. COMMITTEE ACTION: Re jec t . COMMITTEE COMMENT: Chapter 8 o f t h i s d r a f t presents a l te rnat ive guidelines that the Committee feels provide a greater degree of safety.

(Log #3) 68- 5 - (A-5-I (New)): Reject SUBMITTER: Thomas E. Frank, Tacoma, WA RECOMMENDATION: Add new subsection as fol lows:

A-5-1 The fol lowing graph may be substituted for Table C-2 in Appendix C to obtain the spacing between consecutive vents:

.5 data from Table C-2 / X = 2 K = , /

3

D: duct diameter 2 / ~ J L,=distance

/ / / / between L~,~ ~-~'-- -.,~'_ _ consecutive

1 ~ vents I I I I I I I t I

0 4 8 12 16 20 24 28 32 36 L! ,ft

Figure A-5-1 Space and Size o f Vents Along Ducts Conta in ing Moving Gas or A i r M ix tu res .

NOTE: The above curves are p l o t s o f the f o l l o w i n g equat ions :

D = (Lj.) 2 + 25 L z (K=I) 426

D = (Lj.) z + 35 L I (K=2) 234

These equat ions were der ived by f o r c e - f i t t i n g 2nd order equat ions o f the form

D = a(L1) z + b(L1) + c

to the data in Table C-2. This permits an o r d e r l y , logical extrapolat ion to larger duct diameters, which is necessary i f the guidelines are to be usable for indust r ia l -s ized equipment. SUBSTANT]IATION: The guidelines in Table C-2 do not present any data for duct diameters larger than 2 f t 6 in. Many industr ia l size, low pressure pneumatic conveying systems use ducts up to 5 f t diameter, or even larger. Guidelines are needed for these larger systems.

Since the data in Table C-2 is only approximate (as evidenced by the discont inui ty at I f t 6 in. duct diameters), a smooth curve f o r ce - f i t t ed to the tabular data should yield equally acceptable results. Then, the curve can be extrapolated to larger diameters, providing usable guidelines. Several wood dust explosions have been successfully vented in ducts 3 to 4 f t in diameter using these guidelines. Though this is not conclusive evidence, i t does tend to indicate general acceptabi l i ty for use as a guide, which is better than no guidelines at a l l .

COMMITTEE ACTION: ReJect. COMMITTEE COMMENT: The Committee feels that the guidelines in Chapter 8, which are applicable to both large and small diameter ducts, provide a more comprehensive design basis.

(Log #7) 68- 6 - (A-5-2 (New)): ReJect SUBHITTER: Thomas E. Frank, Tacoma, WA RECOMMENDATION: Add new subsection as fol lows:

A-5-2 Where L/D is less than 6, the nomographs in this Appendix may be used in l ieu of the design data by Simmonds and Cubbage (Reference 71) mentioned in Appendix C. This, too, should give a s l igh t overestimation of the necessary vent area. SUBSTANTIATION: The homographs are inferred as being applicable only for vessels with L/D < 5. The work by Simmonds and Cubbage (1961) was done on vessels with L/D ~ 3 as noted in Appendix C. This leads to an overestimation of vent area as L/D approaches 6.

I t would seem the homographs would give at least as accurate an estimation of vent area, i f not more so. They, too, give an overestimation as L/D approaches 6. Since the work by Simmonds and Cubbage is only referenced in Appendix A and not presented in a usable form, i t is necessary to obtain a repr int of the paper to get any usable guidelines. The nomographs should give an equally acceptable guide and are already presented for use. COMMITTEE ACTION: Reject. COMMITTEE COMMENT: Chapter 8"of this d ra f t presents a l ternat ive guidelines that the Committee feels provide a greater degree of safety.

(Log #2) 68- 7 - (Appendix CI Vent Closures and Figure C- l ) : Accept in Principle SUBMITTER: Gerald E. Lingenfelter, American Insurance Association RECOMMENDATION: Revise paragraph to read:

"Diaphragm-vent or panel closures of metal f o i l , f l ex ib le or f rangible heat resistant types of p las t ic sheets, or various other commercially avai lable rupture discs can be used. Preference should be given to the use of a noncombustible material to protect equipment against f i r es external to the equipment. Figure C-I shows a method of constructing a panel closure."

Revise Figure C-1 to delete reference to asbestos millboard (use only "Panel") and revise t i t l e of Figure to "Panel Closure". SUBSTANTIATION: Stat is t ics produced by the Department of Health, Education and Welfare (Health and Human Resources) indicate the potent ial losses to people exposed ind i rec t l y to asbestos in the i ns ta l l a t i on , modification or repair of asbestos products are vast. These exposures to par t ic les of asbestos produce several diseases related to lung ailments. Ailments are being caused by the mere breathing of a i r contaminated by asbestos f ibers below 5-10 micrometers.

Further, use of such materials has already been deleted from NFPA No. 211 for these reasons, and we are not aware of anyone who is presently commercially producing th is product. COMMITTEE ACTION: Accept in Pr inciple. COMMITTEE COMMENT: The Committee believes that the Submitter's concerns are adequately addressed by this complete revision of NFPA 68.

68- 8 - (Entire Document): Accept SUBMITTER: Technical Committee on Explosion Protection Systems RECOMMENDATION: Completely revise the 1978 edi t ion of NFPA 68, Guide for Explosion Venting, as shown in the fol lowing text . SUBSTANTIATION: 1. The revision brings NFPA 68 up-to-date with the current state of the ar t .

2. The revision takes into account much new data that has been published since the last ed i t ion. COMMITTEE ACTION: Accept.

32

Guide for Venting of Deflagrations

NFPA 68-1987

Chapter 1 Gener,~1

I - I Scope.

I-1.1 This Guide applies to the de:;ign and use of devices and systems that wil l vent the combustion gases and pressures resulting from a defl,]gration within an enclosure so that structural and mechanical damage is minimized. The enclosure may be a room, a building, a piece of equipment, or any other type of enclosure. The deflagration may result from the ignit ion of a combustible gas, mist, or dust.

I-1.2 This Guide does not apply to detonations, bulk autoignition of gases, or unconfined deflagrations, such as open-air or vapor cloud explosions.

I-1.3 This Guide does not apply to devices that are designed to protect storage vessels against excess internal pressure due to external f i r e exposure or to exposure from other heat sources. (See NFPA 30, Flammable and Combustible Liquids Code.)

I-1.4 This Guide does not apply to emergency vents for runaway exothermic reactions.

I-1.5 This Guide does not apply to pressure re l i e f devices on equipment such as oi l - insulated transformers. I t also does not apply to pressure r e l i e f devices on tanks, pressure vessels, or domestic (residential) appliances.

I-2 Purpose. The purpose of this Guide is to provide the user with c r i te r ia for venting of deflagrations. I t is important to note that venting wi l l not prevent a deflagration; venting wi l l minimize the destructive effects of a deflagration.

I-3 Definitions. For the purpose of this Guide, the following terms have the meanings given below.

Burning Velocity. The velocity at which a flame front propagates relat ive to the unburned material in a direction perpendicular to the flame front. Burning velocity varies with mixture composition, temperature, pressure, and the turbulence in the. v ic in i ty of the flame front.

Combustible. Capable of undergoing combustion.

Combustion. A chemical process of oxidation that occurs at a rate fast enough to prc, duce heat and usually l ight , ei ther as glow or flames.

Deflagratlon. Propagation of a combustion zone at a velocity which is less than the speed of sound in the unreacted medium.

Detonation. Propagation of a comhustion zone at a velocity which is greater than the speed of sound in the unreacted medium.

Dust. Any f inely divided solid, ,120 microns or less in diameter ( i . e . , material passin!) through a U.S. No. 40 Standard Sieve).

Explosion. Bursting or rupture of an enclosure or a container due to the development of internal pressure by a deflagration.

Flame Speed. The speed of a flame front relat ive to a fixed reference point. Flame speed is dependent on turbulence and the equipment geometry and is not primarily a property of the fuel.

Flammable Limits. The minimum and maximum concentrations of a combustible material, in a homogeneous mixture with a gaseous oxidizer, that wi l l propagate a flame.

Flammable Range. The range of concentrations lying between the lower and upper flammable l imits.

Flashpoint. The minimum temperature at which a l iquid gives o f f vapor in suf f ic ient concentration to form an ign i t ib le mixture with a i r near the surface of the l iquid, as specified by test.

Fog. See def in i t ion of mist.

Fundamental Burning Velocity. The velocity of a laminar (nonturbulent) flame under stated conditions of composition, temperature, and pressure of the unburned gas (normally stoichiometric mixture, 20°C, and one atmosphere absolute, respectively).

Gas. The state of matter characterized by complete molecular mobility and indef in i te expansion. Used synonymously with the term "vapor."

Hybrid Mixture. A mixture of a combustible gas with either a combustible dust or a combustible mist.

Minimum Ignit ion Energy. The minimum amount of thermal energy released at a point in a combustible mixture that wi l l cause indef in i te flame propagation away from that point, under specified test conditions. The lowest value of minimum ignit ion energy occurs at close to the stoichiometric mixture.

Mist. A dispersion of re la t ive ly f ine l iquid droplets in a gaseous medium.

Optimum Mixture. A mixture in which the fuel and the oxidant are in the proportions that yield the most violent deflagration.

Oxidant. Any gaseous material that can react with a fuel (ei ther gas, dust, or mist) to produce combustion. Oxygen in a i r is the most common oxidant.

Rate of Pressure Rise (dp/dt). The rate of increase in pressure over the time interval required for that increase to occur. The maximum rate of pressure rise is computed from the slope of the steepest part of the pressure versus time curve during deflagration in a closed vessel. (See Appendix A, Guidelines for Measuring Oeflagration Indices of Gases and Dusts.)

Stoichiometric Mixture. A mixture of a combustible material and an oxidant in which the oxidant concentration is just suf f ic ient to completely oxidize the fuel.

Vapor. See def in i t ion of Gas.

Vent Ratio. The rat io of the free area of the vent to the volume of the enclosure protected by the vent.

I-4 Conversion Factors. The following conversion factors, to three signif icant figures, wi l l be useful in understanding the data presented in this Guide:

33

Lenath

Area

Volume

lm

1 i n . 1 f t 1 micron

1 m z 1 yd 2 1 i n . z

1 l i t e r 1 f t 3 1 m ~

1 g a l ( U . S . )

Pressure 1 atmosphere

1 psi I Newton/m 2 1 bar

1 kilogram/cm 2 1 kilogram/m 2

Enerav 1 Joule I Btu I Joule

Vent Ratio I f t 2 / f t 3 1 m2/m 3

K~ and Ks_~ Conversion Factors

I bar-meter sec

1 ps i - foo t sec

Concentration I oz. A v o i r . / f t 3

3.28 f t 39 .4 i n .

1 . o g yd 2 .54 cm

30 .5 cm 1.00 x 10 -6 m

10.8 f t 2 0 .836 m 2

6 .45 cm 2

61 .0 i n . 3 7 .48 U.S. gal

35 .3 f t 3 264 U.S. gal

3 .78 l i t e r s 231 in. 3

0.134 f t 3

760 mi l l imeters Mercury (mm Hg) 101 kiloPascals (kPa)

14.7 psi 1.01 bars 6 .8g kPa 1.00 Pascal

100 kPA 14.5 psi 0 .987 atmosphere

14.2 psi 0 .205 l b / f t 2 ( p s f )

1.O0 Wat t -second 1055 Joules

0.738 foot-pounds

3.28 m2/m 3 0.305 f t 2 / f t ~

4 7 . 6 o s i - f o o t sec

0.021 ba r -me te r sec

1000 g/m 3

1-5 Symbols. For the purpose o f t h i s Guide, the f o l l o w i n g symbols have the meanings g i ven below:

A - Area, m z o r f t 2 o r i n . 2

Av - Vent Area, m 2 o r f t 2

C - Constant in Runes equa t i on

Cg - Concen t ra t i on o f Gas in M i x t u r e , pe rcen t by volume

d p / d t - Rate o f Pressure Rise, ba r / sec o r p s i / s e c

Fr . - React ion Force, lb

K O - D e f l a g r a t i o n Index f o r Gases, bar -m/sec

Kr - React ion Force Cons tan t , l b

Kst - D e f l a g r a t i o n Index f o r Dusts , bar -m/sec

Ln - L inear Dimension o f Enc losu re , m or f t (n = 1 ,2 ,3 )

LFL - Lower Flammable L i m i t , pe rcen t by volume

P - Per imeter o f Duct Cross Sec t i on , m o r f t

P - Pressure, bar (gage) o r ps ig

P m a x

Pred

Ps ta t

~P

Su

Se

St

tF

UFL

V

- Maximum A l l o w a b l e Overp ressure OR Maximum Pressure Deve loped, bar (gage) o r ps ig

- Reduced Pressure ( i . e . , t he maximum pressure a c t u a l l y deve loped d u r i n g a ven ted d e f l a g r a t i o n ) , ba r (gage) o r ps i g

- Vent C losure Release Pressure , ba r (gage) o r ps ig

- Pressure D i f f e r e n t i a l , bar o r psi

- Fundamental Bu rn ing V e l o c i t y , cm/sec

- Flame Speed, cm/sec

- T r a n s l a t i o n a l Flame V e l o c i t y , cm/sec

- Du ra t i on o f p ressu re pu l se , sec

- Upper Flammable L i m i t , pe r cen t by volume

- Volume, m 3 o r f t 3

NOTE: A l l p ressures are gage p ressu re un less o t h e r w i s e s p e c i f i e d .

34

Chapter 2 Fundamentals of Deflagration

2-I Prerequisites. The following are necessary for a deflagration to occur:

- fuel, in the proper concentration; - an oxidant, in suf f ic ient quantity to support

the combustion; - an ignit ion source strong enough to i n i t i a te

combustion.

These factors are discussed individual ly in the following sections.

2-2 Fuel. The fuel involved i,n a deflagration may be a combustible gas (or vapor), a mist of a combustible l iquid, a combustible dust, or some combination of these. The most common combination of two fuels is that of a combustible gas and a combustible dust, called a "hybrid mixture."

2-2.1 Fuel Concentration.

2-2.1.1 Gaseous fuels have a lower flammability l imi t (LFL) and an upper flammability l i ~ i t (UFL). Between these l imits, ignit ion is possible and combustion wi l l take place. The optimum concentration usually occurs at s l ight ly richer than the stoichiometric mixture.

2-2.1.2 Combustible dusts also have a lower flammability l imi t , often referred to as the "minimum explosive concentration." For many dusts, this concentration is about 20 g/m 3. Although this concentration can be experimentally determined, i ts practical value is somewhat limited because of the tendency for dust to fa l l out of suspension and sett le on surfaces. However, such deposits can be thrown into suspension, thereby forming a dust cloud having an ign i t ib le concentration. Therefore, the "minimum explosive concentration" can be used to determine the amount of such "stat ic" dust that may be allowed to safely accumulate.

A "maximum explosive concentration" exists but is d i f f i c u l t to evaluate because of problems in achieving adequate dispersion of the dust during testing. Just as with gases, there exists an optimum concentration which yields the maximum rate of pressure rise during combustion.

Experiments show that a combustible dust cloud containing small particles (nominally less than 420 microns) may deflagrate. The maximum rate of pressure rise and the maximum pressure developed both increase as part ic le size is decreased. The maximum rate of pressure rise is more sensitive to part ic le size, and the sensi t iv i ty is most pronounced for part ic le sizes below about 50 microns (Reference 63). The sensi t iv i ty of maximum pressure developed is most pronounced for the larger part icle sizes in the size range of 200 - 420 microns. Minimum ignit ion energy is extremely sensitive to part ic le size. See Figures 2-2(a) and 2 - 2 ( b ) .

I t should be "noted t h a t the average p a r t i c l e d iamete r i s o f t e n reduced as a r e s u l t o f a t t r i t i o n du r ing m a t e r i a l hand l ing and process ing ~tnd t h a t c e r t a i n process ope ra t i ons may cause sepa ra t i on o f f i n e p a r t i c l e s from coarse p a r t i c l e s . This r e s u l t s in the f o rma t i on o f a "zone" o f pa r t i c l es ; which has a sma l l e r average part icle diameter than the bulk of the material and which is no longer protected by the di lut ion effect of a suff ic ient concentration of coarse part icles.

2-2.1.3 A mist of combustible l iquid droplets can also deflagrate. This may happen not only at i n i t i a l temperatures above the flashpolnt, but also at any temperature below the flashpoint. In the extreme case, a cloud of frozen droplets may de~lagrate in the same manner as a dust cloud.

The lower flammable l imi t (LFL) for dispers'ed l iquid mists varies from about 50 mg/l i ter to about 10 mg/liter as the representative droplet diameter increases from about 10 to 100 microns. Fi f ty mg/liter is roughly equal to the LFL for combustible gases in a i r at room temperature. '

Ease of ignit ion of l iquid mists is related principal ly to the representative droplet diameter. The minimum ignit ion energy (MIE) increases in proportion to the cube of droplet diameter.(1') Conversely, the MIE is reduced dramatically as droplet diameter is reduced.

Foams of combustible liquids burn readily and, as a source of finely-dispersed mist, they may exhibit a low MIE. Oxygen is more soluble than nitrogen in most combustible l iquids and i f a foam is produced by a degassing process the oxidant concentration may be enriched.

2-2.2' Hybrid Mixtures.

2-2.2.1 A mixture of a combustible gas and a combustible dust in an oxidant is referred to as a "hybrid mixture." The presence of the gasmay have some effect on the combustion characteristics of the dust. This influence may be considerable and may occur even though the gas is belo V i ts lower flammable l imi t and the dust is below i ts minimum explosive concentration. For example, small amounts of combustible gas may lower the minimum ignit ion energy of a dust cloud, as i l lust rated in Figure 2-2(c). The maximum rate of pressure rise during a deflagratlon may increase considerably, as shown in Figure 2-2(d), and the maximum pressure attained during the deflagration may also increase, as shown in Figure 2-3(e)~ although this la t te r effect is less pronounced.

300

200

100

8O

'~ 6(]

4C E

I [ I I I I I I I

Maximum Pressure

""",,, Maximum Rate

]

30,000

20,000

2G -- 2,000

1 ' 1 ~ - - ' 1,000

"8 - - ' 800

e I I I I I I 1 1 1 600 10 20 30 40 60 80 100 200

Average Particle Diameter, Microns

10,000

8,000

6,000 ~=

4,000

Figure 2-2(a) Effect of average part ic le diameter of a metal dust on the maximum pressure and the maximum rate of pressure rise developed by deflagration in a small, closed vessel. (2)

35

C ~

5O(

4O(

3O(

2O(

10(I

I I I I

B

40 80 120 160 200 Ave~gePaRicle Diameter, Microns

d u s t in h .~orid mixtures C: log[mj ) ~ . , E l ) Honso yellow M < 20pm

• cellulose M - 27 pm

" , PVC M - 20~Jm ] ~ " • PVC . ~ 2 5 ~

l O ] k O propGne turbulent M

°'°I \ "q'° "-- 1-- I

O o • , v ~------O-~ 0.~ t~ Vo) ]

lel prol:~ne content

F igure 2--2(c) Lowest minimum i g n i t i o n energy o f hybr id mix tu res versus propane content . (3)

Figure 2-2(b) Effect of average par t i c le diameter of a • typical agr icu l tura l dust on the minimum ign i t ion

energy. (Unpublished data, courtesy of U.S. Mine Safety and Health Administrat ion.)

"6

o

[~=,,3

2 ~

/ ~ . , . . . . . . . . 3/>..,:,,.....~ • , g Z - ; '>, ' " " " "" /"-:>', ;.",,/;,

," / t~:<..-,..~,;.>:///~.~

m e t h a n e con ten t in t h e air for combustion:

I

I 7 vol % ; i

~ ~ . . . ~ 5 vo~ Ol,

, I t

3 rot °/,

. 1 ~ t %

~ N,,al

PVC dust concentrcztk:n

Figure 2-2(d) Exp los ion data f o r Po lyv iny l C h l o r i d e / methane/a i r mix tures (1 m 3 vesse l ; chemical de tona to r wi th an i g n i t i o n energy o f 10,000 Jou le ) . (4)

3 6

O, 1 4 5 6 "oVol]

p r o p a n e conten'~ in air

dusi in hybrid mixfures

o ~ ,e,lo~ ~ 20~m o ce,uloSe M - 2 : ,, 0 PE M-~O .. 7 ~C M - 20 ,.

[ 1 PVC M -125 7 5 0 b a r / s "~ " • "~ p ' o p 3 r e turbu,ent

.=.

~ 2 s o . , ,

0 1 3 4 5 6 [ '~ Vo|l

p r o p a n e c o n t e n t i n air

Figure 2-2(e) Explosion data for dust/propane/air mixtures (Im 3 vessel; pyrotechnic igni ter with an ignit ion energy of 10,000 Joule). (4)

The minimum explosive concentration of the dust may be reduced and combination formulae have been suggested by both Bartknecht and Field to estimate this lower concentration (5,6). Dusts which have low Kst values seem to be more sensitive to the presence of a combustible gas. Careful evaluation of the ignit ion and deflagration characteristics oi: these mixtures is required; specific testing is strongly recommended, since a hybrid mixture may require more vent area than would be required by either component alone.

2-2.2.2 Situations where hybrid mixtures may occur in industrial processes include f luidized bed dryers, in which combustible dusts wet with solvent are dried in a warm a i r stream; desorption of combustible solvent and monomer vapors from polymers; and coal pulverizing operations.

In many instances the evolution of the gas may be completely unexpected or may be very slow. I t has been shown that the introduction of a combustible gas into a cloud of dust which would normally be a minimal explosion hazard can result in a vigorous combustion of the hybrid mixture. An example of this phenomenon is the combustion of unplasticized polyvinyl chloride dust in an air/methane atmosphere.

2-3 Oxidant.

The oxidant in a deflagration is normally the oxygen in a i r . Oxygen concentrations greater than 21 percent tend to intensify the combustion reaction and increase the probabil i ty of transit ion to detonation. Conversely, concentrations less than 21 percent tend to decrease the rate of reaction. There is for most fuels a l imi t ing oxygen concentration below which combustion wi l l not occur. (See NFPA 69, Standard on Explosion Prevention Systems.) Also, other oxidants, such as the halogens, may have to be considered.

/

2-4 Fundamental Burning Velocity and Flame Speed.

The flame speed is the local veloci ty of a freely propagating flame relat ive to a fixed point. I t is the sum of the burning velocity and the translational veloci ty of the flame front. This is expressed by the equation:

Sf = Su + St

S~ = flame speed, cm/sec;

S, = fundamental burning veloci ty, cm/sec;

St = translational veloci ty, cm/sec.

The fundamental burning veloci ty is the veloci ty at which a plane reaction front moves into the unburned mixture as i t chemically transforms the fuel and oxidant into combustion products. I t is only a fract ion of the flame speed. The translational veloci ty is the sum of the velocity of the flame front caused by the volume expansion of the combustion products due to the increase in temperature and any increase in the number of moles and any flow veloci ty due to motion of the gas mixture pr ior to ign i t ion. Special techniques are needed to accurately measure the fundamental burning velocity.

2-5 Igni t ion Source.

2-5,1 Both the maximum pressure and the maxi:mum rate of pressure rise developed during a deflagration in vessels much smaller than I m s increase as t~e energy of the ignit ion source increases. In larger vessels these increases only occur with powerful sources of igni t ion, such as j e t flames. Thus, the energy released by a point source of igni t ion in a re la t ive ly large vessel w i l l have l i t t l e ef fect on the course of the deflagration. This is because turbulence is induced in the flame front by the deflagration and this turbulence wi l l outweigh any effects of the ignit ion source.

2-5.2 Igni t ion at the geometric center of an enclosure wi l l usually result in the most destructive effects. Of course, the energy of the igni t ion source must be above some minimum value. Values of these minimum ignit ion energies have been reported for gases and for dust clouds (7 through 13). Usually minimum ignit ion energies of gases are much lower than those of dust clouds. However, some recent work has been reported which indicates that dust clouds can be ignited by sources releasing much less energy than has been previously reported (14).

2-5.3 Igni t ion can result from external energy sources such as an electr ical arc, a flame, a mechanically-produced spark (impact or f r i c t i o n ) , or a hot surface. Ignit ion can also result from slow exothermic reactions which may produce spontaneous heating. Simultaneous multiple igni t ion sources may produce turbulence in the fuel/oxidant mixture that wi l l intensify deflagration. An igni t ion source may travel from one zone to another; e.g., a mechanical spark may be transported from a grinding mil l to a dust col lector via ductwork. Similarly, a flame produced by an igni t ion source in one enclosure may i t s e l f become a much larger ignit ion source i f i t propagates to another enclosure.

2-6 I n i t i a l Temperature and Pressure. Any change in the i n i t i a l (absolute) pressure of the fuel/oxidant mixture at a given i n i t i a l temperature, w i l l produce a proportionate change in the maximum pressure developed by a deflagration of the mixture in a closed vessel. Conversely, any change in the i n i t i a l (absolute) temperature at a given i n i t i a l pressure wi l l produce an inverse change in the maximum pressure attained. However, an increase in temperature usually results in an increase in the maximum rate of pressure r ise: (See Figure 2-6.)

37

40O .m

30o E

x ~m

6OO

5 0 0 - -

200 --

100--

0 0.04

Po = 1 5 psia

7~:~8° F 3412°F 13F3+ F

0.08 0.12 0.16 1

Reciprocal of Initial Temperature, °R x 10 .2

0.20

Figure 2-6 Effect of in i t ia l temperature on the maximum pressure developed in a closed vessel for deflagrations of 9.9 percent methane/air mixtures at several in i t ia l pressures. (15)

120

101]

80

~ +o

E

4O

20

I I I I

Maximum Pressure ( T u r b u l e n t ) f ""

i 6 8 lO 12 14

Methane, Percent

30,000

25,000

zo, ooo

_=" 15,000 ¢

10,0oo

5,000

Figure 2-7 Maximum pressure and rate of pressure rise for turbulent and nonturbulent methane/air mixtures in a I cubic foot closed vessel. (16)

2-7 Turbulence. In i t i a l turbulence in closed vessels results in both higher maximum pressures and higher maximum rates of pressure rise than would be obtained i f the fuel/oxidant mixture were at i n i t i a l l y quiescent conditions. This is shown in Figure 2-7.

2-8 Presence of Moisture.

2-8.1 Moisture absorbed on the surface of dust particles wil l raise the ignition temperature of the dust because of the energy absorbed in vaporizing the moisture. However, the moisture in the air (humidity) surrounding a dust particle has no significant effect on a deflagration once ignition has occurred.

2-8.2 There are direct relationships between moisture content and the minimum energy required for ignition, the minimum explosive concentration, the maximum pressure developed during a deflagration, and the maximum rate of pressure rise. For example, the minimum ignition temperature of cornstarch dust may increase by as much as 50°C when the moisture content increases from 1.6 to 12.5 percent, by weight.

2-8.3 As a practical matter, moisture cannot be considered an effective means of preventing a deflagration since most ignition sources wi l l provide more than enough energy to vaporize the moisture and to ignite the dust. For moisture to prevent ignition of a dust by most common sources (such ashot pieces of slag from cutting operations, hot bearing surfaces, etc.) the dust would have to be so damp that a cloud would not readily form. Unfortunately, material containing this much moisture wi l l usually cause processing di f f icu l t ies.

2-9 Presence o f I n e r t M a t e r i a l .

2 - 9 . t I n e r t gases such as n i t r o g e n or carbon d i o x i d e are o f t e n used to prevent i g n i t i o n o f gases and dus ts . The use o f i n e r t gases i s d iscussed in NFPA 69, Standard f o r Exp los ion Preven t ion Systems.

2 -9 .2 I n e r t powder can reduce the c o m b u s t i b i l i t y o f a dust f o r the same reason t h a t mo is tu re does: the powder wil l absorb heat. Unfortunately, the amount of inert powder necessary to prevent a deflagration is considerably greater than the concentration which can usually be tolerated as foreign material. Some inert powders such as s i l ica can be harmful because they increase the dispersibi l i ty of the combustible dust.

2-9.3 Addition of inert powder to a combustible dust/oxidant mixture, when practical, wi l l reduce the maximum rate of pressure rise and wi l l increase the minimum concentration of combustible dust necessary for ignition. Rock dusting of coal mines is one practical application of the use of inert dust to prevent a deflagration. However, enough rock dust is usually added to provide a concentration of at least 65 percent inert dust. See Figure 2-9 for an example of the effect of admixed inert powder.

38

0.8 m

O. N O =- O

e- oJ

O u 0~

E E o~

O .-1

0 . 6

0.4 -

0 . 2 - -

1 20

Cornstarch and calcium carbonate

//if/Co :,::t,: reChrt n d J J Sul fur and

I I I 40 60 80

Admixed inert, %

100

Figure 2-9 Effect of admixed inert powder on the minimum explosive concentration of several dusts. (17)

Chapter 3 - Fundamentals of Venting of Oeflagrations

3-I Deflagration Vents.

A deflagration vent is an opening in an enclosure through which combustion-generated gases may expand and flow. The purpose of the vent is to l imi t the deflagration pressure so that dam~Lge to the enclosure is limited to an acceptable level or eliminated ent i re ly. The vent may or may not be equipped with a cover. In the case of uncovered vents, the maximum pressure attained during venting ~,ill exceed atmospheric pressure, but wi l l be lower than the pressure developed in an unrented enclosure. In the case of covered vents, the maximum pressure developed during venting wi l l be greater them for the case of the uncovered vents (a l l other factors being equal} because of the pressure required to open the vent by bursting the cover or pushing i t out of the way.

3-2 Consequences of a Deflagration.

3-2.1 In any enclosure that is t(po weak to withstand the overpressure from an expected deflagration, extensive damage wi l l occur should there actually be a deflagration. For example, an ordinary masonry wall (8 in. brick or concrete block, I0 f t high) cannot withstand a sustained overpressure of much more than 0.5 psi. Unless an enclosure is designed to withstand the maximum expected overpressure from a deflagration, venting should be considered to minimize damage. The - area of the vent must be great enough to l imi t the deflagration pressure to some predetermined safe level.

3-2.2 Venting of a deflagration implies the need to relieve internal pressure fast enough to maintain a low enough overpressure within the enclosure so that signif icant damage does not occur. The peak overpressure allowed is normally chosen to be less than the rupture pressure of the weakest signif icant structural element. In buildings, this may be a wall, f loor, roof, column, or beam~ in equipment, the weakest element may be a jo in t or seam.

Few data are available on the actual forces experienced by the structural elements of an enclosure during a deflagration. Therefore, designs must be based on the type of enclosure (vessel, equipment, room,'building), i ts material of construction, i ts resistance to mechanical shock, the effects of vents (including consequent thrust forces), and the level and duration of overpressure. In practice, the vent design should be based on withstanding the maximum overpressure attained during venting of the deflagration. I f no venting is provided, the (maximum) overpressures developed during a deflagration wi l l typical ly be between 8 and 12 times the i n i t i a l absolute pressure, assuming complete combustion. In many cases i t is impractical and economically prohibit ive to construct an enclosure'that wi l l withstand or contain such pressures. In some cases, however, i t is possible to design for containment of a deflagration. (See NFPA 69, Standard for Ekplosion Prevention Systems.} I f adequate venting canbe provided, the enclosure need not be constructed so robustly.

3-3 Maximum Rate of Pressure Rise and Maximum Pressure.

3-3.1 The rate of pressure rise is an important parameter in the venting of a def lagrat ion;; ' i t determines the time available for products of combustion to escape from the enclosure and for pressure to dissipate. A rapi d rate of r ise means that only a short period of time is available for successful venting. Conversely, a slower rate of rise-:permits the venting to proceed more slowly, yet s t i l l be effect ive. In terms of required vent area, the more rapid the rate of r ise, the greater the area needed for venting to be.effective, a l l other factors ~eing equal.

3-3.2 The effect of a deflagration depends on the maximum pressure attained, the maximum rate,,of pressure rise, and the duration of the peak overpressure. The total impulse imparted to the enclosure ( i . e . , the integral of the pressure vs. time curve) is reduced as

39

the rat io of vent area to enclosure volume increases. (See Figure 3-3) However, total impulse is not a useful design basis. The stress developed on the enclosure should be calculated on the basis of the stat ic force that is equivalent to the dynamic force developed at the peak pressure reached during venting.

120 1,20C

~80 ~ B~

"a60

E

x

~_ 4o I 4oo

2O 20O

0 0

I I I Dust Concentration - 0.500 oz per cu ft

\\

Maximum Rate

I I I I "~". 2 3 4 5

Ralio of Relief Area to Volume. sq ft per 100 cu ft

12.000

10,0oo

B,ooo ~T

6 . ~

4,000

E

2,000

o

Figure 3-3 Variation of pressures, rates, and impulses with vent ratios in magnesium deflagrations in a vented vessel. (18)

3-4 Vent Variables.

3-4.1 Vent Size and Shape. The maximum pressure developed in a vented enclosure decreases as the available vent area increases. I f the enclosure is re lat ively small and symmetrical, one large vent may be just as effective as several small vents of equal combined area. As an enclosure increases in size, this probably ceases to be true. Rectangular vents are almost as effective as square or c ircular vents of equal area; thus, vent shape has minimal effect on the successful application of venting.

3-4.2 Vent Type. Open or unrestricted vents are the most effective in rel ieving deflagration overpressures. Vents covered with a diaphragm, rupture disc, swinging or hinged cover, or other type of cover present inert ia and a mechanical attachment that must be overcome. Such vents ale inherently less effective. Chapter 9 describes various types of vents and vent closures.

3-4.3 Inertia of Vent Closure. The free area of a vent does not become fu l l y effect ive in rel ieving the deflagration pressure unti l the vent closure moves completely out of the way of the vent opening. Until this occurs, the closure obstructs the combustion gases issuing from the vent. The closure has mass and this mass represents iner t ia that must be overcome by the force of the deflagration. Some f i n i t e period of time is needed for the combustion gases to push the closure completely out of the way.

Since the acceleration of the closure is inversely proportional to i ts mass, the greater the mass of the closure, the longer the closure takes to completely clear the 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.

Experience has shown that the iner t ia of the vent closure is usually not signif icant i f the closure weighs less than 2.5 Ib per sq f t of free vent area.

3-4.4 Vent Operation. Vents must function dependably. Closures must not be hindered by deposits of snow, ice , or debris; neither must they be hindered by build-up of deposits on their inside surfaces. Adequate clear space must be maintained on both sides of the vent to enable operation without restr ic t ion and without impeding the free flow of vented gases.

3-5 Basic Recommendations for Venting. Since venting of deflagrations is a complex subject of many variables on which information is limited, the following provides only general guidelines.

3-5.1 Venting is usually required in buildings, rooms, or equipment that contain an operation or process that may release combustible material in amounts suf f ic ient to create an ign i t ib le mixture with a i r or other available oxidant.

3-5.2 The required vent area wi l l depend on the strength of the enclosure, the maximum rate of pressure rise and maximum pressure developed for the fuel/oxidant mixture in question, and the design of the vent i t s e l f , including presence or absence of a closure device. Empirical methods are presented in la ter chapters to determine the required vent area.

3-5.3 Vents should be evenly distributed over the surface area of the enclosure to the greatest extent practical.

3-5.4 The gases vented from an enclosure during a deflagration must be directed to a safe location to avoid injury to personnel and to minimize property damage. I t may be necessary to insta l l guardrails immediately in front of vent panels in building walls and around vent panels in roofs to prevent personnel from fa l l i ng against or through the panels. Suitable warning signs should also be posted. I t may also be necessary to provide restraining devices to keep vent panels or closures from becoming missile hazards. An alternat ive means of protection is to provide a missile barr ier close enough to the vent to intercept any missiles, but far enough from the vent so as not to impede i ts operation.

3-5.4.1 When a deflagration is vented, a tongue of flame of br ief duration issues from the vent. I f the fuel is a dust, this tongue of flame wi l l usually contain some unburned dust, along with the gaseous products of combustion. This is because the amount of dust present i n i t i a l l y is usually greater than that which the oxidant in the container can burn. This unburned dust wi l l be ignited as i t flows out the vent and can produce a large f i rebal l that wi l l extend not only outward and upward, but also downward from the vent. This has been shown in numerous tests conducted with fu11-scale equipment.

3-5.5 I f vents are f i t ted with closure devices, they should be designed so that they do not allow the development of a vacuum in the enclosure af ter heated gases have cooled.

3-5.6 Interconnections between separate pieces of equipment should be avoided. Where such interconnections are necessary, flashback prevention devices should be considered to prevent propagation of the deflagration from one piece of equipment through the interconnection to other equipment. Such devices may be mechanical or chemical in operation.

3-5.7 Structural damage can also be minimized by locating vented equipment ei ther outside buildings or in isolated areas.

3-5.8 Ducts used to direct vented gases from the vent to the outside of a building must be strong enough to withstand the maximum expected deflagration overpressure and must be able to withstand the maximum anticipated temperature during venting. Ducts should be as short as possible and should preferably not have any bends.

40

3-5.9 Wind may cause a vent to operate falsely or may hinder i ts operation. Vent design must anticipate the problems created by prevailing wind patterns.

3-5.10 Situations may occur in which i t is not possible to provide adequate deflagration venting as described in Chapter 4 through 7 of this Guide. This is not jus t i f i ca t ion for providing no venting at a l l . I t is suggested that the "maximum practical" amount of venting be provided, since some venting wi l l reduce the resulting damage to a limited degree. In addition, consideration should be given to other protection and prevention methods. (See NFPA 69, Standard for Explosion Prevention Systems.)

3-5.11 Reaction forces resulting from venting should also be considered in the design of the equipment and their supports. (See 5-2.9.)

Chapter 4 - Venting of Deflagrations in Low-Strength Enclosures

4-I Introduction.

4-1.1 This Chapter is applicable to the design of deflagration vents for low-strength enclosures capable of withstanding not more than 1.5 psig (0.1 bar ga.), such as rooms, buildings, and equipment enclosures having re lat ive ly low strength-to-volume ratios.

4-1.2 The proper design of deflagratlon vents depends on many variables, only some of which have been investigated in depth. The techniques available for calculating the area needed for deflagration venting are based on a limited number of actual tests and analyses of actual deflagration incidents.

4-1.3 Tests and analyses conducted to date have al lowed cer ta in genera l i za t ions to be made. The ca l cu la t i on techniques presented in th is Guide are based on these genera l i za t ions . The techniques must, there fore , be recognized as approximate on ly . ' The user of th i s Guide is urged to give special a t t en t i on to a11 precaut ionary statements.

4-2 General.

4-2.1 The.reason for prov id ing de f lag ra t ion vent ing for an enclosure is to minimize or eliminate structural damage to the enclosure i t se l f and to reduce the probabil i ty of both damage to other structures and injury to personnel.

4-2.2 Most enclosures of the type addressed by this Chapter cannot be subjected to high internal overpressures without serious damage. Adequate venting can minimize the damage from a deflagration. However, the venting must be suff ic ient to prevent the maximum pressure developed within the enclosure from exceeding the "breaking point" of the weakest structural element, which may be a wall, the f loor, the roof, a column, or a beam.

4-2.3 Care must be taken to ensure that the weakest structural element is recognized. All structural elements must be considered - walls, windows, doors, f loors, ceil ings, roofs, and structural supports. For example, i t must be recognized that floors and roofs are usually not designed for much structural loading from beneath. Furthermore, the structural analysis" must be based on the actual design and the existing condition of the enclosure.

4-3 Calculating the Vent Area.

4-3.1 Numerous methods have been presented for calculating the vent area for an enclosure. (References 19 through 23) The method recommended here is the Runes equation (20):

Where

Av = C(LI • Lz)

Av = vent area, f t z or m2;

C = fuel characteristic constant;

LI = smallest dimension of the enclosure, f t or m;

L2 = second smallest dimension of the enclosure, f t or m;

P = maximum internal overpressure which can be withstood by the weakest structural element, psi or kPa.

4-3.2 Applicable Dimensions. The Runes equation is believed to be applicable to enclosures having a nominal length-to-wldth (L/D) rat io of up to 3. For enclosures having an L/D rat io exceeding 3, the enclosure should be subdivided into units, each having an L/D no greater than 3. Where Lt and Lz are unequal, the effect ive value of D is ~ . Therefore, the Runes equation is constrained as follows:

L3 ~ 3 V~ x L2

where L3 = longest dimension of the enclosure, f t or m.

Where th isconst ra in t cannot be met or where the enclosure is unusally shaped, the enclosure should be "normalized" by subdividing i t into several regular rectangular spaces, then calculating the vent area for each space separately.

4-3.3 Fuel Characteristic Constant. The value of "C" in the Runes equation serves two purposes: i t characterizes the fuel and i t clears the dimensional units. Also, two sets of C values have been derived so that the Runes equation can be used with either English or SI units. Table 4-3 gives some recommended values of C.

Table 4-3 Fuel Characteristic Constant for Runes Equation

Fuel C, Enolish Units

Gases with Fundamental Burning Velocities l ike Propane 1.3 3.4

Vapors of Flammable Liquids 1.3 3.4

Organic Dusts 1.3 3.4

Organic Mists 1.3 3.4

Fast Burning Me£al Dusts 2.0 5.3

4-4 Enclosure Strength.

4-4.1 The term "P" in the Runes equation is defined as the "maximum internal overpressure which can be resisted by the weakest structural e lement . " This term was or ig inal ly derived from an o r i f i ce equation which uses the pressure d i f fe rent ia l across the or i f i ce . Since one side of the vent is always assumed to be atmospheric, the gage pressure within the enclosure can be used.

C. SI Uni ts

41

4-4.2 The Runes equation was tested mathematically to 2 psig and by application to an actual building deflagration incident where the strength of the building was estimated to be within 20 percent of l.O psig. The original presentation of the equation (20) included the restr ic t ion that P should not exceed 2.0 psig. This Guide recommends that the equation be used only for enclosures whose strength does not exceed 1.5 psig.

4-4.3 Theoretically, the force exerted on an enclosure by an internal deflagration is dynamic. However, recent work by Howard and Karabinis (24) indicates that the enclosure may be assumed to respond as i f the peak deflagration pressure is applied as a static loading, provided some inelast ic deformation (but not catastrophic fa i lure) can be accepted. Therefore, any structural element which cannot be permanently damaged or deformed by the deflagration must be designed to withstand the maximum internal overpressure, P, without catastrophic fa i lure .

4-4.4 In designing an enclosure to prevent catastrophic fa i lu re while s t i l l allowing some inelastic deformation, the normal dead and l ive loads should not be rel ied upon to provide adequate restraint . For example, walls should be fastened along top and bottom edges, as well as at a l l corners.

4-4.5 In al l cases, except as noted in 4-4.6, the maximum allowable design stress should not exceed two-thirds of the ultimate strength.

4-4.6 Ductile design practices should be used. For materials subject to b r i t t l e fa i lure , such as cast iron, special reinforcing should be considered. I f such reinforcing is not used, the maximum allowable design stress should not exceed 25 percent of the ultimate strength.

4-4.7 In al l cases, the strength of the enclosure should exceed the vent re l i e f pressure by at least 0.5 psi (72 psf).

4-5 Vent Design.

4-5.1 Where inclement weather is not a consideration, open vents may be used and are recommended. In most cases, however, vents wi l l be covered by some type of lightweight closure or panel. The panel must be designed, constructed, instal led, and maintained so that i t wi l l readily release and move out of the path of the combustion gases. The panel must also not become a missile hazard when i t operates.

4-5.2 The total weight of the panel assembly, including any insulation and permanently-mounted hardware, should be as low as practical, but in no case should i t exceed 2.5 I b / f t z. The purpose of this l imitat ion is to keep the iner t ia of the assembly as low as possible so that the vent opens as rapidly as possible.

4-5.3 The material of construction of the panel should be suitable for the environment to which i t wi l l be exposed. B r i t t l e materials wi l l fragment, producing potent ial ly lethal missiles. Some panels, because of their configuration, may travel some distance from the enclosure. Each insta l la t ion must be evaluated to determine the extent of the hazard to personnel from such missiles.

4-5.4 Vent panels must release at as low an internal pressure as practical, yet stay in place when subjected to external wind forces. The suction effects of wind passing around and over the structure and across the surface of the panel must be considered. In most cases, the vent panel release pressure can be about 20 pounds per sq f t (psf). In areas subject to severe windstorms, the release pressure may have to be as great as 30 psf.

4-5.5 Under the dynamic conditions of deflagration venting, magnetic, sprlng-loaded, or diaphragm-type panels wil l release at overpressures reasonably close to their design values. Release devices that fa i l under tension or shear may require unusually higher forces for operation under dynamic condition than under the static conditions at which they are ususally tested. These higher forces may not be compatible with the design requirements of the vent system.

4-5.6 The panel (or panels) must provide the required vent area for the volume of the enclosure being protected. I f this enclosure is i t s e l f subdivided by walls, part i t ions, f loors, or ceilings into compartments, then each compartment that contains a deflagration hazard must be provided with i ts own vent.

4-5.7 A single large vent should not provide the required vent area for more than one enclosure. This restr ict ion ensures that the pressure developed by a deflagration must only move the mass of vent panel required for venting that enclosure only.

4-5.8 Each panel must be designed and installed to move freely without interference by obstructions such as ductwork, piping, etc. This ensures that the flow of combustion gases is not impeded by a "hung-up" vent panel.

4-5.9 Guardrails must be instal led in front of the panel to keep personnel from leaning against and possibly fa l l i ng through the panel.

4-5.10 A restraining device may be needed to keep the panel from tearing completely free of the enclosure and becoming a missile. This is discussed in Chapter 9.

4-5.11 The c r i te r ia for the design of roof panels are basically the same as for wall panels. Since the panels wil l not l i ke ly be safe to step on or s i t on, access to the roof should be prohibited or guardrails should be installed around each panel. In climates subject to snow and ice accumulation, the panels should not be insulated, thus allowing building heat to thaw any snow and ice. I f building heat alone is not adequate, special heating may have to be provided.

4-6 Sample Calculation.

4-6.1 Consider the following building for which deflagration venting is to be provided:

(see figure following)

The required vent area is to be calculated according . . to the Runes equation. The building is a complex shape and i ts dimensions do not appear to sat isfy the restraint imposed by 4-3.2.

4-6.2 To confirm that this example does not sat isfy the length-to-diameter constraint, use the long, narrow part of the building.

L3 <.. 3 V / L1 x Lz

In this example, LI (the height) varies from 20 to 30 f t ; use the average height of 25 f t . Lz is 30 f t and L3 is 170 f t .

3~/'C;" x L2 = 3Vf -~ x 30 = 82.2

The length, L3, exceeds the constraint by a factor of 2, so the building dimensions must be "normalized".

42

60 fl (18.3 1 ~' ']

~70ft (51 8m~

I o,,

p~--- 50 n-~. I End Wall I IIS25 m)

' .7 ; = so t~- - - . . I

118,3 m)

(Not to tcalel

Figure 4-6(a) Building used in sample calculat ion. (Not to scale.)

4-6 .3 The b u i l d i n g can be assumed to be made up of th ree par ts as shown in Figure 4 -6 (b ) :

170 ft (51.8 m;

Part III

E ~ Wall It

r . . . . . . ~ .~_~ n i3.:~1

i I ..... J i '

19.15 m)

,o,w.,,, ~ , , : ~ ' ~

j , . , " ' ~ l O ' " (3.06 m).

;= ~o ft----.~ i18,3 m)

(Not tO scale)

Figure 4-6(b)

Normalize each part as follows:

(a) Consider the foot of the "L" as one part, designated Part I in the i l l us t ra t i on . The height of end wall I can be normalized to the area of the wall by calculating an average height that w i l l yield an equivalent area.

Area of End Wall I (60 x 20) + (0.5 x 30 x 10) = 1350 f t 2

Normalized height = 1350 = 22.5 f t 60

The required vent area for Part I w i l l be the same as for a structure having dimensions of 50 f t by 60 f t by 22.5 f t high.

(b) The leg of the "L", consisting of Parts I I and I I I , is normalized in a similal~ manner to the area of end wall I f .

Area of End Wall I I = (30 x 20) + (0.5 x 30 x lO) = 750 f t 2

Normalized Height = 750 = 25 f t 30

End Wall II

....... L ..... i J i

(9.1Sin)

r Now, the requ i red vent area is based on a b u i l d i n g having dimensions of 30 f t by 120 f t by 25 f t h igh. However, the length of th i s sec t ion of the b d i l d i n g s t i l l violates the constraint of the Runes equation:

L3 ~ 3 ~LI x L2 i'

In this case, the maximum permissible lengthl for L3 is 82.2 f t , as established in subsection 4-6.2. In essence, the building is too long by 37.8 f t . Therefore, i t must be considered as two sections: Part I I and Part I I I . Parts I I and I I I w i l l require the same vent area according to this calculation. The boundary between the two parts should be located so that the vents are symmetrically distr ibuted over the entire volume represented by the two parts.

4-6.4 The required vent area may now be calculated according to the Runes equation. Assume that the maximum allowable overpressure for this building has been estimated by a structural engineer to be 0.5 psi. The material for which venting is required is a combustible gas whose fundamental burning veloci ty is about the same as that of propane. Thus, the value of C in the Runes equation is 1.3.

43

(a) Applying the equation to Part I :

Av = 1.3(22.5)(50) = 2068 f t z

The area available for venting is:

Wall Area = (60 x 20) + (0.5 x 30 x I0) + (30 x 20) +2(50x20) = 3950 f t 2

Roof Area = (50 x 30) + (50 x v t ~ O ~ - ~ ) = 3081.1 f t z

Total Area = 3081 + 3950 = 7031 f t z

Suff ic ient wall and roof area are available to provide the needed vent area for Part I . Note that only the outer walls and the roof can be used as vent area; a deflagration cannot be vented into other parts of the b u i l d l n g

(b) Apply lng the equat ion to the res t o f the b u i l d i n g invo lves s e t t i n g a boundary between Parts I I and I I I . The boundary is a r b i t r a r i l y chosen so tha t the length o f Part I I i s 50 f t . Thus:

Av = ~ = 1379 f t z

The area a v a i l a b l e f o r ven t ing is :

Wall Area = (50 x 30) + (25 x 30) + (50 x 20) = 3250 f t 2

Roof Area = 50 x ~ r - + - - l ~ = 1581 f t 2

Total Area = 3250 + 1581 = 4831.1 f t 2

Again, suf f ic ient area is available for venting of Part I I

(c) Applying the equation to Part I I I of the building:

Av = 1.3(25)(30) = 1379 f t z p~T

The area available for venting is:

Wall Area = (70 x 30) + (70 x 20) = 3500 f t 2

Roof Area = 70 x ~30 z + 10 z = 2214 f t z

Total Area = 3500 + 2214 = 5714 f t z

Again, there is su f f i c ien t vent area available in Part I I I .

(d) In the above example, the boundary between Part I I and Part I I I was not c r i t i c a l , since there was more than enough wall and roof area available for use as deflagration vents. Had d i f fe ren t boundaries been chosen, one part of the building may not have had suf f ic ient wall and roof area for vent purposes. In such a case, a t r i a l and error approach would have had to be used to f ind a "good" boundary.

4-6.5 Situations may arise in which the roof area or one or more wall areas cannot be used for vents, e i ther because of equipment placement or due to exposure to other buildings or to areas normally occupied by personnel. In such cases i t is necessary to strengthen the structural members of the compartment so that the reduced vent area available is matched to the vent area required. The minimum pressure requirement for the weakest structural member is obtained by subst i tut ing into the Runes equation the available vent area and calculating the minimum overpressure. The vent area must s t i l l be distr ibuted as evenly as possible over the building's "skin".

Chapter 5 Vent ing o f D e f l a g r a t i o n s in High Strength Enclosures - General

5-1 I n t r o d u c t i o n .

5-1.1 This Chapter and Chapters 6 and 7 apply to vesse ls and equipment capable o f w i ths tand ing at l e a s t 1.5 ps ig (0.1 bar ga).

5-1 .2 D e f l a g r a t i o n vent requi rements are dependent on many v a r i a b l e s , on ly some o f which have been f u l l y i n v e s t i g a t e d . The technology o f c a l c u l a t i n g the requ i red vent area in an enc losure sub jec t to d e f l a g r a t i o n i s based on a l i m i t e d number o f t es t s and the analyses o f actua l exp los ion i n c i d e n t s . The t e s t i n g and analyses conducted to date have al lowed c b r t a l n g e n e r a l i z a t i o n s to be made; the recommended c a l c u l a t i o n methods presented in t h i s Guide are based" on these g e n e r a l i z a t i o n s . The c a l c u l a t i o n methods must, t h e r e f o r e , be regarded as approximate on ly . The user o f t h i s Guide is urged to g ive spec ia l a t t e n t i o n to a l l p recau t i ona ry statements.

5 -1 .3 I t i s not poss lb le to vent a de tona t ion s u c c e s s f u l l y .

S-1.4 The maximum overpressure t ha t w i l l be reached dur ing ven t i ng , Pred, w i l l always exceed the pressure a t which the vent dev ice re leases ; in some cases i t w i l l be much h igher . This maximum overpressure is a f f e c t e d by a number o f f a c t o r s . These must be cons idered when des ign ing the vessel o r p iece o f equipment tha t w i l l be p ro tec ted . This Chapter and Chapters 6 and 7 g ive gu ide l i nes f o r determin ing t h i s maximum overpressure .

S-2 Basic P r i n c i p l e s . Cer ta in bas ic p r i n c i p l e s are common to the ven t ing o f d e f l a g r a t i o n s o f gases, mis ts , and dusts . These inc lude but are not l i m i t e d to the f o l l o w i n g :

5-2.1 The vent design must be adequate to prevent the d e f l a g r a t i o n pressure i ns ide the vented enc losure from exceeding t w o - t h i r d s o f the u l t i m a t e s t reng th o f the weakest pa r t o f the enc losure which must not f a i l . This c r i t e r i o n does a n t i c i p a t e tha t the enc losure may bulge or o therw ise deform.

a

S-2.2 Vent c losures must open dependably. The i r p roper ope ra t i on must not be h indered by depos i ts o f snow, i ce , t a r r y o r s t i c k y m a t e r i a l s , polymers, e tc . The i r ope ra t i on must not be prevented by co r ros ion or by ob jec ts which obs t ruc t the opening o f the vent c losu re . Al lowance should be made f o r the r e s t r i c t i o n to f l ow caused by any ob jec ts in the path o f the gas f low.

5-2 .3 Vent c losures must have a low mass per un i t area to minimize i n e r t i a in o rder to reduce opening t ime. The to~al mass o f the c losure d i v i ded by the area o f the vent opening should not exceed 2.5 l b . / f t z (12.S kg/mZ). Greater mass per u n i t a rea r e s u l t s in h igher maximum overpressure dur ing ven t i ng . The vent c losure should have no counterweights ; counterweights add more i n e r t i a . Vent c losures must be designed to have e f f i c i e n t opening c h a r a c t e r i s t i c s to minimize opening t ime.

5 -2 .4 Vent c losures should not become m i s s i l e hazards as a r e s u l t o f t h e i r ope ra t i on . For example, vent panels made o f f r a n g i b l e ma te r ia l l i k e glass f i b e r r e i n f o r c e d p l a s t i c or cement / inorgan ic f i b e r can r e a d i l y break when they opera te . Theb roken pieces w i l l c o n s t i t u t e m i s s i l e hazards. In most cases the vent c losure should be r e s t r a i n e d so tha t i t w i l l not f l y away from the vessel when i t opera tes . (See Sect ion 9-4 f o r two s u i t a b l e methods f o r r e s t r a i n i n g vent c l o s u r e s . )

5 -2 .5 Vent c losures must w i ths tand exposure to the ma te r i a l s and process cond i t i ons w i t h i n the vessel o r enc losure being p ro tec ted . They must a lso wi ths tand ambient cond i t i ons on the non-process s ide .

44

5-2.6 Vent closures must release at overpressures reasonably close to their design release pressures• Therefore, release mechanisms must be properly designed and installed. Magnetic or spring-loaded closures wi l l sat isfy this cr i ter ion. Release devices that fa i l in tension or shear may require much greater forces to break under dynamic conditions than under stat ic test conditions.

5-2.7 Vent closures must re l iably withstand fluctuating pressure d i f ferent ia ls which are below the design release pressure. They must also withstand any vibration or other mechanical forces to which they may b e subjected.

5-2 .8 Vent c losures must be inspected and p r o p e r l y maintained in o rder to ensure dependable ope ra t i on . In some cases t h i s may mean rep lac ing the vent c losure at s u i t a b l e time i n t e r v a l s . See Chapter 10 f o r d e t a i l s .

5 -2 .9 The suppor t ing s t r uc tu re f o r the enc losure must be s t rong enough to wi ths tand any reac t i on fo rces developed as a r e s u l t o f ope ra t i on of the vent . The equat ion f o r these reac t ion fo rces has been es tab l i shed from tes t r e s u l t s (25) , as f o l l o w s :

Fr : 1.2 (A)(Pre¢) where Fr = reaction force resulting from combustion

venting, lb. ; A = vent area, in.2; Pred = maximum pressure developed during

venting, psig.

The total thrust force can be considered equivalent to a force applied at the geometric center of the vent. Instal lat ion of vents of equal area on opposite sides of a vessel cannot be depended upon to prevent thrust in one direction only. I t is always possible for one vent to open before another. Such imbalance should be considered when designing vessel or enclosure restraints for resisting thrust forces.

Reference 25 contains a rule-of-thumb equation that roughly approximates the duration of the thrust force of a dust deflagration. Knowing this duration can aid in the design of certain support structures for vessels with deflagration vents. The duration calculated by the following equation w i l l be quite conservative:

tF = ( lO -2 ) (K~ t ) (V ~/s) (Pred)(A)

where tF = duration of pressure pulse, sec.;

Kst = Deflagration Index for dust (see Chapter 7) ;

V = vessel volume, m3;

Pre~ = maximum pressure developed dur ing ven t ing , bar ga;

A = Area o f vent (w i thou t vent duc t ) , m 2.

5-3 Correlating Parameters for Deflagration Venting.

5-3.1 The technical l i te ra ture reports extensive experimental work on venting of deflagrations in vessels up to lO0 m 3 in volume (References 26 through 31). From this experimental work, Bartknecht and Donat have developed a series of nomographs, Figures 6-2(a) through (d) in Chapter 6 and Figure:~ 7-2(a) through ( f ) in Chapter 7, that can be used for determining the necessary vent areas for vessels and equipment.

5-3.2 The nomographs d i f f e r from e,~rlier techniques in that they are not based on a l inear relationship of vent area to vessel volume.

5-3.3 The selection of the proper nomograph to use is discussed in detail in Chapters 6 and 7.

5-3.4 The nomographs may not exactly predict the vent area required for d i f ferent volumes of vessels. Certain data (32) indicate that the gas venting

nomographs may not be conservative in every case. For the present, however, the use of the venting nomographs is recommended on the basis of successful industrial experience.

5-3.5 The nomographs apply only to enclosures where the length to diameter rat io is less than 5. For long pipes or process ducts or vessels whose L/D rat io is 5 or greater, the deflagration vent design should be based on the information given in Chapter 8.

5-3.6 The homographs for deflagration venting of gases (Chapter 6) and for deflagration venting of dusts (Chapter 7) are based on experimental data. The homographs for gases cannot be used for dusts, and vice versa.

5-4 Effects of Vent Ducts.

5-4.1 Normally, equipment to be vented is placed in a safe outside location and is vented di rect ly to the outdoors .

5-4 .2 In some s i t u a t i o n s equipment o r vesse ls t ha t r equ i r e d e f l a g r a t i o n vents must be located i ns ide b u i l d i n g s . In these cases the vents p r e f e r a b l y should not discharge within the buildings. Flames emerging from the vessel during the venting process may seriously injure personnel and may damage other equipment or the building i t s e l f . Gases discharging from the vent may also cause appreciable overpressure within the building and lead to additional damage. Therefore, vent ducts should be used to direct vented material from the equipment to the outdoors•

5-4.3 Vent ducts wi l l s igni f icant ly increase the pressure developed in the equipment during venting. The vent ducts must have a cross-section at least as great as that of the vent i t s e l f . The increase in overpressure due to the use of vent ducts as a function of duct length is shown in Figures 5-4(a), for gases, and 5-4(b), for dusts. The same phenomenon, as a function of flow velocity through the duct, is shown for both gases and dusts in Figures 5-4(c) and 5-4(d), respectively•

5-4.3.1 The use of vent ducts of larger cross section than the vent wi l l result in a smaller increase in the maximum pressure developed during venting (Pred) than wi l l vent ducts of equivalent cross sect ion. Figure 5-4(e) shows this.

5-4.4 I f vented equipment must be located within buildings, they should be placed close to exter ior walls so that the vent ducts wi l l be as short as possible, preferably not more than 3 m long.

5-4.5 Vent ducts should be as straight as possible• Any bends wi l l cause increases in the overpressure developed during venting• I f bends are unavoidable, they should be as shallow-angled ( i . e . , have as long a radius) as practical.

5-5 Exposure from the Venting Process• Flames emerging from the vessel or equipment during the venting process can seriously injure personnel, can ignite other combustibles in the v ic in i ty , can result in ensuing f i res or secondary explosions, and can result in overpressure damage to adjacent buildings or equipment• For a given quantity of combustible mixture, the amount which wi l l be expelled from the vent and the thermal and overpressure damage which results outside of the equipment wi l l depend on the volume of the equipment and the vent opening pressure. For a given volume of equipment an d a given quantity of combustible mixture, a lower vent opening pressure relat ive to the internal operating pressure wi l l result in more unburned material being discharged through the vent, resulting in a larger f i reba l l outside the equipment• A higher vent opening pressure relat ive to internal operat!ng pressure wi l l result in more combustion taking place inside the equipment pr ior to the vent opening, higher velocity through the vent, and the potential for more overpressure damage.

4s

1¢6

\

c~ e.~

/

/

I N ~ L e ~ W • 3 ~ P, ,~÷ l . e , , , f fk .,~ 3,,.,

Figure 5-4(a) Maximum pressure developed during venting of gases, with and without vent ducts. (33)

x.o

| "~ t.(, .4., u

~ 6.5 -

3

1 a."

/ /

o.I , , o.n

/

o . ~ - o±r I . o Z . o .~. o ~ . o

Figure 5-4(b) Maximum pressure developed during venting of dusts, with and without vent ducts. (33)

5.0

[ 2,5 . ~2 .0

~I~o

0.5

J /

0.1 0.1

i /

/ /

/ / /

/

/ /

/ /

- - velocity _>330 m~s -velocity -~ 3 3 0 m/s

02 04 OG

P r e d w i t h o u t duct, bar (gages)

f ur

/

/

08 1.0 k$ 20

Figure 5-4(c) Maximum pressure developed during venting of gases, with and without vent ducts. (34)

.~o

,o

-o .¢ Qs

/

/

/

/

/

/ 1 /

/

I /

DUSTS velocity ->330 nn/s velocity : 330 m/s

(11 Q2 Q~. qS O~ tO 15 ~0

Pred without d u c t , b a r (gages)

Figure 5-4(d) Maximum pressure developed during venting of dusts, with and without vent ducts. (34)

30

25

~ 20 w

w

~. t5

,o

5

KEY No. Vent dia,in Ductdia,in

I 4.25 4.25 2 5.75 4.25 5 5.75

/ 2

! I I 0 5 I0 15 20

DUCT LENGTH~ ft

Figure 5-4(e) Maximum pressure developed during venting of explosion of cornstarch through various sized ducts. (35)

46

5-6 Location of Deflagration Vents Relative to Air Intakes. Deflagration vents should not be located in such positions that the vented material can be picked up by air intakes.

Chapter 6 Venting of Deflagrations of Gas Mixtures and Mists in High Strength Enclosures

6-I Nomographs for Deflagration Venting.

6-1.1 The nomographs in Figures 6-2(a) through 6-2(d) can be used for determining the necessary vent area for venting methane, propane, coke gas, or hydrogen during a deflagration. I t is important to note that these homographs were developed for in i t i a l conditions of:

no i n i t i a l turbulence in the vessel at the time of ignit ion,

- no turbulence-producing internal appurtenances,

- a low ignition energy of'10 Joules or less, and

- atmospheric pressure.

See later sections of this chapter fo.r effects of changes in these variables.

6-1.2 The homographs apply only to cases where vessel or equipment length-to-diamete r ratio (L/D) is five or less. For venting equipment havin~ an L/D greater than five, refer to Chapter 8. " .

6-2 Deflagration Venting of Gases Other than Those Specified on the Nomographs. The homographs in Figures 6-2(a) through 6-2(d) can he used to establish the deflagration vent requirements for gases other than methane, propane, coke gas, and hydrogen. Three approaches that may be used for other gases are described below.

6-2.1 Use of Deflagratlon Testing to Interpolate Between Nomographs. Deflagration testing, as described in Appendix A, may be used to charzLcterize a specific gas for interpolation between the homographs. The basis for thls interpolation is thaLt i f two gases yield the same maximum rate of pressure rise, (dp/dt)max, when they are ignited in the same closed test vessel, i t can be assumed that they wi l l both require the same vent area to provide protection for" any size of enclosure. ..

The maximum rate of pressure rise of a gas varies with the volume and shape of the test vessel and with the ignition energy. Thus, i f this; technique is to be used for interpolation, the values of the maximum rate of pressure rise for the speci£ic gas, and for the gases used in the homographs, must be determined. These determinationsmust be performed in the same test vessel, using the same ignition energy. For further details of the test procedure see Appendix A. See 6-2.4 for an example of interpolation between the homographs of the "standard" gases having higher and lower maximum rates of pressure rise than the gas in question.

6-2.2 Classification of Gases by Fundamenta ! Burning Velocity. With less dependability, the deflagration venting requirements of certain ga:;es can be determined by'comparing their fundamental burning velocities, Su, with that of propane. Table BI in Appendix B in Appendix B gives values of Su for many common gases. I t should be noted that the values of Su in this table have been derived from a single source, as explained in the Appendix. These values may not be consistent with those from other sources.

I f the fundamental burning v e l o c i t y given in Appendix B for a spec i f i c gas is less than 60 cm/sec, about 1.3 times that of propane, then the propane nomograph (Figure 6-2(b)) may be used. I f the Fundamental burning v e l o c i t y exceeds 60 cm/sec, then the hydrogen nomograph (Figure 6-2(d)) may be used.

6-2.3 Use of Nomographs WithOut Test ing. I f t es t data of the type described in 6-2.1 are unava i lab le , the hydrogen nomograph, Figure 6-2(d) , can be used to estimate the vent requirements. Although th i s approach is conservat ive in many cases, t he .add i t i ona l vent area resulting from its use wil l normally be small.

6-2.4 Example of Determining the Required Deflagration Vent Area by Interpolation. Given a 10 m3 vessel which must be provided with deflagration, venting for a gas that is not specifically covered by a homograph, calculate the required vent area for the following conditions:

- Maximum allowable value of Pred = 0.8 bar

- Pstat. = 0.2 bar

- Maximum rate of pressure rise for gas in question in a particular test vessel = 730 bar/sec

Using the propane and hydrogen homographs (Figures 6-2(b) and (d)), the required vent area to ~rotect the vessel specified wil l be 10.1 m 2 and I I .0 m ~, respectively. The maximum rates of pressure rise for propane and hydrogen are 369 and 2029 bar/sec, respectively, in the same test vessel. By linear interpolation, the required vent area for this vessel and this specific gas i s :

10.1 +F(730 - 369) X (11.0 - 10.1)7 = 10.3 m 2 L (2029 - 369) d

6-2.5 Kc Values. The maximum rate of pressure rise can be normalized to give the KG value (See equation I in Appendix A). I t should, however, be noted that the KG value is'not constant and wi l l vary depending on test conditions. In particular, increasing the volume Of the test vessel and increasing the ignition energy can result in increased KG values. Although the KG value provides a means of comparing tffe maximum rates of pressure rise of various gases, i t

.should only be used-as a basis for deflagration vent s;zing i f the tests are performed in vessels of approximately the same shape, and size, and with the same kind of igniter having the same ignition energy. (See Appendix C for examples of KG values.)

6-3 Effects of In i t ia l Turbulence and Internal Vessel Appurtenances for Enclosures with In i t ia l Pressure Near Atmospheric.

6-3.1 In i t ia l Turbulence. In many items of industrial equipment, the gas phase is present in a turbulent condition. An example is the cont!huous feed of'a combustible gas/oxidant mixture to a catalyt ic partial oxidation reactor. Normally this mixture enters the reactor head as a high velocity turbulent flow through a pipe. As the gas enters the reactor head, s t i l l more turbulence develops due to the sudden enlargement of the flow cross section.

I f the gas system is i n i t i a l l y turbulent, the rate of deflagration is increased (References 36 and 37). In this case the nomog~aphs do not direct ly apply. I t has been found that i n i t i a l l y turbulent methane and propane exhibit (dp/dt)ma, values similar to that of i n i t i a l l y quiescent hydrogen. For this reason the hydrogen homograph shou)d be used for venting i n i t i a l l y turbulent gases which have'(dp/dt)max values, in the quiescent state, that are similar to or less than that of propane.

47

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The susceptibi l i ty of a turbulent system to detonation increases with increasing values of the quiescent (dp/dt)max. In particular, compounds that have (dp/dt)max values close to that of hydrogen are highly susceptible to detonation when ignited under turbulent conditions. I t should be noted that deflagration venting is not an effective method of protecting against a detonation.

6-3.2 Vessel Appurtenances. The presence of internal appurtenances within vented equipment can result in turbulence which may result in transition from deflagration to detonation. When the equipment contains internal appurtenances, an expert should be consul ted to determine i f the potential exists for a detonation to occur. (See Reference 38 for further information.)

6-4 Use of the Nomographs with Hydrogen. The user is cautioned that hydrogen/alr deflagratlons can readily undergo transition to detonations. I t is therefore recommended that, before using the nomograph for hydrogen (Figure 6-2(d)), consideration should be given to the potential for a detonation to occur. This may require test work and consultation with an expert on the subject.

6-5 Effect of High Ignition Energy.

6-5.1 The amount and type of ignition energy can affect the effective flame speed and the venting. The exact amount of ignition energy which may occur in vessels or equipment cannot normally be predicted. In many industrial cases, however, the ignitlon,energy can be quite large.

6-5.2 A typical case is that of two vessels connected by a pipe. Ignition in one vessel wi l l cause two effects in the second vessel. Pressure development in the f i r s t vessel wi l l force gas through the connecting pipe into the second vessel, resulting in an increase In both pressure and turbulence. The flame front wi l l also be forced through the pipe into the second vessel, where i t wi l l become a very large ignition source. The overall effect wi l l depend on the relative sizes of the vessels and the pipe, as well as on the length of the pipe. This has been investigated by Bartknecht who found the effects can be quite significant (39). Pressures developed in the pipeline i tse l f can also be quite high, especially i f the deflagration changes to detonation. When such conditions prevail in equipment design, the reader should refer to Reference 40 or should consult a specialist.

6-6 Extrapolation of Nomographs.

6-6.1 The lowest Pstat value on the homographs is 0.1 bar ga; the lowest Pred value is 0.2 bar ga. I t is sometimes desirable to vent equipment at lower pressures, with resulting lower maximum pressure developed dur ing ven t i ng (Prod) . TO determine the necessary vent area requ i res e x t r a p o l a t i o n o f the nomographs. A g raph ica l approach is shown in F igure 6-6. Such a graph w i l l need to be const ruc ted f o r each vessel s i ze .

6-6 .2 In F igure 6-6 the vent areas f o r a 10 m 3 vessel were taken from the f o u r gas nomographs at constant Pred, but for different values of Pstet . Similar graphs can be constructed for various values of Pred. This graph allows interpolation and extrapolation, thus extending the u t i l i t y of the basic nomographs.

6-6.3 Recently published papers have proposed calculation of vent areas for gases on the basis of fundamental flame and gas flow properties and experimentally determined constants (References 22, 60, 61). These calculation procedures have not yet been fu l ly tested against the venting nomographs. The venting nomographs are to be taken as the final authority within their applicable ranges of Pstat and P r e d .

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The user i s caut ioned not to e x t r a p o l a t e the nomographs below 0.05 bar ga f o r Pstet nor below 0.1 bar ga f o r Pred. For va lues below these r e f e r to Chapter 4. Pred should a lso not be ex t r apo la ted above 2.0 bar ga. , the upper l i m i t in the nomographs. Pstat can be extrapolated upward, but i t must always be less than Pred by at least O.l bar.

6-7 Effect of In i t ia l Elevated Pressure.

6-7.1 The e f f e c t o f i n i t i a l pressure must be c o r r e l a t e d on the basis o f abso lu te pressures . The data from Reference 42 serve as a basis f o r c o r r e l a t i n g pressures developed dur ing ven t ing as a f unc t i on o f the i n i t i a l abso lu te pressure o f gases in the vessel and as a f unc t i on o f the abso lu te pressure a t which the vent opens. I f the r a t i o o f vent bu rs t i ng pressure to i n i t ia l gas pressure is kept constant and i f vessel size and vent size are kept constant, the pressure developed during the venting of propane combustion wi l l vary approximately as the 1.5 power of the in i t i a l pressure. The power exponent for propane varies from about 1.2 for larger vent ratios (A/V ~/3 = 0.3) to about 1.5 for smaller vent ratios (A/V a/3 = 0.1). For hydrogen, the exponent ranges from 1.1 to 1.2.

6-7.2 I t i s recommended tha t the 1.5 power be used in e x t r a p o l a t i n g from the homograph f o r gases having K O va lues c lose to tha t o f propane. For hydrogen, the recommended exponent f o r increased i n i t i a l pressure is 1.2; f o r e thy lene, 1.4. The l a t t e r va lue has not been v a l i d a t e d by t e s t . The c o r r e l a t i o n may apply to i n i t i a l pressures up to 4 atmospheres abso lu te , but t h i s a lso is untested.

6 -7 .3 Based on h is ex tens ive exper imen ta t ion , Bar tknecht (3) main ta ins : "The nomographs are based on an opera t ing pressure o f 1 bar ( a b s o l u t e ) , but they may be used w i thout c o r r e c t i o n f o r ope ra t i ng pressures up to 1.2 bar (abso lu te ) . For h igher opera t i ng pressures, s u f f i c i e n t exper ience is not yet a v a i l a b l e . For the t ime being i t should be assumed tha t when the opera t ing pressure is ra ised above normal (a tmospher ic ) pressure, the reduced exp los ion pressure (Prod) w i l l ' s h o w a p ropo r t i ona l increase f o r a g iven constant r e l i e f ven t ing a rea . "

52

6-8 Effect of I n i t i a l Temperature. The effect of i n i t i a l temperature is discussed in this Guide in Chapter 2. In most cases an increase in i n i t i a l temperature wi l l result in an increase in maximum rate of pressure rise and a decrease in the pressure generated by combustion in an unrented vessel. I t is therefore believed that no adjustment in the estimated pressure developed during venting needs to be made for an increase in i n i t i a l temperature (43). The same may be true for i n i t i a l temperatures below ambient.

6-9 Effects of Combihations of Variables. There are insuff ic ient data to determine precisely how combinations of variables may affect the maximum pressure developed during venting (Pred). On the basis of test work recently conducted (44), i t appears that the effects of i n i t i a l turbulence, ( i . e . , pr ior to ignit ion) may not be signif icant when the i n i t i a l

.pressure is above 1.0 bar ga. In such cases an allowance would only be made for the i n i t i a l pressure above atmospheric, but not for turbulence.

6-10 Deflagration of M~sts of Combustible Liquids. Combustible mists wi l l burn not only at temperatures above the flashpoint temperature of the l iquid but also at temperatures below the flashpolnt temperature (See References 45 through 48). In this sense, mists are similar to dispersed dusts, which may also be ignited at any i n i t i a l temperature. The design of explosion venting for combustible mists can be based on the propane venting homograph. For more deta i l 'on ' combustible mists see Chapter 2.

6-II Deflagratlon of Foams of Combustible Liquids. Foams of combustible l iquids can burn. I f the foam is produced by bubbling a i r through the l iquid, the bubbles wil l contain a i r for burning. Combustion characteristics wil l depend on a nUmber of properties such as the specific l iquid, size of bubble, and thickness of bubble fi lm. There is, however, a more hazardous case. I f a combustible l iquid is saturated with a i r under pressure and i f the l iquid phase is then released from pressure with the formation Of a foam, the gas phase in the bubbles may be preferent ia l ly enriched in oxygen. This is because the so lub i l i t y of oxygen in combustible l iquids is higher than that of nitrogen. The increased oxygen concentration wi l l result in intensif ied combustion. I t is therefore recommended that combustible foams be careful ly tested relat ive to design for deflagratlon venting.

6-12 Venting Deflagrations of Combustible Gases Evolved from Solids. In certain processes combustible gases may evolve from solid materials. These gases may form combustible mixtures with any oxidant present. I f the solid is i t se l f combustible and is dispersed in the gas/oxidant mixture, as might be the case in a f luidized bed dryer, a "hybrid" mixture results. For hybrid mixtures use the homograph that applies to the component that requires the larger' vent area, which is usually the gas. See also Section 7-8 for more detai l .

6-13 Venting of Deflagrations in Conveying and Ventilating Ducts. Most deflagrations of combustible gas mixtures inside ducts occur at i n i t i a l internal pressures of nearly atmospheric. The venting of deflagrations in such ducts is di~,cussed in Chapter 8.

Chapter 7 Venting of Oeflagrations of Dust Mixtures in High Strength Enclosures

7-1 Introduction.

7-1.1 The most comprehensive design bases f o r ven t ing o f dust d e f l a g r a t i o n s are conta ined in VDI R i c h t l i n i e 3673, publ ished in Germany (49). . This work is based on data obta ined from an ex tens ive t e s t program i n v o l v i n g fou r dusts and f ou r vessel s i zes : 1, 10, 30, and 60 m 3. The nomographs developed from the t e s t data are reproduced here as Figures 7 -1 (a ) through 7 - 1 ( f ) . The nomographs apply to vesse ls o f L/D not over 5.

7-1 .2 Figures 7 -1 (a ) , (b ) , and (c) are based on the Kst va lues f o r the i nd i v i dua l dusts , as determined by t e s t procedures descr ibed in Appendix A. F i g u r e s 7 -1 (d ) , ( e ) , and ( f ) are based on the dust c lasses • S t - l , St-2, and St-3, r e s p e c t i v e l y . These dust c lasses represent a range o f Kst va lues , as shown in Table 7-1.

Table 7-1 Hazard Classes o f Dust D e f l a g r a t i o n s 1'2

Hazard Kst, 3 Class bar m/sec

St-! ~ 200 St-2 201 - 300 St-3 > 300

1The a p p l i c a t i o n o f the nomographs is l i m i t e d by an upper Kst va lue o f 600.

ZSee Appendix D f o r examples o f Kst va lues.

3Kst va lues were determined in approx imate ly spher ica l c a l i b r a t e d t e s t vesse ls o f at l e a s t 20 l i t e r capac i t y .

7-1 .3 Combustion ven t ing c h a r a c t e r i s t l c s o f dusts o f the same chemical composi t ion vary wi th the phys ica l p r o p e r t i e s such as s ize and shape o f dust p a r t i c l e , mois ture content , and o thers . See 2 -2 .1 .2 and Appendix A f o r more i n fo rma t ion On t h i s sub jec t .

7-2 Use o f Dust Nomographs.

7 - 2 . i The necessary vent area f o r a dust can be determined from the nomographs as a f unc t i on o f the Kst value or the dust hazard c lass , the vessel volume and s t reng th , and the r e l l e v i n g pressure o f the vent c losu re .

7-2 .2 The vent areas p red ic ted by the two sets o f nomographs descr ibed in 7-1.2 may not complete ly agree. The agreement i s , however, s u f f i c i e n t l y c lose f o r p r a c t i c a l a p p l i c a t i o n s . When exper imenta l va lues o f Kst are a v a i l a b l e , F igures 7 - t ( a ) through 7-1(c ) should p r e f e r a b l y be used to e s t a b l i s h the minimum ven t area r e q u i r e d . The homographs themselves are not exact and the de te rmina t ion o f Kst can in t roduce a d d i t i o n a l e r r o r s . However, the nomographs have been shown to p r e d i c t the requ i red vent area wi th s u f f i c i e h t accuracy f o r dependable use in i ndus t r y .

7-3 E x t r a p o l a t i o n and I n t e r p o l a t i o n o f Nomographs.

7-3.1 The dust nomographs can be ex t r apo la ted and i n t e r p o l a t e d using the g raph ica l techniques descr ibed in Sect ion 6-6.

7-3 .2 The user i s caut ioned not to extrapolate the nomographs below 0.05 bar ga. f o r Pstat nor below 0.1 bar ga- f o r Pred, For va lues below these, use the c a l c u l a t i o n procedure, in Chapter 4. Furthermore, Pred should not be extrapolated above 2.0 bar ga. , the upper l i m i t in the nomographs. Al though Pstat may be extrapolated upward, i t must always be less than Pred by at l e a s t 0.1 bar. The ven t ing homographs are to be taken as the f i n a l a u t h o r i t y w i t h i n t h e i r a p p l i c a b l e ranges o f Pstat and Pred-

7--3.3 The dust nomographs were developed f o r e s s e n t i a l l y atmospheric i n i t i a l p ressure , be fo re i g n i t i o n , and they apply to i n i t i a l pressures up to 0.2 bar ga. No guidance is a v a i l a b l e at present f o r systems opera t ing at h igher i n i t i a l p ressures.

53

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7-4 Bins, Hoppers and S i l os .

7-4.1 Deflagration venting for bins, hoppers, and si los must be from the top or the upper side, above the maximum level of the material contained and must be directed to a safe outside location (see Sections 5-5 and 7-7). In some instances the required vent area may be as large as the vessel cross section. In these cases the entire vessel top can be made to vent. Space must be available above the top to allow i t to open suf f ic ient ly. The top should be as l ight weight as possible. (See 3-4.3 for effects of vent mass.) Large diameter tops of this type cannot be made self-supporting and wi l l require internal supports. Panels which make up the top must not be welded or otherwise attached to the internal roof supports. As an alternative, individual vent closures may he located on the top or the side (above the maximum level of solids). When vent closures are located on the side and top of the vessel, the maximum useful area for venting wil l correspond to the cross sectional area of the vessel.

7-4.2 The vent area requlred is determined by the strength of the vessel. I f this vent area is larger than the vessel cross section, the vessel needs to be strengthened to contain a pressure consistent with the available vent area. In al l cases the total volume of the vessel should be assumed to contain a suspension of the combustible dust in ques t ion . That i s , no c r e d i t should be taken f o r the vessel being p a r t l y f u l l o f settled material.

7-4.3 Deflagration venting is sometimes accomplished by means of vent panels distributed around the wall of the vessel just beneath the top. In such cases care must be taken not to f i l l the vessel above the bottoms of the vent panels. Otherwise, large amounts of dust may be blown out into the atmosphere, be ignited, and form a large f i r eba l l . Furthermore, dust piled above the bottoms of vent panels can hinder vent panel opening and can also result in Pstat values which are higher than design.

7-5 Effects of Vent Ducts. The effects of vent ducts are discussed in Section 5-5.

7-6 Venting of Enclosed Bag Dust Collectors. I t is desirable to design bag f i l t e r vent panels in such a way as to minimize the potential for bags and cages to interfere with the venting process. The f i l t e r medium may not adequately segregate the clean and d i r ty sections of the col lector during the deflagration. Therefore, the entire volume of each section should be used when calculating the vent area for that section. I f the volume of the clean section above the tube sheet is relat ively small, the vent area required may be achieved by placing the vents on the d i r ty section.

7-7 Flame Clouds from Dust Deflagrations. Normally when dust deflagrations occur, there is far more dust present than there is oxidant to burn i t completely. When venting takes place, large amounts of unburned dust are vented from the vessel. Burning continues as the dust mixes with additional a i r from the surrounding atmosphere. Hence a very large and long f i reba l l of burning dust develops, which can extend downward as well as upward. The size of the f i rebal l depends on many factors. In one deflagration venting test a dust f i rebal l extended at least 4 m below the level of the vent and about 15 m horizontal ly. Personnel enveloped by such a f i reba l l would l i ke ly not survive. The potentially large size of the f i reba l l extending from the dust deflagration vent should be considered when locating vents and vent ducts so as to avoid hazards to adjacent equipment and personnel.

7-8 Hybrid Mixtures.

7-8.1 A mixture of a combustible gas and a combustible dust constitutes a hybrid mixture. Such a mixture may be igni t ib le even i f both constituents are below their respective lower flammable l imits. The properties of hybrid mixtures are extensively discussed by Bartknecht (3). Certain dusts which do not form combustible mixtures by themselves may do so i f a combustible gas is added, even i f the la t te r is at a concentration below i ts lower flammable l imi t . The lower flammable

l imi t concentrations of most combustible dusts are decreased by addition of combustible gases, even when the concentrations of the l a t t e r are below their lower flammable l imits. The minimum igni t ion energy is also reduced below that for the dust alone.

For additional information see 2-2.2.1.

7-8.2 The effect ive Kst value of most combustible dusts is raised by the admixture of a combustible gas, even i f the gas concentration is below the lower flammable l imi t . This in turn leads to an increase in the required vent area. For hybrid mixtures, use the nomograph for the component that requires the greater vent area. This is usually the gas.

Chapter 8 Vent ing o f D e f l a g r a t i o n s from Pipes, Ducts and Elongated Vessels Operat ing

At o r Near Atmospheric Pressure

8-1 Scope. This chapter app l i es to systems ope ra t i ng at pressures up to 1.2 bar abso lu te .

8-2 General .

8-2.1 Several f a c t o r s make the design o f d e f l a g r a t i o n vents f o r p ipes, ducts, and e longated vesse ls ( l eng th to d iameter r a t i o s o f 5 or g r e a t e r ) a d i f f e r e n t problem than the design o f d e f l a g r a t i o n vents f o r o r d i n a r y vesse ls and enc losures . These inc lude :

(a) The geometry o f l a rge L/D r a t i o s promotes rap id a c c e l e r a t i o n o f f lames. Acce le ra t i on to very high f lame speeds, o r even de tona t i ons , can occur.

(b) For an i n d i v i d u a l ven t , any vent area in excess o f the cross sec t i ona l area o f the p ipe , duct , o r vessel w i l l not be e f f e c t i v e in reducing the d e f l a g r a t i o n pressure . The cross sec t i ona l area is the maximum e f f e c t i v e vent area a t t a i n a b l e .

(c) Turbu lence-produc ing appurtenances such as valves, elbows, and other f i t t i ngs are frequently present. The turbulence produced can generate sudden flame acceleration and a consequent rapid increase in pressure.

(d) Ignit ion of a combustible mixture in a vessel to which a pipe or duct is attached results in a flame front that generates considerable turbulence ahead of i t s e l f and precompresses the gas in the pipe or duct. When the flame front reaches the entrance to the pipe or duct, i t is f u l l y developed and turbulent. The result is a flame front that propagates into the pipe or duct with much greater i n i t i a l violence than that which would result from spark igni t ion in the pipe or duct i t s e l f .

(e) Conversely, when a flame front propagates through an inadequately vented pipe or duct and then enters an enclosure or vessel containing a mixture in the flammable range, the result ing j e t of flame is such a massive ignit ion source that any deflagration venting in the vessel may be rendered inadequate.

8-2.2 The design of adequate deflagration venting for pipes, ducts, and elongated vessels is further complicated by the fact that there has been re la t ive ly l i t t l e systematic test work publlshed on this subject. The guidelines in this chapter are based on information contained in References 50 through 58 and are thought to provide reasonable protection, but the i r use should be .tempered by sound engineering judgement for specific applications. Any deviation from these guidelines should be in the direction of more, rather than less, vent area.

8-3 Design Guidelines.

8-3.1 For pipes, ducts, or elongated vessels having cross sections other than circular, the hydraulic diameter should be used in the correlations that follow. The hydraulic diameter is equal to 4A/P, where A is the area of the cross section and P is the perimeter of the cross section.

60

8-3.2 The total vent area at each vent location should be at least equal to the cross sectional area of the duct or pipe. The required vent area can be accomplished by using either one or more than one vent at each location.

8-3.3 Any pipes or ducts connected to a vessel in which a deflagration can occur may also require deflagration venting. For gases and Class St-3 dusts, a deflagration vent whose area is equal to the cross sectional area of the pipe or duct should be provided at a l o c a t i o n on the p l p e ' o r duct that i s no more than two diameters d i s t a n t from the po in t o f connect ion to the vesse l . For Class St-2 and St.-1 dusts, eva lua t ions should be made to determine the need f o r any a d d i t i o n a l ven t ing on a case by case bas is .

8 -3 .4 De f l ag ra t i on vents should be located c lose to poss ib le i g n i t i o n sources, when these sources can be i d e n t i f i e d .

8 -3 .5 For systems hand l ing gases, unless approp r ia te t es t s i nd i ca te o therwise, pipes and ducts conta in ing obstac les should be prov ided wi th d e f l a g r a t i o n vents on each side o f the obs tac le . When des ign ing f o r a P~eo o f 0.2 bar or less , two vents , each o f which has an area equal to the cross sec t iona l ,~rea o f the duct or p ipe, should be 'p laced on each side o f the obstac le at d i s t a n c e s f r o m the obs tac le o f 3 diameters and 6 diameters, r e s p e c t l v e l y . When des ign ing f o r a Prod o f g rea te r than 0.2 bar, one vent on each s ide o f the obstac l e at d is tances o f 3 diameters should be s u f f i c i e n t . At the present t ime, lhere is not s u f f i c i e n t i n f o m a t i o n a v a i l a b l e fGr vent ing of dusts. An obstac le is def ined here as an elbow, tee, f low s p l i t t e r , o r i f i c e , va l ve , or any appurtenance tha t b locks more than 5 percent o f the ,:ross sec t iona l area o f the pipe or duct.

8-3.6 The weight of deflagratlon vent closures should not exceed 2.5 Ib/sq f t for each s,~uare foot of free vent area.

8-3.7 The release pressure of vents should be as much below the design value of P~ed as possible, consistent with operating conditions, but should not exceed one half of the design value for Pred. Covers may be held by magnets or springs.

8-3.8 Deflagration vents must discharge to a location that wi l l not endanger personnel.

8-3.9 Consideration should be given to reaction forces that develop during venting. See 5-2.9.

8-4 Determination of Pred for Pipes, Ducts, or Elongated Vessels That Are Vented at One End Only.

8-4.1 The curves in Figure 8-4(a) should be used to determine the maximum a l lowab le length o f a smooth, s t r a i g h t p ipe, duct , o r vessel t ha t i s c losed on one end and vented on the o ther when no a d d i t i o n a l d e f l a g r a t i o n vents a ' re 'p rov ided. I f L/D r a t i o s g r e a t e r than those shown in the f i g q r e are present , there is a r i s k that de tonat ion may occur." In these cases, the con ta ine r should be designed to re:~ist de tonat ion pressures, provided wi th addi t iona ' l vents , or prov ided wi th exp los ion prevent ion measures such as those descr ibed in NFPA 69, Standard on Explos ion Prevent ion Systems. Class St-1 d~sts are an except ion in tha t there is no evidence that large L/l) ratios can lead to a detonation of these dusts.

8-4.2 I n i t i a l Velocity 2 m/sec or Less--Gases. The curves in Figure 8-4(b) shouldbe used to estimate the pressure developed in a pipe, duct, or vessel that is vented at one end only when the pressure results from deflagration of a gas/air mixture i n i t i a l l y flowing at a velocity of 2 m/sec or less. This applies to gas mixtures having properties similar to those of . propane. For diameters oiher than those shown, the curves should be interpolated. I f the pressure developed may exceed the strength of the container, additional vents should be provided as outlined in Section 8-5.

8-4.3 I n i t i a l V e l o c i t y 2 m/sec or Less-Dusts. The curves in F igure 8-4(c ) should be used to es t imate the d e f l a g r a t i o n pressure developed in a p ipe, duct , o r e longated vessel tha t is c losed on one end and vented on the othe r , w i th no add i t i ona l vents , when d u s t / a i r mix tures i n i t i a l l y f lowing a t 2 m/sec or less are i gn i t ed . . I f the pressure developed exceeds the burs t s t rength o f the con ta iner , then a d d i t i o n a l vents should be prov ided as ou t l i ned in Sect ion 8-5.

8 -4 .4 I n i t i a l V e l o c i t y Greater Than 2 m/sec. Flame acce le ra t i on and peak pressures can be g r e a t l y enhanced when the flammable mixture is i n i t i a l l y f l ow ing at v e l o c i t i e s g r e a t e r than 2 m/sec. Consequent ly, p ipes, ducts, or e longated vessels t ha t are vented on ly a t one end should be const ructed to w i ths tand de tona t i on , provided wi th a d d i t i o n a l exp los ion vents , or prov ided wi th exp los ion p ro tec t i on measures such as those descr ibed in NFPA 69, Standard On Exp los ion Prevent ion Systems. In l i e u o f des ign ing f o r de tona t ion pressures, Class St-1 dusts may be handled in systems designed to withstand 10 bar without bursting.

8-5 Explosion Vent Requirements When More Than One Vent Can Be Provided.

8-5.1 Maximum Distance Between Vents. The curves shown in Figure 8-5(a)should be' used to determine the maximum allowable distance between vents. I f distances in excess of those indicated are used, the pipe or duct sfiobld be designed to withstand a detonation or explosion prevention measures such as those described in NFPA 69, Standard for Explosion Prevention Systems, should be used. This l imi tat ion does not apply to Class St-I dusts, since there is no evidence that large L/D ratios can lead to a detonation of these dusts.

8-5.2 I n i t i a l Velocity 2 m/sec or Less. Figure 8-5(a) can be used to determine the increase in pressure caused by a deflagratlon in a pipe or duct when more than one vent can be provided. This figure applies to gases with fundamental burning veloc i t ies no more than 1.1 times that of propane and to dusts ' for which Kst £ 300.

8-5.3 I n i t i a l Velocity Between 2 m/sec and 20 m/sec. To l im i t P~e~ to 2.5 psig or less, the distance between vents can be determined from Figure 8-5(b). This figdFe applies to gases with fundamental burning veloci t ies no more than 1.1 times that of p~opane and to dusts for which Kst < 300.

8-5.4 For Other Gases. The results contained in the preceding paragraphs can be used for gases other than propane, provided the fundamental burning veloci ty does not exceed 1.1 times that of propane. Conversion of the data is accomplished by use of one of the following equations:

P~ = (Sx) 2 , PP ' ( S P )

where: Px = pressure predicted for gas;

Pp = pressure predicted for propane;

Sx = fundamental burning veloEity of gas;

Sp = fundamental burning veloci ty of propane.

Lx = (Sx) 2 Lp (Sp) '

where: Lp = distance between vents for propane;

Lx = distanc e between vents for gas;

Sx = fundamental burning veloci ty of gas;

Sp = fundamental burning veloci ty of propane.

Sl

2011 100

L = Distance between deflagration vents

80 or

x Length of pipe or duct having one end open

f f 60

40

Dusts with Kst 4. 200

20 E ~ ' ~ . . Propane, dusts with Kst :> 200

/ o / l I I u I I I

0 1 2 3

Diameter, meters

Figure 8-4(a) Maximum allowable distance, expressed as length-to-diameter ra t io , for a smooth

st ra ight pipe or duct.

3

J

1 0 D

D

m

5 - -

m

D

B

n

0 0

D 0 2 m

I I' I I I I I I 20 40 60 80

Length - to - diameter ratio

F i g u r e 8 - 4 ( b ) Maximum p r e s s u r e d e v e l o p e d d u r i n g def lagrat ion of propane-air mixtures flowing at 2 m/s or less in a smooth, stra ight pipe closed at one end.

62

10

?

D

m

D

Kst = 300

K s t = 2 0 0

I ! . i I I I i 20 40 60 80

L e n g t h - t o - d i a m e t e r r a t i o

Figure 8-4(c) Maximum pressure developed during def lagrat lon of dust -a i r mixtures flowing at 2 m/s or less in a smooth, straight pipe closed at one end.

Kst = i00

0.3

0.2

=> o ¢u

o J

0~

0

f Kst ~- 300

/ L= Distance between vents . J ' . D= Diameter °f pipe °r.dlct

l I I i I I i 20 40 60 80

Length - to - diameter ratio

Figure 8-5(a) Maximum pressure developed during def lagrat ion of dusts in a pipe or duct when more than one vent is provided.

6 3

10

Propane and dusts with Kst d_ 300

2 m/s and 20 m/s

.0

"0

o

.J

Figure 8-5(bi from exceeding 0.2 bar ga.

15 I

o I I I I 0 1 2

Vent spacing required to keep Pred Diameter, meters

I

8-5.5 In i t i a l Velocity Greater Than 20 m/sec, or Gases Having Burning Velocities More Than I . I Times That of Propane, or Dusts With Kst ~ 300. For these situations, vents should be placed no more than 1 to 2 meters apart or the pipe or duct should have a design pressure capable of withstanding a detonation or exp)osion protection measures such as those described in NFPA 69, Standard on Explosion Prevention Systems, should be employed.

8-5.6 Obstacies. For ducts or pipes containing obstacles as previously described, vents should be placed as specified in 8-3.5. Additional vents, as specified elsewhere in Section 8-5, may also be required.

8-6 Examples.

8-6.1 A dryer handling a dust whose Kst is 190 is two meters in diameter and 20 meters long and is designed so that one end functions as an explosion vent. What pressure wi l l be developed during a vented explosion?

(a) Check maximum allowable length: According to Figure 8-4(a), an L/D of about 25 is allowable. The dryer has an L/D of I0, so this is acceptabl@.

(b) Maximum Pressure: According to Figure 8-4(c), a pressure of about 1.0 bar ga wi l l be developed in this equipment by the deflagration of this dust. Hence, the equipment must have a design pressure of at least this value.

8-6.2 A f lare stack is 0.4 meters in diameter by 40 meters ta l l and is equipped with a water seal at i ts base. What must i ts design pressure be in order to protect i t from the pressure developed by ignit ion of a fue l -a i r mixture having properties similar to those of propane?

(a) Check maximum allowable length: From Figure 8-4(a), a maximum L/D of 28 is allowed. This stack has an L/D equal to 100. Therefore, i t must be designed to withstand a detonation or must be protected by some other means.

8-6.3 A straight duct 1 meter in diameter and 100 meters long is to be protected by explosion vents. I t contains a hydrocarbon/air mixture having properties similar to those of propane. What vent spacing is required to l imi t the deflagration pressure to 2.5 psig (0.17 bar) i f (a) the velocity is less than 2 m/sec, or (b) the velocity is less than 20 m/sec? In both cases, the vents are designed to open at 0.5 bar.

(a) From Figure 8-5(a), the spacing must be about 45 diameters (45 meters) in order to l im i t the increase to 0.12 bar above Pstat. However, this violates the maximum allowable spacing of about 19 diameters, as indicated in Figure 8-4(a). Hence, the vent spacing should not exceed 19 meters for this case. I t is recommended that seven vents be provided, including one at each end.

64

(b) From Figure 8 -5 (b ) , the vents should be placed no more than 7.6 meters apar t . In o rde r to meet t h i s requirement , i t is recommended thai: a vent be placed at each end and tha t 13 a d d i t i o n a l vents be evenly spaced a long the duct .

8 -6 .4 Provide d e f l a g r a t i o n vents f o r the ducts in the system shown in F igure 8-6 .4 . Assume a l l ducts are 0.6 m in d iameter and tha t the dust c o l l e c t o r , d rye r , and a11 ducts have a maximum a l lowab le work ing pressure o f 0.2 bar ga and tha t the maximum opera t i ng pressure anywhere in the system is l i m i t e d t:o 0.05 bar ga. The system handles a c lass St-3 dust . I t i s f u r t h e r assumed tha t the d rye r and dust c o l l e c t o r are equipped wi th adequate d e f l a g r a t i o n vents. Since the system handles an St-3 dust , i t i s recommended tha t the p rov i s i ons o f 8 -2 .3 and 8-2.5 be f o l l owed . There fore , the f o l l o w i n g vents are requ i red :

- A and B, located two and f i v e d iameters d is tance, r e s p e c t i v e l y , from the d ryer o u t l e t .

- C and D, located th ree and s i x d iameters d is tance , r e s p e c t i v e l y , from the f i r s t elbow.

- G, located two diameters upstream o f the dust c o l l e c t o r i n l e t .

- H, I , and 3, located at the midpo in ts , r e s p e c t i v e l y , o f the three 1.5 meter sec t ions . Since these sec t ions are less than th ree diameters in length , the second vents spec i f i ed in paragraph 8-2 .5 ( i . e . , the vents to be located s i x d iameters upstream and downstream o f an obs t ruc t i on ) are not requ i red .

- K and L, located three and s i x d iameters d is tance , r e s p e c t i v e l y , a f t e r the l a s t elbow.

Add i t i ona l vent ing is requ i red f o r the 20 meter sec t i on . At 100 m3/min, the v e l o c i t y in the l i n e s is 6 m/sec. Hence, Figure 8-4(b) should be used. According to t h i s curve, vents should be placed a t i n t e r v a l s o f 6 1/2 meters or less . The d is tance

be tween vents D and G is 15.2 meters. Therefore, two a d d i t i o n a l vents (E and F), located 1/3 and 2/3, r e s p e c t i v e l y , o f the d is tance between D and G, are requ i red .

The t o t a l vent area at each vent l o c a t i o n should be at l e a s t equal to the c ross - sec t i ona l area o f the duct . This w i l l ' r e s u ] t in a va lue o f 0.2 bar ga f o r Pred. According t o ' 8 - 2 . 7 , the vent re lease pressure should be no h igher than 0.1 bar ga.

Duct Lengths Dryer outlet to first elbow 5 rn F i r s t elbow to dust collector 20 m Dust collector to second elbow 1.5 m Second elbow to fan inlet 1.5 m Fan outlet to third elbow 1.5 rn Third elbow to end of duct 5 m

®

f (~= Vent locations

Figure 8-6.4 Diagram f o r example in 8 -6 .4 .

Chapter 9 Desc r ip t i on o f D e f l a g r a t i o n Vents and Vent Closures

9-1 General .

9-1.1 The d e f l a g r a t i o n vents and vent c losures descr ibed in t h i s Chapter have been designed to r e l i e v e the overpressure tha t r e s u l t s from a d e f l a g r a t i o n w i th in an enc losure.

9-1.2 Some types o f vent and c losure assembl ies are cormnercial ly a v a i l a b l e and may be purchased r e a d y - t o - i n s t a l l . Others must be cus tom- fab r i ca ted on s i t e by the user. The f o l l o w i n g d e s c r i p t i o n s may be used as a basis f o r the s e l e c t i o n or design o f s u i t a b l e vent and c losure assembl ies.

9-2 Normal ly Open Vents.

9-2.1 The most e f f e c t i v e d e f l a g r a t i o n vent i s an unobstructed opening tha t has no c losu re . Open vents are an op t ion wherever equipment o r rooms do not need to be t o t a l l y c losed. However, there are compara t i ve ly few s i t u a t i o n s where opera t ions wi th an inheren t d e f l a g r a t i o n hazard can be conducted in open equipment.

9-2.2 Louvered Openings. Openings f i t t e d wi th f i x e d louvers may be considered as open vents . However, the cons t ruc t i on o f the louvers p a r t i a l l y obs t ruc ts the opening, thus reducing the net f r e e vent area. The obs t ruc t i on presented by the louvers decreases the f l o w r a t e o f gases passing through the vent and inc reases the pressure drop across the vent . These f ac to r s must be considered when choosing louvered ven ts .

9-2.3 Hangar- type Doors. Large hangar - type or overhead doors may be i n s t a l l e d in the s ide wa l l s o f "rooms or bu i l d i ngs tha t conta in a d e f l a g r a t l o n hazard. The doors can be opened to p rov ide s i zeab le unobstructed vents dur ing ope ra t i on o f the process o r equipment in which there is an inhe ren t d e f l a g r a t i o n hazard. I t must be recognized t ha t the opening is a vent on ly when the door is not in p lace. S t r i c t superv iso ry and systems con t ro l i s e s s e n t i a l .

9-3 Normal ly Closed Vents f o r Rooms, Bu i l d ings and Other Large Enclosures.

9-3.1 In most cases a c losure must be f i t t e d over the vent opening to p ro tec t aga ins t weather , to conserve heat, to prevent unauthor ized en t r y , to prec lude re lease o f mater ia l o r to prevent contaminat ion.

9-3.2 The vent c losure must be designed to f unc t i on a t as low a pressure as p r a c t i c a l and must be s u i t a b l e f o r the se rv i ce cond i t i ons to which i t w i l l be exposed. The s t a t i c re lease pressure, Pstat, must be i d e n t i f i e d , i d e a l l y by t e s t , and i t must c o r r e l a t e w i th the c a l c u l a t i o n s used to determine the vent area versus the maximum pressure developed dur ing ven t ing , Pred. I f the enc losure w i l l be exposed to temperatures which may a f f e c t the re lease pressure, t h i s must be taken i n to cons ide ra t i on in determin ing Pstat.

9-3.2 .1 The c losure should be permanently marked w i th the re lease pressure.

9-3 .3 The vent c losure must be designed to f u n c t i o n as r a p i d l y as is p r a c t i c a l . Thus, the mass o f the c losu re should be as low as poss ib le to reduce the e f f e c t s o f i n e r t i a . The t o t a l weight o f the movable pa r t o f the c losure assembly should not exceed 2.5 l b / f t z. (The e f f e c t o f i n e r t i a i s i l l u s t r a t e d in Table 9 - 3 . 3 . ) Counterweights should not be used because they add to the i n e r t i a o f the c losure . The c losu re must a lso be designed to wi ths tand na tura l f o rces such as wind or snow loads, opera t i ng cond i t i ons such as i n t e r n a l pressure f l u c t u a t i o n s and i n t e r n a l temperature , and e f f e c t s o f co r ros ion .

65

Table 9-3.3 Maximum Pressure developed by deflagration in enclosures having unrestricted

vents and di f ferent vent closures. (59)

Type of Vent Dust Ratio

sq f t / 100 c u f t

Coal 1.56 Coal 3.52 Aluminum

(Atomized, f ine) 3.52 71 16l

9-3.4 Types of Building or Room Vent Closures. The following types of vent closures are intended for use primarily with re lat ive ly large, re lat ive ly low strength enclosures such as those covered by Chapter 4.

9-3.4.1 Hinged Doors, Windows and Panels. These closures are designed to swing outward and normally have latches or similar hardware that automatically release when influenced by sl ight internal pressure. Friction, spring-loaded, or magnetic latches of the type used for doors on industrial ovens are the usual type of hardware. For personnel safety the door or panel should be designed to remain intact and to stay attached. Materials that tend to fragment, such as glass or mineral/cement boards must not be used. Also, special attention must be given to maintenance of operating mechanisms to ensure proper function.

9-3.4.2 Shear and Pull-through Fasteners. Specially designed fasteners that wi l l fa i l under re la t ive ly low mechanical stress to release a vent closure are commercially available. The shear-type fastener is designed to break from the shear stress that develops in the fastener when the overpressure from a deflagration pushes la te ra l l y on the vent closure. The pull-through type of fastener uses a collapsible or deformable washer to hold the closure panel in place.

Type of Vent or Vent Opening

Unrestricted Light Heavy Opening S w i n g i n g Swinging

Door Door Maximum Pressure, Ib/sq f t

8 1 lOl - - 29 36 55

232

The fo rce o f the d e f l a g r a t i o n on the panel causes the washer to be pu l led through the mounting hole and the panel can then be pushed away from the vent opening. Since these fas teners can be app l i ed to a v a r i e t y o f types and c o n f i g u r a t i o n s o f vent and c losure assembl ies, the response o f a g iven f a s t e n e r to a pressure d i f f e r e n t i a l cannot be p red i c ted f o r any g iven a p p l i c a t i o n based on f a s t e n e r t e s t data a lone. Dynamic t e s t i n g should be ca r r i ed out to e s t a b l i s h the Pstat f o r any g iven f a s t e n e r / v e n t / c l o s u r e combinat ion.

Shear and p u l l - t h r o u g h f as tene rs are s u i t a b l e f o r a p p l i c a t i o n s where the vent design c a l l s f o r very la rge vent areas, such as the entire side wall of a room.

9-3.4.3 Friction-held Closures. Some commercially available vent and closure assemblies use a f lex ib le diaphragm held around i ts edges in a restraining frame. When a deflagration occurs, the pressure deforms the diaphragm, pushing i t from i ts frame. (See Figure 9-3.4(a) and (b).) This type of vent and closure assembly is well suited for large structures such as rooms, buildings, conveyor enclosures, s i los, dust collectors and baghouses, and other large enclosures. I~ is also part icu lar ly suited to ductwork operating at or close to atmospheric pressure.

In locations where personnel or equipment might be damaged by f ly ing diaphragms, tethering of the diaphragm to i ts frame or other safety measures may be necessary.

The material used for the diaphragm should be durable, nonshattering, and should not exceed 2.5 I b / f t 2. The diaphragm should be appropriately dimensioned and attached.

These vent and closure assemblies are capable of being tested by stat ic methods and by simulated deflagrations corresponding to the intended application. I t is recommended that both stat ic and dynamic tests be conducted.

r ~RE

Figure 9 -3 .4 (a ) Exploded view o f manufactured vent c losure .

66

ROOF MOUNT: BUILT- UP ROOF

ROOF- MOUNT: METAL ROOF

~ E N T

WALL-MOUNT:FILTER COLLECTOR HOLDER

DUCI" MOUNT

Figure 9-3.4(b) Typical applications for manufactured vent closures.

9-3.4.4 "Weak" Roof or Wall Construction. A portion of a roof or wall may be deliberately designed to fa i l under sl ight overpressure. In this type of venting, suitable lightweight panels may be located between strong part i t ion walls. In some cases the entire roof area is constructed as a blowout panel. In al l cases the weak wall or roof must be adequately anchored to prevent wind l i f t .

9-4 Restraints for Large Area Pal~els.

9-4.1 When large, lightweight pa~els are used as vent closures (usually over entire wall areas), i t is usually necessary to tether the panels so they do not become missile hazards. The restraining method shown in Figure 9-4.1 i l lus t ra tes one method that is part icular ly suited for conventional single wall metal panels. The key features of the system include a permanent anchor between the panel and the building structural frame using a 2 in. wide, 10 guage bar washer. The length of the bar is equal to the panel width, less 2 in. 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. diameter through-bolts. Shea~ fasteners or collapsible" washers are used at the opposite end of the panel;

9-4.1.1 "Pop" rivets have been used successfully as the fa i lure fastener. During'deflagration tests using this design, the pop rivets fai led within acceptable design l imits to allow rotation of the panel about the plastic hinge formed by the attachment of the panel to the building structural frame.

6-4.1.2 Precautionary Measures for Aluminum Vent Panels. In tests of 21 gauge corrugated aluminum panels, a tendency for the panels to tear out in the v ic in i ty of the through-bolts (see Figure 9-4.1) has been observed. This may be controlled by maintaining at least 3 in. distance between the edge of the panel and the bar washer and by hinging the panels to the lowest building structural member. This l imits the amount of rotation that can occur, thus reducing the chance of tear-out.

9-4.2 When the vent closure panel is a double-wall type (such as insulated sandwich panels) the restraint system described in 9-4.1 is not recommended. The st i f fness of the double-wall panel is much greater than that of a single wall panel. The formation of the plastic hinge wi l l occur more slowly and rotation of the panel may be incomplete. Both factors wi l l tend to delay or impede venting during a deflagration.

The restraint system shown in Figure 9-4.2 is recommended for double-wall panels. For successful functioning, the panel area is limited to 33 f t 2 and i ts mass to 2.5 I b / f t 2.

67

F Vent panel

Building girt

I I Bar washer (tOp,)

1 Girt

L Girt

Girt l

Girt

-H- Lap

Vent panel

I~r washer

/ :+ + +

Elevation showing vent panels and bar wssher assemblies

Figure 9-4.1 Panel restraint system for single wall metal vent panels.

I Vent panel ~ |

Sheetmetal su bgirt (10 GA.) Roof girder

1/4 in. d ia .= / B l i n d rivet thru-bolt ~ L ¢ -

forged eye bolt ~ I . . . . . . bnocK aosoroer-rreedorn to

; = ~ ; ~ ~;/?kin" Thk') m°ve th r °ugh90 ° arc

Bar washer ~ | 1/4 in. d i a . ~ T 4 ~ ' w i r e rope clips ! \ fa,-.fe ?* tether ~ 1/4 in. dia,, 4 ft. long galv.

wire rope tether

~ 1/2 in. die. bolts

...4

Close-up of shock absorber

leg provides add i t i ona l d is tance and time over which the panel is decelerated whi le s imul taneously d i s s i p a t i n g some of the panel 's k i n e t i c energy.

9-5 Equipment Vent Closures.

9-5.1 Hinged Devices. Hinged doors or covers may be designed to funct ion as vent c losures fo r many kinds of equipment. The hinge should be designed to o f f e r minimum f r i c t i o n a l res is tance and to ensure tha t the c losure device remains i n t a c t dur ing vent ing . Closures held shut wi th spr ing, magnetic, or f r i c t i o n latches a r e most f requent ly used fo r t h i s form of p ro tec t i on . Hinged devices can be used on t o t a l l y enclosed mixers, blenders, driers, and similar equipment. I t is d i f f i cu l t to vent equipment of this type i f the shell, drum, or enclosure revolves, turns, or vibrates. Charging doors or inspection ports can be designed to serve this purpose when their action does not endanger personnel. Special attention should be provided to the regular maintenance of hinge and spring loaded mechanisms to ensure proper operation.

9-5.2 Rupture Diaphragm Devices. Rupture diaphragms may be designed in round, square, rectangular, or other shapes to effectively provide vent re l ie f area to f i t the available mounting space. (See Figure 9-5.2.)

q

Figure 9-4.2 Panel r e s t r a i n t system fo r double wal l insu lated metal vent panel.

9-4.2.1 Tests employing fewer than three rope c l i p s have in some instances resul ted in s l ippage of the te ther through the rope c l i p s , thus permi t t i ng the vent panel to become a f ree p r o j e c t i l e .

9-4.2.2 Forged eyebol ts are necessary. A l t e r n a t i v e l y , a 0.5 in. "U" bo l t may be subs t i tu ted fo r the forged eyebol t .

9-4.2.3 A "shock absorber" device wi th a f a i l - s a f e te ther is provided. The shock absorber is a 4 in . wide, 3/16 in. th i ck , L-shaped piece of steel p la te to which the te ther is at tached. During vent ing, the shock absorber w i l l form a p l a s t i c hinge at the juncture in the "L" as the outstanding leg of the "L" ro tates in ~n e f f o r t to fo l l ow the movement of the panel away from the s t ruc tu re . The ro ta t i on of t h i s

Figure 9-5.2 Typlcal rupture diaphragm.

9-5.3 S ta t i c Release Pressure. As in a l l vent c losure designs the s t a t i c vent re lease pressure, Pstat , must be i d e n t i f i e d . Pstat is a funct ion of vent design and mater ia ls of const ruct ion and may vary from l o t to l o t dur ing manufacture. Therefore, a minimum of two samples from each l o t manufactured must be d e s t r u c t i v e l y tes ted. The average of the tes t values is to be considered the s t a t i c vent r e l e a s e pressure.

9-5.4 Ef fects of Temperature. Most mater la ls used fo r rupture diaphragms w i l l be a f fec ted by e levated or reduced operat ing temperatures. I f the opera t ing temperature at the vent c losure device is o ther than ambient, the s t a t i c release pressure should be rated at the co inc ident operat ing temperature. This may be done by performing the two required tes ts of the l o t manufactured at the co inc ident temperature or by using

68

a temperature versus pressure curve established speci f ical ly for the material or materials of the rupture diaphragm which is then applied to the average of the destructive tests performed at the ambient temperature.

9-5.5 Opening Characteristics. Some materials used as rupture diaphragms may balloon, tear away from the mounting frame, or otherwise open randomly, leaving the vent opening par t ia l l y blocked on i n i t i a l rupture. Although such restr ict ions may be momentary, delays of only a few milliseconds i n rel ieving deflagrations of dusts or gases having high rates of pressure rise may cause extensive damage to equipment~ For these reasons only rupture diaphragms with controlled opening patterns which ensure fu l l opening on i n i t i a l rupture should be ut i l ized.

9-5.6 Blowout panels may be held in place by special rubber clamps (see Figure 9-5.6). Pressures developed by a deflagration wi l l push the panel out of the rubber clamp, providing an unrestricted vent opening.

/

V~e n i~~er Exploded view

Figure 9-5.6 Typical blow-out panel.

9-5.6.1 The panel must be secured to avoid missile hazards. Such restraints should be carefully designed. and tested for the type of application.

9-5.6.2 Because the weight of the panel wi l l have a marked effect on i ts performance, any replacement panels must be manufactured from the same material and material thickness as the original design.

9-5.6.3 ,Aging, corrosion, or embrittlement of rubber clamps may cause the Pstat of such vent closure devices to change. Scheduled replacement of the rubber clamp may be necessary to maintain the desired performance.

Chapter 10 Inspection and Maintenance

10-1 General.

10-1.1 This Chapter covers the inspection and maintenance procedures necessary to ensure proper function and operation of devices for venting deflagrations.

lO-l.2 The occupant of the property in which the deflagration vents are located is responsible for inspecting and maintaining such devices,

IO-l.3 Inspection and maintenance should only be performed by persons experienced and knowledgeable in the insta l la t ion and operation of the devices used.

I0-2 Definitions. For the purpose of this Chapter. the following terms have the meanings shown.

Inspection. Verif ication that the venting device is in place and able to function as intended. This is done by ensuring that the device is properly instal led, that i t has not operated or been tampered with, and that there is no condition that might hinder i ts operation.

Maintenance. Repair of any defects noted during inspection and periodic testing, performance of procedures recommended by the manufacturer of the device, or replacement of the device or i ts components.

I0-3 Inspection Frequency and Procedures.

10-3.1 I f required, acceptance inspections and tests should be conducted immediately af ter insta l la t ion to establish that the venting devices have been instal led according to manufacturers' specifications and accepted practices and that al l operating mechanisms wi l l function as intended.

10-3.2 Venting devices should be inspected on a regular basis, the frequency of which wi l l depend on the environment and service conditions to which the devices wil l be exposed. Process or occupancy changes which may introduce signif icant change in condition, such as changes in the severity of corrosive conditions, increase in accumulation of deposits or debris, etc., may require more frequent inspection.

10-3.3 The recommendations of the manufacturer regarding inspection procedures and frequency should be followed.

10-3.4 Inspection procedures and frequency should be in written form and shbuld include provisions for periodic testing, where practical.

10-3.5 To f ac i l i t a te inspection, access to and v i s i b i l i t y of venting devices should not be obstructed.

10-3.6 Any seals or tamper indicators that are found to be broken, any obvious physical damage or corrosion, and any other defects found during inspection should be immediately corrected.

10-3.7 Any structural changes or additions that might interfere with operation of venting devices should be immediately reported.

10-4 Maintenance.

10-4.1 Venting devices should receive appropriate preventive maintenance as recommended by the manufacturer.

10-4.2 Any defects noted during inspection should receive immediate corrective action.

10-5 Record-keeping. A record should be maintained showing the date and results of each inspection and the date and description of each maintenance act iv i ty . The record should be kept at least unti l the completion of the next inspection.

Appendix A Guide l lnes f o r Measuring D e f l a g r a t i o n Ind ices o f Dusts and Gases

A-1 General Comments on Dust Test ing. At the t ime o f the writing of this Guide, work was p~ogressing by standards-setting organizations (such as Committee E27 on Hazard Potential of Chemicals of the American Society for Testing and Materials) towards a standard method for measuring deflagration properties of dusts. This Appendix does not discuss formal proceddres but is a general discussion of test procedures already in use which rely on the same basic principles.

A-t-1 Purpose. The purpose of these measurements is to predict the effect of deflagration of a part icular material (dust or gas) in a large enclosure without carrying out fu l l -scale test work.

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A-2 Basic Principles. The nomographs presented in this Guide and those in VDI 3673 (Reference 49) are based on large-scale tests carried out in vented vessels using a variety of test materials and vessel sizes (References 3, 26). For each test material and vessel volume, the maximum reduced deflagration pressure (Pred) was found for a series of vents with various areas (Av) and opening pressures (Pst~t)- Use of the homographs requires only that a single material c lassi f icat ion (the KG or Kst index) be experimentally obtained by the user. Knowing the volume and mechanical constraints of the enclosure to be protected, the user can then determine the venting requirements from the homographs.

A-2-I The Ko and Kst Indices. The test dusts used during the large-scale test work were classif ied according to the maximum rate of pressure rise that was recorded when each was deflagrated in a 1 m 3 closed test vessel. The maximum rate of pressure rise found in this 1 m 3 vessel was designated "Kst." Kst is not a fundamental material property, but depends on the conditions of the test. The c lassi f icat ion work carried out in the Im 3 vessel provides the only direct l ink between small-scale closed vessel tests and the large-scale vented tests on which the homographs are based.

The Ko index may simi lar ly be dete'rmined in a l m 3 vessel, but published K c values correspond to tests made in smaller vessels. KG is known to be volume-dependent and should not be considered a constant. I ts use is restricted to normalizing (dp/dt)max data gathered under a fixed set of test conditions.

A-2-2 Standardization of a Test Faci l i ty . The objective of standardization is to be able to compare the deflagration behavior of a part icular material with others for which fu l l -sca le test data are available. Without access to the Im 3 vessel in which the original Kst classif icat ions were made, i t is essential to standardize the test conditions employed using samples tested either in the Im 3 vessel or in one standardized against i t . The homographs ident i fy a series of gas mixtures that were used in the fu11-scale tests. In order to cal ibrate for gases, the actual K o values are not c r i t i ca l . This is because one may compare the maximum rate of pressure rise of a part icular gas mixture with that of the gas mixtures ident i f ied in the homographs. I f these (dp/dt)max values are a l l measured under identical conditions, in a vessel meeting certain c r i t e r i a (Section A-3), the nomographsmay be used by interpolation. In order to cal ibrate for dusts, which cannot be ident i f ied by composition alone, i t is necessary to obtain samples having established Kst (Section A-4).

A-2-3 Determination of the Ko and Kst Indices. I f the maximum rate of pressure rise is measured in a vessel of volume other than Im 3, the following relationship is used to normalize the value obtained to a Im ~ vessel.

(dp/dt)max (V) I/3 = K

where p = pressure, (bar) t = time, (s) V = Volume, (m 3) K = Normalized K o Or Kst in index

(bar.m/s)

The measured maximum deflagration pressure Pma~ is not scaled for volume and the experimental value is adequate for design purposes. The maximum rate of pressure rise is normalized to a volume of Im ~ using the above equation. I f the maximum rate of pressure r ise is given in units of bar/sec, and the test volume in units of m 3, the equation defines the Ko or Kst index for the test material.

Example: The volume of a spherical test vessel is 26 l i t e r s (0.026 m 3) and the maximum rate of pressure rise, (dp/dt)max, found from the slope of the

pressure-vs-time curve is 8300 psi/s (572 bar/s.). Substituting these values in the equation above, the normalized index is equal to 572 x (0.026) '13 , or 169 bar-m/s.

A-2-4 Effect of Volume on Ko and Kst. In the case of many i n i t i a l l y quiescent gases, the normalized index KG is found not to be constaht, but increases with vessel volume. Figure A-I shows the variat ion of Ko with vessel volume for methane, propane, and pentane as measured in spherical test vessels (Reference 63). The increase of Ko is related to various flame acceleration effects as described in References 60, 61~ and 62. I t is for this reason that KG values measured in vessels of d i f ferent sizes cannot be direct ly compared, even i f a l l other factors affect ing KG are held constant. Ko measurements should be made in a spherical vessel of at least ten l i t e r s volume and the va lues obta ined should be used only to i n t e r p o l a t e between the ven t ing requirements o f gases i d e n t i f i e d in the nomographs (Sect ion A-3) .

The e f f e c t o f vessel volume alone on Kst va lues obta ined f o r p a r t i c u l a r dusts has not been we l l es tab l i shed . Dusts cannot be suspended in a qu iescent manner and the i n i t i a l tu rbu lence in t roduces a non-sca leab le v a r i a b l e . , However, i t cannot be assumed tha t Kst in the equat ion in A-2-3 is independent o f vessel volume. I t has been found (Reference 26) t ha t Kst va lues obta ined in the o r i g i n a l lm 3 c l a s s i f y i n g vessel cannot be reproduced in spher ica l vesse ls o f less than 16 l i t e r s volume, nor in the c y l i n d r i c a l Hartmann apparatus. A l l existing f ac i l i t i es that have standardized equipment use a spherical test vessel of at least 20 l i t e r s volume or a squat cylinder of larger volume (such as the Im 3 classifying vessel i t s e l f ) . The principle of Kst standardization in such vessels is to adjust test conditions (part icular ly i n i t i a l turbulence) unti l i t can be demonstrated that a series of dusts al l yield Kst values in acceptable agreement with the values that have been established in the Im 3 vessel. I f vessels of volume other than I m 3 are used, the equation in A-2-3 must be used. This may lead to errors which are dependent on Kse. Such errors should be considered when applying test data to vent design (Reference 63).

A-3 Gas Testing. The test vessel used for gas testing should be spherical with a volume of at least ten l i t e r s and preferably 20 l i t e r s or greater. Since the only source of i n i t i a l turbulence is the ignit ion source employed, an important consideration is that the flame front not by unduly distorted by the ignit ion process. The ignit ion source shoduld be centrally-placed and should approximate a point source. A discrete capacitor discharge carrying no great excess of energy above that needed to ignite the mixture is recommended. Fused-wire and chemical igniters may cause multi-point ignit ion and should not be used for routine Ko measurements in small vessels.

Standardization gas mixtures, as ident i f ied in the nomographs, must be i n i t i a l l y tested in the system. Each gas mixture must be ver i f ied to be well-mixed and quiescent immediately pr ior to ignit ion. The maximum rates of pressure rise are measured systematically for several compositions close to the stoichiometric mixture unti l the maximum Ko value has been found. A table of KG values is then established for the standardization gases as measured in t he tes t vessel. These values wi l l not necessarily be the same as the K o values given in the gas,nomographs (see Section A-2-4 ) .

In o rder to subsequent ly app ly the homographs to a t e s t gas, the maximum Ko va lue f o r the t e s t gas must f i r s t be found under i d e n t i c a l cond i t i ons to those used f o r s t a n d a r d i z a t i o n . The t e s t mate r ia l i s compared wi th s t a n d a r d i z a t i o n gases having Ko va lues above a~d below the t e s t va lue as measured in the t e s t vessel and the vent requirements are then found by i n t e r p o l a t i o n between the requirements f o r the s t a n d a r d i z a t i o n gases.

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A data base should be es tab l i shed f o r the t e s t equipment in which Kc values are given f o r a wide v a r i e t y o f gases tes ted under the s tandard ized cond i t i ons . Ko va lues should not be repor ted unless t h i s data base, o r a t a minimum the KG va lues found f o r the s t a n d a r d i z a t i o n gases, are a lso repor ted .

Most combust ib le gas mix tures a t the optimum concent ra t ion may be conven ien t ly i g n i t e d in small vesse ls using a capac i to r spark o f 100 mJ or less and t h i s might be a normal i g n i t i o n source f o r s t anda rd i za t i on . However, the i g n i t i o n requirements f o r ce r t a i n excep t iona l gas mix tures may g r e a t l y exceed t h i s f i g u r e . Before a gas mix ture is des ignated as noncombust ible, i t should be subjected to a s t rong i g n i t i o n source (Sect ion A-S).

Although the nomographs address d e f l a g r a t i o n s o f gases in a i r , i t may be requ i red to p r e d i c t the e f f e c t o f o ther ox idan ts such as c h l o r i n e . I t i s recommended tha t the Kc concept not be extended to such cases except where cons lderab le expe r t i se can be demonstrated by the test fac i l i t y . Many gaseous mixtures wil l be incompatible with the material of the test vessel and trace contaminants within i t , including traces of humidity. Expert opinion should be sought in applying such test data to the protection of large enclosures.

A-3-1 The composition l imits for the coke gas used to develop the gas homographs were:

45-55% Hydrogen 6-10% Carbon Monoxide

25-33% Methane 4.6% Nitrogen 0.1% Carbon Dioxide 2-3% Unspecified Hydrocarbons

There are no available data to indicate whether KG varies signif icantly within these l imits.

A-4 Dust Testing. Dust samples ha.ving the same chemical composition wil l not necessarily display similar Kst values or even similar deflagration pressures (Pmsx). The burning rat(, of a dust depends markedly on the particle size distribution and shape, and on other factors such as surface oxidation (aging) and moisture content. The form in which a dust is tested must bear a direct relation to the form of that dust in the enclosure to be protected. Owing to the physical factors influencing the deflagration properties of dusts, the homographs do not identify the dusts involved in large-scale testing except by their measured Kst values. Although Appendix D of th is Guide gives both Kst and dust identit ies for samples tested in a Im 3 vessel, i t must not. be assumed that other samples of the same dusts wil l yield the same Kst values. These data cannot be used for vessel standardization, but are useful in determining trends. The test vessel to be used for routine work must be standardized using dust samples whose Kst and Pma~ characteristics are known.

A-4-I Obtaining Samples for Standardization. Samples should be obtained having establisl~ed Kst values in Dust Classes St-l , St-2, and St-3. At the time of the writing of this Guide, suitable standard samples wer e not generally available.

A-4-2 Effect of Dust Testing Variables. For a particular spherical test vessel (20 l i te rs or greater) and a particular prepared dust sample, the following factors affect the measured Kst:

o the mass of sample dispersed, or concentration; o the uniformity of the dispersion; o the turbulence at ignition; o the ignition strength.

The concentration is not subject, tostandardization since this must 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. A useful series of concentrations to test are (in g/m:3): 250, 375, 500, 625, 750, 1000. A plot of measured Kst is made

aga ins t concen t ra t ion and t e s t s are cont inued u n t i l the maximum has been found, By t e s t i n g p r o g r e s s i v e l y leaner mix tu res , the mln imumexp los ive concen t ra t i on (lean l imi t or LFL) may similarly be determined. This l imi t may be affected by ignition energy.

A-4-2.1 Obtaining a Uniform Dust Dispersion. The uniformity of dust dispersion is implied by the ab i l i t y to achieve consistent and reproducible Kst values in acceptable agreement with the established valuesfor the Samples tested. Poor dispersion wi l l lead to low values of Kst and Pmax.

A number of dust dispersion methods exist. For small vessels the most common types are the perforated ring and the "whipping hose." The perforated ring (Figure A-3) f i t s around the inside surface of the test vessel and is designed to disperse the dust in m a n y directions. A ring of this type is described in Reference 26 in relation to the dust classifying work in the Im 3 vessel. However, aq inherent problem with this device is a tendency to clog in the presence of waxy materials, low-density materials, and materials that become highly e lectr ica l ly charged during dispersion. To minimize these problems, the whipping hose is recommended. This is a short length of heavy-duty rubber tubing (Figure A-2) which "whips" during dust injection and disperses the dust. These two methods have been compared under otherwise identical conditions (Reference 63) showing that they may not be interchangeable and the dispersion method shoul d be subjectto standardization.

A-4-2.2 Standardizing Turbulence at Ignition. During dust injection, the partially-evacuated test vessel receives a pulse of a i r from the a i r bomb (Figure A-2) which brings the pressure to I atmosphere (absolute) and disperses dust placed below the dispersion system. Some time after the end of injection the igniter is f ired. The following test variables affect turbulence at ignition in the test vessel:

o a i r bomb volume; o a i r bomb pressure; o in i t i a l vessel pressure; o injection time; o ignition delay time.

References 63 and 64 describe combinations for these variables that have yielded satisfactory results. For example, a 26 l i t e r test vessel (Reference 63) employed a l l i t e r a i r bomb at 300 psia. Having established the a l r bomb volume and pressure, the in i t i a l test vessel reduced pressure and injection time are set so that after dust injection the test vessel is at I atmosphere absolute. I t should be noted that the a i r 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 suitable timing circui t . For standardization, reproducibil ity of timing is essential and i t may be found that the optimum ignition delay time is of the order 10 milliseconds. Fast-acting valves and accurate timing devices should be employed.

Standardization using well-characterized samples (Section A-4-I) is complete when samples in Dust Classes St-l, St-2, and St-3 have been shown to yield the expected Kst (to within acceptable error) with no adjustment to the variables l isted in this Section. Also, the mode of ignition (Section A-4-2.3) should not be changed for standardized testing.

A-4-2.3 Ignition Source. The ignition source may affect the Kst values obtained even i f al l other variables are held constant. I t has been found (Reference 26) that in a Im 3 vessel, capacitor discharge sources of between 40 mJ and 16 J gave comparable Kst and Pmax data to those obtained using a 10 kJ chemical igniter. In the same vessel, a permanent spark gap underrated both Kst and Pmax for a range of samples. In References 63 and 65, i t is described how comparable Kst and Pmax values were obtained in vessels of about 20 l i te rs using between I and 6 centrally-placed electr ic match igniters rated at 138 J apiece.

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Various types of e lec t r l ca l l y - in i t ia ted chemical ignit ion source devices have proven satisfactory during routine test work, the most popular being two 138 J electr ic match igniters or two 5 k3 pyrotechnic devices. These ignit ion sources are not interchangeable and standardization must be based on a fixed type of igni ter . The matches have insuff ic ient power to ignite al l combustible dust suspensions. For this reason, any dust appearing to be St-O should be retested using two 5 k3 pyrotechni'c igniters (Section A-5). The routine use of the pyrotechnic igni ter as a standardized source requires a method of correction for i ts inherent pressure effects in small vessels (Reference 63). Neither source is therefore ideal for al l applications.

A-4-3 Dust Preparation for Kst Testing. A dust must be tested in a form which bears a direct r@lation to i ts form in any enclosure to be protected (Section A-4). Only standardization dusts and samples taken from such enclosures are normally tested in the "as-received" state. The following factors affect the Kst:

o s ize d i s t r i b u t i o n ; o p a r t i c l e shape; o contaminants (gas or s o l i d ) .

Although dusts may be produced in a coarse s ta te , a t t r i t i on can generate " f ines." Fines may accumulate in cyclones and baghouses, on surfaces, and in the void space when f i l l i n g large enclosures. For routine testing, i t is assumed that such fines may be represented by a sample screened to sub-2OO mesh (75 l~m). For comprehensive testing, cascade screening into narrow size fractions of constant weight allows Kst to be found for a series of average diameters. Samples taken from the enclosure help in determining representative and "worst case" size fractions to be tested. I f suf f ic ient sample cannot be obtained as sub-200 mesh, i t may be necessary to grind the coarse material. This wi l l introduce a possible error by affecting the shape of the fines produced. The specific surface of a sample, which affects burning ra te , depends on both s ize d i s t r i b u t i o n and p a r t i c l e shape.

When cons ider ing f i nes accumulat ion, the accumulat ion o f add i t i ves must a lso be considered. Many dust handl ing processes may accumulate add i t i ves such as a n t i o x i d a n t s tha t are added as on ly a small f r a c t i o n o f the bulk . Such accumulat ion may a f f e c t Kst and, by reducing the i g n i t i o n energy o f the mix ture , may increase the p r o b a b i l i t y o f a d e f l a g r a t i o n (Reference 63).

Combustible gases may be present in admixtures wi th dusts (hybr id mix tu res) and may accumulate wi th t ime owing to gas desorp t ion from the s o l i d phase. Where this possib i l i ty exists, both Kst and ignit ion energy may be affected. The effect of hybrid mixtures may be synergistic to the deflagration and a gas present at only a fraction of i ts lower flammable l imi t must be considered (Reference 3). Testing of hybrid mixtures may be carried out by injecting the gas-plus-dust mixture into an identical gas mixture already present in the test vessel. The gas concentration (determined on the basis of part ial pressure at the time of ignit ion) should be systematically varied to determine the range of hybrid Ks¢ values that might apply to the practical system.

The use of a whipping hose (Section A-4-2.1) should avoid the necessity of using inert flow-enhancing additives to help dust dispersion in most cases. The use of such additives in testing is not recommended.

A-5 C l a s s i f i c a t i o n as "Noncombust ib le . " A gas or dust mix ture cannot be classed as noncombust ible ( f o r example, Oust Class St-O) unless i t has been subjected repea ted ly to a s t rong chemical i g n i t i o n source o f lO kJ. I f a mater ia l f a i l s to i g n i t e over the range o f concen t ra t ions tested using the standard i g n i t i o n source, then, a f t e r checking the equipment us ing a mate r ia l o f known behav ior , the t e s t sequence is repeated using a 10 k3 chemical i g n i t e r . I t must be es tab l i shed tha t the s t rong i g n i t i o n source w i l l not y i e l d a pressure h i s t o r y in the vessel t ha t may be confused wi th any d e f l a g r a t i o n produced by i t .

An a l t e r n a t i v e to the use o f the s t rong i g n i t i o n source and i t s assoc ia ted pressure e f f e c t s in small vesse ls i s to tes t f i n e r s i ze f r a c t i o n s than the rou t i ne sub-200 mesh. Dust i g n i t i o n energy v a r i e s w i th approx imate ly the cube o f p a r t i c l e d iameter (Reference 63), hence the use o f e l e c t r i c matches may be extended to i d e n t i f i c a t i o n o f St-O dusts . S i m i l a r l y , the dust lean l l m l t concent ra t ion may be sub jec t to i g n i t i o n energy e f f e c t s which decrease wi th decreas ing p a r t i c l e s i ze o f the sample. Such e f f e c t s l a r g e l y d isappear when sub-400 mesh samples are tes ted . In the case o f gases, a s t rong i g n i t i o n source c o n s i s t i n g o f capaci tance discharges in excess o f 10 Joules or fused-w i re sources o f s i m i l a r energy may be used. Such sources are r o u t i n e l y used f o r flammable l i m i t de te rm ina t ion .

A-6 Ins t rumenta t ion Notes. Data may be gathered by analog or d i g i t a l methods, but the ra te at which t h i s i s done must be adequate f o r the purpose. The logg ing equipment should be capable o f r e s o l v i n g a s igna l o f 1 kHz or h igher f requency ( f o r d i g i t a l methods, b e t t e r than 1 data po in t per m i l l i s e c o n d ) . For f a s t - b u r n l n g dusts and gases, p a r t i c u l a r l y in small vesse ls , f a s t e r ra tes o f data logg ing may be requ i red to reso lve (dp/dt)max. Data logg ing systems inc lude o s c i l l o s c o p e s , o s c i l l o g r a p h s , microcomputers and o the r d i g i t a l recorders . An advantage o f d i g i t a l methods is tha t both the system opera t ion and subsequent data reduc t ion can be r e a d i l y automated us ing computer methods (Reference 63). A computer r epo r t from an automated system is shown in F igure A-4. A f u r t h e r advantage o f d i g i t a l methods is tha t expansion o f the t ime ax i s enables a more accurate measurement o f s lope o f the pressure / t ime curve than can be obta ined from an analog osc i l l oscope record. When us ing automated data reduc t ion , i t i s essen t i a l to i nco rpo ra te a p p r o p r i a t e l o g i c to obv ia te the e f f e c t o f spur ious e l e c t r i c a l s i gna l s . Such s igna ls may be reduced by j u d i c i o u s cable placement, grounding, and screen ing, but are d i f f i c u l t to avoid altogether. I t is advantageous to manually confirm automated (dp/dt)ma~ values using the pressure-tlme curve generated.

When making up gas mixtures by the method of part ia l pressures, i t is important to incorporate a suitable gas temperature measuring device, e.g., a thermocouple, to ensure this is done at constant temperature. Gas analysis is preferable where such f a c i l i t i e s exist.

I t has been found that piezoelectric pressure transducers are satisfactory in test systems of this kind owing to calibration s tab i l i t y . The transducer should be flush-mounted to the inside wall of the vessel and coated with sil icone rubber, thus minimizing acoustic and thermal effects.

The entire test system should be routinely maintained and subjected to periodic tests using standard materials of known behaviour. Soon af ter i n i t i a l standardization i t is advisable to prepare large quantities of well-characterized dust samples (Classes St- l , St-2, and St-3) of a type not subject to aging or other effects. When suitably stored, these dusts may be used for periodic system performance tests.

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FIGURE A- t

E F F E C T OF T E S T V O L U M E ON K G

M E A S U R E D IN S P H E R I C A L V E S S E L S

'600

500

400

-" 300 ¢I JD

¢3 200 v

100

LEGEND ( a ) - - 100J SPARK ( b ) - - - 10J SPARK

• BARTKNECHT DATA (c)-10000J PYROTECHNIC • FENWAL DATA ( d ) - - ~ - - 1J SPARK • UNION CARBIDE DATA (e) -250 mJ SPARK o US BUREAU OF MINES (f) (a),(b) or (c)

(R~[-.7839) DATA ,, US BUREAU OF MINESs,,,,,I)")' PENTANE (~'~ROPANE

(R~-7507) DATA / .., ) t (~)~ . I 1 I ~

( a ) t / t " 1 1 I / / (m) ~ . / I ( b )

I"

-- I ( t ) f ~ l l ( f ) _(e __ (d)A .~ . ) . . . . ~ METHANE ~" ~ e ; _ _ . - . ~ - - ~ - - - - - qw(m ) (d) . . . . . . ,w(s ) &(e)

a , ,J .... I . . . . . . . . I , . . . . . . . | . . . . . . . . I . . . . . . . . I . . . . . . . . I I I . . . . . . 0.001 0.01 0.1 1 10 100 1000

V O L U M E (M 3)

FIGURE A - 2

TYPICAL DUST TESTING APPARATUS

ELECTRONIC (~) ~ f R U P T U R E DISC MANOMETER / _ _ ASSEMBLY

VACUUM D<~_ t ~ i ' VENT 'C><] -~ T ST VESSEL

DUST S A M P L e , " ~ - PIEZOELECTRIC L'(~ C-/~ ;r-I'b I~ - - - - -~ \ , , , TRANSDUCER

,

AIR RESE SOLENOID I I I 1

I

~ - - ~ { MICROCOMPUTER 1

I NOTE: TRANSDUCER IS FLUSH-MOUNTED AND COATED WITH I I

OPAQUE SILICONE RUBBER TO PREVENT ACOUSTIC & I PRINTER ] THERMAL EFFECTS.

73

FIGURE

PERFORATED RING

A - 3

DISPERSION SYSTEM

J

74

FIGURE A-4 COMPUTER REPORT

FROM 26 LITER TEST APPARATUS. F I L t NO." 845 TINUVIN .~ |0

LQGGING RATE: 5 . 0 POIHTS/MSEC INJECTION PRESSURE: $00 PSI VACUUM IN SPHERE: 6 . 8 4 P S I .

IGNITIOK DELAY TIME: t0 MSEC SAMPLE INJECTION TIME: 130 MSEC CONCENTRATION: ~00 CMIM!

M&E PRESSURE: I t 7 AT 64 MSEC MAX R&TE: 12694 AT 51 MSEC KST ~ 2 5 6

150

120

P ? O R E S S U R E

60 ( P S I )

30

!

2

i i i i

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i . . . . . . . . . . . . . . . 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . : . . . . . . . . . . . . . . . . . . . . . . "+ . . . . . . . 4

F" i

................. .................. ..................... i .............. i .................................................................... ..................... i ................. .................

.................... i .................. :.-.i ....................... ~ ........................ i .................... i ........................ ! ....................... i .................... '~ .................... ~ ..................

.................. i ............ !i . . . . . . ! - ~ i . ! !

• ! . . . . . . . . . . . . . . .

i i ' ~ i

. ! ! 60 120 180 2 4 0 300

TIME (HSEC)

75

Appendix B Fundamental Burning Ve loc i t i es for Selected Combustible Gases in A i r

The values of fundamental burning v e l o c i t y given in Table 8-1 are based on NACA Report 1300 (66). For the purpose of th is Guide, a reference value of 46 cm/sec fo r the fundamental burning v e l o c i t y of propane has been used. The compi lat ion given in Perry 's (67) is based on the same data (NACA Report 1300) but uses a d i f f e r e n t reference value of 39 cm/sec fo r the fundamental burning v e l o c i t y of propane. The reason fo r using the higher reference value (46 cm/sec) is to gain c loser agreement w l t~more recent ly published data. Gases marked wi th an as te r i sk have been c r i t i c a l l y examined in References 68 or 69 wi th regard to fundamental burning v e l o c i t y . Table B-2 compares the selected values from these references wi th those given in Table B-1. See References (68) and (69) fo r a mope deta i led discussion.

Table B-1 Fundamental Burning Ve loc i t l es of Selected Gases

Gas Fundamental Burnlna Ve loc l t v . cm/sec

Acetone 54 Acetylene 166" Acro le in 66 A c r y l o n l t r i l e 50 Al lene (propadlene) 87 Benzene 48

,n -bu ty l - 37 , t e r t . - b u t y l - 39 ,1 ,2 -d imethy l - 37 , 1 , 2 , 4 - t r l m e t h y l - 39

1,2-Butadiene (methy la l lene) 68 1,3-Butadiene 64

,2 ,3-d imethy l - 52 ,2-methyl- 55

n-Butane 45 ,2 -cyc lopropy l - 47 ,2 ,2-d imethy l - 42 ,2 ,3-d imethy l - 43 ,2-methyl- 43 , 2 , 2 , 3 - t r i m e t h y l - 42

Butanone 42 1-Butene 51

,2 -cyc lopropy l - 50 ,2 ,3-d imethy l - 46 , 2 -e thy l - 46 ,2-methyl - 46 ,3-methyl - 49 2,3-dimethyl-2~butene 44

2-Buten- l -yne (v iny lace ty lene) 89 1-Butyne 68

,3 ,3-d imethy l - 56 2-Butyne 61 Carbon d i su l f i de 58 Carbon monoxide 46 Cyclobutane 67

, e t h y l - 53 , i sopropy l - 46 ,methyl- 52 ,methylene- 61

Cyclohexane 46 ,methyl- 44

Cyclopentadlene 46 Cyclopentane 44

,methyl- 42

Cyclopentene 48 Cyclopropane 56

, c i s - l , 2 - d i m e t h y l - 55 , t r a n s - l , 2 - d i m e t h y l - 55 , e t h y l - 56 ,methyl - $8 , 1 , 1 , 2 - t r i m e t h y l - 52

t rans-Decal in (decahydronaphthalene) 36 n-Decane 43 ]-Decene 44 Diethyl ether 47 Dimethyl ether 54 Ethane 47 Ethene (ethylene) 80* Ethyl acetate 38 Ethylene oxide 108 Ethylenimlne 46 n-Heptane 46 Hexadecane 44 1,5-Hexadiene 52 n-Hexane 46 1-Hexene 50 i-Hexyne 57 3-Hexyne 53 Hydrogen 312" Isopropy] alcohol 41 Isopropylamine 31 Hethane 40*

,d ipheny] - 35 Hethyl alcohol 56 1,2-Pentadiene (e thy la l l ene ) 61 c is - l ,3 -Pentad iene 55 t rans- l ,3 -Pentad iene (p ipery lene) 54

,2 -methy l - (c ls or trans) 46 1,4-Pentadiene 55 2,3-Pentadiene 60 n-Pentane 46

,2 ,2 -d imethy l - 41 ,2 ,3 -d imethy l - 43 ,2 ,4 -d lmethy l - 42 ,2-methyl - 43 ,3-methyl - 43 , 2 , 2 , 4 - t r l m e t h y l - 41

1-Pentene 58 ,2-methyl - 47 ,4-methyl - 48

cis-2-Pentene 51 1-Pentyne 63

,4-methy l - 53 2-Pentyne 61

,4-methyl - 54 Propane 46*

,2 -cyc lopropy l - 50 ,1-deutero- 40 ,1-deutero-2-methy l - 40 ,2-deutero-2-methy l - 40 ,2 ,2 -d imethy l - 39 ,2-methyl - 41

Propene (propylene) 52 ,2-cyclopropyl 53 ,2-methyl - 44

Propionaldehyde 58 Propylene oxide (1,2-epoxypropane) 82 l-Propyne 82 Spiropentane 71 Tetrahydropyran 48 Te t ra l i n ( tetrahydronaphthalene) 39 Toluene (methylbenzene) 41 Gasoline (lO0-octane) 40 Jet f ue l , grade JP-1 (average) 40 3et f ue l , grade OP-4 (average) 41

76

Table B-2 Comparison o f Fundamental Burning Velocities for Selected Gases

Table Gas B-1

Fundamental Burning Velocity, cm/sec Andrews and Bradley (67) France and Pritchard (68) (in air) (in oxygen) (in air)

Acety lene 166 158' 1140 - - Ethy lene 80 79 . . . . Hydrogen 31;! 310 1400 347 Hethane 40 45 450 43 Propane 4(; - - - - 46

Appendix C Deflagration Characteristics of Selected Combustible Gases

As stated in Section 6-2.5 and in Appendix A, the K° value is not constant and wi l l vary depending on test conditions such as type and amount of ignition energy, volume of test vessel, and other test c o n d i t i o n s . Thus a S ing le va lue of K° f o r a particular set of test conditions is but a "snapshot" among a continuum of values which vary over the range of test conditions.

Figure A-I, Appendix A, shows KG values for methane, propane, and pentane over a range of vessel sizes.

Below are listed K° values determined for several gases. The values were determined by tests in a 5 - l i te r Sphere with ignition by an electric spark of approximately 10 3 energy. Where the fuels had sufflcient vapor pressure, the tests were dope at room temperature. Where the fuels did not have suff iciently high vapor pr@ssure, the tests were don e 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 CoL, Basel, Switzerland (private communication).

A Ko value for a combustible gas can be approximated from a known K° value for anothe r combustible gas by the following equation:

(Ko), = (Ko), [(Su), (P.~,), 1 L (Su)l " (Pmax); J

where s u b s c r i p t 1 = gas whose K° i s known

s u b s c r i p t 2 = gas whose K° i s not known

KG = D e f l a g r a t i o n Index f o r gases, bar • m/sec

Su = Fundamental Burn ing V e l o c i t y , cm/sec

Pmax = Maximum Pressure Developed when an optimum m i x t u r e o f gas and a i r , i n i t i a l l y a t a tmospher ic p ressure and tempera tu re , i s burned in a c losed v e s s e l , bars abso lu te .

(See Appendix B f o r va lues o f fundamental burp ing v e l o c i t i e s f o r a number o f gases . )

The va lues f o r Pmax f o r th e two gases can be measured by ac tua l t e s t under c l o s e l y s i m i l a r c o n d i t i o n s , o r they can both be c a l c u l a t e d f o r a d i a b a t i c combustion c o n d i t i o n s . However, one Pmax cannot be c a l c u l a t e d and the o t h e r measured by t e s t . By "optimum m i x t u r e " i s meant a m i x t u r e o f t h a t compos i t ion wh!ch g ives the h ighes t maximum pressure dur ing combust ion. Usua l l y t h i s is not a stolchi0metric mixture but one which is s l ight ly richer in fuel gas than stoichiometric. This equation applies best where the two combustible gases hav e similar values of K°.

Table C-I

FLAHHABLE FLAMHABLE RANGE Pmax bar gage KG GAS L i t . Measured L i t . * Measured bar .m/sec

Methane 5 ~ 15 5 - 14.5 7.2 7.05 64

~hane 3 - 12.5 2.5 - 13 - 7 .8 106

Propane 2.1 - 9 .5 2.0 - 10.0 8 .6 7.9 96

Butane 1.5 - 8 .5 1.75 - 8 .5 8 .6 8 .0 92

Pentane 1.4 - 7 .8 1.5 - 8 .7 7.65 104

E t h y l - benzene 1.0 - 7 ,8 0 .5 - - 6 .6 94

Ace to - phenone 0 .8 - - 6 .9 109

Hydrogen 4.0 - 75.6 5.0 - 72.5 7.4 6 .9 659

" L i t e r a t u r e values are taken from Nabert & Sch~n, S i c h e r h e i t s t e c h n i s c h e Kenzahlen b rennbarer Gase und D~mpfe (Technica l Safe ty C h a r a c t e r i s t i c s o~ Gases and Vapors)

77

Appendix O Deflagration Characteristics of Selected Combustible Dusts

The following tables are based on information obtained from Forschungsberi¢ht Staubexnlosionen: Brenn- und ExolosionsKennarossen von Stauben, published by Hauptverband der gewerblichen Berufsgenossenschaften e.V., Langwartweg 103, 5300 Bonn, l , West Germany, 1980. (Reference 70) For each dust, the tables show the median part ic le size of the material tested as well as the following test results obtained in a Im 3 vessel: minimum explosive concentration, maximum pressure developed by the explosion (Pmax), and the maximum rate of pressure rise (dp/dt)max. Also shown is the Kst value, which is equivalent to (dp/dt)m,, because of the size of the test vessel~ and the Dust Hazard Class as used in the homographs in Chapter 7 of this Guide.

Material

Table D-1

median minimum part ic le explosive

size, concentration pm g/m ~

Agricultural Products

Pmax. bar, ga

Kst Dust (dp/dt)max, bar-m Hazard

bar /sec sec Class

Ce l l u l ose 33 60 9.7

Ce l l u l ose , 42 30 9.9 pulp

Cork 42 30 9.6

Corn 28 60 9.4

Eggwhite 17 125 8.3

Milk, 83 60 5.8 powdered

Milk, 60 - 8.8 nonfat, dry

Soy Flour 20 200 9.2

Starch, 7 - 10.3 corn

Starch, 18 60 9.2 rice

Starch, 22 30 9.9 wheat

Sugar 30 200 8.5

Sugar, 27 60 8.3 milk

Sugar, 29 60 8.2 beet

Tapioca 22 125 9.4

Whey 41 125 9.8

Wood Flour 29 - 10.5

229 229 2

62 62 1

202 202 2

75 75 l

38 38 1

28 28 l

125 125 I

110 110 1

202 202 2

101 101 1

115 115 1

138 138 1

82 82 1

59 59 1

62 62 1

140 140 1

205 205 2

78

Mate r i al

Table D-2 Carbonaceous Dusts

median minimum par t i c le explosive

s i ze , concentration Pmax, Hm g/m 3 bar, ga

Kst Dust (dp/dt)max, bar-m Hazard

bar/sec sec Class

Charcoal , 28 60 ac t i va ted

Charcoal , 14 60 wood

Coal, 2~ 60 bi tuminous

Coke, 15 1 2 5 petro leum

LamPblack <I0 60

Ligni te . 32 60

Peat, 15% HzO 58 60

Peat, 22% HaO 46 125

Soot, p ine <10 -

7.7 44 44 1

9.0 10 10 1

9.2 129 129 1

7.6 47 47 1

8 .4 121 121 1

1 0 . 0 151 151 1

1 0 . 9 157 157 1

8.4 69 69 1

7.9 26 - 26 1

Material

median par t i c le

size, wm

Table D-3 Chemical Dusts

minimum exp los i ve Kst

concentration Pmax, (dp/dt)max, bar--m g/m 3 bar, ga bar/sec sec

Dust Hazard Class

Adipic Acid <10

Anthraquinone <10

Ascorbic Acid 39

Calcium 92 Acetate

" 85

Calcium Stearate 12

Carboxymethyl- 24 cel lulose

Dextrin 41

Lactose 23

Lead Stearate 12

Methylcellulose 75

Paraformalde- hyde 23

Sodium Ascorbate 23

Sodium Stearate 22

Sulfur 20

60

60

500

250

30

60

60

30

60

60

60

30

30

8.0 97 97 1

I0.6 364 364 3

9.0 111 111 I

5.2 9 g 1

6.5 21 21 l

9.1 132 132 1

9.2 136 136 I

8.8 106 106 1

7.7 81 81 l

9.2 152 152 I

9.5 134 . 134 I

9.9 178 178 1

8.4 119 119 1

8.8 123 123 I

6.8 151 151 . I

79

Materlal

median part ic le

size, ~m

Table O-4 Metal Dusts

minimum exp los i ve

concen t ra t ion g/m 3

Pmax, bar, ga

Kst Dust (dp/dt )max, bar-m Hazard

bar/sec sec Class

Aluminum

Bronze

I ron Carbonyl

Magnesium

Zinc

Zinc

29

18

<10

28

10

<10

30

750

125

30

250

125

12.4 415 415 3

4;1 31 31 1

6.1 111 11i 1

17.5 508 508 3

6.7 125 i25 l

7.3 176 176 1

So

/

Material

median particle.

size,

Table D-S Plastics

minimum explosive

concentration g/m 3

Pmax, bar, ga

Kst Dust (dp/dt)m.~, bar-m Hazard

bar/sec sec Class

(po l y )Ac ry la - mlde 10

( p o l y ) A c r y l o n i - t r i l e 25

(po ly)Ethy lene (Low Pressure

Process) <10

Epoxy Resin 26

Melamine Resin 18

Helamine, molded (Wood f l o u r and Mineral F i l l e d Phenol- Formaldehyde) 15

Helamine, molded (Phenol . . . . . . . Cel lu lose) 12

(poly)Methyl 21 Acry la te

(poly) f fe thy l Acry la te , Emulsion Polymer 18

Phenolic Resin <10

(poly)Propylene 25

Terpene-Phenol Resin 10

Urea- Formaldehyde/ Ce l lu lose, Molded 13

(po ly )V iny l Aceta te / Ethylene Copolymer 32

(po ly )V iny l Alcohol 26

(po ly )V iny l Butyral 65

(po ly )V iny l Chlor ide 107

(po ly )V iny l Ch lor ide / Vinyl Acety- lene Emulsion Copolymer 35

(po ly )V iny l Ch lo r ide / Ethy lene/Viny l Acetylene Suspension Copolymer 60

250

30

3O

125

60

6O

3O

30

15

30

1S

60

30

6O

30

2OO

6O

6O

5.9 12 12 1

8 . 5 121 121 1

8.0 156 156 1

7.9 129 129 1

10.2 110 110 1

7.5 41 41 1

10.0 . . . . . 127" 127 1 "

9.4 269 269 2

10.1 202 202 2

9.3 129 129 1

8.4 101 101 1

8.7 143 143 1

10.2 136 136 1

8.6 119

8.9 128

8.9 147

7.6 46

119

128

147

46

8.2 95 95 1

8 . 3 98 98 1

81

Appendix E Referenced Publications

I. Ballal, D.R., and Lefebure, A.H.; "Ignit ion and Flame Quenching of Quiescent Fuel Mists"; Proceedinas Qf the Royal Society, London; Vol 364; 1978; pp 277~294.

2. Jacobson, M., Cooper A.R., and Nagy, J.; Explosibi l i tv of Metal Powders; Report of Investioations 6516; U.S. Bureau of Mines; Pittsburgh; 1964.

3. Bartknecht, W.; Exolosions: Course. Prevention. Protection; Springer-Verlag; New York; 1981.

4. Ibid.; p 51.

5. Ibid.; p 50.

6. Field, P.; Dust Exolosions; 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 Chemle; New York; 1977.

8. Calcote, H.F., Gregory, C.A. Jr., Barrett, C.M., and Gilmer, R.B.; "Spark Ignition, Effect of Molecular Structure"; Industrial and Enaineerina Chemistry; Vol. 44; p 2659; 1952.

9. Jacobson, Cooper, and Nagy; op. c i t .

lO. Nagy, J., Dorsett, H.G. Jr., Cooper, A.R.; Exolo~ibil i tv of Carbonaceous Dusts; Reoort of Investioations 6597; U.S. Bureau of Mines; Pittsburgh; 1965.

11. Dorsett, H.G. Jr. and Nagy, J.; Dust Exolosibi l i tv of Chemicals. Druas. Dyes. and Pesticides; Reoort of Investiaations 7152; U.S. Bureau of Mines; Pittsburgh; 1968.

12. Jacobson, M., Nagy, J., Cooper, A.R. and Ball, F.J.; ExDlosibil itv of Aaricultural Dusts; Report of Investioations 5753; O.S~ Bureau of Mines; Pittsburgh; 1961.

13. 3acobson, M., Nagy, J., and Cooper, A.R.; Exolosibi l i tv of Dusts Used in the Plastics Industry; Report of Investioations 5971; U.S. Bureau of Mines; Pittsburgh; 1962.

14. Eckhoff, R.; "Toward Absolute Minimum Ignition Energies for Dust Clouds?"; Combustion and Flame; Vol. 24; Elsevier Scientif ic Publishing Co.; New York; 1975; pp 53-64.

15. Fenning, R.W.; "Gaseous Combustion at Medium Pressures"; Phil. Trans. Royal Society; London; Serial A, Vol. 225; 1926.

16. Nagy, J., Seiler, E.C., Corm, J.W., and Verakis, H.C.; ~plosion Develooment in Closed Vessels; Reoort Qf Investiqations 7507; U.S. Bureau of Mines; Pittsburgh; 1971.

17. Nagy, J. and Verakis, H.C.; Develooment and Control of Dust Exolosions; Marcel Dekker; New York; 1983; p 55.

18. Hartmann, I. and Nagy, 3.; Effect of Relief Vents on Reduction of Pressures Develooed bY Dust Exolosions; Report of Investioations_ 3924; U.S. Bureau of Mines; Pittsburgh; 1946.

19. Howard, W.B.; "Interpretation of a Building Explosion Accident"; Loss Prevention - Volume 6; American Institute of Chemical Engineers; New York; 1972; pp 68-73.

20. Runes, E; "Explosion Venting"; ib id. ; pp 63-67.

21. Rust, E.A.; "Explosion Venting for Low-Pressure Equipment"; Chemical Enaineerino; McGraw-Hill Co.; New York; Nov. 5, 1979; pp I02-110.

22. Swift, I . ; "Venting Deflagrations - Theory and Practice"; Plant/Ooerations Proaress; Vol. 3, No. 2; American Institute of Chemical Engineers; New York; Apri l , 1984; pp 89-93.

23. Yao, C.; "Explosion Venting of Low-Strength Equipment and Structures"; Loss Prevention-Volume 8; American Institute of Chemical Engineers; New York; 1974; pp 109.

24. Howard, W.B. and Karabinis, A.H.; "Tests of Explosion Venting of Bui ld ings"; Plant/Ooerations Proaress; Vol. 1; American Insti tute of Chemical Engineers; New York; January 1982; pp 51-68.

25. Faber, M.; Symposium on Safetv Aaainst Explosions; Lucerne, Switzerland; 3une 5-7, 1984.-

26. Donat, C.; "Application of Explosion Pressure Relief as a Protective Measure for Industrial Plant Equipment"; Loss Prevention-Volume 11; American Insti tute of Chemical Engineers; New York; 1977.

27. Donat, C.; "Release of the Pressure of an Explosion With Rupture Discs and Explosion Valves"; ACHEMA 73; Frankfurt am Main, Republic of Germany; 1973.

28. Donat, C.; StBub-Reinhaltuna der Luft; Vol. 31, No. 4; Apri l , 1971; pp 154-160.

29. Bartknecht, W. and Kuhner, G.; Forschunasbericht 45; Burdesinstitut fur Arbeitsschutz; 1971.

30. Bartknecht, W.; Exolosions: Course. Prevention. Protection; Sprlnger-Verlag; New York; 1981.

31. Pineau,.J., Gil taire, M., and Dangreaux, J.; "Etude d'exelosions de poussieres en recipients de I, 10 et 100 mJ"; Note No. 1005-83-76, Cahier de Notes Documentaires; No. 83, 2rid Trimestre; 1976.

32. Solberg, D.M., Pappas, J.A. and Skramstad, E. "Observations of Flame Instabi l i t ies in Large Scale Vented Gas Explosions"; Proceedinas of the Eiahteenth ~nternB~ional Svmoosium on Combustion; The Combustion Inst i tute; Pittsburgh; 1981; pp. 1607-1614.

33. Bartknecht, W.; private communication.

34. Bartknecht, W.; op. c i t . ; p 118.

35. Nagy, J. and Verakis, H.C.; op. c i t . ; p. 246.

36. Harris, G.F.P. and Briscoe, P.G. "The Venting of Pentane Vapour - Air Explosions in a Large Vessel"; Combustion and Flame; Vol. 11, No. 4; Butterworths; London; August, 1967; pp 329-338.

37. Bartknecht, W.; op c i r . ; pp 17-18.

38. Lee, J.H.S. and Guirao, C.M.; "Pressure Development in Closed and Vented Vessels"; Plant/Ooerations Proaress; Vol. l , No. 2, American Insti tute of Chemical Engineers; Apri l , 1982; pp 75-85.

39. Bartknecht, W.; op. c i t . ; pp 18-23 and p 124.

40. Ibid; pp 7-26.

41. Ibid; pp. 111.

42. Cousins, E.W., and Cotton, P.E.; "The Protection of Closed Vessels Against Internal Explosions"; American Society of Mechanical Engineers; Paper No. 51-PRI-2; 1951.

43. Halsey, H.R.; "Gaseous and Dust Explosion Venting"; Chemical and Process Engineering; VoI. 46; October, 1965.

44. Chippett, S.; "An Investigation of Vented Explosions at I n i t i a l l y Elevated Pressures for Propane/Air Flames"; Report of Research Project conducted for NFPA Committee on Explosion Protection Systems; February 2, 1984.

82

45. Burgoyne, 3.H. and Cohen, L.; Proceedinas of the Royal Society; Vol. 225; London; 1954; pp 375-392.

46. Browning, 3.A., Tyler, T.L.; and Dra l l , W.G.; "Effect of Par t ic le Size on Combustion of Uniform Suspension"; Industr ial and Engineering Chemistry; Vol. 49; 1957; pp. 142-1zhB.

47. Howard, W.B. and Vincent, G.C.; "Hydrocarbon Mist Explosions - Part I : Prevention b)r Explosion Suppression"; Loss Prevention - Volume 10; American Ins t i t u te of Chemical Engineers; New York; 1976; pp 43-47.

48. Zabetakis, M.; Flammability Characteristics of Combustible Gases and Vaoors; Bulletin 627; U.S. Bureau of Mines; Pittsburgh; 1965; pp 6-7.

49. Pressure Release of Dust Explosions; VDI R ich t l i n ie 3673; Verein Deutscher Zngenieure - Kommission Reinhaltung der Luft, Dusseldorf; i979 and 1983; VDI Verlag GmbH, Dusseldorf.

50. Rasbash, D.3. and Rogowski, Z.W.; "Gaseous Explosions in Vented Ducts"; Combustion and Flame; Vol. 4, No. 4; Butterworth; London; December; 1960; pp 301-312.

51. Rasbash, D.3. and Rogowski; Z.W.; "Relief of Explosions in Propane/Air Mixtures Moving in a Straight Unobstructed Duct"; Second Svmoosium on Chemical ' Process Hazards with Soecial-Reference to Plant Design; Institution of Chemical Engineers; London; 1963.

52. Bartknecht, W.; op. c i t .

53. Palmer, K.N.; "Relief Venting of Dust Explosions in Process Plant"; Svmoosium on Major Loss Prevention in the Process Industries; IChemE !~ymposium Series No. 34; Institution of Chemical Engineers, London; 1971.

54. Palmer, K.N.; Dust Exp1osion~ and Fires; Chapman and Hall; London; 1973.

55. Guide to the Use of Flame Arrestors and Exolosion Reliefs; Ministry of Labour; New Series No. 34; Her Majesty's Stationary Office; London, 1965.

56. Plneau, 3. and Ronchail, G.; "Propagation of Dust Explosions in Ducts"; Proceedings of the Symposium on the Control and Prevention of Dust Explosions~ Basle; 1982.

57. Matsuda, T., Toyonaga, K., Nozima, Y.,'Kobayashi, M. and Shimizu, T.; "Some Observations on Dust Explosibi l i ty in a Pneumatic Transport System"; 3ournal of Powder and Bulk Solids Telchnolo.gJL; Vol. 6, No. 4; 1982; pp 22-28.

• 58. Bjorklund, R.A. and Ryason, P.R.; Detonation Flame Arrestor Devices for Gasoline Caroo Vaoor Recoverv Systems; Publication 80-18; 3et Propulsion Laboratories; Pasadena; 1980.

59. Palmer, K.N.; "Explosion Protection of a Dust Extract ion System"; I ns t i t u t i on of Chemical Engineers; Symposium Series 39; Apr i l , 1974.

60. Swift, I . , "Gaseous Combustion Venting *A Simpl i f ied Approach", The I ns t i t u t i on of Chemical Engineers Symposium Series 82, 4th Internat ional Symposium on Loss Prevention and Safety Promotion in the Process Industr ies, Volume 3 - Chemical Process Hazards (1983).

61. Chippett, S., "Modell lng Vented Explosions", Combustion and Flame; Elsevier Sc ien t i f i c Publlshing Co., Volume 55, No. 1; (1984). .'

• . . - : , , , . ,

62. Solberg, D.M., and Papas, 3.A., "Observation of Flame I n s t a b i l i t l e s in Large Scale Vented Gas. . . . Explosions," 18th Symposium ( I n t . ) on Combustion, (1981).

63. Br i t ton, L.G., and Chippet t , .S. , "Practi:cal Aspects of Dust Deflagration Test ing,".Paper S8d, 17th Annual Loss Prevention Symposium; American I ns t i t u t e of Chemical Engineers; Houston, March 24-28, 1985.

, I I V 64. Swift, I . De elopments in Dust Exp los i ' b i l i t y Testing: The Effect of Test Var iab les," Proceedings of the International Specialists Meeting on Fuel-Air_. Explosions, McGill University, Montreal, November 4-6, (1981) edited by 3.H.S. Lee and C.M. Guirao; University of Waterloo Press (1982).

65. Cocks, R.E., and Meyer, R.C., "Fabrication and Use of a 20 Liter Spherical Dust Testing Apparatus," Loss Prevention. Volume 14; American Inst i tute of Chemical Engineers (1981).

66. NACA Report 1300; National Advisory Committee'on Aeronautics; 1959, Tables 31-32.

67. Perry, R.H. and Chilton, C.H. (Eds.); Chemical Enaineers' Handbook. 5th Edition; McGraw-Hill, New York; 1973.

68. Andrews, G.E. and Bradley, D.; "Determination of Burning Velocities: A Crit ical Review"; Combustion and Flame; Vol. 18; Elsevier Scientif ic Publishing Co.; New York; 1972; pp 133-153.

69. France, D.H. and Prltchard, R.; "Burning Velocity Measurements of Multicomponent Fuel Gas Mixtures"; Gas Warme International; Vol. 26, No. 12; 1977.

70. Forschungsbericht Staubexplosionen: Brenn- und Explosionskenngrossen yon Stauben; Hauptverband der gewerblichen Borufsgenossenschaften e.V.; Bonn; 1980.

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