1424 committee on explosion protection systems …su~cient expertise on the subject, and one member...

1424 C O M M I T T E E ON E X P L O S I O N P R O T E C T I O N SYSTEMS EPs~I

Report of Committee on Explosion Protection Systems

F. W. Wischmeyer , Chairman, Eastman Kodak Co., Kodak Park Div., Rochester, N.Y. 14650

A. R. Albrecht , Manufacturing Chemists Association

C. F. Averlll , National Automatic Sprinkler & Fire Control Assn.

W. L. Bulkley , American Petroleum In- stituto

R. G. F i t z t e r a ld , Factory Insurance As- sociation

G. J . Grabowski , Fire Equipment Manufac- turers Assn.

N. J . Howard, American Insurance Asso- ciation

W . B . Howard, Monsanto Co. P. Jenmen, A. O. Smith Harvestore Prodcuk,

Inc,

J . Na~v, U.S. Bureau of Mines B. Peaetaky, Union Carbide Corp. A. Santoe, Factory Mutual Research Co~. P. J . Schram, Underwriters' LabomtorieQ

] h e . m

R. F. Schwab, Allied Chemical Corp.

R. B. Z ie~e r . Hartford Insurance Group

Alternates.

J. E. Bates, Fire Equipment Manufacturers M, D. Peterson, Underwriters' Labomtotles Assn. (Alternate to G. J. Grabowski) Inc. (Alternate to P. J. Sehram) '

Y. W. Corm, U, S. Bureau of Mines (Alter- D . B . Tucker , Factory Insurance Ass~ nsto to J. Nagy) (Alternate to R. G. Fitzgerald)

J, F. McKenna , American Petroleum In- stituto (Alternate to W. L. Bulkley)

This list represents the mernberslttp at the tlme the Committee ~ balloted on the text of this tdltia~ Since that time, changes in the m~mbtrship may hate occurred.

This report has been submitted to letter ballot of the Committee which consists of 75 voting members. 13 of whom have voted afftrma- tively, one member did not vote 6ecause he felt he did not have su~cient expertise on the subject, and one member did not return his ballot.

The Committee on Explosion Protection Systems presents for adoption, as a Temporary Guide, amendments to the Guide for Explosion Venting, NFPA No. 68, 1954, which is published in the 1973-74 National Fire Codes in separate form.

Eps-2 AMENDMENTS TO NFPA 68 1425

Amendments to Guide for

Explosion Venting

NFPA 68 n 1 9 5 4

1. Revise Page 68-3, Object and Scope of This Guide, to read:

The object of this Guide is to provide useful information for the design and utilization of vents to limit pressures developed by explosions (deflagrations) of dusts, or gases (vapors) in buildings, rooms, bins and equipment in order to prevent or reduce structural or mechanical damage.

Vents are not generally intended to serve as a means of pre- venting explosions, although in some cases, they may be used to retard the development of an explosion, or prevent propagation from one section of a plant to another.

This Guide is not intended to be used for calculating venting or emergency relief for exothermic runaway reactions.

For the purposes of this Guide, a vapor is considered a gas. Also, the term "explosion" shall mean the bursting of a building or container as a result of internal pressure developed by a "deflagration," beyond the confinement capacity of the building or container.

2. Revise Pages 68-5 and 68-6, Definitions, to read:

Definitions Deflagration

Burning which takes place at a flame speed below the velocity of sound in the unburned medium.

Detonation Burning which takes place at a flame speed above the velocity

of sound in the unburned medium.

Dust In this Guide dust or powder refers to any finely divided solid

material whether product or waste.

Explosion The bursting of a building or container as a result of develop-

ment of internal pressure beyond the confinement capacity of the building or container.

Explosive L i m i t s (Flammable Limits) In the case of most flammable or combustible liquids, gases

1426 COMMITTEE ON EXPLOSION PROTECTION SYSTEMS

and dusts, there is a minimum concentration of vapor, gas or dust in air or oxygen below which propagation of flame does not occur on contact with a source of ignition. There is also a maxi. mum concentration of vapor, gas or dust in air above which propagation of flame does not occur. These limit mixtures of vapor, gas or dust with air, which if ignited will just propagate flame, are known as "lower and upper explosive or flammable limits." In the case of vapors and gases limits are usually expressed in terms of percentage by volume of gas or vapor in air. In the case of dusts, limits are usually expressed in terms of ounces of dust per cubic foot of volume. (The upper limit for a dust usually cannot be well defined.)

Explosive Range The concentration lying between lower and upper explosive or

flammable limits, expressed in terms of percentage of vapor or gas in air by volume, comprises the "explosive range," also often referred to as the "flammable range."

Fire Point The fire point is the lowest temperature of liquid in an open

container at which vapors are evolved fast enough to support continuous combustion. (Methods for determining fire point are the Tag Open Cup ASTM-D1310 and, for high temperature, the Cleveland Open Cup ASTM-D92.)

Flash Point The flash point of the liquid or solid is the minimum tempera-

ture at which it gives off sufficient vapor to form an ignitible mixture with air near the surface of the liquid or within the vessel used. By "ignitible mixture" is meant a mixture within the explosive range (between upper and lower limits) that is capable of the propagation of flame away from the source of ignition when ignited. (By "propagation of flame" is here meant the spread of flame from layer to layer independently of the source of ignition.) This is not to be confused with "Fire Point." (Methods for determining flash point are the Tag Closed Cup ASTM-D56, and for viscous liquids, the Pensky-Martin ASTM-D93.)

Ignition Temperature The ignition temperature of a substance, whether solid,

liquid or gaseous, is the minimum temperature required to initiate or cause self-sustained combustion independently of the heating or heated element. The ignition temperature of a solid is influenced by its physical condition and the rate of heating. Figures on ignition temperatures may vary, depending upon the test method, since the ignition temperature varies with

EFS-4 AMENDMENTS TO NFPA. 68

1427

the size, shape and material of the testing container and other factors.

Open Vent Pressure Open vent pressure may be defined as the pressure developed

by an explosion in a container having an unobstructed vent.

o p t i m u m Mixture Optimum mixture is one in which the combustible material and

air are in the proper proportion to give the most violent explosion. Generally this occurs at approximately the stoichiometric proportions.

Rate of Pressure Rise Rate of pressure rise during a particular interval of an ex-

plosion may be expressed as the ratio of the increase in explosion pressure to the time interval (AP/sec.) required to attain that increase of pressure. The "average rate of pressure rise" is the ratio of the maximum pressure to the time interval from the initiation of the explosion until the maximum pressure is reached, and the "maximum rate of pressure rise " (APmax/sec.) is computed from the slope of the steepest portion of the pressure- time curve during the development of the explosion. The rate of pressure rise depends somewhat on the volume of the container.

Vapor As specified herein is a gas.

Vent Ratio Vent ratio is the relationship of the area of the rupture dia-

phragms or explosion panels to the volume of the equipment or room subject to internal explosions. Vent ratio may he expressed in terms of "square feet per 100 cubic feet" or as the reciprocal of the cubic feet of vented volume per square foot of vent.

1428 COMMITTEE ON EXPLOSION PROTECTION SYSTEMS E P S ~ 5

3. Revise Page 68-22, Vent Ratio Recommendations, to read:

CHAPTER $. VENT AREA RECOMMENDATIONS

A. Exothermic Runaway Reactions For calculating venting of process equipment for runaway

exothermic reactions, refer to J. E. Huff, Loss Prevention, Vol. 7, CEP Technical Manual published by American Institute of Chemical Engineers, 1973, pp. 45-47. (Reference 37 - - Ap- pendix I).

Although this paper was written from the standpoint of polymerization reactors, they apply equally to venting for mono. meric exothermie reactions, which usually involve liquids of lower viscosity than do many polymerization reactions.

B. Venting of Combustion of Gas Mixtures and of Gas. Dust Mixtures

1. General Comments At the time of drafting this Guide, the state of technology for

combustion venting is still only partially developed. Com- bustion venting involves many variables, only some of which have been investigated. Even the investigation of any particular variable has been limited in scope. The investigations which have been done do allow certain generalizations to be made. From these have been developed the recommended calculation bases given below. The calculation bases must be recognized as approximate only. An attempt was made to evaluate the data and correlations in recently published papers and to make use of those which were considered reliable. (See Appendix I, Refer- ences.)

Many variables affect the maximum pressure which can develop during combustion venting. This pressure can be sig- nificantly higher than the pressure at which the venting device releases. Among the variables which affect the pressure which will develop during combustion venting are the following:

a. size of vent relative to size of space to be vented; b. vent release pressure; e. vent inertia; d. effective length, cross section, and geometry (e.g., bends)

of pipe or duct, if any, both'upstream and downstream of vent;

e. location of vent on container to be vented;, f. initial pressure in the container before ignition occurs; g. initial temperature; h. initial composition of gas mixture or of gas-dust mixture,

and degree of uniformity of composition throughout the space;

AMENDMENTS TO NFPA 08

1429

i. initial turbulence, and degree of uniformity of that turbulence;

j. size of containing space; k. shape of containing space; 1. identity of fuel; (chemical and physical properties) m. identity of oxidant; (chemical and physical properties) n. number of ignition sources;" o. location of ignition source or sources.

2. Ven t ing of Gas Combustion Inside Buildings Buildings are usually not designed to withstand high pressures

from within. Buildings will often be blown apart by sustained internal pressures of 1 psi (7 k N / m 2) or even less. Combustion venting of buildings therefore normally implies the need to vent in a manner such that the maximum~pressure development is low. This maximum pressure must be lower, by a safety margin, than that pressure which the weakest building member, desired not to break or vent, can withstand. This weakest building member may be not only a wall but also the roof or, if the building be elevated, the floor.

Much consideration needs to be given to building venting panels. Above all they need to have the lowest feasible inertia and no counterweights ;,:counterweights add to inertia. Vent panel release pressure needs to be as low as feasible relative to anticipated wind forces. The release pressure can usually be in the range of 20 to 30 lbf/ft ~ (1 to 1.5 kN/m~). The vent panels may be restrained at one end by a simple hinge or chain or cable. The purpose of this is to prevent the vent panels from flying away from the building when they do open to vent combustion.

Special consideration should be given to the material of con- struction of the vent panel. I t should not be of a material that tends to break into pointed shards; specifically it should not be of glass or the cement-asbestos type board. A material which meets most of the desired criteria fairly well is thin gage cor- rugated paneling made of fiberglass-reinforced plastic with a flame spread rating no higher than 25 according to NFPA 255, Test of Surface Burning Characteristics of Building Materials.

A precautionary note is needed. I t may be necessary to install indoor railings along floor edges near venting panels in order to prevent people from inadvertently knocking the panels open and falling out. Consideration also needs to be given to the space outside the building, into which the vent panels will open.

A number of factors complicate combustion inside buildings as discussed in Appendix 1, in References 36, 43, 51, 55. These references also present two different equations for building venting and discuss limitations of the equations. The equations are presented here for convenience.

1430 COMMITTEE ON EXPLOSION PROTECTION SYSTEMS ~ - 1 } ~ 7

References 36 and 55 discuss the "Runes" equation, which is as follows:

kLiL~ A , - ~/~_ (1)

Av = necessary building vent area, ft. 2 or m s k - constant characteristic of the fuel gas, as discussed

below. NOTE: k is dependent upon the units (English or SI) used.

L, = smallest dimension of therectangular building enclosure to be vented, ft. or m.

L2 = second smallest dimension of the rectangular building enclosure to be vented, ft. or m.

P = maximum internal building pressure which can be withstood by the weakest building member which is desired not to vent or break, lbf/in 2 or k N / m 2

For most gases such as natural gas, propane, gasoline, benzene, acetone, and many others the values of maximum fundamental burning velocity are nearly the same. For such gases the value of k in Equation 1 is approximately 2.6 for use of the English units, 6.8 for use with SI units. Note in this connection that the actual flame speed in a building will be many times the fundamental burning velocity~ The value of k, in fact, allows for the actual flame speed under conditions of flame turbulence increase due to (1) physical obstructions, such as process equipment, piping, building structural members, etc., ( 2 ) f low induced turbulence due to venting, and (3) turbulence induced directly by a large sized flame. For ethylene the suggested value for k is 4 in English units, 10.5 for SI units; for hydrogen, 6.4 in English units, 17 for SI units. The last two values suggested for k have not been tested in actual building venting incidents. The first value suggested for k has been tested in only one building venting incident. Turbulence conditions, and hence flame speeds, can differ as turbulence producing .conditions vary.

It is believed that the venting equation is suitable for combustion venting of spaces in buildings having a nominal length/ width (i.e., L/D) ration up to 3. For a space having an L/D greater than 3 the space should be subdivided into multiple units, each having an L /D of no more than 3. Each unit should then have its own venting area. In a rectangular building, when L1 and L2 are not equal, the effective value of D is V T I L r

Wherever it is possible, combustion vents for a building should be distributed over the walls of that building rather than confined to one wall.

EPS-8 AMENDMENTS TO NFPA 68

1431

References 43 and 51 discuss the Rasbash equation, which is as follows:

Pm --- 1.5Pv q- 0.5K (English units) Pm ~ ll.5Pv q- 3.5K (metric units) (2) where Pm --- maximum pressure reached during venting of the com-

bustion, lbf/in3, or k N / m ~ Pv = pressure within the building space at which the vent

opens, lbf/in3, or kN/m ~ K = venting ratio, A~/A, (dimensionless) Ao = smallest cross-sectional area of the building space Av -~ total area of combustion vents

Reference 51 states the following limitations to Equation 2: a. Maximum and minimum dimensions of the vented space

have a ratio less than 3. b. the value of K is between 1 and 5. c. The inertia of the building vent does not exceed,5 lb / f t 2

(25 kg/m2). d. The pressure needed to move the vent does not exceed

1 lbf/in. 2 (7 k N / m ~) and the pressure is virtually ex- hausted after the vent cover has traveled a few milli- meters. I t was stated that a door held by small springs or latches can serve this function.

Reference 51 also states that when the explosive gases either are turbulent when ignited, or become turbulent durin~ the course of the combustion, pressures considerably in excess of those calculated by Equation 2 may be obtained. Reference 36 points out that considerable turbulefice can develop as a ~result of physical obstructions present within the space to be vented. Examples of physical obstructions are procesgvessels of all kinds, piping, structural members of buildings, and partial floors.

Equation 2 is stated to apply specifically to propane. Reference 43 states that the pressure reached during venting of the combustion, in the case of natural gas and air, will be about 0.8 times that calculated by Equation 2. For town gas (about 60% hydrogen) the pressure reached will be about 2.5 times that calculated by Equation 2.

In planning combustion venting for buildings use the larger of the two venting areas calculated by Equations 1 and 2.

Two important points relative to spill situations should be noted about the venting of combustion inside buildings: (1) Build- ing ventilation rates even as high as one-air-change-per-minute will not-necessarily prevent formation of flammable mixtures inside the building. (2) Concentrations of gas from spills inside buildings can vary greatly throughout the building space. Gas at a concentration above the upper flammable limit can burn be-

1432 COMMITTEE ON EXPLOSION PROTECTION SYSTEMS EPS~. ~

cause the thermal drafts caused by the initial flame will promote further mixing of air into the unburned fuel gas. Furthermore, the initial flame raises the temperature of the unburned mixture, thereby increasing the flammability limits.

Location of flammables handling equipment outdoors obviates many of the problems of gas accumulation in buildings and of necessary building venting. See NFPA 30, Flammable and Com. bustible Liquids Code.

3. Venting of Dust Combustion Inside Vessels

The most comprehensive known data on dust combustion venting are given in Reference 18. These data resulted from a very extensive test program. This program involved combustion venting tests with four dusts in containers of four sizes, 1, 10, 30, and 60 cubic meters. The author graphed the data, and from the graphs, prepared a table of necessary venting areas as a function of the class of dust, the vessel volume and the relieving pressure of the combustion vent.

The tables shown here as Tables la and lb give the necessary venting area to prevent pressures during venting from exceeding a specific value.

Combustible dusts were divided into three classes according to maximum rates of pressure rise obtained in the Hartmann bomb test apparatus of the Bureau of Mines, U. S. Dept. of Interior. Those dusts giving maximum rates of pressure rise up to 7300 psi/sec., or 50,000 kN/ (m s) (sec.), were designated Dust Class St-1. Those giving rates of 7300-22,000 psi/sec., or 50,000- 150,000 kN/ (m ~) (sec.), were designated Dust Class St-2. Those above 22,000 psi/sec., or 150,000 kN/ (m ~) (sec.), Dust Class St-3.

Abbreviations in Tables la and lb are as follows: Pmax = maximum pressure reached during combustion

venting Pv = pressure at which combustion vent opens V = vessel volume

The large boxes of numbers in Tables la and lb are the required combustion vent areas.

The tests were made from initial pressures of essentially atmospheric. On this basis the table, strictly interpreted, applies only to venting of dust combustion in equipment which is at essentially atmospheric pressure at the time of combustion initiation. Most dust handling equipment operates at pressures near atmospheric. No specific correlations have been developed and extensively tested for converting the numbers in Tables la and lb to the case of initial pressure substantially different from

Eps-10 AMENDMENTS TO NFPA 68

1433

T A B L E l a

REQUIRED VENT AREA AS A FUNCTION OF DUST CLASS, R E L I E F pRESSURE (Pv~ CONTAINER VOLUME AND M A X I M U M pRES- sURE DEVELO ED DURING VENTING (Pmax)

(Numbers are in English units)

Class S t . - - 1 Dusts giving maximum rates of pressure rise up to 7,300psi/sec.

- ~ p (max) Dust Class St. - - 1 lbf/ in.* V (ft 8) 2.8 7.1 9.9 14.2 21.3 28.4 35.5 42.6

Jl

35 1.3 0.65 0.52 0.43 0.32 0.29 0.25 0.22 177 4.3 1.9 1.6 1.4 1.01 0.81 0.70 0.65 353 7.5 3.2 2.6 2.1 1.7 1.4 1.2 0.97 530 10.8 4.5 3.5 2.8 2.3 2.0 1.8 1.6 7 0 6 14.0 5.7 4.4 3.6 2.9 2.6 2.3 2.0 883 17.2 6.9 5.4 4.3 3.4 3.0 2.7 2.5

1 0 5 9 19.9 8.1 6.2 5.1 4.2 3.6 3.2 2.9 1412 25.9 10.2 7.6 6.4 5.0 4.5 4.0 3.7 1 7 6 6 31.2 12.3 9.2 7.5 6.0 5.4 4.7 4.3 3532 - - 20.5 15.2 12.4 9.7 8.6 7.5 7.0

II

35 3.9 0.80 0.58 0.48 0.37 0.30 0.26 0.23 177 11.3 2.4 1.7 1.6 1.3 0.90 0.75 0.56 353 17.8 4.1 2.9 2.5 1.9 1.6 1.2 1.1 5 3 0 23.2 5.7 4.0 3.4 2.7 2.3 1.9 1.7 7 0 6 28.0 7.8 5.2 4.2 3.2 2.8 2.6 2.4 885 32.3 8.1 6.2 5.2 3.4 3.2 3.0 2.8

1059 38.6 9.9 7.8 6.0 4.7 3.8 3.5 3.3 1412 43.1 12.5 9.2 7.5 5.6 4.8 4.3 3.9 1 7 6 6 - - 15.1 11.3 8.8 6.7 5.9 5.4 4.8 3532 - - 25.9 19.4 15.1 11.8 9.7 8.6 7.5

II

• 35 - - 3.9 1.1 0.75 0.46 0.35 0.29 0.25 177 - - 10.8 3.4 2.3 1.4 1.0 0.86 0.73 353 - - 17.8 6.5 4.3 3.0 2.2 1.9 1.7 5 3 0 u 23.7 9.1 6.2 4.2 3.1 2.4 2.3 7 0 6 - - 28.5 12.1 8.2 5.4 3.9 3.2 3.0 883 - - 33.4 14.9 10.1 6.4 4.6 4.1 3.5

1059 - - 36.6 17.2 11.8 7.0 5.4 4.7 4.2 1412 - - 43.1 21.5 15.1 9.1 6.9 6.0 5.4 1 7 6 6 - - - - 25.8 18.3 10.8 8.4 7.3 5.8 3 5 3 2 - - - - 38.8 29.1 16.1 12.4 10.8 8.6

Vent Area, Sq. Ft.

1434 COMMITTEE ON EXPLOSION PROTECTION SYSTEMS SeS-1]

T a b l e l a ~ C o n t i n u e d

Class St. ~ 2 D u s t s g iv ing ra tes of 7,300 - - 22,000 p s i / s ee .

P (max) D u s t Class St - - 2 l b f / i n 2 V (ft a) 2 .8 7 .1 9 .9 14.2 21 .3 28 .4 35.5 4 2 . 6 49 .7 56.8

O~

II

35 2.9 1.9 1,6 1.3 177 8.9 5.4 4.8 3.8 2.9 2.4 1.9 1.5 1.3 353 15.3 10.3 8.9 6.8 5.6 4.8 3.8 3.2 2.8 530 20.7 15.1 12.9 10.8 8.1 6.9 5.7 4.8 3.4 706 25.8 19.4 17.2 13.7 10.2 8.6 7.5 5.4 5.7 5.1 883 31.2 23.7 21.0 17.0 12.9 10.8 9.1 8.1 7.0 6.2

1059 36.1 28.0 24.8 1 9 . 9 15.1 12.9 10.8 9.5 8.1 7.3 1412 43.0 36.1 31.8 26.4 18.8 16.2 13,5 12.2 10.8 9.5 1766 ~ 43.1 38.2 32.3 23.7 19.4 16.2 14.3 12.9 11.3 3532 - - 37,7 30.2 25.9 23.7 21.5 18.3

0.97 0.86 0.65 0.54 0.48 0.40

1.1 2,5 3.8

¢q

II

35 4.7 2.4 1.7 1.4 1.1 0.86 0.69 0.58 0.50 0.43 177 14.0 6.5 5.1 4.2 3.1 2.8 2.0 1.9 1.5 1.3 353 21.5 11.3 9.3 7.8 5.9 5.4 4.2 3.7 3.2 2.7 530 28.0 16.1 14.0 11.8 9.5 7.5 6.2 5.4 4.8 4.1 706 33.9 20.5 18.3 14.5 11.3 9.7 8.1 6.8 6.2 5.6 883 39.3 24.7 22.6 18.3 13.8 11.8 9.7 8.6 7.9 6.8

1059 ~ 29.1 25.9 21.1 16.1 14.0 11.6 10.2 9.1 8.1 1412 ~ 37.7 33.4 28.0 20.5 17.2 14.6 12.9 11.8 10.2 1766 39.8 34,5 24.8 20.5 17.2 15.1 14.0 12.4 3532 - - 43.1 34.5 28.0 24.8 22,6 19.4

OQ

II

35 - - 4.7 3.0 2.2 1.3 0.97 0.75 0.62 0.54 0.47 177 ~ 12.9 8.6 6.5 4.3 3.3 2.7 2.3 1.9 1.8 353 - - 21.5 15.1 11.3 7.9 6.2 5.0 4.3 4,0 3.3 530 ~ 28.0 20.5 15.6 11.3 9.0 7.3 6.4 5.7 4.8 706 - - 33.8 25.4 20.2 14.5 11.8 9.7 8,6 7,5 5.4 883 - - 38.4 29.9 24.3 17.5 14.4 11.8 10.6 9.1 8.0

1059 ~ 42.0 34.1 28.2 20.5 16.9 14.0 12.5 10,8 9.5 1412 40.9 35.5 25.9 21.5 17.8 15.9 13.8 12.1 1766 - - 42.0 30.1 25.9 21.0 19.0 16.7 14.7 3532 - - - - 43.1 38.9 32.3 30.1 24.8 21.5

V e n t Area, Sq. Ft.

EPS-12 AMENDMENTS TO NFPA 68

1435

Table l a --- Cont inued

Class St. - - 3 Dus t s giving ra tes above 22,000 psi /see.

p (max) D u s t Class St. - - 3 1 b f / i n ~ V (ft +) 2.8 7.1 9.9 14.2 21.3 28.4 35.5 42.6

0

+:

3 5 3.0 1.7 1.3 1.0 0.86 0.75 0.62 0.56 173 8.1 5.9 4.3 3.6 3.0 2.7 2.3 1.9 3 5 3 12.9 9.7 8.4 7.0 5.9 5.2 4.8 4.3 5 3 0 16.8 13.5 11.8 10.8 9.1 8.2 7.5 6.9 7 0 6 20.5 17.2 15.4 14.0 12.3 11.3 10.8 9.7 883 24.1 20.5 18.3 17.2 15.6 14.5 13.5 12.4

1059 , 26.9 23.7 21.7 20.5 18.3 17.7 16.2 14.5 1412 33.4 29.7 28.0 25.9 23.7 22.6 , 21.5 19.4 1766 38.8 35.5- 33.4 30.1 28.0 26.9 25.9 22.6 3 5 3 2 . . . . . . . .

0 m

II +:

35 6.2 2.7 1.7 1~3 1.0 0.86 0.75 0.62 177 16.1 8.1 5.4 4.3 3.8 3.2 2.7 2.5 353 22.4 12.9 9.1 7.5 7.0 6.2 5.9 4.3 5 3 0 26.9 16.1 12.9 11.3 10.2 9.1 8.6 7.5 7 0 6 30.1 19.9 16.7 14.5 13.4 12.4 11.3 10.2 8 8 3 32.8 23.1 19.9 17.8 16.1 15.4 14.5 12.9

1059 35.0 25.8 22.6 20.7 19.4 18.3 16.7 15.1 1412 38.8 31.2 28.0 26.4 24.8 23.3 21.5 19.9 1766 43.1 36.6 33.4 32.3 30.1 28.0 26.9 24.3 3 5 3 2 . . . . . . . .

tl

35 - - - - 3.8 2.4 1.7 1.4 1.1 0.75 177 - - - - 10.8 7.0 5.4 4.3 3.9 3.2 355 - - - - 15.1 11.3 9.1 7.5 7.0 5.9 550 - - - - 17.8 14.5 12.4" 10.8 9.9 9.1 7 0 6 - - 1 19.9 17.2 15.9 14.0 12.9 11.8 883 - - - - 2L1 19.9 18.8 16.7 15.6 14.5

1059 - - - - 23.7 22.6 21.1 19.4 18.3 17.2 1412 - - - - 29.1 27.6 25.8 24.8 23.7 21.1 1766 - - - - 34.5 32.3 31.2 30.1 29.1 26.9

3 5 3 2 . . . . . . . .

Vent Area, Sq. Ft.

1436, COMMITTEE ON EXPLOSION PROTECTION SYSTEMS

atmospheric. The original tests on which Tables. la a n d 1bare based were made with dusts in mroutent conu|tmn. 1he aato thus apply to the usual industrial equipment handling dusts.

For large vessels such as vertical cylindrical storage hoppe~ or silos for combustible dusts, the:_~equired venting area may be ~ large as, or larger than, the cross section of the vessel. In th~ case the entire vessel roof can be made a venting area by con. strueting it as a weak seam roof as described in American Petro- leum Institute Standards 650 and 2000. Space must .be available above the roof for it is to open sufficiently. Usually such a roof opens only part way around its periphery to vent a dust coin. bustion. Obviously the roof thickness should b e a s small as possible consistent with the strength demands upon it.

If the required vent area be larger than the vessel cross section, the vessel needs to be further strengthened to take a pressure consistent with the vent area that can be provided. In all cases the total volume of the vessel, should be assumed to. contain the combustible dust in suspension. That is, no credit should be taken for the vessel being partly full of settled material:

The use of vent ducts following explosion vents can lead to substantially increased pressure. If the duct is to be of any length at all, its cross section should be substantially greater.than that of the vent itself. The effect of elbows or bends needs to he considered in estimating the effective duct length. Palmer, et al, Reference 46 and 66, found that the maximum pressure developed during combustion venting will increase approximately as the square of the duct length. In one case a duct length of 10 ft. (3 meters) resulted in a pressure increase of 1 lbf/in. 2 (7 kN/m~) over that for no duct; doubling the duct length resulted in a pressure increase of 4 lbf/in 2 (28 kN/m2). Palmer, et al, also found pressure during venting to increase linearly with vent inertia.

Venting panels or "doors" are often used for venting equipment such as bag filters. Such vent panels should have inertias no greater than 2 lb/f t 2 (10 kg/m ~) of effective vent opening.

4. Vent ing of Gas Combust ion Inside Vessels

In the usual industrial situation the venting of gas combustion inside vessels is the most difficult of the different types of combustion venting. Many o f the variables listed above under Section B.1. may come into play in a single case. I t is not at all unusual for the gases to be. at an initial elevated pressure. There may also be a significant degree of turbulence~ and of turbulence nonuniformity, in the gas mixture. Often the turbulence is brought about by the mode of introduction of feed gas streams into the vessel. For various reasons the vent opening pressure

E p s - 1 4 AMENDMENTS TO NFPA 6 8

143.7

TABLE lb

Required vent area as a funct ion of Dust Class, relief pressure (Pv), Container vo lume and m a x i m u m pressure developed during venting

(Pmax)

pmaz S t . - - ! S t d . - - 2 • V N / ms)

(m s ) 20 50 70 I00 150 200 250 300 20 50 70 100

o

z

z

1 0.12 0.05 0.048 0.04 0.03 0.029 0.023 0.02 $ 0.4 0.18 0.145 0.13 0.034 0.075 0.055 0.06

I0 0.7 0.3 0.25 0.2 0.15 0.13 0.11 0.09 15 1.0 0.42 0.33 0.28 0.21 0.19 0.17 0.15 20 1.3 0.53 0.41 0.33 0.27 0.24 0.21 0.19 26 1.5 0.64 0.50 0.40 0.32 0.28 0.25 0.23 30 1.85 0.75 0.58 0.47 0.39 0.33 0.30 . 0.27 4 0 2.4 0.95 0.71 0.58 0.48 0.42 0.37 0.34 50 2.9 1.14 0.85 0.70 0.66 0.50 0.44 0.40

I00 - - 1.9 1.4 1.15 0.9 0.80 0.70 0.68

I 0.36 0.074 0.054 0.045 0.034 0.028 0.024 0.021 5 1.05 0.22 0.18 0.15 0.12 0.086 0.07 0.062

I0 1.65 0.38 0.27 0.23 0.18 0.15 0.115 0.I 15 2.15 0.53 0.37 0.32 0.25 0.21 0.18 0.15 20 2.8 0.68 0.48 0.39 0.3 0.25 0.24 0.22 25 3.0 0.8 0.88 0.48 0.32 0.3 0.28 0.25 30 3.4 0.97 0.68 0.56 0.44 0.35 0.33 0.31 4 0 4.0 1.16 0.85 0.7 0.52 0.45 0.4 0.36 50 - - 1.4 1.05 0.82 0.67 0.55 0.5 0.45

I00 - - 2.4 1.8 1.4 I . I 0.9 0.8 0.7

I - - 0.36 0.105 0.07 0.043 0.033 0.027 0.023 5 - - 1.0 0.32 0.21 0.13 0.090 0.08 0.068

1O - - 1.65 0.6 0.4 0.28 0.2 0.18 0.15 15 - - 2.2 0.8,5 0.58 0.39 0.29 0.22 0.21 20 - - 2.55 1.12 0.76 0.5 0.35 0.3 0.28 25 - - 3.1 1.38 0.94 0.59 0.43 0.38 0.33 30 - - 3.4 1.6 I . I 0.68 0.5 0.44 0.38 4 0 - - 4.0 2.0 1.4 0.8~ 0.64 0.58 0.5 50 2.4 1.7 1.0 0.78 0.68 0.53

100 3.6 2.7 1.5 1.15 1.0 0.8

0.27 0.16. 0.15 0.12 0 . 8 3 0.5 0.45 0.35 1.42 0.95 0.83 0.63 1.93 1.4 1.2 1.0 2.4 1.8 1.6 1.27 2.9 2.2 1.95 1.58 3.35 2.5 2.3 1.85 4.0 3.35 2.95 2.45

- - 4 . 0 3 . 5 5 3 . 0

0.44 0.22 0.16 0.13 1.3 0.6 0.47 0.39 2.0 1.05 0.86 0.72 2.5 1.5 1.3 1.1 3.15 1.9 1.7 1.36 3.66 2.3 2.1 1.7

- - 2.7 2.4 1.96 - - 3.5 3.1 2.6

3.7 3.2

- - 0.44 0.28 0.2 - - 1 .2 0 . 8 0.5 - - 2 . 0 1 .4 1 . 0 5

- - 2 . 6 1 .9 1 . 4 9

- - 3.14 2.36 1.88 - - 3.56 2.75 2.26 - - 3.9 3.17 2.62

3.8 3.3 3.9

Relief surface F (mS)

1438 C O M M I T T E E ON E X P L O S I O N P R O T E C T I O N SYSTEMS EPS-15

N o t e s t o T a b l e l b

( N u m b e r s a r e in S I U n i t s )

Class St . ~ 1 D u s t s g i v i n g m a x i m u m r a t e s of p r e s s u r e r ise u p t o 50 ,000 k N / ( n p ) (sec) Class St . - - 2 D u s t s g i v i n g m a x i m u m r a t e s of p r e s s u r e r ise of 50 ,000 - - 150,000 k N / ( m s) ( sec )

Class S t . - - 3 D u s t s giving m a x i m u m r a t e s of p r e s s u r e r ise a b o v e 150,000 k N / ( m z) ( s ec )

S t . - - 2 S t . - - 3

IM 200 250 300 343 392 100 150 200 250 300 343 892 4 ~

0.09 0.08 0,05 0.05 0.045 0.03l 0.27 0.22 0.18 0.14 0.12 0. I 0.62 0.45 0.35 0.3 0.26 0.23 0.75 0.64 0.53 0.45 0.4 0.35 0.85 0.8 0.7 0.5 0.62 0.47 1.2 1.0 0 .8 ,5 0.75 0.65 0.58 1.4 1.2 1.0 0.88 0.75 0.68 1.75 1.5 1.26 1.12 1.0 0.88 2.2 1.8 1.5 1.33 1.2 1.05 3.6 2.8 2.4 2.2 2.0 1,7

0.I 0.087 0.064 0,054 0.046 0.04 0.29 0.26 0.19 0.18 0.14 0.12 0.55 0.5 0.39 0.34 0.3 0.25 0.88 0.7 0.58 0.5 0.45 0.38 1.05 0.9 0.76 0.53 0.58 0.52 1.26 1.1 0.9 0.8 0.73 0.63 1.6 1.3 1.08 0.95 0.85 0.75 1.9 1.6 1.36 1.2 1.1 0.95 2.3 1.9 1.6 1.4 1.3 1.15 4.0 3.2 2.6 2.3 2.1 1.8

0.125 0.09 0.07 0.058 0.05 0.044 0.4 0.3 0.25 0.21 0.18 0.17 0.73 0.58 0.46 0.4 0.37 0.31 1.05 0.84 0.58 0.6 0.53 0.45 1.35 1.1 0.9 0.8 0.7 0.6 1.63 1.34 1.1 0.98 0.8,5 0.74 1.9 1.57 1.3 1.16 1.0 0.88 2.4 2.0 1.66 1.48 1.28 1.12 2.8 2.7 1.96 1.76 1.65 1.37 4.0 3.8 3.0 2.8 2.3 2.0

0.26 0.16 0.12 0.095 0.08 0.07 0,052 0.052 0.75 0.55 0.4 0.33 0.28 0.25 0.21 0.18 1.2 0.9 0.78 0.65 0.55 0.48 0.45 0.4 1.58 1.25 I . I 1.0 0.85 0.76 0.7 0.64 1.9 1.5 1.43 1.3 1.14 1.05 1.0 0.9 2.24 1.9 1.7 1.6 1.45 1.35 1.25 1.13 2.5 2.2 2.0 1.9 1.7 1.64 1.6 1.36 3.1 2.76 2.6 2.4 2.2 2.1 2.0 1.8 3.6 3.3 3.1 2.8 2.6 2.6 2.4 2.1

0.58 0.25 0.16 0.12 0.093 0.08 0.07 0,055 1.5 0.75 0.6 0.4 0.35 0.3 0.25 0.23 2.08 1.2 0.85 0.7 0.65 0.58 0.55 0.45 2.5 1.5 1.2 1.06 0.95 0.8,5 0.8 0.7 2.8 1.85 1.55 1.35 1.24 1.16 1.05 0.95 3.05 2.15 1.85 1.65 1.6 1 .43" 1,36 1.2 3.25 2.4 2.1 1.92 1.8 1.7 1.55 1.4 3.6 2.9 2.6 2.45 2.3 2.16 2.0 1.85 4.0 3.4 3.1 3.0 2.8 2.6 2.5 2.3

0.35 0.22 0.16 0.13 0.I 0.07 1.0 0.65 0.5 0.4 0.35 0.3 1.4 1.05 0.8,5 0.7 0.55 0.56 1.55 1.33 1.15 1.0 0.92 0.85 1.85 1.6 1.48 1.3 1.2 I . I 2.05 1.85 1.76 1.55 1.45 1.35 2.2 2.1 1.95 ] .8 1.7 1.6 2.7 2.56 2.4 2.3 2.2 2.05 3.2 3.0 2.9 2.8 2.7 2.5

Relief suda~e F (m~)

ffs-16 AMENDMENTS TO NFPA 68

1439

may have to be appreciably above atmospheric. Geometry of the gas space in a vessel may be far from an ideal shape such as a sphere or cube. For various geometry reasons the vent may have to be at a nonpreferred location. The likely location of ignition source usually cannot be determined. Hence it usually cannot be known whether the gases going through a vent, after opening, will be primarily unburned or burned or a mixture of the two.

It is on account of such factors as the above that estimation of necessary combustion venting area for a vessel is complicated. yet the estimation is frequently necessary. Because of uncertain- ties yet existent in combustion venting technology the most conservative case should be assumed. For example, even though a mixture of fuel gas and oxidant in a vessel would normally be outside the flammable limits, for combustion venting it should be assumed that the mixture is such as to give the maximum pressure during combustion venting. Venting is ineffective for detonations. This Guide relates to venting for deflagrations only.

The following guidelines are based on the technology available at the time of writing of this Guide. Because of technology limitations the guidelines cannot be guaranteed to be adequate for every situation:

a. Effect of Vessel Size. The most comprehensive known data on effect of vessel size are given in Reference 17. Combustion venting tests were made for propane-air mixtures in vessels of 1, 10, 30, and 60 cubic meter size. The gas mixtures were initially at atmospheric pressure and were quiescent at times of ignition. The vents had low inertia and no ducts. Vent opening occurred at pressures of 1.4, 2.8, and 7.2 psig. (10, 20 and 50 kN/m~). The results thus obtained are summarized in Tables 2a and 2b. Suit- able interpolations can be made between the numbers in the table.

I t has been proposed by some that the "cubic law" for combustion in unvented vessels be extended to the combustion venting case. For combustion in a closed vessel the cubic law has been stated as:

dp ~ V 1/8 = constant dt ~max

where

V

= maximum rate of pressure rise during combustion in the unvented vessel

= volume of the unvented vessel

1440 COMMITTEE ON EXPLOSION PROTECTION SYSTEMS EPS-17

The data in Tables 2a and 2b do not fully support extension of the cubic law to combustion venting. The data do show, however, tha t the required vent area for a range of vessel size is not a constant vent area per unit volume of vessel, r

T a b l e 2a

Vent areas, ft 2, required for venting of combustion of propane-air mixture inside vessels of L/D approximately 1: C. Donat, Reference 17 (Note: Numbers are in English units).

Max press., during venting, psig 4.35 7.25 14.5 21.8 29.0 36.3

Vessel Vol., ft s

Vent burst pressure - 1.45 psig.

35.3 353

1059 2 1 1 8

35.3 353

1059 2 1 1 8

35.3 353

1059 2118

3.34 2.69 1.61 0.97 0.48 22.6 16.1 8.6 5.9 4.30 3.23 32.3 24.2 16.1 9.7 6.5 4.84 51.6 32.3 12.9 - - - -

Vent burst pressure = 2.9 psig.

3.82 3.23 2.26 1.56 1.02 0.75 23.5 18.3 11.5 8.0 5.1 3.98 29.1 21.8 14.5 10.4 7.8 5.7

40 .5 . . . .

m

I

Vent burst pressure ~- 7.3 psig.

- - 2.10 1.29 • 0.92 0.70 13.0 8.5 6.0 4.52

37.5 20.2 14.5 10.4 7.8 56 23.9 - - - - - -

Data in Reference 22, for venting of pentane combustion in a 60 ft. 3 (1.Tm 3) vessel check fairly well with the data in Reference 17, for venting of propane in a 1 cubic meter vessel.

b. Effect of Fundamenta l Burning Velocity. The pressure developed during combustion venting is determined in par t by the speed with which gases burn before and during venting. This burning speed is determined in par t by the characteristic rate at which flames proceed through different fuel gases. This characteristic rate is normally measured in terms of the maximum fundamental burning velocity, tables of values for which can be found in various handbooks. Some characteristic values are given in Table 3.

E P S - 1 8 AMENDMENTS TO NFPA 68

1441

TABLE 2 b

Vent areas, m s , required for venting of combustion of ~ , . propane-air mixture inside vessels of L /D approximately 1: C. Donat, nelerence 17 (Note: Num- bers are in S1 units)

Max press, during venting, k-N/m s

Vessel Vol. m a

1 10 30 60

30 50 100 150 200 250

Vent burst pressure ---- 10 k-N/m s

0.31 0.25 0.15 0.09 0.045 2.1 1.5 0.80 0.55 0.4 0.3 3.0 2.25 1.5 0.9 0.6 0.45 4.8 3.0 1 . 2 - - - - - -

Vent burst pressure ---- 20 kN/m s

1 10 30 60

1 10 30 60

0.365 0.3 0.21 0.145 0.095 0.07 2.18 1.70 1.07 0.74 0.47 0.37 2.70 2.03 1.35 0.97 0.72 0.53

- - 3.76 . . . .

Vent burst pressure ---- 50 kN/m s

h

0.195 0.12 0.085 0.065 - - 1.21 0.79 0.56 0.42

3.48 1.88 1.35 0.97 0.72 5.22 2.22 - - - - - -

TABLE 3

M a x i m u m f u n d a m e n t a l burn ing velocity

Gas f t . /sec, meters / see .

Methane (natural g ~ ) 1.2 0.37 Propane 1.5 0.46 Butane 1.3 0.4 Hexane 1.3 0.4 Ethylene 2.3 0.7 Acetylene 5.8 1.8 Hydrogen 11.0 3.4

T h e a b o v e v a l u e s a re de f in i t e ly n o t t h e speeds w i t h w h i c h flames wil l m o v e t h r o u g h a f l a m m a b l e m i x t u r e in a vessel . F l a m e speeds in a vesse l a re n o r m a l l y m u c h h ighe r t h a n the f u n d a m e n t a l burn ing ve loc i t y . H o w e v e r , t h e f u n d a m e n t a l b u r n i n g v e l o c i t y is a basic m e a s u r e of t he f lame c h a r a c t e r i s t i c s of a fuel gas.

S ign i f i can t ly t h e gases of m o s t o rgan ic c o m p o u n d s h a v e n e a r l y the s a m e f u n d a m e n t a l b u r n i n g ve loc i t y . T h u s the i r f l ame speeds , and t h e p r e s s u r e d e v e l o p e d d u r i n g ven t ing , wil l be n e a r l y t h e same. I n a d d i t i o n to t h e f i rs t f o u r c o m p o u n d s l i s t ed in T a b l e 3, this i nc ludes gases of such c o m p o u n d s as ace tone , be nz e ne a n d

1442 COMMITTEE ON EXPLOSION PROTECTION SYSTEMS E P S ~ 1 9

ethyl alcohol. Substitution of a halogen, such as chlorine, or certain other atoms in a molecule slows down the combustion. On the other hand, introduction of unsaturation into the molecule, as in the cases of ethylene and acetylene, speeds up the combustion.

Data from References 9 and 17, can be used to compare the pressures developed during combustion venting for propane~air and hydrogen-air mixtures. For the same initial pressure in the vessel, the same vent bursting pressure, and the same sizes of vessel and vent, the pressure developed during combustion venting for hydrogen-air is a maximum of about 1.55 times that developed during combustion venting for propane-air. This ratio decreases somewhat as initial pressure and vent burstin~ pressures are increased. However, to be on the conservative side as mentioned earlier, it is recommended that the 1.55 ratio be used throughout. It should be noted that this ratio has not been tested for vessel sizes above I cubic meter. However, it is recommended that it be used for the larger vessel sizes when extrapolating from Tables 2a and 2b for combustion venting for hydrogen.

The ratio of the fundamentsl burning velocity of hydrogen to that for propane is 11/1.5 -- 7.34. The number, 7.34, must be raised to the 0.22 power to obtain the ratio, 1.55, stated above. If this same power exponent of 0.22 were applied to the ethylene case, the ratio would be 1.1. However, it is recommended that, in calculating combustion venting for ethylene, the pressure during combustion venting be assumed to be 1.2 times that determined from Tables 2a and 2b for propane. It is further suggested that the pressure from combustion venting of gases having about the same fundamental burning velocity as that of propane be assumed to be the same as that for propane. This again is confirmed by comparisons of data from Reference 17, for propane and Reference 22, for pentane.

c. Effect of Initial Pressure. The effect of initial pressure must be correlated on the basis of absolute pressures. The data from Reference 9, serve as a basis for correlating pressures developed during venting as a function of the initial absolute pressure of gases in a vessel and as a function of the absolute pressure at which a vent opens. If the ratio of vent bursting pressure to initial gas pressure in a vessel be kept constant, and if vessel size and vent size be kept constant, the pressure developed during venting of propane combustion will vary as about the 1.5 power of the initial pressure. The power exponent for propane varies from about 1.2 for large vent ratios (6 ft.s/100 ft. 8) to about 1.5 for small vent ratios (2 ft.s/100 ft)) . For hydrogen the exponent ranges from 1.1 to 1.2.

s-2o AMENDMENTS TO NFPA 68 1443

It is recommended that the 1.5 power be used in extrapolating from the data in Tables 2a and 2b, which are for propane-air mixtures at an initial pressure of 1 atmospheric absolute. (See sample calculation - - Appendix V.)

For hydrogen the recommended exponent for increased initial pressure is 1.2. For ethylene it is recommended that an exponent of 1.4 be used; this is untested.

d. Effect of Initial Turbulence. Initial turbulence presents special problems in approximating combustion venting requirements. Industrially it is often difficult to quantify turbulence inside equipment at the time ignition may occur. This is on account of several factors. The turbulence of the gas in a vessel, if that gas be in motion, is apt to be nonuniform. At the time of ignition the gas flows may be at abnormal rates. Ignition may even be caused by breakage of some internal piece of the equipment, and that breakage may result in change in the turbulence pattern.

Some few tests have been reported on the effects of turbulence on pressure developed during combustion venting. However, in most cases the turbulence in the gas mixture was quantified in terms of operation of the device used to create the turbulence. References 1, 22, and 44, serve as bases for estimating effects of turbulence on pressures developed during combustion. All these references give data on turbulence effects for venting aliphatic hydrocarbon-air flames only. The available data show that the absolute pressure resulting from venting initially turbulent gas mixtures is in the range of about 1 to 3 times that resulting from venting initially quiescent gas mixtures. From these numbers it is suggested that a factor of 2 be used for turbulent aliphatic hydrocarbon-air mixtures normally encountered in industry. The same factor is suggested for other gases having similar flame speeds.

For hydrogen-air mixtures it is estimated that the turbulence effect on pressure developed during venting will be less than that in the case of the slow flame speed gases. In the absence of dependable data for hydrogen and acetylene it is suggested that a factor of 1.5 be used. For the case of ethylene it is suggested that a factor of 1.9 be used; again as in the case of hydrogen, this is untested.

e. Effect of Initial Temperature. This Guide, in Appendix 11, gives data for effect of temperature on combustion properties of a number of organic compounds when burned in a closed container. Increase of temperature in most cases results in an increase in maximum rate of pressure rise in the container. However, the

1444 COMMITTEE ON EXPLOSION PROTECTION SYSTEMS E ~

increase in temperature also results in a decrease in number of moles of gas inside the container at a fixed pressure. This de. crease in moles results in a corresponding decrease in the pressure generated from the combustion. I t is found that the decrease in the generated pressure approximately offsets the increase in flame speed. Therefore, for a fixed initial pressure in a vessel, it is believed that no adjustment in estimated pressure develop. ment during combustion venting needs to be made for increase in temperature. The same is probably true for temperature de. crease below ambient, but this is not proved.

f. Effects of Combinations of Variables. At the present state of technology there are insufficient data to tell definitely just how combinations of variables may affect the maximum pressure developed during combustion venting. For the present it is suggested that the effects of the variables be assumed to be cumulative. This pertains to the variables discussed in sectiona a through e above.

g. Effects of Additional Variables. (1) Inertia of Vents. Sometimes a rupture disc will not pro-

vide a suitable vent, and a hinged panel or similar device must be used instead. Again, inertia of such a device must be as low as possible. Tonkin and Berlemont (66) presented test da taon effects of vent panel inertia or pressure developed during the venting of dust combustion. Doubling of the weight per unit area, of a vent panel of a fixed area, i.e., doubling the inertia resulted in a doubling of the pressure developed during-combustion venting. Tripling of the inertia resulted in a tripling of the pressure developed during the combustion venting.

(2) Vent Ducts. There should preferably be no duct in the venting system. If a duct must be employed, it should have just as short an effective length (including effective length of bends in producing pressure drop) as possible. For any ap- preciable duct length the duct cross sectional area should be at least twice that of the vent device. I t should be noted that combustion can take place in the vent duct itself, i.e., unburned gases may be the first to exit from the vent. This has two implications. The first is that the duct should be capable of withstanding a pressure at least as high as that expected to develop in the vessel during venting. The second is that high turbulence can develop in a duct. In the ease of gas burning within the duct, that turbulence could possibly lead to transition of the deflagration to detonation. In that case, far higher pressure could develop in the duct.

zes-2_ AMENDMENTS TO NFPA 68 1445

~ 3o) Ignition Source Location. There are few data on the effec f ignition source location on the pressure developed combustion venting. No allowances for this effect can be recommended.

In cases where multiple simultaneous sources of ignition are probable, i t i ssugges~d that the multiple ignition be assumed to produce the enec~ of initial turbulence.

(4) Other Oxidants. There are little data on effect of other oxidants than air. Other oxidants include oxygen, oxygen- enriched air, oxides of nitrogen, halogens, etc. Oxygen enrich- ment leads to higher pressures and an increased probability to transition into detonations. If direct data are not available for the system being considered, tests are recommended.

(5) Fogs and Mists. I t is noteworthy that flames can propagate through fogs and mists at mixture temperatures below the flash point of the liquid involved. It is suggested that fogs and mists be treated as gases in estimating pressures developed during combustion venting.

5. Venting of Gas Combustion Inside Long Ducts

Most of the cases of flammable gas mixtures inside ducts 'of the ventilation type occur at initial internal pressure of near- atmospheric. The venting of such ducts is covered in Appendix IV.

4. Revise title of C h a p t e r 3. Description of Vents and Vent Closures, to read:

Chapter 4. Description of Vents and Vent Closures No changes in tezt.

1446 COMMITTEE ON EXPLOSION PROTECTION SYSTEMS ]~PS-2~

5. Revise Pages 68-31 through 68-3~, A p p e n d i x I , R e f e r e n c e s , to read:

Appendix I R e f e r e n c e s

1. Bartlmecht, W., and Kuhnen, G., Forschungsberi~ht F45, Bundesinstitut fur Arbeitsschutz (1971).

2. Bonyun, M. E., "Protecting. Pre~, ure V~22(~M~itlhgR,upture Discs.,, Chemical and Metallur',~,~ Enginsermg, vomme 4 ( Y . ,),, P]). 260--263

3. Brown, Hylton, 'Design of Explomon Pressure vents. ~'no/ne~-~ News-Record (October 3, 1946). -°

4. Brown, K. C., et al., Trans. Inst. Mar. Enors. (1962), 74 (8): pp. 261-76. 5. Coffee, R. D. "Dust Explosions: An Approach to Protection Design.,,

Fire Technology, Vol. 4, No. 2 (May 1968), pp. 81-87. ,, 6. Coffee, R. D.; Raymond, C. L.; Crouch, H. W. A LinearYariable

Differential Transformer as a Transducer. Eastman xxoaa~ wompany, Kodak Park Division (1950).

7. Coffee, R. D.; Raymond, C. L.; Crouch, H. W. "TheT~ting of M a.terials for Pressure Relief Vents." Eastman Kodak Company, ~oaa~ rarK Division (1950).

8. Cousins, E. W., and Cotton, P. E. "The Protection of Closed Vessels against Internal Explosions." American Society of Mechanical Engineers, t951, Paper No. 51-PRI-2.

9. Cousins, E. W., and Cotton, P. E., Chemical Engineering, No. 8 (1951), pp. 133-137.

10. Coward and Hersey. "Accuracy of Manometry of Explosions." Bureau of Mines Report of Investigations 3274 (1935).

11. Coward and Jones. "Lindts of Flammability of Gases and Vapors." Bureau of Mines, Bulletin 503 (1952).

12. Creech, M. D., "combustion Explosions in Pressure Vessels Protected with Rupture" Discs." American Society of Mechanical Engineers. Trans- actions, Volume 63, No. 7.

13. Creech, M. D., "Study of Combustion Explosion in Pr~sur,e Vessels." Black, Syvalls and Bryson, Inc., Kansas City, MO (Feprua~ t~av). ~

14. Crouch, H. W.; Chapman, C. H.; Raymond, C. L.; Wmcnmeyer., ~. w. "Maximum Pressures a n d Rates of Pressure Rise Due to .E.xplo.smus of Various Solvents." Eastman Kodak Company, Kodak Park Dtvmm.n (1952!;

15. Cubbage, P. A., "Flame traps for use with town gas/air mixtures. Gas Council Research Communication GC63, London (1959).

16. Dicken. Clinton O., "Dust Explosion Hazards in the Confectionery Industry." National Confectioners' Association of 1 LaSMIe Street, Chicago, IL.

17. Donat, C. "Release of the Pressure of an Explosion with Rupture Discs and. Explosion Valves," paper presented at Achema 73, Frankfurt, West Germany.

18. Donat, C. Staub-Reinhaltung der hft , 31 (4) (April 1971), pp. 154-160. 19. Freeston, H. G., Roberts, J. P. and Thomas, A., Proc. Instn. Mech.

Engrs., (1956), 170 (24), pp. 811-62. 20. Fiock, E. F., "Measurement of burning velocity." High Speed A.e~-

dynamics and Jet Propulsion. Vol. IX., pp. 409-38. Oxford, 1955. Umvermty Press.

21. G~u/de to the Use of Flame Attesters and Explosi~n Reliefs, Ministry of Labour, New Series No. 34, Her Majesty's Stationery Office, Lonoon, (lvv~l.

22. Harris, G. F. P.; Briscoe, P. G. Combustion and Flame I1 (4) (August 1967), pp. 329-338.

A M E N D M E N T S T O N F P A 6 8

1447

~. ~r tmann . "Bureau of Mines Optical Pressure Manometer." Bureau • 7~--l~evort of Investigation 3751 (1935).

o f o A M m ~ n n . "Dust Explosions." Marks Mechanical Engineers' Hand- . -~'~th Edition (1950), pp. 795-800. 00o~, ~gr tmann. "Explosion and Fire Hazards of Combustible Dusts." ,_~'.~-Huciene and Toxicob~gTj, Interscience Publishers, Inc., New York, ~TVol 1, ~hap~r 13, Sectionl2 (1948), pp. 439--454. ,,

~a ~ m a n n . Pressure Re ease for Dust Explosions. NFPA Quarterly, - '~40 No 1 (July 1946) pp. 47-53.

vo~ ~ t ~ a n n . "Recent Research on the Explosibility of Dust Dis- . ~ o n ~ " Industrial and Engineering Chemistry, Vol. 40, No. 4 (April 1948) ~ - 7 5 ~ - 7 5 s ,, pp. Hartmann. The ExPlosion Hazard of Metal Powders and Preventive ,t~'~'-,,res." Metals Handbook, American Society of Metals (1948 ed.), pp. 52-54. 'v~.~l~artma ,nn , Cooper and .J.acobson. "Recent Studies of the Explosibility ^¢ ~ r n Starch. B .ure~u of Mines Repo~ of Inv~tigation 4725 (1950). ~'30 Hartmann anu ~reenwald. "The ~xplceibility of Metal Powder Dust

u'ds" Mining and Metallurg.v Vol. 26, (1945) pp. 331-335. Cl° 1 ~[artmann and Nagy. "~ffeet of Relief Vents on Reduction of Pres- ~ , ~ Developed by Dust Explosions." Bureau of Mines Report of Investi-

~a~fi~n 392~(~46). aa, v "Inflammability and Exploslbility of Powders 32 tla. ~ n n and N_~ . " . . . . Us~ in the Plastic Industry." Bureau of Mines Report of Investigation 3751

(19~)'Hartmann and Nagy. "The Explosibility of Starch Dust." Chem/cal E~'neering News, Vol. 27 (July 18, 1949), P. 2071.

'~ . Hartmann, Nagy and Brown. "Inflammability and Explosibility of Metal Powders." Bureau of Mines Report of Investigation 3722 (1943).

35. Hartmann, Nagy and Jacobson. "Explosive Characteristics of Titanium, Zirconium, Thorium, Uranium and Their Hydrides." Bureau of Mines Report of Investigation 4835 (1951).

36. Howard, W. B., Loss Prevention, Vol. 6, a CEP Technical Manual, Am. Inst. Chem. Engrs., 1972, pp. 68-73.

37. Huff, J. E. Loss Prevention, Vul. 7, a CEP Technical Manual, Am. Inst., Chem. Engrs., 1973, pp. 45-57.

38. Hulsberg, F., Rev. Industr. rain., (1957), 39, pp. 373-76. 39. Jones, W. M., "Determination of Dust Explosion Possibilities."

Factory Insurance Association (1940) Special Hazard Study No. 4. 40. Jones, W. M., "Prevention and Minimizing the Effects of Dust Ex-

plosions in Manufacturing Plants." Factory Insurance Association (1940) ~3pecial Hazard Study No. 5.

41. Jones, Harris and Beattie. "Protection of Equipment Containing Explosive Acetone-Air Mixtures by the Use of Diaphragms." Bureau of Mines Te~nical Paper 553 (1933).

42. Loison, R., Chaineaux, L., and Delclaux, J., "Study of some safety problems in fire damp drainage." Eighth International Conference of Direc~.s of Safety inMines Research. Paper No. 37. Dortmund-Derne, 1954. Safety in Mines Research Establishment.

43. Mainstone, R. J., Current Paper 26/71, Building Research Station, Garston, Watford WD2 7JR, England, 1971.

44. Nagy, J., Bureau of Mines, U. S. Dept., of Interior, Pittsburgh, PA, private communication.

45. Nagy, Zeilinger and Hartmann. "Pressure-Relieving Capacities of __Diaphragms and Other Devices for Venting Dust Explosions." Bureau of Mines Report of Investigation 4636 (1950).

46. Palmer, K. N., Fire Research Note No. 830, "Dust Explosion Venting ~A Reassessment of the Data," Aug. 1970, Fire Research Station, Borcham Wood, Herts., England.


47. Philpott, J. E., Engng., Mater. & Des., (1963), 6 (1), pp. 2A-29. 48. Potter, A. E., "Flame Quenching. ' Progress in Combustion Science and

Technology, Vol. I, pp. 145-81. Oxford, 1960. Pergamon Press. 49. Quinton, P. G., Br/t. Chem. Engng., (1962), 7 (12), pp. 914-21. 50. Radier, H. H., J. Inst. Petrol, (1939), 25, pp. 377-81. 51. Rasbash, D. J., The Structural Engine~., 47 (10) 404-407 (October

1969). 52. Rasbash, D. J. and Rogowski, Z. W., Combat. & F/ame, (1960), 4 (4),

pp. 301-12. 53. Rasbash, D. J. and Rogowski, Z. W., "Relief of Explosions in Dust

Systems." S~nposium on Chemical Process Hazards with Special^Reference to Plant Design. Institution of Chemical En~nesrs, 1961, pp. 5~-~.

54. Rasbash, D. J. and Rogowski, Z.W., Relief of Explosions in Propane/ air Mixtures Moving in a Straight Unobstructed Dust." Second_Symposium on Chemical Process Hazards with Special Reference to Plant Design. In- stitution of Chemical Engineers, 1964.

55. Runes, E., Loss Prevention, Vol. 6, a CEP Technical Manual, Am. Inst., Chem. Engrs., 1972, pp. 63--67.

56. Simmonds, W. A. and Cubbage, P. A., "The Design of Explosion Reliefs for Industrial Drying Ovens." Symposium on Chenncal Proce~ Hazards with Special Reference to Plant Design. Institution of Chemic~ Eng/nsers, 1961, pp. 69-77.

57. Schmidt, H., Haberl, K. and Reclding Hansen, M. K., Tech. Uberwach., (1955). 7 (12), pp. 432-39.

58. Schwab, R. F., and D. F. Othmer, "Dust Explosions," Chem. & Proc. Eng., April, 1964.

59. Smith, J. B. "Explosion Pressures in Industrial Piping System." Fac- tory Mutual Insurance Association (1949).

60. Stecher. "Fire Prevention and Protection Fundamentals." The Spectator (1953).

61. Stretch, K. L., The Structural Engineer, 47 (10) 408-411, October 1969. 62. "Symposium on Bursting Discs." Trans. lnstn. Chem. Engrs., (1953),

31 (2). 63. Thompson and Cousins. "A Direct-Reading Explosion Effect Gage."

Instruments (April 1947). 64. Thompson, N. J. and Cousins, E. "ExplOsion Tests on Glass Windows;

Effect on Glass Breakage of Varying the Rate of Pressure Application." Journal of the American Ceramic 8 ,oc~,ty, Vol. 32, No. 10 (October 1949).

65. Thompson and Cousins. 'Measuring Pressures of Industrial Ex- plosions," Electronics (November 1947).

66. Tonkin, P. S., and Berlemont, C.F.J., Fir~ Research Note No. 942, "Dust Explosions in a Large Scale Cyclone Plant," July 1972, Fire Research Station, Boreham Wood, Herts., England.

67. Tomlinson, W. R., and Audrieth, L. F. "Uninvited Chemical Ex- plosions." Journal of Chemical Education, November 1950.

68. Underwriters' Laboratories, Inc. "A New Type of Bomb for In- vestigation of Pressures Developed by Dust Explosions." Bulletin of Research No. 30 (March 1944).

69. Underwriters' Laboratories, Inc. "An Investigation of Large Electric Motors and Generators of the Explosion Proof Type for Hazardous Locations, Class I, Group D." Bulletin of Research No. 46.

70. Underwriters' Laboratories, Inc. "The Lower Limit of Flammability and the Autogeneous Ignition Temperatures of Certain Common Solvent Vapors Encountered in Ovens." Bulletin of Research No. 43.

71. Underwriters' Laboratories, Inc. "The Spontaneous Ignition and Dust Explosion Hazards of Certain Soybean Products." Bulletin No. 47.

72. Valentine and Merrill. "Dust Control in the Plastics Industry." Transactions of the American Institute of Chemical Engineers. Vol. 38, No. 4, August 1942.

lgPS-26 AMENDMENTS TO NFPA 08 1449

7.~ Yao "Explosion Venting of Low Stren h " d Structures" _. . . - . . . . . . . . . . gt EqmDmentan Technical raper vresen~ea at ~ n ~ Loss Prevention Svmuosium. Phila- delvhia, PA, November 1973, FMRC, Norwood, MA ~ r •

74. Yao, deRis, Bajpai, and Buckley. ,,"Evaluation of Protection from Ex- nlosion Overpressu~ in AEC Gloveboxes L for U.S.A.E.C., Chica~o Operations ~)flice, Argonne, 1~. ~ . ~ ~on~ract AT (11-1)1393, F.M.R.~ Serial No 16215.1, Factory Mutual Research Corp., December 1969. " "

NFPA Publications M a n y s tandards , fire repor ts and o the r publ ica t ions contain°

ing in format ion relat ing to var ious phases of explosion p reven t ion a~d pro tec t ion have been publ ished b y the N F P A . A comple te list of publ ica t ions will be mai led on appl ica t ion to the N a t i o n a l Fire P ro tec t ion Associat ion, 470 At lan t ic Avenue , Bos ton , MA 02210.

1450 COMMITTEE ON EXPLOSION PRO TECTI O N SYSTEMS EPS-27

6. Add A P P E N D I X I V to read as follows:

Appendix IV

Explosion Reliefs for Dusts and Elongated Vessels

(Part II, Anon., "Guide to the Use of Flame Arrestors and Explosion Reliefs," New Series No. 34, Ministry of Labour, HMSO, London, 1965.)

Wherever flammable vapors and gases occur inside an industrial plant there is the danger of a gaseous explosion. The main precaution taken to avoid an explosion is to prevent the concentration of the flammable gas or vapor from falling within the flammable limits in air. Thus when pure methane is passed through a duct there is no danger of explosion unless an accident occurs which results in an approximately seven-fold dilution with air of the methane in the duct; this would bring the concentration of methane down to the upper explosive limit. Conversely, in other systems precautions can be taken to reduce the concentration of flammable gas to well below the lower explosive limit. For many processes it is not possible to be sure that at all times the concentration of flammable gas will be outside the flammable limits. Under these conditions the plant has to be designed so that if an explosion were to occur the minimum amount of damage would ensue. One of the ways in which this is done is to use explosion reliefs. These are provided on the side of the piece of equipment concerned and are designed to open very early in an explosion and allow the harmless release of the products of combustion of the explosion. The area of these vents should be large enough to relieve the explosion gases sufficiently quickly to prevent the maximum pressure from reaching a value greater than the pressure the container can withstand.

Plants in which flammable gases and vapors are handled in industry vary widely in size and shape, and duct systems of differ- ing degrees or complexity are used to connect items for plants in which various processes are carried out. Information on the provision of explosion reliefs for containers approximately cubical in shape have been published elsewhere following the work of the Gas Council on Explosion Reliefs for drying ovens (Reference 56). In this note design data for explosion relief for ducts and elongated vessels are provided. These data are based mainly on work carried out at the Joint Fire Research Organization on the venting of gaseous explosions in ducts (References 52, 53, 54).

EpS-28 AMENDMENTS TO NFPA 08 1451

The ducts varied in dimensions from 3 in. diameter to 12 in. square section and from 6 to 30 ft. long. In most experiments propane/air or pentane/air mixtures were used as the explosive gas, although a few experiments were carried out with ethylene/ air and methane/air mixtures. The correlation of the results of this work and also the inclusion of other sources of information (References 8 and.19) do allow, however, the results to be extrap- olated to ducts with diameters up to about 2 ft. 6 in. and to a number of other gases.

Scope This guide may be used to design explosion reliefs for ducts and

elongated vessels where L /D is equal to or greater than 6 and D does not exceed 2 ft. 6 in. The basic formula given in Section entitled, "Size and Spacing of Explosion Reliefs for Stationary Gases or Gases moving at Speeds of less than 10 ft . /s. ," will provide design data for straight, unobstructed ducts containing propane/air mixtures moving at velocities of less than 10 ft./s.

If the ducts are not straight or contain obstacles, additional relief is required and this may be calculated by using the information given in Section entitled, "Size and Spacing of Ex- plosion Reliefs for Stationary Gases or Gases moving at Speeds of Less than 10 ft . /s."

The Section entitled, "Size and Spacing of Explosion Reliefs for Gases moving at Speeds of 10-60 ft./s. ," deals with propane/air mixtures which are moving at velocities of between 10 ft./s, and 60 ft./s.

Correction factors given in the Section entitled, "Data for Gases other than Propane," should be employed when vessels containing gases other than propane are to be protected.

Where L / D is less than 6 the design data given by Simmonds and Cubbage (Reference 56) may be used, but since this work was carried out on vessels where L / D did not exceed 3, it may lead to the provision of explosion relief with an increased factor of .~fety as L / D approaches 6.

Principle of Relief Venting for Ducts and Long Vessels When a gas is ignited at the center of a long vessel the products

of combustion can first expand freely until the flame reaches the vessel walls. Thereafter the products of combustion expand in two directions along the length of the vessel. During this period, if the flammable gas is hydrocarbon vapor, the flames travel initially at a speed of about 10-20 ft./s. The expansion of the burnt combustion products behind the flame in the duct causes a motion of the unburnt gas ahead of the flame. After a short time

1452. COMMITTEE ON EXPLOSION PROTECTION SYSTEMS ~P.~J~t, -__r~

this moving unburnt gas becomes turbulent and one of the con. sequences of this is that the rate of combustion at the flame front is increased. This process may result in the continued acceleration of the flame to very high speeds. Shock Wave~ associated with the acceleration of the flame may also give rise to a large increase in pressure both in front and behind the flame and . . . . may also play a vital part in the eventual transition to a detonating combustion. Under the latter conditions flame speeds of the order of 6,000 ft./s, and pressures of several hundred lbf./in, s may be obtained.

If the gas is initially in rapid motion the initial propagation of flame is also faster and, other conditions being constant, a more violent explosion occurs than when the gas is initially stationary. Increases in the rate of pressure rise may also be caused by obstacles in the duct. These create local pockets of intense turbulence in the moving unburnt gas and may bring about a very rapid increase in the rate of combustion.

As a general principle relief vents should be cited so that bur~t gas close behind a flame is expelled from the vents; this would minimize the effect of the expansion of this gas on the motion of unburnt gas ahead of the flame. A relief vent should, therefore, be placed wherever there is likely to be a source of ignition. If there is a chance that ignition may occur at any point along the vessel or duct, it follows that relief vents should be installed along the whole length of the duct. It is also necessary that explosion reliefs, particularly those behind and near the flame, should open at a very early stage in the explosion, otherwise high flame speeds and an increased motion in the unburnt gas may quickly result.

In the following sections information is provided on the size and the assumption that the gas may become ignited at any point in the duct, and that the particular mixture of flammable gas and air is that which would give the most violent explosion. The information is for propane/air mixtures except where indicated otherwise, although figures for propane/air mixtures will apply with little modification to many other gases and to most industrial flammable solvents. In general the requirements are given in terms of a relation between the maximum pressure that may be expected during an explosion, the length and diameter of the duct or vessel and the size and separation of the explosion reliefs used. The size of the vents is usually expressed by a factor K which is the ratio of the cross-sectional area of the duct to the area of the vent. Thus K = 1 indicates an explosion relief of area equal to the cross-sectional area of the duct and K = 2 an explosion relief equal to half the cross-sectional area. The term

Eps-30 AMENDMENTS TO NFPA 68 1453

duct or elongated vessel applies to any vessel with a ratio of length L to mean hydraulic diameter D 6; the mean hydraulic diameter being four times the cross-sectional area divided by the perimeter.

The maximum design explosion pressures apply only when an appropriate vent closure is used. Thus, for example, the use of covers heavier than those recommended may result in explosion pressures greater than the maximum design values.

size and Spacing of Explosion Reliefs for Stationary Gases or Gases Moving at Speeds of Less than 10 ft./s.

~traight unobstructed ducts L / D less than 30.

The provision of only one opening as an explosion relief is generally sufficient if L/D, the length of the vessel to the mean hydraulic diameter of the vessel, is less than 30 but not less than 67 The maximum pressure will be given by one of the two following formulae:

Equation (i) a

where K -- 1; P = 0.07 L /D

Equation (i) b

where K is between 2 and 32; P -- 1.8K

For K between 1 and 2 the maximum pressure may be taken as the mean of those given by equations (i) a and (i) b, i.e.

P = 0.035 L /D + 0.gK

P = maximum pressure in lbf./in3

K --- ratio of cross-section of duct to area of vent

If only a single vent is used it should be placed as near as possible to the most likely position of a source of ignition. If no such position can be ascertained it should be placed as near to the center of the vessel as possible. Equations (i) a and (i) b give t he maximum pressure for the most unfavorable relative position of the explosion relief and ignition source.

With this vent system, covers weighing 2 lb./ft3 of vent area and held by magnets or springs may be used. A bursting disc failing at a pressure not higher than half the designed explosion relief pressure given by equation (i) a and (i) b may be used.

EXAMPLE 1 A reaction vessel is 20 ft. long x 2 ft. in diameter and an ex-

1454 jCOMMITTEE ON EXPLOSION' PROTECTION SYSTEMS EPS-3~II

plosion may occur during the empt.ying, or filling of this vessel. What size of explosion relief is reqmred If the maximum pressure that can be allowed is 10 lbf./in3?

ANSWER From equation (i) b the value of K corresponding to a maxi-

mum pressure of P of 10 lbf./in3 is 5.5; therefore, the cross-sec. tional area of the vessel divided by the area of the vent = 5.5, giving a vent of diameter 10.2 in. With this a bursting disc designed to fail at a pressure of 51bof~/~ e maybe used. This vent must of course be put in the e

EXAMPLE 2 A flare stack completely open at the top and with a water-seal

at the bottom is 40 ft. high and 2 ft. in diameter and discharges hydrocarbon vapors to the atmosphere. What is the maximum pressure if a fuel/air mixture is ignited in the stack.

ANSWER Apply equation (i) a; L /D = 20 gives a maximum pressure of

1.4 lbf./in3

S t r a i g h t u n o b s t r u c t e d d u c t s L / D g r e a t e r t h a n 30

For ducts with an L/D ratio greater than 30 it is necessary to provide more than one explosion relief. Even if the L /D ratio is less than 30 a given area of explosion relief is more efficient if it is distributed along the length of the duct. The maximum distance apart at which vents should be placed and the maximum pressure which would result from an explosion depend on the size of the vents and are given in Table 5 A1.

TABLE A1

MAXIMUM DISTANCE BETWEEN EXPLOSION RELIEFS AND MAXIMUM PRESSURES FOR A LONG LENGTH OF

UNOBSTRUCTED DUCT

Maximum '.. pressure for

Size of vents Maximum Formulae giving greatest (K factor for distance maximum pressures spacing

ea_cb vent) apart (lbf'/in's) (lbf'/in'2)

1 60 D .04 L~/D 2.4 2 30 D .06 Lt/D -~- 0.1 1.9 4 20 D .07 L~/D -{- 0.2 1.6 8 15 D .08 Lt/D -{- 0.3 1.5

L~ - - distance apart of explosion reliefs

l~PS-32 AMENDMENTS TO NFPA 08

1455

In designing explosion reliefs for long ducts an open end of a duct may be regarded as an explosion relief of size K = 1. For this purpose an open end may be defined as either an end leadin~

ithout restriction. . . into the 'open atmos, phere., or leading" to a vessel whlc.h !tself is adequately..provlded with explosion reliefs, or leading m~o a room of 200 tLmes greater volume than the volume of the d~ct. . If the ends of.a duct are not open, or may be closed some of t e hme, an explosion relief should be placed near as possible to these ends.

Vent covers weighing not more than 2 lb./ft . 2 should be used. They should be held in position by magnets or springs. If heav ie r covers cannot be avoided then explosion reliefs will need to be closer than indicated in Table A1 if pressures are to be kept below 2 lbf./in.2 If l a te r information on moving gases is followed in this respect than any error will be in the direction of increased safety.

EXAMPLE 3 I t is necessary to protect a straight duct 300 ft. long and 2 ft.

in diameter with explosion reliefs. One end of the duct is open, the other end is often closed or partially closed. How many explosion reliefs are required if the size of each vent is (a) equal to the cross-sectional area of the duct, (b) ~ of the cross-sectional area of the duct. The duct can withstand a maximum pressure of 1 lbf./in3

ANSWER The open end of a duct is a vent of size K = 1 and a vent of

equal size should be placed at the closed end of the duct. Accord- ing to Table A1 when K = 1 and the maximum pressure is 1 lbf./in. ~ the distance apart of explosion reliefs should be 50 ft. This would give a total of five explosion reliefs in addition to the reliefs at each end. Also, when K = 8, the distance apart of explosion reliefs should be 17.5 ft. This gives sixteen openings plus those at each end.

Vessels and ducts containing obstacles

A single obstacle in a duct may increase the maximum pressure in an explosion. Even for an obstacle blocking only 5 % of the cross-sectional area of a duct an increase in pressure by a factor of two to three may be obtained; for obstacles such as sharp right-angled tees or elbows and for orifices or strips blocking about 30% of the cross-sectional area of the duct the factor is about 10. There is insu~cient information to give detailed venting relationships for obstacles of various kinds. Experiments have shown, however, tha t to reduce maximum pressures in ducts containing an obstacle of the above type to 2 lbf./in3, and ex-

1456 COMMITTEE ON EXPLOSION PROTECTION SYSTEMS. EPS~,~3~.3 ~

plosion relief equal to the cross section of the duct needs to be sited every six diameters. I t is essential that an explosion relief also be placed near the obstacle. For a long straight duct con. nected to an obstacle, e.g., a tee-piece or orifice, an explosion relief should be placed as close as possible to the obstacle, and also at six diameters on either side of the obstacle. Thereafter, explosion reliefs should follow as with a straight unobstructed duct. Any bend sharper t h a n a long sweep smooth bend and any obstruction obscuring more than 5% of the cross section of the duct, should be regarded as an obstacle. For obstacles within these limits, it is still advisable that an explosion relief should be sited near the obstacle. Vent closures weighing not more than 3 lb./ft3 should be used. They should be held in position by magnets or springs.

Size a n d S p a c i n g of Explosion Reliefs for Gases Moving at S p e e d s o f 10-60 f t . / s .

Unobstructed ducts

TABLE A2

SPACE AND SIZE OF VENTS ALONG DUCTS CONTAINING MOVING GASES

L/D (Ratio of distance K (Ratio of cross between consecutive section of duct vents to hydraulic

Duct diameter to area of vent) diameter of duct)

I I 12 Up to I ft. 6 in. 2 \ 6

1 ft. 6 in. to 2 ft. 6in. 2 5

Vent systems are given in Table A2 which are designed to limit the maximum explosion pressure to 2 lbf./in. 2 for explosions in duct systems containing flammable mixture moving with velocities up to 60 ft./s. With both systems covers held by magnets or springs should be used. The maximum permitted weight of the covers themselves varies with the velocity of the flammable gas. For gases moving with a velocity of 25 ft./s, the closures should not weigh more than 10 lb./ft3 of vent area and for gases moving with velocity 25-60 ft./s, not more than 5 lb./ft. 2 of vent area.

D u c t s c o n t a i n i n g o b s t a c l e s

Ducts containing obstacles require a greater venting area in the

I~PS-34 AMENDMENTS TO NFPA 68

1457

~eighborhood of the obstacle. Information is available only for ducts up to 1.5 ft. in diameter for which there should be a vent equal to the cross-sectional area of the duct on each side of the obstacle at a distance equal to 3 duct diameters, followed by a further vent for each side spaced at a distance equal to 6 duct diameters. The remainder of the duct should be treated as unobstructed ducts.

The weight of the six covers nearest to the obstacles should not exceed 3 lb./ft3 of vent area for gases moving with velocities below 25 ft./s, and 1-1/~ lb./ft3 of vent area for gases moving with velocities of 25-60 ft./s.

These covers may be held by magnets or springs.

Vent closures

In the majority of applications vent closures must be leak tight, robust and designed in such a way that natural deterioration and lack of maintenance will not result in an increase in the maximum pressure obtained during an explosion.

The use of bursting discs for venting explosions in duct systems is somewhat restricted. These generally are more suitable for higher explosion pressures, and when used with ducts containing obstacles may considerably increase the explosion pressure. They may, however, be used in straight and unobstructed ducts, and relevant data on bursting pressures, mounting, etc., may be found elsewhere (References 47 and 62). They are commercially available either as ready-made units or in the form of materials for fabrication. The majority of disc materials are metals, but graphite is being used where low bursting pressures are required.

If the vents have to withstand a high temperature, asbestos miUboard may be used as a bursting material. There are no reliable design data for this material and bursting pressures of a given batch need to be determined experimentally by subjecting a panel to a static test with compressed air. As a rough guide panels of asbestos millboard 12 in. x 12 in. x ~ in. thick fail at a pressure of approximately 1.4 lbf./in. 2 in a static test; the failure pressure is approximately proportional to the thickness and inversely proportional to the linear dimension. Figure A1 shows a method of constructing an asbestos miUboard closure.

Panels may also be used which are sufficiently strong to withstand rough handling, but which are clamped or retained in such a way that the whole panel is easily pushed out if there is an explosion. I t is important that the panel should be light so that the inertia of moving the panel is reduced to a minimum and the panel itself does not become a dangerous missile. The weight of the panels should not exceed the weight given in the guide. A common way of providing a seal is to retain the panel by light

1458 COMMITTEE ON EXPLOSION PROTECTION SYSTEMS EPS35

friction between two surfaces bearing on a strip of the panel about ~ in. thick at the edge. The drawback of this method is that the pressure at which the friction will be overcome and the panel will fly is uncertain. A more reliable method is to use mag. nets or springs to clamp the panel at the edge. In this method the force holding the panel may be controlled according to the strength of the magnets or the springs. For the applications covered in this guide this force plus the weight of the panel should not normally exceed 30 lb. /f t? of vent area. If, however, there is a permanent slight positive pressure within the duct the mag- netic force or the force of the springs on the vent cover could be correspondingly increased to avoid leakage and displacement of the cover up to a maximum value of 50 lbf./ft. 2, but the weight of the cover must not be increased.

Clamping flanges

Asbestos mill ~board panel ( ~

@ @.@ @ Fig. AI. Asbesto8 mi l l boa rd pane l closur~e

A method of constructing vent closures held by magnets is shown in Figure A2. Essentially it consists of a cover and a magnet assembly. The cover may be constructed from a variety of light and dimensionally stable materials. Fiberboard is one of these. The metal plates, matching the magnets, may be screwed or riveted to the panel. There is no poin~ in making covers heavier than required for strength and heat insulation purposes. Magnets are located on the periphery of the vent and they are held in position by screws or rivets or any other convenient means. The seal may be obtained by the use of soft rubber or other suitable materials.

The total force required to dislodge the panel may be affected by goodness of fit between the steel plates and the magnets and needs to be ascertained by trial. This may be done by loading the cover with weights or pulling it off with a spring balance.

s-36 AMENDMENTS TO NFPA 68 1459

' ~ : Corner detail of cover showing . f ~ ~ interior of resinated kraft paper

Mild steel plates

Wa Oden ~ K~\\\\\~\~\ - ~ - ~ \ \ ~ \ \ ~ \ ~ \ ~ \ \ ~ 1 Permanent _\- _ _

...~.~ in. Clearance

~ " 1 in, Clearance

~ . / / / / / / / / ' / / / / / / ~ / ~ ~ Sectional detail of recessed magnet showing ~ in. allowance /" / ~ for compression of rubber to ensure dust-tight closure

ltig. A2. A duct. fitted with an explosion vent cover held in place by toma~nets. The field Strength of the magnets may be varied according

e 8trength of t h e .plant and the pressure within it.

(This is an exper im en ta l des ign and m a y be modi f i ed where necessary).


A method of retaining the cover by the use of spring clips is shown in Figure A3. The spring must be designed in such a way that the restraining force is no longer active after the cover has traveled s/~ in. This distance is shown on the drawing.

Hinged doors may be used with advantage instead of covers since, if adequately anchored, they do not become dangerous missiles even if they are quite heavy. The weight data given for covers held by springs or magnets also apply to hinged doors. These, however, have to be arranged in such a way that they open to an angle not less than 45 °. In order to obtain a good seal the end opposite the hinges may be held by magnets or spring clips.

Adjusted to disengage after cover has moved more than ¾ in. t

_ , . , ~ @ T h i s end anchored

Metal b r a c k e t - ~ . l ~ / / ) anchored to duct / ~ ~ / / ~ ~

Cover . ~ . ~ - . . / ] ' j , ~ , , , , ~

Duct

][/DUCT WITH THE CLOSURE IN PLACE

Fig. A& Closure held by springs

Data for gases other t h a n propane

It is possible to extrapolate the data given above to ducts containing a flammable mixture of gases other than propane in air. This extrapolation is based on the fundamental burning velocity, a property published in a number of handbooks. A list

• of burning velocities of some common vapors and gases is given in Table A3. Conversion of the data to a gas other than propane is accomplished by the use of equation (ii) :

E P S - 3 8 AMENDMENTS TO NFPA 68

1461

8 2 Equation (ii) P2 - 2.2 PI

where PI is the maximum design pressure for propane

P~ is the maximum design pressure for the gas under consideration

and S is the fundamental burning velocity of gas in ft./s.

On the other ha nd i f the distance between neighboring vents is LI for a given maximum pressure with a propane/air mixture, then for a different gas 'the new distance L~ to give the same maximum pressure would be given by

2.2 Equatio~n (iii) L2 = S~ Ll

The maximum fundamental burning velocity occurs with mixtures near the stoichiometric composition, and these velocities are the values which should be used with any ratio of flammable gas with air. An increase in the temperature of a given gaseous mixture increases the fundamental burning velocity by a factor approximately proportional to the 1.5 power of the absolute temperature. On this basis the maximum pressure should be proportional approximately to the cube of the absolute temperature.

Equations (ii) and (iii) may overestimate the venting required for gas mixtures very much lighter than propane/air mixture since they do not take into account the effect of gas density on the inertia and frictional resistance of the gases.

Mixtures of gases with oxygen or air enriched with oxygen can give substantially higher maximum pressure during an explosion, but there are no data to give an estimate of this increase.

TABLE A$ MAXIMUM FUNDAMENTAL BURNING VELOCITIES

Burning velocity Distance between Gas Mixture ft . /8, vents (propane ---- 1)

Methane/air 1.2 1.5 Propane/air 1.5 1.0 Butane/air 1.3 ~- 1.3 Hexane/air 1.3 1.3 Ethylene/air 2.3 0.4

*Town gas/air 3.7 0.16 Acetylene/air 5.8 -- Hydrogen/air 11.0 --

*Town gas containing 63 % hydrogen

1462 COMMITTEE ON EXPLOSION PROTECTION SYSTEMs EPS-)~

7. Add new A P P E N D I X V to read as follows:

Appendix V Sample Calculation

Assume: (a) The initial pressure is 2 atmospheres absolute. (b) the ratio of vent release pressure to the initial pressure

for this case to be the same as the value read from Table 2, (c) the vessel size and. vent size remain constant.

Solution: The maximum pressure reached during venting twill be (2 atm.abs/1 atm.abs.) 1-5 times the value read from Table 2 or 2.8 times the value read from Table 2.

1424 committee on explosion protection systems …su~cient expertise on the subject, and one member...

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