safety series flame arresters and explosion reliefs and safety executive health & safety series...

Health and Safety Executive Health & safety series booklet HS(G)IJ

Flame arresters and explosion reliefs

London: HMSO

Cd

r This document

[tains C

pages J

C Crown copyright 1980

First published 1980 Fourth impression 1992

HS(G) series The purpose of this series is to provide guidance for those who have duties under the Health and Safety at Work etc Act 1974 and other relevant legislation. It gives guidance on the

practical application of Regulations made under the Act, but it should not be regarded as an authoratative interpretation of the law.

Enquiries regarding this or any other USE publications should be made to USE Information Centres at the following addresses:

Broad Lane Sheffield S3 7UQ Tel: (0742) 892345 Fax: (0742) 892333

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ISBN 0 II 883258 I

Contents

Introduction I

PART I Flame arresters for industrial pipe and duct systems 2 Function of flame arresters 2

Examples of use of flame arresters 3

Merits of flame arresters 3

Principles involved in the specification of flame arresters 4 Types of flame arrester 4 Combustion properties of fuels 9 Effect of design of plant 11

Mechanism of operation of flame arresters 13

Specification procedure for flame arresters 14

Specification data for flame arresters 15 Flame speeds 15 Flame arrester capability 22 Additional factors in the specification of arresters 26 Summary of specification procedure for flame arresters 27

PART 2

Explosion reliefs for ducts and elongated vessels 32

Scope 33

Principle of relief venting for ducts and long vessels 33 Size and spacing of explosion reliefs for stationary gases or gases moving at speeds of less than 10 ft/s. 35 Size and spacing of explosion reliefs for gases moving at speeds from lOto6Oft/s. 38 Vent closures 38 Data for gases other than propane 42

Bibliography 44

1

Introduction

1 The flame arrester and the explosion relief are two well recognised devices that are often used together for the purpose of minimising the spread and effects of gas and vapour explosions in industrial plant. Numerous specific applicatiois of these devices have been investigated in research organisations and a considerable amount of useful information has become available as a result of this work. It was apparent, however, that more information was required concerning the action of flame arresters and explosion reliefs in a wide range of circumstances, before adequate advice concerning their installation and use could be given to firms engaged in processes that were associated with the danger of explosion in the process plant. To obtain this information the Director of the Joint Fire Research Organisation undertook, at the request of the Ministry of Labour, to conduct research investigations concerning the performance of flame arresters and explosion reliefs.

2 In this publication the Joint Fire Research Organisation presents the results of its investigations in the form of a guide to the use of the devices in question. This guide is published in the expectation that it will prove to be of value to a plant designer or to the occupier of a factory faced with the necessity of providing precautions on plant in which a gas or vapour explosion may occur. Any questions concerning the provision of flame arresters and explosion reliefs on plant in a factory should be directed to the Principal Inspector of Factories.

PART 1

Flame arresters for industrial pipe and duct

systems

3 The objectives of this part of the guide are to present the technical

information available on flame arresters and to put forward recommen-

dations for industrial users so that the installation of flame arresters in

industrial plant and equipment can be on a safe and economic basis. It is

addressed principally to those who are concerned with the prevention and mitigation of explosions in flammable gases or vapours present in industrial plant. The use of arresters to protect plant is applicable to a wide range of industries handling flammable solvents or gases and in-

cluding the petroleum, chemical, and gas industries.

4 Unless stated otherwise the technical information on arresters was obtained during investigations at the Joint Fire Research Organisation. The guide is intended to give the maximum amount of information avail-

able but it cannot be comprehensive because there are a number of aspects on which information is lacking. These aspects become apparent below.

Function of flame arresters

5 Most flame arresters, or flame traps, consist of an assembly contain-

ing narrow passages or apertures through which gases or vapours can flow, but which are too small for a flame to pass through. When the flame enters the arrester it is sub-divided into flamelets, which must be

quenched if it is not to propagate through to the other side of the arrester. The various types of flame arrester are described in greater detail below.

6 Flame arresters are built into industrial plant and equipment in order to prevent the unrestricted propagation of flame through flammable gas or vapour mixtures. Depending upon the design of the plant, arresters

may be required to confine the passage of flame to a part of the plant, or to prevent flame from entering or leaving the plant.

7 The uses of flame arresters and of relief vents are often complemen- tary; a flame arrester restricts an explosion to a part of the plant but does not primarily reduce the pressure of the explosion, whereas explosion reliefs can keep the pressure down but do not prevent the explosion from

2

propagating throughout a plant. A combination of both can thus be used to limit the spread and effects of an explosion.

S The type of flame to be quenched will also depend upon the design of the plant, as well as on the combustible gas or vapour present. Apart from ordinary flame propagation, or deflagration, detonations may occur, which are fast flames with associated shock waves, and cool flames, which propagate slowly. The various types of flame, and suitable arresters, are considered in more detail later.

9 Arresters are not suitable for extinguishing open fires in gases or liquids such as may arise from spillages.

Examples of the use of flame arresters

10 The first systematic use of a flame arrester was the wire gauze in Davy's miner's safety lamp, developed early in the nineteenth century. In modern industry the uses of flame arresters are diverse, and because of the large scale of modern plant and equipment the arresters may have to withstand far more arduous explosion conditions.

II The types of plant in which flame arresters are now used cover such a wide range that a complete classification cannot be attempted here. The following are some of the commoner installations:

(a) flare stacks disposing of waste gases;

(b) vent pipes of storage tanks containing flammable liquids; (c) ducting systems carrying solvent vapours to a recovery unit; (d) pipe systems conveying gases to burners, furnaces or torches; (e) crankcases of small internal combustion engines; (f) exhausts of internal combustion engines working in flammable atmospheres.

Merits of flame arresters

12 As a means of protecting plant against explosions, flame arresters have various attractive features. For instance, suitable types are available for use against gas, vapour or mist explosions. Flame arresters are usually easily installed by means of standard fittings into pipework etc. and when installed they require little servicing or maintenance, particularly if the plant is dust free. Should they be damaged by a explosion or during maintenance, they can usually be replaced easily. In general they

3

are cheap compared with the cost of the plant that they protect. It is

important that they are well maintained and securely anchored in an accurate seating so that an explosion does not displace them or propagate past the seating. The protection then given is continuously available, and the majority can be designed so that an explosion flame would be quenched from whichever side the flame approached in either stationary or moving gas streams.

13 The main disadvantages of flame arresters arise because of their necessarily fine structure. There is inevitably some resistance to the flow of gas, but it can be minimised by enlarging the diameter of the arrester and its fittings so that the linear flow rate through the arrester, and hence the pressure drop, is reduced. Because of their tendency to act as filters, flame arresters may block up if the gas passing through them is dirty or contains a substance that can freeze out at ambient temperature, and it is not advisable to use them in dusty atmospheres. In some cases blockage due to dirt etc. can be avoided by using a disposable pre-filter upstream of the arrester. Blockage can also arise from corrosion of the arrester. and it may be necessary to use a more resistant material for construction, at increased cost.

14 Even under clean conditions regular inspection and maintenance of arresters is necessary to ensure continuous safety.

Principles involved in the specification of flame arresters

15 In order to specify a suitable flame arrester for a particular installation three major factors must be taken into account. The factors are: the types of flame arrester available and convenient, the combustible mixture involved and the design and dimensions of the plant or equipment. The three major factors will be outlined separately; they will be followed by short descriptions of the mechanism of operation of flame arresters and of the procedure to adopt in specification of flame arresters for particular installations.

Types of flame arrester

Crimped metal arresters

16 There is a range of commercial flame arresters made from crimped metal. One method of manufacture involves the building-up of the arresters by alternate layers of crimped and flat thin metal ribbon; cellular

4

structures with approximately triangular cells are then obtained (Fig I). The arresters can be made circular, rectangular or square depending upon the shape of the pipe etc. in which they are to be installed, and they are available in a range of crimp sizes, thicknesses or depths, and diameters. The advantages of this type of arrester are that they can be manufactured to within close tolerances, that they are sufficiently robust to withstand mechanical and thermal shock, and that, because usually only about 20% of the face of the arrester is obstructed by the ribbon, any resistance of the arrester to the flow of gas through it is minimised. It is important that the layers of ribbon should not spring apart during routine use or in an explosion, because such movement would in effect increase the crimp size. As the effectiveness in quenching flame diminishes rapidly with thin arresters, they should be at least 05 in thick. Provided that the arresters are properly constructed, they can be

designed to quench violent explosions.

17 The weight of crimped metal arresters depends upon the method and materials of construction, but, as a rough guide, a steel arrester I in thick and 1 ft square in area would weigh about 10 lb including the supporting frame and strengthening bars. Smaller arresters are relatively heavier. The weight of crimped metal alone is best found by calculations based on the size of the arrester, the crimp area, and the metal ribbon thickness.

Fig i Crimped metal arrester

5

Wire gauze arresters 18 Wire gauzes were used in Davy's miners' lamps and they have been used as flame arresters in various applications ever since. The main advantages of gauzes are their cheapness, ready availability, and ease of fitting. Their disadvantages include their limited effectiveness at quenching flames, the ease with which they are damaged and the resistance of fine gauzes to the flow of gas through the meshes. Examples of commer-

cially available gauzes are given in Table I. which also includes values of the proportion of free area of gauze (i.e. area not blocked by wires); with fine gauzes the blockage is well over half the area of the gauze. The calculated weight per unit area of steel gauze is also included.

19 In practice it is unlikely that gauzes finer than 60-mesh could be

installed successfully. because thc mesh is so fine that it would very quickly become clogged by dirt etc. under most industrial conditions.

20 Gauzes can he combined into packs, and if the gauzes are all of the same mesh width they: .arc more effective flame arresters than single gauzes, but the increased effectiveness is limited (see paras 74 to 77). Combined packs of a coarse and a fine mesh gauze are less effective arresters than the fine gauze alone. A disadvantage of gauze packs is that good contact between gauze layers is necessary but may be difficult to guarantee in practice.

21 As gauzes have limited effectiveness in quenching flames, they are

not suitable as arresters for dealing with violent explosions. They have been used for milder explosions, including the case when quantities of hot combustion products are discharged through the flame arrester, e.g. on crank cases of small internal combustion engines.

Tabk I Characteristics of wire gauzes

Mesh width Proportion Standard tWidth of of gauze noi Weight, unit

Gauze wire aperiure Wire biocked by area of mesh gauge in gatize) diameter wires steel gauze number (5.h% g) (in) in) (free area) (1b1/ft)

is 28 0042 0-0i5 056 025 28 28 0021 o-oi 034 040 30 32 0023 0-Oi 034 023 40 34 0-016 0-009 040 0-2i 60 37 0010 0007 034 019 80 39 0008 0005 035 013

As he use of the pound ih( as nuns of torce's stnicils incornec t. th eunit catted the pound-force (thE) has been used tic the force which. site', act,ng o,,a beds ''[mass one potted. vies ii an acceleration equal to that of standard gravity

6

Perforated plate arresters 22 Metal plate perforated with circular holes is a commercial product and is sometimes used as a flame arrester (Fig 2). The diameters of the holes and the thicknesses of the plates that are available cover a fairly wide range, but the perforated plates most easily obtained for flame arresters have hole diameters and thicknesses similar to those of coarse

gauze flame arresters. Perforated plate arresters have greater mechanical

strength and are less liable to overheat than gauze arresters, but the proportion of the area of metal that is available for gas flow is even less than that of the corresponding gauzes. As with wire gauzes, thin perforated plate arresters are not suitable for violent explosions.

Parallel plate arresters 23 These arresters, which are constructed of unperforated metal plates or rings arranged edgewise to the gas flow and are separated from each other by a small spacing, can be made in a range of sizes. The explosion is quenched by the closely separated plates or rings.

24 The advantages of parallel plate arresters are that they are robust, and can withstand violent explosions, and they can be dismantled for cleaning. Their main disadvantages are their weight and their resistance to gas flow. One application for which they are particularly used is in the exhaust systems of internal combustion engines.

Packed tower or pebble box arresters 25 Flame arresters consisting of a tower, or other container, filled with pebbles, Raschig rings etc. have been used for a considerable time. The sizes of the apertures available for flame quenching depend on the sizes of the pebbles or rings, and the effectiveness of the arrangement can sometimes be increased by wetting the packing with water or oil. The advan-

tages of these arresters are that they are easily assembled and can be dismantled for cleaning, and that they can be made sufficiently robust to withstand severe explosions. They have disadvantages in that they may be large, they have a relatively high resistance to gas flow, and the size of the passages through the arresters is not directly controlled. Movement of the packing during an explosion could lead to failure of the arresters.

Sin tered arresters 26 Discs or hollow cylinders of sintered metal or ceramic are available commercially and have been used as flame arresters (Fig 3). The apertures through the sintered material can be made very small and, for this reason, such arresters are able to quench very violent explosions provided that they are given sufficient mechanical strength. Their disadvantages

7

include a high resistance to gas flow and a tendency to block easily. Sintered metal arresters are used in situations where the gases are clean and the flow resistance is unimportant. e.g. in protecting gases being discharged from storage cylinders. Particular care is required to ensure a secure anchorage of the siritered element to prevent leakage round the element caused by the impact of the shock wave.

wisessisli— .isiiisleiiisssiii. n.•isiil.ssiSsssissi I• sill ...sI...ssssseIss I. .iiIliifli•iiiislsiiII I. Is •lii.iIe•Ii5•5 Isles.

•ssil•IsisIsisis•eilsIil is. .sI5i5sisiisIiSs5I•5 C ..s..t................... i.e..... i.s...••iIl•s.s. •esiIsIs .......s...l..ssl •..I......i.s..•i••I••• I•15•.• S••se• I•ssssSs .. ii ..•......... .• 515551.111 ilSl.I5i..5Is leslIe II III •lIllilliissiiIssIil.i.i. •.s............... is sees sili....ii...ii.. is cliii ill ii .i.ii..lIi I •I I.SIs S..

8

Fig 2 Perforated plate arrester (magnified)

Fig 3 Sintered metal arrester (magnified)

Wire pack arresters

27 Flame arresters can be made by compressing a pack of wire that has either been assembled at random or has been knitted. Arresters may also be made from packs of expanded metal. In either case the apertures in the arrester depend on the degree of compression of the pack as well as on its initial state. Arresters made in this way can be reproduced only with difficulty.

Hydraulic arresters 28 Whereas all the flame arresters so far considered have had a solid matrix, the hydraulic arresters contain a liquid, usually water, that breaks up the gas stream into discrete bubbles. Flashback of flame is thus prevented. In addition the arresters may incorporate a mechanical non- return valve. This type of arrester requires regular supervision to ensure that the liquid level is maintained, and is usually effective in quenching flames propagating only in one direction. Because the gas is wetted by the arrester, there may be condensation troubles downstream of the arrester. The gas flow rate has to be limited to ensure that the stream is effectively broken into bubbles.

Combustion properties of fuels

29 The two combustion properties of fuels that must be taken especially into account when dealing with flame arresters are: the standard burning velocity and the quenching diameter. These properties influence respect- ively the severity of the explosion and the ability of arresters to quench flames.

30 The standard, or adiabatic, burning velocity is a fundamental property of a gas mixture and is the velocity with which a plane flat flame

propagates into unburnt gas mixture without heat loss. The burning velo-

city depends on the nature of the gas, the amount present in the mixture, the pressure and the temperature. The burning velocity is a fundamental property but the flame speed is not. The flame speed is the speed at which a flame propagates along a pipe or other system and depends not only upon the burning velocity but also upon the flame shape, turbulence and motion of the gas mixture. In explosions in ducts and pipes the flame speed is much faster than the standard burning velocity. It is generally true that flames propagating through gases or vapours having a high standard burning velocity will be faster than flames propagating under the same external conditions in materials of low burning velocity. Flame

9

speeds in various gases in pipes or ducts are not necessarily in the same ratio as the standard burning velocities of the gases.

31 Maximum values of the burning velocity for some gas mixtures at atmospheric temperature and pressure are included in Table 2; a comprehensive list has been published by Fiock' which enables comparisons to be made between fuels. The majority of saturated hydrocarbons and solvent vapours have burning velocities in air that are close to that of propane. The following common gases, or mixtures of vapours or gases with oxygen or oxygenated air, have higher burning velocities: ethylene, town gas, acetylene, hydrogen. Flame arresters designed for propane/air mixtures are also suitable for the majority of saturated hydrocarbons and solvent vapours in air, but are not suitable for faster-burning gases and vapo urs.

32 Acetylene has had a number of specialised uses for a considerable time. The factors influencing the explosion hazard of acetylene arise partly from the material and partly from the uses to which it is put. Because these factors are not of general application, particularly the possibility of decomposition flames, the provision of flame arresters for acetylene is a specialised topic and is outside the scope of this guide.

33 The quenching diameter is a characteristic of a gas mixture and is the minimum diameter of tube through which a flame in the stationary gas mixture can propagate indefinitely. Under these conditions the flame

speed settles down to a value approximately equal to the standard burning velocity. The quenching diameter does not depend on the material of the tube wall although, for other reasons, a strong material is necessary

Table 2 Standard burning velocities and quenching dtameters of some gas mixtures

Standard burning Quenching diameter Gas mixture velocity (ft s) (in)

Methane/air I 0145 Propane/air 1-5 0-105 Butane/air 1-3 0-110 Hexane/air 1-3 0-120

Ethylene/air 2-3 0-075 Town gas/air 3-7 0-080t Acetylene/air 5-t 0031

Hydrogen/air I -0 0-034 Propane/oxygen 13-0 0-015

Acetylene/oxygen 37-0 0-005

Hydrogen/oxygen 39-0 0-012

Town gas cons-a fling 63' hydrogen Town gas containing 5r hydrogen

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for flame arresters. The importance of the quenching diameter for practical situations is that it represents the upper limit of aperture diameter in flame arresters, beyond which flame quenching cannot reliably occur whatever the thickness of the arrester. In practice, because explosion flames in ducts and pipes usually travel much faster than the standard burning velocity, the apertures in the arrester need to be smaller than the quenching diameter by at least 5O and then the effectiveness of the arrester increases with its thickness.

34 Values of quenching diameters are included in Table 2: the quenching diameter varies with the gas mixture ratio, and the values in Table 2 are the minimum values for the various gas mixtures at atmospheric temperature and pressure. Values for other gases and vapours can be obtained from a comprehensive list by Potter2; his values are for quenching distance, and the quenching diameter is obtained on multiplying by a factor of 154. The majority of saturated hydrocarbons and solvent

vapours have quenching diameters in air that are close to that of propane, but faster-burning gases and vapours have smaller quenching diameters.

35 The quenching diameter must not be confused with the safe gap stipulated for flameproof electrical equipment. The safe gap is the maximum clearance permissible between flanges, etc. to prevent propagation of explosion from within the equipment, and it is appreciably smaller than the quenching diameter because of the high explosion pressures involved.

Effect of design of plant

36 Although the types of plant and equipment using flame arresters vary widely they often have some common features. Frequently plant is not completely enclosed, and it is usual for a part to be open to atmosphere or to a relatively large reservoir either through a duct opening, or a restricted opening such as a nozzle, or through an explosion relief vent. Thus if an explosion occurs there is usually a preferential direction for the gas to move as soon as the pressure begins to increase. This prefer- ence affects the performance required of the flame arrester. Figs 4 to 6 illustrate three simplified systems, represented by a duct sealed at one end and open at the other, and containing a flammable mixture on both sides of the arrester; similar simplification can be applied to more compact or complex systems, but the behaviour in ducting is simpler to visualise.

37 When there is no feed of gas or vapour mixture into the duct, the mixture is stationary when ignited. If ignition of the mixture occurs near

II

the entrance of the duct (Fig 4) or a flame flashes back into the duct, the flame propagates up to the arrester through a stationary or slow-moving mixture. The hot combustion products are mostly discharged through the entrance of the duct, and not through the arrester. Therefore the arrester has only to quench a flash of flame and the amount of heat to be transferred to do this is not large, although the rate of transfer of heat must be high because the time available when the flame is in contact with the arrester is short. If ignition of the mixture occurs at a position remote from the entrance of the duct (Fig 5) the expansion due to the combustion causes the unburnt mixture ahead of the flame to move down the duct through the arrester. As the flame arrives at the arrester it is propagating through a fast-moving mixture tending to carry it through the arrester. When the flame is quenched, most of the hot combustion products will not pass through the arrester but will cool to the walls of the duct. Thus, under the conditions represented in Fig 5 the arrester must be able to quench a fast-moving flash of flame and be sufficiently strong to withstand the pressure arising from the motion of the gas. If ignition of the mixture occurs near the arrester (Fig 6). the flame propagates in two directions. Soon after ignition a slowly moving flame arrives at the arrester; meanwhile another flame propagates towards the closed end of the duct and the hot combustion products generated are expelled through the arrester. The arrester must be sufficiently massive to withstand the hot exhaust and sufficiently strong to withstand the pressure due to the moving gas.

38 In practice the gas or vapour may be in motion when ignited and may continue to flow during and after an explosion. Ifin Figs 4 to 6 the flow is from left to right, then there is a possibility of flames stabilising in the duct. In Fig 4, if the gas is flowing at a high speed, the explosion may not be able to propagate back against the stream; it will then either be swept out of the duct entirely or may stabilise on the entrance of the duct or on a protuberance inside the duct. In any case the flame arrester is not affected. However, if the gas velocity is lower so that the flame can travel back against the flow, the flame will stabilise on the arrester and heat it unless the gas flow is quickly turned off. In Fig 5 the flame could stabilise either on the inlet port of the gas stream or on a protuberance in the duct; in either case a flow of hot combustion products would pass through the arrester. A similar situation can arise in the arrangement shown in Fig 6.

39 Thus, whenever a flame arrester is installed in a system in which

flowing gas or vapour can ignite, the danger of overheating of the arrester may arise. To avoid this risk a detector should be installed which

12

Ignition

Hot combustion products

crr4Tr Unburnt mixture

Fig 4 Ignition at open end olduct

Fig 5 Ignition at closed end of duct

Fig 6 Ignition near flame arrester

will automatically operate a valve to cut off the flow. Automatic detectors are available as commercial products. 40 It shouLd be emphasised that the systems shown in Figs 4 to 6 repre- sent only simplified versions of actual plant, the behaviour of which may be more complex. If it is not obvious at which point ignition is likely to occur, an arrester installed in an actual plant may have to be designed to face a combination of the conditions of Figs 4 to 6.

Mechanism of operation of flame arresters

41 The information available from experiments indicates that flame arresters consisting of an assembly of apertures extinguish flames by cooling. Heat passes from the hot gas in the flame to the cold wall of the aperture. In cases of simple flashback the amount of heat transferred is

13

4- Flame arrester

Ignition

W,H. H_= 4-

Flame arrester

Ignition

Lt__ + Flame arrester

small, it causes little change in temperature of the wall, and is indepen- dent of the material of which the wall is made. However, in cases where hot exhaust gases are forced through the arrester, as in plant systems based on Fig 6, or where flame can stabilise on or near the arrester, the heat transferred to it may be considerable. In such cases, where more than the quenching of a transient flame is involved, the mass and the material of construction of the arrester are important.

42 Hydraulic arresters operate on different principles, relying on dis-

continuity of the gas or vapour stream, and cooling of the flame is not their primary function.

43 When the gas or vapour in the system is initially at a pressure or temperature other than atmospheric, or if the gas pressure can rise appreciably during the course of the explosion, the performance of the arrester is affected. As regards changes in pressure, the process of flame quench-

ing becomes more difficult as the pressure increases, even though flame

speeds may not be much affected. The difficulty arises because more heat is released per unit volume of gas mixture as the pressure is raised. The

increase in the amount of heat released is proportional to the increase in

the absolute pressure. The effect of changes in gas temperature is more

complex. The standard burning velocity increases with temperature, but the density of the gas is reduced and causes the heat released per unit volume of gas to diminish. The quenching diameter is found to be

approximately inversely proportional to the square root of the absolute

temperature: i.e. the quenching diameter decreases as the temperature rises.

Specification procedure for flame arresters

44 The recommended procedure is as follows:

(a) consideration of the performance required of the arrester, taking into account the probable position of the ignition (Figs 4 toG)

(b) estimation of the flame speed at the arrester, which depends on the

plant design and dimensions as well as on the gas or vapour (c) selection of an appropriate flame arrester, capable of quenching the

flame at the estimated speed and being able to withstand the heat transferred to it

(d) consideration of any other factors concerned, e.g. provision of heat detectors and automatic cut-off valves, etc.

14

Specification data for flame arresters

Flame speeds

Ignition near open end

45 Relevant instances include flashback into vent pipes or flare stacks, and the entry of flame into enclosed equipment (Fig 4).

46 In pipes or ducting the flame speed at the arrester depends on the gas or vapour mixture, and also on the distance between the arrester and the point of ignition or the open end of the pipe if the source of ignition is outside. This distance is known as the 'run-up'. Table 3 shows the values of flame speeds to be used for various mixtures and run-up lengths in straight smooth pipes without obstacles. For intermediate run-ups, the higher neighbouring value of flame speed should be used. It is generally advantageous to keep the run-up as short as possible. The flame speeds for propane can be taken as typical of saturated hydro-carbons and many solvent vapours (see Table 2 and Reference I). Where values of flame speeds are given in Table 3 for ignition near the open end of the pipe, the explosion pressures are relatively small and may be neglected.

47 Detonation velocities may be calculated approximately from equation I below. The velocities depend on the gas mixture composition and the pipe diameter, if narrow, but not on the run-up length and only slightly on the gas pressure. Detonations in narrow pipes are slower than predicted by equation 1, because of cooling to the walls, and this fact will provide a margin of safety.

Equation I

V=300\/(y2_ l)Q where V is detonation velocity (ft/s)

y is ratio of specific heats of gas mixture at combustion temperature

and Q is heat of combustion per gram of gas mixture (cal/g)

48 The detonation velocity depends on the composition of the gas mixture; for propane and other saturated hydrocarbons, and for many solvent vapours, a value of 5800 ft/s can be used for mixtures with air. The detonation velocity for town gas/air and for hydrogen/air mixtures can be taken as 7000 ft/s.

49 The values of flame speeds in Table 3 are for pipes and ducts up to I

ft in diameter. Little corresponding information is available for wider

IS

Table 3 Flame speeds for various gases and run-up-lengths; ignition near open end of pipe

(up to I? in diameter)

Flame speeds (U's)

Run-up length Ut) Gas mixture 5 10 35

Propane/air 16 230 330 330

Ethylene/air tOO 230 5 D

Town gas/air tOO D D

Hydrogen/air — D D D

• For a run-up length of lens than 3 In a flane speed o14 fi:s can he assuned — Indicates no values available o indicates no values available but detonation ltkely. wtth de snaIl on velocities up to 7 In

ducts, but the flame speeds are unlikely to exceed those in the Table for ducts up to 3 ft in diameter.

50 For duct lengths greater than 35 ft. where the point of ignition is near the open end, the alternatives are:

(a) assume any gas mixture will detonate and design the pipe and flame arrester to withstand a pressure of 500 lbf/in2*.

(b) install a flame arrester at not more than 35 ft from the point of ignition

(c) install explosion relief vents. e.g. vents equal in area to the cross section of the duct, and spaced every fifty diameters, will keep

propane/air flame speeds down to the values in Table 3 and the

explosion pressure down to 2 lbf/in2. The vents should be sited so

that, as the flame approaches the arrester, the distance between the last vent and the arrester is less than 35 ft.

51 The flame speeds in Table 3 are for straight pipes or ducts contain-

ing no obstacles. For run-up distances of up to 10 ft the propagation of flame past a small obstacle (not more than lO% of the cross section of the duct) or through a single bend or tee would have little effect on

propane/air flame speeds. In branched pipe systems the run-up should be

measured from the likely ignition source, and not from the junction of the branch with the main pipe. For larger or multiple obstacles, longer

run-ups or faster-burning mixtures, the procedure recommended on page 24 for obstacles should be followed. It is a good general rule to site flame arresters away from obstacles, bends, etc. so that the flame does not

* See footnote to Table I

16

propagate past an obstacle before reaching the arrester. If the gas mixture is flowing along the pipe or duct, and continues to flow during and after the explosion, the flame speeds given in Table 3 for propane/air mixtures can still be used. There is no corresponding information to enable guidance to be given for ethylene and other faster-burning gases and vapours. At high flow rates (e.g. about 15 ft/s in a 3 in-diameter pipe) the flame will not be able to propagate against the stream and will stabi- use on the entrance to the duct. At lower flow rates the flame may propagate as far as the arrester and then continue to burn there. It is important that a detector, preferably linked to an automatic cut-off valve, should be installed near the arrester to prevent damage to it (see pages 34 and 35).

52 In many installations it is desirable to use a flame arrester that is wider than the pipe or duct diameter, to reduce flow resistance during normal working. If the expansion piece leading to the arrester housing is too divergent it may lead to the generation of turbulence and to accelerated flames. There is no data available on the velocities of flames under these conditions, but any effect would be minimised by expanding the duct diameter as slowly as possible, preferably at an angle of about 100. With established detonations, however, the propagation velocity may be temporarily reduced; Cubbage3 has shown that with town gas/air detonations a sudden expansion of the pipe causes the detonation momentarily to degenerate, and flame arresting is thus made easier. There is little comparable information available for other gases and vapours.

53 All the flames considered so far have been mixtures of the gas or vapour with air. If, instead of air, pure oxygen is involved, or even air enriched with oxygen, there is a great danger of the flame becoming a detonation. Detonation velocities are up to 12000 ft/s with pressures up to 1000 Ibf/in2. Special precautions must then be taken, including safe siting of the plant and control of access.

Ignition near closed end

54 Relevant instances include ignition within a long pipe system, the flashback through a narrow jet into a pipe, and the discharge of flame from enclosed equipment (Fig 5).

55 As previously, the flame speed at the arrester depends on the gas or vapour mixture and on the run-up. In addition, because flame speeds are generally higher with ignition near a closed end of a pipe than elsewhere it may be necessary to take account of the pressures developed in the

17

TubIs 4 Flame speeds fbr various gases and run-up lengths: igtiitlon near closed end of straight pipe (up to 12 in diameter)

Flame speed (fts)

Run-up length (Ii) Gasmixture I 5 10 35

Propane/air 110 380 420 490

Ethylene/air D Town gas/air D D

Hydrogen/air t) D 0

- indtcates nova luau available

0 indicates no Va i ues ava labia but detonation tikeb. with detonaiton vetoci ties up to 7)0] ft site Equation I - page t 5)

explosion, because these affect the performance of flame arresters. The values of flame speeds to be used for various gases and run-up lengths in

straight, smooth pipes without obstacles are given in Table 4; in the cases where numerical values of flame speed are given the explosion pressures are relatively small and may be neglected.

56 Cubbage3 has shown that with town gas/air mixtures the run-up distance for detonation increases with the diameter of the pipe. With a I

in-diameter pipe the distance was 17 ft and with a 2 in-diameter pipe the run-up was about 24 ft. Similar behaviour may occur with other gases. The values of flame speeds in Table 4 are maximum values for pipes and ducts up to 12 in in diameter. Within the range of diameters tested, maximum flame speeds for a given run-up length are obtained with diameters well below 12 in. No information is available for wider ducts, but the flame speeds are unlikely to exceed those for ducts up to 3 ft in

diameter.

57 When the duct or pipe length exceeds 35 ft and ignition may occur anywhere along its length, the alternatives are:

(a) assume any gas will detonate and design the pipe and flame arrester to withstand a pressure of 500 lbf/in2

(b) install flame arresters at intervals not exceeding 35 ft

(c) use explosion relief venting. The amount of venting required depends on the diameter of the ducting, as well as on its length, and so it cannot be specified simply. Details may be obtained from the guide on the design of explosion reliefs for ducts. The vents should be sited so that, as the flame approaches the arrester, the distance between the last vent and the arrester is less than 35 ft. If the explosion pressures are kept down to I lbf/in2 by the use of vents the flame speeds are

18

not likely to exceed the values in Table 4 which may then be safely used.

58 The flame speeds in Table 4 are for straight pipes or ducts containing no obstacles or bends. The presence of even a small obstacle, about 5 of the cross section of the duct, accelerates the flame and increases the explosion pressure appreciably. If there isa single 90° bend in the pipe or duct the explosion pressure may rise by a factor of 10 and the flame velocity be substantially increased. There is insufficient information available for the increases in flame speed and explosion pressure to be predicted for various obstacles, etc. and gases. Under these conditions the alternative procedures are:

(a) assume detonation occurs and design the pipe and flame arrester accordingly (see paras 50 to 53)

(b) use explosion relief venting liberally; e.g. with propane/air mixtures it is necessary to install an explosion relief equal to the cross section of the duct every six diameters, including one vent very close to the obstacle, etc. No guidance on the economic amount of venting can be given for gases burning faster than propane/air.

59 The necessity of dealing with explosions enhanced in violence by obstacles or bends can often be avoided if the position of the source of ignition is known. Flame arresters should then be placed as close as possible to the source of ignition to prevent the flame from propagating as far as the obstacle, using the data in Table 4.

60 If the gas mixture is flowing along the pipe or duct, and continues to flow during and after the explosion, a modified procedure may be necessary. Experiments have shown that with a pipe of 25 in diameter, and gas flows up to 20 ft/s, the values of flame speed in Table 4 may still be used, provided that the gas velocity is added to the flame speed. With wider ducts and with higher gas velocities there is evidence that the flame speeds and explosion pressures may increase considerably. There is insufficient information for detailed guidance, but the recommended procedure is to install relief venting sufficient to reduce the maximum explosion pressures to below I lbf/in2 by methods specified in the guide on the design of explosion reliefs for ducts. The flame speeds are not then likely to exceed the values in Table 4. There is a danger of flame stabilising within the system and overheating it, if gas mixture continues to flow after the explosion, and suitable preventative measures as outlined in para 91 should be taken.

19

61 When a pipe or duct is expanded to accommodate a flame arrester of greater diameter there is a possibility of flame velocities being increased

by turbulence generated at the expansion piece. There is no data available on the velocities of flames under these conditions, but any effect would be minimised by expanding the duct diameter as slowly as

possible, preferably at an angle of about 10. With established detonations, however, the propagation velocity may be temporarily reduced (see

para 52).

62 With gases or vapours mixed with oxygen-enriched air, or pure oxygen, there is a great danger of detonation occurring. Detonations with velocities up to 12 000 ft.s should therefore be expected, accom-

panied by pressures up to 1000 lbfin. Special precautions must then be

taken, including safe siting of the plant and control of access.

Ignition near arrester

63 A relevant example is involved in preventing flame from leaving an enclosure, following ignition inside the enclosure and near the outlet (Fig 6). The arrester has two functions, as explained in paras 36 to 43: to quench flame and to absorb, without overheating, heat from any burning or burnt gas ejected through the arrester. For the former operation the information in Table 4 should be used.

64 The absorption of heat from the burnt gas, by a flame arrester, cannot at present be dealt with precisely but an approximate method is

available. With propane/air flames the removal from the combustion products of 46 of the heat generated was estimated to be necessary to prevent ignition of unburnt mixture by the hot gases. This percentage is

equivalent under the worst conditions to 400 cal/I. or to 45 BThU/ft3, of unburnt propane/air mixture. Similar values could be expected for other saturated hydrocarbon/air mixtures and for some solvent vapour/air mixtures, excluding those with low ignition temperatures such as carbon disulphide and diethyl ether. The procedure to be adopted is, therefore, to estimate the volume of gas mixture that will be burnt inside the enclosure and hence to derive the heat to be absorbed by the flame arrester. The appropriate flame arrester must be able to absorb the heat without suffering damage and without exceeding an average temperature of 500°C (930°F) for steel or cupro-nickel. For a steel arrester of specific heat 0-1, 05 lb of arrester element is required per 12 in3 of gas mixture.

65 If there is a flow of gas through the enclosure during the explosion then flame may stabilise inside the enclosure and cause further heating of the arrester. Suitable preventative measures are outlined in para 91.

20

Ignition in a closed system 66 A relevant instance is involved in preventing flame from propagating throughout an enclosed plant.

67 If the volume through which the flame can propagate is a small

proportion of the total enclosed volume, then the explosion can vent into a relatively large volume and the data in paras 50 to 62 can be used.

68 If the volume through which the flame can propagate is a substantial proportion of the total volume of the enclosure, then the explosion may be accompanied by intense vibrations of the flames. Thus although there is evidence that with propane/air flames the mean speeds would be less than the values in Table 4, the actual speeds of flames on reaching the arresters could be considerably greater because of the vibratory motions of the flames. In addition, during the course of an explosion in a closed

system there may be substantially increased pressures, so that by the time a flame reaches the arrester it may be propagating through a compressed gas mixture (see paras 69 to 71). Because of the increased vibratory motions and pressures, and a lack of information on the behaviour of flame arresters under these conditions, the only safe procedure for practical situations is to assume that detonation velocities would be attained. Cubbage3 has shown that town gas/air mixtures will detonate in closed pipe systems, and Table 4 can be used for town gas and hydrogen. With gases or vapours mixed with oxygen-enriched air or pure oxygen, detonations should be expected and the system designed accordingly. Special precautions must then be taken, including safe siting of the plant and control of access.

Variation of pressure or temperature 69 If the gas or vapour in the system is initially at a pressure or tem-

perature other than atmospheric, or if the gas pressure can rise appreciably during the course of the explosion, allowance must be made.

70 For moderate increases in pressure, allowance can be made by mul-

tiplying the flame speeds for atmospheric conditions by the ratio of the absolute gas pressure to atmospheric pressure. If the pipe system has little explosion venting so that the explosion pressures are high, it is advisable to use the procedure described above and assume that detonation velocities would be attained.

71 The effect of changes in gas temperature is more complex and, although the standard burning velocity is known to vary regularly with

temperature, there is little data on the variation of flame speeds with

21

temperature. The recommended procedure can only be to use values of flame speeds for atmospheric temperature and then to increase these values by a factor proportional to the square root of the absolute temperature. For practical purposes the effect of temperature may be neglected within the range between atmospheric temperature and 40°C. At temperatures below atmospheric the values of flamespeeds in Tables 3 and 4 may be safely used.

Cool flames 72 Some organic compounds, notably aldehydes and ethers, are able to sustain cool flames. These flames are of low temperature and produce only partial combustion of the vapour; they usually occur in rich vapour/air mixtures. Other compounds are able to sustain cool flames if their vapours are initially at pressures and temperatures other than atmospheric. Cool flames may lead to violent explosions if they propagate into vapour/air mixtures in which normal flame propagation can occur.

73 Information on the quenching of cool flames indicates that flame speeds are low and that the cool flames are about as easily quenched as normal flames propagating at the same velocity. As flame arresters are usually installed to deal with the much more rapid normal flames they should be able to cope with any cool flames that may occur.

Flame arrester capability _________________________________

Crimped metal, wire gauze, perforated plate arresters 74 The maximum speeds of flames other than detonations that these arresters are able to quench can be calculated from equation 2 below.

Equation 2

V = 0'S N where V is the flame speed (ft/s)

a is the free area; i.e. the proportion of arrester surface area not blocked by metal ribbon, wires, etc. (Table I)

y is the thickness of the arrester (in inches); for a single layer of gauze y is twice the wire diameter

d is the diameter of the aperture (in inches)

For a square or circular aperture, d is the width or diameter of the aperture. With crimped metal arresters the apertures are often approxi-

22

mately triangular, and the equivalent hydraulic diameter (_4xarea \ perimeter should be used. For right-angled isosceles triangles the value of d can be taken as 083 x crimp height (measured exclusive of ribbon thickness).

75 There are several importantconditions to be observed in the use of equation 2. These are;

(a) 'd' should not exceed 50% of the quenching diameter (Table 2) (b) y' forcrimped metal arresters should be at leastO5 in

(c) the equation is valid for only single layers of gauze. Multiple layers of gauze of the same mesh are more effective, the value of V increasing by about 20% of the original value for each additional layer up to a maximum of five layers. Further increase gives no extra benefit

(d) if the pressure should rise substantially above atmospheric, then allowance must be made (see paras 69 to 71)

(e) the equation applies to crimped, gauze, and perforated plate arresters, but does not depend upon the material of construction of the arresters provided that it is neitherof low melting point nor mechanically weak

(1) the predicted value offiame speed (V) includes a safety factor to take account of variation in experimental results

Graphical representations of equation 2, for easy reference, are given in

FIg 7.

76 For detonations, which propagate at high veldcities, only the crimped metal arresters need be considered. Cubbage3 hasshown that arresters of crimp height 0017 in. are capable of arresting town gas/air detonations, and the following equa Lion was obtained:

Equation 3

y= 13 V1/5--44 where V and y are measured in the saite units as in equation 2.

77 Arres&ing of town gas/air cMonations was assisted by widening the seetion of the pipe systeni containing the arrester; details were given by Cubbage.3 Apart from the Work on town gas, there is little available Information on the specification of crimped metal flame arresters for detonations.

78 In calculating the mass of the arrester required to absorb heat from combustion products, the procedure would be to divide the quantity of

[Best available

co9

100

80

1 60

V a

40

20

0 0-1

1000

800

1 600 >

a 400

200

0 1

Arrester thickness (yl—tn

Fig 7 Relation between flame speed and arrester thickness, for dtfferent values of free area

(a) and aperture diameter (d). from equatton 2

24

0-02 0-04 0-0 0-08

Arrester thickness Cv) —in.

heat by the product of the permissible temperature rise and the specific heat of the material of the arrester; the quotient is the required mass of the arrester. lilt is practicable to wet or grease the arrester continuously, the added material may be regarded primarily as a coolant, and its effect calculated accordingly.4 The liquid for wetting the arrester is usually water or oil.

Parallel plate arresters

79 The main source of information on these arresters is the work of Loison, Chaineaux, and Delclaux5 on the arresting of methane/air explosions. They showed that an arrester of plate separation 002 in, plate length 2 in and thickness 012 in could arrest methane explosions having a run-up of 32 ft from a closed end of pipe (as in Table 4).

80 It is not at present possible to relate the flame arresting properties to the plate separation and length of these arresters, but for propane/air mixtures a plate separation of 0016 in, thickness 012 in and length 2 in would be expected to arrest flames of run-up 12 in propagating from either an open or a closed end ofa vessel or pipe.

Packed tower, sintered, wire pack arresters

81 There is little information on any of these types of arrester, and what is available is largely ad hoc.

82 I-Iulsberg6 showed that deep layers of spheres of diameter 07 cm (027 in) are able to quench mild propane/air explosions, and that spheres of diameter 04 cm (015 in) are required for town gas containing 43% by volume of hydrogen. No relationship was obtained between the flame speed and the size and thickness of the spheres in the packing.

83 The use of a tower packed with Raschig rings sprayed with water and protected with bursting discs was reported by Schmidt and others7 to quench acetylene decomposition explosions successfully. The recommended size of the rings was 35 x 35mm, the minimum height of the packing was 2 m (66 ft) and the gas velocity in the tower was not to exceed 70cm/s (23 ft/s). The volume of water sprayed should be between 25 and 5 m3/h/m1 of cross section (51 to 102 gal/h/ft2).

84 Radier8 showed that I-in gravel, in a 3-ft layer, was able to quench petroleum/air explosions if the arrester was combined with relief vents, sealed with bursting discs, in the pipelines each side of the arrester. There is no available information on the relations between the speeds of flames quenched by sintered or wire pack arresters and the dimensions of the arresters.

Hydraulic arresters

85 The principal points of design of these arresters are chiefly mechanical; the arresters must break up the gas stream into bubbles, even during an explosion; they should be sufficiently strong not to disrupt; the water supply should not fail, and the water should not be dispersed from the arresters during an explosion.

Additional factors in the specification of arresters

Flow restctance

86 In many cases where a flame arrester is required there are several types of arrester suitable for the application. Under these conditions the most economic arrester may be that with the lowest resistance to gas flow during normal running of the plant, i.e. in the absence of fire or explosion. Data on the flow resistance of most types of arrester have been correlated by Quinton9 and presented in the form of nomograms. From the nomograms the flow resistance may be predicted in terms of the gas velocity, the aperture or particle size of the arrester, its thickness and porosity, etc. The predicted value is for a dry arrester; a heavy coating. particularly of grease (see para 78), is likely to add substantially to the resistance of the arrester to gas flow.

Blockage 87 As the majority of flame arresters contain narrow apertures they may become blocked by dust or condensation carried by the gas, or by corrosion of the arrester material. Blockage due to dust, etc. can be avoided in some cases by inserting an expendable filter upstream and by regular maintenance of the arrester itself. The arrester should be cleaned by blowing air or steam through it, or by washing, and not by poking a tool through it (or 'rodding').

88 If corrosion is troublesome, the material from which the arrester is made should be changed. This procedure is likely to be more successful than attempts to coat the arrester with a protective layer. Most arresters are available in a range of metals.

89 In some instances, blockage caused by dust or by corrosion tends to reduce the aperture size of the arrester and hence to make it more effective in quenching flame. However, the increased flow resistance of the arrester may introduce additional hazards, such as higher explosion pressures and possible bursting of the plant, and so blockage is not desirable.

26

90 When arresters are directly exposed to the weather, as on the vent

pipes of storage tanks, it is advisable to prevent ice and snow from accumulating, e.g. by fitting cowls.

Detection offlame stahilisation

91 If the gas mixture can continue to flow during and after an explosion, the flame may stabilise on the arrester. A typical instance is the feed

of pre-mixed gases to furnaces. In order to prevent overheating detectors should be installed at the arresters; these detectors are available commer-

cially. The detectors should preferably be linked to an automatic cut-off valve which will stop the flow of gas within a few seconds of the flame

becoming stabilised.

Summary of specification procedure for flame arresters

Steps in procedure Page numbers

(a) Performance required of arrester II to 13

(b) Estimation of flame speed, etc. 15 to 22

(c) Selection of flame arrester 22 to 26

(d) Further factors 26 to 27

Examples The following examples illustrate the principles involved in flame arresting.

Example I A flare stack, I ft in diameter, is used intermittently on a plant for disposing of waste propane and butane gases. What flame arrester should be installed at the bottom of the stack to prevent flashback from the pilot flame into the plant, if the stack height is (a) 30 ft or (b) 100 ft?

Answer

(a) The source of ignition is at the open end of the stack. Hence from Table 3, the maximum flame speed is 330 ft/s. To keep the flow resistance of the system as low as possible, use an arrester associated with equation 2 (page 22). Because of the size of the flame speed, a crimped metal arrester is required; a convenient crimp height is 005 in. Then, in equation 2, V=330 ft/s, a=08, d=005 x 083 in. Hence

y= 142 in.

Thus a crimped metal arrester, of thickness 142 in, and crimp height 005 in is suitable.

27

(b) As the run-up is greater than 35 ft. and if it cannot be reduced by installing the arrester within this distance of the top of the stack.

the alternatives are either to design for a detonation or to install

explosion relief vents. The latter is probable a more practical propo- sition. From page 16. 12 in-diameter vents, lightly covered, should be

installed 50 ft from the ground and near ground level but above the

arrester. The arrester specified in (a) can then be used.

Example 2

It is intended to fit a layer of 28-mesh gauze over the end of a vent pipe from a storage tank containing hexane.

(a) Is this a safe procedure?

(b) Could the same gauze be used on a vent pipe from hydrogen?

Answer (a) From equation 2 and Table I

x 0015 = 116 ft/s

But, from Table 3, the flame speed with 1 ft run-up is 16 ft/s. Thus a 28-mesh gauze would not be safe when mounted 1 ft from the end of the pipe. If the gauze could be fixed on the end of the pipe so that the

run-up was less than 3 in then the flame speed would in fact be only about 4 ft/s and the gauze would be safe (see footnote to Table 3). Unless the run-up is less than 3 in. the gauze should not be used.

(b) The mesh width of 28-mesh gauze is 0021 in (Table 1), and the

quenching diameter of hydrogen/air is 0034 in (Table 2). As the mesh width is not less than 50 of the quenching diameter, the gauze should not be used.

Example 3 A 4°/s mixture by volume of propane in air is prepared in a reservoir by a

mixing apparatus and distributed by a network of I in diameter gas pipes to 100 small burner nozzles in a special furnace. What flame arrester should be installed to protect the reservoir and mixing apparatus against a flashback?

Answer

Flashback can occur at any of the nozzles, and it would be impracticable to equip each nozzle with a flame arrester. An arrester should therefore be installed downstream of the reservoir, before the pipe branches. Igni- tion is effectively at a closed end of the pipe system because of the restric-

28

tion to flow of gas caused by the nozzle, and because of the complexity of a pipe network it is advisable to assume that detonation will occur.

The detonation velocity for propane/air mixtures is 5800 ft/s. There is no data specifically on the quenching of propane/air detonations, so equation 3 for town gas should be used instead. The arrester should be of crimped ribbon, crimp height 0017 in, and of thickness 3 in.

An increased safety margin is obtained by using an enlarged diameter of arrester and housing.3 The recommended internal diameter of the arrester housing for a I in-diameter pipe is 35 in, and the face of the arrester exposed to the detonation should be Ito 2 in from the enlargement. On the unexposed side of the arrester, the housing should be reduced to the pipe diameter in as short a distance as possible. Stabilisation of flame on the arrester may also occur, and suitable measures should be taken.

Example 4

A workroom contains ten machines that give off vapours from the acetone used. The vapours are collected by short ventilation ducting from each machine which joins main trunking I ft in diameter and 100 ft in total length. The main trunking is straight and is connected to a blower which passes the vapour/air mixture to a solvent vapour recovery plant. What precautions are advisable in view of the explosion hazard in the ducting and trunking? Answer

The first precaution to take is to ensure that the capacity of the blower is

sufficient to keep the concentration of acetone vapour in the air to less

than 25% of the lower explosive limit during normal working. The acetone concentration in air should not exceed 08% by volume.

Additional precautions should still be taken in case the ducting becomes filled with a flammable acetone/air mixture because of either an unex-

pected increase in the rate of production of vapour or of an accidental reduction in the air flow.

In the system the likely sources of ignition are the machines and the blower. With the machines it is necessary to prevent flame from entering the ducting; with the blower it is good practice to install flame arresters to prevent flame from propagating from it along the trunking. In the former case a flame arrester should be installed near the entry of each of the ventilation ducts. There, from Table 3, a flame speed of 16 ft/s must be expected. Suitable arresters, calculated from equation 2, would be either a 60-mesh gauze or a crimped metal arrester of 05 in thickness,

29

with crimp height 005 in! (Note that the minimum recommended thickness ofa crimped arrester is 05 in,) The blower should be protected by an arrester on each side, and may be regarded as an ignition source at the closed end of a duct. If the arresters are within 5 ft of the centre of the blower, a flame speed of 380 ft/s would be expected under static conditions (Table 4). The blower will, however, probably be operating and causing turbulence which increases the flame speed to an unknown extent. Until more information becomes available an arbitrary increase in the flame speed must be taken. and a reasonable value based on practical experience with arresters would be 1140 ft/s. From equation 2.

crimped arresters of thickness I I in and crimp height 0025 in would be suitable. Any explosion venting of the blower would assist in keeping flame speeds down.

Example S

A portable instrument box 30cm long, 20 x 20cm cross section, can become filled with a petrol vapour/air atmosphere which may be ignited by electrical contacts. What flame arrester should be fitted on the end of the box to avoid discharge of flame?

A nsit'er

The arrester has two purposes, as explained on page 20; to quench flame and to absorb heat from burnt gas. Either parallel plate or crimped metal arresters can be used.

Parallel plate arrester As the available information is so limited it is

necessary to specify that the arrester should cover the entire 20 x 20 cm end of the box. The plate separation should be 0016 in, the plate thickness 012 in and length 2 in. If manufactured from steel plate, the arrester would weigh over 30 lb. and this weight would be ample to prevent melting of the arrester. The weight would, however, be excessive for a

portable instrument.

Crimped metal arrester In order to keep explosion pressures down to a

few lbf/in2 the area of the arrester should be at least half the area of the end of the box, i.e. it should be at least 200 cm2. As there may be some

explosion pressure the flame velocity for a 5 ft run-up (380 ft/s. Table 4) should be taken, rather than that for I ft (30 cm) which would allow for

any safety margin. From equation 2, with, say, a 0030 in crimp height, y=052 in or 133 cm.

An arrester of the above dimensions would prevent flame from propagating out of the box. It is now necessary to determine whether the

30

arrester could be overheated, wherever ignition occurred in the box. The expected rise in temperature of the arrester is now calculated.

The volume of the box 30 x 20 x 20 cm = 12 litres; heat to be absorbed by the arrester is 12 x 400 = 4800 cal.

The volume of the crimped arrester= 200 x 133 = 266 cm3. If the crimp is

10% blocked by the metal ribbon, and the metal is of density 8 g/cm3 the mass of crimped metal is 266 x 01 x 8=213 g (i.e. less than 05 Ib).

Taking the specific heat of the metal as 01, its mean temperature rise

= 4800 =225°C 213 xOl

The temperature rise is thus well within the permitted maximum of 500°C (para 64) and the proposed arrester is therefore acceptable. If the tem-

perature rise had exceeded the permitted maximum, the area and thickness of the arrester would have needed to be increased.

The calculation is based on the free area of crimped ribbon; any areas of ribbon blocked or shielded by supporting bars or bosses are additional.

31

PART 2

Explosion reliefs for ducts and. elongated vessels

92 Wherever flammable vapours 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 vapour 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 occurred which resulted 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. Con- versely, 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.

93 Plant in which flammable gases and vapours are handled in industry varies widely in size and shape, and duct systems of differing degrees or complexity are used to connect items for plant 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)0 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. I 2 I 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 methaneair mixtures. The correlation of the results of this work and also the inclusion of other sources of information14 i5 do allow. however,

32

the results to be extrapolated to ducts with diameters up to about 2 ft 6 in and to a number of other gases.

Scope

94 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.

95 The basic formula given on pages 35 and 36 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 on page 38. Page 38 also deals with propane/air mixtures which are moving at velocities of between 10 ft/s and 60 ft/s. Correction factors given on pages 42 and 43 should be employed when vessels containing gases other than propane are to be protected. 96 Where L/D is less than 6 the design data given by Simmonds and Cubbag&° 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 safety as L/D approaches 6.

Principle of relief venting for ducts and long vessels

97 When a gas is ignited at the centre 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 vapour, the flames travel initially at a speed of about 10 to 20 ft/s. The expansion of the burnt combustion products behind the flame in the duct causes a motion of the unbumt gas ahead of the flame. After a short time this moving unburnt gas becomes turbulent and one of the consequences 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 waves 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 6000 ft/s and pressures of several hundred lbf/in2 may be obtained.

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98 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.

99 As a general principle relief vents should be cited so that burnt gas close behind a flame is expelled from the vents; this would minimise 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.

100 In the following sections information is provided on the size and the spacing of explosion reliefs along elongated vessels or ducts and the methods by which the relief openings may be closed. The data are provided on 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 require- ments 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 = 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 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.

101 The maximum design explosion pressures apply only when an

appropriate vent closure is used. Thus, e.g. the use of covers heavier than those recommended may result in explosion pressures greater than the maximum design values.

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Size and spacing of explosion reliefs for stationary gases or gases moving at speeds of less than 10 ft/s

Straight unobstructed ducts LID less than 30

102 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 6. The maximum

pressure will be given by one of the two following formulae:

Equation 4 where K=l; P=007 L/D

Equation 5 where K is between 2 and 32; p = 18K

103 For K between I and 2 the maximum pressure may be taken as the mean of those given by equations 4 and 5, i.e.

p =0035 L/D+09K P = maximum pressure in lbf/in2 K = ratio of cross section of duct to area of vent

104 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 centre of the vessel as possible. Equations 4 and 5 give the maximum pressure for the most unfavourable relative position of the explosion relief and ignition source.

105 With this vent system, covers weighing 2 lb/ft2 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 equations 4 and 5 may be used.

Example I A reaction vessel is 20 ft long x 2 ft in diameter and an explosion may occur during the emptying or filling of this vessel. What size of explosion relief is required if the maximum pressure that can be allowed is 10

lbf/in2. Answer

From equation 5 the value of K corresponding to a maximum pressure of P of 10 lbf/in1 is 55; therefore, the cross-sectional area of the vessel divided by the area of the vent = 55, giving a vent of diameter I 02 in. With this a bursting disc designed to fail at a pressure of 5 lbf/in2 may be used. This vent must of course be put in the end of the vessel.

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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 vapours to the atmosphere. What is the maximum pressure if a fuel/air mixture is ignited in the stack.

Answer

Apply equation 4: L/D = 20 gives a maximum pressure of 14 lbf/in2.

Straight unobstructed duets with L/D greater than 30

106 For ducts with an L/D ratio greater than 30 it is necessary to provide more than one explosion reliel 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.

107 In designing explosion relief for long ducts an open end of a duct may be regarded as an explosion relief of size K = I. For this purpose an open end may be defined as either an end leading without restriction into the open atmosphere, or leading to a vessel which itself is adequately provided with explosion reliefs, or leading into a room of volume greater 200 times the volume of the duct. If the ends of a duct are not open, or may be closed some of the time, an explosion relief should be placed as near as possible to these ends.

108 Vent covers weighing not more than 2 lb/ft2 should be used. They should be held in position by magnets or springs. If heavier covers cannot be avoided then explosion reliefs will need to be closer than indicated in Table 5 if pressures are to be kept below 2 lb/in. If later information

Table S Maximum distance apart of explosion reliefs and maximum pressures for a long length of unobstructed duct

Maximum

pressure for Size of vents Formulae giving greatest (K factor for Maximum maximum pressures spacing each vent) distance apart (lhf in2) (lbf7in2)

600 04L1:D 24 2 30D L1.D+0l 19 4 200 07 L110+0-2 16 8 150 08 l_i.O+O3 15

Li =d'stance apart oIxpIosio reliefs

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on moving gases is followed in this respect then any error will be in the direction of increased safety.

Example 3

It 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. I-low many explosion reliefs are required if the size of each vent is (a) equal to the cross-sectional area of the duct, (b) one-eighth of the cross-sectional area of the duct. The duct can withstand a maximum pressure of I lbf/in2.

Answer

The open end ofa duct is a vent of size K= 1 and a vent of equal size should be placed at the closed end of the duct. According to TableS when K = I and the maximum pressure is I lbf/in2 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 175 ft. This gives sixteen openings plus those at each end.

Vessels and ducts containing obstacles 109 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 insufficient information to give detailed venting relationships for obstacles of various kinds. Experiments have shown, however, that to reduce maximum pressures in ducts containing an obstacle of the above type to 2 lbf/in2, an explosion relief equal to the cross section of the duct needs to be sited every six diameters, it is essential that an explosion relief also be placed near the obstacle. For a long straight duct connected 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 than 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/ft2 should be used. They should be held in position by magnets or springs.

37

Tibia 6 Space and uize or vents along ducts containing moving gases

L1/D (ratio of distance K (ratio of cross between consecutive vents section olduct to hydraulic diameter of

Duct diameter to are olvent) duct)

Uptolft6in

lftointo2ft6in

Size and spacing of explosion reliefs for gases moving at speeds of 10—60 ft/s ______ __________________

Unobstructed ducts 110 Vent systems are given in Table 6 which are designed to limit the maximum explosion pressure to 2 lbf/in2 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 velo-

city of the flammable gas. For gases moving with a velocity of 25 ft/s the closures should not weigh more than 10 lb/ft2 of vent area and for gases moving with a velocity of 25 to 60 ft/s. not more than 5 lb/ft2 of vent area.

Ducts containing obstacles

ill Ducts containing obstacles require a greater venting area in the neighbourhood of the obstacle. Information is available only for ducts up to 15 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 Co 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/ft2 of vent area for gases moving with velocities below 25 ft/s and 15 lb/ft2 of vent area for gases moving with velocities of 25 to 60 ft/s. These covers may be held by magnets or springs.

Vent closures

112 In the majority of applications vent closures must be leak tight, robust and designed in such a way that natural deterioration and lack of

38

maintenance will not result in an increase in the maximum pressure obtained during an explosion.

113 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.16 7

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.

114 If the vents have to withstand a high temperature, asbestos millboard 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 x 12 x in thick fail at a pressure of approximately 14 lbf/in2 in a static test; the failure pressure is approximately proportional to the thickness and inversely proportional to the linear dimension. Figure 8 shows a method of constructing an asbestos millboard closure.

115 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. It is important

'/, in. radius

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Clamping flanges

FigS Asbestos miliboard panel closure

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 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 magnets 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 10 lb/ft2 of vent area. If, however, there is a permanent slight positive pressure within the duct the magnetic 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/ft2, but the weight of the cover must not be increased.

116 A method of constructing vent closures held by magnets is shown in Fig 9. Essentially it consists of a cover and a magnet assembly. The cover may be constructed from a variety of light and dimensionally stable materials. Fibreboard is one of these. The metal plates, matching the magnets, may be screwed or riveted to the panel. There is no point 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.

117 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.

118 A method of retaining thc cover by the use of spring clips is shown in Fig 10. The spring must be designed in such a way that the restraining force is no longer active after the cover has travelled 0-75 in. This distance is shown on the drawing.

119 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.

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- - L 'Is in. clearance

Fig 9 A duct fitted with an explosion vent cover held in place by magnets. The field strength of the magnets may be varied according to the strength of the plant and the pressure withIn it. (This is an experimental design and may be modified where necessary)

4'

Corner detail of cover showing interior of resinated kraft paper

or aluminium honeycomb /

Sectional detail of recessed magnet

showing 'Is in. allowance for compression of rubber to ensure dust-tight closure,

Adjusted to dIssngi. itter

Fig 10 Closure held by springs

cover has moved more than 3/4 in.

Data for gases other than propane

120 It is possible to extrapolate the data given above to ducts containing a flammable mixture of gases other than propane in air. This extra- polation is based on the fundamental burning velocity. This property is

easily available and a list of burning velocities of some common vapours and gases is given in Table 7.

121 To extrapolate the maximum design explosion pressure for ducts and elongated vessels, this pressure should be multiplied by the square of the ratio of the maximum fundamental burning velocity of gas to be used to that of propane. This gives:

Equation 6 S2

P2=T: P1

where P1 is the maximum design pressure for propane P2 is the maximum design pressure for the gas under consideration

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

42

Metal bracket Cover

anchored to duct This end anchored

Duct with

Force required to dislod the panel <10 lb/ft'

Tuble 7 Maximum fundamental burning velocitie.

Gas mixture Burning velocity (It/s)

Distance between vents

(propane= I)

Methane/air I -2 1-5

Propane/air I-S 1-0

Butane/air 1-3 1-3

Hexane/air 1-3 13 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

On the other hand if the distance between neighbouring vents is L1 for a given maximum pressure with a propane7air mixture, then for a different gas the new distance L2 to give the same maximum pressure would be given by

Equation 7

L2= L1

122 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 15 power of the absolute temperature. On this basis the maximum pressure should be proportional approximately to the cube of the absolute temperature. 123 Equations 6 and 7 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.

124 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.

43

Bibliography I Flock, E F, Measurement of burning veIocity' High Speed Aerodynamics and Jet Pro-

pulsion. Vol. IX., pp. 409—38. Oxford. 1955. University Press 2 Potter, AL, 'Flame quenching.' Progress in Combustion Science and Technology. Vol. I..

pp. 145—81. Oxford, 1960. Pergamon Press 3 Cabbage, P A, 'Flame traps for use with town gas/air mixtures.' Gas Council Research

Communication, GC63. J.,ondon, 1959

4 Browii, K C eta!., Trans. Inst. Mar. Engrs. (1962), 74(8), pp. 261—76

5 Loison, R, Chaineaux, Land Delelaux, J, 'Study of some safety problems in fire damp drainage.' Eighth International Conference of Directors of Safety in Mines Research.

Paper No. 37. Dortmund-Derne. 1954. Safety in Mines Research Establishment 6 Hulsberg, F, Ret. !ndustr. mm., (1957), 39, pp. 373 76 7 Schmidt, H, Habefl, K and Reckling Haasen, NIX, Tech. Uberwach., (1955). 7(12), pp.

423-29 8 Radier, H H, J. Inst. Petrol., (1939), 25, pp. 377--S

9 Qu,intoa,PC,Brit. Chem. Engng.,(1962),7(12), pp.914—2l 10 Simmont, WA and Cubbage, PA, 'The design of explosion reliefs for industrial drying

ovens.' Symposium on Chemical Process Hazards with Special Reference to Plant Design. Instttut ion of Chemical Engineers, 1961, pp. 69—77

II Rasbash, D J and Rogowski, Z W, Combust. & Flame, (1960), 4(4), pp. 301-12 12 Rasbash, D.J. and Rogowaki, Z W, 'Relief of explosions in duct systems.' Symposium on

Chemical Process Hazards with Special Reference to Plant Design. Institution of Chemi. cal Engineers, 1961, pp. 58—68

13 Rasbash, I) J and Rogowski, Z W, 'Relief of explosions in propane/air mixtures moving in a straight unobstructed duct.' Second Symposium on Chemical Process Hazards with Special Reference to Plant Design. Inst itulion of Chemical Engineers, 1964

14 CouSas, LW and Cotton, P F, 'The protection of closed vessels against internal explosions: American Society of Mechanical Engineers. Paper No. SI. Pri, New York, 1951

IS Freeston, H G, Roberts, J P mid Thomas, A, Proc. Instn. Mech. Engrs., (1956), 170(24), pp. 811-62

16 'Symposium on bursting discs.' Trans. Insin. Chem. Engrs., (1953), 31(2) 17 Philpolt, J E, Engng. Mater. & Des. (1963), 6(l), pp. 24—29

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