GB2522476A

GB2522476A - Flame arrester

Info

Publication number: GB2522476A
Application number: GB1401410.4A
Authority: GB
Inventors: Daomin Hong; Mr Lewis Bingham; Mr Peter Evans
Original assignee: ELMAC TECHNOLOGIES Ltd
Current assignee: ELMAC TECHNOLOGIES Ltd
Priority date: 2014-01-28
Filing date: 2014-01-28
Publication date: 2015-07-29
Also published as: GB201401410D0

Abstract

A flame arrester 600 for arresting a flame in a pipeline comprises a housing 601, 602 having an internal cavity 616, 617, an outer end 608 adapted for fitment to a pipeline and a flame arresting element 603 extending across the internal cavity of said housing. Housing 601 further comprises a first aperture 605 at an outer end for passage of gas therethrough and a plate member 614 extending across a first cavity 616 between the outer end and the flame arresting element. The plate member comprises a second aperture and divides the cavity 616 into a first chamber 620 and a second chamber 618, wherein the second aperture is aligned substantially centrally along a main length axis of the housing. The plate member is arranged to block passage of a portion of any incident or reflected pressure waves from the first chamber into the second chamber and to restrict the flow of hot gas. Pressure waves that pass through the second aperture are rarefied by means of expansion into the second chamber.

Description

Flame Arrester

Field of the Invention

[0001] The present invention relates to flame arresters, and particularly although not exclusively to detonation flame arresters.

Background of the Invention

[0002] In various chemical, petrochemical and industrial processes, it is vital to prevent or stop gas explosion propagation in plant, machinery, storage facilities or pipelines. Gas explosions can occur when fuel and oxygen and an ignition source are present together.

[0003] Gas explosions fall into two main types being deflagration and detonation. :15

[0004] Deflagration is a combustion wave or explosion which propagates at subsonic velocity as measured in unburnt gas immediately ahead of a flame front. Flame velocity relative to unburnt gas is typically 0.5 to 100 mIs. However in a pipeline, velocities of the order of several hundred metres per second may be present under normal conditions.

[0005] Combustion overpressure may be in the range up to 10 times the initial gas pressure, which is usually 10 bar overpressure with a gas of an initial atmospheric pressure.

[0006] Detonation is a combustion wave or explosion which propagates at supersonic velocity into unburnt gas, and is accompanied by a shock wave. A flame front and the shock wave front associated with detonation are coupled to each other. The detonation wave or shock wave overpressure may reach 10 to 100 times the initial gas pressure, and may reach over 100 bar overpressure starting with an initial atmospheric pressure. Flame velocities may reach several thousand metres per second.

[0007] Detonations can be further subdivided into two types: firstly, stable detonations, which occur when the detonation progresses through a confined system without significant variation of velocity and pressure characteristics; and secondly, unstable detonations, which occur during the transition of a combustion process from a deflagration into a stable detonation. The transition occurs in a limited spatial zone where the velocity of the combustion wave is not constant and where the explosion pressure is significantly higher than that in a stable detonation.

[0008] The propagation of an explosion in a flammable gas mixture in a container or pipeline system can occur as a detonation or as a deflagration.

Combustion waves may propagate over a wide range of flame velocities, which cover three orders of magnitude for the same gas mixture. :is

[0009] Referring to figure 1 herein, there is illustrated schematically a typical flame acceleration and pressure buildup in a confined pipeline. An explosive gas-air mixture in a pipeline is ignited by a low ignition energy source for example a spark. Flame propagation starts with a deflagration. The deflagration propagates and its velocity is determined by the transfer of heat and mass to the unburnt gas ahead of it. During this period, the combustion occurs behind the pressure wave. The flame front velocity is subsonic and flame acceleration occurs as a result of the expansion of the combustion products driving the flame front forwards along the pipeline. Figure 1 is reproduced from NFPA 69:2008 Standard on Explosion Prevention Systems, page 69-64.

[0010] As a result of flame acceleration, there is a flame front instability, then leading to turbulence. With flame acceleration, a turbulent flame develops due to the flame front instability. The effect of turbulence is to increase the burn rate via faster transport and to increase in burning surface area leading to much more rapid flame acceleration and the formation of shock waves ahead of the flame front. If the appropriate physical and chemical conditions are present, the flame front can accelerate and change the mode from deflagration to detonation.

[0011] Infigurel: v = flame velocity accelerated by increasing turbulence p = pressure increasing by expansion of burnt gases (shock wave) DDT = deflagration to detonation transition L = pipe length from position of the ignition source D = internal diameter of the pipeline [0012] The transition of deflagrationto detonation is usually abbreviated as DDT, and is shown in figure 1 by a rapid and sharp spike of pressure. After reaching a maximum pressure, the unstable detonation turns into a stable detonation. Detonation is a supersonic compression wave in which auto-ignition is achieved and the gas mixture is ignited by the adiabatic compression of the leading shock.

[0013] The DDT usually occurs at a ratio of pipe length from the ignition source L to pipe diameter 0, of greater than 50 (L/D > 50) for typical hydrocarbon-air mixtures, and at a ratio of greater than 30 ([/0 > 30) for hydrogen-air mixtures. The pipe length L from the ignition source is usually called the running up distance".

[0014] Known flame arresters are essential safety devices and can be used as a secondary measure to prevent combustion propagation in equipment and a pipeline system. A flame arrester is fitted either to the opening of an enclosure, or to connecting pipe work of a system of enclosures. The intended function is to allow flow of gas, but to prevent the transmission of flames.

[0015] Deflagration flame arresters (DEF) are flame arresters designed to prevent the transmission of a deflagration.

[0016] Detonation flame arresters (DEl) are flame arresters designed to prevent the transmission of a detonation and a deflagration. Detonation flame arresters are suitable for arresting deflagrations and detonations.

[0017] Flame arresters are commonly required to protect against many types of explosion events within equipment or piping systems. The propagation of an explosion in a pipeline of flammable gas mixture can occur as a detonation or as a deflagration. Detonation flame arresters are usually installed in pipelines where the possibility of a detonation exists.

[0018] Referring to figure 2 herein, there is illustrated schematically in view from one side a prior art flame arrester device 200 fitted into a pipeline. The pipeline comprises a first pipeline piece 201 and a second pipeline piece 202, the flame arrester connecting the first and second pipeline pieces. The flame arrester is symmetrical, so as to achieve bi-directionality.

[0019] The flame arrester comprises a first housing 207 having a first end 203 and flange 209 and a second housing 211 having a second end 204 and flange 213.

[0020] Between flanges 209 and 213 there is provided a flame arrester element 214. The flame arrester element 214 is a gas porous matrix, whose principal function is to prevent flame transmission. The most common porous matrix/medium of the flame arrester element is known as "crimped ribbon element". It has triangular shaped aperture channels which are formed by spirally winding a layer of flat metal foil between layers of foil that have been crimped.

[0021] The flame arrester element 214 is sandwiched between housings 207 and 211. Flanges 209 and 213 are connected to each other using a plurality of external bolts 215.

[0022] Under normal operating conditions it is desirable to minimise resistance to gas flow through the matrix of the flame arrester element. Having a larger diameter flame arrester element compared to the pipe diameter means that there is less resistance to flow for a given flux. The ratio of diameter of the flame arrester element to diameter of pipeline is normally in the range 1.5 to 3, and to minimise resistance to flow, the larger diameter of the flame arrester element, the better. Minimising resistance to flow affects pumping cost and improves the operating conditions. Further, some equipment in a pipeline may have operating pressure limitations.

[0023] In case of an event (explosion), a flame travels down a pipeline from an ignition source and enters an expansion chamber, and the pressure wave and the flame front impinge on the matrix comprising the flame arrester element. :is

[0024] In use, a flame originating from a side, so called the unprotected side, entering into the expanded housing, is incident on the flame arrester element 214. The flame arrester element 214 quenches the flame and prevents the flame from passing through to the other side, so-called the protected side.

[0025] Since detonations generate much higher pressures and are usually much more destructive than deflagrations, detonation flame arresters must be capable of attenuating shock waves and withstand the mechanical effects of shock waves while quenching the flame.

[0026] In a conventional detonation flame arrester, the flame arrester element may be subjected to up to 100 times the initial pressure before ignition.

[0027] Conventional flame arrester devices are usually wider in diameter than the pipelines to which they are designed to be attached. The length of the flame arrester is determined in part by the length of the flame arrester element needed to attenuate a pressure wave/shock wave and to arrest a travelling flame front. The axial length of the flame arrester element required is a function of the initial pressure (maximum allowable operating pressure) and gas mixtures as well as optimized flame quenching channels of the flame arrester element. It is widely acknowledged that the conventional detonation flame arrester cannot be cost effectively manufactured or can be manufactured only at prohibitive cost, especially for large sizes of detonation flame arrester. Pressure drop is significantly dependent on the channel aperture size and channel length. The length of flame arrester element required is dependent on the channel and aperture dimensions.

[0028] US 4,909,730 discloses that a detonation attenuator is provided within the housing of a flame arrester. The attenuator is generally cup-shaped, aligned with the inlet, of greater diameter than the inlet but of lesser diameter than the arrester chamber, and is positioned close to the inlet so as to circumscribe it. The cup-shaped attenuator is to receive and reflect part of the central portion of the detonation wave back into the pipe. The flame arrester element is protected by the attenuator from structural damage from detonation, to a much improved extent. However, the use of the cup-shaped attenuator significantly increases pressure drop under the normal gas flow operating conditions.

[0029] The concept of a pipeline extending into a flame arrester housing appeared at least in DE 44 38 797 Cl. In that document, perforated plates are added to act as a flow rectifier to improve the flow distribution under normal conditions, and to improve the performance of the flame arrester.

[0030] uS 5,905,277, EP 0 765 675 B1, and CA 2 186 652 disclose that "the detonation front is divided up into at least one main front and a secondary front The main front is routed into an expansion space through a longer route than the secondary front, in such a way that when the main front enters into the expansion space, the expansion space contains post-combustion gases of the secondary front which decompose the combustible gases of the main front." The propagation time of the main front is dimensioned relative to the secondary front in such a way that the secondary front will have already decomposed in the expansion space by the time the main front enters the expansion space. "This improved method of the detonation weakening results in the flame arrester being able to have wider and shorter flame-extinguishing gaps, whereby the pressure loss caused by the flame arrester is reduced." [0031] In "Rendering a detonation front harmless" -US 6,342,082 Bi and US 6,409,779 B2; EP 0 951 922 Bi there is disclosed the use of one or more pipe stubs to convey the detonation front to near the flame-arresting device to improve the performance of the flame-arresting device, including the flow resistance (pressure drop). Under normal gas flow, the pipe stubs are used as a flow distributor. Under detonation conditions, the detonation front is split into several partial detonations or is conveyed near the flame arresting device.

[0032] The pipe stub is placed near the flame-arresting device such that the portion of the flame arrester element surface being impinged by the front is essentially equal to the pipeline diameter. The flame-extinguishing operating mode becomes more effective as the end of the pipe stub is placed closer to the flame-arresting device.

[0033] In US 6,409,779 there is disclosed a method for rendering a flame front harmless, along with several designs of flame arrester devices. Some of the devices have a set of pipe stubs inside the flame arrester housing extending in an axial direction between the pipeline and a flame arrester element, which are designed to divide up a flame front into several smaller flame fronts which are directed towards the flame arrester element. The detonation front is split into several partial detonation fronts that impinge on the corresponding portions of the flame arrester element. A distribution space is formed in front of the flame arrester element between a back end wall to which the pipe stubs are attached, and the flame arrester element. This distribution space contains the pipe stubs.

[0034] Under normal gas flow operation, the gas flow is divided by the pipe stubs into several different streams which are distributed uniformly over the area of the flame arrester element.

Summary of the Invention

[0035] An object of the specific embodiments described herein is toovercome the problems of prior art flame arresters. In particular, the present invention disclosed herein addresses the following objectives: * To remove or reduce the powerful reflected shock wave from the reducer wall; * To reduce shock wave impact on the flame arrester element in general; * To increase the operating pressure of a flame arrester, but without increasing the flame quench length; * To increase the operating pressure of the flame arrester without the use of a longer flame arrester element or without reducing the aperture dimensions of the quenching channel * To provide a detonation flame arrester of a minimizing resistance without a significant increase in the diameter of the flame arrester element; * To provide an improved flame arrester performance without increasing the diameter of the flame arrester element.

[0036] According to one aspect there is provided a flame arrester comprising: a housing having a cavity; a flame arresting element; a plate member extending across said cavity within the housing, said plate member positioned between a first end and a second end of the housing; wherein a radially outermost part of said plate member is attached to part of an inner wall of said housing which is at least as radially distant from a main central axis of said housing as said radially outermost part of said plate member; said plate member dividing said cavity into a first chamber and a second chamber; said plate member having at least one aperture; 1.0 said at least one aperture extending through said plate member in a direction transverse to a main surface of said plate member; characterised by :15 there being no guide members extending from said plate member to direct a pressure wave onto said flame arresting element; wherein the plate member blocks a portion of an incident pressure wave in the first chamber and a portion of any reflected pressure wave within the first chamber from passing into the second chamber; said plate member restricts the flow of hot gases into the second chamber; said at least one aperture rarefies said pressure waves by means of expansion of said pressure waves into the second chamber; and said pressure wave passes from said first chamber to said second chamber only via said at least one aperture through said plate member.

[0037] The invention includes a flame arrester comprising: a housing having a cavity; a flame arresting element; a plate member extending across said cavity within the housing, said plate member positioned between a first end and a second end of the housing; said plate member dividing said cavity into a first chamber and a second chamber; said plate member having at least one aperture; said at least one aperture extending through said plate member in a direction transverse to a main surface of said plate member; characterised by there being no guide members extending from said plate member along a direction of a main longitudinal axis of said housing to direct a pressure wave onto said flame arresting element; wherein the plate member blocks a portion of an incident pressure wave in the first chamber and a portion of any reflected pressure wave within the first chamber from passing into the second chamber; said plate member restricts the flow of hot gases into the second chamber; and said at least one aperture rarefies said pressure waves by means of expansion of said pressure waves into the second chamber.

[0038] According to a second aspect there is provided a method of weakening a shock wave in a pipeline, said method comprising: passing said shock wave through a first aperture having a first cross-sectional area into a first chamber having a second cross-sectional area, said second cross-sectional area being larger than said first cross-sectional area; at least partially blocking and absorbing said shock wave by a plate member said plate member having a second aperture; passing said shock wave through said second aperture into a second chamber; and passing said shock wave onto a flame arrester element.

[0039] According to a third aspect there is provided a flame arrester device comprising: :is a housing comprising a first chamber and a second chamber; a wall member extending across said housing between said first and second chambers; and a flame arrester element; said housing having a first aperture having a first aperture cross-sectional area in a plane perpendicular to a main axial length of said flame arrester device; said wall member having a second aperture having a second aperture cross-sectional area in a plane perpendicular to a main axial length of said flame arrester device; characterised in that: said second aperture cross-sectional area is greater than said first aperture cross-sectional area.

[0040] Specific embodiments disclosed herein may provide for a flame arrester having a housing which is divided into a first chamber and a second chamber by a plate member which incorporates an aperture. The plate member incorporating an aperture separates the first and second chambers, and also blocks a portion of the incident shock wave, and most of the reflected shock waves from the housing wall. In addition, the aperture in the plate restricts the flow of hot gases into the second chamber, and also rarefies the shock waves by means of sudden expansion in the second chamber. The effect of this is to reduce the severity of the shock front at the flame arrester element, and in certain cases transform a detonation to a deflagration.

[0041] The plate member incorporating an aperture may perform the following functions: :is (1) restricting flow to reduce the flowing pressure immediately downstream of the plate member incorporating an aperture (2) separating the reflected shock waves from the housing/reducer wall and prevent them from hitting the flame arrester element; (3) to block a portion of incident shock waves and most of the reflected shock waves; (4) to divide the housing into at least two separate chambers, so that at least two separate stages of rarefaction can occur prior to a shock wave hitting a flame arrester element; (5) to create another sudden expansion to reduce shock wave pressure including converting detonation into deflagration; and (6) the plate member incorporating an aperture whose diameter may be optimised does not significantly increase the pressure drop across the flame arrester under normal gas flow operation.

[0042] In various embodiments, different aperture configurations may be present in the plate member as follows: [0043] The aperture of the plate member may be (or may be not) concentric with the plate member.

[0044] It is not always necessary for the aperture of a plate member to be concentric with the plate member even in the case of a cylindrical housing! a concentric embodiment. For example, the aperture may not be concentric when a flame arrester is specified close to pipe bends or close to special equipment such as pumps.

[0045] The aperture may be not concentric to the plate member in particular in an eccentric design!housing or in a right angle design of the h ousi ng!red ucer.

[0046] In the case of concentric embodiments, a plate member incorporating an aperture can be lined up in the line of sight from the flame arrester inlet aperture to the flame arrester element. plate member incorporating an aperture in which the aperture is lined up in the line of sight from an inlet aperture to the flame arrester element, may be still applied to an eccentric design where the aperture is provided eccentrically of the plate member. In addition, the aperture may be so positioned in the plate especially for an eccentric design or a right angle design of flame arrester, in which there may be no line of sight from the flame arrester inlet aperture to the flame arrester element.

[0047] The present embodiments provide a solid ring to substantially prevent the reflected shock waves from the wall reaching the flame arrester element. The ring may be annular.

[0048] There are many possible aperture configurations, including circular, square, triangle and hexagonal (honeycomb) and other polygonal apertures, including segments of those shapes. The ring surrounds the aperture(s).

[0049] The embodiments include the design of eccentrically placed apertures in the plate member.

[0050] In a preferred embodiment, a cross-sectional area of a single aperture, or a combined cross-sectional area of a plurality of apertures in the plate, is greater than a cross-sectional area of an inlet aperture to the flame arrester.

[0051] Preferably the thickness in the main axial direction of the flame arrester of a plate member incorporating an aperture is relatively small compared to a maximum dimension of each aperture in the plate member. The thickness of the plate member incorporating an aperture may be as thin as possible. It needs to be just thick enough so that the plate is strong enough to withstand shock waves, the plate being reinforced if necessary for, so that the aperture does not significantly act as a tubular passage or channel to convey a shock wave a significant distance in the axial direction. It is not necessary for the plate to act as a tube or pipe to convey the shock wave, nearer to the flame arrester element, and preferably is does not act as such.

[0052] In the embodiments presented herein, there is provided a plate member having one or more apertures, where the plate member extends from an internal wall of the flame arrester housing, and extends inwardly towards a centre of the flame arrester, with one or more apertures provided in the plate member for passage of gas there through. Preferably the plate member forms a complete ring around the inside circumference of a flame arrester housing, thereby providing an aperture of cross-sectional area in the plane perpendicular to a main axis length of the flame arrester which is smaller than the cross-sectional area of the part of the housing to which the plate is attached, thereby dividing the housing into a first chamber and a second chamber connected by the aperture.

[0053] In the embodiments described herein, a shock wave entering a flame arrester enters a first chamber where it experiences a first stage of rarefaction, and is partially blocked by a plate member having at least one aperture. Part of the shock wave is reflected back in the first chamber, whilst another part of the shock wave passes through the aperture(s) into a second chamber. The portion of the initial shock wave passing through the apertures into the second chamber experiences a second stage of rarefaction before it hits the flame arrester element. The parts of the shock wave which were reflected in the first chamber may reflect back off the internal walls of the first chamber and some may pass through the aperture(s) into the second chamber, experiencing a second stage of rarefaction in the second chamber as they leave the aperture(s), with a slight delay with respect to the incident shock wave. Hence shock waves entering the flame arrester experience a first stage rarefaction and a second stage rarefaction which acts to defuse energy from the shock wave, prior to reaching the flame arrester element.

[0054] The plate member incorporating an aperture can operate to -Retain some of the detonation waves in the first chamber and allow some to pass to the second chamber; -Block most of the reflected shock waves from the first chamber wall; -Absorb some shock wave energy; -Restrict the hot gas flow from the first chamber into the second chamber; -Convert some of the detonation into deflagration.

[0055] Other aspects are as set out in the claims herein

Brief Description of the Drawings

[0056] For a better understanding of the invention and to show how the same may be carried into effect, there will now be described by way of example only, specific embodiments, methods and processes according to the present invention with reference to the accompanying drawings in which: Figure 1 herein shows flame velocity and pressure curves of an explosion process in a confined pipeline; Figure 2 herein shows schematically in view from one side, a conventional flame arrester; Figure 3 herein shows a schematic cutaway view of a first flame arrester according to a first specific embodiment of the present invention; Figure 4 herein shows schematically in cutaway view from one side, a second flame arrester according to a second specific embodiment of the present invention; Figure 5 herein shows a schematic side view of a third flame arrester in accordance with a third specific embodiment of the present invention Figure 6 herein shows an external perspective view of a fourth flame arrester according to a fourth specific embodiment of the present invention; Figure 7 herein shows the fourth flame arrester in cutaway view from one side; Figure 8 herein shows the fourth flame arrester in cutaway perspective view from above and one side; Figure 9 herein shows the fourth flame arrester in view from one end along a main central axis of the flame arrester; Figure 10 herein shows a fifth flame arrester according to a fifth specific embodiment, having a plate member incorporating an aperture fitted in a reducer section of the flame arrester, to make the flame arrester shorter in length; Figure 11 herein shows in external perspective view, a sixth flame arrester according to sixth specific embodiment of the present invention; Figure 12 herein shows in cutaway view from one side the sixth flame arrester of figure 11, the illustrating a frusto conical plate in a cavity of the housing of the flame arrester; Figure 13 herein illustrates schematically the sixth flame arrester in view from one end along a main length axis of the flame arrester; Figure 14 herein illustrates schematically in perspective view a seventh flame arrester according to a seventh specific embodiment; Figure 15 herein illustrates schematically in cutaway view from the side, the seventh flame arrester of figure 14 herein, showing first, second and third chambers between an inlet port of the flame arrester and a flame arrester element; :15 Figure 16 herein illustrates schematically in cutaway view from one side a bisected section of an eighth flame arrester according to a eighth specific embodiment; Figure 17 herein herein illustrates schematically in cutaway view from one side a bisected view of an ninth flame arrester according to a ninth specific embodiment of the present invention; Figure 18 herein shows schematically a tenth flame arrester according to a tenth specific embodiment, having a plate member comprising a plurality of annular aperture segments arranged concentrically; Figure 19 herein shows in view along a main length axis of the tenth flame arrester, an aperture plate of the tenth flame arrester; Figure 20 herein shows schematically various alternative options for shaping the perimeter of an aperture in an aperture plate as described within the above embodiment flame arresters; Figure 21 herein shows a further alternative shape for a perimeter of an aperture in a plate member; Figure 22 herein shows a further alternative shape comprising a frusto conical perimeter of an aperture in a plate member; and Figure 23 herein shows schematically a yet further alternative cross-sectional shape of an aperture plate in the form of a concave aperture plate.

Detailed Description of the Embodiments

[0057] There will now be described by way of example a specific mode contemplated by the inventors. In the following description numerous specific details are set forth in order to provide a thorough understanding. It will be apparent however, to one skilled in the art, that the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structures have not been described in detail so as not to

unnecessarily obscure the description.

[0058] In this specification, the term "flame arrester" is used to denote a flame arrester device comprising a housing and a flame arrester element.

[0059] In this specification, the term "flame arrester element" is used to denote a gas porous element or component designed to quench a flame, and which is used in aflame arrester device.

[0060] Referring to figure 3 herein, there is illustrated schematically in cutaway view from one side a first flame arrester according to a first specific embodiment of the present invention, being a unidirectional flame arrester designed for arresting flames approaching from one direction in a connected pipeline.

[0061] The first flame arrester 300 comprises a first tubular housing or body 301, having a first end 302 and a second end 303, and a second tubular housing 304 having a first end 305 and a second end. Between the first and second housings is provided a flame arrester element 307. The flame arrester element is of conventional construction.

[0062] First housing 301 comprises an outer tubular cylindrical end portion 308; a frusto conical connecting portion 309; and a tubular cylindrical inner portion 310, the frusto conical connecting portion connecting the outer tubular portion and the inner tubular portion. The outer cylindrical end portion 308 has an internal diameter similar to or identical to an inner diameter of a pipeline to which the flame arrester is intended to be fitted, so that a first inlet aperture where the outer tube tubular cylindrical portion 308 connects to the frusto conical connecting portion 309 has an internal diameter D, the same or similar as the pipeline internal diameter.

[0063] The frusto -conical portion 309 has a smaller diameter aperture at a first end, and a larger diameter aperture at a second end; the inner tubular cylindrical portion 310 having an internal diameter larger than the internal diameter of the outer tubular cylindrical pipe portion 308. Within the housing there is provided a plate member incorporating an aperture 314 extending internally from an inner wall of said inner tubular portion 310, the plate member incorporating an aperture 314 comprising a ring shaped or annular shaped flat plate having a central circular aperture aligned, co-axially with a main central length axis of the housing. The thickness of the plate member incorporating an aperture only needs to be thick enough so that the plate is strong enough to withstand shock waves, the plate being reinforced if necessary, so that the aperture does not signiifcantly act as a tubular passage or channel to convey a shock wave a significant distance in the axial direction.

[0064] There is also provided a first external annular flange at an end of the outer tubular cylindrical pipe, for connecting the housing 308 to a first section of pipeline; and a second external annular flange surrounding an end of the inner tubular cylindrical portion 310, having a plurality of through apertures through which a corresponding plurality of bolts can be fitted, for fitting the second end of the housing to the flame arrester 307 and the other housing.

[0065] The second housing 304 comprises a substantially cylindrical tubular pipe portion having at its first end an annular flange with through -apertures, for connecting the first end of the second housing to the flame arrester element, using a plurality of bolts which extend from the flange on the second housing, to the flange on the second end of the first housing, so that the flame arrester element is positioned between the first and second housings with the bolts arranged circumferentially around the flame arrester element.

[0066] In this asymmetric embodiment, the second housing does not have an internal plate member incorporating an aperture as is provided in the first housing.

[0067] Operation of the first flame arrester will now be described.

[0068] Under normal operation, the first flame arrester is inserted into a pipeline, so that gas can flow from the first section of pipeline, through the flame arrester device and into a second section of pipeline. Taking for example a gas flow in the direction as shown arrowed in figure 3, gas flows from the first housing to the second housing, resulting in a slight pressure drop as it passes through the flame arrester element 307.

[0069] In the case of ignition in the first pipe, a flame front enters the outer cylindrical portion 308, and passes into the frusto conical portion 309. The flame arrester will cope with a deflagration flame, which generally travels at subsonic speeds, or a detonation flame, which travels at supersonic speeds along the pipeline. As the detonation enters the first chamber of the housing, the sudden expansion of the housing will reduce detonation pressure due to the effect of shock wave rarefaction, including some detonation converting into deflagration under special conditions.

[0070] The expansion of the internal dimensions in the passage as the detonation flame enters the cavity and passes throught the frusto conical portion causes the flame front to experience sudden expansion and rarefaction, and a reduction in pressure.

[0071] Plate member incorporating an aperture 314 divides the first housing into two chambers, 315, 316, one being on the side of the frusto conical reducer 309, the other on the side closer to the flame arrester element. The unburnt gas pressure in front of the flame arrester element may be dependent on the plate member incorporating an aperture and fluid flow conditions including the choked flow in case of detonation. The annular plate member incorporating an aperture 314 will effectively block the reflected shock waves from the frusto conical reducer wall 309.

[0072] In a conventional flame arrester, having no internal aperture plate, the reflected shock wave can re-initiate a new detonation and cause flame transmission and flame arrester damage.

[0073] However, the present invention, in this embodiment uses solid or rigid annular plate having a central aperture, to separate and prevent the intense shock waves, in particular the reflected shock waves from the reducer wall (expansion part), reaching the flame arrester element. The dimensions of the solid part of the annular plate is referred to herein as the dam height. The dam height for a circular plate member incorporating an aperture may be the outer diameter of the annular plate minus orifice diameter, divided by two. That is, for a symmetrical circular annular plate having a single concentric circular aperture, the distance of the plate member in the radial direction from the inside of the housing wall to the perimeter of the central aperture is the dam height. The dimension of dam height has a minimum value to effectively separate the reflected shock wave. The orifice also restricts the supersonic flow (choked flow) including the very hot combustion products into the second chamber (Just before the flame arrester element) and rarefies the shock wave due to sudden expansion.

[0074] A portion of the detonation/shock wave entering the first chamber 315 is reflected by the annular plate 314 back towards the direction from which the shock wave originated, and towards the internal frusto conical surface of the first frusto conical reducer portion 309. A central portion of the shock wave passes through the central aperture of the annular plate 314 and impinges directly on the flame arrester element 305. The pressure of any shock waves passing through the aperture in the plate member incorporating an aperture will be reduced again due to further rarefaction effect via sudden expansion.

Furthermore, plate member incorporating an aperture 314 will restrict flow of the combustion products into the second chamber 316 in front of the flame arrester element. Under supersonic conditions, the flow of the combustion products at high temperature is choked. Accordingly, the much less powerful shock waves then impinge on the flame arrester element 305.

[0075] With an optimised/desired location of annular plate member incorporating an aperture 314, shock waves and flames will be decoupled. Since much less combustion products of very high temperature pass the flame arrester element, thus the flame will be easily quenched, and therefore the flame arrester element can be less substantial than in prior art designs. Using a less substantial flame arrester element allows the advantage of reduced flow resistance under normal operating conditions.

[0076] In contrast to the systems shown in US 6,409,779B2, the first flame arrester disclosed herein does not restrict rarefaction into the first and/or second chambers by the use of pipe stubs, or attempt to deliver a shock wave direct to the surface of the flame arrester element. Rather, a detonation shock wave is not restricted to modes which will pass through a cylindrical pipe stub. A portion of an incoming shock wave which passes through an aperture in the plate member can adopt a different and less restricted propagation mode in the second chamber.

[0077] Referring to figure 4 herein, there is illustrated schematically in cutaway view from one side a second flame arrester according to a second specific embodiment of the present invention.

[0078] The second flame arrester 400 comprises a tubular body 401, having a first end 402 and a second end 403, and a central portion 404 between the first and second ends. In the central portion 404 there is provided a flame arrester element 405, which can be a conventional flame arrester element. The flame arrester body is constructed in two housings 406, 407, with the flame arrester element positioned between the two housings. A first housing 406 comprises a substantially funnel shaped shell consisting of a first outer tubular cylindrical pipe portion 408; a first frusto -conical portion 409 having a smaller diameter aperture at a first end, and a larger diameter aperture at a second end; a first inner tubular cylindrical portion 410 having a diameter larger than the internal diameter of the first outer tubular cylindrical pipe portion 408; a first plate member incorporating an aperture 414 extending internally from an inner wall of said first inner tubular portion 410, the first plate member incorporating an aperture 414 comprising a ring shaped plate having a central circular aperture aligned, co-axially with a main central length axis of the housing; a first external annular flange at an end of the first outer tubular cylindrical pipe, for connecting the first housing 406 of the flame arrester to a first section of pipeline; and a second external annular flange surrounding an end of the first inner tubular cylindrical portion 410, having a plurality of through apertures through which a corresponding plurality of bolts can be fitted.

[0079] Second housing 407 comprises a second substantially funnel shaped shell consisting of a second outer tubular cylindrical pipe portion 411; a second frusto conical portion 412 having a smaller diameter aperture at a first end, and a larger diameter aperture at a second end; a second inner tubular cylindrical portion 413 having a diameter larger than that of the internal diameter of the second tubular cylindrical pipe portion; a first annular flange fitted the end of the second outer cylindrical pipe, for fitting the second housing 407 to a second length of pipeline; and a second annular flange located in the central section of the housing, the second annular flange surrounding an end of the second larger diameter tubular inner cylindrical portion 413, the flange being provided with a plurality of through apertures through which a corresponding plurality of bolts can be passed such that a set of bolts are arranged circumferentially around the outside of the housing to connect the two housings being coonected together with the flame arrester element fitted therebetween.

[0080] In this symmetrical bidirectional embodiment, the second housing additionally has a second plate member incorporating an aperture member 415 extending from an inner wall of the second inner cylindrical portion 413, the second plate member incorporating an aperture having a centrally located circular aperture which is aligned to co-axially with a main length axis of the second housing, so that the first and second halves of the flame arrester are identical to each other.

[0081] The plate member incorporating an aperture 414 comprises a plate which extends across the housing, dividing the housing into a first chamber and a second chamber, with gases passing from the first chamber to the second chamber through an aperture in the plate member. The plate member incorporating an aperture is preferably annular, and the aperture is preferably circular, and aligned on a central length axis of the housing. However in other embodiments, the aperture need not be circular, and the aperture can be offset from the main central axis of the housing.

[0082] The first housing 406 provides a first (outer) chamber defined by the inner walls of the first outer cylindrical pipe section 408, the first frusto conical section 409, and the first inner tubular cylindrical portion 410, and a second (inner) chamber defined by the inside of the cylindrical portion 410, the aperture plate 414 and the flame arrester element 405. Similarly, the second housing 407 provides a second outer chamber defined by the inner walls of the second outer cylindrical portion 411, the second frusto conical portion 412, and the second inner cylindrical portion 413, and a second inner chamber defined by the walls of the second inner tubular cylindrical portion 413, the second plate member incorporating an aperture 415, and the other side of the flame arrester element.

[0083] The first plate member incorporating an aperture 414 has a radial thickness between the inner wall of the first inner cylindrical portion 410 and the central aperture of the first plate member so as to provide a circular aperture having diameter of approximately 1.1 times the internal diameter D of the outer tubular cylindrical pipe portion 408. In other words, the plate member has a second aperture having a cross-sectional area in a direction perpendicular to the main length axis of the flame arrester which is larger than the cross-sectional area of an inlet aperture where gas flows into the housing.

[0084] The aperture dimension selection of the second aperture in the plate member should be based on anticipated gas flow rates under normal operating conditions. The principle of operation may rely on the velocity of the gas including combustion products flowing through the aperture reaching the speed of sound. Once this velocity is obtained, the actual flowing volume passing through the aperture cannot increase and is choked. The area of aperture of the plate member determines the gas flow passing through the flame arrester element under the choked condition. Since the diameter of the aperture of the annular plate member is larger than the flame arrester pipe connection size, or the internal diameter of the second outer cylindrical portion 411, under the normal operating conditions, a minimal additional pressure drop will be caused due to the introduction of the plate member. Viewing the housing in an axial direction along a main length axis of the housing, there exists a clear unobstructed open passage extending centrally through the first outer cylindrical portion, and through the first frusto conical portion and the first inner cylindrical portion, direct to the surface of the flame arrester element 405.

[0085] The second internal plate member incorporating an aperture 415 has a central aperture of diameter of the order 1.1 times larger than the internal diameter of the second outer cylindrical portion 411. Since the second half of the housing is identical to the first half of the housing, viewing the device in an opposite direction axially along a main central length of the device, there is a clear passage from the second outer cylindrical portion 411, through the second frusto conical portion 412 and the second inner cylindrical portion 413 directly to the other side of the flame arrester element 405.

[0086] In a preferred embodiment, the inner diameter of the first and second inner cylindrical portions 410, 413 respectively is each around twice the inner diameter of the respective first and second outer cylindrical portions 408, 411.

[0087] The embodiment in Figure 4 has a symmetric line in the centre of the flame arrester element. An annular plate/plate member incorporating an aperture is installed in each housing and thus the arrester can be used to stop or contain explosions coming from either direction along the pipeline.

[0088] Operation of the second flame arrester is substantially as described herein with reference to the first flame arrester, except that the second flame arrester is bidirectional, and capable of arresting flames from either direction in the pipeline.

[0089] Referring to figure 5 herein, there is illustrated schematically in cutaway view from one side a third flame arrester according to a third specific embodiment of the present invention.

[0090] The third flame arrester 500 comprises a first housing 501; a flame arrester element 502; and a second housing 503. The first housing 501 comprises an outer tubular portion 504; an intermediate reducer portion 505; an inner tubular portion 506; and a plate member incorporating an aperture 507 which extends inwardly from an inner wall of the substantially cylindrical inner tubular portion 506, towards a main central axis of the flame arrester.

[0091] The plate member incorporating an aperture 507 divides the interior of the first housing into a first chamber 508, and a second chamber 509, where the first chamber is adjacent an inlet of the first housing, and the second chamber is adjacent the flame arrester element.

[0092] The flame arrester element 502 comprises a solid core 510, around which is wrapped a corrugated metal strip 511, through which gases can flow through under normal operation of the flame arrester. Flow of gases through the flame arrester element results in a slight increase in pressure drop across the flame arrester.

[0093] The embodiment shown in Figure 5 shows a flame arrester with a solid part in the centre of the flame arrester element. This solid part has a diameter of slightly less than the pipe connection diameter and is positioned directly opposite to the pipe inlet. Accordingly, part of a detonation may directly impinge on the solid surface. This unique design combined with the plate member incorporating an aperture can make the flame arrester element work more effectively, in particular, in the case of a larger ratio of flame arrester element area to the pipe connection area. On the other hand, due to the solid area having smaller diameter (less than the pipe connection D), for a larger flame arrester element, the free open area to gas flow will not significantly reduce and thus a very small pressure drop may be increased. Since the flame arrester element with a solid central part can be easily manufactured, this embodiment has substantial manufacturing advantages.

[0094] Operation of the third flame arrester under conditions of detonation upstream of the first housing 501 is similar to operation of the first and second embodiments, and is as follows: [0095] In the case of ignition in the inlet pipe, an explosion or flame front enters the outer cylindrical portion 504, and passes into the frusto conical portion 505. The flame arrester will cope with a deflagration flame, which generally travels at subsonic velocity, or a detonation flame, which travels at supersonic velocity. As the detonation enters the first portion of the housing, the sudden expansion into the wider part of the housing will reduce detonation pressure due to the effect of shock wave rarefaction, including some detonation converting into deflagration under special conditions.

[0096] Expansion of the shock wave as the detonation flame enters the first chamber and passes through the frusto conical reducer portion 505 causes the flame front to experience sudden expansion and rarefaction, and a reduction in pressure.

[0097] The unburnt gas pressure in front of the flame arrester element may be dependent on the position of the plate member, and the size and position of the aperture in the plate member. The annular plate member 507 reflects parts of the main shock wave, and will effectively block the reflected shock waves which reflect off the frusto conical reducer wall 505.

[0098] A portion of the detonation/shock wave entering the first chamber 508 is reflected by the annular plate 507 back towards the direction from which the shock wave originated, and towards the internal frusto conical surface of the first frusto conical reducer portion 505. A central portion of the shock wave passes through the central aperture of the annular plate 507 and impinges directly on the flame arrester element 502. The pressure of shock waves passing through the aperture in the plate member will be reduced again due to further rarefaction effect via sudden expansion. Furthermore, the plate member incorporating an aperture 507 will restrict flow of the combustion products into the second chamber 509 in front of the flame arrester element. Under supersonic conditions, the flow of the combustion products at high temperature is choked.

[0099] In the case that an identical plate member to the plate member 507 is correspondingly installed on the side of a second housing 503, the flame arrester becomes bidirectional and can be operated in any direction of flow and installation.

[00100] Referring to figures 6 to 8 herein, there is illustrated in perspective view, a fourth flame arrester 600. The fourth flame arrester comprises first and second housing portions 601, 602 respectively, spaced apart from each other and having a substantially cylindrical flame arrester element 603 positioned therebetween, the first and second housing portions being connected to each other by a plurality of bolts 604. The housing is symmetrical so that the flame arrester operates bidirectionally.

[00101] First housing portion 601 comprises a first outer tubular cylindrical portion 605; a first cup or bowl shaped portion 606; and a first inner tubular cylindrical portion 607. At one end of the first outer cylindrical portion is a first outer flange plate 608 having a plurality of apertures for fitment of bolts to connect the flange plate to an adjacent pipeline. At one end of the inner cylindrical portion 607 is provided a first inner flange plate 609 having a plurality of apertures through which the set of bolts 604 connecting the two housing portions together are located. The first housing portion 601 defines an internal cavity through which gases pass from an inlet to the housing portion to the flame arrester element 603.

[00102] Second housing portion 602 is of identical construction to first housing portion 601, and comprises a second outer tubular cylindrical portion 610; a second cup or bowl shaped portion 611; and a second inner tubular cylindrical portion 612. At one end of the second outer cylindrical portion is a second outer flange plate 613 having a plurality of apertures for fitment of bolts to connect the flange plate to an adjacent second pipeline section. At one end of the second inner cylindrical portion, there is provided a second inner flange plate 602, which is provided with a plurality of apertures, through which the plurality of bolts 604 fit, to secure the first and second inner flange plates to each other across the flame arrester element 603 therebetween.

[0100] Referring to figure 7 herein, for each housing portion 601, 602, an internal plate member incorporating an aperture 614, 615 in each housing respectively is aligned substantially centrally along a main length axis of the housing, so that there is a line of sight path through the first aperture at the entry of the housing, and through the second aperture in the plate viewed in a direction along a main central axis of the housing. Each internal plate member incorporating an aperture 614, 615 divides a respective internal cavity 616, 617 into an inner chamber 618, 619 and an outer chamber 620, 621, the outer chamber being nearer the pipeline, and the inner chamber being nearer the flame arrester element 603.

[0101] Preferably, each internal plate member incorporating an aperture 614, 615 has a circular aperture aligned on a main central axis of the housing, and preferably aligned coaxially with the inner cylindrical surface of the cavity in which the plate member is contained. A distance in the axial direction of the perimeter surrounding the aperture is the same distance of the thickness of the internal plate member. That is, the substantially cylindrical sides of the aperture do not extend into the chambers either side of the plate.

[0102] The internal plate member incorporating an aperture may be formed by welding an annular plate inside the housing. However, in a preferred embodiment, the internal plate is formed in a same casting as the inner cylindrical portion, and the whole housing portion 601, 602 may be formed in a single casting.

[0103] The diameter of the aperture in the internal plate members is selected so that the distance which the annular plate member incorporating an aperture 614 extends inwardly from the inner surface of the cylindrical part 607 into the cavity is enough for shock wave pressure to be reduced significantly due to the sudden expansion (rarefaction effect), including some detonation converting into deflagration. The plate member acts as a dam to capture reflections of a shock wave, and in particular, the radially extending components, or components travelling transverse to the main direction of travel of the shock wave, which enters the cavity from a pipeline attached to the device, so that most, or a large part of the radially travelling component of the shock wave is arrested or blocked by the internal plate and is reflected or absorbed internally inside the first chamber 620 (outer chamber).

[0104] As the shock wave/detonation enters the first chamber, due to the increasing lateral dimensional of the outer chamber 620 compared to the diameter across the pipe, the shock wave experiences sudden expansion and the rarefaction effect, being a reduction in pressure in the first chamber, before the shock wave encounters the internal plate member incorporating an aperture 614. Therefore, a central part of the detonation/shock wave which passes through the aperture towards the flame arrester element 603 relatively unimpeded is of reduced pressure compared to the pressure which it had on entry into the cavity and some detonation is converted to deflagration.

[0105] Further, the portion of the shock wave/detonation which passes through the aperture in the internal plate member 614 into the second chamber experiences a second stage of sudden expansion, resulting in a second reduction in pressure due to another rarefaction effect and some detonation converting into deflagration, so that the shock wave/detonation which impinges directly on the flame arrester element 603 is significantly weakened compared to its strength or pressure which it had as it passed through the entry aperture into the cavity.

[0106] The annular plate member 614 also sets up back reflections which rebound around the inside of the first (outer) chamber 620. The optimum aperture diameter for a particular diameter of pipeline may be selected experimentally in terms of pressure drop. Further, the effective reduction of detonation/shock wave pressure depends on the type of gas, the flow rate of the gas, the expected operating pressure and temperature in the pipeline and other factors such as the housing configuration and geometry. The combustion process in the outer chamber may be of chaotic nature. . In a preferred embodiment, the diameter of the apertures in the plate members 614, 615 is slightly larger than the internal diameter of the flame arrester connection with a minimum length of the dam height of 0.2 times the internal diameter of the flame arrester connection (pipe connection diameter). Alternatively, the diameter of the apertues is in the range 1.1 to 1.8 times the internal diameter of the flame arrester connection. A diameter of the aperture in the plate member 614, 615 is larger than the diameter of the entry aperture into the cavity but with a minimum length of the dam height of 0.4 times the diameter of the entry aperture, such as 1.2 -1.5 times the diameter of the entry aperture into the cavity. The desired range is dependent on the the ratio of the flame arrester element to the pipe connection and may be determined by experiment.

[0107] The flame arrester element 603 as shown in figure 7 in this embodiment comprises a plurality of circular crimped ribbon disc shaped flame arrester elements sandwiched one after the other in the axial direction. However in other embodiments, a single flame arrester element having channels in the lengthwise axial direction all the way through the flame arrester element may be used.

[0108] Preferably a distance X between the centre of the internal plate member incorporating an aperture 614 and a front surface of the flame arrester 603 is selected to be in the range 0.3 to 1.0 times the internal diameter of the aperture of the flame arrester connection. However, this distance may be determined by the possible uniformly distributed flow over the flame arrester element under normal operating conditions, leading to a minimum pressure drop.

On the other hand, this distance may be also optimised by the diameter of the aperture of the plate member such as 0.2 to 1.0 times the diameter of the aperture of the plate member. The optimised distance can always be determined by experiment.

[0109] Referring to figure 8 herein, there is illustrated schematically the fourth flame arrester in cutaway perspective view, showing the annular plate member incorporating an aperture and the central apertures though the plate member.

[0110] Referring to figure 9 herein, when viewed in a direction along a main length axis of the housing through the outer aperture, which is the same diameter as the internal diameter of the pipe, there is a clear unobstructed path through the housing, through the central aperture in the plate member, to the flame arrester element in the middle of the housing. This means that in normal use where there is no flame present in the pipeline, gas is unobstructed by the parts of the housing other than the flame arrester element itself.

[0111] In use, the fourth flame arrester under normal operating conditions allows for a relatively low reduction in gas pressure through the flame arrester element 603, which is kept as compact as possible due to the weakening effect provided by the first outer housing portion 601 (and similarly for the second outer housing portion 602 in the opposite direction).

[0112] As mentioned previously, a detonation shock wave which enters the housing can be up to 100 times the initial pressure before ignition due to chaotic behaviour when transitioning of the deflagration to detonation occurs. The pressure within the first (outer) chamber is not uniform, but rather is experienced in high-pressure spots which form at unpredictable places in the chamber due to wave reflections inside the chamber. The presence of the internal plate member 614, 615 reflects parts of the shock wave backwardly into the first chamber, in which the shock wave can reflect around and be absorbed, protecting the flame arrester element 603 from those high-pressure spots, and thereby permitting a relatively lower length of flame arrester element than would otherwise be necessary to prevent the shock wave passing through the flame arrester element to the other side of the device and on into the continuing pipeline. This has a space and weight and cost saving effect on the overall size of the flame arrester device and a length of the flame arrester in the axial direction along the pipeline.

[0113] Referring to figure 10 herein, there is illustrated schematically in cutaway view, a fifth flame arrester 1000 according to fifth specific embodiment.

[0114] The fifth flame arrester comprises a first housing 1001; a flame arrester element 1002; and a second housing 1003.

[0115] The first housing 1001 comprises a cylindrical inlet pipe portion 1004 having an inlet aperture 1005; an intermediate frusto conical reducer portion 1006; and an inner tubular portion 1007 of larger diameter compared to the inlet pipe portion 1004. Extending across the frusto conical reducer portion 1006 is provided an annular plate member incorporating an aperture 1008, having a centrally positioned circular aperture which is concentrically aligned with the inlet pipe portion and inner tubular portion 1007 in this embodiment.

[0116] Installing a plate member incorporating an aperture in the frustum shaped reducer part of the housing enables the overall length of the housing to be relatively shorter than if the plate member were provided in the inner, larger diameter cylindrical portion 1007. The plate member incorporating an aperture divides the housing into a first chamber 1009, and a second chamber 1010, the first and second chambers being connected via the aperture in the plate member 1008.

[0117] Operation of the fifth flame arrester under normal operating conditions is similar to that described for the first to fourth flame arresters described above. Operation of the fifth flame arrester under detonation conditions is similar to that described with reference to the first flame arrester as shown in figure 3 herein.

[0118] Referring to figures 11 to 14 herein, there is illustrated in perspective view, a sixth flame arrester 1100. The sixth flame arrester comprises a first and second housing portions 1101, 1102 respectively, spaced apart from each other and having a substantially cylindrical flame arrester element 1103 positioned therebetween, the first and second housing portions being connected to each other by a plurality bolts 1104. The housing is symmetrical so that the flame arrester operates bidirectionally.

[0119] First housing portion 1101 comprises a first outer tubular cylindrical portion 1105; a first cup or bowl shaped or hemispherical shaped portion 1106; and a first inner tubular cylindrical portion 1107. At one end of the first outer cylindrical portion is a first outer flange plate 1108 having a plurality of apertures for fitment of bolts to connect the flange plate to an adjacent pipeline. At one end of the inner cylindrical portion 1107 is provided a first inner flange plate 1109 having a plurality of apertures through which the set of bolts 1104 connecting the two housing portions together are passed.

[0120] Second housing portion 1102 is of identical construction to first housing portion 1101, except that it does not contain an aperture plate. Second housing portion 1102 comprises a second outer tubular cylindrical portion 1110; a second cup or bowl shaped portion or hemispherical shaped section 1111; and a second inner tubular cylindrical portion 1112. At one end of the second outer cylindrical portion is a second outer flange plate 1113 having a plurality of apertures for fitment of bolts to connect the flange plate to an adjacent second pipeline section. At one end of the second inner cylindrical portion, there is provided a second inner flange plate 1114, which is provided with a plurality of apertures, through which the plurality of bolts 1104 fit, to secure the first and second inner flange plates to each other across the flame arrester element 1103 therebetween.

[0121] Referring to figure 12 herein, for the first housing portion 1101 there is provided a frusto conical shaped plate member 1115, respectively in a cavity formed by the inner cylindrical portion and the curved cup, bowl or hemispherical shaped section 1106. The frusto conical shaped plate member divides the cavity into a first chamber 1119, and a second chamber 1121. The frusto conical internal plate member incorporating an aperture is aligned substantially centrally along a main length axis of the first housing, so that there is a line of sight path through the first initial entry aperture at the entry of the housing, and through the second aperture in the plate viewed in a direction along a main central axis of the housing. The diameter of the second aperture in the plate member is smaller than the diameter of the outer perimeter of the plate member incorporating an aperture which connects with the inside wall of the housing, and the plate member incorporating an aperture has its smaller end containing the aperture, towards the inlet of the housing, and its larger diameter end nearer the flame arrester element, so that the frusto conical plate points towards the direction of the inlet end of the housing.

[0122] Operation of the sixth flame arrester under normal conditions of gas flow is much the same as operation of the first flame arresterd described hereinabove. The sixth flame arrester has a conical plate member incorporating an aperture on one side only as shown. Gas passes from the pipeline, through the entry aperture of the housing, through the aperture in the internal plate member in the cavity, through the flame arrester element and onwards through the second housing and onwards on its journey along the pipeline.

[0123] Under conditions of a flame travelling along the pipeline, in this exam pie into the first housing portion 1101, accompanied by a shock wave travelling along the pipeline, the shock wave enters the first cavity lll9through an entry aperture between the outer cylindrical portion 1105 and the cup, bowl or hemispherical portion 1106 into first chamber 1119, experiencing sudden expansion which reduces the pressure of the shock wave. The frusto conical plate member 1115 causes reflection of the part of the shock wave travelling immediately adjacent the inner wall of the inner cylindrical portion 1107, back into the first chamber 1119, whilst a central portion of the shock wave passes through the central aperture in the frusto conical plate member 1115 into the second chamber 1121. The part of the shock wave which passes into the second chamber experiences expansion from the restricted size of the aperture into the wider size of the second chamber, and therefore undergoes a second stage of reduction in pressure prior to impinging on the surface of the flame arrester element 1103.

[0124] The aperture size of the plate member restricts the high-temperature of combustion products entering into the chamber immediately in the front of the flame arrester element. Hence, as with the second flame arrester described hereinabove, the incident shock wave/detonation encounters a first stage of sudden expansion followed by a second stage of sudden expansion within the cavity of the housing between the inlet to the housing and the flame arrester element 1103 and some detonation becomes deflagration.

[0125] In the case that an identical conical plate member to the conical plate member 1115 is correspondingly installed on the side of a second housing 1102, the flame arrester becomes bidirectional and can be operated in any direction of flow and installation.

[0126] Referring to figures 14 and 15 herein, there is illustrated in perspective view, a seventh flame arrester 1400. The seventh flame arrester comprises first and second housing portions 1401 1402 respectively, spaced apart from each other and having a substantially cylindrical flame arrester element 1403 positioned therebetween, the first and second housing portions being connected to each other by a plurality bolts 1404. The housing is symmetrical so that the flame arrester device operates bidirectionally.

[0127] The seventh flame arrester has first and second housing portions which are longer than those of the first to sixth flame arresters, relative to their diameters, since each housing contains a plurality of plate members incorporating an aperture arranged one after the other serially along a main length axis of the housing. The apertures in the respective plate member incorporating an apertures may have the same cross-sectional area as each other, or a different cross-sectional area to each other. In the case of circular apertures, the diameters of the individual apertures in different plate members may be the same as each other, or may be different. In the embodiment shown, all apertures are the same size as each other.

[0128] First housing 1401 comprises a first outer tubular cylindrical portion 1405 a first cup shaped, or bowl shaped or hemispherical shaped portion 1406; and a first inner tubular cylindrical portion 1407. At one end of the first outer cylindrical portion is a first outer flange plate 1408 having a plurality of apertures for fitment of bolts to connect the flange plate to an adjacent pipeline. At one end of the inner cylindrical portion 1407 is provided a first inner flange plate 1409 having a plurality of apertures through which the set of bolts 1404 connecting the two housing portions together are passed.

[0129] Second housing portion 1402 is of identical construction to first housing portion 1401, and comprises a second outer tubular cylindrical portion 1410 a second cup or bowl shaped portion or hemispherical shaped 1411; and a second inner tubular cylindrical portion 1412. At one end of the second outer cylindrical portion is a second outer flange plate 1413 having a plurality of apertures for fitment of bolts to connect the flange plate to an adjacent second pipeline section. At one end of the second inner cylindrical portion, there is provided a second inner flange plate 1414, which is provided with a plurality of apertures, through which the plurality of bolts 1404 fit, to secure the first and second inner flange plates to each other across the flame arrester element 1403 therebetween.

[0130] Referring to figure 15 herein, for each housing portion, there is an internal cavity formed between the flame arrester element and the inlet aperture to the housing, surrounded by the cup, bowl or hemispherical shaped portion 1406 and the inner cylindrical portion 1407. Within the cavity there are provided a first and second internal plates, 1415, 1416 respectively, each having a respective central aperture 1417, 1418 respectively. The plurality of apertures plates divide the interior of the housing up into a plurality of chambers. The aperture cross sectional area of each plate member may be different, but generally larger than the cross sectional area of the pipeline, or the inlet aperture to the housing. Each internal plate in the housing cavity is aligned substantially centrally along a main length axis of the housing, so that there is a line of sight path through the first aperture at the entry of the housing, and through the aperture in the first plate member 1415, and through the aperture in the second plate member 1416 as viewed in a direction along a main central axis of the housing.

[0131] In use, under normal operation where there is no flame present in the pipeline, gas flows through the outer cylindrical section 1405, through a first aperture 1419 into the first housing portion 1401, and into a first chamber 1420 between the first aperture and the first internal plate member 1415. Gas flows through the aperture in the first internal plate member 1415 into a second chamber 1421 between the first and internal plate member 1415 and the second internal plate member incorporating an aperture member 1416. Gas flows through the central aperture 1418 in the second internal plate member 1416 into a third chamber 1422 between the flame arrester element 1403 and the second internal plate 1416.

[0132] Therefore the shock wave entering the housing encounters a first stage of shock wave rarefraction as it expands into a first chamber, followed by a first stage of reflection, separation, and! or attenuation as it impinges on a first internal plate member. A portion of the shock wave which passes through the aperture in a first plate member member encounters the second stage of shock wave rarefaction as to expands into a second chamber beyond the first chamber, and a portion of the resulting residual shock wave passes through an aperture in a second internal plate member and into a third chamber, where it experiences shock wave rarefaction and reduction in pressure, before impinging on a front face of the flame arrester element 1403. In the first chamber, reflections of the shock wave resound around inside the chamber, and some portions of the shock wave pass through the aperture in the first internal plate into the second chamber. In the second chamber, portions of the shock wave rebound around inside the second chamber, and some of the shock wave passes through the aperture in the second internal plate into the third chamber. The incident shock wave is therefore attenuated through the first, second and third chambers before encountering the flame arrester element 1403.

[0133] It will be appreciated by those skilled in the art that in further embodiments, it is possible to include further serial internal plate members, so that a detonation front experiences in series, a plurality of internal annular plates in the cavity of the first housing portion between the pipeline and the flame arrester element.

[0134] In a yet further embodiment, the first housing of the seventh flame arrester can be used on one side of the flame arrester element, with the second housing of the flame arrester shown in figures 11 and 12 herein to create a unidirectional flame arrester having plate members incorporating apertures on one side of the flame arrester element, and no plate member incorporating an apertures in the housing on the other side of the flame arrester element.

Similarly, the first housing of the seventh embodiment flame arrester can be used with a second housing of the third embodiment flame arrester as shown in figures 6 and 7 herein to create an asymmetric flame arrester having two plate members on one side, and a single plate member incorporating an aperture on another side.

[0135] Referring to figure 16 herein, there is illustrated schematically in cutaway view from one side a bisected portion of an eighth flame arrester according to an eighth specific embodiment. The eight flame arrester has a housing having an asymmetric reducer portion, so that the housing is asym metric.

[0136] The eighth flame arrester 1600 comprises a tubular housing 1601 and a flame arrester element 1602. As in other embodiments described herein, the flame arrester element may comprise a spiral wound crimped ribbon metal strip having a plurality of through passages to allow gas to flow through the arrester element under normal flow conditions. The tubular housing comprises an inlet pipe portion 1603 of internal diameter D; a frusto conical reducer portion 1604; a cylindrical portion 1605; and a plate member 1606 comprising an aperture 1607. The plate member 1606 divides an internal cavity of the housing into a first chamber 1608 and a second chamber 1609, such that gas is able to pass between the first and second chambers through the aperture 1607 in the plate member 1606.

[0137] Preferably the aperture 1607 is circular, and is slightly offset from a main length axis 1610 of the flame arrester, so that it is fully to one side of the main centre line of the flame arrester. A diameter d of the aperture may be slightly larger than an internal diameter D of a circular entry aperture to the housing. There may be a plurality of second apertures 1607 in the plate member incorporating an aperture, such that the total area of the apertures in a plane perpendicular to the main axial length of the flame arrester is greater than the cross sectional area of the bore of the pipeline of internal diameter D. In other words, the total combined area of the second apertures in the plate member is greater than the area of the first aperture on entry into the housing, as measured perpendicular to the main length axis of the flame arrester.

[0138] Referring to figure 17 herein, there is illustrated schematically in cutaway view from one side a bisected portion of a ninth flame arrester according to a ninth specific embodiment. The ninth flame arrester has an asymmetric shaped reducer, with an inlet offset from the centre of its flame arrester element.

[0139] The ninth flame arrester 1700 comprises a tubular housing 1701 and a flame arrester element 1702. The tubular housing comprises an inlet pipe portion 1703 of the internal diameter D; a frusto conical reducer a portion 1704; a cylindrical portion 1705; and an plate member incorporating an aperture 1706 comprising an aperture 1707. The plate member 1706 divides an internal cavity of the housing into a first chamber 1708 and a second chamber 1709, such that gas is able to pass between the first and second chambers through the aperture 1707 in the plate member.

[0140] Preferably the aperture 1707 is circular, and is offset from a main slightly larger length axis 1710 of the flame arrester. Preferably a diameter d of the aperture is than an internal diameter D, (d > D)of a circular entry aperture to the housing.

[0141] In the ninth embodiment of figure 17, there may be a plurality of apertures 1707, such that the total area of the apertures in a plane perpendicular to the main axial direction is greater than the total area of the inlet aperture to the housing in a plane perpendicular to the main axial direction. The total area of the plurality of second apertures 1707 is greater than the area of the inlet aperture. :1.5

[0142] Referring to figure 18 herein, there is illustrated schematically in cutaway view from one side a tenth flame arrester according to a tenth specific embodiment.

[0143] The flame arrester comprises a housing 1800, consisting of an outer cylindrical section 1801, a frusto conical reducer section 1802, a cylindrical inner portion 1803, and a flame arrester element 1804. The cylindrical inner section 1803 is of a larger diameter than the cylindrical outer section 1801. At a transition between the cylindrical outer section 1801 and the smaller end of the conical reducer section 1802, there is a first aperture where gases enter the flame arrester. There is provided an annular flat aperture plate 1805, which has a plurality of annular segment apertures arranged concentrically around a circular central plate portion 1806. The aperture plate extends across an inner diameter of the cylindrical inner portion 1803 and is positioned between the inlet aperture and the flame arrester element.

[0144] The aperture plate 1805 divides the space inside the housing into a first chamber 1807, and a second chamber 1808, and gas can pass from the first chamber to the second chamber through the plurality of apertures in the aperture plate.

[0145] Referring to figure 19 herein, there is illustrated schematically in view along a main central axis in the direction of gas flow, the aperture plate of the tenth flame arrester of figure 18 herein. The aperture plate comprises a flat annular plate portion 1805, and a circular plate portion 1806 of smaller external diameter than a diameter of the aperture, and which is concentrically arranged centrally in the annular plate portion 1805. There are provided a plurality of radially extending connector portions 1809-1812 which hold the circular centre plate in position. In practice, the whole plate component can be fabricated from a single sheet of metal or other material. Preferably the total cross-sectional area of the plurality of apertures defined between the outer annular portion 1805, the inner circular portion 1806, and the radially extending connector portions 1809- 1812, is greater than the cross-sectional area of the first aperture at the outer end of the frusto conical reducer section.

[0146] In use, under normal gas flow, gas passes through the outer cylindrical section 1801, expands into the reducer section 1802, and passes through the plurality of apertures in the plate member. Under conditions of detonation, a shock wave enters the first chamber through the first aperture, and experiences a first stage of rarefaction. Portions of the shock wave are reflected in the first chamber by the annular outer ring 1805, by the central circular portion 1806, and by the plurality of connecting portions 1809-1 81 2. A portion of the non-reflected incident shock wave will pass through the apertures, and will experience a second stage of rarefaction as they propagate from the apertures into the second chamber. Reflected waves in the first chamber, are reflected by the aperture plate back onto the inner wall of the housing. These reflected waves also experience a second stage of rarefaction as they pass through the apertures into the second chamber.

Aperture Perimeter Variations [0147] Referring to figure 20 herein, there is illustrated schematically in cross sectional view, different perimeter portions of the plate member incorporating a apertures at the perimeter of the aperture in the plate member.

The shape of the plate at the perimeter of the aperture can be varied with various levels or chamfers or rounding, either on one side or both sides of the plate. The variations shown in figure 20 are applicable to each of the embodiments described hereinabove. The perimeter of the aperture where the aperture passes from one side of the plate to another, is generally cylindrical, but the portions of the plate immediately adjacent the main surfaces of the plate member may be curved or chamfered to allow better gas flow and/or reduce turbulence during normal operation of the flame arrester.

[0148] In figure 20a, there is shown a 90° edge on both sides of the plate member surrounding an aperture.

[0149] In figure 20b, there is shown a perimeter of an aperture having one squared off 90° edge, and another edge which has been chamfered at 45°, connecting an inner cylindrical surface, and an outer flat planar surface of the plate member.

[0150] In figure 20c, there is shown an aperture having a 45° chamfered perimeter edge in a direction upstream of the gas flow, and a 90° edge in a direction downstream of the gas flow direction.

[0151] In figure 20d, there is shown an aperture in an plate member, in which a circular perimeter of the aperture on the side of the plate upstream of the gas flow is rounded off, and a second perimeter of the aperture on the side of the plate member downstream of the gas flow has a 90° edge.

[0152] In figure 20e, there is shown a further aperture perimeter shape in which a perimeter of the aperture on the side of the plate member upstream of the gas flow has a rounded circular edge, and similarly, a perimeter of the aperture on a downstream side of the plate member incorporating an aperture is also similarly rounded with a rounded circular edge.

[0153] In all cases, the aperture sides extend only throught the width of the flat or frustrum shaped plate member, and do not extend beyond the front or rear faces of the plate.

[0154] Referring to figure 21 herein, there is shown in cross-sectional view a further example of a perimeter profile of the plate member incorporating an aperture, showing the edges of the plate around the aperture. The plate member incorporating an aperture 2100 has a frusto conical surface 2101 which extends around a perimeter of an aperture in the plate member, where in this case, a smaller dimension across the aperture is presented on the face of the plate member which is upstream of the gas flow, on the side of the first chamber, and there is a frusto conical surface through the width of the plate extending along a main length axis of the housing, there being a relatively wider dimension edge on the side of the second chamber. Adjacent the first chamber, an edge 2102 of the plate member incorporating an aperture forms an angle of less than 90°, and adjacent the second chamber, an angle of the edge 2103 of the orifice has an angle of greater than 90° as shown in cross sectional view. The sides of the aperture across the width of the plate therefore diverge in the direction of gas flow.

[0155] Referring to figure 22 herein, there is shown in cross-sectional view yet another example of a profile of the plate member incorporating an aperture around a perimeter of the aperture. In this case, a frusto conical surface 2201 has its wider portion facing the first chamber, upstream of the gas flow, and has its narrower portion adjacent the second chamber and downstream of the gas flow. Adjacent the first chamber, an edge of the plate member incorporating an aperture has an angle of greater than 9Q°, and adjacent the second chamber, an angle of the edge of the orifice has an angle of less than 9Q° as shown in cross sectional view. The sides of the aperture therefore, converge in the direction of the gas flow.

[0156] Referring to figure 23 herein, there is shown a cross-sectional view of a further example of a perimeter of an aperture in a plate member incorporating an aperture. In this example, the plate member incorporating an aperture is concave on the side facing the first chamber, and concave on the side facing the second chamber, so that the thickness of the plate member around the perimeter of the aperture is less than the thickness of the plate member nearer the internal walls of the housing. In other words, the plate member becomes relatively thinner towards the centre of the housing. Where the aperture passes through the plate member, there is a substantially cylindrical surface 2301 defining the aperture. In the embodiment shown in figure 23, the plate member becomes gradually thicker in a radial direction extending outwardly from the centre of the aperture.

[0157] In each of the above examples of perimeter profiles for the aperture in the plate member incorporating an aperture, there are no protrusions extending beyond the edges of the perimeters of the orifice or aperture. In contrast to some prior art flame arresters, in the embodiments herein, there are no pipes stubs or conduits to transfer or guide gases or shock waves to a position nearer the flame arrester element.

[0158] To reduce the pressure drop across the plate member incorporating an aperture, the aperture profile such as with the inlet edge rounded is contemplated. In these variations, distance in the axial direction of the perimeter surrounding the aperture is slightly less than or the same as the distance of the thickness of the internal plate member member. That is, the substantially cylindrical sides of the aperture bore do not extend into the chambers either side of the plate and the bevel is limited by the thickness of the plate member.

[0159] Furthermore, the aperture I configurations may include in particular (but not limited to) circular, square, triangle and hexagonal (honeycomb) and other polygonal apertures, including segment of those shapes [0160] In the embodiments described hereinabove, the plate member incorporating an aperture as generally been described as a solid metal plate. The plate extends at least partially across a housing in a plane perpendicular to a main axial length of the housing. However, in other embodiments, the plate member incorporating an aperture may itself be formed of a porous material, for example a plurality of knitted or woven mesh layers sintered together to form a gas permeable plate, which also has sufficient strength and rigidity to reflect a shock wave.

[0161] In the embodiments shown above there have been described flame arresters which have generally circular cross-section in a direction transverse two main axial length of the flame arrester, and most conventional flame arresters have such a circular cross-section. However in the general case it is not essential that the embodiments have a circular cross-section, and other shape cross-sections such as generally square, rectangular, or ovoid may be possible, in which case the inwardly projecting plate member need not be annular, nor have a circular aperture, but may have a corresponding square, rectangular or ovoid shaped aperture. The aperture of the plate member may be off-centre or a segment of circle aperture, which is particularly important for some special designs such an eccentric embodiment.

[0162] In alternative embodiments, the aperture in the plate member may be offset from a central axis of the flame arrester, rather than being symmetrical on the main central length axis of the flame arrester.

[0163] Accordingly, the aperture/bore in the plate member can be shaped or positioned to create advantages for specific applications and to enhance the performance of flow restriction plates.

[0164] In the above embodiments, preferably, a cross-sectional area of a single aperture in the plate member, or where there are a plurality of apertures, a combined cross-sectional area of all the apertures in a single plate member is greater than a cross-sectional area of an inlet aperture to the flame arrester.

[0165] In the above embodiments, a maximum thickness of the aperture plate is relatively small, compared to a maximum dimension of each aperture.

Claims

Claims 1. A flame arrester comprising: a housing having a cavity; a flame arresting element; a plate member extending across said cavity within the housing, said plate member positioned between a first end and a second end of the housing; wherein a radially outermost part of said plate member is attached to part of an inner wall of said housing which is at least as radially distant from a main central axis of said housing as said radially outermost part of said plate member; :is said plate member dividing said cavity into a first chamber and a second chamber; said plate member having at least one aperture; said at least one aperture extending through said plate member in a direction transverse to a main surface of said plate member; characterised by there being no guide members extending from said plate member to direct a pressure wave onto said flame arresting element; wherein the plate member blocks a portion of an incident pressure wave in the first chamber and a portion of any reflected pressure wave within the first chamber from passing into the second chamber; said plate member restricts the flow of hot gases into the second chamber; said at least one aperture rarefies said pressure waves by means of expansion of said pressure waves into the second chamber; and said pressure wave passes from said first chamber to said second chamber only via said at least one aperture through said plate member.
2. The flame arrester as claimed in claim 1, wherein, in use, under conditions of an incident detonation wave, travelling through said first aperture into said cavity, said second aperture acts to defuse said incident detonation wave, prior to said detonation wave encountering said flame arrester element.
3. The flame arrester as claimed in any one of the preceding claims, in which a maximum thickness of the plate member in the axial direction of flow is such that the aperture does not significantly act as a tubular passage or channel to convey a pressure wave to a significant distance in the direction of flow.
4. The flame arrester as claimed in any one of the preceding claims in which a side wall of said aperture through said plate member does not extend beyond a front or rear face of said plate member.
5. The flame arrester as claimed in any one of the preceding claims, wherein said flame arrester element comprises a solid central portion, surrounded by a gas permeable outer portion.
6. The flame arrester as claimed in claim 5, wherein said solid central portion has a diameter slightly less than a diameter D of an inlet aperture to said flame arrester; and wherein said flame arrester element can be viewed directly along a main central length axis of said housing looking into said housing from said outer end of said housing.
7. The flame arrester as claimed in any one of the preceding claims, in which a said aperture is aligned along a main central length axis of said housing, so that there is a line of sight path through said aperture and an inlet aperture to said housing.
8. The flame arrester as claimed in any one of the preceding claims, in which there is a clear unobstructed line of sight path through an inlet aperture to said housing, and through said aperture in said plate member to said flame aresster element.
9. The flame arrester as claimed in any one of the preceding claims, wherein said plate member comprises an annular plate extending at least partly across said internal cavity.
10. The flame arrester as claimed in any one of claims 1 to 6, wherein an aperture in said plate member is positioned eccentrically with respect to a main central length axis of said flame arrester element.
11. The flame arrester as claimed in any one of claims 1 to 6, wherein an aperture in said plate member is positioned offset and away from a main central length axis of said flame arrester element.
12. The flame arrester as claimed in any one of the preceding claims, wherein said plate member becomes relatively thicker in a radial direction outwards from a centre of said aperture, compared to a thickness of said plate member around a perimeter of a said aperture.
13. The flame arrester as claimed in any one of the preceding claims, wherein said plate member comprises a substantially frustum shaped plate member.
14. The flame arrester as claimed in any one of the preceding claims, wherein said plate member comprises a frusto conical shaped member having a smaller diameter end nearer to the outer end of said housing, and having a larger diameter end nearer to said flame arrester element.
15. The flame arrester as claimed in any one of the preceding claims, wherein said aperture plate is formed of a gas porous material.
16. The flame arrester as claimed in any one of the preceding claims wherein said aperture has at least one rounded or chamfered edge.
17. The flame arrester as claimed in any one of the preceding claims, wherein said aperture comprises a circular aperture.
18. The flame arrester as claimed in any one of the preceding claims, wherein said plate member comprises a main aperture and one or a plurality of secondary apertures, each said secondary aperture having dimensions at least one order of magnitude smaller than a maximum dimension of said main aperture.
19. The flame arrester as claimed in any one of the preceding claims, wherein a width dimensiol of said second aperture in a direction transverse to main axial length of said housing is in the range slightly larger than d to 1.8d with a minimum length of dam height of 0.2d, where d is the width dimensional of said first aperture in a direction transverse to a main axial length of said housing.
20. The flame arrester as claimed in any one of the preceding claims, wherein a width dimensional of said second aperture in a direction transverse to main axial length of said housing is in the range slightly larger than d with a minimum length of the dam height of 0.4d, where d is the width dimensional of said first aperture in a direction transverse to a main axial length of said housing.
21. The flame arrester as claimed in any one of the preceding claims, wherein said plate member is positioned an axial length distance from a face of said flame arrester element a distance in the range 0.3 to 1.0 times the width dimension across said first aperture of the housing.
22. The flame arrester as claimed in any one of the preceding claims, wherein said plate member is positioned an axial length distance from a face of said flame arrester element a distance in the range 0.2 to 1.0 times the diameter of the aperture of the plate member.
23. The flame arrester as claimed in any one of the preceding claims, wherein a cross-sectional area of said aperture as viewed in a direction along a main flow direction of said pipeline is greater than an internal cross-sectional area of said pipeline as viewed in a direction along a main flow direction of said pipeline.
24. The flame arrester as claimed in any one of the preceding claims, comprising a second plate member located in said cavity, said second plate member having a further aperture, wherein said second plate member is located in series with said plate member in a direction along a main length axis of said housing.
25. The flame arrester as claimed in any one of the preceding claims, adapted for fitment to an end of a said pipeline, wherein said housing comprises a first end adapted for connection to a first length of pipeline, and a second end adapted for connection to a second length of pipeline.
26. The flame arrester as claimed in any one of the preceding claims, manufactured from a material selected from the following set of materials: stainless steel; a metal alloy; a polycarbonate; a polymer; a plastics material; a porous material.
27. A method of weakening a shock wave in a pipeline, said method corn prising: passing said shock wave through a first aperture having a first cross-sectional area into a first chamber having a second cross-sectional area, said second cross-sectional area being larger than said first cross-sectional area; at least partially arresting said shock wave by a plate member said plate member having a second aperture; passing said shock wave through said second aperture into a second chamber; and passing said shock wave into a flame arrester element.
28. A flame arrester device comprising: a housing comprising a first chamber and a second chamber; a wall member extending across said housing between said first and second chambers; and a flame arrester element; said housing having a first aperture having a first aperture cross-sectional area in a plane perpendicular to a main axial length of said flame arrester device; said wall member having a second aperture having a second aperture cross-sectional area in a plane perpendicular to a main axial length of said flame arrester device; characterised in that: said second aperture cross-sectional area is greater than said first aperture cross-sectional area.