GB2272504A

GB2272504A - Flashback arrestor

Info

Publication number: GB2272504A
Application number: GB9322724A
Authority: GB
Inventors: Trevor Thomas Jenkins
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-11-11
Filing date: 1993-11-04
Publication date: 1994-05-18
Anticipated expiration: 2013-11-04
Also published as: GB9322724D0; GB9223646D0; GB2272504B; DE4338226A1

Abstract

In a flashback arrestor unit for welding equipment or the like, a heat-sensitive valve, through which gas is supplied and which operates in the event of substantial or prolonged overheating of the equipment, comprises a permanent magnet (44) and a valve closure (38, 34, 36) of magnetically susceptible material acted upon by a spring (40) which is normally overcome by the force of attraction of the magnet on the valve closure, thereby to keep the valve open. In the event of overheating which causes the valve closure to lose magnetic susceptibility, the spring (40) is free to urge the valve closure against a valve seat in order to close the valve and shut off the gas supply. The valve closure may be formed by two discs (34, 38) and an intervening spaces (36). An aperture (46) in the disc (34) provides a controlled relief of the pressure differential across the disc, thus delaying reopening of the valve, following cooling of the unit. <IMAGE>

Description

2272504 1_ Flashback Arrestor This invention relates to a flashback

arrestor.

Flashback arrestors are safety devices employed in conjunction with welding, cutting or other equipment using fuel gases such as acetylene with compressed air or oxygen.

In equipment as aforesaid flashbacks can occur in which the flame is returned back through the supply hose(s) towards the regulators and the pressurised gas supply cylinders.

In order to minimise danger from flashbacks, a flashback arrestor is fitted downstream of the supply cylinders, typically on the regulator outlet or in the pipeline downstream of the regulator.

A typical flashback arrestor unit comprises two or more machined metal devices assembled together, in particular including a flame arrestor in the form of a sintered, i.e. porous, stainless steel member, commonly a hollow cylinder or a disc, through which the gas is supplied to the point of use. When a flashback occurs, the flame front is stopped at the sintered filter, which cools gases passing back through it in the upstream direction owing to back pressure, thereby to prevent the possibility of re- ignition on the upstream side of said sintered filter.

A flashback arrestor unit commonly also includes a spring loaded nonreturn valve on the upstream side of the flame arrestor. The back pressure transmitted through the flame arrestor generally causes closure of this valve to shut off the gas supply, in the event of a back flow of gases. This shut down is only temporary; once the flame has been arrested and the back pressure diminishes, the non return valve automatically re-opens under the normal pressure of the upstream gas supply to allow welding, cutting or analogous use of the gases to be resumed.

Sometimes, an arrestor unit also has a pressure relief valve for relieving undue back pressure, to reduce sooting of the sintered filter, amongst other purposes.

A flashback arrestor unit often also incorporates either a pressuresensitive or a heat-sensitive valve, on the downstream side of the flame arrestor, for shutting off the gas supply if there is a sustained flashback or for any other reason the back pressure and/or temperature on the downstream side of the flame arrestor becomes abnormally high. The heat-sensitive valve, in particular, is not intended to be operated in the event of a normal flashback, which is safely handled by the flame arrestor, but only if dangerous conditions continue to develop.

For this reason, when a heat-sensitive valve is used, this is often released by melting solder or adhesive, and it cannot be reset. It constitutes a throw-away component of the unit.

According to the invention, there is provided a flashback arrestor unit incorporating a flame arrestor and, downstream or upstream of said flame arrestor, a resettable heatsensitive valve means which is operated, at a given temperature range below that at which a flame is able to pass through the flame arrestor, to shut off gas supply through the unit, said heat-sensitive valve means comprising at least one element of magneticallysusceptible material disposed in the field of a magnet, the magnetic material being one which effectively loses its magnetic susceptibility when sufficiently heated.

Many magnetically-susceptible materials have this property.

The heat-sensitive valve may reset automatically when the temperature in the region of said valve falls below a given temperature or, after the temperature has fallen, it may be manually resettable.

Preferably, when the magnetically-susceptible valve means reaches a given temperature below that at which a flame is able to pass through the flame arrestor, the valve means is released from the field of the magnet to be displaced against a valve seat and thereby shut off gas supply through the unit. Such displacement preferably takes place under the action of a spring, the spring action normally being overcome by the force of attraction acting on the valve means due to the field of the magnet.

Thus, in the preferred arrangement, the valve means is normally held retracted from the valve seat by the force of attraction of the magnet but when, owing to heating due to a flashback or fire the valve means loses its magnetic susceptibility, the spring is operable to advance the valve means away from the magnet into engagement with the valve seat. The pressure of the gas supply may then assist in maintaining closure of the valve. In one arrangement, when the flame arrestor cools and the valve means regains magnetic susceptibility, the force of attraction produced by the magnet overcomes the spring force again to retract the valve means from the valve seat, thus effecting automatic resetting of the valve.

The pressure of the gas supply may then assist in keeping the valve open.

In an alternative arrangement, it is necessary manually to release the gas pressure acting to hold the valve closed following operation. A reset valve used for this purpose may also act as a pressure relief valve for the system, and is preferably located on the upstream side of the flame arrestor.

In the case of automatic resetting, the valve is required not to re-open until the unit has sufficiently cooled for safe operation of the equipment to be resumed, commonly a period of several minutes. The valve means therefore preferably comprises two valve elements in the form of upstream and downstream valve discs one behind the other and separated by a seal, the downstream disc having a peripheral annular projection which directly abuts the valve seat. The upstream disc is made of magnetically susceptible material such as Monel (Trade Mark) metal and the downstream disc is made of brass. A split steel ring clamp, the diametrical centre of which is aligned with the seal, connects the two valve elements and allows limited freedom of movement between them, sufficient when appropriate to open the seal via a small axial aperture in the downstream brass disc.

When the valve is closed, a differential pressure builds up across the valve seat, and the valve cannot re-open, i.e. reset, until this pressure is substantially relieved. However, the upstream valve disc remote from the valve seat is retained in the valve closed position only by the differential pressure acting over the area of the small aperture in the downstream disc directly abutting the v -5 valve seat, and is thereby retracted to the limited extent permitted by the split steel clamp relatively promptly when the unit cools, thus opening the seal. This then enables the differential pressure across the valve seat to be slowly relieved through the small aperture in the disc abutting the valve seat, so that at an appropriate and predeterminable later time the complete valve means is able to retract fully and thereby open the valve. Preferably a steel or more desirably brass shield carried by the downstream valve disc protects the seal from the direct effect of a flashback through the small axial aperture in said downstream valve disc. Additionally, a baffle in the gas supply passage downstream of the flashback arrestor may act to reduce the velocity of a flashback before it reaches the flashback arrestor.

In the preferred case of manual resetting, the valve means can comprise a single magnetically susceptible valve element, normally held retracted from a valve seat by a permanent magnet, and spring biassed to close against the valve seat when magnetic susceptibility is lost due to the intense heating which arises in the case of an extended flashback.

Again, when the valve is closed in the event of a flashback, a differential pressure builds up across the valve, and the higher pressure on the upstream side has to be relieved before the valve can re-open. it is therefore necessary first to relieve this upstream pressure, for example by partially or wholly disconnecting the gas supply but alternatively in any one of a number of other ways, and then, when the unit has sufficiently cooled, manually open the reset valve on the upstream side of the flame arrestor to relieve the higher pressure on the upstream side of the valve so that the valve element can retract under the influence of the magnet. The gas supply can then be reconnected.

As previously mentioned, in the prior art it is known to provide a springloaded non-return valve on the upstream side of the flame arrestor. Conveniently, in the preferred arrangement according to the invention, the nonreturn valve is also used for resetting. For this purpose, the gas is normally supplied through the non-return valve, which is urged towards the closed position by a suitable spring, the action of which is normally overcome by the supply pressure. However, if there is a substantial pressure build-up on the downstream side of the valve, this pressure acts on the valve member to close the valve by aiding the action of the spring, thus shutting off the gas supply. However, the valve remains manually openable by depressing a member which acts on the valve member through the compressed spring, as required for resetting following operation of the heat sensitive valve.

Two practical arrangements of flashback arrestor unit in accordance with the invention are now described with reference to the accompanying drawings, in which:- Figure I shows a flashback arrestor unit with automatic resetting in axial cross-section; and Figure 2 shows a unit with manual resetting in axial cross-section.

The flashback arrestor unit of Figure 1 comprises three devices of machined metal incorporated into a housing 10.

1 i Thus, the housing contains a sintered stainless steel cylinder 12 which constitutes a flame arrestor. Fitted on the upstream side of the flame arrestor 12, within the housing 10, is a one way valve 14 having an associated resetting spring 16. This valve is conventional and will not be described in detail. However, although shown closed, it is automatically opened when gases under pressure, e.g. oxygen and acetylene, are supplied through the end fitting 18 on the upstream side of the unit, whereby the supply gases enter the chamber 20.

These supply gases are then fed radially outwardly through the wall of the porous flame arrestor 12 into the passageway 22, and pass downstream into radial passages 24 opening into chamber 26 and thence pass downstream, when a heat sensitive valve in accordance with the invention is open, into pipework or hoses leading to the point of use through downstream end fitting 28. In the figure, however, the heat sensitive valve in accordance with the invention, on the downstream side of the flame arrestor 12, is shown closed or operated.

A machined body 30 is accommodated within the housing 10 and at its downstream end incorporates the above-mentioned radial passages 24 opening into chamber 26, which is defined between the downstream end of the body 30 and the end fitting 28. The heat sensitive valve is principally accommodated in the chamber 26, and when closed prevents access of supply gases from said chamber 26 into the end fitting 28.

Thus, the upstream face of the end fitting 28 is grooved to accommodate an 11011 ring seal 32, which constitutes a valve seat. A displaceable valve means constituted by a downstream valve disc 34 of brass, seal 36, and upstream valve disc 38 of Monel (Trade Mark) metal is urged towards a closed position against the valve seat 32 by a spring 40. The valve means also includes a split steel clamp 41 embracing the seal 36 and which permits limited relative axial movement between the valve discs 34 and 38. Fitted within the body 30 is a two-part magnet 44.

Monel (Trade Mark) metal is an example of a magnetically susceptible material which effectively loses its magnetic susceptibility when heated to a given temperature. In practice, this temperature can be approximately predetermined by choice of a suitable magnetically susceptible material from the many available.

During normal operation, therefore, the magnetic attraction of the disc 38 towards the magnet 44 overcomes the force of the spring 40 to retract the discs away from the valve seat 32, opening the valve to passage of supply gases. However, if due to a sustained flashback or possibly an environmental fire the disc 38 is heated to a predetermined temperature at which its magnetic susceptibility is lost, the magnet 44 ceases to be effective and the spring 40 urges the discs against the valve seat 32 to close the valve and stop the gas supply.

It appears obvious that when the discs 34, 38 regain magnetic susceptibility on cooling of the unit, the valve will automatically reopen, i.e. reset. However, in practice the valve is required not to reset for at least a few minutes, while the unit cools to a temperature appropriate for operation of the welding equipment to be resumed. This is made possible partly by the fact that a differential pressure builds up between the upstream and downstream sides of the valve seat 32 when the unit is intensely heated due to the sustained flashback or the like which has caused the valve to close, and partly by the use of the composite valve means constituted by the valve discs 34, 38 and intervening spacer 36.

Thus, the downstream valve disc 34 has a small central aperture 46, shielded by a steel plate 48, so that the differential pressure across the valve seat 32 is applied to the upstream valve disc 38 (via the seal 36) only over the small cross- sectional area of the aperture. Thus, when the unit cools, the valve disc 38 is fairly promptly retracted towards the magnet, taking the seal 36 with it, as far as permitted by the split steel clamp 41. This movement is sufficient to enable the differential pressure across the valve seat 32 to be slowly relieved via the aperture 46 on the downstream valve disc 34, until at an appropriate and selectable (by the dimensions of the aperture) later time the complete valve means is retracted towards the magnet, fully opening the valve. With the heat sensitive valve thus reset, it is safe to re-commence use of the equipment.

The steel shield 48, which is carried by the downstream brass valve disc 34 in a recess therein formed by a peripheral annular projection of the valve disc which engages the valve seat 32, protects the seal 36 from the direct effect of a flashback. Additionally, a baffle 50 in the downstream end fitting 28 acts to reduce the velocity of a flashback before it reaches the flashback arrestor. Finally, shaping 52 in the chamber 26 is important for utilisation of the pressure of the normal gas supply in operation of the flashback arrestor valve means.

The preferred unit of Figure 2, which has a manual instead of automatic resetting facility, is differently arranged.

Normal gas flow from a supply disconnectable at 53 is via a combined spring-loaded non-return and reset valve 54 into a retaining ring 56, which has grooves communicating with spaces outside the flats 58 (see Figures 2(a) and 2(b)) of externally square machined body 60. Normal gas flow is thus downstream past the body 60 into the space 62, radially outwards through flame arrestor 64, and back radially inwards through passages in member 66 into the space 68 and thence into outlet 70 in cap 72. It will thus be appreciated that the heat sensitive valve, which is provided within the machined body 60, is located upstream of the flame arrestor instead of downstream, as in Figure 1. This reduces sooting in the heat sensitive valve.

The heat sensitive valve comprises a permanent magnet 76 which retains in position two disc magnets 78, 80, and a valve disc 82 of magnetically susceptible material (e.g.

Monel metal) acted on by a spring.84. Although two disc magnets 78, 80 are shown, a one-piece magnet could be employed instead. The action of the spring 84 is normally overcome by the force of attraction of the magnets 78, 80, so that the valve disc 82 is normally pulled against the magnets 78, 80, whereby the heat sensitive valve is normally open.

In the event of a substantial and/or sustained overheating of the equipment, the whole assembly incorporating the heat sensitive valve heats up substantially and valve disc 82 loses its magnetic susceptibility, so that the action of the spring 84 is effective to close the valve, shutting off the normal gas supply as the gas is no longer 1, able to flow into the space 62 from outside the externally square- sectioned body 30. Substantial gas pressure also builds up within the heat sensitive valve, as gas is present all round and behind the valve disc 82, which is not a seal fit in the interior of the machined body 30. This high internal gas pressure helps to keep the heat sensitive valve closed, and resetting when the flashback has ceased is only possible by releasing the high internal pressure. It is for this purpose that the non-return valve 54 is provided with a resetting facility. In Figure 2, the valve disc 82 is shown in the closed position.

During normal use, this valve 54 acts in a conventional manner in the case of a high back pressure to prevent back flow of gases. The valve automatically re-opens when the back pressure falls. However, the valve 54 also acts as a reset valve following closure or operation of the heat sensitive valve. The operator is first required to relieve the upstream pressure, possibly by partly or wholly releasing the connector 53 (having switched off the gas flow at a point upstream). These actions are in any case desirable in the case of a prdonged flashback. The valve 54 will then close as a result of back pressure, which overcomes the action of the main spring 86. However, by manually depressing reset button 90 and holding it depressed, the valve 54 is opened via a stack of "0" rings 88 to allow back flow of gases from the inside of the heat sensitive valve, via the outside of the machined body 30, on a reverse path to the normal gas flow when the supply is operative.

Relief of pressure inside the heat sensitive valve then enables this valve to re-open, provided that the valve disc 82 has cooled sufficiently to regain magnetic susceptibility. If not, the heat sensitive valve will re-open when it has sufficiently cooled. Reconnection of the supply will then enable normal use to be resumed.

It will be appreciated that a number of modifications are possible within the scope of the invention hereinbefore defined. For example, a reset valve separate from the upstream non-return valve could be provided on the downstream side of the heat sensitive valve, in particular to replace the automatic resetting facility in the embodiment of Figure 1.

Claims

1. A flashback arrestor unit incorporating a flame arrestor and, downstream or upstream of said flame arrestor, a resettable heatsensitive valve means which is operated, at a given temperature range below that at which a flame is able to pass through the flame arrestor, to shut off gas supply through the unit, said heatsensitive valve means comprising at least one element of magnetically-susceptible material disposed in the field of a magnet, the magnetic material being one which effectively loses its magnetic susceptibility when sufficiently heated.

2. A flashback arrestor unit according to claim 1, wherein the heatsensitive valve resets automatically when the temperature in the region of said valve falls below a given temperature.

3. A flashback arrestor unit according to claim 1, wherein the heatsensitive valve is manua'lly resettable.

4. A flashback arrestor unit according to claim I or claim 2 or claim 3, wherein, when the magnetically susceptible valve means reaches a given temperature below that at which a flame is able to pass through the flame arrestor, the valve means is released from the field of the magnet to be displaced by a spring against a valve seat and thereby shut off gas supply through the unit, the spring action normally being overcome by the force of attraction acting on the valve means due to the field of the magnet.

5. A flashback arrestor unit according to claim 4, wherein, in the closed condition of the heat-sensitive valve, the pressure of the gas supply on the valve means assists in maintaining closure of the valve.

6. A flashback arrestor unit according to claim 2, wherein the valve means comprises two valve elements in the form of upstream and downstream valve discs one behind the other and interconnected by a split ring clamp allowing limited freedom of movement between the valve discs, sufficient when appropriate to release internal pressure via a small axial aperture in the downstream disc.

7. A flashback arrestor unit according to claim 3, having a spring-loaded non-return valve on the upstream side of the flame arrestor adapted to provide a manual resetting facility for the heat-sensitive valve.

8. A flashback arrestor unit according to any of claims 1 to 7, having a baffle in the gas supply passage down- stream of the flashback arrestor acting to reduce the velocity of a flashback before it reaches the flashback arrestor.

9. A flashback arrestor unit substantially as herein- before described with reference to Figure I or Figure 2 of the accompanying drawings.