WO2011121336A1

WO2011121336A1 - Safety valve for flammable gas storage

Info

Publication number: WO2011121336A1
Application number: PCT/GB2011/050595
Authority: WO
Inventors: Vladimir Molkov
Original assignee: University Of Ulster
Priority date: 2010-03-30
Filing date: 2011-03-23
Publication date: 2011-10-06
Also published as: GB201018148D0; GB201005376D0

Abstract

This invention relates to safety valve (10) for gas storage cylinders and in particular a hydrogen storage tank, said valve (10) comprising a piston- like dispersing member (18) provided with at least one vent opening (22) formed therein, said dispersing member (18) being slidably mounted within a body (16) of the valve (10) to be moveable between a retracted position, wherein the at least one vent opening (22) is obscured by a portion of the valve body (16), and an extended position, wherein the dispersing member (18) projects from the body to expose said vent opening (22).

Description

SAFETY VALVE FOR FLAMMABLE GAS STORAGE

This invention relates to safety valve for gas storage cylinders and in particular to a pressure relief valve for a flammable gas storage tank, for example a hydrogen storage tank.

Flammable gases, such as hydrogen, are typically stored under high pressure in metal or composite storage tanks. Onboard storage tanks for hydrogen powered vehicles typically operate at pressures up to 700 bar. In the event of a collision and/or in a fire the hydrogen storage tank represents a considerable hazard due to the risk of explosion. It is therefore necessary to provide a pressure relief device (PRD) to quickly release the gas from the storage tank before the pressure increases to a level where there is a risk of explosion. Known PRDs typically include a fusible member upstream of an outlet vent, whereby the fusible member melts when the tank is exposed to high temperatures to open a vent outlet of the valve above a predetermined temperature. Alternatively, or additionally, a burst disc or frangible member may be provided upstream of the vent outlet whereby the burst disc or frangible member fails above a predetermined pressure.

A problem with existing PRDs, particularly for use in vehicles, is that the vent outlet opening needs to be of sufficiently large size to reduce the pressure in the storage vessel sufficiently rapidly to avoid explosion. At a typical storage pressure of 700 bar, an outlet diameter of 5 mm is required to provide sufficiently rapid blow-down of the hydrogen in the vessel to prevent catastrophic failure of the tank and explosion. However, such outlet diameter and gas pressure can result in a jet flame length of 10 to 15 metres from the open pressure relief valve. Therefore prior art systems have been unable to provide sufficiently rapid pressure release while controlling the flame length from the valve outlet opening.

Furthermore, the vent outlet is typically exposed during normal use of the tank and may become blocked or damaged by debris or by fragments or molten material from the burst disc or frangible/fusible member, reducing or preventing the flow rate of gas through the valve in the event of a fire. This is a particular problem where small outlet orifices are provided, of the order of fractions of a millimetre, required to control flame length.

Another problem with prior art PRDs is that the fusible element or other temperature responsive member typically used to trigger the opening of the PRD at a predetermined temperature may not be directly exposed to a fire, for example where the fire impinges on a hydrogen storage tank at a location remote from the pressure relief valve. This may result in the tank burning through before the PRD is activated, particularly where the tank is made from a composite material, such as carbon fibre reinforced resin. Such could result in catastrophic failure of the tank with disastrous consequences.

According to a first aspect of the present invention there is provided a safety valve for a gas storage tank, in particular a hydrogen storage tank, said valve comprising a piston-like dispersing member provided with at least one vent opening formed therein, said dispersing member being slidably mounted within a body of the valve to be moveable between a retracted position, wherein the at least one vent opening is obscured by a portion of the valve body, and an extended position, wherein the dispersing member projects from the body to expose said vent opening. Preferably said dispersing member comprises a substantially tubular body mounted within a cylindrical opening in the valve body, said at least one vent opening extending through a side wall of said tubular body. Preferably said dispersing member is provided with a plurality of vent openings formed in the side wall of said tubular body. One or more of said vent openings may be angled to impart rotation to the tubular body during the escape of gas therefrom. An upper part of the tubular body may comprise a domed or conical head portion. At least a portion of said one or more vent openings may extend through said domed or conical shaped portion.

In one embodiment, said vent openings may take the form of axially and/or circumferentially extending slots. The individual slot area of each vent opening may be relatively small (in order to create diffusion of gas and reduce gas speed as it is expelled, enhancing dilution of the gas). However the combined area of the slots may be relatively large to ensure adequate flow rate to achieve the required blow down time.

A blocking member may be located within the tubular body of the dispersing head, said blocking member being moveable between a first position, wherein the blocking member obscures at least a portion of said at least one vent opening, and a second position, wherein the blocking member is clear of the at least one vent opening, said blocking member being adapted to move towards its second position under the action of high pressure gas within the gas storage tank when the safety valve is activated.

Preferably said blocking member comprises a piston slidably mounted within the tubular body of the dispersing head. Preferably an outer end of the dispersing head is provided with a vent aperture to allow the release of gas from a region of the dispersing head defined between the piston and the outer end of the dispersing head to allow the piston to move between its first and second positions. Preferably said vent aperture is dimensioned to restrict the flow of gas through said vent opening to restrict the speed of movement of the piston between its first and second positions.

Preferably said one or more vent openings comprise one or more axially extending slots extending through a side wall of the tubular body of the dispensing head such that the effective discharge area of the one or more vent openings gradually increases as the piston moves from its first position to its second position. This reduces the flow rate through the vent openings when the dispersing head first moves to its extended position and avoids excessive flame length.

Preferably the safety valve further comprises at least one heat sensitive trigger means for retaining the dispersing member in its retracted position within the valve body.

Said at least one heat sensitive trigger means may comprise a fusible or combustible member, glass bulb or any other suitable heat sensitive trigger means.

A rupturable or displaceable member may be provided upstream of the dispersing member to isolate the dispersing member from the atmosphere when in its retracted position. The rupturable or displaceable member may serve to protect the dispersing member, and in particular the vent openings, from becoming damaged or clogged. The rupturable or displaceable member may form a seal between the dispersing member and the outlet of the valve. The rupturable or displaceable member may be rupturable or displaced by the dispersing member under pressure when the trigger means is

activated/released and/or by the gas pressure within the gas storage tank. In one embodiment the rupturable or displaceable member may comprise a rupturable disk. In an alternative embodiment the rupturable or displaceable member may comprise a flap or hinged member hingedly attached to the valve body to be moveable between a first position wherein it blocks movement of the dispersing member and a second position wherein the dispersing member may move to its extended position.

The provision of multiple vent outlets enhances dilution of the escaping gas with air while maintaining sufficient flow rate to achieve rapid blowdown.

The blockage problem is solved by locating the at least one vent opening in a piston like dispersing member that is housed inside the valve body until the valve is activated.

In one embodiment said heat sensitive trigger means may be located downstream of said rupturable or displaceable member for supporting said rupturable or displaceable member against the force applied thereto by the piston like dispersing member under the action of gas pressure within the valve body and/or the gas pressure itself. Said heat sensitive trigger means may comprise one or more fusible or combustible elements located downstream of the rupturable or displaceable member supporting the rupturable or displaceable member.

In an alternative embodiment a valve means is provided upstream of the dispersing member, said valve means isolating the dispersing member from high pressure gas from the tank, said valve means being associated with said heat sensitive trigger means for allowing the communication of high pressure gas to the dispersing member to move the dispersing member to its extended position in response to activation of the heat sensitive trigger means. Preferably said valve means comprises a further rupturable or displaceable member adapted to be supported against the force applied thereagainst by said high pressure gas directly or indirectly by the heat sensitive trigger means.

Said heat sensitive trigger means may comprise a movable support member displaceable between a first position, wherein the support member supports said further rupturable member, and a second position wherein said further rupturable member is unsupported and wherein gas can flow through said further rupturable member to act against the dispersing member, said moveable support member being retained in its first position by means of at least one fusible or combustible member.

In one embodiment said heat sensitive trigger means further comprises an elongate tube containing a displaceable material, said elongate tube communicating with a plurality of individual heat sensors, each heat sensor comprising a displaceable piston mounted within a body and moveable between a retracted position and an extended position, a fusible or combustible element being provided for retaining the displaceable piston in its retracted position, said displaceable material engaging the displaceable piston of each heat sensor, whereby, when, the displaceable piston of each heat sensor is in its respective retracted positions, the movable support member of the heat sensitive trigger is retained in its first position by means of said displaceable material, and wherein displacement of the piston of any one of the heat sensors to its extended position due to the melting or burning of its associated fusible or combustible element allows displacement of the displaceable material within the elongate tube such that the moveable support member is able to move towards its second position, causing the rupturable member to rupture and the dispersing member to be exposed to high pressure gas, causing the dispersing member to be displaced to its extended position.

Said displaceable material may comprise a plurality of balls. Said balls may be arranged in groups adjacent each respective heat sensor such that at least one of the balls of each group engages the displaceable piston of a respective heat sensor mounted on a side opening of the tube. Said groups may be separated by elongate spacers or bushes to reduce the overall weight of the heat sensitive trigger means.

In alternative embodiment said heat sensitive trigger means further comprises an elongate fusible or combustible tube containing a blocking member or material, such that said blocking member or material retains the movable support member in its first position to provide support for the further rupturable member until the integrity of the fusible or combustible tube is compromised by a fire acting on a portion of said tube, whereby the blocking member or material is released to allow the support member to move to its second position causing the rupturable or displaceable member to rupture or be displaced and the dispersing member to be exposed to high pressure gas, causing the dispersing member to be displaced to its extended position.

A seal means may be provided against which the support member acts when in its second position to prevent the flow of gas into the tube.

Various embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 is a longitudinal sectional view through a safety valve in accordance with a first embodiment of the present invention in a closed configuration;

Fig. 2 is a longitudinal sectional view through the safety valve of Fig. 1 in an open or activated configuration;

Fig. 3 is a sectional view of the valve of Fig. 1 on line A-A of Fig. 2;

Fig. 4 is a longitudinal sectional view through the dispersing member of the valve of

Fig. 1 with a modified outlet arrangement;

Fig. 5 is a sectional view on line B-B of Fig. 4;

Fig. 6 is a sectional view on line C-C of Fig. 4;

Fig. 7 is longitudinal sectional view through the dispersing member of the valve of Fig. 1 with a further modified outlet arrangement;

Fig. 8 is a sectional view on line D-D of Fig. 7;

Fig. 9 is a sectional view on line E-E of Fig. 7;

Fig. 10 is a sectional view on line F-F of Fig. 7;

Fig. 1 1 is a sectional view through a modified dispersing member wherein the outlets are arranged to impart rotation upon the dispersing member;

Fig. 12 is a longitudinal sectional view through a safety valve in accordance with a further embodiment of the present invention; Fig. 13 is a plan view of the rupture disc of the valve of Fig. 12;

Fig. 14 is a longitudinal sectional view through a safety valve in accordance with a further embodiment of the present invention;

Fig. 15 is a sectional view through a safety valve comprising a dispersing member having a modified shaped in accordance with a further embodiment of the present invention; and

Fig. 16 is a longitudinal sectional view through a safety valve in accordance with a further embodiment of the present invention.

As illustrated in Figs. 1 and 2, a safety valve 10 for releasing gas from a gas storage tank in the event of a fire comprising a valve body 12 defining a gas flow passage therethrough, the valve body 12 and having a threaded end 14 for connection to a vent outlet of a gas storage tank. An upper part of the valve body 12 defines a cylindrical housing 16 within which a piston like dispersing member 18 is slidably mounted for movement between a retracted position, shown in Fig. 1 , wherein the dispersing member 18 is located within the valve body 12, and an extended position, shown in Fig. 2, wherein the dispersing member 18 projects from the valve body 12. A lower end 20 of the dispersing member 18 is dimensioned to be a close sliding fit within the cylindrical housing 16 of the valve 10. A plurality of axially and/or circumferentially extending slots 22 are formed in the sides of the dispersing member 18 to define vent outlets, as will be described below in more detail.

In the retracted position shown in Fig. 1 , an upper face 24 of the dispersing member 18 contacts a rupturable disk 26 which isolates the high pressure gas in the valve body 12 from the atmosphere and protects the dispersing member 18, and its vent outlets 22, from damage or blockage from debris.

A heat sensitive trigger means in the form of a fusible (or combustible) element 28, for example a plug formed from a relatively low melting point alloy, is mounted in a cap member 30 secured to the top of the valve body 12 to support the rupturable disk 26 and to retain the dispersing member 18 within the valve body 12.

When the safety valve 10 is exposed to a temperature over a predetermined maximum, for example in the event of a fire, the fusible element 28 melts and no longer provide support for the rupturable disk 26 which is therefore no longer able to support the force exerted by the dispersing member 18 under the action of the pressurised gas within the storage tank. The disk 26 ruptures and the dispersing member 18 slides within the cylindrical housing 16 of the valve body 12 to move to its extended position, projecting through the cap member 30. The stepped lower end of the dispersing member 18 abuts the cap member 30 to prevent the dispersing member 18 from completely leaving the valve body.

In the embodiment shown in Fig. 3, a plurality of longitudinally extending elongate slots 22 are formed at circumferentially spaced locations around the circumference of the dispersing member 18 defining a plurality of vent opening. The total area of the slots 22 is sufficient to provide rapid blow down of the gas pressure within the tank to which the safety valve 10 is fitted while the small cross section of each individual slot 22 and the spacing of the slots 22 around the dispersing member 18 disperses and dilutes the escaping gas over a large area to resist combustion of the gas and, should combustion occur, preventing the formation of a long jet flame.

As shown in Figs. 4, 5 and 6, in an alternative embodiment the slots 22A may extend circumferentially around the dispersing member 18 instead of longitudinally. In the embodiment shown in Fig. 7, the dispersing member may be provided with a mixture of circumferentially and longitudinally extending slots, in the example shown four

circumferentially extending slots 22A and four longitudinally extending slots 22B.

In an alternative embodiment illustrated in Fig. 1 1 , the elongate slots 22C are angled so that the gas escaping though the slots 22C imparts a rotational force onto the dispersing member 18, whereby the rotation of the dispersing member 18 provides even greater mixing of the escaping gas with air to further dilute the escaping gas, further reducing the risk of the formation of a flame.

In the embodiment shown in Figs. 1 and 2, the fusible element 28 is located directly on top of the rupturable disk 26 to directly support the dispersing member 18. A problem with such arrangement is that the valve will only respond to the temperature conditions directly surrounding the valve.

In an alternative embodiment shown in Fig. 12, the problem of sensing the exposure of a gas storage tank to fire at any location over the whole surface of the tank is solved by providing an elongate sensing tube 40 which accommodates multiple passive fire sensors 42A, 42B, 42C.

Each fire sensor 42A, 42B, 42C comprises a piston 44 slidably mounted in a cylindrical housing 46 to be movable between a retracted position, wherein the piston 44 is located within the cylindrical housing 46, and an extended position, wherein at least a portion of the piston 44 projects from the housing 46, the piston 44 being retained in its retracted position by a fusible or combustible element 48. A first fire sensor 42A is mounted on a distal end of the elongate sensing tube 40. Further fire sensors 42B, 42C are mounted on side openings formed in the side wall of the sensing tube 40 at spaced locations along the length of the sensing tube 40.

A base portion 52 of the sensing tube 40 is connected to threaded connector 54 for connection to a vent outlet of the pressurised gas storage tank. A rupturable disk 56 is located between the base portion 52 of the sensing tube 40 and the threaded connector 54 for isolating the sensing tube 40 from high pressure gas within the storage tank.

A pressure relief passage branches 50 off from the sensing tube 40 immediately upstream of the rupturable disk 56. A safety valve 100 is mounted on an end of the pressure relief passage 50 comprising a cylindrical housing 1 16 within which a piston like dispersing member 1 18 is slidably mounted for movement between a retracted position wherein the dispersing member 1 18 is located within the valve body 1 12, and an extended position wherein the dispersing member 1 18 projects from the valve body 1 12. A lower end 120 of the dispersing member 1 18 is dimensioned to be a close sliding fit within the cylindrical housing 1 16 of the valve 100. A plurality of slots 122 are formed in the sides of the dispersing members 1 18 to define vent outlets 122 are will be described below in more detail. A further rupturable disk 126 is provided upstream of the dispersing member 1 18 for isolating the valve body 1 12 from the atmosphere. A protective cap 130 may be fitted over the end of the valve 100 to protect the rupturable disk 126. The cap may be sufficiently loose fitting to allow the cap 130 to be displaced by the dispersing member 1 18 when the dispersing member 1 18 moved to its extended position when the heat sensitive trigger means is activated.

The safety valve 100 may be essentially the same as that shown in Figs. 1 and 2, with the removal of the fusible element therefrom due to the presence of the rupturable disk 56 located upstream of the safety valve 100 which isolates the dispersing member 1 18 of the valve 100 from high pressure gas within the storage tank until one of the multiple fire sensors 42A, 42B, 42C is triggered.

The rupturable disk 56 is supported by a support piston 60 slidably mounted within the base 52 of the sensing tube 40. An upper end of the support piston 60 engages a plurality of balls 62 tightly packed within the sensing tube 40 and supported by the pistons 44 of the fire sensors 42A,42B,42C such that, when the respective piston 44 of each fire sensor 42A,42B,42C is in its retracted position, such pistons 44 retain the balls 62 in position within the sensing tube 40 to retain the support piston 60 in a first position, shown in Fig. 12, against the rupturable disk 56 to support the rupturable disk 56. At least one ball 62 is located adjacent each respective side opening communicating with each of the fire sensors 42B,42C mounted on the side of the sensing tube to extend into the side opening whereby the respective ball 62 abuts the piston 44 of the respective fire sensor. In the illustrated embodiment groups of three balls 62 are provided adjacent each side opening, lightweight elongate bushes 64 being slidably mounted within the sensing tube between each group of balls to reduce the overall weight of the trigger means.

A shown in Fig. 13, the rupturable disk 56 comprises incisions or lines of weakness extending radially outwardly from a centre of the disk to define openable petals to allow gas to flow therethrough while ensuring that the disk 56 remains intact to prevent parts of the disk from blocking flow passages or vent outlets downstream of the disk.

In the event that any of the fire sensors 42A,42B,42C is exposed to a sufficiently high temperature to melt or burn its respective fusible element 48, for example due to exposure to fire, the piston 44 of the respective fire sensor moves to its extended position, allowing displacement of an adjacent ball 62 and/or bushing 40 within the sensing tube. Such displacement allows the support piston 60 to move away from the rupturable disk 56 to a second position and allows high pressure gas to flow through the rupturable disk. High pressure gas can then escape into the pressure relief passage 50 where such gas acts against the dispersing member 1 18 of the valve 100 to cause failure of the rupturable disk 126 adjacent the dispersing member 1 18 and to urge the dispersing member 1 18 to its extended position whereby the gas may escape through the slots 122 formed in the side wall of the dispersing member 1 18.

A seal 70 is provided against which the support piston 60 may abut when it moves away from the rupturable disk to its second position to prevent high pressure gas from flowing into the sensing tube 40. In an alternative embodiment shown in Fig. 14, a sensing tube 80 is formed from a meltable or combustible material and the support piston 60 is supported by an elongate chain 82 or similar blocking member contained within the sensing tube. When the sensing tube 80 is intact, the support piston 60 is held in its first position against the rupturable disk 56 by the elongate chain. When the integrity of the sensing tube 80 is compromised by fire anywhere along its length, the tube 80 breaks and the chain 82 is released, releasing the support piston 60 to move to its second position and allowing gas to flow through the rupturable disk 56.

Again, a seal 70 may be provided against which the support piston 60 may abut when it moves away from the rupturable disk 56 to its second position within the base 84 of the sensing tube 80 to prevent high pressure gas from flowing into the sensing tube.

The body of the tube 80 may be detachably secured to the base portion 84 of the tube to permit replacement of the body of the tube 80 following activation of the heat sensitive triggering means.

Fig. 15 illustrates a safety valve 200 according to a modified embodiment of the present invention wherein the dispersing member 218 comprises a tubular body having a dome shaped upper portion 224 to facilitate penetration of the rupturable disk 226 by the dispersing member 218. The vent outlets are defined by circumferentially spaced longitudinally extending slots 222, said slots 222 extending through the side wall of the dispersing member 218 and extending into the domed upper portion 224 of the dispersing head to maximise the dispersion and dilution of the gases escaping through the slots 222 when the valve is activated.

As with the previous embodiments, the safety valve 200 comprises a valve body 212 defining a gas flow passage therethrough, the valve body 212 and having a threaded end 214 for connection to a vent outlet of a gas storage tank. An upper part of the valve body 212 defines a cylindrical housing 216 within which the dispersing member 218 is slidably mounted for movement between a retracted position, shown in Fig. 15, wherein the dispersing member 218 is located within the valve body 212, and an extended position wherein the dispersing member 218 projects from the valve body 212. A lower end 220 of the dispersing member 218 is dimensioned to be a close sliding fit within the cylindrical housing 216 of the valve 200. In the retracted position shown in Fig. 15, an upper face of the domed upper section 224 of the dispersing member 218 contacts a rupturable disk 226 which protects the dispersing member 218, and its vent outlets 222, from damage or blockage from debris.

The modified valve 200 of Fig. 15 may be used with a fusible element mounted in the cap 230 of the valve to directly support the rupturable disc 226, similar to the embodiment shown in Figs. 1 and 2, or the valve 200 may be used with a sensing tube arrangement as disclosed in Figs. 12 or 14.

In an alternative embodiment (not shown) the dispersing member may be supported by a displaceable member hingedly attached to the valve body to be moveable between a closed position, wherein the displaceable member isolates the high pressure gas in the storage tank from the atmosphere and retains the dispersing member within the valve body, and an open position, wherein the dispersing member is permitted to move to its extended position under the action of the gas pressure in the storage tank. The displaceable member may be integrally formed with the valve body, the displaceable member being attached to the valve body by a live hinge. Alternatively the displaceable member may comprise a separate part which may be hingedly attached to the valve body on one side of the displaceable member. In a similar manner to the previous

embodiments, the displaceable member is retained in its closed position by one or more fusible or combustible elements mounted upstream of the displaceable member, for example a plug formed from a relatively low melting point alloy.

Fig. 16 illustrates a safety valve 300 according to a further modified embodiment of the present invention. The safety valve 300 is generally similar to that of Fig. 1 , comprising a valve body 312 defining a gas flow passage therethrough, the valve body 312 and having a threaded end 314 for connection to a vent outlet of a gas storage tank. An upper part of the valve body 312 defines a cylindrical housing 316 within which a piston like dispersing member 318 is slidably mounted for movement between a retracted position, shown in Fig. 16, wherein the dispersing member 318 is located within the valve body 312, and an extended position, not shown but similar to that shown in Fig. 2, wherein the dispersing member 318 projects from the valve body 312. A lower end 320 of the dispersing member 318 is dimensioned to be a close sliding fit within the cylindrical housing 316 of the valve 300. A plurality of axially extending slots 322 are formed in the sides of the dispersing member 318 to define vent outlets, as will be described below in more detail.

In the retracted position shown in Fig. 16, an upper face 324 of the dispersing member 318 contacts a rupturable disk 326 or displaceable member which isolates the high pressure gas in the valve body 312 from the atmosphere and protects the dispersing member 318, and its vent outlets 322, from damage or blockage from debris, melting alloy or the like.

A heat sensitive trigger means in the form of a fusible (or combustible) element 328 is mounted in a cap member 330 secured to the top of the valve body 312 to support the rupturable disk 326 and to retain the dispersing member 318 within the valve body 312.

A piston 332 is provided within the dispersing member 318 to be a sliding fit within the dispersing member 318 such that the piston 332 is moveable between an inner position, shown in Fig. 16, wherein the piston 332 obscures the vent outlets 322, and an outer position (not shown) wherein the piston 332 abuts an outer end of the dispersing member 318. When in its outer position the piston 332 is clear of the vent outlets 322 and thus ceases to restrict the flow of gas through the vent outlets 322. An O-ring or similar annular seal 334 may be provided adjacent an outer end of the piston 332 to form a substantially tight seal between the piston 332 and the inner bore of the dispersing head 318.

A relatively small vent 336 is provided in an outer end of the dispersing member 318 venting the space 338 between the piston 332 and the outer end of the dispersing member 318. In use, when the fusible element 328 melts due to high temperature, for example in the event of a fire, such element 328 ceases to provide support for the rupturable disk 326 which is therefore no longer able to support the force exerted by the action of the pressurised gas within the storage tank. The disk 326 ruptures and the dispersing member 318 slides within the cylindrical housing 316 of the valve body 312 to move to its extended position, projecting through the cap member 330. The stepped lower end of the dispersing member 318 abuts the cap member 330 to prevent the dispersing member 318 from completely leaving the valve body. At this time the piston 332 obscures the vent outlets 322. The pressurised gas within the storage tank acts on the inner end of the piston 332, compressing the gas within the space 338 between the outer end of the piston 332 and the outer end of the dispersing member 318, such gas is able to gradually escape through the vent hole 336 in the outer end of the dispersing member 318, allowing the piston 332 to gradually move towards its outer position under the action of the pressurised gas within the tank, gradually uncovering the vent outlets 322.

Thus the area of the gas flow path through the vent outlets 322 is small at the time the valve is initially opened due to the restriction defined by the piston 332, restricting the flow rate of gas through the vent outlet 322 when the gas pressure is at its highest, thus reducing the flame length at such time. As the piston 332 moves towards its outer position, the area of the gas flow path gradually increases. At the same time the gas pressure within the tank is reducing, leading to a substantially balanced gas flow rate through the vent outlets 322 (higher pressure - smaller vent area, lower pressure - larger vent area) and avoiding excessive flame length.

The invention is not limited to the embodiment(s) described herein but can be amended or modified without departing from the scope of the present invention.

Claims

1 . A safety valve for a gas storage tank, in particular a hydrogen storage tank, said valve comprising a piston-like dispersing member provided with at least one vent opening formed therein, said dispersing member being slidably mounted within a body of the valve to be moveable between a retracted position, wherein the at least one vent opening is obscured by a portion of the valve body, and an extended position, wherein the dispersing member projects from the body to expose said vent opening.

2. A safety valve as claimed in claim 1 , wherein the dispersing member comprises a substantially tubular body mounted within a cylindrical opening in the valve body, said at least one vent opening extending through a side wall of said tubular body.

3. A safety valve as claimed in claim 2, wherein an upper part of the tubular body comprises a domed or conical head portion.

4. A safety valve as claimed in any one of claims 2 or 3, wherein the dispersing member is provided with a plurality of vent openings formed in the side wall of said tubular body.

5. A safety valve as claimed in any one of claims 2, 3 or 4, wherein one or more of said vent openings are angled to impart rotation to the tubular body during the escape of gas therefrom.

6. A safety valve as claimed in any one of claims 3-5, wherein at least a portion of said one or more vent openings may extend through said domed or conical shaped portion.

7. A safety valve as claimed in any one of claims 2-6, wherein a blocking member is located within the tubular body of the dispersing head, said blocking member being moveable between a first position, wherein the blocking member obscures at least a portion of said at least one vent opening, and a second position, wherein the blocking member is clear of the at least one vent opening, said blocking member being adapted to move towards its second position under the action of high pressure gas within the gas storage tank when the safety valve is activated.

8. A safety valve as claimed in claim 7, wherein said blocking member comprises a piston slidably mounted within the tubular body of the dispersing head.

9. A safety valve as claimed in claim 8, wherein an outer end of the dispersing head is provided with a vent aperture to allow the release of gas from a region of the dispersing head defined between the piston and the outer end of the dispersing head to allow the piston to move between its first and second positions.

10. A safety valve as claimed in any one of claims 8 or 9, wherein said vent aperture is dimensioned to restrict the flow of gas through said vent aperature to restrict the speed of movement of the piston between its first and second positions.

1 1 . A safety valve as claimed in any one of claims 8, 9 or 10, wherein said one or more vent openings comprise one or more axially extending slots extending through a side wall of the tubular body of the dispensing head such that the effective discharge area of the one or more vent openings gradually increases as the piston moves from its first position to its second position.

12. A safety valve as claimed in any preceding claim, wherein the safety valve further comprises at least one heat sensitive trigger means for retaining the dispersing member in its retracted position within the valve body.

13. A safety valve as claimed in claim 12, wherein said at least one heat sensitive trigger means comprises a fusible or combustible member.

14. A safety valve as claimed in any preceding claim, wherein the safety valve further comprises a rupturable or displaceable member located upstream of the dispersing member to isolate the dispersing member from the atmosphere when in its retracted position.

15. A safety valve as claimed in claim 14, wherein the rupturable or displaceable member protects the dispersing member, and in particular the vent openings, from becoming damaged or clogged.

16. A safety valve as claimed in claims 14 or 15, wherein the rupturable or displaceable member forms a seal between the dispersing member and the outlet of the valve.

17. A safety valve as claimed in any one of claims 14, 15 or 16, wherein the rupturable or displaceable member is rupturable or displaced by the dispersing member under pressure when the trigger means is activated/released and/or by the gas pressure within the gas storage tank.

18. A safety valve as claimed in any one of claims 14-17, wherein the rupturable or displaceable member comprises any one of the following: a rupturable disk, a flap or hinged member hingedly attached to the valve body to be moveable between a first position wherein it blocks movement of the dispersing member and a second position wherein the dispersing member moves to its extended position.

19. A safety valve as claimed in any one of claims 14-18, wherein said heat sensitive trigger means is located downstream of said rupturable or displaceable member for supporting said rupturable or displaceable member against the force applied thereto by the piston like dispersing member under the action of gas pressure within the valve body and/or the gas pressure itself.

20. A safety valve as claimed in any one of claims 14-19, wherein said heat sensitive trigger means comprises one or more fusible or combustible elements located downstream of the rupturable or displaceable member supporting the rupturable or displaceable member.

21 . A safety valve as claimed in any one of claims 12-20, wherein the safety value further comprises a valve means located upstream of the dispersing member, said valve means isolating the dispersing member from high pressure gas from the tank, said valve means being associated with said heat sensitive trigger means for allowing the

communication of high pressure gas to the dispersing member to move the dispersing member to its extended position in response to activation of the heat sensitive trigger means.

22. A safety valve as claimed in claim 21 , wherein said valve means comprises a further rupturable or displaceable member adapted to be supported against the force applied thereagainst by said high pressure gas directly or indirectly by the heat sensitive trigger means.

23. A safety valve as claimed in any one of claims 12-22, wherein said heat sensitive trigger means comprises a movable support member displaceable between a first position, wherein the support member supports said further rupturable member, and a second position wherein said further rupturable member is unsupported and wherein gas can flow through said further rupturable member to act against the dispersing member, said moveable support member being retained in its first position by means of at least one fusible or combustible member.

24. A safety valve as claimed in any one of claims 12-23, wherein said heat sensitive trigger means further comprises an elongate tube containing a displaceable material, said elongate tube communicating with a plurality of individual heat sensors, each heat sensor comprising a displaceable piston mounted within a body and moveable between a retracted position and an extended position, a fusible or combustible element being provided for retaining the displaceable piston in its retracted position, said displaceable material engaging the displaceable piston of each heat sensor, whereby, when, the displaceable piston of each heat sensor is in its respective retracted positions, the movable support member of the heat sensitive trigger is retained in its first position by means of said displaceable material, and wherein displacement of the piston of any one of the heat sensors to its extended position due to the melting or burning of its associated fusible or combustible element allows displacement of the displaceable material within the elongate tube such that the moveable support member is able to move towards its second position, causing the rupturable member to rupture and the dispersing member to be exposed to high pressure gas, causing the dispersing member to be displaced to its extended position.

25. A safety valve as claimed in claim 24, wherein said displaceable material comprises a plurality of balls.

26. A safety valve as claimed in claim 25, wherein said balls are arranged in groups adjacent each respective heat sensor such that at least one of the balls of each group engages the displaceable piston of a respective heat sensor mounted on a side opening of the tube.

27. A safety valve as claimed in claim 26, wherein said groups are separated by elongate spacers or bushes to reduce the overall weight of the heat sensitive trigger means.

28. A safety valve as claimed in any one of claims 12-22, wherein said heat sensitive trigger means further comprises an elongate fusible or combustible tube containing a blocking member or material, such that said blocking member or material retains the movable support member in its first position to provide support for the further rupturable member until the integrity of the fusible or combustible tube is compromised by a fire acting on a portion of said tube, whereby the blocking member or material is released to allow the support member to move to its second position causing the rupturable or displaceable member to rupture or be displaced and the dispersing member to be exposed to high pressure gas, causing the dispersing member to be displaced to its extended position.

29. A safety valve as claimed in claim 28, wherein a seal means is provided against which the support member acts when in its second position to prevent the flow of gas into the tube.

30. A safety valve substantially as hereinbefore described with reference to one or more of the accompanying drawings.