GB2286804A

GB2286804A - Fusible plug for pressure vessels

Info

Publication number: GB2286804A
Application number: GB9503873A
Authority: GB
Inventors: Charles Peter Hammond
Original assignee: Dunlop Ltd
Current assignee: Dunlop Ltd
Priority date: 1994-02-26
Filing date: 1995-02-27
Publication date: 1995-08-30
Also published as: GB9503873D0

Abstract

A fusible plug assembly adapted to be fitted to the wall of a pressure vessel has a body 1 having a stepped cylindrical through bore 8, 9 which is plugged at its outer end 8 by a billet 2 of eutectic material. An integral piston/seal 3 is housed in bore portion 9 and being subject to pressure within the vessel is urged against the billet 2. On reaching the melting point of the eutectic material, the molten billet 2 is expelled from the bore by the ram action of the piston/seal 3 which is itself subsequently expelled under pressure. This results in the bore 8, 9 being cleared to provide an exhaust port through which pressure in the vessel can be released. The annular lip on piston/seal 3 provides pressure-assisted sealing. Composite piston/seals are described. The plug may be used in an aircraft wheel, which is described (Fig. 3, not shown). <IMAGE>

Description

"FUSIBLE PLUG FOR PRESSURE VESSELS" This invention relates to fusible plugs for pressure relief at high temperatures generally and more particularly, but not exclusively, to fusible plugs having a fusible insert and treing adapted to be itounted in a port in the side of a pressure vessel, such as that provided by an aircraft tyre and wheel assembly, for example.

Such plugs are useful because they fail at the nelting point of the fusible insert which is chosen to be considerably lower than that for the material fran which the pressure vessel is fabrited. This enables a pressure vessel to exhaust before rising internal pressure exceeds the declining strength of the vessel.

A prior art fusible plug of this kind is than in Figure 1 of the accompanying Drawings. A body 100 can be fitted in a port in a wall of a pressure vessel (not shown). The bedy 100 has an exhaust port 101 which communicates with an axial bare 102 which is plugged by a fusible insert 103. With the body 100 fitted in said port the exhaust port 101 c=rrunicates with the exterior of the pressure vessel.The insert 103 is made of a fusible or eutectic alloy which at a relatively low temperature (ie: typically between 155 and 202 degrees Celsius) becomes molten and by the action of pressure on its end 104, exposed to the interior of the pressure vessel, is designed to be ejected fran the bore 102, thus leaving a clear Ere through which the pressure vessel can exhaust.

It has been found in use of such a plug that the material of insert 103 may not be completely ejected. This is because once a passage through the molten material is formed fluid exhausting therethrough cools the sining material al so that it once more forms a solid plug albeit one that @ is breached. Consequently, even though the ambient temperature may remain above the melting point of the insert alloy, a restricted passage through the insert 103 will be maintained and the pressure vessel will not be able to exhaust as rapidly as a cleared bore 102 would permit.

Various means for overcoming the aforementioned disadvantage have been proposed. One proposal the subject of aur US Patent 4,221,231 requires that a piston and piston ring arrangement is disposed in a bore with movement of the piston being prevented by a plug of eutectic material situated downstream of the piston. Or=e that plug beeees molten the piston and its associated sealing ring are permitted to move to a position downstream of its original position at which pressurised fluid is able to leak past the piston and exhaust to atmosphere.This proposal has ban found to have the disadvantage that the sealing ring can relatively easily be breached by the pressurised fluid within the pressure vessel particularly if the ring becomes denatured due to continued cycling between relatively high and low te,,peratures.

In an alternative a=aneent a plug of eutectic naterial completely fills and seals the outer end of a cylindrical bore. A metal piston bearing against the upstream end face of the plug is subject to the internal pressure of the vessel by inans of a outer end seal disposed at its inner upstream end. Unfortunately, this too is a complex construction prone to seal failure.

The object of the present invention is to prwide an an improved fusible plug assembly.

wording to a first aspect of the invention a fusible plug assembly, for equalising pressure in a pressure vessel with the ambient pressure adjacent thereto at a pre-determined temperature, cagrises a body adapted to be murlted on or in the vessel wall whereby a through bore of the body will communicate at its inner end to the interior of the vessel and is open at its outer end to the exterior of the vessel, wherein a first outer portion of said bore nearer said outer end is occupied by an insert of a fusible material designed to molt at said pre-determined teteerature with retention means being provided tween said first outer bore portion and said insert and a seond inner portion of said bore nearer said inner end thereof having a diameter no greater than that of the minor diameter of the outer portion is occupied by a plunger which at all operating temperatures is a gas tight fit and slidable therein and which is a unitary moulding of plastics and/or resilient material, with there being a relatively low coefficient of friction between the material of the plunger and that of the body and with the arrangement praviding in use that an inner end face of the plunger confronting the interior of the vessel is subject to the pressure therein thereby urging the outer end face of the plunger against the insert and that an reaching said pre-determined tewerature material of the insert becomes molten and is removed from the bore by the action of the plunger exiting under pressure to result in a clear exhaust port being provided through the outer portion of the bore formerly occupied by material of the insert.

Preferably, said retention neans is prawided by a mechanical interlad)e and may be provided by cosperation between catpleeentary screw-threads of said first outer bore portion and said insert.

Typically, the through bore is of generally stepped cylindrical form having an enlarged outer end portion in which the insert is disposed and further having a snoth cylindrical inner portion in which the plunger is generally disposed.

The minor diameter of the enlarged outer end portion may be greater than the intenal diameter of said cylindrical inner portion and/or a tapered lead niey extend from the distal end of the inner portion of the bore to facilitate ingress of the plunger. The plunger may be of generally cylindrical outer profile and a push fit in said second inner portion of the bore.

Typically, the plunger is formed with a recess at its inner end providing an annular peripheral lip extending rearwardly of the plunger which prawdes a pressure assisted seal against the wall of the second inner portion of the bore. Separate spring means may be incorporated in the plunger to spring urge the peripheral lip against the wall of the bore.

Preferably, the plunger is fabricated fran rubber rubberised material and/or the plunger nay be fabricated fran PmE ar like plastics material. The plunger may have a hardness in the region of 70 to 85 IRBD.

Desirably, the plunger has a heterogeneous material structure and/or composition with the at least a part of the annular peripheral lip being constituted fmn a material al having a first hardness and the remaining material at least around the external periphery or adjacent a ram end face of the plunger having a second hardness. Said heterwensus material composition may be provided by the plunger having a graduated material formulation.

Said first hardness may be substantially within the range 70 to 85 IRHD and/or said first hardness nay not be greater than said second hardness.

Said second hardness may be greater than 80 IRHD.

The plunger nay be reinforeed in a piston region adjacent said outer end face thereof and the reinforcement may comprise a metal core.

According to a second aspect of the invention an aircraft wheel assembly comprises a fusible plug in accordance with the aforesaid first aspect of the invention.

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying Drawings, in which Figure 1 is a sectional elevation of a prior art fusible plug assembly; Figure 2 is a sectional elevation of a fusible plug asselly in accordance with a preferred embodiment of the present invention; Figure 2A is a sectional elevation of the plunger of the asseerbly of Figure 2;; Figure 2B is a sectional elevation of another composition of plunger that can be used in place of the plunger of Figure 2A, and Figure 3 is a partial sectional view of an aircraft wheel assembly in the region of a tyre inflation valve and associated fusible plug assembly generally as illustrated in Figure 2.

In a first embodiment of the invention the fusible plug shown in Figure 2 comprises a body 1, an insert 2 of a fusible material and a plunger in the form of piston/seal 3. The body 1 has an enlarged boss 4 at one end end a reduced diameter, externally screewthreadd portion 5 at the ather. In use, the body 1 can be scrcced into an appropriately threaded port (not shown) in the wall of a pressure vessel and if desired a sealing washer (not shown) may be disposed intermediate the portion 5 and the back face 6 of the boss 4 to effect the required seal therebetween.

Extending inwardly fran end face 7 of the boss 4 an axial bore has an enlarged, internally screw-threaded portion 8 into which the insert 2 can be screwed and a second portion 9 of relatively snaller: diameter. The bore portion 9 is coaxial with an exhaust port 10 which is in the forn of a tapered lead into the bore. In this smooth-walled portion 9 of the bore the piston/seal 3 is disposed.

In other embodiments of the invention the insert 2 may not be screw-threaded into portion 8 of the axial bore. Instead, different mechanical interlocking means may be provided including, for example, deformation of the material of boss 7 or the insert 2 about an adjacent formation of the insert or boss respectively.

The piston 3, as shown in Figure 2A, is wholly of a rubber or like compound and has a generally cylindrical or slightly tapering profile with a solid region 11 providing a ram end face 12 which in use contacts and abuts the fusible insert 2. At its other end it is formed with a frusto-conical recess 14 such that an annular lip 13 provides a pressure assisted seal. The piston/seal 3 is a slight interference or push fit in the bore portion 9 and the recess 14 is at the operating pressure of the vessel. Consequently, the inner peripheral surface 15 of lip 13 is forced radially outwardly thereby urging the outer peripheral surface 16 against the wall of the portion 9 of the bore to effect a fluid tight seal.In addition the piston/seal 3 will be urged udder pressure toward the fusible insert 2.

It will be apeeciated that the fusible plug is readily assembled by screwing in the insert 2 and inserting the piston/seal 3 after which the whole assembly can be screwed into the pressure vessel using screw thread 5.

The insert 2 is fabricated from a suitable euteeelc and/or fusible alloy the melting point of which is considerably lower than that t of the material fran which the pressure vessel is fabricated. In place of screw thread 5 other mechanical fitting means (not shown) may be used.

In use, the boss 4 provides a heat sink which once at the melting point of the insert 2 ensures that the material of the insert at least in the region of its screw thread milts. This results in the insert 2 no longer being restrained by the body 1.Once this occurs, the piston/seal 3 urged under pressure along the narrower-diameter portion 9 of the bore displaces the riiiiiing material of the insert 2 and is ejected fran the bore along its portion 8. Consequently, the portion 8 of the bore is cleared of at least most of the material of the insert 2 and a clear exhaust port is provided through the plug assembly for the escape of fluid from the vessel at least equal in diameter to the narroweer bore portion 9.

To ensure efficient and reliable fail-safe operation of the fusible plug it is desirable that the compound from which the piston/seal 3 is fabricated provides a good seal despite imperfections in the surface 16 at all operating temperatures. In addition, it is required that the coefficient of friction between the said material and the material of surface 16 remains low so that relative movement will occur on melting of the insert 2. It is believed, for example, that traditional rubber compound is not suitable because it does not exhibit the required frictional characteristics across all operating temperatures.Conventional PTFE is also believed to be unsuitable because its rigidity prevents it providing the desired sealing characteristics. Currently, it is understood that a plastics having a formulation similar to PTFE but being definable more akin to a rubber would provide an ideal material for the piston/seal 3. However, it may be that improved formulations of rubber or PTFE would provide the desired characteristics.

In another embodiment of the invention a piston/seal 103, as shown in Figure 2B, is used in place of piston/seal 3 of the first embodiment. By appreciating that the piston/seal 103 requires both rigidity to drive out the eutectic material and in part flexibility to provide a gas-tight seal the piston/seal 103 has a heterogeneous material composition. Structurally, piston/seal 103 has a generally cylindrical or slightly tapering profile with a solid region 111 providing a ram end face 112 which in use contacts and abuts the fusible insert 2 (sililarly to piston/seal 3 than in Figure 2). At its other end it is formed with a frusto-conial recess 114 such that an annular lip 113 provides a pressure assisted seal. In use the piston/seal 103 is a slight interference or Push fit in the bore portion 9 and the recess 114 is at the operating pressure of the vessel.

Consequently, the inner peripheral surface 115 of lip 113 is forced radially outwardly thereby urging the outer peripheral surface 116 against the wall of the portion 9 of the bore to effect a fluid tight seal. In addition the piston/seal 103 will be urged under pressure toward the fusible insert 2.

The composition of piston/seal 103 can be considered to be heterogeneous to the extent that the material composition of the piston region generally defined by solid region 111 is different fran that at the seal region generally defined by annular lip 113. The composition is illustrated by the differential hatching of Figure 23. It will be appreciated that either the whole of lip 113 may have material cOooosition different fran that of solid region 111 or only an user portion closely adjacent the outer extend 117 of the annular lip 113.

Alternatively, the whole of lip 113 and an adjacent portion of solid region 111 may have a differential composition fran the solid region 111 adjacent end face 112 thereof.

Generally, it is desirable that the piston region has a hardness in excess of 80 IRHD (International Rubber Hardness Degrees) whilst the seal region has a hardness of bet 70 and 85 IRHD. In another embodiment (not shown) the plunger comprises an elastcteric material with a hardness of less than 85 IREID, preferably less than 75 TREE), thereby to facilitate the sealing action of the plunger seal region located in the inward region of the through bore, which in the piston region 111 of the plunger is reinforced to ensure sufficient rigidity for the region. The reinforcement may be a solid nepal care encased in the elastaric material of solid region 111, for example.

The aforedescribed embodiments may be used in the eel wall of an aircraft wheel and tyre assembly. This then provides an essential safety feature which allows for controlled decompression of overheating tyres before catastrophic failure occurs. An example of this is shown in Figure 3 in a wheel assembly is adapted to locate a tyre rim (not shown) so that flange 205, barrel 206 and other parts of the wheel assembly (not shown) together with the tyre cooperate as a pneumatic pressure vessel. The flange 205 abuts wheel hub 200 and the interface therebetween is sealed by a sealing ring 204. The flange 205 is secured to the wheel hub 200 by a locking ring 207. Tyre inflation is effected by introducing compressed nitrogen into the interior of the tyre through passage 201 in the wheel hub 200 by way of one-way valve 202. The passage 201 also communicates with fusible plug assembly 203 which corresponds in design to one the aforedescribed embodiments of the invention.

Claims

CLAIMS:

1. A fusible plug assembly, for equalising pressure in a pressure vessel with the ambient pressure adjacent thereto at a pre-determined temperature, the assembly comprising a body adapted to be mounted on or in the vessel wall whereby a through bore of the body will communicate at its inner end to the interior of the vessel and is open at its outer end to the exterior of the vessel, wherein a first outer portion of said bore nearer said aster end is occupied by an insert of a fusible itaterial designed to melt at said pre-determined temperature with retention means being provided between said first outer bore portion and said insert and a second inner portion of said bore nearer said inner end thereof having a diameter no greater than that of the minor diameter of the outer portion is occupied by a plunger which at all operating temperatures is a gas tight fit and slideable therein and which is a unitary moulding of plastics and/or resilient material, with there being a relatively l coefficient of friction between the material of the plunger and that of the body and with the arrangement providing in use that an inner end face of the plunger confronting the interior of the vessel is subject to the pressure therein thereby urging the outer end face of the plunger against the insert and that on reaching said predetermined temperature material of the insert becomes molten and is removed from the bore by the action of the plunger exiting under pressure to result in a clear exhaust port being provided through the outer portion of the bore fortrerly occupied by material of the insert.

2. An assenbly in accordance with claim 1, in which said retention means is provided by a mechanical interlock.

3. An assembly in accordance with claim 2, in which said mechanical interlock is provided by co-operation between complementary screw-threads of said first outer bore portion and said insert.

4. An assembly in accordance with any one of the preceding claims, in which the through bore is of generally stepped cylindrical form having an enlarged outer end portion in which the insert is disposed and further having a smooth cylindrical inner portion in which the plunger is generally disposed.

5. An assembly in accordance with any one of the preceding claims, in which the minor diameter of the enlarged outer end portion is greater than the internal diameter of said cylindrical inner portion.

6. An assembly in accordance with any one of the preceding claims, in which a tapered lead extends fran the distal end of the inner portion of the bore to facilitate ingress of the plunger.

7. An assembly in aceczd2nce with any one of the preceding claims, in which the plunger is of generally cylindrical outer profile and a push fit in said second inner portion of the bore.

8. An assembly in accordance with any one of the preceding claim, in the the plunger is foni with a recess at its inner end providing an annular peripheral lip extending rearwardly of the plunger which provides a pressure assisted seal against the wall of the second inner portion of the bore.

9. An assembly in aordance with claim 8, in which separate spring gleans are incorporated in the plunger to spring urge the peripheral lip against the wall of the bore.

10. An assembly in accordance with any one of the preceding claim, in which the plunger is fabricated fran rubber or ruberised material.

11. An assembly in accordance with any one claims 1 to 9, in which the plunger is fabricated fran PTFE or like plastics material.

12. An assbly in accordance with any one of the preceding claims, in which the plunger has a hardness in the region of 70 to 85 IRHD.

13. An assembly in accordance with any one claim 7 to 11, in which the plunger has a heterogeneous material structure and/or composition with the at least a part of the annular peripheral lip being constituted fran a material having a first hardness and the remaining material at leapt around the external periphery or adjacent a ram end face of the plunger having a second hardness.

14. An assembly in accordance with claim 13, in which said heterogeneous material composition is provided by the plunger having a graduated material formulation.

15. An assembly in accordance with claim 13 Cr claim 14, in which said first hardness is substantially within the range 70 to 85 IRHD.

16. An assembly in accordance with any one of claims 13 to 15, in which said first hardness is not greater than said second hardness.

17. An assembly in accordance with claim 13 or claim 14, in which said second hardness is greater than ao IRHD.

18. An assembly in accordance with any one of the preceding claims, in which the plunger is reinforced in a piston region adjacent said outer end face thereof.

19. An assembly in accordance with claim 18, in which the reinforcement comprises a solid metal core.

20. An aircraft wheel assembly comprising a fusible plug assembly in accordance with any one of the preceding claims.

21. A fusible plug assembly, for relieving excess pressure in a pressure vessel at a pre-determined temperature, substantially as hereinbefore described and as illustrated in Figures 2 and 2A or Figures 2 and 2B of the accompanying drawings.