GB2199373A

GB2199373A - Pressure relief device

Info

Publication number: GB2199373A
Application number: GB08628837A
Authority: GB
Inventors: William Thomas Nicholls
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1986-12-02
Filing date: 1986-12-02
Publication date: 1988-07-06
Also published as: GB8628837D0

Abstract

A pressure relief device, used on a closed vessel containing a flammable medium, has a conventional pressure relief valve 4 and vacuum relief valve 6. The device carriers a meltable polymeric sleeve 10 by means of which it is screwed into a bore in the wall of the vessel. Should the vessel be in the region of a fire, rapid pressure relief occurs when the sleeve melts and the device blows out. <IMAGE>

Description

PRESSURE RELIEF DEVICE This invention relates to a pressure relief device for use on a closed vessel, for example a rail or road tanker, containing a flammable medium such as petrol or fuel oil.

A known pressure relief device for use on such a vessel is located in a hollow boss which defines an aperture through the vessel wall. The device contains a conventional valve.

Pressure relief occurs when the valve member is lifted off its seating against a spring bias by excess pressure inside the vessel. However, such a device may be dangerous if an accident occurs and the vessel is located in the region of a fire. The venting which can take place through the moderately-sized valve member may not be sufficient rapid for risk of an explosion to be obviated. Valve devices which have recently been marketed reduce this risk by employing a large valve member, but are large, cumberscme, and expensive.

In accordance with the present invention there is provided a pressure relief device for use on a closed vessel containing a flammable medium, which device comprises a securement portion for securement in or around an aperture in a wall of such a vessel and a pressure relief valve openable by pressure within such a vessel, wherein at least the part of the securement portion which in use engages in or around the aperture is composed of a material whose melting point is in the range 50 to 3000C.

In a device according to the invention, therefore, pressure relief may take place through a valve, conveniently a conventional valve such as that referred to above, but in the potentially catastrophic situation of a vessel being located in the region of a fire, effective and rapid pressure venting takes place on melting of the said material, the whole pressure relief device then being expelled, to leave the whole aperture free for venting. Medium expelled fran the vessel through the aperture may ignite but can be controlled. The vessel is no longer in a dangerously explosive state.

The melting point of the material is preferably in the range 150 to 2500C.

Preferably, the part of the securement portion which in use engages in or around the aperture is composed of a plastics material, suitably an engineering plastics material, for example nylon or polyacetal.

Preferably, the pressure relief device of the invention also includes a vacuum relief valve, that is, a valve operable in the opposite sense to the pressure relief valve. The presence of such a valve facilitates the rerroval of medium from the vessel, being drawn open to allow the intake of air by the relative negative pressure inside the vessel when an outlet in the vessel is opened. In one convenient eirbodiirent the valve member of one of the valves, preferably the vacuum relief valve, constitutes the seating of and carries the valve member for the other valve.

It is preferred that the pressure relief device of the invention carries a closed, metallic gauze screen on its inboard side, so as to be located between the contents of the vessel and the outside environment at all times, even when the device is open.

The securement portion may comprise a hollow stub, the internal surface of which engages around a hollow boss projecting from the wall of the vessel and defining the said aperture. More conveniently, the securement portion has an externally screw threaded surface, so that the pressure relief device is secured to the vessel by screwing the securement portion into the complementarily-threaded aperture, conveniently defined by a boss projecting from the wall. Preferably, the securement portion tapers towards its leading end.

The meltable material is conveniently formed as a sleeve which may be secured to the remainder of the pressure relief device by any convenient means, for example by adhesive or by bolts. The remainer of the device is preferably made of a metallic material of higher melting point, for example aluminium. If desired, however, the device can be composed substantially wholly of the meltable material.

An existing pressure relief device in which the securement portion is not designed to melt in order to give pressure relief, but which relies solely on the pressure relief valve, may readily be modified to provide a relief device in accordance with the present invention. Thus, in a second aspect of the invention there is provided a method of making a pressure relief device as defined above from an existing pressure relief device in which the securement portion cannot melt to give pressure relief, which method comprises removing the part of the securement portion of an existing pressure relief device which engages in or around the aperture and securing to the device in place of the part removed a sleeve composed of a material whose melting point is in the range 50 to 3000C.

In a third aspect of the invention there is provided a closed vessel for or containing a flammable medium, the vessel having an aperture in its top wall, in or around which aperture a pressure relief device in accordance with the first aspect of the invention is fitted.

The invention will now be further described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows a perspective view of a pressure relief device according to the invention; figure 2 shows a plan view from above (inboard side) of the device of Figure 1; figure 3 shows a plan view from beneath (outboard side) of the device of Figure 1; figure 4 is a cross section along the line AA shown in Figures 2 and 3, of the valve device mounted on a tanker and acting to relieve a relative negative internal pressure; and figure 5 corresponds to figure 4 but shows the device operating to relieve an excess internal pressure.

For clarity, in Figures 1 to 3 a protective cap shown in Figure 4 and 5 has been omitted.

The valve device comprises a hollow body 2 which houses two pressure relief valves. The valve members of the pressure relief valves are shown as 4 and 6.

The inboard end of the body 2 is formed as a stub 8 over the outer surface of which is fitted a sleeve 10 (indicated in solid black shading in Figures 4 and 5). The sleeve 10 has a screw thread on its circumferential outer surface for securement in a complementarily threaded bore 12 welded to the wall 14 of a railway tanker containing petrol. A metallic anti-flash gauze 16 is located on the inboard side of the valve device.

Considering the device in more detail, the outboard portion of the body 2 is of hexagonal external cross section. From each corner of the outboard end of the body 2 extends a leg 18. Each leg is tapped at its end and receives a screw 20, by means of which a protective cap 22 is fitted over the valve device, in order to protect the valves from ice.

The body 2 has an internal wall with six inwardly and longitudinal extending guide pillars 24 to guide the movement of the valve member 6 of the vacuum relief valve. Between each of these pillars the internal wall is scalloped (as at 26) leaving a space through which air can pass to relieve a relative negative pressure inside the tanker (see Fig. 4). The internal wall of the body 2 has a circumferential shoulder 27 which limits the permitted opening of the valve member 6.

To the outboard end of the housing portion 4 is secured, by screws 28, a ring 29 which serves as a seating for the valve member 6 of the vacuum relief valve.

The inboard end 30 of the stub 8 of the body 2 is tapped at three places to receive bolts 32 by means of which the sleeve 10 is secured around the stub 8. The sleeve has a hollow, circumferential, externally screw-threaded part 34 and, at its inboard end, a mounting flange 36 through which the bolts 32 pass and against which the heads of the bolts bear. The part 34 tapers slightly towards the inboard end to match the taper of the bore 12. The circumferential surface of the stub 8 has a roughly-machined finish to aid good contact between the sleeve and the stub.

Thr anti-flash gauze 16 is supported by limbs 35 extending from the stub 8, and integral therewith. The gauze is on the inboard side of the valves and so is located between the contents of the tanker and the outside environment when either valve is open.

The valve member 6 of the vacuum relief valve is biassed by a helical spring 38 towards the seating provided by the ring 29.

The spring 38 is trapped between a shoulder 40 defined on the inboard face of the valve member and a shoulder 42 within the body 2 adjacent the inboard end of the stub 8.

The valve member 6 has a central aperture 44 over which the valve member 4 of the pressure relief valve is located. The valve member 4 is biassed towards its seating, provided by the valve member 6, by means of three pairs of wire springs 46 located around its perimeter, which springs are secured at one end to the valve member 6 and the other end to the valve member 4, and are supported by pillars 48 on the valve member 6.

The body 2 is of aluminium alloy, the valve member 6 of a hard resilient polymeric material, the valve member 4 of steel, and the sleeve 10 of polyacetal. The sleeve is formed by moulding a blank and machining it to form the threads.

In use, when the tanker is being charged with petrol, excess pressure within the tanker is relieved by the valve member 4 rising off its seating on the valve member 6 against the wire springs 46. Air can then pass through the gauze 16, through the hollow body 2 and the aperture 44 within the valve member 6, past the valve member 4 and out past the cap 22 (Figure 5). When the tanker is being emptied the internal relative negative pressure is relieved by the valve member being drawn from its seating to allow air to enter the tanker through the ring 29, around the outside of the valve member 6, along the scalloped spaces 26 within the body 2 between the guide pillars 24, then through the gauze 16.

The valves generally are set to open when the pressure difference between the inside of the tanker and atmospheric is around 0.5 to 5 atmospheres.

Should the tanker be located in the region of a fire more effective pressure venting than can be achieved by the pressure relief valve alone takes place once the sleeve 10 reaches its melting temperature, about 175 C for polyacetal (about 2300C if nylon is used). Heat reaches the sleeve 10 by conduction through the metallic strucure which surrounds it and through convection of the gas within the tanker. When the sleeve melts the device is blown out leaving the whole aperture defined by the boss 12 open.

Fire engulfment test have been carried out, as follows. A device as described above was fitted into a small tank (approximately 1 cubic metre) partly filled with petrol and standing in a tray containing 3 inches (7.6 cm) of water and 3 inches (7.6cm) of gas oil. A thermocouple was attached to the valve boss 12 and the tank was piped for a pressure recording.

During the test the following readings were recorded.

TIME AFTER IGNITION THUMP AT VALVE BOSS OVER PRESSURE (minutes) ( C) (PSI/Pascals) 1 45 0 2 95 0.8/535 3 170 4.3/2870 3.6 200 7.4/4965 * 5 320 0 An identical test was carried out using a device with a nylon thread. During the test the following readings were recorded.

TIME AFTER IGNITION TEMP AT VALVE BOSS OVER PRE (minutes) ( C) (PSI/Pascals) 1 45 0.8/535 2 100 2.2/1470 3 160 6.4/4270 4 225 8.4/5740 * 5 290 0 In these tests the pressure relief valve 4 started to vent after approximately 3 minutes and full venting was achieved on discharge of the valve.

* maximum pressure reached

Claims

ClAIMS 1. A pressure relief device for use on a closed vessel containing a flammable medium, which device comprises a securement portion for securement in or around an aperture in a wall of such a vessel and a pressure relief valve openable by pressure within such a vessel, wherein at least the part of the securement portion which in use engages in or around the aperture is composed of a material whose melting point is in the range 50 to 3000C.
2. A device as claimed in claim 1 in which the melting point of the material is in the range 150 to 2500C.
3. A device according to claim 1 or 2 wherein the part of the securement portion which in use engages in or around the aperture is composed of a plastics material.
4. A device according to claim 3 wherein the plastics material is nylon or polyacetal.
5. A method of making a pressure relief valve device as claimed in any preceding claim from an existing pressure relief valve device in which the securement portion is not designed to melt to provide pressure relief, which method comprises removing the part of the securement portion of the existing pressure relief valve device which engages in or around the aperture and securing to the valve device in place of the part removed a sleeve composed of a material which melts at a temperature on the range 50 to 3000C.
6. A closed vessel for or containing a flammable medium, the vessel having an aperture in its top wall, in or around which aperture a pressure relief device as claimed in any of claims 1 to 4 is fitted.
7. A pressure relief valve device substantially as hereinbefore described with reference to the accompanying drawings.