GB2029050A - Improvements in or Relating to Pressure Relief Valves - Google Patents

Abstract

Pressure vessel explosions due to elevated temperatures generated by, for example, external fires are prevented by a depressurising device (D), for use with a conventional pressure relief valve (P) which opens the valve below its normal set pressure in response to the detection of a temperature above a predetermined value by one or more temperature-sensitive devices (B) which may be in the form of fusible plugs. <IMAGE>

Description

SPECIFICATION Improvements in or Relating to Pressure Relief Valves This invention relates to pressure relief valves.

There have been a number of vessel explosions in the oil refining, petro-chemical and other process industries, where the explosions have not been due to excessive pressure being generated within the vessel, but have been due to fires external to the vessels. These external fires have caused the metal from which the vessels are manufactured to become heated such that it has weakened to a point where it is not able to withstand the normal operating pressure of the vessel, and this has resulted in violent explosions even though the normal safety relief valves have been functioning.

By way of example, the design stress for carbon steel may be 20,000 Ibs per square inch at ambient temperature, but the design stress reduces to 3,000 Ibs per square inch at a temperature of 10000 F. Other metals follow a similar pressure/temperature pattern.

In an attempt to overcome the above problem, it is proposed to provide a depressuring device for use on a safety relief valve, for example a conventional direct loaded or pilot operated relief valve, whereby the fluid pressure in a system including a safety relief valve is relievable down to any predetermined level in the event of an abnormal, substantial increase in temperature caused, for example, by an external fire. It may be underisable to depressurise the vessel down to atmospheric pressure, especially if there is a level of liquid normally present in the vessel which can be useful in absorbing heat, and of course it should be remembered that most liquids will change phase to gas or vapour at lower temperatures if the pressure is also low.

By using a device of the invention, in the event of an external fire developing, the relief valve will automatically depressurise the system down to a predetermined level, such as 75%, 50%, 25% of the normal operating pressure. The degree of depressurisation required is dependent upon the anticipated temperature that the vessel shell is expected to reach under external fire conditions, and the stress carrying capability at higher temperatures of the metal from which the vessel is made.

The provision of the device according to the invention enables a self-regulating safety relief valve, for example, to operate in a normal manner if the pressure should rise due to a process upset or any other circumstances. The normal method of operation is for the safety relief valve to open at a predetermined pressure which is known as the set pressure, and to be so sized that it will discharge the excess pressure which can be generated without the pressure exceeding 1.1 xset pressure. The valve will close in the normal manner at approximately 0.95 of the set pressure. The provision of the device according to the invention enables the safety relief valve to be over-ridden, and the system depressurised to the level where equilibrum is restored between the forces acting on the safety relief valve.

An embodiment of the invention will now be described, by way of example, reference being made to the accompanying drawings, in which Figure 1 is an elevation showing a pressure vessel having a pressure relief valve and a depressurising device according to the invention: and Figure 2 is an enlarged sectional elevation of the device shown in Figure 1.

Referring to the drawings, and firstly to Figure 1, a pressure vessel Vsupported on supports S has a pressure relief valve P with its discharge pipe P1 and an associated depressurising device D which is supplied with compressed air, usually at a pre-set pressure between 1 5 and 20 psi through line L. A plurality of fusible plug blocks B are located on a ring main system as shown in the drawing, this ring main system being interconnected with the instrument air supply.

Referring now to Figure 2, the device D comprises a diaphragm actuator 1 which is in two parts 1A and 1B and which is secured to a block 20 and to a horizontal housing 22. The actuator 1 houses a diaphragm 24 which is spring biassed by means of a diaphragm spring 26 towards the block 20. The block 20, diaphragm actuator part 1A and the diaphragm 24 together define a chamber 26A. The diaphragm 24 is mounted on a spindle 28 which passes through a bush 10 located in a mounting fabrication 8, the spindle 28 being secured to an end fitting 2 by means of a lock nut 3. Said end fitting is secured between two spaced arms of a lever 7, one end of which is mounted about roller bearing 4 by a bearing pin 5 and a bearing nut 6, the other end of the lever being connected by means of spindle 34 and forked arm 30 to contact lifting nut 36 on the spindle 32 of the safety relief valve P.

The lever 7 is biassed towards a neutral position by means of a tension spring 11 secured to the lever 7 by means of attachment screws 1 2.

A cowling 14 is located over the lever 7 to protect the mechanism.

In operation, should one or more of the plugs melt due to an external fire, the compressed air supply is exhausted at a higher rate than it is being admitted into the system and consequently the chamber 26 is depressurised, whereupon the compressed diaphragm spring 26 moves the diaphragm 24, together with spindle 28, leftwards as viewed in Figure 2. The motion of spindle 28 is transmitted to lever 7 via pin 5 and hence to the forked arm 30 which exerts a force on the spindle 32 of the safety relief valve P. This force must be greater than the force exerted by the main safety valve spring (not shown), less any force caused by the outflow of fluid, the amount by which the force exceeds the main spring load determining the degree by which the vessel is depressurised. The rate at which the vessel is depressurised is determined by the bore size of the safety valve.When the force exerted by the diaphragm spring multipiied by the mechanical advantage of lever 7 equals the force exerted by the main safety valve spring minus any force due to out flowing fluid, the valve will begin to close.

In the case of pilot operated valves, a simiiar apparatus to that shown in the drawings could be used, but instead of applying a force to the main spindle of the valve, the force would be applied to the spring loaded pilot.

Should the flexible diaphragm 24 be destroyed by fire, or in any way deteriorate so as to make it significantly permeable, the efficient and safe operation of the valve will not be affected, as the valve will open and depressurise the system down to the predetermined level.

It will be appreciated by those skilled in the art that actuators other than the diaphragm type may be used, and that an actuating fluid other than compressed air may be used, if preferred.

It will further be appreciated by those skilled in the art that the pressure relief valve may be a normal "on/off" valve rather than a self-regulating pressure relief valve, in which case the vessel would usually also be equipped with a valve of the latter type.

Furthermore, whilst the above description of a preferred embodiment refers to a pressure vessel V, a device of the invention could equally well be used in conjunction with a valve to depressurise other containers for containing fluid under pressure, for example a pipeline for conveying pressurised fluid such as oil or a gas.

Also, whilst the device of the invention specifically described above is essentially hydromechanical in operation, functionally equivalent devices may be employed, for example electromechanical devices. Thus, for example, the or each fusible plug block B may be replaced by one or more devices that produce an electrical signal when a predetermined temperature is exceeded, the signal being used to cause the valve to open.

Claims (20)

Claims

1. Apparatus comprising a container for containing fluid under pressure, a pressure relief valve for relieving pressure of fluid within the container, said valve being operable between a normal, closed position and an open position and valve actuating means, including temperature sensitive means, operable to effect opening of said valve when the temperature externally of, and adjacent to, the container exceeds a predetermined value whereby fluid pressure in the container can be relieved in the event of said temperature being exceeded.

2. Apparatus according to Claim 1, wherein said container is a pressure vessel.

3. Apparatus according to Claim 1, wherein said container is a pipeline for conveying fluid under pressure.

4. Apparatus according to any one of Claims 1 to 3, wherein said valve actuating means is operable to effect opening of the valve only when the pressure of fluid in the container is above a predetermined value when said temperature is exceeded.

5. Apparatus according to any one of Claims 1 to 4, wherein the said valve is adapted to close when fluid pressure in the container has been relieved down to a predetermined value.

6. Apparatus according to any one of Claims 1 to 5, wherein the said valve is a pressure relief valve that is automatically openable and closable in dependance upon fluid pressure within the container, the set pressure setting of which valve is adapted to be over-ridden by the said valve actuating means.

7. Apparatus according to any one of Claims 1 to 6, wherein the said valve actuating means include an actuator movable between a first, normal position in which the said valve can assume its normal, closed position and a second position in which the actuator causes the valve to open, a pipeline for feeding fluid under pressure to the actuator to maintain the actuator in its first normal position, biasing means biasing the actuator towards the said second position thereof, and temperature sensitive means effective to depressurise fluid within the pipeline when said temperature exceeds said predetermined value whereupon said biasing means will urge the actuator into said second position to cause the valve to open.

8. Apparatus according to Claim 7, wherein said actuator includes a casing housing a diaphragm said diaphragm being associated with a mechanical linkage interconnecting the actuator with the valve and defining a wall of a chamber within the casing with said pipeline being in communication with the chamber and wherein said biasing means comprises a compression spring for biasing said diaphragm in a direction opposed to the direction of the force applied to said diaphragm by the pressure of fluid fed to the chamber through said pipeline.

9. Apparatus according to Claim 8, wherein said linkage includes mechanical advantage means.

10. Apparatus according to any one of Claims 1 to 9, wherein said temperature sensitive means comprises fusible means connected in said pipeline, said fusible means being adapted to melt when said temperature exceeds said predetermined value thereby enabling pressurised fluid in the pipeline to depressurise and said biasing means to move the actuator into said second position.

11. A pressure relief valve comprising: (a) A body having a chamber therein, an inlet port and an outlet port both communicating with the chamber, a valve seat in the chamber intermediate said inlet port and said outlet port, a valve closure member co-operable with said valve seat and being automatically movable, in dependance upon the pressure of fluid acting therein through said inlet, between closed and open positions, and (b) Valve actuating means to over-ride the set pressure setting of the valve thereby to effect opening of the valve at a pressure below said set pressure, said valve actuating means being automatically operable in response to the detection, by temperature sensitive means, of a temperature in excess of a predetermined value.

12. A pressure relief valve according to Claim 11 of the direct spring-loaded type wherein said valve actuating means is arranged to act on the valve closure thereby to open the valve when said temperature is exceeded.

13. A pressure relief valve according to Claim 11 of the pilot operated type wherein said valve actuating means is arranged to actuate the pilot valve in order to effect opening of the relief valve when said temperature is exceeded.

14. A pressure relief valve according to any one of Claims 11 to 13, wherein said actuating means is as specified in any one of Claims 7 to 9.

1 5. A valve actuating device for association with a direct spring-loaded pressure relief valve and effective in opening the valve below the set pressure thereof, said device comprising a casing housing a diaphragm, said diaphragm defining a wall of a chamber within the casing, mechanical linkage means co-operable with the diaphragm and adapted also to co-operate with the valve closure stem of said relief valve, said casing having a port wherein whereby fluid under pressure may be fed into said chamber to maintain the diaphragm and said linkage means in a first position in which the linkage will not affect normal operation of the relief valve, and a compression spring for biasing said diaphragm and said linkage means against the force of said fluid acting on the diaphragm, said compression spring being effective, upon depressurisation of said chamber, in moving the diaphragm together with said linkage means into a second position whereupon said linkage will lift the valve stem and open the valve.

1 6. A device according to Claim 15, wherein said linkage means includes mechanical advantage means.

17. A valve actuating device according to Claim 1 5 or Claim 16 for association with a pilot operated pressure relief valve wherein said mechanical linkage means is adapted to cooperate with the pilot valve instead of with said valve closure stem.

1 8. A safety relief valve including a device according to any one of Claims 1 5 to 1 7.

1 9. Apparatus substantially as hereinbefore described with reference to and as illustrated in Figures 1 and 2 of the accompanying drawings.

20. A valve actuating device or pressure relief valve including a device substantially as hereinbefore described with reference to and as illustrated in Figures 1 and 2 of the accompanying drawings.