GB2137775A - Improvements relating to pressure and safety relief valves - Google Patents

Abstract

A pressure relief valve for the control of pressure in a pressure vessel comprising a body and a piston 12 and cylinder 17 assembly in the body. The piston is coupled to a valve element 14 controlling pressurised fluid flow past an associated valve seat, and the valve element is biased by a spring 18 to a valve closing position. The piston is at least partially isolated from influence of said pressurised fluid, and pressure sensitive means (30 Fig. 2, not shown) controls application of pressurised fluid to said piston when said valve element is lifted from its seat whereby said valve element is lifted to its full extent from the seat for maximum relief. <IMAGE>

Description

SPECIFICATION Improvements relating to pressure and safety relief valves This invention relates to pressure and safety relief valves. Pressure relief and safety valves each serve the same function -- to prevent the pressure in a pressurized system from exceeding the safe working pressure in a vessel by a factor usually no greater than 10%. The safety relief valve differs from the pressure relief valve in that it is provided with manually operated means to ease the obturator off its seat, however, the fundamental operation is the same.

For the sake of simplicity the term pressure relief valve used throughout will refer equally to either type of valve.

The effective lift or opening of any disc type valve, such as a pressure relief valve, is - of the 4 diameter of the orifice it seals due allowance being made if guides are provided in the orifice.

The obturator of such a valve is normally restrained from opening against the normal or safe working pressure of a pressurized system by a coil spring, or other suitable spring.

When the system pressure exceeds the set pressure of the valve the obturator is urged to uncover the orifice and thus relieve the pressure.

However, due to the spring rate it tends to resist movement of the obturator. Therefore, the relieving movement of the obturator is sluggish and, due to the spring rate increasing the thrust upon the obturator, a pressure rise in the system results. This phenomenon is well known, hence the 10% permissible rise in pressure.

The 10% permissible rise in pressure is not always easy to obtain and requires very careful spring design and manufacture. This is particularly so when the ratio between normal and safe working pressure is small. Furthermore, the larger the valve the more difficult it becomes. This is due to the spring rate per unit movement increasing the thrust upon the obturator and thus accelerating and increasing the pressure rise.

The problem is very much magnified when the valve must lift the full amount. A typical application is when the pressure relief valve is used as a bypass valve on a positive displacement pump. In such cases the valve must be capable of by-passing, the full output of the pump and yet be capable of avoiding an excessive pressure rise.

When the safe working pressure of the pump is at or near the normal working or safe working pressure of the system the conventional type of pressure relief valve cannot cope with such a situation.

Various means have been proposed. One means is to have a pilot operated pressure relief valve. This comprises two valves in series, one smaller and one larger. The smaller is designed to lift first and initiate the opening of the larger valve which, being much larger, releases the pressure much more quickly and with a smaller pressure rise than would normally be the case.

Another known means is to have a pressure control switch in the pressurised system which, when activated by an increase in pressure beyond the normal, electrically and/or pneumatically signals another valve to open. All known methods are expensive and not always convenient.

A further and important situation arises in which the pressure relief valve must be capable of opening fully. This occurs in the food, pharmaceutical, chemical industries, etc. wherein it must be possible to clean the valve.

It is comparatively easy to withdraw the valve from the system. However, this is not always convenient as many systems in the industries mentioned are automatically cleaned-in-place. As the pressure of the cleaning fluids is almost always much less than the normal operating pressure of the system the pressure relief valve as normally installed cannot be opened and the valve must therefore be removed for cleaning. This is advantageous as, in absolutely sterile systems, the system in no way shouid be opened.

The proposal now to be described offers a convenient and economical method of overcoming all known disadvantages of pressure relief valves.

There is provided according to the present invention a pressure relief valve for control of pressurized fluid comprising a body, a piston and cylinder assembly in the body, and a valve element associated with said piston for controlling flow of pressurized fluid through said valve, said valve element being biased to a valve closing position, pressure sensitive means adapted to control flow of pressure fluid to said piston the arrangment being such that upon lifting of said valve element pressure fluid flows to said piston.

In a further aspect of the present invention there is provided a pressure relief valve for the control of pressure in a pressure vessel comprising a body, a piston and cylinder assembly in the body, the piston being coupled to a valve element controlling pressurised fluid flow past an associated valve seat, said valve element being biased, such as by a spring, to a valve closing position, said piston being at least partially isolated from influence of said pressurised fluid, pressure sensitive means for controlling application of pressurized fluid (which may or may not be from the same source) to said piston when said valve element is lifted from its seat whereby said valve element is lifted to its full extent from the seat for maximum relief.

The invention will now be described in greater detail having reference to the accompanying drawings in which Figure 1 is a sectional view of a relief valve. Figure 2 is a schematic drawing of the relief valve system.

Referring to the drawing there is provided within the body of the valve a cylinder, a piston, said piston being attached to the stem 13 of the valve obturator 14. The cylinder 11 and piston 12 is isolated from the valve chamber by an "0" ring gland 1 5. A fluid connection 1 6 is provided to the cylinder 1 Thus the A spring thrust is transferred to the piston and obturator assembly.

The piston 1 2 and associated obturator 14 are biased downwardly by spring 1 8 so that the obturator seals against seat 1 7. The spring 1 8 is captured within upper and lower spring check plates 1 9(a) and 1 9(b) with lower check plate 1 9(b) resting upon an~extension 20 of the obturator stem 13.

The height above the piston 12 in cylinder 11 is such as to allow the valve obturator 1 4 to open at least 4 the diameter of the orifice 21 it controls; the lower face of the spring casing 22 open end acting as a piston stop. Thus, when fluid pressure is admitted to the space below the piston it, together with the obturator 14, will be raised against the spring resistance by a distance equal to at least + the orifice diameter.

In a relieving pressure situation the initial outward movement of the piston-obturator assembly will be assisted by system pressure against the obturator 14. However, this will cease to be effective if the valve is relieving to an unpressurised part of the system such as an overflow or save-all tank. Therefore, it will be appreciated that the effective area of the underside of the piston 1 2 in cooperation with the fluid pressure acting upon it must be such as to overcome the initial and increasing resistance of the spring 1 8. This resistance would vary with the set pressure of the valve. In practice the effective area of the piston is such as to ensure that one size will satisfy a range of set pressures. Beyond that particularly range a large cylinder and/or higher fluid pressure would be required.

In a pressure relieving system the pressure relief valve is activated simultaneously by the pressure against the obturator 14 and a signal from the pressure control switch 30 conveniently iocated in the system. This switch transmits an electrical signal to a 3-way normally closed solenoid valve 31. Upon receiving this signal the solenoid valve opens and admits fluid pressure through passage 1 6 to the effective area of the piston whereupon the valve opens fully.

Immediately the normal pressure of the system is restored the signal is removed, the solenoid valve closes to exhaust the cylinder and the pressure relief valve closes. Preferably any tendency of the valve to slam on to its seat is avoided by restricting the solenoid valve exhaust orifice.

Conveniently the solenoid controlled fluid pressure is obtained from an auxiliary supply.

If the same system has to be automatically cleaned-in-place this could be arranged by fitting a dual switch pressure controller. One switch can be adjusted to open the pressure relief valve at set pressure while the other operates at the cleaning in-place pressure: or a signal can appropriately be taken from the cleaning-in-place control system.

In cases where the pressure relief valve is quite satisfactory but must be opened fully for cleaningin-place manually a fluid manual control valve could be used to activate the pressure relief valve - otherwise it could be operated as previously described.

Whilst the foregoing description relates to pressurised systems what is proposed could be advantageously applied to vacuum relief valves of the inward opening type, i.e. into the vessel or pressure system.

The obturator of such valves are restrained from falling inwards to the system by means of a coiled or other suitable spring and thus suffer the same disadvantages of spring resistance to opening. This is extremely dangerous as such valves should really open fully at or near 50 mm water gauge or less and in a time span of about 1 to 2 seconds, especially when used to protect thin walled vessels against damage caused by sudden pressure reduction.

Whilst initial opening of the valve would still be a function of careful spring design the auxiliary fluid cylinder would ensure the prompt full opening of the obturator in response to a signal from the control switch, the latter being, of course, set to respond to less than 50 mm water gauge.

Claims (6)

1. A pressure relief valve for control of pressurised fluid comprising a body, a piston and cylinder assembly in the body, and a valve element associated with said piston for controlling flow of pressurised fluid through said valve, said valve element being biased to a valve closing position, pressure sensitive means adapted to control flow of pressure fluid to said piston the arrangement being such that upon lifting of said valve element pressure fluid flows to said piston.

2. A pressure relief valve for the control of pressure in a pressure vessel comprising a body, a piston and cylinder assembly in the body, the piston being coupled to a valve element controlling pressurised fluid flow past an associated valve seat, said valve element being biased, such as by a spring, to a valve closing position, said piston being at least partially isolated from influence of said pressurised fluid, pressure sensitive means for controlling application of pressurized fluid to said piston when said valve element is lifted from its seat whereby said valve element is lifted to its full extent from the seat for maximum relief.

3. A pressure relief valve as claimed in claims 1 or 2 wherein the piston forms part of and is directly coupled to said valve element.

4. A pressure relief valve as claimed in any preceding claim wherein said pressurised fluid is from a source independent of that in said pressure vessel.

5. A pressure relief valve as claimed in any one of the preceding claims wherein it is used as a vacuum relief valve into a pressure system.

6. A pressure relief valve for control of pressurised fluid substantially as herein described when having reference to the accompanying drawings.