GB2196722A

GB2196722A - Relief valve

Info

Publication number: GB2196722A
Application number: GB08721643A
Authority: GB
Inventors: John Fletcher
Original assignee: Drum Engineering Co Ltd
Current assignee: Gardner Denver UK Ltd
Priority date: 1986-09-16
Filing date: 1987-09-15
Publication date: 1988-05-05
Also published as: GB8622222D0; GB8721643D0

Abstract

A relief valve comprises a valve closure element (16) biassed towards engagement with a valve seat (18) by fluid pressure in a chamber (31). A conduit (34,38,40,68) extends between the fluid inlet (12) and the chamber (31). A control valve (42) in this conduit responsive to pressure applied to the fluid inlet (12) allows communication between the fluid inlet (12) and the chamber (31) up to a certain value of pressure applied to the fluid inlet, at which pressure the valve (42) isolates the chamber (31) from the pressure in the fluid inlet. As shown, once the chamber (31) is isolated from the fluid inlet (12), the chamber (31) forms a spring whose spring rate is virtually zero, and whose force will not increase as the valve closure element (16) moves. The control valve (42) preferably includes a diaphragm (44) which moves away from a member (62) in the event of excess pressure in chamber (31). <IMAGE>

Description

SPECIFICATION Relief valve The present invention relates to a relief valve.

When air compressors or blowers are used to supply compressed air to a device, it is necessary to provide a relief valve which opens if the pressure becomes too high and which thus ensures that no damage is done to the device.

Presently used relief valves have a valve closure element which is forced into sealing engagement with a valve seat by means of a compression spring. When the air pressure oyercomes the pressure exerted by the spring, the closure element is moved off the valve seat and the air flows through the valve. However, in such a valve the spring produces a larger force as the air pressure increases, and there is also usually some combination of air flow and pressure at which the spring and/or closure element are unstable, resulting in violent oscillation which may cause damage to the valve seat.Furthermore, when the valve is not in use (e.g. when the compressor is switched off or applying only a low pressure) the valve closure element is always urged strongly into sealing contact with the valve seat by the spring, and when the valve is hot the spring relaxes and may result in damage to the valve closure element.

It is an object of the present invention to provide a pilot operated relief valve which overcomes the disadvantages associated with known relief valves.

In accordance with the present invention, a relief valve comprises a fluid inlet, a fluid outlet, a valve closure element and associated valve seat interposed between the inlet and outlet to control the fluid flow through the valve, the valve closure element, in use, being biassed towards engagement with the valve seat by means of fluid pressure in a chamber, a conduit extending between the fluid inlet and the chamber, and a control valve situated in said conduit, the control valve being responsive to pressure applied to the fluid inlet and being adapted to allow communication between the fluid inlet and the chamber up to a certain value of pressure applied to the fluid inlet, at which pressure the valve isolates the chamber from the pressure in the fluid inlet.

In this way, once the chamber is isolated from the fluid inlet, the chamber forms a spring whose spring rate is virtually zero, and one whose force will not increase as the valve closure element moves.

Also, since at low values of pressure at the fluid inlet the control valve causes application of the low pressure to the chamber, there is only a low load on the valve closure element and valve seat when the valve is shut, and thus unnecessary forces on the valve seat are avoided.

In a preferred embodiment, the valve closure element is secured to a diaphragm which partly defines the chamber, and preferably the surface area of the diaphragm is larger than the surface area of the valve closure element.

Preferably, there are means for releasing excess pressure from the chamber when the chamber is isolated from the fluid inlet.

By way of example only a specific embodiment of the present invention will now be described, with reference to the accompanying drawing, which is a cross-section through an embodiment of pilot operated relief valve in accordance with the present invention.

Referring to the drawing, the relief valve comprises a lower valve housing 10 having an air inlet 12, which is connected to a source of pressure, and an air outlet 14, which is connected to a vent. The inlet and outlet are normally isolated from one another by means of a valve closure disc 16 which is normally seated (as will be explained) on an annular valve seat 18 extending from the inlet 12.

An upper valve housing 20 is connected sealingly to the lower valve housing 10, and a flexible diaphragm 22 is secured between the two housings. The centre of the diaphragm is sandwiched between the centre of the valve closure disc 16 on one side and a spring support disc 24 on the other side, and is secured therebetween by means of a securing bolt 26.

The end of the bolt 26 which extends on the spring support disc side of the diaphragm has an enlarged guide head 28 having a cylindrical portion and a frusto-conical tapering portion at its end. The guide head is positioned in, and movable in a blind guide bore 30 which is shaped to receive the cylindrical portion of the guide head 28. Also, a very light spring 32 of extremely low spring rate (which could be substituted by a dead weight attached to the diaphragm in an alternative assembly) extends between a portion of the upper housing 20 and the support disc 24, to bias the valve closure disc 16 downwardly. This spring 32 is merely to provide a light closing force to the element 16 when the pressure in the inlet is low, as will be explained.

A passage 34 connects the inlet 12 to two further, aligned passages 36,38 in the lower and upper housings respectively, which in turn are connected to a further passage 40 in the upper housing, which leads to a conventional, pressure-reducing valve 42. The pressure-reducing valve comprises upper and lower portions, separated by a diaphragm 44. The upper portion comprises a spring seat disc 46 secured to the centre of the diaphragm, which spring seat disc has an aperture 48, with a frusto-conical valve seat pottion. A compression spring 50 extends between a portion of the upper portion and the spring seat disc 46, urging the diaphragm downwardly. The upper portion is connected to atmosphere via a vent 52.

The lower portion of the pressure-reducing valve comprises an annular valve seat 54 in the form of a screw-in, threaded valve seat insert 56 which has a passage 58 in which the stem 60 of the pressure-reducing valve closure member is movable, the stem 60 being narrower than the passage 58. The upper end of the stem is provided with a frustoconical head 62 which engages sealingly with the valve seat portion of the aperture 48 on the spring seat disc 46 by means of a very light spring 64 of very low spring rate, situated in a biind bore in the housing 20 and acting against the opposite end of the closure element. The other end of the stem 60 is provided with a cylindrical valve head 66 which is engageable with the annular valve seat 54 of the screw-in insert 56.The lower portion of the pressure-reducing valve is connected to the chamber 31 defined by the upper valve housing and the main diaphragm 22, by means of a bore 68.

In use, the pressure-reducing valve is normally held in the open position illustrated, since the spring 50 moves the head 66 down off its associated valve seat 54. The spring 64 is only of very low spring rate, and is only to ensure that the opposite head 62 is seated against its associated valve seat. Thus, the inlet 12 is connected to the chamber 31 defined by the upper valve housing 20 and the main diaphragm 22 via the passages 34, 36, 38, 40 the passages 58 in the open pressurereducing valve, and the bore 68. The diaphragm 22 is therefore forced downwardly by air pressure, urging the valve closure member 16 into sealing contact with the valve seat 18, and isolating the inlet 12 and the outlet 14.

At low pressure, the pressure-reducing valve is fully open, and the pressure at the inlet 12 is thus also applied to the diaphragm 22.

Since the area of the diaphragm is approximately twice that of the valve closure element 16, the valve closure element 1 6 is held tightly in contact with the valve seat 18.

As the pressure in the inlet 12 increases, the pressure causes the diaphragm 44 of the pressure-reducing valve to move upwardly, and because of the spring 64 bearing on the lower end 66 of the valve closure element, the valve closure element moves with the diaphragm. Eventually, at a given pressure determined by the spring 50, the pressure causes the head 66 to move into sealing contact with its valve seat 54, thus closing the valve, and the fluid inlet is thus isolated from the chamber 31 formed by the upper valve housing 20 and the main diaphragm 22. At this stage, the valve closure element will still be firmly seated on its valve seat, because of the much larger area of the diaphragm. Increases in the pressure at this stage cause the head 66 to be seated more firmly against its seat 54, but do not change the preaaure in the chamber 31, which is now isolated from the fluid inlet.

Thus, the latter chamber 31, which ia now a cloaed volume, forms a gas spring of constant pressure. When the pressure in the inlet increases sufficiently, the approximately constant force exerted by the gas in the chamber 31 and by the apring 32 is overcome by the fluid inlet pressure, and the closure member 1 6 moves off its seat 18, allowing fluid to flow. The closure member is guided by the guides 28, 30.

Thus, in other words, up to a certain value of pressure, determined by the spring 50, the pressure applied to the inlet is also applied to the diaphragm and the valve is held shut because of the larger area of the diaphragm. At that pressure, the pressure-reducing valve shuts and thus keeps the pressure in the chamber 3 1 conatant. When the pressure in the inlet overcomes the gas and spring forces exerted on the diaphragm, the valve opens.

As the pressure is further increased, the closure member 16 lifts fully off its seat 18, and the air above the diaphragm 22 is compressed. However, any significant rise in pressure is avoided, since a large increase in pressure causes the diaphragm 44 of the pressure-reducing valve to move against the force of the spring 50, and since the head 66 of the pressure-reducing valve is abutting its associated valve seat 54, the spring seat disc 46 moves away from the other head 62 and allows the excess pressure in the chamber 31 to vent to atmosphere through the aperture 48 in the disc 46 and through the vent hole 52.

The invention is not restricted to the details of the foregoing embodiment.

Claims

1. A relief valve comprising a fluid inlet, a fluid outlet, a valve closure element and associated valve seat interposed between the inlet and outlet to control the fluid flow through the valve, the valve closure element, in use, being biassed towards engagement with the valve seat by means of fluid pressure in a chamber, a conduit extending between the fluid inlet and the chamber, and a control valve situated in said conduit, the control valve being responsive to pressure applied to the fluid inlet and being adapted to allow communication between the fluid inlet and the chamber up to a certain value of pressure applied to the fluid inlet, at which pressure the valve isolates the chamber from the pressure in the fluid inlet.

2. A relief valve as claimed in claim 1, wherein the valve closure element is secured to a diaphragm which partly defines the chamber.

3. A relief valve as claimed in claim 2, wherein the effective surface area of the diaphragm is larger than the effective surface area of the valve closure element.

4. A relief valve as claimed in claim 3, in cluding a means for releasing excess pressure from the chamber when the chamber is isolated from the fluid inlet and when the pressure in the chamber exceeds a predetermined level.

5. A relief valve as claimed in claim 4, wherein the control valve comprises an annular valve seat member disposed in said conduit and a coaxial valve member disposed for longitudinal displacement therewith and carrying a first closure member on its one end, and a second chamber disposed in said conduit between said annular valve seat member and said first mentioned chamber, the second chamber being defined in part by a. second diaphragm by which the valve member is normally engaged under spring pressure.

6. A relief valve as claimed in claim 5, wherein said means for releasing excess pressure from the chamber comprises a further valve seat and closure member defined respectively in said second diaphragm and on said valve members of the control valve, the latter valve seat and closure member being adapted to separate only when the pressure in said first mentioned chamber exceeds said predetermined level.

7. A relief valve substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.