PRESSURE RELIEF VALVES

Abstract: A fluid pressure relief valve comprises a housing (1) pro­ vided with a pressure inlet (3) and a relief outlet (4), a chamber (6) in communication with the relief outlet (4), a valve member (5) for controlling flow between the inlet (3) and the chamber (6), the valve member (5) having a seating portion (9) adapted to close the inlet (3) and an area responsive to the pressure in the chamber (6), with resilient means (10) biassing the valve member (5) to close the inlet (3) against pressure applied to the inlet (3), the valve being so constructed and arranged that on initial opening of the valve pressure builds up in the cham­ ber (6), producing a force to act on the valve member (5) in op­ position to the force produced by the resilient means (10). The chamber (6) is in communication with the outlet (4) through passages (7, 8) in the valve member (5). These passages may provide the only communication between the chamber (6) and the outlet (4). Alternatively a further connection (14) is pro­ vided between the chamber (6) and the outlet (4), flow through which is controlled by the valve member (5).

Description: PRESSURE RELIEF VALVES This invention relates to pressure relief valves. Pressure relief valves are widely used in fluid pressure systems, particularly hydraulic systems, to limit the pressure which can build up in the system. One well-known type of pressure relief valve is the poppet type, in which a spring-loaded poppet valve member controls flow between a pressure inlet and a relief outlet. The valve member is biassed to close the inlet, and the spring force is opposed by the pressure at the inlet acting on the valve member. When the inlet pressure reaches a predetermined level it overcomes the spring force, and the valve cracks open to allow fluid to flow to the outlet. The fluid flow forces act on the valve member to open the valve further to allow full flow through the valve. The pressure differential across the valve increases to achieve this, and the difference between the inlet pressure at which the valve cracks open - the crack pressure - and that at which full flow is achieved - the full flow pressure - is known as the operating differential. The operating differential is undesirable since the increase in inlet pressure results in a greater temperature rise in the system, and more arduous conditions for the seals, and in some systems the operating differential may be substantial, up to 40 bar. Furthermore, the fluid flow forces acting on the valve member mean that the inlet pressure at which the valve member reseats - the reseat pressure - is lower than that at which it cracks open. All this results in a substantial variation in the maximum pressure in the system which is undesirable. A further problem with this type of valve is that its maximum opening may only be of the order of 0.025mm, so that it is sensitive to dirt. According to our invention, a fluid pressure relief valve comprises a housing provided with a pressure inlet and a relief outlet, a chamber in communication with the relief outlet, a valve member for controlling fluid flow between the inlet and the chamber, the valve member having a seating portion adapted to close the inlet and an area responsive to pressure in the chamber, and resilient means biassing the valve member to close the inlet against the pressure applied to the inlet, the valve being so constructed and arranged that on initial opening of the valve pressure builds up in the chamber, producing a force to act on the valve member in opposition to the force produced by the resilient means. The pressure in the chamber acting on the valve member moves the valve into its fully open position very rapidly after it has cracked open, so that the operating differential is removed. This avoids the temperature rise in the system, eases the conditions for the seals, and reduces the variation in the maximum pressure. Further, this method of opening the valve means that the maximum opening of the valve can be greater, so that it is not sensitive to dirt. The valve means preferably has passage means providing a fluid connection between the chamber and the outlet. in one embodiment this passage means provides the only connection between the chamber and the outlet, so that once the valve has opened fully in response to the pressure in the chamber it remains open, and fluid flow to the outlet is through the passage means, the valve, and in particular the passage means, being arranged to provide both a suitable pressure build-up in the chamber, and suitable relief flow characteristics. However, this construction has the disadvantage that fluid flow forces acting on the valve member result in the reseat pressure being significantly less than the crack pressure. In a further embodiment, the chamber is provided with a further connection to the outlet, the flow through which is controlled by the valve member. The connection is controlled by movement of the valve member in response to pressure in the chamber, and the force in the resilient means. The connection is closed when the inlet is closed, and when it is cracked open, but opens as the valve opens fully. As the connection opens the fluid pressure in the chamber is relieved, and the resilient means then acts to return the valve member, closing the connection, but not the inlet. Pressure again builds up in the chamber, so the valve member moves to open the connection again, and the cycle repeats. This oscillating motion of the valve member continues until the inlet pressure drops just below the crack pressure, which allows the valve member to reseat on the inlet at the end of its return movement. The valve member is able to reseat at this pressure because of the low pressure in the chamber, and the low fluid flow forces on the valve member. In this embodiment the pressure passes to the outlet in pulses, and not continuously. The valve member moves rapidly in response to the pressure in the chamber, to open the connection, and more slowly on its return, in response to the resilient means. The rapid movement creates pressure waves in the fluid, which provide an audible warning that the valve is operative. This is especially marked in hydraulic systems, where a water hammer effect is produced. In either embodiment the valve member is preferably a spool member which slides in a bore in the housing. The seating portion is preferably conical. Pressure relief valves in accordance with our invention may be used in vehicle hydraulic systems, in which a fixed displacement engine-driven pump is used to supply an accumulator for a braking system, and an hydraulic suspension system. The embodiment with the oscillating valve member is particularly useful, since the valve will reseat without the pump being turned off. iSome embodiments of our invention are illustrated in the accompanying drawings, in which: Figure 1 is a longitudinal section through a pressure relief valve; Figure 2 shows a modified pressure relief valve; and Figure 3 is a scheme of a vehicle hydraulic system incorporating the valve of Figure 2. The pressure relief valve shown in Figure 1 is adapted for incorporation in a fluid pressure system. The valve comprises 8 housing 1 provided with a stepped longitudinal bore 2. At one end of the bore 2 a pressure inlet 3 is defined, while a radial passage 4 leading from the bore 2 communicates with a relief outlet. A spool valve member 5 works in the bore 2, and controls fluid flow between the inlet 3 and the outlet passage 4, fluid flow being through a chamber 6 defined in the bore 2 adjacent the inlet 3. The chamber 6 is in permanent communication with the outlet passage 4 through a bore 7 and an inclined passage 8 in the spool 5, and the bore 2. A conical seating portion 9 on the spool 5 adapted to close the inlet 3 controls fluid flow between the inlet 3 and the chamber 6. The valve spool 5 has an area A responsive to pressure in the chamber 6, and is biassed to close the inlet 3 by a spring 10, which works between the end of the bore 7 in the spool 5, and a stationary plug 11 which is sealingly received in the end of the bore 2 remote from the pressure inlet 3. The plug 11 has an external screw-thread 23 which cooperates with an internal screw-thread 24 in the bore 2, and is provided with a socket 25 to enable it to be screwed into the bore. In operation the pressure relief valve is incorporated in a fluid pressure system (not shown). The valve is preferably screwed onto a suitable part by an external screw thread 26. The annular end of the housing 1 at the open end of the bore 2 has a pair of slots 27 for a screwdriver, enabling the valve to be installed easily. When incorporated in a system the pressure inlet 3 is connected to the source of pressure for the system, and the outlet passage 4 is connected to a reservoir for the fluid. Seals 12, 13 are provided on the housing 1 to seal the connections into the system. When incorporated in the system, pressure at the inlet 3 acts on the conical seating portion 9 to produce a force on the valve member 5, which is opposed by the force in the spring 10. If there is a fault in the system which causes the pressure to rise, at a predetermined level of inlet pressure, known as the crack pressure, the force in the spring 10 is overcome, and the valve cracks open. This allows fluid to flow into the chamber 6, and then out to the outlet passage 4 through the passage 8 and bore 7 in the spool 5, and the bore 2. However the relative sizes of the inlet 3 and passage 8 are chosen so that pressure builds up in the chamber 6. This pressure acts on the relatively large area A of the valve spool 5, and moves the valve spool 5 rapidly towards the plug 11 to open the inlet 3 fully. The spool 5 is then held in position by the pressure in the chamber 6, and the fluid flow forces on the spool 5. The sizes of the various passages are chosen to provide a suitable pressure in the chamber 6, as well as a suitable relief flow through the valve. On rectification of the fault in the system the inlet pressure will decrease, as will the pressure in chamber 6, and at a second predetermined pressure the spring 10 acts to reseat the valve, with the seating portion 9 closing the inlet 3. This second predetermined pressure, the reseat pressure, is likely to be significantly lower than the crack pressure, as the fluid forces act on the valve member 5 against the force in the spring 10. This pressure relief valve overcomes several of the disadvantages of the conventional poppet type valve. For example, a poppet valve in general has a large operating differential, that is, it requires a large increase in inlet pressure after cracking open before it reaches full flow. This operating differential results in a greater rise in the temperature of the system, in more arduous conditions for the seals, and in variation of the maximum pressure in the system. However, in the present valve the operating differential is substantially removed, as the pressure in the chamber ensures that the valve opens fully soon after cracking open. Furthermore, the valve has a maximum opening of approximately imam, so that it is not sensitive to dirt, whereas the conventional poppet type is, as its maximum opening is typically 0.025mm. However, the pressure relief valve of Figure 1 may not reseat automatically when the fault in the system is rectified, due to the reseat pressure being lower than the crack pressure. The pressure source for the system may therefore need to be made inoperative before the valve will reseat, which is a disadvantage. Figure 2 shows a modification of the valve of Figure 1 which overcomes this disadvantage. The valve of Figure 2 includes a second radial passage 14 leading from the bore 2 to the relief outlet, to provide a further connection between the chamber 6 and the outlet. Communication through the connection is controlled by a land 15 on the valve spool 5, and is arranged so that the connection is closed when the inlet is closed and when the valve is cracked open, and opens as the valve opens fully. The rest of the construction is the same as that of Figure 1, and corresponding reference numerals have been applied to corresponding parts. In operation, as before, the valve starts to open at the crack pressure to allow fluid to flow to the outlet via the passage 8, bore 7 and bore 2, and to build up pressure in the chamber 6. The valve opens fully in response to the pressure in chamber 6, and the land 15 uncovers the passage 14, so that the pressure in the chamber 6 flows to the relief outlet. The reduction in pressure in the chamber 6 enables the spring 10 to return the valve spool 5 so that the land 15 closes the passage 14, but the seating portion 9 does not close the inlet 3. Pressure can then build up again in chamber 6 to open the valve fully again, and to open the passage 14, so that the cycle repeats. When the inlet pressure drops to just below the crack pressure the valve will reseat on the inlet 3 at the end of its next return movement. The valve is able to reseat at this pressure because of the low pressure in the chamber 6, and the low fluid flow forces on the valve spool 5. This means that the valve can reseat automatically when the fault in the system is rectified. In the valve of Figure 2 pressure fluid passes to the relief outlet in pulses, and not continuously. The valve spool 5 moves rapidly in response to the pressure in the chamber 6, but more slowly on its return movement in response to the spring 10. The rapid movement creates pressure waves, which provide an audible warning that the valve is operative. This is especially marked where the system is hydraulic, and the valve tends to create a water hammer effect. The frequency of the oscillation of the valve spool 5 will typically be between 1 and 10Hz. Figure 3 shows a vehicle hydraulic system incorporating the valve of Figure 2. The system comprises a fixed displacement engine-driven pump 16 which draws fluid from a reservoir 17, and pumps it to an accumulator 18 through a one-way valve 19 and to a suspension system 20 through a one-way valve 21. The accumulator 18 may operate the braking system (not shown) and other equipment. The pressure relief valve is incorporated in a control valve assembly 22, with the inlet 3 connected at a point between the pump 16 and the one-way valves 19, 21. The passages 4, 14 are connected to the reservoir 17. It is important for the valve inlet to be connected between the pump 16 and the one-way valves, as this ensures that it operates efficiently to relieve the pressure from the pump 16, and not that stored in the accumulator 18 or the suspension system 20. The automatic reseating of the valve of Figure 2 is particularly advantageous in the system of Figure 3, as the engine does not have to be switched off in order to reseat the valve.

Claims: CLAIMS 1. A fluid pressure relief valve comprising a housing (1) provided with a pressure inlet (3) and a relief outlet (4), a chamber (6) in communication with the relief outlet, a valve member (5) for controlling fluid flow between the inlet (3) and the chamber (6), the valve member (5) having a seating portion (9) adapted to close the inlet (3) and an area responsive to pressure in the chamber (6), and resilient means (10) biassing the valve member (5) to close the inlet (3) against the pressure applied to the inlet, characterised in that the valve is so constructed and arranged that on initial opening of the valve pressure builds up in the chamber (6), producing a force to act on the valve member (5) in opposition to the force produced by the resilient means (10). 2. A fluid pressure relief valve according to claim 1, characterised in that the valve member (5) has passage means (8, 7) providing a fluid connection between the chamber (6) and the outlet (4). 3. A fluid pressure relief valve according to claim 2, characterised in that the passage means (8, 7) provides the only connection between the chamber (6) and the outlet (4), so that once the valve has opened fully in response to the pressure in the chamber (6) it remains open, fluid flow to the outlet (4) being through the passage means (8, 7), and the valve being constructed and arranged to allow pressure build-up in the chamber (6), and a suitable relief flow rate from the chamber (6). 4. A fluid pressure relief valve according to claim 2, characterised in that there is a second fluid connection (14) between the chamber (6) and the outlet (4), fluid flow through the second connection (14) being controlled by the valve member (5). 5. A fluid pressure relief valve according to claim 4, characterised in that fluid flow through the second connection (14) is controlled by movement of the valve member (5) in response to pressure in the chamber (6) and the force in the resilient means (10). 6. A fluid pressure relief valve according to claim 5, characterised in that the second connection (14) is closed when the valve member (5) closes the inlet (3), and on initial opening of the valve, and opens as the valve opens fully to relieve the fluid pressure in the chamber (6), the resilient means (10) then acting to return the valve member (5) to close the second connection (14) but not the inlet (3), allowing pressure to build up in the chamber (6) so that the valve member (5) moves to open the valve fully and the second connection (14) again. 7. A fluid pressure relief valve according to any preceding claim, characterised in that the valve member comprises a spool (5) sliding in a bore (2) in the housing (1). 8. A fluid pressure relief valve according to any preceding claim, characterised in that the valve member (5) has a conical seating portion (9). 9. A vehicle hydraulic system comprising a fixed displacement engine-driven pump for supplying an accumulator for a vehicle braking system and an hydraulic suspension system, and incorporating a fluid pressure relief valve according to any preceding claim.

Inventors: GOUDIE, ROBERT JOHN
Applicants: LUCAS INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1984
Main classification: F16K17
IPC codes: F16K17