GB2144519A

GB2144519A - Fluid flow valves

Info

Publication number: GB2144519A
Application number: GB08321217A
Authority: GB
Inventors: David Colin Lewis
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1983-08-05
Filing date: 1983-08-05
Publication date: 1985-03-06
Also published as: GB8321217D0; GB2144519B

Abstract

A plate valve has resilient means 25 to urge a plate 17 against a valve seat 15 against the pressure exerted by the fluid, and means to reduce the force of the resilient moves as the valve is opened to compensate for the reduction of the fluid pressure on the plate. The means may comprise a cam follower 22 acting on the end of resilient means remote from the plate, or the provision of a sloping surface for the valve seat (10, Fig. 3). <IMAGE>

Description

SPECIFICATION Fluid flow valves The present invention relates to plate valves and in particular to a means for maintaining a constant retaining force between the plate and a sealing land.

It is well known to control the flow of a fluid through an orifice by the use of a flat plate valve, which is moved across the orifice to expose more or less of the orifice and so regulate the flow through the valve.

A typical valve is shown in Fig. A. Fluid inlet to a control system is at (1) immediately upstream of a flow regulating plate valve consisting of a plate (2) and seat (3). Contact between the plate and seat is ensured by a light spring (4). The valve is moved by rotating the pivoted arm (5) which is driven by an actuator (6) in response to fluid flow requirements. Fluid pressure adds to the force holding the valve plate 2 onto its seat 3.

A problem with this previously known arrangement is that at low flows, where the valve is almost closed, the fluid pressure urges the plate 2 into contact with its seat 3 and there is a large area of the plate subject to the fluid pressure, which results in a high friction load which must be overcome by the actuator. Due to the restrictions of size and weight imposed by many installations, this can lead to unacceptable response in terms of the flow error signal which is needed to cause the valve to break out of its static friction. As a result the flow can depart from the desired ievel by as much as five or six times the tolerance figure required for the system.

Attempts to alleviate the problem have taken the form of choosing lower friction materials or using more powerful actuators.

These approaches can only modify the effects rather than tackle the cause of the problem and as such can only be of limited use.

A more fundamental change, which tackles the cause of the problem, is to alter the valve layout so that the fluid pressure no longer loads the plate onto its seat, but acts to unseat the valve. This arrangement is shown in Fig. B. In order to maintain the valve on plate 2 the seat 3, a heavier spring 4 is need which will be capable of overcoming the maximum pressure load (when the valve is closed) and still leave a small force holding the valve plate 2 in position. However, when the valve plate 2 is moved to open the orifice the spring load greatly exceeds the pressure loads. The spring load therefore provides a small net force holding the valve plate 2 on its seat 3 under conditions of maximum pressure load.

At this stage, the valve friction is low, leading to high sensitivity. As the pressure load reduces, the amount of unbalanced spring load increases until, at maximum flow, the valve friction is again high, leading to low sensitivity.

An object of the present invention is to provide a means of reducing the load exerted on the valve by the spring in line with the reduction in hydraulic load whilst still maintaining a small net force to hold the valve on its seat.

The invention as claimed employs a compression spring to hold the plate onto its seat but provides means for reducing the force exerted by the spring by allowing the spring to extend in a controlled manner.

Embodiments of the present invention will now be described, by way of examples only, with reference to the accompanying drawings, in which: Figures A and B show two known valves which do not incorporate the present invention.

Figures 1 to 3 illustrate schematically one valve constructed in accordance with the present invention; Fig. 2 is a view of the valve seat of the valve shown in Fig. 1 looking from the downstream side of the valve and Fig. 3 is a cross-section taken along line A-A of Fig. 2.

Figure 4 illustrates schematically a second valve constructed in accordance with the present invention, and shows a cross-sectional view taken along line B-B of Fig. 5 and Fig.

5 is a view of the valve of Fig. 4 looking from the downstream side of the valve.

Referring to Figs. 1 to 3, a valve seat (10) around the fluid duct orifice (12) is given a profile designed to produce the desired flow characteristics. A plate (11) is shaped and positioned such that it covers the fluid duct orifice (12) in a first position and completely uncovers the orifice (12) in a second position.

The plate (11) is retained in, but axially translatable in, a pivoted arm (12) which pivots about an axis (13). A compression spring (14) is located between the plate (11) and the pivoted arm (1 2) to provide a force retaining the plate (11) against the seat (10). The mating face between the seat (10) and the plate (11) is inclined such that when the plate (11) is translated from its first position to its second position, by moving the pivoted arm (12), the plate (1 1) follows the profile of the seat (10) and the compression spring (14) progressively relaxes its retaining force as it expands.

The valve shown in Figs. 1 to 3, whilst partially solving the problems mentioned above, suffers from various disadvantages such as the hysteresis caused by the work done in forcing the valve to the closed position against increasing spring load and the restriction of having to use a constant slope ramp when the pressure load variation may not be linear with angular movement of the arm. Nevertheless the valve of Figs. 1 to 3 is a considerable improvement on other known valves which do not allow the spring to relax.

Referring to Figs. 4 and 5, a more suitable solution will now be described. A valve seating rim (15) around the fluid duct orifice (16) is given a profile designed to produce the desired flow characteristics. A plate (17) is shaped and positioned such that it covers the fluid duct orifice (16) in its first position and uncovers the orifice (1 6) in its second position. The plate (17) is retained in, but free to move axially along a pivoted arm (18) which pivots about a fixed axis (19). A lever arm (20) is mounted on the pivoted arm (18), by means of a pin (21), and the arm 20 pivots about the longitudinal axis of the pin (21).

The other end of the lever arm (20) is fitted with a low friction cam follower (22) which reacts against a surface of a cam (23) which is mounted to a fixed part (24) of the valve. A compression spring (25) provides a force to retain the plate (17) against the seating rim (15) by reacting between the plate (17) and a reaction plate (26) which is mounted on lever arm (20). The cam (22) is profiled to achieve the desired relationship between the position of plate 1 7 and the spring load. Adjustment, to take account of manufacturing tolerances, can be obtained by size assessment of spacers (27) between the spring (25) and reaction plate (26), and between the pivoted arm (18) and fixed point (19).

The centre of pressure on the valve plate (17) will vary with the degree of opening of the valve, whereas the spring force acts at a constant point on the plate (17). This could result in a tipping moment which could lift one end of the plate (17) from the seating rim (15), thus corrupting the valve opening/flow characteristic. In order to prevent this, the valve plate (1 7) is extended so that any tipping has to take place about its extended edge (28). The spring load application point is arranged to fall between the edge (28) and the closest position of the centre of pressure to ensure that edge (28) acts as the fulcrum.

The seat rim (15) is extended (31) to accomodate the larger valve plate (17).

The angular relationship of the valve plate (17) and the lever (20) is determined by pinning the extension arm (29) of the reaction plate (26) to the valve plate (17) with pin (32). This ensures that the metering edge (30) of the valve plate (17) is presented in the correct attitude to the valve sealing rim (15).

Actuation means, shown schematically, and well understood per se, are provided to move the pivot arm (18) such that the valve plate (17) may move between its first position and its second position and thereby selectively open and close the duct orifice to control the fluid flow.

It is to be understood that the spring 14, 25 could be replaced by another form of resilient member such as for example an elastic compressible body made of a deformable material such as rubber, rubber compounds or a plastics material. Alternatively other resilient means may be used such as, for example, a piston which is connected to the plate and operates on a compressible medium such as rubber or a gas contained in a sealed container.

Claims

1. A valve for a fluid flow passage comprising an orifice through which, in operation, a fluid flows, a plate which is movable across the orifice thereby to open and close the orifice, a valve seat against which the plate seals, resilient means operable to exert a force on the plate to urge the plate into sealing engagement with the seat against the action of fluid flow pressure acting on the plate, and means for varying the force exerted on the plate by the resilient means as the plate is moved across the orifice to open and close the valve.

2. A valve according to claim 1 wherein the means for varying the force exerted on the plate by the resilient means comprises cooperating mutually confronting surfaces on the seat and the plate which are inclined to the direction of movement of the plate across the orifice, the surfaces being positioned and arranged so that as the plate is moved across the orifice from a first position, where the orifice is closed, to a second position, where the orifice is opened, the force exerted by the resilient means on the plate is reduced but the plate is retained in contact with the seat by the resilient means.

3. A valve according to claim 1 or claim 2 wherein the means for varying the force exerted on the plate by the resilient means comprises a first arm which is mounted for rotation about a first axis which extends in a direction along the passage and on which is carried the plate, the plate being movable bodily with the arm about the first axis and being movable relative to the first arm in directions extending along the first axis, a second arm which is pivotally mounted on the first arm for rotation about a second axis extending transverse to the first axis, the resilient means being mounted on the second arm and having an end operable on the plate to exert a force thereon, and a cam means for rotating the second arm about the second axis when the first arm is rotated about the first axis thereby to cause the resilient means to become extended or compressed and thereby vary the force exerted by it on the plate.

4. A valve according to any one of claims 1 to 3 wherein the resilient means is a compression spring.

5. A valve according to any one of claims 1 to 3 wherein the resilient means comprises an elastically compressible body.

6. A valve according to any one of claims 1 to 3 wherein the resilient means comprises a piston which operates on a compressible medium.

7. A valve substantially as herein described with reference to Figs. 1 to 3 of the accompanying drawings.

8. A valve substantially as herein described with reference to Figs. 4 and 5 of the accompanying drawings.

9. A valve according to any one of claims 1 to 6 wherein the plate and the first arm extend transversely to one side of the orifice when the plate is in the position where it closes the orifice, and the arm is provided with a fulcrum about which the plate tilts, the fulcrum being spaced from the centre of maximum pressure on the plate, which occurs when the plate closes the orifice, and from the point of application of the force exerted on the plate by the resilient means.

10. A valve according to claim 9 wherein the point of application of the force exerted by the resilient means is located between the fulcrum and the centre of maximum pressure on the plate which occurs when the plate closes the orifice.

11. A valve according to claim 9 or claim 10 wherein the valve seat is extended transversely to one side of the orifice to accommodate the extended plate.