GB2213564A

GB2213564A - Ball valve

Info

Publication number: GB2213564A
Application number: GB8829709A
Authority: GB
Inventors: Brian George Hopkins
Original assignee: FLEXITALLIC INTERNATIONAL VALV
Current assignee: FLEXITALLIC INTERNATIONAL VALV
Priority date: 1988-01-08
Filing date: 1988-12-20
Publication date: 1989-08-16
Also published as: GB8800363D0; GB8829709D0

Abstract

A trunnion-mounted ball valve having at least one annular seal 22 which is spring biassed into engagement with the ball 8 and which is also biassed against the face of the ball with a force generally derived from the fluid pressure in a pipe-line includes fluid jacking means operable to at least partially overcome the biassing force. Preferably the fluid jacking means comprise an annular piston 20 and cylinder defining a chamber 25 into which fluid may be injected. <IMAGE>

Description

Ball valves This invention relates to ball valves of the kind in which a ball having a central aperture thereto is interposed between abutting ends of a fluid pipeline. Rotation of the ball about an axis normal to that of the aperture progressively occludes the pipeline. The invention is particularly concerned with what is known as a trunnion mounted ball valve, in which the ball is supported by bearings; it is not free to float along the axis of rotation.

Such ball valves are extremely well known. The problem of preventing leakage around the ball either to atmosphere or into the mechanism for rotating the ball is usually dealt with by providing annular seals which are located on opposite sides of the ball in a generally co-axial relationship to the aperture through the ball when the latter is rotated to permit fluid flow. It is normal practice to provide means for urging the seal against the ball. Springs are normally used for this, although, in the case of high pressure pipelines, it is also common to provide means whereby the pressure in the pipeline is also effective to bias the seals against the ball,- with an additional a biassing force which is proportional to the fluid pressure in the pipeline. Low pressure sealing is provided by the springs.

The fluid pressure biassing principle is the overbalance", whereby fluid pressure in the pipeline is allowed to access an annular space at least part of which is radially outwardly of an axially slidable annular seat.

The working area of that annular space radially outwardly of the seat is made sufficient to ensure that pressure in the pipeline is always less than the seating force generated by that pressure.

In this type of ball valve the sealing force increases with pressure. Whilst this technique is very satisfactory in preventing leakage, it gives rise to problems. The most significant problem is that extremely high forces are required to rotate the ball, particularly when the valve is in the closed, or no-flow state. This can lead to the use of actuator mechanisms which are indeed at least as large as the rest of the ball valve. It is an object of the present invention to at least mitigate this problem.

According to the present invention, a trunnion mounted ball valve having at least one annular seal means which is spring biassed into engagement with the ball and which is also biassed, in use, against the face of the ball with a force derived from to the fluid pressure in an associated pipeline so as to prevent leakage between said ball and said seal is provided with fluid jacking means operable to at least partially overcome said biassing force.

Preferably, the valve has axially slidable, annular seal means associated with both inlet and outlet to the valve, said seal means being carried by respective seal support means, each provided with auxiliary fluid jacking means operable to at least partially overcome, or relieve the biassing force.

In a particularly preferred embodiment of the invention annular piston and cylinder jacking means are provided to at least partially overcome (when required) the biassing force generated by the fluid pressure in the pipeline.

Miniature piston and cylinder devices, may also be employed.

It will be appreciated that the jacking means need not displace the seal support means very much. It is only necessary to relieve at least some of the force on the ball so that it can be turned more easily than would otherwise be the case. The precise amount of relief required in order to turn the ball will be a matter for experiment, since it is also necessary to maintain effective sealing around the ball to prevent significant leakage.

In order that the invention be better understood an embodiment of it will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a cross-sectional side view of a conventional trunnion-mounted ball valve, Figure 2 is a fragmentary side view, in cross-section, through the seat area of a ball valve according to the invention, and Figure 3 is a view corresponding to that of Figure 1, but showing the device in the operated state.

Referring to Figure 1, a typical ball valve comprises a two-part body 1,2 secured together by bolts 3 and having pipe flanges 4,5 at each end. A passageway 6 extends through the valve, which has a centrally located chamber 7 containing a ball 8. As shown in Figure 1, the valve is open; the passageway 6 extendeds through the ball, which can be rotated by a spindle 9 situated in a column 10 which is secured by bolts 12 to the body parts 1 and 2.

The column 10 is itself in two parts; the upper part is rotatable to rotate the ball 8, but this will not be described here in further detail. At the opposite side of the ball, there is a locating spindle 14 supported in a bearing/seal assembly generally designated 15. The chamber 7 contains axially slidable seal support means 20, each in the form of an annulus carrying annular seals (or valve seats) 21,22. The supports are biassed towards the ball by annular belleville springs 23. The sealing mechanism is best described by referring to Figures 2 and 3 and it will be appreciated that these show only part of the arrangement, in the interests of simplicity.It will be seen that each seal support 20 is not only urged towards the ball 8 by the springs 23, but because there is a radially outward region A, subjected to the pipeline internal pressure there is a overbalance pressure generated on the seal 22 in proportion to the pipeline pressure. As the latter increases so does the overbalance pressure. The valve thus described is entirely conventional; Figures 2 and 3 show part of the ball/seal area of a modified ball valve.

In Figures 2 and 3 the seal support 20 is modified to create an annular cavity 25. This communicates with a further annular cavity 26 through a bore 27. It will be understood that more than one bore 27 will normally be provided around the circumference of the valve. The annular cavity 26 communicates with the exterior of the valve through a bore 28, which enables pressurised fluid to be injected into the cavities 25, 26. It will be seen that the annular support 20 and the annular cavity 25 together constitute a piston and cylinder device with the seal support constituting the piston. By making the annular surface area B of the cavity 25 slightly greater than that of the annular overbalance region A, it is possible to overcome the overbalance pressure by injecting fluid into the chamber 25. This is shown in Figure 3 which illustrates the situation when fluid is injected into the cavity 25 to overcome the overbalance pressure. The effect is to force the seal support 20 backwards away from the ball. In this instance the movement has been exaggerated to show a gap between the seal and the ball, although in practice all that is required is to relieve the pressure against the ball sufficiently for the latter to be turned without at the same time permitting leakage past the seat.

The invention thus provides a simple and readily controllable means of relieving the seat pressure and thereby facilitating rotation of the valve.

Claims

1. A trunnion-mounted ball valve having at least one annular seal means which is spring biassed into engagement with the ball and which is also biassed, in use, against the face of the ball with a force generally derived from the fluid pressure in an associated pipeline so as to prevent leakage between said ball and said seal means, the valve being provided with fluid jacking means operable to at least partially overcome said biassing force.

2. A valve according to claim 1 wherein said seal means are carried by axially slidable annular seal support means associated with both inlet and outlet to the valve, each of said seal support means being provided with fluid jacking means operable to at least partially overcome, or relieve the biassing force.

3. A ball valve according to claim 1 or claim 2 wherein the fluid jacking means are constituted by annular piston and cylinder devices defined between the seal support means and the valve body.

4. A valve according to claim 1 or claim 2 wherein the fluid jacking means are constituted by- discrete piston and cylinder devices.

5. Ball valves substantially as hereinbefore described with reference to and as illustrated by the acocmpanying Figure 2 and 3.