The current working practice in the pipework and plant installation industries (onshore and offshore) is, for safety reasons, to fit at predetermined locations within a fluid pipeline an isolation valve assembly. This often takes the form of what is known in the industry as a"double block and bleed valve". Such a valve has two valve seat surfaces and, in the closed position, prevents flow from each valve end when the cavity between the seating surfaces is vented or drained. Examples of such valves are described in our international patent application WO 02/29303 and UK Patent No.

2271164 in which two single independently operable ball valves are disposed adjacent to one another in the same housing. The ball valve units are both closed, for example, while workers are working downstream of the isolation valve assembly with the safety of the workers being ensured and maintained should one of the ball valve units fail.

Double ball isolation valves of the kind referred to above have proved very effective but nevertheless in certain circumstances it would be more cost effective and desirable to use a single ball valve that could provide the double block feature.

Whilst a conventional isolation valve with a single trunnion-mounted ball valve has two spring biased seats, one disposed on each side of the ball, if flow leaks past the upstream seat it will push the downstream seat away from the ball. It does not therefore provide the double block feature.

It is an object of the present invention to obviate or mitigate the aforesaid problem.

According to the present invention there is provided an isolation valve assembly for use in a fluid pipeline, the assembly comprising a body with a passageway extending therethough, a valve chamber in said passageway and in which is disposed a rotary valve ball, the ball being rotatable to open and close said passageway to control the flow through the valve, a pair of valve seats in said body disposed on opposite sides of the ball, each seat having a first side proximal the ball valve and defining a sealing surface in contact with an outer surface of said valve ball, a second side distal from said ball valve and a second sealing surface in contact with the body, and a passage extending from the valve chamber for redirecting fluid pressure in said chamber towards the second side of the seat.

The term"fluid"is used herein to refer to a gas or liquid.

Any fluid that leaks past one of the valve seats can thus be redirected to force the other seat into greater sealing contact with the valve ball.

There may be provided at least one actuating member between the second side of the seat and the passage, said member serving, in use, to convert the pressure of the fluid in the passage into a force applied to the seat so as to force the first sealing surface towards the valve ball. The actuating member is preferably disposed between an outlet of the passage and the second side of the valve seat and is preferably annular.

The actuating member may have an annular surface for contact with a surface on the second side of the valve seat. In a preferred embodiment it is slidably received in a recess in said body.

At least one seal may be provided between the actuating member and a wall of the recess.

The actuating member may be a piston.

The passage is preferably defined in the body of the valve and one such passage may be provided on both sides of the ball so as to redirect fluid in the chamber towards both of the seats.

The seats are preferably biased into contact with the valve ball without fluid flowing through the valve.

A specific embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic representation of an isolation ball valve assembly in accordance with the present invention; Figure 2 and 3 are diagrammatic representations of part of the valve assembly of figure 1 illustrating its operation in use.

Referring now to figures 1 and 2 of the drawings, the exemplary isolation valve assembly is a single ball valve unit comprising a housing body 1 of standard length for connection into a pipeline by pipe fittings (not shown) at each end, and a valve ball 2 mounted within a valve chamber 3 in the housing 1.

The housing 1 has an inlet 4 and an outlet 5 that are separated by the valve chamber 3 and ball 2. The ball 2 is rotatably disposed in the chamber 3 between a pair of annular valve seats 6,7, one (the upstream seat 6) adjacent to the inlet 4 and the other (downstream seat 7) adjacent to the outlet 5. The seats 6,7 are each generally cylindrical with an outer stepped surface defining a first portion 8a that is in sealing contact with the inner surface 9 of the housing that defines part of the inlet or outlet 4, 5 and a second portion 8b, of larger outside diameter that extends into the valve chamber 3. A O-ring seal 10 is disposed in an annular groove 11 in the first portion 8a so as to provide sealing contact between the seat 6, and the valve housing body 1. The inside surface 12 of each valve seat 6,7 is cylindrical and has a taper 13 that extends radially outwardly at a location adjacent to the ball 2. The taper 13 defines an annular protruding sealing face 14 that seals against the outside surface of the valve ball 2.

Both of the valve seats 6,7 are spring biased (springs not shown in the figures for clarity) into contact with the valve ball 2.

The valve ball 2 is of conventional design with a central bore 15 that when in register with the inlet and outlet 4,5 opens the valve. The ball 2 is rotated through 90 degrees by an actuator (not shown) to close the valve.

An annular piston 16 (removed in figures 2 and 3 for clarity) acts on an outer edge of each seat 6,7 at the inlet or outlet end. The piston 16 is slidably retained in an annular recess 17 for movement towards and away from the seat 6,7. Small bore passages 18 provide fluid communication between the valve chamber 3 and the recess 17 in which the piston 16 is housed. One end 19 of the piston 16 receives the fluid flowing through the passage 18 and further grooves 20,21 on its inner and outer surfaces receipt of O-ring seals 22,23. Axial movement of the piston 16 is effected by the pressure of the fluid flowing through the valve and therefore through the passages 18. As the pressure increases the piston 16 is biased by the fluid against the valve seat 6,7 so as to force it into greater sealing contact with the valve ball 2. The piston 16 and passage 18 arrangement is shown only at the outlet 5 end of the valve in the figures but it is to be appreciated that the same configuration may also be provided at the inlet end 4.

If the upstream seat 6 fails and pressurised fluid leaks past it into the valve chamber 3 it will tend to push the downstream valve seat 7 off the valve ball 2.

However, at the same time the fluid will flow into the passages 18 and act on the end of the piston 16 thus forcing back into sealing contact with the ball 2. Providing the force applied by the fluid pressure on the piston 16 is sufficiently large to exceed the force tending to unseat the valve seat 7, the seal is maintained.

The sealing force applied to the upstream valve seat 6 by the fluid pressure is provided by the diametric offset between the valve seat/valve body seal interface (at diameter A-see figure 3) and the valve seat sealing face/valve ball interface (diameter B). This is illustrated in figure 3. The pressure of the fluid acts on the inlet end of the seat 6 to force the seat towards the valve ball 2 and also in the opposite direction at the tapered portion 13 of inner surface 12 of the valve seat adjacent to the sealing face 14. The resulting sealing force thus effectively acts on the inlet end of the valve seat 6 in the area indicated by the arrow X (the area between diameter A (dA) and diameter B (dB)) and acts in addition to the spring force. This additional sealing force can be represented by the formula : -7dB2 dA Sealing force =-----x Pressure 0 In the event that the upstream seat seal 14 fails and starts to leak the fluid will egress into the valve chamber 3 where it acts on the ball side of the downstream seat 7 in the area defined between diameters A and B as indicated by arrows Y in figure 2.

When the force applied to the seat 7 exceeds the value of the spring sealing force it will tend to force the downstream seat 7 off the valve ball 2. Thus the sealing force on the upstream valve seat 6 becomes the unseating force acting on the downstream valve seat 7. Failures of this kind generally occur at high pressures and the spring force can be considered to be negligible. The reseating force (represented in figure 2 by arrows Z) applied by the piston 16 on the downstream valve 7 seat thus has to be twice the magnitude of the unseating force so as to counter the unseating force and apply a sealing force equal in magnitude to that on the upstream seat.

The end area of the piston required to generate this force is thus calculated by <BR> <BR> <BR> <BR> the formula:<BR> 2x(unseating force)<BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> Piston pressure area =<BR> @ flow pressure Since the unseating force is equivalent to the upstream sealing force the formula can be rewritten as: Piston pressure area (PPA) = 2 2 The inside diameter (dID) of the piston 16 is governed by the wall thicknesses of the valve body and therefore the outside diameter (dot) of the piston can be calculated by the following formula : The arrangement ensures that a double seal can be provided for a single ball valve. Thus if an upstream seal fails, the downstream seal can serve to prevent failure of the valve.

With a piston 16 arrangement at both ends of the valve the double seal can be provided in both flow directions.

It will be appreciated that numerous modifications to the above described design may be made without departing from the scope of the invention as defined in the appended claims.