Displaceable shearing ball valve

A valve typically used in an oil or gas well, has a moveable member such as a ball 5 movable between open and closed configurations and has a cutting device arranged to shear against an anvil member 15 when the ball 5 is moved between the open and closed configuration. A sealing member 10 provides a seat for seating of the moveable member 5 when the moveable member 5 is in the closed configuration. The anvil member 15 and the sealing member 10 are moveable relative to one another in direction A (possibly by use of a cam) when the moveable member 5 is moving from the open to the closed configuration, so that during opening and closing the sealing member 10 is displaced away from the cutting device (typically the inner surface of the bore) as the cutting device engages the anvil member 15. The sealing member 10 is pushed away from the moveable member 5 and the anvil 15 to a maximum separation from the movable member 5 at the point on the stroke when the cutting surface of the moveable 5 member is moving past the anvil member 15, so that the sealing member 10 is moved out of contact with the ball 5 when the cutting surface engages the anvil member 15, and moves back into contact with the ball 5 when the cutting surface has passed the anvil member 15 and the sealing surface of the ball 5 is aligned with the sealing member.