Patent ID: 6899179

Claim:
A bypass valve for selectively isolating the interior of a downhole assembly from the exterior thereof, the bypass valve comprising: a body incorporating a wall provided with at least one opening extending therethrough; a piston slidably mounted in the body and having a longitudinal bore extending therethrough, the piston being movable between a first position relative to the body, in which the bore of the piston is substantially isolated from the exterior of the body, and a second position relative to the body, in which a passage is provided from the bore of the piston to the exterior of the body via said at least one opening; and constraining means for controlling movement of the piston in response to fluid pressure applied to the piston, said constraining means comprising a guide pin and a guide slot for receiving the guide pin; the guide slot being arranged so as to prevent movement of the piston both from the first position to the second position and from the second position to the first position, and the guide slot is further arranged so as to permit said movement after a predetermined number of applications of a given minimum fluid pressure force to the piston.