Hydrostatic referenced safety-circulating valve.

摘要

An annulus pressure responsive downhole tool includes a housing (12) with an operating element (86) disposed in the housing. The operating element is movable from a first element position to a second element position relative to the housing. A hydrostatic referenced annulus pressure responsive first power piston (166) is disposed in the housing, and movable from a first to a second position thereof relative to the housing in response to an increase in well annulus pressure. A lower than hydrostatic referenced annulus pressure responsive second power piston (168) is disposed in the housing and is operatively associated with the operating element for permitting the operating element to move from its first element position to its second element position in response to movement of the second power piston from a first position to a second position thereof relative to the housing. A prevention device (172) is operatively associated with the first and second power pistons for preventing the second power piston from moving to its second position until the first power piston has moved at least part way towards its said second position.