Low pressure responsive downhole tool with cam actuated relief valve

摘要

An annulus pressure responsive downhole tool includes a housing having a power piston slidably disposed therein. First and second pressure conducting passages communicate a well annulus with first and second sides of the power piston. A retarding device is disposed in the second pressure conducting passage for delaying communication of a sufficient portion of an increase in well annulus pressure to the second side of the power piston for a sufficient time to allow a pressure differential across the power piston to move the power piston from a first position to a second position relative to the housing. A pressure relief valve is communicated with the second pressure conducting passage between the power piston and the retarding device for relieving from the second pressure conducting passage a volume of fluid sufficient to permit the power piston to travel to its second position. The pressure relief valve is opened by mechanical action when the power piston starts to move toward its second position.