an assembly adapted to be anchored at a subsurface location within a tubular well conduit to function as a self - closing safety valve . the assembly includes a flapper valve closure element pivotably mounted in a tubular valve housing and controlled by a pressure sensitive control mechanism . an anchoring means having radially expansible anchoring and sealing members is employed to secure the valve in place and to form a seal which forces fluid in the conduit to flow through a flow passage in the valve housing . the expansible anchoring and sealing means may be retracted to permit retrieval of the entire assembly . the control mechanism includes a pressure charged bellows chamber which moves in response to a drop in the pressure within the well conduit to cause the flapper to close the flow passage . an incompressible fluid contained within the bellows of the control mechanism cooperates with a double acting valve to prevent over extension or over compression of the bellows by extremes in pressure differential . the force of a compressed spring is suddenly released by movement of the bellows to produce a short , strong movement which is transmitted to the flapper closure element causing it to snap into closed position .