a well safety valve for controlling the fluid flow through a well tubing in which the valve is biased closed . when fluid control pressure is supplied from the well surface , the valve is opened , but the valve closes when the control pressure exceeds a predetermined value . the tubular member controlling the actuation of the valve member includes a first upper and a second lower telescoping section with releasable locking means normally securing the telescoping sections together . a stop shoulder is positioned in the path of movement of the second section in the opening direction for actuating the release of the locking means when the control fluid force exceeds said predetermined value , and the second telescoping section is moved in a closing direction for closing the valve member when the locking means is released . the locking means may be either a shear pin or a resettable lock .