Pressure-balanced rod piston control system for a subsurface safety valve

A control system for a subsurface safety valve (SSV) comprising of a control line from the surface in fluid communication with the top side of an actuating piston which moves a flow tube downwardly to open the SSV. A balance line runs from the surface to the bottom side of the same actuating piston to put the actuating piston in pressure balance. A buildup of pressure in the control line overcomes a return spring to open the valve, while removal of pressure from the control line allows the return spring to close the valve. Seals and leakpaths are provided through the actuating piston so that, depending on the hydrostatic pressure in the control line and the size of the return spring, the various failure modes of the actuating piston seals and control line or balance line will preferentially result in a fail-closed situation in the SSV.