An apparatus for assisting an actuator (13) to close a valve (107) has a cylindrical booster housing (11) with a central axis. The booster housing includes an upward facing piston cavity (45) and a downward facing lower cavity (51) with internal threads (36) for selectively coupling with external threads of a piston head (23) of a valve actuator. A bulkhead (61) is formed between the piston cavity (45) and the lower cavity (51). The bulkhead has an opening (63) for selectively accepting an actuator stem (35) of the valve actuator therethrough. A booster piston (67) is located within the piston cavity. The booster piston is moveable upwards in an axial direction to move the valve (107) from a lower open position to an upper closed position.
