Abstract: A fluid-powered actuator assembly 10 for gate valves 13 having a slab gate 21 with a port 22 adapted to be placed in registry with the flow passage through the valve wherein the valve is moved to open position by application of fluid pressure to the actuator piston 18 and moved to closed position by a piston return spring 33 when the pressure is removed. The assembly includes an auxiliary energy storage device 40 in the form of a coiled spring 41 which is compressed in the initial stages of the actuator stroke to open the valve 13 and is maintained in the compressed energized state by a releasable latch mechanism 52 which latches to the auxiliary spring housing 42. When the actuator 12 operates to close the valve, the auxiliary coiled spring 41 is released near the end of the valve-closing stroke to provide additional axial thrust to the actuator shaft 17 and the valve gate 21 of a magnitude sufficient to shear wire lines 25 or other members which have been inserted through the gate port.