ACTUATOR FOR EXPANDING GATE VALVES

A valve actuator for an expanding gate valve having a gate and a segment thereon. The actuator includes a piston connected to a gate stem and mounted within a cylinder with a pressurized gas on the upper side of the piston and liquid on the lower side of the piston. The gate valve moves from an open position to a closed position across the flow passage at a rapid rate and decelerates adjacent the end of its downward movement upon a metering of the liquid in the cylinder. The expanding gate valve expands laterally into fluid-tight sealing relation at the end of the downward stroke after moving across the flow passage. The bonnet is mounted internally of the valve body and a pressure seal is positioned between the bonnet and the valve body to permit operation of the valve actuator and gate valve even after failure of the pressure seal.