DUAL STAGE HYDRAULIC ACTUATOR FOR EXPANDING GATE VALVE

A dual stage hydraulic actuator (9) for actuation of an expanding gate valve (10). The actuator is mountable to the valve housing and comprises a housing 40 which defines a piston chamber 41 and a primary piston (61) slidably received therein. An actuator stem (23) connects the piston (61) to the valve gate assembly (20). A secondary piston (77), of smaller piston area than the primary piston (61), is slidably received in a piston receiving bore (75) which is formed in the fluid receiving face of the primary piston. A piston rod (80) connects coaxially with the piston (77) and extends through the end (52) of the actuator housing. The actuator is responsive to hydraulic fluid pressure which when applied through an inlet (71) of the actuator housing to one side of the pistons (61, 77) drives the actuator stem and gate assembly to an operational open or closed position of the valve. A spring means (66, 67) in the piston chamber (41) continuously urges the primary piston (61) in the direction which opposes the hydraulic-pressure. A stop element 85 on the piston rod 80 is adapted to engage the end (52) of the actuator housing to limit the length of the secondary piston stroke to a lesser length than that of the primary piston stroke whereby the effective piston area of the actuator (9) for driving the actuator stem (23) and the gate assembly (20) is reduced by an amount equal to the area of the second piston and there is a corresponding reduction in stress imposed on the actuator stem in the latter stage of actuator thrust.