Hydraulic Actuators:

Hydraulic actuators give for automatic or semi-automatic positioning of the valve, same to the pneumatic actuators. Those actuators use a piston to convert a signal pressure inside valve stem motion. Hydraulic fluid is fed to either side of the piston when the other side is drained or bled. Oil or Water is used as the hydraulic fluid. Solenoid valves are classically used for automatic control of the hydraulic fluid to direct either opening or closing of the valve. Manual valves could also be used for controlling the hydraulic fluid; therefore giving semi-automatic operation.

Figure: Pneumatic Actuator