A valve actuator is a mechanical device that uses a power source to operate a valve. It can be operated manually by a human worker or automatically by an outside power source. The power source can be electric, pneumatic, or hydraulic. Two common types of valve actuators include pneumatic and electric. Depending on its design, it can be used for on-off flow applications and/or throttling flow applications. For more information view https://en.wikipedia.org/wiki/Valve_actuator