Valve actuators mount on valves and, in response to a signal, move a valve to a desired position using an outside power source. There are several basic types of valve actuators such as manual, electric, pneumatic and hydraulic. Manual valve actuators do not require an outside power source to activate them. They use a hand wheel or lever to drive a series of gears whose ratio results in a higher output torque compared to the input (manual) torque. Electric valve actuators use a single-phase or three-phase alternating current (AC) or direct current (DC) motor to drive a combination of gears to generate the desired torque level. Pneumatic valve actuators adjust valve position by converting air pressure into linear or rotary motion. Similarly, electro hydraulic valve actuators and hydraulic valve actuators convert fluid pressure supply into linear or rotary motion.
Valve motion and operation style are important specifications to consider when in the process of selecting valve actuators in most cases. Rotary motion valves (rotary valves) such as ball, plug, and butterfly valves rotate a quarter-turn or more from open to close. Linear motion valves (linear valves) such as gate, globe, diaphragm, pinch and angle-style valves have a sliding stem design that pushes the closure element open or closed. The valve stem may rise during rotation, or may rise without rotation. There are two basic operating styles for valve actuators that can be used.
On/off or isolating devices limit actuator motion to preset or open and closed positions. Modulating devices provide controllable motion so that the valves can be throttled as necessary. Performance specifications for rotary actuators include actuator torque and range of motion. Rotary devices move a quarter-turn (90°), through multiple turns (360°), or a nominal 180° or 270°. Performance specifications for linear actuators include a valve stem stroke length, an actuation time, a number of turns, and an actuator force or seating thrust.
The general specifications for all types of valve actuators includes a control signal input type, voltage, a supply pressure, a valve stem diameter, an actuation time, a fail-safe method, a location type and an operating temperature. There are three basic types of control signal inputs that play a roll in the functioning of valve actuators: milliampere, voltage, and pressure. Devices that use AC voltage or DC voltage are commonly available. Supply pressure is the input pressure needed to achieve a desired torque or thrust output. Companies specify air supply pressure for pneumatic actuators and fluid supply pressure for hydraulic actuators.
There are several fail-safe methods that come into play for valve actuators. Devices can open or close valves in the case of power failure, or in the case of loss of control signal. Valve actuators for hazardous locations are designed for environments with atmospheres that contain combustible or potentially explosive mixtures. The devices for non-hazardous locations are designed for environments without the risk of combustion or explosion. A complete assembly for valve actuators usually consists of an electronic valve actuator, a valve kit, plus the valve itself.
