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Electrohydraulic valve actuators and hydraulic valve actuators convert fluid pressure into motion in response to a signal. They use an outside power source and receive signals that are measured in amperes, volts, or pressure. Some electrohydraulic valve actuators and hydraulic valve actuators move rotary motion valves such as ball, plug, and butterfly valves through a quarter-turn or more from open to close. Other valve actuators move linear valves such as gate, globe, diaphragm, and pinch valves by sliding a stem that controls the closure element. Throttling valves can be moved to any position, including fully open or fully closed, within the stroke of the valve. Typically, valve actuators are added to throttling valves as part of a control loop that includes a sensing device and circuitry.

Electrohydraulic valve actuators and hydraulic valve actuators use several different types of actuators. Diaphragm actuators are used mainly with linear motion valves, but are suitable for rotary motion valves with a linear-to-rotary motion linkage. Rack-and-pinion actuators transfer the linear motion of a piston cylinder actuator to rotary motion. They are ideal for automating manually-operated valves. Scotch yoke actuators also transfer linear motion to rotary motion. With lever and link actuators, a splined or slotted lever attaches to the valve shaft in order to transfer the linear motion of a diaphragm or piston cylinder to rotary motion. Vane actuators are used only with rotary motion valves.

Important specifications for electrohydraulic valve actuators and hydraulic valve actuators include actuation time and hydraulic fluid supply pressure range. Devices that move rotary motion valves vary in terms of actuator torque and range of rotary motion. Devices that move linear motion valves vary in terms of valve stem stroke length and actuator force or sealing thrust. For both types of electrohydraulic valve actuators and hydraulic valve actuators, acting type is an additional specification. With single-acting devices, fluid pressure actuates the valve in one direction while a compressed spring actuates the valve in the other. With double-acting devices, fluid pressure actuates the valve in both directions. Since electrohydraulic valve actuators and hydraulic valve actuators work with multi-turn valves, the number of turns is another important specification.

Features for electrohydraulic valve actuators and hydraulic valve actuators include NEMA enclosures and actuator action. The National Electrical Manufacturers Association (NEMA), a non-profit trade organization, rates enclosures for electrical equipment. Devices with NEMA 4 and 4X ratings are suitable for indoor or outdoor use and provide protection against dirt, rain, sleet, and snow. For manual valve actuators, the actuator action can be direct (clockwise) or reverse (counterclockwise). Other features for electrohydraulic valve actuators and hydraulic valve actuators include overtorque protection, local position indication, and integral pushbuttons and controls. Travel stops or limit stops restrict linear or rotary motion. Manual overrides use handwheels, levers, or hydraulic hand pumps for emergency operation.

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