A control valve actuator is a device used on a valve to accurately position the plug, ball or vane into the stationary seat of the valve as dictated by the control system. Control valve actuators are necessary for adjustable control and are widely used within the chemical process industry. The most common type is the pneumatic actuator, but electric, electrohydraulic and manual actuators also are often used. Pneumatic control valve actuators generally are favored for their simplicity of design and low cost relative to electric and hydraulic models.
A pneumatic control valve actuator can control fluid flow by using supplied instrument air to open or close the valve. Valves that require air pressure to open are known as air-to-open or air-to-retract valves. Valves that require air pressure to close are known as air-to-close or air-to-extend valves. Pneumatic actuator types include the piston model, which can be double-acting and can operate at high pressure, and the diaphragm model, which is single-acting.
Valves with air-to-open actuators also are known as direct-acting valves. In the event that supplied instrument air is lost or disconnected, these valves fail in the closed position. Valves with air-to-close actuators also are known as reverse-acting valves. As expected, these valves will fail in the open position if instrument air is lost. When designing new processes, it is important for engineers to consider the effects of instrument air loss and to choose the correct control valve actuator to ensure process safety in the case of an emergency.
Although pneumatic actuators often are used for their dependability and simplicity, process conditions may dictate that another type of control valve actuator be used. Electrohydraulic actuators, for example, may be used in isolated areas in which instrument air cannot be supplied readily. Electrohydraulic actuators typically are more expensive than pneumatic actuators.
Electric actuators share many benefits with electrohydraulic actuators. Instead of using pumps and hydraulic fluid, however, electric actuators function by supplying current to an electric motor that opens or closes the control valve by means of a gear box. Although not as popular as the pneumatic control valve actuator, electric actuators have been gaining popularity in the chemical process industry since their development.
For processes that do not require rigorous control, a manual control valve actuator may be sufficient. A manual actuator must be adjusted by hand, usually by means of a flywheel. They often are used on bypass lines during maintenance activities and, although they are not as accurate as automatic control valves, they typically are far less expensive.