Control valve actuators precisely position valves for adjustable control in the chemical process industry. Pneumatic actuators are preferred due to simplicity and low cost, but electro-hydraulic and electric actuators are also used. Valves can be air-to-open or air-to-close, and engineers must consider instrument air leakage for process safety. Manual actuators are cheaper but less accurate.
A control valve actuator is a device used on a valve to precisely position the plug, ball or vane in the fixed seat of the valve as required by the control system. Control valve actuators are required for adjustable control and are widely used in the chemical process industry. The most common type is the pneumatic actuator, but electric, electro-hydraulic and manual actuators are also often used. Pneumatic control valve actuators are generally preferred due to their simplicity of design and low cost compared to electric and hydraulic designs.
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. Types of pneumatic actuators 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 are also known as direct acting valves. In the event that supplied instrument air is lost or disconnected, these valves will fail in the closed position. Valves with air-to-close actuators are also 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 leakage and choose the correct control valve actuator to ensure process safety in the event of an emergency.
While pneumatic actuators are often used for their reliability and simplicity, process conditions may dictate the use of another type of control valve actuator. Electro-hydraulic actuators, for example, can be used in isolated areas where instrument air cannot be readily supplied. Electro-hydraulic actuators are typically more expensive than pneumatic actuators.
Electric actuators share many advantages with electro-hydraulic actuators. Instead of using pumps and hydraulic fluid, however, electric actuators work by supplying current to an electric motor that opens or closes the control valve via a gearbox. While not as popular as the pneumatic control valve actuator, electric actuators have gained 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 via a flywheel. They are often used on bypass lines during maintenance and while not as accurate as automatic control valves, they are typically much less expensive.
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