Actuators have a close relationship with the control valves. Like many mechanical components, they also have various types. Pneumatically operated control valve actuators are the most popular type in use. Besides, the electric, hydraulic, and manual actuators are also widely used. The spring-and-diaphragm pneumatic actuator is most commonly specified due to its dependability and simplicity of design. Pneumatically operated piston actuators provide high stem force output for demanding service conditions. Adaptations of both spring-and-diaphragm and pneumatic piston actuators are available for direct installation on rotary-shaft control valves.
Because different types of actuators have different designs and characteristics, it is very necessary to distinguish them from each other. Diaphragm actuators pneumatically operated diaphragm actuators use air supply from controller, positioner, or other source. This kind of actuators is simple, dependable, and economical.
Piston actuators are pneumatically operated using high–pressure plant air to 150 psig, often eliminating the need for supply pressure regulator. This kind of actuators furnishes maximum thrust output and fast stroking speeds. Various accessories can be incorporated to position a double-acting piston in the event of supply pressure failure. These include pneumatic trip valves and lock-up systems.
Compared to other types, electro-hydraulic actuators require only electrical power to the motor and an electrical input signal from the controller. Electro-hydraulic actuators are ideal for isolated locations where pneumatic supply pressure is not available but where precise control of valve plug position is needed. Units are normally reversible by making minor adjustments and might be self-contained, including motor, pump, and double-acting hydraulically operated piston within a weatherproof or explosion-proof casing.
The next one is the manual actuator. It is useful where automatic control is not required, but where ease of operation and good manual control is still necessary. They are often used to actuate the bypass valve in a three-valve bypass loop around control valves for manual control of the process during maintenance or shutdown of the automatic system. Manual actuators are available in various sizes for both globe-style valves and rotary-shaft valves. They are much less expensive than automatic actuators.
Rack and pinion designs provide a compact and economical solution for rotary shaft valves. Because of backlash, they are typically used for on–off applications or where process variability is not a concern. Traditional electric actuator designs use an electric motor and some form of gear reduction to move the valve. Through adaptation, these mechanisms have been used for continuous control with varying degrees of success.
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