Pneumatically operated control valve actuators are the most popular in use, but electric, hydraulic and manual
Actuators are also widely used. The pneumatic spring and diaphragm actuator is most commonly specified due to its reliability and simplicity of design. Pneumatically operated piston actuators provide high stem output power
for demanding service conditions. The adaptations of the pneumatic and spring piston actuators are available for direct installation on rotary shaft control valves. Electric and electro-hydraulic actuators are more complex and more expensive than pneumatic actuators. They offer advantages where an air supply source is not available, where low ambient temperatures could freeze condensed water in pneumatic supply lines, or where unusually large stem forces are needed. Below is a summary in which the design and characteristics of popular actuator styles are analyzed.
Pneumatically operated diaphragm actuators use air supply from controller, positioner, or other source. Various styles include: direct acting (increasing air pressure pushes down diaphragm and extends actuator stem,
reverse-acting (increasing air pressure pushes up diaphragm and retracts actuator stem, see figure
- The net output thrust is the difference between the force of the diaphragm and the force of the opposite spring.
- Molded diaphragms provide linear performance and greater travel.
- The required output thrust and the available supply air pressure dictate the size.
- Diaphragm actuators are simple, reliable and economical.
- Piston actuators are pneumatically operated using 150 psig high pressure plant air, often eliminating the need for a supply pressure regulator.
- Piston actuators provide maximum thrust output and fast stroke speeds.
- Piston actuators are double acting to give maximum force in both directions, or spring return to provide a failed opening or closing operation
- Several accessories can be incorporated to place a double action piston in case of supply pressure failure. These include pneumatic trip valves and locking systems
- Hydraulic shock snubbers, flywheels and units without yokes are also available, which can be used to operate butterfly valves, grilles and similar industrial equipment.
- Other versions for service on rotary shaft control valves include a sliding seal on the lower end of the cylinder.
This allows the actuator stem to move laterally, as well as up and down without cylinder pressure leakage.
This feature allows the direct connection of the actuator stem to the actuator lever mounted on the rotary valve shaft, thus eliminating a lost articulation or source of motion.
- Electro-hydraulic actuators only require electrical power for 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 the position of the valve plug is needed.
- The units are normally reversible by making minor adjustments and can be independent, including the motor, pump and hydraulically actuated double-acting piston inside a weather-resistant or explosion-proof housing.
- Manual actuators are useful when automatic control is not required, but where ease of operation and good manual control are still necessary
- They are often used to activate the bypass valve in a three-valve bypass circuit around the control valves for manual control of the process during maintenance or shutdown of the automatic system.
Manual actuators are available in various sizes for globe valves and rotary shaft valves.
- Marking indicator devices are available for some models to allow accurate repositioning of the valve plug or disc.
- Manual actuators are much less expensive than automatic actuators.
Rack and Pinion Actuators
Rack and pinion designs provide a compact and economical solution for rotary shaft valves (figure 3-38). 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.
- To date, electric actuators have been much more expensive than tires for the same performance levels. This is an area of rapid technological changes, and future designs can cause a shift toward greater use of electric actuators.