Actuators are attached to the control mechanism of a valve to replace a manual lever or handle, and they control the position of a valve (open, closed or specific position) remotely and automatically. In industrial applications, numerous types of actuators are available, and they can be connected to a variety of valve types. This one-page reference outlines the functions of the main categories of valve actuators.
Actuator operations
Actuators control valves through three standard valve operations: quarter-turn, multi-turn and linear operation. The quarter-turn operation involves valves that rotate 90 degrees from the closed position to the open position (for example, ball valves and butterfly valves). The actuator control on quarter-turn valves can be either on/off or modulated. Actuators for these valve operations are usually easy to install and maintain.
On the other hand, multi-turn valve operation requires the actuator to turn the valve mechanism several rotations to move the valve from the open to closed. The mechanism can be a rising non-rotating stem or a non-rising rotating stem. Examples of valve types with this operation include globe valves, gate valves and needle valves.
Some valves are opened and closed through the linear operation of the valve mechanism. Linear operation involves motion in a straight line. Linear valve operation can be driven in a number of ways.
Types of valve actuators
Two main types of actuators, based on their motion, are linear and rotary. However, actuators are generally defined by the source of power that drives the actuator.
Electric actuator. Electric actuators use electrical energy — usually 24-V, 110-V, 230-V, 400-V, single- or three-phase, to drive an electric motor whose rotor is connected to the shaft/stem mechanism of the valve (Figure 1, left). The electric motor can be powered by alternating current or direct current. They are an energy-efficient, clean and quiet method of valve control. Electric motors may be connected to the valve mechanism through gears to increase torque or regulate speed.
Electric actuators are capable of relatively high speeds if needed, but they tend to be slow-reacting if standard-specification actuators are used. Electric actuators often have a declutching mechanism to allow rotation of the drive during a power failure or installation. Emergency power can be provided through a battery to ensure a fail-safe operation.
Linear actuators are a specific category of electric actuators that move a stem attached to a load or a valve in a linear direction. They are used in globe and gate valves.
FIGURE 1. At right, an example of a pneumatically powered control valve is shown for a water pump line. At left is an example of an electrically powered linear actuator and valve (photos from Shutterstock)
Pneumatic actuator. Pneumatic actuators are highly reliable and are popular in industrial applications (Figure 1, right). They convert compressed-air energy into mechanical motion and can be used in locations without electricity. Pneumatic actuators are of two types: single-acting or double-acting. Single-acting pneumatic actuators use a single compressed-air source to turn the valve with a spring to return the valve to the normal position. Double-acting pneumatic actuators have two compressed-air sources that turn the valve and return it to its original position.
Pneumatically controlled valves are easy to install and maintain, and require a supply of clean, instrument-quality air at 60–80 psi. Pneumatic actuators have fast operating speed, often using a cylinder with a mechanism that converts the linear motion from the compressed air into rotational motion. The most common mechanism is the rack and pinion, but it can also be a diaphragm, piston or scotch yoke. Most pneumatic actuators are used for quarter-turn valves. The mechanism can be spring-loaded to return to a normal shut-down position in emergencies. Solenoid valves are used to regulate airflow into the actuator. Electrical signals from a controller energize the solenoid valve position to either open or closed, allowing compressed air to flow through to the pneumatic actuator’s sides.
Hydraulic actuator. Hydraulic actuators convert hydraulic power to achieve mechanical work. They can be used for quarter-turn valves, such as ball valves, or multi-turn valves, such as globe valves. Hydraulic actuators consist of a cylinder and a mechanism for converting linear motion to rotational motion. Hydraulic actuators use high-pressure oil from a hydraulic pump to drive the valve. Hydraulic actuators can be single-acting, with a spring as a fail-safe, or double-acting. They are relatively small compared to pneumatic actuators, but with thicker parts due to the high-pressure operation. They are also more precise than the pneumatic actuators because oil is incompressible. Hydraulic actuators are commonly used in large valve sizes that require more turning force.
Editor’s note: The text for this column has been adapted from the following article: Welsford, G., Valve Actuator Selection Guide, Chem. Eng., June 2022, pp. 37–39. Additional references include: Herrera, R., Valve Selection Best Practices, Chem. Eng., July 2015, pp. 34–42.