Actuator Terminology and Definitions

Double Acting - A pneumatic actuator that uses air to both open and close the valve. Also referred to as an "Air-to-Air" operation.

Duty Cycle - The capability of an electric actuator motor to operate or "cycle" continuously - a 100% duty cycle. Ratings less than 100% mean that the actuator motor is capable of intermittent operation, as with most actuation applications. For example, a 25% Duty Cycle means that for each 1-minute operation, 3 minutes of non-operation is required. Duty Cycles greater than 25% are generally needed only when additional workloads are encountered, such as valve modulation or larger butterfly valves' actuation.

Electric Actuator - An electromechanical device used to actuate (open, close, and modulate) a valve. An electric motor and gear train is used to actuate the valve. Numerous application considerations must be made in using an electric actuator. These include duty cycle, cycle time, supply voltage, operating temperature, and enclosure rating (NEMA) for using electric devices under various environmental conditions.

Fail-Closed - Spring returns in a pneumatic actuator which closes the valve upon loss of air pressure. Also referred to as "Air-to-Spring Close" or Air-to-Close" operation.

Fail-Open - Spring returns in a pneumatic actuator which opens the valve upon loss of air pressure. Also referred to as an "Air-to-Spring Open" or "Air-to-Open" operation.

Limit Switches - Electrical switches which may be applied to either electric or pneumatic actuators to supply a signal that the valve cycle has been completed. In electric actuators, the primary limit switches are used to control the open and closed positions of the valve. Auxiliary limit switches may perform other functions, such as starting or stopping a pump. Limit switches are often mechanical and activated by physical contact with a lever or plunger. Proximity switches are electrically tripped without physical contact and may be used with certain computer or microprocessor controls.

NEMA Rating - National Electric Manufacturers Association standard for rating electrical component enclosures. The most commonly used NEMA ratings for electric actuators and electrical accessory enclosures are NEMA 4 and NEMA 4X.

Click here to see NEMA Ratings -Enclosure Classifications and Types

Pneumatic Actuator - An air-operated mechanical device used to actuate (open, close, or modulate) a valve. The actuator converts air pressure into mechanical force, commonly using a piston or diaphragm. The mechanical force output can be either rotational, as in the actuation of a ball valve, or linear, as commonly used in the actuation of a diaphragm valve. Pneumatic actuators are not significantly limited by duty cycles and cycle times (virtually instantaneous) found with electric actuators. However, they may require additional accessories to control further the speed of operation and a voltage supply for certain electric signal devices, which may be desired.

Solenoid Valve - An electro-magnetically operated valve that enables electrical control of air supply to a pneumatic actuator. Solenoids are wired to a manual switch or any electrical controller. Double-acting actuators require a 4-way solenoid, and Spring Return actuators require a 3-way solenoid for proper air control. As electrical devices, solenoid valves are available in various NEMA ratings.

Spring Return - An actuator using a spring to return the valve to either an open or closed position upon loss of power. Spring returns are most commonly used with pneumatic actuators but can be used on electric actuators where the valve must open or close by design or by system failure (see also Fail-Closed and Fail-Open terms).

General System Terms

Ambient Temperature - The temperature of the air surrounding the actuator. Generally stated as a maximum and minimum for proper actuator operation. Note that the ambient temperature range for an actuator may exceed that of a plastic valve and should not be considered the same as the service temperature.

Maximum Line Pressure - The media pressure against which the valve will have to close.

Maximum Service Temperature - Maximum temperature of the media. Service temperature should be considered when selecting the appropriate thermoplastic valve material for the application.

Media - The material flowing through a valve.

Modulating - Throttling or regulating the flow through a valve by varying the position between open and closed.

On-Off Service - Basic full open or full closed operation to start or stop the flow.

Operating Torque - Force required to operate a valve, expressed in inch-pounds or foot-pounds.

Supply Pressure - Facility air pressure supply required to operate a pneumatic actuator.