A pneumatic actuator's operating type describes how it responds to pneumatic signals within a control system to open, close or modulate a valve. Different types suit different operating conditions and safety requirements. Selecting the correct type is the first step toward system safety and stable operation.
1. Classification by action: single-acting vs double-acting
This is the most fundamental and important classification of pneumatic actuators.
Single-acting pneumatic actuator (spring-return)
A single-acting actuator contains a return spring. Compressed air enters from one side only, moving the piston in one direction. When the air supply is interrupted or the signal is lost, spring force returns the piston to its initial position.
- Characteristics
Automatically returns on loss of air to provide fail-safe protection.
Simple structure and relatively low cost.
The spring affects output force, which differs between directions; spring-side output is lower than air-side output.
Typical air supply pressure range: 0.4-0.8MPa.
- Applications
Applications requiring fail-safe protection, such as petrochemical and natural gas industries with demanding safety requirements. Depending on the need, choose air-to-open, which opens with air and closes on air loss, or air-to-close, which closes with air and opens on air loss.
Double-acting pneumatic actuator
A double-acting actuator has no return spring. Compressed air can enter both sides of the piston, and a directional valve switches the air supply to move the piston in either direction.
- Characteristics
Balanced output force and high thrust.
High control accuracy, suitable for applications requiring precise positioning.
Holds its current position after loss of air and does not return automatically.
Higher output torque than a single-acting actuator at the same air supply pressure.
- Applications
Applications requiring higher output force or where fail-safe protection is unnecessary. In modulating applications, use with a valve positioner enables high control accuracy.
2. Classification by control action: air-to-open vs air-to-close
This classification is based on safety: what position should the valve assume when the control signal or air supply is interrupted?
Air-to-open (FC, fail-closed): the valve opens when a pneumatic signal is present and closes when air or signal is lost. Suitable where the process medium must be shut off on failure.
Air-to-close (FO, fail-open): the valve closes when a pneumatic signal is present and opens when air or signal is lost. Suitable where the medium must be discharged or allowed to flow on failure, such as pressure-relief and venting systems.
Selection advice: the choice primarily depends on process safety requirements. During project design, process engineers issue valve sizing data sheets specifying each valve's required fail-safe position. Actuator specialists then select the appropriate air-to-open or air-to-close configuration.
3. Classification by motion: linear vs quarter-turn
Linear: produces linear displacement, with the piston directly driving the valve stem. Suitable for gate valves, globe valves and other valves requiring linear motion.
Quarter-turn: produces rotation of 90 degrees or more. Suitable for ball valves, butterfly valves, plug valves and similar designs.
4. Classification by control signal: on-off vs modulating
On-off (ON-OFF): switches between two positions, fully open and fully closed. Control is simple, usually with a solenoid valve switching the air circuit, and suits applications that do not need intermediate positions.
Modulating: precisely positions the actuator at any opening within its full travel for continuous flow adjustment. A valve positioner is normally required, receiving a standard 4-20mA or 0-10V control signal. Control accuracy can be within ±0.5%.
5. How to select an appropriate operating type
- First confirm safety needs: is fail-safe protection required, and should the valve open or close on air loss? → Select single-acting with air-to-open or air-to-close action.
- Next consider output requirements: if valve torque is high or air supply pressure is limited, prioritize double-acting operation.
- Then consider accuracy: is on-off control sufficient, or is modulation needed? Fit a suitable valve positioner for modulating service.
- Finally confirm the valve type: select quarter-turn for ball/butterfly valves and linear for gate/globe valves.
Understanding these operating classifications makes actuator selection easier in practical projects.
Zhejiang HES Automatic Instrument & Valve Co., Ltd.
Established in 1989 · Focused on pneumatic actuators for more than three decades
Products include AT and HG series and a full range of single-acting/double-acting models