At chemical, pharmaceutical, water treatment and food production facilities, a small device can often be seen mounted above a valve: the pneumatic actuator. Combining the valve with an actuator and using compressed air as the power source enables fast, precise valve control.

How exactly does a pneumatic valve actuator work? What is the difference between single-acting and double-acting designs? This article explains both.

1. What is a pneumatic valve actuator?

Simply put, the actuator is the valve's power unit.

It receives a compressed-air pressure signal and converts the air's pressure energy into mechanical movement, driving the stem or closure element to open, close or modulate the valve.

Compared with manual valves, pneumatic valves offer fast movement, precise control and convenient remote operation, making them essential components of industrial automation.

2. Main components

A pneumatic valve actuator mainly consists of:

  1. Cylinder: the actuator housing and the chamber in which compressed air performs work.
  2. Piston: reciprocates under air pressure and is the core of energy conversion.
  3. Pinion shaft: converts the piston's linear movement into output shaft rotation.
  4. End caps: close both ends of the cylinder to form sealed air chambers.
  5. Seals: maintain airtightness and prevent compressed-air leakage.
  6. Screws and other fasteners: connect and secure the components.

These parts work together to complete the actuator's operating cycle.

3. Double-acting operating principle

A double-acting actuator has two air ports connected to the chambers at the two ends of the cylinder.

When compressed air enters one port, it drives the piston toward the opposite side. The piston rack rotates the pinion shaft, operating the valve.

When the supply switches to the other port, the piston moves back and the output shaft rotates in the opposite direction, reversing the valve movement.

In simple terms, it pushes to open and pulls to close; compressed air powers both directions.

Its advantages include:

  • Stable output torque and reliable movement in both directions.
  • Continued operation even if electrical power or the signal is lost.
  • Particular suitability for applications requiring high reliability.

4. Single-acting operating principle

A single-acting actuator adds a spring-return mechanism at one end of the cylinder. Air pressure and spring force work together to operate the valve. There are normally closed and normally open versions.

Normally closed (air-to-open)

Incoming compressed air pushes the piston, compresses the spring and opens the valve.

If air pressure disappears or is interrupted, spring force returns the piston and automatically closes the valve.

Characteristic: closes on loss of air, suitable for applications requiring fail-safe closure.

Normally open (air-to-close)

The principle is reversed: without pressure, the spring holds the valve open.

Incoming compressed air compresses the spring through the piston and closes the valve. When air is lost, the spring returns and the valve reopens.

Characteristic: opens on loss of air, suitable for applications requiring fail-safe opening.

5. Choosing single-acting or double-acting

With the principles understood, consider these points when selecting:

  1. Fail-safe requirements: if air loss must automatically return the valve to a defined open or closed position, a single-acting actuator is more suitable.
  2. Torque requirements: a double-acting actuator uses air pressure in both directions and provides more torque; a single-acting actuator must overcome spring force and has relatively lower output torque.
  3. Air supply stability: either type may be used with a stable supply and suitable site conditions; prioritize single-acting designs where interruptions are likely.
  4. Application: chemical and gas services with safety requirements often use single-acting actuators for air-loss protection; double-acting designs are economical and practical for frequent ordinary switching without special safety requirements.

6. Conclusion

Although mechanically simple, pneumatic valve actuators are indispensable in automatic control systems.

Their efficiency and reliability support stable industrial processes and provide important protection for site personnel.

Clarify the operating conditions, fail-safe requirements and air supply conditions, then compare the features of single-acting and double-acting designs to select the most suitable actuator.

Zhejiang HES Automatic Instrument & Valve Co., Ltd.

Established in 1989 · More than three decades dedicated to pneumatic actuators