Differences Between Single-Acting and Double-Acting Pneumatic Actuators

Pneumatic actuators are widely used in industrial facilities to automatically open and close valves, control flow, and enable remote management of processes. Particularly used for the automation of quarter-turn valves such as ball valves and butterfly valves, pneumatic actuators convert compressed air into mechanical motion, providing fast and reliable operation.

One of the most important considerations when selecting a pneumatic actuator is whether to choose a single-acting or double-acting model.

Although both actuator types perform similar functions, they differ in terms of their operating principles, air supply requirements, and behavior when the air supply is interrupted. When making the right selection, not only the actuator's operating force but also the system's safety requirements and operating conditions should be taken into consideration.

What Is a Pneumatic Actuator?

A pneumatic actuator is a drive element that converts compressed air into mechanical motion. In valve automation, an actuator enables valves to be opened and closed automatically instead of manually.

In quarter-turn valves, the linear movement of the pistons inside the pneumatic actuator is converted into rotary motion through a pinion and rack mechanism. This allows valves such as ball valves and butterfly valves to be rotated through a specific angle for opening, closing, or flow direction control.

Pneumatic actuators can be used particularly in:

• Industrial automation systems

• Water and wastewater treatment plants

• Chemical and petrochemical facilities

• HVAC applications

• Compressed air systems

• Process control lines

• Ball and butterfly valve automation

What Is a Single-Acting Pneumatic Actuator?

A single-acting pneumatic actuator is a type of actuator in which movement in one direction is achieved using compressed air, while movement in the opposite direction is achieved using spring force.

For example, when compressed air is supplied, the valve may open, and when the air supply is interrupted, the springs inside the actuator can return the valve to the closed position.

For this reason, single-acting actuators are also commonly referred to as spring-return actuators.

The operating principle of İsimatik's UNOX single-acting pneumatic actuators is based on this design: air pressure moves the piston in one direction, while the spring moves the piston in the opposite direction when the air supply is interrupted.

How Does a Single-Acting Actuator Work?

Its operating principle can be summarized simply as follows:

1. Compressed air is supplied to the actuator.

2. Air pressure moves the piston.

3. The movement of the piston is transferred to the valve stem.

4. The valve opens or closes.

5. When the air supply is interrupted, the spring mechanism is activated.

6. The valve returns to a predetermined safe position.

The most important advantage here is that when the air supply is interrupted, the valve can return to a predetermined position instead of remaining uncontrolled in its current position.

This feature can provide a significant advantage in applications where process safety is important.

What Is a Double-Acting Pneumatic Actuator?

A double-acting pneumatic actuator performs both the opening and closing movements using compressed air.

Instead of a spring-return mechanism, the actuator has two separate air connections. When compressed air is supplied to one connection, the valve moves in one direction; when air is supplied to the other connection, it moves in the opposite direction.

UNOX double-acting pneumatic actuators are particularly used for controlling valves and dampers that operate with a 90° rotary movement. These products may also offer on-off and proportional control options.

How Does a Double-Acting Actuator Work?

Its operating principle can be summarized as follows:

1. Compressed air is supplied to one side of the actuator.

2. The pistons move.

3. The pinion mechanism rotates and opens or closes the valve.

4. The other air connection is used for movement in the opposite direction.

5. Thus, both opening and closing movements are performed using air pressure.

Since double-acting systems do not have a spring mechanism, movement in both directions depends on pneumatic power.

Differences Between Single-Acting and Double-Acting Pneumatic Actuators

The fundamental difference between the two actuator types is how the return movement is performed.

What Happens When the Air Supply Is Interrupted?

This is one of the most critical differences between single-acting and double-acting actuators.

In a Single-Acting Actuator

When the air supply is interrupted, the spring mechanism is activated and returns the actuator to its designed safe position.

This position can be designed as:

• Fail Close (FC): The valve closes when the air supply is interrupted.

• Fail Open (FO): The valve opens when the air supply is interrupted.

For example, if the flow of a hazardous fluid through the line needs to be stopped in the event of an air or power failure, a fail-close configuration may be preferred.

In a Double-Acting Actuator

The movements of a double-acting actuator depend on air pressure. When the air supply is interrupted, the system may behave differently depending on the current position, valve type, and control equipment used.

Therefore, in applications where the safety function is particularly important, it is necessary to consider not only whether a double-acting actuator is sufficient, but also how the system will behave when the air supply is interrupted.

Advantages of Single-Acting Actuators

The key advantages of single-acting pneumatic actuators include:

• Ability to return to a safe position in the event of air or power loss

• Contribution to improved process safety

• Suitability for fail-safe applications thanks to the spring-return design

• Effective use in simple on-off valve automation

• Suitability for processes with high safety requirements

However, because of the spring mechanism, actuator sizing should take into account not only the valve torque but also the spring torque and operating conditions.

Advantages of Double-Acting Actuators

Double-acting pneumatic actuators can provide advantages particularly in continuous and rapid opening and closing operations.

Their main advantages include:

• Pneumatic force can be used in both directions

• Suitable for rapid opening and closing operations

• Can be used in various valve and damper applications

• Availability of compact and symmetrical actuator designs

• Can be integrated into on-off and proportional control systems with suitable equipment

UNOX double-acting actuators feature connection standards suitable for the installation of solenoid valves, limit switches, and other accessories.

Single-Acting or Double-Acting?

The answer depends on the operating conditions of the system.

A single-acting pneumatic actuator may be preferred if:

• The valve needs to return to a specific position when the air or power supply is interrupted,

• Process safety is a priority,

• A fail-open or fail-close function is required,

• The valve automation system includes a safe shutdown scenario.

A double-acting pneumatic actuator may be preferred if:

• Both opening and closing movements are intended to be performed entirely by compressed air,

• Fast and repeatable opening and closing movements are required,

• A suitable and continuous compressed air supply is available,

• Standard automatic valve control is required.

İsimatik's product range includes both UNOX single-acting and UNOX double-acting pneumatic actuators. These actuators can also be used with ball valves and similar quarter-turn valve applications.

What Should Be Considered When Selecting a Pneumatic Actuator?

Selecting the right actuator is not limited to deciding between single-acting and double-acting. The valve and actuator must be compatible with each other.

The following criteria should be evaluated during the selection process:

1. Valve Torque

The torque generated by the actuator should be sufficiently higher than the torque required by the valve, with an adequate safety margin.

2. Operating Pressure

The operating pressure of the compressed air in the system must be compatible with the technical specifications of the actuator.

3. Valve Type

Different equipment, such as ball valves, butterfly valves, or dampers, may require different movement and torque values.

4. Fail-Safe Requirement

It should be determined whether the valve needs to remain open or closed when the air or power supply is interrupted.

5. Operating Frequency

How many times the valve opens and closes during the day should be taken into consideration when selecting the actuator.

6. Environmental Conditions

Factors such as temperature, humidity, corrosion, dust, and chemical environments can affect actuator selection.

7. Control System

It should be determined in advance whether on-off or proportional control will be used. In control valve applications, the requirements of the positioner and other control equipment should also be evaluated along with the actuator.

Conclusion: Which Actuator Should You Choose?

The fundamental difference between single-a