When determining the type of valve required for a specific application, it’s essential to understand which type of valve actuator is needed for your project. In an automated valve assembly, the actuator is the component that causes the valve to open or close. Valve actuators can be triggered by electricity, hydraulic pressure, or pneumatic pressure.

What is a Single Acting Actuator?

A single acting pneumatic actuator, often called a spring return actuator, uses air to open the valve and a mechanical spring (or springs) to close the valve. The actuator can also be configured inversely so that air closes the valve and the spring is used to open the valve; this is called a normally open configuration.

Advantages of Single Acting Actuators

– Reliable safety design: If the power supply or air pressure is lost, the valve will quickly return to the normal position.

– Use in critical applications: Ideal for situations where severe consequences can occur if the valve remains open (or closed) in the event of a power or air pressure loss.

Disadvantages of Single Acting Actuators

– Additional cost: The inclusion of mechanical springs in the design adds extra cost to the valve assembly.

– Reduced durability: Springs are prone to fatigue and will generally be the first component to fail.

– Larger size: The spring modules give the actuator a larger shape that may not be compact enough for certain applications.

What is a Double Acting Actuator?

Unlike single acting actuators, double acting actuators use air pressure to both open and close the valve. This type is the most popular among pneumatic valve actuators. Double acting actuators are chosen for their lower cost and longer lifespan, as the absence of mechanical springs eliminates the least durable component of the actuator.

Advantages of Double Acting Actuators

– Extended lifespan: A high-quality double acting actuator can perform millions of cycles before failing.

– Compact and lightweight: They are more compact and lighter than single acting actuators.

– Lower cost: Generally cheaper than single acting actuators.

Disadvantages of Double Acting Actuators

– No fail-safe position: In case of air supply loss, the double acting actuator will remain in its current position until the supply is restored.

When to Use a Single Acting Actuator?

Single acting actuators are common in low to moderate cycle applications where the failure of the valve in the wrong position could present a serious safety hazard and/or cause damage to surrounding equipment. If power or air pressure is lost, single acting actuators ensure the valve will return to the normal position.

When to Use a Double Acting Actuator?

Double acting actuators are ideal for high-cycle applications and/or applications where the failure of the valve in the open or closed position upon power loss is not critical. They are the most popular choice for their lower cost and longer lifespan.

Double Acting Actuators with Fail-Safe

Gemini Valve now offers a double acting pneumatic actuator with fail-safe. This actuator combines the durability of a double acting actuator with the safety normally provided by spring return models. The actuator operates in double acting configuration until power or air pressure is lost, at which point the springs automatically engage to return the valve to the normal position.

Rack and Pinion Design

To achieve the required rotation, a pneumatic actuator often uses a rack and pinion design. This design includes two pistons in the main chamber that operate linearly and simultaneously in opposite directions. The rack teeth of the pistons remain constantly engaged with the teeth of the pinion or drive shaft.

Operation of Rack and Pinion Design

– Application of force: A force applied to move the pistons toward or away from each other.

– Rotation of the drive shaft: The engaged teeth of the rack and pinion design force the drive shaft to rotate.

– Interface with the valve stem: The drive shaft interfaces with the valve stem to translate the rotary motion to the ball itself, thus opening or closing the valve.

Pneumatic Actuation on Quarter-Turn Devices

The implementation of pneumatic actuation often concerns quarter-turn devices such as a butterfly valve or ball valve. A typical design requires a ninety-degree movement to operate.

Double Acting vs. Spring Return

– Double Acting: Uses air pressure to both open and close the valve.

– Spring Return: Uses air pressure to open the valve and a spring to automatically close it in case of air pressure loss.

Fail Position

A spring return configuration allows for a fail position to be designated so that the butterfly or ball valve can close or open in the event of air pressure loss. However, the double acting design will fail in place. If the valve is mid-stroke when air supply is lost, it will remain in that position until air is restored or until the valve is manually turned to the open or closed position.

Conclusion

Choosing between a single acting and a double acting actuator depends on the specific needs of the application. Single acting actuators are preferred for their safety and reliability in critical situations, while double acting actuators are chosen for their lower cost, durability, and compactness. By understanding the advantages and disadvantages of each type, you can make an informed choice to ensure optimal performance of your valve system.