1. Why Is Proper Pneumatic Actuator Sizing Important?

A pneumatic actuator uses compressed air to rotate a ball valve, typically through 90 degrees, between open and closed positions. To operate reliably, the actuator must generate enough torque throughout the entire stroke.

Incorrect sizing can cause several problems.

• Insufficient torque: The actuator may struggle to start the valve, especially when opening against high differential pressure.

• Incomplete closing: The valve may not reach its fully closed position, potentially affecting shutoff performance.

• Unnecessary cost: An excessively large actuator increases purchase cost and may require more compressed air.

• Mechanical damage: Excessive output torque can damage the valve stem, coupling, or mounting components if their allowable limits are exceeded.

• Unreliable operation: Fluctuating air pressure, changing process conditions, or increased friction can cause an actuator that was marginally sized to malfunction.

Proper sizing helps improve operational reliability, reduce avoidable maintenance, and extend the service life of the complete valve assembly.

2. Understand Ball Valve Torque Requirements

Before selecting an actuator, obtain the valve manufacturer's torque data for the specific valve model and operating conditions.

Ball valve torque is influenced by valve size, pressure differential, seat material, packing friction, temperature, process media, and operating frequency. It should not be estimated from nominal pipe size alone.

Four important torque considerations

For many ball valves, the torque required to start moving the ball is higher than the torque required to keep it moving. However, the highest torque point depends on the valve design and operating conditions.

Important: Use the maximum applicable torque requirement across the opening and closing stroke. Do not select an actuator using running torque alone.

Where available, request the valve manufacturer's torque data at the maximum expected differential pressure and relevant operating temperature.

3. Calculate the Required Actuator Torque

The basic sizing formula is:

Required actuator output torque = Valve torque requirement × Safety factor

The valve torque requirement should represent the governing torque demand for the relevant operating condition. For a ball valve, breakaway torque is often a critical sizing value, but seating and other stroke requirements must also be checked.

How to choose a safety factor

A safety factor provides additional torque capacity for reasonable variations in operating conditions, manufacturing tolerances, and friction. The appropriate factor depends on the application and the project specification.

These values are preliminary engineering guidance, not universal requirements. The final factor should be confirmed for the specific valve design, service conditions, and applicable standards.

Example: Calculating the required torque

Suppose a ball valve manufacturer provides the following data:

• Maximum applicable breakaway torque: 80 N·m

• Running torque: 45 N·m

• Seating torque: 65 N·m

• Preliminary safety factor: 1.5

The calculated design requirements are:

The actuator must satisfy the required torque at all relevant points in the stroke. In this example, the breakaway requirement governs at 120 N·m.

However, 120 N·m is not automatically the actuator model's catalogue rating to select. You must identify an actuator that delivers the required output torque at the minimum available air pressure and verify its output throughout the stroke.

4. Check the Available Air Supply Pressure

Pneumatic actuator output torque depends on the actuator design and the pressure of the compressed air supplied to it.

A model rated for a particular torque at 6 bar may deliver less torque if the available pressure drops to 4.5 bar. Therefore, sizing should be based on the minimum air pressure available at the actuator during operation, not just the compressor's nominal pressure.

Before choosing an actuator, confirm:

• Normal operating air pressure

• Minimum available pressure during peak demand

• Maximum permitted actuator supply pressure

• Air quality and supply stability

• Pressure losses caused by tubing, fittings, solenoid valves, or other components

Next, consult the actuator manufacturer's torque table or performance curve at the relevant supply pressure.

For preliminary selection, the actuator's available output torque must meet or exceed the calculated requirement. The manufacturer's published performance data should be used for the final selection rather than assuming a universal torque-pressure relationship.

5. Choose Between Double-Acting and Spring-Return Actuators

The required operating function is another important factor in actuator sizing.

Double-acting pneumatic actuator

A double-acting actuator uses compressed air to drive the piston in both directions.

Typical characteristics:

• Air pressure drives both opening and closing movements.

• Suitable for general automated on/off valve applications.

• Available output torque must be checked in both directions.

• The required torque can often be met with a more compact actuator than a comparable spring-return configuration, depending on the design.

Double-acting actuators are a common choice when the application does not require a mechanically defined fail-open or fail-closed position upon loss of air.

Spring-return pneumatic actuator

A spring-return actuator uses compressed air for one direction and a mechanical spring for the return stroke.

Typical characteristics:

• Can provide a defined fail-open or fail-closed action when designed and installed for that function.

• Requires separate verification of the air-driven stroke and spring-driven stroke.

• Spring torque varies with stroke position.

• The available spring torque must be sufficient to overcome the valve's required torque when the air supply is lost.

For spring-return models, do not check only the actuator's air-driven torque. Verify the applicable minimum output torque of the spring during the return stroke against the valve's torque requirements.

The correct choice depends on process requirements, failure consequences, and the project's control philosophy.

6. How KINKO Can Support Your Pneumatic Ball Valve Selection

KINKO manufactures pneumatic actuators, pneumatic ball valves, electric actuators, electric ball valves, butterfly valves, and related valve automation products for industrial applications.

When selecting a pneumatic actuator for a ball valve, it is important to evaluate the valve and actuator as a complete assembly rather than as two independent products.

The selection process should consider:

• Valve type, nominal size, and pressure rating

• Required operating torque and service conditions

• Available compressed air pressure

• Double-acting or spring-return operation

• Mounting interface and drive connection

• Required accessories and environmental protection

KINKO can discuss your application requirements and help identify suitable valve and actuator options. Final model selection should be based on the verified valve torque data, actuator performance tables, and the confirmed operating conditions.

Conclusion

Sizing a pneumatic actuator for a ball valve requires more than matching nominal sizes. The correct selection depends on the valve's torque requirements, safety factor, minimum available air pressure, actuator operating principle, and mechanical compatibility.

By verifying these parameters before ordering, you can reduce the risk of actuator failure, improve valve automation reliability, and build a more dependable control system.

Need help selecting a pneumatic actuator for your ball valve?

Contact KINKO with your valve specifications and operating conditions. Our team can help review the requirements and discuss suitable pneumatic actuator and ball valve solutions for your application.