Tips on sizing pneumatic actuators
Pneumatic actuators come in many design types, but all share the same need for basic understanding of how their torque operates a valve. Accuracy is critical regardless of who sizes it. Incorrectly sizing an actuator can lead to a multitude of problems. Sometimes an actuator will have enough torque to open a valve but not enough to close it. Sometimes a valve may get stuck in mid-stroke. Other times, an actuator may provide so much torque that it damages the stem of the valve which it is operating.
When sizing an actuator, first consider the valve’s torque requirements. Users need to consider:
- Break torque
- Running torque
- Closing torque
- Maximum shaft torque.
Different valve types have different torque signatures. For example, a metal seated butterfly valve requires a large amount of torque to break or close and very little torque while traveling between the two positions. A metal seated ball valve requires a large amount of torque to open or close but also has an elevated torque requirement while traveling.
Some basic valve definitions users need to know are:
- Break torque, also known as opening torque. This is the amount of torque required to start the movement of a valve from the closed position.
- Running torque, also known as mid-stroke torque. This is the amount of torque required to maintain movement of the valve between open and closed positions.Seating Torque, also known as closing torque. This is the amount of torque required to seat a valve into its closed position.
- Maximum allowable stem torque (MAST), which is the torque required to permanently deform or break the valve stem.
- Safety factor is a given multiple of the valve’s operating torques to ensure proper operation of the actuator. Many customers ask for a 25% safety factor so if a valve requires 100 in-lb torque to open, the actuator’s break torque would be sized at 125 in-lbs.
- Modulating service, also known as throttling service. A valve that is used as a control valve may have its position continuously increasing, decreasing, or stopping movement through its mid-stroke in order to control the valve’s output. Modulating valves are rarely fully open or fully closed. Modulating valves usually require more torque than simple on-off valves.
- On-off service is for a valve that is used for shut-off. It will either be used fully closed or fully open and will not stop during mid-stroke.
Size an actuator correctly by understanding torque
Before sizing an actuator, check with the valve manufacturer for their valve’s torque requirements in your service.
Most manufacturers will publish a minimum torque required at a given operating pressure to open the valve in water service. In most instances, closing torque is the same as opening torque. Others will only publish the torque required at maximum operating conditions. Rarely do valve manufacturers publish the running torque of their valve. In any case, you may not have enough information to properly size an actuator even with the published torques based on what type of actuator you chose. Be very careful to also consider what media is flowing through the valve. Some services will increase the torque required to operate a valve. Also, consider if your customer requires you to add a safety factor to the valve torque prior to sizing an actuator.
Actuator design will affect the sizing. Rack and pinion actuators produce a constant torque output throughout the stroke. Scotch yoke actuators produce reduced torque in its mid-stroke. Please look at the manufacturer’s published torque outputs and make sure you are aware of the torque signature of the actuator.
A spring return actuator has reduced torque the further it is operated because it is overcoming internal springs that are used to return the actuator to its initial position once air is removed. Users will need to know your customer’s real air supply available at the valve in order to properly size an actuator. Just because their compressor is set at a certain pressure does not mean they have the same pressure at the valve
Some basic actuator definitions users need to know are:
- Single acting: also knows as fail open or fail closed. It is an actuator with internal spring that will return the valve to the initial position on loss of air supply.
- Double acting: Air is required to open or close the actuator. The actuator will fail in place with the loss of air supply. Also known as Direct Acting.Start of AirThe amount of torque produced by the air supply to the actuator to start a movement from the normal position.
- End of spring is the amount of torque that the internal springs provide in the normal position with all air pressure removed. In this position, the spring is considered to be in its relaxed position (with a pre-load).
- Minimum air: It is the lowest torque valve of the actuator produced by the air supply. Depending on actuator type, this may be the same value as end of air. Other times it may be the mid-stroke position. Please refer to actuator’s manufacturer.
- End of air is the amount of torque produced by the air to hold the actuator at its end of travel position.
- Start of spring is the amount of torque produced by the internal spring at the actuator’s end of stroke position. This is the force the actuator will supply as the air is removed and the actuator reverses directions from its end of travel position.
Ronnie Moore is Cross Company‘s resident valve expert working in inside sales and support. A Knoxville native, Ronnie spent time in the Air Force as crew chief during the first Gulf War. This article originally appeared on Cross Company’s Instrumentation blog. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, email@example.com.