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:
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:
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:
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, cvavra(at)cfemedia.com.