Pneumatic actuators come in many design types, but all require a basic understanding of how their torque operates a valve. Regardless of size, accuracy is critical. An incorrectly sized actuator can cause many problems. Sometimes the actuator has enough torque to open the valve but not enough to close it. Sometimes the valve can get stuck in the middle of the stroke. Other times, the actuator may deliver so much torque that it damages the stem of the valve it is operating.
When sizing an actuator, first consider the torque requirements of the valve. Users need to consider:
- Breaking torque
- Operating torque
- Closing torque
- Maximum shaft torque.
Different valve types have different torque characteristics. For example, metal seated butterfly valves require a lot of torque to open or close, and very little torque to move between positions. Metal seated ball valves require a lot of torque to open or close and also have high torque requirements when traveling.
Some basic valve definitions that users need to know are:
- Break-off torque, also known as break-in torque. This is the amount of torque required for the valve to move from the closed position.
- Running torque, also known as mid-stroke torque. This is the amount of torque required to maintain the valve moving between the open and closed positions. Seating torque, also known as closing torque. This is the amount of torque required to place the valve in the closed position.
- Maximum allowable stem torque (MAST), the torque required even if the stem is permanently deformed or broken.
- The safety factor is a given multiple of the valve operating torque to ensure proper operation of the actuator. Many customers request a 25% safety factor, so if the valve requires 100 inch-pounds of torque to open, the actuator breakout torque will be 125 inch-pounds.
- Modulation service, also known as throttling service. A valve used as a control valve can have its position continuously increased, decreased or stopped during its mid-stroke to control the output of the valve. Regulating valves are rarely fully open or fully closed. Regulating valves generally require more torque than simple on-off valves.
- On-off service applies to valves used for closing. It can be used fully closed or fully open and will not stop mid-stroke.
Properly Sizing Actuators by Knowing Torque
Before sizing an actuator, consult the valve manufacturer for the torque requirements of their valve for your application.
Most manufacturers publish the minimum torque required to open a water system valve at a given operating pressure. In most cases, the closing torque is the same as the opening torque. Others will only publish the torque required at maximum operating conditions. Valve manufacturers rarely publish the operating torque of their valves. Regardless, even if torque is posted based on the actuator type you have chosen, you may not have enough information to properly size your actuator. Also, consider the media flowing through the valve very carefully. Certain services increase the torque required to operate the valve. Also, consider if your customer is asking you to add a safety factor to valve torque before sizing the actuator.
Actuator design affects size. Rack and pinion actuators produce a constant torque output throughout their travel. Fork actuators produce less torque at mid-stroke. Check the manufacturer's published torque output and make sure you understand the torque characteristics of the actuator.
The torque of a spring return actuator decreases with further reduction in operation as it overcomes the action of the internal spring which returns the actuator to its initial position once the air is removed. The user needs to know the real air supply available on the customer valve in order to properly size the actuator. Just because their compressor is set at a certain pressure doesn't mean they have the same pressure at the valve
Some basic executor definitions that users need to know are:
- Single acting: Also known as fail open or fail closed. It is an actuator with an internal spring that returns the valve to its original position when the air supply is lost.
- Double acting: Air is required to open or close the actuator. The actuator will fail due to loss of air supply. Also known as direct action. Air Start The amount of torque produced by supplying air to the actuator to initiate movement from its normal position.
- The end of the spring is the amount of torque provided by the inner spring in its normal position with all air pressure removed. In this position, the spring is considered to be in a relaxed position (with preload).
- Minimum air volume: This is the minimum torque valve of the actuator produced by the air source. Depending on the actuator type, this value may be the same as the air end. Other times it may be a mid-stroke position. Please consult the actuator manufacturer.
- Air end is the amount of torque that the air creates to hold the actuator at the end of the stroke position.
- Spring start is the amount of torque developed by the internal spring at the end of the actuator's travel position. This is the force the actuator will provide when the air is removed and the actuator reverses direction from its end-of-stroke position.
The above introduces tips about adjusting the size of pneumatic actuators. If you want to buy an actuator, please contact us.
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