How to Size a Valve Actuator Close-Off

Control valves have one function: to control the rate of water flow through the air handling units or fan coil. The actuator is the link to the valve, and each actuator has a close-off rating -- the maximum pressure allowed to smoothly operate the valve. This rating is used to size valves. Determining the appropriate close-off rating for your application is a straightforward process.

Tape measure
Calculator
Wheel velocity of application

Instructions

- 1
  Measure the outside diameter of the application's impeller in inches. As an example, the diameter is 50 inches.
- 2
  Multiply this diameter by the application's wheel velocity in rpm. The wheel velocity can be found with the application's manual in the specifications section. Continuing the example, multiplying an HVAC system's wheel velocity of 1,000 rpm by its impeller diameter of 50 inches equals 50,000.
- 3
  Divide the result from Step 2 by 1,840. In the example, 50,000 divided by 1,840 equals approximately 27.17.
- 4
  Square the result from Step 3 to determine the valve's shut-off head in inches. In the example, squaring 27.17 equals a shut-off head of approximately 738.21 inches.
- 5
  Multiply the pump shut-off head by 0.5. In the example, multiplying 738.21 by 0.5 equals approximately 369.11.
- 6
  Add the result from Step 5 to the pump shut-off head, to determine the appropriate valve actuator close-off rating in pounds per square inches. In the example, adding 369.11 to 738.21 equals a close-off rating of 1,107.32 pounds per square inch.

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