Calibration of the valve positioner can be performed at the same time as the I/P in a loop calibration. Simply tee in the pressure module at the I/P outlet in the I/P calibration. Record the valve position at each test point.
If calibrating the valve positioner separately, connect an input test pressure regulator or hand pump, and monitor the input pressure applied with a pressure standard. If there is no supply air, connect the required supply air to the positioner. Apply the pressure for the desired test points and record valve position.
For example, assume our valve positioner is 3-15 psig input = 0-100% valve position. In this case, apply 3.0, 6.0, 9.0, 12.0, and 15.0 psig. The expected valve positions should be 0, 25, 50, 75, and 100%, respectively. The valve position indicator on the stem usually marks off in 5% or 10% increments.
Therefore, a best estimate of the valve position may be all you can obtain. In other cases, a valve position detector provides a remote indication to a DCS. In such cases, ensure both indicators are working properly.
Many organizations do not require calibration of valve positioners for these reasons. There’s much documentation that control valve positioner performance is responsible for significant loss in system efficiency and, therefore, increased costs.
To provide guidance on methods for testing positioners and control valve performance, ISA has developed a standard, ANSI/ISA-75.25.01-2000, Test Procedure for Control Valve Response Measurement for Step Inputs.
As to control valve calibration, the process is similar to positioner calibration in that one applies a pressure signal to the actuator and then tallies the resulting valve position. This step can take place with the positioner calibration, if applicable, and it can happen in conjunction with I/P calibration.
Remember to ensure the system is in a safe condition if performing the calibration in the field. In addition, know the correct action, direct or reverse, and fail position before starting.
Also See Functional Testing of Valve Positioners