Different types of Control valve calibration:

Field Instrumentation Calibration
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Control valves are essential components of process industry thus it make really important to calibrate control valve periodically. There are importantly two ways to calibrate a control valve.

  • Stroke Calibration
  • I/P convertor calibration

Stroke Calibration:

Stroke calibration approach demonstrates that the actuator has sufficient force to open and close the valve. Calibration is required when the valve and actuator are first installed to decide the stroke positions 0 and 1 in the valve. Measurement of the stroke position of the valve actuator from 0 to 100% of the valve closure / opening.

Procedure:

  • Connect the control valve and its accessories according to the connection

  • Attach air pressure machine to the regulator

  • Regulator change to demand as per data sheet of POSITIONER output pressure.

  • Set Loop Calibrator to provide POSITIONER with the latest output signal (4-20mA).

  • Apply 4 mA to positioner using loop Calibrator

  • Test the stem motion if there is a mistake in the positioner to change the null calibration screw.

  • Apply 20 mA

  • Test the position of the head, if there is any defect, the span screw in the positioner must be changed.

  • Apply 0%,25%,50%,75%,100% of the signal & observe the corresponding stroke of the valve & stem position.

I/P converter calibration of valve:

I/P converter calibration is comparatively simple process, but it can be done in two ways: Simulate standard 4 to 20 mA as input check output from I / P. Or Simulate std 4 to 20 mA as input and measure the movement of the control valve stroke through the scale (0 to 100%).

Procedure:

  • Remove from the housing the final assembly module.

  • Place the range jumper in the high range Hi position

  • Link air supply via the air supply regulator to the output port.

  • Connect an accurate output indicator to the output signal port.

  • Make sure that an output meter port or threaded connector is mounted. For units shipped without output gages, a threaded plug is given.

  • Apply a 4.0 mA (Vm = 1.0 V) signal and adjust the zero screw to 3.0 psi output. The output increases in the direction of the clockwise direction with zero screw rotation.

  • To check and complete the calibration, repeat the last two moves.

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