How-to do Calibration of Control Valve?

The following procedure should be done for control valve calibration.

Suppose it is spring-loaded and direct action. (Direct action means at top of diaphragm signal and with the increase of signal, the stem will move downwards or the stem will extend). Also, the valve is a signal to close. Check its Bench Set. Suppose it is 6 to 21 PSI.

Control Valve Calibration

  1. Set pressure regulator of positioner at about 30 PSI.

  2. Apply 3 PSI (4 mA if positioner is smart or electronics)
    signal to Positioner.Gradullay increase signal. Note the
    movement of stem.

  3. Apply 6 (8 mA),9 PSI (12 mA), 12 PSI (16 mA) and 15 PSI
    (20 mA) signal. Valve stem should move accordingly and
    indicator must show 25%,50%,75% and 100%.

  4. If error, remove it from zero and span of positioner.

  5. If facing any error problem, give signal 50% and then
    remove error. Rest will be ok automatically.

  6. Suppose valve is normally closed and at any vent service
    and signal to close as I mentioned earlier, then make shut
    close of valve at 14.5, or 14.7 PSI. Valve may start
    closing at 2.75PSI.

  7. If valve is at control operation, like level, pressure
    etc, then calibrate it accurately.No need to calibrate it
    at 2.75 and 14.5 PSI.

In addition to the Calibration, it is also crucial that the Control Valve (CV) response matches the demands of the controller.

Normally, the CV is calibrated and checked as already mentioned in this post, but please appreciate that in real operation, for most of the time the controller output will change in very minor amount to counteract the process upsets or set point changes and the CV need to respond rapidly and effectively to those changes.

Thus if you observe any CV, or note its trend from the Control System, you will see movements in fractions of mA/psi etc. Very few CVs will go to the full close position and valve going to the full open position is even rarer.

Also, it is necessary that the CV overcomes any stiction, has minimum hysteresis as well as little dead band. Therefore, it is important the valve be checked for minor changes.

How to check Control Valve:

  1. Move the valve to any position other than at its
    extremes, somewhere in the middle.

  2. Find a safe spot on the CV stem where you can feel the
    minor CV movement against a fixed part of the CV Body. Put
    your finger firmly on that spot of the CV Stem. I
    repeat “SAFE SPOT,” otherwise if the CV operates suddenly
    your finger may be injured.

  3. If the CV has a digital positioned with a feedback, the
    observation may be simpler and finger feeling may even be
    not necessary at all.

  4. Now closely observe the CV input signal, preferably the
    pneumatic signal on a 6 inch dia pressure gauge of at least
    0.5 % accuracy. If digital pressure indicator of similar or
    better accuracy is used that would be better.

  5. While applying input signal move the CV a small amount,
    then apply reverse signal very gently and slowly, and feel
    the CV stem movement with your finger, the stem should
    move. The numerical value of the input signal against the
    minor can be noted from the factory test data of the valve.
    In case that is not available it would be known by
    experience.

  6. Repeat this exercise at two or different CV openings and
    always check in forward and reverse directions, till you
    are confident of the CV performance.

  7. In case valve sticks, shows excessive dead band etc,
    then loosening of the stem packing or even CV opening for
    inspection to check stem/plug misalignment, or hardened
    stem packing may be the culprit.

Author
Sajid Ali Khan