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Answer Posted / sajid ali khan
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 that:
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.
Regards,
Sajid Ali Khan
engineersajid52@gmail.com
| Is This Answer Correct ? | 7 Yes | 5 No |
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