Answer / aftab ahmed mazari

Following procedure should be done for control valve
calibration.
Suppose it is spring loaded and direct action.(Direct
action means at top of diapgargm signal and with the
increase of signal, stem will move downwards or the stem
will extend).Also valve is signal to close. Check its Bench
Set. Suppose it is 6 to 21 PSI.
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.
aftabmazari@yahoo.com

Answer / r.murugan

IF ITS PNEUMATIC CONTROL VALVE FIRST WE HAVE TO CHECK
INSTRUMENT AIR PRESSURE THEN WE HAVE TO REGULATE THE
INSTRUMENT AIR PRESSURE REGULATOR TO I/P COVERTER AND
PNEUMATIC POSITIONER
BEFORE STROKING THE VALVE WE HAVE TO CHECK VALVE AT NEUTRAL
POSITION IF NEUTRAL POSITION THEN ONLY CAN MOVE THE THE
VALVE
IF AIR TO OPEN VALVE WHEN COMMAND 4ma VALVE POSITION SHOULD
BE CLOSE (DIRECT OPERATION) LOCAL WE HAVE TO MECHANICAL
SCALE CLOSE INDICATION
THEN INCREASE THE CURRENT 4.5ma TO 5ma to check the start
of opening the control valve
then gradually can increase and decrese the current and
corrospondingly check the stroke momemnt based on command
we have to check the feedback

Answer / 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

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