A control valve positioner is used for accurate valve positioning, mitigate wear and tear, and provide precise control.
In this article, I will cover the calibration procedure for the D-shaped control valve positioner which is shown in the figure.
Required Equipment :
1. Control valve
2. Positioner
3. Air pressure source
Follow the below step for successful calibration :
1. Provide 20 psi air supply in the input side.
2. keep follower as per our valve's action (Reverse or Direct).
In Direct acting let assume follower's position is as shown in the figure. When pressure is applied to bellow, it deflates resulting in moving flapper near to nozzle means air to open scenario. (close valve - 3 psi, open valve - 15 psi).
In Reverse acting let assumed the position of the follower is at the opposite side. When pressure applied bellow deflates, resulting in moving flapper away from the nozzle means air to close. (close valve - 15 psi, open valve - 3 psi).
3. Let assume Direct acting case. First, apply 3 psi and check the gauge. To do zero adjustments we have to adjust the nozzle, at this point valve remain close (0%).
4. Apply 15 psi and check the gauge. To do span adjustment move flapper by moving follower. Make sure to move it properly otherwise tiny pressure can produce huge movement, at this point valve will fully open (100%).
5. Every time with a change in flapper assembly make sure to reset nozzle to zero also.
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