At times, we have applications where a large control valve is required at high process pressure.  This combination requires a pneumatic actuator with substantial area and displacement volume in order to operate the valve. When a positioner is used, the ability to move this valve in a reasonable time period becomes a challenge because positioners are not known to have high air flow capacity.

To overcome this, you can use a pilot-operated booster in conjunction with the positioners because the booster has a high Cv or air flow capacity that can deliver large amounts of air to move the actuator faster. The positioner then sends a signal to the pilot port of the booster to stop it.This is where the problems occur, because a positioner using a traditional I/P and spool valve is unable to stop the booster (and actuator) at the set point, with the needed accuracy.  This can cause oscillation at the desired set point.

2012-08-20

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