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Answer: In my experience, inserting a booster between a positioner and valve actuator introduces dead band into the loop, resulting in a limit cycle when controlling pressure and liquid level. I do not recommend this practice.
My solution was to install a smart digital positioner on the valve, which provides faster stroking speed with no deadband. See the article, “Smoothing Out Compressor Control,” Chemical Engineering, Feb. 1999.
Process Control Consultant
Answer: The selection of the boosters to be used is driven by the speed requirement for the open and or close time of the valve. The primary problem in using a booster to feed and/or exhaust an actuator is that instability may be caused in the valve positioning.
In selecting the booster size needed to meet a speed requirement most valve manufacturers would have a computer program to determine the actuator input and output Cv’s. An estimate of the Cv required to achieve a certain stroke speed can be determined by using the volume of the actuator and the time for a full stroke to establish a flow rate, then with the pressure drop estimate, one can ball- park a Cv of the booster needed.
It is rare for the presence of a positioner in combination with the booster to cause an instability that cannot be readily fixed using the booster capacity adjustment, unless the positioner itself has excessive gain relative to the size of the actuator.
Some rules of thumb in selecting a booster follow:
Herbert L. Miller, PE, Consultant
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