- CCST
Summary
CCST question
When should a valve positioner be recommended as an accessory to a control valve?
A. For control valves with actuator volumes greater than 50 cubic inches.B. For globe control valves that are 2 inches in size or greater.
C. For control valves that are experiencing a large degree of hysteresis.
D. For ball or butterfly control valves used to control steam or gases.
CCST Answer
The answer is C, “For control valves that are experiencing a large degree of hysteresis.”
Positioners are used to adjust the air applied to a control valve’s actuator. A positioner adjusts the applied air pressure to the actuator to move the valve stem to the commanded position using position feedback. In essence, a positioner is a controller with the programmable logic controller (PLC) or distributed control system (DCS) signal as its set point, the stem position feedback as the process variable, and the pneumatic output from the positioner as the controller output. Positioners add cost to the control valve, but for valves that exhibit a great degree of hysteresis or other nonlinear behavior, a positioner can improve the performance of the control valve and hence its related control loop.
Reference: Goettsche, L. D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.
Reader Feedback
We want to hear from you! Please send us your comments and questions about this topic to InTechmagazine@isa.org.