May/June 2012

ISA Certified Control Systems Technician (CCST) program

Certified Control System Technicians (CCSTs) calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables.

CCST question

A valve positioner is usually recommended as an accessory to a control valve for which situation described below?

A.        For control valves with bodies over 2" in order to overcome actuator spring forces.
B.        For control valves that are constructed of carbon steel or bronze.
C.        For control valves that are experiencing a large degree of hysteresis.
D.        For split-body control valves.

CCST answer

The correct answer is C. A positioner adjusts the pneumatic output to a valve actuator so that the requested position of the valve from a controller is achieved. A valve positioner can correct for any valve inaccuracies with regard to the controller demand signal versus the actual valve stem position, regardless of the amount of valve travel change or direction of valve travel.

Answer A is not correct. The use of positioners is not dependent upon valve size.
Answer B is not correct. Use of a valve positioner is not related to the materials of construction of a valve.
Answer D is not correct. Use of a valve positioner is not related to the valve body type.

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, 2nd Edition (2005), ISA Publishing.