May 2010

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

In the plot of inherent control valve characteristics shown below, what characteristic does curve C represent?

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A. Equal percentage

B. Leak tight shutoff

C. Linear

D. Quick opening

CCST answer

The correct answer is A, equal percentage. Curve C represents the equal percentage valve characteristic, where each increment of valve movement provides the same percentage increase in flow. 

Leak-tight shutoff is not represented on this chart. Curve B represents the linear valve characteristic, where each increment of valve movement provides the same incremental increase in flow. Curve A represents the quick opening valve characteristic, where the flow increases rapidly with valve movement.

References: Hans D. Baumann; Control Valve Primer: A User's Guide, Fourth Edition, ISA Press, 2009.