- July 31, 2020
CCSTs calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables.
The purpose of the plug within the control valve is to:
A. send a signal to the control valve to maintain the set point.
B. create a flow area that modifies the flow rate.
C. measure the upstream pressure of the property.
D. allow the valve to rotate without any linear motion.
The correct answer is B, “create a flow area that modifies the flow rate.” The plug in a control valve is the part that is positioned inside the valve by movement of the valve stem in response to a change in the control output. The plug is typically tapered or conical in shape and seats or fits into an orifice hole in the body of the valve. As the plug moves and changes position in relation to the orifice, a proportionately greater or smaller flow area is created.
When the plug is fully mated to the seat ring, the orifice is plugged and the valve is fully closed, and no flow can pass. As the plug is positioned away from the seat ring, a larger and larger annular area is exposed around the plug and through the orifice, allowing a greater flow rate to be developed through the valve.
Reference: Goettsche, L. D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.
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