ISA Certified Control Systems Technician (CCST) program

CCST

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

Which of the following valve actuation methods would be the best choice for an application that requires a very large force and a small actuator?

A. pneumatic actuation
B. hydraulic actuation
C. manual actuation
D. electrical actuation

CCST Answer

The correct answer is B, hydraulic actuation. Although many pneumatic actuators can provide a large force, they require either a large diaphragm area (in the case of a diaphragm actuator) or a large cylinder (in the case of a rack and pinion actuator).

Hydraulic actuators are driven by a high-pressure fluid (up to 4,000 psig) that can be delivered to the actuator by a pump that is remote from the actuator itself. Hydraulic cylinders can deliver up to 25 times more force than a pneumatic cylinder of the same size.

Manual actuation is accomplished by turning a valve handle, and is limited to the amount of force that an operator can exert on the lever or hand wheel.

Electric actuation delivers high torques for rotary-style valves, but electric actuators tend to be large and heavy compared to hydraulic actuators.

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

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