1 August 2005

ISA Certified Automation Professional (CAP) Program

This question is from the CAP study guide, Performance Domain III, System Design.

Definition: Design, specify, and procure the hardware/software used in the system.

CAP question

Which of the following control-valve body styles is MOST likely to develop blockage in a paper-pulp slurry service?

A. Pinch

B. Characterized ball

C. Butterfly

D. Globe

CAP answer

The correct answer is D, Globe.

A globe valve consists of a plug on the end of a stem that moves the plug onto a seat. Because of the body design of this valve, cavities in the body can easily fill with materials and develop blockage. Ball-, butterfly-, and pinch valve body style have straight-through flow, which is less likely to develop blockage.

References: Borden, Guy, Control Valves: Practical Guides for Measurement and Control, ISA Press, 1998. Lipták, Bela, Instrument Engineers' Handbook: Process Control, Volume 2. CRC Press, 2001.


Nicholas Sheble (nsheble@isa.org) edits the Certification department. For information about the CAP program, go to www.isa.org/CAP.