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February 2010

ISA Certified Automation Professional (CAP) program

Certified Automation Professionals (CAPs) are responsible for the direction, design, and deployment of systems and equipment for manufacturing and control systems.

CAP question

What is the term used for phenomenon in liquid flow where falling static pressure causes the formation of vapor bubbles that subsequently collapse back into the all-liquid state as the fluid static pressure is recovered?

A.  Cavitation

B.   Pressure Piling

C. Saturation

D.  Vortex Shedding

CAP answer

The correct answer is A, Cavitation. Cavitation is a two-stage phenomenon of liquid flow. The first stage is the formation of vapor bubbles within the liquid system because of the fluid’s static pressure falling below the fluid vapor pressure  The second stage is the collapse or implosion of these cavities back into an all-liquid state as the fluid decelerates and static pressure is recovered.

Answer B is not correct. Pressure Piling is a condition that results from the ignition of pre-compressed gases in compartments or subdivisions other than those in which ignition was initiated.

Answer C is not correct. Saturation is a device characteristic exhibited when a further change in an input causes no further change in the output.

Answer D is not correct. Vortex Shedding is the phenomenon that occurs when fluid flows past an obstruction. The shear layer near the obstruction has a high velocity gradient, which makes it inherently unstable. At some point downstream of the immediate vicinity of the obstruction, the shear layer breaks down into well-defined vortices.

Reference: The Automation, Systems, and Instrumentation Dictionary, Fourth edition, ISA, 2003.