This automation industry quiz question comes from the ISA Certified Automation Professional (CAP) certification program. ISA CAP certification provides a non-biased, third-party, objective assessment and confirmation of an automation professional's skills. The CAP exam is focused on direction, definition, design, development/application, deployment, documentation, and support of systems, software, and equipment used in control systems, manufacturing information systems, systems integration, and operational consulting. Click this link for more information about the CAP program.
Cavitation occurs in liquid flow when:
a) gas and vapor flow become sonic, and the flow rate drops
b) mixtures of fluid and vapor cause erosion of the valve and pipe surfaces
c) fluid pressure drops below the liquid’s vapor pressure, and the vapor pressure is below the outlet pressure
d) fluid pressure drops below the liquid’s vapor pressure, and the vapor pressure is above the outlet pressure
e) none of the above
Click Here to Reveal the Answer
Fluid pressure drops below the liquid’s vapor pressure, and the vapor pressure is below the outlet pressure. In cavitation, when the liquid drops below the vapor pressure around the vena cava, the liquid vaporizes. As the material moves downstream, the pressure recovers to above the vapor pressure again, and the vapor collapses back into a liquid. This can cause mechanical damage to the valve, trim, and piping.
Answers A and B are not correct, because cavitation requires a state change, which is not indicated in sonic flow or two-phase flow. Answer D is not correct, as this describes the condition known as flashing, where the liquid vaporizes as it passes through the valve orifice, and the pressure does not recover to the point of having the vapor collapse back to a liquid state.
The answer is C.
Reference: Nicholas Sands, P.E., CAP and Ian Verhappen, P.Eng., CAP., A Guide to the Automation Body of Knowledge. To read a brief Q&A with the authors, plus download a free 116-page excerpt from the book, click this link.