May/June 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

As a liquid flows through the restriction of the control valve trim, the pressure drops and then recovers to a fraction (<1.0) of the upstream pressure. A common problem in the application of control valves is cavitation, which can cause surface damage to the wetted surfaces of the control valve or connecting pipe. 

Cavitation will occur in a liquid flow stream, like the one described above, when:

A. the fluid pressure remains constant across the trim of the control valve

B. the fluid pressure drops below the liquid's vapor pressure and the vapor pressure is above the outlet pressure

C. the fluid pressure drops below the liquid's vapor pressure and the vapor pressure is below the outlet pressure

D. the fluid viscosity decreases through the control valve trim to the point of cavitation

The correct answer is C, the fluid pressure drops below the liquid's vapor pressure and the vapor pressure is below the outlet pressure.  At the point where the fluid drops below the liquid's vapor pressure, a gas is formed.  As the fluid moves downstream to where the fluid pressure is greater than the vapor pressure, it "collapses" back into a liquid, causing a shock wave that can damage interior valve and piping surfaces.  This phenomenon is called cavitation.

Answer A cannot be correct since cavitation cannot occur without a change in the pressure profile through the valve.

Answer B is the definition of flashing, which will also occur when the fluid pressure drops below the liquid's vapor pressure, but because the vapor pressure is above the outlet pressure, it remains as a gas into the downstream piping. This can also lead to valve or pipe damage and noise issues.

Answer D is incorrect since there is no direct correlation between viscosity and cavitation.

Reference:  Hutchinson, J.W., ISA Handbook of Control Valves, Second Edition, ISA, 1976.