Thursday, October 24, 2013

Defining Cavitation in Control Valves

cavitation in control valves
Formation of cavitation
Cavitation is the formation of vapor spaces (bubbles) in the control valve cavity resulting from a rapid change in pressure. When the bubbles move downstream and are exposed to a larger cross-sectional area, velocity decreases and pressure increases. The surrounding higher pressure causes the lower pressure bubble to implode which causes shockwaves. These shockwaves can cause metal fatigue and cause excessive wear on the internals of the valve. As the bubbles collapse, they make a discernible sound with accompanying vibration and the damage to a valve can happen quickly (weeks or months).

Cavitation is very destructive and may wear out the trim and body parts of the valve in short time. Using hard internal materials provide a little improvement, but the harder materials also cost more and their use must be justified.

How can you minimize cavitation in control valves? You can change the process conditions and reduce operating temperatures. This may lower the vapor pressure enough to stop cavitation. Reducing the differential pressure across the valve can relieve or lower cavitation. Locating a control valve in the lowest elevation of the piping system and keeping the control valve as close as possible to the pump are  also recommended practices.

Here is an excellent and thorough technical paper on cavitation in control valves courtesy of Samson Controls.