Understanding and Predicting Cavitation in Control Valves

Cavitation in control valves occurs only with liquid media, and the principal factors are fluid velocity and pressure drop. Vapor bubbles will form if the liquid’s upstream pressure suddenly drops below its vapor pressure as it flows through the valve. Cavitation is the collapsing of these vapor bubbles as the pressure recovers downstream of the valve’s trim outlet.

Cavitation Damage

Cavitation damage is a form of hyper-erosion that can destroy both control valves and piping, which can result in unacceptable process failures. The vapor bubbles created as a result of a pressure drop will implode – nucleate, grow, collapse, and rebound – as the vapor returns to liquid form. The implosion of vapor bubbles in the cavitation phenomenon inflicts damage in the form of small pits in the metal, which cumulatively wear away surfaces.

Predicting Cavitation

Embedded is the Sigma Cavitation Index for predicting the potential for cavitation given a set of valve process parameters. Sigma is the most widely-accepted and precise cavitation index used to quantify and predict cavitation in control valves. Simply put, Sigma is the ratio of the potential for resisting formation of vapor bubbles to the potential for causing formation of vapor bubbles

Preventing Cavitation with Staged Pressure Reducing Trim

In applications with more severe cavitation (1.0 << σ< < 1.5), the ideal solution is to reduce pressure gradually from the trim inlet to the trim outlet. By staging pressure reduction, the trim can prevent the process pressure from dipping below the vapor pressure – thereby preventing the formation of the damaging vapor bubbles altogether.

How can you avoid cavitation?

There are several ways you can avoid excessive cavitation. Here are a few of them: Make sure you select the right valve for your application. If your valves are the wrong size or the wrong style, their likelihood of cavitation increases. In water and liquid systems with high pressure drop, use an anti-cavitation valve. Use multiple control valves or multistage control valves so that the pressure drop happens gradually rather than all at once. This is called pressure drop staging. Place the control valve at a lower elevation in the system or in an area where the fluid temperature is reduced.

As you can see in the image above, removing bonnet cover of top entry valves, allows access to valve internals for assembly, disassembly, repair, or maintenance without removing the valve from the pipeline.

Ref.

⦁ https://www.flotechinc.com/understanding-predicting-cavitation-control-valves

⦁ https://alliedvalveinc.com/the-valve-expert/cavitation-control-valves-need-know/