The impact of cavitation on the valve and how to solve?

What is cavitation？

of liquids at low pressure to form bubbles, which rupture rapidly when they enter the high-pressure region. This phenomenon occurs mainly in the liquid flow at broadband, like in pumps, propellers and turbines and other mechanical devices. The formation of the cavitation process includes low-pressure vaporization and bubble rupture in the high-pressure region. Liquid in the local pressure is reduced to below the evaporation pressure and will form blisters, and bubbles into the high-pressure region after the rapid rupture, leading to a strong shock wave.

The effects of cavitation on mechanical equipment mainly include material damage, noise and vibration, and performance degradation. The shock wave generated when the bubble ruptures will produce erosion externally of the equipment, and the long-term effect will cause material damage. The cavitation phenomenon will also cause equipment noise and vibration, affecting normal operation. Furthermore, cavitation can disrupt the level of smoothness of liquid flow, reducing equipment efficiency and performance.

The risk of cavitation in valves is influenced by several factors, extending beyond just the cavitation index. The extent to which a valve is opened plays a crucial role—partially opened valves are more susceptible to cavitation.

Other contributing factors include:

• Valve Size: Larger valves are more prone to cavitation.
  
  

• Pressure Class: Higher pressure classes can result in greater pressure drops, increasing cavitation risk.
  
  

• Material: The material composition of the valve impacts its cavitation resistance. Harder materials like 22Cr duplex steel are less likely to suffer from cavitation compared to softer options like austenitic stainless steel. Hard trim materials such as Stellite 6 or 21, and 13Cr martensitic stainless steels, also offer higher resistance.
  
  

• Leakage: Leakage around the valve seat, especially when the valve is closed, heightens cavitation risk.
  
  

• Flow Regime: Turbulent flow and high velocity exacerbate cavitation.
  
  

• Trim Design: Valves with multi-step trim designs distribute pressure drop over multiple stages, minimizing the risk in any single stage and protecting critical areas like the seat and plug sealing.
  
  

Understanding these variables can help in designing and selecting valves that minimize cavitation risks.

Cavitation severity is assessed using a measurement called the cavity index. This index quantifies the intensity and spread of cavitation within valves. A specific formula is utilized to calculate the cavity index, which works as the primary indicator for evaluating cavitation severity.

The rise from the cavity index calculations are then referenced against established benchmarks found in detailed charts, like Table 1. These benchmarks classify levels of cavitation, helping engineers determine the exact severity and potential effect on the valve system.

By counting on the cavity index, professionals can identify whether the cavitation goes to a manageable level or if it requires immediate intervention to avoid further damage.