Noise from Cavitation: Bad for Control Valves(3)

Predicting & Eliminating Cavitation Damage

Especially with rotary valves, the prediction of damaging levels of cavitation is more complex than simply calculating the choked flow pressure drop. Experience has shown that there are likely to be areas of localized vaporization and vapor bubble collapse before the pressure in the main flow stream drops to the vapor pressure of the liquid. Some valve manufacturers predict the beginning of cavitation damage by defining an incipient damage pressure drop. One valve manufacturer’s method of predicting the beginning of cavitation damage is based on the fact that it is vapor bubble collapse that causes both cavitation damage and noise. This manufacturer has determined that if calculated noise levels are below the following limits, significant cavitation damage will be avoided.

• Up to 3 inch valve size – 80 decibels (dBA)
• 4-6 inch valve size – 85 dBA
• 8-14 inch valve size – 90 dBA
• 16 inch and larger valve size – 95 dBA

Methods of eliminating cavitation damage include both valve style selection and process modifications. Special valve designs for eliminating cavitation employ flow division and pressure drop staging, sometimes individually and sometimes together. ‘Flow division’ divides one large flow into a number of smaller flows by designing the flow path in the valve so that the flow passes through a number of small parallel openings. This is effective because the size of the cavitation bubbles is partly a function of the size of the opening the flow is traveling through. Smaller openings make smaller bubbles, which results in less noise and less damage when they collapse.

Pressure drop staging means that the valve is designed to have two or more throttling points in series, so that instead of taking the entire pressure drop in a single step, it is taken in several smaller steps. Smaller individual pressure drops can prevent the pressure at the vena contracta (the point where the velocity is the highest and the local pressure is the lowest) from dropping to the liquid’s vapor pressure, thus eliminating cavitation. Improved cavitation resistance can be obtained by combining flow division and pressure drop staging in the same valve.

Modifying the process to locate the control valve where the pressure at the valve inlet is higher (such as farther upstream or at a lower elevation) can sometimes eliminate a cavitation problem. Also, locating the control valve at a location where the liquid temperature, and thus the vapor pressure, is lower (such as the low temperature side of a heat exchanger) can help eliminate a cavitation problem.

Summary

As has been shown, cavitation in control valves does more than just degrade valve performance and damage the valve. Downstream piping and equipment is also at risk, and process contamination can ruin the product that the process is intended to make. Predicting cavitation and taking steps to eliminate it is the only way to avoid a costly and ongoing problem.

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