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.
Vapor bubble collapse can cause both noise and cavitation damage. It was calculated by one of the valves manufacturers that if noise levels (sound pressure level) are below a certain limits, cavitation damage will be kept to a minimum.
Below are the recommendedexample of pipe sizes and sound levels based on uninsulated schedule 40 pipe.
- Up to 3 inch valve size – 80 dBA
- 4-6 inch valve size – 85 dBA
- 8-14 inch valve size – 90 dBA
- 16 inch and larger valve size – 95 dBA