Cavitation is the formation and collapse of air or gas bubbles in a liquid.
The bubbles are formed when liquid undergoes a rapid change of pressure and falls below the vapor pressure. They collapse when the pressure recovers.
This can all happen in a very short span just after the vena contracta—the point in the valve where the diameter of the flow is at its smallest, and fluid velocity is at its maximum.
What are the Symptoms?
Because it happens inside a valve or pipeline, cavitation is not easy to spot. Here are two symptoms that may be caused by cavitation:
- Valve Trim Pitting. When the vapor bubbles collapse, it creates a strong force. When this happens over and over on the metal surface of the valve trim, the metal will begin to erode and pit.
- Water Hammering. If cavitation is occurring, your valve may repeatedly open and then close sharply. This is called water hammering, and it can lead to an eventual compromise of the valve stem, coupling block and/or valve seat.
How Can I Prevent Cavitation?
There are three things you can do to prevent cavitation in your valves:
- Decrease the pressure drop across your valve.
- Install the valve in a cooler place in the process, thereby decreasing the vapor pressure.
- Make sure your valve is properly sized. Cavitation often happens in applications when a valve is oversized. For tips on how to size your valve, see our video, “How to Select a Valve.”