Wikipedia defines cavitation as:
…the formation of vapour cavities in a liquid – i.e. small liquid-free zones (“bubbles” or “voids”) – that are the consequence of forces acting upon the liquid. It usually occurs when a liquid is subjected to rapid changes of pressure that cause the formation of cavities where the pressure is relatively low. When subjected to higher pressure, the voids implode and can generate an intense shock wave.
These shock waves can result in serious damage to control valves where these rapid changes in pressure can occur. This 5:56 YouTube video on cavitation clearly describes the phenomena, its impact on valves, and ways that it can be eliminated or significantly reduced.
The video shares the fact that the implosion of bubbles can cause local pressure ways of up to 100,000 pounds per square inch (6895 Bar). The combination of pressure waves and liquid micro-jets, which form when the recovering pressure makes indentations in the bubble and burst through it, can cause severe damage to the valve plug, seat and body around the areas where this cavitation occurs. Excessive noise and vibration of the surrounding piping are also problems that can occur.
The video highlights the primary factors in damage caused by cavitation. These include:
- Cavitation intensity
- Materials used in control valve
- Length of exposure to cavitation
- Valve size
- Design of valve and trim
- Leakage when valve is closed
Emerson’s line of Fisher Severe Service control valves incorporate technologies such as hardened materials to protect the valve body, pressure drop staging (ex. Cavitrol III trim), and flow diversion away from the sides where cavitation can cause the most damage.
Hopefully, the few minutes spent watching this video will help provide you a clear picture of what cavitation is and ways to prevent or mitigate its damage in control valves.
You can connect and interact with valve experts in the Valves group in the Emerson Exchange 365 community.