Most people who work on, or around pumps are familiar with the term “cavitation”. It is often used as a catch all phrase for any flow-related vibration on pumps, and is often reported by vibration analysts when:
But there are a handful of flow-related vibration problems on pumps, and only one of those is truly cavitation. But all of them have similar vibration characteristics, and all of them can cause inefficiency of the pumping system, and pump damage, if severe.
Let’s look at the most common reasons for flow-related pump vibration.
CAVITATION – True cavitation occurs when “cavities”, low pressure vapor bubbles, form in the liquid on the suction side of the pump. This can be from:
When these low pressure “bubbles” get into the impeller section, pressure balances, and these bubbles implode, often onto the impeller surface.
This can be quite damaging to the impeller and pump housing. Cavitation is often quite loud, and typically explained as “it sounds like gravel is going through the pump!”.
AERATION, OR AIR ENTRAPMENT – Air entrapment happens when bubbles are in the liquid before reaching the impeller. This can happen when:
While this problem is not always as damaging (or as loud) as cavitation, it can certainly damage the impeller if left unchecked.
RE-CIRCULATION – Re-circulation is most often caused by low flow.
Two types of re-circulation problems can occur:
Vibration analysis is good at detecting flow-related pump problems, but distinguishing which of these problems is present is difficult, if not impossible, to determine by vibration alone. An easy method to determine which of these problems is taking place is to slowly throttle the discharge valve closed, if operations will permit.
As the discharge valve is being throttled closed:
Throttling the discharge valve may seem like an easy fix, but it is merely a diagnosis method. If any of these problems are occurring, there is either an operational or design problem, and it should be addressed by engineering.