This situation is caused by various types of instability, such as the combined action of turbulence, backflow and vortex that may occur in the impeller when the pump operates under the condition of small flow. Sometimes referred to as "de flow" or "hydrodynamic" cavitation, these flow patterns will intensify at small flow rates. The flow rate at which this occurs will vary from impeller to impeller.

2. Outlet reflux

Outlet backflow is a similar situation, which may cause pitting damage at the blade tip and sometimes at the volute tongue of (volute pump). It may also be caused by the pump running at a small flow rate.

(3) entrained air

Entrained air contains a variety of situations in which vapor bubbles are already present in the liquid before reaching the pump. When they reach the impeller inlet, exactly the same thing happens, as if they were created at that point. In other words, after reaching the blade, the steam bubble begins to bear the increasing pressure and thus collapses, causing the same damage as cavitation at the same position.

Since the entrained air causes the same pitting damage to the impeller at the same position as the vaporization cavitation, it may cause confusion in fault diagnosis, especially in the same working condition, the two may occur at the same time. However, rapid comparison of npsha and NPSHr, combined with visual inspection of pipe characteristics, is often helpful to determine the root cause of so-called "cavitation" and solve the problem of entrained air.

4. Similarities and differences

Conventional vaporization cavitation, entrained air and reflux cavitation will all lead to pitting damage on the impeller, which is caused by the formation of bubbles and subsequent collapse. The difference between them lies in the method of forming bubbles and the location of the impeller damage caused thereby.

As the severity of all these conditions increases, so will noise, vibration, and impeller damage. Under severe conditions, pitting damage will spread to the entire impeller and may also extend to the pump housing.

Discrimination of cavitation

In general, the big problem is how to determine which of the above three hydraulic conditions is when encountering the common symptoms of noise and vibration.

The flow rate of the pump can be reduced by adjusting the opening of the outlet valve of the pump, and then it can be judged according to the effect of the flow rate change on the noise and vibration. Adjusting the outlet valve of the pump to reduce the flow through the pump will produce three possible situations:

1) The noise and vibration will be significantly reduced or even completely eliminated.

2) Noise and vibration may become more serious.

3) There is little or no change in noise and vibration.
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