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The Practical Pumping Handbook .........................................................................................................

relatively familiar. They are a unique rumbling/rattling noise, and high
vibration levels. Closer inspection will also reveal pitting damage to the
impeller and a slight reduction in the Total Head being developed by
the pump. In order to consistently avoid or cure these problems, it is
important to understand what cavitation really is and what causes it in a
centrifugal pump.
Cavitation is a two part process caused by the changes in pressure as the
liquid moves through the impeller. As the liquid enters the suction
nozzle of the pump and progresses through, there are a number of
pressure changes that take place as shown in Figure 4.1.
As the liquid enters the pump through the suction nozzle, the pressure
drops slightly. The amount of reduction will depend on the geometry
of that section of the particular pump and will vary from pump to
pump. The liquid then moves into the eye of the rotating impeller
where an even more significant drop in pressure occurs.
The first part of the cavitation process occurs if the pressure falls below
the liquid's vapor pressure in the eye of the impeller. This causes vapor
bubbles to be created in that area (in other words, the liquid boils!).
The second part of the process occurs as the centrifugal action of the
impeller moves the bubbles onto the vanes where they are instantly re-
pressurized and thus collapsed in a series of implosions.
While a single such implosion would be insignificant, their increasing
repetition and severity develops energy levels well beyond the Yield
Strength of most impeller materials. At this stage, the impeller starts to
disintegrate and small cavities arc created in the metal. This condition
also creates the noise and high vibration levels mentioned earlier.

P

x
Suction
Preseure

Vapour
Pressure

Figure 4.1 Pressure gradient in suction/impeller

m s4