Preventing Cavitation
If a centrifugal pump is cavitating, several changes in the system design or operation may be
necessary to increase the NPSHA above the NPSHR and stop the cavitation. One method for
increasing the NPSHA is to increase the pressure at the suction of the pump. For example, if a
pump is taking suction from an enclosed tank, either raising the level of the liquid in the tank or
increasing the pressure in the space above the liquid increases suction pressure.
It is also possible to increase the NPSHA by decreasing the temperature of the liquid being
pumped. Decreasing the temperature of the liquid decreases the saturation pressure, causing
NPSHA to increase. Recall from the previous module on heat exchangers that large steam
condensers usually subcool the condensate to less than the saturation temperature, called
condensate depression, to prevent cavitation in the condensate pumps.
If the head losses in the pump suction piping can be reduced, the NPSHA will be increased.
Various methods for reducing head losses include increasing the pipe diameter, reducing the
number of elbows, valves, and fittings in the pipe, and decreasing the length of the pipe.
It may also be possible to stop cavitation by reducing the NPSHR for the pump. The NPSHR is
not a constant for a given pump under all conditions, but depends on certain factors. Typically,
the NPSHR of a pump increases significantly as flow rate through the pump increases.
Therefore, reducing the flow rate through a pump by throttling a discharge valve decreases
NPSHR. NPSHR is also dependent upon pump speed. The faster the impeller of a pump rotates,
the greater the NPSHR. Therefore, if the speed of a variable speed centrifugal pump is reduced,
the NPSHR of the pump decreases. However, since a pump's flow rate is most often dictated
by the needs of the system on which it is connected, only limited adjustments can be made
without starting additional parallel pumps, if available.
The net positive suction head required to prevent cavitation is determined through testing by the
pump manufacturer and depends upon factors including type of impeller inlet, impeller design,
pump flow rate, impeller rotational speed, and the type of liquid being pumped. The
manufacturer typically supplies curves of NPSHR as a function of pump flow rate for a particular
liquid (usually water) in the vendor manual for the pump.
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