Preventing Cavitation:

Several changes within the system design or operation might be essential to increase the NPSH_A above the NPSH_R and stop the cavitation if a centrifugal pump is cavitating. A method for increasing the NPSH_A is to raise the pressure at the suction of the pump. For instance, if a pump is taking suction from an enclosed tank, either increasing the level of the liquid in the tank or increasing the pressure in the space above the liquid increases suction pressure.

It is also probable to increase the NPSH_A through decreasing the temperature of the liquid being pumped. By Decreasing the temperature of the liquid decreases the saturation pressure by causing NPSH_A to increase. Recall from the earlier module on heat exchangers which large steam condensers commonly subcool the condensate to less than the saturation temperature, known as condensate depression, to avoids cavitation in the condensate pumps.

The NPSH_A will be increased if the head losses in the pump suction piping could be decreased. Several methods for decreasing head losses include increasing the pipe diameter, decreasing the number of elbows, valves, and fittings in the pipe, and reducing the length of the pipe.

It might also be possible to stop cavitation through reducing the NPSH_R for the pump. The NPSH_R is not a constant for a provieded pump under all conditions, but depends on certain factors. Classically, the NPSH_R of a pump increases importantly as flow rate by the pump increases.

Thus, reducing the flow rate through a pump through throttling a discharge valve decreases NPSH_R. NPSH_R is also dependent upon pump speed. The faster the impeller of a pump rotates, the greater the NPSH_R. Thus, if the speed of a variable speed centrifugal pump is decreased, the NPSH_R of the pump decreases. Therefore, because a pump's flow rate is most frequently dictated through the requirements of the system on that it is linked; only limited adjustments could be made without beginning additional parallel pumps, if available.

The net positive suction head need to avoid cavitation is determined by testing by the pump manufacturer and depends upon factors involving kinds of impeller inlet, pump flow rate, impeller design, impeller rotational speed, and the kind of liquid being pumped. The manufacturer classically supplies curves of NPSH_R as a function of pump flow rate for a particular liquid (commonly water) within the vendor manual to the pump.