Centrifugal pumps cavitation, avoidance

To avoid cavitation, the pressure at the pump inlet must exceed the vapor pressure by a certain value, called the net positive suction head (NPSH). The required value of NPSH is about 2 to 3 m (5 to 10 ft) for small centrifugal pumps, but it increases with pump capacity, impeller speed, and discharge pressure, and values up to 15 m (50 ft) are recommended for very large pumps. For a pump taking suction from a reservoir, like that shown in Fig. 8.5, the available NPSH is customarily calculated as... 

To avoid cavitation in centrifugal pumps, the pressure of the fluid at all points within the pump must remain above saturation pressure. 

Avoidance of cavitation. As with reciprocating pumps, the criterion NPSHa > NPSHr should be obeyed. The available NPSH (NPSHa) for a centrifugal pump may be calculated using equations (9.14) and (9.15) developed in section 9.S.2.4 for reciprocating pumps. In this case however //ja = 0 and NPSHa usually falls with the square of the volume flow rate (Figure 9.28). 

Secondary pumps are normally centrifugal and are used to pressurise the whole system by sucking the liquid from the bottom of the kier and feeding it to the main pump. Cavitation, which is a result of steam bubbles forming at high temperatures on the low-pressure side of the pump, may also be avoided if the whole system is pressurised even when dyeing at 100 °C. 

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