PumpsDOE-HDBK-1018/1-93CENTRIFUGAL PUMP OPERATIONThe condition that must exist to avoid cavitation is that the net positive suction head availablemust be greater than or equal to the net positive suction head required. This requirement can bestated mathematically as shown below.NPSH_{A}NPSHRA formula for NPSH_{A} can be stated as the following equation.NPSH_{A} = P_{suction} - P_{saturation}When a centrifugal pump is taking suction from a tank or other reservoir, the pressure at thesuction of the pump is the sum of the absolute pressure at the surface of the liquid in the tankplus the pressure due to the elevation difference between the surface of liquid in the tank andthe pump suction less the head losses due to friction in the suction line from the tank to thepump.NPSH_{A} = P_{a} + P_{st} - h_{f} - P_{sat}Where:NPSH_{A} = net positive suction head availableP_{a}= absolute pressure on the surface of the liquidP_{st}= pressure due to elevation between liquid surface and pump suctionh_{f}= head losses in the pump suction pipingP_{sat}= saturation pressure of the liquid being pumpedPreventingCavitationIf a centrifugal pump is cavitating, several changes in the system design or operation may benecessary to increase the NPSH_{A} above the NPSH_{R} and stop the cavitation. One method forincreasing the NPSH_{A} is to increase the pressure at the suction of the pump. For example, if apump is taking suction from an enclosed tank, either raising the level of the liquid in the tank orincreasing the pressure in the space above the liquid increases suction pressure.It is also possible to increase the NPSH_{A} by decreasing the temperature of the liquid beingpumped. Decreasing the temperature of the liquid decreases the saturation pressure, causingNPSH_{A} to increase. Recall from the previous module on heat exchangers that large steamcondensers usually subcool the condensate to less than the saturation temperature, calledcondensate depression, to prevent cavitation in the condensate pumps.If the head losses in the pump suction piping can be reduced, the NPSH_{A} will be increased.Various methods for reducing head losses include increasing the pipe diameter, reducing thenumber of elbows, valves, and fittings in the pipe, and decreasing the length of the pipe.Rev. 0ME-03Page 13

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