Generally speaking, a difference is made between the NPSHA-value and the NPSHR-value. The NPSHA-value (available net positive suction head) defines the existing pressure reserves at the suction nozzles. The pressure head difference in which the NPSHA-value is measured is made up of the difference between the total pressure in the suction nozzles and the evaporation pressure of the conveyed fluid. The following applies when calculating the NPSH-value:

The NPSHR-value describes the required pressure reserves at the pump inlet, so that the vapour pressure of the medium is not undershot because of pressure losses during entry into the pump. Thus, the following condition must be fulfilled for operation free of cavitation:

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