It is calculated from the absolute energy level minus the evaporation pressure level. The evaporating head shall be calculated with the evaporating pressure corresponding to the temperature prevailing in the inlet cross-section of the pump.
The NPSHavailable value is the system-specific NPSH relating to the given flow rate and the fluid characteristics.
The NPSHrequired is the lowest NPSH value at which definite cavitation criteria (i.e. wear due to cavitation, vapour formation, vibration, noise, head loss) can be contained.