Net positive suction head required (NPSHR). This is the minimum head required to stop the pump from caviating when it is pumping cold water.

The pump curve that came with your pump shows the NPSH required for any given impeller size and capacity.

This number was determined by pumping cold water through the pump while reducing the suction head, until the pump showed a reduction in discharge head of three percent (3%) due to the low suction head and any formation of bubbles within the pump.

This point is called “the point of incipient cavitation”.

ROTARY PUMPS NPIP (Net Positive Inlet Pressure)

Positive displacement or rotary pump people do not use the term “head”, they use the term “pressure” instead so NPSHR would not be an appropriate term.

Rotary pumps are often selected to move liquids with a low vapor pressure point, or fluids with a lot of entrained bubbles.

This means that NPIP required (NPSH in centrifugal pumps) is difficult to test. The Hydraulic Institute establishes the point at the first indication of any of the following.

  • Cavitation noise is heard.
  • A 5% reduction in capacity at constant differential pressure and speed
  • A 5% reduction in power consumption at constant differential pressure and speed.