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What is NPSH

NPSH (Net Positive Suction Head) is a measure of the pressure on the suction (inlet) of a pump. It is used to avoid running a pump under conditions which cause cavitation.

Net Positive Suction Head (NPSH) is the pressure of liquid pushing down on the inlet of a pump.  A lack of enough NPSH is one of the biggest reasons for pump failure.

Most pumps are designed to pump liquids out and into the discharge pipework, but they are completely reliant on the pressure on the inlet (suction) to get enough liquid into their chamber. A pump must be able to get the same amount of liquid in as it is capable of pumping out when this isn’t the case cavitation can occur.

There are 2 NPSH terms.

  1. NPSHa which is the NPSH available in the system.
  2. NPSHr which is the NPSH required by the pump.

NPSHa must always be more than NPSHr and it's the overall process designers repsonsibility to know what the NPSHa is.


How to calculate NPSHa

In simple terms the formula is       NPSHa = H1 + Hss – Hf – Hvp


  • H1 is the absolute pressure over the liquid (m)
  • Hss is the static head (m) of the liquid before the pump– this is negative if there is a suction lift (m)
  • Hf is the friction in the pipe work leading to the pump inlet (m)
  • Hvp is the vapor pressure of the liquid at its operating temperature (m)

Click a HERE for a simple NPSHa calculation tool.


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