What is NPSH? What is the difference between NPSH-A and NPSH-R

NPSH is the measure of suction pressure available at the inlet side of the pump. It says that water at the pump should not vaporize due to which the suction pressure should be always higher than the vapour pressure of the liquid.

The minimum suction conditions are more frequently specified in terms of NPSH. If the pressure at the suction side is lower than the required NPSH vapour, than bubbles can cause cavitation inside the pump. This is very dangerous for a pump. Therefore a calculation will be made with the help of the design of the pump through the manufacturing company of the pump.


It is the relation between the temperature and vapour pressure of water. For example, if the water temperature is 100℃ and its vapour pressure is 2 bar, if the pump creates a suction pressure of 1.8 bar on the suction side the water will be converted into vapour and cavitation will occur. For this reason, the suction pressure should be more than the vapour pressure of the water at this temperature. The vapour pressure of fluids changes with respect to temperature.

We all know that 1.013 bar=760 mm Hg=10.3 metre water column. On the other hand, we know that the density of hot and cold water is different. When the temperature of the water is 0℃ and the pressure generated by the pump is 1 bar and we can suck the water from 10.3 metres below the pump theoretically, but if we consider frictional and other losses then the actual value comes down to 7-8 metres.

As the temperature of the fluid increases then the suction head required will also increase. We always need to know what the suction pressure is over the inlet point of the pump and the temperature of water over here so that liquid does not form vapour or bubbles.

Given below are the different suction pressures needed for different temperatures of water,

                                       The cold water head required is -7.5 m.

                                       64℃ head required is -3 m.

                                       77℃ head required is -2.1m.

                                       94℃ head required is 0m.

                                       110℃ head required is 3m.

                                       123℃ head required 6.7m.


  • NPSHA:-NPSH-A is the total available pressure at the suction port of a centrifugal pump.
  • NPSHR:-NPSH-R is a property of a pump that will be calculated by the manufacturing company. It is called the NET POSITIVE SUCTION HEAD REQUIRED. It is the minimum pressure required at the suction side of the pump to keep the pump from cavitation.

                    NPSH A= HA ± HZ − HF − HVP

where HA=absolute pressure of surface.

            HZ=water head.

            HF=frictional losses.

            HVP=absolute vapour pressure.


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