Pumping Tips NPSH

NPSH Margin: NPSHA-NPSHR

In any application it is desirable that the system NPSHA be greater than the pump NPSHR.

Having a margin between the two figures will help ensure cavitation free operation, act as a safety margin on the calculations and allow for an increase in NPSHR when the pump internal clearances wear.

This margin should not be deducted from the calculated NPSHA as a safety margin. The calculated NPSHA should be advised to the pump vendor and a pump with an acceptably lower NPSHR should be selected. Adding or deducting safety margins may lead to unnecessary considerations in the pump selection and hence higher costs.

The size of the margin between NPSHA and NPSHR will depend on the type of application and the purchaser's preferences and specifications.

The majority of purchasers specify a minimum one (1) metre margin at the rated duty flow rate.

Others may specify the following:

0.6 m margin at the rated duty point
1.0 metre margin at all points from minimum continuous flowrate up to and including the rated duty flowrate
1.0 metre margin at all points from minimum continuous flowrate up to 125% of rated duty flowrate
The margin between NPSHA and NPSHR shall be a minimum of 10% of the NPSHA

Our preference is for the second statement, i.e. a one (1) metre margin at all points from minimum continuous flow rate up to and including the rated duty flow rate. This ensures an adequate margin at all anticipated flowrates.

Many users consider a 0.6 metre margin too small and the third and fourth statements can lead to the requirement of much larger pumps than are really necessary.

For pumps in the following services a larger NPSH margin should be considered:

vacuum services (suction pressure below atmospheric pressure)
low temperature services
hot water services
fluids with entrained gases

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