Net positive suction head (NPSHr)

The net positive suction head (NPSHP, NPSHrequired) is pump-specific and is usually represented by the manufacturers as a curve in the performance curve diagram of the pump.

Generally recognizable is the strong dependence on the pump speed. If the construction is unchanged:

High speed -> High holding pressure head
Low speed -> Low holding pressure head

In order to take account of any uncertainties in the design of the duty point, these values must be increased by a safety margin of 0.5 m when selecting the pump.

By definition a minimum cavitation is permissible at NPSH, whereby the following conditions are allowed:

  • The head of the pump at the nominal point is reduced by 3%.
  • No material damage impairing the function and service life occurs.

Due to the permissible cavitation, cavitation noises can still occur, some of which are perceived as disturbing.

In order to eliminate the residual cavitation, it is necessary to provide the calculated minimum inlet head with a surcharge of approx. + 1 to + 5 m. The minimum inlet head must be calculated with a surcharge of approx. This surcharge depends on the speed and the operating point of the pump.

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System Curve

The system H-Q curve is essentially a parabolic curve representing the relation between the head (pressure) required to handle the respective rate of flow within a pipe system.

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Pump Selection

Centrifugal pumps should be selected such that the specified System Duty Point is located on or as near as possible to the point of maximum efficiency (Pump Duty Point) of the max. speed H/Q-curve.

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