The NPSHrequired is the lowest NPSH value at which definite cavitation criteria (i.e. wear due to cavitation, vapour formation, vibration, noise, head loss) can be contained.
As a function of the volume flow Q, NPSHreq is a characteristic of the centrifugal pump and is specified for many types as pump characteristic curve NPSH(Q). At low volume flow rates, the NPSH value is almost constant, whereas it rises steeply at high volume flows.
The NPSH value of the pump changes with the speed as well as the impeller diameter.
For some pump types, the NPSH value can optionally be reduced by an additional construction. A typical example of this is the inducer, in which an axial impeller with a small number of blades is arranged directly in front of the actual impeller of the centrifugal pump.
- Increasing the available static pressure at the pump location (positioning the pump at a lower level, e.g. from a roof plant room into the basement to benefit from the increased static head from the higher water column. The pump performance remains unchanged).
- Lowering the fluid temperature (reducing the vapour pressure pD).
- Changing the pump characteristics (reducing the driven speed and/or different pump design with a lesser Pump NPSHrequired).
This occurs if the static pressure in the fluid falls below the temperature-related vapour pressure. A forceful implosion of the bubbles occurs when the fluid stream reaches a region of higher pressure.
Cavitation can lead to premature material wear and noise emission. Cavitation should therefore be avoided as far as possible.
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.
