Net Positive Suction Head
At the inlet to a pump, the pressure normally drops as the fluid accelerates
around the blades, and at the exit, it normally increases to a higher value.
This pressure increase is necessary as it helps to overcome the pipe losses. At
the inlet, one can expect the static pressure to drop below the vapor pressure
value. We can therefore expect cavitation to occur new the blades. The
cavitation bubbles that form collapse is fractions of seconds with awesome force
which damages the blades. To avoid cavitation, pumps must operate at an inlet
head which is at least equal to the net positive suction head (NPSH). By
definition:
where Pv is the vapor pressure of the liquid
The NPSH is given from the performance curves so to avoid cavitation, we must
make the right hand side greater.
If the pump is at an elevated position, Zi, above a reservoir
whose free surface is at a pressure pa, we can use Bernoulli's
equations to rewrite the NPSH.
where hfi is the frictional pipe loss between the reservoir and
the pump inlet. Rearranging for the inlet head, we get
Place this back into our original NPSH equation
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