We need to know it to know where and when the cavitation can occur:
The Available NPSH (NPSHA): a measure of how close the fluid at a given point is to flashing, and so to cavitation.
The Required NPSH (NPSHR): the head value at a specific point (e.g. the inlet of a pump) required to keep the fluid from cavitating.
NPSH stands for Net Positive Suction Head.
The physical meaning of this expression is the absolute pressure that must exist at the suction port of the pump in order to pump the liquid without
This can occur when the absolute pressure falls to values likely to allow the formation of vapour bubbles within the fluid, causing the pump to work with reduced head.
Therefore, NPSH can also be seen as the pressure required to compensate load losses in the path between the suction port and the point with the lowest pressure of the impeller.
All this demonstrates the importance of checking that the pump is not producing cavitation, as in addition to creating high noise similar to metal hammering, cavitation will also quickly damage the impeller.
A special formula associates the NPSH value required by the pump with the conditions of the system and with the type of liquid, allowing to calculate the minimum pressure required at the suction, and consequently to determine the position in which to locate the pump in relation to the free surface of the liquid to be pumped.
The general NPSH formula is:
Z1 = the difference in level (in m) between the axis of the pump and the free surface of the liquid to be pumped.
p1 = the possible pressure (in kg/cm2) on the surface of the liquid in the tank from which it is collected. If the liquid is collected from an open tank
and the surface of the liquid is in contact with the atmosphere, p1 will be equal to 0.
pb = atmospheric pressure (in kg/cm2) at the site of installation.
pv = the vapour tension (in kg/cm2) of the liquid at pumping temperature.
γ = the specific weight (in kg/dm2) of the liquid at pumping temperature.
10 = conversion factor of the units of measure used.
Hr = head loss (in m) in the suction pipework.