The difference between inlet pressure and the lowest pressure level inside the pump is called NPSH: Net Positive Suction Head. NPSH is therefore an expression of the pressure loss that takes place inside the first part of the pump housing.
The pressure inside a pump varies from the inlet on the suction side to the discharge port on the discharge side. In the first part of the pump, the pressure decreases before it increases on the discharge side to a level higher than the intake pressure.
Pump NPSH is shown in the figure to the right. The NPSH curve following an NPSH calculation shows H (head) at the Y-axis and Q (flow) at the X-axis.
NPSH will cause the lowest pressure inside the pump to decrease below the evaporation pressure of the pumped liquid, if the inlet pressure is too low. Consequentially, cavitation occurs in the pump, causing noise and leading to breakdowns.
NPSHR (Net Positive Suction Head Required) is provided in the data material for all pumps. NPSHR indicates the lowest inlet pressure required by the specific pump at a given flow to avoid cavitation.
Use the Grundfos sizing tool to find out Grundfos pump NPSHR and to see the NPSH calculation for pressure-boosting and boiler feed applications.