NPSH (Net Positive Suction Head)

The difference between inlet pressure and the lowest pressure level inside the pump is called NPSH (Net Positive Suction Head). NPSH is 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.
 
The difference between inlet pressure and the lowest pressure level inside the pump is called NPSH (Net Positive Suction Head). NPSH is an expression of the pressure loss that takes place inside the first part of the pump housing. The size of the NPSH is shown in the figure to the right.
 
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.
 
Illustration
1. Suction pressure
2. Pressure line
3. Atmospheric pressure
4. Pump inlet
5. Evaporation pressure
6. Pump discharge
7. Vacuum
8. NPSH
9. NPSHR