The term NPSH stands for Net Positive Suction Head.
It represents the pressure drop in between the suction point of the pump and the minimum pressure that is reached at the pump inlet.
If the NPSH available is less than the NPSH required, the pump will not work properly. Either the liquid will not be pumped (not possible to prime the pump) or the operation will generate cavitation.
The NPSH required for a given pump is given by the pump manufacturer. It can be read on the pump characteristic curve.
Figure 1 : Pump NPSH, required and available
The NPSH available needs to be calculated from the actual suction pipe layout of the pump. The following case is considered below :
Figure 2 : Pump circuit
Equation 1 : NPSH calculation [Midoux]
Note : in many cases the velocity head u2/2g is negligible which simplifies the relation [Sihi]
- h1 = height from the level of liquid to centrifugal pump axis (m)
- p1 = pressure of the tank from which liquid is pumped (Pa)
- pv = vapor pressure of liquid pumped (Pa)
- u1 = velocity of liquid in the suction pipe (m/s)
- ΔHf = pressure drop by friction in the suction pipe (m)
- ρ = specific gravity of liquid pumped (kg/m3)
An NPSH available too low will lead to operation issues for the pump, especially a decrease in flowrate and the risk of cavitation.