Water-Lifting Devices – Pumps - Practices of Irrigation and On-farm Water Management - page 444

Eq. ( 12.1 ) . If the water source is located above the eye of the pump impeller (sub-
merged pump), suction head (SH) must be added instead of subtracted. An accurate
determination of NPSHa is critical for any centrifugal pump application.
NPSHa = BP − SH − FL − VP (12.1)

where

BP = barometric pressure at pump level (m)

SH = suction head or lift (m)

FL = friction losses in the suction pipe (m)

VP = vapor pressure of the liquid at given temperature (m)

12.4.2.3 Net Positive Suction Head required (NPSHr)

It is a measure of the head necessary to transfer water into the impeller vanes effi-
ciently and without cavitation. The NPSHr required by a specific centrifugal pump
depends on the pump design and flow rate. It is constant for a given head, flow, rota-
tional speed, and impeller diameter. However, it changes with wear and different
liquids, since it depends on the impeller geometry and on the density and viscosity
of the fluid. For a given pump, NPSHr increases with increases in pump speed, flow
rate, and water temperature.
The value of NPSHr is provided by the manufacturer for each specific pump
model, and it is normally shown as a separate curve on a set of pump characteristic
curves. To avoid cavitation, NPSHa must be always equal to or greater than NPSHr.

12.4.2.4 Static and Dynamic Head
Static Head

Static Suction Head

Static suction head (or static suction lift) is the vertical distance from the static water
level in the suction pipe to the centerline of the impeller.

Static Discharge Head

Static discharge head is the vertical distance from the center of the impeller to the
discharge outlet, or liquid level when discharging into the bottom of a water tank.

Dynamic Head

Dynamic Suction Head

Dynamic suction head is the sum of static suction lift plus friction loss in the suc-
tion pipe, plus the loss of head in the formation (when the pipe is installed in a
aquifer). During pumping, actual water level lowers than the static level due to lift-
ing of water column. This is observed outside the suction pipe by observation well.