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The Practical Pumping Handbook - - - ~ ...... " ........... ~_

Work out
Efficiency =
Work in

For centrifugal pumps, it is the Capacity multiplied by the Total Head
and divided by the Power Input. When working in Gallons per Minute
and Feet of head, the formula is as follows"

USGPM x Head (in feet) x Specific Gravity
Efficiency =
Horsepower x 3960

When selecting a pump, we usually know the Efficiency and need to
find the horsepower in order to size the driving motor; therefore, the
equation is used as follows"

USGPM x Head (in feet) x Specific Gravity
H.P. =
Efficiency x 3960

In this equation an efficiency of 67% would be identified as 0.67.
With the same Head-Capacity
curve for the maximum impeller Effy
diameter, we can establish another
vertical axis and draw in the pump
Efficiency curve.
The flow rate, at which the
highest point on the efficiency I
curve is achieved, is known as the
Best Efficiency Point. This B EP BEP Q
is the most stable operating
condition for that pump. Figure 2.6: Efficiency curve

2.2.2 Net positive suction head
To ensure that Head, Capacity and Efficiency arc fully developed by the
pump, a suitable hydraulic condition is required at the inlet to the
pump. This condition is referred to as the Net Positive Suction Head
Required (NPSHR) and can be drawn against another vertical axis. The
NPSH required by the pump (NPSHR) must be made available from
the system (NPSHA) in order for the pump to fully develop the Head-
Capacity at the efficiency shown on the curves.
The performance curve shown on Figure 2.7 represents the total
hydraulic capability of the pump, when operating at one particular
speed. It shows

m 20 : . . . . . . . . . . . . . . .