Handbook of Civil Engineering Calculations

FIGURE 3. Approximate relative impeller shapes and efficiency variations for various specif-
ic speeds of centrifugal pumps. (Worthington Corporation.)

specific speed giving maximum efficiency for a given pump and is a number used to
identify a pump. Specific speed is important in cavitation and suction-lift studies. The
Hydraulic Institute curves, Figs. 1 and 2, give upper limits of speed, head, capacity and
suction lift for cavitation-free operation. When making actual pump analyses, be certain
to use the curves (Figs. 1 and 2) in the latest edition of the Standards of the Hydraulic
Institute.

SELECTING THE BEST OPERATING SPEED

FOR A CENTRIFUGAL PUMP

A single-suction centrifugal pump is driven by a 60-Hz ac motor. The pump delivers
10,000 gal/min (630.9 L/s) of water at a 100-ft (30.5-m) head. The available net positive
suction head = 32 ft (9.7 m) of water. What is the best operating speed for this pump if the
pump operates at its best efficiency point?

Centrifugal Mixflo Propeller rotation

Center of

rotation

Center of

Values of specific speed Ns

Below lOOgal/min (6.3 L/s)

Effidency,%