Page 126 - Centrifugal Pumps 2E

NPSH 107

Figure 8-24. NPSHR cavitation free compared with NPSHR Hydraulic institute.

values. Above 2,000 SSU, it is necessary to use the pump designer's
judgment or experimentally determine the cavitation characteristics.
Figures 8-24 through 8-26 compare our cavitation-free NPSHR with
NPSHR as permitted by the Hydraulic Institute. The Hydraulic Institute
values shown were established by test and cavitation-free values calcu-
lated. Of 98 pump tests reviewed, 60 were selected for compilation of
data. Criterion for selection was a witnessed suppression test conducted
in accordance with the standards of the Hydraulic Institute. To reduce
scatter, test data were confined to capacities limiting a to a maximum of
l
6 /2°. The pumps used were limited to a maximum impeller blade periph-
eral velocity of 120 ft/sec. At higher velocities, the difference between
cavitation-free and Hydraulic Institute will be more evident. Suction spe-
cific speed values lower than those shown in Figure 8-26 can be ex-
pected. General details of the pumps are:
• Number of pumps—50
• Number of tests—60
• Pump size (discharge nozzle)—! in. to 30 in.
• Impeller diameter—7 in. to 37 in.
• Number of stages—One
• Impeller entry—Single suction: 26 pumps. Double suction: 24 pumps.
• Specific speed range—500 to 4,750