Page 212 - Centrifugal Pumps Design and Application

186 Centrifugal Pumps: Design and Application

Suction specific speed ranges typically from S s = 7,500 to 10,000 for
P.E, pumps, which is in about the same range as for higher specific speed
pumps of single suction, overhung impeller design. High S s values trans-
late into low NPSHR, or good suction performance. The range men-
tioned varies with flow rate where high S s values are associated with low
flow rates.
Solving the specific speed equation for NPSHR yields a suction expres-
sion in terms of speed and flow, which can alternately be converted into
terms of head and specific speed:

Contrary to what might be expected, the inlet radial blade geometry used
in P.E. pumps achieves suction specific speed parity with the higher spe-
cific speed pumps utilizing more sophisticated inlet shapes. It is appar-
ent, however, that high speed pumps will be demanding from the stand-
point of NPSHR. The bracketed term in the latter expression is known as
the Thoma cavitation parameter, usually designated by sigma:

The Thoma parameter, then, states NPSHR as a fraction of pump head
and is a function of the ratio of specific speed to suction specific speed.
Low specific speed thus offsets to a degree the higher NPSHR associated
with high speeds.
The inlet eye size in the prior expressions is assumed to be generously
sized, as is generally done so that only small NPSHR impact exists. The
NPSHR expression expanded to include inlet eye size effect becomes:

Inlet eye size has been found to have an influence on the efficiency po-
tential of the pump, which as we have just seen, affects NPSHR. Avail-
ability of efficiency advantage via eye sizing then in reality hinges on
NPSHA in the application. More will be said on this subject in the section
"Partial Emission Design Evolution."