Page 214 - Centrifugal Pumps Design and Application

188 Centrifugal Pumps: Design and Application

Figure 11-4. Brumfield performance criterion,

design considerations and that many details of inducer design are not ad-
dressed. The angle /3 is the fluid angle at inlet and differs from the blade
angle, ft, by a positive incidence angle, a. Prerotation is assumed to be
zero, and other considerations such as blade shape, blockage, hub geom-
etry, and leakage are simply ignored. The intent here is primarily to show
the fundamental influence of the inlet blade angle on suction perfor-
mance potential.
Optimal inducer design is distinctly a high-tech endeavor which must
conform to hard-earned design guidelines and hydraulic disciplines. A
well-designed inducer should possess a "sharp" breakdown characteris-
tic as illustrated by the solid curve in Figure 11-5, rather than the "gen-
tle" curve shown in broken line. The NPSHR disadvantage with gentle
breakdown is evidenced by the NPSHR differential which exists at the
3% head depression level. It should not go unnoticed that the 3% head
depression level refers to the inducer-pump combination, so the level of
the 3% line on the inducer headrise curve will vary according to the head
of the pump to which the inducer is coupled. Lower head units will suffer
an NPSHR disadvantage with a gentle breakdown inducer compared to
high head units equipped with the same inducer.