Page 214 - Centrifugal Pumps 2E

192 Centrifugal Pumps: Design and Application

Figure 11-8. Diagram of concentric bowl static pressures and flows.

to a basic hydraulic "fault" in the concentric bowl pump exists in the eye-
to-throat flow path described in the section "Terminology." The sketch in
Figure 11-8 illustrates a concentric bowl pump, where the dashed line
represents a polar plot of static pressure within the bowl. The static pres-
sure is depressed in the vicinity of pump discharge, and this depression
increases with increasing flow rate. This indicates unfavorable exchange
of static head for velocity head in the area of discharge, which must be
reconverted to static head in the diffuser. Furthermore, fluid approaches
the discharge throat in the direction indicated by vector c, the vector sum
of U2 and v r, detracting from the diffuser recovery potential.
These adversities can be eliminated by abandoning the concentric bowl
geometry in favor of a volute collector geometry. This modification has
been shown to improve efficiency by about 6 points with specific speeds
in the range of N s = 800 to 1,000, but this advantage fades to parity with
the concentric bowl configuration at specific speeds of about N s = 300
to 400.
Radial side load results from standing pressure variations around the
impeller periphery. These hydraulically imposed radial loads are propor-
tional to the product of pump head times the projected area of the impel-
ler, and must be reckoned with from the standpoint of bearing loads. Side
load trends vary dramatically with the pump design geometry as indi-
cated in Figure 11-9, where magnitudes are shown in the upper plot and
vector direction trends are shown by the polar plots within the lower fig-