Page 204 - Centrifugal Pumps Design and Application

178 Centrifugal Pumps: Design and Application

flow. Very high suction pressures move the cutoff point to higher flow
rates.
Figure 11-2B shows the typical head coefficient and flow coefficient
characteristics of Barske, which is simply a reflection of the head-flow
curve. The tall radial blade impeller geometry produces relatively high
head coefficients typically in the range of ^ = .7 to .75.
The Barske pump does not require close operating clearances to pro-
vide good performance. Open impellers present a leakage path from the
impeller tip back to the eye through the impeller front side clearance, but
only low sensitivity to this clearance has been found. Clearances nor-
mally used in commercial pump sizes range typically from .03 to .05 in.,
which simplifies manufacture and maintenance. The open impeller con-
cept frees the pump from performance decay which can occur with wear
ring construction when erosion or rubbing contact increases the ring
clearance.
Hardware is physically small and geometrically simple allowing pro-
duction by straightforward machining operations. Surface finishes typi-
cal of ordinary shop practice are adequate to avoid excessive losses,
which would be likely to exist with relatively rough cast surfaces. Very
little or no benefit is available through polishing the case surfaces. Impel-
lers are usually made from castings which are trimmed to match the case
geometry. Surface finish on the impellers is unimportant due to the use of
tall impeller blades, which results in low radial flow velocities.

Terminology

The Barske pump design deviates from that of higher specific speed
designs, which are generally referred to as fall emission (RE.) radial or
Francis types. Francis-type pumps are generally suited for relatively high
flow rates and moderate head rise, and meet these objectives with the
highest attainable efficiency of any centrifugal pump type. Full emission
designs almost universally use backswept impellers configured accord-
ing to refined hydraulic practices so as to provide constant meridianal
velocity, to avoid design-point flow separation, to avoid incidence losses
and so forth. These designs are characterized by flow which exits uni-
formly through the full impeller periphery, hence the description: full
emission. But these design procedures become less beneficial with high
stage head and low flow design objectives, i.e., in low specific speed
designs. This occurs because flow passages are being decreased in size
simultaneously with increasing impeller diameter, with an attendant dis-
proportionate increase in friction losses and lowered efficiency.
It has been established through experience that high-flow machines can
be made to work relatively well at low flow rates by simply plugging