Page 220 - Centrifugal Pumps Design and Application

194 Centrifugal Pumps: Design and Application

Change to a volute collector raises the question of terminology, since
fluid exists via the full impeller periphery and it may be asked whether or
not the description "partial emission" should be abandoned. On balance,
tall radial blade geometry is carried over from the concentric bowl de
sign, so the impeller emission area remains much larger than the dis-
charge throat area. Disorderly flow conditions similar to those in concen-
tric bowl design prevail due both to retention of tall blades and the radial
inlet geometry. The volute modification perhaps is most accurately
viewed as a P.E./F.E. transitional design, but we will choose here to re-
main with the P.E. classification, in part because this modification stems
from the original concentric bowl concept to which the P.E. designation
clearly applies.
Further efficiency improvement is possible through optimization of the
inlet eye diameter. By analytic means, this optimum has been established
as:

Test experience has shown that optimum eye sizing can improve effi-
ciency by about four points over that attainable with large eye diameters
on the order of twice the optimum diameter, which are often used in the
interest of minimizing NPSHR. Equation 11-17 shows that eye size influ-
ences NPSHR as a fourth power function of diameter, so freedom to ex-
ploit the eye size efficiency advantage often does not exist. For inducer-
less design and ample NPSHA, near-optimum eye sizing should always
be used. This situation nearly always exists, for example, in stage 2 of
series-staged machines.
Curve shape improvement and noise reduction have been achieved
through use of high-solidity impellers, i.e., by adding more impeller
blades, which increases the ratio of blade cord length to blade spacing.
High-solidity impellers tend toward minimizing flow stratification and
blade loading because the total power is divided between a larger blade
complement. To obtain benefit with high solidity impellers, it is neces-
sary that all blades penetrate equally into the impeller eye. Use of splitter
geometry or blades alternating in length as is often done in turboma-
chines to avoid eye crowding results in dominance of the larger blades
and provides no advantage in P.E. pumps.
A rising-to-shutoff or stable curve shape can be provided with high so-
lidity radial-bladed impellers as shown by the solid curve in Figure 11
10. This is contrary to the generally held view that backswept blade da-
sign is essential to providing a stable curve. The reason that improved
curve shape results with high-solidity impellers is believed to lie with im-