Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

180 Centrifugal Pumps: Design and Application
paper, question arose as to whether inclusion of the Ui 2 /2g term in the
static head expression recognizing the blade inlet diameter is appropriate.
Low through flow and a strong forced vortex might well combine to extend
rotation to the impeller center!ine, i.e., might introduce prerotation
of inlet flow. Measurements indicating that static pressure at the tip is
close to u 2 2 /2g reinforce this view. Also, inlet prerotation is indicated by
quite respectable suction performance despite radial blade inlet geometry.
Thus, the theoretical head normally used in practice simplifies to
When the subscript is dropped, u is taken to indicate the impeller tip
speed. Actual head for P.E. pumps is then stated as:
To further understand the workings of the RE. pump, a somewhat simplistic
exercise involving a mixture of theory and experience is put forth
to establish how actual head is generated. As has been indicated, tall
blade geometry produces a strong forced vortex resulting in a static head
coefficient near unity, so the actual static head produced by the impeller
is simply u 2 /2g. Tests have shown that diffusion efficiency is nearly flat
through much of the flow range, so we state that *? d = .8. Diffusion recovery
potential is in accordance with the diffuser area ratio
And finally, the P.E. flow coefficient is in the vicinity of 0 = .8. So actual
head generation may be written
Say, then, that the diffuser terminates in two throat diameters, i.e., has an
area ratio of 4, so the actual head should be