Page 213 - Centrifugal Pumps Design and Application

High Speed Pumps 187

As an aside, pump users should be aware that overly conservative
statements of NPSHA in an application can work to their disadvantage.
The pump manufacturer must meet the stated NPSHA, so understated
suction conditions can force the design toward lower speed or more and
larger stages, which can result in an efficiency penalty or higher initial
cost.

Inducers

Need to improve suction performance becomes quickly apparent in the
move toward exploitation of high speed advantage. Inducer development
began more than 50 years ago to provide this improvement. An inducer is
basically a high specific speed, axial flow, pumping device roughly in the
range of N s = 4,000 to 9,000 that is series mounted preceding a radial
stage to provide overall system suction advantage. Inducers are charac-
terized by relatively few blades, shallow inlet blade angles, and generally
sophisticated hydraulic design.
The inducer must put up enough head to satisfy the needs of the radial
impeller stage but in itself has a suction level requirement that establishes
a new lower NPSHR for the system. Inducers are an important element
in high speed pump design, and so have been and continue to be the sub-
ject of considerable interest and developmental work. Inducer design
should be such that maximum suction performance is achieved, and such
that cavitation erosion in the inducer itself is avoided in long-term opera-
tion.
Inducer performance is generally taken as the suction specific speed
which corresponds to 3% pump head depression as NPSH is decreased.
Theory exists establishing optimum suction performance in an expres-
sion known as the Brumfield criterion. A form of the Brumfield criterion
developed in a comprehensive document on inducer design developed by
NASA is as follows:

Where 4> is the inlet flow coefficient or the ratio of meridianal flow ve-
locity to inducer tip speed:

A plot of the Brumfield criterion is shown in Figure 11-4. It should be
emphasized that this expression is theoretical but tempered by practical