Page 98 - Centrifugal Pumps Design and Application

Double-Suction Pumps 79

two. Experience shows that this procedure will give misleading design
factors and unfavorable test results.
Design of the suction approach to the pump impeller for double-suction
pumps will differ from the single-suction design. This will be covered in
detail in the following paragraphs.

Double-Suction Impeller

The method for calculating impeller diameter, impeller width, number
of vanes, and vane angularity is identical to the procedure for the single-
suction impeller described in Chapter 3. The method for impeller layout
will also follow Chapter 3, with a double-suction impeller being consid-
ered two single-suction impellers back to back. With double entry the eye
area is greater and the inlet velocity lower, thus reducing NPSHR.

Side Suction and Suction Nozzle Layout

The importance of hydraulic excellence in the design of liquid passage
areas from suction nozzle to the impeller eye or eyes is quite often
minimized or unfavorably adjusted for economic reasons. Experience
shows that this approach leads to many field NPSH problems. The cur-
rent trend in industry is one of reducing NPSHA; therefore, it is essential
for optimum NPSHR that the design of the suction approach to the impel-
ler eye be carefully controlled.
We know from experience that in the design of the side suction inlet a
certain amount of prerotation of the incoming liquid is desirable. To ob-
tain this condition, the baffle (or splitter) is provided. This splitter is ro-
tated 30° to 45 ° from suction centerline in the direction of pump rotation.
The splitter will locate the radial section of zero flow, and the areas will
progressively increase in both directions away from it.
The following drawings and information must be available to design a
side suction.

1. Volute layout.
2. Impeller layout.
3. Shaft or sleeve diameter at the impeller.
4. Suction nozzle size.

Layout of the laterally displaced side suction should be done in two
parts:

1. Sketch an approximate end view and profile (Figures 7-2 and 7-3)
using the following guidelines: