Page 90 - Centrifugal Pumps Design and Application

72 Centrifugal Pumps: Design and Application

To attaie the best hydraulic performance, anti-rotation splitters must be
added to the suction channel at the impeller eye. The best overall results
are obtained by placing two splitters at the casing parting split as shown
In Figure 6-5.
The long crossover is identical to the short or series configuration yp to
the area "B," where the long channel that traverses the pump begins,
This long channel should be designed with a "window" at the top for
cleaning and a properly shaped plate matched to the crossover opening
before welding. The configuration of the long crossover is also shown in
Figure 6-5.

Crossovers with Diagonal Diffusion Sections
The diagonal diffusion type crossover shown in Figure 6-5 leads the
liquid from the volute throat to the suction of the next impeller while
traveling diagonally around the periphery of the volute. This design has
one long radius turn as compared to the "U" bend used in the radial dif-
fusion type. Other than these differences, both types of crossover have
the same diffusion, area progression etc. Figure 6-5 also shows the con-
figuration in which the long crossover channel climbs over the short
crossover.
Even though this crossover has only a single long-radius turn, it is not
as efficient as the radial type. This can be attributed to the diagonally
located channel, which imparts a spiral motion to the fluid leaving the
volute throat, resulting in hydraulic losses larger than those in the "U"
bend.

Mechanical Suggestions

In previous chapters, we have described design procedures for centrif-
ugal pump impellers and volutes applicable to one-stage or multi-stage
units and in this chapter crossovers for multi-stage pumps only. However,
hydraulic considerations alone for multi-stage pumps are not sufficient to
complete a final unit. Mechanical details must be considered. This refers
to patterns, foundry methods, mechanical bolting, and quality controls,

Patterns

Multi-stage casings have quite complex shapes of liquid passages, in-
cluding crossovers, crossunders, double volutes at each stage, etc. For
this reason, pattern equipment must be of high quality sectional design to
allow for variations in number of stages. Normal practice is to make the
first pattern a four stage of hard wood with each stage being a separate