Page 85 - Centrifugal Pumps Design and Application

Design of Multi-Stage Casing 67

However, in a multi-stage casing, the liquid from one stage to the next
stage must be transferred by means of a crossover passage. The term
"crossover" refers to the channel leading from the volute throat of one
stage to the suction of the next. Crossovers leading from one stage to the
next are normally referred to as "short" crossovers and are similar to
return channels in diffuser pumps. These are normally designed in right
hand or left hand configurations, depending upon the stage arrangement,
Crossovers that lead from one end of the pump to the other or from the
center of the pump to the end are normally referred to as "long" cross-
overs.
The stage arrangements used by various pump manufacturers are
shown schematically in Figure 6-2. Arrangement 1 minimizes the num-
ber of separate patterns required and results in a minimum capital invest-
ment and low manufacturing costs. However, with this arrangement a
balancing drum is required to reduce axial thrust. Arrangement 2 is used
on barrel pumps with horizontally split inner volute casings. Arrange-
ment 3 is the most popular arrangement for horizontally split multi-stage
pumps and is used by many manufacturers. Finally, with Arrangement 4
the series stages have double volutes while the two center stages have
staggered volutes. This design achieves a balanced radial load and an ef-
ficient final discharge while requiring only one "long" crossover,
thereby reducing pattern costs and casing weight.

General Considerations in Crossover Design

The principal functions of a crossover are as follows:
• To convert the velocity head at the volute throat into pressure as soon as
possible, thereby minimizing the overall pressure losses in the cross-
over.
• To turn the flow 180° from the exit of one stage into the suction of the
next.
• To deliver a uniformly distributed flow to the eye of the succeeding
impeller.
• To accomplish all these functions with minimum losses at minimum
cost.

Velocity cannot be efficiently converted into pressure if diffusion and
turning are attempted simultaneously, since turning will produce higher
velocities at the outer walls adversely affecting the diffusion process.
Furthermore, a crossover channel that runs diagonally from the volute