Page 82 - Centrifugal Pumps Design and Application

6

Design of

Multi-Stage

Casing

Multi-stage casing discussed in this chapter will be related to one type
of pump, namely, a horizontally split, opposed impellers, double-volute
design, similar to one shown in Figure 6-1. This type of pump offers the
following features:

• Pump casings, when properly bolted, are suitable for high working
pressures. Many pumps in service are operating at 3,000 to 4,000 psi
discharge pressure.
• The pumps are axially balanced allowing the use of standard ball bear-
ings for many services. Only when shaft diameter is too big or rotating
speed too high, will Kingsbury-type bearings be required.
• When NPSHA is low, double-suction first-stage impellers are available
as a part of standard design.
• A completely assembled rotating element can be checked for shaft and
ring run-out over the full length of the shaft.
• The most modern designs have split impeller hub case rings, horizon-
tally split, allowing the assembled rotating element to be dynamically
balanced.
• The pumps are easily designed for high pressure and speeds above
7,000 rpm.
• Removal of the top half of the pump casing exposes the complete rotat-
ing element for inspection or repair.

The impeller design for the multi-stage pump is the same as that for a
one-stage unit, as described in Chapter 3. The double-volute design is
also the same as that for a one-stage pump as described in Chapter 5,

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