Page 69 - Centrifugal Pumps Design and Application

52 Centrifugal Pumps: Design and Application

Types of Volute Designs

There are several different volute designs being manufactured today.

Single-Volute Casing Designs

Single-volute pumps have been in existence from Day thie. Pumps de-
signed using single-volute casings of constant velocity design are more
efficient than those using more complicated volute designs. They are also
less difficult to cast and more economical to produce because of the open
areas around impeller periphery. Theoretically they can be used on large
as well as small pumps of all specific speeds. Stepanoff gives a complete
description of single-volute casing design.
In all volute pumps the pressure distribution around the periphery of
the impeller is uniform only at the BEP. This pressure equilibrium is de-
stroyed when the pump is operating on either side of the BEP, resulting in
a radial load on the impeller. This load deflects the pump shaft and can
result in excessive wear at the wearing rings, seals, packing, or bearings,
In extreme cases, shaft breakage due to fatigue failure can result. The
magnitude of this radial load is given by:

P = KHD 2B 2 spgr/2.31

Values of the experimental constant K are given in Figure 5-2. For a
specific single-volute pump it reaches its maximum at shutoff and will
vary between 0.09 and 0.38 depending upon specific speed. The effect of
the force will be most pronounced on a single-stage pump with a wide b 2
or a large-sized pump.
It is safe to say that with existing design techniques, single-volute de-
signs are used mainly on low capacity, low specific speed pumps or
pumps for special applications such as variable slurries or solids han-
dling.

Double-Volute Casing Designs

A double-volute casing design is actually two single-volute designs
combined in an opposed arrangement. The total throat area of the two
volutes is identical to that which would be used on a comparable single-
volute design.
Double-volute casings were introduced to eliminate the radial thrust
problems that are inherent in single-volute designs. Test measurements,
however, indicate that while the radial forces in a double volute are
greatly reduced, they are not completely eliminated. This is because al~