Page 70 - Centrifugal Pumps 2E

56 Centrifugal Pumps: Design and Application

Circular-Volute Casings

Several pump manufacturers have conducted tests to evaluate the hy-
draulic performance of pumps with circular volutes. A study of the re-
sults of these tests reveals that circular volutes improve the hydraulic per-
formance of small high head or low specific speed units and impair the
performance of high specific speed pumps. Specifically, pump efficiency
is improved below specific speeds of 600. For specific speeds above 600
the efficiency of circular-volute designs will be 95% of that possible with
conventional volute designs. This can be explained by remembering that
in a conventional volute a uniform pressure and velocity distribution ex-
ists around the impeller periphery only at the BEP. At off-peak capaci-
ties, velocities and pressures are not uniform. For circular volutes the
opposite is true. Uniform velocity and pressure exist only at zero flow.
This uniformity is progressively destroyed as the capacity is increased,
Therefore, at BEP the casing losses are generally greater than those of
the conventional volute. For low specific speed pumps, however, there is
some gain in efficiency due to a circular volute since the benefits of the
Improved surface finish in the machined volute outweigh the problems
created by the nonuniform pressure distribution. A comparison of the ef-
ficiency of circular and conventional volutes is shown in Figure 5-4. The
use of a circular volute design should be considered in the following in-
stances:

• For small, high head, low specific speed (N s 500-600) pumps.
» For a pump casing that must accommodate several impeller sizes.
• For a pump in which foundry limitations have dictated an overly wide
impeller b 2.
» For a pump that must use a fabricated casing.
« For a pump that requires that the volute passage be machined in the
case casting.

General Design Considerations

It was pointed out previously that the casing itself represents only
losses and does not add anything to the total energy developed by the
pump. In designing pump casings it is therefore important to utilize all
available means of minimizing casing losses. However, commercial con-
siderations dictate some deviations from this approach, and experience