Page 80 - Centrifugal Pumps Design and Application

Volute Design S3

Manufacturing Considerations

Casings, particularly of the double-volute design, are very difficult to
cast. In small- and medium-sized pumps the volute areas are small and
the liquid passages are long, requiring long unsupported cores. In volute-
type multi-stage pumps the problem is more pronounced since there are
several complicated cores in a single casing.

Casing Surface Finish
To minimize friction losses in the casing, the liquid passages should be
as clean as possible. Since cleaning pump casings is both difficult and
time consuming, an extreme effort to produce smooth liquid passages
should be made at the foundry. The use of special sand for cores, ceramic
cores, or any other means of producing a smooth casting should be stan-
dard foundry practice for producing casings.
Particularly with multi-stage pumps, however, even the best foundry
efforts should be supplemented by some hand polishing at points of high
liquid velocities such as the volute surfaces surrounding the impeller and
the area around the volute throat. Both of these areas are generally acces-
sible for hand polishing. In addition, both cutwater tongues should be
sharpened and made equidistant from the horizontal centerline of each
stage. The same distance should be maintained for each stage in a multi-
stage pump.

Casing Shrinkage

Dimensional irregularities in pump casings due to shrinkage variations
or core shifts are quite common. Shrinkage variations can even occur in
castings made of the same material and using the same pattern. The ac-
ceptance or rejection of these defects should be based upon engineering
judgments. However, knowing that shrinkage and core shifts are quite
common, the designer should allow sufficient space for rotating element
end float. The allowance for total end float should be a minimum of one-
half inch.

Conclusion

Although it is often claimed that casings are very efficient, this is mis-
leading, since the hydraulic and friction losses that occur in the casing
can only reduce the total pump output and never add to it. It is the design-
er's responsibility to do his utmost to minimize these losses.