Page 75 - Centrifugal Pumps Design and Application

58 Centrifugal Pumps: Design and Application

3. All volute areas should be designed to provide a smooth change of
areas.
4. Circular volutes should be considered for pumps below a specific
speed of 600. Circular volutes should not be considered for multi-
stage pumps.
5. The total divergence angle of the diffusion chamber should be be
tween 7 and 13 degrees. The final kinetic energy conversion is ob-
tained in the discharge nozzle in a single-stage pump and in both the
discharge nozzle and crossover in a multi-stage pump.
6. In designing a volute, be liberal with the space surrounding the im-
peller. In multi-stage pumps in particular, enough space should be
provided between the volute walls and the impeller shroud to allow
one-half inch each way for end float and casting variations, A vo-
lute that is tight in this area will create axial thrust and manufactur-
ing problems.

The Use of Universal Volute Sections for Standard Volute Designs

It has been noted that when the volute sections of different pumps are
factored to the same throat area; their contours are almost identical. Any
differences that do exist can be traced to mechanical considerations or the
designer's whim, rather than any important principle of hydraulic design.
Similarly, factoring the impeller width and the radial gap between the im-
peller and the cutwater reveals that the values of these parameters also lie
in a very narrow random range.
In other words, the entire discharge portion of the pump casing when
viewed in cross section and factored to a common throat area has only
minor variations throughout the entire specific speed spectrum. This fact
enables us to eliminate the usual trial-and-error method of designing vo-
lute sections while still consistently producing casings to a high standard
of hydraulic design. To facilitate this process we have prepared a set of
"universal" volute drawings on which the typical volute sections de-
scribed above have been laid out for a 10 sq in. throat area. Once the
designer has chosen his throat area, he can quickly produce the required
volute sections by factoring the sections shown for the "universal" vo-
lute. Sections for a single-volute pump are shown in Figures 5-5 and
5-6, and sections for a double volute pump are shown in Figure 5-7.

The Design of Rectangular Double Volutes
For low capacity (500-600 GPM) or low to medium specific speed
(N s < 1,100) pumps, a rectangular volute design should be considered.