Page 67 - Centrifugal Pumps Design and Application

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Volute Design

The object of the volute is to convert the kinetic energy imparted to the
liquid by the impeller into pressure. The pump casing has no part in the
(dynamic) generation of the total head and therefore deals only with
minimizing losses.
The absolute velocity of the liquid at the impeller discharge is an im-
portant parameter in pump casing design. This velocity is, of course, dif-
ferent from the average liquid velocity in the volute sections, which is the
primary casing design parameter. The relationship between these two ve-
locities is given indirectly in Figure 5-1. This relationship is given in a
slightly different form in Figure 3-7.
Volutes, like all pump elements, are designed based on average veloci-
ties. The average velocity is, of course, that velocity obtained by dividing
the flow by the total area normal to that flow. Designs are usually based
only on the desired BEP flow, and the performance over the rest of the
head-capacity is merely estimated. The results of many tests in which the
pressure distribution within the volute casing was measured indicate that:

1, The best volutes are the constant-velocity design.
2. Kinetic energy is converted into pressure only in the diffusion
chamber immediately after the volute throat.
3, The most efficient pumps use diffusion chambers with a total
divergence angle between 7 and 13 degrees.
4. Even the best discharge nozzle design does not complete the con-
version of kinetic energy. This was indicated on the Grand Coulee
model pump where the highest pressure was read seven pipe diame-
ters from the discharge flange.

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