Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)
General Pump Design 49 designed and test checked. The other sizes that follow specific speed lines can be factored up and their performance accurately predicted, designed and test checked. The other sizes that follow specific speed lines can be factored up as described in Chapter 2 and their performance accurately predicted. After the hydraulic performance range chart is complete, the designer should check the mechanical requirements as outlined by the applicable industrial specification. If a complete line is being designed, the following mechanical features should be checked. • Shaft sizes • Bearing arrangements • Stuffing box design • Bolting for maximum pressures • Suction pressures • Pump axial and radial balance • Bed plates, motor supports, etc. • Gasketing • Lubrication The standardization and the use of existing parts should be considered at this time; however hydraulic performance should never be sacrificed for mechanical or cost reasons. If sacrifice becomes necessary, adjust pump hydraulics accordingly. Mechanical design features will be covered in other chapters.
5 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 important parameter in pump casing design. This velocity is, of course, different from the average liquid velocity in the volute sections, which is the primary casing design parameter. The relationship between these two velocities 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 velocities. 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 conversion of kinetic energy. This was indicated on the Grand Coulee model pump where the highest pressure was read seven pipe diameters from the discharge flange. 50
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5<br />
Volute <strong>Design</strong><br />
The object of the volute is to convert the kinetic energy impart<strong>ed</strong> to the<br />
liquid by the impeller into pressure. The pump casing has no part in the<br />
(dynamic) generation of the total head <strong>and</strong> therefore deals only with<br />
minimizing losses.<br />
The absolute velocity of the liquid at the impeller discharge is an important<br />
parameter in pump casing design. This velocity is, of course, different<br />
from the average liquid velocity in the volute sections, which is the<br />
primary casing design parameter. The relationship between these two velocities<br />
is given indirectly in Figure 5-1. This relationship is given in a<br />
slightly different form in Figure 3-7.<br />
Volutes, like all pump elements, are design<strong>ed</strong> bas<strong>ed</strong> on average velocities.<br />
The average velocity is, of course, that velocity obtain<strong>ed</strong> by dividing<br />
the flow by the total area normal to that flow. <strong>Design</strong>s are usually bas<strong>ed</strong><br />
only on the desir<strong>ed</strong> BEP flow, <strong>and</strong> the performance over the rest of the<br />
head-capacity is merely estimat<strong>ed</strong>. The results of many tests in which the<br />
pressure distribution within the volute casing was measur<strong>ed</strong> indicate that:<br />
1, The best volutes are the constant-velocity design.<br />
2. Kinetic energy is convert<strong>ed</strong> into pressure only in the diffusion<br />
chamber imm<strong>ed</strong>iately after the volute throat.<br />
3, The most efficient pumps use diffusion chambers with a total<br />
divergence angle between 7 <strong>and</strong> 13 degrees.<br />
4. Even the best discharge nozzle design does not complete the conversion<br />
of kinetic energy. This was indicat<strong>ed</strong> on the Gr<strong>and</strong> Coulee<br />
model pump where the highest pressure was read seven pipe diameters<br />
from the discharge flange.<br />
50