Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)
Design of Multi-Stage Casing 69 Figure 6-3. Configurations evaluated during crossover performance study. 7° in the passage between the volute throat and the entrance to the "U" bend. From this point the area was held constant to the impeller eye. To prevent prerotation a splitter was added to the suction channel. Crossovers 2 and 3 were designed maintaining the same areas at sections A, B, and C with the same divergence angle but progressively reducing the radial extent of the crossover. The "U" bend on Crossovers 1 and 2 were cast separately from the casing and highly polished before welding. Crossover 3 was cast as a single piece, and the "U" bend polished only in the accessible areas. The results of testing all three crossover configurations are shown on Figure 6-4. The tests indicated that Crossover 1 yielded a peak efficiency four points higher than Crossover 3. Subsequent testing of commercial units, however, indicated the difference to be only two points. The difference in improvement was attributed to the poor quality of the commercial castings and the use of normal ring clearances. The two-point efficiency loss associated with Crossover 3 was deemed commercially acceptable and was incorporated in multi-stage pumps of up to fourteen stages by all the West Coast manufacturers. These pumps were suitable for higher pressures, easily adaptable to any number of stages, odd or even, and readily castable even in double-volute configurations.
70 Centrifugal Pumps: Design and Application Specific Crossover Designs A successful multi-stage pump development should produce a product that has excellent hydraulic performance, low manufacturing cost, and requires a minimum initial capital investment. These three items become the basic design requirements during the layout of horizontally split multi-stage pumps. Hydraulically, the pump design should achieve the best possible efficiency, as well as the highest head per stage, thereby minimizing the number of stages required. The best available technology should therefore be utilized to produce the most efficient volutes and impellers. Although crossover design has only a secondary effect on pump efficiency, it too should use every available "trick" to achieve the best possible results. Figure 6-5 shows short and long configurations of the two basic types of crossovers normally used on multi-stage pumps. Both have been tested by the West Coast pump companies. Results of these tests indicate that the radial diffusion type is approximately one point more efficient than the diagonal diffusion type. Figure 6-4. Results of crossover performance study.
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<strong>Design</strong> of Multi-Stage Casing 69<br />
Figure 6-3. Configurations evaluat<strong>ed</strong> during crossover performance study.<br />
7° in the passage between the volute throat <strong>and</strong> the entrance to the "U"<br />
bend. From this point the area was held constant to the impeller eye. To<br />
prevent prerotation a splitter was add<strong>ed</strong> to the suction channel. Crossovers<br />
2 <strong>and</strong> 3 were design<strong>ed</strong> maintaining the same areas at sections A, B,<br />
<strong>and</strong> C with the same divergence angle but progressively r<strong>ed</strong>ucing the radial<br />
extent of the crossover. The "U" bend on Crossovers 1 <strong>and</strong> 2 were<br />
cast separately from the casing <strong>and</strong> highly polish<strong>ed</strong> before welding.<br />
Crossover 3 was cast as a single piece, <strong>and</strong> the "U" bend polish<strong>ed</strong> only in<br />
the accessible areas.<br />
The results of testing all three crossover configurations are shown on<br />
Figure 6-4. The tests indicat<strong>ed</strong> that Crossover 1 yield<strong>ed</strong> a peak efficiency<br />
four points higher than Crossover 3. Subsequent testing of commercial<br />
units, however, indicat<strong>ed</strong> the difference to be only two points. The difference<br />
in improvement was attribut<strong>ed</strong> to the poor quality of the commercial<br />
castings <strong>and</strong> the use of normal ring clearances. The two-point efficiency<br />
loss associat<strong>ed</strong> with Crossover 3 was deem<strong>ed</strong> commercially acceptable<br />
<strong>and</strong> was incorporat<strong>ed</strong> in multi-stage pumps of up to fourteen stages by all<br />
the West Coast manufacturers. These pumps were suitable for higher<br />
pressures, easily adaptable to any number of stages, odd or even, <strong>and</strong><br />
readily castable even in double-volute configurations.