Page 89 - Centrifugal Pumps Design and Application

Design of Multi-Stage Casing 71

Figure 6-5. Radial and diagonal diffusion crossovers.

Crossovers with Radial Diffusion Sections

The radial diffusion type crossover shown in Figure 6-5 has a diffusion
section that follows the volute periphery along the impeller centerline,
diffusing with a total divergence angle of 7° up to the point where area at
"B" is four times the volute throat area. This point should be reached
before the "U" bend to the suction channel. The suction channel should
be sized to accommodate the largest capacity impeller that will be used in
the pump. The area of the suction return channel should be consistent
immediately after the "U" bend. Some designers prefer to decelerate
slightly at the impeller eye; however, recent tests indicate that better effi-
ciency is obtained if the liquid is accelerated as it approaches the impeller
eye.
Tests on this type of crossover indicate a total head loss equal to 86% of
the inlet velocity head. The addition of a welded splitter in the "U" bend
will reduce this loss to 65 %. However, the cost of adding this splitter is
generally prohibitive, and it is not generally used.
From a theoretical viewpoint, crossover channels should have a circu-
lar cross section to minimize friction losses. However, all the designs on
the market today have rectangular shapes for practical reasons.