Radial versus Diagonal Diffusion in Multi-Stage Pumps

radialanddiagonaldiffusion
Radial and Diagonal Diffusion

Although crossover design has only a secondary effect on pump efficiency, it too should use every available trick to achieve the best possible results.

This picture (left and below) 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 and the results of these tests indicate that the radial diffusion type is approximately one point more efficient than the diagonal diffusion type. Here’s why:

Radial: The radial diffusion type crossover shown has a diffusion section that follows the volute periphery along the impeller center line, 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.  History shows that it was preferable (by some designers) to decelerate slightly at the impeller eye but recent tests indicate that better efficiency is obtained if the liquid is accelerated as it approaches the impeller eye.

This type of crossover indicates 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.

radialanddiagonaldiffusion

Diagonal: The diagonal diffusion type crossover shown leads the liquid from the volute throat to the suction of the next impeller while traveling diagonally around the periphery of the volute. This design has one long radius turn as compared to the “U” bend used in the radial diffusion type. It also shows the configuration in which the long crossover channel climbs over the short crossover. Even though this crossover has only a single long-radius turn, it is not as efficient as the radial type. This can be attributed to the diagonally located channel, which imparts a spiral motion to the fluid leaving the volute throat resulting in hydraulic losses larger than those in the “U” bend.

Other than these differences, both types of crossover have the same diffusion and the same area progression.

Leave a Reply