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Wednesday, August 25, 2010

Reduction of Cavitation Damage

After the pump has been built and installed, there is little that can be done to reduce cavitation damage. As previously mentioned, sharpening the leading edges of the blades by filling may be beneficial Stepanoff has suggested cutting back part of the blades in the impeller eye together with sharpening the tips, for low specific speed pumps, as a means of reducing the inlet velocity c, and thus lowering parameter s. Although a small amount of prerotation or prewhirtl in the direction of impeller rotation may be desirable, excessive amount should be avoided. This may require straightening vanes ahead of the impeller and rearranging the suction piping to avoid changes in direction or other obstructions. The cavitation damage to the impeller was believed to have been at least partly due to bad flow conditions produced by two 90o elbows in the suction piping. The planes of the elbows were at 90o to each other, and the arrangement should be avoided.

Straitening vanes in the impeller inlet may increase the NPSH requirement at all flow rates. Three or four radial ribs equally spaced around the inlet and extending inward about one quarter of the inlet diameter are effective against excessive prerotation and may require less NPSH then full length vanes. This is very important with axial flow pumps, which are apt to have unfavourable cavitation characteristic at partial flow rates. Operation near the best efficiency point usually minimizes cavitation.

The admission of a small amount of air into the pump suction tends to reduce cavitation noise. This rarely is done, however because it is difficult to inject the right amount to mixing air with the liquid pumped.

If new impeller is required because of cavitation, the design should take into account the most recent advances describe in the literature has suggested:
  1. The use of ample fillets where the vanes join the shrouds
  2. Sharpened leading edges of vanes
  3. reduction of b, in the immediate vicinity of the shrouds
  4. raking the leading edges of the vanes forward out the eye. Increasing the number of vanes for propeller pumps lower s for a given submergence.