Cavitation Erosion

Cavitation erosion is primarily a mechanical process, although it acts synergistically with corrosion and is often considered with other forms of corrosion. Cavitation erosion can be defined as metal removal from the surface caused by high stresses associated with the collapse of vapor bubbles in the fluid. Cavitation occurs in a centrifugal pump when the local pressure of the fluid is reduced to the vapor pressure. In a multistage pump, vapor bubbles form in the low-pressure areas at the impeller inlet and are swept by the flow into regions of higher pressure where they collapse. A great many bubbles may form and collapse in a small area, producing many microjets of high kinetic energy. The energy released by the bubble collapse is expended as impact loading on the
metal surface. This situation is aggravated if protective oxide films are present because these are damaged, exposing fresh metal to the corrosive action of the fluid. This cyclic loading eventually causes the formation of microscopic fatigue cracks. These cracks propagate and intersect, resulting in the removal of metal from the surface and the characteristic spongy or porous appearance of cavitation damage.


Although every effort should be made in the design and application of centrifugal pumps to prevent cavitation, it is not always possible to do so at capacities less than the rated maximum efficiency capacity of the slurry pump. It must be recognized that at a low flow operation, the stated NPSH required curve is not usually sufficient to suppress all cavitation damage. The stated NPSH required is that needed to produce the head, capacity, and efficiency shown on the rating curve. At low flows, some cavitation damage should be expected. It may be impractical to supply an NPSH that would suppress all cavitation at
these low flows, as it could be many times that it is required at the best efficiency point. Therefore, the possibility of cavitation damage frequently becomes a consideration when selecting material for centrifugal pump impellers.