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The most common and troublesome maintenance problem encountered with hydraulic turbines is the erosion caused by cavitation. A study of the cavitation phenomenon has demonstrated that the pressure produced by the collapse of cavitation bubbles is sufficient to cause mechanical erosion of all known materials. However, the resistance to cavitation damage varies with the type of material. This variation can be most readily determined by tests with accelerated cavitation apparatus whereby measurable erosion can be obtained within an hour or two on the most highly resistant material.
Such tests, together with 35 years of field experience with hydraulic machinery, indicate that the most highly resistant materials we know today are stellite, welded stainless steels, welded aluminum bronzes, and stainless and aluminum bronze rolled plates and castings, in that order.
Plastics, while easy to apply for repair, stand up only under conditions of light cavitation.
Recent research in cavitation phenomena has been largely devoted to the effect of corrosion, cathodic protection, and velocity on cavitation damage, and the scale effect on cavitation inception. The most startling discovery has been that the intensity of cavitation damage increases as about the 6th power of the local velocity once a cavity is formed.
In conclusion, we find that the hydraulic turbine industry is interested in both the continuing search for economical materials with high resistance to cavitation damage and research on the effect of corrosion, cathodic protection, scaling, and velocity on cavitation damage.
Rheingans, William J.
Assistant General Manager, Hydraulic Division, Allis-Chalmers Manufacturing Co., York, Pa.