CAVITATION EROSION IN CENTRIFUGAL PUMPS

In addition to furthering basic research at the Swiss Federal Institute of Technology, Lausanne, Sulzer has been systematically evaluating experiences with actual installations in order to develop correlations of material loss from cavitation. Our aim was the development of correlations that include all of the essential parameters that affect loss of material. A correlation has been in use since 1981 in order to determine the inlet head required for high-pressure pumps. The results obtained so far have been good. Our studies were significantly broadened by systematic investigations carried out as part of a research contract from the Electric Power Research Institute (EPRI). Methods for calculating the rate of material loss were developed in which the cavitation intensity is quantified by the cavitation noise or the cavity length. Only investigations involving impellers of sufficient size that are made from cast steel and have a high circumferential speed can be considered as a means of obtaining representative results. The rates of material loss caused by cavitation erosion were investigated using cast steel impellers with an external diameter of 275 mm. The materials and their tensile strengths were as follows: GS-C25, 480 N/mm2; G-X5 CrNiMo 134, 900 N/mm2. The influence of the flow rate was studied at 50, 82, and 100% of delivery at maximum efficiency speed, 5,000-9,030 rev./min; available net positive suction head (NPSH), 50-210 m; cavitation coefficient (σu1), 0.35-0.7; air content, 1.5-29 ppm. In all, 11 erosion investigations were carried out, and the results were used to determine the correlations previously mentioned. Recent work has shown that the onset of cavitation erosion can be determined by calculating the implosion pressure from the measured cavitation noise. This can be accomplished using the laws governing noise radiation in the inlet chamber of a pump. When the implosion pressure exceeds the fatigue strength of the material, the erosion threshold has been reached.