What are the critical differences in cavitation erosion patterns between a Francis turbine runner and a Pelton turbine bucket?

Cavitation erosion occurs when vapor bubbles form and collapse rapidly near a metal surface, causing localized high-pressure impacts that damage the material. The patterns of this erosion differ significantly between Francis turbine runners and Pelton turbine buckets due to their distinct operating principles. In Francis turbines, cavitation typically occurs on the low-pressure side of the runner blades, near the inlet edge and on the back side of the blades. This is because the pressure drops significantly as water accelerates through the curved passages of the runner. The erosion pattern is often characterized by a rough, pitted surface, sometimes appearing sponge-like, concentrated in specific zones where the pressure is lowest. The exact location varies with the turbine's operating point and blade geometry. In Pelton turbines, cavitation is less common because the water jet is exposed to atmospheric pressure. However, it can occur on the inside surface of the buckets, particularly at the splitter (the wedge that divides the water jet) and along the bucket's trailing edge. Cavitation in Pelton buckets typically presents as isolated pits or craters, often concentrated near the splitter where turbulence and pressure fluctuations are highest. The erosion pattern is more localized compared to Francis turbines, reflecting the focused impact of the water jet. The difference in patterns is due to the reaction turbine design of the Francis turbine that has pressure variation across the runner blade compared to the impulse design of the Pelton turbine that has a relatively constant pressure acting on the buckets.