Small bubbles can form at a pressure less than the vapor pressure of the water. If the vapor bubble collapses near the runner surface, highly localized pressure forces can remove runner material. This phenomenon, known as cavitation, can result in significant damage to a turbine runner.

For runners that experience cavitation damage, repair typically begins with an inspection to identify the damaged area(s). Personnel then must grind the damaged area to remove the pitted surface. Frequently, this damaged area can be far larger than what appears upon first inspection. Once surface preparation is complete, the lost volume of the runner is built back up using weld overlay. Finally, the surface is ground back down to best approximate the original runner surface.