Flow Performance Investigation of Francis Turbine Stay Vanes Restoration in Powerful Pumped Hydro Energy Storage Plants

The presented paper discusses and analyzes the methods for restoration of damaged stay rings and vanes of large Francis turbines. The focus is set on the crack damages that were also observed in stay vanes of Francis turbines in “Chaira” Pumped Hydro Energy Storage (PHES) plant, Hydro Unit 4 (HU4), where cracks occurred in all ten stay vanes. Additionally significant cavitation patches where noticed overall the front surface of the turbines stay vanes. Two approaches are examined in the study. The first approach for repairing adds material by welding over the damaged places, which are then polished. Although welding of damaged areas is a widespread method, some adverse effects are observed, mainly because of changes in the characteristics of the base metal and the impact of the high temperatures. The second approach is based on the removal of the damaged material till elimination of the cracks. Virtual models for computer simulation of both methods are prepared. These models are examined by structural mechanics and CFD analyses and compared. Final recommendations are proposed.