A Case Study of Wear in a High Head Francis Turbine Due to Suspended Sediment and Secondary Flow in a Hydropower Plant of Nepal

Abstract

The hydropower plants under Himalayan basins are mostly characterized by heavy sediment load due to geographical and metamorphic constraints. Run-off-river projects with limited size of the desilting basins allow suspended sediments to get carried into the turbine components causing wear due to sediment erosion. In the case of high head power plants consisting of Francis turbines, a large portion of the hydraulic energy is transformed into kinetic energy within the guide vanes. This causes various instabilities in the flow due to high acceleration and velocity. Some recent studies have shown that due to the combined effect of the secondary flow around the guide vanes and sediment carrying flow, the size of the clearance gap increases, which further aggravates the performance of the turbine. This study takes a reference of one of the power plants in Nepal containing high head Francis turbines. An in-depth analysis of the effect of the sediment in this power plant and sediment erosion in the turbine components has been performed. A CFD analysis of the guide vanes and runner blades corresponding to the same turbine has been conducted and the results are used to analyze the erosion pattern on the actual turbine. The detailed erosion analysis is made possible with a 3D scanner, such that the eroded regions can be captured and classified based on the flow behavior at those regions. Guide vanes and runner blades are found to be the predominant components affected by erosion. It has been seen that most of the erosion affected regions are originated from increasing clearance gaps between guide vane and facing plates caused due to continuous leakage flow within the two sides of the guide vanes.