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Abrasive wear in the clearance gap of guide vanes (GVs) increases the gap size, which deteriorates the flow and causes loss of efficiency. This paper investigates the performance of a Francis turbine including erosion-induced clearance gaps on the GVs. The effect of the gap on the performance of the turbine is studied numerically, by using the GV and runner blade passages. The results are compared with an experiment conducted in a single GV rig, developed for the same model. Simulations are performed for GVs with NACA0012, NACA2412 and NACA4412 profiles with each at 11 operating conditions. It is found that the clearance gap induces a leakage flow due to the pressure difference between adjacent sides. The leakage flow mixes with the main flow, forming a vortex filament, which is driven inside the runner. By using an example of a power plant in Nepal affected by sediment erosion, it is found that these vortices containing sediment particles erode the inlet of the runner blade towards hub and shroud. Comparison between the three NACA profiles shows that NACA0012, which is the current shape of GV in the plant, causes a maximum loss due to the leakage flow. The asymmetrical profiles contrarily are found to increase the efficiency of the turbine at all operating conditions. Such profiles are also inferred to have the minimum influence of erosion and pressure pulsations problems at runner inlet. In short, this paper gives an overview of the potential effect of the eroded GV on the turbine's performance and compares different GV profiles to minimize such effects. [ABSTRACT FROM AUTHOR]

Sediment erosion of the hydropower turbine components is one of the key challenges due to the constituent of hard particles in the rivers of Himalayas and Andes. In the case of Francis turbines, previous studies show that the erosion is mostly observed around stay vanes, guide vanes and runner blades. Depending upon the type of flow phenomena in particular regions and operating conditions, the sediment particles having certain geometric and material properties create distinct erosion patterns on those regions. The flow phenomena in Francis turbines are highly unsteady, especially around guide vanes and runner. The unsteadiness arises in the form of leakage through clearance gap, horseshoe vortex, rotor-stator-interaction and turbulences supported by high velocity and acceleration. The erosion on the other hand deteriorates the surface morphology, aggravating the flow. Based on a thorough literature survey, this paper explains the simultaneous nature of the two effects, which in combined, contributes to more losses, vibrations, fatigue problems and failure of the turbine. It also discusses some of the research endeavors to minimize the combined effect by controlling either the erosion or the secondary flow in the turbine. This review paper emphasizes the need of understanding the relationship between the two phenomena and techniques of how the combined effect can be predicted as well as minimized. [ABSTRACT FROM AUTHOR]

Turbine material is heavily eroded when operated in sediment-laden rivers. Erosion reduces the efficiency and increases operational risk. Therefore, welding repair is chosen in Nepal due to low cost to build up the eroded surface. Welding repair of hydraulic turbine does not impair the material property but, it can be catastrophic when carried out improperly. Such problem has been observed in Khimti Hydro-Power (KHP) in Nepal. The Pelton runner at KHP had operated without any major defects for 22,000 h before repair. But, root cracks extending up to 110 mm deep was revealed during inspection of welding repaired runner after 11,000 h of operation. Therefore, quality control during repair of runner is essential to achieve the desired properties and avoid catastrophic failures. Various factors like quality of weld deposit, preheating and post weld heat treatment conditions, and design changes during repair cause failure. The paper presents the possible causes of failure of Runner 1, 3 and 4 with reference to the operation and maintenance history, and quality control measures used in repair of Runner 7 of KHP. [ABSTRACT FROM AUTHOR]