STUDY OF HYDRO-ABRASIVE EROSION OF HYDRO TURBINE UNDER DIFFERENT OPERATING CONDITIONS

Abstract

Hydro-abrasive erosion creates problems for the safe and effective running of hydropower projects. Erosion is of even more significant concern in hilly regions. The considerable losses in the revenue of the hydropower plant have become a burden on the hydropower project developers. These losses are mainly due to the reduced capacity of the generation of electricity units (kWh). Due to extensive hydro-abrasive erosion, the plant load factor is reduced, and the capacity to fulfill the electricity demand suffers. The high amount of sediment load that enters the plant, especially during the monsoon season, leads to severe hydro abrasive erosion in the turbine’s underwater components. The deterioration of underwater plant equipment due to erosion can also lead to a total shutdown of the hydropower plant. Economical restrictions are required to impose on the generating unit by knowing the best operating and silt conditions for enhancing the benefits of the generating unit. Currently, limited studies are available due to the lack of reliable measurement data on suspended sediment properties, turbine erosion, and corresponding efficiency losses. Researchers have developed erosion and efficiency loss models for hydro turbines based on operating and silt conditions. These models have been found in specific field and laboratory conditions; hence application of models requires the calibration for individual hydropower plants for which losses due to hydro-abrasive erosion are to be assessed. Under this work attempt has been made to study and determine the adverse effects of erosion in a hydropower plant. A hydro-abrasive erosion modelling at an existing medium-head hydropower plant was adopted. It is challenging to solve the problem analytically, and some parameters must be determined in situ. The high content of sediments in water is primarily due to an unstable geological environment and porous sedimentary rocks. Such water often passes through hydro turbines of run-of-river-based hydro power plants and, during its movement, produces hydro abrasive erosion in exposed turbine components. The operating data for calculating the extent of erosion in Francis turbine components and its effects on the power plant developers was taken from a medium-scale hydropower plant, Maneri Bhali (Stage-II) (4×76MW). The developed and verified erosion model suggested by IEC 62364 was adopted for this study. The erosion depth and efficiency losses were more severe in part load conditions. The i power generated decreases as the erosion depth increases for all the considered values of silt concentration due to the high recirculation of silt particles with the runner. At the turbine part load (70% load) operation, the erosion rate is found to be maximum for a silt concentration of 4322 ppm. However, at the design capacity operation of turbine (100% load), the erosion rate decreases to the minimum value due to uniform flow around the runner. Further, the hydro-abrasive erosion-associated cost termed sediment-induced cost, i.e., loss in turbine efficiency, annual repair and maintenance, and shutdown, have been computed for the study period (FY 2018-19). The total revenue losses are found as 286.7 Million INR during FY 2018-19, which comes out as 6.27% of total annual revenue. The turbine's performance depends on the sediment and operating parameters like sediment concentration, size, shape, and operating time. Optimization is carried out to get the optimal operating and sediment parameter values set. A preference selection index (PSI) analysis was carried out for this purpose. An attempt has also been made to choose the best conditions for the Francis turbine operation for the data obtained from the hydro abrasive erosion modelling (erosion depth and efficiency loss) and corresponding sediment-induced cost. The performance has been investigated for a Francis turbine, installed at a hydropower plant operating under different suspended sediment concentrations (SSC), silt size, shape factor, velocity, and operating time. The erosion rate, turbine efficiency loss, and sediment-induced cost were determined for SSC as 1000, 2000, 3000, 4000, and 5000 ppm. It has been observed that the gradual loss in turbine efficiency of 1.34 % at SSC as 1,000 ppm, silt size as 90 μm, shape factor as 0.5, velocity as 36 m/sec, and operating time up to 2000 hrs (approx. three months). The efficiency loss increases from 1.34% to 4.2%, sediment-induced cost increases from 2.19 Million INR to 17.18 Million INR as the silt size increases up to 150 μm, shape factor up to 1, velocity up to 38.2 m/sec, and operating time up to 5000 hrs for the same SSC. This effect was found adverse for higher SSC in the range of 5000 ppm SSC, and considerable losses in revenue have been noticed. Continuous operation up to 3000 hrs is profitable; after that, efficiency loss increases as high as 6.72%, and sediment-induced cost increases up to 27.53 Million INR. Beyond 3000 ppm SSC, the high particle load is transported through turbines mainly in the monsoon period resulting in high instability in operation, and runner damage occurs severely. This shows that the turbines are operating under severe sediment conditions which result in ii unstable operations throughout the year. The turbine components deteriorate continuously, resulting in frequent shutdowns and increased repair and maintenance costs. The findings of this study can be helpful to minimize the revenue losses and maximize the overall benefits from generation. Further, this study may help optimize the benefits of the generating units under similar hydro-abrasive erosion conditions in other run-off river hydropower plants operating on sediment-laden rivers to reach the ideal set of working conditions.