Repair and Maintenance of Francis Turbine Runners in Nepal: Issues and Challenges

Rapid growth in hydropower development has resulted in a large number of Francis turbine installations in Nepal. Sediment erosion of turbine components due to high sediment concentration and abundance of hard minerals is a serious challenge to hydropower plants. Repair and maintenance of worn-out turbine components is a more preferred practice than replacement due to low cost. However, poor execution of repair works was found to have decreased the effective service life of turbine components as seen through the study of two different Francis runners operating in the same river. Among the various issues uncovered by this study, improper welding was found to be one of the major and common issues leading to uneven material distribution, porosity, stress build-up and crack formation on the runner. Another major issue was found in the heat treatment of the runner which was being done inappropriately, leading to stress build-up in the runner. Non-destructive testing for quality control of the repair process was also found to be done limitedly, leading to the repaired runners having undetected internal cracks. The process of mass balancing of the runner was also being done insufficiently, resulting in undesired vibrations and eventually leading to increased wear and tear of the runner. Few miscellaneous issues such as ignorance of relevant standards and best practices for repair, scarcity of well-trained manpower, continuous operation of the runner until breakdown etc. were also discussed. All of these issues were found to have been fuelled by the presence of limited number of repair facilities and a lack of stringent regulations for the repair process. This study emphasizes the need for extensive research in the field of turbine repair so as to improve the quality of repair as well as to aid the process of formulating and executing the required regulations.