Repair of cracks detected in cast components of vertical Kaplan turbine rotor hub

Abstract

During the revitalization of the hydroelectric generating station A4 at the hydro power plant 'Djerdap 1', all of its parts are subjected to non-destructive testing. Cracks are detected in cast part of generator rotor hub, in the flange area toward the turbine shaft. Cracks were 10-500 mm long (determined by magnetic particle tests) and 60-100 mm deep (ultrasonic tests). The generator rotor hub is made of cast steel 25L, in accordance with the standard 977-75. The welding/surface welding repair technology in the detected crack zones of the cast part of generator rotor hub includes a large number of details, due to the structural solution and generator rotor function in service. It was also necessary to provide the carefully undertaken activities in order to enable safe operation and continuous use of the rotor hub. Failing to notice, underestimation or incorrect perception of important details could cause significant problems in hydroelectric generating station operation. The paper, due to specifics of reparation technology, contains arranged preparatory activities for the procedure apart from the welding/surface welding technology. It is of importance to mention a huge amount of money savings, The manufacture of a new generator rotor would cost over 4 million EUR (generator mass is over 300 tonnes), not taking into account the time needed for its making (6-12 months), which is directly related to the amount of energy a hydroelectric generating station can produce during that period. This technology of repair welding is also applicable for other components and structures of the turbine and the hydromechanical equipment subjected to various causes of damage during exploitation.