Assessment of integrity and service life of the upper ring of turbine runner guide vane apparatus at hydro power plant ‘Đerdap 1’

Abstract

During the rehabilitation of the hydroelectric generating set A6 at hydro power plant ’Djerdap 1’, state analysis and integrity assessment were carried out for all vital components and structures. After the completion of non-destructive and destructive tests carried out on parent material and welded joints of the structure of the upper ring of guide vane apparatus as part of vertical Kaplan turbine runner, it was decided that it should be replaced with a new one, because parent material and welded joints were degraded beyond repair. Existing upper ring of guide vane apparatus was made of steel St 3, in accordance with GOST standard. New upper ring of guide vane apparatus was manufactured through the use of steel S 235, in accordance with DIN standard. During ultrasonic testing 6 mm high lack of root penetration was detected at the load carrying welded joint along the entire circumference of the guide vane apparatus. It should be noted that there is no possibility of restoring the welded joint, because the occurrence of deformations at the structure has to be avoided. In this paper the results of the check of load carrying capacity of welded joints are presented, taking into account all weakened cross-sections. Determination of load carrying capacity and fatigue strength were carried out in the usual way, as well as the stress state analysis performed through the use of finite element method in order to determine the influence of undercuts. Assessment of integrity and service life of the guide vane apparatus based on the influence of cracks on stress distribution and fracture mechanics parameters was also carried out.