Repair of damages detected at the shaft and ventilation ring vanes of the hydroelectric generating set A2 generator at the hydro power plant ’Pirot’

Abstract

Hydro power plant ’Pirot’, which was built in 1990, is an accumulation-derivative power plant, which consists of 2 above-ground vertical hydroelectric generating sets that contain Francis turbines with nominal power of 41,5 MW, manufactured in Czech Republic, a tunnel and a sunken pipeline with overall length of 2.030 m and diameter that ranges from 3 000 to 3 500 mm. Pipeline, made of 22 mm thick pipes, has been designed and built without anchor blocks at curvatures, which is a rarity elsewhere. Maximum pressure of 2.5 MPa occurs in front of the turbine cover. This paper contains the analysis of the state of the generator shaft on which the mechanical damages were detected, as well as the analysis of the state of vanes of the upper and lower ventilation ring designed for air cooling of the generator rotor where corrosion, erosion and, most ofall, cavitation damages were detected, based on results of non-destructive testing (visual testing, penetrant testing, ultrasonic testing). Procedures for the rehabilitation of damaged components without disassembling them were defined. The execution of these procedures helped in saving approximately 100 000 €. Taking into account the period of time necessary to perform complete disassembling of components, as well as the period of time needed to perform reassembling, which is directly linked with the amount of energy the hydroelectric generating set A2 would produce during that period, the overall save would mount up to approximately 500.000 €.