Analysis of current state and strength evaluation of the pipeline at hydro power plant 'Pirot'

Abstract

The Pirot hydropower plant, which was built in 1990, is an accumulation derivative power plant, which consists of 2 aboveground vertical hydroelectric generating sets that contain Francis turbines with a nominal power of 41.5 MW, manufactured in the Czech Republic, a tunnel, and a sunken pipeline with an overall length of 2,030 m and a diameter that ranges from 3.000 to 3.500 m. Pipes have been made of S275J2G3 steel. The pipe wall is 22 mm thick. Maximum pressure of 2.5 MPa occurs in front of the turbine cover. Geodetic measurements have been conducted permanently from the day the assembly was finished and the pipeline was put into service, both when the pipeline is empty and unloaded by hydrostatic pressure and when it is full. Analysis of obtained data regarding the movements along the pipeline route showed that from the year 2003 there are significantly higher differences in movements comparing the situations when the pipeline is full and when it is empty in comparison with the previous period. This paper contains the analysis of the current state and strength evaluation of the pipeline as a whole on the basis of results of nondestructive tests performed on the vital butt-welded joint in the curvature area at chainage 1+263 m (visual testing, magnetic particle testing, penetrant testing, ultrasonic testing, and radiographic metallographic replication testing).