Structural Integrity Analysis of a Kaplan Turbine Cover

During recent revitalization of Kaplan turbine aggregate made of structural steel St 3 (GOST standard), experimental investigation was performed. Base metal and welded joints were tested by non-destructive methods (NDT), and by testing of mechanical properties, including fracture mechanics parameters. Ultrasound testing (UT) indicated lack of penetration in weld metal and lamellar tearing in base metal. The latter one was in accordance with the reduced contraction of the base metal, as obtained by tensile testing, being significantly lower than the specified value. Also, fatigue crack threshold value was approximately two times smaller than expected for St 3 steel, whereas crack growth rate was significantly higher. In addition, numerical analysis of the cover was performed in different turbine operational modes to get all necessary stress distributions and enable structural integrity and life analysis. Despite significant degradation of the base metal and defects in weld joints, this analysis has shown that structural integrity of turbine cover is not jeopardized, not only because dynamic loading is just 5% higher than the static one, but also because defects in weld root are much shorter than critical crack lengths, as evaluated by LEFM calculation