Information about cracks and failures of rotor shafts is generally kept confidential by the plant management and by the
machine manufacturer; therefore not all the cases have been reported and analyzed in literature, especially in the recent
years [1]. Catastrophic failures in hydro-power plants are rare, mainly due to the fact that main problems during operation
are related to cavitations, erosion and material defects. Serious problems are mainly expected on high head hydro-power
plants having high water fall exceeding 250 m, caused by high water pressures, pressure variations and high water speeds
[2–4]. Moreover, adequate preventive maintenance and clearly defined overhaul procedures in hydro-power plants signifi-
cantly reduce failure occurrences [5].
This paper describes the failure analysis of turbine shaft Kaplan’s 28 MW bulb turbine. The bulb turbine generator’s horizontal
shaft, Fig. 1, is made by joining the forged and cast parts by slag welding, Fig. 2. The shaft is manufactured as hollow,
housing a servomotor inside it, for shifting the runner blades. The flange, on which the crack occurred, is made of steel casting
of 20GSL designation, according to GOST 977-88 [6]. The operating speed of turbine shaft was 62.5 rpm.
After in-service of 163,411 h, sudden oil loss was observed from the control system. Total projected operating life for the
subject shaft is 200,000 h. Upon stopping the turbine and after visual inspection of all the spots where loss could have occurred
a through-thickness crack was discovered of 2100 mm length, through which the turbine oil leaked from the runner
servomotor chamber, Fig. 3. The residues of the anticorrosive coating and oil were detected as well as significant number of
corrosion pits in the shaft-flange transition zone radius, Fig. 4. The location of this crack was on the flange radius, from the