Evaluation of the Last Stage Blades of 37.5 and 110 MW Geothermal Turbines

This paper presents the results of the evaluation and investigation of the last stage blades of 37.5 and 110 MW geothermal turbines. In the investigation, the operational data and the history of the blade failures were analysed for several rotors of the units. The results of previous repairs were reviewed, and metallurgical investigations conducted. The mechanical and metallurgical modes of failures of some of the fractured blades were identified. A finite element analysis (FEA) of the blades was carried out to calculate the natural frequencies and the vibratory stresses on the blades, which were used to estimate their design useful life. Then the natural frequencies of the installed blades were measured in both, on the rotor and in the laboratory. It was observed that deterioration of the blades was caused in general by processes resulting from moisture condensation in the steam: erosion and corrosion. Also, the results of the investigation showed that the fractures of the two kinds of blades were attributed to the errors during the installation and manufacturing. Additionally, general deterioration of the blades contributed to the fracture. The paper discusses the way of this contribution. It also addresses what kind of recommendations were issued to amend the problem, whether repair or replace using the original or new design blades.