After 10 years of operation of a steam turbine with large output power there was an accident during the turbine run-up. One of rotating blade fell off. All 6 LP rotors (two machines) were checked and many cracks on the L-1 blades were found. Due to economic reasons, blades with an identical geometry were manufactured quickly and a new material was used. A better material was chosen in terms of yield limit. The egalization of rotor grooves was performed because of manufacturing accuracy. Tip-timing measurement was installed on two L-1 stages to monitor and protect the blades. After one year of smooth operation new inspections were made. Surprisingly, it was found that the blades made of the new material had comparatively more cracks than the original blades. A new investigation has been started. This article describes measurements including rotor torsional excitation, blade tip-timing measurements, modal analysis and material tests. A computational analysis is presented in Part 2. Application of both approaches revealed what hypotheses should be rejected and, on the other hand, which of them should be analyzed in a deeper way. Consequently, the unstalled flutter has been identified as the most probable cause of blade cracks.

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