ROOT-CAUSE INVESTIGATION ON BLADE FAILURES IN STEAM TURBINES FROM AN ASPECT OF COMPUTATIONAL MECHANICS

摘要

This paper presents the outcomes of computational mechanics applied in the root-cause investigation on blade failures of an auxiliary steam turbine in a domestic power plant, in which cracks were detected at the tenon region on the blades during scheduled outages. First, the calculated data of the finite- element models are benchmarked with the corresponding modal testing results, which show a reasonable agreement between the analysis and experiment. This outcome confirms the suitability of the present models and the methodology. Then, several potentially dangerous vibration modes can be identified in the Campbell diagram, of which eigen-frequencies may trigger resonance phenomena of the structural system and cause damages to the components. Furthermore, forced vibration analyses are performed at the above identified resonance frequencies. It is found from the harmonic response analyses that the 4th & 5th harmonics of the first group of vibration modes generate stress concentration phenomena near the bottom of the tenon, as far as the dynamic stress is concerned. Therefore, it is presumed that the resonance phenomena caused by the above vibration modes should be the root causes responsible for fatigue fracture at the tenon on the blade tip, and some remedy measures are proposed to improve the operational practices. In general, the computational mechanics can be regarded as a valuable tool for the root-cause investigation of component failures in power plant equipment.