This paper presents an application of a non-contacting blade tip timing measurement system using two-sensor method (so-called BSSM) at a low-pressure model steam turbine to investigate dynamic blade stress in extended operation conditions (so-called windage). An analysis method to identify the vibration frequencies and to determine the response amplitudes for the first few excited vibration mode shapes is described in detail. Objective of this paper is to discuss the performance and inherent limitations of the non-contacting measurement system. For that the BSSM results were compared with the blade vibration data obtained from strain gauges. Some experiences and suggestions are also made to improve the reliability and accuracy of this measurement system.

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