Turbine Blade Crack Detection Using Blade Vibration Monitoring: Field Study From an Operating Steam Turbine

Abstract Blade Vibration Monitoring techniques were employed at an operating power-generating steam turbine to measure and analyze blade vibrations and monitor turbine blade health. The blade row of interest was the last row of freestanding low-pressure turbine blades that had a known history of blade root cracking. The monitoring instruments were blade tip timing probes that were permanently fixed to the casing, and recorded time of arrival of each passing blade. During normal operation of the steam turbine, a group of blades was experiencing high vibrations that eventually led to crack initiation and subsequent crack growth in one blade. The blade natural frequency changed as a consequence, which was detectable in the blade health trending data, and triggered an advisory for machine shutdown for unscheduled maintenance in order to inspect the blades. The inspection consisted of Phased Array Ultrasonic Testing for in-situ evaluation, which confirmed the crack in the same blade where the monitoring system indicated. The blade was removed, and Fluorescent Magnetic Particle Inspection also confirmed the crack. The Blade Vibration Monitoring system not only identified the root cause of cracking, but possibly prevented turbine blade failure.