Diagnosis of continuous rotor–stator rubbing in large scale turbine units using acoustic emissions

Abstract Continuous rubbing between the shaft and surrounding seals or end-glands of electricity generating turbine units can escalate into very severe vibration and costly rotor damage. Therefore such rotor–stator contacts require early diagnosis so as to minimize the financial consequences of any unplanned shutdowns. Acoustic emissions (AEs) or stress wave monitoring at the bearings has been identified as a sensitive non-destructive monitoring technique for such rub conditions [Electr. Eng. Jpn. 110(2) (1990); IEEE Proc. 6 (2000) 79; Hall and Mba, 14th International Congress on Condition Monitoring and Diagnostic Engineering Management (COMADEM’2001), Manchester, UK, 2001, p. 21]. However, experimental results from real turbines have been scarce. This paper presents a diagnosis of continuous rotor–stator rubbing in an operational 500 MW turbine unit via high frequency AE measurement within a 100 KHz–1 MHz ultrasonic band. As detailed by Sato [Electr. Eng. Jpn. 110(2) (1990)] and reported in this paper the onset of a continuous rub contact at a seal/gland was revealed by a sinusoidal modulation within the raw ‘rf’ AE response. By synchronous measurement at adjacent bearings, an estimation of the location of the rub was calculated using the phase delay between the adjacent AE modulations. Importantly, the AE diagnosis was closely corroborated by post-inspection of the turbine rotor.