Rubbing between the central rotor and the surrounding stationary components of machinery such as large-scale turbine units can escalate into severe vibration, resulting in costly damage. Although conventional vibration analysis remains an important condition monitoring technique for diagnosing such rubbing phenomena, the non-destructive measurement of acoustic emission (AE) activity at the bearings on such plant is evolving into a viable complementary detection approach, especially adept at indicating the early stages of shaft-seal rubbing. This paper presents a case study on the application of high-frequency acoustic emissions as a means of detecting and verifying shaft-seal rubbing on a 217 MVA operational steam turbine unit. The generation of AE activity is attributed to the contact, deformation, adhesion and ploughing of surface asperities on the rubbing surfaces of the rotor and stator.
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