Intermittent Rub Caused by Carbonized Oil in a Steam Turbine

Light rubs between shaft and stationary parts are one of the most common faults in rotating machines. Sometimes, deposits of carbonized oil may form in the area close to oil-film journal bearings, especially in steam turbines. These deposits may cause the blockage of the available radial clearance between shaft and oil deflectors. In this case light rubs can be generated even starting from rather low vibration levels of the shaft. The friction forces generated by the rubs usually cause a shaft thermal bow and additional synchronous vibrations. Because of the continuous increase of vibration levels and rub-induced contact forces the blocking material can be quickly abraded. This causes a continuous decrease of the shaft thermal bow and vibration amplitudes. Therefore, intermittent onsets of high vibration levels may occur in the operating condition. These fault symptoms are ambiguous and they can make it difficult to diagnose the root cause of the malfunction. This paper shows a case history in which deposits of carbonized oil caused rubbing phenomena, in the area close to oil deflectors, and recurrent temporary peaks of the 1X vibration levels of a steam turbine. A diagnostic method that has allowed identifying the cause of the fault is described. Besides, comparisons between experimental data and numerical results obtained with a diagnostic model-based method are also shown.