Rotor-to-seal rubs in a steam turbine caused by deposits of carbonized oil

Sometimes, deposits of carbonized oil may cause the obstruction of the available radial clearance between shaft and oil deflectors in the area close to oil-film journal bearings of steam turbines. As a result, the friction forces generated by light rotor-to-stator rubs usually cause a shaft thermal bow and changes in the synchronous vibration. Owing to the continuous increase of vibration levels and rub-induced contact forces, the blocking material can be abraded quickly. Therefore, intermittent onsets of high peaks in the shaft vibration may occur also under stable operating conditions. This paper shows a case history in which deposits of carbonized oil caused rubbing phenomena in a steam turbine. A diagnostic method that has allowed the cause of the fault to be identified is described. Besides, comparisons between experimental data and numerical results obtained with a model-based diagnostic method are also shown.