The resolve of journal bearing field problems and the full vibrational assessment of an industrial gas turbine, simple cycle rotor train, ABB’s GTllN2 is presented. Three units experienced several times damage of the journal bearings on the compressor end. The analysis of the damage, including tests, indicated that insufficient jacking oil flow was the cause. The jacking concept was corrected and starting history has proven reliable operation, resulting in not one additional failure.During the same period, two last stage blades failed in one of the units damaging neighboring blades additionally. The machine tripped under the unbalance. Some auxiliary piping and the babbitt of the journal bearing was damaged. However, the rotor came to a safe stop with only some rubbing, and the turbine casing fulfilled its containment function perfectly.In order to better understand the dynamic behavior of the train, an extensive vibration measurement program was decided between Kentucky Utilities and ABB, and executed by Bently Nevada. The results proved excellent rotor balance, verified original design parameters, and no fluid induced instabilities were found. This joint effort allowed to fully assess and prove the rotor dynamic integrity of the gas turbine.

The rotordynamic behavior of an industrial gas turbine rotor train was assessed on site, and the sensitivity to unbalance was quantified. An outline of the measurement procedure is given. Differential data reduction with test unbalances was undertaken to minimize the influence of measurement uncertainty. A test unbalance was installed for one run and then shifted by 180° for the consecutive run. With differential data, the effective dynamic properties of the rotor - support - system can be estimated more accurately. A rotordynamic model was used to identify the support system parameters based on measured data. For the analysis, the anisotropic, elliptical vibration orbits were decomposed into two counter-rotating circular orbits, and the support system parameters identified match the originally predicted values well. The methods of differential data reduction, rotor train mode shape presentation, elliptical orbit decomposition, and the link of measurement to analytical models with parameter definition are described. Examples from on-site measurements are included for illustration.