An industrial gas turbine had a reoccurring failure with its accessory gearbox. The gearbox would run for a few days and then begin to show increased vibrations. The vibration level would gradually increase until the turbine alarm and trip signals operated. Studies at the time suggested alignment and accessory coupling issues were the cause. After many realignments and gears being changed the problem persisted. Eventually the gearbox replaced as it was suspected the original had internal alignment issues. This proved to be unsuccessful and the problem continued.
At the time of the unit’s last overhaul it was discovered the generator’s non-drive-end bearings insulation had failed and could not be rectified in time for its return to operation. It was then decided to install a second rotor earthing brush to the non-drive-end of the generator.
The writer reviewed all the historical and current date including a site inspection of the plant. Initial inspection of the gear damage indicated excessive misalignment and gear tooth overload. Finally, examination of the shell bearing liners had indications consistent with Electrical Static Discharge [ESD]. This had been overlooked; interpretation of the marking was due to unusual misalignment and the gear shaft.