“CGTG-9 is a GE frame 6B gas turbine 31,633 kW, rated power driving a brushless exciter GEC ALstom generator (13,800 kw/60 Hz/0.8 PF, TAC 36-46 Type) through a Flende-Graffenstaden speed reduction load gear (54,000 kW power rating). An overview is shown in Figure 1. The bearings of the unit (turbine, load gear, and generator) are equipped with a 3500 vibration monitoring system with two perpendicular relative (proximity) probes. Both the high-speed shaft (HSS) and low-speed shaft (LSS) are equipped with keyphasors® to measure speed and phase angle. The alarm levels are given in Table 1.

In addition to the proximity probes, the turbine is equipped with two seismic transducers in the outboard and inboard bearings. The load gear is equipped with two seismic sensors and one accelerometer on the GB HSS guide bearing. The generator is equipped with one seismic sensor to read velocity at the outboard bearing (pedestal bearing). The seismic alarm and danger set points are respectively 0.5 IPS and 1 IPS.

To record diagnostics and monitoring data, the gas turbine generator is connected to a Bently Nevada Data Manager Machinery management system.

"Problem:

The unit experienced significant high vibration amplitudes in the gearbox guide bearing (3 mils pk-pk) after it was commissioned in 2001. A hot gas path inspection was conducted in March 2008; no maintenance action was taken on the load gearbox. After the inspection, the machine was put back in service until the vibration level of the vertical Y probe of the same bearing increased to 5.63 mils pk-pk in June 2008.

"Vibration Data Collection and Observation:

Data were recorded using an ADRE 408 DSPi (Automated Diagnostic for Rotating Equipment) data collector. The data were collected at transient (start-up and shutdown) and steady-state operation and at partial and full load operating conditions. The following were observed.