Article
Gearbox Misalignment On Combustion Gas Power Generator

by Hattan S. Mahran

ABSTRACT

This short case history is of a gas turbine generator where misalignment was properly diagnosed and corrected using vibration analysis. The alignment problem was between the turbine & gearbox and caused by thermal growth changes and differences in the turbine support legs. It provides a brief study of the vibration tools used to solve this problem and insight into a less common source of misalignment.

PREVIEW

“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.
• Turbine outboard bearing X/Y showed acceptable vibration amplitudes during transient and steady-state operating conditions up to 35 MW.
• Gearbox HSS inboard bearing X/Y probe and outboard bearing X probe indicated low direct and operating speed vibration levels. The outboard Y probe showed unreliable vibration levels due to low-speed component (5 Hz-10 Hz) activity, which was likely looseness or instrument related.
• The frequency component was at operating speed at almost stable phase angle on the turbine outboard bearing and the gearbox HSS inboard and outboard bearings.
• The gearbox HSS guide bearing showed acceptable dynamics until unit loading. Partial load conditions did not significantly increase vibration levels at direct and operating speed amplitudes. However, above 30 MW load, the vibration level increased until it reached 4.8 mils on the gearbox guide bearing Y (Figure 2).”

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