Applied Diagnostics Ltd - Precision with Pride
During the commissioning stages of this 7.0MW Steam Turbine the Bently Nevada Monitoring System was picking up increased vibration levels as the generator went on load. The increase was only detected on the permanently mounted Velocity Transducers that monitor the axial vibration of the slow speed gearbox shaft. Concerns were raised as the vibration increase was substantial even though only 7% of the maximum electrical load had been reached during testing. When this package is finally installed off shore on an FPSO vessel, generator output would be nearer 100%. This could cause undesirable higher amplitude vibrations.
Gearbox Trend Monitoring
Frequency Spectrum Analysis
When finally installed off shore this is to be a critical unit, therefore it was necessary to investigate the vibration anomaly further. A full vibration test was carried out on all aspects of the package, which include the Steam Turbine, Reduction Gearbox and Generator. The only concern lay with the axial vibration on the gearbox output shaft. A comparison of the frequency spectra shows that the run speed peak increases with 7% load, all gear vibrations appeared to be consistent with a unit in good operational condition. The only other change noted was the slight variation in speed required to synchronise the electrical frequency with the rest of the ship.
Coast Down Test Results
Further testing ruled out balance and misalignment problems, therefore a Coast Down test was performed to investigate the effect of speed variations required when the generator is on load. If a run speed is close to a resonant frequency then vibration amplitudes can be amplified. The graph below plots vibration amplitude against speed. A clear increase in amplitude is seen in the suspected resonant range from 1770 to 1830 rpm.
Conclusions & Recommendations
Additional phase tests confirmed that the cause of the increased vibration was due to resonance of the gearbox casing in the axial plane. The typical operational speed required to generate electricity with a 60Hz line frequency is 1800rpm, which coincides exactly with the resonant frequency. It was suggested that the position of the Velocity transducers was changed from the axial plane to vertical plane.
Gearbox Velocity Transducer Positions