High Vibration On Gas Turbine Load Gear Box & Generator Pedestal Bearing


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Two Frame 5 Model MP 5001 Gas Turbines of site capacity 18 MW are installed at our fertilizer plant to cater for the power requirement of the complex. Each machine is equipped with 03 seismic probes to monitor the casing vibrations of the turbine and the generator. The locations of these seismic probes are: one probe is installed at Bearing # 01, second probe is installed on the compressor inlet plenum and the third one is installed on the pedestal bearing of the generator rotor.

To ensure the reliability of the machines, casing vibrations are also being monitored on monthly basis. These casing vibrations are taken at different points / orientations on load gear box and generator rotor bearing pedestal using hand held CSI vibration meter.

In April 2015 an upward trend has been observed in the casing vibrations of load gear box and pedestal bearing of one of the machine. Total electrical load on this machine is 16.5 MW. Upon comparing the current vibration levels with previous values, the rise has been observed only on the load gear box points which are towards generator rotor side and on the bearing pedestal; no change has been witnessed in the vibration levels on the points on load gear box facing turbine side. In terms of amplitude, previously vibration levels were in the range of 12 to 13 mm/s and 5 to 6 mm/s on load gear box and pedestal bearing respectively; these values have now been increased to 18 to 20 mm/s and 10 to 12 mms on load gear box and pedestal bearing respectively.

Previously the values of seismic probe # 1, 2 & 3 were 1.6, 3, & 7.1 mm/s respectively. Only minor change has been logged in the value of seismic probe # 3 which is presently 8.1 mm/s, the other two probes are showing the same values. Moreover, upon analyzing the spectrum of the vibration data, it has been revealed that the major rise is in the 1 X (generator speed: 3000 rpm) component.
Valuable opinion of the professionals is requested on following points / queries:

1. What are the probable causes of the rise in casing vibrations?

2. What can be done to bring down the vibration levels?

3. Does the current high casing vibration pose any mechanical threat to the machine?