A problem was first identified when high vibration levels during runup began causing the machine to keep running down under automatic control.

A quick remote analysis using the PROTOR system highlighted the generator critical speed. This allowed the problem to be understood and consequently the trip level could be raised.

The machine could then be safely run up and unnecessary down time reduced.

A failure was initially detected when there was a fall in 1st order vibration level. The problem was confirmed by a corresponding increase in higher harmonics. Often this is an indication of a shaft crack.

However, in this case the problem was traced to a worn bearing.

It is believed that the first shaft order levels fall because the worn bearing drastically increase the clearance in the bearing. Thus more relative motion is taking place in the oil film and less is seen on the pedestal.

The greater motion in the oil film generates a higher degree of non-linearity, or even impacting, that generates the increase in the higher shaft harmonics.

All of this proves that a fall in vibration is not always good.

Over a period of time it was noticed that there was a slow rise in vibration accompanied by a phase change.

It was realized that this indicated a potential problem and the machine was taken out of service before a catastrophic failure occurred. Investigation revealed a cracked compressor tie bolt.

By using the vibration level histories stored by PROTOR and taking decisive, informed action it was possible to reduce outage time and the associated lost revenue.

The disk crack is seen as a persistent vector change in vibration levels which is quite small in the first instance. Following a return to service the underlying vibration vector had changed and again continued to show a persistent vector change but now with an exponential rate of change.

When viewed on a Polar plot from within the PROTOR software this showed that the vector change is for both periods is in the same direction.

This shows the difficulty of monitoring built up rotors where, due to movements in the rotor, the base level of vibration can change but be benign.

This is an example of the benefits in using PROTOR remotely. In this case a return to service was monitored remotely without the expense or inconvenience of a vibration engineer visiting the site. The LP vibration levels were found to be excessive.

The vibration data was analyzed using the results presented by PROTOR and advice passed back to station engineers on site to balance the machine in-situ thus avoiding unnecessary extension to outage and hence lost generation.