High vibration at reduced load


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We have GE Frame 9 unit rated 120MW operating in combined-cycle mode. We are observing that recently when we reduced load on the unit from base to 90MW, vibrationof bearing #3 increased from 3.8/3.9 to 5.14/6.15 mm/sec. vibration reduced to almost previous value after the load on unit was increased to base load. A 1.5mm/sec increase in vibration of bearing # 2 was also seen. What can be reason for this phenomenon? All other parameters remained unchanged. Thanks.
When did this begin occurring; after a maintenance outage? How long has this been occurring?

When was the last time the turbine and generator were aligned?

When was the last time the turbine bearings were aligned (internal alignment)?