High unbalance rotor but very low machine vibration! Why ? 1

[bkarim55]

Dear All,
The talk is about GE HDGT (Heavy Duty Gas Turbine) double shaft driving Centrifugal Compressors.
Right now a MI (Major Inspection) is ongoing on this machine. Because this machine is running over 90.000 fired hours and following OEM Instructions, the HP rotor has been sent to the local OEM Shop for deep inspection and repair if necessary.
It’s the second MI on this machine and it has been running very smooth from its installation 15 years ago. SMOOTH I mean the vibration level is very low about 4mm/s, the alarm setting is 12 mm/s (1/2 inch/s) and the Trip setting is one inch/s.

When the HP rotor installed (as received from the plant)on the balancing machine the readings were very high and the rotor needs to be dismantled and each disc of the rotor will be balanced separately

The balancing readings are (Rotor unbalance as received):
* With Turbine buckets:
...................Plane 1:1526 inch.g / 39.3 deg and Plane 2:1300 inch-g /65.5 deg

* Without Turbine buckets:
...................Plane 1:1582 inch.g / 43.4 deg and Plane 2:1088.7 inch-g /52.5 deg

The run Out of the rotor is acceptable.

Note: The balancing machine double checked and recalibrated but the reading still same same.

My questions:
With same level of rotor unbalance, the machine should vibrate but it’s not the case. How to explain this situation?
What could be the root causes of this rotor unbalance?

Thanks InAdvance and Best regards
Karim

 

[georgeverghese]

When was the last time the vibration monitoring system was calibrated ? We have had the same issues with centrifugal gas compressors and often wondered why the vibration levels were so low, and the Bentley Nevada monitoring system was never re calibrated after we took over the plant from the manufacturers.