Dear Friends,

We are facing one vibration issue in one of 660 MW turbo machinery. The vibration peak and phase datas are attached for your reference. 

The problem what we are facing is while performing balancing the FSNL vibration readings are drstically reduced however the full load vibration datas are further increased compared to before balancing datas. We have taken 3 trials.

First trial we have added the weights in LP rotor- B both side( bearing plane 5 & bearing plane 6) We added 885 grams in bearing 5 at 270 degree from the key phasor and 445 grams in bearing 6 at 90 degree.

The vibration levels are increased however the phase angles are nearly same and hence correction weights are calculated and shifted. This trial we run the unit only up to FSNL speed and we have not increased the load.

Second trail weights are added in LP rotor -B. Bearing 5 & 6 ( 322 grams@340 degree in bearing 5 and 258 grams@110 degree in bearing 6). This time vibration in full speed no load are reduced upto satisfactory limits and loaded the unit. However the full load vibration levels are increased in generator Front bearing -7. The load vibrations in beairng 5 & 6 are acceptable level.

Third trial was taken by adding weights in generator coupling 344 grams@350 degree. This time again FSNL vibrations are reduced in generator however the load vibrations are further increased.

So gentlemens your advice in this regard is highly appreciated.

We would like to know that the by shifting the weights in generator coupling alone will improve the vibration levels or not.

Why this much changes in vibration levels between no load to full load vibration.

Thanks and regards

Selva

Selva

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Original Post

I think you should provide more information like history, trend of data, time of conducting field balancing comparing to time of FSNL (ex. 4hours from starting...), section drawing, which plane and position used to conduct field balancing, rotor mass, critical speed....

what is reason making you believe that this is due to unbalance? Have you consider to the other cause.

For the large turbine, I think that it need sometime to get to thermal equilibrium and in the time the phase and vibration could be changed. The balancing should be analyzed and done in steady state.

.......................

Regards,

The point raised by Mr. Ngo Dinh regarding reaching thermal equilibrium is crucial to balancing a turbine.  I have reviewed some who tried to take short cuts by just getting to speed to take the measurements for a balance. Significant errors can occur in the balance calculation due to bad or inconsistent data.

Ensure that the turbine vibration has stabilized - in whatever condition, loaded or unloaded that you choose -- prior to obtaining balance data.

I would like to add some relevant points regarding the background of this vibration problem:

*  The Relative shaft vibration trend of all the bearings were within limits till April 2nd week 2019.  After 2nd Week, without any obvious reason the vibration trend at Bearing NO:5 (LP-B Rotor - Front Bearing) & Bearing No:7 (Generator Front Bearing) suddenly got increased.

*  Bearing No:7 was more susceptible and was getting influenced by Megawatt Loading of the UNit and day by day it increased to about 160 Microns.  Even though it was not a Mechanical Unbalance and the Generator Was "Thermally Sensitive", as a first aid measure, Balancing was attempted to reduce the LP-B Rotor Unbalance.  LP rotor balancing did yield results and vibration got reduced at rated speed and to some extent at Full Load condition.  (Please refer the Excel sheet Attachment).   To reduce the Generator Front vibration, 3rd trial was attempted by adding trial weight at the Coupling between LP-B rotor and Generator.  This weight requires shifting but further permission to Shut the Unit for balancing, could not be taken due to Financial Constraint in stopping a 660 MW Turbine.  (Between Thermal equilbrium & Financial Equilbrium, the thermal equilbrium was compromised.)

Agreed with all, the concern regarding for  proper stabilization time is required to get a correct readings, but still compromized because of time constraint in putting the Unit into operation.

(The point to note is, from Barring to Full Load the time taken can be hardly within 3 hours) .

Members Suggestion Required:

*  During the next shut, taking Trial 2 & Trial 3 into consideration if we Shift the Weight at Coupling to 100 degrees from 350 degrees, will it reduce the vibration at Bearing No:7 in full load codition.  If any alternate suggestions of shifting weights of LP Rotor B, Please suggest.  The phase angle of Bearing No:5 & 7 are almost in phase and between No Load to Full also the variation is between 25 degrees to 50 degrees.

*  Why Bearing No:5 & 7 vibration alone are getting increased significantly in full load condition.  RSP test was conducted on Generator and the results was ok.  No Shorted turns. 

Note:  Customer suspicion  is that the Generator Rotor has gone down and one of the "Insulation Pad" has got damaged.  But no long shutdown is available to verify this point.  (The Generator Front Bearing X - Probe, DC-Gap Voltage has increased up to -17 Volts.  Y probe readings no much change).

The Wisdom of the Members to resolve the vibration issue atleast temporarily without taking a longer shut down shall be highly appreciated.

 

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