38 mw Turbine trips with constant load

This is a 38 mw TG set
Operating speed 5340 rpm,  with gearbox,  generator speed 1500 rpm
Turbine critical speed 2400-2800 rpm
Generator critical speed 1000-1300 rpm
Bearing 5 pad tilting bearing 
Set not connected to grid. Max it can be loaded up-to 6 mw
Set was rolled till rated. All vibrations normal. Load increased to 6 mw,  all vibrations normal 
After running for 4 hours at 6 mw suddenly turbine rear vibs went up from 20 to 90 microns and tripped the set within 15-20 seconds
Initially only phase started changing,  shaft vibs remain constant. After sometime phase continue to change along with increase in vibs. 
Field parameters checked. All remain normal. 
Data shows only 1X. It's a clear case of rubbing between rotor and stator. But why rubbing started after a gap of 4 hours with load and all other parameters being constant. 
Any design issue...clearance issues between stator and rotor....?? Shaft center line plots are indicating that No rub between bearing and shaft.
3250x turbine rear x
3251y turbine rear y
3252x turbine front x
3253y turbine front y
All casing vibrations normal.  However it was observed after trip during coasting down turbine front vibs reached almost 400 microns below 2100 rpm-300 rpm. Bode plots attached. Heavy casing vibs were observed on turbine front. Any thermal bow...??
Any feedback is welcome. 
Nagaraja IMG_20190220_141150IMG_20190220_141612_1IMG_20190220_141612IMG_20190220_151457IMG_20190220_143036IMG_20190220_143036IMG_20190220_143025IMG_20190220_142446IMG_20190220_142227


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Original Post

Is it new? Any work done on it? IMHO part load should normally be possible to run for long time. Was any visible indication found of touching anywhere after the event? What coupling? What gearbox? I have a similar event with coupling problems btw. gbx and generator. Heavy pitting found in coupling after less than 100h operation. Very fast vibration increase at random times but low load, it is a 50MW unit, never passed 15MW so far. Last trip was 5MW after some 10-15 minutes steady state. Increase is only on 2xRPM, 50Hz... Vibration also drop very fast after trip. Strange is that generator oil pressure oscillate at the same time as vibration increase and that drop also at trip, instantly.

Thanks OLI

It's a flexible coupling. Helical gear arrangement. Generator and gearbox vibs normal. No GB mesh frequency seen in casing data. 

In this case only 1x observed. Haven't opened the casing yet. Want to take a roll again. But the phenomena was peculiar. First for few minutes only phase change.then suddenly x and y vibs shoots up and machine trips. From 20-90 microns in 10 seconds. 

Can any thermal expansion between stator and rotor happens so fast. I assume it's a gradual phenomena 

Is it new or did it operate earlier?

If you get a spot heated by touching on the shaft the bend is "instant" if you have help by a resonance it is even faster so that is the case that is fast enough but no proof that it is the case here. Verify that your monitoring and gbx transducer can include the gearmesh, not always the case... It should always be some peak. What kind of flexible coupling? We have a geared coupling and got radial pitting, don't know how it makes that but we likely had random coupling lock up. So normally 1xRPM in such case would be bending. Did you start again after the event? Anything seen at the thrust bearing?

I would like to see the waterfall taken from start to stop to see what change in frequency. and shaft centerline from 14h20 to stop to see what SCL change when vin increased. and if any, I want to see polar plot.

In the other hand, what field parameter do you check? 

Is it steam turbine or gas turbine? how about axial displacement? bearing temp? have you check prestretch of coupling? key way of bearing support? how about governor valve working? (suspected that it is steam turbine), Insulation?


This is a steam turbine. Will try and post some data. Field parameters include any variation in steam inlet temperature , condensate, restriction in bleed lines etc. There was no unbalance , no alignment issues. It's a uneven thermal expansion due to which rubs are happening. Why uneven expansion between rotor and casing is the question 

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