9FA Generator High Vibration Recommended Shutdown


Thread Starter


A 9FA gas turbine generator recommended shutdown because of high vibration on the coupling and collector end bearings just after sync. and loading the turbine.

-Unit returned from CI inspection, offline water washed and dried well before startup.

-During startup from turning gear mode to FSNL the situation was normal all readings are perfect.

-Generator CB closed, preselected load activated and 30 MW targeted.

-Seismic and proximeter vib. sensors on the generator started to read increasing values corresponding to MW.

-Reaching 150 MW seismic on the collector end (BB8) reads 7.5 mm/s and (BB9) reads 15.75 mm/s and proximeter on 91Y and 101Y reads 0.28 exceeding the high limit (0.25) and alarm generated on the HMI.

-All reading are normal except those vib. sensors increased only after loading the CTG and not before.

-All vib. sensors are checked and calibrated and bad sensors replaced before this startup.

-we shutdown the unit before it trips.

if anyone has an explanation for this situation please explain it with us.

thanks for reading
Dear Sayed;
I'm not familiar with frame 9 GT but I am learning :))

What I can say that the data provided here are NOT enough to make diagnostic or troubleshooting..
<b>If all parameters are fine except vibration readings,</b> the most common causes of high vibration on GEN are mechanical imbalance, thermal sensitivity, misalignment, and bearing wear see GER 3809: https://powergen.gepower.com/content/dam/gepower-pgdp/global/en_US/documents/technical/ger/ger-3809-generator-rotor-thermal-sensitivity-theory-experience.pdf

Was there any work on the GEN during CI? Did something abnormal happen during the Startup? Before shutdown recommended; Did you gather vibration data (use of vib analyser)? What is the predominant frequency? Those data are very helpful to reduce trouble causes.



Dear Karim

Thanks for your reply. here my answers for your questions.

Was there any work on the GEN during CI? NO

Did something abnormal happen during the Startup?

NO as i said everything was perfect from startup, firing, acceleration and FSNL all readings are perfect and i mean it, it starts to record increase in GEN vib. just after sync and targeting a constant increasing load rate for tuning the unit by GE engineer.

Before shutdown recommended; Did you gather vibration data (use of vib analyser)?

Vibration was sensible for us because we were in CTG MCC or PEECC just a couple of meters from the turbine compartment it was real and high.

What is the predominant frequency?
Do you mean the grid frequency its here 50 Hz.


how much time you allowed the machine to run in FSNL?

mind one thing, after sync gen frequency i.e. turbine speed is same.
so the problem lies with may be:

1.temp & CPD pressure which increases with load

2. gen field circuit as gen field need to be increased for higher load.

3. Time (may be after start any abnormality development need some time too)
Dear Ahmed;

What is the predominant frequency? One of the most MOST common causes of high vibration on GT (Gas Turbine) or GEN (Generator) is rotor unbalance. And rotor unbalance has 1X <b>predominate vibration frequency</b> and its characteristic response.

The vibration readings on your post are overall values and they are not useful to diagnostic. You need to perform vibration analysis. You need to trend Amplitude and Phase plot of both turbine & generator bearings. Otherwise you need a vibration EXPERT to conduct this analysis..

High vibration level on GEN bearings doesn't, necessarily, mean that troubles are located on the GEN side.

I'm not sure! but I think GEN is running near its 3rd resonance speed.. If it's the case, GEN is running near critical speed, any vibration, even turbine side, will be amplified at the GEN side. So you need also to consider Barindra75 post about Temp & CPD pressure changes.. Maybe there are non-uniform combustion dynamics especially if it's a Frame 9FA with DLN system?! What about exhaust temperature readings, is there any abnormal exhaust temperature spread?



Dear all friends who replied to my post,

thank you all for your answers and patience. sorry to be late in my feedback of the solution of the problem, but you know power generation work so sorry again.

Here is the reason for this trip:

the trip was caused by fault in the generator rotor, GE consultant was here he made his trends, prepared the report, send it to his engineering section head and came with this result so our maintenance stuff pulled out the rotor and shipped it to USA.

thanks again
best regards
Dear Ahmed;

Thank you very much for the feedback...
So, the trouble (high vibration) was coming from the Gen Rotor..

If it's possible! Do you have more details? What's was wrong within the rotor?

Again, thanks for your update