High random Vibration on steam turbine

[Sofistioelevib]

Hi Guys,
i have a steam turbine with big vibration problem random (last two event was about 1 month apart).
These the details:
[ol ]
[li]Steam turbine with generator about 35 MVA[/li]
[li]Steam turbine connected to a paper plant in a cogenerator scheme[/li]
[li]Vibration appear only on generator as in a picture[/li]
[li]No relation apparently with: MVAR, CURRENT, VOLTAGE, MW, Flux Condenser valve[/li]
[li]Vibration appear and disappear within 6 hour (generally)[/li]
[li]The vibration amplitude coming from proximitor[/li]
[/ol]

The vibration amplitude is really big and i can feel it directly onsite on generator.
The maximum amplitude recorder is 165 micron PP and with 6 mm/s on absolute accelerometer mounted on housing bearing

Sincerly i've no other idea how and Where to inspect on generator in electrically and mechanincally parts (even it is possible to see an indication of misalignment even in opposite side of coupling).

The only one assumption should be.... a mechanical problems should be present ever.

 

[Sofistioelevib]

I had a trouble inserting a file.

Here you found the document

http://files.engineering.com/getfil...-b33a-3877551c30e3&file=Vibration_problem.pdf

 

[Strong]

I guess generator speed is 1500 rpm and turbine is about 6810 rpm.
Can you confirm turbine and generator shaft speed with high vibrations present? Does the high vibrations occur a full/normal speed and what is load?
What location has the highest vibrations?

What software is used for display?
Why is a IIR Filter used?
What are Orbit and Spectrum amplitudes not displayed in Microns?
Why are full scale amplitude axis values not simple whole numbers?
Spectrum from only one probe is shown and probe orientation and rotation direction are not indicated.

If I had to guess as the cause of vibrations, I would say intermittent rotor rub. I caution you to get better data before taking corrective actions; including verifying operation of monitoring system.

Walt

 

[Sofistioelevib]

Please Walt see below:

I guess generator speed is 1500 rpm and turbine is about 6810 rpm.
Yes it is
Can you confirm turbine and generator shaft speed with high vibrations present? Does the high vibrations occur a full/normal speed and what is load?
The Vibration occur at full or normal speed with no particular load
What location has the highest vibrations?
The high vibration is only in generator both sides but not at same location every time

What software is used for display?
Dewesoft
Why is a IIR Filter used?
Should be used for very low frequency (less than 2 Hz i think)
What are Orbit and Spectrum amplitudes not displayed in Microns?
Probably this pictures are not rescaled? i need to investigate
Why are full scale amplitude axis values not simple whole numbers?
Spectrum from only one probe is shown (No, there are all the spectra for every sensors and probe orientation and rotation direction are not indicated.

If I had to guess as the cause of vibrations, I would say intermittent rotor rub.
Please Walt, could you please describe better what do you means, this is really interesting for me: your opinion about a problem
I caution you to get better data before taking corrective actions; including verifying operation of monitoring system.

 

[Strong]

I looked at the data again. I am not familiar with DEWESoft, so I am assuming the cross-marker on the Orbit plots is the Key Phasor (Bently terminology) Phase Reference. The high vibration events show about 180-degree phase shift from normal vibration, and both ends of generator are in-phase. The Spectrum indicates the dominant frequency is at 1x generator shaft speed. Generator rotor unbalance is indicated with probably dominant deflection near mid-span (single plane unbalance). The key question is what is cause of temporary rotor unbalance? Possible causes include:
Rotor thermal bow
Loose rotor mass (loose component)
Rotor-stator rub (also causes thermal bow)
Cracked shaft

Other data that coiuld be of value includes:
Shaft centerlines from probes
Vibration 1xSS amplitude and phase during entire event
Bearing temperatures (metal and oil)
Stator temperatures
Audible sound as indicator of rotor-stator rub

Walt

 

[Sofistioelevib]

Thank you Walt
your post was very helpful for me because i have more things were i'm going to work on.

I'll update you and the forum about news but i'm not sure when because this event it is very random and unpredictable.

Anyway i have to say you... no the marker is not key phasor (it was unavailable at time of data collection) but with your reply and making some obvious considerations i wonder the importance of Key phasor in this trouble.
With this i'm not sure if there was a 180 phase shift during the event but i need to investigate deeper.

Regarding unbalaance... i'm according with you this was my first opinion and i had the same your questions in mind, furthermore:
[ul]
[li]seems to be some hysteresis in this phenomenon: could be the unbalance started at beginning (and right now is not clear the source) and stay on even the source will miss?
[/li]

[li]The idea of hysteresys could be fine but considering vibration fluctuate from maximum to minimum (close to zero) when the vibration come to zero, or the source coming up again and restart with the initial vibration or in reality the vibration is not disappear but it is only sum of two vector out of phase
[/li]
[/ul]

Thank you again for your support