Sub vibration Turbo Expander/compressor

[Bachmann]

I have just started to monitor a Turbo Expander/compressor and discovered sub vibrations on the expander side. The sub vibrations does not vary with load, varies between 0.1 to 0.65 x rpm. Sometimes the amplitude is grater than 100% of 1 x rpm
I do not thik its caused by oil whirl since the viscoity is acceptable and steady. Also the bearing temperatures are fine. The overall vibration leves on the expander are approx 20 um pp.

Anybody knows whats causing the sub vibrations?

 

[dgallagher]

Here is a link to an article on seal problem generating subsynchronous vibrations:

http://www.bearingsplus.com/pdf_files/papers/turbomachinery/tms_vol2407_1995.pdf

Dave G.

http://www.ReliabilityDirectStore.com

 

[Bachmann]

Thanks

 

[sms]

This kind of feels like an process side instability. Process fluid in circulation around the back of the wheel or shaft. What is the rotor configuration?

What is the fluid? Is this a cryogenic service? Can you get data as a function of load or speed? What is the speed?

20 micron on the high speed expanders I have messed with would be considered pretty high vibration.

"Why don't you knock it off with them negative waves? Why don't you dig how beautiful it is out here? Why don't you say something righteous and hopeful for a change?" Oddball, "Kelly's Heros" 1970

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[Bachmann]

The shaft configuration is "warm" turbo expander --> copling --> compressor.

The gas runs through two of these machine trains.

The system like this: Dry natural "raw" gas goes through expander 1, enters a separator and a heater. Separates again, then through expander 2. Then separates again before going though both compressors.

Running speed is approx 11000 rpm

 

[dcasto]

Futher question, what are the inlet guide vanes controled by. There could be a process coupling causeing the two units to harmonize at a low frequency.