Sudden high vibration in GTG unit 1

[edison123]

We have a 5100/3000 RPM, 20 MW Gas Turbine-Gear-Generator unit. Velocity probes are mounted on the generator bearings (two) and turbine bearings (two). The gear does not have any probes.

The unit runs normally with vibration velocities at above points not exceeding 6 mm/sec. But, without any appreciable change in noise or vibration, the probe in the generator front bearing trips on high vibration (over 25 mm/sec)when the load exceeds 18 MW. But this does not happen every time. Only occasionally enough to cause concern.

The vibration probes/cards/cables have been interchanged/replaced/calibrated but no use. The generator is certified healthy.

What could be the reason for this sudden high vibration? The low speed gear (3000 RPM) is found to have free vertical play of over 10 mm in decoupled state. Could this free play of gear create sudden high vibration especially at high loads? If so, how ?

 

[bluemax]

Your problem seems repeatable but random, which suggest some components moving or sticking relative to the bearing or shaft. Eighteen megawatts exerts enough threshold torque or force to initiate the movement. You might have a bearing support problem causing the bearing journal to vibrate excessively to to unloading. If the vibration is spiking after 18 mw it could be a very serious rub or loose support. Check frequencies of vibration

 

[Franko]

You must be kidding - 10 mm vertical free play in the gear??
Is the bearing gone?
Load dependent vibration is likely from the gear - transmitting vibration to the generator - gear torque causes radial loads due to gear tooth angles. If this speed decreaser is an upmesh gear, the pinion would be forcing the bull gear up in its bearings. The coupling could be resisting - getting more and more misaligned. If it is an out-dated gear-type coupling you could have torque-lock. Inspections should be done of coupling and bearings and then the gear (for backlash, damage etc.)

 

[McVibe]

I agree with Bluemax in that I think you need to do frequency analysis to figure out what is going on. Its one thing to say you have too much vibration, its a different thing to figure out where it is coming from (hence frequency).
It is also possible that you are getting a feedback from the electrical circuitry. If you have a way to capture the data leading up to the trip, it should have the information you are looking for.

 

[edison123]

Franko,

We also think the gear vertical play is way beyond limits. The gear is more than 30 years old and has never been opened and checked all these years. Thanks for your comments. We will take up the gear inspection asap.

bluemax & Mcvibe,

As I said, the vibration is too sudden to take any signature. It all happens in a trice. Though we have online monitoring (metrix), there is no provision to capture any data. Any suggestions, how we can capture data (like what on line vibration monitoring and data acquisition system we should use) ?

 

[hadi10]

ADRE (portable system) by Bently Nevada can capture such spikes. It has fairly good sampling rate up to milliseconds..

But again, what is the frequency? Is it true vibration or just spike caused by instrument grounding? Frequency will let you know.

 

[Franko]

The ADRE system would work if you have correct data sampling rates. The other alternative is to continuously tape record such as with a TEAC DAT tape recorder, then play the data into youre current FFT analyzer (add an accelerometer to the gear near the coupling to the generator). Tapes aren't expensive and you can tape over 2 or 3 tapes before throwing out the tape until you capture the event (check with TEAC).

http://www.teac-recorders.com/drindex.html

Another transient data recorder should be available from:

http://www.maars.com/

 

[MachineryWatch]

Check out the MAARS Model 5000 portable data acquisition system. It is designed for just such work. Use existing probes and accelerometers or velocity probes. Up to 16 simultaneous channels. Like tape recording, it won't miss anything and does the frequency, time and orbit analysis, along with trends of specific frequencies.

http://www.maars.com

phone +1-865-212-0006
Knoxville, TN

Good luck,
Skip Hartman

http://www.machinerywatch.com

 

[edison123]

Thank you people for your views and ideas.

 

[pepnic]

I have heard of a similar situation in one of our units at increased load (>170MW on a 282 MW unit - excessive vibration indication, but not verified by handheld device). Simple solution of insulation (heat shrink or silicone tape) at the extension cable/probe coupling. This may not be your situation, but it's something to think about. In any event, I would recommend an independent source to focus your troubleshooting before you start to tear apart the unit.

 

[Franko]

The last post jogged my memory - if probes are same as older IRD's, stray magnetic field interference may be the cause of false readings. It is recommended the magnetic shield for IRD equipment be used when taking vibration readings on electrical equipment.
IRD velocity probes aren't made anymore to my knowledge.
Can also be a problem with accelerometers:

http://www.wilcoxon.com/technotes/tn14.html

http://www.skfcm.com/products/sensor/vib/cm2005.pdf

 

[pepnic]

My previous post was referring to Bently Nevada 3300XL 8mm proximitors and probes with a 3500/42 monitor.

 

[Franko]

To Pepnic - I know but original post reported that one of the velocity probes was giving the high readings and "The vibration probes/cards/cables have been interchanged/replaced/calibrated but no use."
It could be that currents are at the front bearing affecting the probe. Perhaps the bearing insulation capability is intermittent. We had rare cases of currents when monitoring the motors driving our PAP industrial air compressors We made a home-made version of a shield, then used one from IRD. Aluminum can extended around entire probe and connector for output cable. We now use an accelerometer and integrate to get velocity / displacement for motor vibration (shaft displacement probes for compressor).