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Mike C.

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We have a large turbine generator set, 770MW that has been experiencing high vibration on the exciter during startup and shutdown.  There has been discussion through various engineering departments, both in-house and out-house that the machine may have a soft foot condition.  I have force hammers and a 2140 analyzer.  The last time I did this work was in 1994 using a 12 pound hammer and a Spectral Dynamics SD390 analyzer on a 750MW generator.  I still have access to the 12 pound hammer but I have a 4 pound hammer at my site.  I know the hammers are titled differently but I am relating them to the approximate size of a sledge hammer. 

I have a question or two.

Can I use my 4 pound hammer to impact the sole plates this exciter sets on or am I going to need the 10 pounder.  The millwrights are going to ring the sole plates with ball peen hammers this weekend but it is not a measurable test and leaves room for doubt to remain in the engineering groups mind.

My 2140 has the Advanced Analysis package.  I have done routine impact testing but have not used it enough to know if it will provide the stiffness and mobility information I need to know.  I feel sure it will, it just needs to crunch the number in a different order.

I don't think I need to be too concerned about the foundation itself.  It is a massive concrete pedestal about 40 feet wide and 60 feet tall and 100 yards long.  I would think it is more about the integrity of the sole plates and grouting.

If anybody has any comments or words of wisdom I would appreciate it.

Thanks Mike C

John from PA

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Reply with quote  #2 
Quote:
Originally Posted by Mike C.
We have a large turbine generator set, 770MW that has been experiencing high vibration on the exciter during startup and shutdown.  

Thanks Mike C


Mike, is the dominant frequency 1X or 2X?  I also assume you are on 60 cycle so let me know if you aren't.
Walt Strong

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"The millwrights are going to ring the sole plates with ball peen hammers this weekend but it is not a measurable test and leaves room for doubt to remain in the engineering groups mind."

Actually you can make this a quantitative test by using a smaller 1 pound impact/impulse hammer with a metal tip and a small accelerometer. The transfer function magnitude and the coherence function can be used as a way of making comparative measurements to detect poor contact between sole plates and grout. It is still a fishing contest, but I have found that the sounding method results were comparable to the impulse response method. Of course there could be a lot of other reasons for high vibrations on the exciter besides the sole plates!

Walt
Mike C.

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Reply with quote  #4 
John the vibration is at 1X and yes it is a 60 cycle machine.

Walt I do know there are a lot of other things that can cause this.  We have done a lot of things to improve the vibration as it passes through its critical at 1740 cpm.  This machine has a large history in a short amount of time.

Here is a quick and dirty history.  We have 2 exciters one is a spare.  Last summer exciter "A" shorted due to a duct bolt that liberated.  We installed exciter "B" it was a fresh overhaul and it blew a chunk out of the rotor at 18 hours runtime.  We inspected and did a quick refurb on exciter "A".  It worked and we did a balance shot to clean up the running speed vibration.  It compromised the critical speed vibration to some extent.  It displayed what I considered an odd vibration pattern passing through its critical.  Going up through the critical the vibration was 5 to 8 times higher than coming down.  Also there is a service bulletin regarding cracked shafts on this style of machine.  We have made a couple of changes related to the loading of the last bearing,  By the way this is a 1 bearing machine with the inboard end of the shaft carried by the outboard coupling on the generator.  These changes to the bearing loading have made some effect on the critical speed vibration but nothing like we are looking for and the roll up to coast down vibration spread continues.  Exciter "B" is back from the shop and we are going to swap it out soon.  I know I haven't mentioned the vibration levels.  They have been 15 to 30 mils going up and 3 to 7 mils coming down depending whether the machine was hot or cold and how much runtime after start up.

I have 1 pound hammer too, not sure if it has the metal tip in the kit.  I'll acquire some standard impact data with both hammers if I have the steel tip and see where we go from there.

Thanks
Ron Brook

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Reply with quote  #5 
Mike,
I wrote this article a few years back.  It illustrates exactly what you want to determine concerning your exciter foundation.  Unfortunately, most of today's analyzers do not give you the ability to view and measure dynamic stiffness.  The data in my article was taken with the Zonic Medallion 'back in the day'.  Hope you find the article informative.
Regards,
Ron Brook

 
Attached Files
docx A Better Understanding of Rotor Dynamics and Support Stiffnes1.docx (701.70 KB, 22 views)

RustyCas

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PDF version for instant viewing.... Right-click, then Save As to download it.

 
Attached Files
pdf A Better Understanding of Rotor Dynamics and Support Stiffnes1.pdf (354.18 KB, 45 views)


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Ron Brook

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Thanks, Rusty!
Ron Stiemsma

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Can you just measure the vibration at the concrete, sole plate and the exciter foot and see if there are any large differences?  Add phase for more info.
Walt Strong

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"Can you just measure the vibration at the concrete, sole plate and the exciter foot and see if there are any large differences?  Add phase for more info."

This would be an operating deflection shape (ODS) test that can be conducted with the machine running. The impact test and the sounding with a ballpeen hammer can best be done with the machine off.

Walt
Mike C.

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Reply with quote  #10 
Ron B. thanks for the technical paper you wrote.  It covers just what I was looking for.  This paper will come in handy for a lot of people that have problems after re-installation of machines.  Sites like this get technical information that has been put out of sight back into view.  The fundamentals haven't changed much we just can't remember where we put them.

Ron S. that would be a good way to work this or even better the ODS Walt speaks of.  Unfortunately this machine is in an enclosure that is locked when running.  It produces very high sound levels and extreme windage.  It is essentially a large open generator with a large fan mounted on the shaft to provide some of the cooling.

So, I expect to do this work tomorrow.  I am going to collect several rounds of impact data using the different hammer and tip configurations I have available.  We'll see what we can get out of it.  I know the 2140 can collect the data I just don't know if it or the MHM software will produce all of the FRF's I want.  I think it will I have seen it in a lot of the older CSI products.

Thanks for the input.
Mike C.

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Reply with quote  #11 
Well the ime has come and past.  They exchanged the exciter and placed the new one on the pedestal.  This exchange is probably about 125 to 150 clock hours to complete.  They did not call me when it happened.  I was told it would be Sunday afternoo but they did it Saturday night.  I am going to visit the job today and see if I can still do something with my equipment.  They concluded the ring test was good enough and went with it.  It wasn't really a necessity for me to do this but I wanted to get the old tools out and see what I got.
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