The unit driver is gas turbine. This is driving two compressors namely HP & LP used for gas compression. The desired RPM for compressor is 17000. Now when we are trying to start the compressor , high vibration on HP DE end and LP NDE end on both X & Y probes(All probes are eddy current probes). The reading obtained is at different RPM is tabulated in the chart. The Major component in the spectrum is 1X of compressor .WE doubted unbalance in the G/B to HP compressor coupling and changed it , but no improvement. Also checked all the alignment, compressor bearings lift etc and found satisfactory. The bearing vibration at HP and LP coupling end is normal. We had a third party analysis on this one and according to him when passing through first critical the LP comp DE vibration appeared to be critically damped. Can we get any result if we misalign a little bit the HP-LP coupling to change the stiffness at these areas I hope this can effect the HP DE bearing and LP NDE bearing and help in bring down the vibration. Your thoughts and advices will be most welcome.

Turbine.xls (19 KB, 34 downloads)

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Can we get any result if we misalign a little bit the HP-LP coupling to change the stiffness at these areas I hope this can effect the HP DE bearing and LP NDE bearing

The problem I have with this is that it is short on logic and long on hope.

You want to mis-align to change the stiffness. Ok, that might happen (or not. Are the bearings already stiff enough or even too stiff?). Why do you want to do this? I see no logic after wanting to make it more stiff; even that lacks follow through analysis or reasoning.

It looks like you would just be experimenting with a trial and ERROR approach with no engineering reasoning behind the experimentation. Often this can look like you are playing with the equipment. Without thought, you risk losing time with little or no positive progress towards problem resolution.

Bill is giving you extremely valuable advise for free.

Do you have valid cold and hot alignment data. Are you sure? Can you furnish. What is your coupling?

You will placing this unit in jeopardy of failing if you monkey with alignments based upon guesses. You must absolutely know.

I agree with Bill and Slim. The alignments will be made by the OEM as per the standards which should not be changed without valid reasons.

You have not mentioned in your post since how long you are facing this high vibs. how old is this unit..?? is the problem exists from the beginning or newly developed. What does the casing vib data says..??

Have you made any attempt to balance the machine at site conditions, may be a correction mass on the coupling between HP comp and GB ... just a thought.

Madhu

Thanks for all who replied. One thing i can feel from this forum the consideration and anxity of the memebers for machines which they never seen and even not to bother at all.I really shocked by some of the comments and think what a fool I am. So I think I had to again consider some good solutiion rather than trial and error.
Now answer to some questions.
This is a very ole unit. But Most of the parts are now newly overhauled. Both HP and LP compressers are newly overhauled. I am not sure wether we can do an onsite balancing on such an RPM machine.Any body done this? Again if it is unbalance why is it not reflecting on both bearing of the same unit? It is high on HP DE and LP NDE.

Given the high speed of this unit, I would suggest that your next step is to pull the bearings and inspect them. It is very likely that you have a wiped bearing on the end of the machine with the high vibration. This is also a simple and fairly quick thing to check.

Without actually seeing so vibration data it is hard for me to assess anything else. But I have experienced a bearing wipe on startup after overhaul on a high speed machine that yielded similar symptoms.

I am in with Steve. You probably have wiped 1 or more bearings. Full inspection would be recommended.