Data collected recently on a gas turbine driven generator indicates high axial vibration components (26 mm/s at 1x RPM; 3000 RPM) at the generator DE. This becomes very apparent at high MVar and after a prolonged period of time (over 1 hour). Radial vibration indicates lower levels of vibration (5-8 mm/s) at all monitored positions. I suspect an electrial problem with the coils/windings. Would be interested in any comments on this and also what would be the best way to pin point the problem(s)? Data collected via portable instrumentation, no radial probes on this machine.
Thanks

Does this happen to be a R&R having a turbine wheel with 2 gasgenerators? We had a case with problems from the exotic S&S sprocket coupling btw. turbine and gen. giving strange things at various operation, when opened the little springs were found all over the place including markings from residual of them inside the coupling gear surfaces, they seem to fatigue after some 20 years or so. If that´s not the case you are probably correct about the generator. Would there be any reason for the axial position to have changed? How is the axial bearing temp? Olov

Thanks Oli,
This is a John Brown set, diesel start, single gas turbine. This is also an old set (20 years) and has very little instrumentation attached (no RTD's!).

Hi VibeGuy - sounds like the old faithful GE Frame 5 unit?
Could well be an impending problem with the generator which is causing the generator rotor to hunt around its electrical centre, with the increase over time/load supporting this possibility.
We had several cases of coils coming lose within the stator (wedge deterioration) and I seem to recall a similar scenario.
What do JB say up there in sunny Scotland?

Hi there Mike,
Thanks for the info, my conclusion is also related to the coils etc. Are there any further checks (non intrusive) which would support this diagnosis?
Also, thanks for the link to this forum.
Steve

Don't think so Steve. Is there any "noise" from the stator? Lose coils would tend to vibrate at line freq. I'm a bit rusty on these guys and was mainly involved with the hot end!

Some of the GE gas turbine units tend to show high axial with a high unbalance force. If the rotor is bowing due to thermal sensitivity, your unbalance will change. The GE procedure and explanation for this test is covered in their GER-3809. Basically you set field currnt steady and vary the load, recording vibration and then set load steady and vary field current all the while recording the vibration vector. This will help you detrermine the cause of the vibration change. Generally, if the vectors change with load, its likely and alignment or other issue. If the vector changes with field current there could be issues like shorted turns, sticking hardware blocked cooling passages, etc.

GER-3809 that Mark mentions is available on the web.

Vibguy,

Thank's to Bill, here is the link:

http://www.gepower.com/prod_serv/products/tech_docs/en/downloads/ger3809.pdf

Walt

All,
Thanks for your responses and esp. the GE link, very informative.