Sign up Calendar Latest Topics Donate
 
 
 


Reply
  Author   Comment  
OLi

Sr. Member
Registered:
Posts: 1,913
Reply with quote  #1 
So a GT about 19.1 MW landbased not aero derivative get 4mm/s on all bearings 95.95 Hz on all bearings and when you clean it it's gone. It comes back when dirty again and it repeats perfectly. Some minor multiples can be seen but very small. It is not a known feature of the machine, so far it is not a obvious frequency to be traced to any obvious machine part so it is a tricky one. I know what I fall back on when things don't add up but on all bearings and that relative hi is something different IMHO. Any idea or seen something similar? It has been known by boroscope inspection to have a minor touch damage of a blade somewhere but that's all that could be found.
__________________
Good Vibrations since early 1950's, first patented vibrometer 1956 in the US.
http://www.vtab.se
Walt Strong

Sr. Member
Registered:
Posts: 889
Reply with quote  #2 
Perhaps combustion related? Perhaps rub-induced natural frequency of stationary component? Are vibrations higher on auxiliary drive components? Is there sound audible or measurable at same frequency, and more noticeable near intake, combustor, or power turbine? Does it occur at same load above compressor bypass operation? I assume you mean blade cleaning by injecting something into intake?

Walt
OLi

Sr. Member
Registered:
Posts: 1,913
Reply with quote  #3 
No noise related change reported or noticed but nobody looked or heard for it either but not obvious noted anyway and they always are kind of noisy . I have not been on site yet. Strange is that change is almost the same vibration level all along the machine and same freq. everywhere so I also think it should be a resonance that is excited and it should be excited from a noise, reasonably broad band source normally in such cases. I also think light touch, rubb or flow related or in this case combustion related excitation.  I think it is not load related, the only thing that changes behavior are cleaning cycle and yes that should be by injecting something in the intake. Machine design is 2 years older than me and in it's 4th generation so some normal behavior are recorded, but not this and as far as I can tell not at that freq. or not recorded by the OEM either.
__________________
Good Vibrations since early 1950's, first patented vibrometer 1956 in the US.
http://www.vtab.se
vogel

Sr. Member
Registered:
Posts: 76
Reply with quote  #4 
Hi Oli, interesting case ...
So this is a Heavy-duty, single-shaft? gas turbine + Reduction gear + Generator. The 4 mm/s at 95,95 Hz are seen in all bearings, but only in the GT bearings, not on the gearbox or the generator, right?

When the vibration is present, is it measured during startup, steady operation and shutdown? How does it change during startup and shutdown? Is it consistent between startups if the there is not cleaning in between? Is it always there and stable or is it sporadic or variable?
I guess that by cleaning you mean that the axial compressor is cleaned/washed, or do you mean the turbine stages?

Then the main question, what's the turbine 1X? Assuming this is a machine similar to an old MS5000 the speed could well be close to 5.757 rpm ...
So first of all I would double-check that the 95,95 Hz is not 1X due to compressor fouling that is gone when cleaned.
Alternatively, I would think about some structural natural frequency or aerodynamic excitation, although I can't find a relationship with the cleaning effect. I think that 95,95 Hz is too high to be combustion-related but I'm not an expert on combustion dynamics.


OLi

Sr. Member
Registered:
Posts: 1,913
Reply with quote  #5 
Yes I have a handful of data "good" and "bad" all from GT bearings likely from full load and I think it is cleaned all the way but not 100% sure, all other peaks seem to be aprx the same only this pops up and it is reported to be consistent and hi whatever you do part from cleaning. So in principle, this is a one off excitation, a special case in flow or touching in bearing or seal or it is something that suddenly became undamped and easy to excite like foundation, shroud or any beam anywhere. 95 Hz is pretty hi a workshop floor is common to be 50Hz and this is twice as stiff, so what crack, loosen to become free to resonate at that freq. to be found on the whole GT or it may be a combination of both odd happenings.... So that is the question. I asked if there are a SSS coupling somewhere in it as they can make very strange features but maybe not that repeating as this seem to be. This is the data I have with harmonics cursor peaklist and good/bad data so, yes 1xRPM do change something so that is maybe enough to rub somewhere.

 
Attached Files
pdf 20190111_093102.pdf (481.20 KB, 11 views)


__________________
Good Vibrations since early 1950's, first patented vibrometer 1956 in the US.
http://www.vtab.se

Walt Strong

Sr. Member
Registered:
Posts: 889
Reply with quote  #6 
"95 Hz is pretty hi a workshop floor is common to be 50Hz and this is twice as stiff"

~2x frequency = ~4x stiffness

What is the software for the spectrum plot?

Walt
OLi

Sr. Member
Registered:
Posts: 1,913
Reply with quote  #7 
It is the 01dB/Acoem Oneprod NEST and XPR analysis software with Oracle database and MVP or Falcon instrument. True, even more off from the floor then, so what could it be? Some machine casing?
__________________
Good Vibrations since early 1950's, first patented vibrometer 1956 in the US.
http://www.vtab.se
Walt Strong

Sr. Member
Registered:
Posts: 889
Reply with quote  #8 
Olov,

Here is one machine rated about 19.1 MW:
https://www.energy.siemens.com/us/pool/hq/power-generation/gas-turbines/SGT-500/downloads/SGT-500_for_PG_EN.pdf

What is your machine? If the compressor and power turbine shaft are not directly connected, then the compressor would typically have higher speed than the PT. It would make some sense that cleaning the compressor would restore rotor balance and reduce 1xSS compressor vibrations. Otherwise, you are chasing a combustion pulsation or flow-induced vibration, but probably not a blade rub. 

Walt
OLi

Sr. Member
Registered:
Posts: 1,913
Reply with quote  #9 
Yes you found it. It is made in my birth town. Actual item may be a little less power as it likely is not the latest version.  So we are ending in the most difficult branch of this tree of vibrations, flow or combustion, fun. I already told my contact before asking here that it could be the case. I was hoping you guys would find something easier....
__________________
Good Vibrations since early 1950's, first patented vibrometer 1956 in the US.
http://www.vtab.se
Walt Strong

Sr. Member
Registered:
Posts: 889
Reply with quote  #10 
My motto is "if it was easy, I wouldn't be working on it"!!

Walt
Previous Topic | Next Topic
Print
Reply

Quick Navigation:

Easily create a Forum Website with Website Toolbox.