Hi,

here is the vib data for all your reference.
its a gas turbine-gearbox-compressor bundle.

GT speed:6500 rpm
compressor speed: 9360 rpm.
gearbox ratio: 1.44

pinion: 47 teeth
Gear: 67 teeth

unbalance in the compressor 3rd stage
site balancing carried out and shaft vibrations were brought down.

earlier unit was running at 6400 rpm,
once there was a gear tooth failure
speed reduced to 6000 rpm

now unit is running at 6000 rpm
actual rated speed 6500 rpm (GT speed)

compressor unable to run above 6200 rpm since GB vertical casing vibs are going high.
see the attachement

at 6000 rpm all is well
at 6100 rpm all is well
at 6200 rpm gearbox HSS DE and NDE vertical not OK.

as soon as the GT speed goes to 6200 RPM with comressor speed being 8928 rpm the vibration levels of gearbox HSS DE and NDE vertical vib levels will increase.

a predominant frequency of 8.2X is seen in the spectrum.
gearbox HSS DE, NDE horizontal and axial vib data are normal.
shaft vibs are normal.
all other casing and shaft vibs are normal.
only vertical vib levels of gearbox high speed increases

from 4 mm/sec at 6000 rpm with 200 rpm increment the vib levels are shooting to 14 mm/sec...???
gearbox HSS DE V: 12 mm/sec RMS
gearbox HSS NDE V: 14 mm/sec RMS

can it be a local resonance...??
no gearmesh frequencies are matching with 8.2X
why increase only in vertical...??? when horizontal and vertical are normal, shaft vibs are normal.
temperatures are slightly high on the bearings...

see the attachement. looking for help.

Madhu

ppt.pps (154 Kb, 27 downloads)

Madhu,

71625 = 8928 x 8.0

So, the factor is very close to 8.0 as oppose to 8.2
Also, 71625cpm has several harmonics. Is 8 the number of blades or something of this nature?
If so, likely this is what is exciting resonance.

David

Also it looks like there is modulation of the offending peak by the compressor's running speed substantiating airdynamic excitation nature of this vibration aggravated by resonance at 6200 RPM input speed.