Gentlemen,

customer has pretty large natural gas 2-stage compressor. Vibrations on LP DE bearing are slowly rising and now reached dangerously high level. vibrations at gearbox DE, which is rigidly coupled to compressor are low, showing some reaction to the vibration changes at compressor. NDE bearing vibrations are also low. No bearing temperature changes. No serious load changes. Vibrations just went down ones, August-September 2010, we were thinking about some temperature influence, but now it is 20-30degC colder than in summer, vibrations are still growing.
Major component is 1X, no phase changes since train was commissioned 3 years ago. I was suspecting bearing deterioration (1X, no changes on other bearings, gradual vibrations growth), but bearings can't improve (!). Contaminations (fouling)will, in theory, produce the same pattern, but shouldn't vectors change
Lost, need your opinion.

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Bill, I'v seen the rotor model and had discussion with manufacturer about a year ago. OEM and customer came to agreement that alarm Sp can be increased, which was done. Now compressor operates in 2-10um margin from trip level and, as it is close to the 2nd bending mode, deflection somewhere in the middle (don't remember which impeller row exactly) may result in rub at lab seals.
I studied results of performance calculations and they look consistent with rotor fouling version. Machine is slowly loosing poly efficiency.
you mentioned relation of the vibration with respect to ambient temperature drop. do you have TCV in your oil system for the bearing oil? 20-30 degC ambient temp drop would affect significant change in oil supply temperature exiting the oil cooler. from the trend, there is sign of cyclic vibration in early nov. try to adjust the oil supply temperature via oil cooler back to what you had during summer.
If affordable, get transient data collected. But be ready for the basic inspection and repair it the machine did not restart.
Bode plot and shaft average centerline plot are important to better understand the case.

Cleaning means opening the case? Before doing so, ensure all basic spares are available. Before that also, review any operational parameter changes line flow rate reduction and higher discharge temperature.
Looks like unbalance.
Possible of fouling.
I have almost the same experience on Gas Turbine.
Only 1 bearing high 1X vibration.
It reduce after engine wash.
So i think better stop for inspection and cleaning.

The other cases was high 1X RPM on DR gas compressor but when detail with orbit it looks like preload. It has been rectified by alignment.
A slowly climbing vibration trend with 1X is pointing towards an increased bearing clearance rather than fouling. we have 10 Trains compressssing natural gas but never found a fouling problem on proces compresor . Also oil stiffness play some role. check the viscocity of oil as well.
quote:
Originally posted by abdulla:
A slowly climbing vibration trend with 1X is pointing towards an increased bearing clearance rather than fouling.

Increasing journal bearing clearance will result in 1X harmonics accompanied by half harmonics, if severe.
Irshad

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