Journal Bearing Temperature Spikes of Centrifugal Compressor

[Afifex]

I work in Ammonia plant and we're experiencing a problem in our Ammonia Compressor. The temperatures of both journal bearings of centrifugal ammonia compressor (which is driven by steam turbine directly) increase in spike form, without any change in bearing temperatures of steam turbine, and without any effect in radial or axial vibration values.
Could anyone suggest a reason for such a behavior?

 

[rotw]

Because there is a flexble coupling (most likely it is not rigid, unless we talk huge frame/power size and typically power gen.), one could assume that the turbine and compressor behave like rotordynamically uncoupled.

If journal bearing metal temperature increases significantly, this increases oil film temperature which in turn effect load carrying and damping capability of bearings. So I am wondering how come this does not reflect on vibrational readings according to your statement.

Could it be an instrumentation problem?
Has there been changes in process which increased loading on compressor and/or its speed?
Lube oil supply, any noticed change in supply temperature, oil quality (via sample analysis)?
Any changes in lube oil system parameters (oil return temperature, heat exchanger temperatures, lube oil supply pressure, etc). Turbine may be subjected to same as compressor but may accomodate better changes in oil supply conditions (e.g. different bearing design etc.).
Has there been start/restart of machine performed recently?

Small note: you mention "axial' vibrations, these are displacements monitored due to axial thrust acting on rotor.

 

[Afifex]

It is worth to mention that the lube oil system is one for both turbine and compressor, hence lube oil pressure and temperature are the same for all four bearings.
We've checked also the instrumentation, each bearing temperature on separate module, so there are no chance of instrumentation malfunction.
If it is a matter of load, axial displacement and axial bearing temperature would have been affected, which does not occur.

 

[rotw]

It was implicit in my post that lube oil system is common. I would not infer process loading from axial displacement info, as you move operating point in compressor map, there is no such a formula that correlates directly displacement to the process conditions as far as I know. Loading info (speed, process variables, etc) should be obtained from control system historian (i.e., control room, DCS, etc.). This is on a side note. Root cause still to be investigated and I think machine monitoring log info, should be investigated in conjunction with the process info at hand.

 

[JJPellin]

We had a similar event recently. Both radial bearings in a large motor driver went into alarm for high temperature. Our event was sustained and not just a spike. There was no other evidence of a real problem. We initially thought it could not be an instrument problem because we were seeing increased temperatures on two bearings. We were wrong.

An instrument engineer found a bad card in the Bently 3500 rack. They changed out the card and we have not had any problems since then. We considered shutting down a gas plant when we thought this was a real bearing failure.


Johnny Pellin

 

[awhicker84]

This doesn't sound mechanical to me. How short / long (duration) and how tall (temperature) are the spikes?

 

[romke]

have you ever checked the temperature of the oil coming out of the bearings? if the bearing temperature rises the oil temperature should also rise, not necessarily in the same order of magnitude though. if you notice spikes in the bearing temperature and no corresponding change in the oil temperature flowing out of the bearing, a instrumentation error would be the most likely cause.

 

[rQuestionEngineering]

looks like an instrument fault