Bearing Failure in Oil Flooded Screw Compressor
Bearing Failure in Oil Flooded Screw Compressor
(OP)
I work for a company who packages a very specialized type of petrochemical/refining chemical process machinery. Occasionally, we will also package associated compressor packages as well if they add benefit to the proposal. An electrical engineer by school, and instrumentation and controls engineer by career, I am finding myself right in the middle of assisting with troubleshooting a bearing failure on a screw compressor package we installed overseas last year sometime. Not my typical expertise, so I thought I would offer it up to you guys.
The screw is a small oil flooded twin rotor screw compressor. This compressor is installed in tandem with a second screw compressor. The two are operated in a redundant fashion (never run at the same time). Excessive vibration led to the removal from operation and dis assembly by the compressor manufacturer. The compressor in question has approximately 4000 hrs runtime. Our customer luckily caught it before it caused any further damage.
Once we removed the casing, we found the problem almost immediately. The female driven rotor (smaller) negative thrust bearing (smallest bearing assisting with axial load toward the shaft) had significant damage. The bearing is an angular contact ball bearing (single row) from FAG and showed very strange wear that appeared like flattening on the actual balls. The attached picture shows that the ball appears to be "eaten" away. The races for the balls showed a very scratched surface similar to common flaking.
Identifying the root cause is the hope. Oil survey came back good per viscosity and metal entrainment. Process conditions seem to be adequate and there was no significant under or over load conditions present. Alignment was very likely satisfactory as I know their mechanical experience and the fact that no other bearings showed any sign of wear. I read about a phenomena called false brinelling which seems like a consideration given the proximity of the two compressors and the pattern of use. Could this lead to flaking and eventual failure?
Thanks in advance,
Brian
The screw is a small oil flooded twin rotor screw compressor. This compressor is installed in tandem with a second screw compressor. The two are operated in a redundant fashion (never run at the same time). Excessive vibration led to the removal from operation and dis assembly by the compressor manufacturer. The compressor in question has approximately 4000 hrs runtime. Our customer luckily caught it before it caused any further damage.
Once we removed the casing, we found the problem almost immediately. The female driven rotor (smaller) negative thrust bearing (smallest bearing assisting with axial load toward the shaft) had significant damage. The bearing is an angular contact ball bearing (single row) from FAG and showed very strange wear that appeared like flattening on the actual balls. The attached picture shows that the ball appears to be "eaten" away. The races for the balls showed a very scratched surface similar to common flaking.
Identifying the root cause is the hope. Oil survey came back good per viscosity and metal entrainment. Process conditions seem to be adequate and there was no significant under or over load conditions present. Alignment was very likely satisfactory as I know their mechanical experience and the fact that no other bearings showed any sign of wear. I read about a phenomena called false brinelling which seems like a consideration given the proximity of the two compressors and the pattern of use. Could this lead to flaking and eventual failure?
Thanks in advance,
Brian





RE: Bearing Failure in Oil Flooded Screw Compressor
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Bearing Failure in Oil Flooded Screw Compressor
The compressor is direct coupled to the motor and has an internal slide valve that is used for capacity control. We start the compressor fully unloaded and ramp the slide valve to get to approximately 90% load during steady state.
RE: Bearing Failure in Oil Flooded Screw Compressor
Assuming the latter conjectural cause, I'd try starting the compressor with a little load on it, and/or ramping the load up faster.
Mike Halloran
Pembroke Pines, FL, USA
RE: Bearing Failure in Oil Flooded Screw Compressor
RE: Bearing Failure in Oil Flooded Screw Compressor
I will discuss the load considerations during startup with the vendor. The ramp from no load to 70% load (where the slide valve controller is set to automatic) is over a time period of a few minutes. I would be surprised if that short of time period would be the cause, but it's certainly a valid concern. Thanks for the help.
Ischgl-
I have attached a picture of the bearing with a more accurate color. I am pretty confident that the bearing always had ample oil (forced lube 5.5 barg @ 55degC) and don't see much indication of overheating. Take a look and let me know what you think. Again, thank you for your opinion.
Brian
RE: Bearing Failure in Oil Flooded Screw Compressor
RE: Bearing Failure in Oil Flooded Screw Compressor
You mentioned the oil viscosity came back ok. Generally recommend minimum oil viscosity of 13mm^2/s at operating temperature to ensure minimum oil film. While the oil viscosity may be ok at room temp, check how it is at operating temperature. If not ok, metal to metal contact could have caused the problem.
RE: Bearing Failure in Oil Flooded Screw Compressor
You would also expect to see indentations or flaking at ball spacing on the inner/outer rings with false brinelling.
RE: Bearing Failure in Oil Flooded Screw Compressor
Johnny Pellin
RE: Bearing Failure in Oil Flooded Screw Compressor
Mike Halloran
Pembroke Pines, FL, USA
RE: Bearing Failure in Oil Flooded Screw Compressor
Is somewhat hard to tell from the photo.
Attached is some pictures of known skidding damage.
Compare these to the bearing and let us know.
RE: Bearing Failure in Oil Flooded Screw Compressor
RE: Bearing Failure in Oil Flooded Screw Compressor