Rotor Seizure due to DG seals

[mthakur]

Dear all, we had an incident of rotor seizure of syn gas compressor in ammonia plant. The compressor tripped on high vibration and came to halt very quickly, internal inspection inspection revealed that impellers and labys were all good on but inboard primay gas seal was damaged and found in pieces also badly entangled with its housing. Have you come across such a scenario of rotor caused by malfunction of DG seals? It will be helpful in understanding the root cuase of this unsual failure.

 

[zdas04]

It would also be helpful if we knew what kind of compressor you're talking about.

David

 

[JJPellin]

We had one instance that seems to match what you are describing. We had a double pressurized dry gas seal in a high speed, single-stage overhung compressor that failed catastrophically shortly after an overhaul. The seal face had broken into pieces and dropped down on the shaft. In our case, the drive had horsepower to spare and kept driving the rotor despite the extra drag. The pieces cut through the sleeve and about 1/4" into the shaft. We shut down because of high vibration and an inability to maintain differential pressure on the seal. When we came down for repair, we found the problem. There were two runs of tubing connected to the area near the seal. One was the seal gas (nitrogen) supply to the seal. The other one was an impulse line to the seal DP indication. Those two lines had been switched. The seal gas was being dumped into the compressor gas reference line and the seal was running with no gas supplied. The seal faces (Silicon Carbide) must have overheated and cracked up.

If there is any chance that you lost seal gas flow or DP in operation, this could cause a failure of this type. Mishandling during installation could cause cracks that would fail the seal faces in service. High vibration from surging or liquids could damage the faces depending on the configuration of your machine.


Johnny Pellin

 

[mthakur]

This was a syn gas compressore LP case of HT ammonia plant. regards

 

[JJPellin]

Your reply seems to be a description of the service, but tells us almost nothing about the machine. I happened to have an opportunity to overhaul a syn gas compressor at an ammonia plant a few years ago. I believe that one was an old Clark, barrel machine with iso-carbon seals. But I don't know if that is typical. At that same plant, they had axially split Demag/Delaval (spelling?) compressors set up in a long train (turbine - turbine - low stage compressor - gearbox - high stage compressor). That was the most challenging alignment I have ever done. But I don't recall much about the compressors. I think that may have been the ammonia train. The other big compressor they had was their air machine. But I really don't remember anything about it.

If the machine you are asking about has continuous process and vibration monitoring, you should examine the history to look for periods of low flow with high vibration that could indicate surge.


Johnny Pellin