We have Steam turbine driven CO2 centrifugal compressor having 2 casings (LP/HP) with 2 stages in each Casing. Recently abnormal decrease in 3rd stage (HP) discharge temperature observed with increase in pressure. Temp reduced from 180 deg C to 140 deg C and pressure increased to 6 bar g. With this flow (Throughput) reduced upto 6 tons/hr.
OEM indicated that Labyrinth seal between 3rd and 4th stage might have damaged which is causing low temp (40 deg C) 4th stage suction gas to mix with 3rd stage discharge gas. There is a cooler between 3rd stage discharge and 4th stage which reduces temp from 180 deg C to 40 deg C. 
I have few queries regarding understanding of this theory and need experts opinion.
1) Is it possible for 4th stage gas to travel back to 3rd stage through damaged labyrinth, when pressure of 4th stage will be slightly lower due to pressure loss in downstream piping/cooler/fittings?
2) Even if gas is travelling from 4th to 3rd stage suction, it is mixing after the impeller tip. Will discharge pressure of 3rd stage will increase by mixing of cold gas, while travelling through diaphragm?
3) Is there any other cause of decreasing the 3rd stage discharge temp and increase of pressure? there are 2 Temp sensing thermowells and both showing same low temp, so instrument malfunction is not the cause.

P. S.

In past we have faced issues with scaling inside this HP casing between stages, due to process issues.

Need your urgent and kind reply so that we can take proper decision. Our process team is insisting to open the compressor (Barrel type) but Maintenance team is reluctant due to heavy job scope and not confirmed root cause of Labyrinth damage.

Thanks and Best regards,

Faisal Ali Khan

UM - Rotating equipment

Fatima Fertilizer Co. ltd.

Original Post

Dear Faisal,

I think your OEM is right.  About half of the pressure is generated in the impeller and the rest is recovered in the diffuser so the pressure your 3rd stage final impeller sees is not the same pressure at the 3rd stage discharge.  If the labyrinths are damaged then the higher pressure from the 3rd stage discharge/4th stage inlet could very well be leaking back into the 3rd stage causing exactly the symptoms you are describing. 

1) Temperature reduced because cooler CO2 from 3rd stage aftercooler leaking into 3rd stage diffuser inlet.

2)  Pressure increase as post-diffuser pressure adding to 3rd stage impeller outlet pressure.

3)  Flow reduced as higher head across 3rd stage impeller.

Sorry but it looks to me like its time to start pulling the 3rd/4th stage rotor.

Keep us posted.

ron.frend@petroskills.com

 

I agree with Ron and support the maintenance activity immediately.  The compressor has a serious problem and is operating well outside the integrity operating window.  Continued operation can only do more damage.  It will not repair itself.  Also, when did the condition change to abnormal?  Was it gradual over time or following an event?  My guess is that a surge or other event created axial displacement of the rotor which damaged the labyrinth seals.  Review recent operating to see if such an event occurred.

I agree with both the comments. Even if you think of scaling, temperature reduction is  not possible. So a mechanical integrity issue between 3 and 4th stage seems to be inevitable. Do you have any vibration monitoring on these units? If it's is there, what is the response, especially in the axial displacement

Thank you all for your valuable comments and direction. I also do think now, from your explanations, that internal inspection should be planned in coming opportunity, which is after 4 months during turnaround.

Currently we are not having any major issue of Production loss or serious threat. Condition is stable there.

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