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.
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,