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[fahd]

i ve been workin on a relocated ammonia plant in pakistan as a trainee engineer.this plant was purchashed from usa where it was being run by olin chemicals in lake charles,lousiana. it is a double train(having 2 synthesis loops working in parallel) plant.its name plate capacity was 1270 tpd.the management decided to enhance its capacity upto 1570 tpd by carrying out its BMR which was completed in two phases and during which lot of modifications were made including replacement of old NG compressor,addition of air booster units,replacement of old tva converter baskets with new s-200 design etc.
after the completion of BMR we started the plant on 20th may but after a couple of days' smooth operation faced a problem.
the whole thing started with the tripping of air booster blower of one of the air compressors.after which unloading of back end was started.one thing that i'd like to mention is that before this BMR we had manual system of introducing quench ammonia to the ammonia compressors during unloading but during the BMR we had installed control valves (of class 5) for this purpose on both the compressors to make operation more sophisticated.these valves control the quench ammonia by calculating the dew pt on the suction of the compressors and keep the suction temperature 3 degrees above dew pt.the old manual system was also retained.
lets come back to the story which started with the blower trip.as soon as unloading was started i.e. by openin the 1st stage antisurge the compressor tripped within a matter of seconds on high axial vibration.and within a few seconds the second compressor also tripped again on high vibration
and the coupling of both compressors (lpc to hpc) were found detached from shaft.one observation was of severe icing on both the anti surges and the body of compressors' lpc which was indicative of liquid ammonia carryover.
one more thing i'd like to mention is that during the BMR couplings of both compressors were converted to dry (shim type) from gear type.also in the pre BMR days we used to isolate quench ammonia after unloading the compressors but it wasnt done this time and checking of needle valves on the quench ammonia line showed excessive passing.the quench ammoina lines inject liquid at the down stream of anti surge.
investigations are being carried out to find out the real problem but nothing conclusive have come up yet.everyone believes that this is due to liquid carryover.
i want people of this forum to share any such experience with me and also tell me if there may be some other cause of this unfortunate incident.also the analysis are being carried out by GE THERMODYN to assess the strength of dry coupling in comparison with the gear type ones.they hevent come up with the conclusion yet but initial study shows that gear type couplings were stronger than the dry ones and this may be a possible cause as well becuse load had been increased after BMR and dry couplings have been found to have low torque tranfer ability and strength.which means that we deprived our system of the strength while at the same time increasing the load.
i have been given the task to look into this matter and come up with results.i expect a positive responce from more experienced people and their guidance.