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We have a CDU with (a preflash tower), the top product in which naphtha vapors are cooled down using a series of horizontal air coolers (vertical air flow). A corrosion problem was noted days ago in some tubes, knowing that this air cooler is only 6 years old in service while another series of air coolers used for the same object but with another CDU with (a preflash drum) for 23 years now and they work well, at least better than the stated one.
We use Ammonia as demulsifiers int desalter in both units.
 
Answers
08/07/2011 A: Lindsay McRae, Pall Corporation, Lindsay_McRae@pall.com
You may be having recycle of sour water from CDU overhead drum back to CDU which can cause corrosion in the head of CDU and in condenser. Sometimes residence time in overhead drum is not sufficient for a decent separation so sour water goes with naphtha to debutaniser and also with reflux back to CDU and perpetuates the corrosion cycle in CDU overhead / condenser . This is more common after CDU is debottlenecked or heavier / sourer crudes are used which makes CDU OH drum separation performance marginal. The use of corrosion inhibitor assist to help reduce corrosion, but also can act to stabilise emulsion in CDU OH drum reducing the interfacial tension and making the separation more difficult again contributing to recycle of sour water back to CDU overhead.
Are you having any issues with debutaniser tray fouling or HDT reactor plugging or feed exchanger fouling? This can also result from corrosion in CDU overhead / condenser and is an indicator of that.
Check desalter operation for a start. If that's OK, then you might want to consider high efficiency Liquid Liquid coalescer on the naphtha steam from the bottom of the CDU overhead drum. By reducing sour water recyle, corrosion can be tackled at its source.
07/07/2011 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Could it be that, in the case of the preflash column, the overhead vapor contains water vapor, and in the case of the preflash drum, it does not? Corrosion typically occurs where the water condenses, unless a good system is in place to retard the corrosion.
07/07/2011 A: Terry Ross, Maintenance Products Australia, terry@maintenance.net.au
can you please advise feed pipe size and flow rates