[updated:LAST EDITED ON Jun-25-05 AT 06:42 AM (CDT)]Pl. specify the following data before any discussion can be started 1) The crude type (source, sweet/heavy, salt content & BSW etc. TAN No. etc) 2) Location of fouling in CPT - is it localized or distributed. If localized, please specify the location and temp range where it is more predominant. 3) Exch. tube matl, especially where corrosion is taking plac. 4) Salt content after Desalter & Na dosing rate. Also whether you have experienced any severe coking in secondary units like VDU/Coker/visbreaker etc. 5) Crude tower O/H corrosion pattern etc. I hope, with above data we all will get a clearer picture of the problem and experts in the forum can participate as well. Regards Chakra

Uncontrolled caustic injection is never recommended. The Na is going to create problems in downstream units - like accelerated coking, poisiong of catalysts etc. The recommended Na limit is 3 PTB or say 10-12 ppm for most of the refineries. However, you may consult your process licensor for the limit in your case. It is one of the reason that I asked you about coking problem in heaters. Also, with such a low salt content, so much NaOH dosing is unnecessary. You must be having a metering pump (as they say) for precise NaOH injection. The answer to fouling problem remains in your question itself. The corrosion is being caused by H2S - creating FeS scales. The crude you are using is a light crude. In that case, S will be present as H2S also. The resultant FeS scale, as it seems, is not stable under the present circumstances. this may be due to high velocity or some other factor. There some references available which indicates that in presence of high pH, aqueous H2S corrosion may increase. (that is one of the reason why they allow lower pH 5.5 at CDU column top in case of high sulphur crudes). Whether this is applicable here or not needs to be seen. One more thing. You have not indicated whether this corrosion + fouling problem is a recent problem or it is present since the beginning of plant commissioning. In case it a recent development, try to find out what change was made prior to this problem. In one of NACE discussion, (Denevr, 2002), a Shell spokesman had talked about similar problem (after desalter & caustic injection, at approx 300 F) after they started blending Eocene crudes with their original crudes. They also discussed that the corrosion had nothing to do with Cl- content. Once you diagonise the problem, the answer will be self evident. As a forum member, I would request you to tell us about your findings. Regards