MASOUD The reason for the failure at the top rows only and at the fourth pass (top pass) - i am unable to guess. Since gas enters the shell at the bottom - preferential corrosion may also occur at the exit where the exit temperature is more due to higher velocity of gas with hot carbonate vapors also. This could be one reason for the failure. It is possible that you had frequent shut downs of ammonia plant and this resulted in some condensation of hot carbonate liquid at the top rows to affect Cr-Mo tubes? S.S 321 or 316 or 347 in solution annealed condition may help in solving the problem but provided the tube to tube sheet area may become over a period of years SCC attack. I guess : You will be using the same tube sheet - which in original design will be carbon steel and this may give problems of galvanic / crevice and also SCC at the tube to tube sheet seal welded joints. You should recognize this problem also before change of material. I suggest that you examine from vapor attack at the top portions, change of material and the likley problems that you may get after som years before change of material alone. Trust this answers your querry C.V.Srinivasan April 24,2005 India Nishi Engineers Pvt Ltd E-mail: nishi@vsnl.com >Dear Sirinvasan, >During the shut down of plant we have inspected the top >portion of absorber.Fortunately the condition of trays and >demister pad at the top of absorber was good and a minor >leakage from manways also corrected before closing the >equipment. It mus be said that gas inter the shell from >bottom and designer considered an impingment plate in this >area however the leaked tubes are located in the uper part >of tubesheet (first row and as I explained upper surface of >tubes in two distinct sectin along the lenght of >tubes).Would you please describe the mechanism of Potasium >carbonate corrosion and what is your comment regarding the >change of material to S.S 321. >masoud