ABSTRACT This paper discusses a successful experience in solving corrosion problems in the overhead system of a sour water stripper. In general, corrosion in this area is manageable unless the water discharged from a fluid catalytic cracking unit, which often contains a significant amount of cyanides, is processed. However, severe corrosion caused by condensed ammonium bisulfide (NH4HS) occurred in sour water stripper overhead condenser tubes fabricated from Type 316L stainless steel (SS). Higher corrosion rates were observed on the upper side of a tubes exposed to the vapor phase. By performing the process analyses using the electrolyte process simulation, the most influential variables determining the localized concentration of NH4HS and resultant corrosion rate were detected. The countermeasures to avoid the localized accumulation of corrosives successfully reduced the corrosion rate to a negligible level.
INTRODUCTION Sour water strippers are commonly equipped in refineries to remove byproducts from the refining process, such as hydrogen sulfide and ammonia. In nonacidified single-stage condensing strippers, the overhead vapor is condensed and can be potentially corrosive when the excessive accumulation of ammonia and hydrogen sulfide takes place. In many units, carbon steel has been used successfully for air-cooler tubes. However, when NH4HS corrosion is particularly severe, it is necessary to use a material with a higher corrosion resistance. In sour water strippers where the feed consists of waste waters that do not contain cyanides, there ha ve been few reports on appreciable corrosion in overhead air-cooler tubes fabricated from Type 316 SS.1 This suggests this material is ideal for the service. We do not agree with the previous sentence. It is suggested that this may be true regionally, but may be affected by globally differing regulatory requirements and period of original construction. However, the occurrence of remarkable thinning in these tubes suggests that there are important factors that must be taken into account in the design and maintenance. This paper discusses the severe corrosion and detailed process analysis of an overhead condenser of a nonacidified condensing stripper for a hydroprocessing unit with type 316L SS tubes. Severe corrosion was noticed in spite of an appropriate design and material choice of equipment.