Corrosion of overhead system in the distillation unit has been a chronic and unresolved issue even with the injection of neutralizer and corrosion inhibitors. Recently, a light naphtha discharge pump (made of UNS J91150 casing and impeller) malfunctioned, which caused severe damage to the impeller. It was subsequently realized that, prior to the malfunction of the impeller, the pH of the water at the second overhead accumulator unexpectedly dropped to 2.6 with simultaneous increase of Fe concentration. Preliminary analysis of the impeller revealed that it had failed from a combination of corrosion and cavitation. To understand the origin of corrosion, extensive water chemistry analysis was performed, which revealed the presence of corrosive species such as sulfate and nitrate ions, presumably from the flue gas, which would have caused pH to drop. Autoclave corrosion tests also revealed that these species would cause significant corrosion of UNS S41000 which is similar to UNS J91150 used in the pump. In-depth electron microscope study (i.e., SEM, EBSD) on corrosion scale and damaged impeller surface was performed to confirm that the damage mechanism was corrosion-induced cavitation. Details of the analysis will be presented and discussed.
Key words: overhead corrosion, naphtha, sulfate, nitrate, cavitation of UNS J91150