(145b) The State-of-the-Art of Naphthenic Acid and Sulfidic Corrosion Evaluation and Prediction

Conventional rules for assessment of naphthenic acid and sulfidic corrosion and operating risk have evolved over the past 40 years, primarily based on experience at individual plants processing certain crude streams. This is the basis for the rules-of-thumb that TAN levels of 1.5 mg KOH/g and higher generally produce elevated corrosion rates in fractionation systems particularly at high velocities up to 100 mps. More recent work has shown that naphthenic acid corrosion can be successfully inhibited on intermediate alloys (e.g. 5Cr and 9Cr steels) by the presence of moderate levels of H2S but can also be aggravated with higher H2S levels. Most recently there has been evidence that the type of naphthenic acid structures is also an important element in determining corrosivity, with some crude feedstocks having lower than expected corrosive tendencies despite high TAN levels. The most referenced specification for assessing corrosivity in high temperature fractionation systems is the API RP 581 base resource document that provides a risk matrix based on conditions of TAN, sulfur, temperature and alloy. However, it has been found that these guidelines are overly conservative in many cases leading to excessive alloy requirements. This paper reviews all of these factors and indicates a path forward for a more rational basis of assessing crude oil corrosivity that includes analysis, testing and corrosion modeling that takes into account critical process parameters for evaluation of operational risk, unit upgrading requirements and inspection/maintenance planning.