New approach for sulphidation prediction in crude oil refineries

摘要

This study shows data from continuous wall thickness measurements with online sensors over 4.5 years. These UT (ultrasonic testing) sensors collect the wall thickness of piping in high temperature areas (> 240°C) in CDUs/HVUs (crude distillation unit/high vacuum unit) of a German crude oil refinery. Monthly mean corrosion rates were calculated for each sensor revealing variations of corrosiveness per corrosion loop and over time. The observed wall thinning was linked to high temperature sulphidation and influencing factors such as TAN (total acid number) or flow velocity were discussed and quantified. In a next step, the corrosion rates were compared to the modified McConomy curves, the common prediction tool for sulphidation. This revealed significant differences between field data and prediction. Accordingly, corrosion was found to peak between 285°C and 320°C and to decrease to a minimum at higher temperatures. The theory (McConomy) also predicts this peak but at much higher temperatures around 415°C, which led to a strong overestimation of sulphidation for this study. Furthermore, carbon steel piping was found at temperatures > 285°C to be much less prone to sulphidation than 5Cr-0.5Mo steel. This is contradictory to existing understanding that Cr-Mo steels generally possess a higher resistance against sulphidation than carbon steel. The observed differences between theory and practice were interpreted to be mainly due to the influence of passive sulphide scales.