51313-02185-Naphthenic Acid Corrosion on an Extra-Heavy Crude Oil Upgrader Medium Vacuum Gasoil Side Cut Piping

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Product Number: 51313-02185-SG

ISBN: 02185 2013 CP

Author: Hon Ng

Publication Date: 2013

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After 5 years service at an average temperature sulfur content and total acid number (TAN) of 200º C (392°F) 3.3%wt and 4.3 mg NaOH/g respectively; grooves and through-wall pits have caused multiple leaks on the horizontal and inclined sections of the pipe with heat conservation insulation that draws 17º API Medium Vacuum Gasoil (MVGO) from the vacuum distillation column fabricated in carbon-steel. Fluid and operating characteristics suggest that the damage mechanism is naphthenic acid corrosion (NAC). Visual inspection of piping and pump damage morphology indicates that they should be caused by condensing phase naphthenic acid corrosion one of the most severe type of NAC reported by the literature. Guided wave inspection of full liquid packed piping showed no damage. This process seems to occur when column operation is interrupted. Historical corrosion rate estimated from through-wall pits increased from 1.5 mm/year (59 mils/year) in 2009 to 50 mm/year (187 mils/year) in 2011. It is still unclear if the damages progress during operation. The characterization of the damage mechanism allowed the improvement of the line’s inspection strategy since it predicted the location of susceptible points. To avoid future leakage temporary shells were welded on sections with localized corrosion and thicknesses below 3.4 mm (0.13”); thickness will be measured on a monthly basis at points with incipient pitting previously identified with automated UT corrosion mapping or manual UT phased array until the line upgrading to SS317L piping. An operational instruction was given to maintain a high liquid level on the MVGO tray during outages until temperature decrease below 250°F.