A large process tower in a petroleum refinery was discovered to have severe corrosion under insulation (CUI) in two areas. The areas were confined to the vicinity of external rings, but spanned 360deg of the tower. The remaining thickness of the shell in these areas ranged from 3mm to 9mm (0.13in. to 0.35in.) (original thickness was 23mm(1516in.)). The tower was assessed for immediate risk of collapse and emergency stabilizing bracing was installed while options for a permanent repair could be defined and selected. An external automated weld overlay technique was selected as a permanent repair method that restored the corroded areas to original thickness. This paper describes the engineering process taken to assess the immediate risk of collapse of the tower, including the emergency repairs taken to stabilize the tower. The review and selection of longer term repair options and the analysis involved in coordinating the removal of the emergency repairs with the installation of the permanent repairs in such a manner that the tower was returned to service well ahead of schedule are also covered.

V. M.
, “
Preventing Corrosion Under Insulation
,” National Board Bulletin, January 1988, National Board of Boiler and Pressure Vessel Inspectors, Columbus, OH (Note: This article may also be found as part of the “National Board Classic Series” at their website, http://www.nationalboard.org/Classics/classic10.htmlhttp://www.nationalboard.org/Classics/classic10.html).
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