Author – Gary Whittaker, Valentus Specialty Chemicals
A major chemical manufacturer located in the southeast US has operated a gas clean up plant that incorporates mostly carbon steel equipment for over 30 years. The nature of the operation is such that the plant cannot be shutdown for more than a few days at a time so as much maintenance as possible must be completed with the plant on line. While most of the plant operates cold, one distillation column operates between 80°C at the topand 100°C at the base. The column is self-supporting and located in an exposed outside position which led to concern about the potential for corrosion under insulation (CUI). The column was de-insulated and CUI was found at each insulation support ring. The original fibrous insulation was saturated with water due to the inability of the original aluminum weather proof jacketing to maintain its integrity.
Both NACE and API recommend the use of protective coatings to prevent CUI in the operating temperature range of the column. The recommended epoxy phenolic requires an abrasive blast surface preparation that could not be done in this case because the column could not be taken out of service. Neither NACE nor API recommend the use of a maintenance surface tolerant epoxy under insulation at temperatures above 60°C. In order to prevent future CUI, the corroded areas were prepared to the SP3 standard and a new highly corrosion resistant surface tolerant epoxy was applied while the column remained in service. This was top coated with a new high-performance coating made by Valentus Specialty Chemicals and based on Eastman TetrashieldTMprotective resin. Both the topcoat and epoxy were formulated for application to elevated temperature surfaces. This coating system has shown outstanding corrosion resistance in laboratory testing and has very good moisture permeation resistance, both factors along with the ability to apply it without blasting were key in choosing it for this application.
As good as the coating system is it was highly likely that reinstallation of a conventional fiberglass or mineral fiber insulation with an aluminum jacket would eventually result in failure and more CUI. As a result, the insulation was changed to a more CUI resistant material and the jacket was changed to an elastomeric Hypalon based material that is adhesively attached to the surface of all jacket penetrations. This belt and suspenders approach reduced the likelihood that water would get through the jacket and reduced the chances of the insulation becoming saturated if it did. All these factors combined result in a system that is much less likely to experience CUI in the future.