07214 Oxygen Reactor External Stress Corrosion Cracking and Repair

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Product Number: 51300-07214-SG

ISBN: 07214 2007 CP

Author: Angela Wensley and Michel Tremblay

Publication Date: 2007

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External stress corrosion cracking (ESCC) was found on two solid 316L stainless steel oxygen reactors that had been in service for 10 years in a pulp and paper mill in eastern Canada. Both reactors had been designed to Section VIII, Division 1 of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code for service under internal pressure and with external circumferential stiffeners for protection against vacuum. ESCC was found beneath wet thermal insulation on the top head and where water had collected on the circumferential stiffeners, but was most severe where water had gained access behind external reinforcing rings for the circumferential stiffeners. Crack depths up to ½ inch (13 mm) deep were encountered in the 316L shell although no cracks went completely through to the process side. Shallow stress corrosion cracking (SCC) was also observed on the process side near the bottom cylinder-to-cone transition. All external and internal cracks were removed by grinding. The National Board Inspection Code (NBIC) was used to evaluate the locations of grinding that went below the minimum values permitted by the ASME Code. Some locations were deemed acceptable to be left as-is, other locations of deeper grinding were built up using type ER309L stainless steel weld metal, and 316L flush patches were installed in locations where extensive grinding below Code minimum had been necessary to remove the cracks. Both reactor shells were restored to a safe condition for the design pressure and new circumferential stiffeners with taller reinforcing rings were installed before the reactors were re-insulated and returned to service.