Materials Performance

SEP 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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46 SEPTEMBER 2018 W W W.MATERIALSPERFORMANCE.COM CM CORROSION MANAGEMENT R This article presents a real-time case study on the interface between risk- based inspection (RBI) and various material damages, including corro- sion, erosion, and cracking of corro- sion-resistant materials. Additionally, it focuses on mitigating risks to as low as reasonably practical, followed by decision-making techniques. In the absence of a sound RBI approach, certain damage mechanisms and their risks might be overlooked, which may lead to irreversible and even cat- astrophic asset failures. Recently, risk-based inspection (RBI) has emerged as a modernized inspection approach to foresee the operational risks for various pressurized process equipment. These risks are attributed to localized corrosion, general corrosion, pitting, crack- ing, and mechanical loading and vibra- tions. Of the available RBI approaches, the qualitative RBI approach, though with few limitations, is simple, economical , and more viable at various levels. Case Study: A Heat Exchanger Failure A cr yogenic ser vice heat exchanger, which carried liquid oxygen on the tube side, suffered seawater leakage from the shell-side f lange, as well as variations in heat transfer. From the pitting damage on the shell, it was suspected that some tubes may be leaking. The heat exchanger was taken out of service and mothballed with nitrogen. Multiple conventional reviews were conducted by the end user to finalize the inspection and testing plan, as well as the scope of work. A repair contract, which included making the equipment ASME R -Stamp compliant, was awarded to a maintenance contractor a few months later. 1 The proposed scope of work involved removing the tube bundle, repairing the shell and outer surfaces of the bundle, cut- ting the dished head, and leak testing the tubes, followed by rewelding the dished head and final pressure testing of the tube bundle. The dished head material was Type 304L stainless steel (SS) (UNS S30403) and the tubes were made of duplex SS (UNS 31254). All conventional reviews for the inspection and testing plan development involved references to "as-built" fabrication drawings, the original data sheet for the h eat e xch an ger, an d hi stori cal re p air knowledge of common shell and tube heat exchangers. There were certain unaddressed areas, however, due to the unique configuration of the tube bundle (i.e., the welded dished head and presence of spiro-vanes inside the tubes), unavailability of in-house/original equipment manufacturer (OEM) repair p r o c e d u re s f o r t h i s h e a t e x c h a n g e r, u n av a i l a b i l i ty of d e t a i l e d i n s p e c t i o n records for the equipment, and absence of any repair history or evidence specific to the welded dished head type of cryogenic heat exchanger with spiro-vanes in the tubes. Moreover, there were challenges associ- ated with the service of this heat exchanger. During normal operation, high-pressure liquid oxygen on the tube side was meant to Mitigating Material Damages Through Risk-Based Inspections a HM ad Ra Z a K H an Rana and Zo H ei R f a RH at, Department of Mechanical Engineering, Dalhousie University, Halifax, Nova Scotia, Canada

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