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A refinery experienced an unexpected shutdown due to a power outage linked to an extreme weather condition. The loss of power lasted for 1 week and the utilities were down for a total of 2 weeks. As a consequence of the steam tracing being down for that long, heavy products like the Vacuum Tower Bottom (VTB) solidified in the piping systems. During the unplugging effort, a leak was discovered on a VTB piping circuit constructed of austenitic stainless steel. Therefore, before beginning to restart the unit and the refinery, an investigation had to be carried out to identify the origin and extent of the problem. The cracking was initiated from the inside and propagated in a short period of time. The failure investigation identified the stress corrosion cracking by chlorides to be the mechanism involved, due to an inappropriate product used during the unplugging of the circuit. The consequences could have been much worse as cracking only affected a limited part of the circuit but could have impacted it entirely which would have resulted in a much higher economic impact for the plant. In circumstances like the one encountered after a major power outage where the resources are stretched in a short timeframe between the different recovery efforts, the need of having guidance for the “unplugging of heavy product lines” is key to ensure the integrity of equipment and the safety of people. This paper describes the investigation that was performed to identify the corrosion mechanism involved and especially the reason why it took place.
Key words: stress corrosion cracking, stainless steel, unplugging, chlorides
This paper will identify and document how these different factors affect the susceptibility of austenitic stainless steel to Chloride-Stress Corrosion cracking based on a review of currently available literature. A review of current industry best practices and a review of how the Oxygen content, the pH and application of stress relief affects Chloride-Stress Corrosion Cracking will be documented and presented.
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In this paper, case studies related to DSS cracking are presented specific to the refining industry. Two cases of chloride stress corrosion cracking (Cl-SCC) of 2205 DSS tubing are presented.