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A Secondary Urea Reactor (R3404) in an urea plant experienced a rapid de-pressure event due to failure and launching of the top flange head. The failure of the top flange head was attributed to environmental stress cracking. The event caused the complete contents of the reactor to be released to atmosphere. The event also caused external damage to the vessel, vessel foundation, and surrounding equipment and buildings. This paper discusses the causes of the failure, the effects on the facility, the process of investigating the incident, and the detailed metallurgical findings, including welding parameters for the low alloy steel reactor.

The authors analyzed factors which caused different cracking categories from material, stress and media, gave advice on the leak detection of urea reactors: either the quality and flow of the leak detection steam should be strictly controlled, or better leak detection method should be used.