Hot Spot Related High Temperature Hydrogen Attack of Secondary ReformerVessel in Ammonia Gas Plant

摘要

The BHPBilliton Yabulu Refinery operates two gas reforming circuits for thernproduction of ammonia and carbon dioxide.rnA hot spot was detected in late 2007 on the manway nozzle of a secondaryrnreformer vessel. The peak temperature measured on the nozzle wasrnapproximately 850 DegC. The cause of the local overheating was confirmedrnto be due to the failure of the refractory detail on the inside of the nozzle.rnA risk based justification was presented to the management team to shutrndown the vessel and assess vessel integrity by conducting Non DestructivernTesting (NDT) to detect for High Temperature Hydrogen Attack (HTHA) andrnHydrogen Assisted Cracking (HAC). The vessel shell and manway arernmanufactured from C-0.5%Mo steel (Grade SA-204-C) which is susceptible tornhigh temperature hydrogen attack when used for hydrogen service at elevatedrntemperatures. The revised Nelson curve provided in API 941 providedrnpractical operational limits and guidelines during the investigation and repairrnprocess.rnInitial non destructive testing conducted by a local NDT company failed torndetect any anomalies that could indicate the area had suffered hydrogenrnrelated high temperature attack. This could have given sufficient grounds tornallow re-instatement of the vessel on the basis that there was undetectablernlevel of HTHA. However, due to the significance of the hot spot and thernspecialised nature of detection, additional specialist assistance was sought tornfurther assess for HTHA. Specialist consultants were flown to site to conductrnthe sophisticated ultrasonic inspection work (velocity ratio, attenuation andrnspectral analysis methods). The investigation of the affected region showedrnextensive HTHA degradation which warranted a replacement of the affectedrnnozzle and confirmed existing manway nozzle was not fit for service. Therndamaged nozzle was metallographically examined offsite.rnThis paper describes the conditions leading to HTHA, the risk assessmentrnprocess, the importance of routine hot spot monitoring and the specialisedrndetection techniques used to assess for HTHA (and the pitfalls of nonrnspecialised techniques that may provide a false sense of security). The repairrnmethod is detailed together with the mitigation measures implemented tornprevent a repeat occurrence. The paper also describes the metallurgicalrnchanges that occurred and that could be used to calibrate the HTHA detectionrnmethods further.