51312-01149-Metal Dusting in the Crossover Piping System at a Petrochemical Plant

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Product Number: 51312-01149-SG

ISBN: 01149 2012 CP

Author: Jeffrey Xie

Publication Date: 2012

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Metal Dusting in the Crossover Piping System at a Petrochemical PlantJeffrey Xie NOVA Chemicals Research & Technology Center 2928 16th Street NE Calgary AB T2E 7K7 CANADA Lorrie Davies David Eisenhawer Randy Saunders Les Benum NOVA Chemicals Corporation Joffre Alberta T4A 6N1 CANADA This manuscript will describe the metallurgical analysis of the failed crossover components due to metal dusting formation of catalytic coke and the unique mechanism of metal dusting in this particular crossover piping system. The external crossover piping in an ethane cracking unit is described as exiting the furnace convection section and distributing the heated ethane feed to 32 radiant coils. The crossovers including pipes butt weld and socket weld connections are constructed from 304H stainless steels. Metal dusting was initially found in one of the butt welds on the pipes and the wall loss due to metal dusting at this weld reached to a point that the pipe wall thickness was no longer strong enough to withhold the operating pressure. A stress analysis estimated the minimum wall thickness to withhold the hoop stress generated by the operating pressure the estimation matched well with the measured thickness of the remaining wall of the failed butt weld. Subsequent investigations following the metal-dusted weld failure also identified material losses in other potential failure locations: low point drains abandoned thermowells and socket welded cap connections etc. The unique aspect of the metal dusting in this crossover piping system is that metal dusting occurred only at such locations where confined spaces were present. These confined spaces are relatively isolated from the process stream and allowed buildup of catalytic coke. These unique features made metal dusting to occur during the decoking process which is under oxidizing atmosphere rather than during the normal ethane cracking process which is under reducing atmosphere. This observation seems contradictory to the formation mechanism of metal dusting which occurs under reducing atmosphere. During the de-coking process however the confined spaces in the various crossover components were believed to establish a localized environment enriched with carbon monoxide due to the presence of catalytic coke in the crevices. This belief was confirmed by the fact that high concentration of carbon monoxide is always detected in the bulk de-coking gases for the initial period of decoking process. The formation of carbon monoxide promoted metal dusting during the de-coking process. Key words: metal dusting stainless steel coke carburization oxides decoking crossover piping