HIGH-TEMPERATURE CRACKING OF 800HT PIGTAILS IN A HYDROGEN UNIT

This paper addresses issues arising from an investigation carried out on several pigtail failures in a steam methane reforming (SMR) furnace at a refinery. Pigtails are tubes which connect the row(s) of vertical furnace tubes to the outlet collector. In order to be able to resist the creep and mechanical stresses which occur at an operating temperature of about 820°C, these pigtails are made from a special alloy (800HT). The bends of these pigtails cracked after only 3 years of operation, even though these pigtails should normally have a service life of at least 10 years. The investigation of the cracked bends highlighted that the cracks originated from a creep phenomenon which started on the outside. This was linked to degradation of the metal microstructure following poor cold working of the tubes, which was also accelerated by a problem with the support of the tubes. To prevent these failures, the pigtails need to undergo hot bending. This process will avoid dramatic precipitation and a grain size reduction as a result of recrystallisation of the matrix during operation and will also stop the formation of nucleation sites which enhance the formation of voids.