Flange Neck Creep Damage Below API 579-1/ASME FFS-1 Creep Damage Threshold Triggered by Ring Joint Dissimilar Flanges

Reduced flange design is commonly used for fixed bed reactor top nozzles due to the easier provision of manway access for down time maintenance. In this design, a dissimilar flange design is often opted for a material break point of the unit to avoid a dissimilar weld in the piping system. This design concept is also adopted our fixed bed reactors. The vessel is made of 2.25Cr steel including 40inch top nozzle. On the 2.25Cr top nozzle, 347SS dissimilar reduced flange was provided for manway access purpose in conjunction with top nozzle. It had been operated for 54,000Hrs until the reduced flange neck experienced minor cracks. As the total operating hours reached 140,000Hrs, cracks were propagated and leaked. According to their inspection record of those cracks, a creep damage like pattern was observed while its operating temperature was 520Deg. C, that was below the 550Deg.C of ASME Sec II Part D[1] allowable stress table time dependent allowable stress range, and also below the 537Deg.C of potential creep damage threshold indicated in API 579-1 / ASME FFS-1[2] Table 4.1 which is the same with API 571 Table 4.3[3]. As we conducted FEM analysis using an isochronous curve based on API 579/ASME FFS-1 Omega Method[2], the results well explained the actual damage and life, and confirmed a creep damage could happen below the creep damage threshold of API 579-1 / ASME FFS-1[2] and API 571[3], depending on the multiaxial stress state. In this paper, the detail of the inspection findings and isochronous model FEM analysis including remaining life assessment as well as comparison between the damage and analysis will be discussed.