As widely recognized in the industry, it is important to evaluate the creep damage of an elevated temperature vessel so that the mechanical integrity of the vessel can be achieved through the adequate repair and replacement planning. This is quite straight forward procedure for internal pressure vessels. For an external pressure vessel, it is not easy to assess the creep damage due to the complexity of the creep buckling analysis. Eventually, creep cavity evaluation technique without identifying the correct stress distribution has been used so often. However, due to the uncertainty of the technique itself plus conservative mindset of the inspectors, it tends to leads to an excessive maintenance most of the cases. In order to conduct a reasonable remaining life assessment, it is desirable to use the creep cavity inspection in conjunction with another assessment technique such as FEM creep analysis as stated in API 579-1/ASME FFS-1 10.5.7. In this paper, comprehensive approach with FEM and field inspection such as creep cavity evaluation to reinforce the uncertainty of each method will be demonstrated.
This is Part 2 of two papers discussing the significance of two key factors of crack like flaw assessment in the Fitness for Service assessment. While FEM analysis technology has been advancing amazingly in recent years, and FEM based fitness-for-service assessment of a damaged components, such as crack like flaws and local metal loss assessment, has become mainstream in assessments, it is still important to understand the reference stress solution based on a limit load analysis and the role of each factor in the failure mode to control the damaged component safely until the end of its life. In API 579-1/ASME FFS-1[1], Part 9, Assessment of Crack like Flaws, those reference stress solutions were developed based on the limit load analysis using Folias factor Mt and surface correction factor Ms. Folias factor Mt and surface correction factor Ms, are factors that account for the bulging effect around flaws. Those factors enable prediction of a maximum allowable pressure of a damaged cylindrical shell from a simple flat plate model that contain same size of a damaged area. As for Folias factor, Mt, it is well known to express the relationship between the reference stress of a through-wall crack flat plate and a through-wall crack cylinder. The application of Mt is clearly defined in ASME/API 579 FFS-1 part 9C, as well as papers by Folias et al. The the significance of the surface correction factor for surface flaw, Ms, has not been commonly understood well enough in general. Unfortunately, API 579-1/ASME FFS-1 also does not clearly mention its significance and how Ms is to be applied in the stress analysis. At a glance, Ms looks like a similar factor to Mt, and it is tempting to simply apply Ms to primary membrane stress term like Mt, but that is not correct. Eventually, an incorrect application of Ms would lead to an incorrect discussion of a flaw characterization. Often, there is a question about ASME/API 579 FFS-1 Part 9C reference stress solutions, especially for ASME/API 579 FFS-1 eq. 9C.76, from the misunderstanding meaning of the Ms factor. Addressing this issue is important to maintain the integrity of the Fitness-For-Service technology. In this Part 2 of two papers, validation of equations obtained in Part 1 are discussed and proven based on FEM analysis.
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.
Due to the aging of facilities, Oil and Chemical industries in Japan has been longing for using API 579-1/ASME FFS-1 [1] Part 4 and Part 5 assessment over decades. However, most of equipment are subjected to Japanese High Pressure Gas Safety Law so our industry needed to pass through the discussion in a local committee. In the local committee, there was a conflict on the significance of Folias Factor, Mt, and surface correction factor, Ms. The conflict had been a stumbling block against the formal permission to use API 579-1/ASME FFS-1 Part 4 and Part 5 assessment technology. In 2021, throughout the long term effort of cross industry task team led by authors supported by API579-1/ASME Joint Committee on Fitness-For-Service members, the conflict has been solved in the local committee. Effective from April 1st, 2022, Oil and Chemical industry in Japan got a formal approval from the government to use API 579-1/ASME FFS-1 Part 4 and Part 5 assessment for equipment subjected to High Pressure Gas Safety Law. Authors noticed similar conflict on the significance of Folias Factor, Mt, and surface correction factor, Ms, is also found in societies outside of Japan occasionally. Those factors are sometimes referred as “bulging factor” that sounds like those are simple conversion factors between flat plate stress and cylinder stress. However, it is not a whole picture of the factor as discussed in this paper. Therefore, it would be beneficial for future improvement of Fitness-for-Service technology to share our outcomes on the correct significance of Mt and Ms including our Lr discussion on API579-1/ASME FFS-1 Part 9 Crack Like Flaw assessment taking this opportunity. In addition, authors studied the relative relationship between plastic zone length and distance to major stress discontinuity associated with the discussion. This might be beneficial to be shared for a future discussion too. Through this paper, significance of Mt and Ms, including derivation of Ms reinforced by FEM analysis, will be discussed. Also, plastic zone size model that can be used for a future discussion on required Lmsd, distance to major stress discontinuity, will be introduced.
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