Reorganize Significance of Mt, Ms and Plastic Zone Size Against LMSD Under Plastic Collapse Regime

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.