Allowable stress for code is determined based on material tensile strength SU using design margin, where the design margin is normally taken to be 5.0, 4.0 or 3.5. ASME B&PV Codes Sections I, III and VIII Division 1 had improved the design margin from 4.0 to 3.5 in 1998. Japanese industrial codes such as JIS (Japanese Industrial Standards), HPI (High Pressure Institute) codes and Japanese government rules has two design margins of 4.0 and/or 3.5. One of the Japanese government rules is discussing to introduce the design margin of 3.5 for thermal power boilers. The allowable stress based on 3.5 increases to 14% larger that based on 4.0. The objective of the paper is to demonstrate that minimum flaws by NDE (non destructive examination) technologies can be well identified, and permissible flaws by NDE are harmless to cause fractures, even when the design stresses increase by 14%.

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