Abstract: To ensure no accident will happen within the service life of the system, the sophisticated piping-valve system connected by insert welding technology is adopted as a structural integrity assessment procedure. Based on engineering practices, we assume more dangerous plane defects in the welding area on the basis of the circumferential defects lacking penetration. First, the stress strain curves and fracture toughness of J-75 in the presence of hydrogen are obtained by combining high pressure gas phase thermal hydrogen pre-charging test with the standard tensile and single edge bend test. Then the FE program is used to calculate the stress distribution under the loading of inertia, internal pressure, and off-surface displacement. Considering the uncertainty in clearance fit, the maximum clearance in the model is changed during multiple simulations, and it is found that the maximum stress position is almost unchanged, where the hypothetical defect will be produced. Finally, the Failure Assessment Diagram (FAD) method is used to assess the structural integrity of the system with four defect modes. By introducing the concept of safety margin, we found the system has more than twice of the safety margin when it contains four types of plane defects in initial size. Using the Engineering Critical Analysis (ECA), as the defect size increases, the system becomes the most vulnerable when it contains the defect 2, which is located on a symmetrical plane at the maximum stress area at the weld of the pipe and extending along the direction of welding depth. When the defect depth exceeds 0.61 mm, the fracture failure happens. Targeting at the most dangerous mode of the system, considering the dispersion of the test data of material properties, and taking the safety margin as the reliability index, the FAD method is combined with reliability assessment. The results show that the confidence level of the reliability of the system exceeds 0.999 5 under with the defect 2 condition of 0.15 mm.

Key words: hydrogen damage, clearance fit, failure assessment diagram (FAD), engineering critical analysis (ECA), reliability