A leak detection assessment at a circumferential weld has been carried out to support a Leak-before-Break argument. This was performed using detailed finite element analysis (FEA) of the component to determine limiting defect sizes and crack opening areas. Leak rates of CO2 were then calculated and were found to be at an acceptable level.
The loading on the component were complex, including thermal, pressure and weld residual stresses. The thermal field originated from both a steady state operating condition and hold point during start up plant state. Computational Fluid Dynamics (CFD) was used to generate the thermal fields for this component and the crack was postulated to occur at the region of highest opening stress along the weld. In addition to the thermal field, the weld was also subjected to a residual stress, which was applied in the model via a novel application method that is described in the paper.
The FEA incorporated temperature dependent Ramberg-Osgood stress strain data that was generated from tensile data. For the limiting defect size calculation, lower bound material properties were used, and for the crack opening area, mean properties were used.
Various crack sizes were considered and J-integrals were calculated and compared against the material toughness of the weld. This enabled the calculation of a limiting defect size, and crack opening area, so that a leakage rate could then be calculated. Contour independence was observed for the J-integral evaluation giving confidence that the finite element meshes were fit for purpose.