Other than referencing flanges as listed components for pressure classification, B31.3 provides little input on flange load limits. B31.3 allows "more rigorous analysis" and evaluating flange stresses in accordance with ASME Section VIII Division 1 Appendix 2 (or equivalent) may be useful. Leakage is best addressed through material selection and bolt tightening. If you want to do more analysis, try FEA methods.