Most designs that follow an ASME code or standard use ASME B16.5 flange ratings.  If unmodified from the dimensions and limits specified therein, then further justification for their use is not required by Regulators.  However, modified ASME B16.5 flanges or those used outside of the design limits specified in ASME B16.5, need further justification.  Either ASME Section II, an acceptable full gasket calculation, or Appendix Y calculation will be needed, depending on your circumstances.  We can assist you with this.