The proper operation a flange requires that it be able to "energize" the seal (typically a gasket), and then maintain that seal when pressure is applied.  When a flange does not properly seal, the clues to the cause are on the sealing surfaces.  Bartlett Engineering had regularly uses the rules of the ASME Code for the design and trouble shooting of joints that use anything from O-rings and conventional gaskets to metal-to-metal (i.e. - "gasketless") joints.