RAISED FACE to FULL FACE: We do not recommend mating a full face flange to a raised face flange especially when the full face flange is cast or ductile iron. Due to the potential for warping the flange, or in the worst case cracking it, the utmost care should be taken.
Even if a spacer that fits on the raised face flange outside the raised face area is used, damage to the flanges can still occur and great care should be taken.
FULL FACE FLANGES: In a bolted joint using ANSI full face (or flat face) flanges it must be remembered that the same bolts used in the corresponding raised face joint are now being asked to seal 3 to 4 times the gasket area with full face flanges. It is almost impossible to create an effective seal and high strength bolts should be considered.
ANSI Class 150 Full face bolted joints are poor design and should only be used for non-critical fluids.