Newie,
The rigidity check for flanges, as far as I know does not differentiate between the classification of flanges, so the answer is yes, the flat face flanges have to be checked for rigidity as per Appx. 2.
cheers,
gr2vessels
Another point of view. The flange rigidity check used to be in non-mandatory Appendix S, Design considerations for bolted flanges. Presumably the general comments of this Appendix would apply to any and all bolted flange designs, but of course the designer must determine if the comments make sense for their application.
Appendix S-2 indicates that flanges designed on the basis of allowable stress limits may not be sufficiently rigid to prevent leakage of the joint.
The flange rigidity check was moved a few years ago into Mandatory Appendix 2 (2-14). But at the same time it was not copied as well into Appendix Y. That may give some indication that the check is not necessary for this type of flange.
The basis for design of Appendix Y flanges is that the gasket is self-sealing (no gasket seating forces are necessary). These gaskets can accept a little bit of relative movement or rotation of the flange faces during operation.
More to the point is that the gaskets are loaded approximately with a point contact; as the flange surfaces rotate the point of contact may change on the self sealing gasket but the seal remains. Whereas the "face" type of gasket in most Appendix 2 designs is subject to breaking its seal if the flanges experience much differential rotation.
Also, the existence of the metal-to-metal contact both inside and outside the bolt circle lends to a more rigid construction that is less susceptible to differential rotation. Appendix Y-9 provides some useful insight into the effect of bolting forces and flange thickness on design of the flanges. Due to the interaction of the flanges "theoretically an unlimited number of designs can be found which satisfy the rules."
