It seems that the most economical rating is the 600#, pressure and temperature rating per ASME B16.5. However, we have had recently to employ some Norsok flanges for the extreme ratings under the 600# class, particularly for those 30" (and larger) flanges, in order to keep the costs under control. I subscribe to the limit of 600#, otherwise the cost goes thru the roof...There is also another limit, for the Austenitic Steel sections, which have lower rating limit or the option of Duplex and higher...
cheers,
gr2vessels