Flange and Gasket Types

I did the opposite without any claim... I think is "better to have but mandatory only with particular codes" thanks

There is a mechanical reason for this. While bolt torqueing you can over-stress a flange and create crack at the bolt holes. Raised face flanges have a raised face for a reason, higher pressure ratings, thus a concentrated force to properly seal the mechanical joint. Typically the flanges are much thicker to handle the allowable bolt torque loads. Flat face flanges (125#/150# rating) operate a lower pressure rating, due to the design have a greater sealing area using a full face gasket, while bolting to higher torque values. Flat face flanges are typically less thick due to the design being a uniform, not concentrated load on the sealing surface. Example a 4" 125# full face flange bolt torqueing value is 180 foot lbs. with a stud length of 3.5" A 4" 150# raised face flange bolt torque is 160 foot lbs. for carbon steel and 110 foot lbs for stainless steel, both raised face stud lengths are 4" long

No, it is not mandatory and if you are careful and you do not over tighten the bolts or studs it will seal and work fine. Flat face flanges are mainly used on cast iron gas and a few other service valves. If you use a raised face flange on these it is very easy to crack the cast iron flange because of the small gap between the two.

I am not sure who makes them at the moment but you can buy or have someone make an adapter ring from plate material (they are usually made from 304 s/s) that is the same thickness as the height (usually 1/16")of the raised face sealing area on the R/F flange. The ring is designed to fill the gap so you will not break or crack the flat face flange when tightening up the two. I have used them many times but I am not sure where we ordered them from. I have also had these cut by local machine shops with both plasma and laser and a water jet would do it as well. A few time when I did not have access to a shop with CNC cutting capabilities I have made them myself on the job with a hand torch/plasma rig from both c/s and s/s. They are very easy to lay out and cut and I make them with bolt holes 1/16" larger than the holes on the flanges to make alignment easier when installing them. These spacer rings are not part of of the flange sealing surface and they do not require a gasket under the spacer ring on the RF flange side but you must use a full face gasket with them or make the ring twice as thick as the raised face sealing surface is.

I have seen people try and cut down a full face gasket to make a spacer ring and they do not work very well because when tightening these rings will compress a little and can cause a crack in the flat face flange.They should be made from hard plate only and s/s is better to make them from because moisture will seep into the tiny space under the spacer and cause carbon steel to corrode pretty fast. If I have them made I have an ear like on a pancake blind left on the spacers to flag the next guy that takes them apart so he will notice that it is a different than a normal setup before he starts. I will check around and see if I can find out who makes and/or sellsthese spacers/adapter rings when I get a chance.