Only join mated flanges. Some -if not most- flat flanges are inherently made of Cast Iron. If you try to match a flat cast iron flange against a steel raised face flange, the steel variety will win the contest. The flat flange will break up when the bolts are torqued.
If the installation falls under one of the pressure piping codes, it's not permitted. Even if it doesn't fall under a code requirement, it's not good practice. I see it all the time in general industrial plants, though. It normally involves 125# flat faced flanges on valves or strainers bolted-up to 150# raised face steel flanges on the piping. Typically, 125 PSIG is the operating steam pressure, with the safety valves set for 150 (or higher) on the boilers. The installation is normally held together using some sort of goofy non-code nuts & bolts. There are some "interesting" installations out there. Don't let yours be one of them.
Interesting question. Obviously if some sort of "ring gasket" is chosen, there will be sort of an essentially open/exposed crack/aperture between the two flange faces from about the bolt holes outward. If a flat full-face gasket is chosen, the rubber say from near the bolt holes outward will at least not be compressed in the fashion of the rubber in the inner annular machined face-to-face/bearing area. If a special sealing gasket design is chosen, I would think it would be best if said special sealing mechanism/bulb etc. falls securely in that machined face-to-face area. I believe many years ago when there was a preponderance of gray cast iron flanges, there was probably more of an incidence of essentially unsupported (not uniformly supported between the faces by gaskets) gray cast iron flanges being broken by high bolting loads. While I guess it is possible some "codes" attempt to control this to assure some more perfect mating in some highly controlled applications; however, from a practical matter at least in some other applications with all manner of pipe, fittings, valves, pumps, meters, manufactured to various standards etc. that someone might want to join out that are "out there", I suspect this sort of condition is VERY hard to totally avoid.
For such reasons, I know that some manufacturers in the water and wastewater industries (e.g. ACIPCO) went with all-ductile iron companion flanges decades ago (even though others insisted on selling and keeping gray cast iron companion flanges in some standards).
I would offer up that in general ductile iron and steel flanges will be much harder to break than say gray cast iron items, when subjected to adverse loading by bolts or otherwise. I also remember there was some additional guidance offered up in some manufacturers literature and at least ANSI/AWWA standards I think with regard to a sort of unsupported bolting (see under "Bolts and Gaskets" heading at
"Also, per the Appendix of AWWA C110 and C115, high-strength bolts should not be used when a gray iron flange is involved in the connection." I guess the rationale (that could conceivably also be of interest here or other situations) is that lower strength bolts might at least make a not-too-delicate contractor etc. to at least work a little harder to break the gray cast iron flanges!
This is a common occurence with pump discharge flanges and the suggestion "don't" is not very helpful. Here's what is commonly done to successfully mate flat face cast iron flanges to the RF CS/SS flanges. Have your gasket supplier fabricate an adapter ring from a hard, compatible gasket material (Teflon, Gylon, Blue Guard, etc). The adapter ring is a full-face gasket with the ID cut 1/32" larger than the RF diameter. Adhere the adapter ring to the RF flange, and you've essentially turned the RF flange into a FF flange. Use a soft, low stress to seal full face gasket, and you're in business. Be aware however that full face gaskets have considerably more surface area than ring gaskets, and thus will require higher bolt loads to seat. Reduced area full face gaskets (Inertex OPRA) are a good fit here to keep the assembly bolt load low.
The common solution is to have the raised face on the 150 flange machined-off, and use a full face gasket. Any machine shop can take the raised face of a flange. Finding a supplier to come up with a custom gasket will be not much short of impossible in many areas.
It depends on the material. For an example: I would not hesitate to mate a 10" #125 cast iron flange with a #150 PVC RF flange. To machine the raised face off the PVC flange would be the wrong/bad advice to give to anyone. The PVC flanges typically deflects so much in the bolting area that it requires the raised face to seal. Even if you could find what one might call a FF PVC SCH 80 flange, it is not really a flat face. It will be a modified flat face witch greatly reduces the highly loaded contact area of the gasket similar to the reasoning behind RF flanges. I do not believe that a blanket statement of never FF to RF and instead machine the RF flat is good advice for all materials.
The original post asked about mating a FF to a RF. Not specifically #125 or #250 cast iron to B16.5 steel RF flanges.
If both RF and FF flanges were #150 ASTM A105 flanges what specific section of an ASME piping code prohibits mating these together?
I certainly do not defend mating FF to RF as a recommended practice; however, sometimes life is not always convenient. What is important is to understand the consequence of the decision to mate FF to RF and decide if the benefit outweighs the risk.
Zapster is correct, it depends on the material. For example if you have cast iron or non ferrous either side then you should have FF to FF with a full face gasket. On the other hand you can mix FF and RF for steel on steel.
one more option:
make a 1/16" thk steel ring plate equal to the height of the raised face and to cover the area outside the bolt hole on the raised flange. So, it makes like a flat face flange. It is working fine on the water service at 150psig.
Just a cautionary note. A couple posts appear to have advocated fabricating or obtaining a special second thin ring (it appears to to more or less fill the outer space created by the raised face) more specifically described in the last post, "make a 1/16" th(ic?)k steel ring plate equal to the height of the raised face and to cover the area outside the bolt hole on the raised flange".
I suspect this has probably "worked" for some folks in some applications based on these posts; however, it could be argued that this concept might in essence defeat any sealing (or bolt torquing?) intent of the original product or standard raised face designer. Also, I think that (at least for some products and per some standards) it is possible the height of the raised face might not necessarily be a specifically (or precisely) controlled dimension, perhaps resulting in unintended joining mismatch or concerns. Therefore, it may be adviseable to carefully consider these points, including the dimensions of the actual raised face and ring etc. to be joined, if an outer annular ring or adaptor s to be placed around same.
What an interesting posting, I should expect a code backed standard solution to this threat, if the application is to be certified to meet international standard, I will suggest the don't option otherwise a safe option of requesting for a tested proven adapter from supplier is the best especially if it involes elevated pressure
