Matching Flanges with Different Materials

Failure to follow the Client's approved piping standards can expose one to significant risk that, in the event of a significant loss to the business, professional indemnity insurance providers may not be willing to cover.

I think #2 has had experience with this condition. If the carbon steel is not exposed to the corrosive medium, the #2 answer will work every time.

Well, It is very easy to bolt the two flanges together but carrying the responsibility of doing something wrong that could get people hurt or killed or destroy expensive equipment may not be to easy. I am sure there is a reason they wanted to use Monel for the flange material. Most of the tight-wad people I know don't spend the extra money for monel if they don't need it.

Without seeing the pipe specification that you must have not seen either I would never install a carbon steel flange on a system fabricated from monel unless I was very educated about the system ,type contents, corrosion factor, pressure, temperature, and all of the other things that caused them to use Monel in the first place. Carbon steel will not have near the corrosion, erosion or wear resistance as well as the strength of monel.

It is hard for me to picture a job that would require a company fabricate and install a piping system without a piping Spec. I would suggest you ask your boss and/or client for a piping spec. and/or drawing because they probably know how to get it and if they can't produce a Spec. see if they would put it in writing that they take full responsibility for having you make such a dramatic Spec. change from monel to carbon material.