FF flange connection with RF flange

Is it allowed to connect FF flange with RF flange in piping design?


Case-1
3" firewater hydrant (End connection- 150# FF) to be installed where concern piping spec. contains RF flanges for 3" size.

Piping Mat - CS, 150#, Design pressure 16 Bar, Design Temp. 65 Degree C.


Case-2
3" RO plate (150#, RF) to be installed in firewater line but concern piping spec has FF flanges for 3" size.

Piping Mat - CS, 150#, Design pressure 16 Bar, Design Temp. 65 Degree C.





If FF to RF connections are not allowed, then we have to raise a request to Client for required flanges addition in respective piping spec.

It looks like you have a cast iron 125# flange with it being a fire line tie-in possibly. is it an existing tie-in.

It's not recommended to mate a RF steel flange to a FF cast iron flange because it can cause it to crack etc when you torque the bolts.

Solution here if you are in a hurry or at a pinch point is to machine down the RF flange and remember to use a full face gasket.

or as Anton says get one added (FF) to the piping spec and order it from your MTO.