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[b]Is it allowed to connect FF flange with RF flange in piping design?
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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.
Generally, no.
For many reasons, including leakage at the gasket, and damage to the flanges etc.
Solutions?
1. Get FF flanges added to your spec.
2. Go "Out of Spec" and use a FF flange.
3. Use a RF flange appropriate to the pressure rating of the FF flange, and machine off the raised face, essentially turning it into a FF flange.
All Subject to project / Client approval, of course.
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.