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RF flange connects to FF flange

RF flange connects to FF flange

RF flange connects to FF flange

We have this issue when connecting instrument to tank nozzle. Both flanges are 150# and made of CS A105.

Manager think it unacceptable and asked to provide adapter flange with one face RF and the other face FF.

I am thinking the connection is OK becasue gasket sealing is the same as between two RF flanges and RF<->FF flanges.

Also when adapter flange used with 2 gaskets, bolt toque has to double to make both gasket yield to create sealing. We have to calculate to verify if 150# flange can sustain that without pivotting around bolt hole and the edge.

I am asking if any code or book has some wording about this. I did see some RF<->FF coonections in power plant, not common , but not rare, as long as steel flanges.


RE: RF flange connects to FF flange

This thing happens if there is an inproper communication within the company. You are still lucky you have flange under the same class and code size on both sides.

I do not thing there is a need for the adaptor flange if the gasket is suitable for both side flanges and liquid/gas to be contained in case to surface finishes of flanges are adequate with the gasket used.

Make it simple principle is always valid, adding more is likely cause more problem if the leakage is concern. However, I do not accept that the torque value of bolts in two gasket in the connection will be twice the value for one gasket configuration. Both gaskets are to be under the same load required, if the contact pressure areas are the same, and therefore the same bolt load is sufficient for the adaptor flange connection.

I hope I understood your question correctly and it helps.

Ibrahim Demir

RE: RF flange connects to FF flange


I think that, with an adaptor, the problem will be worse. You'll have two leakage paths, not one.

We use RF flanges because they are more efficient in sealing, using less bolt forces to squeeze the gasket enough. FF, on the other hand, avoid bending on flanges - what is no a problem with A105. But you'll need a FF gasket and lots more of bolt forces - or a gasket thick enough to allow bendind in flanges ( this is the reason for rising the gasket seat, I think).

I think that saplanti is right - you're lucky having flanges that match, and are  both in CS.

RE: RF flange connects to FF flange

And the system will have the lower pressure rateing of a the FF flange.

RE: RF flange connects to FF flange

Thanks all.

I was wrong on the bolt force. The bolt torque should follow FF flange requirement.

What kind of gasket(FF or RF) I should use if FF flange to RF flange? RF gasket should be enough in my thinking.

RE: RF flange connects to FF flange

The first question to ask is: "Is the FF flange item Cast Iron or Forged Steel?  
Normally the FF (Flat Face Flange) terminology is associated only with Cast Iron items and the RF (Raised Face Flange) terminology is associated only with Forged Steel items.
So if the FF item is Cast Iron then you should indeed do something to eliminate the possibility of breakage of the Cast Iron item.
However, if the FF item is (by chance) actually a Forged Steel item then there should be no problem bolting them together.

RE: RF flange connects to FF flange

pennpiper has this nailed.  If the FF part is of brittle material (this includes cast iron, FRP or plastic materials amongst others), you do have a risk of breakage due to over-torquing of the studs.  If both parts are of carbon- or stainless steel, no problem and no adapter required.

RE: RF flange connects to FF flange

Please note that the original post clearly says that both flange materials are A105.

I would leave gasket selection to the responsible person who knows the content of the vessel and reads ASME B16.20.

Ibrahim Demir

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