B16.5 Lap Joint Flanges / Dissimilar metals 2

[Tuckabag]

Hi.
I am sure I am overlooking something here, but I cannot find reference in B16.5 with regards to pressure/temp rating of lap joint flanges using dissimilar metals.
I am looking to use an A182 F316/316L hub and A105N flange. My assumption is that the P/T rating of this connection will be to the A105 rating as the flange is the pressure retaining component (we are connecting to a W/N A105N flange).
Can anyone help shed some light on this for me? (or point me to the correct section of B16.5)
Cheers.

 

[LittleInch]

I don't believe B 16.5 goes into that much detail.

I've seen this before when people have tried to argue that the A105 backing ring isn't a the same temperature as the lap joint and therefore has a higher rating than if you took the process temperature.

IMHO this is not the way to proceed, especially if the joint is insulated.

So I think you are correct - take the worst case material and use that at the process temperature of the hub flange and you can't go wrong. Anything else is pushing the boundaries too far.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Krausen]

Tuckabag - That's correct. B16.5 has no bearing on stub end/hub fittings. Your P-T rating is governed by the material of the lap flange (A105 - Group 1.1).

 

[moltenmetal]

The lap joint stub end is designed as a piping component. The only thing that matters is its wall thickness, and of course the quality of the weld between it and the pipe.

The flange rating is determined by the material of the backing flange, assuming the correct bolting has been selected as well. The lap joint stub end material has nothing to do with the flange rating.

Unless you can do modelling that demonstrates that the backing flange will be at a temperature substantially cooler than that of the pipe, you can't take credit for heatloss in selecting the MAWP of the flange. That goes for flanges of any construction. Whenever we rely on heatloss to drop a flange class, we put temperature measurement on the flanges, with high temperature alarms.

 

[Tuckabag]

Thanks for your feedback.
This is an odd application. We are having custom forgings of A182 F316/316L for the stub with a 6mm bore (sample probe application). So there us a massive amount of WT and we have a socket weld connection on the other side for a DBB valve.
We need to use 316SS for the sample stream but require A105N flange rating for the P/T requirements.
The lap joint arrangement seemed to fit the bill which is when I hit a wall with B16.5.

Appreciate the clarifications, we'll proceed with offering this design to the client for approval.

Cheers!