Are there conditions which derate Lap joint flanges?

[hollerg]

I have operating scenarios that could overpressure an existing 600 class flanged nickel piping system. The flanges are the weak link rated for 720 psig up to my maximum operating temperature of 500 F. I plan on replacing the RFWN with a lap joint. I would use straight grade 316 Lap Joint backing flanges, and stub ends that meet 16.9 requirements. However in looking at 16.9 and MSS - 43, I did not see the following addressed explicitly.

Since the lap is as thick as the pipe, but made from the softer metal, is the joint derated in any way, or does it match the lessor of the backing flange rating Vs the rating for the pipe components?

 

[moltenmetal]

Far as I know, if the pipe is good for the P&T, so is the lap joint stub end.

If the backing flange is good for the P&T, so is a combination with a lap joint stub end of adequate thickness.

Never seen anything which said otherwise, but then again I haven't seen everything.

 

[XL83NL]

moltenmetal got it right. What you should do is, I think, calculate the required thickness of your lap joint stub end, to match your required/desired p,T-rating for the line class. Express the size just like you spec a pipe. The LJSE is a B16.9 standard component. Make sure to spec the right facing on the stub end, and if you require long or short pattern. (I think differentation is made based on installation required - not sure though)
The LJ flange, which actually is a B16.5 backing ring, also has a p,T-rating, like moltenmetal said.
See which one limits your spec.

I think there's some guidance in B31.3 (or B16.5) on limitations of LJ flanges, e.g. suitability in cyclic servies, but Im not sure. Then again, I'd reckon one uses exotic materials based on corrosion resistance rather than high pressure/cyclic piping.