Creep-Regime Flange Design

[TGS4]

I wanted to poll the members of the forum on their opinions on this questions. We are currently designing a piece of pressure equipment for a design temperature of 1842°F. We are using a hot-wall design, and are using Haynes Alloy 230. There are no Div. 1 allowables at that temperature, nor are there B31.3 allowables at that temperature. Regardless, due to the extremely short life of the component (at that temperature) we have developed allowable stresses. That was the easy part.

Now, for the design of the flanges, or more specifically, the bolting for the flanges. We are also specifying Alloy 230 for the bolts. The question has come up - what "allowable" stress should we use for the bolts?

Option 1 - 2/3 short-term yield at temperature
Option 2 - 0.5% creep at XXX hours life at temperature
Option 3 - Option 1, but at a lower temperature
Option 4 - Option 2, but at a lower temperature
Option 5 - something else

What would you do?

 

[gr2vessels]

Well, I have to start something, otherwise we'll all look overwhelmed by the above description. Since we are out of ASME VIII range, we might as well ignore it all together. But we just can't do that...
1. Assuming that the flange OD and the bolts will not be insulated, ie a perforated mesh will provide personnel protection if needed, then the design = operating temperature can be determined in many ways and will be much less than 1842F, hence higher allowables for bolts than the flange itself.
2. The 2/3 short-term yield at temperature assumes only elastic /thermal expansion of the bolts (refer also to Clause S1), which might be correct for flange, but not for the bolts.
3. The creep of the bolt material would allow first a small leak of the flanged joint, before plastic collapse. Hence I believe that the creep stress at the 'lower' temperature should be used.
Cheers,
gr2vessels