We have had discussions of clad materials before on this site. Please use the search function to find them, if you need to.

Herr Luf is very much correct about the D/t ratio - for your pipe it is over 200 and the research underlying the B31 Codes generally has been limited to components with D/t less than 100. The reason for this is that at high D/t ratios, the pipe may ovalize and there would then be regions near the most highly curved portions of the pipe wall where local stress effects not considered by CAESAR II would be limiting. This is why julius2 is suggsting you use stiffening rings.

Also, your sketch gives no length dimensions. If one or more of your spools has a length about equal to the diameter, that's another warning that CAESAR II may not be the right analysis tool.

But, in general, I don't think that B31.3 would be my preferred design Code for your application. It could be required by your contract, though.

In the absence of a specified design code, I would much prefer to design this as a large piece of ductwork, with fabric expansion joints. With appropriate fabric materials (silicone rubber), you can easily withstand 240 C. Once you get proper fabric expansion joints specified, the rest can then be designed as a large duct, which in my experience is usually designed under the appropriate structural design code. Of course you then have to anchor the duct to limit the strain on the fabric joint.