Here's what B31.3 paragraph 319.2.3(b) says:
... "the algebraic difference between strains in the extreme displacement condition and the original (as-installed) condition (or any anticipated condition with a greater differential effect) remains substantially constant during any one cycle of operation. This difference in strains produces a corresponding stress differential, the displacement stress range, which is used as the criterion in the design of piping for flexibility."...
You are referencing "the extreme displacement condition and the original (as-installed) condition" and I am talking about "any anticipated condition with a greater differential effect".
Again, in my example T1 is above ambient and T2 is below ambient.
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Dave Diehl