Corne,

Just so that things don't get off track, would you please edit your post and change:
L3=L1-L3 (EXP)
to:
L3=L1-L2 (EXP)

Thanks.

By the way, the piping codes do not address this sort of issue directly.
B31.3 para. 319.2.3(b) implies that the expansion stress range is:
"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)".

If L1 & L2 are the extremes, that's what you should use.