According to page 6-26 of the user manual
for version 4.5:
"if the user has an operating temperature below
ambient in addition to one above ambient the user should add another load case as follows:

Case#6 L2-L1 (Exp)

(assuming t1=hot t2=cold)

Am i right in thinking that this effectively
an addition of the cold stress to the hot stress?
ie a Subtraction of a negative stress is an addition?

If this is so, is it correct as the pipe will pass from the cold case (t2) through its relaxed (ambient) state to reach the hot case t1?

Also, is this a code specific requirement as
B31.3 does not require the addition of stress's
that pass through ambient.

I am particulally intrested in this as i am often
doing calculations that pass from cold to ambient to hot.
and i assume the pipe is relaxed as it passes through the ambient stage.