Basis for Thermal Expansion Calculations

The following is from the Linked in “Piping Design” Group discussion about PDS & PDMS.
“All these programs work out the expansion stress in pipe for given piping material, design pressure and design temperature and given principal overall dimensions. They are helpful in checking viability of a preliminary piping layout. Computation of expansion data, modulus of elasticity and allowable stresses are automatic.”

This is not the only place this show s up, it just happens to be the current reminder of something that has always annoyed me. It annoys me because it is more costly to a project.

Background:
We as pipers try to be very precise about the work we do. We base our work on carefully developed P&ID’s, Specifications, Codes and use certified Outlines from vendors. We even have rigid fabrication tolerance (PFI ES3) for our pipe assemblies.

Here are my questions:
Why is “Design Temperature” used for the expansion calculation instead of the actual maximum Operating Temperature?
Why do we base our Thermal Expansion Stress Analysis on such a fictitious premise?

If your answer is "Because the 'Code' says so" then define WHY the Code says so.

I got this response from David Diehl of CAESAR II.

"B31.3 defines design pressure and design temperature to establish wall thickness. That's it.
Stress and load calculations in B31.3 do not mention "design" values.
So use actual, expected temperatures and pressures if you wish. BUT I am concerned that someone, in the future, will read those design values, assuming that the system can be run at those conditions without further evaluation, and that may prove to be trouble. Note that CAESAR II does not explicitly request design pressure and design temperature."
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Dave Diehl