I suppose this topic has been discussed many times in the past (found some relevant discussions) - yet posting it for expert opinion and clear understanding of the subject.

For a piping system being designed as per B31.1, we have following typical temperature conditions:
Average Ambient (expected condition during installation) : 24ºC
Minimum ambient : -4ºC
Design temperature for the piping system: 220ºC

The prevalent practice in the organization I have recently started with is to analyze the model by defining the ambient as 24ºC and operating temperature (T1) as 220ºC. The minimum ambient temperature was not used at all in the analysis.

Considering the fact that there is theoretical possibility of the plant shutdown during the coldest ambient and the metal temperature reaching the minimum ambient (or a nominated temperature, close to the minimum surrounnding), I believe the above philosophy is wrong.

To check for the correct full thermal stress range, I feel we should do one of the followings:
i) Define ambient as the minimum site temperature, ie, -4ºC. The only downside I can think of is that the physical thermal expansion (movement) will be calculated for -4º to 220ºC which is not correct.
ii) Define another operating condition for T2 = -4ºC @ 0 barg pressure, which will enable checking the thermal stress range for T1-T2

Would appreciate suggestions from experts as to what is followed in other organizations for this common situation.

regards,