Excuse me!
Which temperature will be used in stress analysis,flexible temperature or operation temperature?Here Code B31.3 does not show it,it just request that the most severe condition must be considered.The flexible tmperature sometimes much higher than the operation temperature,If we use the flexible temperature to analyse,there must be some conditon distortion.So we can not choose spring and design piping supports according
to the flexible temperature.In my opinion,we shall analyse piping stess using the operation temperature,and the flexible tmeperature just for checking stress.
Any suggestion?

Use the Temperature at Maximum Sustained Condition to analyse the lines.

Nope... Read the code more carefully... the intent of the code is to calcululate the maximum displacement stresses and their range, and the sustained stresses in all possible conditions albeit capture the highest sustained stress.

The use of the terms design pressure and design temperature refer specifically to pressure design, the analysis of displacement and sustained stresses may or may not require the use of these numbers...

Buy some reference books on the intent of the code, and study them.