Installation temperatures provided by the client from a gas processing plant site as below:

Max. Surface Temp. (direct sunlight) = 85 C
Max. Ambient Temp. = 46 C Min.
Ambient Temp. = 0 C
Working Fluid (Propane) Operating Temperature = -20 C

In the Caesar Stress Analysis, design temperature T2 which is 46 C has been used. The question is why 85 C is not considered instead? This line is one of the compressor side load piping.

Any suggestions?

Direct sunlight typically applies to BARE (uninsulated) lines.