Suppress the Pressure load in the buckling analysis. What is your environmental temperature, is it 20 or 22 C? Let’s say it is 20 C. A Temperature of 100 C means a delta T of 80 C. Try a temperature of 21C, which is a delta of 1 C. Now run the buckling analysis. What is the Load Multiplier (eigenvalue)? Let’s assume the multiplier is 2.345. Now set the temperature to 22.345 and run the buckling analysis. Do you get close to a 1.0 Load Muliplier?

Now suppress the Temperature load in the buckling analysis. Set the Pressure load to 1 MPa. Solve the buckling analysis. Get the first positive Load Multiplier for this analysis. Set the new Pressure in MPa to the value of the Load Multiplier. Do you get close to a 1.0 Load Multiplier? Note that you must ignore any negative Load Multipliers since they represent a negative pressure in the pipe (partial vacuum) which causes bucking more easily than positive pressure.

This exercise shows you the size of each load to independently get to the onset of buckling.

I expect a very small temperature increase causes buckling while a very large positive pressure causes buckling.