Dear paldex,
Let us say that the piping has operating temp T1, design temp T2 and minimum ambient temp T3.
The piping will be installed at the installation temp. When the piping will become hot it will see the operating temp T1 and if temp continues to rise it may see temp T2. Similarly the piping will also see T3 if it is cooled. In order to represent the above conditions we require the following load cases.

1) W+T1+P1 (OPE)
2) W+T2+P1 (OPE)
3) W+T3+P1 (OPE)
4) W+P1 (SUS)
5) L1-L4 (EXP)
6) L2-L4 (EXP)
7) L3-L4 (EXP)

The load cases 5), 6) and 7) take care of the three conditions mentioned above. But are these three load cases sufficient enough to satisfy the intent of the code regarding displacement stress range? Answer is No.
To completely satisfy the intent of the code, another load case must be added, as follows:

8) T2-T3 (EXP)

The above load case will take care of the condition that will cause the highest differential effect as mentioned in B31.3 para 319.2.3.b.
Hope it clarifies.