Bom,

The (EXP) Expansion case is a "stress range". You compute a range by subtracting. The Piping Codes require that you check/verify the "extreme" displacement stress range.

In a situation where you have a single operating temperature, then the range is simply "OPE - SUS". However, there are other more complex situations, for instance:

Two Temperatures on either side of Ambient: In this instance, there are three ranges that must be checked (a) T1-ambient, (b) T2-ambient, and (c) T1-T2. In all probability, case (c) will be the "extreme" range that governs here.

Multiple Operating Pump Scenarios: In this instance, you have part of the system hot, part cold, and depending on the pump on/off status, these parts can change. As an exmple, take pumps A, B, C, where pump A is off. Now whem pump A is turned on, and pump B is turned off, you're probably not going to shut down the whole system. So your "range" isn't (T1-ambient), but rather (the Ta condition minus the Tb condition). If this was setup such that T1 is A off, T2 is B off, and T3 is C off, then you need to evaluate the following Expansion cases:

T1 - ambient
T2 - ambient
T3 - ambient
T1 - T2 (turning A on and B off)
T1 - T3 (turning A on and C off)
T2 - T3 (turning B on and C off)

All six cases need to be checked.