I have to disagree with Bob. The (Operating - Thermal) case will calculate a sustained stress based on the hot position of the pipe without changing those +Y's to Y's. Maybe this model change would eliminate questions but if you're going to go throught that work you might as well just remove the +Y's that are not active in the operating case and run the sustained case alone. But as Bob says, a search should uncover several of these conversations.

(Operating - Thermal) will account for the incremental change in the sustained stress (or really, the longitudinal stress due to sustained loads) as the load from thermal strain is incrementally added to the system (that is, as the system heats up).

I have no comment on occasional loads.