You can get the good information from "How to Stress Analyze Jacketed Piping" published by Hydrocarbon Processing on Feb.1978.
Also, you can find more information in this Discussion Forum, posted on Aug.30, 2000 and Mar.04,2003.
The worst differential expansion case must be considered for conservative analysis.
For example; if Core is polyethylene with the temp.(DT=150, OT=70) and Jacket is steam(DT=170, OT=150) with same carbon steel for both pipes,I would model Core=70 and Jacket=170 for conservative design. The case of different pipe materials will be more complicate but you can find the worst case temperature combination.
Regards,
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Sun Wee