In case of analysis using CAESAR II, core and jacketed piping are easily modeled, if you model core pipe first considering spacer location, then copy the core and paste, modify diameter, thickness, pressure, liquid equivalent density and temperature for the jacketed pipe. Finally you can connect core to jacketed with CNode restraint.
Also, you have to take care the interference between core and jacketed at bends or else by reviewing the analysis results unless the restraints with gaps are not provided at these nodes.
The straight pipe with both ends connected to another pipe like jacketed becomes higher stress because the high axial load is generated by differential thermal elongation in case of the dissimilar material.
Of course, in actual jacketed piping has a layout with flexible loops to avoid such a high load for both semi and full jacketed piping.
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