Question: 

Appendix A is not applicable for U-type heater, but UW-20 is excepted from appendix A. If the U-type heater design is full strength or partial strength welds for tube-to-tubesheet joints, UW 20 should be applicable for the weld size calculation. In current version of AutoPIPE Vessel software, it's not allowed to choose applicable codes neither UW20 nor App. A regardless of the joint types. that's why no verification on the tube-tubesheet joint in the reports. If it's seal weld, it can be no necessary to check the weld size, but it is not reasonable no weld size verification if it's full strength weld or partial strength weld.

Answer: 

U-tube cannot have their welds checked with UW-20 because in a U-tube there are no axial loads. There is no Ft. Welds cannot be full nor partial because there is no axial load. Ft is only calculated for non U-tube tubesheets.