How to keep one chamber only with this type of design?


I'm checking the design of an horizontal vessel with an openable head on one side.

I'm starting from a simple horizontal vessel, from which I delete one head. Then I add the body flanges and finally, I add a new head. 

Problem: I now have 2 chambers and the calculation doesn't run properly.

I also tried to keep the original head, but then I cannot locate the body flanges properly.

Does someone know how to proceed?

Thanks in advance for your help!