Yani,

It should be noted that Section VIII-1 calculations for external pressure assume that all other loads are negligable. In the case of piping which is goverend by external pressure design bending, axial load or torsion may have a large impact on the stability of the pipe. The vessel code does not give any guidance as to how to include these loads in the calculations. That is typically the reason that full vacuum design (1 bar external) is called for.

Regards,
EJL