In my opinion there is no problem. The checks for internal and external pressures are different. Say in case of B31.3 the internal pressure effect is checked w.r.t hoop stress eqn. and the external pressure effect is checked w.r.t ASME Sec VIII Div 1 requirements.For your problem both checks are OK.In fact your net external pressure will be the algebraic sum of internal and external pressures.

Regarding longitudinal stress you only need to consider the internal pressure as it is this pressure which tries to open up the pipe in a plane perpendicular to the longitudinal axis, resulting in development of longitudinal stress to balance the internal and external forces.

Anindya Bhattacharya