I´m a spanish Mech. Eng. and I've been working during 10 years in design of heat exchangers and Pressure vessels. Your doubt is totally clearificated in a good book called
Pressure Vessel Design Manual of Robert Moss , in the applicable design chapter about the flanges.
You can obtain Equivalent pressure using the following equation:
Peq = 16*M/(pi()*G**3)+4*F/(pi()*G**2)+P
using M bending moment(in-lb) and F radial load (lb)