Hello Shabeer,

if you go for a plate flange ,design it with the help of appendix 2 for both internal as well as external pressure ,which is the inside pressure of the vessel.

Also there is a clause in 2-11 in appendix 2 of ASME section VIII Div:01 which states that

[/b](b) When flanges are subject at different times during
operation to external or internal pressure, the design shall
satisfy the external pressure design requirements given in
(a) above and the internal pressure design requirements
given elsewhere in this Appendix.[b]

clause says "Different Times" ???

Hope this helps

Regards

ASD23789
(Beginner)