You cannot really change the thickness of a long weld neck flange. They are set by ANSI B16.5. Therefore, to size your long weld neck flange, you simply determine the flange rating from the pressure/temperature using the charts in ANSI B16.5.
On the other hand, you still have to check the normal nozzle reinforcement calculations by doing ASME calculations from the applicable section.
outer diameter is fixed by B16.5. Bore can be amended by agreement with manufacturer between bore for weld neck flange and bore for lapped flange, but default is bore for a weld neck flange
As dimension X is bigger than a normal weld neck flange at the weld connection, you are normally Ok.
If you want to make sure, then use the same calculation as you would for your connecting pipes.
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