The flanges in piping systems are relatively standard components. These flanges are usually not calculated for strength. Standard bolt load tables are used for the assembly bolt force.
The pressure vessel body flanges or shell & tube heat exchangers flanges are usually custom designed flanges. The regular calculations according the Taylor Forge method will assess the stresses in the flanges and bolts. The calculation as per ASME VIII div.1 appendix 2 and the EN 13445-3 Clause 11 are using this calculation method. These calculations are suitable for assessment of the strength of the flanges. Some assumptions, like the bolt load, might lead to stiff design and/or thick flanges.
The comparison with the EN 13445-3 annex G and the EN 1591-1 will show differences and provides the engineer with valuable arguments to further assess the design. There are many dimensional design parameters to be entered for these calculations. A good sketch to scale in the FLG application very quickly shows faulty input, much better then only the values of the calculation report.