The PT rating of flanges depends for some material groups on a reference thickness vR. The definition for the vR might be unclear from the EN 1092-1 standard.
The EN 1092-1 explains the background of vR in F.2.4, however only that for different reference thicknesses there are individual PT ratings.
EN 1092-1 paragraph 5.1.2:
Methods of manufacture: see Table 1.
Mechanical properties are dependent from dimensions of unmachined parts („vR” for forgings, „t“ for flat products).
As the result of the machining this dimensions will be reduced. For machined flanges and collars the flange dimensions C1 to C4 or F shall not be less than 80% of „vR” for forgings, and the flange dimensions C1 to C4 or F shall not be less than 80 % of t for flat products.
EN 1092-1 paragraph F.2.4:
EN material standards are specifying strength values depending on different ranges of product thicknesses. The thickness related material values shall be used in calculation. vR shall be the upper thickness of each nominal thickness range for which a material strength value (ReH, Rp, Rm as appropriate) is given in the material standard. The calculation results of Formulae (F.2) and (F.3) shall be valid for all flanges of this European Standard provided the flanges are made of material with the same or higher material values as the values used for the calculation.
EN 1092-1 paragraph G.2.1:
Thickness related strength values for all thicknesses up to and including 150 mm have been considered, except for material 1.4903 where vR ≤ 130 mm.
The Annex G.1.2 it states that the value of vR depends from the product of material from which a flange is manufactured. For the different methods of fabrication see Table 1 of this European Standard'.
In Tabel 1 you will see that a weld neck flange can be forged, machined or for > DN700 bended and electric welded.
Therefore you have to consider the manufacturing method. In case of a forged flange, the hub might be additional forged.
An important statement is in para. 5.1.2: "... flange dimensions C1 to C4 or F shall not be less than 80 % of „vR” for forgings ...". These dimensions are related to the flange ring thickness. This would lead to the assumption that vR would be the flange thickness (at bolt hole) plus the raised face (optionally divided by 0.8), if applicable. The strange thing is that you can get two PT ratings for a pipe class if the larger flanges have a thickness of, for example, just over 50 mm.
Officially, we don't know the material thickness of the flange when the pipe class is developed. We advise our customers to use the lowest PT rating (largest thickness) because you don't know when you'll switch to a different line size in a piping system.