ASME PCC-2 Para. 3.1, Under-thickness of a raised face

[StaticPressure]

ASME B16.5 Para. 6.3.2 says a raised face may be removed from a raised face flange to convert it to a flat face flange, provided that the required dimension, tf, shown in Fig. 6 (Fig. II-6 of Mandatory Appendix II) is maintained.
ASME PCC-2 Para. 3.1 says under-thickness of a raised face due to refinishing shall be acceptable, provided the minimum finished height of the raised face is 0.8 mm (0.031 in.).

I would appreciate it if somebody could explain to me why PCC-2 wants to keep 0.031" of the raised face when B16.5 allows to complete removal. Is that for serration of the face?

 

[BJI]

B16.5 allows removal of raised face when converting to a flat faced flange. PCC-2 requires maintaining 0.8mm of raised face when refinishing, i.e. still using the flange as a raised face flange. This is to minimize the risk of the flange contacting the centering ring and rotating of the gasket sealing element.

 

[StaticPressure]

Thanks BJI for quick response.