Hi SKK

B16.5 Flange we can use until 2500 pressure rating as stated by B16.5.
Beyond that we need to go for
1)special flange design ( Ex UDHE do that design)
2) Special design Lens ring is used between the flanges
3)There is limitation in force & moment ( Indicated by vendor) in each lense ring
4) High Pressure Piping (B31.3) is ok to use.

Regards

Habib