You dont have to change the piping design code for high pressure piping.
For pressure > 430bar, additional precautions need to be taken, for which you need to refer Chapter IX - High Pressure Piping (B31.3)
Refer Table-K326.1 - Component standards.
It is permissible to use B16.5 Flanges, provided they meet all the requirements of the Chapter IX.
Hope this helps...
Regards,
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SKK