Restriction Orifice

Dear friends,

I need to depressurize a vessel containing natural gas at 270 barg. When sizing the Restriction Orifice ( downstream pressure 1 atm) I got the message that critical flow will occur and maximum pressure drop is 123 bar. I mention that before the Restriction Orifice the pipe class is 2500 # and after the RO is 150 #.Do I need to specify a higher pipe class because of the downstream pressure calculated by the program (147 bar) ?

Thank you for your help!