How to calculate Metering Regulating Station (MR/S) Inlet and Outlet Pipe Diameter?

Where :                                                    

                           
D = Pipe Diameter (mm)                           
Q = Gas Flow Rate (Sm3/h)                                       
V = Gas Velocity (m/sec)
                                   
    Maximum Allowable velocity for unfiltered gas, such as inlet filter is 20 m/sec           
    Maximum Allowable velocity for filtered gas, such as outlet filter is 40 m/sec
                                                   
P = Pressure (Bar gauge)
                                   
    The most critical calculation where the pressure at minimum condition. Based on MR/S model number, critical condition will occurs at Minimum Inlet and Minimum Outlet