What is the equation to calculate gas/pipe flow velocity?

Velocity of fluid in pipe is not uniform across section area. Therefore a mean velocity is used and it is calculated by the continuity equation for the steady flow as:

The answer is   V=Q/S                 V= velocity ( m/sec)

                                                          Q= Flow rate M3/S

                                                           S= pipe section in M2  

the flow calculation is based on Reynolds number 

Re= 123.9 p V D / u

V average velocity in ft/sec

D diameter in inch  

u is viscosity

p is density in lbs/ft^3 

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