Q=volumetric flow (given as 5E4 m^3/hr
A= pipe cross sectional area (A=pi*r^2) in m^2
v= fluid velocity in m/hr
or r=sqrt[Q/(pi*v)] if in the units above r (pipe radius) is in meters)
pipe diameter is 2 times the radius.
Now the problem is to find v (fluid velocity). This is a function of pressure drop, pipe roughness and fluid viscosity, or measured.