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The surface roughness of cast iron uncoated pipe (ε) is 0.00024m
The average velocity of the liquid can be determined from the volume flow rate as:
     V = 4Q/πD2
Substitute the above expression for the velocity into the Darcy-Weisbach equation to yield:
     hL = f(L/D)(V2/2g)
     hL = f(L/D)(4Q/πD)2/2g
Rearrange terms and the diameter of the pipe is given by
     D5 = 8LQ2f / hLgπ2
          = 8(400)(0.12)2f /...