The friction factor has been determined to depend on the Reynolds number for the flow and the degree of roughness of the pipes inner surface.
The quantity used to measure the roughness of the pipe is called the relative roughness, which equals the average height of surface irregularities (e) divided by the pipe diameter (D).
The value of the friction factor is usually obtained from the Moody Chart (Figure B-1 of Appendix B). The Moody Chart can be used to determine the friction factor based on the Reynolds number and the relative roughness.
Determine the friction factor (f) for fluid flow in a pipe that has a Reynolds number of 40,000 and a relative roughness of 0.01.
Using the Moody Chart, a Reynolds number of 40,000 intersects the curve corresponding to a relative roughness of 0.01 at a friction factor of 0.04.