The relative roughness of pipe is defined as the ratio of inside surface roughness (e) to the diameter (D):

Relative roughness = e / D

Typical values of absolute roughness for various types of pipe

Type of Pipe	e (mm)
Glass or plastic	Smooth
Drawn tube	0.0015
Wrought iron	0.046
Commercial steel	0.046
Asphalted cast iron	0.12
Galvanized iron	0.15
Cast iron	0.26
Riveted steel	1.8

 

To determine the values of the friction factor for use in Darcy’s equation, we use the Moody diagram. If we know the relative roughness and Reynolds number, the friction factor can be determined easily. No curves are drawn in the critical zone, Re lies in between 2000 and 4000 because it is not possible to predict whether flow is laminar or turbulent in this region. At the left end of the chart (Reynolds number less than 2000), the straight line curves give the relationship for laminar flow:

f  = 64/ Re (where, R = Reynold number)