The following figure shows the comparison between the Hazen-Williams coefficient and the equivalent friction factor f values for a range of pipe diameters and flow velocities. [ For given CHW, D, v , the slope Sf was calculated from the Hazen-Williams equation and this was used in the Darcy-Weisbach equation to give the equivalent value of the friction factor f.] In this case the points for a given CHW all fall on a single straight line.
Two conclusions can be drawn from this plot :
- In the transition or smooth turbulent zone, the plot for CHW = 140 parallels approximately the relative roughness lines. This means that a given CHW is equivalent to a particular relative roughness not a roughness height and that CHW should increase with increasing diameter.
- For rough pipes, a particular CHW doesn't represent any particular relative roughness buy cuts diagonally across the relative roughness lines.
The Hazen-Williams equation does not describe rough pipes adequately and its use when CHW < 120 is hard to justify.