If you are interested in further references on this subject, I refer you to Pumping Station Design which has an extensive discussion. Below are a few paragraphs from the book.
The basis of the Hazen-Williams C factor in Equation 3-8 has resulted in some confusion. The factor is a function not only of the smoothness of the pipe wall, but also of the difference between the actual ID of the pipe and the nominal pipe size.
Matters are not improved by the apparent increase of C with diameter. According to AWWA Manual M11 [10], the average value of C for pipe with smooth interior linings can be approximated as C = 140 + 0.17J, where d is inside pipe diameter in inches. After a long term of lining deterioration, slime buildup, etc., C = 130 + 0. 16 d. However, above a diameter of about 900 or 1200 mm (36 or 48 in.), there is little increase in C values according to Gros [11], who has had many years of experience in measuring C values in the field. The values of C listed in the first part of Table B-5 reflect this experience.
In addition to the discussion above, there are other limitations on the value of C. Values of C less than 100 are only applicable for velocities reasonably close to 1 m/s (3 ft/s). At other velocities, the coefficients are somewhat in error. For water pipes, Lamont [12] advises the following:
• C values of 140 to 150 are suitable for smooth (or lined) pipes larger than 300 mm (12 in.).
• For smaller smooth pipes, C varies from 130 to 140 depending on diameter.
• C values from 100 to 150 are applicable in the transitional zone (between laminar and turbulent flow), but the scale effect for different diameters is not included in the formula.
• The formula is unsuitable and, hence, not recommended for old, rough, or tuberculated pipes with C values below 100.
• Force mains for wastewater can become coated with grease and C values may vary down to 120 for severe grease deposition.
In the past (before 1940 or 1950), it was common to line steel and cast-iron pipe with hot coal-tar dip, which provided poor protection and allowed C values to drop from 130 for new pipe to 100 or less for pipe in service for 20 yrs or more [13]. The modern use of cement mortar or plastic linings makes pipe very smooth, prevents corrosion and tuberculation, and maintains its smoothness indefinitely. In field measurements [14] made all over the United States on new water pipe with diameters of 100 to 750 mm (4 to 30 in.) lined with cement mortar, the values of C varied from 134 to 151 (median = 149, average = 144). For 150 to 900 mm (6 to 36 in.) pipe in service for 12 to 39 yrs, C varied from 125 to 151 (median = 139, average = 140) — a decrease of only about 5 units.
Water treatment often creates deposits that greatly increase friction in pipes. In one pipeline, lime incrustation reduced the measured value of C to only 80 downstream from the treatment plant. Pipe can, however, be cleaned and relined with cement mortar in situ and restored nearly to its original smoothness. Under some circumstances, deposits of bacterial slime in water pipes can change the smoothest pipe (whatever the material) into very rough pipe. Fortunately, chlorination destroys the slime and restores the former smoothness.
