(0.15-0.01 in.)

(0.04 in.)

common pipe material—clean, new commercial steel or wrought iron—has been found to have an effective roughness of about 0.0018 in. (0.045 mm). Other surfaces, such as concrete, may vary by as much as several orders of magnitude, depending upon the nature of the surface finish. These roughness values are not measured directly but have been determined indirectly. Conduit surfaces artificially roughened by sand grains of various sizes were studied initially by Nikuradse, and measurements of f and NRe were plotted to establish the reference curves for various known values of s/D for these surfaces, as shown on the Moody diagram. The equivalent roughness factors for other materials are determined from similar measurements in conduits made of the material, by plotting the data on the Moody diagram and comparing the results with the reference curves (or by using the Colebrook equation). For this reason, such roughness values are sometimes termed the equivalent sand grain roughness.

C. All Flow Regimes

The expressions for the friction factor in both laminar and turbulent flow were combined into a single expression by Churchill (1977) as follows:

Equation (6-41) adequately represents the Fanning friction factor over the entire range of Reynolds numbers within the accuracy of the data used to construct the Moody diagram, including a reasonable estimate for the intermediate or transition region between laminar and turbulent flow. Note that it is explicit in f.

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