2.14. Friction-Factor Diagram. Factor f in the Darcy formula (2.18) is some function of Reynolds number and the degree of roughness of the inside surface of the conduit.
Moody” has related these factors as shown in Table 2.2 and Fig. 2.8.
The Table 2.2 ROUGHNESS INDICES F OR. VARIOUS TYPES OF PIPE Pipe aerial .
Roughness Factor e Riveted steel 0.003 ~0.03 Concrete 0.001 -0.01 Wood stave 0.0006—0.003 Cast iron 0.00085 Galvanized iron 0.0005 Asphalted cast iron 0.0004 Commercial steel or Wrought iron 0.00015 Drawn tubing 0.000,005 relative roughness factor is the roughness factor c divided by the pipe diameter in feet.
The relative roughness factor for a particular pipe is referred to in Fig. 2.8 and identifies the curve to be used for selecting a satisfactory f value. For example, a 3-in. (inside diameter) commercial steel pipe has a relative roughness of 0.0006. This value identifies the proper curve to be used in Fig. 2.6.
If Reynolds number is found to be 9 >< 104, the friction factor f is 0.021.
Friction factors for water and atmospheric air can be determined from the values of VD from the top of Fig. 2.8. Air is flowing at 900 ft per min in a 20-in. galvanized iron pipe.
The relative roughness is 0.003, which identifies the proper curve of Fig. 2.8. The product of VD is 300, which provides a friction factor 3‘ of 0.027.
If Reynolds number is desired, it can be read directly from the VD position. It is 1.4 >< 105 in this case.
Note that the velocity term V shows in the formula for determining Reynolds number (2.8), in Darcy’s formula (2.20), and in the Bernoulli formula (2.7).
Solution of a probleiri in which the velocity is known is a straightforward arithmetical procedure.
On the other hand, if velocity is to be determined, solution must be by trial and error or by a graphical procedure.
Trial-and-error solutions are usually satisfactory, but the graphical method gives more accurate results.
This method is demonstrated by the following example.