Hydraulic Friction Factors for Pipe Flow

Hydraulic friction loss in pipelines directly affects pipe and pump sizing, as well as the hydraulic balance of networks. All friction head‐loss equations have an uncertainty in the estimation of pipe interior surface roughness. When a single resistance coefficient based only on the pipe material is used, different equations may give significantly different estimates of frictional head losses depending on pipe size and water flow rate. In complex pipe network systems, minor differences in equations or coefficients may accumulate and result in vastly different predicted head losses or flow rates in individual pipes. This study presents mathematical relationships and nomographs to relate the friction factors of three widely used pipe friction equations and to determine the magnitude of the differences in calculated head losses. Friction factors are developed to be used with the Hazen‐Williams and the Manning equations for PVC and cast‐iron pipes that give similar calculated head losses to those of the Darcy‐Weisbach equation. The variation of the friction factors with diameter and flow conditions is presented, and computer implementation of the variable friction factors is discussed.