Limitations and Proper Use of the Hazen-Williams Equation

Abstract

The Hazen-Williams equation is used widely in water supply and sanitary engineering. This equation uses a constant, the Hazen-Williams C, to indicate the roughness of a pipe interior. Because of the empirical nature of the equation, its range of applicability is limited. Many textbooks and software manuals give C values based on pipe type, condition, and age but do not give the range of applicability. Historic experimental data is used to demonstrate that C is a strong function of Reynolds number and pipe size and that the Hazen-Williams equation has narrow applicable ranges for Reynolds numbers and pipe sizes. The level of error when the Hazen-Williams equation is used outside its data ranges is significant. However, a valid C for a given pipe at a specific Reynolds number can be used to estimate a pipe's relative roughness, which then can be used by the rational Darcy-Weisbach equation without the range limitations. A method for doing so is given.