In Fluid mechanics The Hazen–Williams equation is an empirical relationship which relates the flow of water in a pipe with the physical properties of the pipe and the pressure drop caused by friction. It is used in the design of water pipe systems] such as fire sprinkler systems, water supply networks, and irrigation systems. It is named after Allen Hazen and Gardner Stewart Williams.
The Hazen–Williams equation has the advantage that the coefficient C is not a function of the Reynolds number, but it has the disadvantage that it is only valid for water. Also, it does not account for the temperature or viscosity of the water.
Pd = 4.52 × Q1.85 / C1.85 × d4.87
where:
Advantages to Hazen Williams approach
1. Coefficient Chw is rough measure of relative roughness
2. Effect of Reynolds number is included in formula
3. Effect of roughness on velocity are given directly