Hazen-Williams Equation

Introduction

The Hazen-Williams equation is an empirical formula used to model the friction head loss of water flowing through pipe. The accuracy of the Hazen-Williams is less than that of the C.F. Colebrook equation. This equation uses the coefficient C to specify the pipes roughness, which is not based on a function of the Reynolds number, as in other pressure loss equations. 

It is also possible to use Hazen-Williams to model fluids other than water as long as the viscosity is approximately 1.130 centistokes.

Calculating Head Loss

Where:

C      = Friction Factor (Hazan William Constant)

d       = Inside diameter of pipes (in)

Q       = Flow rate in Gallons per minute of water

L      = Length of pipe (ft)

hf     = Friction head loss (ft)

Limitations

1. it should only be used for water between the temperature of 55 degrees Fahrenheit (12.8 deg C) and 65 degrees Fahrenheit (18.3 deg C). The formula is popular with civil engineers who constantly need to make calculations for water flow through pipe in the ambient atmosphere.

2. Hazen William Equation can only be used in equation can only be used when water is flowing within the 'turbulent' flow range

Applications

The Hazen William formula has now become adopted through the world as the pressure loss formula to use for the hydraulic design of fire sprinkler systems and in almost all cases the use of the hazen william formula will provide adequate answers. The Hazen William formula can also be used for the calculation of water mist systems where the system pressure does not exceed 12 bar (low pressure water mist systems) or the water velocity does not exceed 7.6 m/s and the minimum pipe size is 20 mm in the case of intermediate and high pressure water mist systems.