PIPE-FLO Professional and Flow of Fluids offer both the Darcy Weisbach and Hazen Williams equations as calculation options. There can be significant deviation between these methods so it is critical to choose the method most suitable for the application at hand. The Darcy Weisbach is currently the preferred method for most incompressible fluid applications, but some industry standards still require the use of the Hazen Williams.

The Hazen Williams equation, empirically derived with water, was once in wider use as it provided for simple, non-iterative solutions for hand calculations. This simplicity led to its use to approximate losses for fluids other than water, despite the fact that fluid properties are not factored into the equation. With the proliferation of cheap computing, spreadsheets and dedicated piping system software it is just as convenient to use a more complicated, intensive solutions like the Darcy Weisbach method.

PIPE-FLO applies unit conversion of flow rate and headloss to the original Hazen Williams formula to create a version of the equation that incorporates the density of the fluid. This provides for the proper calculation of fluid headloss as well as better correlation with the Darcy Weisbach with changing fluid density. Unfortunately, as the relation between the two is not directly proportional, changes in parameters can have an unpredictable impact on the deviation between the two methods. More obvious, is the effect of viscosity on the results. Hazen Williams does not consider changes in viscosity and as such should only be used for fluids with viscosities around 1.1 x 10-6 m2/s (1.2 x 10-5 ft2/s), similar to water at 60°F. As fluid viscosity increases so should the differential pressure, but as shown in the chart below, the Hazen Williams values remain unchanged.

The Darcy Weisbach method provides superior results across a wide range of incompressible fluid applications and some compressible applications. It is widely accepted and easily implemented with the assistance of software like PIPE-FLO. Keeping that in mind, it is hard to recommend Hazen Williams for any application where it is not implicitly required. Fortunately, in PIPE-FLO, switching from one to the other is a single click on the units tab of the Systems Settings & Properties dialog box.