Ductile Iron Pipe Hazen-Williams Flow Coefficient Declines Over Time

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John Houle: Senior Technical Consultant, PVC Pipe Industry

John Houle holds a Master’s Degree in Civil Engineering from the University of Missouri and an MBA from the University of Oregon. He has more than 25 years of experience in the plastic pipe industry in applications engineering, market development, forensic analysis, technical writing, and standards development.

Biased Results in Study on Water Pipe Materials

Posted By John Houle on Jun 24, 2017

In July 2016 a study was published titled “A Framework to Evaluate the Life Cycle Costs and Environmental Impacts of Water Pipelines.” The study, funded by the Ductile Iron Pipe Association (DIPRA), “aims to develop a Pipe Material Life Cycle Assessment tool (PMLCA) that is capable of analyzing different pipe material scenarios and suggesting the best option to the various decision makers.”

The paper could have been a welcome addition to literature on the subject. However, there is one significant problem area: the design example comparing DI pipe to PVC pipe. The example contains questionable assumptions that have biased the analysis in favor of DI pipe.

Skewed Design Assumptions 
The paper contains three key assumptions that create bias:

  1. DI pipe has an unrealistically long service life
  2. PVC pipe has an unrealistically short service life
  3. DI pipe’s Hazen-Williams “C” factor is constant instead of steadily declining with age for DI pipe

Setting the Record Straight
There are many reasons why the assumptions should be modified (which would result in a much more objective analysis). For my Tech Brief on the subject, click here.

The authors of the study should revise their design example to make it more relevant for utilities that are considering both materials.