Common Pitfalls in Hydraulic Design of Large Diameter Pipelines: Case Studies and Good Design Practice

Abstract

When designing large diameter water transmission pipelines, some engineers rely on design rules-of-thumb or a previous project as a template, without recognizing the inherent differences of each project. For large-scale water supply projects, mistakes in hydraulic design, especially underestimating friction headlosses, can be magnified resulting in reduced system capacities, catastrophic failures, or potential litigation. One of the most common mistakes in hydraulic design of large-diameter pipelines is underestimating pipe resistance and friction headlosses. The Hazen-Williams equation is the most widely used method for calculating headlosses in pipelines because it is simple and easy to use. However, the Hazen-Williams equation is empirical and, for large-diameter pipelines, has a limited range of applicability. Conversely, the Darcy-Weisbach equation provides a better approximation of friction headlosses since it takes into account the pipe roughness and Reynolds Number for different pipe materials, and is valid for all pipe sizes and turbulent flow ranges. Although there is an abundance of evidence of the limitations of the Hazen-Williams equation, it is continually misused in the engineering industry. There are many other design issues that can cause serious performance problems with large-diameter pipeline projects if not taken into consideration during design. Additional common hydraulic design pitfalls include: underestimating effects of sediment and biological material in raw water sources, not accounting for aging of pipeline materials, inadequate pipe pressure class design, improper placement and sizing of air valves, lack of accurate transient and surge analysis, inadequate flow and pressure field measurement, and potential need for pipeline maintenance and cleaning. There have been numerous publications on the topic of hydraulic design and proper calculation of pipeline friction headlosses. However, the focus of this paper is to provide analysis through case studies of several major water supply systems that reaffirms the importance of utilizing proper hydraulic considerations. Common hydraulic design oversights and short-cuts can often result in capacity and maintenance problems for large water supply systems.