Consequences of Under-Estimating Friction Losses in Wastewater Forcemains

An accurate estimation of energy losses caused by pipe friction is important for the analysis and design of forcemain systems. Friction losses in a forcemain are primarily dependent on the hydraulic roughness of the pipe; which is a function of the material, age, size, condition of the pipe, and characteristics of the wastewater. One of the most common approaches used to estimate the friction losses in a pipe is the Hazen Williams Equation, where the Hazen Williams roughness coefficient (C-Factor) is used to represent the roughness parameter and consequently friction losses. Therefore, appropriate selection of C-Factor is critical to hydraulic calculations of a pipeline, as it would impact pipe sizing and can have a significant impact on pump selection. As a best practice, most engineers consider a range of C-Factors in the hydraulic calculations, representing new and aged conditions of the pipe. Hydraulic analyses performed by the Author of more than 14 municipal wastewater forcemains located in the United States and other available literature determined that C-Factors from in-service forcemains are much lower than typical values used in practice. Approximately 60% of forcemain systems evaluated are operating at C factors less than 100. Results suggest that the wastewater slime layer affects forcemains in a similar manner regardless of pipe material. Nearly all lift station pumps evaluated were found to be operating to the left of the design point which increases operational costs due to longer pump run times at lower efficiency. In many cases the actual capacity of the lift station was found to be significantly less than the original design capacity. This paper will compare the actual C-Factors calculated from in-service forcemains to the C-Factors used in the original design and identify the increased energy consumption and operational costs resulting from underestimating friction losses (i.e., over-estimating C-Factors). In addition, the paper will provide recommendations to identify ranges of in-service C-Factor values and proper selection of C-Factors to be used for wastewater forcemain design.