Low Velocities in Force Mains: Impacts and Solutions
- B. Copeland, S. P. O'Rourke - Hazen and Sawyer
Low velocity is a common problem experienced in many wastewater force mains (FMs). The primary concerns with operating FMs at low velocities relate to solids deposition and accumulation of gas pockets, both of which can lead to pipe deterioration and increased life cycle costs. As utilities work to better understand their assets, extend asset life, replace aging infrastructure, and reduce costs, they are often encountering the impacts of low FM velocities—whether or not they recognize it. This presentation will explain the issues associated with low FM velocities, including flow behaviors of solids-liquid flow, relevant standards, consequences, contributing factors, and potential solutions. Case studies will be presented to demonstrate some of these considerations.
The goal for FMs is to operate at adequate velocity to achieve a somewhat-turbulent, heterogeneous suspension without solids deposition and to periodically achieve flushing velocity, while balancing low-velocity concerns with the consequences of high-velocity—e.g. excessive headloss, increased operating pressure, exacerbated surge conditions, and higher pumping costs.
Two well-known references, “Ten States Standards” and “Pumping Station Design”, as well as numerous other standards and publications, recommend that wastewater pumping facilities be designed to maintain a minimum velocity of at least two feet per second (2 fps) to avoid solids deposition in the FM. Many of these sources further recommend that systems periodically achieve flushing velocities of at least 3.5 fps. However, there is some inconsistency between engineers and from project to project in how these guidelines are applied. For example, adjustable-speed pump systems may be designed to achieve 2 fps at minimum pump speed, at average pumping rate, or only at full pump speed; and multi-pump systems may be designed to achieve 3.5 fps at firm capacity or only with all pumps operating.
During this presentation, participants will gain a better understanding of why low velocities in wastewater FMs are undesirable, the typical causes of low FM velocities, the resulting problems, and potential solutions to avoid or remedy low velocities. Proper understanding of these issues will help engineers to make better design decisions; allow utility operators to better understand issues associated with O&M of existing FMs, which will help them to make better decisions; and reduce overall life-cycle costs of wastewater pumping facilities.
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