Flexible Pump Technologies Increase Energy Efficiencies and Reduce Sewer Overflows
The Turtle Park Pump Station in Oak Ridge, Tennessee, is the largest and most critical sewer pump station in the city’s wastewater system. It receives all combined flows from the city’s collection system that feed to the Turtle Park Wastewater Treatment Plant, representing approximately 95% of all sewer flows in the city. By 2017, the station had become outdated, utilizing two 40-year-old intermediate shaft-driven pumps and two direct coupled non-submersible pumps located within a dry pit.
The motors, electrical distribution, and control systems located on the upper floor, ranging from 20 to 40 years old, had also reached the end of their useful life and were no longer reliable. Due to equipment age, frequent failures and limited means of station bypass, the city of Oak Ridge recognized the pump station needed a major overhaul.
By Nazmul Zaman and Dudney Fox, Xylem
Originally built in the late 1970s with a firm capacity of 14.1 million gallons per day (MGD), the Turtle Park Pump Station was upgraded in the late 1990s to a firm capacity of 30 MGD. In lieu of replacing the 1970s pumping equipment during those initial improvements, two 15 MGD pumps were added.
For the latest project, the city of Oak Ridge sought to replace the antiquated systems while protecting the environment from potential releases due to high flows. In addition, the city sought to reduce its electricity usage and carbon footprint with new, more energy efficient technology.
The project team had to overcome several challenges during the project’s design and installation. The existing drywell configuration was challenging, as tight clearances between the pumps made maintenance very difficult. The compact drywell, which measured only 16 feet, also significantly limited available pump and piping configurations despite the need to accommodate a wide range of flows; from a minimum of 1 MGD to a peak of 30 MGD. Additionally, the limited space meant a limited means of bypass pumping at the existing station. Finally, the electrical systems needed to be replaced, all while maintaining service during construction.
The utility’s desire for two tiers of pump capacity, smaller pumps to accommodate dry weather flow conditions and larger pumps for wet weather flow conditions, was a further challenge to the station layout. The average daily flow requirement was 4.2 MGD with a range from 1.0 MGD to 7.6 MGD. By providing different capacity pumps, the team could meet one of the city’s goals and provide maximum hydraulic efficiency and energy savings.
The existing suction isolation valves also required replacement, which necessitated a complete bypass of the station. As existing bypass capabilities were limited to dry weather flow rates, the team needed a new approach to ensure no over-flows occurred during the construction period.
Submersible Pumps Saves Space
FOXPE, the city’s engineer for the project, worked with Xylem’s application engineering team to design a successful pumping solution that overcame the station’s legacy constrictions to meet the utility’s goals.
The Turtle Park Pump Station previously had conventional dry pit pumps with long line shaft and motors located on another floor. The new solution was to provide T-installation vertical dry pit submersible pumps, which are very compact. With a submersible motor, the pumps can be installed under the flood level of the station, allowing them to run even if the station gets flooded.
Flygt NT3356 and NT3315 pumps were selected to meet the station’s desire to be more flexible and efficient. Two small capacity vertical dry pit submersible pumps are now used during dry weather without compromising the station’s efficiency, while the three larger capacity pumps can run during wet weather and peak flow conditions.
Using two different pump sizes proved challenging for FOXPE in terms of piping design, but the compact nature of the pumps allowed them to fit the modern, efficient design into the available space. FOXPE and Flygt were able to fit five new dry pit submersible pumps and associated piping in a space which had previously housed four.
The team accomplished the entire upgrade during a period of repeated heavy rainfall in one of the wettest years on record. The new bypass system was able to handle all incoming flows, and all bypasses were completed without over-flows to the environment. This was especially important due to the location of the bypass immediately adjacent to East Fork Poplar Creek.
The existing electrical switch gear also needed replacement while at the same time maintaining operation during construction. The installation team used a phased approach to ensure uninterrupted operation.
Benefit of Increased Peak Capacity
The project team completed the pump station upgrades in December 2018. Through the first five months of use, the pump station realized a 7% reduction in electrical consumption despite rainfall that resulted in higher than average flows. Thus, the project accomplished one of the city’s goals of becoming more energy efficient.
The greatest impact of the upgraded station, though, has been realized in the reliable and higher firm capacity of 32 MGD from the new pumps; this exceeds the design capacity of 30 MGD. “The station’s ability to deliver 30 MGD to the Turtle Park WWTP during high flow events is crucial to the city’s efforts to reduce and eliminate over-flows in the collection system,” explained Tom Roberts, maintenance superintendent for the City of Oak Ridge.
Other benefits include renewed mechanical reliability, protection of the environment, and reduced station noise. Thus, the team accomplished all of the city’s goals within the confines of the legacy station.
The end result is an efficient, resilient, and robust main wastewater pump station. Early collaboration among key partners enabled the project to be completed under budget and without any over-flows.
About the Authors
Nazmul Zaman is a senior applications engineer with Xylem Inc. Zaman has a Bachelor of Science in mechanical engineering from Bangladesh University of Engineering and Technology. For more information, visit xylem.com.
Dudney Fox is a principal of FOXPE, an engineering firm in Nashville, Tennessee, that provides solutions to the water and wastewater communities. He holds a Bachelor of Science in mechanical engineering from Clemson University and a master’s in civil/environmental engineering from the University of Colorado – Boulder.