In early 2023, the Warren County Utility District (WCUD) in middle Tennessee was faced with a dilemma. The water tank in their Short Mountain (SM) pressure zone required cleaning and repairs as part of federal compliance regulations, but with only one tank in the pressure zone, as is the case with many rural municipalities, they had few options to maintain water service to customers while the tank was out of service during the washout process and while repairs were completed. WCUD maintains a 4.0 MGD Water Treatment Plant, six storage tanks, four remote pump stations, into four pressure zones with approximately 650 miles of mains to serve 9,000 customers in rural portions of Warren and five adjacent counties.
To avoid a catastrophic suspension of service to roughly 400 customers in the SM Zone, WCUD turned to Danfoss and its FC 202 variable frequency drive (VFD). The drive also includes PID (proportional, integral and derivative) control, which allows it to maintain discharge pressure in a closed-loop system. By installing the VFD and discharge pressure transducers on the booster pumps, WCUD was able to maintain water service with proper pressure into the SM zone with the tank out of service for three weeks. Instead of feeding water from the tank, the drive was able to effectively convert the booster pump normally used to fill the SM Tank into a source – supplying all water into the SM Zone and pumping it through the system without needing to send the water through a tank.
The VLT® AQUA Drive FC 202 provides built-in intelligence for water and wastewater applications. Its variable speed technology enables it to match the daily load variation in treatment plants and pumping stations. By providing motor control on pumps and blowers, the drive can lower energy consumption and ultimately, operating costs. The VFD feature also decreases wear and tear on equipment, resulting in lower maintenance costs and increased lifespan.
Dedicated pump-specific features such as cascade control, deragging, drive pump detection, end of curve detection, motor alternation, two-stage ramps (for submersible or vertical turbine pumps), pipe fill mode and smart logic controller provide process control that can be tailored to meet the various demands of water and wastewater plants. Using the VFD’s soft start capabilities in lift station applications helps reduce wear on the pump as it turns on and off to empty the tank, thereby reducing maintenance costs.
Through condition-based monitoring (CBM), the VLT® AQUA drive can act as a smart sensor to monitor the condition of the motor and application to detect early issues so that station personnel can find solutions before performance issues have an impact on the plant’s functioning. This is done through edge computing and features functions such as motor stator winding condition monitoring, mechanical vibration monitoring and load envelope monitoring. Early detection is especially important for municipalities with limited resources. The lack of a cloud subscription requirement reduces cyber-security risks for the plant.
Maintaining the Infrastructure
WCUD was familiar with Danfoss, as they were already using Danfoss drives on multiple pumps at other sites. The familiarity and the service and support available from WASCON, the Danfoss product representative in their area, were key factors in their decision to install the FC 202. Anthony Pelham, WCUD general manager, noted that the installation and programming process required less than one day, and the soft start capability made the operational transition smooth.
For the duration of the cleaning and repairs project, the drive allowed the pump to maintain water supply into the zone at the programmed discharge pressure even with the SM tank out of service. The drive is linked to a discharge pressure transducer to provide a feedback loop to the VFD. With its variable-speed technology, the VLT® drive modulates the motor speed to control the pump flow and maintain the set discharge pressure. When flow increases and pressure drops, the drive speeds up. When the flow decreases and pressure drops, the drive slows down.
Instead of pumping the water to fill a tank and shutting down until the tank drained to low level, the booster pumps ran continuously for the three-week period, effectively becoming the entire source of water and pressure control for the SM zone, thanks to the FC 202. As a result, the 400 customers served by the station continued to receive water with no disruptions.
The FC 202 drive continues to provide benefits post-repairs. With its soft start-up, the drive can reduce the power spike from the motors on start-up and provides a better power factor in daily operations, helping reduce energy consumption. The drive also performs an invaluable role as the system backup: if a problem with the tank should occur, the drive can keep the pump operating and the water flowing to customers.
“The FC 202 drive helped us maintain water service and system pressure without any loss of service to customers, even with our tank out of service,” said Pelham. “It was a perfect fit for our pumping station, user-friendly, with ease of access and installation.”
“Rural municipalities have limited financial resources and equipment and taking tanks out of service for required maintenance can present a challenge,” said Judy Finch, regional sales manager at Danfoss Drives. “The VLT® AQUA DRIVE FC 202 can provide a cost-effective and energy-efficient solution to maintain operations and infrastructure.”