With 450 miles of sanitary sewer-collection lines and 33 lift stations that feed into a central wastewater treatment plant, the City of Lawrence, KS, wastewater management system is extensive. The city has been recognized as one of only two in the nation that have three environmental management certifications for wastewater operations. These certifications demonstrate the city’s ability to provide customers with an efficient level of service, while maintaining a high standard of environmental protection.
Lift station number 16, built in 1958 and located close to downtown Lawrence, is responsible for transporting approximately 600 million gallons of dry weather-flow a year, about 25-30 percent of the city’s wastewater usage. The combination of aging equipment and increasing flow from new residential developments led to the decision to retrofit the station in the spring of 2006.
Dave King, wastewater maintenance manager, turned to the city’s long-time PLC supplier, Logic Inc., to help develop a solution. An ABB distributor since 1997, Logic recommended installing ACS800 drives on each of four new 125 hp pump motors. The goal was to increase efficiency and power factor to improve electricity cost savings; extend the life of the motors and reduce repair costs; and minimize or eliminate an addition of harmonics that can interfere with instrumentation and overheat feeder breakers and transformers.
With the old system, the station ran the pumps based on an on/off level, and used an across-the-line starter. When the level in the wet well reached a designated point, it would trigger the starter to get the pump motors up to maximum speed as quickly as possible. This rapid acceleration created spikes and mechanical stress on both the motor and pump. The station would run at full speed until it reached the “off” level elevation. The constant-speed, starting-and-stopping process caused wear and tear on the bearings and impellers, causing the motors to deteriorate.
The new drives will help stabilize pumping for the wet well. Since the lift station fits in a 900 square-foot space, the ACS800’s compact footprint fit well into the facility.
“By controlling pump speed, an optimum level can be maintained that ensures constant flow between what’s coming in and what needs to be pumped out,” King said. “Installing the ABB drives has enabled us to eliminate unnecessary pump starts.”
The ACS800 drives also incorporate a soft-start function that gently ramps speed to limit potential turbulence - and reduce the need for maintenance. The drives respond to sensors in the wet well that are relayed to a PLC unit via DeviceNet. The wastewater treatment plant communicates with the PLC unit at the lift station via Ethernet to ensure that wastewater levels are being pumped out consistently, based on established parameters. Although three pumps are needed to run the station, the city keeps an installed fourth pump on stand-by at all times. The pumps are alternated to balance wear. The station operates 24-hours a day and cannot afford downtime.
“ABB drives give us the ability to handle the wide range of variations in wastewater flow entering the station due to time of day, time of year and weather conditions, as well as the capacity to accommodate future growth,” King said. “The fact that we can now pump wastewater out at the same rate it enters also helps us reduce odors.”
Since wastewater is constantly running throughout the city, another important feature to the lift station is the by-pass function, which allows a manual override of the pump if a drive were to go offline.
“Even though modern drives are very reliable, if for any reason we were to have multiple drive failures, the bypass gives us the ability to still operate the station,” King said. “At this location, station down-time is not an option.”
Minimizing Harmonics
Because the lift station is located next to a residential district, minimizing harmonic distortion is a necessity. With the amount of dynamic load on the existing transformer, standard drives were not a viable option.
“Excessive harmonics can interfere with pump instrumentation,” King said. “Plus, we didn’t want to cause any interruption to the electrical appliances of our customers.”
The Ultra-Low Harmonic (ULH) ACS800 drives do not require a multi-pulse transformer, external filters or other additional equipment for minimizing harmonics. The drive features an active converter with Direct Torque Control (DTC) to eliminate low-order harmonics. With an active front-end LCL (Inductor, Capacitor, Inductor) line filter to reduce high frequency harmonics, the city did not have to worry about overheating feeder breakers or transformers.
Turnkey Solutions
The Lawrence utility required a variety of features in the cabinet of the drives, such as a by-pass, fuse disconnect, remote I/O and selector switches, King said. ABB was able to provide everything required direct from the factory in a turnkey solution.
Since the drives were shipped with all of the specifications pre-installed, installation was simplified. To maintain constant uptime during the retrofit, two pumps were taken off-line at a time, keeping the other two pumps running.
The lift station retrofit was not the first instance where the city has benefited from using ABB drives. The wastewater treatment facility also has ABB drives equipped with bypass to increase efficiency. Because of the positive experience with the drives at lift station number 16, the city is installing ABB drives in four other wastewater pump stations now under construction.
