With 450 miles of sanitary-sewer collection lines and 33 lift stations feeding into a central wastewater-treatment plant, the City of Lawrence, Kan., wastewater-management system is extensive. Lawrence is one of only two cities in the nation with 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.
NEED FOR RETROFIT
Built in 1958 and located close to downtown Lawrence, Lift Station No. 16 is responsible for transporting approximately 600 million gal. of dry-weather flow — about 25 to 30 percent of the city's wastewater usage — a year. The combination of aging equipment and increasing flow from new residential developments led to the decision to retrofit the station during the spring of 2006. Dave King, the city's wastewater maintenance manager, turned to the city's long-time programmable-logic-controller (PLC) supplier, Logic Inc., for help.
Logic recommended installing ACS800 drives from ABB on each of four new 125-hp pump motors to improve efficiency, extend motor life, reduce repair costs, and minimize or eliminate harmonics that threatened to interfere with instrumentation and overheat feeder breakers and transformers.
ENSURING CONSTANT FLOW
When wastewater enters a lift station, it goes through a course screening process to remove solid particles that can damage pumps. The wastewater then flows into an enclosed wet well, which can be 30- to 40-ft deep. Sensors monitor the depth of the wastewater in the wet well. Pumps are used to displace wastewater from the well to the treatment facility. Drives often play a key role in supporting pump operation.
“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.”
PROBLEMS WITH CONSTANT SPEED
Previously, the station's pumps were based on an on/off level and used an across-the-line starter. When the wastewater 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 led to mechanical stress on the motors and pumps. Motors would run at full speed until reaching “off”-level elevation and then turn off. The constant-speed starting-and-stopping process resulted in wear and tear on bearings and impellers, causing the motors to deteriorate.
PREVENTING WATER HAMMER
In lift stations, hydraulic water hammer often occurs when fluid is pumped vertically or at a slant. Once a pump stops, fluid will come back to the source, jarring the piping. Over time, water hammer can lead to pipe deterioration. Because the ACS800 drives provide acceleration and deacceleration ramp, wear and tear on pump mechanical elements is reduced.
The drives incorporate a soft-start function that gently ramps speed to limit potential turbulence and reduce the need for maintenance. The drives respond to signals from 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 wastewater is being pumped out consistently. Although three pumps are needed to run the station, the city keeps an installed fourth pump on standby at all times, running the pumps alternately to balance wear. The station operates 24 hr 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.”
The drives' bypass function is another important feature, allowing manual override of the pumps.
“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.
PROTECTING POWER GRIDS
With the lift station located next to a residential district, harmonic distortion must be minimized. Considering the amount of dynamic load on the 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.”
ACS800 drives do not require a multipulse transformer, external filters, or other additional equipment used to minimize harmonics. The drives feature an active converter with direct torque control to eliminate low-order harmonics. With an active front-end inductor-capacitor-inductor line filter to reduce high-frequency harmonics, the city does not have to worry about overheating feeder breakers or transformers.
King was impressed with ABB's ability to offer turnkey solutions direct from the factory.
“We required certain features in the cabinet, such as a bypass, fuse disconnect, remote I/O, and selector switches,” King said. “ABB was able to provide all that from a factory service center.”
Featuring a compact footprint, the ACS800 drives fit well inside of the 900-sq-ft lift station.
Because the drives were shipped as specified, installation was smooth and easy. To maintain constant uptime during the retrofit, two pumps were taken offline at a time.
Information and photograph courtesy of ABB.
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