A few months ago, I wrote about...
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., has an extensive wastewater management system. 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.
The Need for Retrofit
Gravity is the driving force for wastewater distribution to the treatment plant, except where there are vertical elevation changes in the topography. In these circumstances, lift stations are used to move wastewater to the treatment plant.
Because of ongoing increases in demand, Dave King, the city’s wastewater maintenance manager, is continually looking for ways to improve efficiency. Lift Station No. 16, built in 1958 and located close to downtown Lawrence, transports about 600 million gal of dry-weather flow per year, about 25% to 30% 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 completely in the spring of 2006. King turned to the city’s long-time PLC supplier, Logic, Inc., to help develop a solution.
ACS800 drives were installed on each of four new 125-hp pump motors to: increase efficiency and power factor, improve electricity cost savings, extend the life of the motors, reduce repair costs and minimize or eliminate harmonics that can interfere with instrumentation.
Ensuring Constant Flow
When wastewater enters a lift station, it initially goes through a course screening process, which removes solid particles that might cause damage to the 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 the wastewater from the well to the treatment facility. Drives often play a key role in supporting the pump operation, ensuring that a constant flow is discharged to maintain optimum wastewater levels.
“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.”
Previously, the site 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. It would run at full speed until it reached the off-level elevation, and then it would turn off. The continuous starting-and-stopping process caused wear and tear on the bearings and impellers, causing the motors to deteriorate.
Preventing Water Hammers
By maintaining a consistent level in the lift station wet well, drives also help to avoid water hammers. When a pump starts or stops suddenly, it slams the check value on the back of the pump and creates a water hammer effect (pressure surges caused by the energy of a fluid in motion when it is forced to stop suddenly). In a lift station, hydraulic water hammers often occur when trying to pump fluid vertically or at a slant. Once a pump stops, the fluid will come back to the source, jarring the piping. Over time, water hammers can lead to pipe deterioration. Because the ACS800 drives provide acceleration and de-acceleration ramp to the pumps, wear and tear on the pump mechanicals is reduced.
Drives & Bypass
Drives also incorporate a soft-start function that gently ramps speed to limit potential turbulence and reduces the need for maintenance. The drives respond to sensors in the wet well that are relayed to a PLC unit via DeviceNet. The 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 standby at all times, running them alternately to balance wear. The station operates 24 hours per 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.”
Because wastewater is constantly running throughout the city, another important feature to the lift station is the bypass function, which allows a manual override of the pump if a drive was 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 downtime is not an option.”
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.”
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 to eliminate low-order harmonics. With an active front-end inductor/capacitor/inductor line filter to reduce high-frequency harmonics, the city did not have to worry about overheating feeder breakers or transformers.
ABB offered 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. ABB was able to provide all that from a factory service center,” King said. “The lift station fits in a 900-sq-ft space and the ACS800’s compact footprint fit well into the facility.”
Because the drives were shipped with all of the specifications pre-installed, installation was smooth and easy. To maintain constant uptime during the retrofit, two pumps were taken offline at a time, keeping the other two pumps running.
A Continued Relationship
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 No. 16, the city is installing ABB drives in four other wastewater pump stations now under construction.
“I anticipate extended pump life, electric cost savings and ultimately better service to our customers as a result of ABB drives,” King said.