Pumps overcome persistent plugging
Located across the Columbia River from Portland, Ore., the City of Vancouver, Wash., has distinguished itself as a leader in effective sanitary sewer system management by maintaining adequate capacity, proactively maintaining sewage infrastructure and minimizing cost for existing and new ratepayers.
As is the case at many municipal wastewater utilities, some popular consumer products have proven problematic for elements of Vancouver’s infrastructure. For the crews that maintain the wastewater collection system of Vancouver, the disposal of household cleaning and personal wipes emerged as a costly and paralyzing issue at a strategic lift station in 2004.
Built in 2000, the Andresen Pump Station is the largest lift station in the Vancouver system. The Andresen facility brought online a 3500 gal per minute low-head facility with a predominantly commercial and residential flow from a gravity line. The low-head station pumps the wastewater into an immediately adjacent high-head, 4400 gpm pump station that is equipped with four 300 hp pumps. From there, the combined wastewater from another gravity facility and the direct flow into the high-head station are pushed through a force main over a hill with a 130 ft elevation before advancing to one of the wastewater treatment plants.
“When the pump station came online just before 2004, we noted routine maintenance incidents involving clogged pumps. As the years passed, the clogging and maintenance increased to a frequency of twice and sometimes three times per week,” recalls Frank Dick, P.E., an engineer with Vancouver’s department of public works. “It became apparent there was a direct correlation between household cleaning and personal wipes and the ragging that steadily sapped the flow and efficiency of the pumps. The increased incidences of clogging at the low-head lift station averaged three unscheduled maintenance dispatches a week. To clear the entangled impeller of a 40 hp pump at the low-head facility normally took two hours by an outsourced crew at a rate of $80 per hour.”
The city’s engineering staff received a recommendation that the Flygt N-pump could help prevent clogging at the low-head Andresen station. In field tests in which recurring blockages were an issue for wastewater collection utilities, the pumps not only eliminated clogging but also delivered significant power savings that would accrue during the expected service-life period of more than 50 years.
The reliability of an N-pump can be attributed to its semi-open, screw-shaped impeller, which prevents clogging and facilitates the unobstructed flow of fibrous material. Without material wrapping around the impeller and sapping pump flow and efficiency, energy savings often exceeded 45% during field tests thanks to the patented component. The leading edge of the rotating impeller passes across a stationary relief groove that is located in the pump suction port. The dynamic action cleans and pumps away any rags, stringy materials and solids from the impeller without compromising the system’s hydraulic efficiency.
The city approved the expenditure for the cost to replace the Andresen facility’s original pumps with Flygt N-pumps after Dick presented an electricity cost-savings analysis and a two-inch-thick file of work orders for unscheduled maintenance at the low-head station. “Each page represented a $160 expenditure,” he said.
By eliminating unscheduled maintenance, the utility stood to save at least $20,000 annually by replacing the original pumps with the highly reliable N-pump units. Electric utility reductions—proven to be at least 45% during the metering period—will further add to the total savings accrued over the life of the equipment.
Additionally, the original two low-head 25 hp Flygt units continued to run without ragging and never exceeded a 32 amp draw for more than a year before receiving their routine maintenance inspections.