North Beach force main maintenance

Project status

King County has completed an updated assessment of the NBFM. The last service life assessment was carried out in 1989. Based on previous coupon sample testing results and anticipated useful life range estimates another set of tests were performed. In August 2012, the County’s contractor acquired cored samples from the NBFM, a pressure pipeline extending from North Beach Pump Station to the Carkeek Park Pump Station/Wet Weather Facility. In addition, a corrosion expert acquired samples of soils and groundwater at the location where the pipeline sample was removed.

Analysis of these samples indicates that the North Beach Force Main could provide 30 - 40 more years of service. The new tests indicated that pitting continues to occur, but based on remaining pipeline wall thickness, the observed pitting rates, and current levels of graphitization, the force main has decades of useful life remaining. They also concluded from the analyses that the conditions do not indicate imminent failure of the pipeline.

King County will continue to monitor the condition of the pipeline over time. The County expects to find that it requires rehabilitation or replacement some time prior to the 30 - 40 years remaining life indicated during the recent assessment, but not in the immediate future. For more information, you can download the final report here.

King County’s latest assessment

King County’s most recent assessment used information provided by an Acoustic Listening Device (ALD) called a SmartBall and GIS technology to identify areas of trapped gas that would likely be the areas of higher internal corrosion. Evaluating these areas can provide a more conservative estimate of remaining service life. The ALD is a technology that was not available in 1989.

Project need

The North Beach Force Main runs parallel to the Burlington Northern Railroad tracks for approximately 4,700 ft within the tidal zone of Puget Sound. Flows from the North Beach Pump Station are conveyed through the 14-inch diameter, 7,490-foot long cast iron force main to the Carkeek Wet Weather Facility where they are either pumped to the county’s West Point Treatment Plant or, in large storm events, treated on site.

The County establishes a design life of about 50 years for pipelines like the North Beach Force Main (NBFM), but in practice, the pipelines often last longer than that. To avoid premature replacement of usable assets, King County performs inspections and condition assessments throughout the life of wastewater pipelines. King County carried out a project to evaluate the condition of the North Beach Force Main, installed in 1963.