Article

Milwaukee Region takes proactive approach to manage metallic force mains

City of Milwaukee Skyline

Milwaukee is a water hub, and not just because of its location along the shores of Lake Michigan, which holds 4.3 percent of the world’s supply of fresh drinking water. The City also boasts of global leadership in water technology, having won a U.S. Water Prize for innovative watershed-based approaches toward water sustainability.

The City takes a proactive approach to water management initiatives, as evidenced in the recent condition assessment of the Franklin-Muskego Force Main. Ownership of the pipeline is shared between the City of Muskego and the Milwaukee Metropolitan Sewerage District (MMSD), the government agency that provides water management services for about 1.1 million people in 28 communities in the Greater Milwaukee Area.

Metallic valves

MMSD and Muskego request detailed structural assessment on metallic force main

In 2015, Pure Technologies (Pure) was contracted to perform a detailed condition assessment of the approximately 25-year old pipeline. The purpose of the assessment was to identify the structural condition of the metallic force main, and included pressure monitoring, a SmartBall® leak and gas pocket detection survey, a PipeDiver® electromagnetic inspection, and structural evaluations of the pipeline.

Notably, the latest investigation used electromagnetic technology delivered on the 24-sensor mini PipeDiver platform to validate inspections conducted the previous year along the same lines.

Ductile iron pipe is a challenging material to assess

The Franklin-Muskego Force Main carries wastewater along approximately 1.6 miles of 24-inch and 1.3 miles of 30-inch ductile iron pipe (DIP). A small section of 20-inch DIP force main was also included in the survey.

One of the challenges in assessing DIP is determining if the pipe has undergone any loss of wall thickness due to internal or external corrosion, which are the primary causes of failure. DIP in water service with a cement mortar lining generally has fewer internal corrosion failure rates, unless damaged during handling and installation, or later as a result of 3rd party damage.

This is not the case when DIP is used in a force main, where internal corrosion is the primary cause of failure. Gas pockets are of significant concern as concentrations of hydrogen sulfide gas within wastewater may be subsequently converted to sulfuric acid by bacteria in the slime layer on the pipe wall.  This may cause corrosion and eventual breakdown of the pipe’s exposed surface.

Gravity mains vs pressurized mains

In a force main, identifying internal areas with potential corrosion is challenging, as traditional gravity pipeline inspection techniques are often not applicable to in-service pressurized pipelines.

One method for assessing gas pockets is to locate air release valves (ARVs) or other high points along the alignment and provide pipe wall assessment in those areas. While this is a valid method for locating potential gas pocket locations, additional gas pockets may occur due to differential settlement, improper installation or non-functioning ARVs.

Therefore, these desktop surveys may not identify and locate all gas pockets along a pipeline, which is why Pure recommends other more precise survey methods.

SmartBall with case and insertion tools

SmartBall inspection summary

In June 2014 and October 2015 Pure performed a SmartBall leak and gas pocket detection survey of the Franklin-Muskego Force Main. Acoustic and sensor data was collected and recorded as the free-flowing SmartBall device traversed the pipeline.

During the 2014 survey, Pure detected zero (0) anomalies characteristic of leaks and one (1) anomaly that characterized a fully developed gas pocket.  During the 2015 survey, the SmartBall tool detected zero (0) anomalies characteristic of leaks and four (4) acoustic anomalies characteristic of fully developed gas pockets on the force main.

PipeDiver tool. insertion

24-sensor PipeDiver electromagnetic inspection

In 2014 Pure conducted a PipeDiver electromagnetic inspection, followed by a re-inspection in 2015, utilizing the new, 24-sensor electromagnetic PipeDiver tool. The technology ascertains a magnetic signature for each pipe section to identify anomalies that are produced by areas of corrosion or reduced wall thickness.

During the 2015 electromagnetic inspection using the mini PipeDiver, 13 pipes were found to have a total of 16 electromagnetic anomalies consistent with localized wall loss.

The electromagnetic inspection conducted in the 2015 inspection used an enhanced exciter coil allowing the electromagnetic field to return a more pronounced response. In addition to the enhanced exciter coil, the tool used in the 2015 inspection had a total of 24 receiving sensors, improving the ability of the tool to identify defects.

Confident conclusions

The results of the condition assessment indicate that the Franklin-Muskego Force Main is generally in serviceable condition, which was confirmed after an excavated pipe established a true baseline condition.

While the assessment recognized several areas with an increased likelihood of failure, overall the data was good, and coupled with Pure’s engineering recommendations, gave all stakeholders confidence in the health of pipeline for the near foreseeable future.