Grand Forks Wasterwater Authority - North Dakota, U.S.A.
|Service:||SmartBall® - Leak & Gas Pocket Detection|
|Client:||Grand Forks Wasterwater Authority|
|Type of Pipeline:||Force Main|
|Diameter:||30- and 36-inch|
|Pipe Material:||PCCP and PVC|
|Results:||6 Gas Pockets|
In late October 2008, the Grand Forks Wastewater Authority (Authority) located in Grand Forks; North Dakota initiated a project to conduct a condition assessment of an 8.8 mile 30- and 36-inch wastewater force main. The Authority retained the services of Webster, Foster and Weston, Inc. (WFW) and Advanced Engineering and Environmental Services, Inc. (AE2S) to conduct a condition assessment study of the force main. Since force mains generally operate under pressurized conditions and lack redundancy or the ability to be removed from service for an extended time, traditional wastewater pipeline assessment techniques such as closed circuit television (CCTV) technologies are not feasible. After researching several pressurized pipeline inspection technologies, WFW/AE2S retained Pure Technologies to conduct a leak detection and air pocket inspection of the force main utilizing their SmartBall Technology. The project specified that the inspection be able to locate the presence and magnitude of leaks and air pockets in large diameter pipelines and all types of pipe material while the force main remained in services.
The force main varies between 30- and 36-inches and is comprised of both poly-vinyl chloride (PVC) and prestressed concrete cylinder pipe (PCCP) over its length. Pure Technologies completed the 8.8 mile inspection in two deployments over two days with a total of 10.5 hours in actual inspection time. The SmartBall technology collects several data parameters including flow velocity and the acoustical profile, which are used to identify and locate anomalies representing leaks and air pockets.
The SmartBall consists of a 7-inch foam ball with a 2.5-inch aluminum core that contains a data acquisition system. The device is released into the water/wastewater stream and travels the pipeline, un-tethered, with the flow. It rolls along the bottom of the pipe at 80 to 90% of the flow velocity. While it traverses the pipe, it records the acoustic activity of the pipe along with positional information. The device is captured at a point downstream and the data is analyzed to determine the presence, approximate size of, and location of leaks and air pockets along the pipeline. During the two deployments, different styles of extraction were used on the Grand Forks survey; for the first deployment, the standard under-pressure net extraction technique was utilized and on the second deployment the SmartBall tool continued in the flow and was removed at the headworks screening facility inside the treatment plant. The survey was performed while the pipe remained was in service.
The position of the SmartBall within the pipeline was critical for locating important features such as leaks and air pockets. The methodology used to track the tool involves obtaining a velocity profile using data obtained from the accelerometers on board the SmartBall. Then, absolute position reference points obtained from the SmartBall Receiver (SBR) are applied to time stamped data. Individual SBRs were able to track the ball’s progress through the pipeline. The result of the rotation profile and SBR tracking is a position versus time relationship for the entire run of the tool. Upon retrieval of the tool, the acoustic data recorded by the SmartBall was analyzed and cross-referenced with the position data from the SBRs to determine the location of the SmartBall throughout the inspection.
After the SmartBall survey was completed and the data analyzed, eight (8) acoustic anomalies were detected resembling leaks and air pockets (two (2) small leaks and six (6) air pockets). After validation of the suspected leak locations by the Authority and further research by Pure Technologies on other low pressure force mains, it was determined that there were no leaks and the anomalies were due to sediment buildup creating background noise that differs from the baseline acoustical profile within the pipeline. Based on this experience, Pure’s SmartBall technicians screened out the sedimentation buildup within acoustical profile. Collection and validation of this information is critical in the effort to better understanding inspection results within wastewater force mains.
Further validation of the SmartBall survey results was conducted utilizing a tradition closed circuit television (CCTV) inspection during a shutdown of the force main due to a repair. While the CCTV survey was not able to inspect all of the reported anomalies, physical evidence of two air pockets was confirmed in corrosion at the crown of a valve and the pipe. Hydrogen sulfide hasprogressively corroded the crown of the pipe through the concrete core and steel cylinder exposing the prestressing wires. Since the prestressing wires are the primary structural member in the composite pipe construction, continued corrosion of the wire is of significant concern.