Managing Atlanta's Water Transmission Main with Transient Analysis: How Making Fine Adjustments Extends Asset Life
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When a surge tank system in operation since 1990 on the 48-inch ductile iron finished water transmission main coming from the Atlanta-Fulton County Water Treatment Plant (AFCWTP) began to fail in 2010, the City of Atlanta’s Department of Watershed Management (DWM) initially looked at a steep repair bill to restore the system. The surge system consisted of two 20,000 gallon underground tanks located in a physically restricted commercial parcel in the town of Johns Creek, both connected via a 36-inch steel surge line to the City’s transmission main. The surge tanks were taken off line in 2010 due to apparent leaks and subsequent repairs needed at the tanks and pipe connecting the tanks to the main. The cause of these leaks spurred an investigation by the DWM and Jacobs’s staff in January 2011 that concluded in May 2012 and included these major activities: Physical inspection of the two surge tanks and the connecting surge line with the assistance of a pipeline contractor who had excavated down adjacent to one of the tanks as well as a portion of the steel line. Recommendations on rehabilitating the tanks to put them back into service as well as further investigation and possible rehab options on the surge line which included short-term additional cathodic protection measures to arrest deterioration of the steel pipe. A transient analysis of the Atlanta main from the AFCWTP through to the connection to the City’s distribution system south of the suburban city of Sandy Springs near the low point at the Chattahoochee River. The final activity, the transient analysis of the main, was done to determine if the surge tanks at the Johns Creek location were needed and should therefore be rehabilitated along with the surge line based on how the system was configured and being operated. Approximately $500,000 went into excavating the facilities there, doing the investigation, cathodic protection, and restoration of the site and the City wanted to ascertain the need to make further investment in those facilities. The transient analysis was revealing in that other operational and maintenance items, like sizes, locations and operability of air release valves (ARVs) as well as interconnects with adjoining systems could have as much impact on protecting the transmission assets as the operation of the surge tanks.