Transmission Main Assessment in the City of Phoenix: Where Have We Been and Where Are We Going?

The City of Phoenix (City) has been an industry leader in transmission main assessment and renewal since the early 2000s. The City operates a water system which includes nearly 7,000 mi of mains and serves nearly 1.5 million customers over more than 500 square miles. The City’s approximately 600 mi of transmission mains range in diameter from 16 to 108-in. Responding to worldwide failures of prestressed concrete cylinder pipe (PCCP), the City began its transmission main program with a focus on assessing all 150 mi of PCCP in its system. The City has now assessed more than half of its PCCP and has an established program in place to complete assessment of all PCCP by 2027. The City’s PCCP assessment program initially focused on mains constructed in the 1970s, when relaxed PCCP standards resulted in pipe that is more susceptible to environmental and operational damage. Data collected over 75 mi of PCCP assessment shows that the City accomplished its goal of focusing first on the most at-risk pipe and that the overall damage profile of City PCCP as a function of installation year correlates well to national averages. The program has improved the operational resiliency of the City’s transmission mains by documenting how to isolate and dewater mains for inspection and how to move water to compensate for mains out-of-service for inspection. Effective mitigating actions and risk-management strategies have emerged which can be applied in future transmission main assessments. For PCCP, the City identifies pipe in need of operational modifications or renewal through a combination of internal inspection techniques and technologies. The worst mains in the City’s system have been identified for renewal using steel sliplining or replacement. The City has completed nearly 10 mi of steel sliplining and approximately 3,000 ft of PCCP replacement as a result of its program. Operational modifications have been identified as an important strategy to reduce risk of pipeline failure. PCCP condition data collected in Phoenix shows a clear link between transients, operating-to-design pressure ratio, and pipe damage. The City has implemented a system-wide pressure and transient monitoring program focused on eliminating surges and reducing pressure in at-risk PCCP. As the City’s PCCP program has become established, the City has expanded its focus to additional transmission mains with a multi-year transmission main program focused on assessment of more than 200 mi of bar-wrapped pipe beginning in 2017. This paper focuses on the evolution of the City’s PCCP management program from 2003 to the present.