City of tarpon springs assesses critical ductile iron force main

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Courtesy of Pure Technologies

Across the United States, there are many thousands of water and wastewater utilities that serve populations less than 50,000. Although the majority of attention surrounding aging infrastructure focuses on the challenges of large utilities, these small utilities are often faced with greater challenges.

Smaller utilities often have fewer resources – both financial and personnel – devoted to managing their water and wastewater systems. At times, this can lead to the utility having  less information available about their system, such as pipe drawings, break and leak history and condition data.

Coupled with having fewer resources, small utilities often have primary mains that are non-redundant and represent the sole source of supply or collection for the population, making a leak, rupture or shutdown of any kind very disruptive.  

The City of Tarpon Springs, FL serves a population slightly less than 25,000. With limited resources and a mandate to provide both reliable water supply and wastewater collection for its customers, the City decided to assess the condition of one of its primary 14-inch force mains that experienced a failure in summer 2013. 

The Dixie Highway Force Main is made of 14-inch ductile iron pipe (DIP) and poly-vinyl chloride (PVC) pipe, which was installed after the failure. In summer 2014, the City decided to complete condition assessment on nearly 1 mile of the force main to identify specific areas of concern before investigating further replacement.

Since internal hydrogen sulfide corrosion is the primary cause of DIP force main failure – and was the cause in 2013 – an inline survey was completed to collect relevant condition data.

For the inspection, the City used the SmartBall tool, which can locate leaks, gas pockets and pipe wall stress in metallic pipelines. Leaks or failures on wastewater pipelines can have a devastating effect on the environment and can lead to litigation and consent decrees. In addition, gas pockets in force mains are of significant concern as hydrogen sulfide gas within the wastewater can be converted to sulfuric acid by bacteria in the slime layer on the pipe wall, which may cause corrosion and eventual breakdown of the pipe’s exposed surface.

While inline leak and gas pocket assessment is a well-developed approach for force main operators, the development of pipe wall assessment (PWA) technology provides a more comprehensive level of condition information – areas of the pipe wall with damage will be under more stress than areas with limited or no damage.

By identifying stress anomalies, it provides operators with a detailed report of areas that warrant a more detailed assessment or testing.

The SmartBall assessment identified no leaks and nine gas pockets along the force main. Three of the gas pockets are located along the PVC section of pipe, indicating that gas pockets re-emerged in the PVC section of pipe in less than a year after replacement. It was recommended that air release valves be installed along the force main to clear gas pockets.

In addition, the PWA survey identified six areas that indicated stress within the pipe wall. One of the stress anomalies corresponds with a transition from buried pipe to exposed pipe, and therefore is caused by the change in load. The remaining five PWA anomalies do not correspond to any known features and could represent pipe degradation. The City during the insertion of the air release valves will be performing some field validation of these pipes.

By assessing the entire force main in advance of replacement, the City of Tarpon Springs is now able to make more informed decisions about its critical asset while avoiding the costly and mostly unnecessary strategy of replacement of the entire force main length. This mentality is an excellent example for other small utilities that are looking for ways to manage aging critical infrastructure, since replacing assets is very expensive within limited capital budgets.

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