Large Diameter Pipes & Critical Sewers

Which of your pipes keeps you up at night? In most cases, large diameter interceptor or trunk lines are the culprits keeping you awake when the rest of us are sleeping. These critical pipes are the arteries of the system that transport large volumes of wastewater from the collection points to the treatment plants for processing.  Often these pipes are located in areas of dense population within busy urban areas, making them difficult to access, inspect, and repair.  Failure of these critical sewers can lead to catastrophic consequences on the surface. How do you best inspect these most important pipes to avoid these kinds of catastrophes.

Critical Sewers: More Equal Than Others

To understand this concept of some pipes being more equal than others (and more important), you can begin by looking at the best practices for pipe inspection. Currently, CCTV is the de-facto industry standard. Dozens of different tractor, camera, and software vendors provide the capability to drive through a pipe and produce images and videos from the pipe interior. Those on the leading edge employ NASSCO PACP 4.2 certified software to produce portable and interchangeable defect databases that interface with a wide range of asset management software. CCTV is suitable for diagnosing a wide variety of pipe features and defects, particularly when acquired by skilled operators, with proper lighting, and when using the right software. However, CCTV provides only a picture – qualitative data at best.  While this is fine for “low consequence pipes,” (the kind that do not keep you up at 3 am), additional information is essential for accurate diagnosis of critical sewers.

A more rigorous approach to inspection is prudent because the stakes are high when dealing with large diameter or trunk pipes (“critical pipes”). Critical sewers require accurate measurements of pipe conditions in order to correctly evaluate and extend life expectancy. While video such as CCTV is a necessary element of any pipe inspection, it is not sufficient for critical pipe. Measurements of sediment volume, H2S gas levels, internal geometry, and pipe inclination and meander combine to provide a much more thorough insight into pipe condition. This makes finding the most effective rehabilitation methods more likely than with video alone.

Here is an example of how combining CCTV with measurable information can reduce costs:  A CCTV inspection may clearly show that a critical sewer pipe is corroded and requires future rehabilitation. However, the correlations of sediment data with measured H2S gas levels will actually show the source of the corrosion and may indicate that periodic cleaning will significantly reduce gas levels and slow or stop corrosion. Further direct measurements of internal geometry may reveal that internal corrosion rates have not yet propagated beyond the rebar and, as such, point repairs to protect exposed rebar are possible.

Often, when faced with the much higher expense of a comprehensive rehabilitation or replacement of a critical asset, a program of point repairs in conjunction with a cleaning regime is a worthy alternative to consider. Whether preventing a catastrophic failure, or enabling better predictions on the likelihood and causes of eventual pipe failure, the upfront investment in newer technologies pays for itself many times over.

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