One of the biggest concerns for water utilities during extremely hot or cold weather is water main breaks. Water mains are expected to last a long time – as long as 100 years in many cases. But with manymiles of pipe buried underground, it’s reasonable to expect a particular section of pipe will fail or break at some point. The challenge for water utilities is to work proactively to minimize the number of breaks and to respond effectively when a main does break.
While the oldest water mains were made of wood, by the late 1800s, a variety of iron pipe was being used to construct water distribution systems. Common iron varieties included cast and galvanized in the early part of the 20th Century, with galvanized used primarily for smaller diameter pipe. Cast iron pipe was used until the late 1950s when stronger, more flexible ductile iron pipe became common. Plastic pipe, including Polyvinyl Chloride (PVC) and High Density Polyethylene (HDPE) became common in the 1970s. The primary difference between these two plastic pipes is that PVC is stiffer than HDPE, which is more flexible.
Even though pipe is expected to last for decades, that doesn’t mean it won’t break at some point. While it is impossible to predict specific pipe breaks, we know that environmental conditions are a major factor in water main breaks.
In the northern and northeast areas of the country where winters are more extreme, cold soils and cold water combine to add stress to pipes, which can—and often do—result in breaks. Iron, like all metals, contracts as temperatures drop. This problem is more common when the source water is surface water (rivers and lakes). These waters are significantly affected by air temperature and can drop to near freezing in the winter. A temperature difference of just 10 degrees in water or air temperatures can cause pipes to contract or expand. Additional stress inside and outside the pipe occurs as temperatures near the freezing point, making the pipe vulnerable to breakage. Water temperature changes more slowly than air temperature changes so the impact of cold water on pipes can cause breakage to take place as many as a couple days after temperatures freeze. Water systems with groundwater sources (wells) have more stable water temperatures because the water is not affected by air temperatures, and therefore, not as significantly impacted.
Just as pipes are adversely affected by cold weather conditions, they are also affected by severe heat. In some groundwater systems in the southern and southwestern states, the soils are like sponges and hold lots of water. However, during extended periods of hot temperature when high demands for water increases water withdrawal from the aquifers, the soil becomes very dry. In these conditions, the soil contracts and subsides, pulling away from the pipe and diminishing support for the water main. The absence of support for the main can cause it to break. This particular problem led the City of Houston, Texas to begin to convert its groundwater supply to surface water.
Although older mains are generally more susceptible to breaks, breaks can occur on newer mains. This is most likely the result of improper installation or a manufacturing issue with that particular section of pipe. By examining trends in water main breaks over time, a utility is better able to identify categories of pipe that are more prone to breaks, and thus proactively target that pipe for replacement. Aqua employs such tactics in determining which mains to replace. By the end of 2013, Aqua expects to have spent $170 million of its $325 million capital improvement program on water main replacement and associated work.