A polyvinyl chloride (PVC) pipe failure occurred during the winter. The pipe, an 8-inch sprinkler main, was located underground and a leak from the pipe was first detected when water was observed coming out of the ground. A plumber was called to the site, and he excavated down to the pipe to identify the source of the leak. The pipe was left in operation and uncovered overnight, during which the ambient air temperature dropped to 26°F from a high of 53°F during the day. Early the next morning, water was found flowing from the pipe at a high rate. The pipe was stamped as PVC 1120 DR18 AWWA Standard C-900, and it had a pressure rating of 150 psi. The water pressure at the time of the site inspection was reportedly 180 psi, but surges as high as 207 psi were also noted on the gages at the check valve for the building’s fire protection system.
The pipe was excavated and later examined in the laboratory. The pipe showed indications of rapid crack propagation that would be consistent with a high-pressure event. The crack origin was located a few inches outside of the bell and spigot connection. The fracture surface at the origin was discolored or stained in an elliptical pattern that extended over a length of approximately 5 inches. The difference in color and texture (fracture topology) suggests that this may have been a pre-existing crack that developed over prior to the leak and rupture incidents. At some point, the crack became unstable and grew suddenly and catastrophically, leading to the large-scale rupture.
In order to understand the effect of the pre-existing crack on the remaining strength of the pipe, a fracture mechanics analysis was performed. The analysis found that, at a pressure of 180 psi and a temperature of 26°F, the pipe would fail catastrophically. Moreover, with the 5-inch long elliptical through-wall crack in the subject pipe, brittle fracture was imminent. The cold temperature also reduced the fracture toughness of the material significantly.
Normally, a through-wall crack would have allowed water to leak out of the pipe in a relatively benign manner. In fact, plastic pipes are generally designed such that they can tolerate the presence of small cracks without rupturing. However, the combination of a relatively long crack and elevated pressure led to a situation in which catastrophic rupture was imminent. The exposure to cold air on the night of the incident further precipitated the rupture.
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