Metallurgy Case Study
The failure of water pipes in a high-rise building can lead to massive damage throughout the building, especially if the failure occurs on an upper floor. Since utilities, like equipment for rooftop based cooling towers, are commonly placed on the upper floors, water releases are not uncommon. Such an event occurred due to the failure of a 1-inch PVC elbow in the cooling water system of a high-rise building in Philadelphia. Significantly, the failure occurred only months after the piping was installed.
Did the failure result from improper installation, e.g., applying excessive forces, or was the PVC elbow itself defective?
An undamaged elbow installed in the system is shown in Figure 1, showing that the pipe joints are threaded and a Teflon type of sealing tape was used in the joints. It is not immediately obvious from this photo, but the joint was also lubricated with a Vaseline-like grease in order to reduce friction and make it easier to tighten the joint.
A photograph of the broken elbow is shown in Figure 2. The failure initiated on a large, pre-existing crack then propagated around the elbow, leading to the separation. The stresses that drove the crack are the same as those that would result from over-tightening the pipe in the elbow. A magnified view of the crack in Figure 3, shows that the pre-existing crack had multiple steps on its surface, indicating that the crack formed in a stepwise fashion. As the crack became sufficiently larger it grew catastrophically, separating the elbow and causing the water release.
Analysis of the PVC from which the elbow was manufactured revealed nothing unusual about the PVC itself. However, certain kinds of grease, like Vaseline, are well known to weaken and embrittle PVC. Also, the presence of the grease greatly increases the likelihood of over tightening. In fact, manufacturers of such fittings recommend against the use of Teflon tape or any kind of lubricant, and specify only a pipe sealant compound (pipe dope) specifically made for use with PVC.
Based on the above evidence, it was concluded that the elbow failed because it was improperly installed as over-tightened. Additionally, the grease in the joint encouraged over-tightening and promoted cracking. The crack most likely formed when the joint was tightened, then it grew, step by step, as the pressure varied inside the pipe during use. When the crack reached a critical size, the joint failed and the water was released into the building.