ANALYSIS OF A WATER PIPE
COMPRESSION FITTING FAILURE
Charles C. Roberts, Jr., Ph.D., P.E.
Figure 1 is a view of a typical plumbing compression fitting used to splice plumbing pipes. In some installations with long pipes and limited access, replacement of a section of pipe is often accomplished with two compression couplings at each end of the replacement pipe.
Care in the installation of compression couplings should be exercised as illustrated in the following case study. Plumbers repaired a pipe that was leaking, as a result of corrosion, on the third floor in an old, historic building. A section of the pipe was removed and replaced along with two compression couplings, one at each end. The plumbers finished the work and left. Approximately 3 days later, one of the compression fittings parted, causing a significant water loss. Figure 2 (arrow) shows the pipe that had parted from the compression fitting.
Figure 3 shows the parted pipe and the outline of the compressed rubber seal of the compression fitting. The compression fitting worked its way off the end of the pipe, allowing water to pour down several floors of the building.
Further inspection of the vertical pipe in the basement yields a clue to the failure of the coupling (Figure 4). The vertical pipe that pulled loose of the coupling (arrow, Figure 4) terminated at a T fitting with a larger pipe. The hangers of the larger pipe had deteriorated, rendering the large pipe virtually unsupported other than the support of the vertical pipe that failed. The friction of the vertical pipe through the concrete floors and the deteriorated pipe hangers were the only restraint holding the large horizontal pipe in place.
Figure 5 illustrates the failure scenario. Figure 5A shows the system with the coupling in place, shortly after the plumbers had left the job site. Figure 5B shows that the vertical load carrying capacity of the pipe has been significantly reduced with the usage of the compression fitting. Because of the limited axial force capacity of the vertical pipe when using compression fittings, the deteriorated hangars failed, shifting the load to the vertical pipe and couplings. The couplings could not support the load, slipped and parted. It should be noted that compression fittings require restraints against axial movement brought on by weight and vibration related forces. Unlike threaded fittings, compression fittings can seal properly but still slip out of position as a result of forces on a pipe. In this case, the plumbers failed to properly restrain the repaired pipe when using compression fittings. Figure 5C shows that a restraint for the vertical pipe and hangar repair should have been performed before job completion. A quick inspection of the pipe in the basement would have revealed the necessity to repair the pipe hangers and restrain the repaired pipe. No defects were noted in the compression fittings that could have caused the failure. Analysis of the evidence suggests that the underlying cause of the failure was improper repair of a water pipe.