Failure of XL Commercial Sprinkler Pipe – Root Cause Analysis

XL Piping Failed at a Tee Junction

After an XL commercial sprinkler pipe system had failed at a pipe-to-pipe tee junction, we were asked to perform a root cause analysis. Allied XL pipe is a strong, thin wall pipe manufactured specifically for sprinkler system applications.

NFPA 13 specifies that sprinkler systems using threaded connections must be threaded per ANSI/ASME B1.20.1, “Pipe Threads, General Purpose (Inch)”. This standard defines the dimensions of pipe threads for general purpose applications including tapered threads appropriate for this application.

The investigation included structural analysis and material/metallurgical testing. Measurement of the threads away from the failure surface clearly show that the threads were cut in accordance with ASME 81.20.1 and that the failure was not a result of as-built thickness deficiencies.

Calculations were performed to determine the additional, external load required to cause tensile overload due to bending on the leg of pipe where the failure occurred. A uniform, distributed load of 29 pounds per foot or a concentrated load of 84 pounds located at the center of pipe span would cause collapse. This type of loading would cause sudden, catastrophic failure of the pipe.

O’Donnell Consulting performs engineering troubleshooting services for clients in various industries.