Piping Failed at a Tee Junction
After the failure of a XL commercial sprinkler pipe-to-pipe at a tee junction had occurred – we were asked to perform a root cause analysis. Allied XL pipe is a strong, thin wall pipe manufactured specifically for sprinkler system applications.
The leg of the sprinkler system where the failure occurred is supplied by a 2-inch header. The total length of the 1-inch leg is approximately 163 inches (13.5 feet) and the failure location is approximately 64.5 inches (5.4 feet) from the header.
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 inspection, material / metallurgical testing and analysis. 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. In general, the sprinkler system piping appeared to be in good condition. However, it was noted that the first section of 1-inch pipe in the leg where the failure occurred was visibly deformed. There is no way to determine when this deformation occurred. However, it is very unlikely that an inspector would approve a sprinkler pipe in this condition during the initial sprinkler system testing, inspection and approval.
The minimum pipe wall thickness at the thread root measured at the fracture surface does not represent the as-installed, minimum pipe wall thickness at that location. The pipe material is a ductile steel which exhibits necking when overloaded in tension. The phenomenon of “necking” can also be described as local thinning and a reduction in cross sectional area in the region of the failure location. Measurements of material thickness at the site of failure will include a component of necking that must be accounted for in any attempt to quantify the original, pre-failed thickness dimensions.
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. The results determined that 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.
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