A natural gas distribution line leaked and gas migrated into a nearby construction site. The migrated gas accumulated inside a newly installed sewer pipe. When the gas concentration in the sewer pipe reached the Lower Explosive Limit (LEL), the natural gas in the sewer pipe ignited and exploded.
Pressure testing of the gas line revealed a perforation in the gas distribution line. The leaking gas line is shown in Photograph A. A white arrow denotes the pipeline leak.
Photograph A View of gas/water leak discovered during leak test.
Visual examination of the natural gas distribution line revealed a through wall, round bottomed corrosion pit. The corrosion penetrated along what visually appeared to be a ferrite weld line along the gas line ERW butt weld. A close-up view of the ferrite weld line failure is shown in Photograph B below.
Photograph B Corrosive attack along weld ferrite line in ERW butt welded gas pipe.
Long-term degradation of the asphaltic coating, used to protect the steel pipeline from corrosion occurred, thereby allowing corrosion to proceed at various locations along the steel gas distribution line. The failure of the gas distribution line was believed to be the result of lack of maintenance by the gas utility owning the distribution line. The failure may have occurred along an incomplete ERW weld used to fabricate the pipe, which resulted from a lack of fusion weld.