Case Studies
Electrofusion Welded Coupling
Background
A recently installed 250 mm diameter electrofusion (EF) welded coupling in a facility’s fire system was reported to have been leaking.
Initial Hypothesis
A welding defect was present from the time of installation.
Data Collection
The fitting was cut on a longitudinal cross section to permit examination of the weld interface. It was noted that one of the revealed weld interfaces showed signs of incomplete fusion (Fig. 1, Fig. 2). Further, the centre of the section had a melt out defect and the pipe ends were not cut square (Fig. 1).
A specimen was extracted from the weld interface with possible lack of fusion for peel decohesion testing to determine the local strength of the weld. The specimen separated under zero applied force, demonstrating complete lack of fusion (Fig. 3). The interface surfaces at the intended fusion location were covered with a light layer of dirt (Fig. 4).
Analysis
The complete lack of fusion in part of the EF weld constituted a welding defect. The dirt on the interface surfaces in this region indicated that the pipe and/or fitting surfaces were likely not appropriately cleaned prior to welding.
The melt out defect indicated that the weld most likely did not reach its intended pressure, further increasing the likelihood of lack of fusion. Because the pipe ends were not cut square, this could have been caused by misalignment within the coupling.
Conclusion
The hypothesis is accepted — a lack of fusion defect was present in the weld and this defect resulted in the observed water leak. The root cause of failure was inadequate cleaning of the interface surfaces in the EF weld joint compounded by possible misalignment from improper pipe cutting.
Recommendation
It is recommended that the client review the cleaning procedures used in EF welding to reduce or eliminate the risk of inadequate cleaning of interface surfaces leading to weld defects. It is also recommended that pipe cutting be conducted using equipment and procedures that produce square cut ends facilitating proper alignment.
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