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Energy-to-Failure to Assess the Quality of Electro Fusion Joints in PE Pipes

 

Frans Scholten

Papers # 2014 Chicago

When electro fused joints in newly installed PE100 pipelines are tested using the Peel test (ISO 13954 [1]) they too often fail in a brittle manner. Contamination in the interface between the surfaces to be fused is usually the cause. About 77 % of the contaminants in 39 cases of brittle test bars could be identified by chemical analysis. Silicones were found in 23 % of the cases. In 54 % of the cases inorganic particles consisting of silicate (sand, clay) or calcium carbonate (from cement dust) had been trapped by electrostatic charges on newly-skinned PE pipe surfaces. Fine particles pose a larger threat to joint quality than coarse particles. In 13 % of the cases the presence of sweat, hand balm and detergent was suspected. In 15 % of the cases the identification was inconclusive.

By integrating the area under the force-displacement curve in the Peel test, the total amount of energy needed to produce failure was determined. This Energy-to-Failure (EtF) method delivers quantitative results and opens up new possibilities for studying correlations with important jointing variables and with external influences, such as contamination of the joint surface.

Commercial cleaning liquids developed to remove silicone contamination before jointing were tested and compared to ethanol using the EtF method. While ethanol is partially effective, the silicone removers are more effective, although they evaporate more slowly. For the least contaminated joints their quality is not only higher but also depends on the gap between pipe surface and coupler surface and therefore on pipe ovality.

Using isopropanol instead of ethanol for cleaning pipe surfaces may lead to the formation of voids. This occurs when the time allowed for drying is not prolonged. The lower volatility and higher solubility in PE of isopropanol lead to more absorption in the skinned outer pipe surface. At the high fusion temperature bubbles filled with isopropanol vapour are locked in by the molten PE, which subsequently remain as highly undesired voids after cooling. GC-MS proves that isopropanol is still present in the voids and is therefore responsible for their formation. A return to ethanol as cleaning liquid removes such voiding.

Related keywords : pipe, joint, peel, energy, electro, fusion, conditions.
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