Investigating Failures in Pressure Pipes

Welding is a common practise when installing and joining pipelines of polyethylene (PE). This is however a complex procedure which require an accurate welding operation.

Failures are not uncommon and Norner has often been involved in failure analysis in such incidents. The failure mode and root cause vary from case to case. Here we show you an example of failure analysis we have carried out recently.Picture 1 shows the location of the failure initiation. It started at the inner wall.

Picture 2 is a close-up picture of the failure initiation. Picture 3 shows the pattern of the surface where the failure started and picture 4 shows the pattern in the area adjacent to this. The failure has thereafter propagated as a brittle fracture around the circumference.The pattern in this area is a replica of the welding tool and it is no or little signs of plastic adhesion. This indicates that the pressure in the welding phase was either too low or unbalanced and thereby created a local area without the right conditions for co-melting and co-crystallisation.

Asymmetrical welding beads both on the inside and outside as well as the replicated pattern of the welding tool reveal a poor fusion where the fracture has started. A visual inspection of the weld bead should reveal this.

The cause of the fracture is therefore concluded to be due to that a sector of the welding zone has been exposed to a too low pressure. This caused a poor fusion of material from the two pipe ends.

There are, however, several issues related to pipe welding which are difficult to discover by visual inspection, such as contaminated weld surfaces, cold-seal due to excessive pressure, long dwell times, material oxidation due to too long heating time or high temperature. Other kinds of initiated fractures in the weld line or notches hidden by the weld bead are also difficult to detect visually.

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