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Welding polyethylene pipes with confidence

Connect, no. 113, July/August 2001, p.7

A close-up of the ultrasonic probe assembly mounted in an automatic butt fusion welding machine
A close-up of the ultrasonic probe assembly mounted in an automatic butt fusion welding machine

A European-funded project has just been completed which developed a butt fusion welding machine with an integral ultrasonic non-destructive examination (NDE) system. This has the potential for providing complete confidence in the long-term quality of each weld produced.

Even with good site practice, it is impossible to eliminate all possible flaws in butt fusion welds in polyethylene (PE) pipes. Such flaws could include airborne dust and sand, water, grease, air pockets and cold welds produced by deviations from set welding parameters. Consequently, there is a need to determine the existence of any weld flaws through NDE and to establish whether they are likely to reduce the service life of the pipe system. Such information would eliminate the need for destructive testing of the welds which would reduce costs and allow the quality of the actual installed PE pipe system to be determined.

This 1m Euro project was managed by TWI and involved twelve other organisations from five European countries. The objectives were to:

  • optimise three ultrasonic NDE techniques (time-of-flight diffraction, tandem and creeping wave) for butt fusion welds in PE pipes.
  • determine the limits of detection of an NDE system that combined the above three techniques.
  • determine critical defect sizes and levels of particulate contamination in PE pipe butt fusion welds, ie sizes/levels above which the long-term integrity of the joint is reduced.
  • design and manufacture a prototype welding/NDE machine.

Results showed that a combination of the three different ultrasonic NDE techniques is capable of detecting planar flaws down to at least 1mm and sand contamination at levels down to at least 3% by area, in butt fusion welds in PE pipes of diameters up to 315mm outside diameter (OD). Neither of these types of flaw can be detected reliably by visual examination or manual testing of the external weld bead.

The work has also shown that ultrasonic NDE cannot detect either fine particulate contamination or cold welds produced by non-standard welding conditions. However both types of flaw can be either detected or eliminated by other techniques. Fine particulate contamination can be detected using a manual bend-back test on the removed external weld bead, and conditions that produce cold welds should not be possible using an automatic butt fusion welding machine, with process control and monitoring.

The project also determined the minimum size of planar flaw and minimum levels of fine and coarse particulate contamination that cause premature failure of butt fusion welds in PE pipes of 125mm and 315mm OD. It used a combination of specimen and whole pipe tensile creep rupture tests on welds containing known sizes/levels of different flaws. This information, together with the combined automatic butt fusion welding machine/ultrasonic NDE system developed in this project, followed by examination of the removed external bead, should allow all possible defects likely to occur in butt fusion welds in PE pipes to be either detected or eliminated.

Although the work in this project used one particular grade of PE, welded using one particular welding procedure, the advantage of a combined TOFD, tandem and creeping wave system is that it should be applicable to any PE pipe material made using any welding procedure since, unlike some other ultrasonic systems, it does not rely on a specific shape or size of weld bead.

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