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Project Outline

State of the art

The current European practices for plastic pipe weld quality assurance involve visual inspection of every weld, supplemented with either hydrostatic pressure testing of a section of welded pipeline or destructive testing of welds on a sample basis using tensile tests or bend tests. However, removing a good weld and replacing it with a weld of unknown quality can be detrimental to the integrity of the pipeline. In addition, visual inspection can only examine the external surface of the pipe weld; it cannot provide evidence of a weld with incomplete fusion or cold fusion.

Volumetric testing using ultrasound will not destroy perfectly good welds, which means that the integrity of every weld in a pipeline can be assured. Currently there are companies in North America and South Korea that offer ultrasonic inspection of PE pipe welds. However, these systems do not have the knowledge of defect reporting criteria, without which the examination of welds has no quality or financial benefit.

TestPEP: State of the Art Previous European-funded projects, WINDEPP and Polytec Systems, developed ultrasonic inspection systems for butt fusion and electrofusion joints in PE pipes, respectively, and also determined critical defect sizes based on the long-term integrity of the welds. However, these systems were specific to certain sizes of pipe. Thermal inspection methods were also examined in the Polytec Systems project and were found to be insensitive to the types of flaws found in plastic welds due to the low thermal conductivity of these materials. Microwave has similar restrictions in that the wavelength is greater than the size of flaws that are required to be detected.

In the European steel pipe industry the primary inspection method for pipe welds is now ultrasonics; radiography having been stopped a number of years ago due to safety implications. There are many phased array ultrasonic inspection systems commercially available for steel pipes. However, none of these systems has either the software or hardware to provide volumetric examination of plastic welds. Furthermore, there are no sealed phased array ultrasonic instruments available for rugged application. Although there are many pipe inspection manipulators, none of these are applicable for operation in a pipe trench environment and none incorporate phased array probes with low frequency, high power and special materials to accommodate the low ultrasonic velocity of plastic pipes.

Limitations of Current Inspection Knowledge

  • The only current commercial ultrasonic inspection systems for plastics pipes are in North America and South Korea. The American system is limited to butt fusion welds and uses conventional time-of-flight-diffraction rather than phased array and, as a consequence it is not applicable to more complex weld configurations such as elbows, reducers and tees. The Korean system is limited to electrofusion joints and does not record data.
  • Within Europe there are no volumetric inspection systems and the reliance is upon visual inspection and mechanical testing.
  • TWI has developed inspection technology with reporting criteria for some sizes of PE pipe but there is no commercial system available.
  • There are currently no commercially available compact and sealed phased array instruments.
  • The available phased array probes are expensive and mechanically vulnerable due to the cabling.