The American Society of Mechanical Engineers (ASME) is revising the published Code Case (N-755) for the use of polyethylene (PE) in nuclear power plant buried pipe systems. However, the United States Nuclear Regulatory Commission (NRC) has not approved this Code Case due to a number of concerns; one of which is the lack of validated non-destructive examination (NDE) techniques for inspecting welded joints in PE pipes. This paper describes the development of an ultrasonic phased array system and procedures for the inspection of welded joints in PE pipes of diameters between 125 and 1000mm. The system includes hardware and software which has been designed specifically for inspecting PE pipe joints. The system has been assessed by a number of organizations and the results of these trials will be presented. Alongside the development of the inspection system, a major program of work has been carried out to develop acceptance criteria for flaws in PE pipe welds. The types of flaws investigated included particulate contamination, planar flaws and cold fusions. The critical flaw sizes and contamination levels were determined based on both long-term and short-term testing of welded joints in PE pipes.
The current practice for assuring the quality of butt fusion joints in polyethylene (PE) pipes during installation is by recording the welding parameters used, together with a visual inspection of the welded joint, supplemented by the destructive testing of welds on a sample basis using a short-term test. However, visual inspection can only examine the external surface of the pipe weld; it cannot provide evidence of embedded flaws or a weld with incomplete fusion or cold fusion. In addition, cutting a specimen from a weld for mechanical testing and then replacing it with a weld of unknown quality does not ensure the integrity of the pipeline. Volumetric non-destructive examination (NDE) will not destroy perfectly good welds and has the added environmental advantage of reduced waste. This paper describes an ongoing European-funded project to develop ultrasonic phased array techniques for the inspection of butt fusion (BF) and electrofusion (EF) joints in PE pipes of diameters between 90 and 1000mm, and to determine critical defect sizes and particulate contamination levels using accelerated long-term testing. In addition, defect recognition and automated defect sentencing software will be developed to allow the system to automatically sentence detected flaws.
A reliable non-destructive evaluation (NDE) approach is required for the inspection of different polyethylene (PE) pipe joints in various material grades and pipe sizes. In February 2010, a European (FP7)-funded project on the development and validation of an automated non-destructive evaluation (NDE) approach for testing welded joints in plastic pipes (TestPEP), involving 13 organisations from seven European countries, was started. Several individual tasks need to be solved before approaching the final solution. The challenging material properties of PE pipes, low speed of sound and high-frequency-dependent attenuation, must be overcome. A flexible scanner with probe and wedge holder incorporated must be adaptable for the variety of pipe joint configurations. Furthermore, a rugged flaw detector instrument will be developed, capable of performing the advanced procedures required for these materials. In this paper, the technical problem, industry needs and the proposed approach to the solution are stated. Initial results show the route for success in the project together with the proposed inspection system and inspection techniques.
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