DIGITAL RADIOGRAPHY OF PE JOINTS

Transcription

1 DIGITAL RADIOGRAPHY OF PE JOINTS Mats Bergman Inspecta Sweden AB Stockholm, Sweden Karin Jacobson Swerea KIMAB Stockholm, Sweden SHORT SUMMARY The Swedish Water & Wastewater Association (SWWA) has funded a R&D project to investigate different methods suitable for installation inspection of PE joints in field. Methods like phased array ultrasonic testing (PAUT), microwave technology and digital radiography were studied. Digital radiography shows promising results in finding defects and imperfections as pores, oblique-angled, toe-in and ovality, incorrect insertion and dirt and impurities. It has also been successfully used to investigate the changes in the weld during a welding cycle. Our conclusion is that digital radiography should be considered as part of a prescribed standard for installation inspection of electrofusion PE joints and that it is a powerful tool to be used for research and development into improving the welding performance. KEYWORDS Electrofusion, PE, polyethylene, joint, digital radiography ABSTRACT The water utility sector faces major challenges as large sections of the drinking water network must be renovated, and new installations of water mains are made on a large scale in many cities. Polyethylene (PE) is the leading material for new installation of drinking water pipes in Sweden and also for sewer systems and gas applications. The material is recognized by the market as a flexible, light weight and cost effective solution. However, there have been a fairly large amount of failures of electrofusion joints after only a few years in service. Failure investigations have shown that in most cases the installation of the joint had not been performed correctly. This has large economic consequences and also, in the case of gas piping, safety concerns. Studies in Sweden show that the total average cost for fixing a leak a water supply system is about 50 keur. So there is clearly a need to improve the quality of the installation. An integral part of this is the quality control at the installation. The Swedish Water & Wastewater Association (SWWA) has funded a R&D project to investigate different methods suitable for installation inspection of PE joints in field. Methods like phased array ultrasonic testing (PAUT), microwave technology and digital radiography were studied. Digital radiography shows promising results in finding defects and imperfections as pores, oblique-angled, toe-in and ovality, incorrect insertion and dirt and impurities. It has also been successfully used to investigate the changes in the weld during a welding cycle. Our conclusion is that digital radiography should be considered as part of a prescribed standard for installation inspection of electrofusion PE joints and that it is a powerful tool to be used for research and development into improving the welding performance. The presentation will give the background of the project, an overview of digital radiography technology and the result as images of different kind of defects. 1 Copyright 2016 by Mats Bergman (mats.bergman@inspecta.com)

2 INTRODUCTION The water utility sector faces major challenges as large sections of the drinking water network must be renovated, and new installations of water mains are made on a large scale in many cities. Polyethylene (PE) is the leading material for new installation of drinking water pipes in Sweden and also for sewer systems and gas applications. The material is recognized by the market as a flexible, light weight and cost effective solution. However, there have been a fairly large amount of failures of electrofusion joints after only a few years in service. Failure investigations have shown that in most cases the installation of the joint had not been performed correctly. This has large economic consequences and also, in the case of gas piping, safety concerns. Studies in Sweden show that the total average cost for fixing a leak a water supply system is about 50 keur. So there is clearly a need to improve the quality of the installation. An integral part of this is the quality control at the installation. Figure 1. Leaking saddle joint (hydrostatic test) and example of consequence of leaking water main. The Swedish Water & Wastewater Association (SWWA) has funded a R&D project to investigate different methods suitable for installation inspection of PE joints in field. Methods like phased array ultrasonic testing (PAUT), microwave technology and digital radiography were studied. Here the use of digital radiography is discussed. Radiography's origins can be traced to November 8, 1895, when German physics professor Wilhelm Conrad Röntgen discovered the X-ray and noted that, while it could pass through human tissue, it could not pass through bone or metal. Röntgen referred to the radiation as "X", to indicate that it was an unknown type of radiation. He received the first Nobel Prize in Physics for his discovery. Radiography is an imaging technique that uses electromagnetic radiation other than visible light, especially X- rays, to view the internal structure of a non-uniformly composed and opaque object i.e. a non-transparent object of varying density and composition. The information in the image consists of absorption contrast, phase contrast, or backscatter contrast. The absorption contrast is the most commonly utilized information carrier. In an absorption contrast setup the object to inspect is placed between the X-ray generator, producing the X-ray beam, and the X-ray detector. The detector can be either a photographic film or a digital detector. A certain amount of X-ray energy is absorbed by the object, which is dependent on the particular density and composition of that object (radiodensity or radiopacity). The digital detectors, for example the digital detector array (DDA), produce a 2-D digital image representing some of the object s internal structures. NOMENCLATURE DDA DRT PE Digital detector array Digital radiography Polyethylene 2 Copyright 2016 by Mats Bergman (mats.bergman@inspecta.com)

3 EXPERIMENTAL The Swedish Water & Wastewater Association (SWWA) has funded a R&D project to investigate different methods suitable for installation inspection of PE joints in field [1]. Methods like phased array ultrasonic testing (PAUT), microwave technology and digital radiography were studied. Here the use of digital radiography is discussed. Digital radiography system A Vidisco digital radiography system with a FlashXPro panel was used in the study. The panel has a 16 bit dynamic range and a resolution of 3,5 line pairs per mm. The image area is 342 x 432 mm which makes the panel well suited also for larger diameter joints. As radiation source an Yxlon Smart 300HP portable x-ray generator was used. See figure 2. The images are obtained more or less immediately after exposure. The possibility of immediate result, a high image quality and an easy interface for evaluation makes the system better suited for this application than film based radiography. Figure 2. Digital radiography system. Objects studied Pipes with a diameter up to DN800 with both socket and saddle joints were studied. See figure 3. Defects and imperfections were either manufactured in the joints or leaking joints were cut out from installation or service. Defects and imperfections studied were: - Voids and pores - Misalignment, - Toe-in and ovality, - Incorrect insertion, - Dirt and impurities. 3 Copyright 2016 by Mats Bergman (mats.bergman@inspecta.com)

4 Figure 3. Example of digital radiograph of socket and saddle joint. The objects were mainly studied in laboratory but field tests were also undertaken. See figure 4. Figure 4. Laboratory and field tests. Results Figures 5-9 show examples of digital X-ray images of electrofusion welds. With this technology it is possible to detect if welding wires are broken, if there is dirt and if there are air voids in the weld. The detection of air in the welds is a real strength for this technology for electrofusion welds since air voids and air gaps often indicate common errors such as: pores, misalignments, ovality and toe-in phenomenon. Figure 5. Pores and voids 4 Copyright 2016 by Mats Bergman (mats.bergman@inspecta.com)

5 Figure 6. Misalignment Figure 7. Toe in and ovality Figure 8. Insertion 5 Copyright 2016 by Mats Bergman (mats.bergman@inspecta.com)

6 Figure 9. Dirt and impurities Swerea KIMAB has, since the Swedish Water & Wastewater Association (SWWA) R&D project discussed above, routinely used digital X-ray as a tool in failure analysis. The following is an example of when digital X-ray has been used in failure analysis of a 350 diameter electrofusion joint. Three images were taken as indicated in Figure 10. Figure 10 The areas marked with BILD shows the different positions around the joint for the X-ray images. Two of the resulting X-ray images are shown in Figure 11 and 12 6 Copyright 2016 by Mats Bergman (mats.bergman@inspecta.com)

7 No fusion 1V Voids Voids 2H Figure 11. Radiograph from failed electrofusion joint showing a large number of voids and where the brittle weld had cracked open. 7 Copyright 2016 by Mats Bergman (mats.bergman@inspecta.com)

8 Figure 12. Different view also showing the large number of voids DISCUSSION Radiography is a very useful technique for electrofusion joints for detecting and depicting cavities such as voids/pores, gaps and cracks in the fusion zones. A picture of the pipe end location and appearance of the socket is generated from which it is possible to determine the thicknesses and distances of different details. For example, it is possible to determine the socket and the pipe thickness and to determine how far as the pipe ends are pushed into the socket. Furthermore, effects such as ovality, toe-in and misalignements of the pipe relative to the socket can be measured. It also gives a very good picture of the position of the metal wires is in the welding zone and if and how they may have been displaced during welding. Moreover, it is possible to detect dirt in the joint in the form of sand and gravel. One advantage of the radiography technique is that qualitative information can easily be extracted from the X-ray images (radiographs), even by a non-professional. For example, the radiographs are quite easy to interpret when it comes to analysis of misalignment and improper insertion compared to e.g. phased array ultrasound. A drawback of the X-ray technique is that it cannot determine the size of the heat affected zone, which can give an indication of that the heat input has been correct. Nor can it determine if there is a cold weld from an unscraped pipe. It is also currently only limited information extractable from radiography of butt-welds, the mina problem being that the x-ray beam is parallel to the weld, see Figure 13. However, this might be solved by using X-ray computed laminography. Laminography can be explained as a tomographic reconstruction method capable of handling planar objects, that is, objects that extend to infinity in some direction/s. The idea is that lack of fusion might be detectable with this method. Such a setup optimized for metal pipe X-ray inspection (called TomoCAR) has been developed by the Federal Institute for Materials Research and Testing in Berlin. Possibly only minor modifications to the TomoCAR system are required for it to work with PE-pipes. Alternatively, a simpler special purpose calibration method could possibly be utilized. For both approaches proof-of-concept experiments will be needed. 8 Copyright 2016 by Mats Bergman (mats.bergman@inspecta.com)

9 Figure 13. Radiograph of a PE butt weld CONCLUSIONS Our conclusion is that digital radiography should be considered as part of a prescribed standard for installation inspection of electrofusion PE joints and that it is a powerful tool to be used for research and development into improving the welding performance. ACKNOWLEDGMENTS The authors want to acknowledge the Swedish Water & Wastewater Association (SWWA) for financing the study. The authors also want to acknowledge Thomas Blomfeldt and Gunnar Bergman at Swerea Kimab, and Thomas Henriksson and Rickard Långh at Inspecta for their work in the study. REFERENCES [1] Bergman, Gunnar and Blomfeldt, Thomas (2014). To ensure tightness and quality in polyethylene piping. Report No (in Swedish). Swedish Water & Wastewater Association. 9 Copyright 2016 by Mats Bergman (mats.bergman@inspecta.com)