Polyethylene pipe has been used widely in gas transportation and nuclear safety-related cooling water applications due to its exceptional resistance to corrosion and erosion. Butt fusion joint is one of the main welding forms for polyethylene pipes. Ultrasonic technique is a typical nondestructive examination technique. To overcome the coupling problems when inspecting butt fusion structures, an inspection technique of ultrasonic phased array using water wedge is proposed to solve the coupling matching and to increase the ultrasound amplitudes. The influences on the imaging of the parameters (such as the angle and height of the water wedge), the array element specifications, and the aperture, were investigated via simulations. The parameter optimization was conducted to establish a suitable detection process. After that, the related probe and a simulated water wedge were designed based on the results and manufactured, in which the simulated wedge as a specific probe holder could adjust its own incident angle and array height. Meanwhile, a typical DN315 pipe of PE 100 was made with some typical artificial defects in it. Experiments were conducted, and the results showed that the proposed water-wedged ultrasonic phased array technique is suitable for butt fusion joint inspection.
Polyethylene (PE) pipe, particularly highdensity polyethylene (HDPE) pipe, has been successfully utilized to transport cooling water for both non-safety-and safety-related applications in nuclear power plant (NPP). Though ASME Code Case N755, which is the first code case related to NPP HDPE pipe, requires a thorough nondestructive examination (NDE) of HDPE joints. However, no executable regulations presently exist because of the lack of a feasible NDE technique for HDPE pipe in NPP. This work presents a review of current developments in NDE technology for both HDPE pipe in NPP with a diameter of less than 400 mm and that of a larger size. For the former category, phased array ultrasonic technique is proven effective for inspecting typical defects in HDPE pipe, and is thus used in Chinese national standards GB/T 29460 and GB/T 29461. A defectrecognition technique is developed based on pattern recognition, and a safety assessment principle is summarized from the database of destructive testing. On the other hand, recent research and practical studies reveal that in current ultrasonic-inspection technology, the absence of effective ultrasonic inspection for large size was lack of consideration of the viscoelasticity effect of PE on acoustic wave propagation in current ultrasonic inspection technology. Furthermore, main technical problems were analyzed in the paper to achieve an effective ultrasonic test method in accordance to the safety and efficiency requirements of related regulations and standards. Finally, the development trend and challenges of NDE test technology for HDPE in NPP are discussed.
With the application of high-density polyethylene (HDPE) pipe with thick wall in nuclear power plant (NPP), great attention has been paid to the safety of the pipeline joints, which can be assessed by phased array ultrasonic testing (PAUT). PAUT creates constructive interference of acoustic waves to generate focused beams according to delay law based on time-of-flight. However, due to the existence of acoustic attenuation and dispersion, waveform distortion occurs when ultrasonic pulse propagates in HDPE, which will accumulate with the increase of propagation distance, and then results in imaging errors. In this paper, the relationship between acoustic attenuation and dispersion in HDPE was obtained by numerical simulation in Field II®, which can be verified by the experiment of our previous work. Then, the investigation of the waveform distortion revealed the linear relation between peak offset and propagation distance. Considering the relation, an improved delay law was proposed to increase the intensity of ultrasonic field. This improved delay law was compared with the conventional one by numerical simulation of ultrasonic field and PAUT experiments, which showed that the improved delay law could increase the image sensitivity.
With the application of High-density polyethylene (HDPE) pipe with thick wall in nuclear power plant (NPP), great attention has been paid to the safety of the pipeline’s joints, which can be assessed by phased array ultrasonic testing (PAUT). PAUT creates constructive interference of acoustic waves to generate focused beams according to delay law based on time-of-flight. However, due to the existence of acoustic attenuation and dispersion, waveform distortion occurs when ultrasonic pulse propagates in HDPE, which will accumulate with the increase of propagation distance, and then results in imaging errors. In this paper, the relationship of acoustic attenuation and dispersion in HDPE was obtained by numerical simulation in Field II®, which can be verified by the experiment of our previous work. Besides, the investigation of the waveform distortion revealed the linear relation between peak offset and propagation distance. Considering the relation, an improved delay law was proposed to increase the intensity of ultrasonic field. This improved delay law was compared with the conventional one by numerical simulation of ultrasonic field and PAUT experiments, which showed that the improved delay law could increase the image sensitivity.
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