Ultrasonic Testing Technique Research And Equipment Fabrication Of Polyethylene Butt Fusion Joint

The pipeline is the main transportation of oil and natural gas, and regarded as "blood vessel" of the national economy. Polyethylene (PE) pipe has been widely used for transportation of gas and water in cities because of the excellent corrosion resistance, appropriate mechanical properties, good welding performance and 50 years’ service life. Fusion welding is one of the main connection methods of polyethylene pipe. However, the defects in butt fusion joints due to circumstances and wrong operation are very common on the work site, which have been one of the most serious risks of pipeline system. Owing to the different material and structural, conventional non-destructive testing (NDT) for metal welds is not applicable to test the PE joint. It is crucial to study the NDT technique and develop the corresponding non destructive testing equipment for PE joint, which exerts a vital significance to healthily promote the development of PE pipe industry.With the financial support from the National Key Technology R&D Program of "Eleventh Five-Year Planning Program" and "Twelfth Five-Year Planning Program", this paper mainly aimed at BF joints of PE pipes. Defects classification, defects acoustic response, phased array ultrasonic technique and coupling focusing ultrasonic technique are studied. Non-destructive testing (NDT) of process defects of BF joint are then systematically studied. Based on this research, automatic ultrasonic testing system is developed. The main tasks of this work are listed below.(1) The classification of the PE butt-fusion joint defects is investigated through theoretical analysis in combination with engineering investigation and experimental study. In consideration of the state-of-art during the welding of PE butt-fusion and the operating loading of PE pipeline, a series of PE butt-fusion joints with different kinds of defects were made. According to the causes of defects, the morphological characteristics and the failure modes, the defects are divided into cracks, holes, defects of fusion face and process defects (including cold welding, over welding, misalignment and fusion face that is too short). The classification will lay the foundation for ultrasonic detecting and safety assessment of PE butt-fusion joints.(2) On the base of the classification of PE butt-fusion joint, the reflection and diffraction law of ultrasonic during the detection of joint defects are investigated, while the study of the reliability of ultrasonic detecting for the direction of area-type defects is placed in the focus. The coupling focusing ultrasonic technique and phased array ultrasonic technique is presented, with the purpose of overcoming the detecting difficulty caused by the attenuation of PE. The technical solution for the developing of detection equipment is given together.(3) Various process defects may occur in the PE butt-fusion, along with the appearance of morphological characteristics of defects. The applicability and reliability of the process defects of nondestructive detection method are studied. This paper also revealed the presence of ultrasonic signal line in the internal of butt fusion joint and presented the method of ultrasonic characteristics lines in cold welding detection of butt-fusion joint.(4) The technology roadmap for developing automatic ultrasonic detecting system, which is applicable for the detecting of PE joints with the fusion face perpendicular to the detecting face and irregular crimping, is provided. This technology presents phased array dynamic focusing instead of mechanical focusing and the longitudinal electronic linear scanning instead of mechanical scanning. Comprehensively integrated impedance matching with acoustic coupling technique, phased array electronic focusing technique, numerical control scanning technique, C scan real-time imaging technique are applied to developed the automatic ultrasonic testing system and realize the automation, digitization, image of the entire system.(5) The reliability and process performance are tested through detecting PE joints with typical defects. It is shown that the detecting system is applicable for the detecting of typical defects in PE joints, which will improve the safety of PE pipeline.