Weican Guo

Safety Assessment of Cold Welding Defect in Electro-Fusion Joint of Polyethylene Pipe

Cold welding defect is the most common defect in electro-fusion (EF) joint for connecting polyethylene (PE) pipe. In our previous study [1], the cold welding defect is successfully inspected by an eigen-line method based on phased array ultrasonic testing technology. However, limited research has been reported on the acceptance criterion of cold welding defect in EF joint. In this paper, the bonding strength of EF joint is measured using a peeling test. The bonding energy of welding interface is calculated both by phenomenological model and deformation energy analysis method. EF joints with different degrees of cold welding are made and used for peeling tests. The results show that the bonding energy of fused interface rises rapidly after bonding and then goes through a plateau region. The starting point of the plateau region in the bonding energy versus welding time curve is regarded as the minimum required welding time of EF joint. Based on bilinear fitting, the acceptance criterion of cold welding defect is proposed.

Formation Mechanism of the Eigen-line in Electrofusion Joints of Polyethylene Pipes

Cold welding is the most dangerous defect in the electrofusion joint of polyethylene (PE) pipes. A proprietary method was developed to detect the degree of cold welding by using an Eigen-line, which was discovered in our previous study. To understand when, where, and how the Eigen-line would appear, the forming mechanism was investigated in this paper. Three factors, i.e., micro-air-bubbles, difference of acoustic impendence of PE in the melted and unmelted region, and small-crowded crystals in the region of the interface of melted and unmelted region, may cause the appearance of the Eigen-line. It was found that the number of Eigen-lines was the same as that of welding times when rewelded with decreasing welding time, and only one Eigen-line could be observed when rewelded with increasing welding time. The result showed that small-crowded crystals in the region of the interface of melted and unmelted region may be the dominating factor. This was then verified by the temperature analysis and differential DSC tests, and the forming process of the small-crowded crystals was discussed in detail.

A Method of Automatic Defect Recognition for Phased Array Ultrasonic Inspection of Polythene Electro-Fusion Joints

Defect classification is the basis of defect safety assessment because defects of different types can lead to failure in different forms. However, the identification of defect type has long been a critical issue in ultrasonic inspection. Wave acoustic was applied in this study to investigate the sound scattering of metal wires in polyethylene (PE), which provided theoretical support for ultrasonic feature extraction. A method of defect recognition for PE electro-fusion (EF) joints was proposed based on pattern recognition of ultrasonic inspection images. According to location, shape, signal intensity, and cluster conditions, typical defects of EF joints of PE pipes were distinguished and identified in phased array ultrasonic images. Furthermore, an automatic defect recognition software was designed based on the proposed approach; the software was improved and verified through defect inspection and identification experiments. Results showed that accuracy can reach 80% for joints with complex defects and 100% for those with single defects.Copyright

Experimental Investigation on Coupling Focusing Ultrasonic Technique for Inspection of Polyethylene Butt-Fusion Joint

The butt fusion process is a widely used method for connecting Polyethylene (PE) pipes. Conventional ultrasonic test is unable to detect defects due to heavy ultrasonic attenuation, disturbance by the structure of the joint, and the weak acoustic response caused by lack of fusion. A new technique named coupling focusing ultrasonic technique used for testing PE butt fusion joints is developed in this paper. The key of the technique is to get a special couplant, which is a mixture of glycerinum, sodium silicate, water, and defoamer, with almost the same sound velocity and acoustic impedance of PE. Through the couplant, sound beam is converged by spherical lens, entering the PE material with focusing zone overlapping the fusion area of the joint. It is proved that the technique has a good testing ability for defects of typical lack of fusion, gas holes with 1mm in diameter and crack with 1mm in height, based on the results of detection experiments of test sample accompanied with typical defect. It is also proved that its sensitivity is better than TOFD technique and traditional contact angle wave ultrasonic technique as well.Copyright

Study on the Allowable Temperature for Preventing Over Welding During Thermal Welding of Polyethylene Pipe

Polyethylene (PE) pipe material may degrade into lower carbon number volatiles quickly during thermal welding when the welding temperature rises up to very high temperature. However, PE may also degrade into lower molecular weight (MW) polymer when subjected to a lower temperature. As a result, determination of the allowable temperature during thermal welding is crucial for guaranteeing the quality of welding joint. In this paper, a typical commercial PE100 material was chosen to conduct thermogravimetic analysis (TGA) and gel permeation chromatography (GPC) tests. The thermal degradation behavior of PE100 was investigated in dynamic and isothermal mode. The composition of the residue after thermal degradation was also analyzed through MW and MW distribution (MWD) measurements. Based on the experimental results, the initial temperature of thermal degradation with volatilization was derived and the thermal degradation process was studied in detail. To limit the degree of thermal degradation within a certain range during thermal welding of PE pipe, the allowable welding temperature for typical commercial PE100 material was determined.

Research on Ultrasonic Testing for Electro-Fusion Joint of Reinforced Thermoplastic Pipes

Reinforced Thermoplastic Pipe (RTP) is generally connected by butt fusion joint or electro-fusion joint with intensified sleeves. Contrast to regular electro-fusion joints for PE (polyethylene) pipe, those for RTP are of larger length and thickness. Based on the ultrasonic inspection research of electro-fusion joint of PE pipe, the authors designed a special phased array probe for the electro-fusion sleeve of RTP considering its complicated structure and large size. A special C-scan technique was proposed including a longitudinal scan carried out with electronic method and circumferential scan carried out with mechanical method. Meanwhile, special software was designed to ensure the synchronization and precision control between longitudinal electronic scan and circumferential mechanical scan, and the consistency control between ultrasonic information and position information. Based on the results of detection experiments of RTP electro-fusion joint with typical defects, it was proven that this technique has the capability to detect typical defects in RTP electro-fusion joint, and high testing sensitivity was obtained.Copyright

Ultrasonic Inspection of Large Diameter Polyethylene Pipe Used in Nuclear Power Plant

Polyethylene (PE) pipe has many advantages such as good flexibility, corrosion resistance and long service life. It has been introduced into nuclear power plants for transportation of cooling water both in U.S. and Europe. Recently, one Chinese nuclear power plant in Zhejiang Province also introduced four polyethylene pipelines in essential cooling water system with operating pressure of 0.6MPa and operating temperature of no more than 60°C. The PE pipes used in this nuclear power plant are DN762 SDR9 (30in OD, 3.3in wall), which are much larger and thicker than traditional natural gas PE pipe. As the pipe wall is so thick that the ultrasonic phased array instrument used in inspection of PE pipe with diameter less than 400mm has been improved. Results of field inspection in the Sanmen nuclear plant are reported, and the presented ultrasonic inspection technique proves to be effective for high density polyethylene (HDPE) pipe of large size in nuclear power plant.Copyright

Investigation on the Allowable Temperature During Electrofusion Welding of Polyethylene Pipe

Polyethylene (PE) pipe material may degrade into lower carbon number volatiles quickly during the electrofusion welding process when the welding temperature rises up to more than about 370°C. Meanwhile, PE may also degrade into lower molecular weight (MW) polymer when subject to a lower temperature. As a result, the allowable temperature during electrofusion welding is uncertain. In this paper, a typical PE100 material was chosen to conduct thermogravimetic analysis (TGA) and Gel permeation chromatography (GPC) test. The thermal degradation behavior of PE100 was investigated in Dynamic and isothermal mode. And the composition of the thermal degradation residue was determined through MW and molecular weight distribution (MWD) measurements of the residue. Based on the experimental results, the initial temperature of thermal degradation with volatilization was derived and the thermal degradation process was studied in detail. To limit the thermal degradation degree of PE in a required range in thermal welding process, the preliminary allowable welding temperature for typical commercial PE100 material was determined. In addition, some regular information was obtained, which could promote the ultimate determination of the allowable welding temperature.Copyright

Classification of Defects in Fusion Joints of Polyethylene Pipes

To establish a comprehensive classification method of the defects in fusion joints of polyethylene (PE) pipes, the geometrical characters of typical defects in electrofusion (EF) and butt fusion (BF) joints were researched by means of ultrasonic phased array inspection and naked-eye observation. Then the results were comparatively analyzed, and a novel classification method of the defects in PE pipe joints was proposed. Typical defects in EF joints were classified as lack of fusion, voids, wire dislocation, and cold welding. And those in BF joints were classified as crack, voids, inclusion on fusion interface, and process defects. Furthermore, characteristics of the classified defects, together with their main causes were briefly described.Copyright

Development of Non-Destructive Test and Safety Assessment of Electrofusion Joints for Connecting Polyethylene Pipes

Polyethylene (PE) pipes are widely used in transporting natural gas since 1970s. However, there is still no method for safety assessment of PE pipes. This paper introduces our recent works in developing ultrasonic NDT equipment and research on safety assessment of electrofusion (EF) joints of PE pipes. According to geometrical characteristic and ultrasonic response, the defects in EF joints can be divided into four categories, i.e. lack of fusion, voids, wire dislocation, and cold welding. Cold welding, which is the most common and dangerous defects, was successfully detected by the developed ultrasonic equipment and assessed with a patented cold welding Eigen-line methods.Copyright