The use of non-destructive examination (NDE) for assessing the quality of butt fusion joints in polyethylene (PE) pipes has been included in the draft Mandatory Appendix XXVI to Section III of the ASME Boiler and Pressure Vessel Code (Rules for construction of Class 3 buried polyethylene pressure piping). However, currently, there are no acceptance criteria for flaws in butt fusion joints in PE pipes. There is an ASME Task Group on flaw evaluation for PE pipe, which is developing a code case using linear elastic fracture mechanics (LEFM) to determine critical flaw sizes. However, the initial experimental crack growth data generated suggests that linear elastic fracture mechanics is not able to adequately describe slow crack growth in PE materials. In addition, this work is only considering planar lack of fusion flaws in the joint; it is not considering other critical flaw types that can occur in butt fusion joints, such as particulate contamination and cold fusion. TWI has developed procedures using mechanical testing to develop flaw acceptance criteria for butt fusion joints in PE pipes. This is based on inserting lack of fusion flaws of known size and particulate contamination flaws of known concentrations into butt fusion joints and determining the effect of these flaws on both the short-term and long-term integrity of the joints. An important aspect of this work is to determine which of the wide array of mechanical tests available for assessing the integrity of butt fusion joints in PE pipes are the most discriminating. This paper describes the procedures developed for inserting simulated flaws into butt fusion joints in PE pipes, the experimental work to compare the results from different standard short-term and long-term tests on flawed and unflawed joints and the procedures developed to determine flaw acceptance criteria. Results have shown that the most discriminating short-term test for butt fusion joints in PE pipes is a tensile test using a waisted specimen, such as those defined in EN 12814-2, EN 12814-7 and ISO 13953, and the most discriminating property is the energy to break the specimen. The most appropriate long-term test for butt fusion joints in PE pipes is the whole pipe tensile creep rupture test, as defined in EN 12814-3; this is the only long-term whole pipe test that consistently generates slow crack growth in the fused joint, even if it contains no flaws.
High-density polyethylene (HDPE) is increasingly used for pipe applications owing to its lower cost and higher resistance to chemical corrosion and biological attack compared with metallic materials. Hence, HDPE with improved mechanical properties for pipe application were developed. In addition, additives are used to improve the longterm performance of PE pipes. Carbon black (CB) is one of the most widely used additives in PE pipes due to its low cost and absorption of UV which contributes to the resistance to photo degradation. However, it is difficult to achieve good homogenisation of HDPE and additives by the single-screw extrusion method. Insufficient homogenisation is observed as black (where HDPE and CB are well mixed) with white striations (only HDPE without CB); the white striations are called "windows". It has been shown that windows could lead to reduction of HDPE pipe performance and several methods have been developed to detect them. However, there is lack of study on the effect of windows on the integrity of butt fusion joints and there is only one method reported in open literatures which can quantify the window. However, this method causes material wastage.
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.
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