Termoplastik Doğal Gaz Borularının Elektrofüzyon Kaynağı Ve Kaynak Parametrelerinin Kaliteye Etkisi

Abstract

Bu çalışmada termoplastik doğal gaz borularının bağlantılarında kullanılan elektrofuzyon kaynak tekniği üzerinde durulacaktır. Termoplastiklerin genel özellikleri ve şekillendirilmeleri özet olarak anlatılacaktır. İkinci aşamada, termoplastik boruların bilinen kaynak teknikleri, donanımları ve kaynak işlemleri anlatılacaktır. Üçüncü aşamada, elektrofuzyon kaynak tekniği tercih edilme sebepleri; teknik nedenler, pratik nedenler ve insan faktörü bakımından incelenecek ve bilinen klasik temoplastik boru kaynak usulleri ile karşılaştırılacaktır. Ayrı zamanda elektrofuzyon fitingsleri, kaynak donanımları, kaynak prensipleri, kaynak uygulamaları ve elektrofuzyon kaynak tekniği ile yapılmış iyi bir bağlantının özellikleri üzerinde durulacaktır. Daha sonra polietilen reçine seçimi, boru tasarımı, boruların özellikleri, avantaj ve dezavantajları detaylı olarak incelenecektir. Son olarakta, elektrofuzyon kaynak tekniği hataları ve nedenleri üzerinde durulacak ve bu çalışmadan elde edilen sonuçlar ve öneriler sunulacaktır.

As a result of their nature, properties, and unique advantages and limitations, nonmetalic materials have always played a significant role in manufacturing. More recently, wood has become less significant, and the term nonmetalic materials now brings forth an extensive listing of plastics, elestomers, ceramics and composites. Most of these are processed materials and wide variety of properties and characteristics can be obtained. İt is helpful to have an understanding of the basic moleculer structure of plastics. Most are based on hydrocarbons in which carbon and hidrogen combine iin therelationship CnH2n+2, known as paraffins. Theoreticaly these hydrocarbons can be linked together indefinitely to form very large molecules etc. Methane, Ethane, Ethylene H H H H H I II II H- C- H H-C-C-H C = C I II II H H H H H Methane Ethane Ethylene More prolyethelene is used than any other plastic materials. It is ubiquitous in conteainers of all sorts, packing and electrical insulation. Polyethylene is atype of polymer. Polymers are very large organic molecules made the successive linking together of small molecules. They are called monomers and provide the repeat units form from which the poly (many) "mers" are built. Converting monomers into a polymer is called polymerization. The use of plastic pipes in the gaz industry was developed in many cointries since the fifties. After a period of indecision, when PVC seemed to be the fittest material for mains, poyethylene has gained more and more ground, becoming the plastic mostly used for gas pipes. XII Up to now, the mechanical characteristics of PE have been improved; this led to a better safety of the plants made of his materials and thus to an increase of demand. In the past they used to classify PE into three families: weight density.942 to.965 g/cm3 weight density.930 to.942 g/cm3 weight density.915 to.930 g/cm3 Polyethelene used in network contraction for gas and water distrubition must have an absolute gravity greater than.930 g/cm3; whereas, as far as PE having density lesser than.930 g/cm3 is concerned, the most common uses are tobe mainly found in construction of sewer outlets and watering systems. Pipes to be used for construction of mains must be made of resins derived from ethelene polymerization with the adition of stabilizers and antioxidizers (carbon black, cadmium sulphide, etc.) Gas pipes must be classsified according to their mechanical characteristics and type A or B PE pipes, as defined by UNI ISO 4437, may be used. Properties of polyethylene are as follows; chamical resistance to gas constituents, resistance to chamical action, resistance of microorganisms of soil, stability to radiation, behaviour in the presence of flame, toxicological properties, stability at weather conditions, resistance to soil corresion, surface electric resistance, resistance to low tempature, resistance to abrasion, flexibility, density-weigh, expansion. Polyethelene development is mainly due to the economy obtained both in investment and network manegement. The benefit comes from lightness and high flexibility of polyethylene pipes which allow to make trenches with smaller cross-section and greater posibility of adaptation to whatever laying profile. Besides, whenever it is possible, and for diameters for which such an application is XIU provided for ND 63 max, the use of coils of pipes long even more than 100 metres, allows to lay pipes at high speed and a smaller number of welds. Further economy is to be found in lack of cost for cathodic protection, because polyethelene is not subject to deterioriotion due to stray currents in subsoil. At present the most used process for polyethelene pipes manufacturing is extrusion because it makes it possible to make weldless pipes and, practically, of infinite length. Thus pipe length is set according to the exigencies for carriage and market requirements. Trasformation of PE grains into fittings takes place by means of injection moulding. The material goes through an extruder that melts, homogenizes and by injection compresses the grain into the mould of the fitting to be manufactured. During the injection phase, the walls of the mould are at temperature of 70° C and are continuously cooled, that way allowing the fitting to cool down from 200-300 degrees to the mould temperature. Sockets for connection of pipes and service lines are available both for network laying nad live lines. These fittings have a built-in electric resistance in the shape of a coil that almost complletely covers the inside surface of the piece. When an electric current goes through it, the electric resistance heats up melting the connecting material surfaces welding the fitting on the pipe. The electroweldable pieces production is characterized by a special moulding process, because the heating electric coil is to be embedded together with its external contacts to the welding machine. By welding it is designed a set of works by means of which the joining by fusion of material is obtained, in order to achive a permanent joint between two stretches of pipe, a stretch of pipe and a fitting or between two fittings. Electrofusion welding is carried out by means of special welding machines that supply current to the electrical resistance embeded in to the socket or tap tee melting the material and welding together the pieces to be joined. XIV In the zone involved in the weld the wall thickness of the pieces to be ioned should generally be equel. Welding must be performed in accordance with the following conditions: - The involved area must be protected from negative atmospheric influence (rain, wind, snow etc.), during all the welding and cooling period; - The welding zone must also be protected from air from inside pipes. For this purpuse it must be guaranted that at least one of the pipe end is closed either by means of plastic plugs supplied to gether with the consigment or other suitablee means (rang, expansion plugs). All this in order to avoidn that air going through the pipes could affect the welding outcome. - Room temperature, as measured on the pipe, must be within 0° and +40°C. - Whatever tensile stress on the joint must be avoided both during welding and through out its cooling period. - The surfaces to be welded together must always be throughly finished and cleaned in accordance with the criteria characteristic of the used welding type. - The equipment used for welding must be suitable for this work. Electrofüsion is currently acknowledged as being the most reliable and bes suited jointing process for gas distributors and for distribution of fluids under presssure. Compered with butt fusion or socket fusion, the advantages of electrofüsion are based on acombination of technical, practical and labor related features. Due to technical reason; In abutt or socket weld, the jointing principle is based on: - Preliminary fusion of the surfaces to be welded using heated tools, - Removel of these tools and relative displacement of the parts to be welded to ensure correct contact of the surfaces. This causes movements of material in fusion, - Holding in contact and under pressure of thees surfaces throughout the cooling phase. Due to displacement of the material in fusion, this type of joint engenders internal stresses in the weld areas which are detrimantal to the long term behavior of assembly. This effect is further aggraveted when two resins of different fluidities are xv used. Due to the geometry of the welded parts(sleeve or branch tapping), socket fusion is particularly subject to this problem. With electrofusion technique, welding is made after assembly, and holding the parts to be welded with respect to each other. Therefore at the moment of welding, there is no movement between the pipe and the fitting. Material movements ar very limited, resulting in low stress factors in the welding plane. This holds true even for two resins of different origins, the melt flow index MI 5 of which is between 0.3 and 1.4 g/10 min. Since the coupling are injected ising compatible resins, total interchangeability between the pipe and the fitting is obtained, and consequently between two pipes of totally different types. Due topractical reason; socket and butt fusion techniques are complementary: - Socket fusion applies only to pipes of diemeters of 75 mm, - Butt fusion requires a minnimum pipe material thichness, and can therefore only be used with pipes of diameters of 50 mm prepared in straight sections only. These multiplies the number of joints and the with of trenches if assebly is to take place in the bottoms of the trenches. Electrofusion enables assembly of pipes over a range of diameters of 20 to 200 mm. This is the only technique enabling the use of long pipes of diametres of up to 125 mm. Also, this is a complete technique which can be esed simultaneously: - Throughout all phases of network constructions, - For under-pressure connection, - For repair work. While other techniques cannot be used alone and must be at least asociated with electrofusion or mechanical fittings to obtain joints in pipes already installed, or for repair work. Moreover, with the electrofusion technique, universal assembly tools enable the operator to dispense with procurement problems, leaving him the the possibility of altering this choice of fittings, in time, without having to acquire new tooling. XVI Due to human factor; when socket fusion and butt fusion are involved, dexterity is of the utmost importance in joint quality. Therefore, in thiscase, specialists are required to define and apply the various pressures and temperatures during the welding phase. Jointquality and therefore long-term behaviour depend directly on the operator. This has two consequences: - Setting up of an important an delicate personnel training program, with periodic checking of welders' skill, - The necessity of a weld inspection prosedüre either in the form of a visual inspection or using ultrasonics, which can only detect volumic defects, and cannot provide total assurance concerning weld quality. The elctrowelding technique dispenses with subsequent inspection. In fact, welding temperature and pressure simply a function of the coupling geomery and welding parameters, which are perfectly known and defined. Ideal welding conditions can therefore be repeated systematically from one weld to another. Welding is then performed using automatic machines. Electrofusion under these conditions therefore simply demand only slight training. At the other hand, control of the jointing is ver important for the quality. The method used for PE velding defines the importance which the visual testing has on the check of the joint quality. For visual testing severel defeccts and their possible origins may be given as follows; - Defects; irregular outline of the bead on the pipe circumference, origins; non satisfactory preparation of the ends to be welded with consequent non-uniform heat distribution. - Defect; dimension on the bead non equal to the foreseen value, origins; bad adjustment of the welding parameters (temperature, pressure, welding time). - Defect; deep carving by the centre of the bead, origins; temperature values or welding pressure lower than estimated. - Defect; superficial inclusions by the bead, origins; non satisfactory cleaning of the ends to be welded. XVII - Defect; exceeding brightness of the the bead surfece, origins; overheating during the welding cycle. - Defect; off-centering higher than 10% of the pipe/fiting thickness, origins; wrong centering or exceeding out-of-round of pipes. There can be no doubt that electrofusion is a reliable assembly technique offering much higher performances than conventional techniques such as socket or butt fusion. The quality of an electrowelded assembly depends not only on the design of the fitting used, but also on the manner in which it is implemented.