Bonded joining in sewer and wastewater engineering using thermoplastics

Sep 07, 2007

In order to build a permanently operational sewer and waste-water system with a long service life, as in gas and water supply, it is necessary to not only select the appropriate materials and processes but also use optimum methods of pipe connection, i.e. joining. Such pipe connection methods include welding to create a permanent bond, an approach that has been used successfully in the field of PE and PP thermoplastics for many decades. The chief processes used are heated-tool butt welding and electrofusion. These methods of joining are processes which are recognised and practised worldwide in accordance with standardised procedures for welding, application, quality control and testing. They provide a documented secure and permanent method of connecting components in sewer construction and sewer repair/rehabilitation. That ensures a high level of reliability and a secure investment for the operators of municipal and industrial wastewater systems.

Welding thermoplastics
The ability of thermoplastics, such as PE and PP, to pass reversibly to a plastic state when warmed and to set again when they become cold makes it possible to achieve a firm, permanently integral and watertight weld between thermoplastic components. In thermoplastic pipeline construction there are chiefly two methods which are used. Heatedtool butt welding and electrofusion welding: the difference between these two methods is the type of thermal energy introduced to the joining plane.
Heated-tool butt welding
Heated-tool butt welding is a method where the surfaces of the components being welded together are contact-heated into the thermoplastic temperature range with the aid of a heated tool and then welded under pressure. The process phases of heated-tool butt welding consist of welding surface adaptation, heating the components without pressure, change-over and joining. The process phases and the sequence of pressure-controlled heated-tool butt welding are illustrated in Figure 1.
Electrofusion welding
In electrofusion welding, heating is performed by means of an electrical resistance wire in the joining plane. The surfaces to be welded, i.e. the surface of the pipe and the interior surface of the fitting, are positioned directly above each other (overlapping) and heated to melting point and welding temperature by applying electricity with the aid of a filament or spiral, which is usually located in the fitting. The welded joint is produced by plasticization and the associated thermal expansion, as a result of which joining pressure is created. The temperature at the boundary surface between the pipe and the fitting, plotted against time, is shown by way of example in Figure 2 . To support the pressure created as a result of thermal expansion, the welding pressure required for a high-quality seam is produced by specific material properties (shrinkage stresses frozen into the component during production) or additional direct and indirect tensioning devices (reinforcements or mechanical tensioning devices).
Fields of application
Use of the most appropriate welding method depends, amongst other things, on the particular construction project and local conditions. Heated-tool butt welding is chiefly used where the overall pipe lengths being welded are long and where it is essential to avoid welding sockets that would increase the outside diameter and interfere with pipe installation operations, for example in slip lining, as illustrated in Figure 3. Electrofusion welding with electrofusion sockets or custom welded joints, such as on pipes with integral electrofusion joints, are used, for example, where for space constraints it is not possible to work economically with large butt-welding machines, as illustrated in Figure 4.
Testing and quality assurance
The key criteria in achieving a quality of seam that is optimised for a particular application are the selection of materials and fitting geometries as well as precise compliance with welding parameters and appropriate welding seam preparation.

The components to be welded in pipeline construction usually consist of pipes, fittings such as bends and branches, and special connection components. The components, which are all mass-produced industrially, are not only subject to the manufacturers' monitoring and quality control systems but also to external certification by neutral, publicly appointed testing institutions. The quality of parts is confirmed by appropriate factory test certificates, providing the user and network operator with consistent quality for the materials used.  
Joining processes constitute the final stage in the construction of a pipeline system. The relevant welding parameters and process algorithms for heated-tool butt welding and electrofusion welding can be found in the appropriate standards, guidelines and information sheets, e.g. DVS Guideline DVS 2207. For custom components and custom processes there are appropriate processing instructions available from the manufacturers. Joints can be tested by two principal methods. These are direct, visual, non-destructive testing and destructive welding seam inspections. Since visual inspections, such as the examination of bead configuration in heatedtool butt welding or the checking of melting/ temperature indicators in the case of electrofusion sockets only provide information about the performance of welding and not about seam quality, appropriate system pressure tests or leak tests have to be conducted after the completion of welding work in order to check fitness for service. Information about the strength of a weld can only be obtained by a destructive welding seam test. However, the tests and inspections are generally only performed in order to determine welding parameters and for the purpose of subjecting pipeline components to factory quality control because destructive testing by the client would not be very practicable. An overview and appropriate requirements profiles for short-term and long-term destructive tests on welded plastic joints can also be found in the relevant standards, guidelines and information sheets, such as DVS 2202 and DVS 2203.
Custom components
In addition to the established standard components used in industrial pipeline construction and gas and water supply, further components have been developed for a specific requirements profile in sewer construction and sewer repair/rehabilitation, the prime objective being to offer systems matched to a particular project. They include pipes with integral electrofusion joints, Figure 5a , as well as shaft and wall connectors which can be welded to create a strong, permanent connection, Figure 5b. There have also been further developments in the important sector of drain connection in sewer construction and repair. They include external saddles for service pipe connection by electrofusion welding, Figure 5c , for installation by means of the open method, and internal saddles, Figure 5d , for laying with the help of the closed method. The processing of these components is performed to meet the quality requirements indicated above.
Conclusion
The further development and application of electrofusion welding, as used in the pressure- pipe sector of industrial pipeline construction and gas and water supply, to wastewater systems made of thermoplastics makes it possible to create pipeline systems in this field of application which are permanently watertight and functionally reliable. By complying with all the necessary standards and guidelines for the construction and repair of municipal and industrial waste-water systems, it is possible to create an infrastructure in the waste-water and drainage sector which is permanently functional, watertight and economical. In order to reliably create a system connection which is optimised for a particular application and service, the selection of materials, compliance with specified processing parameters depending on the type of connection, and appropriate preparations are of vital importance.
Literature
[1] DIN 1910 „Schweißen, Schweißen von Kunststoffen, Verfahren“ (1977-09)

[2] Helmut Potente: Fügen von Kunststoffen, Carl Hanser Verlag

[3] Bachmann/Lohkamp/Strobel: Maschinenelemente, Vogel-Buchverlag

[4] Taschenbuch DVS-Merkblätter und -Richtlinien „Fügen von Kunststoffen”, DVS-Verlag GmbH

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