Electrofusion Technique for Pipes up to d800

Transcription

1 Electrofusion Technique for Pipes up to d800 Dipl.-Ing. Robert Eckert PE pipes have been in use for almost 50 years producing very good results. Today, polyethylene is a wellestablished material for gas and water supply systems, in the fields of waste water disposal and sewage systems as well as in numerous industrial applications. PE piping systems, also in larger nominal diameters, have become the standard both in tenders and specifications of civil engineering firms. Today, already transmission pipelines of DN 500 and above are routinely installed. Even for large dimensions, the electrofusion welding is an indispensable joining method. Process- and application-technical know-how is not only the basis for the safe and reliable handling but also improves cost efficiency both in the construction phase and during operation. POLYETHYLENE: REVIEW OVERVIEW OUTLOOK When the first PE pipes were installed approximately 45 years ago, the main focus was on the questions on reliability and the PE pipes duration of use to be expected. As a pioneer" one can naturally not resort to any operational experience but must rely on theoretical assumptions. A prognosis on the long-term pipe properties was facilitated by the theory already developed by Arrhenius based on which the life expectancy of a plastics pipe can be proven by higher test temperatures at reduced testing times. Looking back, the statements made were true. The statements developed by Arrhenius have in the meantime become the standard in all common basic materials standards as basic proof for life expectancy, operating pressure and operating temperature of pipe materials. At the same time, the knowledge about PE has been enormously extended in the course of the decades which resulted in a permanent improvement of material properties. Today, the material properties can be largely tailor-made depending on the application specific requirements. The material polyethylene has proven itself excellently for the production of pipes and also in other fields of our daily life. As compared to conventional materials, plastics offer numerous advantages, not only from the technical point of view: Normally, the economic advantages also speak "in favour of polyethylene. The handling of polyethylene pipes at the construction site is above all characterised by the low weight, the high flexibility, the excellent notch impact strength as well as a safe and proven joining method. The smooth internal surface guarantees low wall friction losses in operation and the considerable impairment of the hydraulic performance caused by the typical depositions and incrustations do almost not occur in PE pipes in water supply systems. Corrosion is unknown in polyethylene in piping construction. It is specifically also the excellent notch impact strength, the good fusibility and further improved properties which encourage us to forecast a life expectancy of 100 years for PE materials of the third generation. AREA OF APPLICATION OF ELECTRO FUSION FITTINGS IN LARGE PIPE ENGINEERING A particularly important aspect for the use in practice is a joining method which meets the expectations with regard to the pipe. Modern fusion methods make use of the possibility of an optimal, homogeneous material joint using heated tool butt welding and electrofusion. A single, homogeneous pipeline is thus created from individual pipes lengths. The strength of the joint created when using electrofusion is normally even higher than the strength of the pipe itself. This can be impressively shown by bursting strength and tension tests. Electro fusion fittings are today used up to d 800 mm/32" in the form of couplers for axial joints and saddle components for branches and connections. Permissible operating pressures of up to PN 25 (water) can be reached to some extent when using the material PE 100 and a corresponding design of the dimensions. In gas supply, the maximum operating pressure for PE 100 is presently limited to 10 bar (EN 1555, DVGW-G472; German Technical and Scientific Association for Gas and Water) Easy handling, a broad range of applications and of course primarily a reliable and safe processing are in the centre of the product development. DESIGN OF THE FITTING: CRITERIA The design of electro fusion fittings is based on the following criteria (Figure 1): Solid wall thicknesses to absorb the internal pressure load Long pipe insertion depth serves the pipe guiding Broad fusion zones for a reliable joint

2 Broad internal cold zone eliminate the effect of sunken pipe ends Exposed heating coils for an optimal heat transfer Preheating for a high gap bridging capacity Reinforcement of the outer diameter to improve the joint quality (Figure 2) Separated fusion zones for flexibility during installation Matched use of installation aids and devices Fig. 1: Section through a welded joint d 800 mm Fig. 2: Outer fitting reinforcement prevents its expansion during fusion SAFE MELT PRESSURE BUILD-UP THROUGH CONSTRAINED ELONGATION An important parameter for fusion joints is apart from temperature and time the pressure acting in the melt during fusion. In butt welding, this joining pressure is typically applied through hydraulically introduced forces in a complex way via the saddle in which the pipes have to be tightly clamped. In electrofusion welding, the volume increase created during the conversion of the state of aggregation solid liquid is used. The joining gap is filled by the created melt; the joining pressure is build up due to a further volume increase. The forces created here result in an enlargement of the coupler s diameter depending on the size of the annular gap between couplers and pipe. If the elongation is permitted, the melt pressure is reduced at the same time. The outer reinforcement (Figure 2) of the coupler prevents the elongation during the fusion process under the acting melt pressure. The mesh reinforcement at first remains resilient and progressively counteracts the increasing elongation. A stiff, inflexible reinforcement in contrast could provoke melt release, i.e. an impermissible pressure reduction. The effect of the reinforcement can be well observed in practice: After cooling of the joint, the coupler shrinks again, in contrast to the mesh reinforcement. The reinforcement comes loose from the coupler at some points and thus indicates safely that the reinforcement has been active in the joining process and that an optimal melt pressure was given. Typical defect patterns in products in the field of large pipes, for whose development this physical effect has not been considered, are marked shrinkage holes in the fusion zone. PROCESSING STEPS FOR LARGE COUPLERS In the following, the most important processing steps are briefly explained: Cutting off of the pipes in a right angle to the pipe axis Apart from very complex settings, the electrical straight back saw has proven itself in practice. For orientation, the cutting line can be easily marked at a right angle across the entire diameter using a flat strip. A power saw may also be used. In order to avoid residues of lubricants in the area of the fusion zone in principle, the chain may not be lubricated. The manufacturer s instructions and accident prevention measures are to be observed. Marking of the scraping area (pipe insertion depth + approx. 5mm) Removal of the oxide layer with the scraper tool The use of the manual scraper for large pipes is not justified because of the considerable effort, neither for economic nor technical reasons. For a pipe of d 630mm, the uncoiled length of the circumference to be processes is approx. 2,000mm (Fig. 3). If required, pipes have to be repeatedly scraped in the range of the upper tolerance limit. The use of scraper tools is thus urgently recommended or even prescribed. Narrow limits apply to the fit ratios of coupler/ pipe. In case of large pipes, the permissible tolerance range of the plus-tolerated outer

3 diameter (Table 1) also increases. Although the application-friendly mount ability of the fitting must be guaranteed, repeated scraping may be required in practice to reduce the pipe diameter and to facilitate a slipping on of the coupler. If repeated scraping is required it is to be noted that deviations related to out-of-roundness render the installation difficult. Fig. 3: Professional fusion surface preparation on a drinking water line d 355mm with the help of the scraper tool FWSG710L Tab. 1: Comparison of outer diameter tolerances in pipes pursuant to EN 1555 / EN Nominal outer diameter pursuant to EN , EN Mean outer diameter dem [mm] Tolerance [mm] d n [mm] d em, min. d em, max Δd 32 32,0 32,3 + 0, ,0 63,4 + 0, ,0 125,8 + 0, ,0 226,4 + 1, ,0 402,4 + 2, ,0 633,8 + 3,8 Chamfering of the cutting edge, production of a marked chamfer A marked chamfering of the outer pipe cutting edge facilitates the installation of the fitting. By slipping on the first part of the insertion depth, the inching of the coupler, elevated points at the pipe diameter, which might have to be finished locally with the manual scraper, can be determined easily by visual inspection. Marking of insertion depth Mark the length of the insertion depth threefold, displaced by approx. 120 by the pipe diameter. This facilitates an easy and reliable control of a jam-free installation of the fitting. Restoration of the roundness of irregular / oval pipes using the rounding clamp In case of a marked out-of-roundness of more than 3mm, rounding clamps are to be used to facilitate installation. When mechanical rounding clamps are used, the forces are no longer manageable. Thus, tools with hydraulic support are available. The rounding clamps are mounted onto the pipe at the end of the insertion depth mark and thus at the same time serve as stop for slipping on of the coupler. Cleaning of fusion surfaces Cleaning agents which contain ethanol with a purity degree of 99.8% pursuant to DVGW VP 603 are recommended. For cleaning, cloth is used which is lint-free, not dyed and not produced from recycling material. Only the defined fusion surfaces are to be cleaned. Any contaminations from neighbouring, unscraped surfaces may not be introduced into the joining zone. Installation of pipe and fitting (Figure 4) Because of the fit tolerances, the fitting can in most cases not be easily slipped on when it comes to large couplers. Narrow fit ratios are required for an uniform melt pressure build-up. An excessive gap between the coupler and the pipe is to be avoided. In this respect, the processing instructions of the manufacturer are to be observed. If required, carry out a preheating (see "Knowing demands in practice, solving problems": preheating) Carry out the fusion process (Fig. 5)

4 Fig. 4: Connection of a branch to a gas line d500 Fig. 5. Connection of valves and branches to a gas piping system d d 630 for filling of the gas tank Recording of data Recommended is the date, fusion time, name of the welder, and above all the time after fusion to be able to observe the following cooling time reproducibly. Letting the joint cool down The cooling times for the pressure test of the piping are to be observed. KNOWING DEMANDS IN PRACTICE, SOLVING PROBLEMS: PREHEATING METHOD Avoidable mistakes in the processing of large pipes are mainly the result of shape deviations: Out-of-roundness and flattening at the outer diameter. Storage and transport influences may cause these deviations from the ideally round shape of the pipe, above all with regard to the net weight or linear distributed loads as bearing reactions (Figure 6). Fig. 6: Shape deviations in PE pipes. Out-of-roundness or local flattening Principal view of possible deformations of the pipe contour and their effects If the out-of-roundness of the pipe cannot be avoided as a whole, the fitting must provide for a corresponding resistance to shape deviations. The preheating method was thus developed to improve the fit ratios for coupler components. Both the storage on the ground as well as the displaced arrangement of pipe layers in a wooden crate may result in a local flattening of the outer pipe diameter. In butt welding, this might result in an impermissible displacement; in electrofusion, this might result in excessive distances between the coupler and the pipe. A large gap between the pipe and the installed coupler as annular gap or also in locally defined areas may result in an insufficient build-up of the melt pressure in the extreme case. In order to counteract these deviations which are hardly to be completely avoided in practice -, their negative influence is taken into consideration when designing the coupler: By preheating the joining zone below the melt temperature before starting the fusion process, the joint gap is reduced (Figure 7). This utilises the specific material properties of the material polyethylene: The relatively large thermal elongation of PE results in a volume increase, pipe and fitting approach each other, the joint gap is reduced.

5 Stress relief under the influence of heat and memory effect: Frozen stresses (as e.g. out-of-roundness due to storage) are relieved, at the same time, the pipe "remembers" its almost ideally round contour of its production process and tries to restore it. The memory effect of plastics has been known for decades and is used in many areas, e.g. medical engineering. The heat applied to the joining area below the melt temperature increases the energetic level in the joining area before the joining process and thus improves the condition for the following fusion process. 1. Step: Carry out preheating 2. Step: Carry out fusion Fig. 7: Functional principle of preheating. Preheating process: Reduction of the annular gap through heat input utilising the thermal expansion coefficient. The temperature thus remains below the crystallite melting temperature. HOUSE SERVICE CONNECTIONS AND BRANCHES Saddle fittings can be installed and fused to large pipes from d 250mm to d 710mm using the top-loading method (Figure 8). This makes the bottom section superfluous which is unhandy for large pipes. Thanks to the flexibility of the component at the saddle faces and the pneumatically supported, spring-elastic clamping method, the entire dimension range from d 250mm to d 560mm (d 630mm) including all intermediate dimensions can be covered with one single fitting. This method facilitates the production of house service connections and branch lines under operating pressure and without interrupting the supply, e.g. with a tapping ball valve (Figure 9) Fig. 8: Connection to a drinking water line d 400 using the top loading method Fig. 9: Top-loading tapping ball valve: Combination of saddle fitting and shut-off valve for pipes up to d 560

6 CONCLUSION Excellent experiences have been gained in numerous areas of application with PE pipes in large dimensions. The advantages of the material and the joining method, electrofusion, let expect a technical and economic optimum with regard to installation, operation and life. New fittings and innovative installation methods tap the full costsaving potentials and thus in addition increase the attractiveness of PE piping systems. Slide 9 Standards, Regulations and Guidelines GAS ISO 4437: Buried polyethylene (PE) pipes for the supply of gaseous fuels ISO 8085: Polyethylene fittings for use with polyethylene pipes for the supply of gaseous fuels EN 1555: Plastics piping systems for the supply of gaseous fuels EN : Gas supply systems. Pipelines for maximum operating pressure 16 bar. Specific functional recommendations for polyethylene (MOP 10 bar) Remarks: WATER ISO 4427: EN 12201: EN 805: Polyethylene (PE) pipes for water supply Plastic piping systems for water supply. Polyethylene (PE) Water supply. Requirements for systems and components outside buildings PRESSURE DISCHARGE EN 13244: Plastics piping systems for buried and above-ground pressure systems for water for general purposes, drainage and sewerage. Polyethylene (PE) Slide 10 Standards, Regulations and Guidelines FUSION / WELDING ISO : Plastics pipes and fittings -- Equipment for fusion jointing polyethylene systems -- Part 2: Electrofusion EN 13067: Plastics welding personnel - Qualification testing of welders - Thermoplastics welded assemblies DVS : Welding of thermoplastics - Heated tool welding of pipes, pipeline, components and sheets made from PE-HD DVS : Imperfections in thermoplastic welding joints - Features, descriptions, evaluation DVS : Welding of thermoplastics - Machines and devices for the heated tool welding of pipes, pipeline components and sheets EWF EUROPEAN PLASTICS WELDER (EPW): Guideline of the EUROPEAN FEDERATION FOR WELDING, CUTTING AND JOINING Minimum requirements for the education, training, examination and qualification of personnel DVGW GW330: PE Welder DVGW GW331: Supervisor for PE Welder

7 Slide 13 Education and Training, Supervision The Keys to Success: Installation Instruction, Operating Instructions, Manuals Slide 14 Education and Training, Supervision The Keys to Success: Education and Training Workshops Seminars Practical Training Slide 15 Education and Training, Supervision The Keys to Success: Support on Site Training on the Job Handling of Equipment, e.g. scraper tool Technical instruction under practice conditions

8 Slide 36 SA-XL d 315/160: Field test Project: Sony Slowakei Customer: SPP, Slowakia Challange: Branching d 160 of a gas pipe d 315 PE100 SDR17 under 4 bar OP Slide 41 Standards for Tests and Test Preparation ISO/FDIS 11413: ISO/FDIS 11414: ISO 13953: ISO 13954: ISO 13955: ISO/DIS 13956: ISO/WD Plastics pipes and fittings - Preparation of test piece assemblies between a polyethylene (PE) pipe and an electrofusion fitting. Plastics pipes and fittings - Preparation of polyethylene (PE) pipe/pipe or pipe/fitting test assemblies by butt fusion. Polyethylene (PE) pipes and fittings - Determination of the tensile strength and failure mode of test pieces from a butt-fused joint. Plastics pipes and fittings - Peel decohesion test for polyethylene (PE) electrofusion assemblies of nominal outside diameter greater than or equal to 90 mm. Plastics pipes and fittings - Crushing decohesion test for polyethylene (PE) electrofusion assemblies. Plastics pipes and fittings - Determination of cohesive strength - Tear test for polyethylene (PE) assemblies Plastics pipes and fittings Determination of cohesive resistance Strip-bend test for electrofusion assemblies Slide 42 NDT: Non Destructive Testing of electrofusion joints 1. Visual inspection Did the installer follow the installation instructions of standards, e.g. DVS and the manufacturer? (Electronic!) documentation of fusion process data Maintenance of Equipment (Fusion boxes, Scrapers) Welder Pass: Expiration of the validity Correct Pipe preparation Marking of insertion depth Marking of fusion parameters Stress free installation...

9 Slide 43 NDT: Non Destructive Testing of electrofusion joints 2. Ultra sonic, x-ray and other methods Source: TWI Ltd, Plastic Pipes Fittings and Joints 2007, Cologne Source: TWI Ltd, Plastic Pipes Fittings and Joints 2007, Cologne Slide 44 Source: KIWA Gastec Technology, Plastics Pipes XIII, 2006, Washington Destructive Testing of electrofusion joints According to ISO/WD Typical example of ductile fracture between wires side view Typical example of brittle fracture side view Typical example of brittle fracture view on fracture surface