DoandDontsforPolyethyleneJointing

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Jointing of Polyethylene Pipe Systems At the forefront of PE jointing technology is fusion welding, which can be used to join pipes directly or through fittings. Jointing methods for polyethylene pipe also include push-fit systems, mechanical couplings and many others. Fusion Welding There are three methods of fusion butt fusion, electrofusion and hot iron. Butt fusion is a technique which has been used for many years in joining polyethylene pipes above 50mm. Electrofusion has now largely supe
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    1 Jointing of Polyethylene Pipe Systems At the forefront of PE jointing technology is fusion welding, which can be used to join pipesdirectly or through fittings. Jointing methods for polyethylene pipe also include push-fitsystems, mechanical couplings and many others. Fusion Welding There are three methods of fusion butt fusion , electrofusion and hot iron . Butt fusion is atechnique which has been used for many years in joining polyethylene pipes above 50mm.Electrofusion has now largely superseded the hot iron technique – used for jointing pipes withsocket-type fittings.There are a number of unique benefits associated with fusion jointing. These can besummarized as follows: ã   A fused joint is at least as strong as the pipe itself, and this in turn ensures that the corrosionimmunity of the polyethylene pipe system is continuous and unimpaired. Moreover, a fusedpipeline will be end-load bearing throughout its length. ã   Fusion enhances the innate flexibility of polyethylene. With strong joints, flexible pipestrings can be fed easily into position from above ground, whatever installation technique isused. ã   No bore impedance’s need be introduced by fusion joints. Where devices such as pipesupport liners are used, there is unavoidable interference to follow. The smooth internalfinish of a de-beaded fused joint ensures there is no such impedance.In this chapter we are concerned specifically with butt fusion and electrofusion.    2 Butt Fusion In butt fusion the pipe ends to be joined are brought together in a dedicated butt fusionmachine. The end faces are squared up by planing with a mechanical trimmer, then heated withthermostatically controlled non-stick heater plate. When molten, the faces are pushed togetherand allowed to cool.The butt fusion process generates weld beads both inside and outside the pipe. These can beremoved easily to produce a smooth bore or outer surface. Inspection of the beads alsoprovides a useful quality check on the weld.Codes of practice for mains-laying stipulate that this jointing method should be applied to pipesonly, thus all fittings are delivered to site with half-metre ‘pups’ of pipe prefabricated to them.Detailed instructions on butt fusion technique is given under ‘A Guide to Good Welding’ below. Electrofusion This method uses socket-type fittings with integral heating elements to construct a pipe system.Couplers are used to join mains pipes; saddle fittings are used to connect service pipes. Fittingssuch a flange adaptors are spigotted and can be used in conjunction with electrofusion couplersand reducers.Within an electrofusion fitting there is a resistive heating wire connected to surface terminals.An electric current passed through the wire melts the polymer and fuses the fitting to the pipewall.The pipe to be welded is first prepared by scraping away the outer surface layer, then pipe andfitting are clamped together to restrain movement. An electrical current is applied across theterminals from a generator via a control box (generally 40V in the UK). After welding theassembly is allowed to cool thoroughly before under clamping.Detailed instructions on electrofusion jointing technique is provided in ‘A Guide to GoodWelding’ below. A Guide to Good Welding Though PE is relatively simple to weld, only by following the correct procedures can a sound buttfused or electrofusion weld be achieved every time.    3 Safe handling of molten material During fusion welding of PE pipes and fittings, molten PE is formed. If allowed to come intocontact with human skin, it will adhere strongly and burn severely. The thermal capacity of molten PE is high and it will remain hot for some time.Gloves must be worn if there is any risk of skin contact. Small quantities of fumes may be givenoff during welding – more pronounced at higher temperatures. Great care should also be takenwhere there is a risk of PE residues adhering to heated surfaces such as heating plates used forwelding. Some ventilation is important in ensuring safe working conditions. Butt fusion welding Only approved, well maintained machines should be used. There are two basic types of weldingmachine, manual and automatic, the difference being that the welding cycle is computercontrolled in the automatic machine. Modern equipment is hydraulically operated, the oldpneumatic machines need a compressor as well as a generator and should not be used. Equipment ã   Generator to power the machine’s heater, trimmer and hydraulic pump.   ã   Welding machine of suitable size, including clamp frame, clamp shells, trimmer, controller(for automatic machines) and hydraulic pump – an electrically operated pump is best, sinceit enables one person to carry out welds on pipe up to 250mm. ã   Pipe Support Rollers ã   Pipe end covers ã   Welding tent ã   Clean, dry, lint free cloth or paper towel. ã   External/internal debeading tool. ã   Bead gauge ã   Calibrated digital thermometer and probe. ã   Timer    4 Pre-welding checks Before commencing a welding operation, check that: ã   The site is suitable for welding and a tent is used. ã   You know the correct welding parameters for the machine and pipe being welded. Theseare normally on the machine’s data plate. If in doubt, refer to our Technical ServicesDepartment. ã   The machine is complete and undamaged. ã   The generator has enough fuel for the work to be done. ã   The heater plate is clean – if not, was it while cold with lots of clean water and dry with aclean, lint free cloth or paper towel. ã   The trimming tool is clean and sharp. ã   The clamp shells are of correct size and clean, with no embedded grit which could lead topipe misalignment. ã   The heater plate is at the correct temperature. ã   The pipes to be welded are of the same material and SDR (wall thickness). Dummy welds Before a welding session, a dummy weld should be carried out to clean the heater plate. Thewelding process (see instructions below for manual and automatic) should be followed up to theheat-soak stage, at which point the weld may be aborted. Any dirt will be transferred from theheater plate to the pipe end which, when cold can be retrimmed for welding.Pipes larger than 180mm should be dummy welded twice before proceeding.
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