Heating element butt welding

Preparatory work: <p><span/></p><table><tbody><tr><td> <span/> <ul><li>Checking the roundness of the pipes and adjusting if necessary</li> <li>Clamping the pipe ends axially into the welding slide</li> <li>Processing the pipe ends by squaring-off and cleaning them, if necessary</li> <li>Checking the plane parallelism and displaced joints and adjusting, if necessary</li> <li>Cleaning the heating element</li> <li>Checking the temperature of the heating element</li> <li> <p>The pipe ends should be axially aligned before clamping in the welding machine</p></li> <li> <p>Cleaning of the …</p></li></ul></td></tr></tbody></table>
Description of method: <p><span/> <span/></p><ul><li> <b>Alignment:</b> under-pressure adjustment of joint faces of the heating element parts to be welded</li> <li> <b>Heating:</b> heating of joint faces under reduced pressure up to required welding temperature</li> <li> <b>Relocating:</b> Removal of the heating element</li> <li> <b>Joining:</b> joining faces under pressure</li> </ul><table class="picturegroup"><tbody><tr><td> (Image: Heating element butt welding device)</td> <td> (Image: Welding of the HD-PE long pipes into continuous pipes)</td> <td> (Image: Welding of the HD-PE long pipes into continuous pipes)</td> </tr></tbody></table><p/>
Alignment process: <div class="clearfix layout-row columns-2"><div class="layout-column first column-1"><p>The joining areas are pressed against the heating element as long as the complete areas lie coplanar against the heating element. The alignment process is finished when the heights of beads have reached stipulated values along the whole perimeter or the whole top of the panel, respectively. The bead heights are indications that the joining areas lie plane against the heating element. The adjusting pressure is maintained during the whole procedure.</p></div></div>
Heating-up: <span/> <table><tr><td><span/> <p>In order to get heated, the joining areas have to lie plane against the heating element under low pressure. For this purpose, the pressure is reduced to almost zero (≤ 0.02 N/mm<sup>2</sup>). During the heating process, the heat intrudes into the areas to be welded and takes them to the required welding temperature. </p></td><td> (Image: Heating element butt welding ? adjusting the joining areas) </td></tr><tr><td> (Image: Temperature sequence and pressure curve in the adjusting and heating-…</td></tr></table>
Relocating: <span/> <table><tr><td><span/> <p>After heating-up, the joining areas are to be removed from the heating element. The heating element is to be taken out without damaging or soiling the heated joining areas. The joining areas are to be brought together quickly until they almost meet. The relocating time is to be kept as short as possible as otherwise the plastified areas cool down. This would have negative effects on the quality of the welding seam.</p></td><td> (Image: Heating element butt …</td></tr></table>
Joining: <p><span/></p><table><tbody><tr><td> <span/> <p>The areas to be welded are supposed to connect with a velocity of near zero. The necessary jointing pressure is preferably applied in linear increase and is, for instance, 0.15 ± 0.01 N/mm<sup>2</sup> for PE. Sudden use of full pressure would press the hot plastic material out of the welding zone which would result in an inadequate weld factor [Simona11].</p><p>The jointing pressure has to be maintained during the cooling-off period.</p></td> <td> (Image: Heating element butt …</td></tr></tbody></table>
Welding parameters polyethylene: <p>Guidance values for the heating element butt welding of PE-pipes and fittings are provided by the pipe manufacturers. They generally apply at an external temperature of about 20 °C (68°F).</p><p>(Table: Guidance values for the heating element butt welding of pipes, pipelines and sheets made of HD-PE at an external temperature of about 20°C and moderate air flow (interim values have to be interpolated) according to [DVSR2207-1])</p>
Staff and equipment: <span/> <table><tr><td> (Image: Lining with continuous pipes [FI-KMG] - Heating element butt welding) <span/> <p>Devices and materials required for heating element butt welding:</p><ul><li>welding slide</li> <li>insertion shoulders for different pipe diameters</li> <li>facer</li> <li>heating element with control unit</li> <li>cleaning tissues and ethyl alcohol</li> <li>pressures and times are to be taken from indications of manufacturer</li> </ul><span/> <p>According to [GSTT2], only trained personnel in possession of welder's certificates may be used for …</p></td></tr></table>
Equipment: <p><span/></p><p><span/></p><div class="clearfix layout-row columns-2"><div class="layout-column first column-1"><div><ul><li>welding slide</li> <li>data-recording device with external temperature sensor, pressure plant etc.</li> <li>hydraulic aggregate</li> <li>facer</li> <li>heating element</li> </ul></div></div><div class="layout-column last column-2"><div>(Image: Equipment for heating element butt welding [FI-Huern])</div></div></div><p/>
Preparatory work for welding: <div class="clearfix layout-row marginalia"><div class="layout-column side-note"><div>(Image: Temperature)</div></div><div class="layout-column text"><div><p>The welding area is to be protected from unfavourable climatic influences (e.g. moisture and temperatures below +5 °C).</p><p>The following measures may help to achieve this:·</p><ul><li>preheating,</li> <li>covering under canvas</li> <li>heating</li> </ul><p>In order to prevent stresses due to differing temperatures such as the influence of the sun, temperature equalisation should be achieved by covering the region of the welding seams on time.</p></div></div></div><p/>
Preparatory work for welding: <span/> <table><tr><td> (Image: Temperature) </td><td><span/> <p>Before the welding procedure starts, the temperature of the heating element that is required for welding has to be controlled. This is carried out e.g. by means of a temperature measuring device with a fast display used for surface measurements. The control measurement has to be carried out within the area of the heating element that corresponds with the semi-finished product. In order to create a thermal balance, the heating …</p></td></tr></table>
Controlling the welding seam: <p><span/></p><div class="clearfix layout-row columns-2"><div class="layout-column first column-1"><div><p>The visual inspection serves to examine butt weldings with regard to the following aspects:</p><ul><li>The welding beads have to be equally round and clearly visible</li> <li>All beads are supposed to be of equal size approximately</li> <li>The bead surfaces have to be smooth</li> <li>The beads are to be free of notches</li> <li>The displaced joint should not exceed 10% of the pipe wall thickness</li> </ul></div></div><div class="layout-column last column-2"><div>(Image: Welding bead)</div></div></div><p><span/></p><p/>
Ultrasonic testing of HD-PE-welding seams: <div class="clearfix layout-row columns-2"><div class="layout-column first column-1"><div><p>During PE-welding processes, different failures may occur, which can be detected by means of ultrasonic non-destructive testing according to DVS 2203.</p><p>Incomplete fusion at the interior surface, exterior surface or in the cross section, displaced joints.</p></div></div><div class="layout-column last column-2"><div>(Image: Conventional sliplining process with annular space [FI-Teerb] - Testing the welded seam)</div></div></div><div class="clearfix layout-row columns-2"><div class="layout-column first column-1"><div>(Image: Conventional sliplining process with annular space [FI-Teerb] - View on the complete welding joint)</div></div></div>
Evaluation of fracture surface: <span/> <table><tr><td><span/> <p>The fracture surfaces can basically be divided into two types:</p><ul><li>brittle fractures are characterised by smooth surfaces and indicate poor or insufficient welding</li> <li>ductile fracture surfaces have a stretched structure and indicate good welding</li> </ul></td></tr><tr><td><p><b>Brittle fracture</b></p></td><td><p><b>Ductile fracture</b></p></td></tr><tr><td><span/> <p>The molecular chains of smooth fracture surfaces could not or only to a very small extent intermingle in the interfaces of the two join partners (e.g. by join partners that were …</p></td></tr></table>
Documentation: <div><span/></div><p>Welding work has to be monitored. The type and extent of supervision has to be agreed upon between the contract partners. It is recommended to document the method information in welding protocols or on data media [DVSR2207-1].<span/></p><div><div class="clearfix layout-row columns-2"><div class="layout-column first column-1"><div>(Image: Documentation)</div><div><p><b>Welding protocol</b></p><ul><li>Melting temperature</li> <li>Melting time</li> <li>Jointing pressure<span/></li> </ul></div></div><div class="layout-column last column-2"><div><div class="clearfix box important"><div><p>As the welder knows the nominal values from the regulations and standards, these values will always remain within the stipulated tolerances.</p></div></div></div></div></div></div>
Sources of failure: <p><span/><span/></p><ul><li>Wrong calculation of the welding pressure</li> <li>Non-observance of the heating-up time, relocation time or preparation time of joining pressure</li> <li>Non-observance of welding temperature (too high or too low)</li> <li>Machine is defect</li> <li>Machine is not calibrated</li> <li>Chosen welding pressure cannot be maintained (e.g. because of leaky piston connection)</li> <li>Contaminated joint surfaces</li> <li>Wrong material matching</li> <li>Material residues at heated tool</li> </ul><p/>
Finishing work: <div class="clearfix layout-row columns-2"><div class="layout-column first column-1"><div><p>The external and internal welding beads that are formed can be removed if desired.</p><p>If the weld bead requires finishing this is best done before cooling – approx. 2/3 of the cooling time – is complete. There is a danger that cutting work on the cold bead will cause notches [Simona11].</p></div></div><div class="layout-column last column-2"><div>(Image: Device for removing the internal welding bead for HD-PE pipes [FI-WIDOS])</div></div></div><div class="clearfix layout-row columns-2"><div class="layout-column first column-1"><div>(Image: Arrangement for removing the internal welding bead for HD-PE pipes (Locheisen) …</div></div></div>
Quality assurance for the heating element butt welding: <p><span/></p><div class="areaTable">(Table: Quality assurance measures during heated element butt welding)</div>
Standards and regulations for heating element butt welding: <p><span/> (Table: Standards and regulations for heated tool welding (as of 12/2005) [FI-Steina])</p>