P1 = Pipe CS area × Material Thrust Coefficient CS area of machine hydraulic cylinder NOVEMBER 2016 / WELDING JOURNAL 53 Heating Mirror Equipment The equipment was designed per ISO 12176 (Ref. 10). It consisted of a hydraulic unit for holding pipes, hydraulic pump for aligning and applying pressure in the axial direction, a motorized facing tool for trimming the pipe faces, and a heating mirror used to heat the pipe faces. The heating unit consisted of a circular metallic electric heater that heated both faces through a temperature controller. A Model PT 500 machine made by Tecnodue in Italy was used for welding the HDPE pipes in this project. Welding and Testing Procedures Thermoplastic fusion butt-joint welding and inspection procedures for joining the HDPE pipes were prepared based on the specifications given in ASTM D2657 (Ref. 11), PPI-TR-33 (Ref. 12), and IS 7634 (Ref. 13). Procedure Qualification The scope of nondestructive examination in a HDPE weld joint is very limited. Hence visual inspection of the weld joint is important, and this was meticulously carried out by qualified inspectors. The various weld joint defects that could occur during a thermoplastic fusion butt-joint welding process are shown in Fig. 2. However, destructive tests were done on test welds in which the welds were cut across in strips and checked for any voids, porosity, or other discontinuities. Inside face bend, outside face bend, tensile, and elongation tests were conducted as per ASTM-2657 and were compared with tests carried out on similar samples made out of the pipe base material. To qualify the weld procedure, test joints were prepared with various combinations of values of weld parameters. All test samples were subjected to visual and destructive tests. Based on these results, welding pressure and time duration applicable for various steps were finalized and a procedure was prepared. The optimized values of various parameters and observations made on the weld quality during extreme values of welding parameters are shown in Table 1. The same procedure was also used to qualify the welding machine. This qualification exercise was repeated daily during startup of welding work. Preprocessing First, the two pipes to be welded were held in the hydraulic unit and aligned. The motorized facing tool was fixed on the frame of the hydraulic unit between the pipes and was rotated to simultaneously machine the faces of both pipes as shown in Fig. 3. Facing was done to remove any pipe face toe-in and to make the ends into a perfect, aligned square. The facing tool was removed and both the pipes were moved to close the faces with each other. The pipes were visually inspected for alignment, which needed to be <10% of the wall thickness, i.e., 2.7 mm (0.106 in.), and the root opening, which needed to be <2 mm (0.0787 in.), was measured. Welding was done only after the pipes passed this visual test. The surfaces to be fused were thoroughly cleaned to make them free from contaminants, which could cause incomplete fusion and make the joint unacceptable. Welding Process The heating mirror was heated to 210 ±5°C (410 ±9°F), which is around 30°C (86°F) more than the melting temperature of any commercial HDPE pipe material. During the entire process, heating was continued through a temperature controller and temperature was cross verified using thermal chalk. Also, it needed to be checked that the difference in temperature between both the heating faces of the mirror was not more than 5°C. Then the heating mirror was placed between the two pipes as shown in Fig. 4. To make a butt joint, the pipes were moved so their faces were butted against the heating surface from either side of the heating mirror and were kept pressed together by a hydraulic pump at a pressure (P1) for initiation and formation of bead for 300 s (t1). This was called the bead formation step. Pressure applied was calculated using the following formula: The material thrust coefficient for HDPE is 1.5 kg/cm2 (21.34 lb/in.2). The cross-sectional area of the machine hydraulic cylinder given by the equipment manufacturer was 14.13 cm2 (2.2 in.2). Thus P1 was calculated as 30 kg/cm2 (427 lb/in.2). At the beginning of welding, the drag pressure required to move the pipes had to be recorded and added to the applied pressure values. Drag pressure corresponds to the force required to overcome internal friction of the equipment while moving the pipes. The equipment used for this project had a drag pressure of 10 kg/cm2 (142 lb/in.2). Though the faces of the heating mirror are coated with PTFE, which resists sticking of molten HDPE, after a certain period the faces can get contaminated and should be cleaned. Otherwise heat transfer will not be efficient. Fig. 3 — HDPE pipe ends being machined using the facing tool. Fig. 4 — Heating mirror being used for welding HDPE pipe. Fig. 5 — Pressure vs. time graph for welding DN 355 PN 10 PE 100 HDPE pipes.
Welding Journal | November 2016
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