HARMONISATION OF POLYETHYLENE PIPE BUTTFUSION PROCEDURES AND TEST METHODS

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1 HARMONISATION OF POLYETHYLENE PIPE BUTTFUSION PROCEDURES AND TEST METHODS Stephen Beech - SHB Consulting, UK Johannes Grieser - Hessel Ingenieurtechnik GmbH, DE David Lowe - Exova Polymer, UK Philippe Vanspeybroeck - Becetel, BE ABSTRACT Several well established procedures are used for butt fusion welding of PE pipes globally. The most widely used procedures are the European Single Pressure, UK Dual Pressure, and the USA High Pressure procedures. In the future it may be possible to rationalise these procedures. Materials and machinery developments are resulting in an increasing size range of pipe. Confirmation of the appropriate test methods is required to assure weld quality, to fully understand the properties of the welds and to confirm the suitability of the fusion procedure used.. The PE 100+ Association has established this project to evaluate welds in large diameter thick walled pipe. After consultation with the industry, the German DVS Technological Bend test, the UK WIS Tensile test with energy measurement, and the German DVS Creep test to assess long term properties were selected to evaluate welds made with 1200 mm PE 100 pipe in three thicknesses produced by the three procedures. The results of this programme obtained to date are discussed and conclusions made. INTRODUCTION The advantage and ease of fusion welding of polyethylene was recognised during the early days of the introduction of PE pipeline systems. Fusion welding creates a fully end load resistant joint, resulting in a flexible pipeline system which will resist ground movement and even earthquakes, see Nishimura et al (1). The introduction of low sag PE materials is allowing thicker walled pipe to be produced, and it is important that further information to confirm the butt fusion procedures used for such pipe is gained. This paper outlines the efforts being made by the PE100+ Association and the industry to further develop and harmonise global standards for fusion procedures, test methods and non destructive techniques (NDT) suitable for thick walled pipe. CURRENT SITUATION There are several well established procedures and variations of these used for butt fusion welding within Europe and elsewhere. There are distinct differences between these procedures which can be a bit confusing for the contractor using butt fusion for the first time. Procedures have been

2 developed mostly on a national basis, but it is recognised that the industry needs accepted international standards for the fusion procedure(s), test methods, site codes of practice, and NDT techniques in order to be in a stronger position to promote large diameter PE systems. The ISO Butt fusion procedures standard is being developed, which brings the European Single Pressure, the UK Dual Pressure, and the USA High Pressure procedures into one document, (2). There is a significant difference between each of these procedures, but they each have a successful track record, see Fig. 1. These will be referred to as the Single Low Pressure, Dual Low Pressure, and Single High Pressure fusion procedures in future revisions of ISO For the Single Low Pressure (SLP) procedure the pipes are brought together and held at a pressure of 0.15 MPa whilst cooling. The Dual Low Pressure (DLP) procedure is performed at the same pressure, but the pressure is reduced to MPa during cooling. The Single High Pressure (SHP) procedure has some similarity to the Single Low Pressure procedure but a higher pressure of MPa is used initially and during cooling. Fig. 1 European Single Pressure, UK Dual Pressure, and USA Single High Pressure buttfusion procedures. It should be noted that ISO 21307, published in 2009, is being amended and revised because incorrect fusion pressure and cooling times are given for the Dual Low Pressure procedure. The Single Low Pressure procedure specified in this document is not exactly as specified by DVS and will be subject to revision.

3 Test methods for butt fusion welds are perhaps more confusing and potentially misleading than the procedures used! Methods exist in ISO, European and many national and industry standards. Short term destructive tests used include a variety of tensile tests on small specimens cut from the weld, impact tests, pressure tests and tensile tests on whole pipe assemblies. Long term tests include creep tests on full thickness specimens, FNCT and pressure tests. SETTING UP THE PE 100+ PROJECT After an initial discussion with Marcogaz, the European gas suppliers organisation, the PE 100+ Association decided to initiate this project in order to consider harmonisation of test methods and procedures. A steering group of experts was set up with members from the PE 100+ Association, Marcogaz, GDF Suez, Kiwa GasTec, and test laboratories Becetel, Exova Polymer UK (formerly Bodycote PDL), and Hessel Ingenieurtechnik. In addition support has been received from Reinert Ritz (welding and test specimen preparation), Egeplast (supply of pipe), Widos (welding machine), and McElroy (welding to USA procedure). An outline description of the project was presented at Plastics Pipes XIV in Budapest, see Beech et al (3) PROJECT OBJECTIVES The key to success of this project is the evaluation of test methods to ensure that the short term and long term properties of the weld are fully understood. Some of the test methods currently used are misleading, and may not be applicable to the full range of wall thickness of pipe used nowadays. Also it is recognised that accepted site NDT procedures are essential for greater acceptance of large diameter PE piping systems. TEST PROGRAMME TO EVALUATE TEST METHODS An important aim of this project is to evaluate welds in large diameter thick walled pipe with a range of wall thicknesses. To avoid using different extrusion lines, extrusion conditions and batches of material, it was decided to use the same pipe but to machine the inner lip of the bore of the pipe to give three thicknesses for welding rings cut from the pipe. A 1200 mm SDR mm nominal wall thickness pipe produced from a PE 100+ listed material was kindly supplied by Egeplast DE. The expertise of Reinert Ritz DE, a leading producer of fabricated fittings, was offered to prepare the machined rings of the pipe, and also to carry out the extensive task of cutting the welds into multiple test specimens. Rings of full 70.6 mm, 50 mm, and 30 mm wall thickness ends were produced. Preparation of the welds was carried out by Reinert Ritz using a Widos welding machine using the Single Low Pressure (SLP) procedure welds specified in DVS and the UK WIS Dual Low Pressure (DLP) procedure, (4), (5). McElroy USA produced welds using the USA Single High Pressure (SHP) procedure as specified in ISO In addition for comparison a Poor Weld was produced using a lower temperature and heat soak time. See Table 1 for a summary of the welding conditions.

4 Weld Wall mm Plate Temp C Initial bead size mm Heat soak time s Plate removal time s Time to achieve fusion press s Fusion pressure MPa Cooling time under pressure Min SLP SLP SLP DLP /0.025* 20 DLP /0.025* 27 DLP /0.025* 35 Poor SHP , ,52 13 SHP , ,52 22 SHP , ,52 30 SLP - Single Low Pressure, DLP - Dual Low Pressure, SHP - Single High Pressure * Reduced fusion pressure after 10 s SHORT TERM TESTS Table 1 Fusion conditions used to prepare the welds. The reasons for selection of tests to be evaluated in this project was discussed in the paper presented at PPXIV Conference in Budapest, Beech et al (3). The decision of the Steering Group was to select the WIS Tensile test specified in WIS (5) and the DVS Technological Bend test specified in DVS and EN , (6,7). The WIS test includes energy measurement as well as an assessment of the fracture surface. Figure 2 shows the basic geometry of the test specimen. Results for absorbed energy are dependent on the aspect ratio of width of the waist to thickness as reported by Hill et al (8). This means that data obtained for absorbed energy at any aspect ratio may be corrected for the effects of aspect ratio by dividing results for absorbed energy by aspect ratio. This may then be used to compare results in all sample sizes. For thicker walled pipe the overall width is increased to provide sufficient area for gripping. The test is carried out at a speed of 5 mm/min at 23 C. Fig. 2 WIS Tensile test specimen

5 The Bend tests are carried out on both inner and outer weld beads. Decreasing temperature can be used to promote brittle behaviour. The wall thickness of the test pieces with reduced thickness (50 mm and 30 mm) were milled to an equal wall thickness over the total length of the test piece. Distance between the supports for thickness 70 mm, 50 mm and 30 mm is respectively 375 mm, 260 mm, and 160 mm to give the same outer fibre stress. The specimen is deflected at a crosshead speed of 50 mm/min and the test is stopped after reaching a maximum angle of 100 if no failure has occurred. Tests were carried out at 0 C, and -10 C in some cases to try to promote or confirm brittle behaviour. Fig. 3 DVS Technological Bend test LONG TERM TESTS The Tensile Creep test in accordance with EN and DVS , is used extensively in Germany and Scandinavia, and was selected as the principle test to evaluate the long term properties of the welds, (9, 10). In addition the Full Notched Creep Test (FNCT) on standard 10 mm by 10 mm square bars with the notch located at the centre of the weld is being used to help develop a correlation of failure time of the Creep test with lifetime. The FNCT test on cylindrical test specimens specifically developed by GDF Suez Research and Innovation Division for evaluation of buttwelds is also being used to assist this study, Bertier et al (11). The Creep test specimen is subject to a constant tensile load of 2,5 N/mm 2 at 90 C, in an aqueous solution with 2% wetting agent to accelerate failure, ie a mixture of anionic and cationic detergents termed 'NM5'. For this test programme the specimen was milled to give a parallel gauge length of full wall thickness and equal width, both weld beads being retained. Initiation normally occurs in the notch between the weld bead and the pipe, and should propagate into the pipe material. However in poor welds the crack tends to propagate into the fusion zone, Hessel et al (12). The examination of the mode of crack propagation is used to assess the weld but time to failure is taken into account.

$Fig. 4 Tensile Creep test specimen showing crack propagation and growth from the weld bead weld bead into the pipe material and fracture surface PROGRESS - TEST RESULTS At the time of preparing this$ Results of the programme using the WIS Tensile tests and the Bend tests on the full range of welds produced are shown in Table 2 and fig.5.

Results of the programme using the WIS Tensile tests and the Bend tests on the full range of welds produced are shown in Table 2 and fig.5.

6 Fig. 4 Tensile Creep test specimen showing crack propagation and growth from the weld bead weld bead into the pipe material and fracture surface PROGRESS - TEST RESULTS At the time of preparing this paper, the WIS and Bend tests on all welds have been completed. The Creep tests have been completed on the 30 mm and 50 mm Dual Low Pressure and Single Low Pressure welds. Results of the programme using the WIS Tensile tests and the Bend tests on the full range of welds produced are shown in Table 2 and fig.5. The energy values given for the WIS tests are an average of 5 or 6 tests. For the bend tests 4 samples with the inner bead and 4 samples with the outer bead were tested. In both cases samples have been evenly spaced around the circumference of the welds. The results of the Creep tests carried out to date are given in Table 3. Tests are ongoing to evaluate the Poor weld, Single High Pressure procedure welds, and the 70 mm Dual Low Pressure weld. Weld Thickness WIS Energy KJ/m2 Bend Failure Mode (at 0 C) Bend Failure Mode (at -10 C) SLP 70 mm Mixed X 1 Inner - Ductile SLP 50 mm Mixed No failure SLP 30 mm Ductile X 1 Outer - Ductile DLP 70 mm Ductile No failure DLP 50 mm Ductile No failure DLP 30 mm Ductile No failure SHP 70 mm Mixed No failure (x 4) No failure (x 4) SHP 50 mm Mixed No failure (x 4) x 1 Inner - Brittle x 1 Outer - Brittle SHP 30 mm Brittle X 1 Outer - Brittle x 2 Outer - Brittle X 1 Inner - Mixed Poor 50 mm Brittle X 1 Inner - Brittle X 2 Outer - Brittle Poor 50 mm Brittle X 1 Inner - Mixed x 1 Inner - Brittle x 1 Inner - Mixed 1 Weld contaminated (see Discussion of test results) Table 2 Results of the short term tests.

Energy KJ/m 2 800 700 600 500 400 300 200

Low Pressure Single High Pressure Single

7 Energy KJ/m B D D D D M M B M M Poor DSP SLP SHP Fig. 5 Energy measured in the WIS Tensile for the full range of welds 30 mm 50 mm 70 mm D - ductile M - mixed B - brittle Dual Low Pressure Single High Pressure Single Low Pressure 'Poor' weld Fig.6 WIS Tensile test fracture surfaces for 50 mm welds

8 Weld Thickness Failure Mode 1 Time h SLP 70 mm Propagation from both beads, partly 7384 into fusion zone SLP 50 mm Propagation from outer bead 1 Propagation from inner bead SLP 30 mm Propagation from inner bead 1 Propagation from inner bead 1 Propagation from defect in fusion plane 2 DLP 70 mm Test to be performed DLP 50 mm Propagation from inner bead 1 DLP 30 mm Propagation from inner bead 1 Propagation from both beads 1 Propagation from inner bead Propagation from inner bead Propagation into parent pipe indicating good fusion 2 Subject to further investigation Table 3 Failure mode and time to failure in the DVS Creep test DISCUSSION OF TEST RESULTS The WIS Tensile test results show a trend of higher energy values giving more ductile fracture surfaces and brittle behaviour confirmed by lower energy values. These findings are confirmed by the Bend test results in which a mixture of no failure and brittle failures were achieved in welds, indicated to be brittle by low energy and the fracture surface in the WIS test. In view of these variable results when using the Bend test, multiple tests on a weld would be recommended. Both the Poor and the Single High Pressure procedure 30 mm welds gave the lowest energy values and brittle failure in the WIS tests, also confirmed by brittle fractures in some of the bend test specimens. The behaviour of the Poor weld was expected as it is classed as a 'cold weld' made at a lower temperature and heat soak time. However the behaviour of this Single High Pressure procedure weld indicates that something went wrong during the preparation. Subsequently it was found that the 30 mm rings were machined in the USA using a cutting fluid which contaminated the material. Good quality of the 50 mm and 70 mm Single High Pressure procedure welds was indicated by the energy values and no failures in the Bend tests at 0 C, and even at -10 C for the 70 mm weld. The WIS sample fracture surfaces were mixed ductile/brittle in appearance. A similar performance was seen from the 50 mm and 70 mm Single Low Pressure procedure welds, but the 30 mm weld gave higher energy values and a ductile fracture appearance. The reason for a ductile failure in the Bend test on a 70 mm and a 30 mm test specimens for this procedure cannot be explained, but the fact that ductility was exhibited is reassuring. The Dual Low Pressure 30 mm weld gave a slightly lower energy value, but the 50 mm and 70 mm weld energy values were higher than for the other two procedures. The fracture surfaces for the Dual Low Pressure test specimens were generally more ductile than for those made with the other procedures, and no failures occurred in the Bend tests on these welds. The failure mode of the 30 mm and 50 mm test specimens from the Single Low Pressure and Dual Low Pressure welds in the Creep test confirms the expected crack initiation point at the

9 stress concentration between the bead and the parent material. This was followed by crack propagation into the parent pipe material and not into the fusion zone or the interface between the heat affected zone and the pipe. All specimens show behaviour indicative of slow crack growth accelerated by the detergent media. The test performed on one 70 mm specimen (Single low Pressure) failed after a much longer rupture time by propagation from both inner and outer beads and partly into the fusion zone. Referring to the supplementary sheet 5 of the directive DVS 'Service life temperature time limits due to thermal ageing' such a long test at elevated temperature is in the range of the onset of thermal ageing (13). A Dual Low Pressure specimen will be tested to compare failure mode before any conclusions can be made about testing such large specimens which are breaching the limits of test equipment. Comparing the welding procedures, it is apparent that the specimens from the Single Low Pressure welds achieved longer final rupture times compared with the Dual Low Pressure specimens. A 30 mm Single Low Pressure specimen failed prematurely (997 h) from an apparent defect in the fusion plane, which is subject to further investigation. These times cannot be directly correlated with lifetime. To achieve this aim, comparable FNCT tests will be carried out on samples cut from the centre of the welds as well as from the parent material in which it is possible to make a correlation with lifetime. This method is outlined in DVS Supplementary Sheet 3, (10). NDT OF PE BUTT WELDS All the tests discussed in this paper are destructive and therefore only a limited number of site welds can be tested in this way. The question of availability of NDT testing of PE welds is often raised by contractors and end users. The establishment of accepted procedures by the industry will be a major benefit for the promotion of large diameter polyethylene pipeline systems. For many years manufacturers of ultrasonic NDT equipment have tried to develop systems to assess plastics welds. Whilst it might be possible to detect voids in PE butt welds, not normally an issue, it is the structure and behaviour of the material in the heat affected zone of the weld that is important, Scholten (14). Examination and analysis of weld beads does provide a deal of information in experienced hands. The size and uniformity of the weld beads can reveal any misalignment of the pipe, twisting or tearing the beads apart can reveal brittle behaviour, and bubbles or protrusions in the weld beads can indicate the presence of moisture or contamination. Thermal analysis techniques may be useful to assess the material properties of the weld bead and any presence of degradation. These topics are being discussed by the Steering Group. Microwave Imaging is showing promise as an NDT technique for both butt fusion and electrofusion welds, and has been tried on some of the welds made for this project. This technique is the subject of another paper at this conference, Murphy and Lowe (15).

10 CONCLUSION The short term tests used in this study have been shown to be capable of characterising welds in terms of fracture behaviour and ability to detect poor brittle welds. Both the WIS Tensile test and the DVS Bend test detected a purposely made 'cold weld' and a weld which had been contaminated. The WIS test provides a measure of fracture energy related to ductility or brittleness, confirmed by the fracture surface. The Bend test detected brittle behaviour but sufficient samples must be tested from around the circumference of the weld. The short term tests carried out on welds using the Single Low Pressure, Dual Low Pressure and Single High Pressure procedures demonstrate that apparently good welds can be made by each procedure in thicknesses up to 70 mm. The Dual Pressure procedure gave a higher level of ductility in these tests indicated by energy measurement and appearance of the fracture surface. The failure mode of the long term creep tests carried out on the Dual Low Pressure and Single Low Pressure welds confirms good long term fusion properties of these welds. However the Single Low Pressure welds gave longer times to failure in the few tests carried out. A correlation to lifetime is the subject of further study. Although this study is not complete, the results to date do indicate that some future rationalisation and harmonisation of the three procedures could take place if the industry is willing to do this. ACKNOWLEDGEMENT PE 100+ Association members (Borealis, Borouge, Ineos Polyolefins, LyondellBasell, Prime Polymer, Sabic, SCG Chemicals & Thai Polyethylene, and Total Petrochemicals), Marcogaz, GDF Suez Research and Technical Innovation, KiwaGasTec, and test laboratories (Becetel, Exova Polymer UK, and Hessel Ingenieurtechnik). In addition support has been received from Reinert Ritz (welding and test specimen preparation), Egeplast (supply of pipe), Widos (welding machine), and McElroy (welding to USA procedure) REFERENCES 1 Polyethylene Pipes Survive the Great Hanshin-Awaji Earthquake without Failures, H. Nishimura, H. Maeba, T. Ishikawa, H. Ueda, IOM Plastics Pipes X Conference, ISO 21307:2009 Plastics pipes and fittings Butt fusion jointing procedures for polyethylene (PE) pipes and fittings used in the construction of gas and water distribution systems. 3 Harmonisation of polyethylene pipe buttfusion procedures and test methods, S.H.Beech A.Hutten, M.Ritz, Plastics Pipes XIV Conference, DVS Heated tool welding of pipes, pipeline components and sheets made of PE- HD.

11 5 WIS Specification for the fusion jointing of polyethylene pressure pipeline systems using PE 80 and PE 100 materials, published by WRc plc, UK 6 DVS Testing of welded joints of thermoplastics plates and tubes: Technological bend test. 7 EN :1999 Testing of welded joints of thermoplastics semi-finished products Part 1 Bend test 8 Butt fusion welding of large diameter thick walled PE pipes, Hill et al, Plastics Pipes XI, Munich, EN :2005 Testing of welded joints of thermoplastics semi-finished products Part 3 Tensile creep test 10 DVS Testing of welded joints of thermoplastics plates and tubes - Part 4 Tensile creep test. 11 Predicting the Slow Crack Growth Resistance of PE Resins by Means of Full Notch Creep Testing in Air : Confrontation between Experiments and Modellization, Berthier F., Lopitaux A., Gueugnaut D., Constantinescu A., Dang Van K., 17th International Plastic Fuel Gas Pipe Symposium, pp.38, San Francisco (CA), USA, October 20-23, Welding of large diameter heavy wall polyolefine pipes and long term performance, J Hessel, A Lueghamer, M Tsunaga, Plastics Pipes XII International Conference, Baveno IT, DVS Supplement 5, Service life temperature time limits due to thermal ageing 14 Non-Destructive Ultrasonic Inspection to Detect Flaws in Plastic Pipe Walls and Joints Correlation with Lab Testing Frans L. Scholten, Kiwa Gastec Technology, Hyeon Jae Shin, Inde Systems, Plastics Pipes XIII, Washington, Evaluation of a novel microwave based NDT inspection method for polyethylene joints, K.Murphy, D.Lowe, Plastics Pipes XV, Vancouver,2010.

HARMONISATION OF POLYETHYLENE PIPE BUTTFUSION PROCEDURES AND TEST METHODS FINAL CONCLUSIONS

HARMONISATION OF POLYETHYLENE PIPE BUTTFUSION PROCEDURES AND TEST METHODS FINAL CONCLUSIONS Stephen Beech - SHB Consulting, UK Christophe Salles PE 100+ Association, FR Ulrich Schulte PE 100+ Association,