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PE Welding
By Dr Jonathan Morris is a Senior Asset Management Consultant with Opus International Consultants Limited. Jonathan has a background in pipeline materials installation and assessment, and has a doctorate in non-destructive testing.
Temperature
If you want to melt something, you have to heat it up without overcooking it. For welding, that means that the welding machine and controller must work properly. If the temperature is too cold, the material will not melt properly and the ends will not stick together properly. If it is too hot, the melted material might degrade, or it will be squeezed out of the joint.
In butt fusion, the pipe is melted by the heater plate. Heater plates are usually made of aluminium with a PTFE (Teflon) non-stick coating. These need to be kept clean to avoid getting dirt into the joint – avoid scratching them, check them frequently and only clean them when they are cold.
Most heater plates are electrically heated and the melt temperature is controlled by the temperature controller with one or more sensors embedded in the heater plate. If the heater plate or one of the heating elements is damaged, or the temperature sensor is inaccurate, the working temperature will be wrong. A hand-held thermometer with a surface probe is the most accurate way to measure heater plate temperature - these need regular calibration by a certified testing lab.
An uneven heater plate temperature will usually show up through uneven weld bead formation - cold spots will have smaller beads - or through oversized beads and charring of overheated areas.
An undersized power supply can also result in a cold joint, if it can’t supply enough power to maintain the heater plate temperature. Cold joints don’t stick properly, and really bad ones won’t stick at all.
Blocking the ends of the pipes with end caps or sand bags will reduce the risk of air flowing along the pipes and cooling the heater plate while heating, the weld surfaces or when removing the heater plates.
On hot days, especially when the temperature is 30°C or more, the heater settings may need to be adjusted as less heating will be needed (this will also make the pipe cool down more slowly). Pipes also can get very hot in direct sunlight - surface temperatures of about 80°C or more are possible on black pipes.
If the two sides of the pipe are at very different temperatures this could also cause uneven melting and a poor joint could result. Some form of shelter or sheeting should be used to protect stored pipes from direct sunlight, but you need some air flow as otherwise the pipes can overheat very easily.
Pressure
Molten PE is quite thick and does not flow very well, so you have to press the melted surfaces together to ensure there is some mixing but if you press too hard or not hard enough the joint will be faulty.
The correct pressure for butt fusion can be taken from the welding specification, or it might be determined automatically by the machine when you enter the pipe details. You will usually have to take account of the frictional forces of the machine and the drag caused by the pipes within the machine - the bigger the pipes, the larger the drag forces. The table shows an example calculation using the usual PIPA welding specification.
- The pipe sizes have been taken from AS/NZS 4130 (the pipe standard), but these could also be measured on site.
- The drag pressure can vary with each joint, according to the weight of the pipe string, the number and position of pipe rollers and how level the site is.
- The machine cylinder area will depend on the welding machine.
The Following text is based on material developed for PE welding course.
PIPA POP003 Butt Welding Pressure Calculation
This can be written more clearly as:
DN180 pipe, SDR 17
Outside diameter = 180.9 mm
Wall thickness 11.3 mm
Pressure = 175 kPa
Drag pressure = 80 kPa (example value; can vary with each joint)
Cylinder area = 1200 mm ² (example only; depends on welding machine)
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PE also has a large thermal expansion, so it expands a lot when heating and then shrinks again during cooling. Applying pressure during cooling ensures that the joint is not pulled apart as the pipes shrink during cooling and reduces the risk of voids forming in the joint.
Insufficient pressure will result in poor heat transfer to the pipe. The joint will probably not stick. If the pressure is too low during the cooling stage, shrinkage will usually cause voids to form in the joint.
If a joint is pressed together too hard, too much melt will be squeezed out. The over-squeezed joint will have an oversized bead and be very weak. It may also form with a poor weld structure which can weaken it further.
Original pipe with substantial elongation
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Good jointing conditions – elongation 81% of pipe
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Excess welding pressure –
flat fracture with elongation only 4% of pipe.
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Test pieces cut from a thick-walled PE100 pipe showing the effect of welding pressure on fracture surfaces.
The excess pressure used when making the right-hand sample prevented the original structure reforming and resulted in a very brittle joint – the original pipe was more than 20 times tougher. The more elongation in the test, the better the joint.
For more information, contact Jonathan Mackey at the Environmental Training Centre, 0800678738.
Dr Jonathan Morris is a Senior Asset Management Consultant with Opus International Consultants Limited. Jonathan has a background in pipeline materials installation and assessment, and has a doctorate in non-destructive testing. He worked in the Pipeline Technology Group at the UK Water Research Centre for 10 years and then spent seven years with a leading New Zealand risk based inspection company. Jonathan’s speciality is pipeline condition management, which involves identifying what information you really want, finding a practical way of getting it and then doing as much as you can with it.
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