Butt fusion welding involves simultaneously heating the ends of two pipes/fittings to be joined until each contact surface reaches a molten state.
The two surfaces are then brought together under controlled pressure for a specific cooling time to form a uniform fusion joint. The joint thus formed is fully capable of withstanding end loads and performs under pressure comparable to the pipe itself.
An electric heating plate is used to raise the temperature at the end of the pipe to the desired fusion joint temperature.
Butt-welding machines for welding polyethylene pipes have controls to ensure that the welding parameters are strictly adhered to. The control parameters are as follows
- Heating plate temperature
- Ellipticity and alignment
- Joint pressure
- Weld seam width
- Heat soak time
- Conversion time
These factors must be controlled to ensure that the weld does not fail prematurely. Field conditions during the welding of polyethylene pipe can have a significant effect on the strength of the joint. In order to obtain an acceptable weld, three elements are vital:
- Cleanliness: because contamination can damage the joint
- Technique: Most owners of PE pipe systems require that the person carrying out the butt welding of PE pipe is qualified by completing an accredited training course.
- Correct equipment design and maintenance: correct welding temperatures, welding procedures and pipe welding tools must be kept within tolerances and in good condition.
When properly fabricated, a welded joint is as strong as the original pipe and can withstand all the loads of everyday installation and operation.
Internal and external "weld beads" are formed during the fusion process. Techniques have been developed to minimize the size of the weld bead, but since the weld bead does not add to the overall strength, it can be safely removed when needed. Removed weld beads can be inspected as part of a quality control program.
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