Butt Welding - What is it?

This procedure utilises the exceptional weldability of the various polyethylene materials and results in a pipeline with an axial strength close to that of the parent material. Before undertaking the installation of a welded PE pipeline, consideration must be given to the following.

Choice of Equipment

The range of available butt fusion welding machines has expanded rapidly in recent years. The cost of machines for larger pipes is high and the use of specialist welding subcontractors is normal practice. Machine operating characteristics vary with age, with earlier models requiring manual operation and the latest being equipped with fully automatic control to the extent that a printed record of all weld parameters can be produced for each joining operation.

A welding machine is in fact comprised of a number of components mounted on a single base frame or chassis. These include pipe clamps for each side of the joint (i.e. full circle vices) specifically to suit the pipe diameters being joined, a hydraulic pump, hydraulic rams to provide the jointing forces, an electrical lathe with double sided cutter for trimming operations, an electrically heated twin sided welding plate (with non-stick surface) sometimes called a ‘mirror’, pressure gages for the hydraulic system and (welding plate) temperature senses. If the machine is semi or fully automated, there will usually be a computer/printer unit as well.

A power source such as a portable generator or compatible electric mains connection is also necessary. Pipe rollers to reduce drag friction together with lifting equipment will be needed for larger pipes. Temporary shelter for this equipment during the welding operation such as a tent is important, as it will assist in keeping the pipe being welded at a uniform and stable temperature free of moisture and dust. Sunlight and wind can create uneven ambient pipe temperatures affecting the weld quality.

Welding Principals    

Pipes to be joined must be of the same wall thickness (i.e. SDR) and the ends must be cut square, i.e. perpendicular to the pipe axis. Unmatched wall thicknesses will require a machining or chamfering operation of the greater thickness at an angle of fifteen degrees (1:4) or less to give the same face thickness on both sides of the joint. The success of each weld in extremely dependant on cleanliness, temperature control and good equipment, which has been properly maintained. The pipe ends must be properly aligned before machining and heating. They should be dry and free of dust contamination. Mating surfaces must be planned immediately before welding to remove surface material as polyethylene oxidises on exposure to the air. If these prepared surfaces are touched after machining there is a risk of contamination that may be not be visible to the naked eye.

The timing of the welding sequence is most important if consistent weld quality is to be obtained. Times for the simultaneous heating of the pipe ends against the hot plate or the ‘mirror’, mirror removal, the pressing of the melted pipe ends together to give the required amount and shape of bead material, and finally the cooling time whilst maintaining pressure, are all of critical importance.

If the pipe temperature is not uniform, such as for example can occur when welding is carried out in direct sunlight, an uneven pipe wall temperature will exist which could affect the uniformity of the weld. This temperature difference must be equalised by shielding the weld zone well in advance of making the join. Covers on the ends of pipes remote from the weld will prevent cooling air passing through the pipe interior and assist in keeping weld zone temperatures uniform.

Operator Training

As with all welding carried out on any material used for engineering purposes, polyethylene welding is a skilled operation requiring the use of properly qualified operators. Formal courses, which provide accreditation to suitably skilled personnel, are available.  The use of unqualified operators is unacceptable. For information on training courses in New Zealand, go to the link below.

 

Available From

    

 

Back to Polyethylene