Welding plastics pipes
TWI Bulletin, March/April 1988
Martin Watson, BSc, PhD, MWeldI, is Leader of the Plastics Section in the Forge and Precision Processes Department.
Pipes made from plastics are being used increasingly because of their lightness, flexibility, ease of handling and good corrosion and erosion resistance. About 10% of all pipe work is made from plastics, and the market is growing at about 5% per annum. The leader of Abington's Plastics Section looks at available welding techniques for joining such pipe.
Plastics pipes are widely used in gas and water distribution, sewage and effluent handling, building, agricultural and chemical industries. Applications requiring high integrity of pipe systems, such as gas distribution, use polyethylene pipes. Currently in the UK, more than 95% of local service and 80% of low pressure mains gas pipe, up to 180mm diameter, laid each year is polyethylene. About 18% of the present gas distribution piping system in the United States is plastic, most of it polyethylene.
Plastics pipe has increased from 12% of all distribution mains and services installed in 1966 to 83% in 1986. Pipes of up to 800mm diameter have been used in the insertion replacement of old iron mains as well as for direct burial installations. In West Germany and Scandinavia, pipes up to 1600mm are used for gas distribution. Similar trends are occurring in the water industry where polyethylene pipes are beginning to be used for mains water distribution and sewage systems. Polyethylene is also widely used in industrial pipe work, for example in slurry transport and in chemical plant, but here other materials such as ABS, PVC, polypropylene and fluorinated polymers are also important.
Polyethylene pipes
For distribution of gas and water, polyethylene is usually the preferred material, mainly because of its flexibility which makes laying easy. The long term behaviour of a pressurised pipe system is dependent on its creep and stress rupture properties, which have been extensively researched to enable 50 year design stresses to be calculated ( Fig.1).
Fig.1. Typical stress rupture data for polyethylene pipe, indicating the need to design for a known intended life
Plastics pipes are produced by continuous extrusion. Diameters range from 16mm to at least 1600mm, but 800mm is the largest produced in the UK. The pipes are colour coded depending on their end use - yellow is for gas, blue for water ( Fig.2,3). This enables identification during subsequent excavations.
Fig.2. Polyethylene pipe is often used to re-line existing sewers. This method relies on the flexibility of the welded pipe as it is pulled into the existing sewer
Fig.3. 800mm diameter polyethylene pipe
Plastics pipe systems have to be welded. It is normally impractical to extrude the pipe on site, although there have been successful cases, so it is cut to straight lengths or coiled for transport from the factory to site and has then to be butt joined. Also, junctions, fittings, moulded bends, and size reducers must be incorporated into the system. These are produced by injection moulding and are sometimes welded to pup lengths of pipe in the factory to make site welding simple. Nevertheless a very large number of site welds are required.
Joining
Butt joints and T joints of equal or different sizes are required. Butt joints can be made directly between the two pipe ends via a socket coupler which can be welded by heated tool or Electrofusion (resistively heated wire implant) or by butt fusion. Branches can be welded using a heated tool or Electrofusion. The moulded socket usually incorporates a cutting tool so that connection to a live main is possible.
Heated tool welding
This is normally a completely manual process or uses a hand operated machine ( Fig.4). The pipe and fitting are cleaned and checked. The correct size heating tool is then used to heat the outside of the pipe and the inner bore of the fitting. For the grades of PE used in the UK a tool temperature of 275°C is used. Heating times vary depending on wall thickness, but are in the range 5-45sec for pipes in the size range 20-125mm. The tool is then removed and the pipe inserted into the socket under firm, constant pressure and held for two minutes.
Fig.4. Socket fusion using a heated tool:
a) Heated tool, shaped to allow heating of outside of pipe and inside of socket. Removable cheek pieces are used to accommodate different pipe sizes
b) Pipe, socket and heated tool located into the fusion machine. Manual operation provides the welding force
c) Completed joint
For self-tapping saddles a similar procedure is used ( Fig.5), except that the tool is shaped to heat the intended joint area on the outside of the main pipe. The main pipe is usually clamped to ensure it is perfectly round.
Fig.5. Sequence for heated tool welding of self-tapping tee:
a) Heating tool
b) Main pipe is clamped to ensure roundness. Surface of pipe is scraped clean
c) Fitting and pipe are heated
d) Tool is removed and operator quickly checks for even melting before completing the joint
e) Finished joint
Heated tool welding is still widely used, but is being superseded by Electrofusion which is less dependent on operator skill and requires fewer tools.
Electrofusion welding
The recently developed hot wire implant 'Electrofusion' system ( Fig.6), which uses an integral electrical resistance heater within the fitting ( Fig.7), allows socket and saddle joints to be made reliably with less operator involvement and simple equipment. It is widely used for gas pipes, particularly for tie-ins or where the pipe is restrained. A major advantage is that the joint can be pre-assembled prior to welding ( Fig.8). Assembly and welding are thus separate operations which makes the overall process less urgent and critical. This is important in confined spaces or other difficult site conditions. A further advantage is that the weld zone is not exposed to atmospheric cooling effects and a known, consistent quantity of heat is used to make the joint. Approximately two million Electrofusion fittings per year are used by British Gas in sizes 20-180mm. Larger sizes are currently under development.
Fig.6. Electrofusion socket fitting and section through an Electrofusion weld (the internal resistance heating wire is evident). Here 90mm diameter polyethylene pipe is used for gas distribution
Fig.7. Winding of the resistance heating element in an Electrofusion socket coupler
Fig.8. Blue polyethylene water distribution pipe being joined by Electrofusion. The pipe ends are held in a 'window' clamp. Leads from the socket are attached to the electrical power supply
Butt fusion welding
Hot plate welding, more commonly called butt fusion welding when it is used on pipes ( Fig.9-12), is perhaps the simplest welding technique used with plastics. It is very widely used both in mass production and for large structures. Equipment for shop floor and for on-site use is available and is capable of welding components from a few millimetres to more than 1.5m diameter.
Fig.9. Butt fusion welding equipment joining 250mm diameter polyethylene pipe in laboratory trials at The Welding Institute. Illustration shows the hot plate in position, heating the pipe ends.
Fig.10. Section through hot plate butt fusion welded polyethylene pipe, 250mm diameter 25mm wall thickness (for water distribution). The typical double humped appearance of the weld bead is clearly evident.
Fig.11. A very large hot plate welding machine butt joining 800mm diameter gas pipe on site. Note hydraulically operated clamps, and completed weld bead.
Fig.12. The large hot plate welding machine capable of joining 630mm diameter pipe, presently under construction at The Welding Institute.
The principle of the process is simple. The parts are held in fixtures which press them against a heated plate. The heating takes place in two stages. Firstly, in the 'bead up' phase, the hot plate melts the surface and as this material is displaced a smooth surface is obtained. Secondly, a reduction in pressure prevents too much displacement of material, but the parts are still held in contact with the hot plate until they are softened for some distance from the joint.
The welding fixtures then open, the hot plate is withdrawn, and the fixtures reclose so forcing the two pipes together. Again pressure control regulates the amount of deformation that occurs. The parts are held together until the weld is cool. For on-site joining of pipes manually controlled hydraulic equipment is used.
For polyethylene a hot plate temperature of 205°C is typically used. Total welding times can vary between five and 40 minutes, depending on pipe.
Summary
The brief descriptions given above should allow the reader to identify the welding processes used in polyethylene gas and water pipe joining.
This introductory article does not pretend to deal with wider issues such as installation and inspection requirements, mechanical testing and NDT. But it would not be complete without making mention of the plastics activities of The Welding Institute. Two reports are shortly to be made available to research member companies only:
Watson M N: 'A study of the butt fusion (hot plate) welding process for polyethylene pipe'.
Edwards G R: 'The development of non-destructive test procedures for hot plate butt fusion welds in polyethylene'.
These describe work conducted in the period 1986-87. Currently in progress is the commissioning of a large laboratory welding machine to study joining of components such as pipes up to 630mm diameter ( Fig.12). An extensive Group Sponsored Project is also in the final planning stages. This will be conducted on joining techniques and NDT for plastics pipes.
Those interested in the joining of plastics are invited to contact the author at Abington.
