HDPE Jointing Method
One of the greatest features of HDPE pipes is the fact that a wide variety of joining methods is available to suit a whole range of applications. The joining methods can be divided into permanent joining and detachable joining:
Butt welding is a thermofusion process which involves the simultaneous heating of the ends of two components which are to be joined until a melt state is attained on each contact surface. The two surfaces are then brought together under controlled pressure for a specific cooling time and homogeneous fusion is formed upon cooling. The resultant joint is resistant to end thrust and has comparable performance under pressure to the pipe.
Butt welding is a very economical and reliable joining technique for making permanent welded joints, requiring only butt welding equipment. Butt welding or Butt Fusion is a simplest and space-saving jointing method to connect the pipe and fittings of PE.
Butt welding principles
Butt welding is particularly suitable for pre-fabricating pipe sections and special fittings. Fittings with short spigot ends and fittings with long spigot ends are suitable for butt welding.
Only pipes and fittings from the same wall thickness series can be butt welded together. Dissimilar materials and dissimilar wall thicknesses can be jointed by electrofusion. Please note that the maximum working pressure should not exceed the lower of the two pipes.
Butt welding procedure
There are five main steps in the welding process:
- Preparations (on equipment)
- Bead-forming (also known as adapting or equalisation),
- Heating (also known as soaking or pre-heating),
All electrofusion fittings employ the same basic principle. The socket of the fitting incorporates an electrical heating coil. An electrofusion control unit (ECU) supplies the electrical energy necessary to heat the coil. When the coil is energized the material adjacent to it melts and forms an expanding pool which meets the surface of the pipe. The continued introduction of heat energy causes the pipe surface to melt and a mixing of pipe melt and fitting melt takes place; this is vital to produce a good weld. Following the termination of the heat cycle, the fitting and pipe are left to cool and the melted material solidifies to form a sound joint.
General steps to be followed when performing electro fusion joining are:
- Prepare the pipe (scrape, clean).
- Mark the pipe.
- Align and restrain pipe and fitting per manufacturer’s recommendations.
- Apply the electric current.
- Cool and remove the clamps.
- Document the fusion process.
High integrity, consistently reproducible electrofusion joints will only be achieved if the following criteria are met:
- Heating coils are as close to the joint surfaces as possible.
- Wire position is accurately controlled during manufacture and during subsequent fusion process.
- Heat distribution is uniform over the length of the hot zone.
- Melt pressure and temperature are both accurately controlled.
- Coils are protected from damage prior to, during and after fusion.
- Spigot ends are scraped properly.
Socket fusion welding
Socket fusion welding is a widely used technique for assembling plastics piping systems using injection moulded fittings. Operating principles are straightforward and the welding cycle essentially consisting of a heating phase and a cooling / welding phase.
Depending on the size of pipe, socket fusion welding can either be done by hand or carried out on a manually operated machine.
A metal socket mounted on a hot plate heats the outside circumference of the pipe. Heating occurs along a defined length which will vary depending on the size of pipe and fitting being welded. A metal spigot on the opposite side of the hot plate simultaneously heats the inside surface of the injection moulded fitting. The length of the heated region is the same as that for the pipe.
Both the pipe and fitting are heated for a set length of time after which the heated socket/spigot tooling is removed, and the pipe is pushed into the fitting. The pipe and fitting is then left for a predetermined time to cool and form a weld.
For small bore pipe connections up to size 125MM OD, compression type joints are satisfactory. The installation process started with the cutting of pipe ends square or 90deg in relation to its axis. Fit the collar and clinching ring into the pipe and place the rubber O-ring at the tip of the pipe. Then push the body of the fitting until the pipe ends reaches its full stop. Slide the clinching ring and Collar (Nut) until it catches the fitting and tight fully using a belt or chain wrench.
Stub & Flanged Connection
A HDPE Stub is butt welded to the pipe with the loose CS backing flange inserted in the middle of the pipe to establish a detachable joint. A continuous line can then be made by simply bolting the flanges together which are held securely in place. The jointing method “Stub & Flange Both Ends (SFBE) is regularly used in the mining industry as it has numerous advantages if you consider the regular maintenance (easy serviceability) of your pipeline. The fact that you can cut out the “bead” after the weld has been done makes SFBE a very popular jointing method for Slurry & Feed lines. Primarily used where the pipes may need cleaning later or may have to be moved from one place to another after use.
Victaulic fittings are used with Victaulic stubs to establish detachable joints in HDPE pressure pipe systems. Stubs may be butt welded to pipe or spigot ends on butt-weld fittings. Victaulic clamps enable detachable joints to be made with pipes that have Victaulic stubs butt-welded to them.
Tak fittings are used with Tak stubs to establish detachable joints in HDPE pressure pipe systems. Stubs may be butt welded to pipe or spigot ends on butt weld fittings. Tak clamps enable detachable joints to be made with pipes that have Tak stubs butt-welded to them.