The use of plastic pipes is growing in many applications, from chemical plants to lining metal pipes in the oil and gas industry, as well as the more established transporting of water and gas for the utilities sector.
Leaks in these pipes can result in significant cost and environmental damage. Most leaks occur at joints, meaning that improving the quality of plastic pipe welds is the primary way that leaks can be avoided in the future. And plastic pipe weld inspection is now becoming part of the specifications for new installations and routine maintenance, meaning that there will be a growing demand for qualified inspectors.
The two types of weld used for HDPE pipe are thermofusion and electrofusion. The former, also known as heat fusion, is normally used to directly join sections of pipe, also known as butt-fusion. The latter is used to join pipe to fittings, such as branch saddles and tees.
Both are fusion welding techniques, meaning the two parts being joined are heated to melting temperature, allowing the material to mix before it solidifies.
In all types of plastic pipe welding, it is essential that there is no water in the vicinity of the weld. This is to ensure that the correct fusion temperature is achieved and also to prevent voids that can be caused by steam formation. In new installations, dewatering may not be a major consideration, but for repairs and insertion into existing networks, however, dewatering may require considerable effort.
BUTT-FUSION WELDING
Butt-fusion welding is the preferred method of joining sections of pipe. Butt-fusion welding of plastic pipes is covered by ISO 12176-1. In the process, the ends of two sections of pipe are heated using an electrical resistance heating plate, and, once the fusion temperature has been reached, the ends are brought together with a controlled pressure.
Although weld beads are formed during welding, these do not add strength and can therefore be removed. If the weld beads are removed, then the welded joint also causes no disturbance to fluid flow. Because no fittings are required, butt-fusion welding is also a cost-effective way to join sections of pipe.
The butt-fusion welding process has the following steps:
1) The sections of pipe are first clamped into an axial press. This is used throughout the process to hold the pipes in alignment, to prevent ovality, and to generate the axial pressure required at each stage in the process.
2) Next, the ends of the pipes must be prepared by scraping to ensure a good contact with the heating plate and with each other. At least 0.3 mm must be removed to ensure that the outer oxidised layer of plastic is removed. A scraping tool is inserted between the ends of the pipes. The axial press is then used to press the pipes against the scraping tool as it rotates.
3) Once cut square and flat, the ends of the pipes are cleaned using a grease-free cleaner, such as alcohol. They should not be touched after cleaning.
4) An electrical resistance heating plate is then positioned between the ends of the sections of pipe. It is important to check that the plate has reached the correct temperature by taking temperature readings at multiple positions. The axial clamp is used to press the ends of the pipes against this plate for a specified period of time.
5) To form the weld, the heating plate is removed and the molten ends of the pipes are pressed together until they solidify, forming a joint.
6) If required, the weld beads can be removed using a special scraping tool.
Butt-fusion welding is enabling the use of plastic pipes in applications that were previously reserved for steel. For example, a large outfall pipe is currently being constructed on Vancouver Island. It has an outer diameter of 7.5 feet, and it extends 1.2 miles out to sea. Outfall pipes of this type have historically been steel, despite issues with corrosion in salt water and risk of cracking if there is movement, such as seismic activity. The ability to butt-fusion weld HDPE pressure pipe now makes it possible to replace steel. This gives significant benefits in terms of reduced maintenance, resistance to corrosion and ability to withstand earthquakes. The pipe used is part of the Agruline series, produced by Agru America, using a HDPE, PE 100-RC.
“This is considered a homogenous weld, meaning the strength of the weld is the same as the strength of the pipe,” says Melissa Grace, vice president of Agruline. “The joint is also leakproof, which is another benefit of the HDPE pipe solution.”
ELECTROFUSION WELDING
Electrofusion is used to join special fittings that have electrical heating coils integrated into them. The fitting is clamped in position and an electrofusion control unit (ECU) is then connected to the coil, supplying a current that generates the heat to form a fusion joint.
Bar codes on the fittings provide the ECU settings required to form the joint. However, separate scraping tools must be used to prepare the section of pipe before the fitting is attached, and additional clamps may be required to hold the fitting or the pipe in alignment.
One advantage of electrofusion welding is that compact ECUs can be used for in-trench joining. Attaching a butt-fusion welding machine to pipes within a trench may be considerably more difficult. Electrofusion welding of plastic pipes is covered by ISO 12176-2.
PIPE WELD INSPECTION
The established quality assurance methods for plastic pipe welds are to first record the welding parameters used and then perform a combination of visual inspection, and a short-term hydrostatic pressure test.
These methods must be supplemented by destructive testing of sample welds using the same welding parameters. In this approach, the primary method of inspecting plastic pipe welds is visual inspection. This requires a good knowledge of the different methods of welding plastic pipes, as well as the types of defect which can occur.
The established approach is inadequate because only surface defects can be detected by visual inspection and practical sample rates mean there is a low probability of detecting defects during destructive testing. For a more rigorous evaluation of pipe welds, ultrasonic inspection equipment may be used. This is able to identify initial flaws in the weld, such as particulate contamination, lack of fusion, cold fusion and under-penetration. Mechanical damage such as fatigue cracking may also be detected.
The Welding Institute (TWI) has a team of plastic welding experts, based in a purpose-built plastic welder training facility in Middlesbrough, UK. They have been delivering training in plastic pipe welding for over 20 years and are now also offering courses in pipe weld inspection. This includes both visual inspection and the use of their proprietary ultrasonic inspection method PolyTest.