Pipeline pitfalls part 2, joining methods: Electrofusion
Polyethylene (PE) pipe is widely used for the transportation of water and gas, thanks to its chemical inertness and estimated lifespan of more than 100 years. Poor fusion jointing between lengths of PE pipe is the biggest threat to pipeline integrity after third party damage. Last month we looked at the butt weld technique for joining PE pipe lengths – a simple method using a flat heater plate.
We will now consider the more complex jointing method of electrofusion. This technique involves the use of injection moulded PE fittings into which the pipes are inserted. Embedded within the fitting are a series of heating wires just below the surface of the internal bore of the fitting, with terminals on the outside for electrical connection. When energised by a controlled electrical power source for a pre-defined duration, the wires produce the necessary heat to melt the plastic and, once allowed to cool, form a welded joint.
The electrofusion fitting is designed with special ‘cold zones’ located in the centre of the fitting and at each mouth where the pipe enters. When the fitting is energised, the molten plastic is contained within the joint by the cold zones and due to thermal expansion a melt pressure is built up. This promotes mixing of pipe and socket material, and when cooled makes a strong fusion joint.
In cases where the pipe is not fully pushed home into the socket bore, or where there is severe misalignment of the pipe and socket, the wires become exposed and molten material flows away from the joint interface (see image above). This allows the wires to move within the molten material and can lead to the wires touching or bunching together, causing short-circuiting or localised overheating. The result can be thermal degradation, void creation along the fusion interface, or a drop in energy input, resulting in partial fusion. In order to negate these possible failure modes, the pipe must be aligned and restrained during the fusion cycle.
Electrofusion sockets are designed with a clearance to allow pre-assembly, while maintaining an even gap between the socket bore and the outer diameter of the pipe, so that melt pressure is not lost. Pipe ovality can pose a significant threat to the fusion gap and consequently, re-rounding clamps must be used to reduce misshaping. Without the use of clamps, the probability of defective joints is high.
Defects within the fusion zone arising from poor installation practice will act as stress raisers and initiation sites for slow crack growth (SCG). When a joint is subjected to internal pressure, severe misalignment and bending, SCG will propagate through the joint interface. This leads to premature failure and a leakage of gas analogous to that of a leaking cast iron joint.
Scratching the surface
Since electrofusion jointing is not a self-cleaning process, pipe preparation is essential and the removal of surface contamination and oxidised layers requires a scraping device. An unscraped surface will not fuse successfully and all electrofusion joints assembled without adequate scraping are likely to fail.
Typically, simple hand-held scrapers are used and it is down to the operators’ skill as to how well the PE pipe surface is prepared. The development of mechanical scraping devices has not been too successful in that the tooling is considered by some to be cumbersome, difficult to use and expensive to purchase. As a result, the industry has not embraced their use and hand scraping is still very much the norm. At the other end of the size scale, over-zealous scraping of service pipes has also been identified as a problem. Small diameter pipes between 16mm and 32mm are at risk if subjected to intensive scraping resulting in a large gap along the interface between the pipe surface and the bore of the electrofusion socket. The gap can be such that the fusion pressure generated during the jointing cycle is reduced leading to an incomplete fusion.
The maintenance and cleanliness of the scraper is clearly of paramount importance. However, it is not uncommon in the field to observe jointers using blunt, rusty and heavily contaminated scrapers (see above). This demonstrates the poor understanding and noncompliance often seen in the field, which is ultimately undermining electrofusion technology and posing a threat to PE pipeline asset integrity.
The difficulties presented by scraping have driven the development of peelable PE pipe. This consists of a core PE pipe and a sacrificial polypropylene (PP) skin, with the outside diameter and wall thickness of the core PE pipe meeting the requirements of relevant PE gas pipe standards (see below). The pipe is manufactured from a non-pigmented PE100 resin that matches traditional pigmented PE100 pipe in all aspects of dimensions and hydrostatic strength and includes all the usual additives that protect the resin. The PP skin is additional sacrificial material, which can be removed by peeling from the core PE pipe. The design and processing of the pipe is such that the skin can be peeled in a controlled manner. Removing the skin exposes the core PE pipe for the first time after solidification. The no-scrape aspect of peelable pipe is a key attribute for utility companies that wish to increase joint integrity and at the same time reduce installation costs.
In the absence of mature NDT techniques, which can be easily deployed and interpreted by field operatives, in-process destructive testing of joints remains the only way to assess and give confidence in the quality of the pipeline being laid. Cut samples taken from the weld can be subjected to tensile tests in accordance with BS ISO 13954 (see below) The pass criterion is ductile interface tearing along the fusion length over a minimum of 66% of the apparent fusion length.
In order to aid the engineer, new developments include the use of remote cameras that can remotely monitor and record the quality of joint workmanship in the field. The very fact that the operator knows joint quality is being inspected provides an impetus to comply with good practice.
Failed pipe systems can cause catastrophes including floods, explosions and fires. After poor fusion jointing and third party damage, the remaining causes of premature joint failure are a combination of poor training and awareness and non-compliance with industry good practice. A lack of robust in-field quality assurance and spot check auditing methodology also contribute. Front-end investment to combat these issues offers a cost benefit in terms of reducing the risk to public health. The commercial benefits would be greater assurance in the longevity of pipeline lifetime for its full potential of 100 years or more.