Polyethylene pipe fusion explained: Part I
At Squire Energy, carrying out new gas connections is what we do. Designed by our expert engineers and installed in-house by our teams, we create new networks of gas pipe infrastructure using pipe fusion across the UK, working on projects ranging from commercial and industrial consumers to residential and mixed-use developments.
Most of our pipes are made from Polyethylene (PE); a man-made, versatile and flexible plastic material which can sit in the ground for decades without corroding, unlike ferrous metal. However, where metal can be welded or screwed together to form an air-tight – thus gas-tight – seal, PE isn’t suited to these methods of jointing. PE can’t be screwed together like steel, nor can it simply or safely be blasted and bonded with a welding torch to create a seal.
So, how do we form a gas-tight seal between PE pipes and fittings which is safe and reliable? The answer – pipe fusion. There are two types: electro-fusion and butt fusion, with both altering the composition of the PE pipe to allow it physically to bond with suitable fittings/pipes to create a gas-tight seal.
In keeping with our unwavering commitment to gas safety and pursuit of industry knowledge, a contingent from Squire Energy completed a pipe fusion training course in March. Taking place at Develop Training in Derby, the team gained first-hand appreciation of pipe fusion methods, as well as some hands-on training.
We’ve put together a two-part guide* based on the training workshop, aiming to give an informative overview of the pipe fusion processes, some of which will include fusion being applied at a current Squire Energy project.
This is the jointing process usually applicable when jointing together PE pipes >180mm diameter. When merging two pipes of identical diameter and SDR (Standard Dimensional Ratio: the ratio of the nominal outside diameter to the nominal wall thickness), butt fusion is used to ensure a tight seal. This process requires a cradle, which holds the two pipe sections in place, and an automatic butt-fusion machine, which monitors and regulates the timing of the fusion operation itself.
As this type of fusion normally happens above ground, the operation takes place in a fusion tent to protect the joints, machine and operatives from the elements.
[Fusion Machine in tent]
Pre-Joint checks and pipe preparation
A series of ‘pre-joint’ checks are required – a top-to-toe physical inspection and calibration of the equipment by a competent engineer who ensures everything is in working order. This could be anything from checking fuel levels in the generator, scrutinising the effectiveness and cleanliness of the cradle clamps and ensuring that the butt fusion machine has been recently serviced. One key element of pre-joint preparation is the cleaning of the pipe itself. For safe and secure fusion, all potential surface contaminants, such as dirt or moisture, must be avoided. The pipes are physically cleaned with a non-synthetic cloth to avoid contaminants like lint or synthetic fibres from being created by the cleaning process itself. The use of non-synthetic material is also essential to avoid the build-up of a static charge.
1. Connect the heater, chassis and trimmer cables to the machine
2. Select pipe sizes, SDR, pipe lengths
3. Roll the pipes into the clamps.
The two pipes are held in place by clamps within a cradle, and the pipes will sit here for the duration of the pre-joint preparation works and during the fusion process itself. A butt fusion machine is used to input the data of the pipes and it regulates the operation.
[Pipe in the cradle | Pipe with clamps on | Butt Fusion Machine]
The trimmer – a steel plate which sits between the pipes and which contains cutting blades – shaves the ends of each pipe to ensure the two faces are even. The trimmer is loaded into the machine and the pipes are positioned gently against the discs with the blades rotating at speed until strips of PE pipe – the swarf – start to emerge. The amount of swarf from each pipe is monitored by the operator, who looks for a continuous stream of swarf from both pipes to signal the completion of the trim cycle.
The trimmer is then removed and the two pipes are shunted together with the operator performing a visual inspection of the two pipes to make sure they’re sitting square. As a general rule, for pipes <180mm diameter, the operator will allow for a 1mm mismatch allowance, with a maximum mismatch of 10% of wall thickness permitted for pipes >180mm dia.
Making the Joint
The next stage is making the joint. The pipes are separated again and an electronically powered, non-stick heating plate (aka an ‘iron’), is inserted between them.
[heating plate close-up]
It’s crucial that the heating plate is heated to the required temperature (230 degrees Celsius, +/-10 degrees), for a successful fusion – this is indicated by the machine entering the ‘green zone’. As the pipes make contact with the plate, the pipe ends melt slowly in preparation for fusion. The duration of contact with the plate is calculated from the pipe parameters. Once time is up, the heater pops up and the machine immediately shunts the two pipes together for safe and secure fusion.
A good alignment and temperature prior to this are essential. The pipe then needs to be left in the clamps to cool as any movement could compromise the bond.
Post-Fusion Checks and Bead Measuring/Cutting
Once the fusion has cooled, a bead between the two pipes is visible and this must also undergo stringent inspection to ensure that effective fusion has taken place.The bead is measured in-situ with a bespoke measuring tool, as the width of a bead is a good signifier that secure fusion has taken place.
[bead after fusion | bead measuring]
After passing inspection, the bead is cut-off and subsequently examined by the operator. This additional check involves twisting and folding the bead back on itself to see if it cracks or splits.
If the integrity and quality of the cut-off bead seems compromised in any way then the effectiveness of the pipe fusion operation is negated. The solution? Cut out the joint and start over again. No gas pipe goes into the ground until one of our competent engineers is 100% confident that the joint is correctly sealed, bonded and that it is fit for purpose.
Failure of effective pipe fusion can be caused by several factors, such as; the wrong temperature settings, incorrect pipe selection, poor pipe preparation and insufficient time allowed for fusing/cooling. Many of these problems have been overcome since the introduction of automatic butt-fusion machines, which provide effective monitoring and control of the process. Nonetheless, everything – every joint, from start to finish – is checked and monitored by our experienced teams of trained operatives and managers.
Once the joint has cooled and the appropriate joints completed, the section of pipe is carefully lowered in to the pre-excavated trench. The process continues until everything is in situ, from the connection up to termination point, with all valves and fittings suitably installed. The pipe network is then tested for soundness – aka gas tightness – usually with air, before natural gas is introduced and the pipe purged with gas safely to atmosphere. Then it’s fit for purpose, with `as laid` records completed and reinstatement can commence.
In our second and final part of this short guide, we’ll explain the process of electrofusion. If you’d like to view some of the work we’ve already completed, click here.
*This guide is intended for general information purposes only, it has not been written by one of our operatives, as a technical instruction or for any replacement of any manufacturer’s instructions, training or industry procedures. Some parts of the process may also have been omitted, as they are deemed applicable only in certain bespoke situations. We hope you will find this guide of interest.