Getting it right every time - quality consistency of on-site plastic gas pipe joins

TWI Bulletin, May/June 1994

After obtaining a degree in Materials Science, and a PhD for work on the metallurgy of submerged-arc weld metals in constructional steels, Martin joined TWI in 1979. Since then he has been responsible for many developments in laser welding (particularly for joining thick sections) and plastics and polymer composites joining. He has been closely involved in process selection and research projects in industries such as power generation, structural engineering, process plant, aerospace and mass production. Martin is currently Business Development Manager in the Advanced Materials and Processes Department, which provides technical support and R&D for manufacturing industry using adhesives, ceramics, composites, microtechnology and plastics.

Eric Bridgstock obtained a BSc in fuel technology from Leeds University, emigrating afterwards to Canada to work for Du Pont promoting polyethylene pipe systems. In 1971 he returned to his native Sheffield where, with others, he started Fusion Equipment. Eric Bridgstock is now Chairman of Fusion Group plc, a company which manufactures butt fusion machines and electrofusion fittings.

Jeremy Bowman has a BSc in Metallurgy and PhD from Liverpool University for work on plastics processing. He lectured for 11 years at Brunel University where his research centered on plastics processing, artificial bones and the strength of plastics pipe systems. In 1989 he joined the Fusion Group and is now Technical Manager of Fusion Plastics. Jeremy Bowman is a member of the Research Board of TWI.

Gas utilities worldwide seek improvements in the quality of pipe systems they install. One route to raising the quality of polyethylene fuel gas pipe systems still further is via control of on-site butt fusion joining. Here Martin Watson, Eric Bridgstock and Jeremy Bowman highlight the benefits of computer controlled butt fusion joining.

By being able to monitor the joining process and provide traceability of the butt fusion joining conditions, gas utilities can approach ISO 9000 series standards on-site. In addition, use of computer controlled machines not only raises the quality but can increase machine on-site productivity more than 100% and allow introduction of new welding profiles without the need to retrain the operators. It is these advantages that have led several utilities to specify that PE pipes are to be butt fusion joined only by using computer controlled automatic butt fusion machines.

Pipe system quality

A notable and important success in raising system quality has been the awareness of the need to use resins with excellent resistance to slow crack growth (SCG). In this area the Gas Research Institute (GRI) of Chicago has contributed by funding significant work on this topic.

The results of the efforts of the manufacturers, the GRI, the utilities and others is that PE pipes and fittings are now most often manufactured from resins with very good resistance to SCG.

Having tackled the topic of SCG, other areas are being addressed to raise, still further, the already good quality and performance of PE fuel gas pipe systems. An area receiving increasing attention is joining, and in particular the ability to ensure that on-site fusion joining is conducted, every time, to the correct standard.

Concurrent with the need to ensure continuity in joining conditions is the desire of utility companies, and/or their contractors, to have ISO 9001/9002 quality procedures for on-site butt fusion joining. This article shows that computer controlled machines offer a clear way forward.

Butt fusion joining

• Bead-up The trimmed and aligned pipes are (usually) forced against the hot plate to create inner and outer wall weld beads. This stage of the joining process is most often controlled visually by the operative - when the bead attains a critical size (2-3mm depending on pipe size) the bead-up phase has ended.
• Heat soaking This stage, which follows immediately on from bead-up, allows more heat into the pipe, but without any external force being applied to the two pipes. The pressure-less heat soak phase is usually defined and controlled by time alone. The heat soak time is unique to a material and (usually) pipe wall thickness and/or diameter.
• Dwell time After the bead-up and heat soak phases the pipes are at the correct temperature and contain sufficient heat. The heater plate is then removed and the pipe ends brought together; the time required to do this is the dwell or plate removal time. Existing standards on butt fusion joining ^[4,3] stipulate that the time required for plate removal should not exceed a given value, which is usually around 8sec.
• Pipe welding In this phase of the joining process the two pipes are in intimate contact and welding starts. Pressure is applied in the joining phase which, in turn, causes the two pipes to continue to move towards one another after the initial contact. Evidence exists ^[3] of a minimum required flow velocity for obtaining strong butt fusion joints. Despite this it is more common to control this phase of the joining process via the welding pressure, and the time at pressure.
• Joint cooling Following the welding phase the joint is allowed to cool further without pressure being applied. Time is the only process variable to be controlled in this phase of the joining process.

Factors to control in making PE pipe butt fusion joints

The conventional single pressure butt fusion welding process is often represented via a plot of welding pressure against time, see Fig.la. This shows, together with the Table, that by controlling welding pressure and the time for which the various pressures are applied, together with a visual inspection of weld bead size during the bead-up phase and the heater plate temperature, the process can be well controlled. It is this approach that has served the PE pipe industry very well for many years.

a) Conventional single pressure welding:

b) Dual pressure joining

Monitoring pipe movement

At the eleventh 'Plastic Fuel Gas Pipe' symposium meeting in the Autumn of 1989, Bridgstock ^[4] identified the relative movement of the fixed and moving clamps found during butt fusion joining, see Fig.2. By attaching a displacement transducer to a butt fusion machine this movement can be monitored to provide greater information on, and control of, the joining process itself. This is seen to have two advantages.

Enhancing control of the joining process

By fitting a displacement transducer to a butt fusion machine the following gains accrue when joining PE gas pipes using the usual procedure ( Fig.1a).

• Identifying pipe slippage Computer controlled butt fusion machines request the operator to undertake a check cycle after trimming of the pipe ends. This is primarily to check the axial alignment of the pipes is correct, as axial misalignment can reduce joint strength. ^[5] At the same time the displacement transducers look for excessive relative movement of the two clamps.
• Bead size control Material that constitutes the beads that form during the bead-up phase of the joining process clearly comes from the pipe walls. As the beads form and grow in size, so the distance between the fixed and moving clamps changes. By knowing this relative movement the bead size can be predicted.
• Checking welding time If an operative attempts to remove the joined PE gas pipes before the specified welding time is attained, the moving clamp will move towards the fixed. The displacement transducer can identify this movement and alert the operative that the welding time is too short.

Controlling the welding process by displacement

As identified in Fig.1a and earlier in the paper, it is common to control the butt fusion joining process, in part, via the pressures (or forces) applied to the pipe ends. But the forces applied to the pipe ends cause the pipes to move towards one another, see Fig.2. It should thus be possible to control the welding process by controlling the relative displacement of the pipes rather than the forces applied to the pipes. A short study has been undertaken to investigate the feasibility of displacement control in the welding phase.

For 250mm SDR11 pipes manufactured from PE100 resins, the relative movement of the pipes 'one into another' was limited to the values identified. The joints were made with controlled displacements after heat soaking the pipe ends for times between 0 and 180sec, (heater plate temperature 230°C). The quality of the welds was assessed via tensile testing a tapered tensile test sample that focuses the stress at the weld ^[6] see Fig.3a. A pass in this test is a totally ductile rupture, while a failure is a planar brittle failure through the weld region. Figure 3b highlights the results obtained to date. The data show that using the correct heat soak time (> 100sec) and the correct displacement, good ductile joints can be obtained. Monitoring the relative displacement of the two pipes during welding may thus offer another technique by which to assess the quality of a butt fusion joint.

a) Tensile sample used to assess joint quality;
b) The influence of pipe displacement and heat soak time on weld quality, where quality is the appearance of the failed test sample shown in a)

Reductions in welding times

For manually operated butt fusion machines one of the key steps in the joining process is the removal of the hot plate without damaging the softened pipe ends, and the hot plate must also be removed as quickly as possible. To achieve this requires significant skill and two operatives working in harmony. But with manual machines and skilled operatives it is still possible that the time to remove the hot plate can be long, up to 8 or even l0sec.

It has therefore become common to have relatively long heat soak times to ensure that there is sufficient heat within the pipe ends to, in part, compensate for long dwell/plate removal times. With long heat soak times there are then extended welding and cooling times.

A study has been undertaken ^[7,8] on the possibility of reducing heat soak times for computer controlled machines that have reproducible and monitored short dwell times. If the dwell time is short, say ≤3 seconds in a reproducible fashion, then the cooling of the pipe ends is restricted, so allowing reduced heat soaking, reduced welding and cooling times, and reduced total process times. Figure 4 shows the advantages of reduced heat soak times for a 125mm SDR11 PE80 pipe (Note, the beadup times were constant at 28sec). The total process time is the time for the centre line of the weld to cool to 80°C from the initial nominal value of 250°C.

It can be seen that real benefits will follow, in terms of machine productivity, by reducing the heat soak time. Process times can be reduced from 12 down to 4min or even smaller.

The ability of utilities to use shorter heat soak times depends, in part, on having reproducible and measured short dwell times. By fixing a displacement transducer to a butt fusion machine with auto-lift for plate removal, the movement of the pipes away from the hot plate can be accurately limited to reduce dwell time.

And by using a computer controlled machine the dwell time can be monitored, and any excessively long dwell times identified so that welds must be re-made. This ability to identify the dwell time is advantageous for both conventional heat soaking times and for reduced heat soak times. The quality of the joint can thus be raised at the same time as machine productivity is increased.

New butt fusion welding profiles

Published work from Stewarts and Lloyds Plastics of Huntingdon, England highlights the need for a 230°C heater plate temperature with a reduced welding pressure. Joint quality was assessed via a calculation of G _c using Charpy samples notched at the butt fusion joint.

Computer controlled butt fusion machines offer major advantages if one or more complex welding profiles have to be incorporated. First, the machine can be refined to offer good control at both the high and low pressures by software modification. Second, complex profiles can be specified and stored, and these more complex profiles can be reproduced weld after weld. Third, by storing the weld profile in the memory of the computer, no retraining of operatives is necessary.

Data recall of joining conditions

Butt fusion joining is a process necessary to the creation of a PE fuel gas pipe system. ISO 9001 and ISO 9002 require that processes are carried out under documented and controlled conditions (see sections 4.9.1 and 4.8.1 for ISO 9001 and ISO 9002 respectively). Computer controlled butt fusion machines themselves contain documented work instructions that are brought up on the screen. Computer controlled machines monitor and control the joining process. Furthermore, computer controlled butt fusion machines provide continuous monitoring, as required for special processes (see sections 4.9.2 and 4.8.2 of ISO 9001 and ISO 9002 respectively). In addition computer controlled machines maintain records of the joining processes, as required in ISO 9001/9002.

From the above it can be concluded that the process control requirements in ISO 9001/9002 are attained using a computer controlled butt fusion machine. The machine will monitor temperature, time, pressure and now displacement, signals that can be used to both control the process and provide critical information on the joining parameters used.

In conclusion

Gas utilities around the world seek to raise the quality of the installed pipe systems. This has to be tackled on several fronts and one area of particular importance is the quality and strength of PE pipe butt fusion joints. Automatic computer controlled machines have the following advantages:

• The process is fully monitored and controlled via the computer program;
• The displacements of the pipes can be monitored and used to control the bead-up and welding stages;
• Faster joining is possible because the dwell time can be shortened and monitored: this allows reductions in the heat soak time and thus in the total process time;
• New and more complex welding profiles can be introduced without the need for, and expense of, operator retraining;
• Data on the joining conditions can be stored within the computer memory.

All these attributes aid the attainment of the process control requirements listed in both ISO 9001 and ISO 9002. This will help gas utilities to apply quality standards to site butt fusion joining of PE fuel gas pipes. It is advantages such as these that have led some utilities to specify that only automatic butt fusion machines are to be used on-site. Steps such as this are a positive route to improve, still further, the already good performance of PE pipe systems for the transportation of fuel gases.

References