Making ends meet - joining thermoplastic lined pipe

TWI Bulletin, July/August 1994

Nicki Taylor gained a degree in Metallurgy and Materials Science from the University of Birmingham in 1985. Following this she joined the Plastics Joining Department of TWI, where her work has included development of welding techniques for continuous fibre reinforced thermoplastic composite materials, and a wide range of application studies involving welding of thermoplastic and composite materials.

Plastic lined steel pipe combines the corrosion resistance of plastic with the strength and toughness of steel, but how should we join them? Nicki Taylor explains.

As service demands increase, the oil, gas, chemical, marine and mining industries require enhanced performance from a wide range of product and process pipework systems. Improved corrosion resistance is needed to withstand combined sour gas/CO ₂ attack, the conditions in sea water injection lines, and a diverse range of corrosive chemicals and operating environments.

Longer service lives may be sought by increasing steel pipe wall thickness to give a corrosion allowance, or by specifying a corrosion resistant alloy grade. In either case, the resulting pipework may be considerably more expensive than an installation in conventional C-Mn steel.

A new type of pressure pipework system is proposed. This consists of steel pipe which is lined by a thermoplastic layer, and which can be joined in the shop and in the field by butt welding to give continuous lengths of plastic lined steel pipe.

This article gives an insight into a programme directed at developing a new family of thermoplastic lined metal pipes which may be joined by welding.

The development programme is focused on applications involving complex runs and high operating pressures, which are not suitable for alternative multiwall solutions such as dry lining or joining by flanging and mechanical fastening.

Materials

Thermoplastic lining

Use of a plastic liner is intended to reduce the need for high corrosion resistance in the outer pressure resistant metal casing: specification of the lining polymer or polymers is therefore critical and application specific. The following polymer properties are particularly important:

• thermal stability;
• chemical resistance;
• erosion resistance;
• compatibility with steel casing;
• mechanical properties;
• ease of processing into liner form;
• cost.

A wide range of bulk and composite thermoplastic is available as liner material ranging from polyethylene and PVC to high temperature materials such as PEEK. Medium density polyethylene was chosen for the present study, in view of its ready availability.

Outer casing

If adequate corrosion resistance is supplied by the plastic liner, the outer casing material may be specified using the strength and toughness criteria applied for carrying non-corrosive products. It should therefore be possible to use standard pipe steels and standard fabrication methods.

Trials in the present project have been based on carbon steel pipe 274mm OD x 10mm wall thickness.

Lining methods

It is likely that plastic linings will be applied to standard lengths of steel pipe, using methods which will be determined by the polymer and the pipe diameter. Lining techniques include:

• Dry lining of individual pipe lengths by stretching a pre-extruded plastic pipe to reduce its diameter and allow insertion in the outer casing.
• Subsequent recovery and expansion of the plastic would give a tight mechanical fit between the liner and casing. This might be enhanced by an adhesive layer pre-applied to the steel.
• Axial or spiral extrusion of a plastic tube within the steel casing could be used to give a relatively thick plastic lining. Co-extrusion of multiple plastic layers is possible.
• Thin internal plastic coatings could be deposited by hot spraying, spiral tape laying or blow moulding of polymer coatings on to the inner wall of the steel pipe.

These methods may be used individually or in combination.

Joining process development

Outer casing

The methods used for welding the plastic liner and steel outer casing must be reliable and compatible. Both shop and field butt welding procedures are required, supported by NDT and mechanical test specifications to assure product quality.

A basic aim of the present work is that, where possible, the methods used for joining the steel casing should be similar to those commonly used for C-Mn and low alloy steels, i.e. manual or mechanised one sided positional arc welding.

If this is the case, steps must be taken to protect the polymer from the temperatures (>1000°C) developed at the underside of the arc weld. A number of high performance insulating materials are now available, which provide adequate thermal protection when present in only a thin layer.

One sided welding against a non-consumable joint backing strip is an established technique in shipbuilding and large diameter pipe welding.

Initial trials at TWI involving 3mm thick glass fibre weld backing material for butt welding of flat 25mm steel plates using the semi-automatic dip transfer MAG process confirmed the potential for this method of protecting an underlying plastic liner from excessive heating.

Following these trials, further semi-automatic MAG butt welding experiments were carried out using carbon steel pipe 274mm OD x 10mm wall thickness with an inner MDPE liner 250mm OD x 25mm wall thickness. Details of the butt joint preparation and testpiece geometry are shown in Fig.1. To accommodate pipe ovality, the thermal barrier tape was recessed into a 3mm deep groove machined into the plastic liner. Thermocouples attached to the plastic surface at the joint line allowed monitoring of the temperature cycles experienced during welding of the outer casing.

The steel pipe with its plastic liner was set in the 5G position, and butt welded using the semi-automatic dip transfer MAG process with the consumables and procedures shown in the Table. The maximum temperature recorded at the outer surface of the plastic liner during these trials was 176°C, at the 6-8 o'clock position during the root pass. In these regions, the arc, instead of being directed at the leading edge of the descending weld-pool, impinged directly on to the thermal barrier material. The plastic liner was overheated, and a small amount of localised surface melting occurred. The maximum temperature experienced during subsequent filler passes was 168°C, which also produced some local surface melting of the plastic.

Welding procedure and thermal results for 5G pipe welding trials

These results indicate the basic possibility of insulating a plastic liner against the heat produced during arc welding. In practice, a minor modification of the arc welding procedure to ensure continuous contact with the leading edge of the weld pool, and adoption of vertical up stringer runs for the filler passes should reduce heating of the plastic liner still further.

Plastic liner

Of the joining methods potentially available for butt welding thermoplastic lining materials, the two considered in this study were electric resistance implant welding and extrusion welding.

Electric resistance implant welding

This technique is based on the electrofusion systems, commonly used for joining thermoplastic piping systems. A plastic insert (coupler), containing an embedded resistance wire is positioned between the two ends of the liner, as shown in Fig.2. When an electric current is passed through the wire it heats up, melting the surrounding plastic and resulting in a fusion weld. This technique could be operated from either the inside or outside of the pipe. The plastic insert would be moulded or machined, supplied complete with an insulating layer and backing bar for the arc weld root run. The processing sequence for butt welding with this technique is as follows:

• Prepare steel pipe end;
• Cut back and prepare plastic liner;
• Insert resistance welding coupler;
• Jig and clamp pipes;
• Make electrical connections to coupler;
• Complete plastic butt weld;
• Complete weld in outer casing.

Extrusion welding

Extrusion welding is a commonly used manual welding technique for thick section plastic components. An internal or external orbital, automatic welding head deposits molten plastic into the joint preparation. An appropriate joint configuration incorporates the weld and strip of barrier material (positioned after welding the liner, if welded externally).

In-service performance

Factors which may influence the service performance of a plastic lined pipeline have been identified as follows:

• Selection of plastic liner type and steel composition. A number of the liners used on existing systems are based on fluoropolymers because of their excellent chemical resistance. It must be remembered, however, that a number of these, e.g. PTFE, are not weldable. The required steel composition may be dependent on the permeability of the liner.
• Liner thickness: two factors influence the thickness of the liner. First the thickness required for it to meet its specified lifetime and, second, the thickness required to give it the necessary handling properties. This, to some extent, will be determined by the insertion technique used. The possibility of producing a ribbed pipe to increase stiffness should be considered.
• Pipe thickness: again, this is determined by more than one factor. The strength of the pipe, the required weight ( i.e. if the pipe is to sink under its own weight) and any corrosion allowance will determine the thickness of the steel pipe.
• Selection of in-service inspection techniques, both internally and externally, and repair procedures. These two issues are obviously related. The plastic liner may lead to novel NDT techniques, such as some form of selective chemical inspection of the transmitted fluid.
• Creep properties of the liner.
• Effects of differential thermal expansion of the plastic lined system, especially at the ends of the pipeline.
• Effect of a negative pressure on linear adhesion and possible liner collapse.

Cost analysis

A cost analysis has been conducted to compare a number of possible scenarios. The results for the following pipes are summarised in Fig.3. The cost comparisons were built up using pipe cost data and a pipeline project cost breakdown. A base project cost was calculated and the construction cost element varied to accommodate the change in materials. Two pipe diameters were considered, a nominal 6inch (147mm) and 24inch (610mm) pipe, in two wall thicknesses (i.e. thin: 7.9mm for 6inch and 12.7mm for 24inch, and thick; 17.9mm for 6inch and 22.7mm for 24inch). The carbon-manganese standard 'thin' wall pipe was used as the basis for all the comparisons.

• A standard carbon-manganese steel pipe with a 'thin' wall (hydrogen cracking and corrosion may be a problem in sour service).
• A standard carbon-manganese steel pipe with a 'thick' wall (corrosion 'accommodated' by increasing wall thickness, hydrogen cracking may be a problem in sour service).
• A sour service steel with a 'thin' wall (some degree of sour service protection provided by steel composition/treatment).
• A sour service steel with a 'thick' wall (corrosion 'accommodated' by increasing wall thickness, sour service protection provided by steel composition/treatment).
• A standard carbon-manganese steel pipe of 'thin' wall lined with plastic (corrosion and sour service protection provided by the plastic lining).
• A duplex stainless steel pipe of 'thin' wall (corrosion protection provided by alloy content).

In conclusion

Introduction of a new pipe material, together with novel installation and testing methods is a complex and extensive task. Joining technology development provides a convenient entry point to this field, and the results so far encourage the view that welding could provide a practical approach for shop and field installation.