Mike Troughton BSc PhD CPhys MinstP
Paper presented at Non-metallic pressure equipment seminar, 5 June 2006, London, I Mech E.
Thermoplastics, such as polypropylene (PP), polyvinylchloride (PVC), polyethylene (PE) and polyvinylidenefluoride (PVDF) are being used more and more for the storage and transportation of many hazardous chemicals due to their excellent chemical resistance. This paper describes the various techniques that are used for welding thermoplastic tanks and pipelines; and the standards that are available for testing the quality of the welds produced. The paper also summarises the European standard BS EN 13067 on qualification of thermoplastics welding personnel.
1. Welding techniques used in the fabrication of thermoplastic tanks and pipe systems
As the use of plastics in more structurally demanding applications increases so does the need for rapid, reliable and high-quality welding techniques. For this reason, a wide variety of welding techniques have been developed and improved over many years to satisfy these requirements. The main welding techniques used for fabricating thermoplastic tanks and pipe systems are described below.
1.2 Hot Gas Welding
Hot gas welding is mainly used for joining thin (< 6mm) sheets of PP, PVC, PE and PVDF to themselves and also to pipes. The welding equipment is a hand-held welding gun consisting of an integral blower, a heating element with thermostat and a set of interchangeable nozzles for directing hot gas at the workpiece. A filler rod is used and this is made from the same polymer as the parts to be welded.
Usually, the gun is fed with air although for some applications nitrogen gas is used. The temperature of the hot gas stream is typically in the range 200-400°C, depending on the polymer being welded. With the heated gas directed towards the joint, local melting or softening of the components and filler rod take place ( Fig.1). A weld is formed when the joint region and filler rod fuse and then cool to ambient temperature. Since hot gas welding is a manual process, its success depends greatly on operator skill.
1.3 Extrusion welding
Fig.2. Extrusion welding gun
Extrusion welding is mainly used for joining thick (> 6mm) sheets of PP and PE to themselves and also to pipes. The process involves continuously extruding molten thermoplastic material into a weld preparation on the plastic structure being joined. The equipment ( Fig.2) is based on an electric drill with a mini extrusion barrel attached to the front. The extrusion barrel is heated along its length, either by cartridge heaters or hot air. A thermoplastic rod or granule feedstock is fed into the rear of the extrusion barrel and the material is heated as it is drawn through the barrel by the rotating extruder screw. Molten thermoplastic is continuously ejected through a PTFE shoe attached to the front of the extrusion barrel. The PTFE shoe is shaped to match the profile being welded, and defines the shape and size of the final weld. At the leading edge of the PTFE shoe, hot gas is used to heat the substrate material in front of the area where the molten bead is to be laid. This ensures that there is sufficient heat in the substrate material to form the weld. Typical welding speeds are 0.5-1.0 m/min. Again, since extrusion welding is a manual process, weld quality is dependent on the skill of the operator.
1.4 Butt Fusion Welding
The butt fusion welding technique (also known as hot plate welding, butt welding, mirror welding or platen welding) is primarily used for joining PE pipes for the water and gas industries, and PP and PVDF pipes for the chemical industry. It can be carried out on a wide range of pipe sizes, typically between 63 and 1600mm outside diameter (OD).
Fig.3. Butt fusion welding
The welding equipment ( Fig.3) consists of a system for clamping the two pipes to be welded and allowing them to move co-axially, a trimming unit to ensure that the pipe ends are flat and square prior to welding and a heated metal plate.
The welding sequence begins when the hot plate, at a preset temperature, is positioned between the two pipe ends. The pipes are pushed towards each other until the pipe ends come into contact with the hot plate and the pressure is increased to give good thermal contact. The pipe ends melt and the interface pressure forces the molten material outwards to form 'weld beads' at the outside and inside pipe surfaces; hence the term 'bead-up' stage. At the end of this stage, the pressure is reduced to a value sufficient only to maintain the pipe in contact with the hot plate. This allows the melt depth to increase without increasing the size of the weld beads. At the end of this 'heat soak' stage, the pipe ends are pulled away from the hot plate. The hot plate is removed, and the two molten pipe ends are pushed together at the same pressure as used during the initial bead-up stage. This causes further growth of the weld bead and is called the 'bead roll over' stage. The pressure is maintained until the weld is fully cooled.
1.5 Socket Fusion Welding
Fig.4. Socket fusion welding
The socket fusion technique is mainly used for welding pipes made from PE, PP and PVDF for chemical pipework. The process operation is generally manual and can either be carried out by hand (for pipe sizes up to 50mm OD) or on a manual machine for pipe sizes typically between 63mm and 150mm OD.
A socket mounted on a hot plate is used to heat the outside surface of the pipe being welded. On the opposite side of the hot plate, a spigot is used to heat the inside surface of an injection moulded fitting ( Fig.4). Both the fitting and the pipe are heated for a set period, known as the heating time. When the heating time is complete, the heated pipe and fitting are removed from the socket and spigot, and the pipe is pushed inside the fitting, producing the weld.
1.6 Electrofusion welding
The electrofusion (EF) technique is mainly used for welding pipes made from PE, for the water and gas utilities, although PP and PVDF can also be EF welded. This technique permits joining of pre-assembled pipes and fittings to be carried out with minimum equipment. It also offers a number of practical advantages to the installer; it is easy to use for repairs and where the available space and pipe movement is limited.
Fig.5. Electrofusion welding
The EF welding process involves the use of a fitting ( Fig.5), which is basically an outer sleeve with a coil of electrical resistance wire at the bore, which the two pipe ends slide into. An internal stop prevents the pipe ends from meeting. EF fittings are typically available in sizes from 16mm to 500mm. However, sizes up to 710mm are available.
Before welding, the pipe ends are cut square, the pipe surfaces to be joined are scraped to reveal uncontaminated material and the pipes are clamped to eliminate movement between the pipes and fitting. The welding process, where a current is passed through the coil to heat it to a temperature above the melting point of the surrounding polymer, can be divided into three stages: i) initial heating and fitting expansion, ii) heat soak to create the joint and finally iii) joint cooling. The duration of stages i) and ii) is commonly termed the 'fusion time'.
1.7 Infrared welding
The infrared (IR) technique is used primarily for welding pipes made from PP and PVDF for the semiconductor, pharmaceutical and chemical process industries. It can be carried out on pipe sizes typically between 20 and 225mm. The technique uses an electrically heated metal plate, which is typically at a temperature between 320 and 530°C, depending on the material and size of pipe to be welded. The pipes to be welded are brought into close proximity to the hot plate (typically 1.5-2.0mm) but without touching it and heat up due to radiation and convection. When the pipe ends become molten, the plate is withdrawn and the pipes are forced together to form a weld. The resulting joints have smaller weld beads compared with butt fusion joints because there is no 'bead-up' stage.
1.8 BCF welding
The bead and crevice free (BCF) welding technique is used for joining small diameter (20-63mm) PVDF piping systems for the semiconductor, biotechnology, pharmaceutical, food and beverage industries.
The BCF welding technique is based on the use of a rubber inflatable bladder, which is placed at the joint line inside the pipes, before welding commences. The pipes are clamped remote from the joint, and a heated metal collar surrounds the pipes at the joint line. As the polymer around the joint melts, it cannot deform outwards because it is constrained by the collar nor inwards because it is constrained by the bladder.
After a predetermined time period, the heat supply to the collar is switched off and the joint cools. Welds produced in this way exhibit no weld bead, which means that there are no crevices inside the pipe in which bacteria might grow.
2. Mechanical testing standards for thermoplastics welds
There are many mechanical tests available and it can often be difficult to assess which is the most appropriate method to use. As a general rule, the test method(s) chosen should be representative of the type of loading and mode of failure that the weld would be expected to see in service.
Mechanical tests may be used for optimising welding conditions and qualifying procedures. Alternatively they can be used for QA purposes to ensure that welds conform to previously established acceptance criteria.
Since, the mechanical behaviour of plastics is strongly influenced by factors such as strain rate, temperature and environmental conditions, whenever possible, test specimens should be conditioned in a controlled atmosphere prior to testing.
2.2 Tensile tests
The tensile test is probably the most common method used to determine weld quality. There are several variations on the tensile test that may be applied, depending on factors such as the quality of the weld and the form of the material to be tested.
In BS EN 12814-2 two specimen geometries are specified; a constant width (Type 1) specimen and a 'dog-bone' (Type 2) specimen, which are cut perpendicular to butt welds in either sheets or pipes.
A preferred specimen type is not specified. However, if Type 1 specimens consistently fail in the clamps then Type 2 specimens should be employed. Test specimens are extended along their major axis at a constant crosshead displacement rate until fracture or yield occurs.
Both welded and parent material specimens are tested in order to determine the short-term tensile welding factor, f s :
where σ w and σ r are the mean fracture stress of the welded and parent material specimens, respectively. Acceptable values of f s for different materials and welding processes are given in BS EN 12814-8 and range from about 0.8 to 1.0.
Fig.6. Notched specimen geometry for tensile welded joint test
An alternative specimen geometry, where a controlled double edge notch is introduced by carefully drilling two holes either side of a 25mm gauge length, which are then cut to the specimen edges ( Fig.6), can be used which ensures failure occurs within the weld region. This type of specimen is specified in BS EN 12814-7 and ISO 13953, and either the energy to break is calculated or the failure mode is assessed.
Another tensile test that uses a waisted specimen geometry to induce fracture at the weld is the low temperature tensile test, as described in BS EN 12814-6. The test is conducted at a nominal temperature of -40°C, depending on material, in order to induce brittle fracture at the weld.
2.3 Bend tests
A bend test for the assessment of butt welds in plastics materials with a thickness between 3 and 30mm is described in BS EN 12814-1. Rectangular section un-notched specimens are loaded in three-point bend as shown in Fig.7, at ram speeds between 10 and 50mm/min, depending on the material. The bend angle, α, or the ram displacement, H, at which either fracture occurs or a crack initiates is calculated. Graphs of minimum required bend angle/ram displacement against specimen thickness for different materials are given in BS EN 12814-8.
Fig.7. Bend test arrangement, according to BS EN 12814-1
2.4 Peel tests
Fig.8. Peel test arrangement, according to BS EN 12814-4
Peel tests for materials that are joined using some form of overlap joint, such as EF joints, are described in BS EN 12814-4. The test specimen and loading arrangement is shown in Fig.8.
The specimen is pulled perpendicular to the weld until complete separation occurs. Analysis of the fracture surfaces provides qualitative information about the joint integrity. Ductile yielding, with signs of stress whitening and drawn material between the wires indicates a good quality weld. Smooth fracture surfaces indicate a weak brittle failure, characteristic of a poor quality weld.
An alternative test for EF joints is given in ISO 13954. In this test the specimen is clamped in a vertical position by gripping the pipe stub portion of the joint. The joint is then vertically peeled apart. Again, the ductility ofthe fracture surface is assessed.
An indirect peel test for the evaluation of EF welds is the crush test, which is specified in BS EN 12814-4 and ISO 13955. The welded pipe/fitting assembly is cut in half lengthways and held in a vice. The pipe portion close to the weld is squeezed in the vice until the inner surfaces meet. There should be no evidence of cracking or peeling at the fusion interface.
2.5 Tensile creep rupture test
Since polymers are highly strain rate sensitive, the measured tensile strength or toughness for a parent material or weld only characterises the material for short-term load applications. Consequently, these parameters are not applicable to time dependent material behaviour.
Although more expensive to perform than short-term tests, creep rupture tests can provide more useful information when designing components that are under constant load.
Fig.9. Example of creep rupture curves for weld and parent material, and calculation of the long-term tensile weld factor
The tensile creep rupture test for plastic welds is described in BS EN 12814-3. The basis of the test is to subject a tensile test specimen to a constant load at a constant temperature in a known medium. The times to failure of the welded joints are compared to those for the parent material at different test stress levels so that a long-term tensile weld factor f L can be determined ( Fig.9).
The test conditions (temperature, stress) should be selected so that at least 30% of the fracture surface shows 'brittle' fracture. Increasing the test temperature and decreasing the applied stress encourages 'brittle' fracture. Appropriate surfactant solutions may be used as a means to accelerate the test.
2.6 Hydrostatic pressure tests
Hydrostatic pressure tests can be used to determine the time to failure of plastic pipes under constant internal pressure, following test methods such as ASTM D1598 and ISO 1167. Data obtained for a range of internal pressures, giving a pipe lives over two or more logarithmic decades, can be analysed using regression methods.
It should be noted that although these tests can be applied to welded pipes, since the circumferential stress in this test is twice the axial stress, failure will normally occur in the parent pipe away from the weld. This may not be the case for in-service welded pipes, which may fail at the weld, due to a combination of internal pressure and external loading. Consequently, this test method is not recommended to determine the long-term performance of welded joints in plastic pipes.
3. Non-destructive testing standards for thermoplastics welds
Although mechanical tests provide a good indication of the quality of a welded joint, in doing so they destroy the weld. There is therefore a need to obtain the same information without damaging the weld, i.e. using non-destructive testing (NDT) techniques.
Before employing any NDT method it is important to consider the types of flaw that may potentially be introduced during welding. These are described in BS EN 14728 and AWS G1.10M. It is also important to consider the acceptance criteria, i.e. the minimum size of flaw or level of contamination that reduces the integrity of the joint.
3.2 Visual inspection
Although the simplest of all NDT methods, the importance of a careful visual examination, as described in BS EN 13100-1, should not be overlooked. The appearance of the finished weld can often indicate problems with the welding process, reveal signs of weld contamination and highlight any misalignment of the parts. It can detect flaws such as surface cracks, imperfections in weld shape and size, and thermal/mechanical damage. However, it cannot detect embedded flaws such as voids or solid inclusions. Nor can it detect lack of fusion, unless this is apparent on the weld surface. Therefore, although weld shape imperfections indicate that the weld is of poor quality, it does not always follow that a visually perfect weld will be of good quality.
3.3 Bead removal and testing
This technique is used for butt fusion welds in PE pipes and is described in Annex B of EN 12007-2.
The external weld bead (and sometimes also the internal weld bead) of the PE pipe butt fusion weld is removed using an appropriate debeading tool, which removes the weld bead in one piece. The bead is then bent back manually at a number of locations along its length. If the two halves of the weld bead split apart ( Fig.10) this indicates that there is contamination in the weld. However, this technique will not detect embedded flaws and may not detect 'cold' welds or coarse particulate contamination.
Fig.10. Bend-back test of removed external bead from butt fusion weld in PE pipe. The split at the centre of the bead indicates contamination in the weld
3.4 Ultrasonic testing
Ultrasonic testing describes a range of NDT techniques that use high frequency sound waves directed into the object under examination to locate and, under certain circumstances, size embedded flaws. A variety of ultrasonic techniques have been developed that can be used for the inspection of plastics joints; the most important ones being pulse-echo, tandem, time-of-flight diffraction (TOFD) and creeping wave; and, more recently, phased-array. Ultrasonic testing of plastics welds is described in BS EN 13100-3.
The pulse-echo technique is the simplest form of ultrasonic testing, where a single ultrasonic probe is used to send sound energy into the object under test and to monitor the energy reflected from features within the material. This technique has been used to inspect EF, hot gas and extrusion welds, where it can be used to detect voids/porosity and lack of fusion type defects.
The tandem and TOFD techniques both use two probes: a transmitter and a receiver, and are used to look for vertically oriented flaws in butt fusion joints. The tandem technique uses two angled compression wave probes, positioned one behind the other on the same side of the joint. The front probe transmits compression waves into the weld region, while the rear probe monitors any reflections. In the TOFD technique the probes are positioned either side of the joint, facing each other. The transmitting probe sends a broad beam of compression waves into the weld region, which, effectively, 'floods' the entire material thickness with ultrasound and the receiving probe looks for diffracted signals from the edges of any flaws.
Creeping waves propagate just beneath the surface of the object under examination and have been used to detect flaws immediately beneath the outer weld bead in butt fusion welds in PE pipes.
Radiographic testing describes those techniques where electromagnetic radiation, in the form of X-rays, is used to produce a two-dimensional image of an object on photographic film or other form of detector. Its application to plastics welds is described in BS EN 13100-2.
As X-rays pass through the object under examination they are attenuated and only a proportion of the incident energy reaches the photographic film, placed on the opposite side of the test object. The amount of attenuation depends on the thickness and density of material penetrated. For example, where there is less material, more radiation reaches the film and a dark region appears on the radiograph. If a relatively dense material is present, more of the radiation is stopped from reaching the radiographic film and a light area appears on the radiograph.
Radiography is good for detecting volumetric flaws, such as voids, porosity and particulate contamination. It also provides a permanent record of the joint under inspection, which is relatively easy to interpret and which can be stored for future reference. However, it is time consuming and is manpower intensive. It is also important to recognise the hazards associated with the use of ionising radiation and the appropriate safety precautions that need to beapplied (e.g. stopping work during inspection and setting up safety zones).
Another limitation of radiography is its inability to locate the through-thickness position of flaws. If any dirt on the surface of a joint is not removed prior to X-ray inspection, it will appear in the joint area on the radiograph and could easily be mistaken for contamination in the weld. It is therefore essential that the joint surface is thoroughly cleaned before X-ray examination.
3.6 High voltage testing
High voltage, or spark, testing is used to detect discontinuities in thermoplastics linings, and is described in BS 6374:Part1:Appendix B and also in the NACE standard RP-02-74. The equipment generally comprises a unit that can generate a wide range of voltages, an earth wire and a live electrode, typically a brass or steel brush. The earth wire is connected to a bare section of a conducting target layer on the rear surface of the lining. This target layer can take the form of a carbon fibre strip or graphite cloth. Alternatively, if the vessel being lined is electrically conductive, the vessel itself can be used as the target.
Having connected the earth wire to the target layer, a large voltage (10 to 25kV) is then applied across the lining between the live electrode and the target layer. If there is no fault in the lining, no current will flow. However, a break in the lining, such as a pinhole, reduces the electrical resistance locally, sufficient for a spark to jump across the gap between the electrode and target layer, setting off both visual and audible alarms.
4. Qualification of thermoplastics welding personnel
Since many of the welding techniques used in the fabrication of thermoplastic tanks and pipe systems are manual processes it is imperative that the personnel carrying out the welding are properly trained and qualified. The procedure for carrying out qualification of plastics welders is specified in the European Standard BS EN 13067. This standard covers hot gas, extrusion, electrofusion and heated tool (butt, saddle, socket and wedge) welding of sheet, pipe, fittings and lining membranes in PVC, PP, PE, PVDF, and other fluoropolymers. It specifies the qualifications of the examiner, required experience for an operator to be admitted to the test, content of practical and theoretical tests, evaluation of test pieces and the period of validity of the approval.
The standard requires that a welder must have at least two years industrial experience in the relevant welding technique(s) before they can take the examination, which is divided into a theoretical and practical test. The candidate must pass both tests to be awarded a certificate, achieving at least 80% in a 20 multiple-choice paper and producing a welded test piece with minimum required mechanical properties.
The approval is valid for up to four years, depending on certain criteria being met, after which the examination must be retaken.