NDTnet - April 1996, Vol.1 No.04
ULTRASONIC AND RADIOGRAPHIC NDT OF BUTT
FUSION WELDS IN POLYETHYLENE PIPE
Table of contents of this Page:
3. Radiographic NDT
The radiographic inspection of welded joints in metallic structures is well established. Exposure charts exist which enable the radiographer to select the X-ray energy and exposure necessary to produce a good quality radiograph of a particular thickness and type of metal. The radiographic inspection of plastics is less well established and similar exposure charts are unlikely to exist. In comparison to metals, plastics are much less absorbent of X-ray radiation and require the use of 'softer', lower energy, X-rays to achieve good quality results. To illustrate the point, 50mm of steel would typically be radiographed at an X-ray energy of 300kV, while an identical thickness of polyethylene, under approximately the same exposure conditions, only requires an X-ray energy of 16kV to produce the same density radiograph.
3.2. Work At TWI
TWI have pioneered the use of wire type image quality indicators (IQIs) made from polyethylene to the specifications of BS EN462-1 (10). The existence of such IQIs is a major achievement and significantly helps to overcome the problems of describing image quality of radiographs shot at low X-ray energy «26kV).
A range of IQIs were manufactured from polyethylene (7). These included an ASME plaque (ASME V, Article 22), a British step wedge with holes (BS EN462-2:1994), and the aforementioned British wire type. The whole range in common use is illustrated in Fig.11.
These were assessed and compared for their ability to provide a reliable measure of radiograph quality and sensitivity, and for their practical handling. All IQI types were able to achieve sensitivities better than 2%. The ASME type is small and easy to handle, however, a range of thickness plaques are required for different thickness testpieces. Also, spacers are required to cater for the additional thickness of the weld beads, and these are not easily bent to the pipe's curvature. The British wedge type had similar advantages and disadvantages. The wire type achieved the highest sensitivity (0.67% on 37mm of polyethylene), did not require the use of spacers and was able to cover a much wider range of material thicknesses.
The same programme of work (7) considered the effect of X-ray absorption through different grades of polyethylene. It was concluded that the very low absorption by polyethylene of X-ray radiation can vary significantly from one polyethylene type to another but that, for the range of polyethylene Materials shown in Table 1, only two exposure charts are necessary to produce radiographs in the density range specified by BS 2600 (11). Figure 12 illustrates a
typical exposure chart.
These exposure charts have been used to produce prototype radiographic procedures for the inspection of butt fusion welds in polyethylene pipe (7,8). The procedures make use of two double-wall single image radiographic inspection techniques. When the X-ray source is in the plane of the weld (see Fig.13) the technique is referred to as the 'straight' technique, and the weld fusion face appears as a straight line on the radiograph. When the source is offset from the plane of the weld it is referred to as the 'throwing' technique and the weld fusion face appears as a curve on the radiograph. The straight technique was used as a primary method. The throwing technique was used on occasions to provide additional information regarding the shape and character of the flaw.
The prototype procedures discussed here, have been used to detect reliably a range of manufactured and natural flaws in polyethylene pipes including; dust contamination of the fusion face, planar discontinuities in the fusion zone (Aluminium foil discs), lack of fusion and a thinning of the weld bead corresponding to the presence of a cold weld.
3.3. Comparison With Other Studies
In practice, X-ray radiography is used to supplement the visual inspection of larger diameter polyethylene pipe (3,4). The radiographic procedures developed by TWI and those in use at British Gas (12) both stipulate a low energy X-ray, double wall single image technique. British Gas has identified problems with existing IQIs and have sought to overcome the problem by developing their own ERS plastic penetrameter (12). TWI have also addressed this problem by developing polyethylene wire type IQIs in accordance with (10). |Top|
The main conclusions are based on the work undertaken at TWI on a range of polyethylene
materials and pipes with diameters greater than 180mm and thicknesses greater than 12mm.
- Ultrasonic creeping waves can be used to inspect the fusion area immediately beneath the outer weld bead in polyethylene pipes.
- Ultrasonic tandem and TOFD techniques can be used to detect and image perpendicular planar flaws in butt fusion joints in polyethylene pipes.
- Ultrasonic techniques are currently unable to detect nominally cold welds.
- Radiographic intensities of between 16kV and 26kV are optimum when inspecting thicknesses of polyethylene pipe between 5mm and 50mm.
- With radiography it is possible to infer the presence of a cold weld from local radiographic density measurements.
- Polyethylene wire type IQIs have been proven to achieve radiographic sensitivities at least as good as polyethylene plaques and step wedges with holes, and have other practical handling advantages with regard to pipe inspection. |Top|
- Dodin M G: 'Welding mechanisms of plastics - a review'. J. Adhesion, 12 (1981).
- Gas Research Institute: 'Quality assurance of heat fused polyethylene plastic pipe joints'.
GRI 85/0284. Final report November 1983, June 1986.
- Ewing L and Richardson W: 'Polyethylene gas pipe systems: An appraisal of joint design
and construction methods'. British Gas report no. ERS EZ67, November 1980.
- Ewing L and Maine L: 'The electrofusion of PE gas pipe systems in British Gas'. 1983,
ERS report no. ERS E339.
- Gas Research Institute: 'Inspection device for heat fused butt joints'. GRI Technology
Profile, September 1987.
- Edwards G R: 'The development of non-destructive test procedures for hot plate butt fusion
welds in polyethylene'. TWI Members report no. 370/1988.
- Georgiou G A and Macdonald I A: 'Ultrasonic and radiographic NDT of butt fusion joints
in polyethylene'. TWI Members report no,. 465/1993.
- Munns 1 J and Georgiou G A: 'Ultrasonic and radiographic NDT of butt fusion welded
polyethylene pipes'. Draft TWI Members report, March 1995.
- McClinton M: 'How to detect flaws iii PE pipe butt fusion joints'. Pipe Line Industry,
- BS EN 462-1:1994: 'Non-destructive testing - Image quality of radiographs. Part 1:
Image quality indicators (wire type) - Determination of image quality value'.
- BS 2600- Part 1: 1983: 'Radiographic examination of fusion welded butt joints in steel.
Methods for steel 2mm u to and including 50mm thick'.
- Lumb R F, Bennett A and Ward C R: 'A review of NDT at British Gas Engineering
Research Station'. BJNDT, September 1980, pp.217-225. |Top|
Rolf Diederichs 1.March.1996, email@example.com
/DB:Article /AU:Munns_I_J /AU:Georgiou_G_A /CN:UK /CT:UT /CT:RT /CT:weld /CT:plastic /ED:1996-04