|NDT.net Mar 2005 Vol. 10 No.3|
Nondestructive Testing of Fusion Joints of Polyethylene Piping by Real Time Ultrasonic ImagingH. J. SHIN, Y. H. JANG*, INDE System Co., Ltd., Suwon, Kyunggi-do, South Korea
J. R. KWAN, Korea Gas Safety Corporation, Kyunggi-do, South Korea
E. J. LEE, Korea Nondestructive Testing & Internet Co., Ltd., Seoul, South Korea
ABSTRACTPolyethylene is one of the emerging materials that are explosively spreading into many industrial pipes. Regardless of the short history, thousands of kilometers of polyethylene pipes are newly installed every year. In this paper, an innovative nondestructive testing method for electrofusion joints are proposed and the sample test results are presented. The testing method is ultrasonic array technology in which ultrasound array pulses insonify test specimen for imaging the cross section of test specimen such as electrofusion joints. A 5MHz array ultrasound can test various diameter ranges of polyethylene pipes. Also 3.5MHz for the high penetration and 7.5MHz for high resolution can be used for thick and thin walls, respectively. The ultrasonic image is obtained in real time and the resolution is less then 1 mm. The arrangement of heating wires wound at fusion interface is individually identified. Geometrical information of the fusion joints such as thickness, wire location, wire gap, fusion length, and cold zone are also identified and measured. Defects such as void, imperfect fusion interface, inclusions, and wire locations are detected. Since ultrasound is non invasive, the test method can be applied in service as well as pre service. For the test, ultrasonic array transducer is placed outer surface of the fusion joint and the image is captured within one second. The ultrasonic imaging technique is a very promising method for the nondestructive testing of fusion joint in polyethylene pipes.
Keywords: Array ultrasonic, Polyethylene (PE), Electrofusion (EF) joining, Ultrasonic Image, Gas piping.
INTRODUCTIONThe demand of polyethylene (PE) pipeline increased in many field. A non-corrosive characteristic of PE is great benefits for the maintenance and extended periods of service life. Therefore the integrity of PE pipeline greatly depends on the quality of joints. Especially the PE pipeline for the gas energy distribution should be carefully constructed to prevent an explosive gas from leakage, and the rate of failure should be very low. Up to date efforts to have good joints were focused on the control of the joining process to follow proper procedure. A review of the electrofusion joining process for PE piping system was performed to have PE pipe system of a high integrity . And intelligent, knowledge based system for EF welding was proposed . To achieve the rate of failure close to zero, nondestructive testing (NDT) is strongly recommended in addition to joining process control. However, no well-known nondestructive testing methods exist for the PE pipe fusion joints.
This paper introduces a promising NDT method for electrofusion (EF) joining of PE pipe. EF joining is one of the widespread PE pipe weld methods. An estimated annual use of EF joining (in 1993) was over 15 million, and growing . Figure 1 shows a) EF joining of two PE pipes with EF coupler b) a cross section of EF coupler. Spirally wound-heating wires exist between coupler and main pipes. The distance between adjacent wires is usually few (1~3) mm. The fundamental ideas of joining process are to heat the wires and to melt the inside and outside surface of coupler and main pipes, respectively, then to consolidate fusion area.
In this study, ultrasonic array technique was applied to obtain the real time ultrasonic images of the cross section of electrofusion joints. In the use of ultrasonic waves to inspect the electrofusion joint, the most difficult challenge was to distinguish the wound heating wire at the joint interface from the joint failure. An ultrasonic focusing technique was used with ultrasonic phased array system and 96elements of array transducers. Zone based transmission focusing and dynamic receiving focusing was used. And electronic scanning was used to obtain real time two dimension images of the cross section of the EF joints. Different types of flaw samples were made and tested. Finally, an NDT method for the PE pipe EF joining was proposed.
ULTRASONIC ARRAY TECHNOLOGYUltrasonic array is one of the emerging techniques to enhance NDT quality. The preliminary study on the ultrasonic array technology for the EF joint were proposed very recently . A dominant benefit of the array technique over the traditional ultrasonic technique is the capability to control ultrasonic radiation beam in real time by electronics, so that the real time two-dimensional images can be obtained. Figure 2 shows the simulation results of the ultrasonic radiation beam control by electronics for (a) focusing and (b) steering and focusing. The focusing capability increases the inspection resolution dramatically. Figure 3 shows the electronic scanning concepts of ultrasonic beam for (a) lateral scanning by consecutive activation of array elements and (b) dynamic depth scanning by the control of aperture size. The electronic scanning of focal point and beam path provide real time imaging capability. And the zone based transmission and dynamic receive focusing increase the signal to noise ratio [4, 5]. Basically the ultrasonic array technology greatly depends on the ultrasonic array electronics, transmission and reception beam forming algorisms, and array sensors [6-9]. Because the NDT results are obtained by real time 2D images, the NDT process using array technology is a lot faster and easier to interpret the inspection results in comparison with traditional A- and B-scan UT.
BASIC EXPERIMENTS AND EQUIPMENTIn basic experiments, ultrasonic A-scan method was used. An average velocity of the PE was measured as 2.24 Km/s, and attenuation was measured as 3.5dB/cm and 6dB/cm at 1Mhz and 3.5MHz respectively. With a non-focused 5MHz traditional normal incident single element transducer and an ultrasonic pulse receiver, PANAMETRICS 5800, it was hard to detect back wall echoes of a PE piping thicker than 2cm. Also echoes from heating wire was small and the resolution was very poor to tell flaws from heating wires, since the gap between adjacent wires were too small as shown in Figure 1. Acoustic impedance of PE pipe was measured as 2329.6X103 pa s/m. For the NDT of the PE pipe EF joining in this study, an ultrasonic real time system and array transducers were developed. Figure 4 shows ultrasonic array transducers, each has 96 elements. The center frequency of the transducers determined as 3.5MHz, 5MHz, and 7.5MHz for the high penetration, general purpose, and high resolution respectively. Figure 5. shows a battery operated ultrasonic array system that is named AIM33. The system supports 96 array element transducers and has capability of real time 2D imaging, on screen sizing, image saving, and so on. The system could be connected with a B/W print, a PC monitor, and a head mount display. Therefore AIM33 is hand carry real field application system.
INSPECTION CONPTIONS FOR EF JOINTSInspections are performed outside surface of the EF coupler as shown in Figure 5. Ultrasonic waves are focused and scanned underneath of the array transducer by ultrasonic array system. Heating wires reflects ultrasound and the wire signals are indicated in UT images. If the resolution of the ultrasonic beam is good, the heating wires are indicated individually, so that it is possible to count number of wires and to measure the distance between wires. Ultrasound is transmitted very well when the EF joint is good, in this case the fusion interface will not be detected in UT images. Imperfect fusion joints reflect ultrasound and the fusion interface will be detected in UT images. Fusion interface is forming underneath of wires, therefore the imperfect fusion interface signal will be detected underneath of wire signals in UT images. Also dislocation of wires and air or soil inclusions will be indicated in ultrasonic images.
EXPERIMENTAL RESULTSFigure 6 (a) and (b) show a good EF joint and its ultrasonic image that shows clear wire indications. All wires are individually indicated and the fusion interface is not indicated in the image, which means that the ultrasonic wave propagates very well through the interface. And the wire arrangement is well located as it should be. Figure 6 (c) and (d) show an imperfect EF joint and its ultrasonic image that shows the interface indication as well as wire indications. Because the fusion area is underneath of the wires, the imperfect interface signals are located underneath of wire indications. Figure 6 (e) and (f) show a over heated EF joint and its ultrasonic image that shows the dislocation of wire array. Figure 6 (g) and (h) show an EF joint with soil inclusion at the fusion interface and its ultrasonic image that shows the soil indication at the fusion interface area.
CONCLUDING REMARKSArray technology provides good resolution and penetration power for the NDT of EF joint. In results, heating wires are individually identified in ultrasonic real time images, which means that the ultrasonic array technology provide enough resolution to tell wire signals from flaw signals. Imperfect fusion joints and soil inclusion are clearly indicated in the images. Even locations of wires are appeared as is. Therefore this array technology is a very promising method for the nondestructive testing of EF joining of PE pipe. In addition, the capability of real time displays makes the technology transfer to real field easy.
ACKNOWLEDGEMENTSThe author wishes to thank KOREA GAS SAFETY CORPORATION for the financial supports. The author also wishes to thank DAEYOUN MOLD INDUSTRIAL CO., LTD. and POLYTEC CO., LTD. for the making of various EF joining samples.