INSPECTION RESULTS OF DAMAGED POLYETHYLENE ELECTROFUSION JOINTS USING ULTRASONIC IMAGING METHOD

Transcription

1 INSPECTION RESULTS OF DAMAGED POLYETHYLENE ELECTROFUSION JOINTS USING ULTRASONIC IMAGING METHOD Seong Hee Kil and Kee Bong Yoon Korea Gas Safety Corporation Abstract Non destructive testing methods of electrofusion(ef) joints of thermoplastics pipes are required for fusion joint safety and for the long term reliability of a pipe system. Electrofusion joints, which are joined at the proper fusion process and procedures, may encounter defects due to the difference of ovality between pipes and coupling, improper fusion process or porosity result from electrofusion joining. These defects can cause the failure of pipeline and by extension, they can be caused the limit to expand the use of plastics pipes. This paper studies two inspection results using ultrasonic imaging method for damaged polyethylene electrofusion joints. One case, gas was leaking from 250mm diameter polyethylene electrofusion joints which was electrofused at 22, December 1994 and operation pressure was 2.45kPa. First, surface inspection was conducted and then in order to find the types of defects examination using ultrasonic imaging method was performed. Lack of fusion and inappropriate inserting for polyethylene pipes into electrofusion coupling were found and causes of the gas leak were judged that misalignment and insert defect. Destructive inspection was performed and each inspection results were compared to. Results of ultrasonic imaging method and destructive inspection were the same. Second case was for the 400mm diameter electrofusion joints. Study was performed by using ultrasonic imaging method and destructive test. By the result of ultrasonic imaging method, located circumferentially porosities were found at fusion joint and the size of porosities was about 8mm. Note that the width of heating wires was about 80mm so the size of porosity was less than 10% of width of heating wires. At this inspection,3.5mhz phased array sensor was used. INTRODUCTION Electrofusion(EF) joints have been widely used as they are ease to fuse and suitable for high quality joints for polyethylene. But such as voids, cracks, discontinuities, foreign particles, defects can be identified in the heat fusion joints by electrofusion process for large diameter pipes than smaller ones. Because ovality is more likely occurred at large diameter pipes. Non destructive testing methods of electrofusion(ef) joints of thermoplastics pipes are required for fusion joint safety and for the long term reliability of a pipe system. Electrofusion joints, which are joined at the proper fusion process and procedures, may encounter defects due to the difference of ovality between pipes and coupling, improper fusion process or porosity result from electrofusion joining. These defects can cause the failure of pipeline, they can be caused the limit to expand the use of plastics pipes. Ultrasonic imaging method can be done in the field in a minimum amount of time. It does not cause significant construction delay, while still providing an aid to quality control before burial of the pipe. In Korea, KGS(Korea Gas Safety Co.) developed KGS code about non destructive testing for polyethylene electrofusion 1

2 joints in THE EXAMINATION This paper studies defects detection for electrofusion joints of polyethylene piping is utilized by the ultrasonic imaging method. Two cases were studied. One case, gas was leaking from electrofusion joint which is 250mm diameter polyethylene pipes and electrofusion coupler. The damaged electrofusion joint was electrofused at 22, December 1994 and operation pressure was 2.45kPa. Figure I: The picture of electrofusion joint(side view) EXTERIOR EXAMINATION OF FIRST CASE From figure II we knew differences of gap between electrofusion coupler and polyethylene pipes. Figure II: The picture of electrofusion joint(front view) 2

3 Figure III: The picture for detailed geometrical shape of electrofusion joint Table I: The data of gap between electrofusion coupler and polyethylene pipes (Unit : mm) Top Left Right Bottom Gap between electrofusion coupler and polyethylene Observation results of the exterior for electrofusion joint are as follows. First, one side of polyethylene pipes was inserted through the opposite side to heating wires, the other side of polyethylene pipes was inserted less in the same side. Figure IV is the estimated figure based on result of exterior examination of electrofusion joint. Second, polyethylene pipes and electrofusion coupler must be connected to the parallel, but polyethylene pipes were not inserted parallel to the electrofusion coupler. Due to these reasons, electrofusion joint was estimated as shown figure IV. 3

4 Figure IV: Estimated figure based on results of exterior examination of electrofusion joint ULTRASONIC IMAGING METHOD OF FIRST CASE Ultrasonic imaging method was performed in order to find types of defects. 5.0MHz phase array sensor was used. The following figure V is the result of ultrasonic imaging method of electrofusion joint and figure VI is the detailed result of ultrasonic imaging method at a region(200mm to the right on the circumference) of electrofusion joint. As shown in figure VI defects were detected on the left and right side of electrofusion joint. Especially at left part abnormal wire arrangement could be seen from the result of ultrasonic imaging method. So we could notice that fused region between polyethylene pipes and electrofusion coupler was completely separated. This defect was assumed to be the biggest defect of electrofusion joint. The right region of electrofusion joint was better than left region when compared to the performance of electrofusion, but porosity between heating wire and heating wire was detected so porosity could be estimated to grow more and more depending on the course of time. Figure VII is the estimated figure based on results of ultrasonic imaging method of electrodusion joint. Lack of fusion was founded and heating wires were not arranged evenly. DESTRUCTIVE EXAMINATION OF FIRST CASE As shown in figure VIII some results were founded by destructive examination. First, polyethylene pipes should be inserted parallel to electrofusion coupler, but this case was not, so gap between electrofusion coupler and polyethylene was occurred and was different. Also polyethylene pipes should be inserted into the middle of electrofusion coupler, but at one side polyethylene pipes were inserted too much into electrofusion coupler. And the other side polyethylene pipes were inserted too less, so inserting defect was occurred by these reasons Figure IX is the picture of lack of fusion which was found also at electrofusion joint. 4

5 Figure V: The result of ultrasonic imaging method of electrofusion joint Figure VI: Detailed results of ultrasonic imaging method of electrofusion joint 5

6 Figure VII: Estimated figure based on results of ultrasonic imaging method of electrodusion joint(figure VI) Figure VIII: The cross sectional view of the electrofusion joint Figure IX: The picture of defect(lack of fusion) of electrofusion joint 6

7 DISCUSSION OF FIRST CASE Lack of fusion was found and appropriate electrofusion wasn t applied to electrofusion process. Polyethylene pipes should be inserted parallel to electrofusion coupler, but at this case, misalignment was occurred. And inappropriate inserting method was applied. It was estimated that defects occurred at electrofusion joint and would grow depending on the course of time. SEOND CASE Second case is for electrofusion joint which is 400mm diameter polyethylene pipes and electrofusion coupler. 3.5MHz phased array sensor was used for detecting the defect at electrofusion joint. Figure X is the picture of field inspection and figure XI is the result of ultrasonic imaging method. From the result of figure XI, porosity was founded at the same position in electrofusion joint continually when ultrasonic imaging method was applied. So electrofusion joint was cut into the circumferential direction and compared to the result of ultrasonic imaging method. As shown in figure XII porosity was connected circumferentially and two results which ultrasonic imaging method and destructive examination were the same. DISCUSSION OF SECOND CASE From the result of ultrasonic imaging method for electrofusion joint, some parts were attached well to each other but some parts were separated which were estimated to defects. The size of porosity was 8mm and the width of heating wires was 80mm, so the defect size is about 10% of the width of heating wires. 7

8 Figure X: The picture of field inspection a) butt fusion b) scraping of polyethylene piping c) before electrofusion d) electrofusion e) NDT1 f) NDT2 Figure XI: The results of NDT of electrofusion joint 8

9 Figure XII: The picture of cutting the electrofusion joint Figure XIII: The cross sectional view of the 400mm electrofusion joint 9

10 List of references 1. Avi Benatar, Donghun No, Developing NDE flaw criteria for short term performance of electrofusion pipe welds, Final report to Korea Gas Safety Corporation, July (2004) 2. ASTM F , Standard specification for electrofusion type polyethylene for outside diameter controlled polyethylene pipe and tubing 3. Hyunjae Shin, Jeongrock. Kwon and Sungjin Song, Ultrasonic real time imaging technique for the inspection of electrofusion joints for polyethylene piping, 2001 Forum for Gas Safety, edited by Korea Gas Safety Corporation, KGS , September (2001), pp Hyunjae Shin, Yuhyun Jang, Jeongrock Kwon and E. Lee, Nondestructive testing of fusion joints of polyethylene piping by real time ultrasonic imaging, 2004 plastics pipes conference, Session 7a. 5. Seonghee Kil, Youngdo Jo and Heungsik Kim, The case study of phase array ultrasonic for polyethylene pipeline in the electrofusion joints, the 12 th international conference on pressure vessel technology(icpvt-12), September 20-23, Seonghee Kil, Youngdo Jo, Heungsik Kim and Keebong Yoon, Field inspection of phase-array ultrasonic for polyethylene electrofusion joints, 2010 plastics pipes conference, September 20-22, 1