Statistical Reliability Assessment of UT Round-Robin Test Data for Piping Welds

Transcription

1 Key Engineering Materials Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Citation & Copyright (to be inserted by the publisher) Statistical Reliability Assessment of UT RoundRobin Test Data for Piping Welds HyunMook Kim 1, IkKeun Park 2, UnSu Park 2, YoonWon Park, SukChull Kang and JinHo Lee 1 Graduate School of Hanyang Univ., 17 Heangdangdong, Sungdonggu, Seoul, Korea 2 R. I. of NDE Technology Seoul National Univ. of Technology, 172 Gongneungdong, Nowongu, Seoul, Korea Korea Institute of Nuclear Safety(KINS), 19 Kusongdong, Yousonggu, Teajon, Korea Keywords: ultrasonic test, reliability,, weld, piping, roundrobin test Abstract. Ultrasonic NDE is one of important technologies in the lifetime maintenance of nuclear power plant. Ultrasonic inspection system is consisted of the operator, equipment and procedure. The reliability of ultrasonic inspection system is affected by its ability. The performance demonstration round robin was conducted to quantify the capability of ultrasonic inspection for inservice. Several teams employed procedures that met or exceeded with ASME Sec. XI code requirements detected the piping of nuclear power plant with various cracks to evaluate the capability of detection and sizing. In this paper, the statistical reliability assessment of ultrasonic nondestructive inspection data using probability of detection () is presented. The result of using logistic model was useful to the reliability assessment for the NDE hit or miss data. Introduction Throughout the world, most of nuclear power plants as well as chemical and petroleum plants have been in operation beyond their original design life for 040 years. NDE (nondestructive evaluation) techniques for characterizing important mechanical and material conditions, such as structural integrity and pressure boundary defects at welded locations in nuclear power plant, have been developed and implemented from various initiatives. The reliability for safety and cost effective operation of nuclear power plant are based on the effectiveness of NDE. It is well known that inservice inspection (ISI) plays an important role in assuring the integrity of the structure or component of nuclear power plant. Preservice inspection (PSI) and ISI in Korean nuclear power plants are performed according to ASME code Section XI every 10year period, because ASME code mandates that PSI/ISI are performed by NDE. These activities include ASME code section XI requirements [1], NRC(Nuclear Regulatory Commission), legislative requirements, and other specific initiatives. The typical NDT (nondestructive testing) method for the inspection of nuclear power plant is UT (ultrasonic testing). Ultrasonic NDE is one of the important technologies in the lifetime maintenance of nuclear power plant and used to inspect various types of weld. Therefore, retention of its reliability is a key technique for the lifetime maintenance of nuclear power plant. Recently, Appendix VII in ASME code section XI provides requirements for performance demonstration about ultrasonic inspection procedures, including equipment and operator s personality. UT system consists of operators, equipments and procedures. The reliability of results in UT system is affected by its ability. It s has been frequently reported that the existing ISI UT methods can t detect even quite large defects in mockup specimens, fail to size the defect, are dependent upon an operator s skill and physical condition. Furthermore, the reliability of ultrasonic ISI is influenced by the inspection environment, material characteristics and types of defect. Thus, it is very difficult to estimate the reliability of NDE due to various factors. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA09/05/16,2:50:8)

2 1744 Advances in Nondestructive Evaluation Photo. 1. Mechanized UT round robin test Fig. 1. The index of detection In this paper, the performance demonstration roundrobin test was conducted to quantify the capability of ultrasonic ISI and to analyze the level of UT ISI technology in Korea. For the sake of the statistical reliability analysis of roundrobin test results, the model of the reliability of assessment such as logistic probability model [2,] is studied. UT RoundRobin Test (RRT) For approximately two months, nine NDT corporations participated in testing piping weld mockup specimen through manual UT RRT. Using the specimen used in manual UT RRT, we have conducted RRT with identical design variables for mechanized UT RRT. Test results of manual and mechanized tests were compared to assess performance and reliability of Korean UT testing. Four corporations participated in mechanized UT RRT. As for the specimen, pipetopipe specimen identical to the weld specimen used in nuclear power plant was developed on two piping specimen with different dimensions and materials, one of which was processed with different sizes and shapes of EDM notch on two welding parts and the other with fatigue crack and EDM notch. About one grading unit, detect variables were divided into five categories as follows. Defect direction was designed for longitudinal and circumferential directions as shown in Fig. 1. Index of inspection includes defect location, defect depth (a) from the inner wall, and defect length (L). Specimen materials were 04L stainless steel SA12 and TP47 stainless steel. RRT Results Performance of detection. Fig. 2 is display of a curve which assesses the performance of detection () of mechanized inspection. As shown in Fig. 2(a), performance of detection

3 Key Engineering Materials Vols curve 95% Confidence limits 0.2 curve 95% Confidence limits Crack Length [mm] (a) Crack Length & Crack Depth [mm] (b) Crack Depth & Fig. 2. Logistic curve fit of data of all teams with 95% confidence bounds Table 1. Summary of detection performance between mechanized UT and manual UT Defect Size Manual Mechanized Blank FCP Size 1 Size 2 Size Size according to the crack length is approximately 0.66 (66%) for length longer than 15mm. However, for cracks less than 15mm, 95% reliability boundaries were big and fell under 0.5. Such result indicates that detection information on the length less than 15mm shows low level of reliability. In Fig. 2(b), a relation of crack depth and was shown to be an average of 0.6 (60%). However, in defect depth less than 5mm, a width of 95% reliability boundaries were considerably high; thus detection information for this particular area was proven to show low level of reliability. A boundary with low level of reliability has high possibility of false call probability (FCP) since it is very likely to have no defect even if the inspection system reports detect defects. However, most of cracks are not very deep, indication that potential danger caused by misjudgment is not very serious compared to deeper cracks. Table 1 compares detection performance according to the range of defect size of manual and mechanized UT. It was proven that mechanized UT resulted greater detectability compared to manual UT. Effect of defect variables. Table 2 shows detectability of mechanized test according to the defect type variable. As in manual test, mechanized test resulted relatively low detectability in thermal fatigue crack (TFC) compared to EDM notch. Table is a display of detectability of geometrical variables [4]. Because sizes of designed longitudinal direction defects only include ranges that correspond to Size 1 and 2, the comparison was performed in these two ranges only. Compared to

4 1746 Advances in Nondestructive Evaluation Table 2. Summary of detection performance for crack type variables Size Type EDM notch Thermal fatigue Crack (TFC) Size 1 Size 2 Size Size Table. Summary of detection performance for geometrical crack variables Size Type Circumferential Crack Axial Crack Size 1 Size 2 Size Size circumferential direction, of longitudinal direction defect was relatively low, indicating it is difficult to detect defects. Blank : no flaw in a grading unit Size 1 : a/t 0%, L 10mm Size 2 : a/t 0%, L 10mm Size : 0%<a/t 60% Size 4 : 60%<a/t 90% In mechanized UT, scanning direction of the probe is set to scan welding crown vertically. Thus, in longitudinal direction defects, the propagating direction of the ultrasonic wave and area direction of the defect is not vertical, which caused lower detectability compared to circumferential direction. Also, about a defect which two cracks in one grading unit overlapped, most of overlapped defects were reported as one single defect, as shown in Fig., and in four cases, no single defects was detected, showing high frequency. In Fig., One means the case that the overlapped two defects were detected as a singe defect. In case of overlapped defects, defects can develop and integrate two defects into one big defect, which could be crucial in the safety of welding parts. Therefore, it is necessary to develop methods that can improve resolution so overlapped defects could be identified accurately. Sizing performance. Sizing of mechanized UT was assessed with ASME Section XI Appendix VIII, just like manual UT. Fig. 4(a) is the sizing performance of length of crack of mechanized UT. It could be learned that the length sizing performance is relatively high. Such results are also driven in the length sizing of crack of manual UT is excellent. Fig. 4(b) is results of the crack depth sizing performance. Different from the length of crack, deviation form ideal line were relatively big, indicating the sizing is not very accurate. As shown in the figure, with a crossing point within 10mm as a center, defects smaller than 10mm was measured to be bigger than actual defect and those bigger than 10mm was measured smaller than actual defect. It is categorized into two errors. First is an error of reporting defects smaller than critical size to be bigger than critical size and second is an error of reporting defects bigger than critical size. In case of first error, false information of incorrect NDT could cause financial loss in facility of nuclear power plant. In case of second error, we cannot guarantee the safety of facility [5,6]. Therefore, in order to secure the integrity and safety of nuclear power plant through inservice inspection (ISI), a measure to improve crack depth measurement performance of mechanized UT system is necessary.

5 Meas. Length [mm] Meas. Depth [mm] Frequency, N Key Engineering Materials Vols No One Number of detection Two Fig.. The number of detection for overlap defects Regression Line Ideal Line Regression Line Ideal Line True Length [mm] (a) length True Depth [mm] Fig. 4. Regression fit of all teams' sizing measurements (b) depth Team s Performance. Table 4 is a display of results of detection performances of teams which participated in mechanized RRT. As shown in the table, all teams were deficient in judgment standard of ASME Section XI Appendix VIII. However, team 1 and 4 displayed relatively excellent detection performance compared to team 2 and with considerably big difference. Considering the fact that inspection equipments of the participating teams had similar specifications, it is judged that there were problems in the judgment ability of operator and calibration of inspection equipment, the detection performance of manual UT is judged as better than mechanized UT. Table 5 is results of sizing performance of teams participated in mechanized UT. All four teams were deficient in judgment standard of ASME Section XI Appendix VIII. However, team 4 displayed the best performance. Team 2 could be judged excellent since it showed lowest value of RMS, 2.6mm, in measuring the depth. However, it only detected seven defects (refer to Table 4). Therefore, it could not be considered to have excellent sizing performance. Also in terms of length sizing performance, team 1 and 4 showed excellent results, showing big difference from teams 2 and.

6 1748 Advances in Nondestructive Evaluation Table 4. Summary of detection performance of each team Team No. No. of detected No. of false called grading Unit grading unit Criteria Bad Bad 7 5 Bad Bad Mean value No. of good team : 0 No. of bad team : 4 Table 5. Summary of the sizing performance of each team Team No. Sizing performance parameters Mean Deviation of Length [mm] Root mean square of depth [mm] Mean value Conclusions It was proven that mechanized UT was less influenced by defect variables compared to manual UT. However, according to big differences among participating teams, capacity of operator about inspection procedure, equipment calibration, defect information reading and etc. had great effect in the test results compared to manual UT. Thus, when testing with mechanized UT system, it is necessary for operators to be trained about the equipment and tested for their performance through pretests prior to the inspection. In case of performance of sizing, mechanized UT length sizing showed excellent. However, depth sizing was proven to be less accurate. It is necessary to supplement and develop methods that can improve the accuracy of length sizing. When detecting defects in actual cracks, such as thermal fatigue crack, there is higher possibility of which detectability and sizing performance to get lower. Therefore, in order to improve the reliability of inservice inspection of nuclear power plant, it is necessary to induce performance demonstration system and assess uncertainty of UT results to reflect them in the inspection results. References [1] ASME B&PV Code Section XI Appendix VIII: Performance Demonstration for Ultrasonic Examination System (ASME 1995). [2] A.P. Berens: NDE Reliability Data Analysis, in Metals Handbook Vol. 17 (ASM International Metals Park, Ohio 1989). [] V.E. Panhuise, et al.: Quantitative Nondestructive Evaluation, in Metals Handbook Vol. 17 (ASM International Metals Park, Ohio 1989). [4] P.G. Heasler and S. R. Doctor: Piping Inspection Round Robin (NUREG/CR5068 PNNL ). [5] D.E. Bray and R.K. Stanley: Nondestructive Evaluation (McGrawHill, New York 1989). [6] D. Keqin: A quantitative Method for Determining the Flaw Size in the Structure, 15th WCNDT (2000), idn 107. [7] C. Paliou, M. Shinozuka: Reliability and Durability of Marine Structures, Journal of Structural Engineering ASCE 116 (1987), p

7 Key Engineering Materials Vols [8] J.L. Bogdanoff, and F. Kozin: Probabilistic Models of Cumulative Damage (John Wiley & Sons 1985).

8 Advances in Nondestructive Evaluation / Statistical Reliability Assessment of UT RoundRobin Test Data for Piping Welds /