Ultrasonic Phased Array for the Circumferential Welds Safety Inspection of Urea Reactor

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1 Available online at Procedia Engineering 43 (2012 ) International Symposium on Safety Science and Engineering in China, 2012 (ISSSE-2012) Ultrasonic Phased Array for the Circumferential Welds Safety Inspection of Urea Reactor Dong Hu a, Qiang Wang a, *, Kun Xiao a, Yehao Ma a a Department of Quality and Safety Engineering, China Jiliang University, Hangzhou , China Abstract Due to the multi-layer thick wall structure and coarse grain materials of inside liner, the circumferential welds safety inspection of urea reactor is usually not easy. The paper analyzes the problems and difficulties of NDT techniques used for circumferential welds of urea reactor and overviews the theory and advanced point of UPA technology. Then two related test blocks (RB-2/20 and Austenitic stainless steel butt joint test block) with artificial defects are selected to simulate the outer and inner layer. Experiments by UPA are carried out on the test blocks and results are analyzed. It is proved that UPA is effective and efficient in solving the circumferential welds safety inspection Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Capital University of Economics and Business, China Academy of Safety Science and Technology. Open access under CC BY-NC-ND license. Keywords: Urea reactor, Circumferential welds, Ultrasonic phased array testing, Multi-layer thick wall structure, Coarse grain material 1. Introduction Multi-layer structure, widely used in high pressure and ultra-high pressure vessels such as urea reactor, is a common structural style for pressure vessel. But in recent years, several safety accidents of urea reactor have caused concern in the community [1]. In order to avoid such catastrophic accidents, timely inspection should be carried out, especially the inspection of circumferential welds that is easier to be corroded. Non-destructive testing (NDT), which is based on techniques that rely on the application of physical principles to determine the characteristics of materials, always contributes to detect and assess flaws or harmful defects without changing the serviceability or usefulness of said materials [2]. Multiple NDT techniques have been used together in the inspection of circumferential welds of urea reactor such as RT (Radiographic Testing) and UT (Ultrasonic Testing), which not only increases the cost of detection but also reduces the detection efficiency. Deep welds RT detection could only get very poor sensitivity, and traditional UT system has only fixed focus, which is burdensome for layer inspection of welds [3]. In addition, coarse grain materials, which lead to attenuation losses and scattering noise, also bring difficulty to UT inspection [4]. Ultrasonic phased array (UPA) has excellent abilities of electronic steering, deflection and focusing [5]. As the rapid development of electronic and computer technology, it has been widely employed in industry NDT, the nuclear industry [6] *Qiang Wang. Tel.: ; fax: address: wangtulip@yahoo.com.cn, qiangwang@cjlu.edu.cn Published by Elsevier Ltd. doi: /j.proeng Open access under CC BY-NC-ND license.

2 460 Dong Hu et al. / Procedia Engineering 43 ( 2012 ) and chemical industry in particular. In the past few years, UPA inspection of coarse grain materials [7] and UPA imaging technology [8,9] have made great progress. In this paper, the structure of urea reactor and the superiorities of UPA are analyzed firstly. Then, UPA is employed to inspect the defects on two test blocks. The defects are located at RB-2/20 test block s 80mm depth and Austenitic stainless steel butt joint test block s 10mm depth. Finally, conclusions are presented. 2. Structure of urea reactor Urea reactor belongs to pressure equipment, design pressure of which is usually above 20MPa and usually uses multilayer structure. The structure of circumferential welds of urea reactor is as Fig.1, It is clear that the welds are divided into outer and inter layers. The material parameter of each layer is as Table.1 Fig. 1. Structure of circumferential welds of urea reactor Table 1. Material parameter of welds Name Material Thickness (mm) Inside Liner 316L 8~12 Laminate panel 15MnVR 90~ Mathematic model of ultrasonic phased array Seen from Fig.2, UPA testing instrument can electronically steer and focus the transmission ultrasonic beams. These features provide a new testing approach for circumferential welds.

3 Dong Hu et al. / Procedia Engineering 43 ( 2012 ) Fig. 2. Beam focusing and beam deflection of liner UPA In order to keep good directivity and depress energy leakage of the ultrasonic beam, the shape of the focus sound field and the location of the focus area must be controlled. Therefore, precision control of transmission delay for each element is demanded. The object of delay computation is to find the delay difference t n between an arbitrary element ( n the element) and the first element. Take linear UPA for example, if the deflection angle of the ultrasound beam is, the delay time of random adjacent elements 0 satisfies [10]: 0 dsin / c (1) where c is the ultrasound velocity in the medium, d is the center distance between two adjacent elements. Supposing the ultrasonic beam focuses along the line that has an angle of with the normal line. The line is called the beam direction. There are N elements are excited. Relative to the middle element, the delay of element is n(1 n N) [9]: 2 f d N 1 d N 1 tn t0 1 1 ( n ) 2sin ( n ) c f 2 f 2 (2) 4. Experimental procedure Inner and outer layer defects of circumferential welds in urea reactor are simulated through related test blocks. Table.2 presents the introduction of experiments Table 2. Introduction of the Experiments Number 1# 2# Name Inspection experiment of Laminate defect Inspection experiment of Inside Liner defect Instrument OmniScan MX OmniScan MX Probe 5L64-A2 5L64-A2 Test block RB-2/20 Austenitic stainless steel butt joint test block Depth of the simulated defects 80mm 10mm Fig.3 and fig.4 show the specific information of the test blocks:

4 462 Dong Hu et al. / Procedia Engineering 43 ( 2012 ) Fig. 3. RB-2/20 5. Results and discussion Fig. 4. Austenitic stainless steel butt joint test block (a) (b) Fig. 5. Austenitic stainless steel butt joint test block

5 Dong Hu et al. / Procedia Engineering 43 ( 2012 ) Figure.4 shows that inspection of simulated defects in both inner and outer layer by UPA technology get nice effects, the imaging results is intuitive. High SNR UPA images are obtained through one time parallel inspection, which improve the inspection efficiency. In addition, UPA technology has good effects on the coarse-grained material. 6. Conclusions UPA technology has the advantages of convertible beam direction and focus, the results is easy imaging and the detection has better accessibility and applicability. Therefore, it can solve the problems and difficulties of NDT inspection in over-lapped pack structure and coarse grain materials testing. The technology can improve the accuracy of ultrasonic testing and the reliability of test results, ensure the safe and economic operation of urea reactor. Acknowledgements This work is supported by Key Program of Science and Technology Planning Project of Zhejiang Province, China under Grant 2011C References [1] Wu Junfei, Zhu Xiaochen, Zhang Yanfei, Yu Benliang, Gao Wei, Theoretical research & numerical simulation on the effect of urea-reactor layer plate gap on stress distribution. Mechanic Automation and Control Engineering (MACE), International Conference on, pp [2] Raj B., Jayakumar T., Thavasimuthu, Practical Non-destructivetesting, Woodhead Publishing.New Delhi,INDIA. [3] Shili Chen, Xianglin Zhan, Shijiu Jin, Design of Ultrasonic Phased Array System on Pipeline Girth Weld Inspection. Journal of Tianjin Univ 39, p [4] Xianglin Zhan, Shijiu Jin,2005. Ultrasonic Inspection of Stainless Steel Welds. Journal of Nondestructive Inspection 29, p.12. [5] Zheng Zhongxing, A Fractal-Based Flaw Feature Extraction Method for Ultrasonic Phased Array Nondestructive Testing. Proceedings of the 2009 IEEE International Conference on Mechatronics and Automation August 9-12, Changchun, China. [6] Abittan Elie, Inspection of thermal barriers of primary pumps with phased array probe and piezocomposite technology. Journal of Nondestructive Testing 5. [7] Mahauts, Godefroit J. L., Roy O., Application of Phased Array Techniques to Coarse Grain Components Inspection. Journal of Ultrasonic 42, p [8] Steve Mahaut, Olivier Roy, Sylvain Chatillon, et al, 2001.Application of the Simulation to the Conception of Phased Array Mathods, 8th ECNDT, Barcelone. [9] Austeng A.,Holm S.,2002.Sparse 2-D Arrays for 3-D Phased Array Imaging: Design Methods, Journal of IEEE Trans. Ultrason. Ferro. Freq. Contr. 49, p [10] Azar L., Shi Y., Wooh S.C., Beam Focusing Behavior of Linear Phased Array. Journal of NDT & E Intern 33,p.189.