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Failure analysis on abnormal bursting of heat transfer tubes in spiral-wound heat exchanger for nuclear power plant
Engineering Failure Analysis xxx (xxxx) xxxx
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis on abnormal bursting of heat transfer tubes in spiral-wound heat exchanger for nuclear power plant Xiao-Lei Yanga, Yi Gonga, Qi Tongb, Zhen-Guo Yanga,
⁎
a b
Department of Materials Science, Fudan University, Shanghai 200433, PR China Department of Aeronautics and Astronautics, Fudan University, Shanghai 200433, PR China
ARTICLE INFO
ABSTRACT
Keywords: Failure analysis Stress corrosion cracking Spiral-wound heat exchanger Nuclear power plant
A spiral-wound heat exchanger failed to work during the reliability test of nuclear power plant and abnormal bursting of the heat transfer tubes was found after disassembling inspection. To solve this problem, a series of macroscopic and microscopic analysis methods were conducted, and finite element analysis was also used to analyze the stress distribution of the failed heat transfer tube. The results indicated that the stress corrosion cracking (SCC) caused by the interaction of the tensile stress induced by inappropriate fabrication, the aggressive chloride and hot alkaline environment from the media and the susceptible austenite stainless steel materials degraded by improper surface treatment was the root cause of this failure. The failure mechanisms were discussed and the relevant countermeasures were proposed to prevent the occurrence of the similar failure again. Considering that the failure case of spiral-wound heat exchanger was reported rarely so far, the achievement of this paper would provide useful experience for the safety maintenance management of spiral-wound heat exchanger in the future.
1. Introduction Spiral-wound heat exchangers (SWHEs) have been widely applied for heat transfer in nuclear power plants due to the high heat transfer efficiency, high compactness and excellent adaptability to variable loads [1,2]. The special structure of spiral-wound heat exchanger that the wound tubes are wrapped around the core tube in opposite winding directions between two adjacent layers facilitates secondary circulation in tube side flow, which enlarges the heat transfer area and therefore improves the heat transfer efficiency of the spiral-wound heat exchangers [3]. However, this complex spiral structure also makes the flow characteristics more complicated and increases the difficulties in manufacturing and fabrication processes, which will bring hidden threats to safe operation and restrict its further application in nuclear power plants. Therefore, more attention should be paid to understand the flow characteristics and operational reliability of spiral-wound heat exchangers. Previously, some researches have been carried out to investigate the flow characteristics and the optimal design of spiral-wound heat exchangers. Ren et al. [4] investigated the shell side heat transfer characteristic of spiral-wound heat exchanger and the influence of working parameters by a numerical method. Wu et al. [5] discussed the influence of main structural parameters like thickness of the spacer, winding angle and axial tube spacing on heat transfer property and pressure drop in order to optimize the design of spiral-wound heat exchanger. Lu et al. [6] studied the influences of the changes in geometrical factors on flow and heat transfer performances in multi-stream spiral-wound heat exchangers by numerical methods. However, most of the existing studies on spiral-wound heat exchangers were carried out by experimental and numerical methods, while the actual cases from engineering ⁎
Corresponding author. E-mail address: [email protected] (Z.-G. Yang).
https://doi.org/10.1016/j.engfailanal.2019.104298 Received 30 May 2019; Received in revised form 18 July 2019; Accepted 4 November 2019 1350-6307/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Xiao-Lei Yang, et al., Engineering Failure Analysis, https://doi.org/10.1016/j.engfailanal.2019.104298
Engineering Failure Analysis xxx (xxxx) xxxx
X.-L. Yang, et al.
Fig. 1. The appearance of the failed spiral-wound heat exchanger: (a) Appearance, (b) The location of the bursted tube.
practices were rarely reported. In actual working conditions, the improper material selection, inappropriate fabrication, aggressive working environment and untimely maintenance are all likely to cause premature failure of the spiral-wound heat exchangers. Therefore, the comprehensive analysis on the practical failure cases of spiral-wound heat exchangers is a great concern for the safe operation in engineering. In this study, an unexpected failure case of bursting of the heat transfer tubes in a spiral-wound heat exchanger that is used for reliability test of nuclear power plants was reported. During the test, the pressure of the system dropped suddenly. After disassembling of it, the broken heat exchanger tube was found (hereinafter referred as ‘the bursted tube’). The appearance of the failed spiral-wound heat exchanger and the location of the bursted tube are shown in Fig. 1. It is generally believed that the wounded tube is the vulnerable section because of the residual stress induced from the winding process. However, a rare failure case of the straight tube section is shown in Fig. 1, which should be paid special attention to. The heat transfer tube is made of 304L and the relevant working parameters are listed in Table 1. Based on our previous experience of failure analysis on nuclear power plant equipment and heat transfer tubes [7–10], a series of experiments were conducted to find out the root cause of this failure. Some practical countermeasures were also put forward to avoid the similar failure happened again. Since only a few actual cases of failed spiral-wound heat exchangers have been analyzed and reported, the findings of this study can provide direct guidelines for the safe operation and failure prevention of spiral-wound heat exchangers in the future. 2. Experiments and results 2.1. Visual examination Fig. 2 exhibits the appearance inside the failed spiral-wound heat exchanger after disassembling. As seen in Fig. 2 (a) and (b), the heat transfer tubes are covered by surface deposits. It is notable that there are many deposit sediments between the heat transfer tubes and on the bottom of the shell (as shown in Fig. 2 (c) and (d)). The source and the constituent of these deposits would be discussed later. After comprehensive examination, leak point was found 350 mm away from the tube sheet. The appearance of the bursted tube is shown in Fig. 3. As displayed in Fig. 3 (a), the break located on the upper of the bursted tube where abnormal bend deformation occurred. After measurement, we can see the maximum bend deformation is about 20 mm, while the width and the length of the break is 5 mm and 55 mm, respectively. As shown in Fig. 3 (d), the outer wall of the bursted tube was covered by a layer of surface deposits except for the area around the break. Besides, there were many pits and cracks surrounding the break. 2.2. Examination of deposit sediment X-ray diffraction was used to identify the constituent of the deposit sediments and the result is presented in Fig. 4. The results showed that the mixture of FeOOH, Fe, iron oxide and CaCO3 is the main constituent of the deposit sediment. However, no chromium compounds were detected in these deposit sediment. After communication with the personnel from the nuclear power plant, we learned that the upstream of the spiral-wound heat exchanger is the pipeline made of carbon steel, indicating that these deposit sediment might be formed in the upstream and then transported to the heat exchanger by the flow. Table 1 The working parameters of the failed spiral-wound heat exchanger. Working parameters
Medium
Pressure (MPa)
Peak flow rate (m/s)
Temperature of hot end (°C)
Temperature of cold end (°C)
Shell side Tube side
Cooling water Deionized water
< 0.5 15.0
1.05 2.17
33.0 284.3
48.0 251.1
2
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Fig. 2. The appearance inside the spiral-wound heat exchanger: (a) hot end, (b) cold end, (c) deposit sediment between heat transfer tubes, (d) deposit sediment on the bottom of the shell.
Fig. 3. Visual examination of the bursted tube: (a) appearance of the bursted tube, (b) bend deformation, (c) appearance of the break, (d) pits and cracks around the break.
2.3. Metal matrix examination 2.3.1. Chemical compositions Chemical compositions of the failed bursted tube were inspected and the results are listed in Table 2. Compared with the standard, the chemical compositions were in accordance with the requirement of 304L.
3
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Fig. 4. XRD of the deposit sediment. Table 2 The chemical compositions of the bursted tube. Element
C
Si
Mn
P
S
Cr
Ni
304L [11,12] The bursted tube
≤0.03 0.019
≤1.00 0.27
≤2.00 1.67
≤0.035 0.031
≤0.030 0.001
18.00–20.00 18.53
8.00–12.00 8.16
2.3.2. Metallographic structures The specimen was polished and etched to observe the metallographic structure of the bursted tube. As shown in Fig. 5 (a), the inclusion classification of bursted tube specimen is D1, DS0.5 [13,14]. Fig. 5(b) shows that the metallographic structure of bursted tube is composed of austenite, which is the characteristic microstructure of the austenite stainless steel. 2.3.3. Mechanical property Tensile test was conducted and the results are listed in Table 3. The yield strength of bursted tube is 255 MPa, the tensile strength is 576 MPa and the elongation is 56.5%. The results show that the mechanical properties of the bursted tube are qualified, which indicates that the bursted tube should have performed well under normal service condition and there might be some abnormal factors during its operation. As mentioned above, the results of chemical compositions, metallographic morphologies and mechanical properties prove that the material of the bursted tube is qualified 304L, suggesting that unqualified material is not the reason for the premature failure. However, the abnormal bend deformation that should not happen under normal working condition indicates the bursted tube was applied excessively high stress during operation and the source of the stress should be identified later. 2.4. Three-dimensional stereo microscope (3D-SM) 3D-SM was used to further observe the morphologies of the break of the bursting tube. As shown in Fig. 6, the direction of the break parallels to the axial direction of the bursted tube. In addition, branched cracking was observed initiating and propagating along the pits on the outer surface of the bursted tube. These featured morphologies [15] indicate that the burst of the heat transfer tube might be related to stress corrosion cracking (SCC).
Fig. 5. Metallographic structure of the bursted tube: (a) after being polished, (b) after being etched. 4
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Table 3 Mechanical property of the bursted tube. Mechanical property
Yield strength Rp0.2 (MPa)
Tensile strength Rm (MPa)
Elongation A (%)
304L [11,12] Bursting tube
≥175 255
≥480 576
≥35.0 56.5
Fig. 6. 3D-SM of the break of the bursted tube.
2.5. SEM analysis The microscopic morphology of the cross-section of the break is shown in Fig. 7. As shown in Fig. 7(a), the break of the bursted tube could be divided into five segments and after magnification, obvious crackings were observed initiating from outer wall of the bursted tube and propagating through the tube wall (as shown in Fig. 7(b)). These crackings initiated and propagated from different points at the same time and therefore caused such serious bursting of the bursted tube. Based on the results that the crackings all initiated from the outer wall, further analysis of the outer wall should be conducted. The microscopic morphology of the outer surface of the break is shown in Fig. 8. The break exhibits the characteristics of embrittlement and no obvious tube wall thinning was found. As displayed in Fig. 8(b) and (c), the edges of the break propagate along cracks that initiated from the outer wall of the bursted tube, After magnification, it is confirmed that branched cracking near the tips of the crack (in Fig. 8(d)) propagated along the grain boundaries. Rotate the specimen to some angle to observe the area where is away from the break. As shown in Fig. 8(f) and (g), it is surprising that though no micro-cracking was found, obvious grain boundaries were still observed. 2.6. Medium quality inspection As shown in Fig. 9, two kinds of media exist in the spiral-wound heat exchanger, the circulating cooling water transports in the shell side while the deionized water flows in the tube side. Hence, inductively coupled plasma-atomic emission spectrometry (ICPAES) and ion chromatograph (IC) analysis were applied to inspect the quality of these two media. The cation compositions of circulating cooling water and deionized water were testified by ICP-AES. As listed in Table 4, calcium, sodium and magnesium were found in circulating cooling water in the shell side. These high concentration alkaline and alkaline earth element are likely to cause scaling and corrosion of stainless steel. The anion compositions of these two media were analyzed by IC. 1.6 ppm of chloride ion was detected in the tube side flow and 88.2 ppm of chloride ion was found in the shell side flow. The concentration of Cl− is far higher than the generally accepted threshold that will initiate stress corrosion cracking (10 ppm in high temperature [16]). The results further confirmed that the stress corrosion cracking occurred in hot chloride and alkaline solutions is the root cause of the failure of bursting tube. 3. Failure analysis and discussion As exhibited in Fig. 6 and Fig. 8, characteristic stress corrosion cracking morphologies were observed near the break of the bursted tube. Meanwhile, high concentration of chloride ion and alkaline element were detected in the cooling water of shell side and formed corrosive environment. Therefore, it is supposed that stress corrosion cracking (SCC) is the root cause of this failure. It is well known that austenitic stainless steel is susceptible to stress corrosion cracking (SCC) when it is subject to the tensile stress under specific aggressive environment. To be specific, the occurrence of SCC required three factors: the specific material, the corrosive environment 5
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Fig. 7. Microscopic morphology of the cross section of the break: (a) the overall morphology of the cross section, (b) cracking originated from the outer surface.
and tensile stress condition. In this case, the bend deformation occurred on the straight section of the bursted tube under abnormal applied load. And as shown in Fig. 10, the bend deformation phenomenon of heat transfer tubes is universal and the location of the bend heat transfer tubes is not all around the bursted tube but distributed randomly. So we can assume that the bend deformation of the bursted tube is not caused by the bursting but is the result from inappropriate fabrication. Finite element analysis was conducted to further analyze the stress distribution of the bend bursted tube under applied load. The right end of the bursted tube was fixed by tube sheet, axial load was applied on the left side because of inappropriate fabrication and led to the bending deformation of the bursted tube. The simulation was implemented by ABAQUS software. The element type is S4R and the approximate global size of the elements is 0.1. The constitutive model for the material is elastic-plastic with Young’s modulus 210 GPa, Poisson’s ratio 0.3 and yield strength 275 MPa. A postbuckling analysis was conducted by introducing an imperfection based on a pre-step of linear perturbation. The postbuckling results are presented in Fig. 11, it is obvious that the stress near the maximum bend deformation and the two ends are higher than the other areas. The maximum bend deformation area coincides with the break area of the bursted tube. The specific stress schematic diagram of the maximum bend deformation area is shown in Fig. 12, the applied load caused axial compressive stress on the maximum bend deformation area. Due to the volume invariant law, the reduction of the length of axial direction inevitably led to the increase of the length of circumferential direction, which indicated the existence of circumferential tensile stress of the maximum 6
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Fig. 8. Microscopic morphology of the outer wall of the bursted tube: (a) appearance, (b) magnification of the break, (c) cracking originated from the outer surface, (d) branched cracking, (e) magnification of the branched cracking, (f) area away from the break, (g) magnification of the area of (f).
Fig. 9. Schematic diagram of the failed spiral-wound heat exchanger.
bend deformation area. The result coincides with the fact that the direction of the break is perpendicular to the direction of tensile stress. Besides, the material of the bursted tube is 304L stainless steel, which is susceptible to SCC [17]. What’s more, according to Fig. 8, the cracking propagated along the grain boundary and abnormal bared grain boundary could be observed on the outer wall. 7
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Table 4 ICP-AES results of media in shell side and tube side. Element (ppm)
Ca
Na
Mg
Tube side Shell side
< 0.02 71.24
< 0.02 30.16
< 0.02 15.19
Fig. 10. The bend deformation phenomenon of the heat transfer tubes.
Fig. 11. The stress distribution result of finite element analysis.
Fig. 12. The schematic diagram of stress analysis of the maximum bend deformation area.
Considering the large area of the bared grain boundary, we can suspect that the surface treatment process might be improper. Acid pickling and passivation are the common surface treatment methods used to remove the surface impurities and improve the corrosion resistance of the stainless steel by forming protective film [18]. However, if the acid pickling and passivation processes of the stainless 8
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steel is improper, it would even deteriorate the surface condition of the stainless steel and decrease the corrosion resistance. After communication with related manufacturers of the heat transfer tubes, we found that the pickling and passivation process was onestep process instead of two-step process as common. The improper pickling and passivation process could not achieve the expected goal, even break grain boundary and cause defects on the surface, which makes the material more susceptible to SCC. In addition, the media inspection results show large amounts of chloride ions and alkaline ions contained in cooling water in shell side. The chloride ions triggered the electrochemical corrosion on surface defects like bared grain boundaries and formed pits. The aggressive Cl− exert destructive influence on the composition and properties of the passive film and promote the dissolving of steel. The dissolution of matrix from anodic sites could be described by the reaction [19]. 3Fe + 4H2O → Fe3O4 + 8H+ + 8e− +
(1)
−
Ni + H2O → NiO + 2H + 2e +
(2) −
2Cr + 3H2O → Cr2O3 + 6H + 6e
(3)
These anodic reactions were balanced by cathodic reaction: 2H2O + O2 + 4e → 4(OH)−
(4)
The dissolution of matrix formed pits that served as the initial site of crack along grain boundaries. The presence of alkaline elements and the corrosion products from upper steam deposited inside the pits over the time. The layer of incrustation and corrosion product limited the migration and diffusion of chloride ions, which increase the corrosion extent and then degrade the mechanical properties of the bursted tube. Then the pits formed along the grain boundaries on the surface became the stress concentrate sites under applied stress. According to previous research the dissolution of grain boundaries are chemical active paths for crack propagation [20]. Therefore, the crack formed and grew along the pits in a intergranular mode. The growth of crack caused the exposure of new metal surface under corrosive environment and led intensive corrosion. As the interaction of corrosive environment and mechanical effect, branched cracking formed and caused break of the bursted tube eventually. As shown in Fig. 13, the bursted tube underwent four stages: the first stage is the exposure of grain boundary, the second stage is the initiation of pits, the third stage is the formation and growth of crack along the grain boundary, the last stage is the avalanche-like development of the crack and the break of the bursted tube. In a nutshell, the failure of the bursted tube resulted from stress corrosion cracking induced by the applied tensile stress, aggressive and corrosive environment and the degraded susceptible materials. 4. Conclusions and countermeasures In this study, the failure analysis of abnormal bursting of heat transfer tubes in a spiral-wound heat exchanger was performed. To identify the root cause of this failure, optical emission spectrometer, metallographic microscope and tensile test were used to investigate the chemical compositions, metallographic structures and mechanical property of the bursted tube. Three-dimensional stereo microscope and scanning electron microscope were utilized to analyze the morphologies of the bursted tube on both macroscopic and microscopic scales. X-ray diffraction was employed to analyze the constituent of deposit sediments. Besides, medium quality inspection was conducted by inductively coupled plasma-atomic emission spectrometry and ion chromatograph analysis. Finite element analysis was also used to further analyze the stress distribution of the bursted tube. Based on the analysis above, the following conclusions were formulated:
Fig. 13. Schematic diagram of the failure mechanism of the bursted tube. 9
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1. The chemical compositions, metallographic structures and mechanical properties of the bursted tube were in accordance with the requirements of 304L and not the causes of this failure. 2. The stress corrosion cracking caused by the interaction of applied tensile stress induced by inappropriate fabrication, aggressive chloride and hot alkaline environment as well as austenitic stainless steel degraded by improper surface treatment was the root cause of this failure. 3. The corrosion products from upstream carbon steel pipeline deposited on the outer wall of heat transfer tubes and enhanced corrosion extend. That was another essential cause of this failure. In terms of the conclusions mentioned above, the following countermeasures were put forward: 1. The fabrication of the spiral-wound heat exchanger should follow the standards and requirements to avoid the applied stress caused by inappropriate fabrication. 2. Control the quality of media and limit the introduction of aggressive chloride ions. If the media environment is unchangable, the selection of the material of heat transfer tubes should have high corrosion resistant ability and could operate well under the media. 3. Improve the surface treatment process of the heat transfer tubes to make sure the formation of protective films and avoid the unexpected localized corrosion during the process. Declaration of Competing Interest The authors declared that there is no conflict of interest. References [1] Y.D. Chen, X.D. Chen, H.F. Jiang, C.G. Chen, X.A. Zhang, Design technology of large-scale spiral wound heat exchanger in refinery industry, Proc. Eng. 130 (2015) 286–297. [2] Y. Ren, W.H. Cai, J. Chen, L.Y. Lu, Y.Q. Jiang, Numerical study on the shell-side flow and heat transfer of superheated vapor flow in spiral wound heat exchanger under rolling working conditions, Int. J. Heat Mass Transf. 121 (2018) 691–702. [3] X. Lu, X. Du, M. Zeng, et al., Shell-side thermal-hydraulic performances of multilayer spiral-wound heat exchangers under different wall thermal boundary conditions, Appl. Therm. Eng. 70 (2014) 1216–1227. [4] Y. Ren, W.H. Cai, J. Chen, et al., The heat transfer characteristic of shell-side film flow in spiral wound heat exchanger under rolling working conditions, Appl. Therm. Eng. 132 (2018) 233–244. [5] J. Wu, S. Liu, M. Wang, Process calculation method and optimization of the spiral-wound heat exchanger with bilateral phase change, Appl. Therm. Eng. 134 (2018) 360–368. [6] X. Lu, G. Zhang, Y.T. Chen, et al., Effect of geometrical parameters on flow and heat transfer performances in multi-stream spiral-wound heat exchangers, Appl. Therm. Eng. 89 (2015) 1104–1116. [7] Y. Gong, Q. Ding, Z.G. Yang, Failure analysis on premature fracture of anchor bolts in seawater booster pump of nuclear power plant, Eng. Fail. Anal. 97 (2019) 10–19. [8] T.T. Bi, Z.G. Yang, Failure analysis on speed reducer shaft of sluice gate in nuclear power plant, Eng. Fail. Anal. 80 (2017) 453–463. [9] F.J. Chen, C. Yao, Z.G. Yang, Failure analysis on abnormal wall thinning of heat-transfer titanium tubes of condensers in nuclear power plant Part I: Corrosion and wear, Eng. Fail. Anal. 37 (2014) 29–41. [10] F.J. Chen, C. Yao, Z.G. Yang, Failure analysis on abnormal wall thinning of heat -transfer titanium tubes of condensers in nuclear power plant Part II: Erosion and cavitation corrosion, Eng. Fail. Anal. 37 (2014) 42–52. [11] GB/T 13296-2013, Seamless stainless steel tubes for boiler and heat exchanger, CN-GB, China, 2013. [12] ASTM A312/312M-17 Standard specification for seamless, welded, and heavily cold worked austenitic stainless steel pipes. [13] GB/T 10561–2005 Steel- Determination of content of nonmetallic inclusions- Micrographic method using standards diagrams, CN-GB, China, 2005. [14] ISO 4967 Steel- Determination of content of non-metallic inclusions- Micrographic method using standard diagrams, 2013. [15] S.G. Xu, C. Wang, W.Q. Wang, Failure analysis of stress corrosion cracking in heat exchanger tubes during start-up operation, Eng. Fail. Anal. 51 (2015) 1–8. [16] S.K. Wang, S.G. Xu, S.J. Huang, Failure analysis of authentic stainless steel tubes in a vertical fixed shell-tube heat exchanger, J. Fail. Anal. Prev. 18 (2018) 405–412. [17] M. Hamzeh, M.M. Karkehabadi, R. Jalali, Failure analysis of stress corrosion cracking of 316L structured packing in a distillation tower, Eng. Fail. Anal. 79 (2017) 431–440. [18] S.N. Geng, J.S. Sun, L.Y. Guo, Effect of sandblasting and subsequent acid pickling and passivation on the microstructure and corrosion behavior of 316L stainless steel, Mater. Des. 88 (2015) 1–7. [19] D.H. Du, K. Chen, H. Liu, et al., Effects of chloride and oxygen on stress corrosion cracking of cold worked 316/316L austenitic stainless steel in high temperature water, Corros. Sci. 110 (2016) 134–142. [20] M. Ananda Rao, R. Sekhar Babu, M.V. Pavan Kumar, Stress corrosion cracking failure of a SS 316L high pressure heater tube, Eng. Failure Analy. 90 (2018) 14–22.
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Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis on abnormal bursting of heat transfer tubes in spiral-wound heat exchanger for nuclear power plant Xiao-Lei Yanga, Yi Gonga, Qi Tongb, Zhen-Guo Yanga,
⁎
a b
Department of Materials Science, Fudan University, Shanghai 200433, PR China Department of Aeronautics and Astronautics, Fudan University, Shanghai 200433, PR China
ARTICLE INFO
ABSTRACT
Keywords: Failure analysis Stress corrosion cracking Spiral-wound heat exchanger Nuclear power plant
A spiral-wound heat exchanger failed to work during the reliability test of nuclear power plant and abnormal bursting of the heat transfer tubes was found after disassembling inspection. To solve this problem, a series of macroscopic and microscopic analysis methods were conducted, and finite element analysis was also used to analyze the stress distribution of the failed heat transfer tube. The results indicated that the stress corrosion cracking (SCC) caused by the interaction of the tensile stress induced by inappropriate fabrication, the aggressive chloride and hot alkaline environment from the media and the susceptible austenite stainless steel materials degraded by improper surface treatment was the root cause of this failure. The failure mechanisms were discussed and the relevant countermeasures were proposed to prevent the occurrence of the similar failure again. Considering that the failure case of spiral-wound heat exchanger was reported rarely so far, the achievement of this paper would provide useful experience for the safety maintenance management of spiral-wound heat exchanger in the future.
1. Introduction Spiral-wound heat exchangers (SWHEs) have been widely applied for heat transfer in nuclear power plants due to the high heat transfer efficiency, high compactness and excellent adaptability to variable loads [1,2]. The special structure of spiral-wound heat exchanger that the wound tubes are wrapped around the core tube in opposite winding directions between two adjacent layers facilitates secondary circulation in tube side flow, which enlarges the heat transfer area and therefore improves the heat transfer efficiency of the spiral-wound heat exchangers [3]. However, this complex spiral structure also makes the flow characteristics more complicated and increases the difficulties in manufacturing and fabrication processes, which will bring hidden threats to safe operation and restrict its further application in nuclear power plants. Therefore, more attention should be paid to understand the flow characteristics and operational reliability of spiral-wound heat exchangers. Previously, some researches have been carried out to investigate the flow characteristics and the optimal design of spiral-wound heat exchangers. Ren et al. [4] investigated the shell side heat transfer characteristic of spiral-wound heat exchanger and the influence of working parameters by a numerical method. Wu et al. [5] discussed the influence of main structural parameters like thickness of the spacer, winding angle and axial tube spacing on heat transfer property and pressure drop in order to optimize the design of spiral-wound heat exchanger. Lu et al. [6] studied the influences of the changes in geometrical factors on flow and heat transfer performances in multi-stream spiral-wound heat exchangers by numerical methods. However, most of the existing studies on spiral-wound heat exchangers were carried out by experimental and numerical methods, while the actual cases from engineering ⁎
Corresponding author. E-mail address: [email protected] (Z.-G. Yang).
https://doi.org/10.1016/j.engfailanal.2019.104298 Received 30 May 2019; Received in revised form 18 July 2019; Accepted 4 November 2019 1350-6307/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Xiao-Lei Yang, et al., Engineering Failure Analysis, https://doi.org/10.1016/j.engfailanal.2019.104298
Engineering Failure Analysis xxx (xxxx) xxxx
X.-L. Yang, et al.
Fig. 1. The appearance of the failed spiral-wound heat exchanger: (a) Appearance, (b) The location of the bursted tube.
practices were rarely reported. In actual working conditions, the improper material selection, inappropriate fabrication, aggressive working environment and untimely maintenance are all likely to cause premature failure of the spiral-wound heat exchangers. Therefore, the comprehensive analysis on the practical failure cases of spiral-wound heat exchangers is a great concern for the safe operation in engineering. In this study, an unexpected failure case of bursting of the heat transfer tubes in a spiral-wound heat exchanger that is used for reliability test of nuclear power plants was reported. During the test, the pressure of the system dropped suddenly. After disassembling of it, the broken heat exchanger tube was found (hereinafter referred as ‘the bursted tube’). The appearance of the failed spiral-wound heat exchanger and the location of the bursted tube are shown in Fig. 1. It is generally believed that the wounded tube is the vulnerable section because of the residual stress induced from the winding process. However, a rare failure case of the straight tube section is shown in Fig. 1, which should be paid special attention to. The heat transfer tube is made of 304L and the relevant working parameters are listed in Table 1. Based on our previous experience of failure analysis on nuclear power plant equipment and heat transfer tubes [7–10], a series of experiments were conducted to find out the root cause of this failure. Some practical countermeasures were also put forward to avoid the similar failure happened again. Since only a few actual cases of failed spiral-wound heat exchangers have been analyzed and reported, the findings of this study can provide direct guidelines for the safe operation and failure prevention of spiral-wound heat exchangers in the future. 2. Experiments and results 2.1. Visual examination Fig. 2 exhibits the appearance inside the failed spiral-wound heat exchanger after disassembling. As seen in Fig. 2 (a) and (b), the heat transfer tubes are covered by surface deposits. It is notable that there are many deposit sediments between the heat transfer tubes and on the bottom of the shell (as shown in Fig. 2 (c) and (d)). The source and the constituent of these deposits would be discussed later. After comprehensive examination, leak point was found 350 mm away from the tube sheet. The appearance of the bursted tube is shown in Fig. 3. As displayed in Fig. 3 (a), the break located on the upper of the bursted tube where abnormal bend deformation occurred. After measurement, we can see the maximum bend deformation is about 20 mm, while the width and the length of the break is 5 mm and 55 mm, respectively. As shown in Fig. 3 (d), the outer wall of the bursted tube was covered by a layer of surface deposits except for the area around the break. Besides, there were many pits and cracks surrounding the break. 2.2. Examination of deposit sediment X-ray diffraction was used to identify the constituent of the deposit sediments and the result is presented in Fig. 4. The results showed that the mixture of FeOOH, Fe, iron oxide and CaCO3 is the main constituent of the deposit sediment. However, no chromium compounds were detected in these deposit sediment. After communication with the personnel from the nuclear power plant, we learned that the upstream of the spiral-wound heat exchanger is the pipeline made of carbon steel, indicating that these deposit sediment might be formed in the upstream and then transported to the heat exchanger by the flow. Table 1 The working parameters of the failed spiral-wound heat exchanger. Working parameters
Medium
Pressure (MPa)
Peak flow rate (m/s)
Temperature of hot end (°C)
Temperature of cold end (°C)
Shell side Tube side
Cooling water Deionized water
< 0.5 15.0
1.05 2.17
33.0 284.3
48.0 251.1
2
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Fig. 2. The appearance inside the spiral-wound heat exchanger: (a) hot end, (b) cold end, (c) deposit sediment between heat transfer tubes, (d) deposit sediment on the bottom of the shell.
Fig. 3. Visual examination of the bursted tube: (a) appearance of the bursted tube, (b) bend deformation, (c) appearance of the break, (d) pits and cracks around the break.
2.3. Metal matrix examination 2.3.1. Chemical compositions Chemical compositions of the failed bursted tube were inspected and the results are listed in Table 2. Compared with the standard, the chemical compositions were in accordance with the requirement of 304L.
3
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Fig. 4. XRD of the deposit sediment. Table 2 The chemical compositions of the bursted tube. Element
C
Si
Mn
P
S
Cr
Ni
304L [11,12] The bursted tube
≤0.03 0.019
≤1.00 0.27
≤2.00 1.67
≤0.035 0.031
≤0.030 0.001
18.00–20.00 18.53
8.00–12.00 8.16
2.3.2. Metallographic structures The specimen was polished and etched to observe the metallographic structure of the bursted tube. As shown in Fig. 5 (a), the inclusion classification of bursted tube specimen is D1, DS0.5 [13,14]. Fig. 5(b) shows that the metallographic structure of bursted tube is composed of austenite, which is the characteristic microstructure of the austenite stainless steel. 2.3.3. Mechanical property Tensile test was conducted and the results are listed in Table 3. The yield strength of bursted tube is 255 MPa, the tensile strength is 576 MPa and the elongation is 56.5%. The results show that the mechanical properties of the bursted tube are qualified, which indicates that the bursted tube should have performed well under normal service condition and there might be some abnormal factors during its operation. As mentioned above, the results of chemical compositions, metallographic morphologies and mechanical properties prove that the material of the bursted tube is qualified 304L, suggesting that unqualified material is not the reason for the premature failure. However, the abnormal bend deformation that should not happen under normal working condition indicates the bursted tube was applied excessively high stress during operation and the source of the stress should be identified later. 2.4. Three-dimensional stereo microscope (3D-SM) 3D-SM was used to further observe the morphologies of the break of the bursting tube. As shown in Fig. 6, the direction of the break parallels to the axial direction of the bursted tube. In addition, branched cracking was observed initiating and propagating along the pits on the outer surface of the bursted tube. These featured morphologies [15] indicate that the burst of the heat transfer tube might be related to stress corrosion cracking (SCC).
Fig. 5. Metallographic structure of the bursted tube: (a) after being polished, (b) after being etched. 4
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Table 3 Mechanical property of the bursted tube. Mechanical property
Yield strength Rp0.2 (MPa)
Tensile strength Rm (MPa)
Elongation A (%)
304L [11,12] Bursting tube
≥175 255
≥480 576
≥35.0 56.5
Fig. 6. 3D-SM of the break of the bursted tube.
2.5. SEM analysis The microscopic morphology of the cross-section of the break is shown in Fig. 7. As shown in Fig. 7(a), the break of the bursted tube could be divided into five segments and after magnification, obvious crackings were observed initiating from outer wall of the bursted tube and propagating through the tube wall (as shown in Fig. 7(b)). These crackings initiated and propagated from different points at the same time and therefore caused such serious bursting of the bursted tube. Based on the results that the crackings all initiated from the outer wall, further analysis of the outer wall should be conducted. The microscopic morphology of the outer surface of the break is shown in Fig. 8. The break exhibits the characteristics of embrittlement and no obvious tube wall thinning was found. As displayed in Fig. 8(b) and (c), the edges of the break propagate along cracks that initiated from the outer wall of the bursted tube, After magnification, it is confirmed that branched cracking near the tips of the crack (in Fig. 8(d)) propagated along the grain boundaries. Rotate the specimen to some angle to observe the area where is away from the break. As shown in Fig. 8(f) and (g), it is surprising that though no micro-cracking was found, obvious grain boundaries were still observed. 2.6. Medium quality inspection As shown in Fig. 9, two kinds of media exist in the spiral-wound heat exchanger, the circulating cooling water transports in the shell side while the deionized water flows in the tube side. Hence, inductively coupled plasma-atomic emission spectrometry (ICPAES) and ion chromatograph (IC) analysis were applied to inspect the quality of these two media. The cation compositions of circulating cooling water and deionized water were testified by ICP-AES. As listed in Table 4, calcium, sodium and magnesium were found in circulating cooling water in the shell side. These high concentration alkaline and alkaline earth element are likely to cause scaling and corrosion of stainless steel. The anion compositions of these two media were analyzed by IC. 1.6 ppm of chloride ion was detected in the tube side flow and 88.2 ppm of chloride ion was found in the shell side flow. The concentration of Cl− is far higher than the generally accepted threshold that will initiate stress corrosion cracking (10 ppm in high temperature [16]). The results further confirmed that the stress corrosion cracking occurred in hot chloride and alkaline solutions is the root cause of the failure of bursting tube. 3. Failure analysis and discussion As exhibited in Fig. 6 and Fig. 8, characteristic stress corrosion cracking morphologies were observed near the break of the bursted tube. Meanwhile, high concentration of chloride ion and alkaline element were detected in the cooling water of shell side and formed corrosive environment. Therefore, it is supposed that stress corrosion cracking (SCC) is the root cause of this failure. It is well known that austenitic stainless steel is susceptible to stress corrosion cracking (SCC) when it is subject to the tensile stress under specific aggressive environment. To be specific, the occurrence of SCC required three factors: the specific material, the corrosive environment 5
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Fig. 7. Microscopic morphology of the cross section of the break: (a) the overall morphology of the cross section, (b) cracking originated from the outer surface.
and tensile stress condition. In this case, the bend deformation occurred on the straight section of the bursted tube under abnormal applied load. And as shown in Fig. 10, the bend deformation phenomenon of heat transfer tubes is universal and the location of the bend heat transfer tubes is not all around the bursted tube but distributed randomly. So we can assume that the bend deformation of the bursted tube is not caused by the bursting but is the result from inappropriate fabrication. Finite element analysis was conducted to further analyze the stress distribution of the bend bursted tube under applied load. The right end of the bursted tube was fixed by tube sheet, axial load was applied on the left side because of inappropriate fabrication and led to the bending deformation of the bursted tube. The simulation was implemented by ABAQUS software. The element type is S4R and the approximate global size of the elements is 0.1. The constitutive model for the material is elastic-plastic with Young’s modulus 210 GPa, Poisson’s ratio 0.3 and yield strength 275 MPa. A postbuckling analysis was conducted by introducing an imperfection based on a pre-step of linear perturbation. The postbuckling results are presented in Fig. 11, it is obvious that the stress near the maximum bend deformation and the two ends are higher than the other areas. The maximum bend deformation area coincides with the break area of the bursted tube. The specific stress schematic diagram of the maximum bend deformation area is shown in Fig. 12, the applied load caused axial compressive stress on the maximum bend deformation area. Due to the volume invariant law, the reduction of the length of axial direction inevitably led to the increase of the length of circumferential direction, which indicated the existence of circumferential tensile stress of the maximum 6
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Fig. 8. Microscopic morphology of the outer wall of the bursted tube: (a) appearance, (b) magnification of the break, (c) cracking originated from the outer surface, (d) branched cracking, (e) magnification of the branched cracking, (f) area away from the break, (g) magnification of the area of (f).
Fig. 9. Schematic diagram of the failed spiral-wound heat exchanger.
bend deformation area. The result coincides with the fact that the direction of the break is perpendicular to the direction of tensile stress. Besides, the material of the bursted tube is 304L stainless steel, which is susceptible to SCC [17]. What’s more, according to Fig. 8, the cracking propagated along the grain boundary and abnormal bared grain boundary could be observed on the outer wall. 7
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Table 4 ICP-AES results of media in shell side and tube side. Element (ppm)
Ca
Na
Mg
Tube side Shell side
< 0.02 71.24
< 0.02 30.16
< 0.02 15.19
Fig. 10. The bend deformation phenomenon of the heat transfer tubes.
Fig. 11. The stress distribution result of finite element analysis.
Fig. 12. The schematic diagram of stress analysis of the maximum bend deformation area.
Considering the large area of the bared grain boundary, we can suspect that the surface treatment process might be improper. Acid pickling and passivation are the common surface treatment methods used to remove the surface impurities and improve the corrosion resistance of the stainless steel by forming protective film [18]. However, if the acid pickling and passivation processes of the stainless 8
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steel is improper, it would even deteriorate the surface condition of the stainless steel and decrease the corrosion resistance. After communication with related manufacturers of the heat transfer tubes, we found that the pickling and passivation process was onestep process instead of two-step process as common. The improper pickling and passivation process could not achieve the expected goal, even break grain boundary and cause defects on the surface, which makes the material more susceptible to SCC. In addition, the media inspection results show large amounts of chloride ions and alkaline ions contained in cooling water in shell side. The chloride ions triggered the electrochemical corrosion on surface defects like bared grain boundaries and formed pits. The aggressive Cl− exert destructive influence on the composition and properties of the passive film and promote the dissolving of steel. The dissolution of matrix from anodic sites could be described by the reaction [19]. 3Fe + 4H2O → Fe3O4 + 8H+ + 8e− +
(1)
−
Ni + H2O → NiO + 2H + 2e +
(2) −
2Cr + 3H2O → Cr2O3 + 6H + 6e
(3)
These anodic reactions were balanced by cathodic reaction: 2H2O + O2 + 4e → 4(OH)−
(4)
The dissolution of matrix formed pits that served as the initial site of crack along grain boundaries. The presence of alkaline elements and the corrosion products from upper steam deposited inside the pits over the time. The layer of incrustation and corrosion product limited the migration and diffusion of chloride ions, which increase the corrosion extent and then degrade the mechanical properties of the bursted tube. Then the pits formed along the grain boundaries on the surface became the stress concentrate sites under applied stress. According to previous research the dissolution of grain boundaries are chemical active paths for crack propagation [20]. Therefore, the crack formed and grew along the pits in a intergranular mode. The growth of crack caused the exposure of new metal surface under corrosive environment and led intensive corrosion. As the interaction of corrosive environment and mechanical effect, branched cracking formed and caused break of the bursted tube eventually. As shown in Fig. 13, the bursted tube underwent four stages: the first stage is the exposure of grain boundary, the second stage is the initiation of pits, the third stage is the formation and growth of crack along the grain boundary, the last stage is the avalanche-like development of the crack and the break of the bursted tube. In a nutshell, the failure of the bursted tube resulted from stress corrosion cracking induced by the applied tensile stress, aggressive and corrosive environment and the degraded susceptible materials. 4. Conclusions and countermeasures In this study, the failure analysis of abnormal bursting of heat transfer tubes in a spiral-wound heat exchanger was performed. To identify the root cause of this failure, optical emission spectrometer, metallographic microscope and tensile test were used to investigate the chemical compositions, metallographic structures and mechanical property of the bursted tube. Three-dimensional stereo microscope and scanning electron microscope were utilized to analyze the morphologies of the bursted tube on both macroscopic and microscopic scales. X-ray diffraction was employed to analyze the constituent of deposit sediments. Besides, medium quality inspection was conducted by inductively coupled plasma-atomic emission spectrometry and ion chromatograph analysis. Finite element analysis was also used to further analyze the stress distribution of the bursted tube. Based on the analysis above, the following conclusions were formulated:
Fig. 13. Schematic diagram of the failure mechanism of the bursted tube. 9
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1. The chemical compositions, metallographic structures and mechanical properties of the bursted tube were in accordance with the requirements of 304L and not the causes of this failure. 2. The stress corrosion cracking caused by the interaction of applied tensile stress induced by inappropriate fabrication, aggressive chloride and hot alkaline environment as well as austenitic stainless steel degraded by improper surface treatment was the root cause of this failure. 3. The corrosion products from upstream carbon steel pipeline deposited on the outer wall of heat transfer tubes and enhanced corrosion extend. That was another essential cause of this failure. In terms of the conclusions mentioned above, the following countermeasures were put forward: 1. The fabrication of the spiral-wound heat exchanger should follow the standards and requirements to avoid the applied stress caused by inappropriate fabrication. 2. Control the quality of media and limit the introduction of aggressive chloride ions. If the media environment is unchangable, the selection of the material of heat transfer tubes should have high corrosion resistant ability and could operate well under the media. 3. Improve the surface treatment process of the heat transfer tubes to make sure the formation of protective films and avoid the unexpected localized corrosion during the process. Declaration of Competing Interest The authors declared that there is no conflict of interest. References [1] Y.D. Chen, X.D. Chen, H.F. Jiang, C.G. Chen, X.A. Zhang, Design technology of large-scale spiral wound heat exchanger in refinery industry, Proc. Eng. 130 (2015) 286–297. [2] Y. Ren, W.H. Cai, J. Chen, L.Y. Lu, Y.Q. Jiang, Numerical study on the shell-side flow and heat transfer of superheated vapor flow in spiral wound heat exchanger under rolling working conditions, Int. J. Heat Mass Transf. 121 (2018) 691–702. [3] X. Lu, X. Du, M. Zeng, et al., Shell-side thermal-hydraulic performances of multilayer spiral-wound heat exchangers under different wall thermal boundary conditions, Appl. Therm. Eng. 70 (2014) 1216–1227. [4] Y. Ren, W.H. Cai, J. Chen, et al., The heat transfer characteristic of shell-side film flow in spiral wound heat exchanger under rolling working conditions, Appl. Therm. Eng. 132 (2018) 233–244. [5] J. Wu, S. Liu, M. Wang, Process calculation method and optimization of the spiral-wound heat exchanger with bilateral phase change, Appl. Therm. Eng. 134 (2018) 360–368. [6] X. Lu, G. Zhang, Y.T. Chen, et al., Effect of geometrical parameters on flow and heat transfer performances in multi-stream spiral-wound heat exchangers, Appl. Therm. Eng. 89 (2015) 1104–1116. [7] Y. Gong, Q. Ding, Z.G. Yang, Failure analysis on premature fracture of anchor bolts in seawater booster pump of nuclear power plant, Eng. Fail. Anal. 97 (2019) 10–19. [8] T.T. Bi, Z.G. 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