gineering Failure Analysis 37(2014)29-41 Contents lists available at ScienceDirect ENGINEERING Engineering Failure Analysis ELSEVIER journalhomepagewww.elsevier.com/locate/engfailanal Failure analysis on abnormal wall thinning of heat-transfer Cross Mark titanium tubes of condensers in nuclear power plant Part 1: Corrosion and wear Fei-Jun Chen, Cheng Yao, Zhen-Guo Yang Department of Materials Science, Fudan University, Shanghai 200433, PR China ARTICLE IN FO A BSTRACT Titanium tubes used in condensers in a nuclear power plant in China encountered abnor ACcepable 24 March 20 mal wall thinning, and was thus forced to temporarily stop operation or it could bring 19 November 2013 online 4 December 2013 about catastrophic safety problems. Most of the wall thinning happened at quite regula positions on the tubes and these failure tubes were located similarly in the condensers, indicating some common problems. To find out the root cause and mechanism of the thin- ning failure, we conducted surface deposit analysis, appearance inspection, microstructure hinning nalysis and composition analysis of the diffraction(XRD). ste. reo microscope, scanning electron microscope( SEM)and Energy Dispersive Spect (EDS). The results revealed that the wall thinning was primarily caused by eccent Failure analysis tact wear and three-body contact wear rooted in processing defect of internal boril rosion products deposit and sagging, and foreign particles. Finally, countermeasure proposed for repair and preventio e 2013 Elsevier Ltd. All rights reserved. 1 Introduction Under the background of power crisis, people are relentlessly looking for new and highly effective powers among which nuclear power is the most popular and feasible one. So the safety of nuclear power stations has ever become the priority of ll China has over a dozen of nuclear power stations under operation, one of which located in the southeast has two 700 Mw CANDU units(unit 1 and unit 2) imported from Atomic Energy of Canada Limited(AECL), the only two pressurized heavy water reactor(PHWR)units in the country. Each unit has a vertical structure with four condensers, shown in Fig. 1. The med- ia inside and outside the tubes (also called tube side and shell side)in the condenser are sea water and high purity water steam respectively. Each condenser has two independent shells connected by steam balance channel. within each shell there are two separated horizontal rows of one way heat transfer tube bundles, each of which has independent water inlet and utlet chambers as well as inlet and outlet dynamoelectric isolation valves. So each tube bundle can be isolated for main- tenance and leak emergency treatment. the working parameters of the condensers are listed in Table 1 Each condenser has 9922 heat transfer tubes that fit together as a tower-like structure shown in Fig. 2. The tubes are made of industrial pure titanium in correspondence to Chinese brand TAl, with the length of 17, 370 mm, and specifications of 25. 4 mm x 0.5 mm(outside diameter x wall thickness). Corresponding author. Tel: +86 21 65642523: fax: +86 21 65103056 1350-6307/s- see front matter o 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.11.003
Failure analysis on abnormal wall thinning of heat-transfer titanium tubes of condensers in nuclear power plant Part I: Corrosion and wear Fei-Jun Chen, Cheng Yao, Zhen-Guo Yang ⇑ Department of Materials Science, Fudan University, Shanghai 200433, PR China article info Article history: Received 24 March 2013 Accepted 19 November 2013 Available online 4 December 2013 Keywords: Titanium tube Wall thinning Corrosion Wear Failure analysis abstract Titanium tubes used in condensers in a nuclear power plant in China encountered abnormal wall thinning, and was thus forced to temporarily stop operation or it could bring about catastrophic safety problems. Most of the wall thinning happened at quite regular positions on the tubes and these failure tubes were located similarly in the condensers, indicating some common problems. To find out the root cause and mechanism of the thinning failure, we conducted surface deposit analysis, appearance inspection, microstructure analysis and composition analysis of the samples by means of X-ray diffraction (XRD), stereo microscope, scanning electron microscope (SEM) and Energy Dispersive Spectrometer (EDS). The results revealed that the wall thinning was primarily caused by eccentric contact wear and three-body contact wear rooted in processing defect of internal borings, corrosion products deposit and sagging, and foreign particles. Finally, countermeasures were proposed for repair and prevention. 2013 Elsevier Ltd. All rights reserved. 1. Introduction Under the background of power crisis, people are relentlessly looking for new and highly effective powers among which nuclear power is the most popular and feasible one. So the safety of nuclear power stations has ever become the priority of all concerns. China has over a dozen of nuclear power stations under operation, one of which located in the southeast has two 700 MW CANDU units (unit 1 and unit 2) imported from Atomic Energy of Canada Limited (AECL), the only two pressurized heavy water reactor (PHWR) units in the country. Each unit has a vertical structure with four condensers, shown in Fig. 1. The media inside and outside the tubes (also called tube side and shell side) in the condenser are sea water and high purity water steam respectively. Each condenser has two independent shells connected by steam balance channel. Within each shell there are two separated horizontal rows of one way heat transfer tube bundles, each of which has independent water inlet and outlet chambers as well as inlet and outlet dynamoelectric isolation valves. So each tube bundle can be isolated for maintenance and leak emergency treatment. The working parameters of the condensers are listed in Table 1. Each condenser has 9922 heat transfer tubes that fit together as a tower-like structure, shown in Fig. 2. The tubes are made of industrial pure titanium in correspondence to Chinese brand TA1, with the length of 17,370 mm, and specifications of 25.4 mm 0.5 mm (outside diameter wall thickness). 1350-6307/$ - see front matter 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.11.003 ⇑ Corresponding author. Tel.: +86 21 65642523; fax: +86 21 65103056. E-mail address: zgyang@fudan.edu.cn (Z.-G. Yang). Engineering Failure Analysis 37 (2014) 29–41 Contents lists available at ScienceDirect Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
F-f. Chen et aL/ Engineering Failure Analysis 37(2014)29-41 E: K2B 2A HF1B 1A G界 NERATOR IEH TURBINE SI ⊥:」 GEN ARRANGEMENT(S-1/20c) T/B ocation of Condenser Fig. 1. Schematic diagram of the arrangement of four condensers in a unit. Working parameters of the condenser Parameters Media Flow rate Q(m/s) Flow velocity Pressure P(kPa) Inlet temperature Outlet temperature Sea water 130.1 Shell side Steam Main steam flow rate 1033 kg/s(3718 T/h) 4-49 Note: The steam temperature in the condenser in operation was 47C(steam drain temperature of low pressure cylinder). Temperature of condensed water around30° Every tube is sustained by 22 perforated support plates made of carbon steel (IS SS400)with thickness of 13 mm and interval distance of 755 mm, and the diameter of the internal borings is 25.6 mm. Two ends of the tubes are welded with titanium clad carbon steel plates(ASTM B265 Gr 1 [ 1+ A515 Gr 65[))whose thickness is 40 mm. It is the titanium steel side that contacts sea water he units started commercial operation in August 2002 with actual runtime of about 8 years. During the 5th overhaul in 2010, part of the tubes were found to have suffered from serious wall thinning and thus temporarily stopped operation. After spection of the failure tubes, it was discovered that the wall thinning mainly happened near the support plates but to var- ious extents and at different positions of the tubes. If the thinned tubes had continued to be used, leaking caused by perfo- ration would have been very likely to happen, which could have brought about catastrophic safety problems. Therefore, were asked by the plant to conduct failure analysis on the abnormal wall thinning of the titanium tubes Previous work on mechanical performance of thin tubes 3, 4 and failure analysis of tubes used in condensers and plants [5,6 has provided some clues to this problem. Our team [7, 8 completed the failure analysis of leakage on titanium tubes ithin heat exchangers in a different phase of the same power plant as in our case. The failure was ascribed to electrochem cal corrosion and mechanical degradation. However, in our case, the thinning dominantly happened on the outer wall of the tubes at quite regular positions, which implied a different story To find out the root cause and mechanism of the thinning failure, we conducted a number of experiments for material, microstructure and chemical composition characterization based on previous successful failure analysis experiences 9-12 2. Experiments and results 2.1. Visual inspection and sampling To find out the cause of abnormal wall thinning of titanium tubes, we investigated condenser 1B of unit 2 under repair on the spot with the focus on the appearance of the support plates, their connection with the tubes and the surface condition of them both in the water inlet chamber. Some obvious defects found are shown in Fig 3. Corrosion extent varied among dif- ferent support plates and the most serious corrosion happened between the tube sheet and the first support plate( ig. 3(a)). oducts deposit was seen on the surface of many tubes near the support plates(ig. 3(a)and(b))
Every tube is sustained by 22 perforated support plates made of carbon steel (JIS SS400) with thickness of 13 mm and interval distance of 755 mm, and the diameter of the internal borings is 25.6 mm. Two ends of the tubes are welded with titanium clad carbon steel plates (ASTM B265 Gr.1 [1] + A515 Gr.65 [2]) whose thickness is 40 mm. It is the titanium steel side that contacts sea water. The units started commercial operation in August 2002 with actual runtime of about 8 years. During the 5th overhaul in 2010, part of the tubes were found to have suffered from serious wall thinning and thus temporarily stopped operation. After inspection of the failure tubes, it was discovered that the wall thinning mainly happened near the support plates but to various extents and at different positions of the tubes. If the thinned tubes had continued to be used, leaking caused by perforation would have been very likely to happen, which could have brought about catastrophic safety problems. Therefore, we were asked by the plant to conduct failure analysis on the abnormal wall thinning of the titanium tubes. Previous work on mechanical performance of thin tubes [3,4] and failure analysis of tubes used in condensers and plants [5,6] has provided some clues to this problem. Our team [7,8] completed the failure analysis of leakage on titanium tubes within heat exchangers in a different phase of the same power plant as in our case. The failure was ascribed to electrochemical corrosion and mechanical degradation. However, in our case, the thinning dominantly happened on the outer wall of the tubes at quite regular positions, which implied a different story. To find out the root cause and mechanism of the thinning failure, we conducted a number of experiments for material, microstructure and chemical composition characterization based on previous successful failure analysis experiences [9–12]. 2. Experiments and results 2.1. Visual inspection and sampling To find out the cause of abnormal wall thinning of titanium tubes, we investigated condenser 1B of unit 2 under repair on the spot with the focus on the appearance of the support plates, their connection with the tubes and the surface condition of them both in the water inlet chamber. Some obvious defects found are shown in Fig. 3. Corrosion extent varied among different support plates and the most serious corrosion happened between the tube sheet and the first support plate (Fig. 3(a)). Besides, corrosion products deposit was seen on the surface of many tubes near the support plates (Fig. 3(a) and (b)). Fig. 1. Schematic diagram of the arrangement of four condensers in a unit. Table 1 Working parameters of the condenser. Parameters Media Flow rate Q (m3 /s) Flow velocity V (m/s) Pressure P (kPa) Inlet temperature T (C) Outlet temperature T (C) Tube side Sea water 1,30,100 1.97 – 18.8(note) 27.8 Shell side Steam Main steam flow rate 1033 kg/s (3718 T/h) – 4–4.9 – – Note: The steam temperature in the condenser in operation was 47 C (steam drain temperature of low pressure cylinder). Temperature of condensed water was around 30 C. 30 F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 29–41
F-A. Chen et aL/ Engineering Failure Analysis 37(2014)29-41 look from the out let: count fron left to right o ber number of tubes in a 406p 16 found in 2010 0动43 failure tubes in Part I Fig. 2. Arrangement of heat transfer tubes in the condenser and the location of the failure tubes a Fig 3. Appearance of support plates in the condenser (a)corrosion products on support plates (b) deposition and sagging of corrosion products
Fig. 2. Arrangement of heat transfer tubes in the condenser and the location of the failure tubes. Fig. 3. Appearance of support plates in the condenser (a) corrosion products on support plates (b) deposition and sagging of corrosion products. F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 29–41 31
F-f. Chen et aL Engineering Failure Analysis 37(2014)29-41 ig. 4. Black deposit on the outer wall of titanium tubes and its distribution(a) position near the support plate(b) deposit at the bottom. 2Theta [deg J Fig. 5. XRD results of the black deposit on the surface of titanium tubes. 2.2. XRD analysis of the black deposit Black deposit was found at the bottom and the contact part with the support plates of nearly all failure tubes, shown in Fig 4. We scraped some and conducted XRD for composition analysis. The results are shown in Fig. 5. The black deposit inly consists of Fe3 Oa according to the standard powder diffraction file(pDf)card. The Fe3 O4 mainly came from the galvanic corrosion of support plates made of carbon steel under xygen-dehcient con- ditions. But why did it dominantly appear at the contact part between the tubes and the support plates? It can be deduced
2.2. XRD analysis of the black deposit Black deposit was found at the bottom and the contact part with the support plates of nearly all failure tubes, shown in Fig. 4. We scraped some and conducted XRD for composition analysis. The results are shown in Fig. 5. The black deposit mainly consists of Fe3O4 according to the standard powder diffraction file (PDF) card. The Fe3O4 mainly came from the galvanic corrosion of support plates made of carbon steel under oxygen-deficient conditions. But why did it dominantly appear at the contact part between the tubes and the support plates? It can be deduced Fig. 4. Black deposit on the outer wall of titanium tubes and its distribution (a) position near the support plate (b) deposit at the bottom. Fig. 5. XRD results of the black deposit on the surface of titanium tubes. 32 F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 29–41
F-A. Chen et aL/ Engineering Failure Analysis 37(2014)29-41 Fig. 6. Schematic diagram of the cross section of a tube. a Fig. 7. Appearance of defect on the outer wall of titanium tube 1B134027-1 (a)o(b)90(c)180(d)270%. hat this phenomenon was because the support plates were vertical, the corrosion products deposited and sagged into the internal borings by the action of gravity 3. Failure analysis of defective tubes early all the failure tubes are located at the periphery of the lower part of the heat transfer tube tower. So they must from the ones in the current Part L, and they will be separately discussed in Part ll [13] of the failure analyer te different share some common problems. However, there are a few exceptions whose failure mode and location are q To describe defect distribution and location on the tubes, we drew a schematic diagram for illustration As shown in Fig. 6 the top of the tube is defined as o and the bottom is 180. Clockwise rotation goes from 0 to 180 and counterclockwise rotation from 0 to-180. The arrow direction perpendicular into the paper represents the flowing direction of sea water side the tube. The numbering of the tubes is based on their locations in the condenser, which is provided by the plant
that this phenomenon was because the support plates were vertical, the corrosion products deposited and sagged into the internal borings by the action of gravity. 3. Failure analysis of defective tubes Nearly all the failure tubes are located at the periphery of the lower part of the heat transfer tube tower. So they must share some common problems. However, there are a few exceptions whose failure mode and location are quite different from the ones in the current Part I, and they will be separately discussed in Part II [13] of the failure analysis. To describe defect distribution and location on the tubes, we drew a schematic diagram for illustration. As shown in Fig. 6 the top of the tube is defined as 0, and the bottom is 180. Clockwise rotation goes from 0 to 180 and counterclockwise rotation from 0 to 180. The arrow direction perpendicular into the paper represents the flowing direction of sea water inside the tube. The numbering of the tubes is based on their locations in the condenser, which is provided by the plant. Fig. 6. Schematic diagram of the cross section of a tube. Fig. 7. Appearance of defect on the outer wall of titanium tube 1B134027-1 (a) 0 (b) 90 (c) 180 (d) 270. F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 29–41 33