Case Studies in Engineering Failure Analysis 3(2015)52-61 Contents lists available at Science Direct Case Studies in Engineering Failure Analysis ELSEVIER journalhomepagewww.elsevier.com/locate/csefa Case study Failure analysis on unexpected wall thinning CrossMark of heat-exchange tubes in ammonia evaporators Shi-Meng Hu, Sheng-Hui Wang Zhen-Guo Yang Department of materials Science, Fudan University, Shanghai 200433, PR China Shanghai Institute of Special Equipment Inspection and Technical Research, Shanghai 200333, PR China ARTICLE INFO ABSTRACT Article h A failure incident of heat-exchange tubes in ammonia evaporators, which suffered from Received 7 october 2014 expected wall thinning after only one-year service with respect to their original design Accepted 13 January 2015 lifetime of fifteen years, was reported and carefully analyzed. After overall inspection, many Available online 28 January be walls in the evaporators were found to experience severe degradations at both sides with distinct corroded defects and general cracking of corrosion layers. Thus, ce nvestigations including external appearance, microscopic morphology and chemical omposition were carried out by using a series of characterization methods. The analysis results demonstrated that the unexpected wall thinning of tubes was primarily ascribed to Carbon steel ultiple corrosion factors including uniform corrosion, pitting and interaction behavior Failure analysis between them. Relative failure mechanisms were discussed in detail and prevention Corrosion neasures were also proposed for ammonia evaporators under similar operating condition. @2015 The Authors Published by Elsevier Ltd. This is an open access article under the cc By-nC-ndlicense(http://creativecommonsorg/licenses/by-nc-nd/4.0/). 1. Introduction Recently there occurred an unexpected failure incident of ammonia evaporators in a polyurethane plant, which is located at the coastal area of China and managed by a foreign company. These ammonia evaporators play critical roles for the whole system of circulating cooling process in core plant, mainly used to manufacture 2, 4-tolylene diisocyanate(tDi). Herein, the evaporators are employed to exchange heat between ammonia in shell side with phase change and evaporation, and o-dichlorobenzene(odB)in tube side, which has been utilized in upstream process to cool various products in advance. The accurate operation parameters of evaporators are listed in Table 1. Each evaporator is a type of tubular heat xchanger with 6850 heat-exchange tubes(10 carbon steel, 25 mm x 2 mm) arranged as a horizontal tube bundle in the shell. They are mounted by tubesheet(16Mnlll) welded to at both ends, as well as 8 perforated baffle plates(Q345R steel) sustained in the middle with interval distance of 800 mm evenly, shown in Fig. 1 The unit was put into service in April 2011 with design lifetime of about fifteen years. But within only one year, two evaporators encountered with unexpected failure to different extent in succession, namely sudden leakage and serious tube- wall thinning. Particularly, some tube-wall thickness has decreased up to 40% in localized defect area detected by X-ray on- site inspection. This premature failure affected whole circulating system gravely and enormous losses in finance and energy cannot be avoided In addition, it appeared to be more severe for the units given that they had operated for such a short time Corresponding author. Tel: +86 21 65642523: fax: +86 21 65103056 mail address: zoya 2213-2902/e2015TheAuthorsPublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCcBy-nC-ndlicense(http://creativecommons.org
Case study Failure analysis on unexpected wall thinning of heat-exchange tubes in ammonia evaporators Shi-Meng Hu a , Sheng-Hui Wang a,b , Zhen-Guo Yang a, * aDepartment of Materials Science, Fudan University, Shanghai 200433, PR China b Shanghai Institute of Special Equipment Inspection and Technical Research, Shanghai 200333, PR China 1. Introduction Recently there occurred an unexpected failure incident of ammonia evaporators in a polyurethane plant, which is located at the coastal area of China and managed by a foreign company. These ammonia evaporators play critical roles for the whole system of circulating cooling process in core plant, mainly used to manufacture 2,4-tolylene diisocyanate (TDI). Herein, the evaporators are employed to exchange heat between ammonia in shell side with phase change and evaporation, and o-dichlorobenzene (ODB) in tube side, which has been utilized in upstream process to cool various products in advance. The accurate operation parameters of evaporators are listed in Table 1. Each evaporator is a type of tubular heat exchanger with 6850 heat-exchange tubes (10 carbon steel, 25 mm 2 mm) arranged as a horizontal tube bundle in the shell. They are mounted by tubesheet (16MnIII) welded to at both ends, as well as 8 perforated baffle plates (Q345R steel) sustained in the middle with interval distance of 800 mm evenly, shown in Fig. 1. The unit was put into service in April 2011 with design lifetime of about fifteen years. But within only one year, two evaporators encountered with unexpected failure to different extent in succession, namely sudden leakage and serious tubewall thinning. Particularly, some tube-wall thickness has decreased up to 40% in localized defect area detected by X-ray onsite inspection. This premature failure affected whole circulating system gravely and enormous losses in finance and energy cannot be avoided. In addition, it appeared to be more severe for the units given that they had operated for such a short time. Case Studies in Engineering Failure Analysis 3 (2015) 52–61 A R T I C L E I N F O Article history: Received 7 October 2014 Received in revised form 13 January 2015 Accepted 13 January 2015 Available online 28 January 2015 Keywords: Ammonia evaporator Wall thinning Carbon steel Failure analysis Corrosion A B S T R A C T A failure incident of heat-exchange tubes in ammonia evaporators, which suffered from unexpected wall thinning after only one-year service with respect to their original design lifetime of fifteen years, was reported and carefully analyzed. After overall inspection, many tube walls in the evaporators were found to experience severe degradations at both sides with distinct corroded defects and general cracking of corrosion layers. Thus, comprehensive investigations including external appearance, microscopic morphology and chemical composition were carried out by using a series of characterization methods. The analysis results demonstrated that the unexpected wall thinning of tubes was primarily ascribed to multiple corrosion factors including uniform corrosion, pitting and interaction behavior between them. Relative failure mechanisms were discussed in detail and prevention measures were also proposed for ammonia evaporators under similar operating condition. 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). * Corresponding author. Tel.: +86 21 65642523; fax: +86 21 65103056. E-mail address: zgyang@fudan.edu.cn (Z.-G. Yang). Contents lists available at ScienceDirect Case Studies in Engineering Failure Analysis journal homepage: www.elsevier.com/locate/c sefa http://dx.doi.org/10.1016/j.csefa.2015.01.002 2213-2902/ 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).
S-M. Hu et al/Case Studies in Engineering Failure Analysis 3(2015)52-61 Table 1 peration parameters of ammonia evaporators. arameters velocity. P(MPa temp,T(°) temp,T(°C) Tube sid 4019763.6 Shell side 155573 24.7(L)96234(V) Actually, some failure incidents of ammonia heat exchangers have been reported at both abroad and home in the past 1-8. showing that inadequate thermal treatment, stress corrosion cracking and strain aging embrittlement were general failure causes. However, those studies mostly dealt with incidents in extreme process conditions like elevated temperatures and pressures. Whereas the unexpected wall thinning in our case happened under quite different environment as relative low operation temperature(below 10C)and current non-aggressive medium(oDB)in tube side. Owing to diverse situations, it became hard to explain this wall-thinning case by above mechanisms described in literatures. Hence, based on our recent work on failures analysis of various heat-exchange tubes [9-14, systematic investigations were conducted to find out the real cause of this incident, including external appearance, microscopic morphology and chemical compos d then practical prevention measures were also proposed. Overall, the analysis in this paper will provide a reference with significant engineering value to prevent failure recurrence of ammonia s unger sin r operating condition 2. Experiments and results 2.1. Visual observation and sampling Fig 2 shows several tube samples taken from evaporators. Particular attention should be paid to the tube ends where I typical trace of physical expansion exists( Fig. 2(b)). It's common that to achieve a tight-fit joint, the tube end has to be expanded radially at room temperature by hydraulic process and then welded so the joint between tube and tubesheet is permanently strained and secured. while here, the tube ends apparently hadn 't experienced such treatment before weld. So the joints would be prone to cracking when subjected to the fluctuation of operation conditions and thus brought about potential hazards for the In terms of tested dat It tubes by the plant, we chose two failed tubes with severe degradations to study in detail, arked as2and10°( ns shown in Fig 3). 中中中中最 中 Fig. 1 Schematic diagram of the structure of ammonia evaporators
Actually, some failure incidents of ammonia heat exchangers have been reported at both abroad and home in the past [1–8], showing that inadequate thermal treatment, stress corrosion cracking and strain aging embrittlement were general failure causes. However, those studies mostly dealt with incidents in extreme process conditions like elevated temperatures and pressures. Whereas the unexpected wall thinning in our case happened under quite different environment as relative low operation temperature (below 10 8C) and current non-aggressive medium (ODB) in tube side. Owing to diverse situations, it became hard to explain this wall-thinning case by above mechanisms described in literatures. Hence, based on our recent work on failures analysis of various heat-exchange tubes [9–14], systematic investigations were conducted to find out the real cause of this incident, including external appearance, microscopic morphology and chemical composition, and then practical prevention measures were also proposed. Overall, the analysis in this paper will provide a reference with significant engineering value to prevent failure recurrence of ammonia evaporators under similar operating condition. 2. Experiments and results 2.1. Visual observation and sampling Fig. 2 shows several tube samples taken from evaporators. Particular attention should be paid to the tube ends where no typical trace of physical expansion exists (Fig. 2(b)). It’s common that to achieve a tight-fit joint, the tube end has to be expanded radially at room temperature by hydraulic process and then welded, so the joint between tube and tubesheet is permanently strained and secured. While here, the tube ends apparently hadn’t experienced such treatment before weld. So the joints would be prone to cracking when subjected to the fluctuation of operation conditions and thus brought about potential hazards for the whole unit. In terms of tested data about tubes by the plant, we chose two failed tubes with severe degradations to study in detail, marked as 2# and 10# (positions shown in Fig. 3). Table 1 Operation parameters of ammonia evaporators. Parameters Media Pressure, P (MPa) Inlet temp., T (8C) Outlet temp., T (8C) Flow rate, Q (kg/h) Flow velocity, V (m3 /h) Tube side ODB 1.2 11 17 4019763.6 3000.0 Shell side NH3 2.0 8.7 19 15557.3 24.7(L)/9623.4(V) Fig. 1. Schematic diagram of the structure of ammonia evaporators. S.-M. Hu et al. / Case Studies in Engineering Failure Analysis 3 (2015) 52–61 53
S-M Hu et aL/Case Studies in Engineering Failure Analysis 3(2015)52-61 西? physical expansion Fig. 2. Appearance of heat-exchange tube samples(a) failed tubes;(b) no trace of physical expansion on the tube end 1314011们110 Fig. 3. Schematic illustration about relative positions of 2* and 10" tubes
Fig. 2. Appearance of heat-exchange tube samples (a) failed tubes; (b) no trace of physical expansion on the tube ends. Fig. 3. Schematic illustration about relative positions of 2# and 10# tubes. 54 S.-M. Hu et al. / Case Studies in Engineering Failure Analysis 3 (2015) 52–61
S-M. Hu et al/Case Studies in Engineering Failure Analysis 3(2015)52-61 Table 2 Chemical composition of 2" and 10 failed tubes (wt%). Element Si 0016 0.028 GB9948-200610·15l007-0.130.17-0.3 35-0.65 <0.020 <0.20 t Fig 4 Metallographic structures of tubes in circumferential direction 500x pit Fig. 5. Appearance of 2" tube imaged by stereo microscope(a)outer wall; (b) inner wall
Table 2 Chemical composition of 2# and 10# failed tubes (wt.%). Element C Si Mn S P Cr Ni Cu 2# 0.11 0.27 0.44 0.004 0.019 0.027 0.008 0.015 10# 0.12 0.26 0.47 0.008 0.016 0.028 0.008 0.019 GB 9948-2006 10* [15] 0.07–0.13 0.17–0.37 0.35–0.65 0.020 0.030 0.15 0.25 0.20 Fig. 4. Metallographic structures of tubes in circumferential direction 500. Fig. 5. Appearance of 2# tube imaged by stereo microscope (a) outer wall; (b) inner wall. S.-M. Hu et al. / Case Studies in Engineering Failure Analysis 3 (2015) 52–61 55
S-M Hu et al /Case Studies in Engineering Failure Analysis 3(2015) 52-61 2.2. Material examination of heat-exchange tubes Photoelectric direct reading spectrometer was applied to investigate chemical compositions of tube materials, listed in Table 2. The actual compositions of materials are in correspondence to the specifications of 10 carbon steel in GB 9948-2006 Chinese National Standards [15. inclusions. Therefore, the material could be regarded as qualified to the requirement in genet xialgrar ial direction.As it Fig 4 shows the microstructure of matrix imaged by metallographic microscope(MM)in circumferential direction. As it exhibits, the material has typical feature of low-carbon steel: consisting of ferrite and pearlite equia s with no visible 2.3. Failure analysis of heat-exchange tubes 2.3.1.2 heat-exchange To start with, stereo microscope was used to observe the morphologies of 2" tube. Fig. 5(a) is the appearance of outer wall with obvious defects due to uniform corrosion, including rufous corrosion products, red translucent substances and several irregular shallow holes With respect to the inner wall, there are some different morphologies which imply another story. As Fig 5(b)reveals, distinct failure phenomena referring to visible tiny and deep pit has taken place there, in accordance t typical pitting characteris fterwards, microscopic morphologies were imaged by scanning electron microscope(SEM). In Fig. 6(a), corrosion holes by the cut edge of outer wall appear grey in color surrounded by fish-scale lines, indicating that those holes might act as b Fig. 6. Microscopic morphology of defect zones at 2"tube(a)grey-colored pits by the edge of outer wall; (b) cracking of corrosion layers on the inner wa
2.2. Material examination of heat-exchange tubes Photoelectric direct reading spectrometer was applied to investigate chemical compositions of tube materials, listed in Table 2. The actual compositions of materials are in correspondence to the specifications of 10 carbon steel in GB 9948-2006 Chinese National Standards [15]. Fig. 4 shows the microstructure of matrix imaged by metallographic microscope (MM) in circumferential direction. As it exhibits,the material has typical feature of low-carbon steel: consisting of ferrite and pearlite equiaxial grains with no visible inclusions. Therefore, the material could be regarded as qualified to the requirement in general. 2.3. Failure analysis of heat-exchange tubes 2.3.1. 2# heat-exchange tube To start with, stereo microscope was used to observe the morphologies of 2# tube. Fig. 5(a) is the appearance of outer wall with obvious defects due to uniform corrosion, including rufous corrosion products, red translucent substances and several irregular shallow holes. With respect to the inner wall, there are some different morphologies which imply another story. As Fig. 5(b) reveals, distinct failure phenomena referring to visible tiny and deep pit has taken place there, in accordance to typical pitting characteristic. Afterwards, microscopic morphologies were imaged by scanning electron microscope (SEM). In Fig. 6(a), corrosion holes by the cut edge of outer wall appear grey in color surrounded by fish-scale lines, indicating that those holes might act as Fig. 6. Microscopic morphology of defect zones at 2# tube (a) grey-colored pits by the edge of outer wall; (b) cracking of corrosion layers on the inner wall. 56 S.-M. Hu et al. / Case Studies in Engineering Failure Analysis 3 (2015) 52–61