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
S-M. Hu et al/Case Studies in Engineering Failure Analysis 3(2015)52-61 C Fig. 7. Composition analysis of the micro-areas on the 2 tube(a)sites a and B at outer wall; (b)site C at inner wall. vulnerable sites and endure localized whirlpool erosion. while within a severe corroded area at the inner wall( Fig. 6(b)). micro cracking just initiated from the pit, then propagated along corrosion layers which would definitely lead to serious failure in a long term. Energy disperse spectroscope(eds)analyzed chemical compositions of micro sites at both tube walls(Fig. 7). To our urprise, existences of impurity elements s and cl in the outer and inner wall were found respectively along with corrosion products(Table 3). The significant excess contents of them were detected with approximately 2.0%(wt%)in certain sites, far more than the specified contents of material (less than 0.01%) 2.3.2. 10" heat-exchange tube %o As Fig8 depicts, the ellipse pit located at the inner wall of 10* heat-exchange tube is found with maximum depth of 4 um measured by 3-D synthesis technology, up to 25% thickness of the tube wall (original value is 2 mm). Obviously, the localized degradation is rather threatening Table EDS results of micro-areas at 2 tube(wt %) Site a 8858 182 2.37
vulnerable sites and endure localized whirlpool erosion. While within a severe corroded area at the inner wall (Fig. 6(b)), micro cracking just initiated from the pit, then propagated along corrosion layers which would definitely lead to serious failure in a long term. Energy disperse spectroscope (EDS) analyzed chemical compositions of micro sites at both tube walls (Fig. 7). To our surprise, existences of impurity elements S and Cl in the outer and inner wall were found respectively along with corrosion products (Table 3). The significant excess contents of them were detected with approximately 2.0% (wt.%) in certain sites, far more than the specified contents of material (less than 0.01%). 2.3.2. 10# heat-exchange tube As Fig. 8 depicts, the ellipse pit located at the inner wall of 10# heat-exchange tube is found with maximum depth of 494 mm measured by 3-D synthesis technology, up to 25% thickness of the tube wall (original value is 2 mm). Obviously, the localized degradation is rather threatening. Fig. 7. Composition analysis of the micro-areas on the 2# tube (a) sites A and B at outer wall; (b) site C at inner wall. Table 3 EDS results of micro-areas at 2# tube (wt.%). Element C O Cl S Fe Site A 1.08 5.66 0.49 1.97 88.58 Site B 1.82 13.66 0.65 2.05 78.89 Site C 2.37 7.54 1.77 1.01 83.43 S.-M. Hu et al. / Case Studies in Engineering Failure Analysis 3 (2015) 52–61 57
S-M Hu et aL/Case Studies in Engineering Failure Analysis 3(2015)52-61 a Fig 8. Structure of corrosion pit at inner wall and depth measurement(a)the pit; (b)3-D synthesis image. Furthermore, SEM pictures of 10" tube exhibit the corroded defects caused by multiple corrosion ore clearly In Fig 9(a), micro cracking engendered on the corrosion layers and small pieces have scaled surface After cut apart and etched, the cross-section morphology of the pit in Fig 8 is imaged and it is easy the localized wall-thinning degree( Fig 9(b)). From EDS results of micro areas in Fig. 10. unusual presences of s and CI were verified there in deed (Table 4). In general, these strong discoveries of 10"tube are all in line with previous of 24 3.1. Sources of impurity elements On the basis of our analysis, abnormal existences of two impurity elements(Cl and s) were discovered at the defect zones(tables 3 and 4). It is generally known that chloride ion and sulfides can be regarded as corrosive media in certain situations. So in order to figure out what actually happened to the tubes, the sources of them turned to be the priority for us to identify. After referring to operation information, clues started to surface gradually. Although the medium ODB stable under current condition, it had flowed through other heat exchangers in the upstream before and was possible to bring in some foreign substances, which has been affirmed by the plant. Among them, phosgene(cocl2)is the one that we cannot ignore. It is worth noting that the chemical plant is situated in coastal area, and thus CoCl2 can hydrolyze into ydrochloric acid and carbon dioxide under this humid environment(Eq (1)), which precisely explained the source of chloride CoCl2+H20→2HCl+CO2 With regard to sulfur, it originated from high-pressure industrial cleaning water and mainly remained on the outer tube wall. In addition, for the connection between tubes and tubesheet is seal welding but without physical expansion(Fig. 2), tiny cracking took place at the welded joints during operation. Then media would penetrate those defect sites motivated by
Furthermore, SEM pictures of 10# tube exhibit the corroded defects caused by multiple corrosion factors more clearly. In Fig. 9(a), micro cracking engendered on the corrosion layers and small pieces have scaled off the surface. After cut apart and etched, the cross-section morphology of the pit in Fig. 8 is imaged and it is easy to judge the localized wall-thinning degree (Fig. 9(b)). From EDS results of micro areas in Fig. 10, unusual presences of S and Cl were verified there in deed (Table 4). In general, these strong discoveries of 10# tube are all in line with previous discussion of 2# one. 3. Discussion 3.1. Sources of impurity elements On the basis of our analysis, abnormal existences of two impurity elements (Cl and S) were discovered at the defect zones (Tables 3 and 4). It is generally known that chloride ion and sulfides can be regarded as corrosive media in certain situations. So in order to figure out what actually happened to the tubes, the sources of them turned to be the priority for us to identify. After referring to operation information, clues started to surface gradually. Although the medium ODB is stable under current condition, it had flowed through other heat exchangers in the upstream before and was possible to bring in some foreign substances, which has been affirmed by the plant. Among them, phosgene (COCl2) is the one that we cannot ignore. It is worth noting that the chemical plant is situated in coastal area, and thus COCl2 can hydrolyze into hydrochloric acid and carbon dioxide under this humid environment (Eq. (1)), which precisely explained the source of chloride. COCl2 þ H2O ! 2HCl þ CO2 (1) With regard to sulfur, it originated from high-pressure industrial cleaning water and mainly remained on the outer tube wall. In addition, for the connection between tubes and tubesheet is seal welding but without physical expansion (Fig. 2), tiny cracking took place at the welded joints during operation. Then media would penetrate those defect sites motivated by Fig. 8. Structure of corrosion pit at inner wall and depth measurement (a) the pit; (b) 3-D synthesis image. 58 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 D Fig 9. Microscopic morphology of defects at 10# tube(a)corrosion products and cracking: (b)pit in view of cross section. pressure gradient from shell side(2.0 MPa) to tube side(1. 2 MPa ). Hence, sulfur was also found in site C at inner wall (Table 3). As is shown in Figs. 5 and 6, shallow holes and corrosion deposits ascribed to uniform corrosion distributed unevenly on he tube surface. Because the electrode potential of carbon steel is-0.6V[16. tubes could act as anode and endure anodic dissolution in humid environment. The half-cell and total reaction are presented here Anode:Fe→Fe2++ Cathode: H,O+1/202+2e- 20H Total reaction: 2Fe O2 +2H20- 2Fe(oH)2 4Fe(OH)2+O2+2H20- 4Fe(OH) (5) urthermore, CI in the medium would initiate pitting on the tube owing to its strong permeability, absorbability and migratory aptitude, and thus accelerated the rate of localized corrosion, seen in Fig. 8. This auto-catalyst cycle of degradation could repeat again and again. The mechanism is expressed as follows Fe2++2Cl-→FeCl
pressure gradient from shell side (2.0 MPa) to tube side (1.2 MPa). Hence, sulfur was also found in site C at inner wall (Table 3). 3.2. Multiple corrosion As is shown in Figs. 5 and 6, shallow holes and corrosion deposits ascribed to uniform corrosion distributed unevenly on the tube surface. Because the electrode potential of carbon steel is 0.6 V [16], tubes could act as anode and endure anodic dissolution in humid environment. The half-cell and total reaction are presented here: Anode : Fe ! Fe2þ þ 2e (2) Cathode : H2O þ 1=2O2 þ 2e ! 2OH (3) Total reaction : 2Fe þ O2 þ 2H2O ! 2FeðOHÞ2 (4) 4FeðOHÞ2 þ O2 þ 2H2O ! 4FeðOHÞ3 (5) Furthermore, Cl in the medium would initiate pitting on the tube owing to its strong permeability, absorbability and migratory aptitude, and thus accelerated the rate of localized corrosion, seen in Fig. 8. This auto-catalyst cycle of degradation could repeat again and again. The mechanism is expressed as follows: Fe2þ þ 2Cl ! FeCl2 (6) Fig. 9. Microscopic morphology of defects at 10# tube (a) corrosion products and cracking; (b) pit in view of cross section. S.-M. Hu et al. / Case Studies in Engineering Failure Analysis 3 (2015) 52–61 59
S-M Hu et aL/Case Studies in Engineering Failure Analysis 3(2015)52-61 b of micro-areas on the 10 tube(a)site E: (b) site F. FeCl2+2H20- Fe(OH)2+2(H*CI-) Meanwhile, COz emerged according to Eq (1)could be introduced into unit. It is regarded that the attack of carbon dioxide solutions on steel is stronger than that of diluted mineral acids with the same pH value [ 17, 18. So its presence in ammonia evaporators would aggravate corrosion extent as well In our case, uniform corrosion and pitting occurred at the same time and both of them caused material degradation. Apart from what we, ve discussed above, the interaction behavior of them was also one ruling cause. On one hand, uniform corrosion led to metal dissolving to form Fe*, which was easy to hydrolyze and engender H*. When local pH dropped to 4.0 with concentration of H, it favored pitting at the weak sites of tubes, such as corrosion holes, crevices, and cracks On the other hand, since tubes experienced serious corrosion, brittle corrosion layers consequently formed on the tube surface. While subject to temperature fluctuation and slight media erosion during operation, self-detachment of the corrosion layers from matrix metal happened due to the difference of their CtEs(coefficients of thermal expansion). What's more, corrosion pits could act as stress concentration and promote an initial cracking growth( Fig. 6). EDS results of micro-areas at 10 tube(wt%). 1.35 8.23 1.05 Site F
FeCl2 þ 2H2O ! FeðOHÞ2 þ 2ðHþClÞ (7) Meanwhile, CO2 emerged according to Eq.(1) could be introduced into unit. It is regarded thatthe attack of carbon dioxide solutions on steel is stronger than that of diluted mineral acids with the same pH value [17,18]. So its presence in ammonia evaporators would aggravate corrosion extent as well. In our case, uniform corrosion and pitting occurred at the same time and both of them caused material degradation. Apart from what we’ve discussed above, the interaction behavior of them was also one ruling cause. On one hand, uniform corrosion led to metal dissolving to form Fe2+, which was easy to hydrolyze and engender H+ . When local pH dropped to 4.0 with concentration of H+ , it favored pitting at the weak sites of tubes, such as corrosion holes, crevices, and cracks. On the other hand, since tubes experienced serious corrosion, brittle corrosion layers consequently formed on the tube surface. While subject to temperature fluctuation and slight media erosion during operation, self-detachment of the corrosion layers from matrix metal happened due to the difference of their CTEs (coefficients of thermal expansion). What’s more, corrosion pits could act as stress concentration and promote an initial cracking growth (Fig. 6). Fig. 10. Composition analysis of micro-areas on the 10# tube (a) site E; (b) site F. Table 4 EDS results of micro-areas at 10# tube (wt.%). Element C O Cl S Fe Site D 1.35 8.23 0.65 1.05 87.10 Site E 0.21 10.06 2.70 – 86.52 Site F 6.11 25.71 2.10 – 65.47 60 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 4. Conclusions 1. The material of heat exchange tubes in ammonia evaporators is confirmed to be 10 carbon steel and all testified to be qualified to requirements 2. The connection between tubes and tubesheet is seal welding but without physical expansion. As a result, the welded joints are prone to cracking when subject to fluctuation of operating conditions. 3. Heat-exchange tubes in ammonia evaporators exhibit serious corroded defects and localized tube wall has thinned badly. The failure concretely happened as following steps: irstly, the heat-exchange tubes suffered uniform corrosion and pitting in humid environment with presence of Cl, sand CO2. Besides, the interaction behavior of them accelerated material degradation process in weak sites, bringing about serious localized wall thinning of tubes. Then affected by temperature fluctuation and slight me sion, corrosion layers gradually got cracking and some micro cracking initiated right from the pits. with its ion, corrosion layer subsequently split into small pieces and scaled off the surface. As a result, the tube wall co thinned, leading to premature failure of ammonia evaporators in the end. 5. Prevention measures 1. The tube ends should be seal welding to tubesheet with physical expansion so as to increase their reliabilities. 2. Strictly control the contents of chloride ions and water vapor in ammonia evaporators introduced by equipment upstream to avoid pitting on the heat exchange tubes caused by chloride ions 3. Clean up the corrosion products on the tubes during shutdown maintenance of evaporators in case of under-deposit corrosion References 12 Jahromi SAL, AliPour MM, Beirami A Failu is of 101-C ammonia plant heat exchange. Eng Fail Anal 2003: 10(4): 405-21 Abbasfard H, Ghanbari M, Ghasemi A, ghad sajani HH, Ali Moradi A Failure analysis and modeling of super heater tubes of a waste heat boiler ermally coupled in ammonia oxidation Eng Fail Ana2012:26:285-92. [3] Bhaumik SK. Ramgaraju R, Parameswara MA, Bhaskaran TA Venkataswamy A, Raghuram AC, et al. Failure of reformer tube of an ammonia plant. Eng il Anal2002:95):553-61 Arevalo A, Esparza P, Gomis Bas C, Morales J. Gonzalez S, de Sanchez SR Corrosion on steam-side heat exchange tubes. Mater Perform 1996: 35(1): 67-8. Ueda S Onishi H, Okubo M, Takegawa T. Corrosion of am ant heat exchanger. Ammonia Plant Saf Relat Facil 1978: 20: 98-102. [8] Li Al. Wang wQ, Wang XM, Zhao D Fatigue and brittle fracture of carbon steel process pipeline. Eng Fail Anal 2005: 12: 527-36. Yang ZG, Gong Y, Yuan JZ Failure analysis of leakage on titanium tubes within heat exchangers in a nuclear power plant. Part I: Electrochemical corrosion. Mater Corros 2012: 63(1): 7-17. [11] Gong Y, Yang ZG. Failure analysis of one peculiar"Yin-Yang corrosion morphology on heat exchanger tubes in purified terephthalic acid (PTA)dryer. g Fail Anal2013:31:203- [12] Gong Y, Yang C, Yao C, Yang ZG. Acidic/caustic alternating corrosion on carbon steel pipes in heat exchanger of ethylene plant. Mater Corros [13] Chen FJ. Yao C, Yang ZG. Failure analysis on abnormal wall thinning of heat-transfer titanium tubes of condensers in nuclear power plant. Part I orrosion and wear. Eng Fail Anal 2014: 37: 29-41 [14 Chen F], Yao C, Yang ZG. Failure analysis on abnormal wall thinning of heat-transfer titanium tubes of condensers in nuclear power plant. Part ll [15] GB 9948-2006 Standards specification for seamless steel tubes for petroleum cracking(in Chinese). [18] Nasirpouri F, Mostafaei A, Fathyunes L Jafari R. Assessment of localized corrosion in carbon steel tube-grade AlSI 1045 used in output oil-gas essel of desalination unit in oil refinery industry Eng Fail Anal 2014: 40: 75-88
4. Conclusions 1. The material of heat exchange tubes in ammonia evaporators is confirmed to be 10 carbon steel and all testified to be qualified to requirements. 2. The connection between tubes and tubesheetis seal welding but without physical expansion. As a result, the welded joints are prone to cracking when subject to fluctuation of operating conditions. 3. Heat-exchange tubes in ammonia evaporators exhibit serious corroded defects and localized tube wall has thinned badly. The failure concretely happened as following steps: Firstly, the heat-exchange tubes suffered uniform corrosion and pitting in humid environment with presence of Cl, S and CO2. Besides, the interaction behavior of them accelerated material degradation process in weak sites, bringing about serious localized wall thinning of tubes. Then affected by temperature fluctuation and slight media erosion, corrosion layers gradually got cracking and some micro cracking initiated right from the pits. With its propagation, corrosion layer subsequently split into small pieces and scaled off the surface. As a result, the tube wall constantly thinned, leading to premature failure of ammonia evaporators in the end. 5. Prevention measures 1. The tube ends should be seal welding to tubesheet with physical expansion so as to increase their reliabilities. 2. Strictly control the contents of chloride ions and water vapor in ammonia evaporators introduced by equipment in upstream to avoid pitting on the heat exchange tubes caused by chloride ions. 3. Clean up the corrosion products on the tubes during shutdown maintenance of evaporators in case of under-deposit corrosion. References [1] Jahromi SAJ, AliPour MM, Beirami A. Failure analysis of 101-C ammonia plant heat exchange. Eng Fail Anal 2003;10(4):405–21. [2] Abbasfard H, Ghanbari M, Ghasemi A, Ghader S, Rafsajani HH, Ali Moradi A. Failure analysis and modeling of super heater tubes of a waste heat boiler thermally coupled in ammonia oxidation reactor. Eng Fail Anal 2012;26:285–92. [3] Bhaumik SK, Ramgaraju R, Parameswara MA, Bhaskaran TA, Venkataswamy A, Raghuram AC, et al. Failure of reformer tube of an ammonia plant. Eng Fail Anal 2002;9(5):553–61. [4] Arevalo A, Esparza P, Gomis Bas C, Morales J, Gonzalez S, de Sanchez SR. Corrosion on steam-side heat exchange tubes. Mater Perform 1996;35(1):67–8. [5] Ueda S, Onishi H, Okubo M, Takegawa T. Corrosion of ammonia plant heat exchanger. Ammonia Plant Saf Relat Facil 1978;20:98–102. [6] Sivaprasad S, Narang SK, Singh R. Failure of high pressure ammonia line in a fertilizer plant—a case study. Eng Fail Anal 2006;13(6):867–75. [7] Cui HX, Wang WQ, Li AJ, Li ML, Xu SG. Failure analysis of the brittle fracture of a thick-walled 20 steel pipe in an ammonia synthesis unit. Eng Fail Anal 2010;17:1359–76. [8] Li AJ, Wang WQ, Wang XM, Zhao D. Fatigue and brittle fracture of carbon steel process pipeline. Eng Fail Anal 2005;12:527–36. [9] Yang ZG, Gong Y, Yuan JZ. Failure analysis of leakage on titanium tubes within heat exchangers in a nuclear power plant. Part I: Electrochemical corrosion. Mater Corros 2012;63(1):7–17. [10] Gong Y, Yang ZG, Yuan JZ. Failure analysis of leakage on titanium tubes within heat exchangers in a nuclear power plant. Part II: Mechanical degradation. Mater Corros 2012;63(1):18–28. [11] Gong Y, Yang ZG. Failure analysis of one peculiar ‘Yin-Yang’ corrosion morphology on heat exchanger tubes in purified terephthalic acid (PTA) dryer. Eng Fail Anal 2013;31:203–10. [12] Gong Y, Yang C, Yao C, Yang ZG. Acidic/caustic alternating corrosion on carbon steel pipes in heat exchanger of ethylene plant. Mater Corros 2011;62(10):967–78. [13] Chen FJ, Yao C, Yang ZG. 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 2014;37:29–41. [14] Chen FJ, Yao C, Yang ZG. 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 2014;37:42–52. [15] GB 9948-2006 Standards specification for seamless steel tubes for petroleum cracking (in Chinese). [16] LaQue FL. Marine corrosion, causes and prevention. Hoboken, NJ, USA: John Wiley & Sons Inc.; 1975. 179. [17] Schutze M, Isecke B, Bender R. Corrosion protection against carbon dioxide. Frankfurt: Wiley-VCH; 2011. p. 63–92. [18] Nasirpouri F, Mostafaei A, Fathyunes L, Jafari R. Assessment of localized corrosion in carbon steel tube-grade AISI 1045 used in output oil-gas separator vessel of desalination unit in oil refinery industry. Eng Fail Anal 2014;40:75–88. S.-M. Hu et al. / Case Studies in Engineering Failure Analysis 3 (2015) 52–61 61