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Failure analysis and modeling of super heater tubes of a waste heat boiler thermally coupled in ammonia oxidation reactor
Engineering Failure Analysis 26 (2012) 285–292
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis and modeling of super heater tubes of a waste heat boiler thermally coupled in ammonia oxidation reactor Hamed Abbasfard a,⇑, Mehdi Ghanbari b, Amin Ghasemi c, Sattar Ghader a, Hasan Hashemipour Rafsanjani a, Ali Moradi a a
Department of Chemical Engineering, Shahid Bahonar University of Kerman, Kerman, Iran Department of Chemical Engineering, Yasuj University, Yasuj, Iran c Department of Material Science and Engineering, Azad University of Najaf Abad, Najaf Abad, Iran b
a r t i c l e
i n f o
Article history: Received 10 November 2011 Received in revised form 5 June 2012 Accepted 21 June 2012 Available online 5 September 2012 Keywords: Failure analysis Waste heat boiler Super heater tubes Overheating Dynamic modeling
a b s t r a c t Repeated failures have been observed in the outlet region of a few super heater tubes made of 1Cr–0.5Mo steel integrated in a waste heat boiler thermally coupled in an ammonia oxidation reactor. A case study of a failed tube running at around 37,440 h was presented. The bulging of some parts of super heater tubes were observed while the tubes were under normal operational temperature of 480 °C and internal pressure of 44 bar. The tube hardness measurements were carried out on the selected super heater tubes including those of ruptured, unruptured and headers. The relatively high value of hardness, 260 HV, of ruptured part compared to other parts indicates that some phase transformation probably occurred for such a low-alloy 1Cr–0.5Mo steel which led to lower thermal resistance. The dynamic modeling including conservation of energy has been developed in order to show the tube overheating. The main root cause of repeated failures of the super heater tubes was shortterm overheating following rapid cooling of the tubes due to sudden stoppage of steam flow inside connecting tubes in the event of unit shut down which reduced the thermal resistance of the tubes over time. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Material failures can lead to many potentially disastrous consequences, including poor product quality, necessary repair or component or equipment replacement, production down time losses, environmental impact, and even loss of life. Furthermore, failures may arise from various causes, including design, material composition, and improper thermal processing, in the case of metals such as steel. Therefore, it is critically necessary not only to identify these failures but also to determine their root cause when failures do occur. This is considered as the primary objective of this study [1]. Failure analysis is the process of collecting, examining, and interpreting damage evidence. The objective is to understand the possible conditions leading to a failure and perhaps to prevent similar failures in the future. Many researchers [2–7] described the guidelines and methods of failure analysis. In the literature, many scholars have focused on various aspects of failure analysis. However, there are relatively few that have emphasized on steel failures arising from thermal processing as well as thermally integrated systems. Waste heat boiler (WHB) is a heat recovery unit which is thermally coupled in an ammonia oxidation reactor in order to recover the released heat and lower the temperature of reaction products. The WHB consists of a water-tube evaporative
⇑ Corresponding author. Tel.: +98 9173082258; fax: +98 7112314323. E-mail addresses: [email protected], [email protected] (H. Abbasfard). 1350-6307/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2012.06.012
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H. Abbasfard et al. / Engineering Failure Analysis 26 (2012) 285–292
AIR/ AMMOMNIA
The vent valve SUPER HEATED STEAM
STEAM DRUM
FEED WATER Fig. 1. Schematic diagram of ammonia oxidation reactor thermally coupled with waste heat recovery unit.
Fig. 2. Uninstalled connecting tubes of super heater and evaporator.
unit connected in forced circulation flow with a steam drum and a steam super heater which is schematically shown in Fig. 1. There are numerous articles in literature on failure analysis of super heater tubes mostly occurred in power plants. This study is concerned about a different environment under different operation conditions. As reported by French [8], super heater tubes experience the highest probability of failure and have finite life contributing to prolonged exposure to high temperature, stress and aggressive environment which was assessed by Ray [9]. Jones [10] also mentioned that creep failure problems including super heater tube failure and revealed that under which circumstances the failures took place. Detailed discussions on super heater tube failures have been elaborated by French, Das et al. [11], Port and Herro [12] and Purbolaksono et al. [13] considering failure analysis by visual inspection, measurements of hardness and microstructure analysis. Kim et al. [14] added the fatigue life analysis while Othman et al. [15] and Purbolaksono supported their work by finite element method.
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H. Abbasfard et al. / Engineering Failure Analysis 26 (2012) 285–292
This survey presents a case study of the unexpected failure of the super heater SA213-T12 tube of WHB by visual inspection, chemical and dimensional analysis, in situ measurements of hardness and creep analysis. The dynamic modeling including heat transfer equations also was developed in order to support the claims. It was easily detectable that super heater tube was failed with window rupture burst due to over range value of hardness. Complete access to the ruptured point was not possible due to packed installation (Fig. 2). However, some examinations were carried out on failed tube some parts close to the ruptured point. This work includes helpful findings and analysis for safe running of identical operations. 2. Operation description and condition of failure Heat released by the ammonia oxidation reaction, added to air/ammonia mixture’s sensible heat, raise the temperature of the catalysis gauzes and gases up to about 880 °C. In order to recover the heat as well as lower the temperature of gas, the WHB with the capacity of 27 ton/h is arranged vertically consists of a top, middle and bottom vessel section, as depicted in Fig. 3. Top section contains two evaporative tube rows arranged under catalyst basket, middle section is made up of four super heater tube rows and bottom section is composed of seven evaporative tube rows. The WHB consists of a water-tube evaporative unit connected in forced circulation flow with a steam drum and a steam super heater which is schematically illustrated in Fig. 1. The saturated steam produced through evaporative section is gathered in the two phase constant level steam drum at a pressure of 44 bar and temperature of 260 °C. Such wet steam is then poured into the super heater tubes. The high temperature superheated steam at 480 °C and the same pressure exits the super heater and enters a steam turbine and also to heat the unit’s apparatus after it is desuperheated. New SA213-T12 super heater tubes were fitted in the WHB unit in August 2005. The tubes have outer diameter of 38 mm and thickness of 4 mm. The failure occurred during normal operation of unit which had been shut down in less than 3 days (70 h) before that time in January 2011. According to the record of unit, the failed tube had run at around 37,440 h. 3. Failure analysis 3.1. Visual inspection Visual inspection of the failed tube shown in Fig. 4 was carried out. The access to the ruptured point at the bend of the tube was not possible. However, the examination of the adjacent region to the ruptured point indicated the following findings: – – – –
Finding internal and external (0.5 mm) oxide scale (Figs. 5 and 6). Increasing diameter (bulging) of tube obviously. Finding no evidence of the active corrosion on both internal and external surfaces of the tube. Finding window rupture burst as the type of tube rupture.
Catalyst gauzes 1 Reaction zone Flux of heat by radiation from ceramic raschig ring
2 Ceramic raschig ring 3 Top evaporative section 12 cm 4 Middle super heater section 5 Bottom evaporative section
Fig. 3. Description of different sections inside reactor.
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H. Abbasfard et al. / Engineering Failure Analysis 26 (2012) 285–292
Fig. 4. Ruptured tube in the form of window rupture.
Fig. 5. Oxide scale on the external surface of tube (0.5 mm).
Fig. 6. Oxide scale on the internal surface of tube.
3.2. Chemical analysis During ignition of the platinum catalyst gauzes, ingress of unburnt ammonia gas might occur and thereby causes ammonium nitrates to be formed close to the tubes and corrosive attack may take place. In order to determine the chemical composition of the tubes, a chemical analysis technique of the Energy Dispersive Microscopy (EDS) was performed. The EDS results, as shown in Table 1, indicated that the chemical composition was complied with the SA213-T12 and SA335-P12 original composition of the tube and header, respectively.
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H. Abbasfard et al. / Engineering Failure Analysis 26 (2012) 285–292 Table 1 Chemical composition of material (wt.%).
Tube Header
Cr
C
Mo
P
S
Mn
Si
Fe
0.80–1.25 1.00–1.50
0.15 0.15
0.44–0.65 0.44–0.65
0.045 0.030
0.045 0.030
0.30–0.61 0.03–0.06
0.50 0.50–1.00
Balanced Balanced
Table 2 Dimensional measurements of the overheated tubes. Original outer diameter (mm) Changed outer diameter (mm) a
38 39.7
38 39.5
38a 37.7
38 38.9
38 39.6
38 38.4
38 39.4
Bending area.
Table 3 Hardness measurements data. Northern tube (HV)
Southern tube (HV)
Header (HV)
Hardness measurements on outlet of superheater 170 260
Ruptured tube (HV)
162
131–137
Hardness measurements on inlet of superheater 129–131 129–131
129–131
131–137
3.3. Tube dimensional measurements Measurements of tube diameter were conducted on seven locations along the ruptured tube; see Table 2. According to the internal and external oxide layer thickness, the slight tube wall thickness reduction was obvious. The creep deformation (bulging) of some part of super heater tubes was observed. 3.4. Hardness measurements In situ measurements of hardness were carried out on the tubes and headers of the super heater at different locations and are tabulated in Table 3. The hardness of ruptured tube is 260 HV which has exceeded the recommended values for this grade of steel while other tubes are among the allowable range. The minimum and maximum hardness limit of this grade of steel are 135 HV and 184 HV respectively [16]. 4. Mathematical modeling of overheating The dynamic behavior of super heater tube temperature was examined by developing the mathematical model in the event of unit shut down. Model assumptions are as following:
Lumped formulation was employed. Ideal gases were assumed. Conduction heat transfer in tubes wall was considered to be negligible. Heat transfer mechanism was supposed to be free convection and radiation. The temperature of evaporation section was assumed to be constant. No steam flow inside the super heater tubes was considered. The energy equation for the super heater can be written as follow:
cps ms
dT s ¼ hf As ðT g T s Þ þ Q gr þ reAs ðT 4b T 4s Þ reAs ðT 4s T 4e Þ dt
ð1Þ
where Ts and ms are the temperature and mass of super heater tubes respectively. Qgr is the amount of heat transferred by gas radiation inside reactor which is defined as following:
Q gr ¼ eg ðT g ÞrT 4g ag ðTÞrT 4 A
ð2Þ
where eg (Tg)sis the gas emittance at Tg and ag (T) is the gas absorptance for the radiation from the black enclosure at T and is a function of both Tg and T. The highest volume fraction of mixture is related to water vapor which is the most important
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H. Abbasfard et al. / Engineering Failure Analysis 26 (2012) 285–292 Table 4 Unit specifications used in the modeling. Plant specifications used in modeling
Value
Unit
Heat capacity of tubes (cps) Heat capacity of ceramic (cpb) Heat capacity of gas (cpg) Weight of super heater (ms) Weight of catalyst basket (mb) Heat transfer coefficient (hf) Stefan–Boltzmann constant (r) Emissivity (e) Gas volume (Vg) Super heater surface area (As) Evaporator surface area (Ae) Catalyst basket surface area (As) Gas emitance (eg) Gas absorptance (ag)
500 850 35 905 500 5 5.669 108 0.9 35 120 360 200 0.5 0.1
kj/kg K kj/kg K kj/kg K kg kg W/m2 °C W/m2 K4 – m3 m2 m2 m2 – –
source of gas radiation. Though for a mixture including water vapor empirical relations exist in literature and were used in this model [18]. The energy equation for the catalyst basket (ceramic packing) and gas inside the reactor can be written as following:
cpb mb
dT b ¼ hf Ab ðT b T g Þ þ Q G:R reAb ðT 4b T 4s Þ reAb ðT 4b T 4e Þ dt
ð3Þ
dT g ¼ hf Ab ðT b T g Þ þ hf As ðT s T g Þ þ hf Ae ðT e T g Þ Q G:R dt
ð4Þ
cpg qg V g
The modeling data requirements were tabulated in Table 4. 5. Numerical solution The model consists of three ordinary differential equations. The most widely used method of integration for ordinary differential equations are the series of methods called Runge–Kutta second, third and fourth order. We used the fourth order method for solving the system of equations. The initial values of temperatures (°C) are:
t ¼ 0 : T s ¼ 480;
T b ¼ 840;
T g ¼ 700
ð5Þ
6. Creep analysis Creep is a time-dependent phenomenon that causes a part failure if it is under both quasistatic load and temperatures higher than 0.3–0.5 Tm (absolute melting temperature). Creep strain may produce sufficiently large deformation or distortion that a part can no longer perform its intended function. There are three basic mechanisms of creep failure. At low stresses, the most likely mechanism is intergranular creep fracture. At high stresses failure, it is usually occurred by transgranular creep fracture. At high temperatures and stresses, dynamic recrystallization operates. Creep in a material would be taken place in three stages: – Where a rapid creep rate is seen at the onset of load application, then it gradually decreases. – Where creep remains at a steady-state rate. – Where the creep rate shows an increasing rate until failure occurs [17]. 7. Results and discussion 7.1. Causes of failure As it was mentioned before, sampling from ruptured bending area was not possible. However, it is generally acceptable that destructive effects of overheating at bending area of ruptured tube can be predicted from the adjacent straight sample which suffered less. According to the visual inspection, external and internal tube oxide layer was detected which was negligible with respect to form a considerable thermal resistance. The diameter increase of some parts of super heater tubes was observed while the tubes were under normal operational temperature of 480 °C and internal pressure of 44 bar during normal unit operation. Chemical analysis revealed the fact that (Table 1) there was no indication of the failure due to any defect in the material itself.
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H. Abbasfard et al. / Engineering Failure Analysis 26 (2012) 285–292
Regarding Table 2, short-term overheating coupled with internal pressure of tubes led to increase in outer diameter of tubes (bulging). On the other hand, bending area of tube showed a slight reduction in diameter at which repeated failures occurred incidentally. Hardness measurements indicated that the phase transformation on the ruptured tube was inevitable. High value of Vickers hardness (260 HV) of ruptured tube is in a good agreement with that of austenitized and then rapidly quenched carbon steel with the same carbon content of 0.15% [19]. Fig. 7 illustrated the dynamic behavior of system which supported the short-term overheating of tube in the event of unit shut down. The failure due to creep was not possible because there were no sign and report of exceeding temperature and pressure (stress) during the normal operation of unit over long time. Considering the hoop stress of 18 MPa, the fracture by creep should be occurred at a temperature around 900 °C for such low-alloy carbon which was not possible physically [10]. Moreover, the modeling findings revealed the fact that the creep could not be happened during the relatively low time under which the tube was overheated. Steam flow blockage through super heater tubes resulted in short-term overheating at the time of shut down. This led to transformation products to bainite or martensite or a combination of both which were followed by increasing hardness of the tube (Table 3) and remarkably decreased the tube thermal resistance. The special form of rupture (window rupture) was another sign of relatively high hardness of ruptured bending area.
7.2. Causes of overheating Heat released by the ammonia oxidation reaction is supplied to the outer surfaces of the evaporator and super heater tubes and removed from the inner surfaces by steam flow. Obviously, anything prevents the steam to be flown results in the heat concentration which in turn ends to overheating. It should be noted that there must be enough steam flow through super heater tubes in order to protect them against overheating. In case of unit shut down, the super heater must be protected against the heat radiation of the catalyst basket (ceramic raschig ring). Fig. 7a showed that the temperature of gas
a
900
800
Temperature (°C)
Gas temperature Ceramic raschig ring temperature
700
Super heater tube temperature
600
500
400
300 0
0.1
0.2
0.3
Time
b
0.4
0.5
(s)
900
800
Temperature (°C)
Gas temperature Ceramic raschig ring temperature
700
Super heater tube temperature
600
500
400
300 0
50
100 Time
150
200
(s)
Fig. 7. Dynamic behavior of system temperature including super heater, gas and catalyst basket in the event of unit shut down.
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H. Abbasfard et al. / Engineering Failure Analysis 26 (2012) 285–292
dropped sharply due to its low heat capacity (Table 4) while the tube and catalyst basket temperature changed slowly. According to Fig. 7b, the super heater tubes experienced the maximum temperature of about 600 °C during the first 25 s after event of shut down. Short-term overheating and bulging of tubes occurred when the heat accumulated inside the pressurized tubes without internal flow by the time of unit shut down when the inlet steam to the turbine was automatically closed. The points mentioned above are considered as the main root causes of overheating. 8. Conclusion Failure analysis on the SA213-T12 super heater tube of a WHB internally coupled with an industrial reactor was presented by visual site inspection, chemical analysis, dimensional measurement, hardness measurements and creep analysis. The lumped dynamic mathematical model considering free convection and radiation was also developed to show the temperature changes in case of unit shut down to support the claims. The failure due to creep was not possible because there were no sign and report of exceeding temperature and pressure (stress) during the normal operation and shut down of unit over long time. Stoppage of flow inside super heater tubes in case of shut down resulted in heat accumulation inside tubes and ended to short-term overheating. The main root cause of repeated failures of the super heater tubes was short-term overheating following rapid cooling of the tubes which lowered the thermal resistance of the super heater tubes in the course of time. It is highly recommended that there must be adequate steam flow for safe cooling of super heater tubes which can be supplied by depressurizing the steam drum through a vent valve on the outlet line. This vent is schematically illustrated in Fig. 1 and can be designed to be opened automatically when unit suddenly shut down. Acknowledgements The authors wish to thank Shiraz Petrochemical Complex and the relevant inspection staff on utilizing all the facilities and resources and their helpful recommendations while conducting this study. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Canale LCF, Mesquita RA, Totten GE. Failure analysis of heat treated steel components. ASM International; 2008. Dennies D. How to organize a failure investigation. ASM International; 2005. Wulpi DJ. Techniques of failure analysis. Understanding how components fail. ASM International; 2000. Chapter 1, p. 1–11. Brooks CR, Choudhury A. Introduction. Metallurgical failure analysis. McGraw-Hill; 1993. Chapter-1, p. 1–72. Graham R. Strategies for failure analysis. Adv Mater Proc Aug 2004:45–50. Ryder DA, Davies TJ, Brough I, Hutchings FR. General practice in failure analysis, failure analysis and prevention. Metals hand book, Vol. 11, 9th ed.. American Society for Metals; 1986. p. 15–46. Vander Voort GF. Conducting the Failure Examination, Prac. Fail. Anal., Vol 1 (No 2), April 2001, p 14–46 and Failure Analysis and Prevention, Vol 11. ASM Hand book, ASM International; 2002. French David N. Metallurgical failures in fossil fired boilers. New York: A Wiley Interscience Publication, John Wiley & Sons Inc.; 2000. Ray AK, Sahay SK, Goswami B. Assessment of service exposed boiler tubes. Eng Fail Anal 2003;10:645–54. Jones DRH. Creep failures of overheated boiler, super heater and reformer tubes. Eng Fail Anal 2004;11:873–93. Das G, Chowdhury SG, Ray AK, Das S, Bhattacharaya DK. Failure of a super heater tube. Eng Fail Anal 2002;9:563–70. Port Robert D, Herro Harvey M. The NALCO guide to boiler failure analysis. Nalco Chemical Company, McGraw-Hill Inc.; 1991. Purbolaksono J, Ahmad J, Beng LC, Rashid AZ, Khinani A, Ali AA. Failure analysis on a primary super heater tube of a power plant. Eng Fail Anal 2010;17:158–67. Kim BS, Chang JC, Jung YK, Jung JJ. Failure analysis of the super heater tubes of S-combined power plant. Eng Fail Anal 2010;17:864–72. Othman H, Purbolaksono J, Ahmad B. Failure investigation on deformed super heater tubes. Eng Fail Anal 2009;16:329–39. Smith GV. Evaluation of the elevated temperature tensile and creep-rupture properties of 1/2Cr–1/2Mo, 1Cr–1/2Mo, and 5/4Cr-1/2Mo steels. Philadelphia (PA): ASTM; 1973. Finnie I, Heller WR. Creep of engineering materials. New York: McGraw-Hill; 1959. Hilpert R. Warmeabgabe von geheizen Drahten und Rohren. Forsch Geb Ingenieurwes 1933;4:220. Golozar MA. Basics and application of heat-treating process of steels. Isfahan University of Technology; 2002.
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis and modeling of super heater tubes of a waste heat boiler thermally coupled in ammonia oxidation reactor Hamed Abbasfard a,⇑, Mehdi Ghanbari b, Amin Ghasemi c, Sattar Ghader a, Hasan Hashemipour Rafsanjani a, Ali Moradi a a
Department of Chemical Engineering, Shahid Bahonar University of Kerman, Kerman, Iran Department of Chemical Engineering, Yasuj University, Yasuj, Iran c Department of Material Science and Engineering, Azad University of Najaf Abad, Najaf Abad, Iran b
a r t i c l e
i n f o
Article history: Received 10 November 2011 Received in revised form 5 June 2012 Accepted 21 June 2012 Available online 5 September 2012 Keywords: Failure analysis Waste heat boiler Super heater tubes Overheating Dynamic modeling
a b s t r a c t Repeated failures have been observed in the outlet region of a few super heater tubes made of 1Cr–0.5Mo steel integrated in a waste heat boiler thermally coupled in an ammonia oxidation reactor. A case study of a failed tube running at around 37,440 h was presented. The bulging of some parts of super heater tubes were observed while the tubes were under normal operational temperature of 480 °C and internal pressure of 44 bar. The tube hardness measurements were carried out on the selected super heater tubes including those of ruptured, unruptured and headers. The relatively high value of hardness, 260 HV, of ruptured part compared to other parts indicates that some phase transformation probably occurred for such a low-alloy 1Cr–0.5Mo steel which led to lower thermal resistance. The dynamic modeling including conservation of energy has been developed in order to show the tube overheating. The main root cause of repeated failures of the super heater tubes was shortterm overheating following rapid cooling of the tubes due to sudden stoppage of steam flow inside connecting tubes in the event of unit shut down which reduced the thermal resistance of the tubes over time. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Material failures can lead to many potentially disastrous consequences, including poor product quality, necessary repair or component or equipment replacement, production down time losses, environmental impact, and even loss of life. Furthermore, failures may arise from various causes, including design, material composition, and improper thermal processing, in the case of metals such as steel. Therefore, it is critically necessary not only to identify these failures but also to determine their root cause when failures do occur. This is considered as the primary objective of this study [1]. Failure analysis is the process of collecting, examining, and interpreting damage evidence. The objective is to understand the possible conditions leading to a failure and perhaps to prevent similar failures in the future. Many researchers [2–7] described the guidelines and methods of failure analysis. In the literature, many scholars have focused on various aspects of failure analysis. However, there are relatively few that have emphasized on steel failures arising from thermal processing as well as thermally integrated systems. Waste heat boiler (WHB) is a heat recovery unit which is thermally coupled in an ammonia oxidation reactor in order to recover the released heat and lower the temperature of reaction products. The WHB consists of a water-tube evaporative
⇑ Corresponding author. Tel.: +98 9173082258; fax: +98 7112314323. E-mail addresses: [email protected], [email protected] (H. Abbasfard). 1350-6307/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2012.06.012
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H. Abbasfard et al. / Engineering Failure Analysis 26 (2012) 285–292
AIR/ AMMOMNIA
The vent valve SUPER HEATED STEAM
STEAM DRUM
FEED WATER Fig. 1. Schematic diagram of ammonia oxidation reactor thermally coupled with waste heat recovery unit.
Fig. 2. Uninstalled connecting tubes of super heater and evaporator.
unit connected in forced circulation flow with a steam drum and a steam super heater which is schematically shown in Fig. 1. There are numerous articles in literature on failure analysis of super heater tubes mostly occurred in power plants. This study is concerned about a different environment under different operation conditions. As reported by French [8], super heater tubes experience the highest probability of failure and have finite life contributing to prolonged exposure to high temperature, stress and aggressive environment which was assessed by Ray [9]. Jones [10] also mentioned that creep failure problems including super heater tube failure and revealed that under which circumstances the failures took place. Detailed discussions on super heater tube failures have been elaborated by French, Das et al. [11], Port and Herro [12] and Purbolaksono et al. [13] considering failure analysis by visual inspection, measurements of hardness and microstructure analysis. Kim et al. [14] added the fatigue life analysis while Othman et al. [15] and Purbolaksono supported their work by finite element method.
287
H. Abbasfard et al. / Engineering Failure Analysis 26 (2012) 285–292
This survey presents a case study of the unexpected failure of the super heater SA213-T12 tube of WHB by visual inspection, chemical and dimensional analysis, in situ measurements of hardness and creep analysis. The dynamic modeling including heat transfer equations also was developed in order to support the claims. It was easily detectable that super heater tube was failed with window rupture burst due to over range value of hardness. Complete access to the ruptured point was not possible due to packed installation (Fig. 2). However, some examinations were carried out on failed tube some parts close to the ruptured point. This work includes helpful findings and analysis for safe running of identical operations. 2. Operation description and condition of failure Heat released by the ammonia oxidation reaction, added to air/ammonia mixture’s sensible heat, raise the temperature of the catalysis gauzes and gases up to about 880 °C. In order to recover the heat as well as lower the temperature of gas, the WHB with the capacity of 27 ton/h is arranged vertically consists of a top, middle and bottom vessel section, as depicted in Fig. 3. Top section contains two evaporative tube rows arranged under catalyst basket, middle section is made up of four super heater tube rows and bottom section is composed of seven evaporative tube rows. The WHB consists of a water-tube evaporative unit connected in forced circulation flow with a steam drum and a steam super heater which is schematically illustrated in Fig. 1. The saturated steam produced through evaporative section is gathered in the two phase constant level steam drum at a pressure of 44 bar and temperature of 260 °C. Such wet steam is then poured into the super heater tubes. The high temperature superheated steam at 480 °C and the same pressure exits the super heater and enters a steam turbine and also to heat the unit’s apparatus after it is desuperheated. New SA213-T12 super heater tubes were fitted in the WHB unit in August 2005. The tubes have outer diameter of 38 mm and thickness of 4 mm. The failure occurred during normal operation of unit which had been shut down in less than 3 days (70 h) before that time in January 2011. According to the record of unit, the failed tube had run at around 37,440 h. 3. Failure analysis 3.1. Visual inspection Visual inspection of the failed tube shown in Fig. 4 was carried out. The access to the ruptured point at the bend of the tube was not possible. However, the examination of the adjacent region to the ruptured point indicated the following findings: – – – –
Finding internal and external (0.5 mm) oxide scale (Figs. 5 and 6). Increasing diameter (bulging) of tube obviously. Finding no evidence of the active corrosion on both internal and external surfaces of the tube. Finding window rupture burst as the type of tube rupture.
Catalyst gauzes 1 Reaction zone Flux of heat by radiation from ceramic raschig ring
2 Ceramic raschig ring 3 Top evaporative section 12 cm 4 Middle super heater section 5 Bottom evaporative section
Fig. 3. Description of different sections inside reactor.
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H. Abbasfard et al. / Engineering Failure Analysis 26 (2012) 285–292
Fig. 4. Ruptured tube in the form of window rupture.
Fig. 5. Oxide scale on the external surface of tube (0.5 mm).
Fig. 6. Oxide scale on the internal surface of tube.
3.2. Chemical analysis During ignition of the platinum catalyst gauzes, ingress of unburnt ammonia gas might occur and thereby causes ammonium nitrates to be formed close to the tubes and corrosive attack may take place. In order to determine the chemical composition of the tubes, a chemical analysis technique of the Energy Dispersive Microscopy (EDS) was performed. The EDS results, as shown in Table 1, indicated that the chemical composition was complied with the SA213-T12 and SA335-P12 original composition of the tube and header, respectively.
289
H. Abbasfard et al. / Engineering Failure Analysis 26 (2012) 285–292 Table 1 Chemical composition of material (wt.%).
Tube Header
Cr
C
Mo
P
S
Mn
Si
Fe
0.80–1.25 1.00–1.50
0.15 0.15
0.44–0.65 0.44–0.65
0.045 0.030
0.045 0.030
0.30–0.61 0.03–0.06
0.50 0.50–1.00
Balanced Balanced
Table 2 Dimensional measurements of the overheated tubes. Original outer diameter (mm) Changed outer diameter (mm) a
38 39.7
38 39.5
38a 37.7
38 38.9
38 39.6
38 38.4
38 39.4
Bending area.
Table 3 Hardness measurements data. Northern tube (HV)
Southern tube (HV)
Header (HV)
Hardness measurements on outlet of superheater 170 260
Ruptured tube (HV)
162
131–137
Hardness measurements on inlet of superheater 129–131 129–131
129–131
131–137
3.3. Tube dimensional measurements Measurements of tube diameter were conducted on seven locations along the ruptured tube; see Table 2. According to the internal and external oxide layer thickness, the slight tube wall thickness reduction was obvious. The creep deformation (bulging) of some part of super heater tubes was observed. 3.4. Hardness measurements In situ measurements of hardness were carried out on the tubes and headers of the super heater at different locations and are tabulated in Table 3. The hardness of ruptured tube is 260 HV which has exceeded the recommended values for this grade of steel while other tubes are among the allowable range. The minimum and maximum hardness limit of this grade of steel are 135 HV and 184 HV respectively [16]. 4. Mathematical modeling of overheating The dynamic behavior of super heater tube temperature was examined by developing the mathematical model in the event of unit shut down. Model assumptions are as following:
Lumped formulation was employed. Ideal gases were assumed. Conduction heat transfer in tubes wall was considered to be negligible. Heat transfer mechanism was supposed to be free convection and radiation. The temperature of evaporation section was assumed to be constant. No steam flow inside the super heater tubes was considered. The energy equation for the super heater can be written as follow:
cps ms
dT s ¼ hf As ðT g T s Þ þ Q gr þ reAs ðT 4b T 4s Þ reAs ðT 4s T 4e Þ dt
ð1Þ
where Ts and ms are the temperature and mass of super heater tubes respectively. Qgr is the amount of heat transferred by gas radiation inside reactor which is defined as following:
Q gr ¼ eg ðT g ÞrT 4g ag ðTÞrT 4 A
ð2Þ
where eg (Tg)sis the gas emittance at Tg and ag (T) is the gas absorptance for the radiation from the black enclosure at T and is a function of both Tg and T. The highest volume fraction of mixture is related to water vapor which is the most important
290
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Value
Unit
Heat capacity of tubes (cps) Heat capacity of ceramic (cpb) Heat capacity of gas (cpg) Weight of super heater (ms) Weight of catalyst basket (mb) Heat transfer coefficient (hf) Stefan–Boltzmann constant (r) Emissivity (e) Gas volume (Vg) Super heater surface area (As) Evaporator surface area (Ae) Catalyst basket surface area (As) Gas emitance (eg) Gas absorptance (ag)
500 850 35 905 500 5 5.669 108 0.9 35 120 360 200 0.5 0.1
kj/kg K kj/kg K kj/kg K kg kg W/m2 °C W/m2 K4 – m3 m2 m2 m2 – –
source of gas radiation. Though for a mixture including water vapor empirical relations exist in literature and were used in this model [18]. The energy equation for the catalyst basket (ceramic packing) and gas inside the reactor can be written as following:
cpb mb
dT b ¼ hf Ab ðT b T g Þ þ Q G:R reAb ðT 4b T 4s Þ reAb ðT 4b T 4e Þ dt
ð3Þ
dT g ¼ hf Ab ðT b T g Þ þ hf As ðT s T g Þ þ hf Ae ðT e T g Þ Q G:R dt
ð4Þ
cpg qg V g
The modeling data requirements were tabulated in Table 4. 5. Numerical solution The model consists of three ordinary differential equations. The most widely used method of integration for ordinary differential equations are the series of methods called Runge–Kutta second, third and fourth order. We used the fourth order method for solving the system of equations. The initial values of temperatures (°C) are:
t ¼ 0 : T s ¼ 480;
T b ¼ 840;
T g ¼ 700
ð5Þ
6. Creep analysis Creep is a time-dependent phenomenon that causes a part failure if it is under both quasistatic load and temperatures higher than 0.3–0.5 Tm (absolute melting temperature). Creep strain may produce sufficiently large deformation or distortion that a part can no longer perform its intended function. There are three basic mechanisms of creep failure. At low stresses, the most likely mechanism is intergranular creep fracture. At high stresses failure, it is usually occurred by transgranular creep fracture. At high temperatures and stresses, dynamic recrystallization operates. Creep in a material would be taken place in three stages: – Where a rapid creep rate is seen at the onset of load application, then it gradually decreases. – Where creep remains at a steady-state rate. – Where the creep rate shows an increasing rate until failure occurs [17]. 7. Results and discussion 7.1. Causes of failure As it was mentioned before, sampling from ruptured bending area was not possible. However, it is generally acceptable that destructive effects of overheating at bending area of ruptured tube can be predicted from the adjacent straight sample which suffered less. According to the visual inspection, external and internal tube oxide layer was detected which was negligible with respect to form a considerable thermal resistance. The diameter increase of some parts of super heater tubes was observed while the tubes were under normal operational temperature of 480 °C and internal pressure of 44 bar during normal unit operation. Chemical analysis revealed the fact that (Table 1) there was no indication of the failure due to any defect in the material itself.
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Regarding Table 2, short-term overheating coupled with internal pressure of tubes led to increase in outer diameter of tubes (bulging). On the other hand, bending area of tube showed a slight reduction in diameter at which repeated failures occurred incidentally. Hardness measurements indicated that the phase transformation on the ruptured tube was inevitable. High value of Vickers hardness (260 HV) of ruptured tube is in a good agreement with that of austenitized and then rapidly quenched carbon steel with the same carbon content of 0.15% [19]. Fig. 7 illustrated the dynamic behavior of system which supported the short-term overheating of tube in the event of unit shut down. The failure due to creep was not possible because there were no sign and report of exceeding temperature and pressure (stress) during the normal operation of unit over long time. Considering the hoop stress of 18 MPa, the fracture by creep should be occurred at a temperature around 900 °C for such low-alloy carbon which was not possible physically [10]. Moreover, the modeling findings revealed the fact that the creep could not be happened during the relatively low time under which the tube was overheated. Steam flow blockage through super heater tubes resulted in short-term overheating at the time of shut down. This led to transformation products to bainite or martensite or a combination of both which were followed by increasing hardness of the tube (Table 3) and remarkably decreased the tube thermal resistance. The special form of rupture (window rupture) was another sign of relatively high hardness of ruptured bending area.
7.2. Causes of overheating Heat released by the ammonia oxidation reaction is supplied to the outer surfaces of the evaporator and super heater tubes and removed from the inner surfaces by steam flow. Obviously, anything prevents the steam to be flown results in the heat concentration which in turn ends to overheating. It should be noted that there must be enough steam flow through super heater tubes in order to protect them against overheating. In case of unit shut down, the super heater must be protected against the heat radiation of the catalyst basket (ceramic raschig ring). Fig. 7a showed that the temperature of gas
a
900
800
Temperature (°C)
Gas temperature Ceramic raschig ring temperature
700
Super heater tube temperature
600
500
400
300 0
0.1
0.2
0.3
Time
b
0.4
0.5
(s)
900
800
Temperature (°C)
Gas temperature Ceramic raschig ring temperature
700
Super heater tube temperature
600
500
400
300 0
50
100 Time
150
200
(s)
Fig. 7. Dynamic behavior of system temperature including super heater, gas and catalyst basket in the event of unit shut down.
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dropped sharply due to its low heat capacity (Table 4) while the tube and catalyst basket temperature changed slowly. According to Fig. 7b, the super heater tubes experienced the maximum temperature of about 600 °C during the first 25 s after event of shut down. Short-term overheating and bulging of tubes occurred when the heat accumulated inside the pressurized tubes without internal flow by the time of unit shut down when the inlet steam to the turbine was automatically closed. The points mentioned above are considered as the main root causes of overheating. 8. Conclusion Failure analysis on the SA213-T12 super heater tube of a WHB internally coupled with an industrial reactor was presented by visual site inspection, chemical analysis, dimensional measurement, hardness measurements and creep analysis. The lumped dynamic mathematical model considering free convection and radiation was also developed to show the temperature changes in case of unit shut down to support the claims. The failure due to creep was not possible because there were no sign and report of exceeding temperature and pressure (stress) during the normal operation and shut down of unit over long time. Stoppage of flow inside super heater tubes in case of shut down resulted in heat accumulation inside tubes and ended to short-term overheating. The main root cause of repeated failures of the super heater tubes was short-term overheating following rapid cooling of the tubes which lowered the thermal resistance of the super heater tubes in the course of time. It is highly recommended that there must be adequate steam flow for safe cooling of super heater tubes which can be supplied by depressurizing the steam drum through a vent valve on the outlet line. This vent is schematically illustrated in Fig. 1 and can be designed to be opened automatically when unit suddenly shut down. Acknowledgements The authors wish to thank Shiraz Petrochemical Complex and the relevant inspection staff on utilizing all the facilities and resources and their helpful recommendations while conducting this study. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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