- Email: [email protected]
Failure investigation on rear water wall tube of boiler
Engineering Failure Analysis 16 (2009) 2325–2332
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure investigation on rear water wall tube of boiler J. Ahmad a, J. Purbolaksono b,*, L.C. Beng a, A.Z. Rashid b, A. Khinani b, A.A. Ali b a b
Kapar Energy Ventures Sdn Bhd, Jalan Tok Muda, Kapar 42200, Malaysia Department of Mechanical Engineering, Universiti Tenaga Nasional, Km 7 Jalan Kajang-Puchong, Kajang 43009, Selangor, Malaysia
a r t i c l e
i n f o
Article history: Received 4 February 2009 Received in revised form 7 March 2009 Accepted 7 March 2009 Available online 16 March 2009 Keywords: Rear water wall tube Failure analysis Fly-ash Visual inspection Wall thinning
a b s t r a c t This paper presents failure investigation on the SA210-A1 rear water wall tube by visual site inspection, tube wall thickness measurements and microscopic examination. The rear water wall tube has failed with wide open burst and was situated at the boiler nose lower bent region. On-site wall thickness measurements were performed on all the rear water wall tubes located at the same level of the ruptured tube. The tubes were observed to have experienced significant wall thinning. Microscopic examinations on the failed rupture region and some distance away region of the as-received tubes are also conducted in order to support in determining the failure mechanism and failure root cause. Failure mechanisms are discussed and the findings obtained from the site inspection, wall thickness measurements, microscopic examinations and creep analyses may finally reveal the failure mechanism and main root cause of the failure. The failure mechanism is identified as a result of the combination of the significant localized wall thinning of the rear water wall tube due to fly-ash erosion and a thermally activated process of creep problem due to increasing of temperatures. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Failures in boiler tubes have been discussed and summarized in Refs. [1,2]. As reported in Ref. [1], erosion is listed in top ten failure causes, and based on samples submitted to Davis N. French Inc., it gives 6.5% of the all the failure causes. According to the failures by location, water wall tubes are the second highest failure location after superheater tubes. However, according to the failures by material, carbon steel tubes statistically lead as the most frequent material causing failures. Failure investigation on the SA210-A1 rear water wall tube by visual site inspection, tube wall thickness measurements and microscopic examinations is presented in this paper. The tube originally has outer diameter of 76.2 mm and thickness of 8 mm. A rear water wall tube with a wide open burst was found and was situated at the boiler nose lower bent region. Onsite wall thickness measurements are also performed on all the rear water wall tubes located at the same level of the ruptured tube. The tubes had operated at temperatures of 350–360 °C. The operational steam pressure is reported at 16.5 MPa. The tubes are observed to have experienced significant erosion. In order to support in determining the failure mechanism and failure root cause, microscopic examinations on the failed rupture region and some distance away region of the as-received tubes are also conducted. At the corresponding operating tube temperature the operational hoop stresses are determined and compared with the allowable stress for SA210-A1 tube stated in ASME Code [3]. Creep analyses are also carried out to check the possibility of a thermally activated process involved in failure mechanism. Failure mechanisms are discussed and the main root cause of the failure may be deduced from the findings obtained from the visual site inspection, wall thickness measurements, microscopic examinations and creep analyses.
* Corresponding author. Tel.: +60 3 89212213; fax: +60 3 89212116. E-mail address: [email protected] (J. Purbolaksono). 1350-6307/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2009.03.012
2326
J. Ahmad et al. / Engineering Failure Analysis 16 (2009) 2325–2332
2. Historical backgrounds of the boiler It was reported that the boiler started in service since August 1988 and the rear water wall tube at the boiler nose lower bent region failed in February 2007. Hence, it was estimated that the tube had been in service for 146,000 h. Several mandatory preventive maintenance practices for inspection purposes were carried out at every 30 months prior to failure. During the outages the inspection was carried out to observe conditions and any possible abnormalities especially in the refractory and the surrounding tubes. The activities were also conducted to remove the accumulations of the fly-ash and to perform appropriate maintenances on the refractory. It can be estimated that the failure of the rear water wall tube occurred during the service after the last preventive maintenance was conducted. It is also estimated that the erosion on the rear water wall tubes might have taken place approximately in the last 16,000 h prior to failure by considering that six mandatory outages for inspections and maintenances had been conducted. 3. Visual inspections During site inspection it was found that the rear water wall tube experienced a wide open burst with thin edges as shown in Fig. 1. It may indicate that the tube has significant localized wall thinning and is eventually followed by a sudden failure. Close visual inspection on site also revealed that all the rear water wall tubes located at the boiler nose lower bent region at the same level of the failed tube also experienced significant wall thinning (see Fig. 2). Based on this finding, on-site wall thickness measurements on the 167 tubes at the boiler nose lower bent region need to be carried out. The thickness readings obtained are then used to identify the tubes which might need to be replaced. Further important finding during the site inspection is discovered. It revealed that the refractory behind along the rear water wall tubes at the boiler nose lower bent region as shown in Fig. 3 is badly damaged. It subsequently leaves empty
Fig. 1. The SA210-A1 rear water wall tube experienced a wide open burst with thin edges.
Fig. 2. Wall thinning on the rear water wall tubes located at the boiler nose lower bent region at the same level of the failed tube.
J. Ahmad et al. / Engineering Failure Analysis 16 (2009) 2325–2332
2327
Fig. 3. The damaged refractory behind the rear water wall tubes of the boiler nose lower bent region.
space between the refractory and the rear water wall tubes. The damaged refractory is then suspected to be the root cause of the problem. Visual examinations on the failed tube shown in Fig. 1 revealed findings as follows: – The dimensions of the opening burst of the tube are 41 cm in length and 9.5 cm in width, and no swelling was found elsewhere. – There was no significant scale formed on both internal and external surfaces of the tube. – There was no evident of the active corrosion on both internal and external surfaces of the tube. – There were significant signs of the localized wall thinning of the tube due to erosion as noticed by the smooth surface. The thinning is indicated by the thickness readings shown in Fig. 2. The findings might suggest that the tube has suffered fly-ash erosion prior to failure, and they can be used to describe the failure mechanism of the problem. 4. Wall thickness measurements Wall thickness measurements at various locations of the failed tube are performed as shown in Fig. 1. The thickness readings are summarized as follows: – At the rupture lips/edges, the wall thickness is in the range of 1.10–2.06 mm. – At the rupture ends of bottom and top, the wall thickness are 2.80–2.11 mm, respectively. Fig. 1 and the thickness readings show that the tube suffers a sudden failure after severe wall thinning acted for period of time. On-site wall thickness measurements on the 167 lower bent tubes of boiler nose region are carried out. The thickness readings for the tubes are obtained and plotted in Fig. 4. It can be seen from Fig. 4, all the readings are significantly lower than the required wall thickness (7.58 mm) to be safely used. The average wall thickness of the tubes is 3.94 mm. It clearly leads to the operating hoop stress becoming higher. Immediate remedy actions were taken to replace all the tubes which have the wall thickness less than 3 mm and to rectify the broken refractory. The boiler unit was forced to continue in service with a bad condition until the next planned outage (11 months since the rectification of the failed rear water wall tube was taken). However, no failure of the thinned rear water wall tubes was reported until the next outage. 5. Microscopic examinations Microscopic examinations are carried out on the as-received tube at the rupture edges and some distance away from the rupture region. The metal structure of the failed tube as shown in Fig. 5 indicates that the microstructure has normal ferrite structures and spheroidization of carbide from pearlite does not appear. However, it does not mean that the local overheat-
2328
J. Ahmad et al. / Engineering Failure Analysis 16 (2009) 2325–2332 9 original wall thickness, 8 mm
8
Wall thickness of tubes, mm
minimum wall thickness required, 7.58 mm
7 6 5 4
average thickness, 3.94 mm
3 2 1 1
11 21 31 41 51 61 71 81 91 101 111 121 131 141 151 161
No. of tube Fig. 4. Wall thickness readings on the 167 rear water wall tubes at the same level of the failed tube.
Fig. 5. Photomicrograph of metal structure of the failed tube showing normal pearlite and ferrite.
ing has not occurred. The overheating which may have occurred has not heated the tube metal to the temperature of around 600 °C at which spheroidization would commence. It indicates that the tube might have operated at higher temperature than the normal operating temperature at the time of failure. Hence, it may suggest that the creep analyses need to be carried out. 6. Discussion In addition to indication obtained from the microscopic examination for performing creep analysis, further attentions are addressed to the findings from the wall thickness measurements and the site inspections. The wall thickness measurements revealed that all the rear water wall tubes at the boiler nose lower bent region experienced significant wall thinning. The tubes consequently have a higher operating hoop stress. The estimated hoop stress developed in the tube may be determined as
rh ¼ P
ðr þ 2t Þ t
ð1Þ
where P is operational internal pressure; r and t are inner radius and wall thickness of the tube, respectively. The operational and allowable stresses of the tubes at the normal operating temperature of 360 °C for different wall thickness are presented in Fig. 6. Most tubes have operating hoop stress exceeding the allowable stress for SA210-A1 tube as stated in ASME Code [3]. It indicates that the tubes would have potential failure.
J. Ahmad et al. / Engineering Failure Analysis 16 (2009) 2325–2332
2329
The operating hoop stress, MPa
250
200
150
100
The allow able stress at operating temperature of 3600C [3]
50
0 1
11 21 31 41 51 61 71 81 91 101 111 121 131 141 151 161
No. of tube Fig. 6. The operating hoop stresses of the thinned rear water walled tubes.
During site inspection, in addition to findings of significant wall thinning at all the rear water wall tubes located at the boiler nose lower bent region, another important evident of the finding of the damaged refractory may be used to complete the investigation. The damaged parts of the refractory are exactly behind the rear water wall tubes of boiler nose lower bent region. According to the Refractory Institute [4], refractory is used to constitute the linings for high temperature furnace and other processing units in boiler. In addition to being resistant to thermal stress and other physical phenomena induced by heat, refractory must also withstand physical wear and corrosion by chemical agents. The damaged refractory behind the rear water wall tube at the boiler nose lower bent region is identified to have caused short circuiting of the flue gas flow into the economizer-primary superheater region. It consequently causes continuous striking of flue gas containing fly-ash onto surfaces of the rear water wall tubes. This condition leads to fly-ash erosion caused by particulate matter entrained in high-speed flue gases striking metal surfaces. High gas velocity and large amount of abrasive components in the fly-ash accelerate erosive metal loss by increasing the amount of kinetic energy per impact and by increasing the number of impacts per unit time on a given area of surface. In addition, the short circuiting of the flue gas flow into the failure region reasonably results in potential local overheating of the tubes. The tube metal might be subjected to the increasing of temperatures over period of time. As found from the visual inspection on the failed tube, the localized wall thinning of the tube due to erosion as noticed by the smoothly polished surface agrees with the identification of fly-ash erosion problem. It is also supported by findings showing that there is no evident of active corrosion and significant scale formed on both inner and outer surfaces of the tube. However, finding from the microscopic examination indicates that the failed tube might have been exposed to higher than the normal operating temperature at the time of failure. Hence, it might indicate that there is possibility of the involved creep problem as a result of the increasing of the tube temperatures. In order to check possibility of the creep problem as a thermally activated process over period of time prior to failure, creep damage estimations need to be performed. The most common approach for calculating the cumulative creep damage is computing the amount of life expended by using time fraction as measures of damage. When the fractional damages add up to unity, then the failure is postulated to occur. The most prominent rule is given as [5]
X t si t ri
¼1
ð2Þ
where tsi is the service time and tri is the time to rupture. Diagram of Larsen–Miller parameter with stress variation to rupture of carbon steel (ASTM) [6] as shown in Fig. 7 is utilized to determine the rupture time. The average curve correlating stress variation and Larsen–Miller parameter is used. According to the historical background of the boiler and wall thinning measurements, two scenarios for estimating the creep rate under the service temperatures and the local stress at the thinned region may be made. The first scenario is the creep rate for the tube to be estimated under the normal operating temperature of 360 °C. The second scenario is the tube to be estimated under increasing of temperatures. The increasing of temperature may be assumed to be moderately higher than the normal operating temperature, e.g. 415 °C. The service time is divided into 10 periods of time, i.e. 1600 h each, for both scenarios. The increments of the increased temperature are linearly assumed over the service period prior to failure, i.e. 5.5 °C at every 1600 h. The average wall thickness taken from the rupture ends of bottom (2.80 mm) and top (2.11 mm) of the rupture region is used as a reference of wall thinning rate in order to estimate the operating hoop stress. It also considers that the thinned wall tube did not occur before the last outages/inspections were conducted. Hence, the linear wall thinning rate may be obtained equal to 5.54/16,000 mm/h.
2330
J. Ahmad et al. / Engineering Failure Analysis 16 (2009) 2325–2332
Fig. 7. Diagram of Larsen–Miller parameter with stress variation to rupture of carbon steel (ASTM) [6] (1 ksi = 6.895 MPa).
The estimations of cumulative creep damage under the normal operating temperature (the first scenario) and under the increasing of the tube temperatures are presented in Tables 1 and 2, respectively. It can be seen from Table 1, the creep rupture should not take place since the cumulative fractions of life remain less than one. The condition is compromised to be good in service even though the operating hoop stress exceeds the maximum allowable stress. It also agrees with the fact that no failure of the rear water wall tubes was reported after the boiler unit was forced to continue in service until the next planned outage as mentioned earlier. Next, it can be seen from Table 2, the failed rear water wall tube is confirmed by the estimations of the cumulative creep damage. The estimations are made relatively only based on the increase of 55 °C. However, the temperature increase of the tube could be higher than 55 °C. It can be deduced from all the findings that the failure mechanism is identified as a result of the combination of the significant localized wall thinning of the rear water wall tube much lower than the required minimum thickness due to fly-ash erosion and a thermally activated process of creep problem due to increasing of temperatures. Consequently it results in the tube subjected to the increasing hoop stresses under increasing of temperature. The damaged refractory behind the rear
Table 1 Estimations of cumulative creep damage under the normal operating temperature of the tube. Period of service, h
Wall thickness, mm
Hoop stress, MPa
Tube temperature, °C
Larsen–Miller parameter
Time to rupture, h
Cumulative creep damage
130,000 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
8 7.446 6.892 6.338 5.784 5.230 4.676 4.122 3.568 3.014 2.460
70.33 74.95 80.31 86.61 94.12 103.21 114.46 128.74 147.45 173.03 210.14
360 360 360 360 360 360 360 360 360 360 360
33,400 33,200 32,700 32,400 31,900 31,400 31,000 30,500 29,600 28,600 27,200
1,987,218,465 1,326,804,750 483,293,024 263,665,090 96,040,882 34,983,209 15,594,895 5,680,492 922,385 122,382 7239
6.54E05 6.66E05 6.99E05 7.6E05 9.27E05 0.000138 0.000241 0.000523 0.002257 0.015331 0.236371
2331
J. Ahmad et al. / Engineering Failure Analysis 16 (2009) 2325–2332 Table 2 Estimations of cumulative creep damage under increasing of the tube temperatures. Period of service, h
Wall thickness, mm
Hoop stress, MPa
Tube temperature, °C
Larsen–Miller parameter
Time to rupture, h
Cumulative creep damage
130,000 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
8 7.446 6.892 6.338 5.784 5.230 4.676 4.122 3.568 3.014 2.460
70.33 74.95 80.31 86.61 94.12 103.21 114.46 128.74 147.45 173.03 210.14
360.0 365.5 371.0 376.5 382.0 387.5 393.0 398.5 404.0 409.5 415.0
33,400 33,200 32,700 32,400 31,900 31,400 31,000 30,500 29,600 28,600 27,200
1,987,218,465 744,862,024 156,500,704 50,050,694 11,042,450 2,498,309 701,870 166,419 18,976 1858 90
6.54E05 6.76E05 7.78E05 0.00011 0.000255 0.000895 0.003175 0.012789 0.097104 0.958219 18.77921
Fig. 8. Chronological root of the failure in the rear water wall tube region.
water wall tubes at the boiler nose region is found to be the main root cause the failure. The chronological root of the failure may be sketched and shown in Fig. 8. The refractory should be inspected frequently and maintained periodically to ensure the system working properly. 7. Conclusions Failure investigation on the SA210-A1 rear water wall tube by visual site inspection, tube wall thickness measurements, microscopic examinations and creep analyses was presented. On-site wall thickness measurements were performed on all the rear water wall tubes located at the same level of the ruptured tube. The tubes were observed to have experienced sig-
2332
J. Ahmad et al. / Engineering Failure Analysis 16 (2009) 2325–2332
nificant wall thinning. Microscopic examinations on the failed rupture region and some distance away region of the as-received tubes are also conducted in order to support in determining the failure mechanism and failure root cause. The combination of the significant localized wall thinning of the rear water wall tube much lower than the required minimum thickness due to fly-ash erosion and a thermally activated process of creep problem due to increasing of temperatures was identified as the failure mechanism of the problem. The damaged refractory behind the rear water wall tubes at the boiler nose region was found chronologically to be the main root cause of the failure. Acknowledgements The authors wish to thank Kapar Energy Ventures Sdn Bhd, Malaysia and Universiti Tenaga Nasional, Malaysia for permission of utilizing all the facilities and resources while conducting this study. References [1] [2] [3] [4] [5] [6]
French David N. Metallurgical failures in fossil fired boilers. New York: A Wiley-Interscience Publication, John Wiley & Sons Inc.; 2000. Port Robert D, Herro Harvey M. The NALCO guide to boiler failure analysis. Nalco Chemical Company, McGraw-Hill Inc.; 1991. ASME code for pressure piping, B31. ASME B31.1-2001 power piping. Table A1. Carbon Steel; 2001, p. 105. The Refractory Institute, Centre City Tower, 650 Smithfield Street, Suite 1160, Pittsburgh, PA 15222 (). Robinson EL. Effect of the temperature variation on the creep strength of steels. Transactions ASME 1938;160:253–9. Smith GV. An evaluation of the elevated tensile and creep–rupture properties of wrought carbon steel DSIISI. Philadelphia (PA): ASTM; 1970.
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure investigation on rear water wall tube of boiler J. Ahmad a, J. Purbolaksono b,*, L.C. Beng a, A.Z. Rashid b, A. Khinani b, A.A. Ali b a b
Kapar Energy Ventures Sdn Bhd, Jalan Tok Muda, Kapar 42200, Malaysia Department of Mechanical Engineering, Universiti Tenaga Nasional, Km 7 Jalan Kajang-Puchong, Kajang 43009, Selangor, Malaysia
a r t i c l e
i n f o
Article history: Received 4 February 2009 Received in revised form 7 March 2009 Accepted 7 March 2009 Available online 16 March 2009 Keywords: Rear water wall tube Failure analysis Fly-ash Visual inspection Wall thinning
a b s t r a c t This paper presents failure investigation on the SA210-A1 rear water wall tube by visual site inspection, tube wall thickness measurements and microscopic examination. The rear water wall tube has failed with wide open burst and was situated at the boiler nose lower bent region. On-site wall thickness measurements were performed on all the rear water wall tubes located at the same level of the ruptured tube. The tubes were observed to have experienced significant wall thinning. Microscopic examinations on the failed rupture region and some distance away region of the as-received tubes are also conducted in order to support in determining the failure mechanism and failure root cause. Failure mechanisms are discussed and the findings obtained from the site inspection, wall thickness measurements, microscopic examinations and creep analyses may finally reveal the failure mechanism and main root cause of the failure. The failure mechanism is identified as a result of the combination of the significant localized wall thinning of the rear water wall tube due to fly-ash erosion and a thermally activated process of creep problem due to increasing of temperatures. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Failures in boiler tubes have been discussed and summarized in Refs. [1,2]. As reported in Ref. [1], erosion is listed in top ten failure causes, and based on samples submitted to Davis N. French Inc., it gives 6.5% of the all the failure causes. According to the failures by location, water wall tubes are the second highest failure location after superheater tubes. However, according to the failures by material, carbon steel tubes statistically lead as the most frequent material causing failures. Failure investigation on the SA210-A1 rear water wall tube by visual site inspection, tube wall thickness measurements and microscopic examinations is presented in this paper. The tube originally has outer diameter of 76.2 mm and thickness of 8 mm. A rear water wall tube with a wide open burst was found and was situated at the boiler nose lower bent region. Onsite wall thickness measurements are also performed on all the rear water wall tubes located at the same level of the ruptured tube. The tubes had operated at temperatures of 350–360 °C. The operational steam pressure is reported at 16.5 MPa. The tubes are observed to have experienced significant erosion. In order to support in determining the failure mechanism and failure root cause, microscopic examinations on the failed rupture region and some distance away region of the as-received tubes are also conducted. At the corresponding operating tube temperature the operational hoop stresses are determined and compared with the allowable stress for SA210-A1 tube stated in ASME Code [3]. Creep analyses are also carried out to check the possibility of a thermally activated process involved in failure mechanism. Failure mechanisms are discussed and the main root cause of the failure may be deduced from the findings obtained from the visual site inspection, wall thickness measurements, microscopic examinations and creep analyses.
* Corresponding author. Tel.: +60 3 89212213; fax: +60 3 89212116. E-mail address: [email protected] (J. Purbolaksono). 1350-6307/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2009.03.012
2326
J. Ahmad et al. / Engineering Failure Analysis 16 (2009) 2325–2332
2. Historical backgrounds of the boiler It was reported that the boiler started in service since August 1988 and the rear water wall tube at the boiler nose lower bent region failed in February 2007. Hence, it was estimated that the tube had been in service for 146,000 h. Several mandatory preventive maintenance practices for inspection purposes were carried out at every 30 months prior to failure. During the outages the inspection was carried out to observe conditions and any possible abnormalities especially in the refractory and the surrounding tubes. The activities were also conducted to remove the accumulations of the fly-ash and to perform appropriate maintenances on the refractory. It can be estimated that the failure of the rear water wall tube occurred during the service after the last preventive maintenance was conducted. It is also estimated that the erosion on the rear water wall tubes might have taken place approximately in the last 16,000 h prior to failure by considering that six mandatory outages for inspections and maintenances had been conducted. 3. Visual inspections During site inspection it was found that the rear water wall tube experienced a wide open burst with thin edges as shown in Fig. 1. It may indicate that the tube has significant localized wall thinning and is eventually followed by a sudden failure. Close visual inspection on site also revealed that all the rear water wall tubes located at the boiler nose lower bent region at the same level of the failed tube also experienced significant wall thinning (see Fig. 2). Based on this finding, on-site wall thickness measurements on the 167 tubes at the boiler nose lower bent region need to be carried out. The thickness readings obtained are then used to identify the tubes which might need to be replaced. Further important finding during the site inspection is discovered. It revealed that the refractory behind along the rear water wall tubes at the boiler nose lower bent region as shown in Fig. 3 is badly damaged. It subsequently leaves empty
Fig. 1. The SA210-A1 rear water wall tube experienced a wide open burst with thin edges.
Fig. 2. Wall thinning on the rear water wall tubes located at the boiler nose lower bent region at the same level of the failed tube.
J. Ahmad et al. / Engineering Failure Analysis 16 (2009) 2325–2332
2327
Fig. 3. The damaged refractory behind the rear water wall tubes of the boiler nose lower bent region.
space between the refractory and the rear water wall tubes. The damaged refractory is then suspected to be the root cause of the problem. Visual examinations on the failed tube shown in Fig. 1 revealed findings as follows: – The dimensions of the opening burst of the tube are 41 cm in length and 9.5 cm in width, and no swelling was found elsewhere. – There was no significant scale formed on both internal and external surfaces of the tube. – There was no evident of the active corrosion on both internal and external surfaces of the tube. – There were significant signs of the localized wall thinning of the tube due to erosion as noticed by the smooth surface. The thinning is indicated by the thickness readings shown in Fig. 2. The findings might suggest that the tube has suffered fly-ash erosion prior to failure, and they can be used to describe the failure mechanism of the problem. 4. Wall thickness measurements Wall thickness measurements at various locations of the failed tube are performed as shown in Fig. 1. The thickness readings are summarized as follows: – At the rupture lips/edges, the wall thickness is in the range of 1.10–2.06 mm. – At the rupture ends of bottom and top, the wall thickness are 2.80–2.11 mm, respectively. Fig. 1 and the thickness readings show that the tube suffers a sudden failure after severe wall thinning acted for period of time. On-site wall thickness measurements on the 167 lower bent tubes of boiler nose region are carried out. The thickness readings for the tubes are obtained and plotted in Fig. 4. It can be seen from Fig. 4, all the readings are significantly lower than the required wall thickness (7.58 mm) to be safely used. The average wall thickness of the tubes is 3.94 mm. It clearly leads to the operating hoop stress becoming higher. Immediate remedy actions were taken to replace all the tubes which have the wall thickness less than 3 mm and to rectify the broken refractory. The boiler unit was forced to continue in service with a bad condition until the next planned outage (11 months since the rectification of the failed rear water wall tube was taken). However, no failure of the thinned rear water wall tubes was reported until the next outage. 5. Microscopic examinations Microscopic examinations are carried out on the as-received tube at the rupture edges and some distance away from the rupture region. The metal structure of the failed tube as shown in Fig. 5 indicates that the microstructure has normal ferrite structures and spheroidization of carbide from pearlite does not appear. However, it does not mean that the local overheat-
2328
J. Ahmad et al. / Engineering Failure Analysis 16 (2009) 2325–2332 9 original wall thickness, 8 mm
8
Wall thickness of tubes, mm
minimum wall thickness required, 7.58 mm
7 6 5 4
average thickness, 3.94 mm
3 2 1 1
11 21 31 41 51 61 71 81 91 101 111 121 131 141 151 161
No. of tube Fig. 4. Wall thickness readings on the 167 rear water wall tubes at the same level of the failed tube.
Fig. 5. Photomicrograph of metal structure of the failed tube showing normal pearlite and ferrite.
ing has not occurred. The overheating which may have occurred has not heated the tube metal to the temperature of around 600 °C at which spheroidization would commence. It indicates that the tube might have operated at higher temperature than the normal operating temperature at the time of failure. Hence, it may suggest that the creep analyses need to be carried out. 6. Discussion In addition to indication obtained from the microscopic examination for performing creep analysis, further attentions are addressed to the findings from the wall thickness measurements and the site inspections. The wall thickness measurements revealed that all the rear water wall tubes at the boiler nose lower bent region experienced significant wall thinning. The tubes consequently have a higher operating hoop stress. The estimated hoop stress developed in the tube may be determined as
rh ¼ P
ðr þ 2t Þ t
ð1Þ
where P is operational internal pressure; r and t are inner radius and wall thickness of the tube, respectively. The operational and allowable stresses of the tubes at the normal operating temperature of 360 °C for different wall thickness are presented in Fig. 6. Most tubes have operating hoop stress exceeding the allowable stress for SA210-A1 tube as stated in ASME Code [3]. It indicates that the tubes would have potential failure.
J. Ahmad et al. / Engineering Failure Analysis 16 (2009) 2325–2332
2329
The operating hoop stress, MPa
250
200
150
100
The allow able stress at operating temperature of 3600C [3]
50
0 1
11 21 31 41 51 61 71 81 91 101 111 121 131 141 151 161
No. of tube Fig. 6. The operating hoop stresses of the thinned rear water walled tubes.
During site inspection, in addition to findings of significant wall thinning at all the rear water wall tubes located at the boiler nose lower bent region, another important evident of the finding of the damaged refractory may be used to complete the investigation. The damaged parts of the refractory are exactly behind the rear water wall tubes of boiler nose lower bent region. According to the Refractory Institute [4], refractory is used to constitute the linings for high temperature furnace and other processing units in boiler. In addition to being resistant to thermal stress and other physical phenomena induced by heat, refractory must also withstand physical wear and corrosion by chemical agents. The damaged refractory behind the rear water wall tube at the boiler nose lower bent region is identified to have caused short circuiting of the flue gas flow into the economizer-primary superheater region. It consequently causes continuous striking of flue gas containing fly-ash onto surfaces of the rear water wall tubes. This condition leads to fly-ash erosion caused by particulate matter entrained in high-speed flue gases striking metal surfaces. High gas velocity and large amount of abrasive components in the fly-ash accelerate erosive metal loss by increasing the amount of kinetic energy per impact and by increasing the number of impacts per unit time on a given area of surface. In addition, the short circuiting of the flue gas flow into the failure region reasonably results in potential local overheating of the tubes. The tube metal might be subjected to the increasing of temperatures over period of time. As found from the visual inspection on the failed tube, the localized wall thinning of the tube due to erosion as noticed by the smoothly polished surface agrees with the identification of fly-ash erosion problem. It is also supported by findings showing that there is no evident of active corrosion and significant scale formed on both inner and outer surfaces of the tube. However, finding from the microscopic examination indicates that the failed tube might have been exposed to higher than the normal operating temperature at the time of failure. Hence, it might indicate that there is possibility of the involved creep problem as a result of the increasing of the tube temperatures. In order to check possibility of the creep problem as a thermally activated process over period of time prior to failure, creep damage estimations need to be performed. The most common approach for calculating the cumulative creep damage is computing the amount of life expended by using time fraction as measures of damage. When the fractional damages add up to unity, then the failure is postulated to occur. The most prominent rule is given as [5]
X t si t ri
¼1
ð2Þ
where tsi is the service time and tri is the time to rupture. Diagram of Larsen–Miller parameter with stress variation to rupture of carbon steel (ASTM) [6] as shown in Fig. 7 is utilized to determine the rupture time. The average curve correlating stress variation and Larsen–Miller parameter is used. According to the historical background of the boiler and wall thinning measurements, two scenarios for estimating the creep rate under the service temperatures and the local stress at the thinned region may be made. The first scenario is the creep rate for the tube to be estimated under the normal operating temperature of 360 °C. The second scenario is the tube to be estimated under increasing of temperatures. The increasing of temperature may be assumed to be moderately higher than the normal operating temperature, e.g. 415 °C. The service time is divided into 10 periods of time, i.e. 1600 h each, for both scenarios. The increments of the increased temperature are linearly assumed over the service period prior to failure, i.e. 5.5 °C at every 1600 h. The average wall thickness taken from the rupture ends of bottom (2.80 mm) and top (2.11 mm) of the rupture region is used as a reference of wall thinning rate in order to estimate the operating hoop stress. It also considers that the thinned wall tube did not occur before the last outages/inspections were conducted. Hence, the linear wall thinning rate may be obtained equal to 5.54/16,000 mm/h.
2330
J. Ahmad et al. / Engineering Failure Analysis 16 (2009) 2325–2332
Fig. 7. Diagram of Larsen–Miller parameter with stress variation to rupture of carbon steel (ASTM) [6] (1 ksi = 6.895 MPa).
The estimations of cumulative creep damage under the normal operating temperature (the first scenario) and under the increasing of the tube temperatures are presented in Tables 1 and 2, respectively. It can be seen from Table 1, the creep rupture should not take place since the cumulative fractions of life remain less than one. The condition is compromised to be good in service even though the operating hoop stress exceeds the maximum allowable stress. It also agrees with the fact that no failure of the rear water wall tubes was reported after the boiler unit was forced to continue in service until the next planned outage as mentioned earlier. Next, it can be seen from Table 2, the failed rear water wall tube is confirmed by the estimations of the cumulative creep damage. The estimations are made relatively only based on the increase of 55 °C. However, the temperature increase of the tube could be higher than 55 °C. It can be deduced from all the findings that the failure mechanism is identified as a result of the combination of the significant localized wall thinning of the rear water wall tube much lower than the required minimum thickness due to fly-ash erosion and a thermally activated process of creep problem due to increasing of temperatures. Consequently it results in the tube subjected to the increasing hoop stresses under increasing of temperature. The damaged refractory behind the rear
Table 1 Estimations of cumulative creep damage under the normal operating temperature of the tube. Period of service, h
Wall thickness, mm
Hoop stress, MPa
Tube temperature, °C
Larsen–Miller parameter
Time to rupture, h
Cumulative creep damage
130,000 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
8 7.446 6.892 6.338 5.784 5.230 4.676 4.122 3.568 3.014 2.460
70.33 74.95 80.31 86.61 94.12 103.21 114.46 128.74 147.45 173.03 210.14
360 360 360 360 360 360 360 360 360 360 360
33,400 33,200 32,700 32,400 31,900 31,400 31,000 30,500 29,600 28,600 27,200
1,987,218,465 1,326,804,750 483,293,024 263,665,090 96,040,882 34,983,209 15,594,895 5,680,492 922,385 122,382 7239
6.54E05 6.66E05 6.99E05 7.6E05 9.27E05 0.000138 0.000241 0.000523 0.002257 0.015331 0.236371
2331
J. Ahmad et al. / Engineering Failure Analysis 16 (2009) 2325–2332 Table 2 Estimations of cumulative creep damage under increasing of the tube temperatures. Period of service, h
Wall thickness, mm
Hoop stress, MPa
Tube temperature, °C
Larsen–Miller parameter
Time to rupture, h
Cumulative creep damage
130,000 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
8 7.446 6.892 6.338 5.784 5.230 4.676 4.122 3.568 3.014 2.460
70.33 74.95 80.31 86.61 94.12 103.21 114.46 128.74 147.45 173.03 210.14
360.0 365.5 371.0 376.5 382.0 387.5 393.0 398.5 404.0 409.5 415.0
33,400 33,200 32,700 32,400 31,900 31,400 31,000 30,500 29,600 28,600 27,200
1,987,218,465 744,862,024 156,500,704 50,050,694 11,042,450 2,498,309 701,870 166,419 18,976 1858 90
6.54E05 6.76E05 7.78E05 0.00011 0.000255 0.000895 0.003175 0.012789 0.097104 0.958219 18.77921
Fig. 8. Chronological root of the failure in the rear water wall tube region.
water wall tubes at the boiler nose region is found to be the main root cause the failure. The chronological root of the failure may be sketched and shown in Fig. 8. The refractory should be inspected frequently and maintained periodically to ensure the system working properly. 7. Conclusions Failure investigation on the SA210-A1 rear water wall tube by visual site inspection, tube wall thickness measurements, microscopic examinations and creep analyses was presented. On-site wall thickness measurements were performed on all the rear water wall tubes located at the same level of the ruptured tube. The tubes were observed to have experienced sig-
2332
J. Ahmad et al. / Engineering Failure Analysis 16 (2009) 2325–2332
nificant wall thinning. Microscopic examinations on the failed rupture region and some distance away region of the as-received tubes are also conducted in order to support in determining the failure mechanism and failure root cause. The combination of the significant localized wall thinning of the rear water wall tube much lower than the required minimum thickness due to fly-ash erosion and a thermally activated process of creep problem due to increasing of temperatures was identified as the failure mechanism of the problem. The damaged refractory behind the rear water wall tubes at the boiler nose region was found chronologically to be the main root cause of the failure. Acknowledgements The authors wish to thank Kapar Energy Ventures Sdn Bhd, Malaysia and Universiti Tenaga Nasional, Malaysia for permission of utilizing all the facilities and resources while conducting this study. References [1] [2] [3] [4] [5] [6]
French David N. Metallurgical failures in fossil fired boilers. New York: A Wiley-Interscience Publication, John Wiley & Sons Inc.; 2000. Port Robert D, Herro Harvey M. The NALCO guide to boiler failure analysis. Nalco Chemical Company, McGraw-Hill Inc.; 1991. ASME code for pressure piping, B31. ASME B31.1-2001 power piping. Table A1. Carbon Steel; 2001, p. 105. The Refractory Institute, Centre City Tower, 650 Smithfield Street, Suite 1160, Pittsburgh, PA 15222 (