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Failure analysis of MVR (machinery vapor recompressor) impeller blade
Engineering Failure Analysis 10 (2003) 307–315 www.elsevier.com/locate/engfailanal
Failure analysis of MVR (machinery vapor recompressor) impeller blade Tae-Gu Kima,*, Hong-Chul Leeb a
Department of Safety Engineering, INJE University, Gimhae, Gyeongnam, 621-749, Republic of Korea b Engine Division, ATRI(Aero-Tech Research Institute), ROKAF, PO Box 304-160, Kumsa dong, Dong gu, Deagu, 701-799, Republic of Korea Received 11 October 2002; accepted 14 October 2002
Abstract This dissertation studies an accident resulting from the breakdown of an MVR impeller blade. Visual, stereoscopic and, SEM examinations have been carried out to find out the causes of the MVR blade defects. The results show that failure of the MVR blade was due to material casting defects. An initial crack started from casting defects where stress intensified and then, a fatigue crack progressed to the critical length of this crack. In addition to these defects, overstressing caused by overspeed accelerated the catastrophic failure of the blade. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Impeller failures; Fatigue; Casting defects; Vaibration; Non-destructive testing
1. Introduction The compressor is one of the important pieces of equipment at a sugar factory where reduced productivity was experienced as a result of damage to the impeller blade. This paper emphasizes the importance of taking preventative measures through accident investigation. The damaged cast impeller blade was used for 13 years and showed signs of a crack due to fatigue from exterior examination. It is thus assumed that there is a possibility of casting defects during manufacture and also, the change of operating conditions caused by external factors that effect rotating speed. Therefore, this paper will focus on the analysis of visual, stereoscopic, and SEM examinations to find out the exact causes of the impeller breakdown. There are three identified causes from analysis of impellor accidents: The first cause is defects, such as avoids or cavities during the casting process [1,2]. The second is increasing speed stimulated by the formation of cavity and rapid spread of rust [3]. The third is rust created by the intensity of friction and the velocity of the moving fluid across each part [4]. * Corresponding author. Tel.: +82-55-320-3539; fax: +82-55-325-2471. E-mail address: [email protected] (T.-Gu Kim). 1350-6307/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S1350-6307(02)00077-8
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2. Nature of accident The MVR, as a mechanical compressor, is used to increase the concentration of the refined liquid sugar for crystallization. The factories were supplied with power from two private electric generators (5 MW, 3 MW). The MVR was powered by the generators, and turned at 11,750 RPM. When the accident happened, there was a problem with the speed control system. The frequency of power supply increased from 60 Hz to 64 Hz, causing the speed of theimpeller to increase from 11,750 RPM to 12,530 RPM. As the result of that increase, the electric current and the vibration went up instantaneously, and the impeller blade broke. A picture of the broken impeller is shown in Fig. 1. The process at the factory is shown in Fig. 2 and the accident occurred during the concentration stage.
3. Results of analysis 3.1. Visual examination Beach marks as a broad distinctive feature are identified clearly just by visual observation. The fatigue crack progressed to 10.5 cm and then failed completely. As shown in Fig. 3, cracks existed on the fracture
Fig. 1. Photograph showing the failed MVR impeller blade.
Fig. 2. The workflow of the factory.
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Fig. 3. Visual examination of fracture surface.
surface and several abrasive wear areas were considered as artificial damage after final separation or relative motion between the fracture surfaces. The chemical composition analysis of the failed impeller blade using the ICP (Inductively Coupled Plasma Spectroscopy) shows that it is similar to CB-7Cu-2 of cast stainless as shown in Table 1. There was not sufficient information about the material because the broken impeller was old and the model is no longer in production. 3.2. Stereoscopic examination From a wide-angle view, observation of the fracture surface permits it to be classified very clearly into two parts: The first is a rough and brittle fracture surface The second is a semicircle of the beach marks.
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Table 1 Chemical analysis of failed blade Item
CB-7Cu-2 Blade
Composition C
Si
Mn
Ni
Cr
Cu
Nb
Fe
0.07 0.06
1.00 0.34
0.70 0.44
5.007.00 5.21
14.0 15.5 12.67
2.53.2 1.33
0.20.35 0.21
Other Other
Fig. 4. Stereoscopic examination of initial flaw and fatigue fracture.
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When the semicircle of the beach marks enlarged, it is easy to identify a 1.5 mm initial crack from which the crack propagated, Fig. 4. 3.3. SEM examination Fig. 5 shows the morphology of the fracture surface through the use of SEM to find the cause of the initial crack. As seen in Fig. 6, the fracture surface created by the fatigue crack is smooth, but the initial crack looks coarse (Fig. 7).
Fig. 5. SEM micrograph showing appearance of fracture origin.
Fig. 6. SEM micrograph showing difference between origin and fatigue fracture.
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Fig. 7. SEM micrograph showing initial flaw nucleated by surface defects.
The side of the initial crack has a surface crack that is considered to exist before the fatigue crack. EDS analysis of part of the initial flaw and fatigue fracture surface (Fig. 8) shows that there is a big difference in the quantity of the elements C, O and F, which are compared to the corrosive oxide Na and Ca which do not exist on the fatigue fracture surface, but are detected in the initial flaw. There are lots of surface cracks, as seen in Figs. 9 and 10 that were discovered on the fracture surface of the impeller blade, which occurred during the casting process. The nucleation of the fatigue crack in the impeller blade is considered the result of casting defects. The surface crack became the initial crack and then, as stress intensified on this area, a fatigue crack progressed into a critical crack. In addition to these
Fig. 8. EDS spectrum analysis of initial flaw (left) and fatigue fracture surface(right).
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Fig. 10. SEM micrograph showing material defects (pores, cavity) in the surface of the failed component.
defects, overstressing from overspeed of the impeller accelerated the catastrophic failure of the MVR impeller blade.
4. Discussion No fault was found in the MVR involved in the accident through Non-Destructive Testing (NDT) of the blade during factory overall. NDT on the blade was carried out 2 weeks before the accident. The defect could not be detected through NDT. It is assumed that the accident happened from a fatigue crack through continued operation and changes in operational conditions.
314
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Observation of the polished surface to check the material for porosity, which is easy to find in castings, is shown in Fig. 11. Fig. 12 shows the results of observation of the columnar structure. It confirms that the defect occurred when the impeller blade was produced. There is a need for improvements in the production technique of the impeller blade in order that cracks are avoided during the casting process, and a close examination is necessary to find fatigue cracks. When regular inspections are conducted it is necessary to evaluate the accuracy of the examination carefully. It is
Fig. 11. Metallography showing porosity found in the material.
Fig. 12. Metallography showing the columnar structure.
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315
also necessary to make a close examination of the crack on the fragile part to prevent the recurrence of similar accidents. It is suggested that providing vibration recording monitoring and an auto trip system would be a sensible precaution
5. Conclusions From the investigation and analysis on the failure of MVR impeller blade, the conclusion can be summarized as follows; First, the initial crack in the MVR impeller blade was nucleated by the material-casting defect after a prolonged period of surface crack prorogation. Second, it is assumed that the MVR fractured due to a fatigue crack after a long period of peak operation (about 13 years). Third, overheating caused by the over-speed of the impeller blade in the MVR due to an increase in the oscillation frequency, is considered to be the final cause of the MVR breakdown.
Acknowledgements This work was supported by the 2002 Inje University research grant.
References [1] van Bennekom A, Berndt F, Rassool MN. Pump impeller failures—a compendium of case studies. Engineering Failure Analysis 2001;8(2):145–56. [2] Colangelo VJ, Heiser FA. Analysis of metallurgical failures. New York: Wiley; 1974. [3] Aiming F, Jinming L, Ziyun T. Failure analysis of the impeller of a slurry pump subjected to corrosive wear. Wear 1995;181183(2):876–82. [4] Prakash O, Pandey RK. Failure analysis of the impellers of a feed pump. Engineering Failure Analysis 1996;3(1):45–52.
Failure analysis of MVR (machinery vapor recompressor) impeller blade Tae-Gu Kima,*, Hong-Chul Leeb a
Department of Safety Engineering, INJE University, Gimhae, Gyeongnam, 621-749, Republic of Korea b Engine Division, ATRI(Aero-Tech Research Institute), ROKAF, PO Box 304-160, Kumsa dong, Dong gu, Deagu, 701-799, Republic of Korea Received 11 October 2002; accepted 14 October 2002
Abstract This dissertation studies an accident resulting from the breakdown of an MVR impeller blade. Visual, stereoscopic and, SEM examinations have been carried out to find out the causes of the MVR blade defects. The results show that failure of the MVR blade was due to material casting defects. An initial crack started from casting defects where stress intensified and then, a fatigue crack progressed to the critical length of this crack. In addition to these defects, overstressing caused by overspeed accelerated the catastrophic failure of the blade. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Impeller failures; Fatigue; Casting defects; Vaibration; Non-destructive testing
1. Introduction The compressor is one of the important pieces of equipment at a sugar factory where reduced productivity was experienced as a result of damage to the impeller blade. This paper emphasizes the importance of taking preventative measures through accident investigation. The damaged cast impeller blade was used for 13 years and showed signs of a crack due to fatigue from exterior examination. It is thus assumed that there is a possibility of casting defects during manufacture and also, the change of operating conditions caused by external factors that effect rotating speed. Therefore, this paper will focus on the analysis of visual, stereoscopic, and SEM examinations to find out the exact causes of the impeller breakdown. There are three identified causes from analysis of impellor accidents: The first cause is defects, such as avoids or cavities during the casting process [1,2]. The second is increasing speed stimulated by the formation of cavity and rapid spread of rust [3]. The third is rust created by the intensity of friction and the velocity of the moving fluid across each part [4]. * Corresponding author. Tel.: +82-55-320-3539; fax: +82-55-325-2471. E-mail address: [email protected] (T.-Gu Kim). 1350-6307/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S1350-6307(02)00077-8
308
T.-G. Kim, H.-C. Lee / Engineering Failure Analysis 10 (2003) 307–315
2. Nature of accident The MVR, as a mechanical compressor, is used to increase the concentration of the refined liquid sugar for crystallization. The factories were supplied with power from two private electric generators (5 MW, 3 MW). The MVR was powered by the generators, and turned at 11,750 RPM. When the accident happened, there was a problem with the speed control system. The frequency of power supply increased from 60 Hz to 64 Hz, causing the speed of theimpeller to increase from 11,750 RPM to 12,530 RPM. As the result of that increase, the electric current and the vibration went up instantaneously, and the impeller blade broke. A picture of the broken impeller is shown in Fig. 1. The process at the factory is shown in Fig. 2 and the accident occurred during the concentration stage.
3. Results of analysis 3.1. Visual examination Beach marks as a broad distinctive feature are identified clearly just by visual observation. The fatigue crack progressed to 10.5 cm and then failed completely. As shown in Fig. 3, cracks existed on the fracture
Fig. 1. Photograph showing the failed MVR impeller blade.
Fig. 2. The workflow of the factory.
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309
Fig. 3. Visual examination of fracture surface.
surface and several abrasive wear areas were considered as artificial damage after final separation or relative motion between the fracture surfaces. The chemical composition analysis of the failed impeller blade using the ICP (Inductively Coupled Plasma Spectroscopy) shows that it is similar to CB-7Cu-2 of cast stainless as shown in Table 1. There was not sufficient information about the material because the broken impeller was old and the model is no longer in production. 3.2. Stereoscopic examination From a wide-angle view, observation of the fracture surface permits it to be classified very clearly into two parts: The first is a rough and brittle fracture surface The second is a semicircle of the beach marks.
310
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Table 1 Chemical analysis of failed blade Item
CB-7Cu-2 Blade
Composition C
Si
Mn
Ni
Cr
Cu
Nb
Fe
0.07 0.06
1.00 0.34
0.70 0.44
5.007.00 5.21
14.0 15.5 12.67
2.53.2 1.33
0.20.35 0.21
Other Other
Fig. 4. Stereoscopic examination of initial flaw and fatigue fracture.
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311
When the semicircle of the beach marks enlarged, it is easy to identify a 1.5 mm initial crack from which the crack propagated, Fig. 4. 3.3. SEM examination Fig. 5 shows the morphology of the fracture surface through the use of SEM to find the cause of the initial crack. As seen in Fig. 6, the fracture surface created by the fatigue crack is smooth, but the initial crack looks coarse (Fig. 7).
Fig. 5. SEM micrograph showing appearance of fracture origin.
Fig. 6. SEM micrograph showing difference between origin and fatigue fracture.
312
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Fig. 7. SEM micrograph showing initial flaw nucleated by surface defects.
The side of the initial crack has a surface crack that is considered to exist before the fatigue crack. EDS analysis of part of the initial flaw and fatigue fracture surface (Fig. 8) shows that there is a big difference in the quantity of the elements C, O and F, which are compared to the corrosive oxide Na and Ca which do not exist on the fatigue fracture surface, but are detected in the initial flaw. There are lots of surface cracks, as seen in Figs. 9 and 10 that were discovered on the fracture surface of the impeller blade, which occurred during the casting process. The nucleation of the fatigue crack in the impeller blade is considered the result of casting defects. The surface crack became the initial crack and then, as stress intensified on this area, a fatigue crack progressed into a critical crack. In addition to these
Fig. 8. EDS spectrum analysis of initial flaw (left) and fatigue fracture surface(right).
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313
Fig. 10. SEM micrograph showing material defects (pores, cavity) in the surface of the failed component.
defects, overstressing from overspeed of the impeller accelerated the catastrophic failure of the MVR impeller blade.
4. Discussion No fault was found in the MVR involved in the accident through Non-Destructive Testing (NDT) of the blade during factory overall. NDT on the blade was carried out 2 weeks before the accident. The defect could not be detected through NDT. It is assumed that the accident happened from a fatigue crack through continued operation and changes in operational conditions.
314
T.-G. Kim, H.-C. Lee / Engineering Failure Analysis 10 (2003) 307–315
Observation of the polished surface to check the material for porosity, which is easy to find in castings, is shown in Fig. 11. Fig. 12 shows the results of observation of the columnar structure. It confirms that the defect occurred when the impeller blade was produced. There is a need for improvements in the production technique of the impeller blade in order that cracks are avoided during the casting process, and a close examination is necessary to find fatigue cracks. When regular inspections are conducted it is necessary to evaluate the accuracy of the examination carefully. It is
Fig. 11. Metallography showing porosity found in the material.
Fig. 12. Metallography showing the columnar structure.
T.-G. Kim, H.-C. Lee / Engineering Failure Analysis 10 (2003) 307–315
315
also necessary to make a close examination of the crack on the fragile part to prevent the recurrence of similar accidents. It is suggested that providing vibration recording monitoring and an auto trip system would be a sensible precaution
5. Conclusions From the investigation and analysis on the failure of MVR impeller blade, the conclusion can be summarized as follows; First, the initial crack in the MVR impeller blade was nucleated by the material-casting defect after a prolonged period of surface crack prorogation. Second, it is assumed that the MVR fractured due to a fatigue crack after a long period of peak operation (about 13 years). Third, overheating caused by the over-speed of the impeller blade in the MVR due to an increase in the oscillation frequency, is considered to be the final cause of the MVR breakdown.
Acknowledgements This work was supported by the 2002 Inje University research grant.
References [1] van Bennekom A, Berndt F, Rassool MN. Pump impeller failures—a compendium of case studies. Engineering Failure Analysis 2001;8(2):145–56. [2] Colangelo VJ, Heiser FA. Analysis of metallurgical failures. New York: Wiley; 1974. [3] Aiming F, Jinming L, Ziyun T. Failure analysis of the impeller of a slurry pump subjected to corrosive wear. Wear 1995;181183(2):876–82. [4] Prakash O, Pandey RK. Failure analysis of the impellers of a feed pump. Engineering Failure Analysis 1996;3(1):45–52.