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Thermal fatigue analysis on cracked plenum barrier plate of open-cycle gas turbine frame
Engineering Failure Analysis 17 (2010) 579–586
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Thermal fatigue analysis on cracked plenum barrier plate of open-cycle gas turbine frame A.Z. Rashid a, J. Purbolaksono a,*, A. Ahmad a, S.A. Ahmad b a b
Department of Mechanical Engineering, Universiti Tenaga Nasional, Km 7 Jalan Kajang-Puchong, Kajang 43009, Selangor, Malaysia Putrajaya Power Station Sdn Bhd, Precint 13, Putrajaya 43009, Malaysia
a r t i c l e
i n f o
Article history: Received 23 June 2009 Accepted 11 October 2009 Available online 3 November 2009 Keywords: Cracking Failure analysis Gas turbine Thermal fatigue Finite element
a b s t r a c t Failure investigation is carried out following repeating findings of several obvious surfacecrack spots on the weld joint zone of a plenum barrier plate of open-cycle gas turbine frame. The exhausted hot compressed air will flow through the cracks entering the load compartment which contains temperature sensitive instrumentations and shaft bearings. The modified model of the barrier plate is presented for redesign consideration. Visual inspection, microscopic examination on the cracked barrier plate and thermal fatigue analyses based on strain–life approach are carried out. The life expectancies of the original and modified models for the barrier plate design are evaluated. The finding indicates that the modified model could withstand against the thermal fatigue longer than that of the original model. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction An open-cycle gas turbine is a combustion turbine plant fired by liquid fuel to turn a generator rotor that produces electricity. The residual heat is commonly exhausted to atmosphere at about 550 °C. This technology is similar in design to the combustion turbines, or jet engines, used in the aviation industry. A compressor absorbs air from the atmosphere and compresses it through a number of compressor stages. Fuel is pumped into a combustion chamber and mixed with the compressed air. The fuel/air mixture is then ignited to form hot and high velocity gas. The gas is passed through turbine blades that rotate the shaft that is attached to the rotor of the generator. The rotor rotates inside the stator and electricity is generated. Schematic illustration of the exhaust stage arrangement in the gas turbine is shown in Fig. 1. Post-diffuser exhaust compartment and pre-generator load compartment of the gas turbine are separated by the plenum plate. This study is concerned with the cracked problem in open-cycle gas turbine frame of Putrajaya Power Station Malaysia. The power station is a peaking power plant serving the load center of an area in Malaysia comprising Kuala Lumpur and its suburbs, and adjoining cities and towns in the state of Selangor. Its operating regime is of two shift cycles, operating between 12 and 16 h daily mainly to meet the load demand during peak hours and stabilize the grid line voltage. This work presents failure investigation following repeating findings of several obvious crack spots in a plenum barrier plate of gas turbine frame in Putrajaya Power Station Malaysia. Welding works were always blamed to cause cracks in the plenum barrier plate. Even though the welding works had been taken carefully, the crack problems still persisted. Then, the redesign of the barrier plate of the gas turbine frame is concerned. The thermal fatigue analyses based on strain–life approach on the models of original and modified designs need to be carried out. Thermal and strain analyses are evaluated by using finite element software package of ANSYS [1]. If the surface cracks become through thickness cracks, it will lead to serious situation by
* Corresponding author. Tel.: +60 3 89212213. E-mail address: [email protected] (J. Purbolaksono). 1350-6307/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2009.10.013
580
A.Z. Rashid et al. / Engineering Failure Analysis 17 (2010) 579–586
Fig. 1. Exhaust segment arrangement in gas turbine.
which the exhausted hot compressed air will enter the load compartment. It may destroy the control and instrumentation wirings, the shaft bearings and the other surrounding components in the compartment. Fig. 2 shows the studied area and exhausted hot gas invasion in case of the cracked weld. Close inspections on the barrier plate are conducted. Specimens are taken from the cracked barrier plate and examined by the scanning electron microscopy (SEM) for microstructure analyses. The results are presented and discussed in order to support the investigation. The gas turbine investigated is a 110 MW General Electric Frame 9E model. 2. Visual inspections Fig. 3 shows exhaust region and the schematic illustration of GE Frame 9E gas turbine. There are surface cracks that can be seen obviously at several locations of the barrier plate. The component containing obvious cracks is shown in Fig. 4. It was reported that the operating temperature of the hot compressed air is approximately 550 °C. The cracks are found at the barrier plate as the base metal and at weld metal. The base metal and weld metal are made of RA 253 MAÒ. Material of RA 253 MAÒ is an austenitic heat resistant alloy with high strength and outstanding oxidation resistance [2]. Chemical compositions of RA 253 MAÒ are shown in Table 1. Shielded metal arc welding (SMAW) method is used for the welding process in assembling the plenum barrier plate of the gas turbine frame. SMAW method is welding process to produce metal coalescence by the heat from an electric arc that is maintained between the tip of flux-coated, stick electrode and the surface of the base metal being welded [3]. The number and size of slag inclusions developed during welding process can be minimized by maintaining sufficiently high weld-metal temperatures, maintaining weld metal at low viscosity and preventing rapid solidification. 3. Microscopic examinations Specimen for microstructure analysis is taken from cracked zone at the bottom part of the barrier plate as shown in Fig. 4. This location is chosen due to the accessibility in obtaining the specimen. The specimen is used to analyze the detailed
Fig. 2. Studied area and hot exhaust gas invasion in case of the cracked weld.
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A.Z. Rashid et al. / Engineering Failure Analysis 17 (2010) 579–586
Fig. 3. Exhaust region and the schematic illustration of GE Frame 9E gas turbine.
Fig. 4. The barrier plate where several surface cracks are found (left and middle) and the close-up of obvious cracks (right).
Table 1 Chemical composition of RA 253 MAÒ (%) [2]. Alloy
Ni
Cr
C
Fe
Mn
Si
N
Ce
RA 253 MA
10
21
0.07
66
0.6
1.6
0.16
0.05
Fig. 5. Cracks at weld joint as result of thermal fatigue.
features of materials at the cracked base metal and weld metal. It represents the heat-affected zone (HAZ). Examination is carried out in order to observe any mechanical behaviour process of the materials.
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A.Z. Rashid et al. / Engineering Failure Analysis 17 (2010) 579–586
Fig. 6. Geometry of the original design.
Fig. 5 shows cracks developed at the interface between the base metal and weld metal. The cracks likely indicated that thermal fatigue had occurred. The differences of the thermal expansion between the base metal and weld metal encourage development of mismatch thermal stress due to the fluctuated operating temperatures. It agrees with the operational backgrounds that the gas turbine had been subjected to frequent start-ups and shut-downs for servicing peak duty. However, fatigue analysis on the component under fluctuated operating temperatures needs to be carried out to evaluate the reliability of the barrier plate structure of the original model as well as the modified model as proposed. Finite element analyses are carried out to compute the temperature distributions and the corresponding strains developed in the metals of the component. 4. Finite element analysis Models of the original and modified designs are evaluated by finite element method in order to determine the metal temperature distributions. Next, the metal temperature distributions are then used as the thermal loading to obtain the corresponding thermal elastic strains. The residual stresses developed in weld joints are omitted for simplification. The components are modeled as two-dimensional axisymmetric solid. Two-dimensional triangle elements are used to discretize the models. Data of the original design from the Putrajaya Power Station are used to generate the geometry of the model and as a reference for the modified design.
Fig. 7. Geometry of the modified design.
583
A.Z. Rashid et al. / Engineering Failure Analysis 17 (2010) 579–586 Table 2 Material properties of RA 253 MA [2]. Properties
Temperature (°C)
Modulus of elasticity (109 Pa) Poisson’s ratio Specific heat capacity (J/kg K) Thermal conductivity (W/m C) Thermal expansion coefficient (106 °C1) Ultimate tensile strength (106 Pa) Yield tensile strength (106 Pa) Elongation percentage (%) Reduction of area (%) Density (kg/m3)
20
204
427
649
760
982
200 0.31 440 14.5 16.8 600 310 40 50 7800
185
168
150
139
121
490 17.5 17.7
544 20.2 18.3
595 22.5 18.5
624 24.2 19.4
687 28.7
Table 3 Material properties of dry air [4]. Properties
Specific heat capacity (J/kg K) Thermal conductivity (103 W/m C) Thermal expansion coefficient (103 °C1) Density (kg/m3)
Temperature (°C) 0
20
100
200
500
1000
1006 24.18 3.674 1.275
1007 25.69 3.421 1.188
1012 31.39 2.683 0.9329
1026 37.95 2.115 0.7356
1093 55.64 1.293 0.4502
1185 80.77 0.785 0.2734
Table 4 Thermal conductivity of the insulating material [5]. Temperature (°C)
Thermal conductivity (W/m C)
260
538
816
982
0.06
0.14
0.24
2.09
4.1. Geometrical models Fig. 6 shows the cross-sectional area of the original design of the plenum barrier plate. A simplified weld representation (in circle of Fig. 6) is used for all the weld joints (Zones 5 and 9). This original model is discretized with 2900 elements in total. The model of the modified design is shown in Fig. 7. Improvements are expected from the model by providing a buffer zone (Zone 8) to anticipate the flowing hot air entering directly to the compartment in case of the cracked weld (Zone 2). The number of element for analyzing the modified model is 3000 elements.
4.2. Material properties Properties of RA 253 MAÒ [2] is listed in Table 2. In this study, weld material also uses RA 253 MAÒ to illustrate a better scenario of analyzing thermal fatigue for the original design. Thus, the dissimilar weld issue becomes less. Table 3 lists dry air physical properties obtained from VDI-Wärmeatlas [4]. The insulating material properties [5] are listed in Table 4.
4.3. Boundary conditions In this work, while performing thermal analyses, it is considered that the rate of the hot compressed air flow is constant. However, since the gas flow has high velocity, the phenomenon of the contact condition between the flowing hot air and the metals is considered as forced convection heat transfer. The convection coefficient for the hot exhausted air film of 5 W/ m2 °C is used in this study. The rest of heat transfer in the system is conduction process. Transient heat transfer analyses are carried out until temperature of the hot compressed air achieves the steady state condition (550 °C) for the peak duty period. In order to carry out structural analyses, two displacement constraints in the radial and axial directions are applied to the models as indicated in Figs. 6 and 7.
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5. Fatigue analysis In order to estimate the life expectancy of the plenum barrier plate, a simple approximation is carried by utilizing a technique proposed by Muralidharan and Manson [6]. The method called Universal Slopes technique is a common way in fatigue life analysis in cases where cyclic life properties are unknown. The equation for the Universal Slopes technique is as follows [6]:
0:832 0:155 Su 0:53 De Su ð2 N f Þ0:09 þ 0:0196 ef ð2 N f Þ0:56 ¼ 0:623 2 E E
ð1Þ
Universal Slopes method describes the relationship between De which is two times the cyclic strain amplitude and the cyclic strain–life Nf. The ultimate tensile strength Su, the Young’s modulus E and the true fracture strain ef are obtained from the monotonic material properties listed in Table 2. The true fracture strain ef is taken as the percent reduction is area (% RA) [7] and may be expressed as
ef ¼ ln
100 ð100 RA%Þ
ð2Þ
9
2
1
3 167.455 165.791 164.126 162.462 160.797 159.133 157.468 155.805 154.139 152.475
5 7
6 4
Fig. 8. Temperature distribution (°C) of the original model.
Fig. 9. Temperature distribution (°C) of the modified model.
8
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A.Z. Rashid et al. / Engineering Failure Analysis 17 (2010) 579–586
1 8
10 9
3 0.00568 0.00517 0.00465 0.00414 0.00362 0.00311 0.00260 0.00208 0.00157 0.00105
5 2
4
Fig. 10. Von Mises elastic strain distribution of the modified model.
Substituting data for RA 253 MA from Table 2 into Eq. (1), the strain–life equation may be expressed as
De ¼ 0:004660 Nf0:09 þ 0:2730 N0:56 f 2
ð3Þ
This relationship gives an approximated life cycle for any given cyclic strain amplitude. The life cycles may be then calculated by iterative technique using Newton Raphson method. 6. Results and discussion Fig. 8 shows the temperature distribution of the original model. It can be seen from Fig. 8, the high temperature is concentrated in the area of the overlap weld joint (Zone 2). This is identified due to direct continuous contact with the hot exhausted gas and the shape of the barrier plate design. The high temperatures are also found in the overlapping RA 253 MA sheets (Zones 1 and 3) in addition to the weld fillet (Zone 2). The high temperatures are not expected in the insulator region (Zone 6) and the air region (Zone 7). Fig. 9 indicates a better temperature distribution in the modified model. Although the higher maximum temperature is found in the modified design, the higher temperatures are distributed in a single sheet metal (Zone 1). This would not lead to a critical condition compared to those distributed at connections of multiple parts. The temperature at the tee weld joint (Zone 5) also shows a significant improvement by decreasing of 10 °C compared with that of the original design.
9 1 3 0.142 0.127
5 7
0.111 0.095 0.079 0.063 0.048 0.032
2
4
0.016 0.000 Fig. 11. Von Mises elastic strain distribution of the original model.
8
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A.Z. Rashid et al. / Engineering Failure Analysis 17 (2010) 579–586
Fig. 12. Strain–life curve for barrier plate designs.
Structural analyses show promising findings from the modified model. It can be seen from Fig. 10, thermal strain developed at weld joint (Zone 2) is reduced by 60% compared with that of the original design (Fig. 11). The life expectancies obtained by using Eg. (3) are presented in Fig. 12. It is shown that the estimated life for the modified model by considering the most critical location (Zone 2 in Figs. 10 and 11) is improved by more than 20 times than that of the original model. It is clear that redesigning the plenum barrier plate of the gas turbine frame is necessary to be carried out. Residual stress developed in HAZ as result of the welding process is however unavoidable and it might worsen the susceptibility of the crack problems. Hence, welding process needs also to be given serious attention and requires fair operators for best results. 7. Conclusions Visual inspection, microscopic examination on the cracked barrier plate and thermal fatigue analyses based on strain–life approach were carried out. The modified model of the barrier plate was presented for redesign consideration and the comparison to the original design was made. The life expectancies of the original and modified models for the barrier plate design were evaluated. The finding indicated that the modified model could withstand against thermal fatigue longer than that of the original model. Acknowledgements This work is supported by Universiti Tenaga Nasional Malaysia through the research project of UNITEN Seeding Fund J510050137 and Ministry of Science, Technology and Innovation Malaysia through research project of Science Fund 04-02-03-SF0003. The authors also wish to thank Putrajaya Power Station Malaysia for permission of utilizing all the facilities and TNB REMACO Sdn Bhd Malaysia for assisting tasks for visual inspections during completion of this study. References [1] [2] [3] [4] [5] [6]
ANSYS workbench version 11.0. ANSYS, Inc. Southpointe 275, Technology Drive Canonsburg, PA 15317; 2008. Davies JR. Alloys digest sourcebook: stainless steels. ASM International; 2000. Messler Jr RW. Principles of welding: processes, physics, chemistry and metallurgy. John Wiley & Sons, Inc.; 1999. Hering E, Martin R, Stohrer M. Physik für Ingenieure. 7th ed. Springer Verlag; 1999. Thermal ceramics. SuperwoolÒ bulk and blanket product information; 2008. Muralidharan U, Manson SS. Modified universal slopes equation for estimation of fatigue characteristics. Trans ASME, J Eng Mater Technol 1988;110:55–8. [7] Stephens RI, Fatemi A, Stephens RR, Fuchs HO. Metal fatigue in engineering. 2nd ed. John Wiley & Sons, Inc.; 2001.
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Thermal fatigue analysis on cracked plenum barrier plate of open-cycle gas turbine frame A.Z. Rashid a, J. Purbolaksono a,*, A. Ahmad a, S.A. Ahmad b a b
Department of Mechanical Engineering, Universiti Tenaga Nasional, Km 7 Jalan Kajang-Puchong, Kajang 43009, Selangor, Malaysia Putrajaya Power Station Sdn Bhd, Precint 13, Putrajaya 43009, Malaysia
a r t i c l e
i n f o
Article history: Received 23 June 2009 Accepted 11 October 2009 Available online 3 November 2009 Keywords: Cracking Failure analysis Gas turbine Thermal fatigue Finite element
a b s t r a c t Failure investigation is carried out following repeating findings of several obvious surfacecrack spots on the weld joint zone of a plenum barrier plate of open-cycle gas turbine frame. The exhausted hot compressed air will flow through the cracks entering the load compartment which contains temperature sensitive instrumentations and shaft bearings. The modified model of the barrier plate is presented for redesign consideration. Visual inspection, microscopic examination on the cracked barrier plate and thermal fatigue analyses based on strain–life approach are carried out. The life expectancies of the original and modified models for the barrier plate design are evaluated. The finding indicates that the modified model could withstand against the thermal fatigue longer than that of the original model. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction An open-cycle gas turbine is a combustion turbine plant fired by liquid fuel to turn a generator rotor that produces electricity. The residual heat is commonly exhausted to atmosphere at about 550 °C. This technology is similar in design to the combustion turbines, or jet engines, used in the aviation industry. A compressor absorbs air from the atmosphere and compresses it through a number of compressor stages. Fuel is pumped into a combustion chamber and mixed with the compressed air. The fuel/air mixture is then ignited to form hot and high velocity gas. The gas is passed through turbine blades that rotate the shaft that is attached to the rotor of the generator. The rotor rotates inside the stator and electricity is generated. Schematic illustration of the exhaust stage arrangement in the gas turbine is shown in Fig. 1. Post-diffuser exhaust compartment and pre-generator load compartment of the gas turbine are separated by the plenum plate. This study is concerned with the cracked problem in open-cycle gas turbine frame of Putrajaya Power Station Malaysia. The power station is a peaking power plant serving the load center of an area in Malaysia comprising Kuala Lumpur and its suburbs, and adjoining cities and towns in the state of Selangor. Its operating regime is of two shift cycles, operating between 12 and 16 h daily mainly to meet the load demand during peak hours and stabilize the grid line voltage. This work presents failure investigation following repeating findings of several obvious crack spots in a plenum barrier plate of gas turbine frame in Putrajaya Power Station Malaysia. Welding works were always blamed to cause cracks in the plenum barrier plate. Even though the welding works had been taken carefully, the crack problems still persisted. Then, the redesign of the barrier plate of the gas turbine frame is concerned. The thermal fatigue analyses based on strain–life approach on the models of original and modified designs need to be carried out. Thermal and strain analyses are evaluated by using finite element software package of ANSYS [1]. If the surface cracks become through thickness cracks, it will lead to serious situation by
* Corresponding author. Tel.: +60 3 89212213. E-mail address: [email protected] (J. Purbolaksono). 1350-6307/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2009.10.013
580
A.Z. Rashid et al. / Engineering Failure Analysis 17 (2010) 579–586
Fig. 1. Exhaust segment arrangement in gas turbine.
which the exhausted hot compressed air will enter the load compartment. It may destroy the control and instrumentation wirings, the shaft bearings and the other surrounding components in the compartment. Fig. 2 shows the studied area and exhausted hot gas invasion in case of the cracked weld. Close inspections on the barrier plate are conducted. Specimens are taken from the cracked barrier plate and examined by the scanning electron microscopy (SEM) for microstructure analyses. The results are presented and discussed in order to support the investigation. The gas turbine investigated is a 110 MW General Electric Frame 9E model. 2. Visual inspections Fig. 3 shows exhaust region and the schematic illustration of GE Frame 9E gas turbine. There are surface cracks that can be seen obviously at several locations of the barrier plate. The component containing obvious cracks is shown in Fig. 4. It was reported that the operating temperature of the hot compressed air is approximately 550 °C. The cracks are found at the barrier plate as the base metal and at weld metal. The base metal and weld metal are made of RA 253 MAÒ. Material of RA 253 MAÒ is an austenitic heat resistant alloy with high strength and outstanding oxidation resistance [2]. Chemical compositions of RA 253 MAÒ are shown in Table 1. Shielded metal arc welding (SMAW) method is used for the welding process in assembling the plenum barrier plate of the gas turbine frame. SMAW method is welding process to produce metal coalescence by the heat from an electric arc that is maintained between the tip of flux-coated, stick electrode and the surface of the base metal being welded [3]. The number and size of slag inclusions developed during welding process can be minimized by maintaining sufficiently high weld-metal temperatures, maintaining weld metal at low viscosity and preventing rapid solidification. 3. Microscopic examinations Specimen for microstructure analysis is taken from cracked zone at the bottom part of the barrier plate as shown in Fig. 4. This location is chosen due to the accessibility in obtaining the specimen. The specimen is used to analyze the detailed
Fig. 2. Studied area and hot exhaust gas invasion in case of the cracked weld.
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A.Z. Rashid et al. / Engineering Failure Analysis 17 (2010) 579–586
Fig. 3. Exhaust region and the schematic illustration of GE Frame 9E gas turbine.
Fig. 4. The barrier plate where several surface cracks are found (left and middle) and the close-up of obvious cracks (right).
Table 1 Chemical composition of RA 253 MAÒ (%) [2]. Alloy
Ni
Cr
C
Fe
Mn
Si
N
Ce
RA 253 MA
10
21
0.07
66
0.6
1.6
0.16
0.05
Fig. 5. Cracks at weld joint as result of thermal fatigue.
features of materials at the cracked base metal and weld metal. It represents the heat-affected zone (HAZ). Examination is carried out in order to observe any mechanical behaviour process of the materials.
582
A.Z. Rashid et al. / Engineering Failure Analysis 17 (2010) 579–586
Fig. 6. Geometry of the original design.
Fig. 5 shows cracks developed at the interface between the base metal and weld metal. The cracks likely indicated that thermal fatigue had occurred. The differences of the thermal expansion between the base metal and weld metal encourage development of mismatch thermal stress due to the fluctuated operating temperatures. It agrees with the operational backgrounds that the gas turbine had been subjected to frequent start-ups and shut-downs for servicing peak duty. However, fatigue analysis on the component under fluctuated operating temperatures needs to be carried out to evaluate the reliability of the barrier plate structure of the original model as well as the modified model as proposed. Finite element analyses are carried out to compute the temperature distributions and the corresponding strains developed in the metals of the component. 4. Finite element analysis Models of the original and modified designs are evaluated by finite element method in order to determine the metal temperature distributions. Next, the metal temperature distributions are then used as the thermal loading to obtain the corresponding thermal elastic strains. The residual stresses developed in weld joints are omitted for simplification. The components are modeled as two-dimensional axisymmetric solid. Two-dimensional triangle elements are used to discretize the models. Data of the original design from the Putrajaya Power Station are used to generate the geometry of the model and as a reference for the modified design.
Fig. 7. Geometry of the modified design.
583
A.Z. Rashid et al. / Engineering Failure Analysis 17 (2010) 579–586 Table 2 Material properties of RA 253 MA [2]. Properties
Temperature (°C)
Modulus of elasticity (109 Pa) Poisson’s ratio Specific heat capacity (J/kg K) Thermal conductivity (W/m C) Thermal expansion coefficient (106 °C1) Ultimate tensile strength (106 Pa) Yield tensile strength (106 Pa) Elongation percentage (%) Reduction of area (%) Density (kg/m3)
20
204
427
649
760
982
200 0.31 440 14.5 16.8 600 310 40 50 7800
185
168
150
139
121
490 17.5 17.7
544 20.2 18.3
595 22.5 18.5
624 24.2 19.4
687 28.7
Table 3 Material properties of dry air [4]. Properties
Specific heat capacity (J/kg K) Thermal conductivity (103 W/m C) Thermal expansion coefficient (103 °C1) Density (kg/m3)
Temperature (°C) 0
20
100
200
500
1000
1006 24.18 3.674 1.275
1007 25.69 3.421 1.188
1012 31.39 2.683 0.9329
1026 37.95 2.115 0.7356
1093 55.64 1.293 0.4502
1185 80.77 0.785 0.2734
Table 4 Thermal conductivity of the insulating material [5]. Temperature (°C)
Thermal conductivity (W/m C)
260
538
816
982
0.06
0.14
0.24
2.09
4.1. Geometrical models Fig. 6 shows the cross-sectional area of the original design of the plenum barrier plate. A simplified weld representation (in circle of Fig. 6) is used for all the weld joints (Zones 5 and 9). This original model is discretized with 2900 elements in total. The model of the modified design is shown in Fig. 7. Improvements are expected from the model by providing a buffer zone (Zone 8) to anticipate the flowing hot air entering directly to the compartment in case of the cracked weld (Zone 2). The number of element for analyzing the modified model is 3000 elements.
4.2. Material properties Properties of RA 253 MAÒ [2] is listed in Table 2. In this study, weld material also uses RA 253 MAÒ to illustrate a better scenario of analyzing thermal fatigue for the original design. Thus, the dissimilar weld issue becomes less. Table 3 lists dry air physical properties obtained from VDI-Wärmeatlas [4]. The insulating material properties [5] are listed in Table 4.
4.3. Boundary conditions In this work, while performing thermal analyses, it is considered that the rate of the hot compressed air flow is constant. However, since the gas flow has high velocity, the phenomenon of the contact condition between the flowing hot air and the metals is considered as forced convection heat transfer. The convection coefficient for the hot exhausted air film of 5 W/ m2 °C is used in this study. The rest of heat transfer in the system is conduction process. Transient heat transfer analyses are carried out until temperature of the hot compressed air achieves the steady state condition (550 °C) for the peak duty period. In order to carry out structural analyses, two displacement constraints in the radial and axial directions are applied to the models as indicated in Figs. 6 and 7.
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A.Z. Rashid et al. / Engineering Failure Analysis 17 (2010) 579–586
5. Fatigue analysis In order to estimate the life expectancy of the plenum barrier plate, a simple approximation is carried by utilizing a technique proposed by Muralidharan and Manson [6]. The method called Universal Slopes technique is a common way in fatigue life analysis in cases where cyclic life properties are unknown. The equation for the Universal Slopes technique is as follows [6]:
0:832 0:155 Su 0:53 De Su ð2 N f Þ0:09 þ 0:0196 ef ð2 N f Þ0:56 ¼ 0:623 2 E E
ð1Þ
Universal Slopes method describes the relationship between De which is two times the cyclic strain amplitude and the cyclic strain–life Nf. The ultimate tensile strength Su, the Young’s modulus E and the true fracture strain ef are obtained from the monotonic material properties listed in Table 2. The true fracture strain ef is taken as the percent reduction is area (% RA) [7] and may be expressed as
ef ¼ ln
100 ð100 RA%Þ
ð2Þ
9
2
1
3 167.455 165.791 164.126 162.462 160.797 159.133 157.468 155.805 154.139 152.475
5 7
6 4
Fig. 8. Temperature distribution (°C) of the original model.
Fig. 9. Temperature distribution (°C) of the modified model.
8
585
A.Z. Rashid et al. / Engineering Failure Analysis 17 (2010) 579–586
1 8
10 9
3 0.00568 0.00517 0.00465 0.00414 0.00362 0.00311 0.00260 0.00208 0.00157 0.00105
5 2
4
Fig. 10. Von Mises elastic strain distribution of the modified model.
Substituting data for RA 253 MA from Table 2 into Eq. (1), the strain–life equation may be expressed as
De ¼ 0:004660 Nf0:09 þ 0:2730 N0:56 f 2
ð3Þ
This relationship gives an approximated life cycle for any given cyclic strain amplitude. The life cycles may be then calculated by iterative technique using Newton Raphson method. 6. Results and discussion Fig. 8 shows the temperature distribution of the original model. It can be seen from Fig. 8, the high temperature is concentrated in the area of the overlap weld joint (Zone 2). This is identified due to direct continuous contact with the hot exhausted gas and the shape of the barrier plate design. The high temperatures are also found in the overlapping RA 253 MA sheets (Zones 1 and 3) in addition to the weld fillet (Zone 2). The high temperatures are not expected in the insulator region (Zone 6) and the air region (Zone 7). Fig. 9 indicates a better temperature distribution in the modified model. Although the higher maximum temperature is found in the modified design, the higher temperatures are distributed in a single sheet metal (Zone 1). This would not lead to a critical condition compared to those distributed at connections of multiple parts. The temperature at the tee weld joint (Zone 5) also shows a significant improvement by decreasing of 10 °C compared with that of the original design.
9 1 3 0.142 0.127
5 7
0.111 0.095 0.079 0.063 0.048 0.032
2
4
0.016 0.000 Fig. 11. Von Mises elastic strain distribution of the original model.
8
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A.Z. Rashid et al. / Engineering Failure Analysis 17 (2010) 579–586
Fig. 12. Strain–life curve for barrier plate designs.
Structural analyses show promising findings from the modified model. It can be seen from Fig. 10, thermal strain developed at weld joint (Zone 2) is reduced by 60% compared with that of the original design (Fig. 11). The life expectancies obtained by using Eg. (3) are presented in Fig. 12. It is shown that the estimated life for the modified model by considering the most critical location (Zone 2 in Figs. 10 and 11) is improved by more than 20 times than that of the original model. It is clear that redesigning the plenum barrier plate of the gas turbine frame is necessary to be carried out. Residual stress developed in HAZ as result of the welding process is however unavoidable and it might worsen the susceptibility of the crack problems. Hence, welding process needs also to be given serious attention and requires fair operators for best results. 7. Conclusions Visual inspection, microscopic examination on the cracked barrier plate and thermal fatigue analyses based on strain–life approach were carried out. The modified model of the barrier plate was presented for redesign consideration and the comparison to the original design was made. The life expectancies of the original and modified models for the barrier plate design were evaluated. The finding indicated that the modified model could withstand against thermal fatigue longer than that of the original model. Acknowledgements This work is supported by Universiti Tenaga Nasional Malaysia through the research project of UNITEN Seeding Fund J510050137 and Ministry of Science, Technology and Innovation Malaysia through research project of Science Fund 04-02-03-SF0003. The authors also wish to thank Putrajaya Power Station Malaysia for permission of utilizing all the facilities and TNB REMACO Sdn Bhd Malaysia for assisting tasks for visual inspections during completion of this study. References [1] [2] [3] [4] [5] [6]
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