Engineering Failure Analysis

Transcription

1 Engineering Failure Analysis 16 (2009) Contents lists available at ScienceDirect Engineering Failure Analysis journal homepage: Failure investigation on deformed superheater tubes H. Othman a, J. Purbolaksono b, *, B. Ahmad a a TNB Research, No. 1 Lorong Air Hitam, Kajang 43000, Malaysia b Universiti Tenaga Nasional, KM 7 Jalan Kajang-Puchong, Kajang 43009, Malaysia article info abstract Article history: Received 8 May 2008 Accepted 26 May 2008 Available online 18 June 2008 Keywords: Failure analysis Finite element method Superheater tube Deformation Investigation is conducted following the discovery of significant deformations and the presence of cracks in the vicinity of welded joints at the superheater tubes of a heat recovery steam generator (HRSG) in a Malaysian Electricity Company s power plant. This study performs the finite element (FE) analyses in order to identify the possible root cause failure of the deformed superheater tubes using software package of MSC PATRAN/NASTRAN. The locations of maximum stress induced by the deformed tube are determined. The correlation between the maximum stress and allowable restriction condition is presented, and it shows good correlations with the findings obtained from the visual inspection on site. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Superheater tubes as described by Daniel et al. [1] are surfaces for heat exchange, where the steam is heated to a higher value than saturation. As the steam produced must be at the highest possible temperature, the superheater tubes bundle is placed at the beginning of the flue gas path or at the lower part of the boiler. In regard to the failure problems in boiler tubes, many researchers have reported their studies using a finite element method and experimental analysis. Finite element analysis for bursting failure prediction in bulge forming under combined internal pressure and independent axial feeding is reported by Jeong et al. [2]. Finally, through a series of bulge forming simulations with consideration of the welding, it is concluded that the proposed method would be able to predict the bursting pressure and fracture initiation. Creep rupture lifetimes of boiler tubes damaged due to erosion and/ or corrosion which, cause local thinning, are studied by Basu [3]. He modeled the elastic/creep behavior of axisymmetric pressurized component using finite volume program and conventional FE analysis. Karamanos et al. [4] conducted an investigation on pressurized pipe elbows under bending loads using advanced finite element analysis tools, supported by real-scale test data. This study investigates the effect of the diameter-to-thickness ration on the ultimate capacity of 90 degree elbows, focusing on the failure mode and the maximum bending strength. In this work the locations of the maximum stress induced by the deformed tube are identified, and the correlation between the maximum stress and allowable restriction condition is presented. It can provide a better understanding of the behavior of a water tube boiler, especially HRSG at elevated temperature. 2. Models of the problem The model for FE analysis has been made to represent as close as the actual superheater tubes circuit. Better understanding on the design of the under-studied component is important prior to the establishment of FE model. * Corresponding author. Tel.: ; fax: address: judha@uniten.edu.my (J. Purbolaksono) /$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi: /j.engfailanal

2 330 H. Othman et al. / Engineering Failure Analysis 16 (2009) The ASTM 213 T22 superheater tubes were installed at a waste heat boiler with a capacity of 55 MW. The studied components have been in operation for approximately 1,60,000 h. The components consist of two sections, i.e. bent tubes with- Fig. 1. Drawing of superheater tube circuit (courtesy of Paka Power Station, Malaysia). Fig. 2. Condition of failed tubes: deformed and cracked. Fig. 3. The restriction condition on the fin of identified tubes.

3 H. Othman et al. / Engineering Failure Analysis 16 (2009) Fig. 4. Tube displacement measurement shows 45 mm away from original position. out fin and straight tubes with fin. The bent tubes without fin section are placed in the header compartment and act as a medium to deliver the superheated steam from straight tubes to the header. At the same time, these also act as a loop for thermal expansion compensation. However, the straight tubes with fin section are located in the flue gas path and are supported by several tube sheets. In straight tubes section, heat energy is absorbed directly by a gas turbine exhaust at 540 C. The superheated steam at temperature 520 C and pressure 67 bar is flowed through from 273 mm main steam pipe diameter to steam turbine. The actual superheater tubes circuit is shown in Fig. 1. Visual inspection on site has found two (2) of connecting superheater tubes at outlet header experiencing significant deformation and crack. The crack was circumferential and location was approximately 5 mm from the welded joints. The condition of the failed tubes is shown in Fig. 2. Visual inspection has also been carried out at second boiler wall (opposite bent tube section). It was found that the fin of the identified tubes was restricted at second boiler wall. The restriction condition on the fin of the identified tubes and tube displacement measurement are shown in Figs. 3 and 4, respectively. 3. Finite element models The FE modeling was carried out using MSC Patran/Nastran. Generation of 3D elements from 3D model is based on the following information: Element shape: tetrahedral Mesher: Tetmesh Topology: Tet10 Global edge length: 0.01 Total element, generated: 96,237 Element refinement has been made on a potential cracked area, especially at the welded joint and at its vicinity for better and accurate analysis. Fig. 5 shows the 3D elements generated for this FE analysis and elements refinement on potential cracked area Material properties The material specified is SA 213 (Grade T22) seamless ferritic low-alloy steel tube. Table 1 lists the chemical composition of the material. Mechanical properties assumed for the analysis in this work are as follows: 1. At 520 C: the mechanical properties of the material are derived from Mat/PRO 2.0 [6] which is according to ASME Section 2 Part D. Young s modulus: MPa Yield strength: MPa Tensile strength: MPa

4 332 H. Othman et al. / Engineering Failure Analysis 16 (2009) Fig. 5. Meshing and its refinement on potential cracked area. Table 1 List of chemical composition for SA 213 T22 (Source: Bringas [5]) Carbon Manganese Phosphorus, max Sulfur, max Silicon Chromium Molybdenum Chemical composition Thermal expansion coefficient: mm/mm/ C Poisson s ratio: 0.3 Density: kg/m 3 2. At room temperature: the mechanical properties of the material are derived from Mat/PRO 2.0 [6] which is according to ASME Section 2 Part D. Young s modulus: MPa Yield strength: MPa Tensile strength: MPa Thermal expansion coefficient: mm/mm/ C 3. At room temperature: the mechanical properties of the material are taken from tensile test on as-received tube samples. Young s modulus: MPa Yield strength: MPa Tensile strength: MPa 3.2. Boundary conditions The boundary conditions (see Figs. 6 12) are set according to cases as follows: Fig. 6. No restriction on the straight finned tube.

5 H. Othman et al. / Engineering Failure Analysis 16 (2009) Fig. 7. Restriction on the straight finned tube at 1st boiler wall. 1000mm Fig. 8. Restriction on the straight finned tube at 1 m away from 1st boiler wall. 2000mm Fig. 9. Restriction on the straight finned tube at 2 m away from 1st boiler wall. 1. Boundary condition for all the cases: at the end of 45 connecting tube which is supposed to be attached to the outlet header was set fixed by applying constraints for all six degrees of freedom (translation and rotation), i.e. TX, TY, TZ, RX, RY and RZ. For the straight finned tube, constraints were set at boiler walls and tube sheets at Y direction to act as sliding support. 2. Boundary conditions for case by case: the purpose is to understand the behavior of the tube deformation under different constraints by simulating seven (7) different cases. At each case, the constraint is applied to the straight finned tube at certain point by fixing all six degrees of freedom (TX, TY, TZ, RX, RY and RZ). The applied loads for this FE analysis are temperature and pressure. The temperature load is metal temperature, in which the straight finned and bent tube sections are under T = 520 C and T = 519 C, respectively, as shown in Fig. 13. The pressure

6 334 H. Othman et al. / Engineering Failure Analysis 16 (2009) mm Fig. 10. Restriction on the straight finned tube at 3 m away from 1st boiler wall. 4000mm Fig. 11. Restriction on the straight finned tube at 4 m away from 1st boiler wall. Fig. 12. Restriction on the straight finned tube at 2nd boiler wall as observed during visual inspection on site. load is the internal pressure of 67 bar applied in the internal surface of a superheater tube. Fig. 14 shows the pressure load applied to the finite element model. For the FE analysis, the loading conditions altogether with the constraints as shown in Figs are analyzed separately under operating internal pressure of 67 bar and temperature at 520 C.

7 H. Othman et al. / Engineering Failure Analysis 16 (2009) ºC 519 ºC 2 nd Wall Fig. 13. Temperature load applied in the finite element model. Fig. 14. Pressure load applied in the finite element model Assumptions During the performance of the FE analysis of deformation failure on the superheater tubes, the following assumptions are made: The tubes are subjected to a constant uniform internal pressure P inside the tube and a constant temperature T throughout the boiler wall as studied by Daniel et al. [1] and Basu [3]. The various considerations involved with the superheater tubes being exposed to fluctuating internal pressure and temperature as well as the effectiveness of the finned tube during the operation are beyond the scope of this study. The metal tube temperature is calculated based on an overall heat transfer coefficient, U, through composite resistance, where the various resistance values used are as specified by Ganapathy [7], Robert and Harvey [8] and Lienhard [9]. 4. Numerical results Several numerical results corresponding to the material properties and boundary conditions as described in the previous section are presented and compared with the findings during inspection on site. Comparison of stress levels with regard to constraint distances is made based on three (3) identified spots on the tube which has crack and experiences deformation. The identified spots as indicated in Fig. 15 are Node (crack area), Node (slant portion) and Node (straight portion). The results for the FE analysis are shown in Figs

8 336 H. Othman et al. / Engineering Failure Analysis 16 (2009) Fig. 15. Stress distribution for the tube samples under temperature at 520 C in atmospheric pressure and sub case: restriction at 2nd wall. Stress Vs Constraint Distance for Operating Temperature 520 C & Pressure 67 bars (Material: Actual) 2.50E E E+09 Stress Value, Pa 1.50E E E E E E E E E E E E E E E E E E E E E+06 Free 1 st wall 1m 2m 3m 4m 2nd wall Constraint Distance Crack Area (Node ) Slant Portion (Node ) Straight Portion (Node ) Sy (Yield Strength) St (Tensile Strength) Fig. 16. Stress versus constraint distance of the tube sample (as-received) subjected to internal pressure of 67 bar at operating temperature, 520 C. As per the FE analysis results shown in Figs , the deformation of a tube is similar to the findings of the site visual inspection. The tube is experiencing deformation on two sections: straight portion and slanting portion as a result from tube restriction on 2nd boiler wall during thermal expansion. The crack location on a tube sample is also similar to the highest stress in the simulation. Figs. 19 and 20 show the similarity of the deformation on tube sample and simulation. The results indicate that temperature is the main factor of the deformation due to restriction to the tube. For all the FE analysis results which involve operating temperature of 520 C, there are stress levels on tube exceeding the tensile strength for three types of mechanical properties when the constraint distance is more than 1m from 1st boiler wall, depending on

9 H. Othman et al. / Engineering Failure Analysis 16 (2009) E+09 Stress Vs Constraint Distance for Operating Temperature 520 C & Pressure 67 bars (Material: Standard) 7.70E E E+09 Stress Value, Pa 5.00E E E E E E E E E E E E E E E E E E E E E E E+06 Free 1 st wall 1m 2m 3m 4m 2nd wall Constraint Distance Crack Area (Node ) Slant Portion (Node ) Straight Portion (Node ) Sy (Yield Strength) St (Tensile Strength) Fig. 17. Stress versus constraint distance of the tube of standard material subjected to internal pressure of 67 bar at operating temperature, 520 C. 9.00E+09 Stress Vs Constraint Distance for Operating Temperature 520 C and Pressure 67 bars (Material: 520 C) 8.00E E E E+09 Stress Value, Pa 5.00E E E E E E E E E E E E E E E E E E E E E E E+06 Free 1 st wall 1m 2m 3m 4m 2nd wall Constraint Distance Crack Area (Node 19120) Slant Portion (Node ) Straight Portion (Node ) Sy (Yield Strength) St (Tensile Strength) Fig. 18. Stress versus constraint distance of the tube of standard material subjected to internal pressure of 67 bar at operating temperature, 520 C (using mechanical properties at 520 C) the case. Tube restriction has caused channeling the straight tube expansion to one direction at the connecting tube. The undesired straight tube expansion produced bending stress to the welded joint between the connecting tube and the stub tube. Thus, constraint distance at 2nd boiler wall gives the maximum expansion to the straight tube and is producing the highest stress level on welded joint region for all the cases. This undesired high stress can promote low cycle fatigue tube

10 338 H. Othman et al. / Engineering Failure Analysis 16 (2009) Fig. 19. Deformation on tube sample. Fig. 20. Stresses and deformed shape of the tube obtained from the simulation. failure. Furthermore, the existence of groove feature at the region of the welded joint between stub and connecting tubes has encouraged the failure of tube. 5. Conclusions Finite element (FE) analyses on the deformation of superheater tubes were presented. It was found that temperature was the main factor of the deformation due to restriction to the tube. The locations of the maximum stress induced by the deformed tube were determined. The results presented the correlation between the maximum stress and allowable restriction condition. The finite element results showed a good correlation with the findings obtained during site inspection. It may be used as the guidance for plant inspector in making their decision during the inspection. Acknowledgments The authors wish to the thank Ministry of Science, Technology and Innovation, Malaysia for the financial support through project Grant of IRPA EA001. Special gratitude goes to Universiti Tenaga Nasional and TNB Research Sdn. Bhd, Malaysia for permitting the authors to utilize all the facilities in conducting this study. References [1] LCN, Jansen RCS, Ediberto BT, Adriana CR, Ibrahim CA. Stress and integrity analysis of steam superheater tubes of a high pressure boiler. Materials Research, Rio de Janeiro RJ, Brazil; [2] Jeong K, Yong WK, Beom SK, Sang MH. Finiteelement analysis forbursting failure prediction inbulge forming of a seam tube. Finite elements in analysis and design. Elsevier; [3] Basu A. The finite volume analysis of damaged boiler tubes. PhD Thesis, New South Wales; 2002.

11 H. Othman et al. / Engineering Failure Analysis 16 (2009) [4] Karamanos SA, Tsouvalas D, Gresnigt AM. Ultimate capacity of pressurized 90degree elbows under bending, design and analysis of pressure vessels. In: Heat exchangers and piping components conference, PVP; [5] Bringas JE. Handbook of comparative world steel standards ASTM DS67A. 2nd ed [p. 217]. [6] MAT/PRO 2.0 material properties software based on ASME 2007 Section II, Part D Table 1A; [7] Ganapathy V. Steam plant calculations manual. 2nd ed. Marcel Dekker; [p ]. [8] Port Robert D, Herro Harvey M. The NALCO guide to boiler failure analysis. McGraw-Hill Inc: Nalco Chemical Company; [p. 6 10, 29 36, 47 52]. [9] Lienhard JH. A heat transfer textbook. 2nd ed. Cambridge Massachusetts: Phlogiston Press; p