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H. Othman et. al.
ICCBT 2008 - F - (26) – pp283-294 285
Figure 1. Drawing of superheater tube circuit (Courtesy of Paka TNB Power Station).
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
approximately 5 mm from welded joints. The condition of the failed tubes is shown in Figure
2.
Figure 2. Condition of failed tubes: deformed and cracked.
Visual inspection has also been carried out at second boiler wall (opposite bend tube section).
It was found that the fin of the identified tubes were restricted at second boiler wall. The
restriction condition on the fin of the identified tubes and tube displacement measurement are
shown in Figures 3 and 4 respectively.
Inlet
45º
Connecting
Tube
90º
Connecting
Tube
Outlet
Bend Tube Section Straight Tube Section Bend Tube Section
1st wall 2nd wall
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Failure Analysis on Deformed Superheater Tubes by Finite Element Method
ICCBT 2008 - F - (26) – pp283-294286
Figure 3. The restriction condition on the fin of identified tubes.
Figure 4. Tube displacement measurement shows 45 mm away from original position.
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
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H. Othman et. al.
ICCBT 2008 - F - (26) – pp283-294 287
Element refinement has been made on potential cracked area especially at the welded joint
and its vicinity for better and accurate analysis. Figure 5 shows the 3D elements generated for
this FE analysis and elements refinement on potential cracked area.
Figure 5. Meshing and its refinement on potential cracked area.
3.1 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.
Table 1. List of chemical composition for SA 213 T22 (Source: Bringas [5])
Chemical Composition
Carbon Manganese Phosphorus,
Max
Sulfur,
Max
Silicon Chromium Molybdenum
0.05–0.15 0.30-0.60 0.025 0.025 0.50 1.90-2.60 0.87-1.13
Mechanical properties assumed for the analysis in this work are as follows:
1. At 520ºC: 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 : 1.72 x 105
MPa
• Yield Strength : 183.0 MPa
• Tensile Strength : 378.9 MPa
• Thermal Expansion Coefficient : 14.46 x 10-6
mm/mm/ ºC
• Poisson Ratio : 0.3
• Density : 7,833.44 kg/m3
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Failure Analysis on Deformed Superheater Tubes by Finite Element Method
ICCBT 2008 - F - (26) – pp283-294288
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 : 2.11 x 105
MPa
• Yield Strength : 206.1 MPa
• Tensile Strength : 413.7 MPa• Thermal Expansion Coefficient : 11.52 x 10
-6mm/mm/ ºC
3. At Room Temperature: The mechanical properties of the material are taken from tensile
test on as-received tube samples.
• Young’s Modulus : 6.36 x 104
MPa
• Yield Strength : 271.87 MPa
• Tensile Strength : 557.78 MPa
3.2 Boundary Conditions
The boundary conditions (see Figures 6 and 7) are set according to cases as follows:
1. Boundary condition for all cases: at the end of 45º connecting tube which supposed to be
attached to the outlet header was set fixed by applying constraints for all six degree 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 constraint by simulating seven (7) different cases. At
each case, the constraint is applied to the straight finned tube at certain point by fixing allsix degree of freedom (TX, TY, TZ, RX, RY and RZ ).
Figure 6. No restriction on the straight finned tube.
1st
Wall2nd Wall 14836mm
Fixed
SlidingSliding
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H. Othman et. al.
ICCBT 2008 - F - (26) – pp283-294 289
Figure 7. Restriction on the straight finned tube at X m away from 1st
boiler wall.
The applied loads for this FE analysis are temperature and pressure. The temperature load ismetal temperature, in which the straight finned and bend tube sections are under T = 520 ºC
and T = 519 ºC respectively, as shown in Figure 8. The pressure load is internal pressure of 67
bar applied in internal surface of superheater tube. Figure 9 shows the pressure load applied to
the finite element model. For the FE analysis, the loading conditions altogether with the
constraints as shown in Figures 6 and 7 are analyzed separately under operating internal
pressure of 67 bar and temperature at 520ºC .
Figure 8. Temperature load applied in the finite element model.
Figure 9. Pressure load applied in the finite element model.
1st
Wall2nd Wall 14836mm
Fixed
Fixed
X mmSliding
Sliding
1st
Wall2nd Wall
520 ºC 519 ºC
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Failure Analysis on Deformed Superheater Tubes by Finite Element Method
ICCBT 2008 - F - (26) – pp283-294290
3.3 Assumptions.
During performing FE analysis of deformation failure on superheater tubes, the following
assumptions are made as
- The tubes are subjected to constant uniform internal pressure P inside the tube andconstant temperature T throughout the boiler wall as studied by Daniel et al. [1] and
Basu [3].
- The various considerations involved with superheater tubes being exposed to
fluctuating internal pressure and temperature as well as effectiveness of finned tube
during operation is beyond the scope of this study.
- The metal tube temperature is calculated based on overall heat transfer coefficient, U
through composite resistance where the various resistance values used 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 Figure 10 are Node 191270 (crack area), Node 191397 (slant portion) and
Node 115914 (straight portion). The results for the FE analysis are shown in Figures 11, 12
and 13.
Figure 10. Stress distribution for the tube samples under temperature at 520 ºC in atmospheric
pressure and sub case: restriction at 2nd
wall.
Node 115914
Node 191397
Node 191270
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Stress Vs Constraint Distance for Operating Temperature 520 C & Pressure 67 bars
(Material: Actual)
7.35E+08
2.59E+09
2.72E+08
5.58E+06
5.58E+06 5.36E+06
3.71E+083.71E+083.77E+08
5.36E+065.58E+06
5.58E+06 5.58E+06 4.45E+06
3.77E+08
4.45E+065.58E+06
5.58E+08
0.00E+00
5.00E+08
1.00E+09
1.50E+09
2.00E+09
2.50E+09
Free 1 st wall 1m 2m 3m 4m 2nd wall
Constraint Distance
S t r e s s
V a l u e ,
P a
Crack Area (Node 191270) Slant Portion (Node 191397) Straight Portion (Node 115914)
Sy (Yield Strength) St (Tensi le Strength)
Figure 11. Stress versus constraint distance of the tube sample (as-received) subjected to
internal pressure of 67 bar at operating temperature, 520 ºC .
Stress Vs Constraint Distance for Operating Temperature 520 C & Pressure 67 bars
(Material: Standard)
2.54E+09
7.70E+09
2.06E+085.57E+06
4.27E+08
8.49E+08
2.12E+09
1.69E+09
2.96E+09
3.84E+063.84E+06
4.27E+08 4.27E+08
8.51E+08
1.78E+09
4.28E+084.27E+08 4.14E+08
0.00E+00
1.00E+09
2.00E+09
3.00E+09
4.00E+09
5.00E+09
6.00E+09
7.00E+09
8.00E+09
Free 1 st wall 1m 2m 3m 4m 2nd wall
Constraint Distance
S t r e s s
V a l u e ,
P a
Crack Area (Node 191270) Slant Portion (Node 191397) Straight Portion (Node 115914)
Sy (Yield Strength) St (Tensile Strength)
Figure 12. Stress versus constraint distance of the tube of standard material subjected to
internal pressure of 67 bar at operating temperature, 520ºC.
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Failure Analysis on Deformed Superheater Tubes by Finite Element Method
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Stress Vs Constraint Distance for Operating Temperature 520 C and Pressure 67 bars
(Material: Standard @ 520 C)
5.57E+06
7.74E+09
5.57E+06
2.21E+09
5.57E+06
1.66E+09
1.83E+08 1.83E+083.79E+08
4.28E+08
8.51E+081.27E+09
2.12E+09
2.54E+09
8.51E+08
5.57E+06
5.41E+06
4.28E+08 4.28E+08
4.28E+080.00E+00
1.00E+09
2.00E+09
3.00E+09
4.00E+09
5.00E+09
6.00E+09
7.00E+09
8.00E+09
9.00E+09
Free 1 st wall 1m 2m 3m 4m 2nd wall
Constraint Distance
S t r e s s
V a l u e ,
P a
Crack Area (Node 19120) Slant Portion (Node 191397) Straight Portion (Node 115914)
Sy (Yield St rength) St (Tensile St rength)
Figure 13. Stress versus constraint distance of the tube of standard material subjected to
internal pressure of 67 bar at operating temperature, 520 ºC (using mechanical
properties at 520 ºC )
As per FE analysis results shown in Figures 11, 12 and 13, the deformation of 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 tube sample is also similar to the
highest stress in the simulation. Figures 14 and 15 show the similarity of the deformation on
tube sample and simulation.
Figure 14. Deformation on tube sample.
A
B
Crack
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Figure 15. Stresses and deformed shape of the tube obtained from the simulation.
The results are indicating that temperature is the main factor of the deformation due to
restriction to the tube. For all FE analysis results which involve operating temperature of 520
ºC , 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 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 weld joint between the connecting tube and 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
weld joint region for all cases. This undesired high stress can promote low cycle fatigue tube
failure. Furthermore, the existence of ‘groove feature’ at the region of weld joint between stub
and connecting tube has encouraged the failure of tube.
5. CONCLUSIONS
Finite Element (FE) analyses on 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 maximum stress induced by the deformed tube were determined. The resultsshowed the correlation between the maximum stress and allowable restriction condition. The
finite element results showed 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 thank Ministry of Science, Technology and Innovation, Malaysia for
financial support through project grant of IRPA 09-99-03-0033 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.
A
B
HighestStress
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REFERENCES
[1] Daniel L. C. N., Jansen R. C. S, Ediberto B. T., Adriana C. R., Ibrahim C. A. Stress and
Integrity Analysis of Steam Superheater Tubes of a High Pressure Boiler , Materials
Research, Rio de Janeiro - RJ, Brazil, 2004
[2] Joeng K., Yong W.K, Beom S.K, Sang M.H, Finite Element Analysis for Bursting
Failure Prediction in Bulge Forming of a Seam Tube, Finite Elements in Analysis and
Design, Elsevier, 2003
[3] Basu A., The Finite Volume Analysis of Damaged Boiler Tubes, PhD Thesis, New South
Wales, 2002
[4] Karamanos S.A., Tsouvalas D., Gresnigt A.M., Ultimate Capacity of Pressurized 90
Degree Elbows Under Bending, Design and Analysis of Pressure Vessels, Heat
Exchangers and Piping Components Conference, PVP 2004.
[5] Bringas J.E., Handbook of Comparative World Steel Standards ASTM DS67A, 2nd
Edition, pg. 217, 2002
[6] MAT/PRO 2.0 – Material Properties Software based on ASME 2007 Section II, Part D
Table 1A, 2007
[7] Ganapathy.V, Steam Plant Calculations Manual, 2nd
Edition, Marcel Dekker, Inc, pg
234-263, 1994
[8] Robert D. Port, Harvey M. Herro, The NALCO Guide to Boiler Failure Analysis, Nalco
Chemical Company, McGraw-Hill Inc, pg. 6-10, 29-36, 47-52, 1991
[9] Lienhard J.H, A Heat Transfer Textbook , 3rd Edition, Phlogiston Press, Cambridge
Massachusetts, pg. 74-82, 2002