ICCBT 2008 - F - (26) – pp283-294 ICCBT2008 Failure Analysis on Deformed Superheater Tubes by Finite Element Method H. Othman, TNB Research Sdn. Bhn,MALAYSIA J. Purbolaksono, Universiti Tenaga Nasional, MALAYSIA B. Ahmad, TNB Research Sdn. Bhn,MALAYSIA ABSTRACT Significant deformations and the presence of cracks in the vicinity of welded joints have been discovered at the superheater tubes of a Heat Recovery System Generator (HRSG) in a TNB (Tenaga Nasional Berhad) 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 results of this study show the correlation between the maximum stress and allowable restriction condition, and indicate good correlations with the findings obtained during site inspection. Keywords: Superheater tube, Deformation, Failure Analysis, Finite Element Method*Correspondence Author: J. Purbolaksono, Universiti Tenaga Nasional, MALAYSIA E-mail:[email protected].
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UNITEN ICCBT 08 Failure Analysis on Deformed Super Heater Tubes by Finite
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8/8/2019 UNITEN ICCBT 08 Failure Analysis on Deformed Super Heater Tubes by Finite
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
8/8/2019 UNITEN ICCBT 08 Failure Analysis on Deformed Super Heater Tubes by Finite
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
8/8/2019 UNITEN ICCBT 08 Failure Analysis on Deformed Super Heater Tubes by Finite
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
8/8/2019 UNITEN ICCBT 08 Failure Analysis on Deformed Super Heater Tubes by Finite