uniten iccbt 08 failure analysis on deformed super heater tubes by finite

8/8/2019 UNITEN ICCBT 08 Failure Analysis on Deformed Super Heater Tubes by Finite

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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



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



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




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.


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.


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



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




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



H. Othman et. al.

ICCBT 2008 - F - (26) – pp283-294 291

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


Constraint Distance

S t r e s s

V a l u e ,

P a


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.




ICCBT 2008 - F - (26) – pp283-294292

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


Constraint Distance

S t r e s s

V a l u e ,

P a


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



H. Othman et. al.

ICCBT 2008 - F - (26) – pp283-294 293

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




ICCBT 2008 - F - (26) – pp283-294294

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

uniten iccbt 08 failure analysis on deformed super heater tubes by finite

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