Mechanical Stress Analysis of a 600MW Supercritical Boiler Superheater Outlet Header

doi:10.4236/eng.2013.51B012

Paper Menu >>

Journal Menu >>

Engineering, 2013, 5, 66-68

doi:10.4236/eng.2013.51b012 Published Online January 2013 (http://www.SciRP.org/journal/eng)

Mechanical Stress Analysis of a 600M W Supercritical Boi-

ler Superheater Outlet Header

Dongyi Li, Zhongguang Fu, Qiang Zhang, Ting He

School of Energy,Power and Mechanical Engineering;North China Electric Power University;Beijing,China

Email: aiolos-[email protected]m

Received 2013

ABSTRACT

The mechanical stre ss distribution and the stress concentrations of the superheater outlet header of a 600MW supercrit-

ical boiler we r e analyzed by the finite element method. The results showed that the stress concentrated at the inside

conjunction area between the pipe and the header cylinder , and the value of the maximum mechanical stress concentra-

tion factor is 2.51.

Keywords: Superheater Outlet header; Finite Element Method; Stress Concentration

1. Introduction

With t he d evel opment of economy and the rapid increase

of electricity demand, large-capacity high-parameter

thermal power generating unit takes part in the peak load

operation. Since the boiler load changes greatly during

the peak load operating process, the stress of the bearing

pressure parts changes rapidly. Because of that, the stress

of the boiler load section changes drastically which

eventually shorten the ori ginal operating time very muc h

1,2]. Therefore, the stress analysis of the bearing com-

ponents becomes essential for operating the power plant

safely and economically.

The total stress is mainly impacted by the internal

pressure on the basis of previous studies[3,4].This paper

takes a superheater outlet header of a 600MW boiler as

the research object to analyze the distribution and varia-

tion law of mechanical stress with the finite element

analysis software-ANSYS. Besides, the pressure data

during the process of load pick-up is passed as boundary

conditions.

2. Model and Boundary Conditions

2.1. Structure of Header

Figure 1 shows the structure of the superheater outlet

header of a 600MW supercritical boiler.

This research considers the header as a thick walled

cylinder which diameter and wall thickness are 635mm

and 136mm. On the cylinder, 68 rows of nozzles are

evenly distributed with 304.8mm between each two rows.

The diameter and wall thickness for each nozzle are

45mm and 11mm. The metal material of the header is

T92/P92[5,6] .

Three hypotheses are proposed for the following anal-

ysis: (1) all physical parameters of the cylinder are iso-

tropic; (2) a half part of the cylinder and the inserted

nozzles was taken by the model because of the symme-

trical structure of the cylinder; (3) One nozzle was ran-

domly taken by the model because all nozzles are evenly

distributed on the whole cylinder. The model which con-

tains 52994 units and 79843 nods is analyzed by an algo-

rithm of free mesh SOLID98 element in this research.

Check the mod el on F ig ure 2.

2.2. Load and boundary conditions

The right cr oss-section(B-surface) on Y-axi s shifti ng and

the two longitudinal section(A-surface) on X-axis shift-

ing needs to be constrained. At the same time, restrain

the shifting of the Z-axis on a critical point-D. When

load i ng the inte rnal-p re s sur e o n the i nne r wal l, the t e nsi l e

load also needs to be loaded on the left cross-section and

five nozzle cross-section surfaces in the meantime. The

equations to calculate the tensile load are shown below

[7].

Figure 1 . The structure of header.

D. Y. LI ET AL.

Figure 2. T he model and the grid divis ion.

PP −

(1)

iD DD

PP −

(2)

Where PC=cylinder end cross-section uniform ten-

sion,mm;PD=nozzles end cross-section uniform ten-

sion,mm;Pi=internal pressure,MPa;do=nozzles outer

diameter,mm;di=nozzles inner diameter,mm; Do = cy-

linder outer diameter, mm; Di = cylinder inner diame-

te r ,mm.

The data of load that used for analysis are the actual

ope rat ing da ta in a po wer pl ant . The c hangi ng p ath o f the

inter nal pressure i s shown in Figure 3.

3. Results and Discussion

3.1. Distribution of Mechanical Stress

Figure 4 shows the distribution of mechanical stress.As

shown in Figure 4, mechanical stress changed greatly at

the inside conjunction area between the pipe and the

header cylinder when it changed very little at the area

which far from the intersecting line area. The phenome-

non of stress centralization at the intersecting line area is

in evidence. The stress at the area far from the intersect-

ing line area can be considered as the membrane stress of

the cylinder. It can be calculated with the follow equa-

tion[8]:

SDP

)( +

(3)

where P = internal pressure,MPa; Di = cylinder inner

dia meter, mm; S = wa ll t hic kne ss, mm. Co mpa riso n of the

ANSYS analysis to the theoretic calculation is shown in

Figure 5. Two results are basically the same. According

to the co mparison, the reliab ility of ANSYS is verified.

3.2. Maxi mum Stre ss Poi nt

During the running process, the maximum stress ap-

peared mainly at point-N as shown in Figure 6. The

point is on the intersectin g line.The variation of mechan-

ical stress at point-N sho ws in Figure 7.

Figure 3 . Pressure change curve in pick-up l oa d process .

Figure 4 . Cloud picture of mech anical stress.

Figure 5. Comparison of the ANSYS analysis to the theo-

retic calculat ion.

Figure 6 . Position of p oint-N.

D. Y. LI ET AL.

Figure 7. Variat ion of s tres s at point-N.

Figure 8 . Changing c urve of stress concentr ation fact or.

Impacted by the internal pressure, the mechanical

stress and the internal pressure are sharing the similar

changing tendency.The value of mechanical stress is va-

rying from 56.61MPa to 121.01MPa.

3.3. Stress Concent ration Factor

The mechanical stress concentration factor is indepen-

dent of ru nni n g co nd it ion[ 7] . T he str ess o n t he s tudy a re a

is directly proportional to the value of the internal pres-

sure. Define stress concentration factor as follow[9]:

max

(4)

where

max

= maximum stress. The changing curve of

the stress concentration factor during the process of load

pick-up at point-N is shown i n Figure 8.

Based on above computations, in the later period of

the load pick-up process, the stress concentration factor

with its value 2.51 can be considered keeping stable dur-

ing the whole process because of the tiny changes on the

amplitude o f the factor.

4. Conclusion

● Refers to the computations, the phenomenon of stress

centralization at the inside conjunction area between

the pipe and the header cylinder is obvious. Far from

the intersecting line area changes less in stress; it can

be considered as the membrane stress of the cylinder.

● The maximum stress is distributed mainly at point-N

which is focu sed o n the inters ectin g line. Accor din g to

the numerical calculation, the stress concentration

factor of point-N is 2.51.

● These analysis results, especially the stress concentra-

tion factor, can be used in life expenditure on-line

monitoring system in following research.

REFERENCES

[1] D. Q. ZHANG, S. J. YANG, G. Z. LIU and G. C. Zhang,

"Fatigue Life Assessment of Plant Drum in service,"

Shandong Electric Power, vol. 2, NO. 6, 1997, pp.

19-21+25.

[2] Y. G. PEI, Y. F. SHEN, T. L. YANG, Z. D. Cao, J. Wu, S.

X. Zhuang and H. D. Wang, "Management System of

Boiler Drum s Fatigue Life," Journal of Xi An Jiao-

tong University, vol. 5, NO. 16, 1999, pp. 64-67.

[3] L. B. HE, "Service Life Analysis of The High Tempera-

ture S uperheat er Outlet Header in a 6 00MW Su percrit ical

Boiler," Master, Thesis, North China Electric Power

University, Beijing, 2011.

[4] Q. Z. YIN, J. S. XIAO, A. L. Jiang, Y. P. Chen and Z. L.

Zhao, "Finite Element Analysis of Three Dimensional

Transient Stress in 300MW Boiler Drum," Journal of

Wuhan University of Technology(Transportation Science

& Engineering), Vol. 1, No. 17, 2007, pp.60-62+69.

[5] Vallourec & Mannesmann Tubes. "T92/P92 steel Hand-

book," In: (Ultra-) supercritical boiler steel and welding

technical symposium, 2005, pp. 285-304.

[6] Z. W. SHI, "Numerical simulation of temperature field

for T92/12Cr1MoV dissimilar steel welding," Mechanical

research and Application, Vol. 4, No. 41, 2010, pp.

99-101+104.

[7] Z. J. WANG, "Boiler structure finite element analysis,"

Chemical Industry Press, Beijing, 2005.

[8] T. Liu, "Life Estimation and On-line Monitoring for

Pressu rt e-bearing Part of Utility Bolier," Ph.D. Thesis,

North China Electric Power University, Beijing, China

2007.

[9] D. B. WANG, X. L. WEI, S. XIANG, C. X. Guo, J. X.

Wu, "Stress analysis and design for tangential opening

tubing connection on pressure vessels," China Petroleum

Machinery, Vol. 4, No. 2, 2006, pp. 5-7+85.