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Engineers, Part E: Journal of ProcessProceedings of the Institution of Mechanical

http://pie.sagepub.com/content/224/4/269The online version of this article can be found at:

DOI: 10.1243/09544089JPME332

269 2010 224:roceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering

J Ahmad, J Purbolaksono, L C Beng and A AhmadFailure Evaluation on a High-Strength Alloy SA213-T91 Super Heater Tube of a Power Generation

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CASE STUDY 269

Failure evaluation on a high-strength alloy SA213-T91super heater tube of a power generationJ Ahmad 1, J Purbolaksono 2 ,LCBeng 1 , and A Ahmad 2

1Kapar Energy Ventures Sdn Bhd, Kapar, Malaysia2Department of Mechanical Engineering, Universiti Tenaga Nasional, Kajang, Selangor, Malaysia

The manuscript was received on 3 December 2009 and was accepted after revision for publication on 12 April 2010.

DOI: 10.1243/09544089JPME332

Abstract: This article presents failure investigation on a high-strength alloy SA213-T91 super-heater tube. This failure is the rst occurrence involving the material in Kapar Power StationMalaysia. The investigation includes visual inspections, hardness measurements, and micro-scopic examinations. The failed super-heater tube shows a wide open rupture with thin andblunt edges. Hardness readings on all the as-received tubes are used for estimating the operating metal temperature of the super-heater tubes. Microstructures of the failed tube show numerouscreep cavities consisting of individual pores and chain of pores which form micro- and macro-cracks. The ndings conrmed that the super-heater tube is failed by short-term overheating.Higher temperatures of the ue gas due to the inconsistent feeding of pulverized fuels into theburner is identied to cause overheating of the failed tube.

Keywords: high-strength alloy steel, failure analysis, hardness measurements, overheating,micro-structure examinations

1 INTRODUCTION

The enhanced ferritic steels such as grades 91,92, and 122 have recently been utilized for powerplant applications [ 1]. As these materials have very good creep strength, the applications of the mate-rials signicantly contributed to fabrication prac-tices, particularly in terms of appropriateness of the micro-structures in components [ 2]. In particu-lar, a chromium–molybdenum–vanadium steel tube

(SA213-T91)hasbeenavailableinthemarketsincetwodecades ago. The SA213-T91 material has better high-temperature strength and oxidation resistance thanthose of the widely used materials such asT11 and T22[3]. The 9Cr–1Mo–1V steel was co-developed by theCombustion Engineering and the Oak Ridge NationalLaboratory in the late 1970s [ 4].

Recently, the rst occurrence of the boiler tubefailure involving a high-strength alloy SA213-T91occurred in Kapar Power Station Malaysia.This article

Corresponding author: Department of Mechanical Engineering,Universiti Tenaga Nasional, Km 7 Jalan Kajang-Puchong, Kajang,Selangor 43009, Malaysia.email: judha@uniten.edu.my; j.purbolaksono@gmail.com

presents the failure investigation comprising visualinspections, hardness measurements, and micro-scopic examinations. Hardness readings on all theas-received tubesare usedforestimatingthe operating temperature of the super-heater tubes. Discussionson the failure mechanism and the root cause arepresented and the recommendations for preventing similar failure in the future are made.

2 OPERATIONAL BACKGROUNDS

After 38 012 h in service, the boiler unit had under-gone a planned outage for 45 days. Next, the boilerunit was returned to service with an average operating steam pressure of 163 bar (16.3 MPa) for serving load-ing of 500 MW. Unfortunately, the unit was forced toshut down, due to the tube leakage involving materialof T91 (9Cr–1Mo–1V steel) at the super-heater region just after running for around 38 h. The failed tubeand 11 other neighbour tubes were then removed andreplaced. The neighbour tubes experienced a local-ized wall thinning, due to the steam impingement

which was escaping from the failed tube. Figure 1shows thefailedsuper-heater tube after removalof theneighbour tubes. The super-heater tubes had an outerdiameter of 50.8 mm and a wall thickness of 7.2 mm.

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270 J Ahmad, J Purbolaksono, L CBeng, and A Ahmad

Fig. 1 The ruptured super-heater tube in the locationafter the removal of the neighbour tubes

3 VISUAL INSPECTIONS

Visual examination on the as-received failed super-heater tuberevealedthe followingfeaturesas follows.

1. The tube was failed by a wide open rupture in thedimension of 13 cm length and 7 cm width andleaving thin edges (see Location 4, Fig. 2).

2. A half-width of the tube wall with a length of 30 cm was blown out, and the remaining ruptured part was leaving blunt edges (see Locations 2 and 3,Fig. 2).

3. There was no swelling at other parts of the tube.4. Therewere no unusual deposits on theexternal and

internal surfaces.5. There was no evidence of active corrosion on either

the external or internal surfaces.

6. There was no evidence of wall thinning at somedistance away of the rupture region.

7. There was the presence of numerous longitudinalmarks/lines along the side of the ruptured edges.

8. Theinformation most likelyindicates that thefailedtube had experienced short-term overheating.

4 MICROSCOPIC EXAMINATIONS

The metal structures at different locations of the as-received failed tube are examined. Figure 2 showsthe locations of the samples taken for the examina-tions (i.e. at the failure region (Locations 2 and 3)and at 60 cm away from the rupture end (Location1)). Figures 3 and 4 reveal the presence of numerouscreep cavities either by individual pores or a coales-

cence of pores. The coalescences have formed micro-and macro-cracks. Figure 5 shows numerous creepcavities and a crack propagating towards the innersurface of the tube (Location 3). The ndings of themicroscopic examinations conrmed that the failedtube had operated at a high temperature prior tofailure.

5 HARDNESS MEASUREMENTS

Hardness measurements on the tube samples thatexperienced a localized wall thinning due to steam

impingement are carried out. The measurement onthe failed tube was also conducted. At the time of failure, the steam poured out of the boiler tube, lead-ing to an immediate rapid cooling (fast quenching)process which would signicantly inuence the hard-ness value of the tube metal in the vicinity of therupture region becomingsignicantly higher than thatof the other parts. Thus, the sample of the failedtube was representatively taken from the location of

Fig. 2 The failed T91 super-heater tube with blunt and thin edges

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Failureevaluationon a high-strength alloy SA213-T91 super heatertube ofa power generation 271

Fig. 3 Photomicrographs for Locations 1 and 2 showing micro-structural changes in cavity formations

Fig. 4 Photomicrographs for Location 3 showing crack with some micro-cracks (in circles) andcreep cavities

Fig. 5 Photomicrographs showing numerous creep cavities and a crack propagating towards the

inner surface of tube (Location 3)

60 cm away from the rupture region. The hardnessreadings are then used for estimating the operating metal temperature at the time of failure, considering that the operating service is 38 h.

Correlation between hardness (HV) andtheLarsen–Miller parameter for 9Cr–1Mo–1V steel in the as-quenched condition may be expressed as [ 5]

Hardness (HV ) = 933 − 0.018 25 P (1)

where P is theLarsen–Miller parameteranddenedas

P =95

T C + 492 (C + log t ) (2)

in which T C is the temperature in degrees Celsius,t is the rupture time in hours, and C is a constantequal to 20.

Equations (1) and (2) are used for estimating theoperating metal temperatures of the as-received tubesbased on the hardness readings, and the estimationsare tabulated in Table 1. It can be seen from Table 1,the estimated metal temperatures indicated that thetubes had experienced higher-temperature operation.

The hoop stress σ h developed in the tube may bedetermined as

σ h = p(r + (h / 2))

h (3)

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272 J Ahmad, J Purbolaksono, L CBeng, and A Ahmad

Table 1 Estimated operating tube metal temperatures

Number Tube samplesHardness(HV)

Operating temperatures ( ◦ C)

1 1 246 695.82 2 255 683.13 3 260 676.04 4 250 690.15 5 235 711.36 6 235 711.37 Failed tube 246 695.88 7 262 673.29 8 271 660.5

10 9 250 690.111 10 249 691.512 11 248 693.0

Table 2 The maximum allowable stresses for differ-ent temperature [ 6]

Temperature ( ◦ C) 566 593 621 649Maximum allowable

stress (MPa)88.95 71.02 48.27 29.65

where p is the operational internal pressure, and r and h are inner radius and wall thickness of the tube,respectively. Thus, the operating hoop stress can becalculated equal to 49.35MPa. The maximum allow-able stresses for SA213-T91 for different temperatures[6] are presented in Table 2. Referring to Table 1for the estimated metal temperature and Table 2for the maximum allowable stresses, the operating hoop stress in the super-heater tubes is found to beexcessive.

Diagram of Larsen–Miller parameter with stressvariation to rupture for T91 steel [ 1] as shown in Fig. 6may also be utilized to estimate the metal temper-ature at the rupture region, considering that thereis no scale on the internal surface. Parameter T inFig. 6 is the absolute temperature in degrees Kelvin(◦ C + 273). For the operating stress of 49.35 MPa, theLarsen–Miller parameter obtained by referring to theminimum curve in Fig. 6 is 32 750. Hence, the metal

temperature at the rupture region may be estimated ataround 732 ◦ C. It indicates that the temperature of thefailed tube had gone extremely high.

6 DISCUSSION

The thin-edge rupture of the failed tube andtheseveredeterioration of the metal structure indicated that thefailure was failed by the short-term overheating. Theestimated metal temperature at the time of failureindicated that the failed tube had operated at much

higher metal temperature than the temperature alarmof 570 ◦ C. The overheating is also conrmed by thepresence of numerous creep cavities, either by indi-vidual pores or by chain of pores. The coalescences of

Fig. 6 Diagram of Larsen–Miller parameter with stressvariations to rupture for grade 91 steel [ 1]

the pores have formed both micro- and macro-cracks,resulting in tube rupture.

The estimated temperature of the failed tube basedon the hardness reading is about 695 ◦ C (Table 1).However, as reported by Jones [ 7] the rupture regionmust signicantly have a higher temperature. Hard-

ness readings of the 11 neighbour tubes also show high metal temperature operation. It may indicatethat the high-temperature operation had generally occurred in the super-heater region. Therefore, thecombustion issue is reasonably suspected to be theroot cause of the failure. It is a more reasonable causethan that by the tube blockage which usually leads tosteam starvation and overheating. Inconsistent feed-ing of pulverized fuels into the burner is identied tocause the uctuation and uneven distribution of heatcausing a sudden high ue gas temperature.

Even though the failure is most likely caused by the combustion problem, the issue of tube blockageshould not be totally ruled out. Therefore, in order toensure that the internal tube is free from any leftoverof the foreign materials, video boroscope inspectionsat the bottom region of vertical super-heater headerare strongly recommended during the next outage.However, after the feeding of pulverized fuels into theburner is cautiously taken care of, the boiler unit may have run normally till the next planned outage.

Several recommendations could be considered toavoid a similar failure in the future as follows.

1. Combustion issue that includes a pulverized mill

and combustion system inside the boiler furnaceshould be addressed correctly.2. Consistent feeding of the pulverized fuels into the

burner should be maintained.

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Failureevaluationon a high-strength alloy SA213-T91 super heatertube ofa power generation 273

7 CONCLUSIONS

Failure investigation by visual inspections, hardnessmeasurements, and microscopic examinations on a

high-strength alloy SA213-T91 super-heater tube waspresented. The estimated metal temperatures basedonthehardness readings of thetubesamples indicatedhigher-temperature operation at the time of failure.Findings of the microscopic examinations showednumerous creep cavities consistingof individualporesand chain of pores forming micro- and macro-cracks.It may be deduced from the ndings that the super-heater tube was failed by short-term overheating.The inconsistent feeding of pulverized fuels into theburner was identied to cause the uctuation anduneven distribution of heat, resulting in a suddenhighue gas temperature or overheating.

ACKNOWLEDGEMENT

The authors wish to thank Kapar Energy VenturesSdn Bhd Malaysia for permission of utilizing all thefacilities and resources while conducting this study.

© Authors 2010

REFERENCES

1 Masuyama, F. Creep rupture life and design factors forhigh-strength ferritic steels. Int. J. Press.Vessel Pip. , 2007,84 , 53–61.

2 Perrin, I. J. and Fishburn, J. D. A perspective on thedesign of high-temperature boiler components. Int. J.Press. Vessel Pip. , 2008, 85 , 14–21.

3 French, D. N. Metallurgical failures in fossil red boil-ers , 2nd edition, 1993 (John Wiley and Sons Inc.,New York).

4 Sikka, V. K., Ward, C. T., and Thomas, K. C. Modied9Cr–1Mo steel – an improved alloy for steam genera-tor application. In Ferritic steels for high-temperature applications , Proceedings of the ASM International Con-ference on Production, fabrication properties, and appli-cations of ferritic steels forhigh temperature applications ,Metals Park, Ohio, 1983, pp. 65–84.

5 Viswanathan,R.,Foulds,J. R., and Roberts,D.A. Methodfor estimating the temperature of reheater and super-heatertubes in fossils boilers. InProceedingsof theInter-national Conference on Life extension and assessment ,The Hague, 1988.

6 ASME. ASME international electronic stress table, Table1A: The maximum allowable stress values for ferrousmaterials,Section II,Part D ofTheASME boiler andpres-sure vessel code, 1998 (ASME International, NewYork).

7 Jones, D. R. H. Creep failures of overheated boiler, super-heater and reformer tubes. Eng. Fail. Anal. , 2004, 11,

873–893.

APPENDIX

Notation

C constant used in Larsen–Miller parameterh wall thicknessp operational internal pressureP Larsen–Miller parameterr inner radiust rupture timeT operating temperature

σ stress

Subscripts

C degrees Celsiush hoopR rupture

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