lly exfailur

(9.0%), high temperature (short-term overheating) (8.8%), erosion (6.5%), oxygen pitting (5.6%), caustic attack (3.5%) andstress corrosion cracking (2.6%). In general, 30% of all tube failures in boilers and reformers are caused by creep [57].

1350-6307/$ - see front matter 2010 Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: +60 3 89217269; fax: +60 3 89212116.E-mail address: mujibur@uniten.edu.my (M.M. Rahman).

Engineering Failure Analysis 17 (2010) 14901494

Contents lists available at ScienceDirect

Engineering Failure Analysisdoi:10.1016/j.engfailanal.2010.05.005originally specied [1]. Tube metal temperature may increase gradually over many years due to the growing oxide scalesinside tube or elevate rapidly caused by loss of internal steam or water-coolant ow. Internal pressurized tubes are criticalcomponents in water-tube boiler and steam superheater elements. Tubes in such application are vulnerable to high temper-ature upset condition, undergoing severe creep deformation or even nal rupture. Therefore, boiler tubes in power plantshave nite life because of prolonged exposure to high temperature, stress, aggressive environment, corrosive degradation,etc. However, uses of suitable boiler tube material in thermal power plants are required to ensure that the materials aresafely used under higher temperatures and pressures for a long period of operation [2]. Many works concerning the failureproblems in water-tube boiler have been reported.

The North American Electric Reliability Council (NERC) reported that the coal-red boilers are among the highest eco-nomic risk components in any other power plant. By far, the greatest number of forced outages in all types of boiler is causedby failures [3]. Elimination of boiler tube failure could save the electric power industry about $5 billion a year [4]. Metallur-gists from French, Inc. [1] published data of the top 10 causes of failures where creep (long-term overheating) is 23.4%, fol-lowed by fatigue (13.9%) (thermal 8.6%, corrosion 5.3%), ash corrosion (12.0%), hydrogen damage (10.6%), weld failuresKeywords:Superheater tubeCoal-red boilerLocalized overheatingVisual inspectionMetallurgical analysis

1. Introduction

Coal-red boiler tubes are generacommon cause of any metallurgicalposed to high internal pressure and high temperature of steam and ue gas. Thee of a superheater tube is due to the tube metal temperature higher than that asShort communication

Root cause failure analysis of a division wall superheater tubeof a coal-red power station

M.M. Rahman a,*, J. Purbolaksono a, J. Ahmad b

aDepartment of Mechanical Engineering, Universiti Tenaga Nasional, Km 7 Jalan Kajang-Puchong, Kajang 43009, Selangor, MalaysiabKapar Energy Ventures Sdn Bhd, Jalan Tok Muda, Kapar 42200, Malaysia

a r t i c l e i n f o

Article history:Received 27 January 2010Received in revised form 3 May 2010Accepted 23 May 2010Available online 26 May 2010

a b s t r a c t

journal homepage: www.elsevier .com/locate /engfai lanal

Ray et al. [8] conducted assessment of service exposed to boiler tube of the superheater and reheater made of 2.25 Cr-1Mosteels in a 120 MWboiler of a thermal power plant. The results showed that although therewas degradation of ultimate tensilestrength (UTS) and stress rupture of the boiler tubes due to increasing temperature and prolonged service exposure. But at theoperating condition of 540 C and 40 MPa in pressure, all these service exposed tubes have a remaining life of more than100,000 h, provided that there are no defects in the materials due to long-term service exposure. Husain and Habib [9] inves-tigated the steel tubes failure in a superheater boiler at one of the Kuwait Electrical and Power plants which suffered localizedoverheating. The tube was made of low alloy steel, SA 213-T 12 and it has been in operation for 109,415 h before failed. Theinvestigation indicated that the failure was attributed to the formation of thick scale of magnetite at the inner surface of thetubewall. This phenomenonprevented the accessibility of heat to the tubematerials and consequently localized andprolongedoverheating took place, in which the temperature raised up to 700 C in a frequent manner for long period of time. The prop-erties of the tubematerials changed from its original design values due to the effect of the localized and prolonged overheating.

Baoyou et al. [10], analyzed a boiler tube rupture through chemical analysis, scanning electron microscope, and energydispersion spectroscopy. The results showed that the tube burst due to overheating and excess temperature caused byobstruction of steam ow associated with the bubble clusters on the surface of local regions. Khajavi et al. [11] conductedinvestigation through visual examination, optical microscope, scanning electron microscope (SEM), and X-ray diffraction(XRD), to reveal the root causes of the boiler tube failure due to water-side corrosion problems. The results showed that cor-rosion failures are caused by a combination of ineffective control of water chemistry, deciencies in design and materialselection and operational problems such as inadequate water-side circulation which led to the formation of deposits in local-ized zones along a water line.

Srikanth et al. [12] conducted failure analysis of several evaporator tubes during commissioning and trial run of a waste

Chattoraj et al. [13] have investigated the corrosive degradation and failures of vertical furnace wall tubes of a co-gener-

M.M. Rahman et al. / Engineering Failure Analysis 17 (2010) 14901494 1491ation boiler. The investigations included chemical analysis of the corrosion deposit and microstructure observations. The re-sults showed that the most probable degradation mechanism is acid corrosion and under deposit corrosion due to thepresence of deposits, leading to localized heating (due to scale formation), and eventual rupture assisted by overheatingand decarburization.

Recently, several works on the failure analysis of boiler tubes that included superheater tube, reheater tube as well aswater wall tube have been reported [1421]. The reported works are conducted either through experimentation or numer-ical simulation. Most of the boiler tubes reported in the previous works failed after thousands of hours in operation.

Thisworkpresents failure investigationonadivisionwall superheater tubeof BoilerUnit 4 at SultanShalahuddinAbdulAzizShah Power Station by visual inspections, metallurgical examination and temperature estimation using the empirical formula.The ndings obtained from the investigation are discussed to deduce the failure mechanisms and the root cause of the failure.

2. Boiler operational backgrounds

Boiler unit number 4 at Sultan Shalahuddin Abdul Aziz Shah Power Station was noticed to have operated normally untilring of Bontang coal. It is a medium standard coal that was imported from East Kalimantan, Indonesia. The coal started to be

Fig. 1. Massive clinkers covering superheater region.heat recovery boiler using visual inspection, chemical analysis, X-ray radiography, fractography, microscopic examination atvarious locations, mechanical properties measurement and analysis using SEM. The results showed that the failure of theevaporator tubes at the bent tubes have been initiated by lamellar tearing because of inherent defects in the material, im-proper design of welding, and the absence of stress relieving treatment after the cold bending and welding operations.

1492 M.M. Rahman et al. / Engineering Failure Analysis 17 (2010) 14901494used on June 2007, and few weeks later a tube rupture was occurred at a division wall superheater region. It was reportedthat the failed tube had only operated at around 394 h with the average steam pressure of 122.5 bar (12.25 MPa). The failedsuperheater tube material is Chrome Molybdenum alloy steel (SA213-T 22). Based on site condition of the failure, Bontangcoal had signicantly caused formation of heavy slag and clinker as for example depicted in Fig. 1.

3. Visual inspections

Generally, Bontang clinkers have covered the empty spaces in the division wall superheater area. However, there weresporadic spots which were not covered or lightly covered by clinker in the division wall superheater region. The failed tubewas taken from the plant for further inspection. The as-received failed tube with a total length of 190 mm as shown in Fig. 2was inspected visually and dimensions of the cross section were measured by verniar caliper. The measurements showedthat outer diameter and thickness of the failed tube were 45 mm and 6.2 mm, respectively. Measurement of the scale thick-

Fig. 2. The as-received failed tube: (a and b) longitudinal view and (c and d) cross sectional view.ness inside tube was carried out through a metallographic sectioning and observed by an illuminated microscope. It wasfound that the thickness was about 0.197 mm (7.74 103 in.).

Evidence of localized coal-ash corrosion was seen at the rupture region of the failed tube sample (Fig. 2a and b). The crosssection of the sample was found in proper round shape (Fig. 2c and d) indicating that there were no deformations that trans-formed the tube metal to oval shape.

Fig. 3. Metal structure of SA213-T 22 showing complete stage of spheroidization.

whereBas

M.M. Rahman et al. / Engineering Failure Analysis 17 (2010) 14901494 1493where p is operational internal pressure; r and h are inner radius and wall thickness of the tube, respectively. Based on theaverage operating steam pressure of 12.25 MPa and the geometry of the tube, hence the operating hoop stress can be cal-culated equal to 38.33 MPa. According to Section 2, Part D of the ASME Boiler and Pressure Vessel Code [23], the maximumallowable stress for different operating temperature of seamless tube SA213-T 22 may be tabulated as shown in Table 1. Itcan be referred to Table 1 that the operating hoop stress with regard to the operating temperature at the time of failure ismuch higher than that as specied in ASME Code [23].

6. Discussion

Based on the information for the boiler operational backgrounds and on site inspection, it is obvious that the ring of Bon-tang coal which had low ash fusion temperature had caused massive heavy clinker formation, leading to obstruction of theue gas ow path in superheater tube bank. The small remaining portion of areas not covered by clinker around the failed

superhsevere5667the mresultRecennume

Higshowier temrh pr h2

h3ature at the time of failure may be estimated at around 636 C. It is clear that the operating temperature is higher than thedesign operating temperature of 540 C.

The estimated hoop stress developed in the tube may be determined as:T is the temperature in degree Celsius; t is the service time in h; C is a constant equal to 20.ed on the measured scale thickness of 0.197 mm and the tube operating hours of 394 h, thus the operating temper-where X is scale thickness in mm and P is the Larson-Miller parameter. In the Larson-Miller method, time and temperatureare related by the following equation:

P 95T 492

C log t 2logX

0:0254

0:00022 P 7:25 14. Metallurgical examination

The metal structures at the rupture region of the as-received tube were examined by the metallurgical microscope formicro-structural assessment. It can be seen from Fig. 3 that the metal structure was found to have signicant evidencesof the complete stage of spheroidization where the carbide particles have dispersed uniformly throughout the microstruc-ture. It indicates the expected changes in morphology from a normalized structure of ferrite and pearlite as normally exhib-ited by the virgin tube metal.

5. Operating temperature and hoop stress

An empirical formula correlating scale thickness and Larson-Miller parameter as reported by Rehn et al. [22] is utilized toestimate the operating temperature as:

Table 1The maximum allowable stress for different operating temperature of seamless tube SA213-T 22 [23].

Temperature (C) Max. allowable stress (MPa)

537.78 55.16565.56 39.30593.33 26.20621.11 16.55648.89 9.65eater tube was then exposed to higher ue gas temperature, causing the higher operating metal temperature. In thiscondition, coal-ash corrosion is expected to occur on metals whose surface temperatures are in the range of

32 C [24]. When fuel supply or fuel type is changed, it will result in production of an aggressive ash. As consequence,olten ash would be carried over from the furnace to the tube superheater bank, and the high velocity gas ow wouldin a higher metal temperature above the safe operating limit and subsequently caused the tube to fail by overheating.tly, the phenomenon of a boiler tube exposed to higher temperature and higher velocity of ue gas was also describedrically by Purbolaksono et al. [25,26].her temperature operation is also conrmed by the nding from the metallurgical examination on the failed tube,ng a complete spheroidization. If the pearlite has spheroidized, then the rupture has almost certainly occurred at high-perature operation above 600 C [6]. Spheroidization in ferritic tube structures would usually commence as the carbon

tube metal temperature is around 600 C. The estimated operating temperature based on the measured scale thickness uti-lizing the empirical formula [22] also showed a higher temperature operation. At this particular temperature, the operatinghoop stress becomes much higher than that as specied in ASME Code [23].

7. Conclusions

Failure analysis on the failed division wall superheater tube of a boiler unit through visual inspections, metallurgicalexaminations and estimation of the operating temperature utilizing an empirical formula were presented. It was found thatthe failed SA213-T 22 tube was caused by localized overheating due to the formation of clinker because of the burning of lowquality coal. In addition, the molten ash had worsened the situation by synergically causing coal-ash corrosion.

Acknowledgements

1494 M.M. Rahman et al. / Engineering Failure Analysis 17 (2010) 14901494The authors would like to express the best gratitude to Kapar Energy Ventures Sdn Bhd Malaysia for permission of uti-lizing all the facilities and resources during this study.

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Root cause failure analysis of a division wall superheater tube of a coal-fired power stationIntroductionBoiler operational backgroundsVisual inspectionsMetallurgical examinationOperating temperature and hoop stressDiscussionConclusionsAcknowledgementsReferences