D. Ghosh*, S. Ray, A. Mandal and H. Roy

Failure Investigation of Radiant Platen Superheater Tube of Thermal Power Plant Boiler

Abstract: This paper highlights a case study of typical premature failure of a radiant platen superheater tube of  210 MW thermal power plant boiler. Visual examina­tion, dimensional measurement and chemical analysis, are conducted as part of the investigations. Apart from these, metallographic analysis and fractography are also conducted to ascertain the probable cause of failure. Finally it has been concluded that the premature failure of  the super heater tube can be attributed to localized creep at high temperature. The corrective actions has also  been suggested to avoid this type of failure in near future.

Keywords: boiler, failure, superheater, creep, metallo­graphy

DOI 10.1515/htmp-2013-0128Received December 12, 2013; accepted April 18, 2014;published online May 30, 2014

1  Introduction The thermal power plant boiler is a critical component for production of electric power. The function of boiler is to produce the superheated steam which is fed on steam turbine. Pulverized coal is the common fuel used in boiler along with preheated air. The boiler consists of different critical components like, economizer, water wall, super heater and reheater [1]. The bunch of superhetaer tubes are connected with header to form the superheated steam.

Radiant platen superheater is one of the critical compo­nent of drum type utility boiler and located in the furnace zone as pendent coil. These coils are directly exposed to radiant flux zone for superheating the steam for desired temperature and pressure [2].

Boiler tube failure is common unscheduled outage in thermal power plant component. Different damage mech­anism like creep, fatigue, erosion and corrosion are re­sponsible for the failure of different pressure parts tubes in boiler [3]. In spite of the best efforts of design engi­neers  and material scientists, engineering components fail in service. In some cases failure may lead to affect the reliability, availability and safety of the equipment. This finally leads to huge financial loss in different indus­tries. In the event of a failure, it is, therefore, essential to investigate the root cause of failure in terms of design, quality of material and fabrication procedure. The root cause of the failure of the different engineering compo­nents have been carried out to prevent the repetitive failure in near future and at the same time remedial mea­sures are also given to avoid similar failure in near future [4–10].

In this paper, an attempt has been made to understand the reasons of premature failure of the radiant platen superheater tube used in thermal power plant boiler. The  material specification, design and operating para­meters of the tube as obtained from the plant are given as follows.1. Material specification of the tube: SA­213 T22 (2.25 Cr­1

Mo steel).2. Working temperature and pressure of the tube: 763 K

& 130 kg/cm2.

3. Location of failure: Inlet leg of radiant platen super­heater tube

4. Effective running time: 1,20,000 hours5. Nominal dimension of the tube: 38 mm öutside

diameter × 8 mm thickness.

The exact location of failure in boiler is shown in Fig. 1.The visual examination, dimensional measurement

and chemical analysis are carried out on the failed mate­rials. Apart from these tests, metallographic analysis and fractography are also conducted to ascertain the root cause of the failure.

*Corresponding author: D. Ghosh: NDT & Metallurgy Group, CMERI, Durgapur 713209, India. E-mail: dghosh@cmeri.res.inS. Ray, H. Roy: NDT & Metallurgy Group, CMERI, Durgapur 713209, IndiaA. Mandal: Central Research Facility (CRF), CMERI, Durgapur 713209, India

High Temp. Mater. Proc. 2015; 34(2): 171 –175

   D. Ghosh et al., High-temperature Creep Failure

2 Tests and results

2.1  Visual examination and dimensional measurement

Visual examination is conducted on the failed superheater tube and it reveals small thick lip fish mouth failure (Fig. 2). The length of the failure zone is found to be 55 mm. Longitudinal cracking of outer scale is observed at the vi­cinity of failure zone. The inner surface of the failed tube contains no abnormal deposits (Fig. 3). The longitudinal cracking of scale is observed at the inner surface. The outer diameter (OD) and wall thickness are measured by using Vernier caliper and ultrasonic wall thickness gauge (Type: DM­3, Krautkramer, Germany). The results shows that the outside diameter near failure zone is increased

to  15.78% of the nominal diameter. The thickness at the failure zone is reduced to 20.32% of the nominal value.

2.2  Chemical analysis

Drilled chips from the failed tubes are subjected to analyze chemically by spectrometer and the estimated chemical composition is given in Table 1.

2.3  Oxide scale thickness measurement

The samples prepared from the failure locations are suit­ably polished and examined in optical microscope (Model:

Fig. 1: Location of failure in radiant platen superheater

Fig. 2: As received failed tube showing failure region and longitudinal scale cracking

Fig. 3: Inner surface of the failed tube

Table 1: Chemical composition of the failed tube

C Mn Si S P Cr Mo Fe

0.11 0.48 0.22 0.008 0.012 2.03 0.99 Balance

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D. Ghosh et al., High-temperature Creep Failure   

MeF3, Reichert Jung, Austria) for oxide scale thickness measurement. The oxide scale at the inner surface con­sists of double layer scale (Fig. 4). The measured thickness is found 375 µm. The mean tube metal temperature is cal­culated from the measured oxide scale thickness by using the following formula:

log x = −6.8398 + 2.83 × 10−4T (13.62 + log t) [11]

where x = scale thickness in mils (1 mil = 40 µm), T = temperature in Rankine (°R), °R = °F + 460, and t = service exposure time in hours. In this case, x = 375 µm (15 mil), t = 1,20,000 hours, and T = 1514.71 °R = 567 °C = 840 K.

2.4  Metallographic examination

Metallographic specimens are sectioned from fracture tip  of failed tube by precision abrasive cutter (Model: Abrasimet, Buehler Ltd., USA). These samples are then subjected to grinding in 120, 220, 400, 600 and 800 grade SiC papers followed by diamond polishing up to 1 µm. The polished specimens are etched using 2% nital and are ex­amined Scanning electron microscope (Model: S­3000N, Hitachi Limited, Japan). The microstructure at the frac­ture tip of the failure zone shows complete break down of bainite into coagulated alloy carbides along with the in­tergranular microcracks at the grain boundaries (Fig. 5a). At the same time, microstructure away from the failure zone indicates the precipitated alloy carbides along with  few isolated creep cavities at the grain boundaries (Fig. 5b).

2.5 Fractographic analysis

The fracture surface of the failed tube is carefully cleaned in ultrasonic cleaner for fractographic analysis in SEM. The fractographic image shows the sign of brittle fracture (Fig. 6).

3 DiscussionThe failure region suggests the fish mouth opening frac­ture at the flue gas side of radiant platen superheater zone (Fig. 2). The outer scale at the vicinity of the failure zone shows longitudinal scale cracking. The inner surface of the failed region also reveals the scale cracking at inner surface. Apart from this, no abnormal deposit is found in inner surface. The failed region also shows bulging, as the outer diameter is increased to a considerable extent. The reduction of wall thickness around the failed region is not  much. This justifies thick lip rupture at the failure region. The chemical composition confirms the specifica­tion 2.25 Cr­1.0 Mo Steel (Table 1). The mean tube metal temperature estimated from oxide scale thickness mea­surement (Fig. 4) indicates the higher tube metal tempera­ture (840 K). This further indicates that the tube is oper­ated in higher tube metal temperature for extended period of time (1,20,000 hours) before failure. Such a failure results from a relatively continuous extended period of slight overheating (differential between design and actual temperature). This type of failure is designated as high temperature creep failure.

The microstructure at the vicinity of the failure region shows breakdown of bainite into precipitated alloy car­bides and microcracks at the grain boundaries. The micro­cracks form by preferential linking of creep cavities along the grain boundaries (Fig. 5a). The microstructure indi­cates the severe creep damage (tertiary creep) for which the premature failure of the tube occurs. The microstruc­ture away from the failure indicates formation of isolated creep cavities which also indicates the initiation of creep damage of the tube material (Fig. 5b). At the same time the original microstructure of the new tube before service exposure reveals the presence of bainitic colony along with ferrite (Fig. 5c). The fractographic analysis of the fracture surface (Fig. 6) shows the intergranular brittle fracture, which justifies that the failure is taken place by high temperature creep in intergranular brittle fracture mode.

High temperature creep develops from the insufficient boiler­coolant circulation, elevated boiler gas tempera­ture or insufficient material properties that are inadequate

Fig. 4: Oxide scale at the inner surface of the tube

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for actual operating conditions. This abnormal operating conditions may exist due to the following reasons.1. Restriction of tube coolant flow by scale debris and

condensate inside the tube.2. Overfiring or uneven firing of fuel burners.3. Reduction of heat transfer capability due to develop­

ment and growth of steam side oxide scales or chemi­cal deposits.

4. Operation of tube material at higher than the allow­able temperature.

4 ConclusionsReviewing the technical data on tube failure, physical evidence and laboratory experimental results, it is con­cluded that the premature failure of the radiant platen su­perheater occurs due to localized creep at high tempera­ture. The growth of steam side oxide scale reduce the heat transfer capability and increases the mean tube metal

Fig. 5: Microstructure (a) near fracture surface of failed tube showing intergranular microcraks at the grain boundaries (b) away from the failure zone showing isolated creep cavities at the grain boundaries and (c) new unused tube showing banite and ferrite

Fig. 6: Fractographic analysis of fracture surface showing the evidence of intergranular brittle fracture

   D. Ghosh et al., High-temperature Creep Failure174

temperature for extended period of time. The high tem­perature causes localized creep at tertiary range and finally leads to premature failure of the radiant platen superheater tube.

The premature failure can be avoided by removal of scale, debris or deposits that have accumulated inside the tube. These will create internal flow restriction and reduc­tion of tube heat transfer, which finally results growth of steam side oxide scale and higher tube metal temperature. High pressure fluid flushing or chemical cleaning may be adopted to restore the designed coolant flow or tube heat transfer characteristics. At the same time, periodic overfir­ing or uneven firing of the fuel burner and operating tube material at higher than allowable temperature should be avoided.

Acknowledgments: The authors gratefully acknowledge Mr. C. Mukherjee (NDT & Metallurgy Group), Mr A. Mandal and Mr D. Karmakar (CRF) for their continuous effort in the characterization part of paper. The authors are also indebted to Director, CSIR­CMERI for his constant source of inspiration for writing this paper.

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