TECHNICAL NOTE

Failure Analysis of Bed Coil Tube in an AtmosphericFluidized Bed Combustion Boiler

M. Venkateswara Rao • S. U. Pathak •

D. R. Peshwe • Y. Y. Mahajan • S. A. Paranjape

Received: 6 September 2012 / Accepted: 21 August 2013 / Published online: 13 November 2013

� Indian Institute of Metals 2013

Abstract The Atmospheric Fluidized Bed Combustion

Boilers have advantages as compared to conventional coal

fired boilers in respect of heat transfer, SOx, NOx, emis-

sions and efficiency. In the present study, the failure

analysis of bed coil tube, used in atmospheric fluidized bed

combustor was studied. The failure studies included

physical, chemical and metallurgical analysis and the

results analyzed reveal that the failure is typically due to

caustic gouging. The cause of failure has further been

substantiated using scanning electron microscopy with

energy dispersive X-ray analysis as well as X-ray diffrac-

tion analysis.

Keywords Metallurgical failure analysis � Power plant �Boiler tube � Corrosion � Caustic gouging

1 Introduction

The fluidized bed combustion (FBC) technology is being

used in thermal power plants for steam generation. FBC

plants are more flexible than conventional plants wherein

coal, oil and biomass, among other fuels may be used for

firing. The FBC technology has important advantages over

conventional pulverized coal boilers like excellent heat

transfer, good combustion efficiency, low emission of

contaminants and good fuel flexibility [1]. This results in

burning of various fuels at lower temperatures with more

efficiently and also has the advantage of reduction in

effluents like SOx and NOx is reported [2, 3]. FBC systems

essentially fit into two major groups, atmospheric systems

(AFBC) and pressurized systems (PFBC), and two minor

subgroups, bubbling or circulating fluidized bed.

Boiler tubes are subjected to high internal pressure and

temperature during their operation as well as harsh environ-

ment of high temperature combustibles externally [4]. The

presence of corrosive elements would cause severe damage to

the inside and outside of the tubes. Many research investiga-

tions [5–8] have indicated that for safe and corrosion free

operations of boiler requires proper water monitoring and

treatment procedures. The disturbances in the water regime of

the boiler will lead to waterside deposition and subsequently

manifest itself into different corrosion problems such as

caustic gouging, erosion, stress corrosion cracking, galvanic

corrosion, pitting, etc. [9, 10]. Alkaline compounds like

Sodium Hydroxide, Hydrazine, Ammonia and Sodium Sulfite

can all contribute to dissolve the protective layer of Magnetite

on the metal surface. The reaction kinetics in respect of

hydroxide corrosion is given below [7, 10]:

Fe3O4 þ 4NaOH ! 2NaFeO2 þ Na2FeO2 þ 2H2O

ð1Þ

Further, the sodium hydroxide can react with the iron

according to the following equation [7]:

Fe þ NaOH ! Na2FeO2 þ H2 ð2Þ

The critical factors that contribute to caustic corrosion

are the presence of sodium hydroxide or alkaline producing

salts in boiler water, malfunctioning of chemical feed

equipment and the mechanism of concentration of sodium

M. V. Rao (&)

Central Power Research Institute, Bangalore, India

e-mail: mvraocpri1@gmail.com

S. U. Pathak � D. R. Peshwe � Y. Y. Mahajan

Visvesvaraya National Institute of Technology,

Nagpur 440010, India

S. A. Paranjape

Welmet Technologies Pvt. Ltd., Hingna, Nagpur 440016, India

123

Trans Indian Inst Met (2014) 67(3):437–442

DOI 10.1007/s12666-013-0351-x

hydroxide. The susceptibility of steel attacked by sodium

hydroxide is based on the amphoteric nature of iron oxides;

that is, oxides having low-pH and high-pH levels [11]. The

other deposits like phosphates, carbonates and silicates

formed in boiler tubes depends upon the water chemistry

and the quality at different temperature levels/regimes [12].

The present work focuses on the AFBC boiler tube

failure and the strategies to find out the root cause of failure

through laboratory investigations. The lab studies included

visual inspection, thickness measurement, chemical anal-

ysis, optical microscopy, Scanning Electron Microscopy

(SEM) with energy dispersive X-ray analysis (EDAX) and

X-ray Diffraction analysis (XRD).

2 Brief Construction Features of AFBC Boiler

The bed coil tube of AFBC boiler operated for about 1,000 h

has the dimensions of 51.0 mm OD and 7.0 mm thickness.

The fuel used in the boiler is coal of size 6 mm. In the AFBC

system, the water from the boiler drum is made to run

through six down comers. It enters the inlet header of bed

coils and then passes through water-wall to outlet header.

There is an air box with eight compartments installed under

the bed coils. Its top cover is made of 25 mm thick

distribution plate. From this distribution plate primary air

goes to the furnace along with coal through 32 Coal-Air

nozzles. The secondary air also goes into the furnace through

about 5,000 air-nozzles. A 400 mm thick fluidized bed of

alumina and silica (35 ? 65 %) is created with help of

secondary air. Particle size of fluidized bed is between

0.85 and 2.4 mm. The working pressure of boiler is

8.43 MPa.

3 Experimentation

The visual examination is done to check the physical

damage like change in color, distortion, nature of puncture,

etc. Thickness measurements are done at different locations

using digital micro-meter. The failed tube was cut into

cross section, then mounted, mirror polished by emery and

cloth polishing and etched to study the microstructural

features in optical microscope (Olympus make). The

samples were also checked for the hardness by Vickers

Hardness Tester (Instron make). To assess the internal

surface condition of the tube, the tube was also cut into two

longitudinal sections without affecting the base structure

properties. The internal deposits were carefully drawn for

phase analysis by XRD (PANanalytical PW 3040/60

Fig. 1 Schematic location of

affected area of Bed coil Tube

Fig. 2 Photograph of damaged

tube showing fracture of the

tube and cross sectional view

438 Trans Indian Inst Met (2014) 67(3):437–442

123

diffractometer) and samples drawn from the internal sur-

face were examined by SEM with EDAX attachment

(JEOL JSM 6380A).

4 Results

4.1 Visual Inspection

The failure has occurred in lower bend of the tube and at 12

o’ clock position i.e. top portion of the tube as shown

schematically in the Fig. 1. The visual inspection of the

failed tube reveals narrow split opening at the central

region of the outer coil with lot of deformation (rupture)

marks seen (Fig. 2a). Further, there is a wall thinning as

seen from the Fig. 2b (ovality). The inner surface of the

tube at 12 o’ clock position shows heavy pitting and cor-

rosion product. The Outer surface of the tube doesn’t show

any scaling or deposition.

4.2 Wall Thickness Measurements

The wall thickness measurements of the tube were con-

ducted on four sites (A, B, C, D sites) around the rupture as

shown in Fig. 1. The tube thickness near location B was

4.0 mm against the original thickness of 7.0 mm i.e.

around 43 % material loss. In all the other locations, the

thickness was found to be close to the original thickness

values. The localized reduction in wall thickness was

observed in the form of groove type which may be prob-

ably due to corrosion on the internal surface of the tubes.

4.3 Chemical Analysis

The chemical analysis of the failed tube material was

carried out by using optical emission spectroscopy and the

results are tabulated in Table 1. The chemical analysis

results confirms to the design specification of the bed coil

tube i.e. as per the ASTM A210 Gr.1 steel material.

The phase analysis of deposits collected from the in-

ternals of the bed coil tube is presented in Table 2.

The deposit predominantly contains the corrosive oxides

of sodium and traces of phosphorus and chlorine.

Table 1 Chemical composition of tube material

Tube No % C % Si % Mn % Cr % Mo % S % P

Failed tube 0.21 0.21 0.45 – – 0.016 0.021

Table 2 Phase Analysis of Deposits (Wet chemical analysis and

XRD)

Sample No. Fe2O3 Al2O3 CuO SiO2 Na2O MgO

Internal deposit

of bed coil tube

88.80 0.78 0.10 3.50 6.11 0.49

Fig. 3 Microstructure of the

bed coil steel—a away from the

fracture at 9100 magnification.

b at punctured location at 9100.

c Internal Corrosive products

and material wastage at

punctured location at 9100

Trans Indian Inst Met (2014) 67(3):437–442 439

123

Fig. 4 a SEM photograph of inner surface of the tube. b SEM and EDAX analysis of the inner surface of the failed tube. c SEM and EDAX

analysis of the inner surface another location of the tube. d SEM and EDAX analysis of the inner surface another location of the tube

440 Trans Indian Inst Met (2014) 67(3):437–442

123

4.4 Hardness Test

The hardness of the bed coil tube measured at surface as

well as at the core is in the same range. It is observed that

the hardness is in the range of 140–150 HV, which con-

firms to the standard grade ASTM A210 Gr. 1 steel [13].

4.5 Metallographic Examination

Metallographic examinations were conducted on the failed

tube from the rupture site and from a site away from the

rupture along the unaffected location of the tube. Figure 3a

shows the microstructure of the tube far away from the

rupture site, unaffected region. The microstructure of the

Fig. 3a revealed banding with alternate layers of ferrite and

pearlite. It is reported that banding formation in steel

depends upon the heat treatment temperature, soaking time

and cooling rate. Slow cooling from austenitic temperatures

gives intense banding and the intensity decreases with

increasing cooling rate [14]. Figure 3b shows the micro-

structure of the tube from the rupture site. The microstruc-

ture contains similar features of ferrite and pearlite with

banded nature. Figure 3c shows the microstructure of the

tube at inner side of the tube, where the corrosion has

occurred. The microstructure contains corrosive deposits

and fluxing of base metal of banded structure is clearly seen.

There is severe localized material loss from the inner surface

of the tube which is most likely due to internal corrosion.

4.6 SEM and EDAX Analysis of the Internals

of the Tube

A photomicrograph of the SEM of the corroded region of

the inner surface of the tube is shown in Fig. 4a. The SEM

photomicrograph shows the presence of distinct hemi-

spherical or elliptical depressions which would have

occurred due to the corrosive interaction of sufficiently

concentrated sodium hydroxide with a metal. The depres-

sions have been filled with dense corrosion products that

contain predominantly magnetite [7]. The EDAX was

carried out at different locations of the corroded regions

and results were presented in Fig. 4b–d. Figure 4b, c shows

the surface has features like cavities which are wider than

pits and also has globule type features. Figure 4d shows the

presence of large quantity of carbon and oxygen (attributed

to presence of carbonate) and small quantity of sodium and

chlorine in the deposit. The EDAX spectrums illustrate that

the deposits consist mainly iron and small amounts of

sodium, phosphorous, magnesium, silicon and chlorine.

The presence of sodium, phosphorous elements in the

deposits indicates that the tube has suffered localized

Fig. 5 XRD analysis of deposit sample from boiler drum

Trans Indian Inst Met (2014) 67(3):437–442 441

123

internal corrosion damage probably as a result of distur-

bance in the water chemistry and operating conditions of

the boiler [15].

4.7 Deposit Analysis by XRD

The results of XRD analysis of the deposits are shown in

Fig. 5. The XRD spectrum shows the presence of iron

oxide (Fe3O4) predominantly and minor amounts of oxides

of silicon, sodium, magnesium, calcium, etc. The results

are almost matching with the SEM EDAX results which

contain iron, silicon, sodium, magnesium, etc. It seems that

considerable corrosion has occurred during the operation

and resulting products got accumulated in localized pref-

erential locations.

5 Discussion

It is obvious from the experimental data that the material of

the tube has suffered localized severe internal corrosion,

probably due to caustic gouging phenomena. The banded

nature of microstructure is comparatively weak as compared

to uni-axial ferrite and pearlitic structure in terms of

mechanical and chemical properties, which might have also

contributed in internal corrosion [14]. The visual examina-

tion and thickness measurements clearly indicate the loss of

material in the inner surface especially at 12 o’ clock posi-

tion. The internal corrosion of metal is clearly shown in the

microstructures of the tube at the rupture site, where corro-

sive deposits and fluxing of base metal had taken place; see

Fig. 3c. The corrosion mechanism is also substantiated with

the results of SEM EDAX and XRD analysis of deposits.

Consequent to this corrosion, the metal thickness of the tube

has come down to a level where it could not withstand the

hoop stress due to internal water pressure. As the thickness

reduction takes place the hoop stress in the tube rises con-

siderably and finally failure had occurred. Similar cases of

failure were observed and documented in literature on dif-

ferent types of boiler tube steels [7, 12, 15].

6 Conclusions

To summarize, the bed coil tube of AFBC boiler had failed

due to internal corrosion specifically by caustic gouging

nature due to the presence of sodium compounds in the

internal deposits and material wastage from internal sur-

face. Also the banded nature of base microstructure con-

tributed in corrosion process. When the availability of

sodium hydroxide or alkaline-producing salts and the

mechanism of concentration exist simultaneously, they

govern susceptibility to caustic corrosion. This also occurs

when stratification of the steam water mixture produces

high concentration of caustic. Also steam blanketing can

occur when the fluid velocity is insufficient to maintain

turbulence and produce phase mixing especially in low

sloped tubes where the heat flux is high [7, 15].

The following remedies could be followed to eliminate

this type of corrosion:

• Reduce the amount of available free sodium hydroxide.

• Prevent inadvertent release of caustic regeneration

chemicals from makeup-water demineralizers.

• Prevent in-leakage of alkaline-producing salts into

condensers.

• Prevent contamination of steam and condensate by

process streams.

• Prevent departure from nucleate boiling (DNB) by

usually eliminating the hot spots, which is accom-

plished by controlling the boiler’s operating parame-

ters. Hot spots may be caused by excessive over-firing

or under-firing, misadjusted burners, change of fuel, gas

channeling, and excessive blow-down.

• Prevent excessive water-side deposition and creation of

waterlines.

• Prevent the use of tubes having banded microstructure.

Acknowledgments The authors are grateful to the Materials

Technology Division, Central Power Research Institute, Bangalore

and Metallurgical and Material Engineering Department, Visvesva-

raya National Institute of Technology, Nagpur for their kind per-

mission to publish this paper.

References

1. V. Kain, K. Chandra, B.P. Sharma, J Eng Fail Anal 15 (2008)

182.

2. Oakey JE, Pinder LW, Vanstone R, Henderson M, Osgerby S,

National Physical Laboratory. UK (2003).

3. Chacon-Nava JG, Stott FH, Martinez-Villafane A, ASM J Mater

Perf Eng 10 (2001) 699.

4. Khalil Ranjbar, Eng Fail Anal (2006).

5. Heyes AM. Eng Fail Anal 8 (2001) 123.

6. Chattora I, Das SK, Ravi Kumar B, Bhattacharya DK. Eng Fail

Anal 4 (1997) 274.

7. Deport Robert, Hero HM. The Nalco guide to boiler failure

analysis. Hill Inc (1944).

8. British electricity International. Modern power station and prac-

tice, vol. E. In: Chemistry and metallurgy. Pergamon Press

(1992).

9. Baffordi BP, Mater Perform 23 (1984) 17.

10. Fontana MG, Corrosion engineering. New York: McGraw-Hill

(1987) p 525.

11. Montrone ED, Blough JL, Mater Perform 28 (1989) 49.

12. S.A.J. Jahromi, M.M. AliPour, A. Beirami, Eng Fail Anal 10(2003) 405.

13. American Society for Metals, Metals Handbook Vol. 1(1996).

14. A. Mustapha, E. A. Charles, and D. Hardie, J Metall (2012).

15. Dooley R.B. and McNaughton, Water touched tubes, Electric

Power Research Institute, 2 (1996).

442 Trans Indian Inst Met (2014) 67(3):437–442

123