INVESTIGATIONS ON THE FAILURE OF BOILER TUBESIN SWCC JEDDAH POWER PLANT1

Anees U. Malik*, Nausha Asrar*, Mohammad F. Al-Ghamdi**and Abdullah Hasan Hodhan**

* SWCC Research & Development Center, P.O.Box 8328, Al-Jubail 31951** SWCC, Jeddah Desalination & Power Plant, P.O.Box 6538, Jeddah 21452

ABSTRACT

Leakages were observedfrom the roof tubes on the secondpass of boilers # 6, 7 and 8 ofJeddah Phase-3 plant. Saturated steam flowed through the roof panel tubes made ofcarbon steel, the temperature outside the tube was around 1280°C and insidetemperature was about 450°C. The tubes were having surficial cracks and pits on thefire-side of the tube wall. Methods of ultrasonic testing, optical and scanning electronmicroscopy, X-ray diffraction analysis and energy dispersive X-ray analysis wereapplied during investigations. The causes of the failure have been determined and themechanism of corrosion has been discussed.

BACKGROUND

Five samples of roof panel tubes of boiler #6, 7 and 8 of Jeddah Power and Desalinationplant were received at R&D Center, SWCC, Al-Jubail, for failure analysis. These tubeswere having surficial cracks and pits on the fire-side of the tube wall. Thinning downof the inner surface of the tube or bulging out of the outer surface of the tubes were thefamiliar features of the failure.

EXPERIMENTAL

Techniques Used

Failure investigations were carried out using both non-destructive (ND) as well asdestructive (D) testings. The techniques employed during failure analysis included:ultrasonic testing, optical microscopy, optical image analysis, scanning electronmicroscopy, X-ray fluorescence spectroscopy, X-ray diffraction analysis, atomicabsorption spectroscopy and energy dispersive X-ray analysis. Using these techniques

1 Presented in the Second Saudi Symposium on Energy Utilization and Conservation,KFUPM, 2 7-30 November, 1994, Dhahran

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visual examination of the failed tubes, analysis of steel and oxide scales andfractography of the failed areas of the tubes were carried out.

RESULTS AND DISCUSSION

In recent years owners and operators of industrial boilers strive to extract maximum heatfrom the fuel, and therefore, more efficient high pressure and temperature steam cyclesare selected to optimize the amount of electrical power and steam produced. Thisindustrial trend towards higher pressure (up to 1800 psig) and temperature in boilers,sometimes results into unprecedented failures and presents new challenges for the boilerusers.

Overheating, hydrogen damage, pitting corrosion and corrosion of weldments are themain causes to which the failure of boiler tubes of Jeddah Power and Desalination Plantcan be attributed. On the basis of destructive and non-destructive testings mechanisminvolved in the failure is discussed below:

Overheating

Visual Inspection:

One of the roof panel tubes of boiler .#6, designated as 6A in Fig. la, had a swollenportion with many circumferential cracks on it. Longitudinal section of the failed area,cleaned with inhibited 3% hydrochloric acid solution, revealed that the cracks hadinitiated at the steam -side of the tube. Some of the cracks had crossed the entirethickness of the tube and had opening at the fire-side (Fig. 1 b and c).

Failure of roof panel tube of boiler #7 also appears to be due to overheating of the tube.Tube 7A had a blown up portion near the tapered end (Fig. 2a) and tube 7B showslocalized circumferential cracks and pits around the crack opening (Fig. 3).

Analysis of Oxide Scales:

Thick black oxide scales deposited on the steam-side surface of the tube and brownoxide scale formed on the fire-side surface of the tube were identified by X-rayDiffraction technique and EDAX. It was found that the oxide scales deposited on thesteam-side surface is composed of magnetite (Fe3O4 while the scales on fire-sidesurface contains haemetite and wustite (Fe2O3 + FeO)

Metallographic Evaluation:

Metaliographic analysis is an important tool for analyzing material temperature history.When carbon steels are exposed to temperatures above 482°C but less than the lowercritical temperature (723°C) this steel is subjected to spheroidization. The carbidespresent in the steel in annealed condition are not in their lower energy state and exposureto higher temperature results in the coalescence of these carbides into spheroidal form.

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If the tube wall gets ruptured and temperature is more than the lower criticaltemperature, the rapid cooling effect of the escaping steam will result in a distinctquenched microstructure form. Microscopic studies of cross-section of failure zone ofboiler tube 6A and 7B show quenched microstructure with small amount of carbidespresent at the grainboundaries (Fig. 4a and b), while microstructure of failure zone oftube 7A shows coalescence of the carbides into a spheroidal from (Fig. 5) andelongation of the grains at the end of the knife-edge failure opening (Fig. 6). Oncomparing the above microstructures of failed area with unfailed area of tube 6A, 7Aand 7B (Fig. 7a and c), the former appears as quenched overheated microstructures.This shows that the exposure temperatures of the above failed areas have exceeded thelower critical temperature and were exposed to this temperature for quite long time.

These roof panel tubes of ST 35.8 III type steel (C < 0.17, Si 0.1-0.3, Mn 0.4-0.8, S0.05 max., P 0.05 max) are heated up at a temperature of 1285oC in the combustion chamber and are cooled by steam flowing through it at a temperature around 400oC.The strength of steel is nearly constant upto about 454oC. When the tube metaltemperature is gradually increased beyond this temperature, then the influence ofcombined effect of loop stress (related to internal pressure of steam and tube dimension)and temperature will result into plastic deformation (swelling) and rupture of the tube.

Boiler tubes are overheated due to the internal deposits insulating the tube metal fromthe cooling effect of steam. As the deposits do not form uniformly along the tube,overheating is always localized in nature.

Overheating failures are of two types: short-term and long-term. Short-term overheatingfailures result when (i) tube metal temperature increases hundreds of degrees abovedesign temperature (ii) there is blockage of the tube and (iii) loss of drum water level.This type of tube failure exhibits considerable creep elongation resulting inconsiderable reduction in wall thickness. Sometimes the tube blows up into the furnaceand shows a knife edged failure opening.

In view of the above discussion of overheating failure cases, blowing up of tube 7Aappears to be a short-term overheating failure which might have occurred due toblockage of the tapered end and sudden localized rise in temperature. Tube 7A withblown up portion is shown in Fig.2a and a comparison imprint with dimensions ofknife edged failure opening is shown in Fig. 2b. These figures reveal that the tube hasbursted after a considerable reduction in the wall thickness.

As the severity of the overheating is lessened, the time frame of failure is increased.This type of failure is termed as long-term overheating failure. Creep elongationeffects (swelling and wall thickness reduction) are much less pronounced in thesefailures. These failures usually show evidence of secondary cracking in addition to themain crack associated with the rupture. Cracks and ruptures of tube 6A and 7B withless creep elongation appear to be the cases of long-term overheating failure.

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Hydrogen Damage

A serious and catastrophic failure of carbon steel tubings of boilers, exposed to hightemperature (> 500°C) and high pressure of steam (>1500 psig) for a long time is thatassociated with the decarburization of steels resulting into hydrogen damage.Characteristically a deposit plays a key role in: (a) overheating of metal and (b) creatinga site with high availability of elemental hydrogen. The damage mechanism requiresrelatively high rates of carbon diffusion which is evident by the decarburization of thesteel on both steam and tire-side surfaces (Fig. 8a and b). But the cause ofdecarburization of steam-side surface is expected to be steam while that of fire-side it isthe CO2 gas.

Decarburization at the steam-side surface is represented by the equations:

3 Fe + 4H 2 O = F e 3 O 4 + 4 H 2 [ l ]

Fe3C + 2H2 = 3Fe+ CH4 [ l l ]

Decarburization at the fire-side surface is represented by the equation:

Fe3C+4CO2 = 3FeO+5CO [lll]

An illustration of the environment that results in hydrogen damage is shown in Fig. 9.

A study of the Fe-H2-H2O and Fe-CO-CO2, equilibrium diagrams2,3 and presence ofblack oxide scale composed largely of Fe3O4 on the steam side surface (H2/H2Oatmosphere) and brown oxide scales, Fe2O3+ FeO on the fire-side surface (CO/CO2/O2)of the tube, supports a H 2 - induced failure mechanism.

As the partial pressure of hydrogen [I] increases, it diffuses into the steel and reacts withcarbides forming methane gas [II] which exerts an internal pressure. The decarburizedmetal gets embrittled and fails to withstand this internal pressure. As the size ofmethane molecule is bigger than CO molecule and there is a high pressure of steaminside the tube, more cracks have initiated at steam-side surface of the tube. The reasonfor cracks being circumferential in nature appears to be thermal residual stresses whichdeveloped due to localized heating and acted parallel to the length of the tube.

Pitting Corrosion

Tubes 6C, 7B and 8B had large crack openings and deep pits around it. From thefigures 10a and b, it is evident that pitting started at the tire-side surface of the tube nearthe crack and progressed downwards undercutting the surface. It was also observedthat depth of the pit increased as one moves away from the crack. No chemical attackwas found on the steam-side of the tube wall.

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Pitting corrosion of the fire-side surface of these tubes does not appear to be due todifference in oxygen concentration, because for such type of pits presence of crevicesand/or deposits is essential. Visual inspection of the pits and their size indicate that theyhave formed under highly acidic condition. When the steam flowing in the tube at apressure of around 70 bar escapes through the narrow cracks, it will cool down near thefire-side end of the crack due to the pressure gradient (Joule-Thomson effect). Thiscooling of the steam will result in:

(1)(ll)

Condensation of steam into water near the crack.Dissolution of sulfurous gases of the fire product and formation of acidicwater. EDAX results of the oxide scales formed on the fire-side surface of thetube 7B and 8B show the presence of vanadium and sulfur in the oxide scales(Fig. 11).

As the temperature of the fire-side surface of the tube will be less, the condensation ofescaping steam will be more, and consequently pitting corrosion will be more. If we tryto analyze the temperature history of all the failed tubes under study on the basis of theirextent of swelling, reduction in the wall thickness and spheroidized and quenchedmicrostructures, it seems that the fire-side temperature experienced by the tubes 6C, 7band 8B was less than other failed tubes. Therefore, severity of the pitting corrosion ofabove mentioned tubes is more.

Rupture of Welded Joint

Tube 6C of boiler #6 had a single circumferential crack at the welded joints (Fig. 12)and pits around it. The fine grained microstructure of the failure site (Fig. 13) revealsthat the welded joint of tube 6C was not stress-relieved. It appears that the long andheavy tube welded with the bifurcated tube of the header generated high stresses in thetube wall. The corrosion fissures at the welded joint grew slowly over the life time ofthe tubing, and the section ruptured when fissure penetration reached to a critical depth.Thermal fatigue may also have contributed to the penetration of the fissures.

CONCLUSIONS

On the basis of the above discussion following conclusions can be drawn:

(1)

(2)

(3)

(4)

Tubes 6A and 7A of boiler #6 and #7, respectively have failed due tooverheating.Failure of tube 7B and 8B of boiler #7 and 8 respectively, appears to be due tohydrogen damage.Rupture of the tube 6C at the weldment site may be due to residual stressesgenerated during fabrication and assembly.The high sulfur containing fire products are deposited on the outer surface ofthe tubes making the condensed water around the cracks acidic. Inconsequence, tire-side surfaces of the tubes 6C, 7A and 8B of boiler #6, 7 and8 respectively , undergo pitting corrosion.

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RECOMMENDATIONS

(1) Chromium and molybdenum additions to steel improve its resistance tocracking and decarborization by forming carbides which are morethermodynamically stable than iron carbides5. Therefore, carbon steel tubesused in boilers should be replaced by steels containing Cr and Mo. Type ofCr-Mo steel will depend upon the condition of the area where these tubes willbe used.

(2) Overheating of the tubes should be avoided by effective cleaning of the steam-side surface of the tubes and removing all the tenacious deposits.

(3) During fabrication and assembly of the pipes with welded joints, care should betaken to avoid generation of stresses at the weldments.

REFERENCES

1.2.

3.

4.

5.

Combustion Engineering, Inc. Combusion 3rd, Windsor, CT. 198 1.Metals Handbook, Vol. 2, p.68, American Society for Metals, Metals. Park,Ohio, 1964.W.D.Jones, Fundamental Principles of Powder Metallurgy, p. 537, EdwardArnold (Publishers) Ltd.H.A.Klein and H.A.Grabowski, Corrosion and Hydrogen Damage in HighPressure Boilers, Presented at the Second Annual Education Forum onCorrosion , NACE, Drexel Institute of Technology, Philadelphia, Sept. 15 to17, 1964.V.P.Murphy, “Industrial Water Treatment”, presented at TAPPI,Jacksonville, Florida, March 11, 1966.

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Figure 1. Boiler tube 6A showing (a) swollen area (b) laboratory cleaned fire-sidesurface and (c) laboratory cleaned steam-side surface

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I I

-------130 -(b)

Figure 2. Boiler tube 7A (a) as received with failure opening (b) actual dimensions ofthe opening (in mm).

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Figure 3. As received boiler tube 7B with pits and cracks.

Figure 4. Micrograph of cross-section of boiler tubes (a) 6A and (b) 7B, showingquencned microstructure and transgranular cracking originating at steam-sidesurface of the tubes (X 200).

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Figure 5. Micrograph of cross-section of failure zone of tube 7A showing spheroidizedmicrostrucure of carbon steel by overheating (X 200).

Figure 6. Micrograph of knife edge failure opening of tube 7A showing elongated grains(X 200).

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(a!

Fig.7 : Micrograph of the cross-section of unfailed area of the tube (a) 6A (b) 6C (c)7A & 7B (d) 8B showing pearlitic structure (x 200).

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Fig.8 : Micrograph of cross-section of failure zone of the tube 8B (a) steam-sidesurface and (b) fire-side surface showing decarburized microstructure (x 200).

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Fig. 9 : Sketch illustrating hydrogen damage mechanism in boiler tube.

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(a>

Fig.10 : Magnified view of (a) fire-side surface and (b) steam-side surface of thetube 8B after laboratory cleaning.

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Fig.11 : EDX analysis showing distribution in the oxide scale formed on the fire-sidesurface of the tube 8B.

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Fig.12 : Boiler tube 6C showing crack (marked by mow) at the welded area.

Fig.13 : Micrograph of cross-section of ruptured welded area f tube 6C showing firegrains near the crack (X 100)

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