swcc investigation.pdf

8/12/2019 SWCC INVESTIGATION.pdf

1/16

INVESTIGATION ON THE FAILURE OF BOILER No 1

ECONOMIZER TUBES, SHUQAIQ PLANT 1

Anees U. Malik, Mohammed Mobin and Mohammad Al-Hajri

Research and Development CenterSaline Water Conversion Corporation (SWCC)P.O.Box 8328, Al-Jubail 31951, Saudi Arabia.

E-mail: [email protected]

INTRODUCTION

In a letter (Ref No. 2500/0852 dated 23.3.1426) addressed to the Manager, R&D

Center, the Manager, SWCC Shuqaiq Plant informed about the failure of Boiler # 1

Economizer tubes. The Manager, Shuqaiq Plant requested RDC to carry out failure

analysis and sought advice on any further operation and maintenance strategy. Two

damaged tubes (tube #2 from row #13 and tube #2 from row # 14) were sent for

investigations by the plant. The RDC decided to take up the task.

BACKGROUND

The T & I Department, Shuqaiq Plant provided the following first-hand information

about the failure incidence took place related to economizer tube:

During the normal boiler operation, on 24 January 2005, a heavy water leakage was

observed at the flue gas outlet duct (upstream of economizer). Subsequently, the boiler

was brought to shut down and the manhole was opened for economizer inspection.

Heavy water leakage was observed from the bottom of the economizer coils at the 13 th

and 14 th rows 2 nd tubes from the top rows of the tube bank, approximately 300 mm

away from the water wall tube.

The washing of the external boiler tubes was carried out by the contractor followed by a

detailed visual testing by the T & I department. The important observations made were

as follows:

(i) One tube ruptured (fish mouth opening) by 40 mm length and a 3 mm

diameter hole and another having a pin hole (leak).

(ii) There were defects at the original welding in 17 other tubes.

1 Issued as Troubleshooting Technical Report No. TSR 3804/05007 in July 2005.


2/16

(iii) Dimensional checking at random locations reveals tube thinning at the

external surface.

(iv) Tube thickness reduced by 1.2 2.5 mm as against standard thickness of

3.2 mm.

(v) Thickness reduction was extended upto the water wall tubes.

The schematic of boiler # 1 showing 3 dimensional views of steam / water circulation

system is illustrated in Fig. 1 and the schematic of boiler # 1 showing economizer

tube leak location is represented in Fig. 2. Fig. 3 (supplied by the Shuqaiq Plant

engineers) shows the location of rupture of inlet tube #2, row 13. Subsequent actions

taken by plant authorities included replacement of 4 tubes starting from feed water inlet

header and repairing of welding defects on the other 15 tubes. It is worth mentioning

that uptill 2 years back, the fuel for the boilers had been crude oil which was replaced

by Bunker C.

Table 1 provides details of Boiler # 1 which include information about operating

parameter, tube configuration and material.

PHYSICAL EXAMINATION

The two samples of economizer tubes provided by the Shuqaiq Plant were visually

examined in as received condition. The external surface of the tubes appeared to be

cleaned without any deposits. Figure 4 shows two feed water inlet economizer tube # 2

samples from rows # 13 and 14, respectively. The magnified views of tube # 2, row 13

shows a large fish mouth rupture near the feed water inlet, a pin hole is also present

slightly away from the rupture (Fig. 5). The cross section of tube # 2, row 13 showswall thinning at the side of the rupture (Fig. 6). The shape of fish mouth rupture

indicates that rupture occurred from inside of the tube. Fig. 7 shows a closer view of

tube # 2, row 14. The tube appears to be without any pin hole, pit or rupture.

METALLOGRAPHIC STUDIES

The microstructures of the cross sections of economizer tube provide information about

the matrix structure and scaling. The fire side of the tube # 2, row 14 (inside furnace)


3/16

shows matrix structure which is pearlitic ferritic without scaling but the side appears

to be fissured (Fig. 8). The outer side of the tube # 2, row 14 (outside furnace) shows a

coarsened peartliticferritic with no scaling but some decarburization (Fig. 9).

ENERGY DISPERSIVE X-RAY (EDX) STUDIES

The EDX studies have been carried out to determine the elemental compositions of the

matrix and the deposits/scales on the boiler tubes.

The EDX profile (Fig. 10) of the fire side of inlet tube # 2, row 14 (inside furnace)

shows iron in very high concentrations (81.3%) with low vanadium (4.7%). The EDX

(Fig. 11) of the inner most layer has some vanadium (2.9%) with very low sulfur

(0.22%). The results of EDX studies indicate that there is substantial incorporation of

vanadium compounds in the economizer tube during boiler operation. The source of the

vanadium compounds is flue gas.

ANALYSIS OF FLY ASH SAMPLES

Due to cleaning of economizer tubes after failure, the boiler samples received by the

RDC had no deposits on the external surface. This information is very important in point of view of failure investigation. However, Shuqaiq Plant authorities sent fly ash

sample from boiler # 1, economizer area, which was collected during third economizer

tubes failure on 26/6/2005. This ash sample reflects the nature of ash deposited on

economizer tube during first failure on 24/1/2005. The composition of the ash sample is

given in table 2. EDX profile (Fig. 12) indicates significant concentration of sulfur

(12.8%), low concentration of vanadium (1.5%) and high concentration of iron

(34.0%). The high iron contents indicate that the ash is rich in iron sulfide / sulfatecontaining corrosion products.

MEASUREMENTS OF ECONOMIZER TUBES OUTER AND INNER DIA

The average reduction in the outer and inner diameter of economizer tubes measured on

the same tube at two different locations, namely, inside the furnace and outside the

furnace is listed in Table 3. A reduction in the outer dia of the tube indicates the

thinning (loss in wall thickness) from outside (fireside) whereas a reduction in inner dia

is indicative of the inside (steam side) scaling. The portion of both the tubes located


4/16

inside the furnace show appreciable reduction (4 -5%) in outer dia indicating the

initiation of wall thinning from fire side. The inner dia of the tube are almost unaffected

at both the locations; this indicates negligible attack from steam side.

DISCUSSION

When a firing fuel containing sulfur or sulfur compounds is subjected to combustion,

the sulfur present in the fuel oxidizes to form SO 2 and to a small extent of SO 3. CO 2

and water vapors are invariably present in the combustion gases. The SO 3 combines

with water vapor to form H 2SO 4. At certain temperature H 2SO 4 condense on the

metallic surfaces and initiate corrosion. The temperature at which H 2SO 4 first

condenses is known as H 2SO 4 dew point. The H 2SO 4 dew point depends upon the

concentration of SO 3 and water vapor in the flue gas. In general, the dew point

increases with an increase in the concentration of SO 3 and moisture content in the flue

gas. The concentration of SO 3 in the flue gas increases with increase in the level of

excess air, gas residence time, gas temperature and sulfur level in the fuel. The moisture

content in the flue gas depends upon many factors. The source of moisture is attributed

mainly to the fuel, leaks in boiler tubes and steam from soot blowing.

When the temperature of a metallic component drops below the H 2SO 4 dew point of the

flue gas, the H 2SO 4 is condensed on the metal surface and corrodes the metal. This type

of corrosion, known as cold end corrosion, is mainly encountered in relatively low

temperature boiler components such as economizer, air preheater, stack, etc. The

critical factors governing the cold end corrosion include the presence of sufficient

concentration of SO 3 and moisture in the flue gas and a metal surface whose

temperature is below the H 2SO 4 dew point. It should be noted that in this type of

corrosion it is the temperature of metal which is critical and not the temperature of flue

gas. Even if the temperature of flue gas is above the dew point, the corrosion is very

likely to occur wherever the metal temperature is less than dew point.

Considering the present failure the economizer feed inlet temperature appears to be on

lower side keeping in view the high sulfur content in the flue gas. As the H 2SO 4 dew

point is directly dependent upon the sulfur content of the flue gas, a higher dew point

than the feed inlet temperature is most probable. This in turn, favored the condensationof H 2SO 4 on the economizer tubes outer surfaces causing severe corrosion. This, in


5/16

consequence, resulted in the thinning of metal to a stage when it could not bear the

inside pressure of the feed water and ultimately got ruptured. Further, the external

deposits on the tubes helped in lowering down the tube metal temperature and thus

favoring acid condensation over the deposit. A reduction in the wall thickness of the

tubes, located inside the furnace support the initiation of corrosion from fire side as a

result of acid condensation.

CONCLUSIONS

1. The thinning and perforation in the economizer tubes are due to H 2SO 4 dew point

corrosion.

2. The relatively low temperature of feed water caused the lowering of the tube metal

temperature and promoted the condensation of H 2SO 4.

3. The external deposits on the tubes, as a result of bunker oil firing, further helped in

lowering down the tube metal temperature, thus promoting H 2SO 4 condensation

over the deposit.

RECOMMENDATIONS

1. An increase in the economizer feed inlet temperature will help in reducing the

severity of cold end corrosion.

2. An exact feed inlet temperature can be determined by knowing the acid dew point

temperature of the flue gas.

3. The possibility of carrying out condition/remnant life assessment of economizer

tube banks and header should be explored.

4. Sulfur content should be reduced to minimum which can help in preventing the

acid dew point corrosion.

5. An efficient fuel chemical additive program will help in reducing the possibility of

acid dew point corrosion.

6. The efficiency of soot blowing process should be improved.


6/16

Table 1. Details of Boiler # 1, Shuqaiq Plant

Boiler Type: Hitachi Zosen/Steinmuller Type HS5-400, Natural Circulation, Single

Drum,Radiant, Semi-Outdoor Type.

Firing System (at present) : Bunker CMax. Working Pressure Drum & Superheater / Economizer : 70 / 74 barg.Evaporation at MCR (FSH outlet) : 105.4 Kg/s.Steam Condition at MCR (FSH outlet) : 56 barg., 496 deg. CTotal Operating Hours (approx.) from

its inception up to Dec. 2004 (G) : 116123 Hrs.

Economizer Type :

Design Pressure : 74 barg.Outside Diameter of Tubes : 38.1 mmSpacing of Tubes Vertical : 100 mm

- Horizontal : 75 mmTube Wall Thickness - Minimum : 3.2 mm

- Nominal : 3.2 mm Number of Elements : 200 Number of Tubes in Parallel : 2Material : SA 210 Gr. A1Heating Surface : 1529 m 2

Table 2. Chemical Composition of the Ash Deposited on Economizer tubes asdetermined by EDX analyses.

Element Element Atomic% %

C 11.72 21.84O 38.04 53.22Mg 1.61 1.48Si 0.26 0.20S 12.84 8.96V 1.52 0.67Fe 34.00 13.62Total 100.00 100.00


7/16

Table 3. % reduction in Economizer tubes # 2, Outer and Inner diameter .

Tube LocationTuberow #

Average Tubedia (mm) Out side the

FurnaceInside theFurnace

Averagereduction in

dia (mm)

% Reductionin dia

Outer 38.18 36.22 1.96 5.13

13

Inner 30.46 30.36 0.10 0.32

Outer 38.09 36.46 1.63 4.28

14

Inner 30.11 30.01 0.10 0.33


8/16


9/16

Figure 2. Economizer tube leak location in Boiler # 1 Shuqaiq Plant


10/16

Figure 3. Photograph showing failure of Economizer inlet tube # 2, Row 13


11/16

Figure 4. Photograph of the samples from economizer inlet tube # 2, row 13 and 14

Figure 5. Closer view of economizer inlet tube # 2, row 13 showing fish mouth openrupture and a hole


12/16

Figure 6. Thinning of the economizer inlet tube # 2 row 13

Figure 7. Economizer inlet tube # 2, row 14


13/16

Figure 8. Photomicrograph of the fire side scales on economizertube # 2, row 14 (inside furnace) X 200

Figure 9. Photomicrograph of the outer side scales on economizertube # 2, row 14 (outside furnace) X 200


14/16

Figure 10. EDX profile of a cross section of economizer tube # 2, row 14(fire side) located inside furnace

0 5 10 15 20Energy (keV)

0

20

40

60

cps

O

Ca

V

Fe

Fe


15/16

Figure 11. EDX profile of a cross section of economizer tube # 2, row 14 (fire side)located inside furnace

0 5 10 15 20Energy (keV)

0

20

40

60

cps

C

O

S CaV

Mn

Fe

Fe

Ni


16/16

0 5 10 15 20Energy (keV)

0

10

20

30

40

50

cps

C

O

Mg

Si

S

V

Fe

Fe

Figure 12. EDX profile of the ash deposits collected from economizer outer

surface

swcc investigation.pdf

Documents