b-1c tube failure metallurgical assessment by betz

8/18/2019 B-1C Tube Failure Metallurgical Assessment by BETz

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GE Power & WaterWater & Process Technologies

Metallurgy Services 9669 Grogan’s Mill Road The Woodlands, Texas 77380281-367-6201281-363-7794 Fax

METALLURGICAL LAB REPORT

Representative: Robert Stewart Plant: Petrojam Ltd.Location: Kingston. JamaicaUnit: B1C, 150 psigReport No.: 2014-0030Date: March 13, 2014

BACKGROUND

Two tube sections from the B1C Boiler at the subject account were submitted for failure

analysis. When the tubing was removed on January 13, 2014, the time in service was five

years.

RESULTS

Figure 1 is a photograph showing the submitted tubing. The samples were not labeled

and, for the purposes of this report, were arbitrarily identified as Tubes A and B. Tube A

exhibited perforation (Figures 2 and 3). The external surface of Tube B was covered with

tan and gray deposit, no failure was observed (Figure 4).

The tubes were split lengthwise to facilitate examination of the internal surface. The

internal surface were covered with light-gray, white and brown deposit (Figures 5 and 6).

Test sections were removed from both tubes to determine the deposit weight density

(DWD) values. Micrometer measurements taken before and after the deposit removal

step were used to measure the deposit thickness (Figures 7 and 8). The following results

were obtained:


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DWD (g/ft 2)

Maximum Deposit

Thickness (in.)

Tube A 33 0.006

Tube B 41 0.007

Scanning electron microscope-energy dispersive x-ray analysis (SEM-EDXA) was used

to determine the elemental composition of the external and internal deposit in the

submitted tubes. The semi-quantitative results (Table 1) indicate the external deposits

were composed of mostly iron, calcium and sulfur species. Minor amounts of vanadium,

sodium, chlorine, and silicon were also detected. The presence of sulfur, chlorine,

sodium, and vanadium are known to cause fireside corrosion via molten salt corrosion.

The internal deposit on the waterside (Table 2) consists primarily of iron, calcium, and

sulfur compounds. Minor amount of sodium, magnesium, chlorine, vanadium, and silicon

were also detected, along with trace amounts of other constituents.

.

The external and internal surfaces were mechanically cleaned to facilitate examination of

the underlying metal. There was evidence of corrosion pitting on the internal surface after

cleaning (Figures 9 – 11). The maximum internal surface pit depth measured in Tubes Aand B was 0.014 inches and 0.018 respectively. Figures 13 and 14 showed the external

surface of Tubes A and B after mechanical cleaning, which exhibited general/fireside

corrosion.

At the failure edge of Tube A, the wall thickness was measured to be 0.003 inches, which

represents an approximate 95% loss of metal thickness as compared to the tube nominal

thickness of 0.055 inches. The combined wall loss of Tube B due to external surface

corrosion and internal pitting was 0.030 inches, or 57%, when compared to the maximum

thickness of 0.070 inches.

Transverse sections were cut out from selected areas of the tubes and prepared for

metallographic inspection. Figures 15 – 19 show the microstructure along the internal and


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external surface of the tubing, which exhibited general corrosion/corrosion pitting. The

internal pitting appeared consistent with dissolved oxygen corrosion in the tubing. The

mid-wall microstructures of the tubing consisted of ferrite/pearlite, normal for carbon

steel boiler tubing in the as-manufactured condition. No overheating was observed in the

tubing.

CONCLUSIONS

1. The failure of Tube A was caused by combination fireside corrosion (over 95%

loss of wall thickness estimated in the failed region), and general internal

corrosion pitting within the thinned area.

2. Tube B also exhibited combined wall loss due to external surface corrosion and

internal corrosion pitting (57% wall loss, when compared to the maximum wall

thickness).

3. The maximum internal pitting was 0.014 inches and 0.018 inches respectively in

Tubes A and B. The pitting was likely caused by dissolved oxygen corrosion.

4. The composition of the fireside deposit in the thinned regions was varied, withsome vanadium, sulfur-compounds, and trace chloride salts noted to be present.

Vanadium, sulfur, sodium, and chlorides can combine to produce aggressive

molten salts, to flux protective iron oxide scales on the tube fireside surfaces,

resulting in accelerated metal loss.

5. Microstructural analysis indicated that no overheating occurred in the tubing.

Michael AdeosunMetallurgical Engineer


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Element

Tube A ODWhite(wt %)

Tube A ODBrown(wt %)

Tube B ODTop

(wt %)

Tube B ODBottom(wt %)

Iron 93.4 38.7 41.5 96.4

Calcium – 27.1 26.2 –

Sodium 3.1 1.1 0.5 –

Vanadium – 3.0 2.9 –

Sulfur 2.6 26.3 26.2 2.8

Silicon – 2.4 0.9 –

Chlorine 0.6 – – –

Chromium – – – 0.5

Aluminum 0.4 1.4 1.8 0.4Data is normalized with carbon and oxygen excluded

TABLE 1. SEMI-QUANTITATIVE ELEMENTAL SEM-EDXA OF THETUBES EXTERNAL SURFACE DEPOSIT


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Element

Tube A IDWhite(wt %)

Tube A IDBrown(wt %)

Tube B IDBrown(wt %)

Tube B IDGray

(wt %)

Calcium 31.7 9.2 0.7 0.5

Iron 30.0 80.1 94.5 93.1

Sulfur 27.9 3.2 – 0.8

Vanadium 3.4 3.8 – –

Silicon 2.3 1.9 3.2 4.5

Nickel 1.9 – – –

Aluminum 1.6 1.3 0.1 0.5

Chromium – – – 0.7

Sodium 0.5 – 0.6 –

Magnesium 0.3 0.6 0.5 – Data is normalized with carbon and oxygen excluded.

TABLE 2. SEMI-QUANTITATIVE ELEMENTAL SEM-EDXA OF THETUBES INTERNAL SURFACE DEPOSIT


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Figure 1. Photograph showing thesubmitted tube samples as-received forevaluation.

Figure 2. Photograph showing theexternal surface of Tube A, at the failure.

Figure 3. Photograph showing a crosssection of Tube A, at the failure (arrow).

GE Power & Water Water & Process Technologies

Tube A

Tube B


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Figure 4. Photograph showing theexternal surface of Tube B.

Figure 5. Photograph showing theinternal surface of Tube A at the failure(arrow).

Figure 6. Photograph showing theinternal surface of Tube B.



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Figure 7. Photograph of the internalsurface of DWD test section, Tube A.

Figure 8. Photograph of the internalsurface of DWD test section, Tube B.

Figure 9. Photograph of the internalsurface of DWD test section, Tube A,after mechanical cleaning.



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Figure 10. Photograph of the internalsurface of DWD test section, Tube B,after mechanical cleaning.

Figure 11. Photograph showing theexternal surface of Tube B, aftermechanical cleaning.

Figure 12. Photograph showing theinternal surface of Tube B, aftercleaning.



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Figure 13. Photograph showing theexternal surface of Tube A, at the failure(arrow), after cleaning

Figure 14.. Photograph showing theexternal surface of Tube B, aftermechanical cleaning.

Figure 15. Photomicrograph showing thesteel microstructure at the failure edge ofTube A. Nital Etch. 50x.



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Figure 16. Photomicrograph showing theexternal and internal surfaces of Tube A,adjacent to the failure. Nital Etch. 100x.

Figure 17. Photomicrograph showingthe mid-wall steel microstructure ofTube A. Nital Etch. 500x.

Figure 18. Photomicrograph showing theinternal surface steel microstructureobserved on Tube B. Nital Etch. 50x.



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Figure 19. Photomicrograph showing themid-wall steel microstructure of Tube B.Nital Etch, 500x.



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Notes

b-1c tube failure metallurgical assessment by betz

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