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EBT 423 Materials Selection Design

Mid Semester 1 Test

Academic Session 2009/ 2010

Time : 60 minit

This question paper has ONE question. AnswerALL questions.

Question

A schematic diagram of a tube illustrated in Fig .

1 is one of numerous tubes that failed in the

boiler located at first stage superheater tubes.

The tubes were found cracked after 3 months of

service. It had been moderately cold-bent during

installation, and was not stress-relief-annealed.

The steam drum lacked adequate devices for

separation of steam and water, and load swings

were frequent, possibly causing carryover ofboiler water. Microstructural analysis revealed

plastically deformed grains from the cold

bending. The cracks were highly branched and

ran between the grains (intergranular) as theypassed through the tube wall. The corrodent was

sodium hydroxide from boiler-water carryover.

The tube was then removed and sends to your

consultancy firm for further failure investigation.

Figure 1: Schematic diagram shows the transverse

crack in a bent carbon steel superheater tube. Note

small "window" that has been blown out of the wall.

i) To initiate the failure investigation, you have to make a site visit to the plant to get some

background information. Identify the important information needed from plant engineers thatcould helps you getting some primary clues.

[20 marks]

1. Location,

2. 2.name,

3. 3.Part No,4. 4.Description,

5. 5.Manufacturer or fabricator

6. 6.Function of item7. 7.Maintenance procedures

8.8.

Maintenance history9. 9.Cleaning procedures10. 10.Cleaning history

11. 11.Operational procedures (temp./

pressure etc)12. Type of materials (Chemical

composition)

13. 13.Operational documentation

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(standard, temp./ pressure etc)

14. 14.Service deviations15. 15.Normal stress orientations

16. 16.Extent of incident

17. 17.Precipitating events

18.18.

Drawings

19. 19.Photographs

20. 20.Previous inspections reports,21. 21.Opinions of related personnel

## 1 POINT = 1 MARK

ii) From the above statement, what your hypothesis regarding the root cause of the tube failure?

[10 marks]

Stress-corrosion cracking

Exactly

AccurateAcceptable

Less

AcceptableConfuse Nonsense

10 5 3 1 0

iii) Give four (4) reasons to justify your hypothesis.

[20 marks]

1. Moderately cold-bent during installation results residual/ internal stress

2. Not stress-relief-annealed results residual/ internal stress still present in the tube

3. The steam drum lacked adequate devices for separation of steam and water, and load

swings were frequent, possibly causing carryover of boiler water results hot water

enter the tube & forces the acceleration of oxidation reaction

4. Micro structural analysis revealed plastically deformed grains from the cold bending

shows an elongated grain proves the presents of residual/internal stress

5. The cracks were highly branched shows the presents of stress

6. ran between the grains (intergranular) shows the presents of corrosion

7. The corrodent was sodium hydroxide from boiler-water carryover shows hydroxide

ion from NaOH acts as active corrosion agent

Given statementExactly

AccurateAcceptable Confuse Nonsense

2 1 0.5 0

Reason

Exactly

AccurateAccurate Acceptable

Less

AcceptableConfuse Nonsense

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3 2 1 0

iv) Concisely, plan the procedures that should be carried out to implement failure analysis on the

failed tube and briefly give reason for each of procedures

[30 marks]

Procedures Reason

1) Obtaining background data To investigate the basic information to be a primary clue

of the root cause analysis.

2) Visual examination

followed by closed-up

photographs by digitalcamera.

To record of the original & locations condition

3) Chemical compositionanalysis by using XRF or

spark emission spectrometer

To make sure whether the material meets the specified

composition limits.

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5) Metallographic analysis by

metallurgical microscope. As-polished microstructure, to identify the presence of

defects such as inclusions and voids or microstructural

anomalies

Etched microstructure, to determined and the relationship

between the microstructure and the type of materials,manufacturing process and/ or the damage mechanism

6) Fractographis analysis

(SEM) & microanalysis

(EDS)

SEM analysis: to identify of the fine scale features fracture

mode.

EDS: to Identify of the chemical species present in thecorrosion deposits

7) Determination of failuremechanism &

recommendations

Failure mechanism: to determine the root cause of thefailure,

Recommendations to determine whether the components

should be replaced or not and to eliminate or reduce the

frequency of future failures.

7) Report writing For documentation

To be send to customer

Procedure

Procedure &equipment

stated

OnlyProcedure

stated

Onlyequipment

stated

Nonsense

3 1 1 0

ReasonExactly

AccurateAccurate Acceptable

Less

AcceptableConfuse Nonsense

4 3 2 1 0.5 0v) Considering the root cause of failure, recommends

a) two (2) types of tube materials that should be replacing carbon steel, and

1. Killed carbon steel (3 marks)

2. Stainless steel (3 marks)3. High chromium steel (1 marks)

4. Superalloys (0.5 marks)

Max = 6 marks

b) three (3) corrective actions that should be taken in order to prevent the same future failure.

[20 marks]

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1. Reduce the concentration of chemical species that promotes corrosion. (2)

2. Avoid cold bending during installation process (3)

3. Avoid bending in the plants design. (3)

4. Proper stress-relief-annealing (4)5. Intelligent corrosion monitoring and smart sensors (3)

6. Signal analysis and artificial intelligence (3)7. First-class practice in inspection management (3)

8. Implement fitness for service test before operation (3)

9. Expert system on failure analysis (3)

10. Mind setting aim for zero failure (3)

Max = 14 marks