Failure Analysis: MidTest - Marking Schema

A process engineer responsible for the operation of petrochemical plant noticed a significant pressure drop which indicates that the boiler tubes might have experienced some damage problem. Primary witness report by inspection engineers found that the water content in steam tanks was reduced by as much as 15%. There are the some pitting marks were observed at tensioned side at the bend area of the boiler tube (refer Figure 1) results the vaporizing out of water vapour. The tube was then removed and sends to your consultancy firm for further investigation.

To initiate the failure investigation, you have to make a site visit to that petrochemical plant to get some background information. Identify the important information needed from plant engineers that helps you getting some primary clues. [30 marks]

1. Location, 2. name, 3. Part No, 4. Description, 5. Manufacturer or fabricator 6. Function of item 7. Maintenance procedures 8. Maintenance history 9. Cleaning procedures 10.Cleaning history 11.Operational procedures (temp./ pressure etc)

12. Type of materials (Chemical composition) 13. Operational documentation (standard, temp./ pressure etc) 14. Service deviations 15. Normal stress orientations 16. Extent of incident 17. Precipitating events 18. Drawings 19. Photographs 20. Previous inspections reports, 21. Opinions of related personnel

ii) Considering chemical composition of boiler tube as shown in Table 1,

a) What your hypothesis regarding the root cause of the tube failure [25 marks]

1. Residual stress resulted from bending works 2. Corrosion attacks originate from water, believed resulted from application of untreated water 3. Lower content of Mo (lower pitting index no.) contributes to pitting corrosion 4. All of the above factors contribute to the stress corrosion cracking (SCC).

b) Calculate the tube pitting index number [20 marks]

Pitting Index (PI) = wt.% Cr + (3.3)(wt.% Mo) = 17.585 + 3.3(0.396) = 18.89

c) Recommend the new materials to prevent the same future failure. [25 marks]

Further upgrades are out of the realm of the stainless steels and into high-alloys steel. Eg Pitting resistance metals such as Ø high-molybdenum SS Ø Incoloys, Ø Inconels, or Ø Hastelloys