What are objectives of failure analysis and explain how it can be implemented ?
There are 3 primary objectives;
i)
The first objective is to examine the objective evidence presented by the failed components and, from that evidence, determine the failure mechanism.
ii)
The second objective is to determine the primary cause, or what is commonly called the root cause, of the failure. This is accomplished by examining design and operational issues to determine what specific factor, or factors, was responsible for the failure.
iii)
The third objective, recommendation of corrective actions that will prevent similar failures, can be accomplished once the root cause of the failure has been identified.
“Microstructural examinations were conducted on boiler tube from local petrochemical plant for assessment on the reusability of the tube. Tube diameter was measured along the tube and it was shows that there is no bulges on the as received tube. The boiler tube was found bending at fire side area (Figure 1). The structure has equated grains consisting of pearlite and ferrite. Microstructure examination revealed that the grain size at the bend was slightly bigger as compared to grains at far from than the bend side area. There is no change in structure of grain at both areas; it means that the temperature applied to the tube was not higher than the limit of permissible temperature of the tube. According to the different in grain size, it means that temperature at the bent area was also different with the straight area of the tube. Temperature gradient should be existed along the tube due to the different in temperature. The bending in the tube was caused by the temperature gradient.”
Slide 25
Figure 1 Schematic diagram shows the condition of bending and metallographic spot on the tube surface marked as R1, R2, R3 & R4
From the statement above, plan your investigation procedures to implement failure analysis on the boiler tube. Give your reason for each procedures
To investigate the basic information that possible to be a primary clues of the root cause analysis.
These importance information are including its location, manufacturer, function of component, operational condition, maintenance and cleaning history as well as opinions of related personnel
2) Visual examination followed by closed-up photographs by digital camera.
Since many of the subsequent steps in the evaluation will involve destructive examinations, the as-received photographs provide an archival record of the original condition of the failed component prior to cutting and sectioning
The as-received photographs also provide an effective place to indicate the locations from which the various samples used in the destructive examination were obtained
3) Chemical composition analysis by using XRF or spark emission spectrometer
Analysis of the chemical composition of the component is a routine part of a failure analysis
The analysis is used either to determine if the material meets the specified composition limits (in this case, boiler tube is carbon steel) or to identify an unknown material
4) Dimension measurement by vernier calliper
The operational condition of boiler tube normally involves a high temperature stress. Thus as-received boiler tubes should have to document their dimension along the tube to investigate the occurrence of bulging on the tubes resulted by its operational condition. However there’re no bulging occur on the sample
5) The straightness of the tubes is carried by rolling the tube on the flat & smooth surface.
Any bending or uneven on the tube can be identified when unstable rolling is observed resulted by high temperature operational condition that probably hits the one side of the tubes.
6) Determination of failure mechanism & recommendations
Without identification of the failure mechanism, it is usually impossible to determine the root cause of the failure, which forms the basis for recommendations for immediate action whether the components should be replaced or not and to eliminate or reduce the frequency of future failures.
Determination of the failure mechanism is considered a critical step in the failure investigation. In this particular case, the failure was caused by the occurrence of temperature gradient on the tube.
Visual observation, optical microscope and scanning electron microscope are three (3) major tools that play an important role in determining the success of a root-caused analysis. Explain the each their roles in determining the (a) ductile and (b) brittle failure
[Pemerhatian mata kasar, mikroskop optik dan mikroskop elektron imbasan adalah tiga (3) alat utama yang memainkan peranan penting dalam menentukan kejayaan analisis punca-penyebab. Terangkan peranan setiap alat tersebut dalam patahan mulur dan rapuh]
i)Allow for the penetration action of corrosion along with the mechanical strength requirements when determining the appropriate metal thickness
ii)Weld rather than rivet containers to reduce crevice corrosion. If rivets are used, choose a rivet material that is cathodic to the materials being joined.
iii)If possible, use galvanically similar metals for the entire structure. Avoid dissimilar metals that can cause galvanic corrosion. If galvanically dissimilar metals are bolted together, separate them with nonmetallic gaskets and washers.
iv)Avoid excessive stress and stress concentrations in corrosive environments to prevent stress-corrosion cracking, especially when using susceptible materials such as stainless steels and brasses.
v)Avoid sharp bends in piping systems to prevent erosion corrosion.
vi)Design tanks and other containers for easy draining and cleaning.
vii)Design systems for easy removal and replacement of parts that are expected to fail in service, such as pumps in chemical plants.
viii)Design heating systems such that hot spots do not occur.
A process engineer found a significant pressure drop and realizes one of the plant heat exchanger might have experienced some problem. After several inspections, he found that one of the tubes in the E-1406A heat exchanger was experiences leaking. The tube was then removed and a group of UniMAP final year student was given a task to further investigate this failure problem. According to the daily report made by personnel incharge, the tube side was contains vapor and liquid of hydrochloride acid (HCl), ethylene dichloride (C2H4Cl2), and vinyl chloride monomer (C2H5Cl2), while the shell side contains the flow of untreated cooling water (Figure 1).
After some investigation is done, several findings were found;
i)The tube side was coated by 25 mm polypropylene
ii)The shell side of the tube was covered by brownish deposit layer
iii)Thetube material was carbon killed steel as stated in plant design.
iv)The failure of the tube was resulted by pitting corrosion that initiated from shell side surface.
v)No microcracking or branching cracks were observed and it confirms that the failure was not due to overloading or any excessive external stresses on the tube.
vi)There are the reductions of the solute atoms in the stainless steel matrix during corrosion attack.
vii)Microanalysis at pitting region shows the present of chloride and the failure was believed resulted by untreated cooling water.
From the statement above, plan root-caused analysis procedures that should be taken and give reason for each of procedures. Suggest your own recommendation to prevent the future failure.
[Seorang jurutera proses mendapati kejatuhan tekanan yang tinggi dan menyedari salah satu daripada penukar haba logi berkemungkinan telah mengalami masalah. Selepas beberapa pemeriksaan, beliau mendapati salah satu tiub dalam penukar haba E-1406A telah mengalami kebocoran. Mengikut laporan harian yang dibuat oleh kakitangan bertugas, sebelah dalam tiub mengandungi wap dan cecair asid hidroklorik (HCl), etilina diklorik (C2H4Cl2) dan monomer vinil klorida (C2H5Cl2), semetara sebelah luar tiub mengandungi aliran air penyejukan tak terawat (Rajah 1). Tiub tersebut kemudiannya dikeluarkan dan satu kumpulan mahasiswa tahun akhir telah diberi tugas untuk siasatan lanjut masalah kegagalan ini. Setelah siasatan dilakukan, beberapa penemuan telah dijumpai;
i)Tiub sebelah dalam disaluti dengan 25 mm polipropilena
ii)Tiub sebelah luar diselaputi dengan lapisan mendakan keperangan
iii)Bahan tiub adalah keluli carbon termati seperti yang dinyatakan dalam rekabentuk logi
iv)Kegagalan tiub adalah disebabkan oleh kakisan lubang yang bermula daripada permukaan sebelah luar
v)Tiada mikrorekahan atau rekahan cambah diperhatikan pada mikrostruktur tiub dan mengesahkan bahawa kegagalan tidak disebabkan oleh lebihan berat atau kewujudan tekanan luar pada tiub
vi)Terdapat penurunan atom-atom terlarut dalam matriks keluli kalis karat semasa serangan kakisan.
vii)Mikroanalisis pada rantau lubang menunjukkan kehadiran klorida dan kegagalan adalah dipercayai disebabkan oleh air penyejukan tak terawat.
Daripada pernyataan di atas, rancang prosedur-prosedur analisis punca-penyebab yang sepatutnya diambil dan berikan alasan untuk setiap prosedur. Cadangkan tindakan pencegahan untuk mengelakkan kegagalan sama di masa hadapan.
Figure 1: Schematic diagram shows the pitting corrosion on the heat exchanger tube.
•To investigate the basic information including its location, manufacturer, function of component, operational condition, maintenance and cleaning history as well as opinions of related personnel that possible to be a primary clue of the root cause analysis.
2) Visual examination followed by closed-up photographs by digital camera.
§The as-received photographs provide an archival record of the original condition of the failed component prior to cutting and sectioning
§Also provide an effective place to indicate the locations from which the various samples used in the destructive examination were obtained.
§In this case, visual examination found the failed tube was experiences pitting corrosion which exhibited by large transverse cracks, holes and pitting profile.
3) Chemical composition analysis by using XRF or spark emission spectrometer
§Analysis of the chemical composition of the component is a routine part of a failure analysis
§The analysis is used either to determine if the material meets the specified composition limits.
§In this case, heat exchanger tube materials is meets the specified composition limits of 316L stainless steel.
5) Metallographic examination by optical microscope.
i)As-polished microstructure
ii)Etched microstructure
Prior to metallographic viewing, there are several works need to carried out such as cutting, sectioning, mounting, grinding, polishing & etching.
§In as-polished condition, the presence of defects such as inclusions and voids or microstructural anomalies at the crack initiation site is examined.
§In this case, as-polished microstructure is presented to investigate the direction pitting attack whether it comes from tube side or shell side. In this case, pitting corrosion from shell side.
§Specific etchant has to be developed to delineate specific microstructural features.
§Once etched, the microstructure of the material can be determined and the relationship between the microstructure and the damage can be examined.
§In this case, no transgranular or intergranular microcracking is detected. Hence, the failure is not due to overloading or any excessive external stresses on the tube.
6) Fractographis analysis (SEM) and microanalysis (EDS)
§SEM analysis is carried out to identify of the fine scale features fracture mode.
§However in this case, no plastic deformation were observed and the surface fracture shows the tube material is ductile. It confirms that the failure is not due to overloading or any excessive external stresses on the tube.
§Identification of the chemical species present in the corrosion deposits plays a critical role in identifying the root cause of the corrosion and in this case, the present of chloride element in EDS result is strongly believed result by untreated that used as a coolent.
7) Determination of failure mechanism & recommendations
§Without identification of the failure mechanism, it is usually impossible to determine the root cause of the failure, which forms the basis for recommendations for immediate action whether the components should be replaced or not and to eliminate or reduce the frequency of future failures.
§Determination of the failure mechanism is considered a critical step in the failure investigation. In this particular case, the failure was caused by the occurrence of temperature gradient on the tube.
•The specimen elongates, forming a necked region in which cavities form.
•The cavities coalescein the neck center, forming a crackwhich propagates toward the specimen surface in a direction perpendicular to the applied stress.
•As the crack approaches the surface, its growth direction shifts to 45ºwith respect to the tension axis. This redirection allows for the formation of the cup-and-coneconfiguration and facilitates fracture.
