Failure Analysis - Q6

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) The tube 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.

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Procedures	Reason
1) Obtaining background data	• 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.