Failure analysis of gas sweetening tower absorber packing

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ECF22 - Loading and Environmental effects on Structural Integrity ECF22 - Loading and Environmental effects on Structural Integrity

Failure analysis of gas sweetening tower absorber packing Failure analysis of gas sweetening tower absorber packing

XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal

M. Newishy, S. A. Khodir, H. Abdel-Aleem M. Newishy, S. A. Khodir, H. Abdel-Aleem

Central metallurgical research and development institute,EGYPT Thermo-mechanical modeling of a high pressure turbine blade of an Central metallurgical research and development institute,EGYPT airplane gas turbine engine Abstract

Abstract a b c P.inBrandão , V. Infante A.M. Deus * installation. Pieces from the failed Gas sweetening tower packing was failed the form of corrosion damage,after 3 months from Gas sweetening tower packing was failed in the form of corrosion damage after 3 months from installation. Piecesmaterial from theaspects failed packing as well as unused one were received for investigation to establish whether its failure was due to specific a Department of Mechanical Engineering, Instituto Superior Técnico, Universidade deitsLisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, packing as well unused one were receivedthat for the investigation to establish whethersteel failure due to specific material aspects or improper use.asThe investigation showed packing was 316L stainless which was failed due to intergranular chloride Portugal or improper use.cracking. The investigation showed thatfailure the packing 316Lpacking stainless steelbe which failed due to Pais, intergranular chloride b corrosion stress root cause of the ofSuperior the was received could attributed the1,material residual IDMEC, Department of The Mechanical Engineering, Instituto Técnico, Universidade de mainly Lisboa, Av. Roviscoto 1049-001 Lisboa, stress bands corrosion cause of theprocess. failure of theshear received packing could be mainly attributed to the material residual shear aftercracking. packing The sheetroot manufacturing The bands were formed due to lack of solution annealing after cold Portugal c shear bands after packing manufacturing process. shearions bands were formed due to lack ofRovisco solutionPais, annealing after cold CeFEMA, Department ofsheet Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. 1049-001 Lisboa, deformation process. Corrosion is mainly controlled byThe chloride and residual stress. It is recommended to 1,use high quality Portugal deformation Corrosion is mainly controlled by annealing chloride ions residual stress. is recommended use high quality material free process. from residual stresses by applying solution heatand treatment after coldItdeformation of the to packing sheets and material free from residualions stresses by applying annealing heat treatment after cold deformation of the packing sheets and removing of the chloride by controlling thesolution inlet water. removing of the chloride ions by controlling the inlet water. Abstract © 2018 The Authors. Published by Elsevier B.V. © 2018 The Authors. Published by Elsevier B.V. © 2018 The Authors. Published by B.V. Peer-review underresponsibility responsibility of Elsevier the ECF22 organizers. Peer-review under of the ECF22 organizers. During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, Peer-review under responsibility of the ECF22 organizers. especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent Keywords: Gas sweetining, asorber packing, corrosion, stress corrosion cracking, failure analysis degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict Keywords: Gas sweetining, asorber packing, corrosion, stress corrosion cracking, failure analysis the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model 1. needed Background for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were 1. obtained. Background The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block in order topacking better establish thefailed model,after and then with the realthe 3Dreplacement. mesh obtainedThe fromexpected the bladelife scrap. The Sweetening gasshape, unit absorber has been 3 months from time overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such Sweetening gas unit absorber packing has been failed after 3 months from the replacement. The expected life time should be 6-8 years. The remaining failed packing samples as well as new packing samples were received for failure a modelbe can6-8 be years. useful in theremaining goal of predicting turbine blade life, given a set FDRpacking data. samples were received for failure should The failed packing samples as well asofnew

analysis. The sweetening gas unit absorber packing had been manufactured to confirm to Stainless steel AISI 316L. analysis. Thehad sweetening gas unit to absorber packing hadonbeen to confirm to Stainless steelisAISI 316L. The packing been constructed serve horizontally the manufactured sweetening tower with operating pressure 85 bar and ©o 2016 The Authors. Published by Elsevier B.V. The packing had been constructed to serve horizontally on the sweetening tower with operating pressure is 85 bar and C. The packing layers are in contact with Benfield solution (28% wt of K CO , 2%wt V O and 70%wt water), 80Peer-review 2 3 2 5 under responsibility of the Scientific Committee of PCF 2016. o C. The packing layers are in contact with Benfield solution (28% wt of K CO , 2%wt V O and 70%wt water), 80 2 3 2 5 natural gas with the CO2 analysis at inlet and outlet of the absorber tower shown in table 1. atCreep; inlet and ofMethod; the absorber tower shown in table 1. natural gas High withPressure the COTurbine 2 analysis Keywords: Blade; Finiteoutlet Element 3D Model; Simulation. Table1. Gas analysis at inlet and outlet of the absorber tower Table1. Gas analysis at inlet and outlet of the absorber tower

Components Components Carbon Dioxide Carbon Dioxide

inlet inlet% Mol. Mol. 8.039% 8.039

outlet outlet% Mol. Mol. 3.322% 3.322

2452-3216 © 2018 The Authors. Published by Elsevier B.V. 2452-3216 © 2018 Authors. Published Elsevier B.V. Peer-review underThe responsibility of theby ECF22 organizers. Peer-review underauthor. responsibility the ECF22 organizers. * Corresponding Tel.: +351of218419991. E-mail address: [email protected]

2452-3216 © 2016 The Authors. Published by Elsevier B.V.

Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216  2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the ECF22 organizers. 10.1016/j.prostr.2018.12.059

inlet inlet% Wt. Wt. % 16.447 16.447

outlet outlet Wt. % Wt. % 7.147 7.147

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2. Introduction Nearly all engineering structures experience some form of alternating stress and are exposed to harmful environments during their service life. [1-3] The environment plays a significant role in the fatigue of high strength structural materials like steels. Chloride stress corrosion cracking (CLSCC) is one the most common reasons why austenitic stainless steel pipework and vessels deteriorate in the chemical processing and petrochemical industries. [4-6] Deterioration by CLSCC can lead to failures that have the potential to release stored energy and/or hazardous substances. Failures of plant can be prevented by an awareness of the onset and evolution of CLSCC, and by periodic inspection to monitor the extent of cracking. CLSCC initiates from sites of localized pitting or crevice corrosion. [7-11] CLSCC propagation occurs when cracks grow more quickly from the pit or crevice than the rate of corrosion. For fabricated structures containing tensile residual stresses, the critical depth of localized corrosion to initiate CLSCC would be <1mm. The rate of crack propagation is strongly dependent on temperature but is relatively unaffected by stress intensity. [13-15] Rates of CLSCC propagation can vary from 0.6 mm. yr-1 at near ambient temperatures to >30mm.yr-1 at temperatures ~100 0C. In laboratory tests CLSCC has been observed in samples at temperatures between 250°C and 40°C The majority of the reported practical instances of CLSCC have occurred where temperatures ≥60 ºC. However, a significant number of failures below 60°C have also been reported. Although in these instances there appear to have been other contributory factors which include the use of highly cold worked and/or free-machining grades, Iron contamination of the surface and the presence of a highly corrosive atmosphere containing chloride compounds. [16-17] 3. Investigation Methodology The packing sheets were visually examined in the as-received condition. The sheets were cut and sectioned for destructive tests. Chemical analysis of the material was carried out to identify the chemical composition. Macro and microstructures investigations of the failed packing were carried out for different specimens, using well recognized methods for metallographic preparation; mechanical grinding down to 1200 grade emery paper followed by polishing using 0.1µm agglomerated alpha alumina suspension, rinsed and degreased with acetone, and then electro-etched using 10% oxalic acid at 6V for 45 sec. Scanning electron microscope equipped with EDS analysis was used for in-depth examination of the damaged section and existing phases. Hardness measurements were carried out under a load of 10kg for 15 sec. loading time, to determine hardness values for used and unused samples. 4. Results 4.1. Visual inspection and chemical composition General and close up views of damaged packing sheets are shown in Figs. 1-2. The most relevant observation is that the received packing has different colors as a result of the formation of corrosion scale and the black color one showed many perforated attack at the stressed region zones as shown in Fig. 2. The holes in the sheets have irregular shapes. Localized thinning in wall thickness was relatively observed on the black sheet compared with the brown sheet and unused one. Chemical composition by Spark Ignition Spectroscopy of the unused sheet material given in Table 2 showed the conformity of sheet composition to the 316L. It can be noticed that the Cr and Mo contents, characterizing corrosion resistance austenitic stainless steel (316L), lies within the lower specified range. 4.2. Metallographic examinations Metallographic examinations were conducted on cut pieces from the damaged packing. The microstructures of the unused sheet are shown in Fig.3. No localized attack or corrosion scales were observed on the outer surfaces of the packing sheet. Optical microstructure of the un-used sheet material has shown austenitic grains with numerous twins. Elongated inclusions in the rolling direction of the steel plates together with remaining deformation bands are clearly observed. Fig. 4 indicates that there are several other cracks had initiated at the stressed zones. The cracks initiated at the pitting area. The morphology of the cracks resembles the intergranular stress corrosion cracking (IGSCC) in nature. In order to investigate the effect of cold working process during the manufacturing of the sheets on the mechanical characteristics, microhardness measurements were done on several locations at the plate. The average hardness value



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of the sheets was (185Hv) which is higher than the value of solution annealing (160 HV). This may be attributed to the presence of few remaining shear bands related to the plate deformation. [8-9]. Table. 2. Chemical composition of the unused packing material. Sample

C 0.025

Si 0.67

Mn 1.03

P 0.004

S 0.003

Cr 16.6

Mo 2.34

Ni 12.8

Cu 0.19

Co 0.23

Fig. 1. As-received packing sheets having different colors as results of corrosion damages

Fig. 2. Close up views of the brown color sheet.

(left) and black color sheet (right) showing the perforated attack the at stressed zones

Fig. 3. Optical microstructure at different points of the unused sheet showing austenitic grains with remaining of the shear bands (white wavy lines) resulted from deformation.

Fig. 4. Optical microstructure of black sheet showing severe intergranular corrosion started from the outer surface and detachment of grains

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4.3. SEM and EDS examinations Scanning electron microscope (SEM) examination has been used for in-depth examination of the microstructure of the black and brown sheet materials. Typical austenitic grains with deformation bands (white wavy lines) were observed in Fig. 5. Fig. 6 shows the fine intergranular corrosion cracks initiated from the outer surface in the direction normal to the cold deformations. Shear bands are more visible at the cross-section of the sheets as shown in Fig. 6. Energy dispersive spectroscopy (EDS) spot analysis was carried out on 9 different locations as shown in the Fig. 7. Example of The result of EDS are given in Figs. 8-10. The obtained result shows that the matrix is close to 316 austenitic stainless steel composition; (point 6). Meanwhile, the other point showed compositions away from the composition of 316 stainless steels. Most importantly, the corrosive chloride (Cl) element was detected in many positions (points 1, 2, 3, 4 and 7) which means that Cl is introduced to the metal from the working environment (probably from water) especially at the crack site. The scales from the packing sheets were also analyzed using XRF analysis and the results are given in Table 3. High percentage of Si, Al, S, Ca, Cu, Zn and K were detected in corrosion scales. Cl was also detected in the corrosion scales. This means that the water used was not cleaned enough before starting the operation. 5. Discussion General visual examination of the received segment of packing sheets showed that the corrosion scale covered all the surfaces with many perforated attacks on high stressed location for sheets having black color. Chemical analysis by spark ignition spectroscopy showed conformity of the un-used material with the chemical composition of 316L. Microstructure investigations, SEM images, EDS analysis and hardness measurements indicated the presence of Cl and residual few shear bands related to sheets manufacturing process (cold deformation). deformation on corrosion is related to additional energy. It leads in consequences to decreasing the thermodynamic durability of the material. The occurrence of (CLSCC) depends mainly on three factors: susceptibility of the material, environment and considerable residual stresses.

Fig. 5. SEM microstructure of the corroded black sheet showing austenitic grains with remaining shear bands resulted from deformation. Table 3. XRF analysis of the deposits on the corroded packing Elem. % Elem. % Elem. %

Na 0.87 Cr 3.83 Ni 28.436

Si 6.7 Mn 0.44 Mg 0.7

P 0.14 Fe 25.8

S 3.41 Al 5.43

Cl 0.4 Cu 1

K 3.99 Zn 2.8

Ca 1.28 Mo 11.09

Ti 0.8 Ba 0.46

V 0.8 Sr 0.99



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Fig. 6. SEM of the corroded black sheet showing coarse austenitic grains with shear bands and inter granular cracks.

Fig. 7. SEM-EDS spot analysis of the black corroded packing.

According to the results obtained, failure of these sheets could be mainly attributed to intergranular chloride stress corrosion cracking (CLSCC). Most studies indicated that cold deformation produces a harmful effect on the corrosion resistance of the material. [8,9,15,17] The accepted mechanism explains the deterioration effect of shear bands or cold deformation on corrosion is related to additional energy. It leads in consequences to decreasing the thermodynamic durability of the material. The occurrence of (CLSCC) depends mainly on three factors: susceptibility of the material, environment and considerable residual stresses.

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Fig 8. The result of EDS analysis of point 1 and 2 on Fig. 7.

Fig 9. The result of EDS analysis of point 3 and 4 on Fig. 7.



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Fig 10. The result of EDS analysis of point 5 and 6 on Fig. 7.

The pattern of the branched tree cracks observed in the microstructures of the received sheets is a regular multibranched intergranular crack of chloride stress corrosion cracking (CLSCC) as shown in Figures 11, 12. The (CLSCC) is one of the most common reasons of failures of austenitic stainless steel. This type of failure requires residual or applied stress and chloride ions, while its onset at the localized pitting corrosion initiated at the outer surface of the sheets. Therefore, cracks propagate at stress levels much lower than those required to cause normal tensile failure. In addition, the crack propagation makes almost right angle to the direction of the stresses. The pitting resistance of stainless steel in a chloride-containing environment is primarily determined by its composition while the elements that have significant effects are chromium, molybdenum, and nitrogen. The pitting resistance can be evaluated as a function of these elements by; PREN = 1 × %Cr + 3.3 × %Mo + 16 × %N

(1)

where, PREN is the pitting resistance equivalent number that reflects the relative pitting corrosion resistance of stainless steels. When PREN exceeds 32, the steel is considered to be chloride pitting corrosion resistant and the higher the PREN value, the more corrosion resistant the steel is 6. Conclusions and Recommendations Based on the results obtained in this investigation, the packing sheets failed due to intergranular chloride stress corrosion cracking. The root cause of the failure of the received packing sheets could be mainly attributed to the material residual shear bands after sheet manufacturing process (lack of solution annealing after cold deformation process) and presence of chloride irons (probably from water). Therefore, corrosion in this condition is mainly controlled by chloride irons and residual stresses (remaining of shear bands). It is recommended to look for a better quality material within the same grade free from residual stresses (applying solution annealing heat treatment after cold deformation of the sheets) and removing of the chloride ions by controlling the inlet water).

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