DETERMINATION OF THE ROOT CAUSES OF THE PREMATURE DEGRADATION OF A REBOILER

Transcription

1 Proceedings of the 5th International Conference on Integrity-Reliability-Failure, Porto/Portugal July 2016 Editors J.F. Silva Gomes and S.A. Meguid Publ. INEGI/FEUP (2016) PAPER REF: 6270 DETERMINATION OF THE ROOT CAUSES OF THE PREMATURE DEGRADATION OF A REBOILER Filipe Nascimento (*), Paula Gorjão Ferreira, Celso Araújo Instituto de Soldadura e Qualidade (ISQ), Porto Salvo, Oeiras, Portugal (*) fmnascimento@isq.pt ABSTRACT This work focuses on the determination of the root causes of the premature degradation of a reboiler based on a material characterization methodology. The results were supported by historical data of the component. An evaluation of the component was also performed to suggest corrective measures to minimize or even to avoid the problem in the future. Keywords: Reboiler, sulfidation, thinning, petroleum industry. INTRODUCTION In petroleum refining industry the materials are in contact with a variety of corrosion agents that can affect their performance. It has been identified a high number of corrosion problems and protection measures are taken into account in the design stage of a factory. However, in certain cases it is hard to predict if a certain part will corrode and the determination of the root causes is important in order to protect and prevent failure of the part in a working factory. A compilation of the usual problems that can occur in petrochemical industry have been carried out which includes mechanical, metallurgical and corrosion problems that can occur (API, 2011). Jahromi and Janghorban tried to determine the corrosion causes of the corrosion of aircoolers in an oil refinery. It was reported that there is a decrease in the crude oil quality with the increase of sulfur and salt content and that the desalter did not have the capacity to remove the sulfur and salt content. The author identified that the observed corrosion was probably due to hydrogen chloride and sulfide corrosion. Ghasemi (Ghasemi, 2011) studied the effects of sulfidation and erosion on carbon steel tubes used in petroleum refining industries. Some authors have focused their studies in the effects of different environments on the materials properties of steel. Otero et al. (Otero, 1995) have studied the effect of different aggressive environments in steels and suggested a mechanism of the coating stability and conditions for coating according to the tested conditions. Otero et al. (Otero, 1996) have also proposed surface treatments to carbon steels P5 and P22 to increase its resistance to oxidation and sulfidation phenomena. Zhou et al. (Zhou, 2013) have studied the effects of hydrogen permeation in an H2S environment in API-X52 pipeline steels. Zhou et al. (Zhou, 2014) have also tested the variation of the hydrogen permeation in different solutions. Lee et al. (Lee, 1996) studied the effectiveness of Nb-Al-Si to sulfidation. The authors observed the formation of a bilayer, an outer layer of Al 3 S 2 and an inner layer of NbS 2, and confirmed that the inner layer was very likely responsible for the higher resistance to sulfidation. Qu et al. (Qu, 2006) evaluated the interaction naphthenic acid corrosion and sulphidic corrosion on different steels tested in synthetic refining media. The high variety of problems in this -433-

2 Topic_H: Mechanical Design and Prototyping industry has ranged a high variety of research areas due to its need to solve these impending problems. This case study focuses on a reboiler working in a debutanizer unit whose tube bundle had been considered not to be threatened by a critical damage mechanism under normal working conditions. However, after a short working period leakage was detected. It was observed that the leakage was related to high corrosion of the tubes at a specific location of the tube bundle, mainly on the top tubes, where an accumulation of scale was also observed. This paper presents the root causes that lead to the damage mechanism and suggests some changes to avoid this problem. The methodology for the determination of the root cause, focusing on the metallurgical and chemical analysis of the corroded tubes and its scale products, was implemented to identify the corrosion mechanism associated to the tube bundle of a reboiler type Kettle. PROBLEM DESCRIPTION The present paper presents a failure analysis of a tube related to a leakage found in a tube bandle installed inside of a reboiler type Kettle (Fig.1). This reboiler belongs to a debutanizer unit, which has a fractional distillation column used to separate butane from natural gas during the refining process. Reboilers are used to generate a flux of vapor to feed, in general, a tower. (Hewitt, 2011) Reboilers type Kettle are widely used in petroleum and chemical industries which operation consists of the entrance of a liquid, that comes from a column, into the shell in which there is a horizontal tube bundle. The liquid will boil when in contact with the outside surface of this tube bundle and vapour will flow back to the column. Tubesheet Fig. 1 - Schematic of a Kettle reboiler (Hewitt, 2011) The present case study focus on the replacement of a tube bundle of a reboiler due to leakage detected during a hydro-test. A visual analysis was performed on the replaced tube bundle and it was found that, top half of the bundle, starting from tube sheet to half of the length, where the leakage took place, was found jammed with tube external corrosion scales and slug and further inspection revealed that tubes in that location were thinned with 90% metal loss

3 Proceedings of the 5th International Conference on Integrity-Reliability-Failure BACKGROUND HISTORY The historical composition of the inlet / outlet streams was checked for presence of any corrosive components which might be the cause for corrosion. Primarily the deviations in the levels of the chemical compounds like H2S, RSH and chlorides were checked. The following observations were made: - the average H2S content of sour gas remained at an average of 5.5 mole % against design composition (design case) of 7.1 mole %; - the RSH (mercaptans) in natural gas liquids (NGL) have shown an increasing trend reaching a value of 870 ppm against the design composition (design case) of 900 ppm; - the condensate sample was analysed for RSH content and found to have 544 ppm against the design composition of 1330 ppm; - it was also confirmed that there were multiple low level excursions resulting in to unsubmergence and overheating of tubes up to deg C. RESULTS Characterization of the corroded tubes According to the design, the tubes of the tube bundle are made of SA334 grade 6 steel (Table 1) with a diameter of 25.4mm and 2.67mm in thickness. A set of tubes comprised of degraded tubes (from the top half of the tube bundle) and non-degraded (from the bottom half of the tube bundle) were received. It was confirmed that the tubes had signs of thickness loss (Fig.2). Table 1 - Chemical composition of the SA334 grade 6 steel Chemical composition (%) C Mn P S Si 0.3max (max) (max) 0.10 (min) Fig. 2 - An example of the received tubes with no visible thickness loss, on the left-hand side, and one with visible thickness loss, indicated by arrows, on the right hand-side

4 Topic_H: Mechanical Design and Prototyping Table 2 presents the results of the diameter measurements carried for three of the tubes. On the basis of the diameter design value (25.4mm), the results clearly show a loss of thickness from the outside surface in two of them (tubes 1 and 2). Table 2 - Values of the outer diameter (in mm), measured by a caliper and their locations. a b c 90 0 o 0 o a b c 90 Section / Position Tube a/0 o a/90 o b/0 o b/90 o c/0 o c/90 o Metallographic analysis Samples from tube 1 and 3 were prepared for metallographic analysis. As shown in Fig.3, tube 1 presents a significant thickness reduction associated with an irregular outer surface profile as a consequence of a corrosion degradation mechanism, whereas tube 3 presents a smoother profile without a visible degradation mechanism. Values of thickness of these two tubes are shown in Table 3. It can be verified that the corroded tube (tube 1) presents an average thickness loss of 1.8 to 2mm when compare to the design thickness value (2.67 mm). With respect to tube 3, the measured values where similar or even higher than the design one. These tubes show a similar microstructure of ferrite and pearlite (Fig.4 a). Tube 1 (Fig.4 b) shows some small corrosion pits in the outer surface Table 3 - Thickness measurement of the tubes in different locations Tube Position/Thickness (mm) o

5 Proceedings of the 5th International Conference on Integrity-Reliability-Failure (a) (b) Fig. 3 - Profile of a transversal section of: a) tube 1 and b) tube 3. (a) (b) Fig. 4 - Micrographs of the tube: a) representative image of the tubes microstructure and b) irregular surface profile in the outer tube diameter. Hardness testing Vickers Hardness test were performed on a Shimadzu HSV-20 durometer, according to ISO :2005, in the metallographically prepared cross sections of tubes 1 and 3. Average values of 140HV for tube 1 and 130HV for tube 3 were obtained and they are within the ASTM SA334 grade 6 specification, which established hardness values lower than 190HV

6 Topic_H: Mechanical Design and Prototyping X-ray diffraction (XRD) of scales Fig. 5 - Analysis of the received deposits with the identification of hematite (in grey) and pyrite (in green). XRD analysis of scales was performed on a Rigaku Miniflex equipment. This test was performed to identify the composition of the scales found on the surface of the tube bundle tubes. The analysis allowed the identification of iron sulfide (FeS 2 ) and iron oxide (Fe 2 O 3 ) in crystalline phase (Figure 5). Scanning Electron Microscopy (SEM) coupled with Electron Dispersive Spectroscopy (EDS) Fig. 6 - Analysis in a deposit inside a crack near the surface showing a high content in sulfur: a) identification of the crack and b) EDS analysis The cross section of tube 1 prepared for metallography was analysed with a Scanning Electron Microscope (SEM) JEOL JSM-6500F coupled with an Oxford Instruments EDS analyzer. An EDS analysis on an unidentified deposit inside the crack showing a very high content in sulfur (S) and iron (Fe). The MEV imaging has shown the presence of several pits on the outer surface of the tube with the propagation of small cracks inside the tube (Figure 6)

7 Proceedings of the 5th International Conference on Integrity-Reliability-Failure CONCLUSION The analysis performed in the tubes showed that the corrosion of the analysed reboiler tubes is typical of high temperature known as sulfidation, characterized by the thinning of the tubes together with the presence of sulphur on the outer surface and in the inside of the cracks nucleated from the outer surface. Furthermore, the corrosion scales are composed by iron sulphide amongst other compounds, which are typical features related to the sulfidation damage mechanism. It was identified that the core of the increased corrosion rate by sulfidation is related to the increase of mercaptans on the system. The specific location for the corrosion phenomenon, as referred for the top half of the tubes of the tube bundle, was due to the multiple low level liquid excursions resulting into unsubmergence and overheating of tubes up to deg C. Overheating up to deg C creates the possibility of getting RSH converted catalytically into H 2 S and corrodes the tubes at much faster rate than expected. The damaged identified is a consequence of the tubes exposure to the vaporized liquid due to unsubmergence of the top half of the bundle as a result of an insufficient height of the weir plate, leading to overheating and concentration of sulphur compounds on the outer surfaces. There is a possible relationship between the accumulation of scale and corrosion occurring only in half tube length. Assuming that such correlation exists, then one can expect that the liquid outside the tubes will clean up the scale only on half the tube length and hence decrease/avoid corrosion in that part of the bundle. This situation occurs due to the flow pattern of the liquid on the shell side and the position of the inlet nozzle, which creates a stagnant zone of fluid on the part of the bundle closer to the tubesheet. The scale will then accumulate on the stagnant zone, creating corrosion on that part of the bundle. In order to minimize or even prevent the reoccurrence of sulfidation on the outer surface of the tubes the following recommendations were drawn: guarantee that the reboiler tubes should be always submerged, completely washed by the liquid by: increasing the height of the weir plate to guarantee adequate volume of liquid inside of the reboiler along with the installation of an automatic control for temperature and liquid level; upgrading to a higher chromium alloy can also be considered to increase the resistance to sulfidation. REFERENCES [1]-American Petroleum Institute. Damage Mechanisms Affecting Fixed Equipment in the Refining Industry. API Recommended Practise 571, [2]-Ghasemi H. High temperature sulfidation of carbon steel heater tubes in gas condensate containing sulfur compounds. Engineering Failure Analysis, 2011, 18, p [3]-Hewitt, GF. Reboilers. 2011, DOI: /AtoZ.r.reboilers Retrieved March 04, 2016, from [4]-Jenabali Jahromi SA and Janghorban A. Assessment of corrosion in low carbon steel tubes of shiraz refinery air coolers. Engineering Failure Analysis, 2005, 12, p

8 Topic_H: Mechanical Design and Prototyping [5]-Lee DB, Mitsui H, Habazaki H, Kawashima A, Hashimoto K. The high temperature sulfidation behavior of Nb-Al-Si coatings sputter-deposited on a stainless steel. Corrosion Science, 1996, 38, p [6]-Otero E, Pardo A, Perosanz FJ, Orts J,. Maffiotte CA, Perez Trujillo FJ. Surface modification of several steels after their exposure at high temperature to oxygen and sulfur mixtures. Surface and Coatings Technology, 1995, 76-77, p [7]-Otero E, Pardo A, Perez FJ, Perosanz FJ, Parra A, Maffiotte CA. Coating protection of several steels after their exposure at high temperature to oxygen and sulfur mixtures. Surface and Coatings Technology, 1996, 86-87, p [8]-Qu DR, Zheng YG, Jing HM, Yao ZM, Ke W. High temperature naphthenic acid corrosion and sulphidic corrosion of Q235 and 5Cr1/2Mo steels in synthetic refining media. Corrosion Science, 2006, 48, p [9]-Zhou C, Zheng S, Chen C, Lu G. The effect of the partial pressure of H2S on the permeation of hydrogen in low carbon pipeline steel. Corrosion Science, 2013, 67, p [10]-Zhou C, Chen X, Wang Z, Zheng S, Li X, Zhang L. Effects of environmental conditions on hydrogen permeation of X52 pipeline steel exposed to high H2S-containing solutions. Corrosion Science, 2014, 89, p