Corrosion Fatigue Cracking of Tube Coils in an Actifier Column Catalytic

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Engineering Failure Analysis 10 (2003) 297–306 www.elsevier.com/locate/engfailanal

Corrosion fatigue cracking of tube coils in an actiﬁer column catalytic cracker

Xuan Shia, Yaowu Shib,* aDepartment of Engineering, Babcock & Wilcox Beijing Company Ltd. Bajiaocun, Shijingshan District, Beijing 100043, PR China bSchool of MaterialsScience and Engineering, Beijing Polytechnic University,100 Ping Le Yuan, Chaoyang District, Beijing 100022, PR China

Received 26 July 2002; accepted 15 October 2002

Abstract Tube coils made of 25Cr–20Ni austenitic stainless steel were horizontally installed in the fluidized bed of an actifier column of a catalytic cracker installation in an oil refinery unit. Catalyst particles and flue gases were moved in the fluidized bed. When the catalyst lost activity, carbon in the catalyst was burned out in the fluidized bed to recover the activity of the catalyst. Meanwhile, a steam–water mixture was formed with a pressure of 4 MPa and saturation temperature of 250 C in the tube coils by the heating of the flue gases. Thus, the heat in the fluidized bed was utilized to generate steam. However, after the installation had been in service for about 40 days, leakage occurred in the tube coils. In general the positions of leaks were in the upper part of the tubes within about 6 m of the inlet. Microscopic analyses indicated that cracks initiated at local corrosion pits where chloride ions present in the feedwater enriched and accumulated. The crack propagated in an intergranular or transgranular manner. Obvious striations were found on the crack surfaces at some positions. Based on the failure analysis and heat transfer calculation, failure of the tube coils was mainly caused by the effects of corrosion fatigue. The lifetime of the tube coils can be prolonged by changing the steam–water flow conditions. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Chemical-plant failures; Heat-exchanger failures; Corrosion fatigue; Thermal stress; Striations

1. Introduction

In oil refinery and petrochemical industries energy saving and reutilization are very important problems. An energy saving measure has been used in a catalytic cracker installation of a refinery in the West of China. Tube coils were horizontally installed in the fluidized bed of an actifier column of the catalytic cracker installation to use the surplus heat. Catalyst particles and flue gases were moved in the fluidized bed. When the catalyst lost activity, accumulated carbon in the catalyst was burned out in the fluidized bed

* Corresponding author. Tel.: +86-10-6739-2265; fax: +86-10-6739-2523. E-mail address: [email protected] (Y. Shi).

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to recover the activity of the catalyst. In this process a large quantity of heat was produced to heat the tube coils, in which a steam–water mixture was formed with a pressure of 4MPa and saturation temperature of 250 C. Thus, the heat in the fluidized bad was utilized to generate steam. The diameter of the shell of the actifier column was 9200 mm. Nine sets of tube coils were horizontally installed in the actifier column, as shown in Fig. 1. The curvature radius of the tube coil was 4200 mm. Each tube coil was turned around 315 then rotated back through a curvature of 150 mm and 180. Seven supports were uniformly distributed along each coil except the inlet and outlet positions. The curvature radius of the connection part between the inlet/outlet and the coil was 350 mm. The tube coils were made

Fig. 1. Schematic of tube coils in the actiﬁer column. 中国科技论文在线 http://www.paper.edu.cn

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from 25Cr–20Ni austenitic steel, with diameter of 114 mm and wall thickness of 8 mm. The tube coils were cold formed, then welded with inlet and outlet terminals. The catalyst and process gases were inside the shell of the actifier, while the steam–water mixture was circulated within the tube coils. Actually the process was that water entering in the inlet end was converted to a steam–water mixture leaving at the outlet end. A water pump was used to circulate the water into the nine sets of tube coils in parallel. Due to actification of the catalyst, the temperature of the flue gases out- side the tube coils was 680 C to 700 C. The main compositions of the flue gases were CO2,N2, CO and residual 2–3% O2. The catalyst was basically composed of Al2O3.SiO2. The density of the dense phase in the fluidized bed was high, while the velocity of the catalyst was low, usually being equal to 1.2 to 1.5 m/s. At the inlet the temperature of the water was below the boiling point at this pressure. The velocity of the water was 2 m/s at the inlet. Impurities and additives in the feed water were controlled within closely defined limitations. The level of dissolved oxygen was 5 ppb. The concentration of chloride ion was 4–5 ppm, occasionally up to 8 ppm. The pH value in the water was larger than 10. The failure in one tube coil occurred about 40 days after commissioning of the unit. The occurrence of the failure was found from the loss of the catalyst or white smoke with steam from the chimney. Three months later three more coils failed by leaking. The aim of this investigation is to find the reason for failure and give suggestions to prevent future failure. Actually this work includes two parts for the failure analysis. One is the examination in the field and the metallographic examination. The other is the calculation of flow dynamics. The failure analysis will be mainly given from the viewpoint of metallography in the present paper.

2. Experimental procedure

During periodical major overhaul, features and locations of the leaks were carefully observed to inves- tigate the reason for leakage and failure of the tube coils. It was found that the outer surface of the tube coils had a bright appearance. Leaks occurred in the upper wall of the horizontally installed tube coils, and basically the leaks were located within 6 m of the inlet. A typical picture of the leaks is shown in Fig. 2.

Fig. 2. Breach in upper wall of tube coil. 中国科技论文在线 http://www.paper.edu.cn

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From the results of measurement it was found that there was no obvious reduction in the thickness of the tube wall except at the regions near the leaks. Parts of the tubes with leaks were cut oﬀ to carry out further visual and metallographic examinations. In addition, fractographic analysis and examination of corrosive products were done as per Refs. [1,2].

3. Results

3.1. Visual examination

It was observed that many ﬁne cracks occurred along one tube coil. Through careful visual examination it was found that there existed a large number of circumferential shallow cracks in the inner upper wall, as shown in Fig. 3. These crowded cracks were roughly distributed along three bands. A center band is wide, and two narrow bands are located at the two sides of the center band. The position of the crack bands is schematically shown in Fig. 4. The two narrow crack bands might be just located at the steam–water boundary layer. The distance between the two crack bands is about 40 mm. It is clear that the circumferential cracks were ﬁrst formed in the inner upper wall of the tube coils, then propagated in depth to the outer wall of the tube coils. Finally, the leak formed in the wall due to the local erosion of the high pressure steam–water mixture. Fig. 5 shows a developing leak on the outer wall of a tube coil, but the crack shown in the center of the unformed leak has penetrated.

3.2. Metallography

Using a scanning electron microscope (SEM) to observe the inner wall leaks, it was found that the crack initiation took place at some local corrosion pits, then some pits propagated to become a trench in shape, as shown in Fig. 6. In order to observe the bottom of the pits and cracks in detail, metallographic samples were prepared and sections along the axis direction of the tube examined. Electrolytical etching was performed by means of 10% CrO3 aqueous solution to reveal the grain boundaries. Cracks initiated and grew from the bottom of pits or trenches, as shown in Fig. 7.

Fig. 3. Extensive circumferential shallow cracks in inner upper wall. 中国科技论文在线 http://www.paper.edu.cn

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Fig. 4. Schematic of crack band position.

Fig. 5. Crack in developing breach center seen from outer surface. 中国科技论文在线 http://www.paper.edu.cn

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Fig. 6. (a) Pits and (b) cracks formed in inner wall.

It should be pointed out that the cracks mainly initiated along the intergranular boundaries, as shown in Fig. 7(a). Some cracks, however, initiated in a transgranular manner, as shown in Fig. 7(b) and the short crack to the left of Fig. 7(c). It is usually difficult for such a short trangranular crack to propagate a long distance. The occurrence of the shallow cracks may relate to the local destruction of passive film on the steel. Crack propagation may also have different paths, as shown in Fig. 8. Fig. 8(a) shows a crack along the grain boundaries with a very sharp tip. Fig. 8(b) shows a transgranular crack. Fig. 8(c) shows intergranular cracks with blunted tips. In this case further crack propagating is difficult. Fig. 8(d) shows a crack along a slip line, and a corrosion pit occurred in the preceding austenitic grain boundary.

3.3. Fractography

Typical crack surfaces are shown in Fig. 9. Some corrosion products and deposit substances occurred on the fracture surfaces. Fig. 9(a) shows a typical intergranular fractograph. It is worthy of note that fatigue striations [3], as shown in Fig. 9(b) and (c), can be clearly found on some parts of the crack surfaces, although the fracture surface was rather rough due to the eﬀects of corrosion. Secondary cracks can also be seen. Thus, it is clear that fatigue has an important role in the failure of the coils.

3.4. Energy spectrometry

Cracks and their surrounding regions on the longitudinal section of the tube coils were examined using an X-ray energy spectrometer. This investigation showed that the corrosive products were concentrated and accumulated at the pits or crack initiation sites. Fig. 10 shows the positions examined and indicated as letters A,B, and C. They represented the tube matrix, bottom, and side of a pit, respectively. From Fig. 10 it is clear that the color of the areas B and C is quite diﬀerent from that of the matrix. The composition results are given in Table 1. This semi-quantitative investigation shows the obvious chloride

Table 1 Compositions by X-ray energy spectrometer (wt.%)

Mn Si S P Cr Ni Fe Al Ca Cl V Cu Mo

A 1.00 2.21 27.71 17.11 51.90 0.10 0.05 B 1.74 9.78 0.23 0.38 21.29 9.32 49.96 4.04 1.56 0.29 1.35 0.07 C 1.02 4.21 0.14 0.34 17.17 14.67 58.75 1.68 0.24 1.19 0.43 0.16 中国科技论文在线 http://www.paper.edu.cn

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Fig. 7. Metallographic features of crack initiation; magniﬁcations: (a) 160Â; (b) 128Â; (c) 128Â.

ion-rich bottom of the pit. Based on the examination the maximum chloride ion concentration is found to be 4900 ppm.

4. Discussion

Damage or failure always occurred in the upper wall of the tube coils near to the inlet, where surface boiling or saturated nucleation boiling took place. Under conditions of repeated vaporization, the concentration and deposit of salt present in the water was rapid, and this process was much easier in the upper wall than in the lower wall. This might cause pitting, stress corrosion cracking or other local corrosion, as found by the microscopic examination. The local corrosion was the origin of corrosion fatigue cracking. Because the calculation of the ﬂow dynamics is very complicated, only the results of our previous cal- culations are given here for discussion. The stress analysis indicates that there are two kinds of stresses in the wall of the tube coils. One is the thermal transient stress, which is caused from the temperature diﬀer- ence between the inner and outer walls. The thermal tensile stress on the inner surface of the wall, as cal- 中国科技论文在线 http://www.paper.edu.cn

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Fig. 8. Metallographic features at crack tip; magniﬁcations: (a) 640Â; (b) 640Â; (c) 640Â; (d) 128Â.

culated, may get to 63.6 MPa circumferential, and 111.5 MPa axial. Combined with the environment of the high temperature water containing the dissolved chlorides, the thermal stress may produce cracking in the colder inner wall of the tube. This is why the cracks are first generated in the inner wall and with a circumferential direction, as observed in Fig. 3. The other is thermally generated cyclic stress, which proved to be the more important stress for corrosion fatigue. During operation of the tube coils, the wall temperature in the boiling region changed repeatedly owing to the water power instability and fluid pulsation [4]. Wall temperature will be changed with the fluid condition to be contacted. The cyclic thermal stresses corresponding to the wall temperature change will lead to circumferential fatigue cracks. Large temperature fluctuations take place at the boiling initiation point which may reach 100–150 C. The thermal fatigue frequency may be about 1–20/min. In addition, as the tube coils were horizontally installed, the wall temperature difference between the upper wall and the lower wall may reach 150 C, when steam–water layer flow took place. Thus, the axial cyclic thermal stress for the tube coils is calculated to be 227–341 MPa, and the occurrence of the maximum cyclic thermal stress was just located in the position near the inlet of the tube coil. As the yield stress of the 25Cr–20Ni steel is 190 MPa, the thermal stress exceeds the yield stress of the steel. Thus, this may lead to strain fatigue in the environment. Moreover, it is also proved from the fractography that fatigue occurred. Corrosion might only accelerate this process. Based on the measurements, the spacing of the striations observed on the intergranular frac- 中国科技论文在线 http://www.paper.edu.cn

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Fig. 9. Typical SEM crack surfaces: (a) intergranular, (b) intergranular with striations, (c) transgranular with striations.

Fig. 10. Pit examined by X-ray energy spectrometer: A—tube matrix; B—bottom of pit; C—side of pit. 中国科技论文在线 http://www.paper.edu.cn

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ture surface is about 3 micrometer, and on the transgranular fracture surface is about 10 micrometer. It should be pointed out that the fatigue striations on the fracture surfaces are easily lost, as the tube coils remained at high temperature for several weeks to several months until overhaul started. Besides corrosion fatigue, the cyclic stress may also cause stress corrosion as long as the value of the maximum cyclic stress exceeds the stress corrosion threshold. However, from the above failure analyses, it is indicated that the main reason for the failure is corrosion fatigue. This can be proved by the features such as beach marks, multiple origins, and fatigue striations on the fracture surfaces. Although the striations only occur in a certain range of stress intensity, it is found that the striations correspond closely to the macroscopic crack growth rate. In order to prevent failure caused by corrosion fatigue, the fluctuation of the wall temperature has to be decreased, then the cyclic thermal stress is decreased. Therefore, it is proposed that the parameters of operation should be changed so as to improve the conditions of steam–water flow, and the water power stability. At the same time this change of the operation parameters is a most convenient and easy method. In subsequent service it has been shown that no damage occurred in an identical tube coil after two over- hauls, during which the water supply flow rate was increased to improve the water power stability and restrain the steam–water layer flow. This is also indicated that the control of the thermal stresses is very important for preventing the corrosion fatigue.

5. Conclusions

1. Failure occurred in the upper wall of the tube coils near to the inlet. Pits or other local corrosion were the original sites of corrosion fatigue cracking. Failure analysis indicated that failure of the tube coils was mainly caused by the effects of corrosion fatigue. 2. Changes of operational parameters may easy change the condition of steam–water flow, and the water power stability. The lifetime of the tube coils can be prolonged considerably by changing the steam–water flow conditions.

References

[1] Failure analysis and prevention. Metals handbook, vol. 11. 9th ed. Metal Park (OH): American Society for Metals; 1986. [2] Corrosion. Metals handbook, vol. 13. 9th ed. Metal Park, (OH): American Society for Metals; 1987. [3] Deng ZJ, Zhou JE. Fracture and fatigue of engineering materials. Beijing: Mechanical Industry Press; 1995. [4] Lin ZH, Chen LX. Process in boilers. Xi’an: Xi’an Jiaotong University Press; 1990.