Corrosion Fatigue Cracking of Tube Coils in an Actifier Column Catalytic
Total Page：16
File Type：pdf, Size：1020Kb
http://www.paper.edu.cn 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 tem- perature 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). 1350-6307/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S1350-6307(02)00074-2 中国科技论文在线 http://www.paper.edu.cn 298 X. Shi, Y. Shi / Engineering Failure Analysis 10 (2003) 297–306 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 X. Shi, Y. Shi / Engineering Failure Analysis 10 (2003) 297–306 299 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 fail- ure. 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 300 X. Shi, Y. Shi / Engineering Failure Analysis 10 (2003) 297–306 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 X. Shi, Y. Shi / Engineering Failure Analysis 10 (2003) 297–306 301 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 302 X. Shi, Y. Shi / Engineering Failure Analysis 10 (2003) 297–306 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.