焦炭塔塔体失效及寿命评估研究
摘要
延迟焦化是发展最快、应用最为广泛的石油焦化工艺之一,而焦炭塔是延迟焦化装置的核心设备,其可靠性和完整性关系到整个延迟焦化系统的安全运行。
本文首先对焦炭塔整个完整循环的工作过程进行瞬态热分析以及热-结构耦合分析,基于有限元分析结果,探讨焦炭塔焊缝开裂及塔体鼓胀机理。为了实现全过程的瞬态仿真分析,建立结构上相对完整的焦炭塔三维有限元模型,考虑了各工艺阶段变化的内边界对流换热系数,随时间和高度变化的动态对流换热温度边界条件,采用生死单元法实现焦炭逐渐生成的动态过程,得到仿真度较高的循环温度场以及应力、应变场变化情况。
瞬态热分析以及热-结构耦合分析结果表明：给水冷焦阶段塔壁各节点温度及温度梯度变化情况复杂,温度梯度的极值水平都较高,而该阶段被焦炭覆盖的下部塔体区域等效应力平均水平可达280MPa,远大于相应温度下塔体材质的屈服强度,出现大面积屈服,给水冷焦阶段是焦炭塔应力、应变场发展的关键阶段。且第一道筒体焊缝、裙座焊缝、裙座本身、锥形封头以及下部筒体区域是焦炭塔受力和塑性屈服的关键部位。
基于数值模拟的塔体失效研究表明：水冷阶段由于焦炭的存在阻碍了被焦炭覆盖的下部塔体区域的收缩,使得下部塔体区域的应力水平急剧上升,又由于材料不连续性以及几何不连续性,造成焊缝热影响区的局部应力集中,所以往往在被焦炭覆盖的下部塔体区域的焊缝处裂纹扩展最为迅速；而进油生焦时塔体的热胀沿轴向不均匀,造成生成的焦炭沿轴向也不均匀,焦炭本身的不均匀必将导致下部塔体收缩后塑性变形沿轴向也是不均匀的,在经历多年的工作循环后,塔体沿轴向不均匀的塑性应变逐渐累积放大,最终导致塔体不可恢复的鼓胀变形。
再通过对在役焦炭塔取材的光滑圆柱形缺口试样的热机械疲劳寿命试验,结合该试样的有限元分析结果,依据Manson-Coffin公式采用等效塑性应变范围法,拟合得到该塔体损伤材料20g的剩余寿命评价方程。将焦炭塔工作过程仿真得到的循环最大等效塑性应变范围带入剩余寿命评价方程,并考虑寿命预测的安全系数,预测得到该在役焦炭塔的剩余寿命。
Delayed coking technology develops quickly, and it is one of the most widely used petroleum coking technology. Coke drum is the core equipment of delayed coking unit, the reliability and integrity of which impacts the whole delayed coking system.
In this paper, the work process throughout the complete cycle of the coke drum is simulated, based on the results of finite element analysis, mechanism of weld cracking and drum bulging is analyzed. The transient analysis is considering the different convective heat transfer coefficient of inner boundary in the process, the changing temperature boundary conditions with time and height, and the dynamic generation process of coke by life and death element method.
Transient thermal analysis and thermal-structural coupling analysis shows that:
The temperature and temperature gradient of drum wall nodes is changing complexly, and the overall level of TG is higher in water cooling stage; at the same time, the average equivalent stress of the lower drum covered by coke is much larger than material yield strength, which caused extensive yield. In conclusion, the water cooling stage is the key stage of stress and strain field development. And the key points of stress concentration and plastic yield mainly occur on the first cylinder weld, skirt weld, the skirt itself, conical head and the lower part of the cylinder area.
The failure mechanism analysis based on numerical simulation results shows that:
In water cooling stage, the coke hinders the contraction of the lower drum area covered by coke, resulting in a sharp rise of stress level in this area, and because of the material discontinuity and geometric discontinuity, the weld heat-affected zone happens stress concentration, so the cracks of lower drum weld expand most rapidly under high cyclic stress amplitude. In oil filling stage, the linear thermal expansion of drum is not uniform along axial, and the coke generated is uneven, which will inevitably lead to nonuniform plastic strain of lower drum after water cooling contraction, with many years of work cycle, the nonuniform plastic strain along axial accumulates gradually, eventually drum body happens irrecoverable bulging deformation.
The life assessment of20g is based on thermo-mechanical fatigue life test and finite element analysis of cylindrical notched specimen. The specimen material is from the service coke drum, the dangerous stress and strain of cylindrical notched specimen is analyzed by FEM. And then according to Manson-Coffin formula, using equivalent plastic strain range method, the life evaluation equation of the damaged material is fitted. Finally, combining with the maximum equivalent plastic strain range of coke drum, the remaining life of the coke drum is predicted.
本文首先对焦炭塔整个完整循环的工作过程进行瞬态热分析以及热-结构耦合分析,基于有限元分析结果,探讨焦炭塔焊缝开裂及塔体鼓胀机理。为了实现全过程的瞬态仿真分析,建立结构上相对完整的焦炭塔三维有限元模型,考虑了各工艺阶段变化的内边界对流换热系数,随时间和高度变化的动态对流换热温度边界条件,采用生死单元法实现焦炭逐渐生成的动态过程,得到仿真度较高的循环温度场以及应力、应变场变化情况。
瞬态热分析以及热-结构耦合分析结果表明：给水冷焦阶段塔壁各节点温度及温度梯度变化情况复杂,温度梯度的极值水平都较高,而该阶段被焦炭覆盖的下部塔体区域等效应力平均水平可达280MPa,远大于相应温度下塔体材质的屈服强度,出现大面积屈服,给水冷焦阶段是焦炭塔应力、应变场发展的关键阶段。且第一道筒体焊缝、裙座焊缝、裙座本身、锥形封头以及下部筒体区域是焦炭塔受力和塑性屈服的关键部位。
基于数值模拟的塔体失效研究表明：水冷阶段由于焦炭的存在阻碍了被焦炭覆盖的下部塔体区域的收缩,使得下部塔体区域的应力水平急剧上升,又由于材料不连续性以及几何不连续性,造成焊缝热影响区的局部应力集中,所以往往在被焦炭覆盖的下部塔体区域的焊缝处裂纹扩展最为迅速；而进油生焦时塔体的热胀沿轴向不均匀,造成生成的焦炭沿轴向也不均匀,焦炭本身的不均匀必将导致下部塔体收缩后塑性变形沿轴向也是不均匀的,在经历多年的工作循环后,塔体沿轴向不均匀的塑性应变逐渐累积放大,最终导致塔体不可恢复的鼓胀变形。
再通过对在役焦炭塔取材的光滑圆柱形缺口试样的热机械疲劳寿命试验,结合该试样的有限元分析结果,依据Manson-Coffin公式采用等效塑性应变范围法,拟合得到该塔体损伤材料20g的剩余寿命评价方程。将焦炭塔工作过程仿真得到的循环最大等效塑性应变范围带入剩余寿命评价方程,并考虑寿命预测的安全系数,预测得到该在役焦炭塔的剩余寿命。
Delayed coking technology develops quickly, and it is one of the most widely used petroleum coking technology. Coke drum is the core equipment of delayed coking unit, the reliability and integrity of which impacts the whole delayed coking system.
In this paper, the work process throughout the complete cycle of the coke drum is simulated, based on the results of finite element analysis, mechanism of weld cracking and drum bulging is analyzed. The transient analysis is considering the different convective heat transfer coefficient of inner boundary in the process, the changing temperature boundary conditions with time and height, and the dynamic generation process of coke by life and death element method.
Transient thermal analysis and thermal-structural coupling analysis shows that:
The temperature and temperature gradient of drum wall nodes is changing complexly, and the overall level of TG is higher in water cooling stage; at the same time, the average equivalent stress of the lower drum covered by coke is much larger than material yield strength, which caused extensive yield. In conclusion, the water cooling stage is the key stage of stress and strain field development. And the key points of stress concentration and plastic yield mainly occur on the first cylinder weld, skirt weld, the skirt itself, conical head and the lower part of the cylinder area.
The failure mechanism analysis based on numerical simulation results shows that:
In water cooling stage, the coke hinders the contraction of the lower drum area covered by coke, resulting in a sharp rise of stress level in this area, and because of the material discontinuity and geometric discontinuity, the weld heat-affected zone happens stress concentration, so the cracks of lower drum weld expand most rapidly under high cyclic stress amplitude. In oil filling stage, the linear thermal expansion of drum is not uniform along axial, and the coke generated is uneven, which will inevitably lead to nonuniform plastic strain of lower drum after water cooling contraction, with many years of work cycle, the nonuniform plastic strain along axial accumulates gradually, eventually drum body happens irrecoverable bulging deformation.
The life assessment of20g is based on thermo-mechanical fatigue life test and finite element analysis of cylindrical notched specimen. The specimen material is from the service coke drum, the dangerous stress and strain of cylindrical notched specimen is analyzed by FEM. And then according to Manson-Coffin formula, using equivalent plastic strain range method, the life evaluation equation of the damaged material is fitted. Finally, combining with the maximum equivalent plastic strain range of coke drum, the remaining life of the coke drum is predicted.
引文
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[7]马颖,贾桂茹,许伟,等.延长焦炭塔疲劳寿命的措施及设计技术的进步[J].石油化工设备技术,2007(2)：1-5.
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[3]吕倩,郭淑芝,夏恩冬,等.我国延迟焦化技术现状及发展趋势[J].炼油与化工,2009(1)：5-7.
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[6]顾月章.焦炭塔的材料与结构[J].炼油技术与工程,2011(11)：17-20.
[7]马颖,贾桂茹,许伟,等.延长焦炭塔疲劳寿命的措施及设计技术的进步[J].石油化工设备技术,2007(2)：1-5.
[8]Shargay C, Singh A, Munsterman T, et al. Coke Drum Design and Fabrication Issues[J]. ASME Conference Proceedings,2010,2010(49224):367-381.
[9]Antalffy L P, Malek D W, Pfeifer J A, et al. Innovations in delayed coking coke drum design[C]. Boston, MA, USA:ASME,1999.
[10]顾一天,龙秀兰,贾桂茹.大型焦炭塔的设计[J].石油化工设备技术,2001(1)：4-8.
[11]Bagdasarian A, Horwege J, Kirk S, et al. Integrity of coke drums (Summary of 1998 API coke drum survey)[J]. American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP,2000,411:265-270.
[12]贾桂茹,顾一天.焦炭塔设计中几个问题的探讨[J].石油化工设备技术,2003(6)：1-5.
[13]顾一天,贾桂茹,赵现峰,等.焦炭塔的新型保温结构[J].石油化工设备技术,2005(2)：7-8.
[14]崔其山.焦炭塔的保温设计[J].石油化工设备技术,2005(6)：4-7.
[15]申海平,刘自宾,范启明.延迟焦化技术进展[J].石油学报(石油加工),2010(S1)：14-18.
[16]陈治强,李彬.延迟焦化装置焦炭塔生焦周期优化方向分析[J].炼油技术与工程,2010(8)：11-14.
[17]蒋朝阳.延迟焦化装置—焦炭塔的安全分析与寿命评估[D].大连理工大学,2003.
[18]Ignaccolo S, Cousin M, Jullien J F, et al. Interaction of mechanical and thermal stresses on the instability of cylindrical shells[J]. Res mechanica,1988,24(1): 25-33.
[19]Penso J A. Fundamental study of failure mechanisms of pressure vessels under thermo-mechanical cycling in multiphase environment[D]. The Ohio State University, 2001.
[20]Allevato C, Richard S, Boswell P E. Assessing the structural integrity and remaining life of coke drums with acoustic emission testing, strain gaging, and finite element analysis[Z]. Houston, Texas:1999.
[21]李一玮.延迟焦化装置焦碳塔的变形、开裂机理和安全分析[J].压力容器,1989(4)：61-66.
[22]Satapathy A K. Thermal analysis of an infinite slab during quenching[J]. Communications in Numerical Methods in Engineering,2000,16(8):529-536.
[23]Kim D S, Boswell R S. Residual stress measurements of coker drum welding coupon[C]. San Diego, CA, USA:ASME,1998.
[24]陈孙艺.焦炭塔内焦床影响塔壁变形的作用机理研究[J].石油化工设备技术,2008(6)：16-19.
[25]Boswell R S, Wright B. State-of-the-art improvements in coke drum design and life extension practices[C]. San Antonio, TX, United states:American Society of Mechanical Engineers,2008.
[26]Penso J A, Lattarulo Y M, Sei jas A J, et al. Understanding failure mechanisms to improve reliability of coke drums[C]. Boston, MA, USA:ASME,1999.
[27]Paris P, Erdogan F. A Critical Analysis of Crack Propagation Laws[J]. Journal of Basic Engineering,1963,85(4):528-533.
[28]Church J M, Lim L B, Brear J M, et al. Crack growth modelling and probabilistic life assessment of coke drums operating under fatigue conditions [J]. International Journal of Pressure Vessels and Piping,2001,78(11-12):1011-1020.
[29]Dover W D. Fatigue crack growth under C.0. D. cycling[J]. Engineering Fracture Mechanics, 1973,5(1):11-21.
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