多层包扎尿素合成塔检验与剩余寿命评估方法研究
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摘要
尿素合成塔是尿素合成装置的核心设备。目前,国内大部分尿素合成塔采用的是多层包扎结构。多层包扎结构由于层板间紧配合的作用,在不增加壁厚的情况下,提高了厚壁圆筒的强度,被普遍认为是非常安全的结构形式,在石油精炼、石油化工和化学肥料行业中得到了广泛应用。
然而,由于多层包扎尿素合成塔(以下简称“尿塔”)筒节之间采用的是深环焊缝结构,并且焊后不进行热处理,在焊接和使用的过程中容易产生缺陷。此外,在设计中一般将多层包扎结构视作单层厚壁容器,忽视了多层结构在制造过程中产生的层板间隙,从而也忽略了多层结构在温度和压力载荷作用下各层板之间变形和受力的非协调性,使得深环焊缝等这些层板变形受约束的部位在操作条件下产生很大的应力集中。另外,国内90%以上的尿素生产企业采用水溶液全循环法尿素生产工艺,而绝大多数化肥企业采用蒸汽进行尿素合成塔检漏。由于检漏管自身结构原因或外力的作用往往发生蒸汽泄漏,使层板间隙内串汽,造成层板的应力腐蚀开裂,严重的甚至导致尿素合成塔的爆破事故,造成了重大人员伤亡和财产损失。
针对尿塔层板和环焊缝出现的应力腐蚀开裂,国内外开展了声发射、超声和射线等无损检测技术的研究工作。但是,这些方法的裂纹检出率和可靠性需要深入研究。此外,对于检查出的在用设备缺陷如何进行评价,进而判定含缺陷尿塔的安全性,是一个迫在眉睫的问题。作者从尿塔结构和尿素生产工艺分析入手,对不同内径的尿塔进行了有限元分析,得到了有层板间隙下尿塔的应力分布规律。在此基础上对发生严重应力腐蚀开裂的尿塔进行了解剖分析,研究了尿塔的应力腐蚀开裂机理。针对大量在用尿塔层板和焊缝裂纹缺陷,进行了无损检验研究工作,并进行了现场解剖验证,得到了较为可靠的尿塔检验方法。对于检验中发现的裂纹,结合有限元分析的结果,给出了尿塔的安全评定方法。
对尿塔结构、生产工艺和检漏工艺进行了分析。研究表明,由于尿塔的特殊结构和工艺介质的强烈腐蚀性,使得尿塔可能产生内衬腐蚀和外部强度层板腐蚀。内衬的腐蚀主要是内部工作介质和工艺变动引起的。外部层板的腐蚀主要是蒸汽介质引起的。检漏蒸汽中携带的碱、氯离子等介质在层板间隙处聚集,引起外部层板的腐蚀和开裂。另外,内部工作介质泄露可引起外部层板的强烈腐蚀,而蒸汽介质含有的氯离子也可以引起内衬层外侧的应力腐蚀开裂。
对尿塔进行了有限元分析。在分析过程中,考虑了热应力、材料非线性、接触等本构关系和层板间隙、焊缝结构等结构因素的影响。结果表明,在操作压力下,尿塔筒节内部层板应力有限元值和Pimshtein公式值呈径向从内至外逐渐降低的趋势。内部层板应力值高于Lame公式值,而外部层板应力值低于Lame公式值。随着层间间隙的增大,内部层板的应力值增大,外部层板的应力值减小,更多外部层板不承受内压,承受内压的内部层板发生破裂的可能性增大。对于间隙为0.25mm的尿塔,仅承压时,在弹性状况下,仅有7层层板承受内压,当壁厚大于7层层板后,层板应力趋近于零。考虑温度的影响时,除内衬外,层板的应力普遍升高,最高应力值向盲板层转移。弹塑性分析结果表明,内部部分层板的应力值已经超过材料的屈服极限,呈塑性屈服状态。
由于尿塔层板间存在间隙,在内压作用下,层板在焊缝处呈内侧受拉外侧受压的状态。越靠近焊缝,层板两侧拉应力和压应力越大,在层间焊趾处产生很大的应力集中。间隙处的应力呈现很强的局部性,离开间隙顶端,应力值迅速下降。在内压力和温度共同作用下时,应力重新分布,内衬层的应力得到缓解,应力最集中的部位仍是层间隙焊趾处。内衬层、盲板层及第一强度层板焊趾处发生局部屈服。屈服区由内到外逐渐减小,高应力区的几何形状与实际解剖发现的裂纹扩展方向相吻合。
对尿塔强度层和焊缝开裂成因进行了分析。对尿塔内衬腐蚀泄漏、强度层开裂和尿塔爆破等失效形式进行了总结。通过宏观检查、解剖分析、显微组织分析以及腐蚀产物和沉积产物分析,指出尿塔的强度层裂纹是应力腐蚀开裂造成的。对尿塔强度层外表面裂纹长度和开裂部位进行了统计,发现裂纹的分布呈现向焊缝部位集中的特点,与焊缝部位的应力分布规律一致。从材料、应力、环境等角度对尿塔强度层开裂原因进行了研究。结果表明,尿塔开裂是在材料缺陷、局部应力集中和检漏介质冷凝浓缩等多种因素作用下产生的。多层包扎筒体深环焊缝焊接时产生了粗大魏氏组织,降低了材料的组织均匀性,而层板间隙的检漏蒸汽发生冷凝浓缩,产生了发生应力腐蚀的介质条件。焊缝组织在缝隙中浓缩的检漏液的作用下产生应力腐蚀和电偶腐蚀等腐蚀形态,从而使尿塔发生开裂,并沿与主应力垂直的方向向外扩展。尿塔强度层的开裂形式与检漏蒸汽压力、检漏蒸汽污染程度、检漏孔结构及层板间隙是否与外界连通有关。检漏蒸汽压力较高、检漏孔串汽且层板间隙不与外界连通的情况下应力腐蚀开裂情况最严重。当检漏蒸汽压力比较低时,若层板之间发生串汽,进入内部层板间隙的蒸汽不具备凝结的条件,只在外部层板间隙发生腐蚀介质浓缩,应力腐蚀开裂只在外部层板和焊趾处发生。当检漏蒸汽露点温度低于尿塔最外层层板温度时,层板的开裂将被遏制。当检漏蒸汽压力高时,进入内部层板间隙的蒸汽具备凝结的条件,焊趾部位多处发生腐蚀介质浓缩,内部层板和焊缝将发生应力腐蚀开裂。另外,内衬泄漏和工艺系统泄漏引起的检漏蒸汽污染仍然可能引起层板腐蚀开裂。
利用声发射、超声相控阵、γ射线和金属磁记忆等检测方法对尿塔存在的缺陷进行了检测和整体性状况评价。首先,根据尿塔检验的特点对声发射、相控阵和射线等无损检测方法进行了对比分析,表明只有充分发挥这些检测方法的优点,才能完成尿塔的完整性检测。其次,利用声发射对尿塔进行了试验研究。研究表明,随试验压力的升高,尿塔的应力集中部位尤其是环焊缝部位出现了明显的定位信号,与有限元分析的应力分布规律一致。随后的解剖结果证实声发射集中定位源处为径向扩展裂纹。对尿塔声发射信号的参数特征进行了统计分析,得出了尿塔缺陷的特征参数。利用超声相控阵检测系统对尿塔深环焊缝截面进行了超声成像研究,结果表明超声相控阵检测方法能够清晰地显示焊缝截面内裂纹类反射体的部位和埋藏深度,因而层间焊趾裂纹、焊缝埋藏缺陷及层板错层等情况都被直观地检测出来,并能够测量出裂纹埋藏深度和长度,为设备维修决策和缺陷评定提供数据支持。该方法适用于尿塔声发射检测后的焊缝局部复验。γ射线检测试验结果表明,射线对较大倾斜角裂纹的检出率较低,只有在层板上有声发射定位信号或焊缝声发射信号无法检出时,利用射线进行复验。金属磁记忆方法测量的是局部应力集中,对尿塔内部埋藏裂纹检测的可靠性还需进一步研究。以上研究结果表明,利用声发射检测技术对尿塔进行整体性状况评价,然后结合超声相控阵和射线测量缺陷的无损检验方法,具有较高的可靠性和检验效率,能够有效检测出尿塔环焊缝中存在的严重缺陷,该技术对与尿塔结构类似的多层包扎压力容器检验具有较好的应用前景。
对含缺陷尿塔进行了剩余寿命评估方法研究。首先,以偏于安全的原则,对解剖发现的尿塔中的裂纹进行了简化处理,将尿塔中的裂纹规则化为裂纹面垂直于轴向的圆周裂纹和裂纹面平行轴向的轴向裂纹。然后,根据有限元计算取得的应力值和尿塔解剖取得的实际裂纹尺寸,计算得出了尿塔裂纹的名义启裂应力强度因子,从而推导出了含裂纹尿塔的初始裂纹尺寸和临界裂纹尺寸表达式,从而可估算出剩余使用寿命。以此为基础,得出了尿塔剩余使用寿命曲线。在尿塔的声发射试验过程中,根据声发射集中定位源出现的压力对照该曲线可估算出尿塔的使用寿命。
总之,通过尿塔结构和尿素生产工艺分析、有限元分析、裂纹成因分析、无损检测方法研究和剩余寿命评估方法研究,掌握了尿塔的应力分布规律和容易失效部位,为尿塔的检验和完整性评价提供了一套完整的解决方案,保证了尿塔的安全运行,具有较高的经济效益和社会效益。但是,由于尿塔的失效与其设计、制造、安装、使用、检验、修理以及改造等都有密切的关系,每一个环节对尿塔的失效都可能产生重要影响。因此,还需要在以上领域开展进一步的研究工作,以便采取相应的措施,预防尿塔事故的发生。
Urea reactor is the core equipment of urea synthesis devices. Presently, Most of the domestic urea reactors have multilayer structures. The strength of the thick-walled cylinder was enhanced without increasing wall thickness due to the tight joint of the layers. Therefore, this kind of vessel has been generally considered as very safe equipment and widely used in the oil refining, petrochemical and chemical fertilizer industry.
However, the multilayer urea reactor cylinders were connected by heavy welds, defects can be easily initiated during welding and operation due to the use of multilayer welding in the heavy weld structure and abolishment of post-weld heat treatment. In addition, multi-layer structure was generally taken as a single layer during design. Thus, interlayer clearances were often neglected in the manufacturing process, and the layer stress and deformation non-coordination under operation temperature and pressure loads were overlooked accordingly, which leading to layer deformation constrained parts such as heavy weld endure enormous stress concentration under the operating conditions. Also, more than 90 percent of the current domestic fertilizer plants use urea aqueous full recycling production process and the vast majority adopt steam as leak detection media. Steam leakage was often observed at the leak detection system due to its structural causes or outside effects; Steam will fill in the interlayer clearances during the course of leakage and induce stress corrosion cracking of layers. Series cracking may lead to serious urea reactor blasting accidents, which caused heavy casualties and property losses.
Nondestructive testing technology such as acoustic emission, ultrasonic andγ-ray had been applied to investigate the layer and heavy weld stress corrosion cracking of urea reactors both at home and abroad. However, the crack detection rate and reliability of these methods need further study. In addition, how to carry out evaluation work of checked out flaws in the in-service vessel, and further assess the safety condition of the vessel is a very urgent problem. The anthor started work from structure and urea production process factors which affected urea reactor corrosion and stress distribution, and investigated urea reactors with different diameters using finite element analysis. The urea reactor stress distribution with interlayer clearances was acquired. On the basis of upper work, dissection was conducted to a serious stress corrosion cracked urea reactor, and the stress corrosion cracking mechanism was studied. Nondestructive testing research and followed site verification was carried out for the lots in-service urea reactor layer and weld crack defects, and acquired a reliable multilayer urea reactor detection method. As for the cracks found during inspection, urea reactor structural integrity assessing method was refined in light of the results of finite element analysis.
Multilayer urea reactor structure, urea production process and leak detection process were analyzed. Results showed that due to the urea reactor special structure and the strong corrosive of process media, corrosion may occur at the liner and the external layers of the urea reactor. Internal liner corrosion is caused by the changes of reaction medium and processing technology. External layer corrosion was aroused mainly by steam. Corrosive deposits such as alkaline ions and chloride ions brought by leak steam would concentrate at the interlayer clearances, which would induce corrosion and cracking of outer layers. In addition, the internal reaction medium leakage would generate strong corrosion of external layers, and the steam medium containing chloride ions can also lead to outer side stress corrosion cracking of the liner.
Finite element analysis (FEA) of a urea reactor was conducted. Factors of heat stress, material nonlinearity, friction constitutive relations and interlayer clearances as well as heavy weld structures were considered during the analysis. The results showed that under the operating pressure, the internal FEA stress values and Pimshtein formulas calculation results of the cylinder layers had a gradually decreasing trend radially from the inside to the outside surface. The stresses of inner layers were higher than Lame formulas calculation results, whereas outer layer stresses were lower than Lame formulas calculation results. With the increasing of clearances between layers, the internal layer stresses increased, and the stress of outer layers decreased, which lead more external layers to an idle state. Therefore, the cracking possibility of inner layers which endure working pressure increased. As for urea reactors with 0.25 mm gaps, only seven layers endure the inner pressure. When the thickness exceeds more than seven layers, the stresses of layers nearly tend to zero. When considering the impact of temperature, the heavy weld stress generally increased and the maximum stress transfer to the annulus layer. Elastic-plastic analysis of reactors with the clearances of 0.25 mm showed that the stress states of some internal layers had exceeded the material yield limit to a plastic state.
Since urea reactor has gaps between layers, the layers showed tension states in the inner side and compression states in the outer side under pressure. The stress states between two sides became larger when nearing the heavy weld, and eventually generated great stress concentrations at the weld toe. The stress states showed strong locality in the weld toe, and decline rapidly when leaving away from it. Under both operation pressure and temperature, the stresses redistributed, and the inner layer stress state relieved, but the weld toe was still the most stress concentrated area. Local yield zone occurred at the weld toe of the liner, the annulus and the first strength layer. The yield zone decreased gradually from inner surface to the outside, and the geometrical shape conform to the actual crack propagation direction found during dissection.
The reason of crack initiation of the urea reactor strength layer and the weld were analyzed. Lining corrosion and leakage, strength layer cracking and the explosion type of the urea reactor were summarized. The macro inspection, dissection analysis, microstructure analysis, as well as deposition and corrosion product analysis results showed that the strength layer cracks were caused by stress corrosion cracking. Statistics of crack lengths and cracking positions showed that cracks have the characteristics of initiating at the weld parts. The reasons for urea reactor cracking were studied from the view of material, stress, corrosion environment. The results showed that the cracks were initiated by a variety of factors such as material defects, local stress concentration and leak detection medium condensation. Multi-cylinder deep seam welding produced large widmanstaten organizations, which reduced the material structure uniformity. On the other hand, leak steam condensation in the layer clearances occurred, which produced stress corrosion cracking conditions. Heavy weld happened to stress corrosion and galvanic corrosion under the effect of leak detection liquid concentrated in the crevice of layers. Therefore, urea reactor cracking occurred, and expanded outward along the direction perpendicular to the main stress. The strength layer cracking form of urea reactors have some relation to leak detection steam pressure, leak detection steam contamination condition, leak detection structure and connectivity condition between interlayer clearances and outside environment. The most serious stress crossing cracking would occur when the following conditions happened simultaneously: high leak detection steam pressure, interlayer clearances filling with steam and the clearances were blinded to the outside environment. When Leak steam pressure was relatively low, the steam which accessed into the interlayer clearances did not have the condensation conditions; stress corrosion cracking would occur only in the external layers and weld toes. When the leak detection steam dew point temperature is below the outmost layer temperature, layer cracking will be arrested. When the leak detection steam pressure was relatively high, the leak detection steam which entered the clearances has the condensation conditions. Multi-points corrosion concentration would happen at the weld toe, which would induce stress corrosion cracking of inner layers and welds. In addition, the liner leakage and process system leakage caused by leak detection steam pollution also can cause layer corrosion cracking.
The existed defects of the urea reactor were detected and comprehensively evaluated using acoustic emission, ultrasonic phased array examination,γ-ray detection and metal magnetic memory methods. First, comparison analysis of acoustic emission, phased array,γ-ray examination and metal magnetic memory was conducted according to the characteristics of urea reactor inspection. Results indicated that only give full play to the advantages of these detection methods, can the urea reactor integrity detection be completed. Second, acoustic emission examination was conducted to a urea reactor. Results showed that, with increment of the test pressure, apparent acoustic emission location signals come forth in the stress concentration sites especially in the heavy weld toe, which were in consistent with the stress distribution pattern of FEA results. Subsequent dissection results confirmed that the acoustic emission source locations were heavy weld radial expansion cracks. Acoustic emission signal characteristics were statistically analyzed, and defects characteristic parameters were acquired. Heavy weld sections of the urea reactor were imaged using ultrasonic phased array system. Results showed that the ultrasonic phased array detection method can clearly show crack type reflectors and crack latent depth, thus weld toe cracks, weld defects and layer mismatch etc. could be directly detected. The length and depth of cracks could be measured through this method, which could provide data for decision-making of equipment maintenance and defect assessment. This method was especially useful for acoustic emission source location re-inspection acquired by acoustic emission examination.γ- ray testing results showed that theγ-ray crack detection rates were very low to the large tilt angle cracks, only when the acoustic emission signals positioned on layers or weld acoustic emission signal could not efficiently detected,γ-ray re-inspection was needed to be utilized. Metal magnetic memory measurement was used for the testing of the localized stress concentrations; the buried crack detection reliability of urea reactor welds needs further study. Results showed that the use of acoustic emission testing technology combining ultrasound phased array andγ-ray non-destructive testing method had higher reliability and efficiency for urea reactor integrity situation evaluation. Combination of these methods can effectively find out heavy weld defects, and had prosperous advantages for testing multilayer structures similar to multilayer urea reactors.
Safety evaluation of urea reactors containing defects was studied. First, cracks in the urea reactor had been simplified on the principle of safety. Cracks in the urea reactors were ruled into two categories: Circumferential cracks with surfaces perpendicular to the axial and axial cracks with surfaces parallel to the axial. Then, the urea reactor crack stress intensity factors were calculated in accordance with the FEA results and actual crack sizes acquired during urea reactor dissection. Therefore, the initial crack size and critical crack size expression formulas of urea reactors were acquired. The urea reactor remaining life could also be estimated. On this basis, urea reactor remaining life curve was given. The remaining life of the urea reactors could be estimated during the acoustic emission testing process in accordance with the acoustic emission source location appearance pressures.
In short, urea reactor stress distribution and liable failure location were studied by urea reactor structure and finite element analysis, crack initiation analysis, non-destructive testing method study and safety assessment method investigation. This work provided a complete set of solutions for urea reactor inspection and integrity evaluation to ensure the urea reactor safety operation, and had tremendous economic and social benefits. However, urea reactor failures are closely related to their design, manufacture, installation, usage, inspection, repair and transformation. Each factor could have important impact on the failures of urea reactors. Therefore, it is also necessary to carry out further research in these areas, so as to take corresponding measures to prevent urea reactor accidents.
然而,由于多层包扎尿素合成塔(以下简称“尿塔”)筒节之间采用的是深环焊缝结构,并且焊后不进行热处理,在焊接和使用的过程中容易产生缺陷。此外,在设计中一般将多层包扎结构视作单层厚壁容器,忽视了多层结构在制造过程中产生的层板间隙,从而也忽略了多层结构在温度和压力载荷作用下各层板之间变形和受力的非协调性,使得深环焊缝等这些层板变形受约束的部位在操作条件下产生很大的应力集中。另外,国内90%以上的尿素生产企业采用水溶液全循环法尿素生产工艺,而绝大多数化肥企业采用蒸汽进行尿素合成塔检漏。由于检漏管自身结构原因或外力的作用往往发生蒸汽泄漏,使层板间隙内串汽,造成层板的应力腐蚀开裂,严重的甚至导致尿素合成塔的爆破事故,造成了重大人员伤亡和财产损失。
针对尿塔层板和环焊缝出现的应力腐蚀开裂,国内外开展了声发射、超声和射线等无损检测技术的研究工作。但是,这些方法的裂纹检出率和可靠性需要深入研究。此外,对于检查出的在用设备缺陷如何进行评价,进而判定含缺陷尿塔的安全性,是一个迫在眉睫的问题。作者从尿塔结构和尿素生产工艺分析入手,对不同内径的尿塔进行了有限元分析,得到了有层板间隙下尿塔的应力分布规律。在此基础上对发生严重应力腐蚀开裂的尿塔进行了解剖分析,研究了尿塔的应力腐蚀开裂机理。针对大量在用尿塔层板和焊缝裂纹缺陷,进行了无损检验研究工作,并进行了现场解剖验证,得到了较为可靠的尿塔检验方法。对于检验中发现的裂纹,结合有限元分析的结果,给出了尿塔的安全评定方法。
对尿塔结构、生产工艺和检漏工艺进行了分析。研究表明,由于尿塔的特殊结构和工艺介质的强烈腐蚀性,使得尿塔可能产生内衬腐蚀和外部强度层板腐蚀。内衬的腐蚀主要是内部工作介质和工艺变动引起的。外部层板的腐蚀主要是蒸汽介质引起的。检漏蒸汽中携带的碱、氯离子等介质在层板间隙处聚集,引起外部层板的腐蚀和开裂。另外,内部工作介质泄露可引起外部层板的强烈腐蚀,而蒸汽介质含有的氯离子也可以引起内衬层外侧的应力腐蚀开裂。
对尿塔进行了有限元分析。在分析过程中,考虑了热应力、材料非线性、接触等本构关系和层板间隙、焊缝结构等结构因素的影响。结果表明,在操作压力下,尿塔筒节内部层板应力有限元值和Pimshtein公式值呈径向从内至外逐渐降低的趋势。内部层板应力值高于Lame公式值,而外部层板应力值低于Lame公式值。随着层间间隙的增大,内部层板的应力值增大,外部层板的应力值减小,更多外部层板不承受内压,承受内压的内部层板发生破裂的可能性增大。对于间隙为0.25mm的尿塔,仅承压时,在弹性状况下,仅有7层层板承受内压,当壁厚大于7层层板后,层板应力趋近于零。考虑温度的影响时,除内衬外,层板的应力普遍升高,最高应力值向盲板层转移。弹塑性分析结果表明,内部部分层板的应力值已经超过材料的屈服极限,呈塑性屈服状态。
由于尿塔层板间存在间隙,在内压作用下,层板在焊缝处呈内侧受拉外侧受压的状态。越靠近焊缝,层板两侧拉应力和压应力越大,在层间焊趾处产生很大的应力集中。间隙处的应力呈现很强的局部性,离开间隙顶端,应力值迅速下降。在内压力和温度共同作用下时,应力重新分布,内衬层的应力得到缓解,应力最集中的部位仍是层间隙焊趾处。内衬层、盲板层及第一强度层板焊趾处发生局部屈服。屈服区由内到外逐渐减小,高应力区的几何形状与实际解剖发现的裂纹扩展方向相吻合。
对尿塔强度层和焊缝开裂成因进行了分析。对尿塔内衬腐蚀泄漏、强度层开裂和尿塔爆破等失效形式进行了总结。通过宏观检查、解剖分析、显微组织分析以及腐蚀产物和沉积产物分析,指出尿塔的强度层裂纹是应力腐蚀开裂造成的。对尿塔强度层外表面裂纹长度和开裂部位进行了统计,发现裂纹的分布呈现向焊缝部位集中的特点,与焊缝部位的应力分布规律一致。从材料、应力、环境等角度对尿塔强度层开裂原因进行了研究。结果表明,尿塔开裂是在材料缺陷、局部应力集中和检漏介质冷凝浓缩等多种因素作用下产生的。多层包扎筒体深环焊缝焊接时产生了粗大魏氏组织,降低了材料的组织均匀性,而层板间隙的检漏蒸汽发生冷凝浓缩,产生了发生应力腐蚀的介质条件。焊缝组织在缝隙中浓缩的检漏液的作用下产生应力腐蚀和电偶腐蚀等腐蚀形态,从而使尿塔发生开裂,并沿与主应力垂直的方向向外扩展。尿塔强度层的开裂形式与检漏蒸汽压力、检漏蒸汽污染程度、检漏孔结构及层板间隙是否与外界连通有关。检漏蒸汽压力较高、检漏孔串汽且层板间隙不与外界连通的情况下应力腐蚀开裂情况最严重。当检漏蒸汽压力比较低时,若层板之间发生串汽,进入内部层板间隙的蒸汽不具备凝结的条件,只在外部层板间隙发生腐蚀介质浓缩,应力腐蚀开裂只在外部层板和焊趾处发生。当检漏蒸汽露点温度低于尿塔最外层层板温度时,层板的开裂将被遏制。当检漏蒸汽压力高时,进入内部层板间隙的蒸汽具备凝结的条件,焊趾部位多处发生腐蚀介质浓缩,内部层板和焊缝将发生应力腐蚀开裂。另外,内衬泄漏和工艺系统泄漏引起的检漏蒸汽污染仍然可能引起层板腐蚀开裂。
利用声发射、超声相控阵、γ射线和金属磁记忆等检测方法对尿塔存在的缺陷进行了检测和整体性状况评价。首先,根据尿塔检验的特点对声发射、相控阵和射线等无损检测方法进行了对比分析,表明只有充分发挥这些检测方法的优点,才能完成尿塔的完整性检测。其次,利用声发射对尿塔进行了试验研究。研究表明,随试验压力的升高,尿塔的应力集中部位尤其是环焊缝部位出现了明显的定位信号,与有限元分析的应力分布规律一致。随后的解剖结果证实声发射集中定位源处为径向扩展裂纹。对尿塔声发射信号的参数特征进行了统计分析,得出了尿塔缺陷的特征参数。利用超声相控阵检测系统对尿塔深环焊缝截面进行了超声成像研究,结果表明超声相控阵检测方法能够清晰地显示焊缝截面内裂纹类反射体的部位和埋藏深度,因而层间焊趾裂纹、焊缝埋藏缺陷及层板错层等情况都被直观地检测出来,并能够测量出裂纹埋藏深度和长度,为设备维修决策和缺陷评定提供数据支持。该方法适用于尿塔声发射检测后的焊缝局部复验。γ射线检测试验结果表明,射线对较大倾斜角裂纹的检出率较低,只有在层板上有声发射定位信号或焊缝声发射信号无法检出时,利用射线进行复验。金属磁记忆方法测量的是局部应力集中,对尿塔内部埋藏裂纹检测的可靠性还需进一步研究。以上研究结果表明,利用声发射检测技术对尿塔进行整体性状况评价,然后结合超声相控阵和射线测量缺陷的无损检验方法,具有较高的可靠性和检验效率,能够有效检测出尿塔环焊缝中存在的严重缺陷,该技术对与尿塔结构类似的多层包扎压力容器检验具有较好的应用前景。
对含缺陷尿塔进行了剩余寿命评估方法研究。首先,以偏于安全的原则,对解剖发现的尿塔中的裂纹进行了简化处理,将尿塔中的裂纹规则化为裂纹面垂直于轴向的圆周裂纹和裂纹面平行轴向的轴向裂纹。然后,根据有限元计算取得的应力值和尿塔解剖取得的实际裂纹尺寸,计算得出了尿塔裂纹的名义启裂应力强度因子,从而推导出了含裂纹尿塔的初始裂纹尺寸和临界裂纹尺寸表达式,从而可估算出剩余使用寿命。以此为基础,得出了尿塔剩余使用寿命曲线。在尿塔的声发射试验过程中,根据声发射集中定位源出现的压力对照该曲线可估算出尿塔的使用寿命。
总之,通过尿塔结构和尿素生产工艺分析、有限元分析、裂纹成因分析、无损检测方法研究和剩余寿命评估方法研究,掌握了尿塔的应力分布规律和容易失效部位,为尿塔的检验和完整性评价提供了一套完整的解决方案,保证了尿塔的安全运行,具有较高的经济效益和社会效益。但是,由于尿塔的失效与其设计、制造、安装、使用、检验、修理以及改造等都有密切的关系,每一个环节对尿塔的失效都可能产生重要影响。因此,还需要在以上领域开展进一步的研究工作,以便采取相应的措施,预防尿塔事故的发生。
Urea reactor is the core equipment of urea synthesis devices. Presently, Most of the domestic urea reactors have multilayer structures. The strength of the thick-walled cylinder was enhanced without increasing wall thickness due to the tight joint of the layers. Therefore, this kind of vessel has been generally considered as very safe equipment and widely used in the oil refining, petrochemical and chemical fertilizer industry.
However, the multilayer urea reactor cylinders were connected by heavy welds, defects can be easily initiated during welding and operation due to the use of multilayer welding in the heavy weld structure and abolishment of post-weld heat treatment. In addition, multi-layer structure was generally taken as a single layer during design. Thus, interlayer clearances were often neglected in the manufacturing process, and the layer stress and deformation non-coordination under operation temperature and pressure loads were overlooked accordingly, which leading to layer deformation constrained parts such as heavy weld endure enormous stress concentration under the operating conditions. Also, more than 90 percent of the current domestic fertilizer plants use urea aqueous full recycling production process and the vast majority adopt steam as leak detection media. Steam leakage was often observed at the leak detection system due to its structural causes or outside effects; Steam will fill in the interlayer clearances during the course of leakage and induce stress corrosion cracking of layers. Series cracking may lead to serious urea reactor blasting accidents, which caused heavy casualties and property losses.
Nondestructive testing technology such as acoustic emission, ultrasonic andγ-ray had been applied to investigate the layer and heavy weld stress corrosion cracking of urea reactors both at home and abroad. However, the crack detection rate and reliability of these methods need further study. In addition, how to carry out evaluation work of checked out flaws in the in-service vessel, and further assess the safety condition of the vessel is a very urgent problem. The anthor started work from structure and urea production process factors which affected urea reactor corrosion and stress distribution, and investigated urea reactors with different diameters using finite element analysis. The urea reactor stress distribution with interlayer clearances was acquired. On the basis of upper work, dissection was conducted to a serious stress corrosion cracked urea reactor, and the stress corrosion cracking mechanism was studied. Nondestructive testing research and followed site verification was carried out for the lots in-service urea reactor layer and weld crack defects, and acquired a reliable multilayer urea reactor detection method. As for the cracks found during inspection, urea reactor structural integrity assessing method was refined in light of the results of finite element analysis.
Multilayer urea reactor structure, urea production process and leak detection process were analyzed. Results showed that due to the urea reactor special structure and the strong corrosive of process media, corrosion may occur at the liner and the external layers of the urea reactor. Internal liner corrosion is caused by the changes of reaction medium and processing technology. External layer corrosion was aroused mainly by steam. Corrosive deposits such as alkaline ions and chloride ions brought by leak steam would concentrate at the interlayer clearances, which would induce corrosion and cracking of outer layers. In addition, the internal reaction medium leakage would generate strong corrosion of external layers, and the steam medium containing chloride ions can also lead to outer side stress corrosion cracking of the liner.
Finite element analysis (FEA) of a urea reactor was conducted. Factors of heat stress, material nonlinearity, friction constitutive relations and interlayer clearances as well as heavy weld structures were considered during the analysis. The results showed that under the operating pressure, the internal FEA stress values and Pimshtein formulas calculation results of the cylinder layers had a gradually decreasing trend radially from the inside to the outside surface. The stresses of inner layers were higher than Lame formulas calculation results, whereas outer layer stresses were lower than Lame formulas calculation results. With the increasing of clearances between layers, the internal layer stresses increased, and the stress of outer layers decreased, which lead more external layers to an idle state. Therefore, the cracking possibility of inner layers which endure working pressure increased. As for urea reactors with 0.25 mm gaps, only seven layers endure the inner pressure. When the thickness exceeds more than seven layers, the stresses of layers nearly tend to zero. When considering the impact of temperature, the heavy weld stress generally increased and the maximum stress transfer to the annulus layer. Elastic-plastic analysis of reactors with the clearances of 0.25 mm showed that the stress states of some internal layers had exceeded the material yield limit to a plastic state.
Since urea reactor has gaps between layers, the layers showed tension states in the inner side and compression states in the outer side under pressure. The stress states between two sides became larger when nearing the heavy weld, and eventually generated great stress concentrations at the weld toe. The stress states showed strong locality in the weld toe, and decline rapidly when leaving away from it. Under both operation pressure and temperature, the stresses redistributed, and the inner layer stress state relieved, but the weld toe was still the most stress concentrated area. Local yield zone occurred at the weld toe of the liner, the annulus and the first strength layer. The yield zone decreased gradually from inner surface to the outside, and the geometrical shape conform to the actual crack propagation direction found during dissection.
The reason of crack initiation of the urea reactor strength layer and the weld were analyzed. Lining corrosion and leakage, strength layer cracking and the explosion type of the urea reactor were summarized. The macro inspection, dissection analysis, microstructure analysis, as well as deposition and corrosion product analysis results showed that the strength layer cracks were caused by stress corrosion cracking. Statistics of crack lengths and cracking positions showed that cracks have the characteristics of initiating at the weld parts. The reasons for urea reactor cracking were studied from the view of material, stress, corrosion environment. The results showed that the cracks were initiated by a variety of factors such as material defects, local stress concentration and leak detection medium condensation. Multi-cylinder deep seam welding produced large widmanstaten organizations, which reduced the material structure uniformity. On the other hand, leak steam condensation in the layer clearances occurred, which produced stress corrosion cracking conditions. Heavy weld happened to stress corrosion and galvanic corrosion under the effect of leak detection liquid concentrated in the crevice of layers. Therefore, urea reactor cracking occurred, and expanded outward along the direction perpendicular to the main stress. The strength layer cracking form of urea reactors have some relation to leak detection steam pressure, leak detection steam contamination condition, leak detection structure and connectivity condition between interlayer clearances and outside environment. The most serious stress crossing cracking would occur when the following conditions happened simultaneously: high leak detection steam pressure, interlayer clearances filling with steam and the clearances were blinded to the outside environment. When Leak steam pressure was relatively low, the steam which accessed into the interlayer clearances did not have the condensation conditions; stress corrosion cracking would occur only in the external layers and weld toes. When the leak detection steam dew point temperature is below the outmost layer temperature, layer cracking will be arrested. When the leak detection steam pressure was relatively high, the leak detection steam which entered the clearances has the condensation conditions. Multi-points corrosion concentration would happen at the weld toe, which would induce stress corrosion cracking of inner layers and welds. In addition, the liner leakage and process system leakage caused by leak detection steam pollution also can cause layer corrosion cracking.
The existed defects of the urea reactor were detected and comprehensively evaluated using acoustic emission, ultrasonic phased array examination,γ-ray detection and metal magnetic memory methods. First, comparison analysis of acoustic emission, phased array,γ-ray examination and metal magnetic memory was conducted according to the characteristics of urea reactor inspection. Results indicated that only give full play to the advantages of these detection methods, can the urea reactor integrity detection be completed. Second, acoustic emission examination was conducted to a urea reactor. Results showed that, with increment of the test pressure, apparent acoustic emission location signals come forth in the stress concentration sites especially in the heavy weld toe, which were in consistent with the stress distribution pattern of FEA results. Subsequent dissection results confirmed that the acoustic emission source locations were heavy weld radial expansion cracks. Acoustic emission signal characteristics were statistically analyzed, and defects characteristic parameters were acquired. Heavy weld sections of the urea reactor were imaged using ultrasonic phased array system. Results showed that the ultrasonic phased array detection method can clearly show crack type reflectors and crack latent depth, thus weld toe cracks, weld defects and layer mismatch etc. could be directly detected. The length and depth of cracks could be measured through this method, which could provide data for decision-making of equipment maintenance and defect assessment. This method was especially useful for acoustic emission source location re-inspection acquired by acoustic emission examination.γ- ray testing results showed that theγ-ray crack detection rates were very low to the large tilt angle cracks, only when the acoustic emission signals positioned on layers or weld acoustic emission signal could not efficiently detected,γ-ray re-inspection was needed to be utilized. Metal magnetic memory measurement was used for the testing of the localized stress concentrations; the buried crack detection reliability of urea reactor welds needs further study. Results showed that the use of acoustic emission testing technology combining ultrasound phased array andγ-ray non-destructive testing method had higher reliability and efficiency for urea reactor integrity situation evaluation. Combination of these methods can effectively find out heavy weld defects, and had prosperous advantages for testing multilayer structures similar to multilayer urea reactors.
Safety evaluation of urea reactors containing defects was studied. First, cracks in the urea reactor had been simplified on the principle of safety. Cracks in the urea reactors were ruled into two categories: Circumferential cracks with surfaces perpendicular to the axial and axial cracks with surfaces parallel to the axial. Then, the urea reactor crack stress intensity factors were calculated in accordance with the FEA results and actual crack sizes acquired during urea reactor dissection. Therefore, the initial crack size and critical crack size expression formulas of urea reactors were acquired. The urea reactor remaining life could also be estimated. On this basis, urea reactor remaining life curve was given. The remaining life of the urea reactors could be estimated during the acoustic emission testing process in accordance with the acoustic emission source location appearance pressures.
In short, urea reactor stress distribution and liable failure location were studied by urea reactor structure and finite element analysis, crack initiation analysis, non-destructive testing method study and safety assessment method investigation. This work provided a complete set of solutions for urea reactor inspection and integrity evaluation to ensure the urea reactor safety operation, and had tremendous economic and social benefits. However, urea reactor failures are closely related to their design, manufacture, installation, usage, inspection, repair and transformation. Each factor could have important impact on the failures of urea reactors. Therefore, it is also necessary to carry out further research in these areas, so as to take corresponding measures to prevent urea reactor accidents.
引文
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