SRT-Ⅲ型裂解炉炉管弯曲变形原因及其对策

通过对裂解炉辐射段炉管组的结构及炉管悬吊形式分析，指出裂解炉辐射段炉弯曲变形主要原因，并根据这些原因提出相应的对策。     

GK-Ⅵ型裂解炉辐射段炉管弯曲原因分析及防范措施

GK-Ⅵ型裂解炉在投料运行3年后,发生了严重的炉管弯曲现象。文中就此问题进行多方面的分析探讨,得出：炉管金属表面的周向温度差不是炉管弯曲的主要原因,炉管弯曲是材料高温下的蠕变引起的。并对此问题提出整改方案和预防措施。     

天然气蒸汽转化炉运行中存在的问题及解决措施

通过对天然气蒸汽转化顶部烧嘴转化炉在运行中出现的转化管在距顶部2～4米范围蠕变破裂、转化管弯曲变形、烧嘴火焰刚性差、火焰局部添管等异常现象和长周期运行时间短的问题进行技术分析,在此基础上从燃烧系统、转化炉炉管材质和维护保养、炉管恒力弹簧支吊架的载荷与热位移方面提出了切实有效的解决措施,并予以实施,取得了良好的效果,延长了装置长周期运行时间。     

SC-1型裂解炉炉管变形原因与对策

文章以SC-1裂解炉为研究对象,首先介绍了某企业运用该裂解炉生产情况,并对该设备进行了详细说明,其次对炉管情况进行了分析,内容主要包括设备参数、变形情况,最后围绕炉管变形原因及对策展开了讨论,指出致使炉管出现变形问题的原因较多,包括但不限于渗碳、蠕变等,要想将问题出现概率降至最低,关键是要酌情采取相应的解决措施,使炉管运行状态趋于稳定。     

辐射段炉管常见失效形式的分析与预防

总结了乙烯装置裂解炉辐射段炉管的常见失效形式,如：渗碳造成的损伤、热冲击造成的损伤、热疲劳造成的损伤、冲刷造成的炉管减薄、高温蠕变损伤、炉管弯曲等。针对各失效形式分别进行了分析,并就此在炉管选材、机械设计、制造、包装及现场安装、裂解炉运行管理等各方面提出了相应的改进与预防措施,以期对延长炉管的使用寿命有所裨益。     

测定炉管蠕变损伤分布的新方法

根据薄板弯曲自由振动理论，利用差分法从数值上计算了悬臂薄析及其附加集中质量时的基本频率，导出了基本频率间的相关关系，给出了用加速度计测定炉管材料的弹性模量的方法，确定了炉管蠕变损伤沿径向的分布。其结果与超声波探伤结论相吻合。     

用C形试样研究HK——40炉管蠕变裂纹扩展

高温下工作的炉管因蠕变裂纹的扩展导致了炉管的失效.本文采用C形试样进蠕变裂纹扩展试验,并用弹性应力强度因子K,净截面应力σ_(net)以及蠕变条件下的能量积分C~*,对获得的不同温度和不同载荷下的裂纹扩展数据进行处理,结果表明:C~*能够较好地描述蠕变裂纹扩展的整个过程.把材料的蠕变性能与裂纹扩展性能联系在一起,进一步讨论了温度的影响.对炉管受力情况进行简化,把断裂力学的方法应用到炉管的残余寿命估计中去,获得了初步的结果.     

裂解炉用36XS炉管开裂的原因分析

通过对裂解炉管进行宏观变形检查、化学成分分析、显微组织分析和力学性能分析等,找出炉管开裂的失效原因。结果表明：裂解炉管变形开裂的主要原因是由于该处实际使用温度偏高,因而造成渗碳和氧化较严重。氧化减薄使炉管膨胀蠕变,渗碳不均匀,由此造成炉管严重鼓胀变形直至开裂。     

加热炉固定式炉管管排结构弯曲变形的研究

本文分析了某加热炉固定式炉管管排产生轴向弯曲变形的原因，提出炉管改进意见，并已在生产中实施。     

蒸汽过热炉炉管稳定性安全分析

为分析一蒸汽过热炉炉管弯曲变形原因,对辐射过热段炉管进行传热、柔度计算和稳定性分析,得出炉管失稳原因是烘炉时管壁温度过高,自由膨胀受到约束;还就该失效炉管能否继续使用进行了简单的安全性分析。     

Creep and Creep Fracture in Hot-Pressed Alumina

The creep and creep fracture behavior of two hot-pressed aluminas are presented, for both flexural and tensile testing. Steady-state power-law creep is observed with a stress exponent of about 2 for each material. Three distinct fracture regimes are found. At high stress in flexure, fracture occurs by slow crack growth with a high stress dependence of the failure time. At intermediate stresses, in both flexure and tension, creep fracture occurs by multiple microcracking after modest strains. Failure times exhibit a modest stress dependence (stress exponent of 2.5 in tension and 3 in flexure), with a constant failure strain equal to 0.09. The failure times are considerably longer in flexure than in tension, because of the constraint imposed on crack growth by the bending geometry. We conclude that flexure cannot be used for creep lifetime assessment, even in simple, single-phase materials such as Al₂O₃. At low stresses, in tension, failure also exhibits a modest stress dependence but with a much higher failure strain. The material shows the onset of super-plastic behavior.     

Creep Behavior of a SiC-Whisker-Reinforced Alumina

Creep tests in four-point flexure loading configuration in air employing applied stresses of 37 to 300 MPa at temperatures of 1200°, 1300°, and 1400°C were performed on 20-vol%-SiC-whisker-reinforced alumina and unreinforced single-phase polycrystalline alumina. The creep rate of polycrystalline alumina was significantly reduced through the addition of SiC whiskers, although strain to failure was lower. Transmission and scanning electron microscopy results suggest that substantial increase in the creep resistance in flexure of alumina composites originates from the retardation of grain-boundary sliding by the SiC whiskers.     

Creep Behavior of Si3N4-Whisker-Reinforced Si3N4-Matrix Composites Using an In Situ Dynamic Beam Profiling Method

A four-point flexure test, capable of simultaneously measuring load-point and beam-center displacements and permitting in situ profiling of the beam throughout the test, has been developed. Compressive and tensile strains in the outer fibers, together with the neutral axis displacement, were determined in real time for Si₃N₄-whisker-reinforced Si₃N₄-ceramic-matrix composites. Combining these data with the closed form solution of Chen and Chuang gave excellent correspondence between the creep exponents determined from flexure tests with those determined independently from tensile and compressive creep tests.     

Effect of creep on the mode II residual fracture energy of adhesives

Viscous flow that often occurs in adhesive materials leads to a permanent deformation when adhesives are subjected to creep loading. Creep loading has a significant influence on the strength of bonded structures. Due to the viscous behavior, the fracture energy also may change with time for joints that experience creep loading in service. In this work the effects of two creep parameters (creep load and time) on the residual mode II fracture energy of an adhesive was investigated using end notched flexure (ENF) specimens. To achieve this, ENF samples were subjected to different creep loading levels at different creep times followed by quasi static tests to obtain the residual shear fracture energy of the adhesive. Experimental results showed that pre-creep loading of the bonded structures can significantly improve the fracture energy and the static strength of the joints.     

Role of Oxidation in the Time-Dependent Failure Behavior of Hot Isostatically Pressed Silicon Nitride at 1370°C

Dynamic fatigue studies were conducted on a hot isostatically pressed silicon nitride in ambient air and inert (argon or nitrogen) environments using four-point flexure at 1370°C. Specimens tested in ambient air exhibited a stressing rate dependence with decreased flexure strength with decreased stressing rates. All fracture surfaces of specimens tested in ambient air possessed a sweeping stress-oxidation damage zone that originated at the tensile side of each bend bar. In addition to this stress-oxidation damage, creep damage (e.g., cavitation) was concurrently observed in the specimens tested at the slower stressing rates, which appeared to further weaken the material. However, tests conducted in argon or nitrogen revealed flexure strength to be independent of the stressing rate. Creep damage was present at the slower stressing rates, but no stress-oxidation damage was evident similar to that observed on the specimens tested in ambient air. By decoupling the effects of oxidation and creep, it was evident that the former contributed to the formation of a detrimental stress-oxidation damage zone which significantly reduced the strength of this material at 1370°C.     

Depletion of Grain-Boundary Phase and Elastic Creep Deformation Upon Ultra-Long Flexural Stress Rupture Experiments of NC-132 Silicon Nitride

A unique grain-boundary structure evolution was observed in two MgO-doped silicon nitride specimens (Norton, NC-132) that were tested in ultra-long flexure stress rupture experiments with an applied stress of 266 MPa and fractured at 14 941 and 17 376 h. Transmission electron microscopy showed that, although the starting material had a secondary glass phase both at multi-grain junctions and along grain boundaries, the tested specimens contained no residual glass phase. Concurrent with the elimination of the secondary glass phase, a continuous network of cracked grain boundaries was observed after long-term flexure testing consistent with the concept of elastic creep. It is, therefore, concluded that at ultra-long annealing times, this material is affected by creep deformation via microcrack nucleation and growth due to the depletion of the amorphous siliceous grain-boundary phase, which is seen as a truly transient, fugitive secondary phase.     

Delayed yielding of epoxy resin. II. Behavior under constant stress

Creep tests were carried out on epoxy resin specimens at room temperature and at different high stress levels under tension, compression, and flexure. Compared with the behavior at constant strain rate (CSR) reported in Part I of this work, creep strain–time curves revealed a distinct delayed yielding region of constant minimum rate (secondary creep) followed by a post-yielding region of increasing slope (tertiary creep). In all cases, results indicate linearity between creep stress and log secondary creep rate, which is almost coincident with the corresponding relationship between yield stress and strain rate obtained in subsequent CSR loading cycles with the same specimens. The similarity in behavior under both the creep and CSR modes conforms to Eyring's theory of non-Newtonian viscous flow at high stress levels and low temperature. Theoretical analysis yields reasonable values of the activation volume, which is unaffected by the loading and test modes or by loading history, and could thus be regarded as an intrinsic parameter of the microstructure, inherently related to the viscoplastic process involved. The above considerations indicate a deviatoric stress-biased diffusional mechanism as the predominant factor in the yielding of an amorphous glassy epoxy system.     

Observed and Theoretical Creep Rates for an Alumina Ceramic and a Silicon Nitride Ceramic in Flexure

Observed creep curvature rates are compared to theoretical rates for both an alumina ceramic at 1000°C and a silicon nitride ceramic at 1200°C in four-point flexure. The observed rates have been calculated from published rise-displacement rates, and the theoretical rates have been calculated from published power-law parameters for compressive and tensile creep, which differ appreciably for these ceramics. Although both compressive and tensile creep measurements are easier to analyze than flexural creep measurements, the latter are usually less expensive and easier to conduct. The present work shows the usefulness of flexural creep tests to verify the accuracy of compressive and tensile creep tests.     

Creep Deformation of Ceramics in Four-Point Bending

Flexural testing was investigated as a method of studying the creep of ceramic materials at elevated temperatures. Three techniques were used to evaluate the steady-state stress exponent: two of them were based on the measurement of the surface curvature of a flexure specimen after testing; the third was based on the more conventional technique of measuring displacement rate as a function of applied load and exposure time. Applied to a grade of commercial vitreous-bonded alumina, the techniques yielded disparate results for the steady-state stress exponent. This discrepancy in results is believed to be a consequence of the fact that ceramics tend to creep more readily in tension than in compression, leading to a shift in the neutral plane for stress and strain in flexural specimens, which results in extended primary creep. A local enhancement of creep under the loading points of the test specimens was observed in the materials tested; this creep enhancement was attributed to contact stresses at the loading points.     

Some mechanical properties of rigid polyurethane structural foam

The mechanical response of integral-skin rigid polyurethane foam, with an average density of 300 to 700 kg/m³, to constant rate and creep loading was determined. Sandwich specimens were modeled by layers of a core material and two skins, whose secant moduli had been determined experimentally by separate tests and approximated by linear functions of the density. The effective rigidities of the sandwich in tension and flexure were calculated and compared favorably to experimental measurements. The sandwich structure improved the flexural rigidity of homogeneous foam by a factor of more than 2.20. Tensile creep tests of sandwich specimens at relatively low stress levels (up to about 38 percent of their strength) showed that the creep was nonlinear, but a single creep curve could represent creep of specimens of various densities, provided the relative load on them was the same. A limited number of flexural creep tests led to similar conclusions, but the creep rate was smaller than in tension. Results from torsion tests of core material, compressive tests of sandwich specimens, and tension and compression tests of nonskin rigid foam are included in this article.