直硝法生产浓硝酸反应釜内筒及附件的腐蚀和结构的研究/study on corrosion and structure about the reacting ke

2019-01-06 13:47:33

腐蚀 corrosion inner vessel C8



直硝法生产浓硝酸工艺源至前苏联。经过近五十年的发展,反应釜内筒一直采用高纯铝材。其使用寿命不足1.5年,腐蚀速度快,焊缝经常发生穿孔腐蚀,安全性能差。前人虽然在制造方面做了一些探索,但在材料应用上没有发生根本性的变化,效果仍不是很理想。考虑C8钢的设计条件与反应釜内筒工艺条件有相似之处,将这已有的研究成果进行深度的开发和利用就有重大意义。
本课题是结合企业的生产特点所做的应用研究,它涉及两个研究领域:金属腐蚀与选材;压力容器结构分析。基于上述内容,建立了C8钢腐蚀模型,以A、B两组作为对照组,每组17根规格为Ф32×2.5的短管,均匀分布在反应釜内筒里。实验按照工业生产条件,历经3672小时,取得如下一些成果:
1)运用一个简单的腐蚀率计算公式,近似地算出反应筒内的腐蚀率,绘出了腐蚀率曲线,并揭示出C8钢在反应釜内筒的腐蚀规律。
2)通过C8钢腐蚀后的外观照片发现,A、B两组腐蚀都很严重,但A组(固溶态)要好于B组(敏化态);反应筒下部的C8钢管比上部的腐蚀更严重。
3)通过观察金相组织发现,母材为单相孪晶奥氏体,焊缝组织为奥氏体+δ铁素体。由此说明在加工坡口的时候或焊接过程中有铁素体污染,造成焊缝晶间腐蚀。
4)通过腐蚀产物的化学成分分析发现,腐蚀产物主要是SiO2,证实了C8钢中的硅是主要的钝化元素。
课题的第二方面内容就是利用有限元对反应釜内、外筒进行结构分析。按照设计条件,计算了内、外筒位移变形,并校核了外筒的强度,得到如下结果:
5)内、外筒的最大位移都小于1mm,对于设备而言在工况下变形非常小。
6)在外筒的危险截面上进行路径分析,结果是计算应力强度远小于许用应力强度,截面强度合格。




The process of concentrated nitric acid which produced using direct method started in the former Soviet Union. During the nearly 50 years development, inner vessel of the reacting kettle is still made by the high pure aluminum material, which service life is less than 1.5 years, which corrosion speed is very quick and which welded joint corroded frequently to perforation, so the degree of safty is lower. The predecessors have made some explorations in the manufacture aspect, but made less progress in the applying of material. The effect still was not very satisfactory. Since there is similarity between the C8 steel’s design conditions and the process conditions in the inner vessel of reacting kettle, it will have the great significance to develop the research results deeply.
This task is the applied research which according to the enterprise condition, it involves two research fields: Metal corrosion and selection; Pressure vessel structure analysis. Based on the above contents, we has established the C8 steel corrosion model. The writer took A group and B group as comparison, and each group has 17 short pipes which uniformly distributed in the reacting kettle’s inner vessel. The experiment has been for 3,672 hours according to the industrial condition. the following results are obtained :
1) Using a simple corrosion rate formula, the writer approximately figured out the corrosion rate in the reacting kettle’s inner vessel, drew the corrosion rate curve, and promulgated the C8 steel’s corrosion rule in the reacting kettle’s inner vessel.
2) Through the appearances of C8 steel pipes which have been corroded, we discovered that: both A group and B group corroded very seriously, but A group (melting condition) is better than B group (sensitized condition); the C8 steel pipe in the bottom of reacting kettle’s inner vessel is more serious than near top.
3) Through observing microstructure, we discovered that: parent metal is single-phase twin crystals austenite, and welded joint organization is austenite + δ ferrite. There is the ferrite pollution in the beveling process or the welding process, which creates the welded joint intercrystalline corrosion.
4) Through analysis of the corrosion product, we discovered that: the corrosion product mainly composed of SiO2, which confirmed that silicon is the main passivation element of the C8 steel.
The second aspect of the task is structure analysis about the reacting kettle’s inner vessel and outer vessel by using the finite element method. Under the design conditions, the writer calculated the displacement, and examined the outer vessel’s strength. We obtain the following results:
5) Both outer vessel’s and inner vessel’s maximal displacement are smaller than 1 mm, which tell us that the equipment deformation under the operating case is extremely slight.
6) After the analysis through the outer vessel’s dengerous section,the result is that the calculating stress intensity is smaller than the allowable stress intensity, so the section intensity is qualified.