表面蒸发式空冷器管束失效分析与防护研究

Failure Analysis of the Tube Bundle on Air Cooler of Alkylation Equipment

目的查明表面蒸发式空冷器管束失效原因,提出避免再次失效的对策。方法目测管束失效部位宏观形貌,用金相显微镜得到管束基体的金相组织,用电子显微镜观察管束失效部位的微观形貌,用能仪谱获得管束基体及其失效部位的化学成分等。结果目测可见失效部位管件表面有红褐色腐蚀产物存在,且管壁减薄非常明显,远离失效部位的管件表面完好;金相显微分析表明,管件基体金相组织与标准20#钢金相组织吻合;电子显微镜观察表明,管壁表面腐蚀产物疏松多孔;能谱分析表明,管件基体化学成分主要缺少了Cr、Ni、Cu这3种合金元素,管壁表面腐蚀产物化学成分由Fe、Zn、O、S组成。结论Cr、Ni、Cu合金元素的缺失导致了管基体耐蚀性能降低,管程内烃类介质的含硫组分和管壳外换热介质的氧成分成为腐蚀源,电偶效应下的全面腐蚀导致管件基体快速减薄而穿孔泄露。使用化学成分符合国标的20#钢生产管束,并保证管束表面镀锌层的完整性,尽量减少烃类介质的含硫组分和换热介质的氧等含量,可以避免管束再次失效。

In order to find out the failure mechanism of the tube on the air cooler and avoid its failure again.The macroscopic morphology of the tube was visually inspected,the metallographic morphology of the tube was analyzed with a metallographic microscope,the microscopic morphology of the tube was observed with a scanning electron microscope,and the chemical composition of the tube matrix and the corrosion products on the tube were analyzed with an energy spectrometer.Visual inspection showed that the thickness of the tube wall was significantly reduced and reddish-brown products were produced on the tube,but the surface of the tube away from the failure location was intact.The metallographic morphology of the tube matrix is consistent with that of standard 20#steel,and the products on the tube are loosely porous,with a chemical composition consisting mainly of Fe,Zn,O and S.The chemical composition of the tube matrix lacks three elements,Cr,Ni and Cu,and the material used for tube bundle manufacture is unsuitable,which reduces the corrosion resistance of the tube material and possesses the material factor for corrosion to occur.The tube bundle at the junction with the tube plate has both internal stress and local breakage of the surface galvanized layer caused by manufacturing factors,the latter leading to the formation of small anode and large cathode electric coupling pairs;under the combined effect of internal stress and electric coupling effect,the overall corrosion of the tube substrate is accelerated and eventually leaks due to thinning and perforation.The outer wall of the tube is in contact with oxygen-rich air or cooling water medium,O_(2) becomes the depolarizing agent of the electrochemical cathodic process;while the inner wall of the tube is a hydrocarbon medium containing sulfur components,H+ions become the depolarizing agent of the electrochemical cathodic process.In the oxygen-rich medium,the surface of the outer wall of the tube is very easy to occur oxygen depolarization electrochemical corrosion,while in the sulfur-rich medium,the surface of the inner wall of the tube is very easy to occur hydrogen depolarization electrochemical corrosion.Therefore,when manufacturing the air cooler,ensure that the material of the air cooler tube bundle is consistent with the design material to avoid large deviations.Avoid local damage to the galvanized layer as much as possible,and carefully check the galvanized layer on the outer surface of the tube bundle after manufacture,and repair any local damage as soon as possible;in addition,eliminate the internal stress at the connection as much as possible.Regularly check the connection between the air cooler bundle and the tube plate to ensure that there are no gaps in the connection and that the galvanized layer is not damaged;regularly remove the dust from the air cooler bundle and drain the water and dirt in the dead ends and low places to prevent the generation of corrosive media.