减压塔顶水冷器支撑梁开裂失效分析

Analysis of Cracking Failure of Water Coolers' Support Beam in Overhead of Vacuum Tower in Petrochemical Plant

某石化公司进行常规检修过程中发现减压塔顶水冷器发生严重腐蚀。该水冷器设备材质为304不锈钢,在运行过程中发生明显孔蚀,并且伴有支撑梁的开裂行为。该水冷器管内介质为循环水,壳程介质为油气,入口温度为95℃,出口温度为47℃。通过金相显微镜和扫描电镜对失效件裂纹及孔蚀形貌进行表征,并通过电子能谱和洛氏硬度仪对支撑梁失效件、腐蚀产物进行了电子能谱扫描和材质硬度分析。结果表明,支撑梁材质Ni和Cr元素含量偏低,不符合304不锈钢标准,材质硬度为HRC 32,远超过设备硬度设计要求,在腐蚀产物中检测出硫元素和氯元素的存在。综合以上结果,减压塔顶水冷器支撑梁开裂行为主要是由于设备材料硬度大于HRC 22,在油气中的HCl及H_2S遇到冷凝水形成HCl-H_2S-H_2O酸性环境下,发生了典型的湿H_2S环境下的应力腐蚀开裂。针对该失效机理,提出了相应的防护措施。

Serious corrosion was found in the water cooler of the vacuum tower overhead in a petrochemical plant. Obvious pitting appeared on water cooler which was made of stainless steel 304,accompanied by the support beam cracking. The flow media in the tube side of the cooler was cooling water and that in the shell side the oil vapor. The inlet temperature was 95 o C and outlet temperature was 47 o C. Morphology of crackings and pittings are characterized by Metallurgical Microscope and the Scanning Electron Microscopy( SEM). Meanwhile the base metal and the corrosion products were analyzed by Energy- dispersive X- ray( EDX) and the Rockwell Hardness Tester. The results show that the Ni and Cr in the material of the beam is low and can not meet the specifications of 304 stainless steel,and that the hardness of the material is HRC 32 which greatly exceeds the hardness requirement in equipment design. There are sulfur and chlorine in corrosion products. It is concluded that the cracking and pitting corrosion of water coolers are mainly due to the hardness of the equipment materials which is higher than 22 HRC. Typical stress corrosion cracking,namely hydrogen embrittlement,occurred in wet hydrogen sulfide environment which is formed by hydrogen chloride( HCl),hydrogen sulfide( H_2S) and condensate water in the oil vapor. Appropriate corresponding protection measures are recommended according to the mechanisms.