合成氨二段转化废热锅炉失效研究
|
摘要
本文研究合成氨二段转化废热锅炉管板-管子连接部位的裂纹穿透失效及其影响因素，为废热锅炉的运行、检修、更新提供依据。
论文首先对合成氨二段转化废热锅炉失效特征、原因进行了分析，发现失效的直接原因是水质、列管温度和水的局部循环状态共同作用下产生的大面积结垢和严重腐蚀穿孔；而列管更换中焊接质量差亦是造成废热锅炉多次泄漏的重要原因。
其次，对管板及其与管子连接处角焊缝焊接应力进行了有限元模拟计算；根据疲劳裂纹扩展判据和列管-管板角焊缝焊接应力形成原理，用断裂力学方法计算了焊接接头裂纹尺寸、临界裂纹尺寸和疲劳寿命，进一步说明疲劳裂纹扩展速度，表明气侧焊缝区与近焊缝区存在很高的焊接残余应力；在交变载荷作用下，高应力区易发生疲劳裂纹扩展，导致废热锅炉多次泄漏。
同时还对废热锅炉进行了热力计算与分析，发现废热锅炉的热负荷设计是保守的，未超过临界负荷，水循环平均特性合理，但局部水循环不尽合理，导致局部结垢、升温和腐蚀。
根据所分析的废热锅炉失效原因，提出了防止废热锅炉失效的措施。在运行时，确保水质指标，控制开车、停车速度，保持装置运行稳定；在列管出现大量泄漏后需要更换列管时，注意取管和列管焊接顺序，保证焊接质量，焊后热处理时，防止出现加热板断电现象，造成管板上存在巨大焊接应力；在管板上出现少量泄漏时，采用金相、着色、磁粉、测厚、气密性试验、水压试验等方法进行查漏，消除裂纹后补焊，并进行焊后热处理；在结构上，建议调整下降管位置，增加中间隔板，改善局部水循环；在管口保护上，保护套管长度增长；在管箱保护上，将刚玉质浇注料改为刚玉砖。
The paper studied the failure caused by the crack propagation occurring in the welding joints between tube-sheet and tube-bundle of the waste heat boiler(WHB) in the secondary reformer of synthetic ammonia plant, so that correct measures and procedures can be provided for the operation, maintenance and renovation of WHB.
The failure characters and reasons of WHB were analyzed first, showing that the direct reason for the failure is due to the serious corrosion and fouling resulted from unqualified supply-water, high tube-temperature and bad water-circulation in the local region. It is found that the improper welding procedure is also an important reason for the repeated leakages of WHB.
Next, a finite element computation was performed for the stress in welding joint and the tube-sheet. According to fatigue crack extension and the principle of formation of welding stress in the welds between tubes and tube-sheets, author calculated the weld-crack size, the critical crack size and the estimated fatigue life by means of fracture mechanics and, further, described the speed of the fatigue crack extension. It is shown that there exists very high welding residual stress on the welding joints and their adjacent area in gas side. Under the alternative load, fatigue crack extension tends to happen in the high-stress area, resulting in repeated leakages of WHB.
The thermal calculation and analysis were also performed and shown that the thermal load design for WHB is conservative, less than the critical load, and the average characteristics for water cycle is reasonable. However, the local circulation of water is not in a normal condition, leading to temperature rise, fouling and corrosion happened in the local area.
Based on the above work, the measures to prevent the failure of WHB were proposed. In order for the operation of WHB to be steady, the quality of supply-water and the start-up/stop speed of the unit should be controlled within the designed conditions. In the case that serious leakage appears in tubes, the correct procedures should be followed to replace the broken tube and weld the new tubes on the tube-sheet. During the welding process and post heat treatment, the measures should be taken to prevent the power failure of heating plates, in order not to result in high welding stresses on tube-sheet. In the case of slight leakage founded on tube-sheet, the checking measures such as metallurgic-graphics, MT, PT, thickness checking and gas tight test or water tight test must be used to find the specific leakage locations, and then perform the repair welding and post heat treatment to eliminate the cracks. Regarding the structure of WHB, it is suggested to adjust the location of the down-comer tubes and add a sep
aration plates among them to improve the partial water cycle. It is also suggested to use a longer protective housing of tube for the protection of the tube end, and to replace the corundum castings with corundum brick to protect the tube box.
论文首先对合成氨二段转化废热锅炉失效特征、原因进行了分析，发现失效的直接原因是水质、列管温度和水的局部循环状态共同作用下产生的大面积结垢和严重腐蚀穿孔；而列管更换中焊接质量差亦是造成废热锅炉多次泄漏的重要原因。
其次，对管板及其与管子连接处角焊缝焊接应力进行了有限元模拟计算；根据疲劳裂纹扩展判据和列管-管板角焊缝焊接应力形成原理，用断裂力学方法计算了焊接接头裂纹尺寸、临界裂纹尺寸和疲劳寿命，进一步说明疲劳裂纹扩展速度，表明气侧焊缝区与近焊缝区存在很高的焊接残余应力；在交变载荷作用下，高应力区易发生疲劳裂纹扩展，导致废热锅炉多次泄漏。
同时还对废热锅炉进行了热力计算与分析，发现废热锅炉的热负荷设计是保守的，未超过临界负荷，水循环平均特性合理，但局部水循环不尽合理，导致局部结垢、升温和腐蚀。
根据所分析的废热锅炉失效原因，提出了防止废热锅炉失效的措施。在运行时，确保水质指标，控制开车、停车速度，保持装置运行稳定；在列管出现大量泄漏后需要更换列管时，注意取管和列管焊接顺序，保证焊接质量，焊后热处理时，防止出现加热板断电现象，造成管板上存在巨大焊接应力；在管板上出现少量泄漏时，采用金相、着色、磁粉、测厚、气密性试验、水压试验等方法进行查漏，消除裂纹后补焊，并进行焊后热处理；在结构上，建议调整下降管位置，增加中间隔板，改善局部水循环；在管口保护上，保护套管长度增长；在管箱保护上，将刚玉质浇注料改为刚玉砖。
The paper studied the failure caused by the crack propagation occurring in the welding joints between tube-sheet and tube-bundle of the waste heat boiler(WHB) in the secondary reformer of synthetic ammonia plant, so that correct measures and procedures can be provided for the operation, maintenance and renovation of WHB.
The failure characters and reasons of WHB were analyzed first, showing that the direct reason for the failure is due to the serious corrosion and fouling resulted from unqualified supply-water, high tube-temperature and bad water-circulation in the local region. It is found that the improper welding procedure is also an important reason for the repeated leakages of WHB.
Next, a finite element computation was performed for the stress in welding joint and the tube-sheet. According to fatigue crack extension and the principle of formation of welding stress in the welds between tubes and tube-sheets, author calculated the weld-crack size, the critical crack size and the estimated fatigue life by means of fracture mechanics and, further, described the speed of the fatigue crack extension. It is shown that there exists very high welding residual stress on the welding joints and their adjacent area in gas side. Under the alternative load, fatigue crack extension tends to happen in the high-stress area, resulting in repeated leakages of WHB.
The thermal calculation and analysis were also performed and shown that the thermal load design for WHB is conservative, less than the critical load, and the average characteristics for water cycle is reasonable. However, the local circulation of water is not in a normal condition, leading to temperature rise, fouling and corrosion happened in the local area.
Based on the above work, the measures to prevent the failure of WHB were proposed. In order for the operation of WHB to be steady, the quality of supply-water and the start-up/stop speed of the unit should be controlled within the designed conditions. In the case that serious leakage appears in tubes, the correct procedures should be followed to replace the broken tube and weld the new tubes on the tube-sheet. During the welding process and post heat treatment, the measures should be taken to prevent the power failure of heating plates, in order not to result in high welding stresses on tube-sheet. In the case of slight leakage founded on tube-sheet, the checking measures such as metallurgic-graphics, MT, PT, thickness checking and gas tight test or water tight test must be used to find the specific leakage locations, and then perform the repair welding and post heat treatment to eliminate the cracks. Regarding the structure of WHB, it is suggested to adjust the location of the down-comer tubes and add a sep
aration plates among them to improve the partial water cycle. It is also suggested to use a longer protective housing of tube for the protection of the tube end, and to replace the corundum castings with corundum brick to protect the tube box.
引文
[1] 古大田，方子凤编．废热锅炉[M]．北京：化学工业出版社，2002
[2] N.M. Nimue Energy recovery in Ammonia Synthysis Plants in The Chemical Engineer[C], July/Auge.,1987, No. 210. 156～162
[3] 何汉机．火管锅炉的设计与选型[J]．大氮肥，1993，16(2)：81～86
[4] 姚灿龙．第一废热锅炉换热管爆裂原因分析及对策[J]．大氮肥，2001，4：249～252
[5] 董进礼．第一废热锅炉承压壳体爆裂原因分析及修复[J]．大氮肥，2001，4：253～255
[6] 化工部化工机械研究院．中国核工业建峰化工总厂转化气废热锅炉事故分析[M]．1994
[7] 吴鸿欣．一段转化废热锅炉炉管腐蚀的探讨[J]．大氮肥，1996，19(1)：55
[8] 蔡哲高．金属磨损与断裂[M]．上海交通大学出版社，1985
[9] 钱在中．焊接技术手册[M]．山西科学技术出版社，1999
[10] 机械工程材料性能数据手册[M]．机械工业出版社，1995
[11] 韩玉林．100E8转化废热锅炉无损检测与寿命评估[M]．东南大学
[12] 法国TP公司提供，大连理工大学译．建峰化工总厂化肥厂100E8二段废热锅炉机械计算、材料证明书、焊接数据及检验、检查和试验报告(原文及译文)，第Ⅵ卷第7．3～8．3部分[M]
[13] 英国标准协会．PD6493-91焊接结构缺陷验收评定方法指南[S]
[14] 田锡唐主编．焊接手册3[M]．北京：机械工业出版社，1992
[15] T．G．Gurney著，周殿群译．焊接结构疲劳[M]．机械工业出版社，1988。
[16] 法国TP公司．建峰化工总厂化肥厂100E8数据手册[M]
[17] Chang Woo Lee. Waste-heat boilers[M]. 1987, 2—112
[18] R. W. Horton. Boilers in the Process Industries[J]. 1995, Auguest: 451—454
[19] 清华大学锅炉教研组．二段转化气废热锅炉设计计算(四)、(五)[M]．石油化工设计，5：12～19，1984
[2] N.M. Nimue Energy recovery in Ammonia Synthysis Plants in The Chemical Engineer[C], July/Auge.,1987, No. 210. 156～162
[3] 何汉机．火管锅炉的设计与选型[J]．大氮肥，1993，16(2)：81～86
[4] 姚灿龙．第一废热锅炉换热管爆裂原因分析及对策[J]．大氮肥，2001，4：249～252
[5] 董进礼．第一废热锅炉承压壳体爆裂原因分析及修复[J]．大氮肥，2001，4：253～255
[6] 化工部化工机械研究院．中国核工业建峰化工总厂转化气废热锅炉事故分析[M]．1994
[7] 吴鸿欣．一段转化废热锅炉炉管腐蚀的探讨[J]．大氮肥，1996，19(1)：55
[8] 蔡哲高．金属磨损与断裂[M]．上海交通大学出版社，1985
[9] 钱在中．焊接技术手册[M]．山西科学技术出版社，1999
[10] 机械工程材料性能数据手册[M]．机械工业出版社，1995
[11] 韩玉林．100E8转化废热锅炉无损检测与寿命评估[M]．东南大学
[12] 法国TP公司提供，大连理工大学译．建峰化工总厂化肥厂100E8二段废热锅炉机械计算、材料证明书、焊接数据及检验、检查和试验报告(原文及译文)，第Ⅵ卷第7．3～8．3部分[M]
[13] 英国标准协会．PD6493-91焊接结构缺陷验收评定方法指南[S]
[14] 田锡唐主编．焊接手册3[M]．北京：机械工业出版社，1992
[15] T．G．Gurney著，周殿群译．焊接结构疲劳[M]．机械工业出版社，1988。
[16] 法国TP公司．建峰化工总厂化肥厂100E8数据手册[M]
[17] Chang Woo Lee. Waste-heat boilers[M]. 1987, 2—112
[18] R. W. Horton. Boilers in the Process Industries[J]. 1995, Auguest: 451—454
[19] 清华大学锅炉教研组．二段转化气废热锅炉设计计算(四)、(五)[M]．石油化工设计，5：12～19，1984