Failure Study Of Waste Heat Boiler In The Secondary Reformer Of Ammonia Plant

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 separation 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.