Failure Analysis And Anti-failure Design Of Ammonia Cooler For Raw Gas

The troubles happened to the ammonia coolers for raw gas during its work time in Honghua Ammonia Synthesized Plant were discussed. This kind of ammonia coolers for raw gas is a horizontal heat-exchanged apparatus. It was found that "inner leakage" appeared within two-month application after its installation. All the destroyed parts were examined and analysed. It was concluded that the phenomenon of "inner leakage" was caused by expansion and fracture through the axes of the heat-exchanged tubes.Methods, such as analysis, experiments, exclusion and calculation, were applied to find out the possible reasons of the failure of the ammonia cooler for raw gas. Relative anti-failure design was performed according to the likely reasons.Macro-structure examination, composition analysis and mechanical property tests were conducted to the destroyed tubes, and it was found that the latter two showed little difference between the invalid and uninvalid materials.The structures and the forming conditions of ball-shape pearlite of steel 20# were analysed. The results indicated that ball-structure forming in the ammonia cooler tubes couldn't cause failure. Then, factors, such as themetallurgical defects of the materials, the process of machining and molding, the corrosion in the ammonia coolers, water in the mediums applied during the work of ammonia coolers, the operating conditions (including natural gas provided, electricity provided, accidents of the equipments, variation of operating conditions and the skills of the workers), and others, which might be the possible reasons leading to the failure of the ammonia cooler for raw gas, were discussed. It was concluded that the main factors causing the failure of the ammonia coolers for raw gas include the following:Water in the medium applied during the work of the ammonia coolers leads to partial corrosion of 20# steel tube; Material 0Crl8Ni9 shows corrosion at the grain boundaries; Ice-block is another factor to cause the fracture of the tubes.Additionally, the amount of water produced by the reaction of raw gas to methane alkylation and by the condensation of raw gas going through water cooler and ammonia cooler, was calculated.In order to guarantee the stability of the conditions provided and the operation process, anti-failure design of the ammonia coolers for raw gas was made. Heat-rolled 20# steel was applied to make ammonia cooler for raw gas tubes. Something must be changed in the working process, which helps to separate water in the gas before it goes through the ammonia cooler. Accordingly, a special separating apparatus with the diameter of 1.2m was designed at the entrance of the cooler. Simultaneously, reforming designs to the ammonia cooler for raw gas were conducted. For example, an exit of the waste with the diameter of 25mm was added to separate the cooled water from the gas. Thus, the water in the gas, which caused corrosion of the tubes, and the conditions helping to form ice-block were forbidden. As a result, the failure of ammonia cooler for raw gas was prevented.In 2003, the program to prevent the failure of the ammonia cooler for raw gas was put into practice, and the effect of water-separation was observed and tested. It showed that 90% of the condensed water, which wasformed after the raw gas went through the ammonia cooler and after the reaction of methane alkylation, was separated at the entrance of the ammonia cooler. The condensed water formed in the ammonia cooler was separated at the exit of it. The seeper in the ammonia cooler tubes was given off in timing through the exit of the waste. The above measurements made the intake gas into the nitrogen/hydrogen compressor clean, and thus made the compressor work well. At the same time, the forming conditions of corrosion and ice-block were get rid of, which guaranteed a longer life of the ammonia cooler.Whatever kinds of materials the ammonia cooler was made, failures happened to the former one when the load of the system was beyond 4800 Nm3/h in summer. The equipment was changed in June, 2003. And no de...