Failure Analysis Of Outlet Pigtail Pipe Cracking In Hydrogen Production Reformer

Reformer is the core equipment of hydrogen plant,and the pigtail pipe at the outlet of converter is the core part of the converter.The bending pipe connecting the lower part of the reformer tube with the outlet collecting pipe is called the outlet pigtail pipe.It is named for its similar shape.It is used to transfer the process medium and absorb the thermal expansion of the furnace tube and the lower collecting pipe so as to reduce the deformation stress of the furnace tube and the lower collecting pipe at high temperature.Hydrogen production reformer is the key equipment in hydrogen production plant.As far as the outlet pigtail pipe is the most vulnerable part to leakage and ignition.Therefore,it is of great significance to analyze the causes of cracks and take corresponding countermeasures and measures for the long-term safety production of hydrogen plant.Firstly,we draw a conclusion through the accident cases of pigtail pipes in similar units in several domestic refineries that the problem is a common problem and a difficult one to solve.Many people have done research and tried to find out the essence of the problem,but each enterprise's problems are different which we can learn from,but can not copy.So learning from other people's experience to analyze the problems of our own devices is also a shortcut.First of all,we can analyze the working conditions,the heat transfer mode and temperature distribution of the reformer from the technological principle and conditions.Through the structural analysis of reformer,the stress state of furnace tube and pigtail tube was studied.The failure reasons of the pigtail tube in this device were analyzed by means of appearance inspection,microcosmic morphology of crack section,energy spectrum analysis,metallographic structure analysis,chemical composition and surface oxide analysis.Then the causes of cracking failure of pigtail pipes are analyzed theoretically: congenital design defects lead to thermal stress concentration,structural deformation stress leads to cracking of weak links,poor welding quality resulting in thermal cracking,carbide precipitation leads to chromium depletion,and chromium is an important element of intergranular corrosion resistance.Its reduction will inevitably lead to a decline in corrosion resistance and eventually lead to corrosion along grain boundaries.Hydrogen embrittlement occurs when hydrogen accumulates and precipitates in the defect zone of high temperature conversion gas under high stress,creep deformation and creep fracture of metal materials at high temperature.The most important thing is that the abnormal fluctuation in production results in a great change in furnace temperature,which can aggravate and accelerate all the above-mentioned effects,resulting in cracks in the equipment and a sharp shortening of its life.Finally some improvement measures are proposed to reduce the possibility of failure: we need to weld pigtail pipe with the specified welding process,materials,processing methods,technical requirements.and then inspect the welding quality,carry out heat treatment to ensure the quality of welding with no cracks,no defects,no carbides precipitation;upgrading the material to Incoloy 800 HT with better high temperature performance,in order to ensure the quality of welding.Under the same technological conditions,the service life is longer and the probabilities of problems are smaller;using the elastic mechanics formula of the strength of common pressure pipes,it is necessary to increase the wall thickness to meet the strength requirements at high temperature;Carrying out structural optimization and analysis,changing the shape of pigtail pipes to reduce the stress of the two welds that are most likely to fracture;The production is stable,there is no large fluctuation,no increase in stress,and no aggravation of the impact on equipment.Considering the intrinsic safety,cold-wall collecting pipes are adopted and pigtail pipes are not used to avoid such accidents from the source.The repeated accidents of pigtail are not neglected,but not thoroughly and comprehensively analyzed.There are many reasons for the cracks.We should not ignore one thing or the other,we should focus on the point,however we should not neglect one of them.Corresponding measures should also be taken into account in the light of the causes and prescribe appropriate medicines.The crack case of pigtail tube in hydrogen plant of Jingmen Branch of Sinopec is due to the weakening of grain boundary by structural stress and precipitation of carbide,resulting in intergranular corrosion and cracking under the combined action of high temperature creep and hydrogen embrittlement.At the same time,abnormal production results in large fluctuation of furnace temperature,which accelerates and enhances the generation of cracks.Therefore,in order to ensure long-term safe operation of the converter,there must be no defects in welding.Secondly,the corresponding structure should be optimized to reduce stress.The most important thing is to ensure stable production and the furnace temperature can not fluctuate greatly.