Investigation On Fatigue Features Of Ultra-high Pressure Tubular Reactor With Defects Post Autofrettage

The ultra-high pressure tubular reactor is the key equipment for LDPE production.It's inevitable to cause damage or defect on its operation.Its operation condition is high temperature,ultra high pressure with corrosion.The impact of damage and defects will gradually expand.Once fatigue feature occurs,it will cause a catastrophic accident.There are many sets of ultrahigh pressure(UHP)polyethylene devices in China which have been operating for many years and these facilities are in its middle or late period of design life.It has great significance to investigate the fatigue feature of the UHP tubular reactor with defects to ensure its long-term,stable and safe operation.In this article,an UHP reactors with long-term service life in a petrify company will be the research object.Based on theoretical analysis,formula derivation,basic experiment and autofrettage research,combining with the standard of UHP vessel,the UHP reactors external wall's defect situation is investigated by theoretical calculation of fatigue features,finite element analysis and impact factors analysis.The main contents are as follow.(1)Firstly,the fatigue invalidation analysis and fracture mechanics analysis of the UHP tubular reactor were carried out,and the fracture mechanics parameter calculation method was presented.Then,based on the bilinear strengthening model,the stress analysis of the UHP tubular reactor is carried out,and the optimal autofrettage pressure calculation formula,residual stress calculation formula and three-way stress calculation formula under different working conditions are derived,and the stress distribution rule is observed.(2)The basic test and autofrettage test were carried out on the ultra-high pressure tubular reactor,and the autofrettage treatment process was monitored and analyzed,and the residual stress distribution of the external wall was obtained.According to the standard of UHP vessel and the theory analysis,the fatigue crack propagation life was increased by 7.06 times,7.47 times and 7.91 times,respectively.(3)An UHP tubular reactor model with external wall defects was created by using ANSYS software,and the stress distribution near the defects under different working conditions was analyzed.In addition,the stress intensity factor and fatigue crack propagation life were analyzed with Fracture Tool and Fatigue Tool.The results showed that the change of stress intensity factor decreased by 30.8%,31.2% and 31.6% respectively after the 634 MPa,665MPa and 690 MP autofrettage treatment.The fatigue crack propagation life was then increased by 6.98 times,7.41 times and 7.86 times,respectively.With the comparison of the theoretical calculation,the max deviation is 4.16% which proves that finite element simulation is feasible.(4)To establish different sizes of the external wall crack,and do the finite element simulation and theoretical calculation,the data were analyzed through the graph,and the results show that the crack size increase and the decrease the ovality which will cause the increase of the stress intensity factor and reduce of the fatigue crack propagation life.When the crack length reaches a certain dimension(for example 2c = 4.5 mm),depth reaches a certain size(for example a = 2 mm),the trend of stress intensity factor variation will increase and the trend of fatigue crack propagation life.Also,based on the analysis of other factors affecting fatigue characteristics,some countermeasures were put forward.In this article,the fatigue features of external wall defect autofrettage UHP tubular reactor are investigated to support the safe operation and safety assessment of such UHP reactors.It is of significance to provide reference value to ensure the UHP tubular operate safely.