Study On Failure Mode Of Ultra-high Pressure Polyethylene Reactor

During the service of high pressure polyethylene reactive tube,the transient overtemperature process caused by thermal decomposition of polyethylene will make the stress field of the reactor very complex and cause the autofrettaged residual stress relaxation,which reduces the fatigue strength of the reactor obviously.The stress corrosion of the outer wall of the reactive tube under the joint action of the working stress and the corrosive medium also seriously affects the safety of the reactor.Studying the failure mode of reactor is of great significance for ensuring the safe production of LDPE tubular reactor and improving its service life.Based on the finite element method and the stress corrosion electrochemical test,the main works in this paper are as follows:(1)Used ABAQUS finite element software to establish 2D finite element model of reaction tube cross-section,simulated the temperature and stress fields of the reactor under coupled impact loads on pressure-temperature.At the same time,the temperature field and stress field of the high pressure polyethylene reactive tube with or without autofrettage were compared and analyzed.The results show that,the over-temperature decomposition of polyethylene will produce a large amount of heat,the heat would cause a sharp increase in the temperature and stress of the inner wall in a short period of time,with the opening of the relief valve,the injection of cold material and the enhancement of the thermal conductivity inside the reactor,the temperature and stress of the reactor continue to decrease and eventually stabilize at about 20s.The maximum equivalent stress of the self-reinforced reactive tube is lower than that of non-autofrettage reactive tube at the time of thermal shock.(2)By analyzing the change of stress and strain before and after the thermal shock,the reason and law of the relaxation of the autofrettaged residual stress are obtained.The effects of thermal shock times,self-tightness and the opening time of valve on the relaxation of residual stress of self-reinforced tubes were analyzed.The results show that,the relaxation of the residual stress occured within 2mm of the inner wal,and the relaxation rate of the inner wall was the highest,The reason for the relaxation of residual stress is the superposition of the thermal stress with the increase of temperature and the autofrettaged residual stress,so that compressive tangential plastic strain occurs near the inner wall,resulting in redistribution of residual stress.The greater the self-tightness,the greater the residual stress relaxation rate near the inner wall,and the maximum relaxation rate of the inner wall can reach about 50%.The longer the valve is opened,the greater the relaxation rate of the residual stress near the inner wall.(3)Electrochemical experiments of stress corrosion for C-ring specimen after loading stress,the influence of the water quality of the working medium on the corrosion performance of the reactor material was analyzed.The results show that the increase of acidity and chloride ion solubility will increase the corrosion current density and the rate of increasing of corrosion current density.(4)In view of the experimental research and theoretical calculation of a UHP unit,the failure mechanism and mode of the reactor was discussed in combination with the accident situation in the domestic related devices.The failure of the inner wall of the reaction tube is mainly due to the thermal shock of the short time overtemperature and the instantaneous cooling of the cold material,and the failure of the outer wall of the reacting tube is mainly due to the compound effect of stress corrosion and corrosion fatigue.