Stress Analysis Of Polyethylene Reaction Tube Under Thermal Shock

When the ultrahigh pressure polyethylene reactor is in service,the instantaneous temperature of the inner wall of the reactor will increase to 700～1000 ℃ rapidly as a result of the phenomenon of over temperature decomposition.There is a great temperature gradient in the direction of wall thickness of the reactor which causes a huge thermal stress.Meanwhile,pressure fluctuation under various operating conditions will make the stress state of the reactor very complicated.It is extremely essential to study the temperature field,stress field and residual stress during the thermal shock to control the risk factors in the process of over temperature decomposition,the stable operation of production equipment and the life safety of the staff.Based on the finite element analysis method,the main work in this paper is as following.(1)Beginning with the research of steady state thermal stress,the distribution curves of the total stress of thick wall cylinder under the action of temperature difference and internal pressure are discussed by the finite element method,and theoretical derivation.The correctness of the finite element model,the boundary conditions and loading is verified.(2)For the thick walled cylinder which is impacted by the transient ultra-high temperature decomposition,the temperature field of the cylinder thermal shock is analyzed with the finite element method,and the maximum error is 7.78%compared with difference results.By the direct coupling method,the temperature field and stress field of thick walled cylinder subjected to the thermal shock and fluctuating internal pressure coupling with the thermal shock are analyzed with bilinear kinematic hardening model.According to the results,temperature and stress rise sharply at the area near the inner wall of reactor in the thermal shock;After 5 seconds of the thermal shock,the area 5 mm away from the inner wall are high stress concentration region,the maximum equivalent stress occurs in the inner wall surface,which is the most dangerous area.For the cylinder under the thermal structure coupled shock load,the higher the shock temperature is,the greater the stress value will be near the inner and outer walls.Compared with the cylinder subjected to the thermal shock individually,the equivalent stress is much lower in the inner wall,but in the area far from the inner wall,the equivalent stress is larger overall,the region affected by thermal shock is wider as well.(3)The effect of self-reinforcement on the stress of thick wall cylinder is analyzed,the result shows that in the first 18 seconds when the thermal shock has the most obvious influence on the reactor,the maximum value of equivalent stress for the thick wall cylinder with self-reinforcement is lower than that of thick wall cylinder without self-reinforcement,but the higher the self-reinforced pressure is,the larger the magnitude of the residual stress and plastic strain will be after the thermal shock.According to the effect of internal pressure and thermal shock on residual stress,the residual stress of the reactor after self-reinforcement is basically consistent with the residual stress after the internal pressure unloading.After the first thermal shock and the internal pressure coupling with the thermal shock,the residual stress has changed.After 20 times,60 times and 90 times thermal shocks,these residual stress curves are the same as the residual stress curve after the first time thermal shock,and there is no new plastic deformation.