Research On Fatigue Life Of Ldpe Ultrahigh Pressure Tubular Reactor

LDPE ultra pressure tubular reactor is the key equipment in high pressure polyethylence production system, which is autofrettaged to enhance elastic loading capabilities and extend fatigue life before operating, the residual stresses gained by autofrettage are critical to the safety of tubular reactor. It is the major threaten to safety of facility that residual stresses relax gradually the decrease of residual stress will reduce fatigue lifetime of the reactor, the ultra pressure devices imported in the 1980s were in the later-stage of design lifetime, the consequence of leakage and explosion is tragedy, and it will also cause emormous propoty loss and casualty. It's significant for safety production and reduction of economic losses to study the relaxation of residual stresses and calculate remaining life.The paper based on the LDPE ultra pressure tubular reactor under service, the mechanism of autofrettaged residual stress are studied, and distribution of autofrettage residual stress was calculated based on Mises yielding criterion and ANSYS simulation.Fatigue test of LDPE ultra pressure tubular reactor were conducted to simulate the operation condition, then residual stresses distribution across the cross section of the tubular reactors after different fatigue history were measured with X-ray diffraction stress analyzer, and relaxion laws of residual stresses were obtained.According to the Shigley method, fatigue lifetime of the reactor post 20000 cycles are 393000 times, compared with lifetime of the reactor post autofrettage treatment without residual stresses relaxation, it reduced 4900 times because of residual stress relaxation.Formulas of inner and external wall surface residual stresses under operate condition were conducted according to a physical and mathematic model developed on the principle of autofrettage residual stresses mechanism accepted by scholars, which were compared with the formulas fitted on the basis of fatigue test results, theoretic basis for safety monitoring of LDPE tubular reactor was acquired.