In order to determine the impacts of residual stress relaxation and autofrettage pressure on the in-service pressure vessel safety, the relaxation rule of the residual stress was showed by the test results. The equivalent stress distributed along the vessel wall was calculated under the coupling conditions of the operating pressure and residual stress; the safety coefficient was simulated under the different operating pressures and autofrettage pressures, then the optimum autofrettage pressure was deduced. In this case, a high-pressure polyethylene reaction tube was serviced for 10 years. The results show that the vessel tangential stress in the near wall region decayes from -600MPa to -333MPa fast with the attenuation rate reaching 45%. Attenuation was measured in the elastic region and the residual stress peak location was relocated but a little change in its peak value. For the auto-frettaged pressure vessel under the coupling conditions of operating stress and residual stress, with the residual stress relaxation, the equivalent stress of inner wall surface is increased and the largest equivalent stress is at the junction of the elastic and plastic body, where the safety factor should be calculated. According to this case polyethylene pipe size, the operating pressure is at 180MPa, 280MPa, 380MPa respectively, then the best autofrettage treatment pressure is at 606MPa, 677MPa, 743MPa respectively, the safety factor is larger 16%, 26%, 37% respectively than residual stress all attenuation to 0. With the higher operating pressure, the vessel safety factor increased by optimal autofrettage process is higher, but impact caused by the residual stress attenuation is greater.
