Research Of Lifetime Prediction Method Of Urban Gas Polyethylene Pipes By Thermal-oxidative Aging

Natural gas is a kind of high-quality and clean energy,which plays an important role in improving people’s life,supporting national economic development and protecting the environment.Natural gas pipeline transportation system is an essential link between the process of natural gas production and application,among which urban natural gas pipeline transportation system is closely related to social production and people’s life.Polyethylene(PE)pipes have the advantages of low density,corrosion resistance,long service life,high impact resistance,superior flexibility and convenient connection,therefore,have been widely used in urban gas engineering.There are a large population living nearby urban gas PE pipes,so the failure of PE pipes would cause serious loss of life and property.With the increase of service life,life of some PE pipes is close to design life,therefore the aging of PE pipes has become a safety issue that needs to be solved.The results show that the aging lives of PE are affected by environmental factors and aging rate can be reflected by the change of its microscopic properties.Therefore,under the actual working conditions of urban gas PE pipes,effects of temperatures and pressures on urban PE pipes are focused on to research in the dissertation,the relationship between the macro performance and the microscopic mechanism is explored,the existing PE pipes ageing test platform and calculation methods are improved,and in-service lifetime prediction model is put forward,which can provide technical support and security for urban gas pipeline safety operation and prolong life.The paper mainly studies in the following aspects:(1)Thermal oxygen aging(TOA)test platform of pressured gas PE pipes was improved and tested under different temperatures(353 K,363 K,373 K and 383 K)and different pressures(0 MPa,0.1 MPa,0.2 MPa,0.3 MPa and 0.4 MPa)by this platform.Therefore,aging PE pipes were obtained.(2)The maximum tensile force(TF)of the aging pressure PE pipes was measured by tensile test.Temperature and pressure effects on TF of the aging PE pipes were discussed under different aging times.According to test data,the maximum tensile force was shown to be calculated by Arrhenius equation.Aging rate of the PE pipes was calculated by Arrhenius equation to analyze the relationship between the TF and aging rate.Pressured thermal oxygen aging lifetime prediction model was built by tensile test.But this method is not suitable for in-service inspection.(3)Oxidative induction time(OIT)of the aging PE pipes was measured by Differential Scanning Calorimeter(DSC).According to the test data,OIT from inner,middle,and external for the aging PE pipes was tested to analyze effects of test site for the aging PE pipes.Temperature and pressure effects on OIT of the aging PE pipes were discussed under different aging times.According to test data,OIT was shown to be calculated by Arrhenius equation.Aging rate of the PE pipes was calculated by Arrhenius equation to analyze the relationship between the OIT and aging rate.Pressured thermal oxygen aging lifetime prediction model was built by DSC.This method is suitable for in-service inspection,but it is based on the calculation of single temperature factor,and the calculation method based on pressure and temperature factor is still lacking.(4)The aging pressured PE pipes were tested by melt flow rate(MFR).According to the test data,temperature and pressure effects on MFR of the aging PE pipe were discussed under different aging times.The density of the aging PE pipes was analyzed under different aging times,temperatures and pressures.Moreover,based on the molecular dynamics simulation method,microscopic molecular model of PE pipes was built to simulate pressured TOA process under different temperatures(313 K,323 K,333 K,343 K,353 K,363 K,373 K and 383 K)and different pressures(0 MPa,0.1 MPa,0.2 MPa,0.3 MPa and 0.4 MPa).The simulation results were analyzed effects of temperatures and pressures for the density of the aging PE pipes and explored the relationship between micro and macro performance to establish relationship macro and micro lifetime prediction.(5)Pressure effects on in-service lifetime of aging gas PE pipes under different temperatures(318 K,328 K,338 K and 348 K)and different pressures(0 MPa,0.1 MPa,0.2 MPa,0.3 MPa and 0.4 MPa)by pressured TOA test platform were researched.Aging PE pipes were tested by tensile test,DSC and MFR to analyzed relationship among TF,OIT and MFR.The results showed that OIT of test data was fit for the residual lifetime prediction for urban gas PE pipes.Therefore,a residual lifetime prediction method contained pressure was proposed.Based on the pressure and temperature factors,this method is used to predict the residual life of urban gas PE pipes.(6)A method for evaluating in-service lifetime of urban gas PE pipes was studied.According to the usage of in-service gas PE pipes,the residual lifetime predicting method was used to carry out engineering verification.A residual lifetime predicting method under actual urban gas working condition was proposed for in-service gas PE pipes.This method is applicable not only to gas PE pipe,but also to other plastics pipes under the similar conditions.