Research On Fretting Wear Characteristics And Tightness Assessment Of Flange Sealing Connection In Gas Station

Natural gas is currently the most widely used fuel gas resource,and as an important clean energy,it is also a step forward of energy transition to achieve the goal of carbon neutrality.In natural gas gathering and transportation,pipelines and stations are the most common ways to ensure safe and efficient transportation of energy.It is worth noting that due to the complex process flow in the gas station,the number of flange joints is far greater than the number of any other instruments,and its reliability significantly determines the safe operation of the entire pipeline system.Moreover,the existence of many pipe fittings also affects the integrity of the pipeline to a large extent,so that the vibration generated by the pipeline cannot be avoided and eliminated when the high-pressure gas is transported.The bolt-gasket-flange connection（BGFC）system,as the main form of pipe connection in natural gas station,is prone to fretting wear behavior between the sealing components under operation conditions,resulting in the decrease of tightness for the BGFC system,thus leading to leakage failures.Therefore,the fretting wear characteristics and the tightness evaluation of the BGFC system of natural gas station are studied in this paper,which not only has important academic value but also has profound practical significance.The main research contents and corresponding conclusions of this study are as follows:（1）A Noisy-OR gate based fuzzy fault tree approach for micro-leakage evaluation of BGFC was proposed to solve the problem of insufficient field data of gas station.By mapping the micro-leakage failure tree model of BGFC system to Bayesian Network,the corresponding minimum cut set and highest risk factor of micro-leakage failure of BGFC were calculated.Five basic events,such as gasket sealing surface wear,were identified as the key factors leading to micro-leakage failure of BGFC in gas station by sensitivity analysis.Furthermore,according to the analysis results,corresponding improvement suggestions and measures were given respectively for high risk factors.（2）Fretting wear tests on the sealing surface of flexible graphite spiral wound gasket were carried out,and the fretting friction characteristics and wear failure mechanism at different operating parameters were investigated.The results show that the flexible graphite transferred layer and gasket winding structure are the important factors causing the reduction and small fluctuation of friction coefficient.In addition,the wear mechanism at small displacement amplitude is basically abrasive and adhesive wear,while the radial wear under large displacement amplitude and high load is plastic deformation.Noting that,the most serious wear occurs in the circumferential fretting with small displacement amplitude and the radial fretting with large displacement amplitude.The transport and transfer of wear debris could lead to the reduction of sealing stress by thinning the thickness of gaskets,and many wear scars generated at the sealing interface may destroy the flatness of the sealing surface of gaskets,thus increasing the possibility of sealing interface leakage.（3）On the basis of fretting friction and wear test of flange gasket,a"star"type micro-leakage path unit was established to simplify the gasket sealing interface leakage model.For the micro-leakage path unit,three leakage modes were defined according to the number of leak ports.The results show that,first-level leakage mode has the highest inlet velocity but weak diffusion capacity,while third-level leakage mode’s outlet velocity meets the maximum value.In addition,the volume flow rate at the inlet/outlet interface develops with the increase of inlet pressure.As for the shear stress of flexible graphite wall,the maximum value for the third-level leakage mode appears in the central acceleration zone and the leakage channel,which has a greater impact on the erosion of the flexible graphite wall.（4）The deformation coordination equation of BGFC in gas station was constructed.On this basis,the relationship between gasket stress and bolt pre-tightening force was established under operating conditions,also the theoretical derivation of gasket stress considering creep and wear of bolt/gasket,as well as external load was improved.Additionally,the tightening diagram method and PVRC method were respectively used to evaluate the tightness of the BGFC system qualitatively and quantitatively.The tightness assessment process of the BGFC system is summarized and designed.Furthermore,a gasket design method based on leakage rate was proposed according to the current domestic method.For different operating conditions and tightness requirements,the corresponding modified gasket coefficient could be obtained,making the gasket design gradually reasonable.（5）The structure parameters and sealing material optimization of flange gasket were studied.According to the"V"type model commonly used,a simplified model based on R（R₁,R₂）parameter section profile was proposed.The contact pressure p,equivalent stressσ,and compression resilience s of flexible graphite sealing tape were taken as the key performance indexes to determine the optimal structure of metal wound gaskets under constraints.In addition,fretting tests of resin-impregnated graphite and antimony-impregnated graphite under different working conditions without lubrication were carried out respectively.The fretting wear characteristics of the two kinds of impregnated graphite were investigated,and the corresponding wear mechanism was revealed,which provided a theoretical basis for the selection of materials for different sealing conditions from the fretting wear point of view.