Fractal Analysis Of The Sealing Performance Of Metal O-rings

Metal o-rings have been widely used in the nuclear power industry to prevent leakage of internal high temperature,high pressure and radioactive coolant in reactor pressure vessels.Inconel 718 alloy self-tightening O-ring is often used in reactor pressure vessel.In the past.the relationship between the rebound amount of metal o-ring and the stagger amount of upper and lower flanges under various working conditions is usually used to indirectly reflect its sealing performance.The purpose of sealing is to prevent leakage,and absolute seals do not exist,just the size of the leakage degree is different,this paper uses the leakage amount to directly reflect the sealing performance of metal o-ring.Therefore,this paper studies the leakage model of metal o-ring.The main research work and conclusions are as follows:(1)The flow state of the fluid in the seal clearance,the structure of the leakage channel and the existing typical leakage model are analyzed;Combined with the applicable pressure range of metal o-ring,the size of sealing clearance and the interface leakage of its metal properties,a leakage model of metal o-ring sealing structure was established based on the metal flat gasket sealing model.(2)The basic concepts of fractal theory,fractal dimension and fractal characterization of rough surface contour are briefly introduced;The deformation mechanism of the micro-convex body on the rough sealing surface and the contact fractal model between the flange and the metal o-ring on the rough sealing surface are analyzed emphatically.The relation between the average compressive stress and the real contact area of the sealing surface is established.(3)The leakage rate of metal o-ring sealing structure is calculated.Firstly,the finite element analysis and regression analysis of the contact width and the average contact stress of the sealing surface of the metal o-ring were carried out by using finite element software and mathematical software,and the relationship between the contact width,the average contact stress and the compressibility of the metal o-ring was obtained.Then the relation between real contact area and compressive stress and fractal parameters and modified parameters are combined.Finally,the leakage rate of metal O-ring seal is calculated based on the model.The calculated value of leakage rate is compared with the existing test value,and it is found that the calculated value is several orders of magnitude different from the test value.and the calculated result is much larger than the test result.The analysis may be caused by the fractal parameters and correction parameters of the metal gaskets used in the leakage calculation.Therefore,the influence of contact parameters on leakage rate needs to be further discussed.(4)Mathematical software was used to analyze the influence degree of fractal parameters and correction parameters on the leakage rate in the leakage model.The results showed that the influence degree of fractal dimension D,scale coefficient C and correction parameter C1 on the leakage rate was far less than that of correction parameter C2.Therefore,the remaining parameters remain unchanged.When the compression rate of the metal o-ring is 10%,the modified parameter C2 is adjusted to reduce the gap between the calculated value of the leakage rate and the test value to within one order of magnitude.Then,the leakage rate under different compression rates was calculated with the adjusted parameters for verification analysis.The results show that the calculated value of the leakage rate under different compression rates is in the same order of magnitude as the test value.It shows that the leakage model after adjusting parameters can provide a reference for predicting the leakage rate of metal o-ring.(5)The leakage rate of the upper seal assembly and the lower seal assembly of the pressure shell of a nuclear energy conversion unit is calculated by using the leakage model after adjusting parameters.According to the design parameters of the pressure shell,the finite element analysis and regression analysis were carried out on the contact width and average contact stress of the sealing surface of the outer ring of the sealing element with finite element software,and the leakage rate of the pressure shell under the design pressure was calculated by combining the modified parameters.Results:in the upper outer ring in the design of the sealing compression ratio is 12%,the environment temperature is 125? and the design pressure of 7 Mpa,under the condition of leak rate for 1.09×10-9 MPa · cm3/s;In the lower outer ring in the design of the sealing compression ratio is 12%,the environment temperature is 350? and the design pressure of 8 Mpa,under the condition of leakage rate is 1.11×10-7 MPa·cm3/s.In this paper,the leakage rate is limited below 10-7MPa·cm3/s according to strict requirements.Therefore,the upper sealing structure and the lower sealing structure meet the requirements under severe working conditions.Therefore,the leakage prediction model in this paper can provide a reference for the design,manufacture and operation of the sealing structure.