Sealing Performance Assessment And Failure Analysis Of Flange Connection System With Octagonal Ring Gasket

As a common seal structure with excellent sealing performance and widespread application,the octagonal gasket seal can be used in the case with high-temperature load,sharply fluctuating load and corrosion condition,such as petrochemical and hydrogenated fields.However,the sealing failure of the octagonal gasket is a frequent phenomenon in the practical engineering.Until now,some assessment methods for the sealing failure have been proposed,however,the existing assessment process has shortcomings of systematization and convenience.Meanwhile,it is difficult to prevent the sealing failure as the mechanism of it hasn't been clearly investigated and verified.Thus,it is meaningful to study the mechanism of sealing failure and establish a convenient evaluation system for it.In order to solve the problem mentioned above,the flange connection system with octagonal gasket was investigated,and the main work and conclusions are listed as follows:(1)Based on the analysis of the design method for the sealing structure of octagonal gasket in GB150 and EN13445,an assessment principle based on the compressive force of gaskets for sealing performance of octagonal gasket was summarized,and an assessment method for it was proposed with using numerical simulation.The assessment method in this paper can be used to predict and assess the sealing failure of flange joint under actual working conditions.And it can also be used in reference to the design and application of other gaskets.(2)With the utilization of the finite element(FE)software,the FE models for the mechanical and thermal analysis were established.The results showed that the numerical solution to this agreed well with the theoretical solution and the numerical solution in existing literature,which illustrated the correctness of the FE model in this work.In addition,the modeling method of the air layer in the bolt hole was dealt with in detail.(3)In order to study the influence of internal pressure for the contact pressure,the FE models of two forms of load conditions,applying the internal pressure on the inner face of the octagonal gasket or not,were modeled.The numerical solution demonstrated that the relative discrepancy of contact pressure between them was not larger than 3%.As a consequence of it,the sealing performance of octagonal gasket couldn't be improved effectively by the pressure-energized effect of the gasket.(4)Based on the completed work cycle which included the start,steady operation and stops phases of equipment,the reasons leading to the sealing failure of the flange were investigated and analyzed.The result showed that the sealing failure in the stop phase was mainly caused by the plastic deformation of the gasket and the slide between the gasket and sealing surface of the flange trapezoidal groove,and this point could help the engineer to avoid the sealing failure of the gasket.(5)On the basis of simulation for the flange under fluctuated thermal load,the influence of the hysteresis of temperature on the sealing performance of the gasket was investigated.The results of simulation indicated that the influences of the hysteresis of temperature in the heating and cooling process were different.Especially,the fit between flange and gasket weakens in the cooling process,which would cause the sealing failure.Thus,it is suggested to avoid sharply change of temperature and decrease the cooling rate to avoid the decline of compressive force result of the hysteresis of temperature.