| Bolt flange connection is the most common form of connection in petrochemical equipment and is widely used in pressure vessels and pipelines in petrochemical and pharmaceutical sectors.As the bolts are important fasteners on the flange,the level of preload of the bolts and the load change after heating and pressurization affect the sealing of the flanges.When the flange is initially tightened,a reasonable bolt preload can guarantee the sealing of the flange under pre-loading conditions and working conditions.After the flange is initially preloaded,it enters the working state.Due to working conditions such as high temperature and high pressure,the load on the bolt and the gasket are changed,which may result in the flange being unable to guarantee sealing during operation.To deal with the reduction of the load on the flange bolts,the project mainly uses the methods of improving the preload of the bolt and the secondary heat tightness.In this paper,the bolt load of the B type flat welded flange of container with the winding pad is calculated by the design method of EN 13445 Annex G,and analyzed.the calculated bolt preload and the variation of the load on the bolt of the working state.The analysis shows that the calculated preload of flange bolt increases with the increase of working temperature,and it tends to increase irregularly with the increase of DN(nominal diameter).The calculated bolt load of the same specification flange at the same working pressure and working condition at different temperatures is a fixed value.This is because the influences of temperature are offset when calculating the working condition bolt force,so the bolt load of the working state of a certain specification flange calculated by Appendix G in this paper is a certain value.The bolts used in the calculation of the working conditions should be smaller than the required bolt tensions for pretensioning.This is due to thermal expansion and the neglect of external forces and external moments.Considering the thermal expansion of the flanges connecting components after heating,ignoring the influence of other factors,the second thermal tightening forces method of the secondary thermal tightness of the container B welded flange was deduced.The secondary thermal tightening forces method provides ideas.And using the Visual Basic program to develop a program to calculate the secondary thermal tightening forces of the Type B flat welded flange of container with wound gasket,which simplifies the secondary thermal tightening forces calculation process.Through further tests,the preload of the bolt sample under different high temperatureconditions was studied.The test results show that the bolt warms up after the initial tensile stress is applied,and the bolt force begins to drop.After warming for 2 to 3 hours,the bolt force is basically stable,and the bolt force no longer significantly decreases.Therefore,in the engineering,it is feasible to perform the secondary thermal tightening after the flange preloading 3 to 4 hours.It has been found through tests at different high temperature conditions under different initial stresses that the applied initial stress and the test temperature have an effect on the residual stress of the bolt specimen.Tests with higher temperatures have less residual tensile stress.When the initial stress is greater applied to the bolt sample,the residual stress is also greater;but when the initial stress exceeds the yield strength of the sample material at the test temperature,the plastic flow occurs inside the sample,and the trend of stress change is no longer as the elastic condition of time.Therefore,it is necessary to select a reasonable bolt preload force for the high temperature conditions of the flange. |
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