Thread Content
In the technical specifications for pressure vessels around the world, seal calculations are all included within the framework of flange design or flange bolt connections, with the stress analysis and calculation of flanges and bolts being the main focus. The calculations related to flanges are not repeated here; instead, emphasis is placed on the calculation of gaskets and the verification of sealing performance. I. Walters’ calculation method: At present, China’s ‘Design Code for Steel Pressure Vessels in the Petrochemical Industry’ follows, similar to the relevant pressure vessel standards in the UK and Japan, the American ASME standards; the Walters’ method is used for the design of flanges and seals. This method emphasizes the strength of the bolts in the calculation of sealing performance. Wallster believes that, under all circumstances, as long as the bolt strength is sufficient and the force exerted by the bolts on the gasket is not less than the design value, a tight connection between the gasket and the sealing surface can be ensured. 1. The minimum bolt load required during operation, Fm1 (N), and the minimum bolt load required when tightening the bolts, Fm2 (N). 2. Calculation of the sealing width using gaskets: The sealing width b of the gasket can be determined as follows: when bo ≤ 0.0064 m, b = bo. As can be seen from Table 3-5, the effective sealing width bo of the gasket is not equal to the actual contact width between the gasket and the pressing surface, N. This is because when the washer is placed on the inside of the bolt hole, the bolt force causes the flange to deflect to a certain extent. After internal pressure is established, the axial force generated by the medium pressure exacerbates the deflection. Therefore, the compressive force is not evenly distributed across the entire contact surface; the outer edge is compact while the inner edge is loose. The medium may penetrate a certain width into the washer, and this phenomenon becomes more severe as the width of the washer increases. Hence, the calculated width b should be ≤ bo, and the method for calculating DG also changes depending on bo. 3. Calculation of the total cross-sectional area of bolts II. The West German DIN2505 method: In the West German standard DIN2505, “Calculations for flange connections,” the procedure for calculating gaskets differs from that specified in China’s current standards. The steps are as follows: (9) After the calculations are completed, it is also necessary to create a force diagram. The deformation amounts of flanges, bolts, and gaskets during the pressurization and heating process are calculated and shown in a single graph, so as to determine whether, under operating conditions, excessive relaxation requires a higher bolt force during pre-tightening or the use of different gaskets. III. Coefficient method: Relevant domestic organizations have carried out extensive work in exploring design methods for the sealing performance of gaskets. A brief introduction to this calculation method is provided below. IV. Discussion on the three calculation methods (1) The ASME Code, as a **standard** in the United States, has a significant influence worldwide. The core of the calculation for the gasket sealing performance lies in determining the preload specific pressure y and the gasket coefficient m. Although the ASME codes are revised every 3 years, the values of y and m have not changed much over the past forty years. For example, the y-value for gasketing filled with asbestos was previously 4500 pounds per square inch; it was changed to 7000 pounds per square inch in the 1977 edition of the ASME Code. Since 1957, various scientists have carried out extensive qualitative or quantitative investigations on the characteristic parameters of gaskets recommended in the standards, and it has been shown that the values of y and m depend not only on the material and structure of the gasket, but also on factors such as the gasket width, the surface finish of the flanges, the stress on the gasket, the internal pressure, the medium in use, and the allowable leakage level. In particular, the leakage amount and the medium have a significant impact on the y and m values. This is a major advancement achieved in the research of modern sealing technology. ①The current standards specify values for m and y; there is no concept of leakage amount. Since leakage is a continuous process that progresses from small to large, without specifying a leakage amount threshold, it is impossible to determine the \"critical leakage point\" mentioned in our definition. The sealing performance of a gasket can only be measured by the minimum amount of leakage that the gasket is capable of preventing under operating conditions. ②Pressure, temperature, and the medium also have an impact on the m and y values. For example, if the leakage rate is kept the same, when using asbestos rubber gaskets, the y and m values for hydrogen are 150 times and 25 times those respectively for other gases. The m and y values for whom are relatively close to the values recommended by the standards. The experiments also showed that, for a given leakage value, the gasket coefficient m is exponentially related to the internal pressure. ③Scientists have long been aware of the impact of the surface roughness of flanges on sealing performance. In the earliest gasket tightness tests (1934), flanges with different surface roughnesses were used. In 1979, Raut mentioned in his paper that experiments were conducted to seal flanges using asbestos-wrapped gaskets with four different surface roughness levels; the y-values of the surfaces were the same, but the leakage amounts varied. The one with the lowest surface roughness has the smallest leakage. (2) The DIN2505 method (which essentially expresses the graphical parts in DIN2505 as mathematical formulas) treats the flange, bolts, and gaskets as a system, focusing on the reaction force of the gaskets under design temperature and pressure conditions, as well as the effects of operating cycles. Through verification, a conclusion can be drawn as to whether the sealing is reliable, which reduces uncertainty to a certain extent. Regarding the characteristic parameters of gaskets, DIN 2505 takes into account the differences between liquids and gases, which is why the values vary; this is not the case in the ASME codes. Furthermore, by taking into account the force exerted by gaskets and the medium, the deflection of the flange, the stretching of bolts, the compression of gaskets, the decrease in the elastic modulus of various materials at high temperatures, as well as thermal expansion and contraction, the calculation results become closer to the actual conditions. In the Walters method, although certain factors are taken into account—such as the effect of flange deformation on the effective contact width when calculating the sealing width b— it does not rely on rigorous mathematical formulas for derivation like the latter two methods; instead, it employs qualitative analysis, so the effects considered are also approximate. The disadvantages of the DIN2505 method and the coefficient method are that the calculation process is relatively complex, involves many parameters, and some of them require the use of computers. The premise for using these two methods is that the actual preload applied during installation must exactly match the calculated value; otherwise, they lose their meaning, and it is very difficult to achieve this at present. Furthermore, during the calculation process, it is still impossible to avoid using the two characteristic parameters of shims; as a result, factors such as leakage rate, surface roughness, and assembly stress cannot be taken into account during the calculation. For the above reasons, the DIN2505 method has not been widely adopted in many cases. As for the coefficient method, it is also rarely used in domestic engineering design. (3) The Walters method, used for sealing calculations, is a strength calculation approach that focuses on the strength of the bolts, without considering whether the reaction force from the gasket is sufficient to prevent medium leakage. Long-term practice has shown that the current values of m and y are \"generally considered satisfactory\" in use; the flange and bolt connection systems are safe, but leaks of varying degrees do occur. The advantage of the Walters method is its simplicity and ease of use, which is why it has been widely adopted in our country and many other places for a long time. The re-determination of m and y values both domestically and internationally is undoubtedly a refinement of the Waters method. But at present, people often take some remedial measures as a precaution. For example, abroad, for flange connections under harsh conditions, the values of y and m are often specified to be several times higher when placing an order. In China, the method of upgrading using flange bolts is employed to address issues in certain leak-prone areas, with the aim of increasing the y value as well.