How to distinguish between dynamic seals and static seals, and how to calculate their quantities?

Is the sealing of pipelines mainly related to the sealing of flanges? Please repost this article so the original poster can take a look. The link is http://bbs.hcbbs.com/thread-424845-1-1.html. For more information on the concepts of dynamic sealing and static sealing, please refer to relevant sources. Another suggestion is to consult Volume 3 of the Chemical Engineering edition of the \"Mechanical Design Handbook\", which contains regulations regarding sealing. 1. In the technical specifications for pressure vessels around the world, seal calculations are all included within the design of flanges or the bolt connections of flanges, 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 designed 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 compressing 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 varies 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 involved 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 presented 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 significantly over more than 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 define 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 can achieve 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 seal is reliable, which reduces uncertainty to a certain extent. Regarding the characteristic parameters of gaskets, DIN2505 takes into account the differences between liquids and gases, which results in different values; this is not specified in the ASME codes. Furthermore, by taking into account the force exerted by gaskets and media on the flange’s deflection, 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 more accurate and 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, involving many parameters, some of which 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 gaskets; 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 engineering design in China. (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, as a precaution, people often take some remedial measures. For example, abroad, for flange joints under harsh conditions, the y and m values 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 areas prone to leakage, with the aim of increasing the y value as well. II. What is dynamic sealing? Dynamic sealing mainly refers to stem sealing. Preventing the medium inside the valve from leaking as the valve stem moves is the core challenge of valve dynamic sealing. 1) Packing box type: At present, for the dynamic sealing of valves, packing boxes are the primary method used. The basic forms of stuffing boxes are: (1) gland type, which is the most commonly used form. The same form can have many differences in details. For example, in terms of compression bolts, they can be divided into T-bolts (used for low-pressure valves with a pressure ≤ 16 kilograms per square centimeter), double-headed bolts, and swivel bolts, among others. In terms of gland covers, they can be divided into integral and modular types. (2) Compression nut type: This type has small external dimensions, but its compressive force is limited; it is used only for small valves. 2) Packing: Inside the packing box, the packing is in direct contact with the valve stem and fills the packing box, preventing the medium from leaking out. The filler must meet the following requirements: (1) Good sealing performance ； (2) Corrosion resistance ； (3) Low friction coefficient ； (4) Adapt to medium temperature and pressure. Common packing materials include: (1) Asbestos packing: Asbestos packing has excellent heat and corrosion resistance, but its sealing performance is poor when used alone; therefore, it is always impregnated or combined with other materials. Oil-impregnated asbestos packing: It comes in two basic structural forms, one being twisted and the other being braided. It can also be divided into circular and square shapes. (2) Polytetrafluoroethylene braided gasket: Polytetrafluoroethylene strips are woven into a gasket; it has excellent corrosion resistance and can also be used with cryogenic media. (3) Rubber O-ring: Provides good sealing performance at low pressures. The operating temperature is limited; for example, natural rubber can only be used at 60°C. (4) Plastic molding fillers: Generally made in a three-piece design, but can also be made in other shapes. Polytetrafluoroethylene is the most commonly used plastic, with nylon 66 and nylon 1010 also being used. Furthermore, the using units often explore various effective forms of fillers according to their own needs. For example, in steam valves at 250°C, alternating layers of asbestos gasket and lead rings can reduce steam leakage ； For some valves where the medium changes frequently, using an combination of asbestos packing and PTFE tape yields a better sealing effect. To reduce friction on the valve stem, molybdenum disulfide (MoS2) or other lubricants can be used in some cases. Currently, research is being conducted on novel fillers. For example, by impregnating polyacrylonitrile fibers with polytetrafluoroethylene emulsion, followed by pre-oxidation and then sintering and pressing them in a mold, shaped fillers with excellent sealing properties can be obtained ； For example, wave-shaped packing made of stainless steel sheets and asbestos can withstand high temperatures, high pressures, and corrosion. 3) Bellows sealing: With the rapid development of the chemical and nuclear industries, there has been an increase in flammable, explosive, highly toxic, and radioactive substances, which has led to stricter requirements for valve sealing. In some applications, packing seals can no longer be used; hence, a new type of seal was developed – bellows sealing. This type of seal does not require a filler, which is why it is also called a fillerless seal. The two ends of the bellows are welded to other components. As the valve stem moves up and down, the bellows expand and contract; as long as the bellows themselves do not leak, the medium cannot escape. As a precaution, a dual seal using a bellows and packing is often employed. III. What is a static seal? A static seal generally refers to a seal between two stationary surfaces. The main method of sealing is to use washers. 1) Gasket material (1) Non-metallic materials: such as paper, hemp, cowhide, asbestos products, plastics, rubber, etc. Materials such as paper, linen, and cowhide have capillaries and are permeable; therefore, they need to be coated with oil, wax, or other impermeabilizing materials when in use. They are rarely used in ordinary valves. Asbestos products include asbestos tapes, ropes, sheets, and asbestos rubber sheets. Among them, asbestos rubber sheets have a dense structure, excellent pressure resistance, and good temperature resistance; they are widely used in both the valves themselves and in the flange connections between valves and pipes. Plastic products have excellent corrosion resistance and are widely used. The types include polyethylene, polypropylene, soft polyvinyl chloride, polytetrafluoroethylene, nylon 66, nylon 1010, etc. Rubber products are soft in texture, and different types of rubber possess varying degrees of resistance to acids, alkalis, oils, and seawater. The varieties include natural rubber, styrene-butadiene rubber, nitrile rubber, neoprene, isobutylene rubber, polyurethane rubber, fluororubber, and others. (2) Metal materials: Generally speaking, metal materials have high strength and excellent heat resistance. But lead is not like this; only its resistance to dilute sulfuric acid is utilized. Commonly used materials include brass, copper, aluminum, low-carbon steel, stainless steel, Monel alloy, silver, nickel, etc. (3) Composite materials: such as metal-clad (asbestos-filled) washers, composite wave washers, spiral washers, etc.