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Factors affecting flange sealing There are many factors affecting flange sealing. Mainly include: Bolt pretightening force ； Sealing surface type ； Gasket performance ； Flange stiffness ； operating conditions. They are discussed separately below. (1) Bolt pre-tightening force Bolt pre-tightening force is an important factor affecting sealing. The preload force must compress the gasket and achieve the initial sealing condition. At the same time, the preload force cannot be too large, otherwise the gasket will be crushed or extruded. Since the preload force is transmitted to the gasket through the flange sealing surface, to achieve good sealing, the preload force must act on the gasket evenly. Therefore, when the pretightening force required for sealing is constant, it is beneficial to sealing by increasing the number of bolts and reducing the diameter of the bolts. (ii) The sealing performance of the sealing surface flange connection is directly related to the sealing surface type, so the shape of the sealing surface must be selected reasonably. The selection of flange sealing surface type mainly considers pressure, temperature and medium. The types and characteristics of flange sealing surfaces commonly used in pressure vessels and pipelines are shown in the flange sealing surface comparison table. Flange sealing surface type table name features description diagram raised surface (RF) surface is a smooth plane, and dense water lines can also be machined. The sealing surface has a simple structure, is easy to process, and is easy to carry out anti-corrosion lining. However, the contact area of this kind of sealing surface gasket is large. When pre-tightening, the gasket is easily squeezed to both sides and difficult to compress. (Figure (a)) The concave and convex surface (MFM) sealing surface is composed of a convex surface and a concave surface. Placing a gasket on the concave surface can prevent the gasket from being squeezed out, so it can be used in situations with high pressure. (Figure (b)) The tongue and groove (TG) sealing surface is composed of a tenon and a groove. The gasket is placed in the groove and will not be squeezed. The gasket can be narrower, so the bolt force required to compress the gasket is correspondingly smaller. Even when used in high pressure locations, the bolt size is not oversized. Therefore, it is easier to obtain good sealing effect than the above two sealing surfaces. The disadvantage of the sealing surface is that the structure and manufacturing are relatively complex, and it is difficult to replace the gasket squeezed in the groove. In addition, the tenon surface part is easily damaged, so care should be taken during disassembly, assembly or transportation. Tongue and groove sealing surfaces are suitable for flammable, explosive, toxic media and higher pressure situations. When the pressure is not high, it can seal well even if the diameter is larger. (Figure (c)) Full plane (FF) and ring connection surface (RJ) full plane seals are suitable for situations with low pressure (PN ≤ 1.6MPa) ； The ring connection surface is mainly used on neck butt welding flanges and integral flanges, and the applicable pressure range is (6.3MPa≤PN≤25.0MPa). (Figures (d) and (e)) Other types of sealing surfaces For the sealing of high-pressure vessels and high-pressure pipelines, the sealing surface can use a tapered sealing surface or a trapezoidal groove sealing surface, which are matched with spherical metal gaskets (lens gaskets) and oval or octagonal cross-section metal gaskets respectively. The sealing surface can be used in situations with higher pressure, but requires high dimensional accuracy and surface finish, which is difficult to process. (Figure 4-37) (Figure 4-38) Attached Figure 1: http://www.cngspw.com/Doc/data.WebNoteBooks/20060517191238/4-36.gif Flange seal diagram http://www.cngspw.com/Doc/data.WebNoteBooks/20060517191238/4-37.gifhttp://www.cngspw.com/Doc/data.WebNoteBooks/20060517191238/4-38.gif (iii) Gasket performance Gasket is an important component of sealing. Appropriate gasket deformation and rebound ability are necessary conditions for forming a seal. The most commonly used gaskets can be divided into non-metal, metal, and non-metal and metal mixed gaskets. Non-metallic gasket materials include rubber asbestos sheets, polytetrafluoroethylene, etc., as shown in Figure (a). The advantage of these materials is that they are soft. Its temperature and pressure resistance is worse than that of metal gaskets, and it is usually only suitable for flange sealing of normal, medium and medium- and low-pressure equipment and pipelines. http://www.cngspw.com/Doc/data.WebNoteBooks/20060517191238/4-39.gif Gaskets made of mixed metal and non-metal include metal-clad gaskets and spiral wound gaskets, see Figures (b), (c) and (d). Metal-clad gaskets are made of non-metallic materials wrapped with thin metal sheets (galvanized steel sheets, 0Cr18Ni9, etc.) ； Metal spiral wound gaskets are made of thin low carbon steel strips (or alloy steel strips) and asbestos strips. This kind of wound gasket is available in two types: without locating ring and with locating ring. Metal-clad gaskets and spiral wound gaskets have better performance than simple metal gaskets, and can adapt to a higher temperature and pressure range. Metal gasket materials generally do not require high strength, but require softness and toughness. Commonly used are soft aluminum, red copper, iron (mild steel), Monel alloy (containing Ni67%, Cu30%, Cr4~5%) steel, etc. Metal gaskets are mainly used for flange connection sealing at medium and high temperatures and medium and high pressures. The selection of gasket material should be determined based on the temperature, pressure and corrosion of the medium. At the same time, the form of the sealing surface, the size of the bolt force, and the loading and unloading requirements should also be considered. See Table 4-15 for gasket material selection. Table 4-15 Gasket Material Selection Material Sealing Surface Pressure MPa Temperature Medium Pressure Rubber Asbestos Plate Smooth, Concave-convex ≤ 2.5 ≤ 150 Compressed Air, Inert Gas, Ammonia Smooth ≤ 1.6 ≤ 300 Steam, Water Polytetrafluorochloroethylene Light (Concave-convex) ≤ 1.6 ≤ 200 kinds of oil products, oil and gas, solvents, petrochemical raw materials and products Oil-resistant rubber asbestos sheets (concave-convex) 2.5 ≤ 200 ≤ 1.6 ≤ 50 Liquefied petroleum gas metal spiral gasket 0Cr1 3 (0Cr19Ni9) steel strip + special asbestos (graphite) concave and convex 4.0 ~ 6.4 ≤ 60 inert gas light (convex and convex) 2.5 ≤ 450 kinds of oil products, oil gas, solvents, petrochemical raw materials and products concave and convex 4 .0≤450 hydrogen gas, hydrogen gas and petroleum mixed gas 6.4≤450 metal clad gasket iron sheet (aluminum sheet) + special asbestos, 0Cr13 (0Cr19Ni9) + special asbestos light (concave-convex) 2.5≤450 Concave-convex 4.0 ≤ 450 Hydrogen, hydrogen and petroleum mixture 6.4 ≤ 450 Flexible graphite hybrid pad Graphite + metal skeleton (0Cr13, 0Cr19Ni9, etc.) Light (concave-convex) 2.5 ≤ 450 kinds of oil products , oil and gas, solvents, petrochemical raw materials and products concave and convex 4.0 ≤ 450 hydrogen, hydrogen and petroleum mixed gas 6.4 ≤ 450 metal ring gasket 10, 0Cr13, 0Cr19Ni9 trapezoidal groove 6.4 ≤ 450