The combined use of flanges, gaskets, and bolts

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1. Selection of flanges, gaskets, and bolts: Flange connection is a combination of three main components – flanges, gaskets, and fasteners – with the ultimate goal of achieving a secure seal. Obviously, the role of each component in the assembly is the only reason for the leakage. When selecting gaskets, it is necessary to consider not only the functions and properties of the gasket itself but also its interaction with other components in the flange connection. Therefore, there is an issue of rational selection of flanges, gaskets, and fasteners, including the type of flange, material, design of the sealing surfaces, as well as their applicable ranges and the specific operating conditions they are subject to, along with the maximum allowable operating pressure under the working temperature and flange material ； Gasket types and application ranges (pressure, temperature, medium), as well as compatibility with flange types, seal surface types, and their surface roughness ； The type, material, or performance grade of fasteners, along with the selection based on operating temperature, flange pressure rating, and gasket type, as well as the fit between bolts and nuts. 2. Others (1) Thickness: The thickness of gaskets is related to factors such as their shape, material, diameter, condition of the sealing surfaces, and the sealing medium. For most non-metallic sheet materials, thinner gaskets with a thickness of 1.5 mm or 2 mm are typically used. This is because the thinner gasket has a smaller surface area exposed to internal pressure, resulting in less leakage of the medium along the body; thus, the leakage rate is lower, and the force exerted by the gasket outward is also smaller ； Thinner gaskets exhibit lower creep relaxation ； The thinner the gasket, the lower the price. However, gaskets must fill irregular flange sealing surfaces; thicker gaskets have better compressibility and can cope to some extent with sealing surfaces that are slightly damaged or warped. Therefore, the minimum thickness of the gasket depends on factors such as the surface roughness of the flange, the compressibility of the gasket, the stress on the gasket, and the degree of deflection of the flange. The recommended values for gasket thickness can serve as a reference for choosing the thickness of non-metallic sheet gaskets, with appropriate adjustments made based on the actual conditions of the flanges and the operating environment. For example, if the roughness of the flange sealing surfaces exceeds the standard values, if there is poor flatness or surface condition, if pressure fluctuations are significant, or if the diameter of the flanges is large, the gasket thickness should be increased. On the other hand, if the fluid viscosity is low, the temperature is high, the pressure is high, or the fluid has high chemical reactivity, the gasket thickness should be reduced. Although the thickness of metallic and semi-metallic gaskets is more closely related to their structural characteristics, these selection principles still apply. (2) Width: The width of the gasket should be such that, under the specified bolt load, the stress in the gasket is equal to or greater than the minimum gasket sealing stress. Obviously, for the same bolt load, the narrower the gasket width, the higher the gasket stress ； However, the gasket stress must not approach or exceed the gasket’s fracturing or crushing pressure. Furthermore, the gasket stress becomes higher ； However, the gasket stress must not approach or exceed the gasket’s fracturing or crushing stress. Furthermore, the gasket width must ensure the necessary length of radial sealing channel and be sufficient to resist the force exerted on the sealing surface. (3) Inner and outer diameters: The inner diameter of the gasket should not be smaller than the inner diameter of the pipe. If the gasket extends into the pipeline, on one hand, the fluid inside the pipeline erodes the gasket, contaminating it, and on the other hand, it increases the flow resistance of the fluid. Furthermore, the compression gasket may be damaged due to the swelling of the gasket material by the medium. As specified in ASME B16.20, for wound gaskets, the inner diameter of the inner ring must extend into the flange or pipe connection by no more than 1.5 mm. For flanges with flat or raised sealing surfaces, when the gasket is located within the circle centered on the bolt holes, it is usually required for installation and positioning that the outer diameter of the gasket (or the outer diameter of its outer ring) be taken as the diameter of the circle centered on the bolt holes minus the diameter of the bolt holes (or bolts). It should be noted, however, that different flange standards use various methods to determine the outer diameter of such gaskets; for example, flanges in the class series according to ASME B16.5 and EN1759.1 use imperial units for the sizes of their bolts and bolt holes. When using the corresponding standard wound gaskets or flanged metal gaskets with a cover layer (ASME B16.20, EN12560-2, and EN12560-6), the outer diameter of the retaining ring (or outer ring) is the diameter of the circle centered on the bolt holes minus the nominal diameter of the bolt. Therefore, if flanges conforming to this standard are used but metric bolts are employed, the diameter of some metric bolts being larger than that of imperial bolts may cause the gasket positioning ring to come into contact with the bolts. Furthermore, there is no need to extend the gasket beyond the flange, as the gasket will be damaged by exposure to an oxidizing or corrosive atmosphere.