[Sealing Knowledge] Gasket and bolt flange connection systems

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Gasket: A gasket is a single sealing component that is placed between the sealing surfaces of a pair of flanges. During installation, an external force is applied to the flange by tightening the bolts; this force is transmitted to the gasket, causing it to compress and deform. Starting from the concept of energy, the work done by an external force on the gasket, if no energy losses are considered, is entirely converted into deformation energy, which is stored within the gasket. During operation, the pressure of the sealing fluid forces the flange sealing surfaces to separate, partially relieving the load on the compression gasket. The deformation energy stored in the gasket is recovered by doing work, that is, the gasket releases its energy. If there is still residual stored strain energy, the gasket remains in contact with the flange sealing surface, and the flange will not leak. Therefore, a gasket is also described as a “device that is sealed between two surfaces and stores energy.” 1. I. In bolt-flange connection systems, if it is technically and economically feasible to create very smooth and precise mating surfaces between the two components to be connected, and these surfaces are joined using bolts such that they remain in constant contact, then there is no need to use gaskets between them. Such bolt connections are known as non-gasketed bolt joints, and they are commonly used in industries such as automobiles, aircraft, steel structures, and heavy equipment. However, due to the limitations of the equipment’s structure and size, it is difficult to achieve an ideally smooth joint surface during processing and installation ； Secondly, the joint surface can also become corroded or abraded over time, losing its smoothness. Therefore, gaskets need to be installed between these joint surfaces as transitional elements, so as to establish tight contact with the joint surfaces, provide the sealing necessary for the connection, and enable sealing at lower bolt loads compared to bolt joints without gaskets. Such bolt connections are known as gasketed bolt joints, and they represent the most widely used and frequently applied detachable connections in pressure pipelines, pressure vessels, and pressure-bearing equipment. Since the connecting elements typically use a pair of flanges, one end of each flange is connected to the housing or pipe, and the two flanges are fastened together with a set of bolts. Such bolted connections are generally referred to as bolted flanged connections (BFC) or bolted flanged joints (BFJ), as shown in Figure 3.18. It can be seen that BFC is a assembly composed of components such as flanges, bolts, and gaskets. The two flanges can be flanges of the same type and material, or they can be flanges of different types and/or materials; alternatively, one of them can be a flat cover connected by bolts. Gaskets are always made of softer materials than those used for flange sealing. II. According to the sealing principle, the two types of bolt flange connections can be divided into two major categories: the floating type (FL), which is a non-metal-to-metal contact type, and the metal-to-metal contact type (MMC) bolt flange connection. As shown in Figure 3.19(a), in a floating bolt flange connection, the gasket is sandwiched between the two mating surfaces of a pair of flanges. From the moment the tensioning bolts are installed until the end of the flange connection’s service life, there is no contact between the flange surfaces; as a result, the bolt tensioning force acts entirely on the gasket. Once the system is in operation, the initial load on the gasket changes depending on the external loads applied, and the load distribution across the gasket is also uneven. In the metal-to-metal contact type (hereinafter referred to as “MMC”) bolt flange connection, the same flanges as those used in the floating-type bolt flange connection can be employed; the only difference is that certain types of compression limiting devices are placed on the flange mating surface within the bolt’s central circle. These secondary compression limiting devices can either be part of the gasket or a separate component that serves as a limiter, as shown in Figure 3.19(b). Therefore, after the bolt is pre-tightened, mechanical contact occurs between the metals of the two mating surfaces of the flange, or between the metal of the flange mating surfaces and the metal of the compression limiter. Compared to the floating type, the main feature of the MMC-type bolted flange connection is that the bolts must be subjected to a high enough load in order to ensure metal-to-metal contact; furthermore, it is necessary to guarantee that under any external loads applied thereafter, metal contact is maintained throughout the entire life cycle of the flange connection without any separation, so that the load on the gaskets remains constant and evenly distributed circumferentially. The compression limiting device serves to bear external loads, providing optimal sealing performance for the flange joint. Therefore, the main difference between floating-type and MMC-type flange bolt connections is that any change in the gasket environment does not pose a risk of interfering with the magnitude and distribution of the gasket load, as such interferences – including loads such as self-weight, fluid pressure, external bending moments, and temperature changes, along with their fluctuations – are transferred to the bolts and the mechanically contacting components. Secondly, the MMC flange bolt connection also reduces the risk of fatigue or loosening of the bolts due to exposure to cyclic loads. Furthermore, it also increases the stiffness of the flange and reduces its deflection. III. Loads on bolted flange connections As shown in Figure 3.20, the loads acting on bolted flange connections are as follows. ①Pressure load P: The pressure of the medium acting uniformly in the radial direction on the inner wall of the flange and the inner circular surface of the gasket. ②End load W: The axial load resulting from the pressure in the medium at the flange ends, which causes the flanges to separate axially. ③Bolt load F: The force that stretches the bolt when the nut is tightened; this reaction force is used to compress the flange. ④Gasket load: The compressive force exerted by the flange on the gasket. The compressive load applied to the gasket is used, in part, to cause the gasket material to flow and fill in the pits and other irregularities on the flange sealing surfaces; the remaining part is used to compensate for separations between the sealing surfaces caused by internal pressures, differences in thermal expansion between the flanges and bolts, etc., thereby preventing leakage of the sealing medium. ⑤External force T and external moment M: These are the axial forces or (and) tangential moments exerted on the flange by the equipment and pipes connected to it. Typically, gasket leakage occurs either due to leakage paths within the gasket itself (internal leakage), or through pathways between the gasket surface and the flange sealing surface (interfacial leakage). When the flange bolts are tightened, the upper and lower surfaces of the gasket are compressed, causing the internal leakage pathways to close, while the deformation of the surface blocks the leakage pathways at the interface. Therefore, the magnitude and distribution of the bolt pre-tightening load or gasket pre-stress are the most direct and important factors affecting the sealing performance of bolted flange connections.