Causes of flange leakage and treatment methods

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Flange sealing generally relies on the preload generated by the connecting bolts. Through various solid gaskets (such as rubber gaskets, asbestos-rubber gaskets, plant fiber gaskets, spiral-wound metal gaskets filled with asbestos, corrugated metal gaskets filled with asbestos, corrugated metal-clad gaskets filled with asbestos, corrugated metal gaskets, flat metal-clad gaskets filled with asbestos, grooved metal gaskets, raised-face flat metal gaskets, metal ring gaskets, octagonal metal gaskets, etc.) or liquid gaskets (which transform into solid gaskets of a certain shape under specific conditions or over a certain period of time), a sufficient operating sealing pressure is achieved to prevent any leakage of the fluid medium being sealed. This type of sealing falls under the category of forced sealing, as shown in Figure 2-1. The common types of leaks in this type of sealing structure are as follows. 1. The main causes of interface leakage are insufficient clamping force on the sealing gasket, inappropriate roughness on the flange mating surfaces, thermal deformation of pipes, mechanical vibrations, etc. All these factors can lead to poor sealing between the gasket and the flange surface, resulting in leakage. Furthermore, after flange connection, issues such as bolt deformation and elongation, as well as plastic deformation of the gasket over time, reduced rebound strength, aging, cracking, and deterioration of the gasket material, can also lead to a poor fit between the gasket and the flange surface, resulting in leaks. As shown in Figure 2-2, therefore, we refer to this type of leakage that occurs due to a poor fit at the interface between the metal surface and the gasket as \"interface leakage\". Leakage at flange connections can occur regardless of the type or material of the gasket used; in most cases, it is interface leakage. Interface leakage accounts for over 80% to 95% of all flange leaks. 2. Penetration and leakage: Plant fibers (cotton, linen, silk), animal fibers (wool, rabbit hair, etc.), mineral fibers (asbestos, graphite, glass, ceramics, etc.), and chemical fibers (various plastic fibers such as nylon and polytetrafluoroethylene) are all commonly used raw materials for manufacturing gaskets. Leather and paperboard are also frequently utilized as gasket materials. The structural composition of these base materials is relatively loose, with poor density; there are countless tiny gaps between the fibers. As a result, they can be easily penetrated by fluid media. Under the pressure exerted by such fluids, the fluid being sealed can seep through these microscopic gaps to the low-pressure side, as shown in Figure 2-3. Therefore, we refer to this leakage phenomenon, which occurs when the fibers of the gasket material have certain gaps between them and the fluid medium can pass through these gaps under certain conditions, as \"permeation leakage\". Osmotic leakage is generally related to the operating pressure of the fluid medium being sealed; the higher the pressure, the greater the leakage rate. Furthermore, permeation leakage is also related to the physical properties of the fluid medium being sealed; media with low viscosity are prone to permeation leakage, while those with high viscosity are less likely to experience it. Permeation leakage generally accounts for about 8% to 12% of flange seal leakage incidents. With the rapid development of materials science, new types of sealing materials have emerged. These new materials possess excellent density, and gasket made from them experience less and less permeation leakage. With further advancements in materials science and technology, penetration leakage incidents involving gaskets will one day be completely resolved. 3. Human factors account for a large proportion of the incidents that cause leaks and damage. During installation, gaskets often become misaligned, resulting in insufficient local sealing pressure or excessive preload. This exceeds the design limits of the gasket, causing it to lose its resilience. Furthermore, uneven tightness of the flange connection bolts and offset between the center lines of the two flanges can lead to the aforementioned issues during the process of tightening the flanges, as shown in Figure 2-4. Therefore, we refer to leaks that occur due to excessive compression of the gasket or insufficient sealing pressure resulting from poor installation quality as \"damage-induced leaks\". This kind of leakage depends to a large extent on human factors. The management of construction quality should be strengthened, such as by selecting a structural form with high sealing reliability. Generally speaking, for low-pressure systems, wide-face flanges are easier to center and align than narrow-face flanges; as shown in Figure 2-5, this results in fewer leaks. Additionally, the convex-concave flange sealing structure is preferable to the flat flange sealing structure. Adopting some effective methods on existing equipment and pipeline flanges can also significantly improve the quality of installation. During the installation of flat flanges, the use of positioning non-drying adhesive can effectively prevent the gaskets from shifting, thereby reducing the workload for the workers. Damage-related leakage incidents generally account for about 1% to 5% of all leakage incidents. The leakage rates for both interface leakage and damage-induced leakage increase significantly over time, whereas the relationship between the leakage rate of permeation leakage and time is not very apparent. Regardless of the type of leak, measures should be taken immediately once it is detected. First, use a wrench to check whether the connection bolts are loose, then tighten them evenly until the leakage stops. If the leakage does not go away after tightening the bolts, some method from dynamic sealing technology should be considered to address the issue. It is better to eliminate leaks using dynamic sealing technology at an early stage rather than later. Attempting to address leaks only after they have significantly increased will complicate the dynamic sealing process and inadvertently increase the construction difficulty. In addition to the three types mentioned above, the corrosion of the medium is also a factor that can cause leakage at the flange sealing surface. This type of corrosion is galling corrosion, which occurs primarily in the tiny gaps at the flange joints. There, the oxygen supply in the medium is insufficient, which creates a potential difference between it and the medium outside the gaps, leading to electrochemical corrosion. This type of chemical corrosion is known as “gradient electrochemical corrosion”. Corrosion leakage occurs gradually; only when corrosion pitting connects to form a passage can the sealed fluid medium leak out. During on-site maintenance, we often find many spots on the flange sealing surface; some of these spots even form distinct small pits. These are the results of differential electrochemical corrosion, yet no leakage has occurred. Corrosion-induced leakage is relatively rare; and even when it occurs, its form is very similar to interfacial leakage, as it takes place at the contact interface between the flange sealing surface and the gasket, in a manner akin to interfacial leakage, so no further details are provided here.