Flange gasket sealing principle and leakage form

Basic Concepts of Gasket Sealing

Gasket sealing is the most important sealing structure for detachable joints such as pressure vessels, process equipment, power machines and connecting pipes in industrial installations. It usually consists of flanges, gaskets, connecting bolts and nuts, collectively referred to as flange sealing joints . The structural form of the flange sealing joint is shown in Figure 1.

Figure 1 Structural form of flange sealing joint

1-nut 2-flange 3-gasket 4-bolt

Gasket Sealing Mechanism

Leakage is the undesired phenomenon of a medium flowing from inside a confined space to the outside, or from outside into a confined space. The medium flows through the interface between the inner and outer spaces, that is, the sealing surface leaks. The root cause of leakage is that there is a gap in the contact surface, and the pressure difference and concentration difference on both sides of the contact surface are the driving force for leakage. Due to the influence of factors such as the form of the sealing surface and the machining accuracy, the sealing surface is not completely matched, that is, there will be a gap on the sealing surface, resulting in leakage. To reduce leakage, it is necessary to maximize the fitting of the contact surface, that is, to reduce the cross-sectional area of ​​the leakage channel and increase the leakage resistance, making it greater than the leakage driving force. Applying a compressive load to the sealing surface can generate compressive stress and improve the contact degree of the sealing surface. When the stress increases enough to cause obvious plastic deformation of the surface, the gap of the sealing surface can be filled to block the leakage channel. The purpose of using the gasket is to use the characteristics of the gasket material to easily produce plastic deformation under the action of the compression load, so that it can fill the small unevenness of the flange sealing surface, so as to achieve sealing. In flanged sealing joints, the force of compressing the gasket deforms the gasket material, thereby filling the micro-gap between the flange sealing surfaces.

Leakage Forms of Gasket Sealed Connections

In flanged sealing joints, gaskets are the main sealing element. For non-metallic gaskets, the sealing of the connection is by tightening the bolts, resulting in a large compression stress on the contact surface between the flange and the gasket and inside the gasket. The micro-gap on the blue surface, on the other hand, reduces the porosity of the gasket material, that is, reduces the leakage channel of the sealed fluid. Since it is impossible to form an absolutely smooth ideal surface by any processing method, and it is impossible to achieve complete fitting between the sealing surfaces and complete blocking of the pores of the seal itself, there is always between the contacting sealing surfaces and the interior of the seal. with tiny gaps or channels. Thus, for gasket seals, leakage is always inevitable. When the medium passes through the bolt-flange connection with a certain pressure, there is always leakage at the sealing point. Analyzing this phenomenon, it can be found that leakage occurs in two forms, namely "interface leakage" and "osmotic leakage", as shown in Figure 2.

1. Insufficient compression stress of the interface leakage gasket, rough flange sealing surface, thermal deformation, mechanical deformation and vibration of the pipeline will cause leakage between the gasket and the flange sealing surface. In addition, under the action of temperature and pressure, the flange joint will deform and elongate the bolt, the creep relaxation of the gasket, the reduction of the resilience, and the aging and deterioration of the gasket material will also cause the flange and the flange sealing surface. leakage between. This kind of leakage between the gasket and the flange sealing surface is called "interface leakage".

2. Penetration leakage

Non-metallic gaskets are usually made of plant fibers, animal fibers, mineral fibers or chemical fibers bonded and pressed with rubber, or made of porous materials such as flexible graphite. Due to its loose organization and poor compactness, there are countless tiny gaps between fibers, so it is easy to be saturated by the medium, especially under the action of pressure, the medium will penetrate through the pores inside the material. This type of leakage that occurs inside the gasket material is referred to as "Infiltration Leakage".

Figure 2 "Interface Leakage" and "Infiltration Leakage"

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