The Sealing Principle and Leakage Form of Gaskets

Sealing Principle and Leakage Form of Gaskets 

Basic Concepts of Gasket Sealing

Gasketed sealing is the main sealing structure for detachable connections in industrial devices such as pressure vessels, process equipment, power machines, and connecting pipelines. It is usually composed of flanges, gaskets, connecting bolts, and nuts, which is collectively known as flange sealing joints. 

Sealing Mechanism of Gaskets

Leakage is a phenomenon that people do not want the medium to flow from the inside of a limited space to the outside, or enter the inside of a limited space from the outside. The medium flows through the interface between the inner and outer spaces, i.e. the sealing surface leaks. The fundamental cause of leakage is the presence of gaps in the contact surface. The pressure and concentration differences on both sides of the contact surface are the driving force behind the leakage. Due to factors like the form of the sealing surface and processing accuracy, the sealing surface may not fully match, resulting in gaps on the sealing surface and leakage. To reduce leakage, it is necessary to maximize the integration of the contact surface, that is, reduce the cross-sectional area of the leakage channel, increase the leakage resistance, and make it greater than the leakage driving force. Applying pressure and tightening load to the sealing surface can generate compression stress to improve the contact degree of the sealing surface. When the stress increases enough to cause significant plastic deformation on the surface, it can fill the gap of the sealing surface and block the leakage channel. The purpose of using gaskets is to use the characteristics of gasket materials that are more prone to plastic deformation under compression load, so as to fill the small unevenness of the flange sealing surface and achieve sealing.

In flange sealing joints, the force of compressing the gasket causes deformation of the gasket material, thereby filling the micro gap between the flange sealing surfaces. 

Leakage forms of gasket sealing connections

In flange sealing joints, gaskets are the main sealing components. For non-metallic gaskets, the sealing of the connection is achieved by tightening the bolts, causing significant compression stress on the contact surface between the flange and the gasket, as well as inside the gasket. On the one hand, this makes the gasket surface tightly adhere to the flange surface, filling the micro-gaps on the flange surface. On the other hand, it reduces the porosity of the gasket material, that is, it reduces the leakage channel of the sealed fluid. Due to the impossibility of any machining method to form an absolutely smooth ideal surface, complete fitting between sealing surfaces, and complete blockage of the pores of the sealing components themselves. There are always small gaps or passages between the sealing surfaces in contact with each other and inside the sealing components. Therefore, leakage is always inevitable for gasket seals. When the medium passes through a bolt flange connection at a certain pressure, leakage always occurs at the sealing point. Analysis of this phenomenon reveals that leakage occurs in two forms, namely "interface leakage" and "permeation leakage".

1. Interface Leakage

Insufficient compression 
Stress on the gasket, rough flange sealing surface, thermal deformation, mechanical deformation, and vibration of the pipeline can all cause inadequate adhesion between the gasket and flange sealing surface, leading to leakage. In addition, under operating conditions, flange joints may experience bolt deformation, elongation, gasket creep, relaxation, decreased rebound ability, aging, deterioration of gasket materials, leakage between the flange and flange sealing surface due to temperature and pressure. This type of leakage that occurs between the gasket and the flange sealing surface is called "interface leakage".

2. Permeation Leakage

Non-metallic gaskets are usually made by bonding and pressing plant fibers, animal fibers, mineral fibers, or chemical fibers with rubber, or by making porous materials such as flexible graphite. Due to its loose organization and poor density, there are countless small gaps between fibers, making it easy for the medium to penetrate. Especially under pressure, the medium will penetrate through the pores inside the material. This type of leakage that occurs inside the gasket material is called "permeation leakage".

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