First, the gasket sealing mechanism
Leakage from the limited space inside the media flow to the outside, or the limited space inside from the outside. Medium flowing through the external space of the seal surface at the interface leak. The fundamental cause leakage due to the contact surface there is a gap, and the pressure difference on both sides of the contact surface, the concentration difference is the driving force of leakage. Due to leakage in the form of surface machining accuracy and other factors, there is a gap sealing surface is inevitable, which will cause the sealing surface is not completely consistent, so that leakage occurs. To reduce leakage, the contact surface must be fitted to the maximum extent, the leakage path cross-sectional area decreases, increasing leakage resistance, leakage and make greater driving force. Compression load is applied to the sealing surface, to produce a compression stress, can improve the degree of contact of the sealing surface, when the stress increases to produce significant enough to cause plastic deformation of the surface, the sealing surface of the gap can be filled, plug leak paths. The purpose is to take advantage of using shims gasket material under the action of the compression load more prone to plastic deformation characteristics, so that filled the tiny uneven flange sealing surface in order to achieve seal.
Second, the gasket seal leakage form
Bolt connection flange gasket, gaskets sealing element is the most important. For non-metallic gaskets, the connection is sealed by tightening the bolt, causing the contact surface of the flange and gasket gasket inside the larger compression stress, which on the one hand and the surface of the flange gasket surfaces fit closely , fill the flange surface of the micro-gap, on the other hand reduce the porosity of gasket material, which reduces leakage of sealed fluid passage. Since any manufacturing or processing methods are not ideal for absolutely smooth surface is formed, it is impossible to achieve full engagement between the sealing surface and the sealing member itself completely blocks the pores, so that the sealing surfaces in contact with each other and the seal between total internal is the existence of a small gap or channel. Thus, the gasket seal, the leakage is always inevitable. When the medium pressure with a flange by a bolt, always there is a leak in the sealing point, analysis of this phenomenon can be found which is based on two forms, namely the so-called "screen leak" and "penetration leak" [1] Figure 3-2
1 Interface Leak
Gasket compression stress enough flange sealing surface roughness, thermal deformation of the pipe, mechanical deformation and vibration can cause gasket and flange sealing surface bonding between the lax leak. In addition, the operating conditions flanged bolt due to temperature, pressure, elongation of the bolt deformation, creep relaxation gaskets, resilience down the aging gasket material, such as also cause deterioration of the flange gasket leakage between the seal faces. This occurs between the sealing surface and the flange gasket leak referred to as "interface leak."
(2) penetration leakage
Non-metallic gaskets are usually made from plant fibers, animal fibers, mineral fibers or chemical fibers and rubber bonded repression, flexible graphite or other porous material produced. Due to its loose tissue density difference, between the fiber and the fiber innumerable slight gap, it is easy to be saturated media, especially under pressure, through the media inside the material permeates the pores. This occurred in the gasket material internal leakage phenomenon known as "penetration leakage."
Third, the gasket sealing process
In the loading process, the leak rate with gasket compression deformation varies. When the axial compression load is smaller than a certain value, although the gasket has produced a certain amount of compressive deformation, but the leak is still very serious, substantially no sealing capacity; continue to increase compression loads, the compression deformation of the gasket along increases, the leakage rate decreases; when the axial compression load to a certain extent, the leakage rate is almost constant. During the uninstall process, gasket compression deformation decreases with the compression load decreases, along with the corresponding leak rate increases, but at the same axial load, unload leak rate than when loaded under stress corresponding gasket leak rate is small. When the axial load is reduced to a certain extent, although the elastic deformation of the gasket has not completely disappeared, still has some resilience, but the leak rate increases sharply.
Loading and unloading the change of the leak rate of the seal face can be explained by the microstructure. Sealing surface microstructure shown in Figure 3-3, the initial surface consists of the following components [2], where: A - the largest flange surface roughness;
     B - flange surface defects (cracks, scratches, etc.);
    a - the largest gasket surface roughness;
    b - gasket surface defects
    c - sealing surface between the impurities, such as burr.
In the loading process, the leak rate with gasket compression varies. With the contact surface between the first surface of the projecting portion is such as glitches, particulate impurities, as 3-3 (a) in Fig. Beginning of the loading process, because the local load is large, the convex portion is flattened or embedded quickly recess until Fig 3-3 (b) state where the gasket has produced although a certain compression, but the leak still serious. In this phase, with most of the surface in a free state, the gap is large, substantially no sealing capacity, yet not form an initial seal. The 3-3 (b) the state continues to load, with the surface between the peaks and troughs interspersed with each other, fitted with the micro-gap side gradually reduced until the anastomosis, as shown in 3-3 (c) in Fig. In this phase, the flow channel cross-section decreases with increasing compression force, flow path resistance increases, the leak rate is reduced accordingly, an increase of the load can be effectively controlled compression leakage, it is usually said that the stage is normally sealed phase. From Figure 3-3 (c) shows that when basic agreement with the surface, if the load continues to increase compression, increased compression gasket minimal leakage rate is almost unchanged. This time from the initial surface roughness formed by micro-gap has been largely blocked, most of the mating surface fitting, leak paths mainly by surface defects such as cracks, scratches, etc., and to further eliminate this part of the gap is very difficult. Unloading process, the sealing surface due to mutual fitting plastic deformation is not generated because the uninstallation and recovery time, as long as the gasket is fully compacted, the spacer is sufficient to compensate for springback caused by pressure medium sealing surface separated from each other between the connection always has a sealing capability. This is the same compression load, unload the leakage rate is much smaller than the load causes the leakage rate. However, due to the initial surface roughness, the sealing surface is uneven stress distribution, the fitting process of the entire surface of the gasket is not formed with the plastic deformation of the flange face anastomosis, a small part of the force trough Department is still in elastic state. This part of the elastic deformation of the compression load decreases with recovery, so part of the uninstall process microgap reappeared, leakage rate and decreases with the compression load gradually increases.
Fourth, the main factor in the leak
On the gasket seal, its leakage status and physical properties of the medium to be sealed, working conditions, flange sealing surface roughness of the gasket compression stresses and basic characteristics, dimensions, loading and unloading process, and many other factors.
1 medium to be sealed affect the physical properties of
Sealed connection using the same form, in the same working conditions, the gas leak
Rate is greater than the fluid leakage rate, the leakage of hydrogen gas leakage rate is higher than the nitrogen. This was mainly due to the medium to be sealed caused by different physical parameters. In the physical properties of the medium to be sealed, the viscous greatest impact. Viscosity is a measure of fluid friction, the viscosity of the medium, the leakage resistance, leakage rate is small; small viscosity of the medium, the leakage resistance, leakage rate is large.
(2) the impact of working conditions
Gasket working conditions including rings pressure, temperature and so on. Different pressure, temperature, the leakage rate of different sizes. Differential pressure across the sealing surface is the main driver of leakage, the greater the pressure difference, the more easily overcome the media leak channel resistance, leakage easier. Temperature sealing properties of the connection structure has a great influence. Studies have shown that the elastic gasket, plastic deformation volume increases with temperature, and the temperature increased and resilience decrease the creep increases with increasing temperature. And as the temperature rises, the gasket aging, weight loss, creep, relaxation phenomenon will become increasingly serious. In addition, the viscosity of the medium temperature also has a great influence, as the temperature increases, the viscosity of the liquid decreases the viscosity of the gas increases. The higher the temperature, the more prone to leakage.
3 flange surface roughness
Shim preload than the same pressure, the flange surface roughness different, not the same leak rate. Typically, the surface roughness is smaller, the smaller the amount of leakage. Milled flange sealing surface sealing effect than its unground flange sealing surface of the seal effect. This is mainly due to the small roughness ChengSu uneven sealing surface which is easily filled, so that the size of the interface to reduce leakage.
4 gasket compression stress
The greater the stress on the gasket compression, the greater the deformation. Gasket deformation on the one hand effectively fill the flange surface irregularities, making the interface leakage is greatly reduced; hand makes the gasket itself is compressed within the pores, reducing the cross section of the channel leakage leakage resistance increases, thereby Leak rate is greatly reduced. However, if the gasket compression stress is too large, then crushing the gasket and thus lose resilience, can not compensate for temperature 'pressure-induced separation of the flange surface, resulting in a sharp increase in the leak rate. Therefore, to maintain a good seal, the compression stress of the spacer must be kept in a certain range.
5.washer properties and geometrical dimensions of
   (A) the basic properties of gasket
Gasket fundamental mainly consists of two parts, one is the mechanical ability gasket, which includes gasket compression rebound characteristics, creep and stress relaxation characteristics; rather basic gasket sealing performance. The basic properties of the connection pads sealing performance greatly.
Bolt connection flange gasket sealing gasket is deformed essentially to increase the flow resistance to be achieved. Gasket compression characteristics of the surface portion of the response is fitted with the flange surface, the ability to form an initial seal. In operating conditions, due to the elongation of the bolt and the deformation of the flange, the flange and gasket surfaces produce relatively isolated (gasket compression stress suffered decrease) the tendency then depends largely on whether or not the close connection resilience in the gasket. Creep and stress relaxation are linked, it reflects the gasket material at a certain temperature and deformation under load increases over time, the stress decreases with an increase over time the variation. Creep and relaxation leading to high temperature sealing connection leak rate increases of important reasons. Characterization gasket seal leakage rate and media pressure, gaskets residual clamping force and temperature relationship is the ability of a gasket seal comprehensive index.
(2) Size of gaskets
 ① thickness of spacer
 In the same compression load, the same media pressure, the leakage rate increases with the thickness of the gasket is reduced [3]. This is because in the same axial loads, the thickness of the compression gasket has a large springback sealed condition has been reached in the initial case, the elastic spacers larger reserves, more than compensating shim As medium pressure induced relative separation between the sealing surface, and to keep large residual surface gasket compression stress, so that the leak rate decreased. But can not say the thicker gasket, the sealing performance is better. This is because the gasket thickness, the conditions for establishing the initial seal is different, since the end face of the effects of friction, the spacer surface of the three-way compression stress, deformation resistance of the material is relatively large; the central gasket, less affected by the end portion, the deformation resistance is small. In the same preload, the gasket surface of the central spacer are more prone than the plastic deformation, this time, to establish the initial sealing surface is also more difficult, so when the gasket thickness reaches a certain value after the sealing performance does not change even worse. In addition, the thicker shim, osmotic leak cross-sectional area larger. Greater penetration leakage rate.
 ② gasket width of
 In a certain range, with the increase in the width of the spacer, the leakage rate decreases linearly [3]. This is because the effective width of the gasket leak resistance and leakage of the medium length of the channel (width proportional to the spacer) is proportional. But can not say, the spacer as widely as possible, because the wider spacer, the spacer, the greater the surface area, so that the gasket to produce the same compression stress, the gasket width narrower than the pad bolt force necessary larger pieces.

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