1. Leakage of gasket
There are four forms of gasket leakage: interface leakage, penetration leakage, diffusion leakage, and damage leakage.
Because the gasket is not tight enough, the flange joint surface roughness is not good, the flange deformation and so on will produce leakage on the sealing interface. For the interface leakage, the use of soft material clamping in the flange surface can be prevented.
Because the material of soft material is mostly fiber, it is easy to be soaked by medium, especially pressure effect. Because of capillarity, the medium will permeate into the low pressure side to form seepage.
Under the driving of the concentration difference, the medium leaks through the seal gap or the material produced by the capillary in the sealing material. Diffusion is the result of molecular motion, and is not unidirectional. In the sealing pressure difference or vacuum state, or sealing nitrogen, hydrogen and other permeable substances, as well as highly toxic, radioactive substances, we should pay special attention to the two kinds of leakage and diffusion.
The sealing part of the working condition is bad, often due to thermal deformation, vibration, shock, improper installation, fastening force is too large, repeated use, compression deformation, stress concentration, over fatigue strength and damage, material aging, deterioration will cause damaging leaks. This leakage has the greatest impact and the harm is great, we should strengthen the prevention.
2. Sealing mechanism of gasket
The function of the gasket is caused by the blocking effect of the sealing material on the joint surface and the elastic restoring force of the fastening force to the internal medium.
3. Classification and structure of gasket
The gasket is divided into non metal gasket, metal and non-metallic composite gasket and metal gasket three categories, the common materials are rubber, leather, asbestos board, cork, PTFE, tinplate, steel, aluminum, copper and stainless steel etc..
4. Selection of gasket
The selection principle of gasket: according to the working pressure, working temperature, corrosion of sealing medium and the form of sealing surface.
In the normal temperature and low pressure, selection of non metal soft honey seal; pressure at high temperatures, the combination of metal and nonmetal or metal gasket seals; in temperature, pressure fluctuations are larger, with good elasticity or self sealing pad; at low temperature, corrosive medium or vacuum conditions, should be considering the seal with special properties.
Influence of flange form
Different gaskets are required to use different gaskets. Flange generally smooth surface for low pressure, with a thin soft gasket; under high pressure, if the flange of sufficient strength, smooth surface can also use the flange, but it should be soft with a thick pad, or use the band to strengthen ring or enhance wound gasket ring. In this case, the metal gasket is not applicable, because the pressing force required at this time is too large, resulting in greater deformation of the bolt, so that the flange is not easy to seal. If the metal gasket is used, the sliding surface should be reduced so that the contact area of the gasket is reduced. Thus, when the bolt tension is the same, the compression stress of the narrow smooth surface will be increased after the reduction.