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There are many types of valves, but the basic role is the same, that is, to connect or cut off the medium flow. Therefore, the valve sealing problem is very prominent. to ensure that the valve can be a good cut-off medium flow, no leakage, we must ensure that the valve seal is intact. The causes of valve leakage for many reasons, including structural design of the unreasonable, sealing contact surface defects, fastening parts loose, the valve body and the valve cover is not tight, etc., all of these problems may lead to the valve sealing is not good, thus generating leakage problems. Therefore, the valve sealing technology is related to the performance and quality of the valve is an important technology, the need for systematic and in-depth research.

    Valve from the generation to the present, its sealing technology has also experienced great development. So far, the valve sealing technology is mainly reflected in two major aspects, namely, static sealing and dynamic sealing.As shown in the figure is an indication of the valve's dynamic and static seals in a butterfly valve：

   一、Static seal: the so-called static seal, usually refers to the seal between two stationary surfaces, the sealing method of static sealing is mainly the use of gaskets. There are many types of washers, often used washers including plain washers, O-rings, Jacketed gasket, Shaped washers, wave washers and winding gasket and other major categories, each type below there can be based on different materials and further division.

   a, Plain washers: plain washers is plain paste in the two stationary cross-section between the washers, generally according to the use of materials can be divided into plastic plain washers, rubber plain washers, metal plain washers and in line with the material plain washers, each material plain washers have their scope of application.

   b, O-ring: O-ring refers to the cross-section shape of the O-shaped washers, because of its cross-section shape is O-shaped, there is a certain self-tightening effect, so the sealing effect is better than the plain washers.

   c、Jacketed gasket：It is a gasket that wraps a certain material around another material, and such a gasket generally has good elasticity to enhance the sealing effect.

   d、Shaped washers: shaped washers are those irregularly shaped washers, including oval washers, diamond-shaped washers, gear-type washers, dovetailed washers, etc. These washers generally have a self-tightening effect, mostly in the high school pressure valves used.

   e、Wave washer: Wave washer is a washer with a wave shape, this type of washer is usually a combination of metallic and non-metallic materials to form, generally has a small compression force, sealing effect is good.

   f、Winding gasket: Winding gasket refers to the very thin metal and non-metallic tape is tightly pasted together, winding the formation of the gasket, this kind of gasket has good elasticity and sealing.

   二、Dynamic sealing, mainly refers to the stem sealing, that is, to prevent the medium inside the valve with the stem movement and leakage, sealing method of dynamic sealing is mainly the use of stuffing box. There are two basic forms of stuffing box, namely, gland type and compression nut type. Gland type is currently the most used form, generally from the form of the gland, can be divided into a combination of two types and the whole type, each form of difference, but basically contains a compression bolt; compression nut type is generally used for smaller valves, due to this form of small size, so the compression force is limited. In the packing box, because the packing is in direct contact with the valve stem, all the requirements of the packing sealing, friction coefficient is small, can adapt to the pressure and temperature of the medium, and corrosion resistance. Currently more commonly used fillers include rubber O-rings, PTFE preparation packing, asbestos packing and plastic moulding packing, etc. Each kind of packing has its own adaptable conditions and scope, according to the specific needs to be selected.

   Sealing is to prevent leakage, then the valve sealing principle is also studied from the prevention of leakage. There are two main factors causing leakage: one is the main factor affecting the sealing performance, that is, there is a gap between the sealing pair, and the other is the pressure difference between the two sides of the sealing pair. The sealing performance of the valve affects the stable operation of the entire piping system. The sealing performance of the valve is directly related to the normal operation of the pipeline system. Good sealing performance can be avoided because of water leakage and air leakage caused by material loss and environmental pollution. Therefore, to maintain good sealing of the valve is an important guarantee for the normal use of the valve.