When choosing a valve stem packing, it should be determined according to the specific conditions. The method we generally choose is: between pressure and temperature, only one *** value can be satisfied; When choosing a high-temperature valve packing, the temperature should be considered first; For corrosive media with low temperature and pressure, materials with good corrosion resistance should be preferred. The sealing packing of the valve stem is divided into three categories: non-metallic, semi-metallic and metallic. The common conditions of use are as follows:
(1) Plastics mainly have PTFE and impregnated PTFE, both of which have strong corrosion resistance, suitable for sealing gases and high-fluidity liquids, the temperature is 200-200 degrees Celsius, and the pressure is below 32 MPa, and is often used in soft seal gate valves, cast iron butterfly valves and other low-pressure valves at room temperature.
(2) Graphite Flexible graphite has the advantages of high temperature resistance, corrosion resistance, excellent self-lubrication, etc., and is often used in high-pressure steam, water and oil media and seals with high requirements for packing. In addition to strong oxidizing media such as oleum acid and concentrated nitric acid, flexible graphite can withstand all media. The operating temperature is between 200 and 600 degrees Celsius, suitable for working conditions with nominal pressure not higher than 32 MPa, and is often used for high temperature and high pressure ball valves, gate valves, globe valves, etc.
(3) Carbon fiber has the advantages of high strength, high temperature resistance, wear resistance, and low relative density. At the same time, it also has excellent elasticity, flexibility and excellent self-lubricability, and can work stably under the working conditions of temperature - 120-350 degrees Celsius and pressure not more than 32 MPa.
(4) Asbestos fiber has good heat resistance, can withstand high temperature of 480 °C, can resist weak acid, strong alkali, has the advantages of high strength and good adsorption performance. However, it has poor acid resistance, high friction coefficient and poor thermal conductivity. Therefore, it is often impregnated with some PTFE emulsion, lubricating grease and some metal wire in asbestos fiber to reduce the friction coefficient and increase its wear resistance and heat resistance.
(5) Bellows sealing: The sealing structure is that one end of the bellows is fixed on the valve stem, and the other end is fixed on the bonnet, so that the valve stem and the bonnet are in a fully closed state. Therefore, the sealing performance is better, and this seal is often used in bellows stop valves used in toxic, flammable and other media, such as carbon disulfide solution in hydrogenation units and liquefied petroleum gas in gas separation units.
(6) Metal can be divided into five types: metal foil tape winding filler, metal foil kneading laminating filler, metal corrugated filler, lead wire twisting filler and lead ring filler. The first two have the advantages of high temperature resistance, erosion resistance, wear resistance, high strength, good thermal conductivity, etc., and are used for high temperature and high pressure working conditions; Metal corrugated packing is a good high-temperature and high-pressure packing; Lead wire twisting filler is mainly used in strong oxidizing media such as concentrated sulfuric acid at a temperature of no higher than 90 °C; The lead ring packing is used in a staggered manner with asbestos packing, and is suitable for gate valves for conveying oil products at a temperature of no higher than 550°C and high-temperature and high-pressure steam shut-off valves for electric vehicles.
In addition to the above packings, there is also a liquid seal at the stem. The use conditions of the packing are closely related to the structure of the packing box, the working conditions, the material, form, number of layers and the installation quality of the filler.
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