Selection of the hottest flange sealing gasket in

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Selection of flange sealing gasket in gas system

gasket seal is the most common static sealing form of industrial devices, and leakage is the most common failure form of production devices in many industrial production processes. According to statistics,% of the unplanned maintenance workload in petrochemical enterprises is used to repair sealing devices. With the development of modern industrial production, all industrial departments, especially large petrochemical enterprises, thermal power plants, nuclear power plants and other industrial enterprises, put forward higher and higher requirements for the performance of sealing gaskets. The fuel gas system is no exception. Due to the flammable, explosive and toxic characteristics of fuel gas, the fuel gas system also puts forward higher requirements for the gasket sealing used in it. This paper introduces the performance characteristics of asbestos rubber gasket, polytetrafluoroethylene gasket, winding gasket, high-strength graphite gasket, corrugated tooth composite gasket and other gaskets commonly used in the fuel gas system at present, and puts forward the problems worthy of attention when selecting them in combination with the situation of the fuel gas system

1 gaskets commonly used in gas systems and their characteristics

1.1 asbestos rubber sheet gaskets

this is the earliest traditional sealing material, including ordinary type and oil resistant type. Among them, there are various brands of high voltage, medium voltage and low voltage. Asbestos rubber sheet is made of asbestos fiber, rubber and other filling materials by mixing, heating and rolling, among which oil resistant sheet is also subject to heating and vulcanization treatment. Because the asbestos fiber has high strength and can withstand high temperature (the temperature resistance of asbestos is usually within 480 ℃, and the crystalline water in the asbestos fiber will decompose and lose strength when it is higher than this temperature), it has a wide range of materials, which is easy for industrial production. It can be made into gaskets of various shapes and specifications on site according to needs. It is convenient to use and cheap. Therefore, it has always been a widely used traditional sealing material. However, the sealing performance of this material is not high (its sealing performance is usually 1 ×~ 1 x cm3n/s), especially the rubber and other components of this gasket are easy to be damaged by the action of temperature and medium (such as solvent), and it is easy to have stress relaxation during use, affecting its sealing performance and service life. In addition, because asbestos is a recognized carcinogen, since the 1970s, many countries in the world have successively formulated laws to include it in the scope of prohibition, With the improvement of people's requirements for equipment sealing and the development of sealing technology, asbestos rubber materials have been gradually replaced by other new materials, and the amount of use is gradually decreasing. Generally, it is only used in non important equipment occasions with low temperature and pressure

1.2 PTFE gasket

PTFE material has excellent chemical corrosion resistance and will be favored by more and more businesses and has broad temperature resistance characteristics. It can resist the corrosion of various chemical media including acid, alkali, solvent, etc. its temperature resistance can range from -250 ℃ ~ 10 250 ℃, and it is clean and pollution-free. Therefore, it is commonly used as a sealing gasket in chemical, food, pharmaceutical and other enterprises, However, the biggest disadvantage of this material is its cold flow performance. Even at room temperature, its contact stress will decrease with the passage of time, that is, it is easy to produce stress relaxation, which will affect the sealing life of the gasket

1.3 expanded graphite spiral wound gasket

spiral wound gasket has been used abroad in the 1930s, and China began mass production and promotion after the 1960s. In the early days, asbestos was used as the filling material for spiral wound gaskets. Because the tightness of spiral wound gaskets filled with asbestos was not as good as that of expanded graphite, and asbestos was a carcinogen, now spiral wound gaskets mostly use expanded graphite as the filling material. The expanded graphite spiral wound gasket has good sealing performance, can meet the requirements of high temperature and high pressure parameters, and its resilience is also good. It can still maintain good sealing performance in the occasion of temperature and pressure changes (its sealing performance index can usually reach 1 x cm3n/s). It is a common gasket used in petroleum, chemical industry, electric power and other systems in the past. However, due to structural and manufacturing reasons, the defects of spiral wound gasket are also obvious. As some experts have pointed out, "this kind of gasket is used the most at present, but it also has the most problems", which is what people usually call "fragmentation" and collapse (or "instability"). The so-called "falling apart" includes two cases: one is that the gaskets are scattered and damaged during manufacturing, transportation and installation. This is because the spiral wound gasket structure is only formed by winding and biting each other, so that the gasket cannot form a solid structure as a whole, so it is easy to be scattered and damaged, which makes it difficult to transport and install the gasket, especially the large-size gasket (such as the gasket on the cold exchange equipment). Second, the spiral wound gasket is subject to a great pressing force in the process of use, and the welded joint used to fix the outer layer is prone to pull off and damage under the action of circumferential force, which makes the whole gasket "scattered" and eventually leads to seal failure. Since the circumferential stress is proportional to the diameter, the larger the size of the gasket, the easier it will be to be pulled out and damaged. The so-called "crushing" (or "instability") refers to that the gasket is subjected to a great compression specific pressure when in use, and the internal stress generated by the compression of the filler (expanded graphite) acts on the inner steel belt in the direction of the inner diameter, so that the steel belt is subjected to external pressure. When the external pressure exceeds a certain value, the steel belt ring surrounded by the steel belt will lose stability like a thin-walled container under the action of external pressure, and local parts will sag inward, causing damage to the gasket, resulting in seal failure. The higher the pressing force is, the larger the gasket size is, the more likely it is to be "crushed". According to the on-site investigation, many spiral wound gaskets used in cold exchange equipment were found to have been "scattered" or "crushed" when disassembling the flange after use, so the leakage often occurred from these parts. Therefore, in order to overcome the defects of "falling apart" and "crushing", inner and outer strong rings can be added at the inner and outer diameters of the winding pad to make the winding pad with inner ring, outer ring or inner and outer ring

spiral wound gasket belongs to semi-metallic gasket, and its compression rigidity is relatively large. Therefore, it is required to achieve its good sealing and service life under sufficient bolt force. It is usually suitable for high temperature, high pressure and other occasions

1.4 high strength graphite composite gasket

high strength graphite composite gasket is composed of a sprinted metal sheet as the skeleton and expanded graphite on its surface: the metal sheet as the skeleton only plays the role of increasing strength and overall rigidity, so that it is convenient to manufacture and use, but the expanded graphite material itself still plays the role of sealing. Expanded graphite material (also known as flexible graphite) is a new type of sealing material invented by the United carbon company in the 1960s. The electromechanical system is not properly matched. The phenomenon of big horse pulling small car is serious. It has extremely excellent temperature resistance (from -260 to +1500 ℃), corrosion resistance (meaning that the manufacturing industry needs to develop higher technical capabilities to withstand most chemical media), low friction coefficient, self lubrication, excellent thermal conductivity, radiation resistance, and low stress relaxation rate, Made of seals have good sealing performance. Therefore, its emergence is considered to be a revolution in the field of sealing materials. The disadvantage of expanded graphite is that its mechanical strength is not high enough, so it is limited when used alone. Therefore, in the development and application, it is more to composite with other materials to make composite seals. The expanded graphite spiral wound gasket mentioned above is a composite gasket made of expanded graphite as a filling material and wound with metal tape. High strength graphite gasket is also a gasket composed of expanded graphite and metal framework. Therefore, it has the good sealing performance of expanded graphite material, wide temperature resistance range, and can withstand the erosion of most chemical media and various solvents. It is a sealing gasket with good performance. However, due to the low strength of the material, its safety is not enough when used under high pressure. Therefore, it is generally used as a non asbestos sealing material to replace the traditional asbestos rubber plate at home and abroad

1.5 wave tooth composite gasket

wave tooth composite gasket is a new generation of flange sealing gasket developed since the 1990s. Since it was introduced to the market in the 1990s, it has been widely used in petroleum, chemical industry (including gas), electric power and other industrial departments and achieved good benefits. At present, the national standard of corrugated composite gasket has been officially promulgated and implemented

corrugated composite gasket is composed of metal skeleton with special structure and expanded graphite material. The upper and lower surfaces of the metal skeleton are provided with concentric circular grooves of special shapes staggered with each other, which are compounded with a layer of expanded graphite material of appropriate thickness, and finally form the gasket of the overall structure. Because the metal framework of the composite gasket is both toothed and corrugated, it is called corrugated composite gasket. When in use, due to the compression of the flange, the graphite material compounded on the gasket is compressed into the groove. The annular tooth peaks on the upper and lower surfaces of the metal skeleton are in close contact with the flange surface and produce elastic deformation under the further compression of the flange, so that the expanded graphite is highly compressed and closed in the annular confined space formed between the metal skeleton and the flange surface. Thus, the unique performance of the corrugated composite gasket is formed. The sharp tooth peaks of the metal skeleton and the highly compressed expanded graphite material form a tight seal. The whole composite gasket actually has the combined effect of multiple metal seals and expanded graphite material seals. The metal framework with special structure is like an elastic element, which makes the composite gasket have good elasticity. Due to the special structure of corrugated composite gasket, it has a series of more excellent performance characteristics compared with other gaskets

(1) excellent sealing. The corrugated composite gasket has the dual functions of tooth shaped metal seal and non-metallic expanded graphite seal, and its sealing belt is completely separated (which is different from the spiral belt of spiral wound gasket), so it has particularly excellent sealing performance. The air tightness test shows that the corrugated composite gasket can not only achieve extremely high sealing performance (the leakage rate can reach cm3/s level) under 35MPa compression, but also has excellent sealing performance under low pressure. Under 15MPa specific pressure, the air tightness still reaches the cm3/s index (the standard of asbestos rubber plate is only cm3/s)

(2) excellent resilience and long sealing life. Because the corrugated composite gasket has a corrugated elastic skeleton with special structure, and the metal and expanded graphite materials that make up the composite gasket have excellent high temperature resistance, fluid erosion resistance and no aging, the elasticity of the gasket is mainly produced by the metal skeleton with special structure, so there is no need to worry about stress relaxation during use, so it can maintain excellent sealing performance for a long time

(3) high safety and reliability. When the corrugated composite gasket is used, the annular tooth peak of the metal skeleton is in close contact with the flange surface, and its expanded graphite material is sealed by the solid metal skeleton and flange surface. Therefore, there is no need to worry that the expanded graphite material will be washed away by the high-pressure fluid, and there is no need to worry that it will be "scattered" or "crushed" ("unstable") like the spiral wound gasket. The corrugated composite gasket actually has the same safety and reliability as the metal gasket

(4) easy to use and install. The overall structure of corrugated tooth composite gasket with metal framework does not need to worry about "falling apart" like spiral wound gasket in transportation, use and installation. In addition, the thickness of corrugated tooth composite pad is relatively thin (usually 2 ~ 4mm). When it is used for convex concave or tongue and groove flange, enough depth can be reserved as the convex flange. How to solve the online failure of computer tension machine for positioning during installation? Therefore, it is convenient to install, which can ensure the installation quality and avoid wrapping

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