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How to choose valve flange gasket seals, PTFE gasket seals

How to choose valve flange gasket seals, PTFE gasket seals

2022-06-16

Gasket seals are the most common form of static sealing in industrial installations, and leakage is the most common failure form of production installations in many industrial production processes. According to statistics, 60-80% of the unplanned maintenance workload in petrochemical enterprises is used to maintain sealing devices. With the development of modern industrial production, various industrial sectors, 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 gas system is no exception. Due to the flammable, explosive and toxic characteristics of gas, the gas system also puts forward higher requirements for the gaskets used in it. Gaskets such as vinyl fluoride gaskets, wound gaskets, high-strength graphite gaskets, corrugated tooth composite gaskets, etc., introduce their performance characteristics and put forward some issues that should be paid attention to when selecting them in combination with the situation of the gas system.


1. Gaskets commonly used in gas systems and their characteristics


1.1 Asbestos rubber sheet gasket


 This is the earliest traditional sealing material, including ordinary type and oil-resistant type. Among them, there are various grades of high pressure, medium pressure and low pressure. The asbestos rubber sheet is made of asbestos fiber, rubber and other filling materials by mixing, heating and rolling, and the oil-resistant sheet is also heated and vulcanized. Due to the high strength of asbestos fibers and high temperature resistance (the temperature resistance of asbestos is usually within 480 ℃, when the temperature is higher than this temperature, the crystal water in the asbestos fibers will decompose and lose strength), the material sources are wide, and the manufacturing is convenient for industrialization. It can be made into gaskets of various shapes and specifications on site according to the needs of production. It is more 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 (the sealing performance is usually around 1×10-2~1x10-3Cm3N/S), especially the rubber and other components of this gasket are easily affected by temperature and media (such as solvents). In addition, since asbestos is a recognized carcinogen, since the 1970s, many countries in the world have successively formulated laws to restrict its sealing performance and life. Included in the prohibited scope, 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 usage is gradually decreasing. Generally, it is only used in non-critical equipment occasions where the temperature and pressure are not high.


 1.2 PTFE gasket


 PTFE material has excellent chemical corrosion resistance and broad temperature resistance characteristics. It can resist corrosion of various chemical media including acids, alkalis, solvents, etc. Its temperature resistance can range from -250 °C to +250 °C, and It has the characteristics of cleanliness and no pollution, so it is often used in chemical, food, pharmaceutical and other enterprises as sealing gaskets, but the biggest disadvantage of this material is its cold flow performance. Even at normal temperature, its contact stress will change over time. If it is reduced, it is easy to produce stress relaxation, which affects the sealing life of the gasket.


 1.3 Expanded graphite wound gasket


 Wound-wound gaskets have been used abroad in the 1930s, and after the 1960s in my country, mass production and popularization began. The early spiral wound gaskets used asbestos as the filling material. Because the sealing performance of asbestos-filled spiral wound gaskets is not as good as that of expanded graphite, and asbestos is a carcinogen, the current spiral wound gaskets mostly use expanded graphite as the filling material. Expanded graphite wound gasket has good sealing performance, can meet the requirements of high temperature and high pressure parameters, and its resilience is also good, and it can still maintain good sealing performance in the case of temperature and pressure changes (the sealing performance index is usually up to 1x10-4Cm3N/S), which is a relatively common gasket used in petroleum, chemical, electric power and other systems in the past. However, due to structural and manufacturing reasons, the defects of wound gaskets are also obvious. As some experts pointed out, "this gasket is currently used the most, but also has the most problems", which is what people usually call the "falling apart" and crushing (or "instability") problems. The so-called "falling apart" includes two situations: one is that the gasket is broken and damaged during manufacturing, transportation and installation. This is because the spiral wound gasket is only formed by winding and interlocking with 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 the transportation and installation of the gasket difficult. Gaskets (such as those on cold-change equipment) are even worse. The second is that the spiral wound gasket is subjected to a large pressing force during use, and the outer layer of the welding point used to fix it is prone to pull-off damage under the action of circumferential force, so that the entire gasket "falls apart" and eventually leads to seal failure. Since the size of the circumferential stress is proportional to the diameter, the larger the size of the gasket, the easier it is to pull off. The so-called "crushing" (or "unstable") is that the gasket is subjected to a large specific compression pressure during use, and the internal stress generated by the compression of the filler (expanded graphite) acts on the inner steel strip in the direction of the inner diameter, so that the The steel strip is subjected to external pressure. When the external pressure exceeds a certain value, the steel belt ring enclosed by the steel belt is like a thin-walled container that is subjected to excessive external pressure and loses its stability, and is partially dented inward, causing damage to the gasket and sealing failure. The higher the compression force, the more prone to "crush" the larger the gasket size. According to the on-site investigation, many spiral wound gaskets used for cold exchange equipment are found to have been "fallen apart" or "crushed" when the flange is disassembled after use, so leakage often occurs from these parts. Therefore, in order to overcome the defects of "falling apart" and "crushing", inner and outer reinforcing rings can be added to the inner and outer diameters of the winding pad to make a winding pad with an inner ring, an outer ring or an inner and outer ring. Spiral wound gaskets are semi-metallic gaskets, and their compression rigidity is relatively high, so they require sufficient bolt force to achieve their good sealing performance and service life, and are usually suitable for high temperature, high pressure and other occasions.


1.4 High-strength graphite composite pad


 The high-strength graphite composite gasket is composed of a stamped metal sheet as a skeleton and composite expanded graphite on its surface: the metal sheet as a skeleton only plays the role of increasing the strength and overall rigidity, so that it is more convenient to manufacture and use, but it plays a sealing role. It is still the expanded graphite material itself. Expanded graphite material (also known as flexible graphite) is a new type of sealing material invented by United Carbon Corporation in the 1960s. It has excellent temperature resistance (from -260 to +1500 ℃), corrosion resistance (resistant to most chemical media) ) has a very low coefficient of friction, self-lubricating, excellent thermal conductivity, radiation resistance, low stress relaxation rate, and good sealing performance as a seal. Therefore, its appearance 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. For this reason, in the development and application, it is more to be compounded with other materials to make a composite seal. The expanded graphite wound gasket mentioned above is a composite gasket made of expanded graphite as a filling material and wrapped with a metal belt. piece. High-strength graphite gasket is also a gasket composed of expanded graphite and metal skeleton. Therefore, it has the good sealing performance of expanded graphite materials, a wide temperature resistance range, and can withstand the erosion of most chemical media and various solvents. It is a sealing gasket with good performance, but due to the low material strength, under high pressure conditions It is not safe enough to use. Therefore, it is generally used as a non-asbestos sealing material at home and abroad to replace the traditional asbestos rubber sheet.


 1.5 corrugated composite gasket


 The corrugated 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 various industrial sectors such as petroleum, chemical Very good benefit. At present, the national standard for corrugated composite gaskets has been officially promulgated and implemented.


 The corrugated tooth composite gasket is composed of a special structure of metal skeleton and expanded graphite material. The upper and lower surfaces of the metal skeleton are provided with concentric circular grooves of special shape that are staggered from each other, and a layer of expanded graphite material of appropriate thickness is compounded on them to form the gasket of the overall structure. Because the metal skeleton of the composite gasket is both sharp and corrugated, it is called a corrugated composite gasket. During use, due to the compression of the flange, the graphite material compounded on the gasket is compressed into the groove, and the annular tooth peaks on the upper and lower surfaces of the metal skeleton are in close contact with the flange surface and generate elasticity under the further compression of the flange. Denatured, the expanded graphite is highly compressed and enclosed in the annular closed space formed between the metal skeleton and the flange surface. As a result, the unique properties of the corrugated composite gasket are formed. The sharp tooth peaks of the metal skeleton together with the highly compressed expanded graphite material form a tight seal. The entire composite gasket actually has multiple metal seals and expanded graphite. Combined action of material sealing. The specially constructed metal skeleton acts like an elastic element to make the composite gasket have good elasticity. Due to the special structure of the corrugated composite gasket, it has a series of more excellent performance characteristics compared with other gaskets.


 (1) Excellent sealing performance. Because the corrugated composite gasket has the dual function of toothed metal seal and non-metallic expanded graphite seal and its sealing band is completely separated (this is different from the spiral band of the wound gasket), it has a special Excellent sealing. The air tightness test shows that the corrugated tooth composite gasket can not only achieve extremely high sealing performance under 35MPa compression (leakage rate can reach 10-5cm3/s level), but also has excellent sealing performance under low pressure. Under the specific pressure of 15MPa, the air tightness still reaches the level of 10-3cm3/s (asbestos rubber sheet standard stipulates that it is only 10-2cm3/S).


 (2) Excellent resilience performance and long sealing life. Because the corrugated composite gasket has a specially constructed corrugated elastic skeleton, and the metal and expanded graphite materials that constitute the composite gasket have excellent high temperature resistance, fluid erosion resistance, and will not age, the elasticity of the gasket is mainly composed of special The metal frame of the structure is produced, and 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 the expanded graphite material is closed by the solid metal skeleton and the flange surface, so there is no need to worry that the expanded graphite material will be washed away by high-pressure fluid, and also There is no need to worry about "falling apart" or "crushing" ("buckling") like spiral wound gaskets. Corrugated composite gaskets actually have the same safety and reliability as metal gaskets.


 (4) Easy to use and install. The overall structure of the corrugated composite gasket is tied to the metal frame, and there is no need to worry about "falling apart" like the spiral wound gasket during transportation and installation. In addition, the thickness of the corrugated tooth composite gasket is relatively thin (usually 2-4mm), when it is used for convex-concave or tongue-and-groove flanges, enough depth can be reserved for positioning when the convex flange is installed, so it is easy to install and can ensure the installation. Quality, to avoid the phenomenon that wound gaskets are prone to pressure deviation due to their large thickness and affecting positioning.


 (5) Wide adaptability. The corrugated composite gasket can be applied to almost most occasions, including high and low temperature (-200℃~+700℃) and high and low pressure (vacuum~25MPa). The corrugated composite gasket made of the corresponding metal skeleton material can be used Suitable for all applications including most corrosive fluids.


 (6) Economical. The corrugated composite gasket is an economical gasket. Especially in petrochemical and power plants where various oil, steam and other non-corrosive fluids are used, the use of corrugated composite gaskets made of carbon steel instead of wound gaskets or toothed gaskets can save costs for users.


2 General recommendations for the selection of gas system gaskets


 The working conditions of the gas system medium are relatively special. Although the actual operating parameters of the gas system such as temperature and pressure are not high, many of the gas system media are flammable, explosive or even toxic gases. If the sealing is poor, it will cause serious damage. as a result of. Therefore, sufficient attention must be paid to the selection of sealing gaskets from the perspective of safety.


 Although the traditional asbestos rubber sheet has a certain degree of sealing, it has a wide range of material sources, is cheap, and is easy to make on site. Therefore, it has been widely used in many occasions in various industrial sectors in the past, including gas systems. With the development of modern industry, various industrial sectors, especially petrochemical systems (including gas systems), have put forward higher requirements for the performance of sealing gaskets. Because the material of asbestos rubber sheet is easily eroded by the medium of petrochemical system, which will eventually affect the service life of the seal. Therefore, with the continuous emergence of new materials in the field of sealing, the previous application of asbestos rubber sheet in petrochemical and gas systems has been There are fewer and fewer, so the use of asbestos rubber as a gasket in the gas system should be comprehensively considered and selected carefully.


 As a sealing gasket, the stability of PTFE to the medium is its most prominent advantage, and the sealing performance is also very good under normal circumstances. The problem is that its use is largely affected by its inherent cold-flow properties. Especially in the case of high temperature and high pressure or when the pressure, temperature fluctuation or pipeline configuration is complicated, the stress relaxation caused by cold flow will eventually affect its sealing life. Therefore, this gasket can usually be used under normal temperature, low pressure or stable temperature and pressure, depending on the level of its sealing requirements.


 The corrugated tooth composite gasket has a series of distinctive characteristics that make it have a wide range of adaptability. It is suitable for both high temperature occasions, low temperature and even ultra-low temperature occasions. It can be used under high pressure up to 25.0MPa, and can also be used in It is used under low pressure and vacuum, because its double sealing mechanism contains pure graphite non-metallic material sealing layer, these graphite layers enable it to achieve good sealing even when the flange surface processing requirements are not high, and it is under compressive stress. A very low level of tightness is maintained, so that it can be used satisfactorily in gas systems regardless of its parameters. Since the corrugated composite gasket entered the market in the early 1990s, it has been widely used in various industrial sectors such as petroleum, chemical industry, electric power, and light textile. The working parameters of practical application have been from low pressure to 26MPa high pressure, and temperature from -96 ℃ air separation system to 700 ℃ high temperature cracking system. Gasket specifications range from DN10 pipeline flange to DN2500 container flange, and the application of corrugated composite gaskets in these fields has been highly praised by users.


 In theory, the expanded graphite wound gasket can meet the requirements of the gas system in terms of material and performance, especially when the pressure and temperature are high and the parameters fluctuate, the effect of the expanded graphite wound gasket is better, but as the previous As mentioned above, because the wound gasket has the defects of being easy to "fall apart" and "crushed", the corresponding gasket type should be selected according to the type of flange when selecting. Ring type or outer ring type wound gasket, concave and convex surface flange should be selected with inner ring type wound gasket. Basic spiral wound gaskets are only suitable for tongue and groove flanges. Special attention should be paid to the selection of spiral wound gaskets in low-pressure pipeline systems. Whether the stress of flange bolts can meet the requirements of gasket compression must be considered, because spiral wound gaskets belong to Semi-metallic gaskets require high bolt force. Therefore, spiral wound gaskets are generally only suitable for flanges above medium pressure. If used on low pressure flanges, due to insufficient bolt force, the proper sealing may not be obtained. Effect.


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