Selection of flange sealing gasket in gas system
Gasket seal is the most common static seal form of industrial devices, and leakage is the most common failure form of production devices in many industrial production processes. According to statistics, 60-80% of the unplanned maintenance workload in petrochemical enterprises is used to repair sealing devices. With the development of modern industrial production, various 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 sealing of gaskets used in it. This paper aims at asbestos rubber gasket, polytetrafluoroethylene gasket, winding gasket, high-strength graphite gasket, wave tooth composite gasket and other gaskets commonly used in the fuel gas system, This paper introduces its performance characteristics, and puts forward some problems worthy of attention in selection combined with the situation of gas system.
Common gaskets and their characteristics in gas system
Asbestos rubber sheet gasket
This is the earliest traditional sealing material, including ordinary type and oil resistant type. It is also divided into various brands of high voltage, medium voltage and low voltage. Asbestos rubber plate is made of asbestos fiber, rubber and other filling materials by mixing, heating and rolling, and the oil resistant plate should also be heated and vulcanized. Because the asbestos fiber has high strength and can withstand high temperature (the temperature resistance of asbestos is usually within 480 ℃. When the temperature is higher than this temperature, the crystalline water in the asbestos fiber will decompose and lose strength), it has a wide range of materials and is easy for industrial production. It can be made into gaskets of various shapes and specifications on site as required. It is convenient to use and cheap, 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 1×10-2～1×10-3Cm3N/S), especially the rubber and other components of this gasket are easy to be damaged by temperature and medium (such as solvent), and stress relaxation is easy to occur during use, affecting its sealing performance and service life. In addition, since asbestos is a recognized carcinogen, many countries in the world have successively formulated laws to prohibit it since the 1970s, 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 use is gradually decreasing. Generally, it is only used in non important equipment occasions with low temperature and pressure.
Polytetrafluoroethylene material has excellent chemical corrosion resistance and wide temperature resistance. It can resist the corrosion of various chemical media including acid, alkali and solvent. Its temperature resistance can range from – 250 ℃ ~ 10 250 ℃, and has the characteristics of cleanness and no pollution. Therefore, it is commonly used as sealing gasket in chemical, food, pharmaceutical and other enterprises, but the biggest disadvantage of this material is its cold flow performance, Even at normal temperature, the 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.
Expanded graphite spiral wound gasket
Spiral wound gasket has been used abroad in the 1930s, and China began mass production and popularization after the 1960s. In the early days, asbestos was used as the filling material for the spiral wound gasket. Because the tightness of the spiral wound gasket filled with asbestos was not as good as that of expanded graphite, and asbestos was a carcinogen, most of the spiral wound gaskets now use expanded graphite as the filling material. The expanded graphite spiral wound gasket has good sealing performance, can adapt to the requirements of high temperature and high pressure parameters, and has good resilience. It can still maintain good sealing performance when the temperature and pressure change (its sealing performance index can usually reach 1×10-2～1×10-3Cm3N/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 there are also the most problems”, which is commonly referred to as “fragmentation” and collapse (or “instability”). The so-called “scattered” includes two cases: one is that the gasket is scattered and damaged during manufacturing, transportation and installation. This is because the spiral wound gasket is only formed by winding and meshing with each other, so that the gasket cannot form an overall firm structure, so it is easy to be scattered and damaged, resulting in difficulties in the transportation and installation of 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. The solder joint used to fix the outer layer is easy to pull off and damage under the action of circumferential force, which will “scatter” the whole gasket and eventually lead to seal failure. Since the circumferential stress is directly proportional to the diameter, the larger the specification of the gasket, the easier it will be to pull off and damage. The so-called “crushing” (or “instability”) refers to that the gasket is subjected to a large compression specific pressure during use, and the internal stress generated by the compression of the filler (expanded graphite) acts on the inner steel strip in the inner diameter direction, so that the steel strip is subjected to external pressure. When the external pressure exceeds a certain value, the steel belt ring surrounded by the steel belt is like a thin-walled container, which loses stability due to the action of external pressure, and some parts are sunken inward, resulting in damage to the gasket and seal failure. The higher the pressing force, the larger the gasket size, the more prone to “crushing”. From the field investigation, many spiral wound gaskets used for cold exchange equipment are found to have been “scattered” or “crushed” when dismantling the flange after use, so the leakage often occurs 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 diameter of the winding pad to make the winding pad with inner ring, outer ring or inner and outer ring.
Spiral wound gasket is a semi-metallic gasket with large compression rigidity. 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.
High strength graphite composite pad
The high-strength graphite composite gasket is composed of a sprint 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 more 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 sealing material invented by United carbon in the 1960s. It has excellent temperature resistance (from – 260 to + 1500 ℃), corrosion resistance (able to withstand most chemical media), low friction coefficient, self-lubricating, excellent thermal conductivity, radiation resistance and low stress relaxation rate. It has good sealing performance when made into sealing gasket. 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 sealing gaskets. The expanded graphite winding gasket mentioned above is a composite gasket made by winding expanded graphite and metal belt. High strength graphite gasket is also a gasket composed of expanded graphite and metal skeleton. Therefore, it has good sealing performance of expanded graphite material, wide temperature resistance range, and can resist 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, it is not safe to use it 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.
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 wave tooth composite gasket has been officially promulgated and implemented.
The corrugated tooth composite gasket is composed of a specially constructed metal skeleton and expanded graphite material. The upper and lower surfaces of the metal skeleton are provided with mutually staggered concentric circular grooves with special shapes, which are compounded with a layer of expanded graphite material with appropriate thickness to finally form the gasket of the overall structure. Because the metal framework of the composite gasket has sharp teeth and corrugated shape, it is called wave tooth 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 wave tooth composite gasket is formed. The sharp tooth peaks of metal skeleton and highly compressed expanded graphite material form a tight seal. The whole composite gasket actually has the combined effect of multi-channel metal seal and expanded graphite material seal. The specially constructed metal skeleton is like an elastic element, which makes the composite gasket have good elasticity. Due to the special structure of wave tooth composite gasket, it has a series of better performance characteristics compared with other gaskets.
- (1) Excellent sealing performance. Because the wave tooth composite gasket has the dual functions of tooth 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), it has particularly excellent sealing performance. The air tightness test shows that the corrugated tooth composite gasket can not only achieve very high sealing performance under 35MPa compression (the leakage rate can reach 10-5cm3 / s), but also has excellent sealing performance under low pressure. Under the specific pressure of 15MPa, the air tightness still reaches the index of 10-3cm3 / S (the standard of asbestos rubber plate is only 10-2cm3 / s).
- (2) Excellent resilience and long sealing life. Because the corrugated tooth composite gasket has a specially constructed corrugated tooth elastic skeleton, and the metal and expanded graphite materials constituting the composite gasket have excellent high temperature resistance, fluid erosion resistance and no aging, the elasticity of the gasket is mainly generated by the specially constructed metal skeleton, 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 tooth 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 closed 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, nor will it be “scattered” or “crushed” (instability) like the wound gasket. Wave tooth composite gasket actually has the same safety and reliability as metal gasket.
- (4) Easy to use and install. The overall structure of the corrugated tooth composite pad tether metal framework does not need to worry about “falling apart” like the spiral wound gasket in transportation, use and installation. In addition, the thickness of wave tooth composite gasket is relatively thin (usually 2 ~ 4mm). When it is used for convex concave or tenon groove flange, sufficient depth can be reserved for positioning during the installation of convex flange. Therefore, it is convenient to install, can ensure the installation quality, and avoid the pressure deviation of spiral wound gasket due to its large thickness and affecting positioning.
- (5) Wide adaptability. The wave tooth composite gasket can be applied to almost most occasions, including high and low temperature (- 200 ℃ ~ + 700 ℃) and high and low pressure (vacuum ~ 25MPa). The wave tooth composite gasket made of corresponding metal skeleton materials can be used in various occasions including most corrosive fluids.
- (6) Economy. Wave tooth composite gasket is an economic gasket. Especially in petrochemical industry and power plant, where various oil products, steam and other non corrosive fluids are used, the corrugated tooth composite pad composed of carbon steel can be used to replace the spiral wound gasket or toothed pad, which can save costs for users.
General suggestions on the selection of sealing gasket for gas system
The working condition of the medium of the gas system is relatively special. Although the actual operating parameters of the gas system, such as temperature and pressure, are not high, many media of the gas system themselves belong to flammable, explosive and even toxic gases, which will cause serious consequences if they are poorly sealed. Therefore, enough attention must be paid to the selection of sealing gasket from the perspective of safety.
Although the traditional asbestos rubber sheet has a certain sealing property, it has a wide range of materials, low price and is convenient for on-site production. Therefore, in the past, it was widely used in many occasions in various industrial sectors, including gas systems. With the development of modern industry, various industrial departments, especially petrochemical system (including gas system), put forward higher requirements for the performance of sealing gasket. As the material of asbestos rubber sheet is easy to be damaged by the medium of petrochemical system, and ultimately affect the service life of the seal, with the continuous emergence of new materials in the sealing field, the previous application of asbestos rubber sheet in petrochemical and gas system has become less and less. Therefore, the use of asbestos rubber as gasket in gas system should be comprehensively considered and carefully selected.
Polytetrafluoroethylene as a sealing gasket, its stability to the medium is its most prominent advantage, and under normal circumstances, the sealing performance is also very good. The problem is that its inherent cold fluidity greatly affects its use. Especially in the occasions with high temperature and pressure, or when the pressure and temperature fluctuation or pipeline configuration are complex, the stress relaxation caused by cold fluidity will eventually affect its sealing life. Therefore, this kind of gasket can usually be used under normal temperature, low pressure or stable temperature and pressure, depending on its requirements for sealing.
Theoretically, the expanded graphite spiral wound gasket can meet the use requirements of gas system in terms of material and performance, especially in the case of high pressure, temperature and parameter fluctuation. However, as mentioned above, due to the defects of easy “falling apart” and “crushing” of spiral wound gasket, When selecting, the corresponding gasket type shall be selected according to the flange type. For plane and convex flanges, the spiral wound gasket with inner ring, outer ring or outer ring shall be selected, and for concave convex flanges, the spiral wound gasket with inner ring shall be selected. The basic spiral wound gasket is only applicable to tenon flange. Special attention should be paid to whether the flange bolt stress can meet the gasket compression requirements when selecting spiral wound gasket in low-pressure pipeline system. Because spiral wound gasket is a semi-metallic gasket, it requires high bolt force. Therefore, spiral wound gasket is generally only applicable to flanges above medium pressure, If it is used on the low-pressure flange, the due sealing effect may not be obtained due to insufficient bolt force.
High strength graphite gasket has good sealing performance. Due to its thick expanded graphite layer and low requirements for flange surface processing, it is suitable for low-pressure occasions, especially to replace the asbestos rubber plate used in general parameter occasions in the past.
Wave tooth composite gasket has a series of unique characteristics, which makes it widely adaptable. It is suitable for high temperature, low temperature or even ultra-low temperature. It can be used not only under high pressure up to 25.0mpa, but also under low pressure and vacuum, because its double sealing mechanism contains non-metallic material sealing layer of pure graphite, These graphite layers enable it to achieve good sealing when the flange surface processing requirements are not high, and it can maintain high sealing when the compressive stress is very low. Therefore, it can obtain satisfactory results regardless of its parameters when used in the gas system. Since the wave tooth composite gasket entered the market in the early 1990s, it has been widely used in various industrial departments such as petroleum, chemical industry, electric power, light textile and so on. The actual operating parameters have ranged from low pressure to 26mpa high pressure, and the temperature from – 96 ℃ air separation system to 700 ℃ high temperature cracking system. The gasket specifications range from dnl0 pipe flange to dn2500 container flange. The application of wave tooth composite gasket in these fields has been highly praised by users. In recent years, the wave tooth composite gasket has also been popularized and applied in the gas system. For example, Foshan Gas Corporation has used the wave tooth composite gasket since 1999 and achieved good results. At present, more than 10 gas system units in the province, including Guangzhou, Foshan, Shaoguan, Dongguan, Zhongshan and Baiyun New Airport, have used the wave tooth composite gasket as the flange gasket of the gas system.
Source: China Sealing Gaskets Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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