3. Gasket selection
1. Precautions when selecting
The correct selection of sealing gaskets is the key to ensuring that the equipment does not leak. For the same working condition, there are generally several types of gaskets to choose from. Gaskets must be selected rationally based on the physical properties, pressure, temperature, equipment size, operating conditions, continuous operation cycle length, etc. of the medium, to maximize strengths and avoid weaknesses, and give full play to the characteristics of various gaskets. When selecting a gasket, the following factors should be fully considered:
(1) It has good elasticity and recovery and can adapt to pressure changes and temperature fluctuations;
(2) It has appropriate Soft and can fit well with the contact surface;
(3) Does not pollute the process medium;
(4) Has enough toughness not to be affected by pressure and tightening force Causes breakage;
(5) Does not harden at low temperatures and has small shrinkage;
(6) Good processing performance, easy installation and compaction;
(7) The sealing surface does not stick and is easy to disassemble;
(8) It is cheap and has a long service life.
In the use of gaskets, pressure and temperature are mutually restrictive. As the temperature increases, after the equipment has been running for a period of time, the gasket material will soften, creep, and stress relax. , the mechanical strength will also decrease and the sealing pressure will decrease. vice versa. For example, the manual lists the use of high-pressure asbestos rubber sheet of steam for 30 minutes). However, in long-term actual use, if the temperature reaches 450°C, the sealing pressure can only be 0.3~0.4MPa. For gas media with strong permeability, it is only 0.1~0.2MPa.
The above situations should be fully considered when selecting.
2. Selection of domestic gaskets
(1) Industrial rubber sheet
Natural rubber is suitable for water, seawater, air, inert gases, alkalis, Salt aqueous solution and other media, but it is not resistant to mineral oil and non-polar solvents. The long-term use temperature does not exceed 90°C. It has excellent low-temperature performance and can be used above -60°C.
Nitrile rubber is suitable for petroleum products, such as petroleum, lubricating oil, fuel oil, etc. The long-term use temperature is 120℃. For example, it can withstand 150℃ in hot oil and the low temperature is -10~-20 ℃.
Neoprene rubber is suitable for seawater, weak acid, weak alkali, and salt solutions. It has excellent oxygen and ozone aging resistance. Its oil resistance is inferior to nitrile rubber but better than other general-purpose rubbers. The long-term use temperature is lower than 90 ℃, the maximum operating temperature does not exceed 130 ℃, and the low temperature is -30~-50 ℃.
There are many varieties of fluorine rubber, which have good acid resistance, oxidation resistance, oil resistance, and solvent resistance. It can be used in almost all acidic media as well as some oils and solvents, and the long-term use temperature is lower than 200°C.
Rubber sheets are used as flange gaskets, mostly used in pipelines or manholes and hand holes that are frequently disassembled. The pressure does not exceed 1.568MPa. Because among all types of gaskets, rubber gaskets are the softest, have good fitting properties, and can exert a sealing effect with a small preload force. Because of this, when subjected to internal pressure, the gasket is easily extruded due to its thickness or low hardness.
When rubber sheets are used in organic solvents such as benzene, ketone, and ether, they are prone to swelling, weight gain, softening, and stickiness, leading to sealing failure. Generally, it cannot be used if the swelling degree exceeds 30.
Under low pressure (especially below 0.6MPa) and vacuum conditions, it is more appropriate to use rubber pads. Rubber material has good density and low air permeability. For example, fluorine rubber is most suitable as a sealing gasket for vacuum containers, with a vacuum degree of up to 1.3?10-7Pa.
When the rubber pad is used in the vacuum range of 10-1~10-7Pa, it needs to be baked and degassed.
The price of asbestos rubber sheets is lower than other gaskets and easy to use; the biggest problem is: although the gasket material is added with rubber and some fillers, it still cannot completely fill the tiny pores that are connected, and there are trace amounts penetration. Therefore, it cannot be used in extremely polluting media even if the pressure and temperature are not high. When used in some high-temperature oil media, usually in the later stages of use, due to carbonization of the rubber and filler, the strength is reduced and the material becomes loose, causing penetration at the interface and inside the gasket, causing coking and smoking. In addition, asbestos rubber sheets tend to adhere to the flange sealing surface at high temperatures, causing a lot of trouble in gasket replacement.
In the heated state, the pressure of the gasket used in various media depends on the strength retention rate of the gasket material. There are crystallization water and adsorbed water in asbestos fiber materials. At 110°C, 2/3 of the adsorbed water between fibers has precipitated, and the tensile strength of the fiber has been reduced by approximately 10%; at 368°C, all the adsorbed water has precipitated, and the tensile strength of the fiber has been reduced by approximately 20%; above 500°C, the crystallization water It begins to precipitate and the strength is lower. The medium also has a great influence on the strength of asbestos rubber sheets. For example, the transverse tensile strength of No. 400 oil-resistant asbestos rubber sheets differs by 80 between aviation lubricating oil and aviation fuel. This is because aviation fuel swells the rubber in the sheet more seriously than aviation lubricating oil. Taking into account the above factors, the recommended safe use range of domestic asbestos rubber sheet XB450 is: temperature 250℃~300℃, pressure 3~3.5MPa; the use temperature of No. 400 oil-resistant asbestos rubber sheet should not exceed 350℃.
Asbestos rubber sheets contain chloride ions and sulfides, which can easily form corrosive cells with metal flanges after absorbing water. In particular, the sulfur content in oil-resistant asbestos rubber sheets is several times higher than that of ordinary asbestos rubber sheets. Not suitable for use in oily media. Gaskets will swell in oil and solvent media, but within a certain range, they have basically no impact on sealing performance. For example, No. 400 oil-resistant asbestos rubber sheet is subjected to a 24-hour immersion test in normal temperature aviation fuel, and it is required that the weight increase due to oil absorption should not exceed 15.
Polytetrafluoroethylene is prone to cold flow and creep under pressure and high temperature, so it is generally used in low pressure, medium temperature, highly corrosive and media that do not allow pollution, such as strong acid, strong alkali, halogen, and pharmaceuticals. wait. The safe operating temperature is 150℃ and the pressure is below 1MPa. Although filled PTFE has higher strength, its operating temperature does not exceed 200°C, and its corrosion resistance decreases. The maximum operating pressure of PTFE pads generally does not exceed 2MPa. Figure 2-8 shows the usage curve of PTFE gaskets and pads in a certain test.
From 2-8(a), it can be seen that due to the increase in temperature, the material creeps, resulting in a significant decrease in sealing pressure. Even if the temperature does not rise, as time goes by, the compressive stress of the sealing surface will decrease, resulting in "stress relaxation" (see Figure 2-8(b)). This phenomenon occurs in various gaskets, but the stress relaxation phenomenon of polytetrafluoroethylene gaskets is more serious and should be paid attention to.
The friction coefficient of PTFE is small (the compression stress is greater than 4MPa, the friction coefficient is 0.035~0.04), and the gasket easily slips outward during pre-tightening, so it is best to use concave and convex flanges. noodle. When a flat flange is used, the outer diameter of the gasket can be contacted with the bolt, and the bolt can be used to prevent the gasket from sliding outward. Figure 2-3(a) and Figure 2-3(b) The two polytetrafluoroethylene pads can be used with a diameter of no more than 300mm. Figure 2-3(c) can be used for equipment with a diameter of more than 300mm, especially glass-lined equipment. and pipelines. Since the glass-lined equipment is sprayed with a layer of enamel on the metal surface and then sintered, the glaze layer is very brittle. In addition, the spraying is uneven and the glaze layer flows, resulting in poor flatness of the flange surface. The use of metal composite gaskets can easily damage the glaze layer, so it is recommended to use a polytetrafluoroethylene gasket with a core material of asbestos board and rubber. This kind of pad is easy to fit on the flange surface and is corrosion-resistant, so it has good use effect.
Many factories use asbestos rubber sheets wrapped with polytetrafluoroethylene raw material tapes for frequently dismantled manholes and pipes in highly corrosive media with low temperature and pressure. It is very popular because of its ease of production and use.
(4) Others
Asbestos resin sheets and impregnated asbestos sheet gaskets are mostly used in pipelines, pumps, valves, and inlet and outlet flanges of various acidic media. Their operating temperature is 80 ℃, pressure below 0.6MPa.
Asbestos woven gaskets are suitable for low-pressure and high-temperature working conditions where the pressure is below 0.1MPa and the temperature does not exceed 800°C. And according to the specific requirements of the equipment, we can weave gaskets of different widths, thicknesses and diameters. Or cut the asbestos tape and apply it directly on the flange surface. It is used at the interfaces of large sulfuric acid and nitric acid oxidation furnaces and some unprocessed equipment, and its effect is far better than that of the original asbestos rope.
(5) Metal-coated asbestos pad
The asbestos board or asbestos rubber board is covered with a metal sheet so that it is not in direct contact with the medium and avoids the decrease in the strength of the asbestos fiber. Customer service leakage phenomenon, thereby expanding the use scope of asbestos rubber sheets. Generally, the operating temperature of metal-clad asbestos mats is 450°C (sometimes it can reach 600~700°C, such as in flue gas with normal pressure ~0.16MPa), and the operating pressure is 4MPa, up to 6MPa. If the pressure increases further, the gasket is prone to cross flow and the core material is extruded from the overlap.
Since metal-clad asbestos pads require large bolt tightening force, even when the operating pressure is lower than 2.45MPa, flanges below pg25 kg cannot be used. Otherwise, the stiffness of the flange and bolts will be insufficient, resulting in deformation and seal failure. Some people believe that if the core material is changed to synthetic rubber with better elasticity, its fastening force will decrease. In fact, this is not the case, because after the core material is softened, the fastening force is absorbed by the core material and cannot provide the fastening force required for the metal plate to fit on the flange surface, and the padding is easily damaged. In addition, in media containing more chloride ions and acidic media, crevice corrosion is prone to occur at the overlap between stainless steel pads and iron pads.
When the temperature is higher than 450℃, ceramic fiber or carbon fiber can be used as the core material. A steel plant uses metal-clad ceramic fiber gaskets for high temperatures of 1100°C, and they have not been damaged after two years of use. Flexible graphite is the most suitable core material. At present, metal-clad flexible graphite pads have been mass-produced in China. Its use effect is better than that of metal-clad asbestos pads. Metal pads can be made into various shapes and are widely used in various heat exchangers, large covers of reactors, loading and unloading holes, manhole flanges, etc. Iron-clad pads with a diameter of 2m have been produced in China and are in good condition.
A layer of flexible graphite sheet is pasted on the surface of the metal pad. Compared with similar metal pads that are not coated on the surface, this kind of gasket has a smaller preload specific pressure and better sealing performance. Figure 2-9 shows the relationship between compression stress and leakage.
At present, there is no such product in China, so some units attach the existing flexible graphite wrinkle tape to the surface of metal pads, metal flat pads, toothed pads and even asbestos rubber pads, solving many leakage problems. . For example, the heat exchanger in a certain factory has a pressure of 5.88MPa, a temperature of 450°C, and the medium is hydrogen/oil and gas. I have used metal flat gaskets and toothed gaskets, and they all leaked. Later, a flexible graphite wrinkle tape was attached to the flat pad to solve this problem. It should be pointed out that this form of gasket is a simple measure to solve flange gasket leakage. The workability quality of the flexible graphite tape directly affects whether the equipment can operate normally. If you apply a layer of glue on the back of the strip, the quality of the application can be improved.
(6) Metal wound gasket
Metal wound gasket cleverly utilizes the heat resistance, resilience and strength of metal and the softness of non-metallic materials, so it has better sealing performance. Among them, the stainless steel tape wrapped around the flexible graphite pad has the best performance. Its preload specific pressure is smaller than that of asbestos wound mats, and there is no disadvantage of asbestos fiber capillary leakage. Figure 2-10 shows the relationship between preload specific pressure and leakage.
In oil media, 0Cr13 is mostly used for metal strips, while 1Cr18Ni9Ti is recommended for other media.
The stainless steel tape flexible graphite winding gasket can be used in gas media with a pressure of 14.7MPa (up to 19.6MPa in China), and can be used in liquids up to 30MPa. Temperature -190~600℃ (can reach 1000℃ under anaerobic and low pressure conditions).
Polytetrafluoroethylene has good low-temperature resistance, and its yield strength at low temperatures is much higher than that at normal temperature. Therefore, PTFE wound pads can be used for low-temperature media, such as liquid hydrocarbons. At the same time, due to the addition of metal strips to improve thermal conductivity, the operating temperature of the PTFE spiral mat can reach 250°C, and can be used in acidic media up to 9MPa and 200°C.
Wound gaskets are suitable for heat exchangers, reactors, pipes, valves, and pump inlet and outlet flanges with large pressure and temperature fluctuations. For those with medium or higher pressure and temperatures exceeding 300°C, the use of inner rings, outer rings or inner and outer rings should be considered. If a concave and convex flange is used, a wound gasket with an inner ring is more effective.
A good sealing effect can also be achieved by laminating flexible graphite sheets on both sides of the flexible graphite winding gasket. The waste heat boiler of a large fertilizer plant is a key piece of high-temperature and high-pressure equipment. It uses a flexible graphite winding pad with an outer ring. It does not leak when the load is full, but leaks when the load is reduced. A 0.5mm thick flexible graphite plate is added to both sides of the gasket, which is cut into an arc shape. The joint part is made of oblique overlap and is in good condition.
(7) Metal flat pads, corrugated pads and toothed pads
Metal flat pads and metal corrugated pads are generally used for medium and high pressure valves, pipes and equipment with smaller diameters. Orchid. The operating pressure depends on the temperature. The former is 1.568~31.36MPa and the latter is 1.568~3.92MPa. The gasket material is selected according to the medium and temperature.
(8) Octagonal gasket and oval gasket
Octagonal gasket and oval gasket (commonly known as "earth steel ring" in the oil refining industry) used for trapezoidal groove flanges have good sealing performance. On the tapered surface of the groove, the octagonal pads are in surface contact and the elliptical pads are in line contact. Therefore, elliptical pads have good fit under low tightening force, but need to be tightened twice; while octagonal pads are generally less likely to leak after being tightened once. Their disadvantage is that they require larger bolt tightening force. When used in low pressure and high temperature conditions, the flange grade must be above pg25 kg.
Gasket installation requirements
1. The sealing surfaces of the gasket and flange should be clean, and there should be no scratches, spots or other defects that affect the sealing performance of the connection.
2. The outer diameter of the sealing gasket should be smaller than the outside of the flange sealing surface, and the inner diameter of the sealing gasket should be slightly larger than the inner diameter of the pipe. The difference between the two inner diameters is generally 2 times the thickness of the sealing gasket to ensure After tightening, the inner edge of the sealing gasket will not extend into the container or pipe, so as not to hinder the flow of fluid in the container or pipe.
3. The pretightening force of the sealing gasket should not exceed the design requirements to prevent the sealing gasket from being over-compressed and losing its rebound ability.
4. When tightening the sealing gasket, it is best to use a torque wrench. For large bolts and high-strength bolts, it is best to use hydraulic tighteners. The tightening torque should be calculated based on the given sealing gasket compression, and the oil pressure of the hydraulic tightener should also be determined through calculation.
5. When installing the sealing gasket, tighten the nuts in order. But it should not be tightened once to reach the design value. Generally, it should be cycled at least 2 to 3 times so that the stress of the sealing gasket is evenly distributed.
6. For pressure vessels and pipelines with flammable and explosive media, safety tools should be used when replacing sealing gaskets to avoid sparks from the tools colliding with flanges or bolts, resulting in fire or Explosion accident.
7. If there is leakage in the pipeline, it must be depressurized before replacing or adjusting and installing the sealing gasket. Operation under pressure is strictly prohibited. Scope of application of gasket
The choice of gasket material mainly depends on the following three factors: temperature pressure medium
1. Metal gasket material
1. Carbon steel: It is recommended that the maximum operating temperature does not exceed 538°C, especially when the medium is oxidizing.
High-quality thin carbon steel plates are also not suitable for use in equipment that manufactures inorganic acids, neutral or acidic salt solutions. If carbon steel is subjected to excessive stress, the accident rate of equipment used in hot water conditions will be very high. Carbon steel gaskets are commonly used in high concentration acids and many alkaline solutions. Brinell hardness is about 120.
2. 304 stainless steel 18-8 (chromium 18-20, nickel 8-10), the recommended maximum working temperature does not exceed 760°C. In the temperature range of -196~538℃, stress corrosion and grain boundary corrosion are prone to occur. Brinell hardness 160.
3. The carbon content of 304L stainless steel does not exceed 0.03. The recommended maximum operating temperature does not exceed 760°C. Corrosion resistance is similar to 304 stainless steel. The low carbon content reduces the precipitation of carbon from the crystal lattice, and its grain boundary corrosion resistance is higher than that of 304 stainless steel. Brinell hardness is about 140.
4. 316 stainless steel 18-12 (chromium 18, nickel 12), about 2 molybdenum is added to 304 stainless steel. When the temperature increases, its strength and corrosion resistance improve. When the temperature increases, it has higher creep resistance than other ordinary stainless steels. The recommended maximum operating temperature does not exceed 760°C. Brinell hardness is about 160.
5. The recommended maximum continuous working temperature of 316L stainless steel does not exceed 760℃~815℃. The carbon content does not exceed that of 316 stainless steel, which has better resistance to stress and grain boundary corrosion. Brinell hardness is about 140.
6. 20 alloy 45 iron, 24 nickel, 20 chromium and a small amount of molybdenum and copper. It is recommended that the maximum operating temperature does not exceed 760℃~815℃. Especially suitable for manufacturing equipment resistant to sulfuric acid corrosion, with a Brinell hardness of about 160.
7. Aluminum (content is not less than 99). Aluminum has excellent corrosion resistance and processability, making it suitable for manufacturing double-clamp gaskets. Brinell hardness is about 35. It is recommended that the maximum continuous operating temperature does not exceed 426°C.
8. Red copper The composition of red copper is close to pure copper, and it contains trace amounts of silver to increase its continuous working temperature. It is recommended that the maximum continuous operating temperature does not exceed 260°C. Brinell hardness is about 80.
9. Brass (copper 66, zinc 34) has good corrosion resistance under most working conditions, but is not suitable for acetic acid, ammonia, salt and acetylene. It is recommended that the maximum continuous operating temperature does not exceed 260°C. Brinell hardness is about 58.
10. Hastelloy B-2 (26-30 molybdenum, 62 nickel and 4-6 iron). It is recommended that the maximum operating temperature does not exceed 1093°C. It has excellent resistance to corrosion by hot concentrated hydrochloric acid. It also has excellent resistance to corrosion by wet hydrogen chloride gas, sulfuric acid, phosphoric acid and reducing salt solutions. Has high strength under high temperature conditions. Brinell hardness is about 230.
11. Hastelloy C-276 16-18 molybdenum, 13-17.5 chromium, 3.7-5.3 tungsten, 4.5-7 iron, the rest are nickel). It is recommended that the maximum operating temperature does not exceed 1093°C. Has excellent corrosion resistance. It has excellent corrosion resistance against various cold nitric acid or boiling nitric acid with a concentration of 70%, good hydrochloric acid and sulfuric acid corrosion resistance and excellent stress corrosion resistance. Brinell hardness is about 210.
12. Inconel 600 nickel-based alloy (77 nickel, 15 chromium and 7 iron). It is recommended that the maximum operating temperature does not exceed 1093°C. It has high strength under high temperature conditions and is usually used in equipment that needs to solve stress corrosion problems. Under low temperature conditions, it has excellent processing properties. Brinell hardness is about 150.
13. Monel 400 (copper 30, nickel) recommended maximum continuous operating temperature not to exceed 815°C. Except for strong oxidizing acids, it has excellent corrosion resistance to most acids and alkali. In hydrofluoric acid, Mercury chloride and mercury media are prone to stress corrosion cracking, so they are not suitable for use in the above media. Brinell hardness is about 120. Titanium. p>
It is recommended that the maximum operating temperature does not exceed 1093°C and has excellent corrosion resistance under high temperature conditions.
It is well known to be resistant to chloride ion corrosion and has excellent resistance to nitric acid corrosion over a wide temperature and concentration range. Titanium is rarely used in most alkaline solutions and is suitable for oxidation conditions. Brinell hardness is about 216.
2. Non-metallic gasket materials
1. Natural rubber NR has good corrosion resistance to weak acids and alkalis, salts and chloride solutions, and corrosion resistance to oil and solvents is poor and not recommended for ozone media. Recommended operating temperature -57℃~93℃.
2. Neoprene CR Neoprene is a synthetic rubber that is suitable for corrosion by moderately corrosive acid, alkali and salt solutions. Good corrosion resistance to commercial oils and fuels. However, its corrosion resistance is poor in strongly oxidizing acids, aromatic hydrocarbons and chlorinated hydrocarbons. Recommended operating temperature -51℃~121℃.
3. Cyanide butadiene rubber NBR is a kind of synthetic rubber that is suitable for use in materials with good resistance to oil, solvents, aromatic hydrocarbons, alkaline hydrocarbons, oil and natural gas in a wide temperature range. Corrosion properties. Good corrosion resistance to hydroxides, salts and nearly neutral acids. However, its corrosion resistance is poor in strongly oxidizing media, chlorinated hydrocarbons, ketones and lipids. The recommended working temperature is 51℃~121℃.
4. Fluorocarbon rubber is compounded by mixing binary and ternary fluorocarbon rubber with compounding agents and vulcanizing agents. In addition to excellent heat resistance, medium resistance, and good physical and mechanical properties, it also has low compression permanent deformation, good elasticity, and ultra-long service life. Fluorine rubber has outstanding heat resistance (200~250℃) and oil resistance. It can be used to manufacture cylinder liner seals, rubber bowls and rotating lip seals, which can significantly extend the service life. Recommended operating temperature -40℃~232℃.
5. Chlorosulfonyl polyethylene synthetic rubber has good corrosion resistance to acids, alkalis and salt solutions, and is not affected by climate, light, ozone, commercial fuels (such as diesel and kerosene, etc.) )Influence. But it is not suitable for aromatic hydrocarbons, chlorinated hydrocarbons, chromic acid and nitric acid. Recommended operating temperature -45℃~135℃.
6. Silicone rubber Silicone rubber has outstanding high and low temperature resistance and can be used for a long time at 150℃ without performance change; it can be used continuously at 200℃ for 10,000 hours and at -70~260℃ It can maintain its unique elasticity, ozone resistance, weather resistance and other advantages within the working temperature range. It is suitable for making sealing gaskets required in thermal mechanisms, such as sealing rings, valve gaskets, oil seals (suitable for water media), etc. Special silicone Rubber can be used to make oil seals.
7. Ethylene-propylene rubber has good corrosion resistance to strong acids, strong alkali, salt and chloride solutions. But it is not suitable for oils, solvents, aromatic hydrocarbons and hydrocarbons. Recommended operating temperature -57℃~176℃.
8. Graphite is a fully graphite material that does not contain resin or inorganic substances. It can be divided into graphite materials with or without metal elements. This material can be bonded to produce gaskets for pipes with diameters exceeding 600MM. It has excellent corrosion resistance to many acids, alkalis, salts, organic compounds and heat transfer solutions, even high temperature solutions. It cannot melt, but will sublimate when it exceeds 3316℃. Caution should be exercised when using this material in strongly oxidizing media under high temperature conditions. In addition to being used for gaskets, this material can also be used to make fillers and non-metallic spiral tape in spiral wound gaskets.
9. Ceramic fiber. Ceramic fiber molded into strips is an excellent gasket material suitable for high temperature and low pressure conditions and light flange conditions. The recommended working temperature is 1093°C and can be made into windings. Non-metallic wrapping tape in gaskets.
10. Polytetrafluoroethylene combines the advantages of most plastic gasket materials, including temperature resistance from -95°C to 232°C. Except for free fluorine and alkali metals, it has excellent corrosion resistance to chemicals, solvents, hydroxides and acids. PTFE material can be filled with glass, the purpose of which is to reduce the cold flow and creep properties of PTFE.