1. Industrial rubber sheet
Natural rubber is suitable for media such as water, seawater, air, inert gas, alkali and salt water solutions, but it is not resistant to mineral oil and non-polar solvents. The long-term use temperature does not exceed 90°C, and it has excellent low-temperature performance. Use above.
Nitrile rubber is suitable for petroleum products, such as petroleum, lubricating oil, fuel oil, etc. The long-term use temperature is 120°C, and it can withstand 150°C in hot oil, and the low temperature is -10°C~-20°C.
Chloroprene rubber is suitable for seawater, weak acid, weak alkali, salt solution, excellent oxygen and ozone aging resistance, oil resistance is inferior to nitrile rubber but better than other general rubber, long-term use temperature is lower than 90 ℃, the maximum use temperature is not more than 130°C, the low temperature is -30°C~-50°C.
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 acid media and some oils and solvents, and the long-term use temperature is lower than 200°C.
As a flange gasket, the rubber sheet is mostly used for pipelines or frequently dismantled manholes and hand holes, and the pressure does not exceed 1.568MPa. Because among all kinds of gaskets, rubber gaskets are the softest and have good bonding performance, and can exert a sealing effect under a small pre-tightening force. Because of this, when under internal pressure, the gasket is easy to be squeezed out due to its thickness or low hardness.
The rubber sheet is used in organic solvents such as benzene, ketone, ether, etc., which is prone to swelling, weight gain, softening and stickiness, resulting in seal failure. Generally, if the swelling degree exceeds 30%, it cannot be used.
Under low pressure (especially below 0.6MPa) and vacuum, it is more appropriate to use rubber pads. The rubber material has good density and low air permeability. For example, fluorine rubber is most suitable as a sealing gasket for vacuum containers, and the vacuum degree can reach 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 pumped.
2. Asbestos rubber sheet
The price is lower than other gaskets, and it is easy to use. The biggest problem is: Although rubber and some fillers are added to the gasket material, it still cannot completely fill the tiny pores that are in collusion, and there is a small amount of penetration. Therefore, in highly polluting media, even if the pressure and temperature are not high, it cannot be used. When used in some high-temperature oil medium, usually at the end of use, due to the carbonization of rubber and fillers, the strength is reduced, and the material becomes loose, and penetration occurs at the interface and inside the gasket, resulting in coking and smoking. In addition, the asbestos rubber sheet is easy to stick to the flange sealing surface at high temperature, which brings a lot of trouble to the replacement of the gasket.
In a heated state, the pressure of the gasket in various media depends on the strength retention of the gasket material. Crystallization water and adsorption water exist in asbestos fiber materials. At 110°C, 2/3 of the adsorbed water between the fibers has precipitated, and the tensile strength of the fiber has decreased by about 10%; at 368°C, all the adsorbed water has precipitated, and the tensile strength of the fiber has decreased by about 20%; over 500°C, The water of crystallization begins to precipitate, and the strength is lower. The medium also has a great influence on the strength of asbestos rubber sheet. For example, in aviation lubricating oil and aviation fuel, the transverse tensile strength of No. 400 oil-resistant asbestos rubber sheet differs by 80%, which is because the swelling of the rubber in the sheet by aviation fuel is more severe than that of aviation lubricating oil. Considering the above factors, the recommended safe range of use of domestic asbestos rubber sheet XB450: temperature 250 ℃ ~ 300 ℃, pressure 3 ~ 3.5MPa; No. 400 oil-resistant asbestos rubber sheet should not be used at a temperature exceeding 350 ℃.
Asbestos rubber sheet contains chloride ions and sulfides. After absorbing water, it is easy to form a corrosion battery with metal flanges. Especially, the sulfur content in oil-resistant asbestos rubber sheet is several times higher than that of ordinary asbestos rubber sheet, so it is not suitable for use in non-oily media. The gasket will swell in oil and solvent medium, but within a certain range, it has basically no effect on the sealing performance. For example, No. 400 oil-resistant asbestos rubber sheet is subjected to a 24-hour immersion test in aviation fuel at room temperature, requiring that the weight increase due to oil absorption should not exceed 15%.
3. Polytetrafluoroethylene
PTFE is easy to cold flow and creep under pressure and high temperature, so it is generally used in low pressure, medium temperature, strong corrosion and medium that does not allow pollution, such as strong acid, strong alkali, halogen and pharmaceuticals. The safe operating temperature is 150°C and the pressure is below 1MPa. Although the strength of filled PTFE is higher, the service temperature does not exceed 200°C, and the corrosion resistance decreases. The maximum operating pressure of PTFE pads generally does not exceed 2MPa.
As the temperature rises, the material creeps, resulting in a significant drop in sealing pressure. Even if the temperature does not rise, as time goes by, the compression stress of the sealing surface will decrease, resulting in the phenomenon of "stress relaxation". This phenomenon occurs in all kinds of gaskets, but the stress relaxation phenomenon of PTFE gaskets is more serious, so it should be paid attention to.
The friction coefficient of PTFE is small (the compression stress is greater than 4MPa, and the friction coefficient is 0.035~0.04), and the gasket is easy to slide outward when preloaded, so it is best to use a concave-convex flange surface. 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. Because the glass-lined equipment is sintered after spraying a layer of enamel on the metal surface, the glaze layer is very brittle, coupled with uneven spraying and glaze layer flow, the flatness of the flange surface is poor. The use of metal composite gaskets is easy to damage the glaze layer, so it is recommended to use polytetrafluoroethylene gaskets with asbestos board and rubber as the core material. This kind of packing pad is easy to fit with the flange surface and corrosion-resistant, and the use effect is good.
In a strong corrosive medium with low temperature and pressure, many factories wrap asbestos rubber sheets with polytetrafluoroethylene raw material tapes for manholes and pipelines that are often dismantled. It is very popular because of its convenient production and use.
4. Asbestos resin board and impregnated asbestos board gasket
It is mostly used for pipelines, pumps, valves and inlet and outlet flanges of various acidic media. The operating temperature is 80°C and the pressure is below 0.6MPa.
Asbestos woven gaskets are suitable for low-pressure and high-temperature working conditions with pressure below 0.1MPa and temperature not exceeding 800°C. And according to the specific requirements of the equipment, it can be woven into gaskets of different widths, thicknesses and diameters. Or cut the asbestos tape and apply it directly on the flange surface. It is used in large-scale sulfuric acid, nitric acid oxidation furnaces and some unprocessed equipment interfaces, and its effect is far better than that of the original asbestos rope.
5. Metal clad asbestos pad
The asbestos board or asbestos rubber board is covered by a metal sheet so that it does not directly contact the medium, avoiding the decrease in the strength of the asbestos fiber and overcoming the leakage phenomenon, thus expanding the use of the asbestos rubber board. Generally, the use temperature of metal-coated asbestos pad is 450°C (some can reach 600°C~700°C, such as in the flue gas with normal pressure of 0.16MPa), and the use pressure is 4MPa, up to 6MPa. If the pressure increases again, the gasket is prone to cross flow, and the core material is extruded from the lap.
Since the metal-clad asbestos gasket requires a large bolt tightening force, even when the operating pressure is lower than 2.45MPa, flanges below pg25kg cannot be used. Otherwise, the rigidity of flanges and bolts will be insufficient, resulting in deformation and seal failure. Some people think that if the core material is changed to a 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, which cannot provide the fastening force required for the metal plate to fit on the flange surface, and the padding is easy to break. In addition, in the medium containing more chloride ions and the acidic medium, the overlap between the stainless steel pad and the iron pad is prone to crevice corrosion.
When the temperature is higher than 450°C, 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 it has not been damaged after two years of use. Flexible graphite is the most suitable core material. At present, metal-coated flexible graphite pads have been mass-produced in China, and its use effect is better than that of metal-coated asbestos pads.
Metal pads can be made into various shapes, and are widely used in various heat exchangers, reactor covers, loading and unloading holes, manhole flanges, etc. Iron-clad pads with a diameter of 2m have been produced in China, and they are in good use.
A layer of flexible graphite thin plate is pasted on the surface of the metal packing gasket. Compared with the same kind of metal packing gasket without covering the surface, the pre-tightening specific pressure is smaller and the sealing performance is better.
At present, there is no such product in China, so some units paste the existing flexible graphite wrinkle tape on the surface of metal pads, metal flat pads, toothed pads and even asbestos rubber pads, which solves many leakage problems. For example, a heat exchanger in a factory has a pressure of 5.88MPa, a temperature of 450°C, and the medium is hydrogen or oil and gas. Metal flat pads and toothed pads have been used, all of which leaked. Finally, a flexible graphite corrugated tape was pasted on the flat pad to solve this problem. It should be pointed out that this type of gasket is a simple measure to solve the leakage of the flange gasket, and the workability quality of the flexible graphite tape directly affects whether the equipment can operate normally. If a layer of glue is applied on the back of the strip, the quality of pasting can be improved.
6. Metal winding pad
The metal winding gasket skillfully utilizes the heat resistance, resilience and strength of metal and the softness of non-metallic materials, so the sealing performance is better, and the performance of flexible graphite gasket wrapped with stainless steel tape is the best. Its preload specific pressure is smaller than that of asbestos winding mat, and there is no defect of asbestos fiber capillary leakage.
In oil medium, 0Cr13 is mostly used for metal strips, while 1Cr18Ni9Ti is recommended for other mediums.
Stainless steel with flexible graphite winding mat in gas medium, the working pressure is 14.7MPa (the domestic one is as high as 19.6MPa), and it can be used in liquid up to 30MPa. Temperature -190~600°C (up to 1000°C in the absence of oxygen and low pressure).
PTFE has good low temperature resistance, and its yield strength at low temperature is much higher than that at room temperature. Therefore, PTFE wound mats can be used in low temperature media, such as liquid hydrocarbons. At the same time, due to the addition of metal strips to improve thermal conductivity, the use temperature of PTFE winding mats can reach 250°C, and can be used in acidic media up to 9MPa and 200°C.
Wound gaskets are more suitable for heat exchangers, reactors, pipes, valves, pump inlet and outlet flanges with large pressure and temperature fluctuations. For those with medium or higher pressure and temperature exceeding 300°C, it should be considered to use inner ring, outer ring or inner and outer ring. If a concave-convex flange is used, it is better to use a wound gasket with an inner ring.
A good sealing effect can also be obtained by pasting flexible graphite sheets on both sides of the flexible graphite winding pad. The waste heat boiler of a large chemical fertilizer plant is a high-temperature and high-pressure key equipment. A flexible graphite winding pad with an outer ring is used. It does not leak when the load is full, and it leaks when the load is reduced. A 0.5mm thick flexible graphite plate is added on both sides of the gasket, which is cut into an arc shape, and the joint part is lapped with a bevel, which is in good use.
7. Metal flat pads, corrugated pads and toothed pads Metal flat pads and metal corrugated pads
It is generally used on medium and high pressure valves, pipes and equipment flanges with small diameters. 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 pads and oval pads
Octagonal pads and elliptical pads (commonly known as "soil steel rings" in the oil refining industry) used for trapezoidal groove flanges have good sealing performance. On the conical surface of the groove, the octagonal pad is in surface contact, while the oval pad is in line contact. Therefore, oval pads have good fit under low fastening force, but secondary fastening is required. The octagonal pad is generally fastened once, and it is not easy to leak. Their disadvantage is that they require a large bolt tightening force. When used in low pressure and high temperature conditions, the flange grade must be above pg25 kg.
