1. Industrial rubber
Natural rubber is suitable for water, seawater, air, inert gases, alkali, salt aqueous solutions and other media, but it is not resistant to mineral oils and non-polar solvents. The long-term service temperature does not exceed 90°C and has excellent low temperature performance. It can be used above -60°C.
Nitrile rubber is suitable for petroleum products, such as petroleum, lubricating oil, fuel oil, etc. The long-term service temperature is 120°C, such as 150°C in hot oil and -10~-20°C in low temperature.
Neoprene is suitable for seawater, weak acid, weak alkali and salt solutions. It has excellent oxygen and ozone aging resistance. Its oil resistance is second to nitrile rubber and better than other general-purpose rubber. The long-term service temperature is less than 90°C, the maximum operating temperature does not exceed 130°C, and the low temperature is -30~-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, as well as some oils and solvents, and the long-term service temperature is less than 200°C.
Rubber sheets, as flange gaskets, are mostly used in pipes or frequently removed human holes and hand holes, with a pressure of no more than 1.568MPa. Because among all kinds of gaskets, rubber gaskets are the softest and have good fit performance, which can have a sealing effect under less pre-tightening force. Because of this, when under internal pressure, it is easy to be squeezed out due to the thickness or low hardness of the gasket.
Rubber sheets are used in organic solvents such as benzene, ketones and ether, which are prone to swelling, weight gain, softening and sticky, leading to sealing failure. Generally, the solubility of more than 30% cannot be used.
Rubber pads are more suitable for low pressure (especially below 0.6MPa) and vacuum. Rubber materials have good compaction and low breathability. For example, fluorine rubber is the most suitable sealing gasket for vacuum containers, with a vacuum of up to 1.3×10-7Pa. Rubber pads need to be baked and pumped when used in a vacuum range of 10-1~10-7Pa.
2. Asbestos rubber board
The price is lower than that of other gaskets and easy to use; the biggest problem is that although rubber and some fillers are added to the gasket material, it still cannot completely fill the small pores in collusion and there is a slight penetration. Therefore, in highly polluting media, it cannot be used even if the pressure and temperature are not high. When used in some high-temperature oil media, usually in the later stage of use, due to the carbonization of rubber and fillers, the reduced strength and the loosening of materials, it will penetrate on the interface and inside the gasket, resulting in coking and smoke. In addition, the asbestos rubber sheet is easy to bond to the flange seal at high temperature, which brings a lot of trouble to replacing the gasket.
In a heated state, the pressure of the gasket in various media depends on the strength retention rate of the gasket material. Crystalline water and adsorbent water exist in asbestos fiber materials. At 110°C, two-thirds of the adsorption water between the fibers has been precipitated, and the tensile strength of the fiber is reduced by about 10%; at 368°C, all the adsorbed water is precipitated, and the tensile strength of the fiber is reduced by about 20%; beyond 500°C, the crystalline water begins to precipitate, and the strength decreases.
The medium also has a great influence on the strength of asbestos rubber sheet. For example, in aviation lubricants and aviation fuels, the transverse tensile strength difference of No. 400 oil-resistant asbestos rubber board is 80%, which is due to the serious dissolution of rubber in the sheet by aviation fuel than that of aviation lubricants. Considering the above factors, the recommended safe use range of domestic asbestos rubber sheet XB450: temperature 250°C ~ 300°C, pressure 3~3.5MPa; the temperature of No. 400 oil-resistant asbestos rubber board should not exceed 350°C.
Asbestos rubber board contains chloride ions and sulfides. After absorbing water, it is easy to form corrosive original batteries with metal flanges. In particular, the sulfur content in oil-resistant asbestos rubber board is several times higher than that of ordinary asbestos rubber boards, so it is not suitable for use in non-oil media. The gasket will swell in oil and solvent media, but within a certain range, it basically has no effect on the sealing performance. For example, the No. 400 oil-resistant asbestos rubber plate is tested in a 24-hour immersion test in normal aviation fuel, requiring an increase of no more than 15% due to oil absorption weight.
There is still a certain gap between domestic asbestos rubber board and foreign famous brand products. Statistics from a unit show that in oil media above 350°C, most of the asbestos rubber sheets are imported products. Japan Petroleum Industry Standard JPI-7S-71 Standard for the Use of Asbestos Rubber Plates for the Petroleum Industry, with a service life of about one year.
3. Polytetrafluoroethylene
Polytetrafluoroethylene is prone to cold current and creep under pressure and high temperature, so it is generally used in low pressure, medium temperature, strong corrosion and media that do not allow pollution, such as strong acids, strong alkali, halogens, drugs, etc. The safe use temperature is 150°C and the pressure is below 1MPa. Although the strength of filled polytetrafluoroethylene is high, the operating temperature does not exceed 200°C and its corrosion resistance is reduced. The maximum operating pressure of polytetrafluoroethylene pads generally does not exceed 2MPa.
Due to the increase in temperature, the material creeped, resulting in a significant decrease in the sealing pressure. Even if it does not heat up, the compression stress of the sealing surface will decrease over time, resulting in stress relaxation. This phenomenon occurs in various gaskets, but the stress relaxation of polytetrafluoroethylene pads is relatively serious and should be noted.
The friction coefficient of polytetrafluoroethylene is relatively small (compression stress is greater than 4MPa, and the friction coefficient is 0.035~0.04). When pre-tightening, the gasket is easy to slip outward, so it is best to use concave and convex flanges. When using a plane flange, the outer diameter of the gasket can be contacted with the bolt, and the bolts can be used to prevent the gasket from slipping.
Because the enamel equipment is sintered after spraying a layer of porcelain glaze on the metal surface, the glaze layer is very brittle, coupled with uneven spraying and the flow of the glaze layer, the surface of the flange is relatively poor. Metal composite gaskets are easy to damage the glaze layer, so it is recommended to use polytetrafluoroethylene pads with core materials made of asbestos board and rubber. This kind of cushion is easy to fit with the flange and is corrosion-resistant, and it works well.
In many factories, in strong corrosive media with low temperature and pressure, asbestos rubber sheet outer polytetrafluoroethylene raw material strips are used for frequent disassembly on human holes and pipes. Due to its easy production and use, it is very popular.
4. Asbestos resin board and impregnated asbestos board gasket
Pipes, pumps, valves and inlets of various acidic media are mostly used at a temperature of 80°C and a pressure of less than 0.6MPa.
Asbestos pads are suitable for low-pressure and high-temperature conditions with a pressure of less than 0.1MPa and a temperature not exceeding 800°C. According to the specific requirements of the equipment, it can be woven into pads with no width, thickness and diameter. Or cut off the asbestos tape and apply it directly to the flange surface. It is used in large sulfuric acid, nitrate oxidation furnaces and some unprocessed equipment interfaces, which is far more effective than the original asbestos rope.
5. Metal-clad asbestos pad
Asbestos board or asbestos rubber board is coated with metal sheet, so that it does not come into direct contact with the medium, avoids the decline in asbestos fiber strength and customer service leakage, thus expanding the use range of asbestos rubber board.
Generally, the operating temperature of metal-clad asbestos pads is 450°C (some of them also reach 600~700°C, such as in flue gas with atmospheric pressure ~0.16MPa), and the operating pressure is 4MPa, up to 6MPa. If the pressure increases again, the gasket is prone to crossflow, and the core material is extruded from the connection.
Because metal-clad asbestos pads require greater bolt tightening force, even if the pressure is less than 2.45MPa, flanges below the pg25 kg class cannot be used. Otherwise, the stiffness of flanges and bolts will not be enough, resulting in deformation, resulting in seal failure. Some people believe that if the core material is replaced with more elastic synthetic rubber, its fastening force will decrease. In fact, it is not the same, 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 cushion is easy to be extremely damaged. In addition, in mediums with more chlorine ions and acidic media, the connection between stainless steel pads and iron pads is prone to gap corrosion.
The temperature is above 450°C, and ceramic fiber or carbon fiber can be used for core materials. A steel mill uses metal-packed ceramic fiber gaskets for high temperatures of 1100°C, which has not been damaged for two years. Flexible graphite is the most suitable. At present, metal-packed flexible graphite pads have been mass-produced in China. Its performance is better than that of metal-clad asbestos pads.
Metal pads can be made into various shapes, which are widely used in various heat exchangers, large covers of reactors, loading and unloading holes, human hole flanges, etc. Iron pads with a diameter of 2 meters have been made in China, and the use is good.
Apply a flexible graphite sheet on the surface of the metal pad. Compared with similar metal pads that are not coated on the surface, the pre-tightening ratio is smaller and the sealing performance is better. Figure 2-9 is the relationship curve between the compression stress and the leakage between the two.
At present, this product is not available in China, so some units attach existing flexible graphite wrinkles to the surface of metal pads, metal flat pads, tooth pads and even asbestos rubber pads, solving many leakage problems. For example, a heat exchanger in a factory has a pressure of 5.88MPa, a temperature of 450°C, and a medium of hydrogen/oil. Metal flat pads and tooth-shaped pads have been used, all of which have leaks. Later, flexible graphite wrinkles were pasted on the mat to solve this problem. It should be noted that this form of gasket is a simple measure to solve the leakage of flange gaskets. The working quality of the flexible graphite belt directly affects the normal operation of the equipment. If a layer of glue is applied to the back of the strip, the coating quality can be improved.
6. Metal winding pad
Metal winding pads cleverly utilize the heat resistance, resilience and strength of metals and the softness of non-metallic materials, so the sealing performance is better, among which stainless steel winding flexible graphite pads are the best. Its pre-tightening ratio is smaller than that of asbestos winding pads, and there is no disadvantage of asbestos fiber pores leakage. Figure 2-10 shows the relationship curve between the pre-tightening specific pressure and the leakage between the two. In oil media, metal strips mostly use 0Cr13, while other media are recommended to use 1Cr18Ni9Ti.
Stainless steel strip flexible graphite winding pads are in gas media at a pressure of 14.7MPa (up to 19.6MPa in China) and can be used up to 30MPa in liquids. Temperature -190~+600°C (available to 1000°C in anaerobic and low pressure).
Polytetrafluoroethylene has good low temperature resistance, and its yield strength at low temperature is much higher than that at room temperature. Therefore, polytetrafluoroethylene winding pads can be used in cryogenic media, such as liquid hydrocarbons. At the same time, due to the addition of metal strips to improve thermal conductivity, the use temperature of the polytetrafluoroethylene winding pad can reach 250°C, which can be used as 9MPa and 200°C in acidic media.
Winding pads are more suitable for heat exchangers, reactors, pipelines, valves, pumps and inlet flanges with large pressure and temperature fluctuations. For people with medium pressure above and temperature exceeding 300°C, consideration should be given to using inner rings, outer rings or inner rings. If you use a concave and convex flange, the winding pad with an inner ring works better.
A good sealing effect can also be achieved by pasting flexible graphite plates on both sides of the flexible graphite winding pad. The waste heat boiler of a large fertilizer plant is a key equipment of high temperature and high pressure. It uses a flexible graphite winding pad with an outer ring, which does not leak when the load is full, and when the load is reduced. A 0.5mm thick flexible graphite plate is added on both sides of the gasket, cut into a curved shape, and the joint part is connected with an inclined lap, which is well used.
7. Metal flat pad and metal corrugated pad
It is generally used for medium and high-pressure valves, pipelines and equipment with small diameters. The operating pressure varies according to the temperature. The former is 1.568~31.36MPa, and the latter is 1.568~3.92MPa. The material of the shim is selected according to the medium and temperature.
8. Octagonal pad and oval pad
The octagonal pad and elliptical pads (commonly known as "earth steel rings" in the oil refining industry) for trapezoidal groove flanges have good sealing performance. On the cone of the groove, the octagonal pad is surface contact, and the elliptical pad is linear contact. Therefore, the elliptical pad fits well under low fastening force, but secondary fastening is required, while the octagonal pad is generally not prone to leakage after one-time fastening. Their disadvantage is that they require a large bolt tightening force. When used in low pressure and high temperature, the flange grade must be above the pg25 kg class.
