1. Industrial rubber sheet
Natural rubber is suitable for water, sea water, air, inert gas, alkali, brine solution and other media, but not resistant to mineral oil and non-polar solvents. Long-term use temperature does not exceed 90℃, low temperature performance is excellent. Can be used above -60℃.
Nitrile rubber is suitable for petroleum products, such as petroleum, lubricating oil, fuel oil, etc. Its long-term use temperature is 120 ° C, such as 150 ° C in hot oil, and the low temperature is -10~-20 ° C.
Neoprene rubber is suitable for seawater, weak acid, weak alkali and salt solutions. It has excellent oxygen resistance and ozone aging resistance, oil resistance is lower than nitrile rubber but better than other general rubber. Its long-term use temperature is lower than 90 ° C, higher use temperature is lower than 130 ° C, and low temperature is -30~-50 ° C.
There are many kinds of fluorine rubber, with good acid resistance, oxidation resistance, oil resistance and solvent resistance. It can be used in almost all acidic media and some oils and solvents, and the long-term use temperature is below 200℃.
As a kind of flange gasket, rubber is mostly used for pipelines or manholes and hand holes that are often disassembled in the oil resource network, and the pressure is less than 1.568MPa. Among all kinds of gaskets, rubber gaskets are the softest, have good bonding performance, and can play their sealing role under a small preload force. Because of this, when the gasket is subjected to internal pressure, it is easy to be squeezed due to its thicker or lower hardness.
Rubber is used in organic solvents such as benzene, ketone and ether. It is easy to expand, gain weight, soften and become sticky, resulting in seal failure. Generally, if the expansion exceeds 30%, it cannot be used.
Under low pressure (especially below 0.6MPa) and vacuum conditions, rubber pads are more suitable. The rubber has good density and low permeability. For example, fluorine rubber is the most suitable gasket for vacuum empty containers, and the vacuum emptiness can reach 1.310-7Pa. When the rubber pad is used in the vacuum range of 10-1~10-7Pa, it needs to be baked and vacuumed.
2. Asbestos rubber sheet
Compared with other gaskets, the price is lower, easy to use; The bigger problem is that although rubber and some filler are added to the gasket material, it still cannot completely fill the tiny pores that are well connected, and there is slight penetration. Therefore, in high-pollution media, even if the pressure and temperature are not high, it can not be used. When used in some high-temperature oil media, usually in the later period of use, due to the carbonization of rubber and filler, the strength is reduced, and the material becomes loose, resulting in the penetration of the interface and gasket, resulting in coking and smoke. In addition, the asbestos rubber sheet is easy to adhere to the flange sealing surface at high temperature, which brings a lot of trouble to the replacement of the gasket.
Under heating conditions, the pressure of the gasket in various media depends on the strength retention rate of the gasket material. There are crystal water and adsorbed water in asbestos fiber material. At 10℃, 2/3 of the adsorbed water between the fibers has been precipitated, and the tensile strength of the fibers decreases by about 10%. At 38℃, all the adsorbed water precipitated, and the tensile strength of the fiber decreased by about 20%. When the temperature exceeds 500 ° C, the crystal water begins to precipitate and the strength decreases. It also has a great influence on the strength of the asbestos rubber sheet. For example, in aviation lubricating oil and aviation fuel oil, the horizontal tensile strength difference of No. 400 oil-resistant asbestos rubber sheet is 80%, because the swelling of the rubber in the board of aviation fuel oil is more serious than that of aviation lubricating oil. In combination with the above factors, the recommended safe use range of domestic asbestos rubber board XB450 is: temperature 250℃~300℃, pressure 3 ~ 3.5MPa40 oil-resistant asbestos rubber board temperature shall not exceed 350℃.
Asbestos rubber sheet contains chloride ions and sulfide, and is easy to form corrosion galvanic cells with metal flanges after absorbing water. In particular, the sulfur content of oil-resistant asbestos rubber sheet is several times higher than that of ordinary asbestos rubber sheet, and should not be used in non-oily media. Gaskets will expand in oil and solvent media, but within a certain range, it has little effect on sealing performance. For example, 400 oil-resistant asbestos rubber sheet is soaked in air fuel at normal temperature for 24 hours, requiring oil absorption weight gain of no more than 15%.
There is still a certain gap between domestic asbestos rubber sheet and foreign famous brand products. Statistics of a unit show that in the oil medium above 350 ° C, most of the non-leaking asbestos rubber sheets are imported products. Japanese stone industry standard JPI-7S-71 "Standard for the use of asbestos rubber sheet for Petroleum industry" the service life is about one year.
3. Teflon
Ptfe is easy to cold flow and creep under pressure and high temperature, so it is generally used for low pressure, medium temperature, strong corrosion and pollution-free media, such as strong acids, strong bases, halogens, drugs and so on. Safe operating temperature 150℃, pressure below 1MPa. Although filled PTFE has high strength, the use temperature is lower than 200 ° C, and the corrosion resistance is decreased. The greater use pressure of the PTFE packing pad is generally less than 2MPa.
As the temperature rises, the material wriggles, resulting in a significant drop in seal pressure. Even if the temperature does not rise, with the extension of time, the compression stress of the sealing surface will decrease, resulting in "stress relaxation". This phenomenon will appear in all kinds of gaskets, but the stress-friendly resource network of polytetrafluoroethylene gaskets is more relaxed, which should be paid attention to.
The friction coefficient of PTFE is small (compressive stress is greater than 4MPa, the friction coefficient is 0.035~0.04), and the gasket is easy to slide outward when pre-tightening, so it is better to use the concave and convex flange surface. When using a flat flange, the outer diameter of the washer can contact the bolt, and the bolt can be used to prevent the washer from slipping out. Because the glass lining equipment is sintered by spraying a layer of enamel on the metal surface, the glaze layer is very brittle, the spraying is not uniform, and the flowing glaze layer makes the flatness of the flange surface poor. The use of metal composite gasket is easy to damage the glaze layer, and it is recommended to use PTFE gasket with asbestos board and rubber as the core material. This packing pad is easy to adhere to the flange surface, corrosion resistance, good application effect.
Many factories use polytetrafluoroethylene tape to wrap asbestos rubber sheets in low temperature and low pressure corrosive media for frequently disassembled manholes and pipes. Because of its advantages and ease of use, quite popular.
4. Asbestos resin board and impregnated asbestos board gasket
For all kinds of acid medium pipes, pumps, valves, import and export flanges, the use of temperature 80℃, pressure 0.6MPa below.
Asbestos gasket is suitable for low pressure and high temperature conditions with pressure lower than 0.1MPa and temperature lower than 800℃. According to the specific requirements of the equipment, it can be woven into different widths, thicknesses and diameters. Or cut the asbestos tape and attach it directly to the flange. Used in the joints of large ***, nitric acid oxidation furnaces and some unprocessed equipment, the effect is far more than the original asbestos rope.
5. Metal coated asbestos pad
The asbestos sheet or asbestos rubber sheet is covered with metal sheets, so that it is not in direct contact with the medium, to avoid the reduction of the strength of asbestos fiber, to overcome the leakage phenomenon, thus expanding the application range of asbestos rubber sheet. Generally, the use temperature of the metal-coated asbestos pad is 450 ° C (some are 600~700 ° C, such as in the gas of atmospheric pressure ~0.16MPa), and the use pressure is 4MPa, up to 6MPa. If the pressure is increased, the gasket is easy to cross flow, and the core material is extruded from the lap.
Because the metal coated asbestos gasket requires a large bolt tightening force, even if the use pressure is lower than 2.45MPa, the flange below pg25 kg can not be used. Otherwise, the rigidity of the flange and bolt is insufficient, resulting in deformation and seal failure. Some people believe that if the core material is replaced by synthetic rubber with better elasticity, its fastening force will decrease. Otherwise, because the fastening force is absorbed by the core material after the core material is softened, the fastening force required for the metal plate to be bonded to the flange surface cannot be provided, and the packing pad is easily damaged. In addition, in the media containing more chloride ions and acidic media, the overlap of stainless steel packing pad and iron packing pad is prone to crack corrosion.
When the temperature is higher than 450 ° C, ceramic fiber or carbon fiber can be used as the core material. Metal-coated ceramic fiber gasket has been used in a steel mill at 1100℃ high temperature, and has not been damaged after two years of use. Flexible graphite is the most suitable core material. At present, the flexible graphite cushion with gold plating has been mass-produced in China. The application effect is better than that of metal coated asbestos pad.
Metal pads can be made of various shapes, widely used in various heat exchangers, reactor covers, loading and unloading holes, manhole flanges and so on. A diameter of 2m iron pad made in China, the effect is good.
A flexible graphite sheet is attached to the surface of the metal packing pad. Compared with similar metal fillers with no coating on the surface, the gasket has a smaller preload specific pressure and better sealing performance.
At present, there is no such product in China, and some units stick the existing flexible graphite corrugated tape on the surface of the metal packing pad, the metal flat pad, the toothed pad and even the asbestos rubber pad, solving a lot of leakage problems. For example, the heat exchanger of a factory has a pressure of 5.88MPa, a temperature of 450℃, and the medium is hydrogen/oil and gas. The metal flat pads and the tooth pads, they're all leaking. This problem is solved by attaching flexible graphite corrugated tape to the flat back pad. It should be pointed out that this gasket form is a simple measure to solve the flange gasket leakage, and 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 tape, the quality of the coating can be improved.
6. Metal wrap pads
The metal winding pad cleverly utilizes the heat resistance, elasticity and strength of the metal and the softness of non-metallic materials, so the sealing performance is better, and the stainless steel tape winding flexible graphite pad has better performance. The preload specific pressure is less than that of asbestos winding pad, and there is no defect of asbestos fiber capillary leakage.
In the oil medium, 0Cr13 is mostly used for metal strip, and other media are recommended to use 1Cr18Ni9Ti.
Stainless steel with flexible graphite winding pads can be used in gas media with a pressure of 14.7MPa(up to 19.6MPa at home), and up to 30MPa in liquids. Temperature -190~+600℃(can be used to 1000℃ under anaerobic low pressure).
PTFE has good low temperature resistance, and the yield strength at low temperature is much higher than that at normal temperature. Therefore, PTFE winding 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 use of PTFE winding pad temperature can reach 250℃, in the acidic medium can reach 9MPa and 200℃.
Suitable for winding heat exchangers, reactors, pipelines, valves, pumps import and export flanges with large pressure and temperature fluctuations. For medium pressure and temperature above 300 ° C, the inner ring, outer ring or inner ring should be considered. If the concave and convex flange is used, the winding pad with the inner ring will be better.
Sticking flexible graphite plates on both sides of the flexible graphite winding pad can also achieve good sealing effect. Waste heat boiler is the key equipment of high temperature and high pressure in large fertilizer plant. It is wrapped with flexible graphite pad with outer ring, which does not leak when full load, and leaks when load is reduced. A flexible graphite plate 0.5mm thick is added on both sides of the gasket, cut into an arc, and the joint part is bonded with a diagonal joint, which has a good effect.
7. Metal flat pad, corrugated pad and toothed pad. Metal flat pad and metal corrugated pad
Generally used for small diameter medium and high pressure valves, pipes and equipment flanges. The pressure of the former is 1.568 to 31.36 mpa, and the pressure of the latter is 1.568 to 3.92 mpa, depending on the temperature. Choose gasket material according to medium and temperature.
8, octagonal pad and oval pad
The octagonal pad and oval pad for trapezoidal groove flange (commonly known as "earth steel ring") have good sealing performance. On the tapered surface of the groove, the octagonal pad is a surface contact, while the oval pad is a line contact. Therefore, the oval pad has a better fit when the fastening force is low, but it needs to be fastened twice. But the octagonal pad is not easy to leak after a fastening. Their disadvantage is that they require a lot of bolt fastening force. When used in low pressure and high temperature environments, the flange grade must be higher than pg25.