There are many types of sealed gaskets, and different materials and different shapes of gaskets have different functions. If the washer is wrong, not only can it be well sealed, it may also be a potential safety hazard, and it may be dangerous at any time. Do you know what are sealed gaskets and what are their characteristics?
Industrial rubber plate

Natural rubber is suitable for water, seawater, air, inert gas, alkali, salt solution and other media, but it is not resistant to mineral oil and non-polar solvents. Its long-term usage temperature is no more than 90 degrees centigrade, its low temperature performance is excellent, and it can be used above -60 C.
Nitrile rubber used in petroleum products, such as oil, lubricating oil, fuel oil, long-term use temperature of 120 DEG C, such as in the hot oil to 150 DEG C, low temperature of -10~-20 DEG C.
Neoprene is suitable for seawater, weak acid, weak alkali and salt solution. It has excellent oxygen and ozone aging properties. Its oil resistance is inferior to nitrile rubber and better than other general rubber. Its long-term usage temperature is less than 90 degrees centigrade, the highest usage temperature is no more than 130 degrees, and its low temperature is -30~-50 C.

There are many varieties of fluoro rubber. They have good acid resistance, oxidation resistance, oil resistance and solvent resistance. It can be used in almost all acid media, as well as in some oil and solvents, and the long-term use temperature is below 200.
Rubber sheet as flange gasket, mostly used in pipes or often dismantled manholes, hand holes, the pressure is not more than 1.568MPa. In all kinds of gaskets, the rubber gasket is the softest, the sealing performance is good and the sealing effect can be brought into play under the smaller pretightening force. Because of this, when the internal pressure is under, it is easily extruded because of the thickness of the gasket or the low hardness.
The rubber sheet is used in the organic solvents such as benzene, ketone, ether and so on. It is easy to appear swelling, weight increase, softening and sticky, resulting in sealing failure. The general swelling degree of more than 30% can not be used.
Under the condition of low pressure (especially below 0.6MPa) and vacuum, it is more suitable to adopt rubber pads. The rubber material has good compactness and low permeability. For example, fluoro rubber, the most suitable for vacuum container sealing gasket, the highest vacuum of up to 1.3× 10-7Pa. When the rubber pad is used in the range of 10-1~10-7Pa vacuum, it is required to be baked and pumped.
Asbestos rubber sheet

The price is lower than that of other gaskets, and it is easy to use. The biggest problem is that although the rubber material and some fillers are added to the gasket material, it is still impossible to completely fill the tiny pores that are colluded, and there is micro penetration. Therefore, in very polluted medium, it can not be used even if the pressure and temperature are not high.
When used in some high temperature oil medium, usually in the later stage, because of the low strength of the rubber and filler and the loose material, it will permeate in the interface and gasket, and there will be coke and smoke. In addition, the asbestos rubber plate is easily bonded on the flange seal face at high temperature, which brings many troubles to the replacement of the gasket.
The use pressure of the gasket in a variety of media depends on the strength retention of the gasket material in the heated state. The crystalline water and adsorbed water are found in the asbestos fiber. At 110 C, the adsorptive water of the fiber has been separated from 2/3, and the tensile strength of the fiber has been reduced by 10%. When the temperature is 368, the adsorption water is completely precipitated, and the tensile strength of the fiber is reduced by 20%. When the temperature exceeds 500 C, the crystal water begins to precipitate and the strength is lower. The influence of medium on the strength of asbestos rubber plate is also great.
For example, in aviation lubricating oil and aviation fuel, the transverse tensile strength of 400 oil resistant asbestos rubber sheet is 80%, which is due to the serious swelling of aviation fuel to the rubber in the board than aviation lubricating oil. Considering the above factors, the recommended safe use range of the asbestos rubber sheet XB450 is 250 ~300, 3~3.5MPa and 400 3~3.5MPa.
Asbestos rubber sheet contains chloride and sulfide. After absorbing water, it is easy to form corroded primary batteries 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 oil medium. The gasket will be swelling in the medium of oil and solvent, but in a certain range, there is little effect on the sealing performance.
For example, No. 400 oil resistant asbestos rubber plate is soaked for 24 hours at normal temperature aviation fuel, and it is required that the weight of oil absorption should not exceed 15%.
Compared with foreign famous brand products, the domestic asbestos rubber plate still has a certain gap. The statistics of a unit show that in the oil medium over 350 C, most of the imported products are not leaked with asbestos rubber plates. Japanese petroleum industry standard JPI-7S-71 "use standard of asbestos rubber plate for petroleum industry", the term of use is about one year.
Teflon

Teflon is easy to cool and creep under compression and high temperature, so it is generally used for low pressure, medium temperature, strong corrosion and non polluting media, such as strong acid, strong alkali, halogen, medicine and so on. The safe use temperature is 150 degrees C, the pressure is below 1MPa. The filled polytetrafluoroethylene, though higher in strength, is not more than 200 c, and the corrosion resistance is reduced. The maximum use pressure of polytetrafluoroethylene pads is generally not more than 2MPa.
As the temperature increases, the creep of the material causes the seal pressure to decrease. Even if it is not heated, the pressure stress of the sealing surface will decrease with the prolongation of time, resulting in &ldquo and stress relaxation. This phenomenon will come into being in all kinds of gaskets, but the stress relaxation of polytetrafluoroethylene pad is more serious and should be paid attention to.
The friction coefficient of PTFE is smaller (compression stress is greater than 4MPa, the friction coefficient is 0.035~0.04). When the preload is tight, the gasket is easy to slide outwards, so it is better to use concave convex flange surface. When the plane flange is used, the outer diameter of the gasket can be exposed to the bolt, and the bolt can be used to prevent the outer slip of the gasket.
Because the glass lined equipment is on the surface of the metal spraying a layer of enamel after sintering, the glaze layer is very brittle, and sprayUnevenly coated and glazed layer flow, flanges surface flatness is poor. The metal composite gasket is easy to damage the glaze layer, so it is recommended to use the PTFE pads with the core material and rubber on the asbestos board. This kind of bag cushion is easy to be fitted with the flange face and corrosion resistance, and the use effect is good.
Many factories in the strong corrosive medium with low temperature and pressure, use the asbestos rubber sheet wrapped around the Teflon tape, for the often dismantled manholes and pipes. It is very popular because it is convenient to make and use.
Asbestos resin plate and impregnated asbestos mat
Many kinds of acid medium pipes, pumps, valves, inlet and outlet flanges are used. The temperature is under 80, and the pressure is below 0.6MPa.
The asbestos compiling gasket is suitable for the low pressure and high temperature conditions under the pressure of less than 0.1MPa, and the temperature is not more than 800. And according to the specific requirements of the equipment, the width, thickness, diameter of the gasket can not be woven.
Or the asbestos belt is cut down directly on the surface of the flange. It is used at the interface of large sulphuric acid, nitric acid oxidizer and some unprocessed equipment, which is far more effective than the original asbestos rope.
Asbestos pad

The asbestos sheet or asbestos rubber sheet is coated with metal sheets, so that it does not directly contact with the medium, so that the asbestos fiber strength is reduced and the leakage phenomenon is overcome, thus expanding the application scope of asbestos rubber sheet.
The general metal asbestos pad use temperature is 450 degrees (individual has 600~700 temperature, such as in atmospheric ~0.16MPa flue gas), the pressure is 4MPa, the highest can reach 6MPa. If the pressure is increased again, the gasket is prone to cross flow, and the core is extruded from the lap.
As the metal asbestos pad requires a larger bolt tightening force, even when the use pressure is lower than 2.45MPa, it is not possible to use a flange under the pg25 kilogram level. Otherwise, the stiffness of flanges and bolts is not enough, resulting in deformation and sealing failure. Some people think that, if the core material is changed into an elastic synthetic rubber better, the fastening force will decline.
In fact, because the core material is softened, the fastening force is absorbed by the core material, which can not provide the fastening force for the metal plate to fit on the flange surface, and the cushion is easily damaged. In addition, in the medium with more chloride ion and acid medium, the lap joint of stainless steel pads and iron pads is easy to produce crevice corrosion.
The temperature is higher than 450 C, and the core material can use ceramic fiber or carbon fiber. A metal - coated ceramic fiber gasket is used in a steel plant for high temperature at 1100 degrees C, which has not been damaged for two years. Flexible graphite core is the most suitable. At present, the mass production of metal coated graphite pads has been produced in China. The use effect is better than that of the metal asbestos pad.
Metal pads can be made of various shapes, which are used in a large number of heat exchangers, large cover of reactor, loading and unloading holes, manhole flanges, etc. In China, an iron bag with a diameter of 2m has been made in good use.
A flexible graphite sheet is pasted on the surface of the metal cushion. Compared with the uncoated similar metal cushion, the gasket has smaller pretightening pressure and better sealing performance.
At present, there is no such product in China. Therefore, some units paste the existing flexible graphite wrinkle belt on the surface of metal cushion, metal flat pad, toothed cushion and asbestos rubber pad, and solve many leakage problems. For example, the heat exchanger of a factory is 5.88MPa, the temperature is 450 C, and the medium is hydrogen / oil and gas.
It has been used for metal pads and toothed pads, and has leaks. The flexible graphite wrinkles were attached to the cushion to solve the problem. It should be pointed out that this kind of gasket is a simple way to solve the leakage of flange gaskets. The quality of workpieces attached to flexible graphite directly affects the normal operation of the equipment. If a layer of glue is painted on the back of the strip, the quality of the cladding can be improved.
Metal winding gasket

The metal winding pad cleverly utilizes the heat resistance, resilience and strength of the metal, as well as the softness of the non-metallic materials, so the sealing performance is better, and the performance of the flexible graphite pad with the stainless steel belt is best. Its pre tightening and specific pressure is smaller than the asbestos winding pad, and there is no defect in the leakage of asbestos fiber pores.
In the oil medium, the metal strip uses more 0Cr13, while the other media recommends the use of 1Cr18Ni9Ti.
Stainless steel with flexible graphite winding mat in the gas medium, the use pressure of 14.7MPa (domestic up to 19.6MPa), in the liquid can be available to 30MPa. Temperature -190~+600 C (at no oxygen, low pressure can be available to 1000 degrees).
Polytetrafluoroethylene has good low temperature resistance, and its yield strength at low temperature is much higher than that at normal temperature. So the polytetrafluoroethylene twining pad can be used in low temperature medium, such as liquid hydrocarbon. At the same time, the thermal conductivity of the metal band is increased. The temperature of the PTFE winding pad is up to 250 C, and can be used in the acid medium to 9MPa and 200 c.
The winding pad is suitable for the heat exchanger, reactor, pipe, valve and pump inlet and outlet flange for high pressure and temperature fluctuation. The use of inner ring, outer ring or inner and outer ring should be considered for those with medium or above pressure and temperature above 300. If the concave convex flange is used, the effect of the winding cushion with the inner ring is better.
Good sealing effect can be obtained by cladding flexible graphite plates on two sides of flexible graphite winding mats. The waste heat boiler of a large chemical fertilizer plant belongs to the key equipment of high temperature and high pressure. With the flexible graphite winding pad with outer ring, it does not leak when the load is full, and the load is leaked when the load is reduced. Increase the thickness of flexible graphite gasket 0.5mm in both sides, cut into circles, the joint part of the beveled lap, in good condition.
Metal cushion, wave pad and tooth shaped metal flat pad, metal wave pad
It is generally used on medium and high pressure valves, pipes and flanges with smaller diameter. The use of pressure is based on the temperature, the former is 1.568~31.36MPa, and the latter is 1.568~3.92MPa. The material of the gasket is selected according to the medium and temperature.
Octagonal pads and oval pads

Octagonal pads and oval pads for trapezoid flanges (commonly known in the refining industry)“ soil steel ring ”) good sealing performance. On the cone surface of the grooves, the octagonal mats are surface contact while the elliptical mats are line contact.
Therefore, good fit in the elliptical pad low clamping pressures, but need two fastening; a fastening and octagonal pad in general, is not easy to leak. Their shortcomings require a larger bolt tightening force. When used for low pressure and high temperature conditions, the flange grade must be above pg25 kilograms.
The source of this article: chemical 707;

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