1. Industrial rubber sheet

Natural rubber is suitable for water, sea water, air, inert gases, alkalis, salt solutions 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 at -60°C. Used above.

Nitrile rubber is suitable for petroleum products, such as petroleum, lubricating oil, fuel oil, etc. The long-term use temperature is 120℃, such as 150℃ in hot oil, and the low temperature is -10~-20℃.

Chloroprene rubber is suitable for seawater, weak acid, weak alkali, and salt solution. It has excellent resistance to oxygen and ozone aging. Its oil resistance is inferior to nitrile rubber but better than other general rubbers. The long-term use temperature is less than 90℃, and the use temperature does not exceed 130℃, low temperature is -30~-50℃.

There are many varieties of fluorine rubber, which respectively have 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°C.

Rubber plates are used as flange gaskets, which are mostly used in pipelines or frequently disassembled manholes and hand holes, and the pressure does not exceed 1.568MPa. Because of all types of gaskets, rubber gaskets are the softest and have good fitting performance. They can exert a sealing effect under a small pre-tightening force. Because of this, it is easy to be squeezed out due to the thicker or lower hardness of the gasket when it is subjected to internal pressure.

Rubber sheets are used in organic solvents such as benzene, ketones, ethers, etc., which are prone to swelling, weight gain, softening, and stickiness, resulting in seal failure. Generally, it cannot be used if the swelling degree exceeds 30%.

Under low pressure (especially below 0.6MPa) and vacuum, it is more appropriate to use rubber pads. The rubber material has good compactness and low air permeability. For example, fluorine rubber is most suitable as a sealing gasket for vacuum containers, and the vacuum degree* can reach 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 exhausted.

2. Asbestos rubber sheet

The price is lower than other gaskets, and it is easy to use; *The problem is: Although the gasket material is added with rubber and some fillers, it still cannot completely fill the tiny pores that collude, and there is a trace of penetration. Therefore, it cannot be used in highly polluting media even if the pressure and temperature are not high. When used in some high-temperature oil media, usually in the later period of use, due to the carbonization of rubber and fillers, the strength is reduced, and the material becomes loose, which will cause penetration in the interface and the interior of the gasket, causing coking and smoking. In addition, the asbestos rubber sheet is easy to adhere to the flange sealing surface at high temperatures, which brings a lot of trouble to the replacement of the gasket.

In the heated state, the use pressure of the gasket in various media depends on the strength retention rate of the gasket material. Crystal water and adsorbed water exist in asbestos fiber materials. At 110°C, 2/3 of the adsorbed 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%; more than 500°C, Crystal water begins to precipitate, and the strength is lower. The medium also has a great influence on the strength of the asbestos rubber sheet. For example, in aviation lubricants and aviation fuels, the transverse tensile strength of No. 400 oil-resistant asbestos rubber sheets differs by 80%. This is because aviation fuel swells the rubber in the sheet more seriously than aviation lubricants. Taking into account the above factors, the recommended safe use range of domestic asbestos rubber sheet XB450: temperature 250°C~300°C, pressure 3~3.5MPa; the use temperature of No. 400 oil-resistant asbestos rubber sheet should not exceed 350°C.

Asbestos rubber sheets contain chloride ions and sulfides, which will easily form corrosive galvanic cells with metal flanges after absorbing water. In particular, the sulfur content of oil-resistant asbestos rubber sheets is several times higher than that of ordinary asbestos rubber sheets, so it is not suitable for use in non-oily media. The gasket will swell in oil and solvent media, but within a certain range, it will basically have no effect on the sealing performance. For example, the No. 400 oil-resistant asbestos rubber sheet is subjected to a 24-hour immersion test in aviation fuel at room temperature, and the weight increase due to oil absorption must not exceed 15%.

There is still a certain gap between domestically produced asbestos rubber sheets and foreign* products. Statistics from a certain unit show that in the oil medium above 350°C, most of the products that use asbestos rubber sheets that do not leak are imported products. The Japanese Petroleum Industry Standard JPI-7S-71 "Standard for the Use of Asbestos Rubber Sheets for the Petroleum Industry" has a service life of about one year.

3. PTFE

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, medicine, etc. The safe use temperature is 150℃ and the pressure is below 1MPa. Although filled with PTFE has higher strength, its use temperature does not exceed 200°C, and its corrosion resistance is reduced. The working pressure of the PTFE cushion* is generally not more than 2MPa.

As the temperature rises, the material creeps, causing the sealing pressure to drop significantly. Even if the temperature is not increased, the compression stress of the sealing surface will decrease with time, resulting in a "stress relaxation phenomenon. This phenomenon occurs in all kinds of gaskets, but the stress relaxation phenomenon of PTFE gaskets is more serious. Attention should be paid.

The friction coefficient of PTFE is relatively small (the compression stress is greater than 4MPa, and the friction coefficient is 0.035~0.04), and the gasket is easy to slip outward during pre-tightening, so *concave-convex flange surface is adopted. 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. 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 uneven spraying and the flow of the glaze layer make the flange surface flatness poor. The use of metal composite gaskets is easy to damage the glaze layer, so it is recommended to use a PTFE gasket with a core material of asbestos board and rubber. This kind of packing pad is easy to fit with the flange surface and is also corrosion-resistant, and the use effect is good.

Many factories use asbestos rubber sheets to wrap raw polytetrafluoroethylene tapes in strong corrosive media where the temperature and pressure are not high, which are used in manholes and pipelines that are frequently dismantled. It is very popular because of its easy 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 service temperature is 80℃ and the pressure is below 0.6MPa.

Asbestos woven gaskets are suitable for low pressure and high temperature working conditions where the pressure is below 0.1 MPa and the temperature does not exceed 800 ℃. And according to the specific requirements of the equipment, it can be woven into gaskets with different widths, thicknesses and diameters. Or cut the asbestos tape and apply it directly on the flange surface. It is used at the interface of large-scale sulfuric acid, nitric acid oxidation furnace and some unprocessed equipment, and the effect is far beyond the original asbestos rope.

5. Metal-clad asbestos mat

The asbestos board or asbestos rubber board is covered by the metal sheet so that it does not come into direct contact with the medium, which avoids the asbestos fiber strength reduction, overcomes the leakage phenomenon, and expands the use range of the asbestos rubber board. Generally, the use temperature of metal-clad asbestos mat is 450℃ (some 600~700℃, such as in the flue gas of normal pressure ~0.16MPa), the working pressure is 4MPa, and the maximum can reach 6MPa. If the pressure increases again, the gasket is prone to cross flow, and the core material is extruded from the overlap.

As the metal-clad asbestos gasket requires a large bolt tightening force, even when the working pressure is lower than 2.45MPa, flanges below pg25 kilograms cannot be used. Otherwise, the rigidity of flanges and bolts will be insufficient, resulting in deformation and sealing 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 easily damaged. In addition, in media containing more chloride ions and acidic media, crevice corrosion is likely to occur at the overlap of the stainless steel pad and the iron pad.

The temperature is higher than 450°C, ceramic fiber or carbon fiber can be used as the core material. A steel plant used 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, China has mass-produced metal-clad flexible graphite mats. Its use effect is better than that of metal-clad asbestos mat.

Metal cladding 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. An iron-clad pad with a diameter of 2m has been made in China, and it is in good condition.

A layer of flexible graphite sheet is pasted on the surface of the metal cladding gasket. Compared with similar metal cladding gaskets that are not covered on the surface, this type of gasket has a smaller pre-tightening pressure and better sealing performance.

At present, there is no such product in China, so some units stick 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 5.88 MPa, a temperature of 450 ℃, and the medium is hydrogen/oil gas. Metal flat gaskets and toothed gaskets have been used, and they all leaked. Later, a flexible graphite wrinkle 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 flange gasket, and the workability quality of the flexible graphite tape directly affects the normal operation of the equipment. If a layer of glue is applied to the back of the strip, the quality of the paste can be improved.

6. Metal wound gasket

Metal wound gaskets cleverly use the heat resistance, resilience and strength of metals and the softness of non-metallic materials, so the sealing performance is better. Among them, the performance of the flexible graphite gasket wound with stainless steel tape is *. The pre-tightening ratio is smaller than that of asbestos winding pads, and there is no shortcoming of asbestos fiber capillary leakage

Among the oil media, 0Cr13 is mostly used for metal strips, while 1Cr18Ni9Ti is recommended for other media.

The stainless steel belt flexible graphite wound gasket is used in a gas medium, and the working pressure is 14.7MPa (up to 19.6MPa in China), and it can be used up to 30MPa in liquid. Temperature -190~+600℃ (can be used up to 1000℃ 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 its strength at room temperature. Therefore, PTFE wound gaskets can be used in low-temperature media, such as liquid hydrocarbons. At the same time, due to the addition of metal tape to improve the thermal conductivity, the temperature of the PTFE wound gasket can reach 250°C, and it can be used up to 9MPa and 200°C in acidic media.

Winding gaskets are suitable for heat exchangers, reactors, pipes, valves, pump inlet and outlet flanges with large pressure and temperature fluctuations. For those with medium or above pressure and temperature exceeding 300℃, consider using inner ring, outer ring or inner and outer ring. If a concave-convex flange is used, a winding gasket with an inner ring has a better effect.

A good sealing effect can also be achieved by sticking flexible graphite sheets on both sides of the flexible graphite winding gasket. A waste heat boiler in a large chemical fertilizer plant is a key high-temperature and high-pressure equipment. It uses flexible graphite wound gaskets with an outer ring, which does not leak when the load is full, and 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 overlapped with an oblique mouth, which is in good condition.

7. Metal flat pads, wave pads and toothed pads, metal flat pads and metal wave pads

Generally used on medium and high pressure valves, pipelines and equipment flanges with smaller diameters. The working pressure depends 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.

8. Octagonal pad and oval pad

Octagonal gaskets and oval gaskets (commonly known as "earth steel ring" in the oil refining industry) for trapezoidal groove flanges have good sealing performance. On the tapered surface of the groove, the octagonal pad is in surface contact and the oval pad is in line contact. Therefore, the oval pad has a good fit under low tightening force, but it needs to be tightened twice; while the octagonal pad is generally not easy to leak after being tightened once. Their shortcomings are that they require a large bolt tightening force. When used in low pressure and high temperature conditions, the flange level must be above pg25 kg.