Eight kinds of gaskets
Table of Contents
Natural rubber is suitable for water, sea water, air, inert gas, alkali, salt aqueous solution and other media, but it is not resistant to mineral oil and non-polar solvents. Its long-term service temperature is not more than 90 ℃, and its low-temperature performance is excellent. It can be used above – 60 ℃.
NBR is suitable for petroleum products, such as petroleum, lubricating oil, fuel oil, etc. the long-term service temperature is 120 ℃, if in hot oil, it can withstand 150 ℃, and the low temperature is – 10 ~ – 20 ℃.
Chloroprene rubber is suitable for seawater, weak acid, weak alkali and salt solution. It has excellent oxygen and ozone aging resistance. Its oil resistance is inferior to that of nitrile rubber and superior to other general rubber. Its long-term service temperature is lower than 90 ℃, the maximum service temperature is not more than 130 ℃, and the low temperature is – 30 ~ – 50 ℃.
There are many varieties of fluororubber, which have good acid resistance, oxidation resistance, oil resistance and solvent resistance. It can be used in almost all kinds of acid media and some oils and solvents, and its long-term use temperature is lower than 200 ℃.
As flange gasket, rubber plate is mostly used for pipeline or manhole and handhole which are often disassembled, and the pressure is not more than 1.568 MPa. Because in all kinds of gaskets, rubber gasket is the softest and has good fitting performance. It can play a sealing effect under a small preload. Because of this, when bearing internal pressure, the gasket is easy to be extruded due to its thick or low hardness.
When rubber sheet is used in organic solvents such as benzene, ketone and ether, swelling, weight gain, softening and stickiness are easy to occur, leading to sealing failure. Generally, it can not be used if the swelling degree is more than 30%.
Under the condition of low pressure (especially below 0.6MPa) and vacuum, rubber pad is more suitable. The rubber material has good compactness and low air permeability. For example, fluororubber is the most suitable sealing gasket for vacuum container, and the vacuum degree can reach 1.3 × 10-7 PA. When the rubber pad is used in the vacuum range of 10-1 ~ 10-7 PA, it needs to be baked and pumped.
Asbestos rubber sheet
The price is lower than other gaskets and easy to use. The biggest problem is: Although the gasket material is added with rubber and some fillers, it still can’t completely fill the micro pores of those collusion, and there is a trace of permeability. Therefore, in the highly polluting medium, even if the pressure and temperature are not high, it can not be used. When used in some high-temperature oil medium, usually in the later stage of use, due to the carbonization of rubber and filler, the strength is reduced, and the material becomes loose, which causes penetration at the interface and inside the gasket, resulting in coking and smoke. In addition, the asbestos rubber plate is easy to stick on the flange sealing surface under high temperature, which brings a lot of trouble to replace the gasket.
Under the condition of heating, the service pressure of gasket in various media depends on the strength retention rate of gasket material. There are crystal water and adsorbed water in asbestos fiber materials. At 110 ℃, 2 / 3 of the adsorbed water was precipitated, and the tensile strength of the fiber was reduced by 10%; at 368 ℃, all the adsorbed water precipitated, and the tensile strength of the fiber decreased by 20%; when the temperature exceeded 500 ℃, the crystallized water began to precipitate, and the tensile strength of the fiber was lower.
The medium also has great influence on the strength of asbestos rubber sheet. For example, in aviation lubricating oil and aviation fuel, the transverse tensile strength difference of No. 400 oil resistant asbestos rubber sheet is 80%, which is due to the swelling of rubber in the plate by aviation fuel is more serious than that of aviation lubricating oil. Considering the above factors, the recommended safe use range of domestic asbestos rubber sheet xb450 is as follows: temperature 250 ℃ ~ 300 ℃, pressure 3 ~ 3.5Mpa; the service temperature of 400 oil resistant asbestos rubber sheet should not exceed 350 ℃.
Asbestos rubber sheet contains chloride ion and sulfide, and it is easy to form corrosion galvanic cell with metal flange after absorbing water. Especially, the sulfur content of oil resistant asbestos rubber sheet is several times higher than that of ordinary asbestos rubber plate, so it is not suitable for use in non oil medium. The gasket will swell in oil and solvent media, but in a certain range, it has little effect on the sealing performance. For example, No. 400 oil resistant asbestos rubber sheet is required to increase its weight by no more than 15% when it is immersed in aviation fuel at room temperature for 24 hours.
Polytetrafluoroethylene is easy to cold flow and creep under pressure and high temperature, so it is generally used in medium with low pressure, medium temperature, strong corrosion and no pollution medium, such as strong acid, strong alkali, halogen, medicine, etc. The safe use temperature is 150 ℃ and the pressure is below 1MPa. Although the strength of filled polytetrafluoroethylene is higher, the service temperature is not more than 200 ℃, and the corrosion resistance is reduced. The maximum working pressure of Teflon gasket is generally not more than 2MPa.
As the temperature rises, the material creeps and the sealing pressure decreases obviously. Even if the temperature is not raised, the compression stress of the sealing surface will also decrease with the extension of time, resulting in “stress relaxation phenomenon”. This phenomenon can occur in all kinds of gaskets, but the stress relaxation of PTFE gasket is serious, which should be paid attention to.
The friction coefficient of polytetrafluoroethylene 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 during pre tightening, so concave convex flange surface is better. When the flat flange is used, the outer diameter of the gasket can be contacted with the bolt to prevent the gasket from sliding.
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 flange surface is poor. The use of metal composite gasket is easy to damage the glaze layer, so it is recommended to use polytetrafluoroethylene gasket with asbestos board and rubber as core material. This kind of packing gasket is easy to fit with the flange surface and has good corrosion resistance.
In the strong corrosive medium with low temperature and pressure, many factories use asbestos rubber plate wrapped with polytetrafluoroethylene (PTFE) raw material belt, which is used in the manhole and pipeline which are often dismantled. Because of the convenience of making and using, it is very popular.
Asbestos resin board and impregnated asbestos board gasket
Asbestos resin board and impregnated asbestos plate gasket are mostly used in pipelines, pumps, valves and inlet and outlet flanges of various acid media. The service temperature is 80 ℃ and the pressure is below 0.6 MPa.
Asbestos braided gasket is suitable for low pressure and high temperature working conditions with pressure below 0.1MPa and temperature not exceeding 800 ℃. According to the specific requirements of the equipment, the gasket with different width, thickness and diameter can be woven. Or cut the asbestos tape and apply it directly on the flange surface. It is used in large sulfuric acid, nitric acid oxidation furnace and some unprocessed equipment interface, the effect is far better than the original asbestos rope.
Metal clad asbestos pad
The asbestos board or asbestos rubber plate is coated by metal sheet, which can not directly contact with the medium, so as to avoid the decline of asbestos fiber strength and solve the leakage phenomenon, so as to expand the application range of asbestos rubber plate.
Generally, the service temperature of metal clad asbestos pad is 450 ℃ (some of them can reach 600 ~ 700 ℃. For example, in the flue gas with atmospheric pressure ~ 0.16mpa), the service pressure is 4MPa, and the maximum is 6Mpa. If the pressure increases again, the gasket is prone to cross flow, and the core material is extruded from the lap joint.
Due to the large bolt tightening force required by the metal clad asbestos gasket, even when the service pressure is lower than 2.45mpa, the flange below pg25kg cannot be used. Otherwise, the rigidity of flange and bolt is not enough, which will lead to seal failure due to deformation. Some people think that if the core material is changed to synthetic rubber with better elasticity, its tightening force will be reduced. In fact, it is not because after the core material is softened, the fastening force is absorbed by the core material, which can not provide the fastening force required for the metal plate to fit on the flange surface, and the gasket is easily damaged. In addition, in the medium containing more chloride ions and acid medium, crevice corrosion is easy to occur at the lap joint of stainless steel and iron sheet.
When the temperature is higher than 450 ℃, ceramic fiber or carbon fiber can be used as core material. The metal clad ceramic fiber gasket was used at 1100 ℃ in an iron and steel plant. It has not been damaged for two years. Flexible graphite is the most suitable core material. At present, flexible graphite pads with metal cladding have been produced in batch in China. Its application effect is better than that of metal clad asbestos pad.
The metal clad gasket can be made into various shapes and is widely used in various heat exchangers, reactor covers, loading and unloading holes, manhole flanges, etc. The ladle mat with a diameter of 2m has been made in China and has been used well.
A layer of flexible graphite sheet is pasted on the surface of the metal clad gasket. Compared with the similar metal clad gasket without surface coating, this kind of gasket has smaller pre tightening specific pressure and better sealing performance. Figure 2-9 shows the relationship between the compression stress and leakage.
At present, there is no such product in China. Therefore, some units have pasted the existing flexible graphite corrugated tape on the surface of metal clad gasket, metal flat gasket, tooth shaped gasket and even asbestos rubber gasket, thus solving many leakage problems. For example, the heat exchanger in a factory has a pressure of 5.88MPA, a temperature of 450 ℃, and a medium of hydrogen / oil gas. Metal flat gasket and toothed gasket have been used, and all of them have leakage. After that, flexible graphite wrinkling belt was pasted on the flat pad to solve the problem. It should be pointed out that this kind of gasket is a simple measure to solve the leakage of flange gasket. The working quality of flexible graphite tape directly affects the normal operation of the equipment. If a layer of glue is applied on the back of the strip, the sticking quality can be improved.
Metal wound gasket skillfully uses the heat resistance, resilience and strength of metal and the softness of non-metallic materials, so it has better sealing performance, and the stainless steel belt wound flexible graphite gasket has the best performance. The pre tightening specific pressure is smaller than that of asbestos winding gasket, and there is no defect of asbestos fiber capillary leakage. Fig. 2-10 shows the curve of the relationship between the pre tightening pressure and the leakage. In oil medium, 0Cr13 is usually used for metal strip, while 1Cr18Ni9Ti is recommended for other media.
The working pressure of stainless steel with flexible graphite winding gasket is 14.7mpa in gas medium (up to 19.6Mpa in China), and 30MPa in liquid. The temperature is – 190 ~ + 600 ℃ (up to 1000 ℃ in the case of no oxygen and low pressure).
The yield strength of PTFE at low temperature is much higher than that at normal temperature. Therefore, PTFE winding gasket can be used in low temperature medium, such as liquid hydrocarbon. At the same time, due to the addition of metal tape to improve the thermal conductivity, the service temperature of PTFE wound gasket can reach 250 ℃, and it can be used up to 9Mpa and 200 ℃ in acid medium.
Spiral wound gasket is suitable for heat exchanger, reactor, pipeline, valve, pump inlet and outlet flange with large fluctuation of pressure and temperature. For medium and above pressure and temperature over 300 ℃, it should be considered to use inner ring, outer ring or inner and outer ring. If concave convex flange is used, the effect of winding gasket with inner ring is better.
Good sealing effect can also be achieved by sticking flexible graphite plate on both sides of flexible graphite winding gasket. The waste heat boiler of a large chemical fertilizer plant is a key equipment of high temperature and high pressure. The flexible graphite winding gasket with outer ring is used, which will not leak under full load and during load reduction. The flexible graphite plate with thickness of 0.5mm is added on both sides of the gasket, which is cut into a circular arc shape. The joint is lapped with inclined mouth, and the service condition is good.
Flat and corrugated metal gaskets
It is generally used for medium and high pressure valves, pipelines and equipment flanges with small diameter. According to the different 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.
Octagonal gasket and elliptical gasket (commonly known as “earth steel ring” in oil refining industry) have good sealing performance. On the conical surface of the groove, the octagonal pad is in surface contact while the elliptical pad is in line contact. Therefore, the ellipse cushion has a good fit under the low tightening force, but it needs to be tightened twice, while the octagonal gasket is not easy to leak after one-time tightening. The disadvantage of them is that they require a large bolt tightening force. When used in low pressure and high temperature conditions, the flange grade must be above pg25kg.
Source: China Gaskets Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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