Industrial rubber
Natural rubber is suitable for water, seawater, air, inert gases, alkalis, salts, aqueous solutions 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 ℃.
NBR is suitable for petroleum products, such as petroleum, lubricating oil, fuel oil, etc., long-term use temperature of 120 ℃, such as in hot oil can be resistant to 150 ℃, low temperature of -10 ~ -20 ℃.
Chloroprene rubber is suitable for seawater, weak acid, weak alkali, salt solution, excellent resistance to oxygen and ozone aging performance, oil resistance is second to nitrile rubber and better than other general-purpose rubber, long-term use temperature is lower than 90 ℃, the maximum use temperature does not exceed 130 ℃, low temperature of -30 ~ -50 ℃.
There are several varieties of fluoroelastomers, which have good resistance to acid and oxidation, as well as oil and solvent resistance. They can be used in almost all acid media as well as some oils and solvents, and the long-term use temperature is lower than 200℃.
Rubber sheet as a flange gasket, mostly used in pipelines or frequently dismantled manhole, handhole, pressure does not exceed 1.568 MPa. Because in all types of gaskets, rubber gasket is the softest, good fitting performance, in the smaller preload can play a sealing effect. Because of this, when subjected to internal pressure, the gasket is easily extruded due to its thickness or low hardness.
Rubber sheet used in benzene, ketone, ether and other organic solvents, easy to dissolve, weight, soften, sticky phenomenon, resulting in sealing failure. Generally swelling degree more than 30% can not be used.
In low pressure (especially below 0.6MPa), vacuum, the use of rubber gasket is more appropriate. Rubber materials are dense, low permeability. For example, fluoroelastomer is the most suitable gasket for vacuum containers, and the vacuum degree can be up to 1.3×10-7 Pa. When the rubber gasket is used in the vacuum degree range of 10-1~10-7 Pa, it needs to be baked and pumped.
Asbestos Rubber Sheet
The price is lower than other gaskets, easy to use; the biggest problem is: gasket material, although adding rubber and some fillers, still can not fill those stringing tiny pores completely, there is a trace of infiltration. Therefore, in the highly polluting medium, even if the pressure, temperature is not high can not be used. When used in some high-temperature oil media, usually in the late use, due to the rubber and filler carbonisation, strength reduction, material becomes loose, then in the interface and the gasket inside the infiltration, coking and fuming phenomenon. In addition, asbestos rubber sheet is easy to bond on the flange sealing surface at high temperature, which brings a lot of trouble to the replacement of gaskets.
In the heated state, the gasket in a variety of media in the use of pressure depends on the strength of the gasket material retention rate. Asbestos fibre material in the presence of crystalline water and adsorbed water. 110 ℃, between the fibres of adsorbed water has been 2 / 3 precipitation, the tensile strength of the fibre is reduced by about 10%; 368 ℃, all the adsorbed water precipitation, the tensile strength of the fibre is reduced by about 20%; more than 500 ℃, the crystalline water began to precipitate, the strength is even lower.
The medium also has a great influence on the strength of asbestos rubber sheet. For example, in aviation lubricant and aviation fuel oil, 400 oil-resistant asbestos rubber sheet lateral tensile strength difference of 80%, which is due to aviation fuel oil on the rubber in the sheet of the swelling than the aviation lubricant is serious. Considering the above factors, the recommended safe use range of domestic asbestos rubber sheet XB450: temperature 250℃~300℃, pressure 3~3.5MPa; the use temperature of No.400 oil-resistant asbestos rubber sheet should not exceed 350℃.
Asbestos rubber sheet contains chlorine ions and sulphide, it is easy to form corrosive primary battery with metal flange after absorbing water, especially the sulphur content in oil-resistant asbestos rubber sheet is several times higher than that of ordinary asbestos rubber sheet, so it is not suitable to be used in non-oil medium. Gasket in oil and solvent media will occur in the swelling phenomenon, but within a certain range, the sealing performance is basically no effect. For example, No. 400 oil-resistant asbestos rubber sheet in room temperature aviation fuel for 24 hours immersion test, the requirements due to oil absorption weight increase shall not exceed 15%.
Domestic asbestos rubber sheet compared with foreign brand-name products still have a certain gap. A unit of statistics show that more than 350 ℃ in the oil medium, where the use of asbestos rubber sheets do not leak most of the imported products. Japan's petroleum industry standard JPI-7S-71 "petroleum industry asbestos rubber sheet use standard", the use of the period of about one year.
Polytetrafluoroethylene
Polytetrafluoroethylene 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 do not allow the contamination of the medium, such as strong acid, strong alkali, halogens, pharmaceuticals and so on. Safe use of temperature at 150 ℃, pressure 1MPa or less. Filled PTFE although the strength is higher, but the use of temperature is not more than 200 ℃, and corrosion resistance is reduced. The maximum use pressure of PTFE gaskets generally does not exceed 2MPa.
As the temperature rises, the material creep, resulting in a significant drop in sealing pressure. Even if the temperature does not rise, with the extension of time, the compressive stress of the sealing surface will also decline, resulting in "stress relaxation phenomenon. The phenomenon in a variety of gaskets will produce, but the PTFE pad stress relaxation phenomenon is more serious, should be noted.
PTFE friction coefficient is small (compression stress greater than 4MPa, the friction coefficient of 0.035 ~ 0.04), pre-tensioned gaskets easy to slip outward, so it is best to use a concave-convex flange. When the plane 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 slipping outward.
As the glass lining equipment is sprayed on the metal surface of a layer of enamel after sintering, the glaze layer is very brittle, coupled with uneven spraying and glaze flow, flange surface flatness is poor. The use of metal composite gasket is easy to damage the glaze layer, so it is recommended to use the core material from the asbestos sheet plus rubber PTFE gasket. This gasket is easy to fit with the flange surface and corrosion resistance, the use of good results.
There are many factories in the temperature, pressure is not high in the strong corrosive media, the use of asbestos rubber sheet wrapped PTFE raw material tape, used for frequent dismantling of manholes, pipelines. Due to the production, easy to use, quite popular.
Asbestos resin sheet and impregnated asbestos sheet gasket
They are mostly used for pipelines, pumps, valves, inlet and outlet flanges of various acidic mediums, whose operating temperature is at 80℃ and pressure is below 0.6MPa.
Asbestos preparation gaskets are suitable for low-pressure and high-temperature working conditions with pressure below 0.1MPa and temperature not exceeding 800℃. And according to the specific requirements of the equipment, it can be woven into gaskets without width, thickness and diameter. Or the asbestos tape is cut and laid directly on the flange surface. It is used in large sulphuric acid, nitric acid oxidation furnace and some unprocessed equipment interface, the effect is far more than the original asbestos rope.
Metal clad asbestos mat
Asbestos sheet or asbestos rubber sheet is covered by metal sheet, so that it is not in direct contact with the medium, avoiding the decline in the strength of asbestos fibre, customer service leakage phenomenon, thus expanding the scope of use of asbestos rubber sheet.
Generally, the temperature of metal-clad asbestos gasket is 450℃ (individually up to 600~700℃, such as in the flue gas at atmospheric pressure ~ 0.16MPa), the use of pressure is 4MPa, up to 6MPa, such as the pressure increases, the gasket is prone to cross-flow phenomenon, the core material from the overlap of the extrusion.
As the metal-clad asbestos gasket requires a large bolt tightening force, even when the use of pressure is lower than 2.45MPa, can not be used pg25kg level below the flange. Otherwise, the stiffness of flanges and bolts becomes insufficient, producing deformation and leading to seal failure. Some people think that if the core material is changed to better elasticity of synthetic rubber, its fastening force will be reduced. In fact, not, because the core material to change the soft, fastening force is absorbed by the core material, can not provide the metal plate in the flange surface affixed to the required fastening force, the package gasket counter is easy to be extremely bad. In addition, in the medium containing more chloride ions as well as acidic media, stainless steel cladding and iron cladding gasket lap is prone to crevice corrosion.
Temperature higher than 450 ℃, the core material can use ceramic fibre or carbon fibre. An iron and steel plant using metal-clad ceramic fibre gaskets for 1100 ℃ high temperature, the use of two years has not been damaged. Flexible graphite as the core material is the most appropriate. At present, the domestic mass production of flexible graphite metal pack gasket. The use of its effect is better than the metal pack asbestos pad.
The metal pack mat can be made into a variety of shapes, a large number of heat exchangers, reactors, large cover, loading and unloading holes, manhole flange, etc.. Domestic has produced a diameter of 2m iron gasket, the use of good.
In the surface of the metal cushion coated with a layer of flexible graphite sheet, this gasket and the surface is not coated with the same type of metal cushion compared to the preload pressure is smaller, better sealing performance. Figure 2-9 for the two compression stress and leakage of the relationship between the curve.
At present, there is no domestic products, so some units will be the existing flexible graphite wrinkled tape affixed to the metal cushion, as well as metal flat cushion, toothed cushion and even asbestos rubber cushion surface, to solve many leakage problems. For example, a factory heat exchanger, pressure of 5.88MPa, temperature 450 ℃, the medium for hydrogen / oil and gas. Have used metal flat pads, toothed pads, are leaking. After the flat gasket pasted on the flexible graphite wrinkled tape, solved the problem. It should be noted that this form of gasket is a solution to the flange gasket leakage of easy measures, pasted with flexible graphite tape workability quality directly affects the normal operation of the equipment. If the back of the strip is coated with a layer of glue, it can improve the quality of the pasting.
Metal Wound Gasket
Metal wound gaskets cleverly use the heat resistance, resilience and strength of metal and the flexibility of non-metallic materials, so the sealing performance is better, which is the best performance of stainless steel tape wound flexible graphite gasket. Its preload pressure is smaller than asbestos winding pad, there is no asbestos fibre capillary leakage shortcomings. Figure 2-10 for the two preload pressure and leakage of the relationship between the curve. In the oil medium, the metal strip more use 0Cr13, while other media are recommended to use 1Cr18Ni9Ti.
Stainless steel belt flexible graphite winding pad in the gas medium, the use of pressure 14.7MPa (up to 19.6MPa), in the liquid can be used up to 30MPa. temperature -190 ~ +600 ° C (in the absence of oxygen, low-pressure case can be used up to 1000 ° C).
The low temperature resistance of PTFE is good, and its yield strength at low temperature is much higher than that at room temperature. Therefore, PTFE wound pads can be used for low temperature media, such as liquid hydrocarbons. At the same time, due to the addition of metal tape to improve thermal conductivity, PTFE wound pads can be used at temperatures up to 250 ℃, in acidic media can be used up to 9MPa, 200 ℃.
It is suitable for heat exchangers, reactors, pipelines, valves, pumps and import/export flanges with large fluctuations in pressure and temperature. For medium or higher pressure and temperature over 300℃, it should be considered to use with inner ring, outer ring or inner and outer ring. If use the concave-convex surface flange, with the inner ring of the winding gasket is more effective.
In the flexible graphite winding gasket on both sides of the flexible graphite plate can also achieve good sealing effect. A large fertiliser plant waste heat boiler is a high temperature and high pressure key equipment, with the outer ring of flexible graphite winding gasket, full load is not leakage, load reduction is leakage. On both sides of the gasket increased 0.5mm thick flexible graphite plate, cut into a circular arc, the joint part of the use of bevelled lap, the use of good condition.
Metal flat gaskets and metal wave gaskets
They are generally used on medium and high pressure valves, pipelines and equipment flanges with smaller diameters. The use pressure depends on the temperature, the former is 1.568~31.36MPa, the latter is 1.568~3.92MPa. gasket material is selected according to the medium and temperature.
Octagonal and oval gaskets
Octagonal and elliptical gaskets for trapezoidal groove flanges (commonly known in the refining industry as "dirt rings") have good sealing properties. On the tapered side of the groove, the octagonal gasket is in face contact while the elliptical gasket is in line contact. Therefore, the elliptic pad in the low tightening force under the fit is good, but need to carry out the second fastening; and octagonal pad is generally a fastening, not easy to leak. Their shortcomings for the requirements of the larger bolt tightening force, when used for low pressure and high temperature, the flange grade must be in the pn25kg level or more.
