The choice of gasket
1, the choice of attention
Proper selection of gasket is the key to ensuring that equipment is not leaked. For the same kind of conditions, there are generally a number of gaskets to choose from. Must be based on the physical properties of the media, pressure, temperature and equipment size, operating conditions, the length of continuous operation cycle, etc., a reasonable choice of gasket, avoid weaknesses, give full play to the characteristics of various gaskets.
When selecting a gasket, the following factors should be taken into account:
(1) have good flexibility and restorability, can adapt to pressure changes and temperature fluctuations;
(2) have appropriate softness, can be a good fit with the contact surface;
(3) does not pollute the process medium;
(4) have sufficient toughness without causing breakage due to pressure and tightening forces;
(5) do not harden at low temperature, shrinkage is small;
(6) processing performance, installation, compact and convenient;
(7) non-adhesive sealing surface, easy to disassemble;
(8) cheap, long service life.
In the use of gasket, the pressure and temperature are mutually constrained, with the temperature rise, the equipment running for some time, the gasket material softening, creep, stress relaxation phenomenon, mechanical strength will decline, The pressure of the seal is reduced. vice versa. For example, the manual listed high pressure asbestos rubber sheet XB450 in water, steam medium, the use of temperature 450 ℃, pressure <6MPa (the material for the sealing performance test, in the 440 ℃ ~ 450 ℃, 12MPa steam for 30 minutes) The But in the long-term practical use, the temperature reached 450 ℃, can seal the pressure of only 0.3 ~ 0.4MPa. For the strong permeability of the gas medium is only 0.1 ~ 0.2MPa.
The above situation should be taken into account in the selection.
2, the choice of domestic gasket
(1) industrial rubber sheet natural rubber for water, sea water, air, inert gas, alkali, salt and other media, but not resistant to mineral oil and non-polar solvent, long-term use temperature does not exceed 90 ℃, low temperature performance , Can be used in -60 ℃ above.
Nitrile rubber for petroleum products, such as oil, lubricants, fuel oil, long-term use temperature of 120 ℃, such as in hot oil can withstand 150 ℃, low temperature -10 ~ -20 ℃.
Chloroprene rubber for seawater, weak acid, weak base, salt solution, oxygen and ozone aging performance, oil resistance than nitrile rubber and better than other general rubber, long-term use temperature below 90 ℃, the maximum temperature does not exceed 130 ℃, low temperature of -30 ~ -50 ℃.
Fluorine rubber has a variety of varieties, they have a good acid, oxidation resistance and oil resistance, solvent resistance. Can be used in almost any acid medium and some oil and solvent, long-term use temperature below 200 ℃.
Rubber sheet as a flange gasket, used for pipes or often demolition of the hole, hand hole, the pressure does not exceed 1.568MPa. Because in all kinds of gaskets, the rubber gasket the most soft, good fit performance, in the smaller preload can play a sealing effect. For this reason, when subjected to internal pressure, because the pad is thick or low hardness and easy to be squeezed out.
Rubber sheet in benzene, ketone, ether and other organic solvents used, prone to swelling, weight gain, soft, sticky phenomenon, resulting in sealing failure. The general swelling of more than 30% can not be used.
In the low pressure (especially 0.6MPa below), vacuum case, the use of rubber pad is more appropriate. Rubber material has good compactness and low air permeability. Such as fluorine rubber, the most suitable for the vacuum container gasket, vacuum up to 1.3 × 10-7Pa. Rubber pad in the 10-1 ~ 10-7Pa vacuum range, the need for baking and pumping treatment.
(2) asbestos rubber sheet prices lower than other gaskets, easy to use; the biggest problem is: gasket material, although the addition of rubber and some fillers, still can not be filled with those tiny pores completely filled, there is a trace of penetration. Therefore, in highly polluting medium, even if the pressure, the temperature is not high can not be used. When used in some high-temperature oil medium, usually in the late use, due to rubber and filler carbonation, strength, material becomes loose, then the interface and gasket inside the penetration, coking and smoke phenomenon. In addition, asbestos rubber sheet at high temperatures easily bonded to the flange sealing surface, to replace the gasket brought a lot of trouble.
In the heated state, the use pressure of the gasket in various media depends on the strength retention of the gasket material. Asbestos fiber material exists in the crystal water and adsorption water. 110 ℃, the adsorption of water between the fiber has 2/3 precipitation, the tensile strength of fiber reduced by about 10%; 368 ℃, the adsorption of water precipitation, fiber tensile strength reduced by about 20%; more than 500 ℃, Crystal water began to precipitate, the intensity is lower. The impact of medium on the strength of asbestos rubber sheet is also great. For example, in the aviation lubricants and aviation fuel, 400 oil-resistant asbestos rubber sheet transverse tensile strength difference of 80%, which is due to aviation fuel on the plate of rubber swelling than aviation lubricants serious. Taking into account the above factors, the domestic asbestos rubber sheet XB450 recommended safe use: temperature 250 ℃ ~ 300 ℃, pressure 3 ~ 3.5MPa; 400 oil asbestos rubber sheet temperature should not exceed 350 ℃.
Asbestos rubber sheet containing chloride and sulfide, easy to form after the metal flange corrosion of the original battery, especially oil asbestos rubber sheet sulfur content higher than ordinary asbestos rubber sheet several times, so in non-oily media should not be used. Gasket in the oil and solvent-like medium will occur in the swelling phenomenon, but within a certain range, the sealing performance of the basic no effect. For example, No. 400 oil-resistant asbestos rubber sheet in the room temperature aviation fuel for 24 hours soaking test, requiring the increase in oil weight shall not exceed 15%.
Domestic asbestos rubber sheet and foreign brand-name products are still a certain gap. A unit of statistics shows that in more than 350 ℃ of oil medium, where the use of asbestos rubber sheet does not leak most of the imported products. Japan Petroleum Industry Standard JPI-7S-71 "Standard for the use of asbestos rubber sheet for petroleum industry" with a term of about one year.
(3) Teflon PTFE in the pressure and high temperature easy to cold flow and creep, it is generally used for low pressure, medium temperature, strong corrosion and not allowed to contaminate the media, such as strong acid, alkali, halogen, drugs Wait. Safe use of the temperature at 150 ℃, the pressure below 1MPa. Filled polytetrafluoroethylene, although the strength of higher, but the use of temperature does not exceed 200 ℃, and corrosion resistance decreased. Polytetrafluoroethylene package pad maximum pressure is generally not more than 2MPa.
As the temperature increases, the material creep, resulting in significant reduction in sealing pressure. Even if not heated, with the extension of time, the sealing surface of the compressive stress will decline, resulting in "stress relaxation phenomenon." The phenomenon will occur in a variety of gaskets, but the stress relaxation phenomenon of polytetrafluoroethylene pad is more serious, Should be noted.
Teflon friction coefficient is small (compression stress greater than 4MPa, the friction coefficient of 0.035 ~ 0.04), preload when the gasket is easy to slip out, so it is best to use concave and convex flange surface. When the use of flat flange, the gasket can be outside the contact bolts, the use of bolts to prevent the slide outside the slide. As the glass-lined equipment is sprayed on the metal surface of a layer of enamel after sintering, 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, it is recommended to use the core material from the asbestos board plus rubber PTFE packing. This pad is easy to fit with the flange surface and corrosion resistance, the use of good results.
There are many factories in the temperature, the pressure is not high strong corrosive media, the use of asbestos rubber sheet wrapped around the PTFE raw material, for the frequent demolition of the manhole, the pipeline. Due to the production, easy to use, popular.