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) with the appropriate flexibility, with the contact surface is a good fit;
(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 is good, easy to install, compact;
(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.
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 petroleum, 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 inferior to 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 all acid media and some oils and solvents, 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 decreased, the 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 the various media depends on the strength retention of the gasket material. Asbestos fiber material exists in the crystal water and adsorption of water. 110 ℃, the adsorption of water between the fibers have 2/3 precipitation, the tensile strength of fiber reduced by about 10%; 368 ℃, the adsorption of water precipitation, the tensile strength of fiber reduced by about 20%; more than 500 ℃, Crystal water began to precipitate, the intensity is lower. The effect 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 rubber swelling than aviation lubricants serious. Taking into account the above factors, 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 ℃.