Gasket is a mechanical seal between two objects, which is usually used to prevent pressure, corrosion, and pipeline natural thermal expansion and contraction leakage. Since the machined surface cannot be perfect, the irregularity can be filled with a gasket.
1. The correct selection of sealing gasket is the key to ensure no leakage of valve. For the same working condition, there are several kinds of pads
Notes on Selection
Correct selection of valve sealing gasket is the key to ensure no leakage of equipment. For the same working condition, there are several kinds of gaskets to choose from. According to the physical properties, pressure, temperature, equipment size, operating conditions and continuous operation period of the medium, the gasket should be selected reasonably to make full use of the characteristics of various gaskets.
When selecting gaskets, the following factors should be fully considered:
(1) It has good elasticity and resilience, and can adapt to pressure change and temperature fluctuation;
(2) It has proper softness and fits well with the contact surface;
(3) It does not pollute the process medium;
(4) It has enough toughness without damage due to pressure and tightening force;
(5) At low temperature, it does not harden and its shrinkage is small;
(6) Good processing performance, easy installation and pressing;
(7) The sealing surface is not bonded and easy to disassemble;
(8) Low price and long service life.
In the use of gasket, the pressure and temperature are mutually restricted. With the increase of temperature, after the equipment operates for a period of time, the gasket material will soften, creep and stress relaxation, and the mechanical strength will also decrease, and the sealing pressure will be reduced. vice versa. For example, it is listed in the manual that the use temperature of high-pressure asbestos rubber sheet xb450 in water and steam medium is 450 ℃ and the pressure is less than 6Mpa (when the material is used for sealing performance test, the pressure shall be kept at 440 ℃ ~ 450 ℃ and 12MPa steam for 30 minutes). But in the long-term practical use, if the temperature reaches 450 ℃, the sealing pressure is only 0.3 ~ 0.4MPa. For the gas medium with strong permeability, it is only 0.1 ~ 0.2MPa.
The above situations should be fully considered.
2. Selection of domestic gasket
(1) Industrial rubber plate
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.
(2) Asbestos rubber sheet
The price is lower than other gaskets, easy to use; the biggest problem is that although the gasket material is added with rubber and some fillers, it still can not completely fill those micro pores in collusion, and there is micro 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; temperature of 400 oil resistant asbestos rubber sheet