Which Is The Best Choice For Sealing Gaskets?
Sep 30, 2024
There are many types of sealing gaskets, and gaskets made of different materials have different applications and performance differences. So which material is better? How to choose specifically? The editor has selected 8 common sealing gaskets for analysis and comparison. Only by understanding the detailed parameters can we make a more accurate choice!
1. Industrial rubber sheet
Natural rubber is suitable for media such as water, seawater, air, inert gases, alkalis, salt water solutions, etc., but it is not resistant to mineral oil and non-polar solvents. Its long-term use temperature does not exceed 90 ℃, and its low-temperature performance is excellent. It can be used above -60 ℃.
Nitrile rubber is suitable for petroleum products such as petroleum, lubricating oil, fuel oil, etc. Its long-term use temperature is 120 ℃, and it can withstand 150 ℃ in hot oil, with a low temperature range of -10~-20 ℃.
Chloroprene rubber is suitable for seawater, weak acids, weak bases, and salt solutions. It has excellent resistance to oxygen and ozone aging, and its oil resistance is inferior to nitrile rubber but superior to other general rubbers. Its long-term use temperature is below 90 ℃, and the maximum use temperature does not exceed 130 ℃. The low temperature is -30~-50 ℃.
There are multiple varieties of fluororubber, each with excellent acid and oxidation resistance, as well as oil and solvent resistance. It can be used in almost all acidic media as well as some oils and solvents, with a long-term usage temperature below 200 ℃.
Rubber sheet is commonly used as a flange gasket for pipelines or frequently disassembled manholes and handholes, with a pressure not exceeding 1.568MPa. Among various types of gaskets, rubber gaskets are the softest and have good adhesion performance, which can achieve sealing effect with small pre tightening force. Therefore, when subjected to internal pressure, the gasket is prone to extrusion due to its thickness or low hardness.
Rubber sheets used in organic solvents such as benzene, ketones, and ethers are prone to swelling, weight gain, softening, and sticking, leading to seal failure. Generally, if the swelling degree exceeds 30%, it cannot be used.
Under low pressure (especially below 0.6MPa) and vacuum conditions, it is more suitable to use rubber pads. Rubber materials have good density and low air permeability. For example, fluororubber is the most suitable sealing gasket for vacuum containers, with a maximum vacuum degree of 1.3 × 10-7Pa. When using rubber pads in the vacuum range of 10-1~10-7Pa, they need to be baked and evacuated.
2. Asbestos rubber sheet
Lower price compared to other gaskets, convenient to use; The biggest problem is that although rubber and some fillers are added to the gasket material, they still cannot completely fill the small interconnected pores, resulting in trace infiltration. Therefore, it cannot be used in highly polluting media even if the pressure and temperature are not high. When used in some high-temperature oil media, usually in the later stage of use, due to the carbonization of rubber and fillers, the strength decreases, the material becomes loose, and infiltration occurs at the interface and inside the gasket, resulting in coking and smoking. In addition, asbestos rubber sheets are prone to sticking to the flange sealing surface at high temperatures, which brings a lot of trouble to replacing gaskets.
The usage pressure of gaskets in various media under heating conditions depends on the strength retention rate of the gasket material. Crystalline water and adsorbed water exist in asbestos fiber materials. At 110 ℃, 2/3 of the adsorbed water between fibers has already precipitated, and the tensile strength of the fibers decreases by about 10%; At 368 ℃, all adsorbed water precipitates, and the tensile strength of the fibers decreases by about 20%; Beyond 500 ℃, crystal water begins to precipitate and the strength decreases.
The medium also has a significant impact on the strength of asbestos rubber sheets. Asbestos rubber sheets contain chloride ions and sulfides, which can easily corrode the original battery with metal flanges after absorbing water. Especially, the sulfur content in oil resistant asbestos rubber sheets is several times higher than that in ordinary asbestos rubber sheets, so they are not suitable for use in non oily media. Gaskets may swell in oil and solvent media, but within a certain range, it has little effect on sealing performance. For example, the 400 oil resistant asbestos rubber sheet is subjected to a 24-hour immersion test in ambient temperature aviation fuel, with a requirement that the increase in oil absorption weight should not exceed 15%.
3. Polytetrafluoroethylene
Polytetrafluoroethylene is prone to cold flow and creep under pressure and high temperature, so it is generally used in low-pressure, medium temperature, strongly corrosive, and non polluting media such as strong acids, strong bases, halogens, drugs, etc. The safe operating temperature is 150 ℃ and the pressure is below 1MPa. Although filled with polytetrafluoroethylene has higher strength, its usage temperature does not exceed 200 ℃, and its corrosion resistance decreases. The maximum operating pressure of polytetrafluoroethylene pads generally does not exceed 2MPa.
Due to the increase in temperature, the material undergoes creep, resulting in a significant decrease in sealing pressure. Even without heating up, as time goes on, the compressive stress on the sealing surface will decrease, resulting in a "stress relaxation phenomenon". This phenomenon occurs in various gaskets, but the stress relaxation phenomenon of PTFE gaskets is more serious and should be noted.
The friction coefficient of polytetrafluoroethylene is relatively small (with a compressive stress greater than 4MPa and a friction coefficient of 0.035-0.04), and the gasket is prone to sliding outward during pre tightening, so it is best to use a concave convex flange surface. When using a flat flange, the outer diameter of the gasket can be in contact with the bolt to prevent the gasket from sliding outward.
Due to the fact that the enamel equipment is made by spraying a layer of porcelain glaze on the metal surface and sintering it, the glaze layer is very brittle. In addition, the uneven spraying and flow of the glaze layer result in poor flatness of the flange surface. The use of metal composite gaskets can easily damage the glaze layer, so it is recommended to use a PTFE pad with asbestos board and rubber as the core material. This type of cushion is easy to adhere to the flange surface and corrosion-resistant, with good performance.
Many factories use asbestos rubber sheets wrapped with polytetrafluoroethylene tape in highly corrosive media with low temperature and pressure, which are used on manholes and pipelines that are frequently disassembled. Due to its convenient production and use, it is quite popular.
4. Asbestos resin board and impregnated asbestos board gasket
Commonly used for pipelines, pumps, valves, inlet and outlet flanges in various acidic media, with a usage temperature of 80 ℃ and a pressure below 0.6MPa.
Asbestos gaskets are suitable for low-pressure and high-temperature conditions with a pressure below 0.1MPa and a temperature not exceeding 800 ℃. And according to the specific requirements of the equipment, it can be woven into gaskets of different widths, thicknesses, and diameters. Or cut off the asbestos tape and apply it directly on the surface of the flange. It is used at the interface of large sulfuric acid and nitric acid oxidation furnaces, as well as some unprocessed equipment, and its effect far exceeds that of the original asbestos rope.
5. Metal wrapped asbestos pad
Covering asbestos sheets or asbestos rubber sheets with metal sheets prevents direct contact with the medium, avoiding a decrease in asbestos fiber strength and preventing leakage, thereby expanding the range of use of asbestos rubber sheets.
The general temperature for using metal wrapped asbestos pads is 450 ℃ (some may reach 600~700 ℃, such as in flue gas at atmospheric pressure~0.16MPa), and the operating pressure is 4MPa, with a maximum of 6MPa. If the pressure increases further, the gasket is prone to cross flow, and the core material is extruded from the overlap.
Due to the high bolt tightening force required for metal wrapped asbestos pads, even when the pressure is below 2.45 MPa, flanges with a weight of less than 25 kg cannot be used. Otherwise, the stiffness of flanges and bolts will appear insufficient, causing deformation and resulting in seal failure. Some people believe that if the core material is changed to synthetic rubber with better elasticity, its tightening force will decrease. In fact, this is not the case because after the core material is softened, the fastening force is absorbed by the core material, which cannot provide the necessary fastening force for the metal plate to adhere to the flange surface, and the gasket is easily damaged. In addition, in media with a high concentration of chloride ions and acidic media, the overlap between stainless steel pads and iron pads is prone to crevice corrosion.
If the temperature is above 450 ℃, ceramic fiber or carbon fiber can be used as the core material. A certain steel plant uses metal clad ceramic fiber gaskets for high temperatures of 1100 ℃, which have not been damaged after two years of use. The use of flexible graphite as the core material is more effective than metal wrapped asbestos pads.
Metal gaskets can be made into various shapes and are widely used for large covers, loading and unloading holes, manhole flanges, etc. in various heat exchangers and reactors.
A layer of flexible graphite sheet is applied to the surface of the metal gasket. Compared with similar metal gaskets without surface coating, this gasket has a smaller preload ratio and better sealing performance
Applying the existing flexible graphite wrinkling tape to the surface of metal pads, metal flat pads, toothed pads, and even asbestos rubber pads has solved many leakage problems. For example, the heat exchanger in a certain factory has a pressure of 5.88MPa, a temperature of 450 ℃, and a medium of hydrogen/oil and gas. I have used metal flat pads and toothed pads before, both of which have leaks. Later, flexible graphite wrinkling tape was applied on the flat pad to solve this problem. It should be pointed out that this gasket form is a simple measure to solve flange gasket leakage, and the working quality of the flexible graphite tape directly affects whether the equipment can operate normally. If a layer of glue is applied on the back of the tape, it can improve the quality of the coating.
6. Metal winding pad
Metal wrapped pads cleverly utilize the heat resistance, resilience, and strength of metals, as well as the flexibility of non-metallic materials, resulting in better sealing performance. Among them, the performance of stainless steel wrapped flexible graphite pads is the best. Its pre tightening ratio is smaller than that of asbestos wrapping pads, and there is no disadvantage of asbestos fiber capillary leakage. Figure 2-10 shows the relationship curve between the pre tension ratio and the leakage rate of the two. In oil media, 0Cr13 is commonly used for metal strips, while 1Cr18Ni9Ti is recommended for other media.
Stainless steel strip flexible graphite winding pad can be used in gas medium at a pressure of 14.7MPa (up to 19.6MPa), and can be used up to 30MPa in liquid. Temperature -190~+600 ℃ (can be used up to 1000 ℃ under anaerobic and low-pressure conditions).
Polytetrafluoroethylene has good low-temperature resistance, and its yield strength at low temperatures is much higher than that at room temperature. So PTFE winding pads can be used for low-temperature media such as liquid hydrocarbons. At the same time, the addition of metal strips improves thermal conductivity, and the use temperature of polytetrafluoroethylene winding pads can reach 250 ℃, which can be used up to 9MPa and 200 ℃ in acidic media.
Spiral wrap pads are suitable for heat exchangers, reactors, pipelines, valves, and pump inlet and outlet flanges with significant pressure and temperature fluctuations. For medium to high pressure and temperatures exceeding 300 ℃, consideration should be given to using an inner ring, outer ring, or inner outer ring. If a concave convex flange is used, a wrapping pad with an inner ring will have a better effect.
Applying flexible graphite sheets on both sides of the flexible graphite winding pad can also achieve good sealing effect. The waste heat boiler of a large fertilizer plant is a key equipment for high temperature and high pressure. It uses flexible graphite wrapped pads with an outer ring, which does not leak at full load but leaks at reduced load. A 0.5mm thick flexible graphite plate was added on both sides of the gasket, cut into a circular arc shape, and the joint part was overlapped with a slanted mouth, which worked well.
7. Metal flat pads, waveform pads, and tooth shaped pads Metal flat pads and metal waveform pads
Generally used on flanges of medium and high pressure valves, pipelines, and smaller diameter equipment. The pressure used varies depending on the temperature, with the former ranging from 1.568 to 31.36 MPa and the latter ranging from 1.568 to 3.92 MPa. The gasket material is selected based on the medium and temperature.
8. Octagonal and elliptical pads
The octagonal gasket and elliptical gasket (commonly known as "earth steel ring" in the refining industry) used for trapezoidal groove flanges have good sealing performance. On the conical surface of the groove, the octagonal pad has surface contact while the elliptical pad has line contact. Therefore, the elliptical pad has good adhesion under low fastening force, but requires secondary fastening; And octagonal pads are generally less prone to leakage after being tightened once. Their drawback is that they require a large bolt tightening force, and when used in low-pressure and high-temperature situations, the flange grade must be above pg25 kg.