Did You Choose Right Flange Gaskets for The Installation of The Electromechanical System?
In the flange connection of industrial pipelines and equipment, the material selection of the sealing gasket directly determines the sealing effect, equipment operation safety, and service life. The sealing gaskets made of different materials have significant differences in temperature resistance, pressure resistance, and medium corrosion resistance. Only by selecting the right material can they accurately adapt to the working conditions.
This article summarizes the core parameters, applicable scenarios, and usage precautions of eight mainstream flange gasket materials. It is recommended to keep them as backup to provide reference for the installation and selection of mechanical and electrical systems.
1. Industrial rubber sealing gaskets
The industrial rubber gasket is the most widely used universal sealing gasket, with the core advantages of good adhesion and low pre-tightening force requirements. Different rubber categories have their own emphasis on medium and temperature resistance, making it suitable for low-pressure and frequently disassembled flange interfaces as a whole.
1. Natural rubber
It is suitable for water, seawater, air, inert gases, alkaline and salt aqueous solutions, not resistant to mineral oil and non-polar solvents. The long term use temperature is less than 90 ℃. It has excellent low-temperature performance and can work in environments above -60 ℃.
2. Nitrile rubber
It is fit for petroleum, lubricating oil, fuel oil, etc. The long term use temperature is 120 ℃, and it can withstand up to 150 ℃ in hot oil. It is suitable for a low temperature range of -10 to -20 ℃.
3. Chloroprene rubber
It is suitable for seawater, weak acids, weak bases, and salt solutions, with outstanding resistance to oxygen and ozone aging. Its oil resistance is inferior to nitrile rubber but superior to other general rubbers. Long term use temperature is lower than 90 ℃, and the maximum one is less than 130 ℃. The low temperature applicable range is from -30 to -50 ℃.
4. Fluororubber
It is suitable for corrosive media in all scenarios, with excellent acid resistance, oxidation resistance, oil resistance, and solvent resistance. It can be used in almost all acidic media and some oils and solvents. Long term use temperature is lower than 200 ℃, and it has excellent vacuum sealing performance.
General usage requirements
The applicable pressure is less than 1.568MPa, with the best adaptability under low pressure (≤ 0.6MPa) and vacuum conditions. The highest vacuum degree of fluororubber can reach 1.3 × 10 ⁻⁷ Pa (baking and vacuum treatment are required when using a vacuum degree of 10 ⁻¹~10 ⁻⁷ Pa).
It is commonly used in disassembled parts such as pipelines, manholes, and handholes, but due to its soft material, it is prone to being squeezed out due to its thickness or low hardness.
It is strictly prohibited to use in organic solvents such as benzene, ketones, and ethers.
2. Asbestos rubber gaskets
The asbestos rubber gasket is inexpensive and easy to use, and is a commonly used sealing gasket in traditional industry. However, it has inherent defects such as trace penetration and high-temperature performance degradation, and its usage scenarios need to be strictly limited.
2.1 Core defects
The material's tiny pores cannot be completely filled, and there is trace infiltration. It is strictly prohibited to use it in highly polluting media. In the later stage of use in high-temperature oil media, carbonization of rubber and fillers can lead to a decrease in strength, material looseness, interface and internal infiltration, resulting in coking and smoke emission. It is easy to bond flange sealing surfaces at high temperatures, increasing the difficulty of replacement. Containing chloride ions and sulfides, it is easy to form corrosive primary batteries with metal flanges after absorbing water. Oil resistant materials have higher sulfur content and are not suitable for use in non-oil media.
2.2 Performance degradation law
At 110 ℃, 2/3 of the adsorbed water precipitates, and the tensile strength decreases by 10%. At 368 ℃, the adsorbed water completely precipitates and the tensile strength decreases by 20%, Crystalline water precipitates at temperatures exceeding 500 ℃, resulting in a significant decrease in strength. The medium has a significant impact on strength, for example, the transverse tensile strength of 400 oil resistant asbestos rubber sheet differs by 80% between aviation fuel and lubricating oil.
2.3 Recommended parameters for use
Domestic XB450 asbestos rubber sheet: temperature 250-300 ℃, pressure 3-3.5MPa
400 # oil resistant asbestos rubber sheet: operating temperature ≤ 350 ℃ (imported products have better sealing effect in oil media above 350 ℃)
Soaking in aviation fuel at room temperature for 24 hours results in an increase of ≤ 15% in oil absorption weight.
The service life of the Japanese Petroleum Industry Standard (JPI-7S-71) is approximately one year.
3. PTFE sealing gaskets
Polytetrafluoroethylene pads are resistant to strong corrosion and have no medium pollution, but they are prone to cold flow, creep, and significant stress relaxation under high temperature and high pressure, making them suitable for strong corrosion conditions at low pressure and medium temperature.
3.1 Basic version
It is suitable for strongly corrosive and non-polluting media such as strong acids, strong alkalis, halogens, and pharmaceuticals. The safe operating temperature is 150 ℃ and the pressure is less than 1MPa.
3.2 Fill in funds
The strength has slightly increased, with a usage temperature less than 200 ℃ and a slight decrease in corrosion resistance.
3. 3 PTFE pad
The maximum operating pressure is less than 2MPa, and it is recommended as a sealing gasket for enamel glass equipment (the core material is asbestos board+rubber, with good adhesion and corrosion resistance, to avoid damage to the enamel layer by the metal gasket).
Notes for use:
When the compressive stress is greater than 4MPa, the friction coefficient is only 0.035 to 0.04, and it is prone to external sliding during pre-tightening. The concave convex flange surface is preferred, and the flat flange needs to be prevented from sliding by bolts,
Demolished manholes and pipelines with strong corrosion and low temperature and pressure can be easily made and used by wrapping asbestos rubber sheets with polytetrafluoroethylene tape.
4. Asbestos resin board and impregnated asbestos board gaskets
Both types of gaskets are specialized for acidic media, with a single adaptability and only suitable for low-temperature and low-pressure acidic working conditions. The extended asbestos gasket is designed specifically for low-pressure and high-temperature acidic working conditions, with the following specific parameters.
4.1 Basic version (asbestos resin board/impregnated asbestos board)
Adaptation scenarios: inlet and outlet flanges of pipelines, pumps, and valves for various acidic media,
Usage parameters: temperature ≤ 80 ℃, pressure ≤ 0.6MPa.
4.2 Extension payment (asbestos woven gaskets)
Adaptation scenarios: Low pressure and high temperature working conditions, large sulfuric acid and nitric acid oxidation furnaces, and unprocessed equipment interfaces (with sealing effects far exceeding asbestos ropes),
Usage parameters: pressure ≤ 0.1MPa, temperature ≤ 800 ℃,
Customization advantages: The width, thickness, and diameter can be customized according to equipment requirements, and asbestos tape can also be directly applied to the flange surface.
5. Metal wrapped asbestos pad
Made of metal sheet wrapped asbestos board/asbestos rubber board, it isolates the medium from contact with the core material, solves the problem of asbestos pad leakage, expands the scope of use, and is suitable for various industrial conditions at medium temperature and medium pressure.
5.1 Basic usage parameters
The standard operating temperature is 450 ℃ (up to 600 to 700 ℃ in flue gas at atmospheric pressure 0.16MPa), and the standard operating pressure is 4MPa, with a maximum pressure less than 6MPa (excessive pressure can easily cause cross flow of core material extruded from the overlap).
5.2 Flanges and fastening requirements
A large bolt tightening force is required. Even if the pressure is less than 2.45 MPa, flanges below PN25 cannot be used, otherwise the flange and bolt stiffness will be insufficient and prone to deformation, resulting in seal failure. The core material cannot be replaced with elastic synthetic rubber, otherwise the fastening force will be absorbed by the core material, and the metal plate cannot be guaranteed to adhere to the flange surface, which may damage the gasket.
5.3 Optimization of high temperature and corrosion resistance
When the temperature is above 450 ℃, the core material can be replaced with ceramic fiber or carbon fiber (a certain steel plant's metal wrapped ceramic fiber mat has been used for 2 years without damage at a high temperature of 1100 ℃). Flexible graphite is the optimal core material, and domestically, metal wrapped flexible graphite mats have been mass-produced, with better performance than metal wrapped asbestos mats,
In media with a high concentration of chloride ions and acidic media, the overlap between stainless steel and iron sheet pads is prone to crevice corrosion and requires proper protection.
5.4 Sealing performance improvement and applicable scenarios
Enhancement technique: The surface of the metal cushion is covered with a flexible graphite sheet, which can reduce the pre-tightening pressure and improve the sealing performance. There are currently no finished products in China. You can apply flexible graphite wrinkling tape to the surface by yourself (apply glue on the back to improve the quality of the application), which can solve the leakage problem of metal flat pads, tooth shaped pads, etc,
Applicable scenarios: It can be made into various shapes and widely used for large covers, loading and unloading holes, manhole flanges, etc. in heat exchangers and reactors. A 2m diameter iron pad has been produced domestically and has good performance.
6. Metal winding pad
Metal wrapped gaskets combine the heat resistance, rebound, high strength of metals with the flexibility of non-metallic materials, and have excellent sealing performance. They are the preferred sealing gasket for medium and high pressure, temperature and pressure fluctuation conditions.
6.1 Material and Performance Grading
Stainless steel strip wrapped flexible graphite pad (best performance)
Pre-tightening ratio is small, and there is no problem with asbestos fiber capillary leakage, 0Cr13 is preferred for metal strips in oil media, while 1Cr18Ni9Ti is recommended for other media. The pressure used in gas media is 14.7MPa (the highest in China is 19.6MPa), and in liquids it can reach 30MPa. The temperature range is from -190 to +600 ℃ (up to 1000 ℃ under anaerobic and low-pressure conditions).
PTFE winding pad (low temperature specific)
Utilizing the excellent low-temperature resistance of polytetrafluoroethylene to adapt to low-temperature media such as liquid hydrocarbons, metal strips enhance thermal conductivity, with a usage temperature of up to 250 ℃, and can be used up to 9MPa and 200 ℃ in acidic media.
6.2 Use selection techniques
Adaptation scenarios: Heat exchangers, reactors, pipelines, valves, and pump inlet and outlet flanges with large temperature and pressure fluctuations.
Upgrade selection: For medium to high pressure (above medium pressure) and temperatures exceeding 300 ℃, styles with inner, outer, or inner and outer rings should be selected. The combination of concave convex flange and wrapped gasket with inner ring provides a better sealing effect.
Leak prevention technique: Two sides of the flexible graphite winding pad are covered with 0.5mm thick flexible graphite plates (cut in a circular arc shape and overlapped with oblique openings), which can solve the leakage problem under high and low loads (a large fertilizer plant waste heat boiler adopts this solution and has good sealing effect).
7.Metal flat pad and metal waveform pad
Both types are rigid metal gaskets, suitable for medium and high pressure, small-diameter flange connections, with outstanding pressure resistance performance. The material needs to be selected accurately according to the medium and temperature, and the core parameters are as follows.
Applicable parts: flanges for medium and high pressure valves, pipelines, and equipment with smaller diameters.
Pressure resistance parameters: The metal flat pad uses a pressure between 1.568 to 31.36MPa, and the metal waveform pad uses a pressure from 1.568 to 3.92MPa (both adjusted with temperature changes).
8. Octagonal pad and elliptical pad
Octagonal pads and elliptical pads (commonly known as "earth steel rings" in the refining industry) are designed specifically for trapezoidal groove flanges, with excellent sealing performance and are specialized for high-pressure flange connections. The structural differences between the two lead to different usage methods.
8.1 Differences in sealing contact and fastening
Octagonal pad: in contact with the groove cone surface, it is not easy to leak after one fastening, and the sealing stability is better.
Elliptical pad: In line contact with the groove cone surface, it has good adhesion under low tightening force, but requires secondary tightening.
8.2 General usage requirements
Both require a large bolt tightening force, and even for low-pressure and high-temperature conditions, the flange grade must be PN25 or above.
