Selection of Sealing Gaskets
(1) Natural rubber sealing sheets are ideal for water, seawater, air, inert gases, alkaline solutions, and salts, but they are not resistant to mineral oils or non-polar solvents. They should be used at temperatures below 90°C, and they perform excellently in low temperatures down to -60°C.
Nitrile rubber is well-suited for petroleum products like oil, lubricants, and fuel. It can operate at a continuous temperature of 120°C, and up to 150°C in hot oil environments, with a lower limit of -10°C to -20°C.
Neoprene performs well in seawater, weak acids, weak bases, and salt solutions. It offers excellent resistance to oxidation and ozone, outperforming many standard rubbers but falling short of nitrile rubber. It functions best at temperatures below 90°C, with a maximum of 130°C, and operates effectively down to -30°C to -50°C.
Fluorocarbon elastomers are versatile, offering strong acid and oxidation resistance, as well as resilience against oils and solvents. These materials can be used across nearly all acid-based media and certain oils and solvents, with a continuous operating temperature up to 200°C.
As flange gaskets, rubber sheets are commonly employed in frequently dismantled pipes or manholes under pressures not exceeding 1.568MPa. Among various gaskets, rubber gaskets are soft and adaptable, achieving a seal with minimal preload. However, this flexibility makes them prone to extrusion under thick gaskets or low hardness conditions.
When exposed to organic solvents like benzene, ketones, and ethers, rubber sheets may swell, gain weight, soften, and become sticky, leading to seal failure. Typically, if the swelling exceeds 30%, the gasket is unsuitable.
At low pressures (below 0.6MPa) and in vacuum environments, rubber gaskets are preferable due to their excellent density and low permeability. For instance, fluorocarbon elastomers are ideal for sealing vacuum containers with a vacuum degree up to 1.3×10-7Pa. When used in ultra-high vacuum ranges (10-1 to 10-7Pa), rubber gaskets need to be baked and degassed.
Asbestos rubber sheets are economical and user-friendly compared to other gaskets. However, despite adding rubber and fillers, these gaskets still leave microscopic pores unfilled, allowing slight seepage. Consequently, they are unsuitable for highly contaminating media, even at low pressures and temperatures. In high-temperature oil media, the gaskets tend to weaken and become loose over time due to rubber and filler carbonization, causing leaks and potential smoke production. Additionally, asbestos rubber gaskets may bond to flange sealing surfaces at elevated temperatures, complicating replacement.
(2) Synthetic fiber rubber sheets are eco-friendly, heat-resistant, and pressure-resistant, offering broad media compatibility. Bode's synthetic fiberboard can handle pressures up to 120 bar and operate within -200°C to 800°C. Its sealing performance can be tailored to specific applications, making it a leading product in the sealing market.
(3) Polytetrafluoroethylene (PTFE) is prone to cold flow and creep under high pressure and temperature, so it’s typically used in low-pressure, medium-temperature environments with strong corrosive media or those requiring contamination-free operation, such as strong acids, bases, halogens, and pharmaceuticals. The safe operating temperature is 150°C and pressure below 1MPa. Filled PTFE has higher strength but an operating temperature not exceeding 200°C with reduced corrosion resistance. The maximum operating pressure of PTFE gaskets generally does not exceed 2MPa.
(4) Asbestos resin plates and non-asbestos impregnated gaskets are predominantly used in acidic media pipelines, pumps, valves, and flanges. Their operating temperature is 80°C, and pressure is below 0.6MPa.
(5) Metal-clad asbestos gaskets feature an asbestos board or rubber board covered with a thin metal layer to prevent direct contact with the medium, preserving the strength of asbestos fibers and preventing leaks, thus broadening the application scope. Metal-clad asbestos gaskets typically operate at temperatures up to 450°C (some up to 600-700°C, such as in flue gas at normal pressure ~0.16MPa) and pressures up to 4MPa, with a maximum of 6MPa. Increasing pressure further risks cross-flow and extrusion of the core material.
Since these gaskets require significant bolt tightening forces, flanges below pg 25 kg should not be used when the pressure is less than 2.45MPa. Otherwise, the flange and bolt stiffness may be insufficient, leading to deformation and seal failure. Replacing the core material with more elastic synthetic rubber doesn't reduce tightening forces since the core absorbs the force, reducing the required clamping force on the metal plate, and increasing the risk of damage. Moreover, in media rich in chloride ions and acidic environments, the overlap between stainless steel and iron pads can lead to crevice corrosion.
(6) Flat metal gaskets, corrugated gaskets, and toothed gaskets are typically used on medium-to-high-pressure valves, pipes, and smaller diameter equipment flanges. The operating pressure depends on the temperature: flat gaskets at 1.568–31.36MPa and corrugated gaskets at 1.568–3.92MPa. Gasket material selection depends on the medium and temperature.
(7) Octagonal and elliptical gaskets excel in trapezoidal groove flanges and elliptical flanges (known as “earth rings†in the oil refining industry). Octagonal gaskets offer surface contact, while elliptical gaskets provide linear contact. Elliptical gaskets fit well under low tightening forces but require two tightenings; octagonal gaskets usually prevent leaks after one tightening. Both require substantial bolt tightening forces. When used in low-pressure, high-temperature conditions, the flange level must be above pg25 kg.
In summary, selecting the right gasket depends on specific conditions like temperature, pressure, and the nature of the medium.
Steel box girder and steel and concrete composite box girder are used for large spans, which need to minimize the weight of the bridge, and their excellent high torsional stiffness is particularly useful. The clean lines of the box girder bridge usually have no obvious external hardening. It is generally considered to have excellent appearance and durability, and it has no traps for dust and moisture. Box-shaped cross-sections are sometimes used as compression members, such as in cable support structures or arched towers.
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