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The pressure-temperature rating of a flange is an important parameter that is determined by taking into account a number of factors, mainly through the following aspects:
Material Properties
Different flange materials have different mechanical properties and thermal stability. For example, stainless steel, copper-nickel alloy, carbon steel and other materials have different strength, toughness and creep properties at different temperatures. In general, the strength of the material decreases at high temperatures, while the material may become brittle at low temperatures. These material properties are the basis for determining flange pressure-temperature ratings. In a high temperature environment, a material's ability to resist deformation and damage is reduced, so the pressure a flange can withstand decreases accordingly as the temperature increases. For example, carbon steel flanges can withstand high pressure at room temperature, but at high temperatures, their allowable pressure will be significantly reduced, while stainless steel flanges, due to their good high temperature resistance, can still maintain good mechanical properties at higher temperatures and have relatively high allowable pressure.
Design standards and specifications
There are corresponding flange design standards and norms in various countries, such as the B16.5 standard of American Society of Mechanical Engineers (ASME), the EN 1092 standard in Europe and the GB/T 9112 standard in China. These standards specify pressure-temperature ratings for different types and sizes of flanges. The standards usually have detailed pressure-temperature rating tables or formulas based on material type, flange type (e.g., plate weld flanges, butt weld flanges, etc.), nominal diameter, and other factors. For example, ASME B16.5 standard gives the maximum allowable working pressures at different temperatures for flanges made of different materials. Designers can refer to the corresponding standards to determine the pressure-temperature ratings according to specific engineering requirements and flange parameters.
Experimental verification
Experimental tests are carried out to verify the performance of flanges under different pressure and temperature conditions. Common tests include hydraulic test, pneumatic test, high temperature creep test, fatigue test and so on. Hydraulic test and pneumatic test are used to verify the sealing performance and strength of flanges at room temperature to ensure that they will not leak or break under the design pressure. High-temperature creep test is to observe the deformation of flange material under high temperature and certain pressure for a long time to determine its stability under long-term high-temperature environment. Fatigue tests are used to simulate the performance of flanges under repeated loading and unloading conditions to assess their reliability in actual operation. Through these tests, actual performance data of flanges under different pressure-temperature combinations can be obtained to provide a basis for determining their ratings. For example, when testing a flange made of a new material, the high-temperature creep test reveals that at a specific temperature and pressure, after a certain period of time, the flange shows significant creep deformation that exceeds the allowable range, and then this pressure-temperature combination cannot be used as the rated value of the flange.
Safety Factor
In order to ensure the safety of the flange in actual use, a safety factor is taken into account when determining the pressure - temperature rating. The size of the safety factor is usually determined by the importance of the project, the complexity of the environment in which it is to be used, and the reliability of the material. Generally speaking, for some critical industrial piping systems, such as petrochemical and electric power industries, the safety factor will take a larger value to prevent safety accidents caused by flange failure. For example, in petrochemical installations, due to the flammable and explosive, toxic and hazardous properties of the medium in the pipeline, the safety coefficient of the flange may be taken to 1.5 - 2.0, i.e., on the basis of the calculation of the strength of the material, the permissible values of pressure and temperature will be reduced by 1.5 - 2.0 times as the rated value, so as to ensure that the flanges can be operated safely under various working conditions.
To summarize, the pressure-temperature ratings of flanges are determined after comprehensive consideration of material properties, compliance with design standards and norms, verification through tests, and consideration of safety coefficients, etc., so as to ensure the safety and reliability of flanges in actual engineering applications.