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The pressure-temperature rating of flanges is determined after comprehensive consideration of many factors, mainly based on relevant standards and norms, and finally clarified through material performance evaluation, test verification, etc., as follows:
Based on standards and norms
Different countries and regions have their own standards for flanges, such as ASME B16.5 of the U.S.A., EN 1092-1 of Europe and HG/T 20592 of China, etc. In these standards, there are clear provisions and classifications for flanges. The pressure and temperature levels of flanges are clearly defined and categorized in these standards. According to different materials, nominal pressures and temperature ranges, corresponding pressure-temperature rating tables are formulated in the standards to provide a basis for the selection and design of flanges.
Considerations
Material Properties: The mechanical properties of flange materials are the key factors in determining the pressure-temperature rating. Different materials have different strength, toughness and creep properties at different temperatures. For example, carbon steel may appear cold brittle phenomenon at low temperature, and the strength will be significantly reduced at high temperature; while stainless steel can still maintain good strength and corrosion resistance at high temperature. The permissible stress of the material will change with the change of temperature, and the standard will give the permissible stress value at different temperatures according to the characteristics of the material, so as to determine the pressure resistance of the flange at the corresponding temperature.
Nominal pressure: Nominal pressure is the maximum allowable working pressure of the flange at the reference temperature. It reflects the structure size and bearing capacity of the flange. Generally speaking, the higher the nominal pressure, the higher the pressure-resistant capacity of the flange, but at different temperatures, its actual pressure-resistant capacity will be adjusted according to the change of material and temperature. For example, a flange with a nominal pressure of PN16 can withstand a pressure of 1.6MPa at room temperature, but its allowable working pressure will be lower than 1.6MPa at high temperature.
Determination method
Theoretical calculation: Based on the data of mechanical properties of the material, structural dimensions of the flange and the relevant mechanical model, the distribution of stress and deformation of the flange under different combinations of temperature and pressure are evaluated by calculation. For example, by using finite element analysis and other methods, the stress concentration area and deformation of the flange under various working conditions can be accurately calculated, so as to determine its maximum permissible pressure at a specific temperature.
Test verification: A series of tests are conducted to verify the performance of flanges under different temperature and pressure conditions. Including pressure test (such as hydraulic test, pneumatic test), high temperature creep test, fatigue test and so on. Pressure test is used to check the sealing and strength of the flange under the specified pressure; high-temperature creep test examines the deformation of the material under prolonged high temperature and a certain pressure, in order to determine its reliability under long-term operation conditions; fatigue test is used to assess the performance of the flange under cyclic load and simulate the pressure fluctuation in actual use. Through these tests, data on the actual pressure resistance of flanges at different temperatures can be obtained to provide an experimental basis for the determination of the pressure-temperature level.