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According to ASME B16.5, steel flanges have 7 pressure classes: The pressure rating of Class150 - 300 - 400 - 600 - 900 - 1500 - 2500 flange is very clear. Class300 flange can withstand greater pressure than Class150 because Class300 flange needs to be made of more materials to withstand greater pressure. However, the pressure-bearing capacity of the flange is affected by many factors. The pressure rating of the flange is expressed in pounds, and a pressure rating has different expression methods. for example: 150Lb, 150Lbs, 150# and Class150 have the same meaning. Examples of pressure levels: Depending on the temperature, the pressure-bearing capacity of the flange is also different. As the temperature rises, the pressure-bearing capacity of the flange decreases. For example, the Class150 flange has a pressure bearing capacity of 270 PSIG at normal temperature, a pressure bearing capacity of 180 PSIG at 400℉, a pressure bearing capacity of 150 PSIG at 600℉, and a pressure bearing capacity of 75 PSIG at 800℉. In other words, when the pressure drops, the temperature is allowed to rise and vice versa. Since flanges can be made of different materials, such as stainless steel, cast iron, ductile iron, carbon steel, etc., each material has a different pressure rating. The picture below is an example of an NPS12 flange. As you can see, the diameters of the inner hole and the raised surface of the flange are the same. However, the outer diameter of the flange, the diameter of the bolt hole center circle, and the diameter of the bolt hole become larger as the pressure level increases. The number and diameter of bolt holes are: CL 150 - 12 x 25.4 CL 300 - 16 x 28.6 CL 400 - 16 x 34.9 CL 600 - 20 x 34.9 CL 900 - 20 x 38.1 CL 1500 - 16 x 54 CL 2500 - 12 x 73 Pressure-Temperature Ratings - Examples Pressure-temperature ratings are the maximum allowable operating gauge pressure (pressure units in Bar) for a material and grade at rated temperature (Celsius). For intermediate temperatures, linear interpolation is allowed. However, insertion between flange levels is not allowed. A flange joint may be rated for pressure-temperature if the bolts and gaskets of the flange joint comply with the relevant limitations and the alignment and assembly of the flange joint are in accordance with good practice. Use of the latter for flange joints that do not comply with these limitations is the responsibility of the user. The rated temperature corresponding to the rated pressure refers to the shell temperature of the pressure vessel of the flange and flange fittings. Typically, this temperature is the same as the temperature of the storage fluid. Responsibility for selecting pressure ratings other than storage fluid temperature rests with the user. When the temperature is below -29℃ (-20℉), the rated value should not be greater than the rated value at -29℃ (-20℉). As an example, here are two groups of materials according to ASTM and their pressure-temperature ratings according to ASME B16.5. 2. Do not use above 455℃. 3. Do not use above 370℃. 4. Do not use above 260℃. 5. Do not use above 425℃.