Steel flanges are available in 7 pressure classes according to ASME B16.5: Class150 – 300 – 400 – 600 – 900 – 1500 – 2500
The pressure rating of the flange is very clear. The flange of Class 300 can withstand greater pressure than that of Class 150, because the flange of Class 300 needs to be made of more materials, so that it can withstand greater pressure. However, the pressure capacity of the flange is affected by many factors. The pressure rating of the flange is expressed in pounds, and there are different ways of expressing a pressure rating. For example: 150Lb, 150Lbs, 150# and Class150 mean the same thing.
Examples of pressure levels
Different temperatures have a different pressure-bearing capacities of flanges. As the temperature rises, the pressure bearing capacity of the flange decreases. For example, a Class 150 flange has a pressure capacity of 270 PSIG at room temperature, 180 PSIG at 400°F, 150 PSIG at 600°F, and 75 PSIG at 800°F.
In other words, when the pressure drops, the temperature is allowed to rise, and vice versa. Since flanges can be manufactured from different materials, such as stainless steel, cast iron, ductile iron, carbon steel, etc., each material has a different pressure rating.
The figure below is an example of an NPS12 flange. As you can see, the diameter of the flange bore and raised face are the same. However, the outer diameter of the flange, the diameter of the center circle of the bolt hole, and the diameter of the bolt hole become larger as the pressure level increases.
The number and diameter of the 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 – Example
Pressure-temperature ratings refer to the highest allowable working gauge pressure (pressure in Bar) at the rated temperature (Celsius) for the material and grade. For intermediate temperatures, the linear interpolation method is allowed to be determined. But insertion between flange levels is not allowed.
Flanged joints may be suitable for pressure-temperature ratings if their bolts and gaskets meet the relevant limits and if they are aligned and assembled in accordance with good practice. It is the user’s responsibility to use the latter for flanged joints that do not meet these limitations.
The rated temperature corresponding to the rated pressure refers to the shell temperature of the pressure vessel of the flange and flanged fittings. Typically, this temperature is the same as the temperature of the storage fluid. Responsibility for selecting a pressure rating other than the storage fluid temperature is the responsibility of the user. When the temperature is below -29°C (-20°F), the rating shall not be greater than the rating at -29°C (-20°F).
As an example, the following 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℃.