Introduction To Flange Pressure Classes
Is there a pressure class for the flange? Answer: Yes! Let's take a look at the pressure class of flange:
According to ASME B16.5, steel flanges are available in 7 pressure classes: Class150-300-400-600-900-1500-2500
Flange pressure class is very clear, Class300 flange can withstand more pressure than Class150, because Class300 flange needs to be made from more materials, thus can withstand more pressure. However, the compression capacity of a flange is affected by many factors. The pressure class of flange is expressed in pounds. There are different ways to express a pressure class. For example: 150Lb, 150Lbs, 150# and Class150 mean the same thing.
The pressure bearing capacity of flanges varies with temperature. As the temperature rises, the bearing capacity of the flange decreases. For example, the pressure bearing capacity of Class150 flanges at room temperature is 270PSIG, 180 PSIG at 400℉, 150 PSIG at 600℉, and 75 PSIG at 800℉.
In other words, as the pressure drops, the temperature is allowed to rise. 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 class.
Below is an example of a flange of the NPS12. As you can see, the flange bore and the face diameter are the same. However, the outside diameter of the flange, the center circle diameter of the bolt hole and the diameter of the bolt hole increase with the increase of the pressure class.
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
The pressure-temperature rating is the maximum allowable worksheet pressure (in Bar) for the material and class at the rated temperature (Celsius). For the intermediate temperature, linear insertion method is allowed. But no insertion between flange classes is allowed.
Such flanged joints may be suitable for pressure-temperature ratings, provided that the bolts and gaskets of the flanged joints comply with the relevant constraints and that the alignment and assembly of the flanged joints comply with good codes. The user is responsible for the use of the latter in flanged joints which do not meet these qualifications.
The rated temperature corresponding to the rated pressure refers to the shell temperature of the pressure vessel of the Flange and the flange fitting. In general, this temperature is the same as the temperature at which the fluid is stored. The user is responsible for the selection of pressure ratings other than those for storage fluid temperatures. When the temperature is below -29 ° C (-20 ° F), the rating shall not be greater than the rating at -29 ° C (-20 ° F).
