According to ASME B16.5, steel flanges are available in 7 pressure classes: Class150 – 300 – 400 – 600 – 900 – 1500 – 2500

The pressure rating of flanges is clear. Class 300 flanges can withstand higher pressures than Class 150 because Class 300 flanges need to be manufactured from more material to withstand higher pressures. However, the pressure capacity of a flange is influenced by a number of factors. The pressure rating of flanges is expressed in pounds, and there are different ways to express a pressure rating. For example: 150Lb, 150Lbs, 150# and Class150 mean the same thing.

Example of pressure class

The pressure capacity of flanges varies with temperature. As the temperature rises, the pressure capacity of the flange decreases. For example, a Class150 flange has a pressure-bearing 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 a NPS12 flange. As you can see, the flange bore and the diameter of the protruding surface are the same. However, the outer diameter of the flange, the center circle diameter of the bolt hole, and the bolt hole diameter become larger as the pressure grade increases.

The number and diameter of bolt holes are as follows

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 working gauge pressure (pressure in Bar) for the material and grade at the rated temperature (degrees Celsius). For intermediate temperatures, a linear insertion method is permitted for determination. However, insertion between flange grades is not permitted.

If the bolts and gaskets of a flange joint meet the relevant qualifications and the flange joint is set and assembled in accordance with good regulations, such a flange joint may be applied to the pressure-temperature rating. If the latter is used for flange joints that do not meet these qualifications, the user is responsible.

The temperature rating corresponding to the pressure rating is the shell temperature of the pressure-bearing vessel of the flange and flange fitting. In general, this temperature is the same as the temperature of the stored fluid. The responsibility for not selecting the pressure rating according to the temperature of the stored fluid is at the user’s own expense. When the temperature is below -29°C (-20°F), the rating should be no greater than the rating at -29°C (-20°F).

As an example, the following are two groups of materials classified according to ASTM and their pressure-temperature ratings according to ASME B16.5.

 Note: 1. Long-term exposure to temperatures above 425°C may transform the carbonized phase in the steel into graphite. Long-term operation at temperatures above 425°C is permitted but not recommended.

2. Not to be used above 455°C.

3. Not to be used above 370°C. 4.

4. Not to be used above 260°C.

5. Not to be used above 425°C.