Do flanges have pressure ratings?

Do flanges have pressure ratings?

According to ASME B16.5, steel flanges have 7 pressure levels: Class 150-300-400-600-900-1500-2500

The pressure rating of a flange is very specific. A Class 300 flange can withstand greater pressure than a Class 150 flange because it requires more material to withstand the higher pressure. However, the pressure resistance of a flange is affected by many factors. The pressure rating of a flange is measured in pounds, and there are several ways to express the pressure rating.

Pressure Rating Example

The pressure capacity of a flange varies with temperature. As the temperature rises, the flange's pressure capacity 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, as the pressure decreases, the temperature increases, and vice versa. Because flanges can be made of a variety of materials, such as stainless steel, cast iron, ductile iron, and carbon steel, each material has a different pressure rating.

How are common flanges classified into pressure ratings?

Common flanges have varying pressure ratings due to their diverse applications. For example, large stainless steel flanges are primarily used in high-temperature pipelines in chemical engineering, placing high demands on the material's pressure-bearing properties.
Therefore, customers often request forged flanges, as forging increases the material's density and pressure-bearing capacity.
Domestic and international standards clearly define the pressure ratings for large stainless steel flanges. Large stainless steel flanges are generally classified into PN25, PN6, PN10, PN16, PN25, and PN40, among others. PN10 and PN16 are the most commonly used.
To illustrate, the design pressure of a flange for an internal pressure vessel with a safety valve should be determined based on the valve's cracking pressure. The design pressure should be equal to or slightly greater than the valve's cracking pressure, which is 1.05 to 1.1 times the operating pressure.
If the working pressure is 0.8 MPa, the opening pressure of the safety valve can be 0.88 MPa, and therefore the design pressure can be 0.9 MPa. Therefore, it's reasonable to choose a 1.0 MPa-class flange with a maximum allowable working pressure of 0.9 MPa (1C4 material) at 50 degrees Celsius for the container pipe flange.