The maximum pressure (measured in psi or bars) that a flange can withstand as temperature rises is referred to as "flange rating" (or "class"). Flanges with higher ratings (classes) may withstand more pressure at hotter temperatures, making them stronger than flanges with lower ratings. Different materials used to make flanges exhibit varying pressure-temperature characteristics at the same rating. The pressure rating tables for carbon, alloy, and stainless steel flanges are provided in the article, along with advice on how to choose the right rating for your piping application.
What is the difference about “pressure rating”, “class”, “#”, “Lb” or “Lbs”?
The terms “pressure rating”, “class”, “#”, “Lb” or “Lbs” are interchangeable, i.e. they all refer to the same exact concept of pressure/temperature performance of a flange (and other equipment like valves, fittings, etc).
ANSI FLANGE PRESSURE RATING EXPLAINED
The maximum permitted pressure that a flange can withstand at rising temperatures is known as pressure rating. There are seven flange pressure ratings listed in the ANSI/ASME B16.5 specification: 150, 300, 400, 600, 900, 1500, and 2500.
Let's use an illustration to illustrate this:
The lower rated flange (class 150) will be smaller, lighter, and less durable than the higher rated flange (class 300) if two flanges have the same bore size (for instance, 6 inches), the same material (for instance, A105), but two different pressure ratings (for instance, one flange is class 150, the other 300). (class 300). In the image, the following is illustrated:
Ratings for Flange Pressure
Let's examine what this means for pressure rating now:
At a temperature of 600 degrees Fahrenheit, the class 150 flange can only tolerate 140 psi (as per the rating chart below)
At the same temperature of 600 degrees F, the larger and more robust class 300 flange can resist 570 psi.
Last but not least, a class 2500 flange of the same size can withstand 34 times greater pressure than a class 150 flange, with a rating of staggering 4730 psi at 600 °F.
