What is a pressure class?

In other words, a 2” 150 A105 blind flange will have the same pressure/temperature limits as a 24” 150 blind flange. It is important to note the pressure/temperature limits are dictated by the combination of the material grade and design specification. The design specification will state the allowable pressures at a given temperature for each material. This is referred to as the pressure/temperature chart.  
 
It is also important to note that not all the pressure-temperature ratings will be the same from different specifications. So a class 150 flange will not have the same pressure-temperature rating as a class 150 malleable iron.