As a process chemical process you should know that piping class refers to valves, flanges, fittings and gaskets (ASME B16) and not to the pipe pressure rating.
Pressure rating is a relationship between temperature and pressure for standardized components mentioned above. This tells us what is the maximum allowed working pressure (MAWP), for a non-shock process, that a material can keep its integrity in a given temperature.
For instance, in the picture, if carbon steel is exposed to a higher temperature, it will fail or rupture in a lower pressure condition. And vice-versa.
Each material has its own curve for a given rating. Be aware that the material can fail also if exposed to temperature below its minimum design temperature.
It is important to mention that class rating (ex.: 150# or 150lb) is a nominal identification that does NOT correspond to the pressure rating value in psig.
