Is anyone who can explain me, what are PN and PT ratings of valves and flanges ?

All Pipes and various pipe fittings are most commonly classified based on their pressure temperature rating or commonly known as P-T Rating. The most common way of specifying pipe pressure temperature rating is given by ASME 16.5, using pound ratings (or lb ratings) – #150, #300, #400, #600, #900, #1500 and #2500. The pressure rating or pound rating for a pipe is determined using the design pressure and design temperature for the pipe. Among other methods of classification by using pressure temperature ratings is ‘Pression Nominal’ or ‘Pressure Nominal’ or PN number method. This PN number is a rough indicator of pressure rating in bar.