Pipes and various pipe fittings are most commonly classified based on their pressure temperature 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.
Pressure rating or pound rating of a pipe is dependent on the pipe material and design temperature.
The pipe pressure rating for the same material changes at different temperatures. For the same material and constant design pressure, different pressure ratings are applicable over different ranges of design temperatures.
As the design temperature increases the pressure rating requirement for the pipe also increases for the same design pressure.
It should be noted that the pound rating for the whole piping system is equal to the pound rating of the weakest part, having the lowest pressure rating in the system. The weakest part may be any piping component or fitting which contains the pressure in the system and has the lowest pound rating due to any possible reasons.