The pressure and temperature together decide the class of the pips. ie class1500 does not mean it is for 1500 PSI pressure per se.

Check the ASME B31 they will answer your question in detail.

http://www.engineeringtoolbox.com/a106-carbon-steel-pipes-d_370.html

http://www.engineeringtoolbox.com/ansi-flanges-pressure-temperature-d_342.html

for sample.

yes, there is a relation between pressure and temperature.

see book of thermodynamic and also material.

when you buy pipe, you will inform the spect of that pipe, type

this standards is depend on pressure and temperature.

Also in ASME 16.11 below the table we find this note as shown below .

"Pipe actually used may be thinner or thicker in nominal wall than that shown in Table 2. When thinner pipe is used, its strength may govern the rating. When thicker pipe is used (e.g., for mechanical strength), the strength of the fitting governs the rating."

Could ne one explain.

Thanks in Advance.

(experienced people please bare with me)

That is to say that when you connect two piping components together (such as pipe to a fitting) with different thicknesses, the component with thinner thickness shall govern our selection for the rating.

That is can be obvious when you connect two pipes with different thicknesses, the design calculations shall be based on the thinner pipe not on the thicker one.