3.2　Design Criterion

3.2.1　Pressure-temperature ratings of piping components can be expressed in terms of the nominal pressure in addition to the design pressure and design temperature.

3.2.2　The nominal pressure of piping components shall comply with the current national standard GB/T 1048 Pipework Components-Definition and Selection of PN.

3.2.3　Unless otherwise specified，for pipe fittings only shown with the nominal pressure，the allowable pressure at the design temperature shall be calculated using the following formula:

Where，Pt——allowable working pressure at design temperature，MPa；

KPN——nominal pressure conversion coefficient，KPN=0.1MPa；

PN——nominal pressure；

［σ］t——allowable stress of material at design temperature，MPa；

［σ］s——reference stress corresponding to the nominal pressure.It refers to the allowable stress（MPa）of material at a specified temperature.

3.2.4　The allowable stress of steel used for fabricating pipe and pipe fittings shall be taken as the minimum of the following three values according to the strength characteristics of the steel:

Where， ——minimum tensile strength of steel at 20℃，MPa；

——minimum yield strength of steel at design temperature，MPa；

——minimum proof strength at 0.2% non-proportional elongation of steel at design temperature，MPa；

———105h average creep rupture strength of steel at design temperature，MPa.