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The commonly used terms PN and Class are both ways of expressing pressure; the difference lies in the reference temperature at which the corresponding pressures are defined. The PN system used in Europe refers to the pressure at 120°C, whereas the Class system used in the United States refers to the pressure at 425.5°C. Therefore, in engineering exchanges, pressure conversion alone cannot be sufficient; for example, for Class 300#, the pressure value based solely on conversion would be 2.1 MPa. However, when the operating temperature is taken into account, the corresponding pressure increases, and according to the material’s temperature resistance tests, it is equivalent to 5.0 MPa. There are two systems for valves: one is the \"nominal pressure\" system, represented by Germany (including China), which is based on the allowable operating pressure at normal temperatures (100 degrees in China and 120 degrees in Germany). One is the “temperature-pressure system” represented by the United States, which is based on the allowable operating pressure at a certain temperature. In the U.S. temperature-pressure system, except for 150LB which uses 260 degrees as the reference, all other levels use 454 degrees as the reference. For a 25-grade carbon steel valve rated at 150 pounds per square inch (150 psi = 1 MPa), the allowable stress at 260 degrees is 1 MPa; whereas at room temperature, the allowable stress is much higher, around 2.0 MPa. Therefore, generally speaking, the nominal pressure rating corresponding to US standard 150LB is 2.0 MPa, while that corresponding to 300LB is 5.0 MPa, and so on. Therefore, the nominal pressure and temperature-pressure class cannot be arbitrarily changed using pressure conversion formulas. PN is a code related to pressure, expressed in numerical form; it is a convenient round integer used as a reference. PN represents the pressure rating in MPa, adjusted to approximate normal temperature conditions, and it is the nominal pressure commonly used for valves in China. For control valves with carbon steel bodies, it refers to the maximum operating pressure permitted when used at temperatures below 200°C ； For cast iron valve bodies, it refers to the maximum operating pressure allowed when used at temperatures below 120°C ； For control valves with stainless steel valve bodies, it refers to the maximum operating pressure permitted when used at temperatures below 250°C. As the operating temperature rises, the pressure resistance of the valve body decreases. American standard valves have their nominal pressure indicated in pounds; this value is determined based on the melting temperature and pressure of a certain metal, in accordance with the ANSI B16.34 standard. The main reason why the pound rating and the nominal pressure do not correspond one-to-one is that they have different temperature references. We usually use software for calculations, but we also need to know how to use tables to check weight classes. Japan primarily uses the K value to indicate pressure ratings. Regarding the pressure of gases, in China we generally use its mass unit \"kilogram\" to describe it (instead of \"jin\"), with the unit being kg. The corresponding pressure unit is “kg/cm2”; one kilogram of pressure means one kilogram of force applied to one square centimeter. Similarly, as is the case abroad, the commonly used unit for measuring gas pressure is “psi”, which stands for “1 pound/inch2”, that is, “pounds per square inch”; the full English name for this unit is Pounds per ****** inch. But it is more common to refer directly to its unit of mass, that is, pounds (LB.), which is actually LB. It’s the pound-force mentioned earlier. By converting all units to metric units, the conversion can be calculated: 1 psi = 1 pound/inch2 ≈ 0.068 bar, 1 bar ≈ 14.5 psi ≈ 0.1 MPa. Countries in Europe and the Americas generally use psi as the unit of measurement. For Class600 and Class1500, there are two different values corresponding to the European and American standards: 11 MPa (corresponding to the 600-pound class) is the value specified by the European system, as stated in \"ISO 7005-1-1992 Steel Flanges\" ； 10 MPa (corresponding to the 600-pound class) is specified in the American system, as outlined in ASME B16.5. Therefore, it cannot be said absolutely that a 600-pound class corresponds to 11 MPa or 10 MPa; the regulations vary depending on the system. Due to the different temperature references for nominal pressure and pressure class, there is no strict correspondence between the two. The approximate correspondence between the two is shown in the table below. Pound rating and nominal pressure comparison table