Conversion between PN nominal pressure and Class in pounds (Lb)

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Nominal pressure (PN) and American standard weight class (Lb) are both ways of expressing pressure; the difference lies in the reference temperature at which the corresponding pressures are defined. PN in the European system refers to the pressure at 120°C, whereas the American standard weight class refers to the pressure at 425.5°C. Therefore, in engineering conversions, it is not sufficient to simply perform pressure conversions; for example, the pressure corresponding to Class 300 based on a simple pressure conversion would be 2.1 MPa. However, when the operating temperature is taken into account, the corresponding pressure increases, and according to the temperature resistance tests of the material, 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 150 Lb with 260 degrees as the reference, all other levels use 454 degrees as the reference. For Grade 25 carbon steel valves rated at 150 pounds per square inch (150 psi = 1 MPa), the allowable stress at 260 degrees Celsius is 1 MPa. At room temperature, however, the allowable stress is significantly higher than 1 MPa—approximately 2.0 MPa. Therefore, generally speaking, the nominal pressure class corresponding to US standard 150 Lb is 2.0 MPa, while that corresponding to 300 Lb 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 for 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 permitted 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 particular 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 look up pound values. Japan primarily uses the K value to indicate pressure ratings. Regarding gas pressure, in China, we generally use the mass unit “kilogram” to describe it (rather than “jin”), with the unit being kg. The corresponding unit of pressure 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 square inch. But it is more common to refer directly to its unit of mass, that is, pounds (Lb.), which is actually the Lb. It’s the pound-force mentioned earlier. By converting all units to metric units, the conversion can be calculated: 1 psi = 1 pound per inch2 ≈ 0.068 bar; 1 bar ≈ 14.5 psi ≈ 0.1 MPa. Regions such as Europe and the United States commonly use psi as the unit of measurement. For Class 600 and Class 1500, 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 outlined in the standard \"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. There are mainly 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 this system, aside from 150 Lb having 260 degrees as its reference point, all other levels use 454 degrees as their reference. For example, for a 25-grade carbon steel valve weighing 150 lbs, the allowable stress at 260 degrees is 1 MPa; whereas at room temperature, the allowable stress is much higher than 1 MPa, around 2.0 MPa. Therefore, generally speaking, the nominal pressure class corresponding to US standard 150 Lb is 2.0 MPa, while that corresponding to 300 Lb is 5.0 MPa, and so on. Therefore, the nominal pressure and temperature-pressure class cannot be arbitrarily changed using pressure conversion formulas. 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. Table of comparison between class ratings and nominal pressures (for reference): Class rating: 150, 300, 400, 600, 800, 900, 1500, 2500, 3500, 4500. Nominal pressure: PN20, 50, 63, 100, 130, 150, 250, 420, 560, 760