Explanation of the conversion between US standard pound rating Lb and Chinese standard nominal pressure Mpa for valves

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When purchasing and using valves, users inevitably encounter the issue of converting between the pressure unit pounds (Lb) used for American standard valves and the pressure unit megapascals (Mpa) used for Chinese standard valves. So what is exactly the conversion relationship between them? What we usually refer to as 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, while the Class system used in the United States refers to the pressure at 425.5°C. Therefore, when performing conversions between these systems, it is not sufficient to simply convert the pressure values; for example, Class 300# corresponds to a pressure of 2.1 MPa if only pressure conversion is considered. However, taking the operating temperature into account increases this pressure value to 5.0 MPa, as determined by tests on the material’s tolerance to temperature. 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 this system, with the exception of the 150LB grade for which 260 degrees serves as the reference point, all other grades use 454 degrees as their reference. For a 150-pound grade (150psi = 1MPa) carbon steel valve, the allowable stress at 260 degrees is 1MPa; however, at room temperature, the allowable stress is much higher, around 2.0MPa. Therefore, generally speaking, the nominal pressure class corresponding to US standard 150LB is 2.0 MPa, that corresponding to 300LB is 5.0 MPa, and so on. Therefore, the nominal pressure and temperature-pressure class cannot be arbitrarily converted 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 the result of calculations based on the melting temperature and pressure of a certain metal, and it is determined in accordance with ANSI B16.34 standards. The pound rating and the nominal pressure are not in a one-to-one correspondence, mainly because 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. 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 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, namely pounds (LB); in fact, LB refers to the pound-force mentioned earlier. By converting all units to metric units, the following conversions can be made: 1 psi = 1 pound/inch2 ≈ 0.068 bar; 1 bar ≈ 14.5 psi ≈ 0.1 MPa. Countries in Europe and the United States typically use psi as their 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: Table of correspondence between class and nominal pressure (for reference). Class: 150, 300, 400, 600, 800, 900, 1500, 2500, 3500, 4500; Nominal pressure PN (MPa): 25, 6.3, 10, 13, 15, 25, 42, 56, 76