[Science Sticker] How to convert the nominal pressure of the valve between the national standard and the American standard?

   For the commonly used identification system for valve “nominal pressure”, the European Standard System (PN) is a “nominal pressure” system based on allowable working pressure at room temperature (120°C) represented by Germany. It is mostly used in ISO, DIN, EN, BS standards. The American Standard System (Class) is a "temperature-pressure" system represented by the allowable working pressure at a certain temperature, represented by the United States, and is mostly used in standards such as AMSE.

   The Class series (namely pounds) in the American standard valve is a result of calculation based on ASME B16.34 after combining temperature and pressure for a certain metal. Because the temperature reference between the pound level and the nominal pressure is different, there is no one-to-one correspondence between the pound level and the nominal pressure, so the pressure conversion cannot be performed only in the engineering conversion. For example, Class300 should be 2.1MPa simply converted by pressure, but if the use temperature is taken into account, the corresponding pressure will increase accordingly. According to the temperature and pressure test of the material, it is equivalent to 5.0MPa. At the same time, in the US "temperature-pressure" system, other levels are based on 454°C, while Class150 is based on 260°C. The allowable stress of Class150 No. 25 carbon steel valve at 260℃ is 1MPa, and the allowable stress at room temperature is much larger than 1MPa, which is about 2.0MPa. Therefore, the nominal pressure and temperature and pressure levels cannot be changed casually according to the pressure conversion formula.

    At present, my country’s “nominal pressure” system mainly refers to the European standard. However, it is slightly different from that in Germany. The normal temperature reference used in Germany is 120℃, while in my country it is 100℃. my country’s national standard GB/T 1048-2019 The "Definition and Selection of Nominal Pressure of Piping Components" is drafted with reference to ISO 7268:1983 "Definition of Nominal Pressure of Piping Components". The degree of consistency between the national standard and the ISO standard is not equivalent.

The following table is taken from GB/T 1048-2019 "Definition and Selection of Nominal Pressure of Pipeline Components" (non-corresponding relationship)

    Since the nominal pressure and the temperature reference of the pressure grade are different, there is no strict correspondence between the two. The approximate correspondence between the two is shown in the table below.

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