The PN and Class we usually use are a representation of pressure. The difference is that the pressure they represent corresponds to different reference temperatures. The PN European system refers to the pressure at 120 ℃, while the CLass American standard refers to the pressure at 425.5 ℃.
Therefore, in engineering exchange, pressure conversion cannot be simply performed. For example, CLass300 # should be converted to 2.1MPa using pressure alone. However, if temperature is considered, the corresponding pressure increases. According to the temperature and pressure resistance test of the material, it is equivalent to 5.0MPa.
There are two types of valve systems:
One is the "nominal pressure" system represented by Germany (including China), which is based on the allowable working pressure at room temperature (100 degrees in China and 120 degrees in Germany).
One is the "temperature pressure system" represented by the United States, which is represented by the allowable working pressure at a certain temperature. In the temperature pressure system of the United States, except for 150LB, which is based on 260 degrees, all other levels are based on 454 degrees. The allowable stress of the No. 25 carbon steel valve in the 150 pound class (150psi=1MPa) is 1MPa at 260 degrees Celsius, and the allowable stress at room temperature is much greater than 1MPa, approximately 2.0MPa.
So, generally speaking, the nominal pressure level corresponding to the American standard 150LB is 2.0MPa, and the nominal pressure level corresponding to the 300LB is 5.0MPa, etc.
Therefore, the nominal pressure and temperature pressure level cannot be arbitrarily changed according to the pressure transformation formula. PN is a numerical code related to pressure, which is a convenient circular integer for reference. PN is approximately the pressure resistance MPa equivalent to room temperature, and is the nominal pressure commonly used in domestic valves. For control valves with carbon steel valve bodies, it refers to the maximum allowable working pressure for applications below 200 ℃; For cast iron valve bodies, it refers to the maximum allowable working pressure for applications below 120 ℃; For control valves with stainless steel valve bodies, it refers to the maximum allowable working pressure for applications below 250 ℃. When the working temperature increases, the pressure resistance of the valve body will decrease.
American standard valves represent nominal pressure in pounds, which is the calculation result of the combined temperature and pressure of a certain metal. It is calculated according to the standard of ANSI B16.34. The main reason why pound scale and nominal pressure do not correspond one-to-one is that the temperature reference for pound scale and nominal pressure is different.
We usually use software to calculate, but we also need to know how to use tables to check the scale. Japan mainly uses K value to indicate pressure level. For gas pressure, in China, we generally use the unit of mass "kilogram" to describe it more commonly (rather than "jin"), with the unit of kg. The corresponding pressure unit is "kg/cm2", and one kilogram of pressure is the force of one kilogram acting on one square centimeter.
Similarly, compared to foreign countries, the commonly used unit of pressure for gases is "psi", with the unit being "1 pound/inch2", which is "pounds per square inch" in English. But more commonly used is to directly refer to its unit of mass, which is pounds (LB.), which is actually LB. It is the pound force mentioned earlier.
Replacing all units with metric units can calculate: 1 psi=1 pound/inch2 ≈ 0.068 bar, 1 bar ≈ 14.5 psi ≈ 0.1 MPa, and countries such as Europe and America commonly use psi as the unit. In Class600 and Class1500, there are two different values corresponding to the European and American standards. 11MPa (corresponding to the 600 pound class) is a European system regulation, which is stipulated in ISO 7005-1992 Steel Flanges; 10MPa (corresponding to the 600 pound class) is an American system regulation, which is stipulated in ASME B16.5. Therefore, it cannot be absolutely said that the 600 pound level corresponds to 11MPa or 10MPa, and the regulations for different systems are different.
Due to the different temperature benchmarks for nominal pressure and pressure level, there is no strict correspondence between the two. The approximate correspondence between the two is shown in the table below.
Comparison Table for Pound Scale and Nominal Pressure
