# PN nominal pressure and Class pounds ( Lb )

Nominal pressure (PN), Class American standard pound level ( Lb ), is a way of expressing pressure, the difference is that the pressure they represent corresponds to a different reference temperature, the PN European system refers to the pressure at 120 ° C The corresponding pressure, while CLass American Standard refers to the corresponding pressure at 425.5 ° C. Therefore, in engineering interchange, it is not possible to simply perform pressure conversion. For example, CLass300 should be 2.1MPa by simple pressure conversion. However, if the operating temperature is considered, the corresponding pressure will increase. According to the temperature and pressure resistance test of the material The measurement is equivalent to 5.0MPa.

There are two types of valve systems: one is the “nominal pressure” system represented by Germany (including China) based on the allowable working pressure at room temperature (100 degrees in my country and 120 degrees in Germany). One is the “temperature and pressure system” represented by the United States, which is represented by the allowable working pressure at a certain temperature. In the temperature and pressure system of the United States, except for 150Lb, which is based on 260 degrees, other levels are based on 454 degrees. . The allowable stress of the 150-psi class (150psi=1MPa) No. 25 carbon steel valve is 1MPa at 260 degrees, and the allowable stress at room temperature is much larger than 1MPa, about 2.0MPa. Therefore, generally speaking, the nominal pressure level corresponding to the American Standard 150Lb is 2.0MPa, and the nominal pressure level corresponding to 300Lb is 5.0MPa, etc. Therefore, the nominal pressure and temperature and pressure grades cannot be converted casually according to the pressure conversion formula .

PN is a code related to pressure represented by numbers, and it is a convenient round integer for reference. PN is the pressure-resistant MPa number approximately equivalent to normal temperature, which is the nominal pressure usually used by Chinese valves. For control valves with carbon steel valve bodies, it refers to the maximum allowable working pressure when used below 200°C; for cast iron valve bodies, it refers to the maximum allowable working pressure when used below 120°C; Maximum allowable working pressure for applications below 250°C. When the working temperature rises, the pressure resistance of the valve body will decrease. The American standard valve expresses the nominal pressure in pounds , which is the calculation result of the combination temperature and pressure of a certain metal , which is calculated according to the standard of ANSI B16.34. The main reason why the pound class and the nominal pressure are not one-to-one correspondence is that the temperature base of the pound class and the nominal pressure is different. We usually use software to calculate, but we also need to know how to use tables to check scales. Japan mainly uses the K value to indicate the pressure level. For the pressure of gas, in China, we generally use its mass unit “kg” to describe (rather than “jin”), and the unit is kg. The corresponding pressure unit is “kg/cm2″, and one kilogram of pressure means that one kilogram of force acts on one square centimeter. Similarly, corresponding to foreign countries, for the pressure of gas, the commonly used pressure unit is “psi”, and the unit is “1 pound/inch2″, which is “pounds per square inch”. The full English name is Pounds per square inch. But it is more commonly used to directly call its mass unit, that is, pound (Lb.), which is actually Lb. That’s the pound-force mentioned earlier. It can be calculated by changing all the units into metric units: 1 psi=1 pound/inch2 ≈0.068bar, 1 bar≈14.5psi≈0.1MPa, countries such as Europe and the United States are used to using psi as the unit. In Class600 and Class1500, there are two different values corresponding to the European standard and the American standard. 11MPa (corresponding to the 600-pound class) is the European system regulation, which is specified in “ISO 7005-1-1992 Steel Flanges”; 10MPa (corresponding to the 600-pound class Class) is the American system regulation, which is the regulation in ASME B16.5. Therefore, it cannot be absolutely said that the 600-pound class corresponds to 11MPa or 10MPa, and the regulations of different systems are different.

There are mainly two types of valve systems: one is the “nominal pressure” system represented by Germany (including my country) based on the allowable working pressure at room temperature (100 degrees in my country and 120 degrees in Germany). One is the “temperature and pressure” system represented by the United States, which is represented by the allowable working pressure at a certain temperature. In the temperature and pressure system of the United States, except for 150Lb, which is based on 260 degrees, other levels are based on 454 degrees. benchmark. For example, the allowable stress of 150Lb. 25 carbon steel valve is 1MPa at 260 degrees, and the allowable stress at room temperature is much larger than 1MPa, which is about 2.0MPa. Therefore, generally speaking, the nominal pressure level corresponding to the American Standard 150Lb is 2.0MPa, and the nominal pressure level corresponding to 300Lb is 5.0MPa, etc. Therefore, the nominal pressure and temperature and pressure grades cannot be converted casually according to the pressure conversion formula .

Since the temperature bases of the nominal pressure and pressure rating are different, there is no strict correspondence between the two. The rough correspondence between the two is shown in the table.

Post time: Aug-31-2023