Comparison Table of Nominal Pressures: Chinese and American Standards

The PN and Class designations we commonly use are both methods of expressing pressure; the difference lies in the reference temperatures at which the pressures they represent apply. The European PN system refers to the pressure at 120°C, whilst the American Class standard refers to the pressure at 425.5°C.
Consequently, in engineering applications, one cannot simply perform a pressure conversion. For example, a Class 300 valve, when converted purely by pressure, would correspond to 2.1 MPa; however, when operating temperature is taken into account, the equivalent pressure increases. Based on temperature-pressure resistance tests on the material, this is equivalent to 5.0 MPa.
There are two valve classification systems:
One is the ‘nominal pressure’ system, represented by Germany (and China), which is based on the allowable working pressure at ambient temperature (100°C in China and 120°C in Germany).
The other is the ‘temperature-pressure system’, typified by the United States, which is based on the allowable working pressure at a specific temperature. In the US temperature-pressure system, with the exception of the 150 lb class, which uses 260°C as the reference, all other classes use 454°C as the reference. For a Grade 25 carbon steel valve rated at 150 psi (150 psi = 1 MPa), the allowable stress at 260°C is 1 MPa, whereas at room temperature the allowable stress is significantly higher, at approximately 2.0 MPa.
Therefore, it is generally stated that the nominal pressure rating corresponding to the American standard 150 lb is 2.0 MPa, that for 300 lb is 5.0 MPa, and so on.
Consequently, one must not arbitrarily convert nominal pressure and temperature-pressure ratings using pressure conversion formulas. PN is a numeric code related to pressure; it is a convenient, rounded figure provided for reference purposes. PN approximates the pressure resistance in MPa at room temperature and is the nominal pressure commonly used for valves in China. For control valves with carbon steel bodies, this refers to the maximum allowable working pressure when used at temperatures below 200°C; for cast iron bodies, it refers to the maximum allowable working pressure when used at temperatures below 120°C; for control valves with stainless steel bodies, it refers to the maximum allowable working pressure when used at temperatures below 250°C. As the working temperature rises, the pressure resistance of the valve body decreases.
American Standard valves express nominal pressure in pound-class ratings. The pound-class rating is the result of calculating the combination of temperature and pressure for a specific metal, based on the ANSI B16.34 standard. The primary reason for the lack of a one-to-one correspondence between pound-class ratings and nominal pressure is that they are based on different temperature reference points.
We typically use software for calculations, but it is also important to know how to consult tables to determine the pound class. In Japan, pressure ratings are primarily expressed using the K-value. For gas pressures in China, we generally use the mass unit ‘kilogram’ (rather than ‘jin’), denoted as kg. The corresponding unit of pressure is ‘kg/cm²’, where one kilogram of pressure represents a force of one kilogram acting on one square centimetre.
Similarly, in other countries, the commonly used unit of pressure for gases is ‘psi’, which stands for ‘pounds per square inch’ (1 pound/inch²). However, it is more common to refer directly to the unit of mass, namely the pound (lb), which in practice corresponds to the pound-force mentioned earlier.
Converting all units to the metric system yields the following: 1 psi = 1 lb/in² ≈ 0.068 bar; 1 bar ≈ 14.5 psi ≈ 0.1 MPa. Countries in Europe and America typically use psi as the unit. In Class 600 and Class 1500, there are two different values corresponding to European and American standards: 11 MPa (corresponding to the 600-pound class) is specified by the European system, as set out in ISO 7005-1-1992 Steel Flanges; 10 MPa (corresponding to the 600-pound class) is specified by the American system, as set out in ASME B16.5. Therefore, it cannot be stated categorically that the 600-pound class corresponds to either 11 MPa or 10 MPa, as the specifications differ between the two systems.
As the temperature bases for nominal pressure and pressure classes differ, there is no strict correspondence between the two. Please refer to the table below for a general correspondence between the two.

Pound Class and Nominal Pressure Comparison Table