In the design, procurement, and construction of pipelines, valves, instruments, and other components, you may often hear terms like “how many kilograms of pressure,” “what class (Lb) rating,” or “how many PSI.” These terms are closely related to the units of pressure we use, and more specifically to the concept of nominal pressure (PN). But how are these terms interconnected?

Definitions and Key Differences

1. Nominal Pressure (PN):
   • Represented by the letter PN followed by a dimensionless number.
   • Commonly used in European and Chinese standards.
2. Class (Lb):
   • Represented by the word Class or the abbreviation CL followed by a dimensionless number (e.g., Class 900 or 900 Lb).
   • Commonly used in American standards.
3. Japanese System:
   • Uses the K rating to represent pressure levels.

These terms do not correspond directly because the temperature basis for these pressure ratings varies between systems:

• PN (European system): Pressure is defined at 120°C.
• PN (Chinese system): Pressure is defined at 100°C.
• Class (American system): For most ratings, pressure is defined at 454°C, except for Class 150, which uses 260°C as the baseline.

Conversions and Key Considerations

While it may seem simple to convert pressure values directly (e.g., Class 300 would theoretically equal 2.1 MPa), the actual pressure changes when temperature is factored in. For instance:

• Class 300: Under practical temperature conditions, the pressure can reach up to 5.0 MPa, determined through material pressure-temperature testing.

Since the temperature references differ, PN and Class ratings cannot be strictly mapped to one another. The approximate relationship is provided in reference tables, but it’s critical to account for temperature during conversions.

Common Units of Pressure

1. China (Metric System):
   • Pressure is commonly described in kilograms (kg) of force per square centimeter (kg/cm²).
   • One kilogram of pressure equals the force exerted by 1 kg over an area of 1 cm².
2. Foreign Standards:
   • Pressure is often expressed in pounds per square inch (psi), where: 1psi ≈ 1pound/inch² ≈ 0.068bar
   • Conversion to metric units: 1bar ≈ 14.5psi ≈ 0.1Mpa
3. Usage in Europe and America:
   • PSI is the standard unit for expressing pressure in these regions.

Standards and Regional Differences

1. Class 600:
   • European System: Defined as 11 MPa per ISO 7005-1-1992 Steel Flanges.
   • American System: Defined as 10 MPa per ASME B16.5.

These differences highlight the importance of recognizing that Class 600 does not universally correspond to either 11 MPa or 10 MPa. The exact value depends on the governing system.

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