When working with pipelines, fittings, and flanges, it is often necessary to switch between different pressure rating systems depending on the region and standards used. PN (Nominal Pressure) and Class Ratings are two of the most widely used pressure standards. However, converting between these two systems isn’t as straightforward as a mathematical formula. The conversion depends on temperature, material, and design specifications. This guide provides a detailed overview of how these two systems compare, the key factors that affect conversion, and how to avoid common mistakes.
1. Understanding PN and Class Ratings
What is PN (Nominal Pressure)?
- PN (Pressure Nominale) is a pressure rating system used predominantly in Europe.
- It is defined by ISO 7005 and DIN standards (such as DIN 2501).
- PN numbers represent the maximum allowable pressure in bars that a fitting or flange can withstand at a reference temperature of 20°C (68°F).
Examples:
- PN 10 means the system can handle 10 bars (~1 MPa) of pressure.
- PN 16, PN 25, and PN 40 are also common in European systems.
What is Class Rating?
- Class Ratings are commonly used under ANSI/ASME standards, especially in the United States and internationally.
- The ratings are based on pound force per square inch (psi) at a reference temperature.
- Common Class Ratings include Class 150, 300, 600, 900, 1500, and 2500.
Examples:
- Class 150 is rated for 150 psi (~1.03 MPa) at 20°C.
- Higher classes (e.g., Class 600 or 1500) are used for systems under higher pressures.
2. Approximate Conversion: PN vs. Class
Although PN and Class systems reflect pressure capacities, they arise from different design principles and are not directly convertible. However, the table below provides a practical comparison based on commonly accepted practice at room temperature (20°C).
| PN Rating | Class Rating | Pressure (MPa) |
|---|---|---|
| PN 2.5 | Class 150 | 2.0 MPa |
| PN 6 | Class 150 | 2.0 MPa |
| PN 10 | Class 300 | 5.0 MPa |
| PN 16 | Class 300 | 5.0 MPa |
| PN 25 | Class 400 | 6.3 MPa |
| PN 40 | Class 600 | 10.0 MPa |
| PN 63 | Class 900 | 15.0 MPa |
| PN 100 | Class 1500 | 25.0 MPa |
| PN 250 | Class 2500 | 42.0 MPa |
Key Observations:
- Lower PN ratings (like PN 2.5 and PN 6) often align with Class 150.
- As pressure capacity increases, higher PN ratings (e.g., PN 100) correspond with Class 1500.
- Class 2500 corresponds with the highest pressure levels (up to 42 MPa), comparable to PN 250.
3. Factors Affecting Conversion
While the table provides a basic equivalency, several factors can alter the actual pressure rating for a given class or PN rating:
1. Temperature:
- Higher temperatures reduce the allowable pressure capacity for both PN and Class systems.
- For example, Class 150 flanges rated at 2 MPa (at 20°C) may only handle 1.5 MPa at 200°C.
2. Material:
- The material of the fitting or flange affects pressure capacity.
- Carbon steel, stainless steel, and alloy steel all perform differently under pressure.
- Both PN and Class ratings assume specific material properties under standardized testing conditions. Always refer to specific material standards (e.g., ASTM or EN).
3. Safety Factors:
- European and American standards apply different safety margins.
- ANSI/ASME Class ratings typically involve higher safety factors than DIN PN ratings.
- It is essential to check the relevant code or standard for each project to ensure compliance.
4. Practical Guidelines for Conversion
1. Consult Standards Documents:
- When performing an actual conversion for design or procurement purposes, always consult:
- ASME B16.5 for flanges and pressure-temperature ratings.
- ISO 7005 or DIN 2501 for PN-rated systems.
2. Use Manufacturer Data Sheets:
- Manufacturers often provide detailed pressure-temperature charts for their products, showing exact correlations between PN and Class ratings under various conditions.
3. Avoid Direct Mathematical Conversion:
- As demonstrated, there is no linear conversion formula between PN and Class because each system has unique assumptions. The right rating depends on the specific operating environment, temperature, and material.
4. Design for the Worst Case:
- When in doubt, design to the higher rating to ensure safety and compliance.
- For example, if a PN 40 system must interface with a Class 300 flange, verify that the system will not experience overpressure.
5. Conclusion
Converting between PN (Nominal Pressure) and Class Ratings is not a one-size-fits-all process. Both systems represent pressure capacities, but they arise from different standards with distinct safety factors, material assumptions, and temperature limitations.
To ensure safe and effective operation, it is critical to:
- Consult relevant standards like ASME B16.5 and ISO 7005.
- Account for temperature and material changes in the operating environment.
- Use manufacturer data sheets for precise equivalency checks.
By following these guidelines, engineers and procurement professionals can confidently navigate the complexities of international pressure standards and ensure their systems meet all performance and safety requirements.