Introduction
Selecting the correct flange pressure rating is essential for safe and reliable piping system operation. A rating that is too low risks catastrophic failure, while an excessively high rating adds unnecessary cost. This FAQ explains how flange pressure ratings work and how to choose the correct one for your application.
What Do Pressure Ratings Mean?
Flange pressure ratings are nominal designations (Class or PN) that indicate the maximum allowable pressure at a given temperature. The rating does not directly equal a fixed pressure - the actual allowable pressure decreases as temperature increases.
ASME (US) system: Class 150, 300, 600, 900, 1500, 2500. The numbers are not pressure in psi at all temperatures - they are dimensionless designations. For example, Class 150 can handle approximately 150 psi at ambient temperature but less at elevated temperatures.
DIN/EN (European) system: PN6, PN10, PN16, PN25, PN40, PN63, PN100, PN160. PN stands for "Pressure Nominal" in bars. PN16 means the flange is designed for nominal 16 bar pressure at ambient temperature.
JIS (Japanese) system: 10K, 20K, 30K, 40K, 63K. The K rating approximates kgf/cm².
Factors Affecting Pressure Rating Selection
Operating pressure: The system's design pressure is the starting point. Always select a flange rating whose pressure-temperature rating exceeds the maximum expected operating pressure with an appropriate safety margin.
Operating temperature: As temperature increases, the allowable pressure of all flange materials decreases. Always check the pressure-temperature rating chart for your specific material and rating at the actual operating temperature.
Fluid medium: Hazardous, flammable, or toxic fluids require additional safety margin and often higher-rated flanges. Vacuum services also require careful consideration.
Material grade: The same Class rating has different pressure-temperature capabilities depending on material. Stainless steel flanges may have different ratings than carbon steel at elevated temperatures.
Applicable code: The project code (ASME B31.3, B31.4, B31.8, etc.) specifies minimum design factors and safety margins.
Common Pressure Rating Applications
Class 150 / PN10 / PN16: Most common for water, HVAC, low-pressure oil, general utility piping. The most widely used flange rating.
Class 300 / PN40: Used for medium-pressure oil and gas, steam systems, and process piping.
Class 600 / PN100: High-pressure process systems, refinery units, high-temperature services.
Class 900-2500: Critical high-pressure applications such as oil and gas transmission, high-pressure steam, and special process services.
Important Considerations
Never assume a Class 150 flange can handle 150 psi at 400°C - always consult the pressure-temperature rating table for your specific material. For carbon steel (ASTM A105), the allowable pressure drops significantly at elevated temperatures. Also, the gasket type and bolt material must match the flange rating - upgrading the flange alone is not sufficient.
Conclusion
Choosing the right flange pressure rating requires understanding the system's design pressure, operating temperature, fluid properties, and applicable code. When in doubt, consult a piping engineer or refer to the pressure-temperature rating tables in ASME B16.5 or EN 1092-1.
References
1. ASME B16.5, Pressure-Temperature Rating Tables.
2. EN 1092-1, Flanges and their joints.
