ASME B16.5 Flange Pressure-Temperature Ratings — Complete Selection Guide

Why Pressure-Temperature Ratings Matter

A flange’s pressure rating is not a fixed number — it changes with temperature. A Class 150 flange that can handle 275 psi at room temperature may only handle 50 psi at 400°C. Selecting a flange based solely on its room-temperature rating can lead to catastrophic failures. This guide explains how to read and apply ASME B16.5 pressure-temperature ratings correctly.

Understanding ASME B16.5 Pressure Classes

ASME B16.5 defines pressure classes from 150# to 2500#. The class number does not represent the actual pressure in psi at any temperature — it is a rough indicator of pressure capacity at room temperature (approximately 10× the class number in psi for Classes 150–600).

Key pressure classes and their typical room-temperature ratings for carbon steel:

• Class 150# — ~275 psi at ambient temperature
• Class 300# — ~740 psi
• Class 600# — ~1,480 psi
• Class 900# — ~2,220 psi
• Class 1500# — ~3,750 psi
• Class 2500# — ~6,250 psi

Pressure-Temperature Rating Tables

ASME B16.5 provides pressure-temperature rating tables for different material groups. The most common material groups are:

• Group 1.1: Carbon steel (A105, F1-F22)
• Group 1.5: Ferritic steel (F11, F22, F5, F91)
• Group 2.1: Stainless steel (F304, F304L, F316, F316L)
• Group 4: Nickel alloys (WNM, WNM-L, etc.)

A carbon steel Class 150 flange (Group 1.1) has a rating of 285 psi at -20°F to 100°F, but this drops to 115 psi at 500°F and 50 psi at 800°F. The same flange in stainless steel (Group 2.1) retains 285 psi up to 200°F and 180 psi at 500°F — significantly better high-temperature performance.

How to Select the Right Flange

Step 1: Determine your system’s maximum operating pressure and temperature.

Step 2: Identify the flange material (carbon steel, stainless steel, alloy steel).

Step 3: Look up the pressure-temperature table in ASME B16.5 for your material group.

Step 4: Find the pressure class where your operating temperature intersects a rating equal to or greater than your design pressure.

Step 5: Add a corrosion allowance and safety margin (typically 10–15%).

Common Mistakes

• Assuming Class 150 = 150 psi at all temperatures: The class number only approximates room-temperature capacity
• Using carbon steel at high temperatures: Carbon steel loses strength rapidly above 400°C; use alloy steel instead
• Ignoring gasket and bolt ratings: The flange rating assumes proper gasket and bolt selection per the system design
• Confusing ASME B16.5 with DIN/EN ratings: PN ratings use a different scale (PN10 = ~10 bar at 20°C)

Conclusion

Proper flange selection requires understanding pressure-temperature derating, not just picking a class number. Always reference ASME B16.5 tables for your specific material and temperature, and consult with your engineering team for critical applications.

PipingFitting.com supplies ASME B16.5 flanges in all pressure classes and material grades. Contact us with your specifications for a quote.