Understanding how to read and interpret flange pressure class is essential for engineers, procurement specialists, and installers working in pipeline systems. Flange pressure class, often referred to as pressure rating, defines how much internal pressure a flange can handle safely at a given temperature.

In this article, we’ll break down what pressure class means, how it is labeled, how temperature affects it, and how to choose the right flange for your application. Whether you’re working with ASME, EN, or AWWA standards, knowing how to read pressure classes is a basic yet critical skill.

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What Is Flange Pressure Class?

A flange pressure class indicates the maximum pressure a flange can withstand under specific conditions. It is represented by a numeric designation, such as:

• Class 150

• Class 300

• Class 600

• Class 900

• Class 1500

• Class 2500

These numbers do not directly mean PSI (pounds per square inch). Instead, they relate to a set of pressure-temperature charts defined by international standards like ASME B16.5 or ASME B16.47.

For example, a Class 150 flange made from carbon steel may have a maximum pressure rating of 285 PSI at 100°F, but this rating will drop at higher temperatures.

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How to Read a Flange Pressure Class Label

When you look at a flange or its documentation, you may see a marking such as:

“ASME B16.5 2” CL300 A105”

This tells you:

• ASME B16.5 – the flange standard used

• 2″ – the nominal pipe size (NPS)

• CL300 – the pressure class (Class 300)

• A105 – the material grade (carbon steel)

The CL300 here means the flange is rated as Class 300, and its pressure-temperature limits are defined in ASME tables based on the material A105.

At sasaflange, all our flanges are clearly stamped with pressure class, size, standard, and material for full traceability and compliance.

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Why Pressure Class Depends on Temperature

One of the most important things to remember is that flange pressure class is temperature-dependent.

For example, a Class 300 flange in A105 carbon steel can handle:

• 570 PSI at 100°F

• 370 PSI at 500°F

• 190 PSI at 1000°F

As temperature increases, metal weakens, and pressure capacity drops. That’s why material selection and operating conditions are just as important as the class rating.

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Different Standards Use Different Class Systems

Most flange pressure ratings follow specific standards, including:

• ASME B16.5 / B16.47

Used in oil, gas, chemical, and industrial piping. Pressure classes range from 150 to 2500.

• EN 1092-1

European standard that uses PN ratings (e.g., PN10, PN16, PN40), which correspond to pressure ratings in bar.

• AWWA C207

Used in waterworks. Uses Classes B, D, E, F instead of numbers like ASME. Class D, for example, handles up to 175 PSI.

If you’re unsure which standard to use, the experts at sasaflange can help match your application with the correct flange type and pressure rating.

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How to Choose the Right Pressure Class

Here are simple steps to read and choose the right pressure class for your project:

1. Know Your Operating Pressure and Temperature

Always check the maximum pressure and temperature your system will experience.

2. Determine the Flange Material

Different materials like carbon steel, stainless steel, or alloy steel perform differently at high temperatures.

3. Refer to Standard Pressure-Temperature Charts

Use ASME or EN charts to find the pressure limit for your material at the operating temperature.

4. Select the Correct Class

Choose a class that offers a pressure limit higher than your system’s pressure at the working temperature.

5. Verify Compatibility

Ensure the flange class matches the pipe schedule, gasket, bolts, and mating flange.

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Common Pressure Class Examples

• Class 150 – Suitable for low-pressure water, HVAC, or general industrial use.

• Class 300 – Used in systems with moderate pressure such as steam lines and chemical plants.

• Class 600 and above – Required for high-pressure systems, refineries, or critical services.

Choosing an unnecessarily high class increases cost and weight. Choosing too low is a safety risk.

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Conclusion

Reading and understanding flange pressure class is fundamental to building safe, leak-proof, and compliant piping systems. By recognizing class labels, consulting temperature-pressure charts, and selecting the right material, you can avoid installation problems and ensure long-term performance.

Manufacturers like sasaflange provide full documentation, pressure testing, and expert support to help you choose the right flange every time. Whether your system runs at 150 PSI or 1500 PSI, sasaflange delivers precision-engineered flanges that meet or exceed global standards.