Pressure ratings determine the maximum pressure a flange can safely withstand at a given temperature. Getting the rating right is not optional — an undersized flange in a high-pressure system is a safety hazard. This article explains how ASME pressure classes are established, how temperature affects the allowable pressure, and what to consider when specifying flanges for your system.
What Is a Pressure Class?
ASME B16.5 defines six standard pressure-temperature classes for pipe flanges: 150, 300, 600, 900, 1500, and 2500. These numbers are often written as "Class 150" or "#150" (pounds). The class number itself does not equal the allowable pressure in psi — it is simply a designator that maps to a pressure-temperature table based on material group.
For example, a Class 300 carbon steel flange (Material Group 1.1, ASTM A105) has a pressure rating of approximately 740 psi at ambient temperature. A Class 300 stainless steel flange (304/316) has a lower rating at the same temperature because stainless has a lower allowable stress than carbon steel at room temperature.
How Temperature Affects Pressure Ratings
All metals lose strength as temperature increases. ASME B16.5 pressure-temperature tables reflect this by providing allowable pressures at temperatures ranging from -20°F to as high as 1500°F, depending on the material group. As temperature goes up, the allowable pressure goes down.
This means a flange's nameplate class alone is not enough information. You must know the operating temperature to determine the actual allowable working pressure. A Class 600 flange that is rated at 1480 psi at 100°F may only be rated at 930 psi at 700°F.
The Six Standard ASME Pressure Classes
Class 150
The lightest standard class. Used in low-pressure services such as cooling water, instrument air, and low-pressure steam. Carbon steel Class 150 flanges are rated at approximately 285 psi at ambient temperature. The relatively thin flange face and bolt pattern make these the most economical option but unsuitable for anything beyond moderate service.
Class 300
Rated at approximately 740 psi at ambient for carbon steel. Common in general process piping, moderate-pressure steam, and water systems that need more margin than Class 150 provides. Class 300 is the next step up when the system pressure or thermal cycling rules out Class 150.
Class 600
Approximately 1480 psi at ambient for carbon steel. Used in high-pressure process lines, medium-pressure steam, and hydrocarbon systems. Class 600 flanges are substantially heavier than 300 class and require larger-diameter, higher-torque bolting.
Class 900
Approximately 2220 psi at ambient for carbon steel. Common in high-pressure steam, high-temperature hydrocarbon service, and chemical plant applications. Less common than 600 and 1500 because many designers jump from 600 to 1500 rather than specifying 900.
Class 1500
Approximately 3705 psi at ambient for carbon steel. Found in high-pressure gas, hydrogen, and supercritical steam applications. Flanges at this class are heavy and bolt patterns require careful torquing sequences to achieve proper gasket seating.
Class 2500
The highest standard class under ASME B16.5, rated at approximately 6170 psi at ambient for carbon steel. Used in ultra-high-pressure services — high-pressure injection systems, wellhead equipment, and specialty chemical processes. Flanges at Class 2500 are extremely massive and expensive, and are only specified when operating conditions genuinely require it.
Material Groups and Their Effect on Ratings
ASME B16.5 divides flange materials into material groups (1.1, 1.2, 1.3, 2.1, 2.2, etc.) based on allowable stress. Within the same pressure class, different material groups have different allowable pressures:
- Group 1.1 — Carbon steel (A105, A515 Gr 70). The reference group; highest ratings within each class.
- Group 1.9 — Austenitic stainless steels (304, 316, 321). Lower strength than carbon steel; ratings are 15–20% lower than Group 1.1 at room temperature.
- Group 2.3 — Chrome-moly alloys (F11, F22). Intermediate ratings, generally better than stainless at elevated temperatures.
- Group 3.2 — Austenitic stainless steels with higher allowables (316H, 304H). Higher ratings at elevated temperature than standard 316/304.
How to Determine the Right Pressure Class
The selection process follows a straightforward sequence. First, establish the design pressure and design temperature for the piping system. Design conditions typically include a margin above normal operating conditions — consult the applicable piping design code (ASME B31.1, B31.3, etc.) for guidance on margins.
Second, select the flange material based on fluid compatibility, temperature range, and project specifications. Third, look up the pressure-temperature table in ASME B16.5 for that material group and find the class where the rated pressure at the desion temperature exceeds the design pressure. That is your minimum pressure class.
It is common practice to move up one class from the calculated minimum as a margin, particularly in systems with significant pressure transients, steam hammer, or water hammer potential.
Hydrostatic Test Pressure
Flanges must also handle hydrostatic test pressure, which is typically 1.5 times the design pressure per ASME codes. Ensure that the selected pressure class can accommodate test conditions without exceeding the pressure-temperature rating at the test temperature (usually ambient).
Large Diameter Flanges: ASME B16.47
For flanges larger than NPS 24, ASME B16.47 applies. This standard covers Classes 75, 150, 300, 400, 600, and 900 in both Series A (MSS SP-44) and Series B (API 605). The two series are not interchangeable — always confirm which series is specified on drawings and purchase orders for large-bore flanges.
Frequently Asked Questions
Does pressure class change with pipe size? No. Pressure class is independent of pipe size within ASME B16.5. A Class 300 flange has the same pressure-temperature rating whether it is NPS 2 or NPS 20, assuming the same material group.
What does the pound symbol (#) mean on a flange? The # symbol is shorthand for the pressure class designator. "#150" and "Class 150" refer to the same thing. The number does not directly represent pounds per square inch.
Can I use a higher class flange in a lower-pressure system? Yes. A Class 600 flange will work perfectly in a Class 150 service. The only downsides are higher cost and heavier weight. In practice, engineers sometimes specify a higher class for future flexibility or because it is what is available.
Conclusion
Pressure ratings for piping flanges are not a single number — they depend on the material group, the operating temperature, and the applicable standard. Always consult the pressure-temperature tables in ASME B16.5 (or B16.47 for large-bore flanges) and apply an appropriate design margin. When in doubt, go up a class.
Need pressure class guidance for your project? Visit Texas Flange & Fittings — we stock all pressure classes and can provide MTRs and pressure-temperature data sheets on request.