What Is Flange Pressure Rating?
A flange pressure rating is the maximum allowable working pressure (MAWP) for a given flange class and material at a specified temperature. As temperature increases, the material strength decreases, so the maximum allowable pressure drops accordingly. Each ASME B16.5 pressure class corresponds to specific flange dimensions, bolt circle sizes, number of bolts, and bolt diameters. Higher classes feature thicker flanges, larger bolt circles, and more bolts to withstand greater internal pressure.
The pressure-temperature rating is material-dependent. Carbon steel (ASTM A105), stainless steel (ASTM A182 F304/F316), and alloy steel (ASTM A182 F11/F22) all have different rating curves for the same class. When selecting a pressure class, you must check the specific rating table for your chosen material at the maximum design temperature.
ASME B16.5 Pressure Class Overview
| Class | Max Pressure at Ambient (psi) | Typical Applications | Common Facing |
|---|---|---|---|
| 150 | 285 | Low-pressure water, air, steam | RF |
| 300 | 740 | Medium-pressure process | RF |
| 400 | 990 | Medium-high pressure | RF |
| 600 | 1480 | Hydrocarbon processing | RF/RTJ |
| 900 | 2220 | Refinery, petrochemical | RTJ |
| 1500 | 3705 | Feedwater, injection | RTJ |
| 2500 | 6170 | Hydroprocessing | RTJ |
Class 150
Class 150 is the lowest pressure class defined by ASME B16.5, with a maximum allowable pressure of approximately 285 psi at ambient temperature. These flanges are the most economical option for non-critical applications including low-pressure water, compressed air, low-pressure steam, and general utility services. Class 150 flanges are widely available in all material grades and facing types. However, the pressure rating drops significantly above 500°F, so they are not suitable for high-temperature service.
Class 300
Class 300 flanges offer a maximum pressure of approximately 740 psi at ambient temperature. They are common in medium-pressure process applications across refineries, chemical plants, and power generation facilities. Class 300 provides a good balance of cost and performance, serving as a bridge between low-pressure and high-pressure systems. They are available with both raised face and ring joint facings, though raised face is standard for most applications at this level.
Class 600
Class 600 flanges handle up to 1480 psi at ambient temperature and are the standard choice for medium-to-high-pressure hydrocarbon service. They are widely used in oil and gas gathering, transmission, and processing facilities. The thicker flange body and larger bolt circle require more bolting torque than lower classes. Class 600 is often the transition point where ring type joint facings become common, though raised face is still used for many applications.
Class 900
Class 900 flanges are rated for approximately 2220 psi at ambient temperature and are used in high-pressure refinery and petrochemical applications. Heavy-wall pipe compatibility typically requires weld neck flanges at this class. Ring type joint facings are standard for Class 900 to ensure leak-tight metal-to-metal sealing. High-strength bolting materials such as ASTM A193 B7 are required to achieve the necessary bolt preload.
Class 1500
Class 1500 flanges provide a maximum pressure of approximately 3705 psi at ambient temperature. They are specified for very high-pressure service including boiler feedwater, chemical injection, and high-pressure gas systems. Large bolt diameters and high torque requirements demand hydraulic torque tools for proper assembly. Alloy steel and stainless steel materials are commonly used at this class to maintain strength at elevated temperatures.
Class 2500
Class 2500 represents the highest standard pressure class in ASME B16.5, with a maximum allowable pressure of approximately 6170 psi at ambient temperature. These flanges have the thickest cross-section of any class and are used in extreme pressure applications such as high-pressure hydroprocessing reactor circuits. They require specialized welding procedures, post-weld heat treatment, and extensive nondestructive examination. Class 2500 flanges are almost exclusively weld neck type with ring joint facings.
EN 1092-1 PN Rating Comparison
The European standard EN 1092-1 uses PN (Pressure Nominale) ratings instead of ASME classes. Common PN ratings include PN6, PN10, PN16, PN25, PN40, PN63, PN100, PN160, PN250, and PN400. The PN value approximates the maximum pressure in bar at ambient temperature. ASME Class 150 is roughly equivalent to PN20, Class 300 to PN50, and Class 600 to PN100. However, dimensional differences between the standards prevent direct interchangeability, and pressure-temperature curves differ at elevated temperatures.
| ASME Class | Approx. EN PN Equivalent | Max Pressure (bar) |
|---|---|---|
| 150 | PN20 | 20 |
| 300 | PN50 | 50 |
| 600 | PN100 | 100 |
| 900 | PN150 | 150 |
| 1500 | PN250 | 250 |
| 2500 | PN400 | 400 |
How to Select the Right Pressure Class
Determine the design pressure and temperature from your process conditions, including any surge, hydrostatic test, or transient conditions that could exceed normal operation. Select a material group and verify its pressure-temperature limits against the design conditions. Always account for future capacity increases or derating requirements. The key takeaway is to always check the pressure-temperature rating for your chosen material at the maximum design temperature, never rely on the class number alone.
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