ANSI/ASME B16.5 and B16.47 standards define standardized pressure–temperature limits for flanged piping systems. Each pressure class corresponds to a specific rating that varies based on material group and operating temperature.
For industrial buyers, the basic rule is simple: the higher the pressure class, the greater the pressure capability, but the correct class must always be selected according to the actual design conditions.
What Are Flange Pressure Classes?
ANSI / ASME pressure classes establish standardized limits governing how flanged piping systems perform under combined pressure and temperature conditions. These classifications are applied across industries such as oil and gas, petrochemical processing, power generation, water treatment, and heavy industrial service.
- ASME B16.5 includes pressure classes 150, 300, 400, 600, 900, 1500 and 2500. For the six classes commonly specified in industrial purchasing, Class 150 is the lower-pressure option, while Class 2500 is intended for much higher-pressure service.
- ASME B16.47 is limited to Classes 75, 150, 300, 400, 600, and 900, intended for large-diameter pipelines.
Note: A Class 600 flange under B16.5 and B16.47 has the same pressure rating, but dimensions and bolt patterns differ, meaning they are not interchangeable.
Standard ANSI / ASME Pressure Classes
| ASME Pressure Class | Typical Pressure Range | Common Applications and Service Characteristics |
|---|---|---|
| Class 150 | Low pressure (up to ~285 psi) | Water systems, HVAC, low-pressure steam, fire protection, and general industrial piping. Commonly used where operating pressures and temperatures are relatively low. |
| Class 300 | Moderate pressure (up to ~740 psi) | Process piping, compressed air, hydrocarbons, and light chemical service. Widely used in industrial facilities which require higher pressure capability than Class 150. |
| Class 400 | Intermediate pressure (up to ~990 psi) | A less common class typically specified for specialized or legacy systems where Class 300 is insufficient but Class 600 is not required. |
| Class 600 | High pressure (up to ~1480 psi) | Oil and gas processing, steam systems, chemical plants, and power generation. Designed for elevated pressures and temperatures with heavier flange construction. |
| Class 900 | Very high pressure (up to ~2220 psi) | Refinery service, high-pressure process lines, and severe hydrocarbon applications. |
| Class 1500 | Severe pressure and temperature (up to ~3705 psi) | High-pressure reactors, critical steam systems, and aggressive chemical environments. Requires careful material and bolting selection. |
| Class 2500 | Extremely high pressure (up to ~6170 psi) | Specialized ultra-high-pressure systems with strict engineering controls and limited application scope. |
* Pressure values shown are approximate reference values at a temperature of 100°F for ASTM A105 carbon steel.
Class 150 Flanges
Class 150 flanges are commonly used for relatively low-pressure industrial piping systems. Typical applications include water systems, general utilities, HVAC, fire protection and some process piping.
Class 150 is often selected when the operating pressure and temperature are within its allowable pressure-temperature rating. Choosing a higher class without a technical requirement can increase flange weight and project cost without providing a practical benefit.
Class 300 Flanges
Class 300 flanges provide a higher pressure capability than Class 150 and are widely used in process plants, refineries, chemical facilities, steam systems and general industrial piping.
For distributors and engineering contractors, Class 300 is one of the commonly requested flange ratings because it offers a practical balance between pressure capability, weight and cost.
Class 600 Flanges
Class 600 flanges are designed for higher-pressure piping applications and are frequently specified for oil and gas, petrochemical, refinery, power generation and high-pressure process systems.
ASTM A105 forged carbon steel is a common material choice for Class 600 flanges, subject to the project material specification and pressure-temperature requirements.
Class 900 Flanges
Class 900 flanges are intended for high-pressure industrial service where Class 600 is not sufficient.
They can be found in demanding oil and gas, petrochemical, power and process applications. At this pressure level, correct flange material, gasket selection, bolting and installation are particularly important.
Class 1500 Flanges
Class 1500 flanges are used for very high-pressure service, including demanding oil and gas and process applications.
These flanges are substantially heavier than lower-rated classes. Therefore, Class 1500 should be selected based on the actual design pressure and temperature rather than simply choosing a higher rating as a safety margin.
Class 2500 Flanges
Class 2500 flanges represent one of the highest pressure classes covered by ASME B16.5 and are intended for specialized, extreme-pressure applications.
They are typically associated with high-pressure process systems, oil and gas equipment and other applications where standard lower-pressure flange classes cannot meet the required design conditions.
For Class 2500 purchasing, the complete specification should be confirmed carefully, including NPS, material grade, facing, bore, pressure-temperature rating and applicable standard.
Does a Higher Flange Class Always Mean Higher Working Pressure?
No.
This is one of the most important points when specifying industrial flanges.
The pressure rating changes with temperature, and different materials have different pressure-temperature ratings. For example, an ASTM A105 carbon steel flange rated at a particular pressure at ambient temperature will have a lower allowable pressure as temperature increases.
Therefore, flange selection should consider:
- Design pressure
- Design temperature
- Flange material
- Nominal pipe size
- Flange type
- Facing type
- Applicable piping standard
- Gasket and bolting requirements
How to Choose the Right Flange Pressure Class
| Factor | |
| Operating Pressure | Identify the maximum operating pressure and maximum operating temperature for the needed piping system. These values should reflect the most severe expected service conditions (including system testing, possible surges, or system upsets), rather than normal operating averages. |
| Flange Material |
Choose a flange material compatible with the process fluid (e.g., A105 carbon steel or F316 stainless), then identify its corresponding material group as defined in ANSI/ASME B16.5. Before finalizing the selection, carefully review the “Notes” section within the standard; these contain critical limitations on temperature ranges and specific service conditions that may restrict a material’s allowable use.
B16.5 and B16.47 generally only permit forged, one-piece flanges. The exceptions are blind flanges or reducing flanges without hubs, which may be fabricated from plate material (ex. ASTM A240 316/316L).
|
| Environment | Navigate to the table corresponding to your Material Group. Locate your Design Temperature and move across the row until you find a pressure rating that exceeds your Design Pressure. The pressure class identified through this process defines the flange’s allowable pressure-temperature rating for the specified material and service conditions. |
Final Takeaway
Class 150, 300, 600, 900, 1500 and 2500 are not simply pressure values in psi. They are standardized pressure classes whose actual allowable pressure depends on temperature, material and the applicable ASME requirements.
For buyers comparing Class 150 vs Class 300, Class 300 vs Class 600, or Class 600 vs Class 900/1500/2500, the right approach is to match the flange class to the project’s actual pressure-temperature conditions rather than selecting the highest class available.
JS FITTINGS supplies industrial flanges including Weld Neck, Slip On, Blind, Plate and Socket Weld flanges, with pressure classes and dimensions manufactured according to applicable international standards and project requirements.
Need help specifying the right industrial flange? Confirm your size, material, pressure class, flange type and application with JS FITTINGS before ordering.
Email: admin@jsfittings.com
WhatsApp: +86 180 0311 9682
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