Specifying the wrong flange pressure class is one of the more costly procurement errors in industrial piping. A flange that is underrated for system pressure will eventually fail. One that is over-specified drives up costs, weight, and installation complexity without adding any additional function. Understanding what ANSI flange classes represent and how ASME B16.5 translates those class designations into working pressure limits across different materials and temperatures is key to getting installation right.
ASME B16.5 defines pressure-temperature ratings for pipe flanges in sizes NPS 1/2 through NPS 24 and establishes seven pressure classes: 150, 300, 400, 600, 900, 1500, and 2500. Each class corresponds to a different range of maximum allowable working pressures depending on the flange material and the operating temperature of the system. These class numbers are not pressure limits by themselves. Class 150 does not mean 150 psi. The number is a class label, and the actual rated pressure is determined by looking up the applicable material group and temperature in the ASME B16.5 pressure-temperature tables. Procurement teams sourcing industrial flanges and flange bolts need to understand these distinctions before placing any order against a piping specification.
What ASME B16.5 Actually Governs
ASME B16.5 is the primary standard for pipe flanges and flanged fittings used in pressure-containing systems. Published by the American Society of Mechanical Engineers, it establishes dimensional requirements, material group classifications, pressure-temperature rating tables, facing type specifications, bolting requirements, and marking requirements for flanges from NPS 1/2 through NPS 24. For large-bore flanges above NPS 24, the governing standard shifts to ASME B16.47.
The standard is referenced by major piping codes, including the ASME B31.3 process piping code and ASME B31.1 for power piping. Any flange used in code-governed piping must conform to the dimensional tolerances and pressure-temperature ratings of B16.5 for the applicable class and material group. Procurement documentation should always reference the standard, the class, and the material group together. A class designation alone does not provide enough information to confirm suitability for a given service condition.
The Seven Standard Pressure Classes at a Glance
ASME B16.5 defines seven pressure-temperature rating classes. Each class represents a progressively higher pressure capability, reflected in heavier wall sections, larger bolt circles, more bolts per joint, and greater overall joint weight. The table below shows approximate maximum allowable working pressures at two common temperature reference points for Group 1.1 carbon steel, which includes materials such as ASTM A105 forgings and ASTM A350 LF2.
| Flange Class | Approx. Max Pressure at 100°F | Approx. Max Pressure at 400°F |
| Class 150 | 285 psi | 230 psi |
| Class 300 | 740 psi | 600 psi |
| Class 400 | 985 psi | 800 psi |
| Class 600 | 1,480 psi | 1,200 psi |
| Class 900 | 2,220 psi | 1,795 psi |
| Class 1500 | 3,705 psi | 2,995 psi |
These values are representative of ASME B16.5 tabulated ratings for Group 1.1 and are shown here for comparison purposes. Actual maximum allowable pressures must be confirmed against the current edition of the standard for the specific material grade being used. Class 2500 extends beyond this range, reaching approximately 6,170 psi at ambient for Group 1.1 carbon steel, and falls outside the typical specification range covered here. Classes 150 through 1500 account for the vast majority of industrial applications, from low-pressure water service to high-pressure gas and process systems.
How Pressure-Temperature Ratings Work
The most frequently misunderstood aspect of ANSI flange classes is that the pressure rating is not a fixed number. It is temperature-dependent. As operating temperature increases, the maximum allowable working pressure for any given class decreases. This inverse relationship is built directly into the ASME B16.5 pressure-temperature tables, and it applies to every material group.
A Class 150 carbon steel flange rated at 285 psi at 100°F drops to approximately 230 psi at 400°F and as low as 140 psi at 700°F. If a system is operating near elevated temperature limits, the effective pressure capacity of a Class 150 flange may be far below what a buyer expects when reading only the class number. The operating temperature must always be evaluated alongside the operating pressure.
The practical implication is that temperature-driven derating can push a specification into a higher class than the operating pressure alone would require. A steam distribution system running at 250 psi and 600°F would actually exceed the Class 150 pressure-temperature rating at that temperature and would require, at a minimum, a Class 300 flange from the appropriate material group. This is not an edge case. It comes up regularly in refinery, power generation, and chemical process environments where elevated temperatures are routine.
Material Groups and Their Impact on Flange Ratings
ASME B16.5 assigns flange materials into numbered material groups, and each group carries its own set of pressure-temperature ratings for every class. The most commonly referenced groups are Group 1.1 for carbon steel forgings such as ASTM A105 and ASTM A350 LF2, Group 1.2 for low and intermediate-alloy steels, and Group 2.1 for austenitic stainless steel grades such as 304 and 316.
Austenitic stainless steel flanges in Group 2.1 generally have lower pressure ratings at elevated temperatures compared to carbon steel flanges of the same class. A Class 300 stainless flange in Group 2.1 is rated lower at 700°F than a Class 300 carbon steel flange in Group 1.1 at the same temperature. This gap widens at higher temperatures, making material group selection a consequential part of the specification process in high-temperature service.
Specialty alloy materials used in sour gas service, hydrogen service, cryogenic applications, and high-corrosion environments fall into their own assigned groups with distinct ratings. Buyers working in offshore, refinery, or aggressive chemical service should always confirm the exact material group and consult the corresponding P-T table in ASME B16.5 before finalizing class selection. A class number without a material group is an incomplete specification.
Selecting a Pressure Class Based on Operating Conditions
Matching a flange class to a given application requires three known inputs: maximum operating pressure, maximum operating temperature, and the flange material or material group. The appropriate pressure class is the lowest class whose rated pressure at the operating temperature meets or exceeds the system’s maximum operating pressure, with sufficient margin to satisfy the applicable piping code.
Class 150 flanges are typically used in low-pressure utility services, water distribution systems, non-critical process lines, and applications operating at moderate pressure and ambient or near-ambient temperatures. Class 300 handles medium-pressure service and is common in process plants, refinery piping, and gas handling systems. Class 600 is frequently specified in gas transmission pipelines, high-pressure steam systems, and upstream oil and gas production, where both pressure and thermal demands increase. Class 900 and Class 1500 appear in high-pressure wellhead systems, critical service applications, and processes where cyclic loading and fatigue resistance are primary design concerns.
Beyond pressure and temperature, class selection also drives downstream decisions. Higher class flanges require larger bolt diameters, more studs per joint, and heavier gasket types. These factors affect total joint weight, torquing procedures, maintenance access, and installed cost. For a petrochemical transfer line, the total installed cost difference between a Class 600 and Class 300 joint is significant and should be evaluated at the design stage. Proper class selection from the outset is what supports a leak-free industrial piping system across its full service life.
Flange Face Types and Bolting Requirements by Class
Pressure class influences both the appropriate flange face type and the bolting grade required to achieve the designed joint load. These are not independent decisions. Face type, gasket selection, and bolting must all be compatible with the class and service conditions.
Face Type Considerations
Raised face (RF) is the standard facing across all ASME B16.5 classes and is used in the majority of industrial flanged connections. The concentrated gasket seating area of a raised face joint performs reliably across a wide range of service conditions. Ring-type joint (RTJ) flanges are used in higher-pressure and high-temperature service, most commonly in Classes 600, 900, and 1500 in oil and gas production and refinery applications. The metallic ring gasket used in RTJ connections provides a more reliable seal under high stress and thermal cycling. Flat face (FF) flanges are used in Class 150 and Class 300 service where the mating equipment, such as cast iron valves or pumps, cannot withstand the unbalanced bolt load imposed by a raised face connection.
Bolting Grade and Torque Requirements
Bolting requirements increase with class. Carbon steel piping across Class 150 through 300 typically specifies studs and nuts to high-pressure bolting material standards, with ASTM A193 B7 alloy steel studs being the most common grade for general carbon steel flange service. A194 Grade 2H heavy hex nuts are the standard pairing. Class 600 and above in demanding service may require the same B7 grade but with closer attention to torque specification, stud stretch, and bolt pattern sequence to achieve uniform gasket seating. Stainless steel and corrosion-resistant alloy piping systems typically use ASTM A193 B8 or B8M studs paired with compatible nut grades. The application of ANSI flange ratings to specific service environments also affects bolt coating, end treatment, and thread lubricant selection, all of which should align with the specified pressure class and the process fluid being handled.
Flanges and Bolting Supplied to Industrial Specifications
Coastal Resources Group supplies industrial flanges and flange bolts across the full range of ASME B16.5 pressure classes, serving petrochemical, midstream, downstream, oilfield, construction, and water treatment operations across Texas and nationally. Every order is matched to the full specification: material group, pressure class, face type, and dimensional standard, not just the class number on the requisition.
Here is what that supply relationship covers:
- Flanges from Class 150 through Class 1500 in carbon steel, stainless steel, and specialty alloy material groups
- Raised face, flat face, and ring-type joint (RTJ) facing options matched to pressure class and service conditions
- Consolidated flange bolting packages, including ASTM A193 B7 and B8 stud bolts with compatible A194 nut grades, sourced alongside the flanges in a single order
- 24/7 sales availability with fast logistics, including dedicated trucking and air freight for urgent field requisitions
- Delivery to remote oilfield and job site locations across Texas and beyond, with no minimum order friction
Pressure class selection matters, from the first line of the specification to the last. Contact us today to request a quote on flanges and bolting rated to the class and material requirements of the job.