Flange Ratings Explained: How to Select the Right Class for Your Piping System

What Is a Flange Rating? A Plain-Language Overview

A flange is a disc-shaped mechanical fitting that connects two sections of pipe, a valve, or equipment. It works by bolting two matching flanges together with a gasket in between, forming a tight, pressure-resistant seal that can later be unbolted for inspection or maintenance.

Every flange carries a pressure class - a number that tells engineers the maximum pressure the joint can safely handle at a given temperature. The higher the class number, the higher the allowable pressure. Getting this number wrong can lead to catastrophic failure, regulatory non-compliance, or costly downtime.

Flange pressure classes in North America are standardized by the American Society of Mechanical Engineers (ASME). The governing document for pipe flanges is ASME B16.5 (NPS 1/2 through NPS 24).

This bulletin explains what each class number means, how pressure ratings change with temperature, which face types and materials to pair with each class, and how to match a flange class to your actual operating conditions.

Governing Standards: Who Sets the Rules?

Industrial flanges are not designed by individual engineers from scratch. They are manufactured to well-established dimensional and pressure standards, which define everything from bolt-hole patterns to pressure–temperature ratings.

ASME B16.5 - The Primary Standard

ASME B16.5, Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24, is the dominant standard in North America and widely adopted internationally. It covers seven pressure classes - 150, 300, 400, 600, 900, 1500, and 2500 - and defines pressure–temperature (P-T) ratings for 22 material groups. All comparative data in this bulletin for pressure ratings is sourced from ASME B16.5-2017.

ASME B16.47 - Large-Diameter Flanges

For flanges NPS 26 through NPS 60, ASME B16.47 applies. It defines two series: Series A (derived from MSS SP-44) and Series B (derived from API 605). Series A flanges are heavier and suited to pipeline systems; Series B are lighter, common in process plant use.

ASME B16.20 - Ring Joint Gaskets

ASME B16.20 governs metallic gaskets for Ring Type Joint (RTJ) flanges - the oval and octagonal rings used in high-pressure, high-temperature, and sour gas applications.

ASME B31.3 - Process Piping Code

ASME B31.3 specifies design, fabrication, and testing requirements for process piping. It assigns piping service categories (B through F) that directly influence minimum flange class requirements. Engineers must align their flange class selections with the B31.3 fluid category of the service.

NACE MR0175 / ISO 15156 - Sour Service

For services containing hydrogen sulfide (H2S) - known as sour service - NACE MR0175 (now published jointly as ISO 15156) mandates specific material hardness limits, heat treatment requirements, and material selection criteria that supersede standard pressure class decisions.

Seven ASME Pressure Classes: What the Numbers Really Mean

A pressure class is not a single pressure value - it is a family of pressure–temperature curves. The allowable pressure decreases as temperature increases, because metals become less strong at higher temperatures. This is why the same Class 150 flange rated at 285 psi at 100°F can only handle 170 psi at 800°F.

Class 150

Class 150 is the lowest and most common flange class, used in water systems, HVAC, low-pressure steam, and light-duty process lines. Its maximum rated pressure is 285 psi at ambient temperature (100°F) for carbon steel A105. Class 150 uses 4-bolt patterns on smaller sizes. It is the most cost-effective choice and should only be selected when operating conditions clearly fall within its P-T envelope.

Class 300

Class 300 roughly doubles the pressure capacity of Class 150 (740 psi at 100°F for A105). It is the standard choice for medium-pressure process services, steam systems, and general refinery applications. Class 300 flanges are physically larger and heavier than Class 150.

Class 400

Class 400 is less commonly specified - it sits between 300 and 600 and is used mainly in elevated-temperature steam applications where the Class 300 P-T envelope is exceeded but Class 600 would be over-engineered. Maximum pressure is 990 psi at ambient for A105.

Class 600

Class 600 (1,480 psi at ambient, A105) is the standard class for high-pressure process piping, refinery units, and compressed gas systems. It is often the minimum class specified in NACE sour-gas service. Class 600 and above typically use Ring Type Joint (RTJ) face to achieve a metal-to-metal seal.

Class 900

Class 900 (2,220 psi at ambient) is used in natural gas pipelines, high-pressure steam, and other demanding process services. Its larger bolt pattern and greater wall thickness provide a significant safety margin for cyclic or high-vibration applications.

Class 1500

Class 1500 (3,705 psi at ambient) is specified for power-generation boiler feedwater lines, high-pressure steam turbines, and similar severe applications. Flanges at this class are significantly heavier and require precision machined gasket seating surfaces.

Class 2500

Class 2500 (6,170 psi at ambient) is the highest class defined in ASME B16.5. It is used in hydrogen service, ultra-high-pressure process lines, and wellhead connections. Class 2500 flanges are extremely heavy and costly. Any specification at this level must be reviewed by a licensed pressure vessel engineer.

Table 1: ASME B16.5 Pressure–Temperature Ratings - Carbon Steel A105 and 316L Stainless Steel

Table 2: Flange Class Comparison - Pressure, Weight, Cost, and Application

Flange Face Types: How Sealing Is Achieved

The face of a flange is the machined surface on which the gasket sits. Different face types are suited to different pressure levels, fluid types, and gasket materials. Mismatching face types - for example, bolting a raised face flange against a flat face cast-iron valve - can cause the cast-iron flange to crack under bolt load.

Raised Face (RF)

The most common face type. A concentric circular raised area (typically 1/16 inch high for Class 150/300, 1/4 inch for higher classes) concentrates gasket load. Used with spiral wound or ring gaskets in most process applications.

Flat Face (FF)

The flange face is flush with the pipe end. Used when mating to equipment with flat-face flanges, such as pumps with cast iron bodies or ASME Class 125 equipment. Requires a full-face gasket to distribute bolt load and prevent brittle equipment flanges from cracking.

Ring Type Joint (RTJ)

A groove machined into the flange face accepts a solid metal ring gasket (oval or octagonal cross-section). The ring deforms under bolt load to create a metal-to-metal seal suitable for Class 600 and above, high-temperature steam, and H2S sour service. Governed by ASME B16.20.

Tongue and Groove (T&G)

One flange has a raised ring (tongue); the mating flange has a matching recess (groove). The gasket is fully confined and cannot be blown out, making this face type preferred for toxic, lethal, or highly volatile services. Used for pump covers and vessel nozzles.

Male and Female (M&F)

Similar to T&G but with a flat gasket ring seated in the female face recess. Used in critical or lined piping systems where gasket position must be precisely controlled.

Table 3: Flange Face Types - Standards, Gaskets, and Applications

Flange Materials: Matching Metal to the Process

The correct pressure class alone is insufficient to define a flange. The material must be compatible with the process fluid, operating temperature, corrosion environment, and any applicable code requirements. ASME B16.5 organizes flange materials into numbered groups, each with its own P-T rating tables.

Carbon Steel Flanges (Groups 1.1 and 1.2)

ASTM A105 (forged) is the most widely used flange material globally. It covers ambient to moderate temperature service in non-corrosive or mildly corrosive media. For low-temperature service (down to -50°F), ASTM A350-LF2 (Group 1.2) is specified, with impact-tested forgings as required by ASME B31.3.

Stainless Steel Flanges (Groups 2.2, 2.3, 2.4)

316L (A182-F316L, Group 2.3): The industry standard for corrosion-resistant flanges. The low-carbon 'L' grade prevents weld sensitization. Suitable for chemical processing, pharmaceutical, food/beverage, and marine splash zone environments. PREN (Pitting Resistance Equivalent Number) approximately 24.

317L (A182-F317L, Group 2.3): Higher molybdenum content than 316L improves resistance to sulfuric and acetic acids. Specified in pulp and paper mills and acid-handling systems. PREN approximately 28.

904L (A182-F904L, UNS N08904, Group 2.4): A super-austenitic grade with 4.5% Mo and 1.5% Cu, specifically designed for sulfuric acid resistance. Outperforms 316L significantly in dilute H2SO4. PREN approximately 36. EETA stocks and supplies 904L flanges.

254 SMO / F44 (A182-F44, UNS S31254, Group 2.4): A 6% molybdenum super-austenitic grade offering outstanding resistance to seawater pitting, crevice corrosion, and chloride-induced stress corrosion cracking (SCC). PREN approximately 43. Used extensively in desalination, offshore, and bleach plant service. EETA stocks and supplies 254 SMO flanges.

Nickel Alloy Flanges (Groups 3.1–3.4)

Monel 400 (B564-N04400, Group 3.2): Outstanding resistance to hydrofluoric acid and seawater. Used in HF alkylation units, marine pump trim, and saltwater desalination.

Hastelloy C-276 (B564-N10276, Group 3.3): The benchmark material for severe corrosion service - aggressive acids, wet chlorine, flue gas desulfurization (FGD). Rated to 1,000°F. EETA supplies C-276 flanges to Class 150 through 1500.

Inconel 625 (B564-N06625, Group 3.3): Exceptional high-temperature strength and oxidation resistance to 1,800°F. Specified in offshore subsea, aerospace-grade chemical plants, and high-temperature gas service. EETA supplies Inconel 625 flanges in all standard classes.

Table 4: Flange Material Groups, ASTM Specifications, and Corrosion Performance

How to Select the Right Flange Class: A Step-by-Step Decision Framework

Selecting the correct flange class requires a systematic approach. Engineers must not simply round up to the next class - both over-specification (unnecessary cost and weight) and under-specification (safety failure) are serious engineering errors.

Step 1 - Define Maximum Allowable Operating Pressure (MAOP)

Establish the highest pressure the system can reach, including surge, upset, and relief valve set conditions. Add a design margin per your applicable code (typically 10–33% above normal operating pressure).

Step 2 - Define Maximum Allowable Operating Temperature (MAOT)

Determine the maximum process temperature and account for excursions. Remember that pressure ratings decrease with temperature - always check the P-T table at your MAOT, not at ambient.

Step 3 - Identify the Material Group

Select a flange material compatible with the process fluid. Use corrosion tables (NACE, Outokumpu Handbook, vendor data) to confirm chemical compatibility. Identify the corresponding ASME B16.5 material group number for your selected alloy.

Step 4 - Look Up the P-T Rating

Cross-reference your MAOP, MAOT, and material group in the ASME B16.5 P-T rating tables. Select the lowest class whose P-T rating exceeds your design pressure at design temperature.

Step 5 - Check Code Requirements

Confirm the selected class satisfies ASME B31.3 piping class minimums for the fluid category. For sour service, apply NACE MR0175 / ISO 15156 requirements. For fire case or lethal service, additional restrictions may apply.

Step 6 - Specify Face Type and Gasket

For Class 150–300 in clean, non-lethal service: Raised Face with spiral wound gasket is standard. For Class 600 and above, or in H2S, high-temperature steam, or lethal service: Ring Type Joint (RTJ) is preferred. Consult ASME PCC-1 for gasket seating stress requirements.

Table 5: Flange Class & Material Selection Matrix by Service Condition

Frequently Asked Questions

Q1: Can I mate a Class 300 flange to a Class 150 flange?

Answer  You can physically bolt them together since bolt hole patterns match within certain size ranges, but the assembly is rated to the lower class (Class 150). This is sometimes done intentionally during a system upgrade, but the lower-class component becomes the limiting factor. ASME B31.3 requires the joint to be rated to the weakest component.

Q2: What does the 'L' in 316L mean, and does it matter for flanges?

Answer  The 'L' stands for Low Carbon - maximum 0.03% carbon content versus 0.08% for standard 316. Low-carbon grades prevent chromium carbide precipitation in the heat-affected zone during welding, which causes a condition known as sensitization that dramatically reduces corrosion resistance. For welded flange connections, always specify the L-grade.

Q3: What is PREN and why does it matter for flange selection?

Answer  PREN (Pitting Resistance Equivalent Number) is a formula-based index that predicts a stainless steel's resistance to pitting corrosion: PREN = %Cr + 3.3 × %Mo + 16 × %N. A PREN above 40 is typically required for submerged seawater service. For context: 316L has PREN ~24, 904L has PREN ~36, and 254 SMO has PREN ~43. Selecting a flange material solely by pressure class without considering PREN in corrosive services leads to premature failure.

Q4: When is a Ring Type Joint (RTJ) flange required?

Answer  RTJ flanges are required or strongly recommended in four scenarios: (1) Class 600 and above, (2) temperatures above 750°F (400°C), (3) H2S sour service per NACE MR0175, and (4) lethal or toxic fluid service. RTJ creates a metal-to-metal seal independent of gasket material degradation, providing superior blowout resistance.

Q5: What is the difference between ASME B16.5 and ASME B16.47?

Answer  ASME B16.5 covers pipe flanges from NPS 1/2 to NPS 24. ASME B16.47 covers large-diameter flanges from NPS 26 to NPS 60. B16.47 Series A flanges (from MSS SP-44) are heavier and used in pipelines; Series B flanges (from API 605) are lighter and used in process plants. If you are specifying flanges larger than NPS 24, you must reference B16.47.

Q6: Can a Class 150 flange handle full vacuum?

Answer  Yes - Class 150 flanges are routinely used in vacuum service. The concern in vacuum service is gasket blowout risk (which does not apply) and potential flange distortion under full vacuum differential. For deep vacuum (< 1 mmHg), consult the equipment manufacturer and verify the bolted joint design using ASME PCC-1 methodology.

Q7: How do I confirm the pressure rating if my material is not in ASME B16.5?

Answer  If your exact material is not listed, locate the closest ASME B16.5 material group based on minimum tensile and yield strength. In such cases, the rating may be calculated per ASME B16.5 Appendix II, or the flange may be designed as a special flange per ASME Section VIII, Division 1, Appendix 2. Always involve a licensed pressure vessel engineer for non-standard material applications.