What are the pressure ratings for pipe flanges according to ASME B16.5?

Selecting the correct flange pressure class is one of the most fundamental decisions in piping system design. ASME B16.5, the standard titled "Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24," defines the dimensional and pressure-temperature (P-T) requirements for steel pipe flanges used across oil and gas, petrochemical, power, marine, and process plant industries. Getting the pressure rating wrong leads to chronic joint leakage, gasket blowout, or in the worst case, a pressure-boundary failure that endangers personnel. This article explains the seven ASME B16.5 pressure classes, how P-T ratings work, and how to read the standard's tables so that you can choose the right flange for your service conditions.

A flange "class" is a dimensionless designation, not a pressure value. The seven classes recognized by ASME B16.5 are 150, 300, 400, 600, 900, 1500, and 2500. The actual allowable working pressure at any given temperature depends on two factors: the flange class and the material group. A Class 150 carbon steel flange, for example, is rated for 19.6 bar at 38 °C, but only 8.1 bar at 400 °C. This is because material strength decreases as temperature rises, and above roughly 370 °C, creep becomes a controlling failure mechanism for carbon steels. For a complete overview of how flanges are categorized by face type and application, see our guide on steel flanges and their material families.

What ASME B16.5 Governs and Its Companion Standard

ASME B16.5 applies to pipe flanges in sizes NPS 1/2 through NPS 24. For larger flanges (NPS 26 through NPS 60), ASME B16.47 Series A or Series B is used. Both standards use the same class designations, but dimensional requirements and bolt patterns differ and the components are not interchangeable. The standard is referenced by ASME B31.1 (Power Piping), ASME B31.3 (Process Piping), and API 570, making it the de facto basis for flange specification in most process plant engineering worldwide.

Section 303 of ASME B31.3 explicitly states that flanges manufactured to ASME B16.5 are deemed suitable for the pressure-temperature limits specified in the standard, eliminating the need for separate pressure calculations as long as the correct class is selected. The standard covers seven flange facing and hub configurations that all share the same P-T rating for a given class and material group:

• Weld Neck Raised Face (WNRF) – Full-penetration butt weld; the preferred choice for high-pressure, high-temperature, and cyclic service because the tapered hub provides a smooth stress transition between the flange body and the pipe wall.
• Slip-On Raised Face (SORF) – Two fillet welds; suitable for low-to-medium pressure utility service.
• Socket Weld Raised Face (SWRF) – Fillet weld on small-bore piping (typically NPS ≤ 3 in); used in high-pressure service.
• Threaded (Screwed) (TNRF) – Threaded engagement; for non-hazardous, low-temperature utilities.
• Lap Joint (LJRF) – Stub end plus loose backing flange; for frequent disassembly or lined pipe systems.
• Blind Raised Face (BLRF) – Used to blank off vessel nozzles and equipment connections.

ASME B16.5 Material Groups

The most important concept in ASME B16.5 pressure rating selection is the material group. The standard organises every acceptable flange material into groups based on their mechanical properties at elevated temperature. Flanges in the same group carry identical P-T ratings; flanges in different groups have different ratings even at the same class. There are three broad categories:

• Groups 1.1 through 1.18 – Carbon and low-alloy steels. Group 1.1 covers ASTM A105, A350 LF2, and A216 WCB, the standard carbon steel for general industrial use up to roughly 800 °F (427 °C).
• Groups 1.5 and 1.7 – Chromium-molybdenum alloys (5Cr-0.5Mo and 9Cr-1Mo), used in power plants and steam systems up to 1100 °F (593 °C). These map to ASTM A182 F5, F9, A335 P5, and P9.
• Groups 2.1 through 2.12 – Austenitic stainless steels. Group 2.1 covers 304/304H (ASTM A182 F304, A351 CF8), Group 2.2 covers 316/316H, and Group 2.3 covers low-carbon grades 316L and 304L.
• Groups 3.1 through 3.19 – Non-ferrous alloys including copper-nickel (90/10 and 70/30), Monel, Inconel, and titanium. For seawater service, our copper nickel pipe for marine use (EEMUA 234) is manufactured from these alloys.

A critical detail: low-carbon stainless grades (Group 2.3) carry lower allowable stresses than their standard-carbon counterparts (Groups 2.1 and 2.2) because low-carbon grades are annealed at lower temperatures and have reduced hot-strength properties. A Class 150 flange in Group 2.3 is rated to 13.8 bar at 38 °C versus 15.1 bar for Group 2.1. Never substitute Group 2.1 ratings for a Group 2.3 flange without verifying.

How Pressure-Temperature Ratings Are Calculated

The maximum allowable working pressure (MAWP) for an ASME B16.5 flange is not a single number; it is a function of both temperature and material group. For Class 300 and higher, the rated pressure at any temperature is calculated from the relationship defined in Appendix A of ASME B16.5:

P_t = (C₁ × S₁ × P_r) / 8750

Where P_t is the rated pressure at temperature t (bar or psi), C₁ is the stress multiplier for the material group, S₁ is the allowable stress at temperature t per ASME Section II Part D, and P_r is the pressure-temperature rating designation (300, 600, 900, etc.). For Class 150, a separate formula applies with a smaller denominator (140) reflecting its special dimensional basis. In practice, engineers do not need to apply these formulas directly; the standard presents the results in ready-to-use tabular form (Tables 2-1.1 through 2-3.19 in the SI appendix), and linear interpolation between listed temperature values is permitted. Interpolation between class designations, however, is not permitted, which is why selecting the next-higher class is the only safe approach.

Reference P-T Tables for the Most Common Material Groups

The following tables reproduce indicative rated pressures (bar gauge) at key temperatures for the three most common material groups in process plant engineering. These values are for orientation; always verify against the current edition of ASME B16.5 for actual design work.

Group 1.1 – Carbon Steel (ASTM A105, A216 WCB, A350 LF2)

Group 2.1 – Austenitic Stainless 304/304H (A182 F304, A351 CF8)

A useful observation from these tables: stainless steels retain their strength at high temperature much better than carbon steel. At 400 °C, a Class 150 Group 1.1 flange is rated to only 8.1 bar, while a Group 2.1 flange of the same class is still rated to 11.5 bar. For high-temperature piping systems, selecting a stainless or Cr-Mo alloy flange can permit a lower class rating and thinner, lighter components, which is one of the reasons why carbon, stainless, and alloy steel flanges are often specified by service temperature rather than pressure alone.

The Three-Step Class Selection Process

ASME B16.5 class selection follows a logical three-step procedure. This process is systematic and leaves no room for assumption or approximation between classes.

Step 1 – Confirm Design Pressure and Design Temperature. Use the design pressure and design temperature, not the normal operating values. Design conditions must represent the most severe credible combination of pressure and temperature that the system could experience simultaneously, including upsets, startups, shutdowns, and transient events such as blocked-in heating. Many field joint failures trace back to engineers using operating conditions when selecting flange class, then encountering a thermal trip or overpressure transient that the flange could not handle.

Step 2 – Identify the Material Group from Table 1A. Open ASME B16.5 Table 1A and find the ASTM material specification for your flange. The table lists the material group in the first column and the applicable P-T rating table number in the third column. Do not rely on memory or generic references for this step; the material group determines everything about your ratings.

Step 3 – Read the P-T Table and ｓｅｌｅｃｔ the Minimum Sufficient Class. In the P-T rating table for your material group, locate your design temperature in the left column. Read across the row, starting from Class 150 and moving right until you find the first class whose rated pressure equals or exceeds your design pressure. That is your minimum required class. If your design pressure exceeds the Class 2500 rating at the design temperature, the material is unsuitable for this service at that temperature and you must either ｓｅｌｅｃｔ a higher-strength material group or reconsider the design. Many engineering specifications apply a 1.25 times multiplier to the design pressure before selecting the flange class, particularly for cyclic service or applications with frequent thermal swings. This protects against bolt relaxation, gasket creep, and the uncertainty inherent in a stepped-class system where you must round up rather than interpolate.

Worked Example 1 – Carbon Steel, Steam Service

A process steam line requires a flange in ASTM A105 carbon steel. The design pressure is 50 bar(g) and the design temperature is 250 °C.

• Material: ASTM A105 → Group 1.1 (Table 1A of ASME B16.5).
• Design pressure (P_d): 50 bar(g). Design temperature (T_d): 250 °C.
• Class 150 at 250 °C → 12.1 bar. Insufficient.
• Class 300 at 250 °C → 43.4 bar. Insufficient.
• Class 600 at 250 °C → 86.9 bar. Sufficient.

Minimum required class: Class 600, with a pressure margin of 73.8 % above design. For steam service of this severity, the appropriate pipe itself must also be specified correctly; high-temperature seamless steel pipe such as ASTM A335 alloy steel pipe for high temperature and pressure is typically paired with Class 600 weld neck flanges.

Worked Example 2 – Stainless Steel, Chemical Service

A corrosive chemical service requires A182 F316L flanges. Design conditions are 25 bar(g) at 200 °C. Confirm the required class with a 1.25 safety factor.

• Material: A182 F316L → Group 2.3.
• Design pressure: 25 bar(g). Safety factor: 1.25. Effective design pressure = 25 × 1.25 = 31.25 bar.
• Class 150 at 200 °C (Group 2.3) → 11.3 bar. Insufficient.
• Class 300 at 200 °C (Group 2.3) → 29.8 bar. Insufficient.
• Class 600 at 200 °C (Group 2.3) → 59.6 bar. Sufficient.

Minimum required class: Class 600. Without the safety factor, Class 300 (29.8 bar) would appear to be sufficient at 25 bar. Applying the 1.25 safety factor correctly reveals that Class 600 is required. This is exactly the type of step where procurement teams save money by skipping the safety factor and end up facing joint failures in the field. Stainless flanges for chemical service pair naturally with ASTM B466 copper-nickel tubing or austenitic stainless pipe of the same material group for welded transitions.

The Special Case of Class 400

Class 400 occupies a unique place in ASME B16.5 history. The rating was carried in earlier editions of the standard but has seen diminishing use in new designs. In the 2003 edition, flanged end fittings conforming to Class 400 and higher were listed only in U.S. Customary units in Annex G of the standard, reflecting the limited commercial demand. New plants rarely specify Class 400 flanges today, and most procurement specifications simply round to either Class 300 or Class 600. If you encounter a Class 400 flange in service, verify the P-T rating data against the edition of ASME B16.5 that was current when the unit was built, as the rating values have been recalculated several times to reflect updated ASME Section II Part D allowable stresses.

Built-in Safety Margins and Conservative Design Practice

ASME B16.5 P-T ratings already incorporate conservative safety factors. The ratings are based on a 4:1 safety factor relative to tensile strength and account for stress concentration effects (notches, transitions), manufacturing tolerance (material variations), small welding defects (voids, inclusions), and installation variables (bolt torque, alignment). Even with these built-in margins, industry best practice often adds additional margin in three common ways: using one pressure class higher than the minimum required, specifying materials with higher temperature capability than the calculated need, or reducing the design pressure below the calculated requirement before looking up the class.

Additional margin provisions that should be considered alongside the class include corrosion allowance (subtract from nominal thickness), creep resistance (for high-temperature service), fatigue life (for cyclic pressure and temperature), vibration effects, and code-specific requirements from ASME Section VIII, API 570, or other applicable codes. In marine and shipbuilding applications, for example, classification society rules (LR, DNV, ABS) often require an additional 1.5 to 1.8 safety factor on top of the ASME rating, which usually pushes the class selection up by one or two steps.

Common Selection Errors and How to Avoid Them

Three mistakes account for the majority of flange-related field failures. The first is using normal operating pressure and temperature instead of design conditions. If a thermal trip or blocked-in heating transient pushes the line to 220 °C at 24 bar, a Class 150 flange rated for 14.5 bar at that temperature will leak. The second is mixing up material groups, particularly substituting Group 2.1 ratings for a Group 2.3 low-carbon stainless flange. The third is interpolating between class designations rather than rounding up to the next class. ASME B16.5 explicitly prohibits interpolation between classes, so the answer is always to ｓｅｌｅｃｔ the next-higher class.

When in doubt, the practical guidance is to apply a 1.25 to 1.5 multiplier to the design pressure before looking up the class, verify the material group against the actual ASTM specification printed on the flange (not what the drawing says), and include the bolted-joint assembly – gasket, studs, and nuts – in the same review. A spiral-wound or RTJ gasket matched to the flange class and service fluid is just as critical as the flange itself.

Conclusion

ASME B16.5 organizes flange pressure ratings into seven classes (150, 300, 400, 600, 900, 1500, and 2500), each with a defined pressure-temperature envelope that depends on the flange material group. Selecting the right class requires identifying the design pressure and design temperature, the material group from Table 1A, and the minimum class whose rated pressure at that temperature meets or exceeds the design pressure. Reference tables for Groups 1.1, 2.1, and 2.3 are included above, and the three-step selection procedure is straightforward to apply. When transients, cyclic service, or aggressive corrosion are involved, round up one class and pair the flange with the correct gasket, stud bolt, and pipe material. With a systematic approach and a clear understanding of how P-T ratings work, flange class selection becomes a routine engineering task rather than a source of field problems.