ASME B16.5 Flange Classes —
Pressure-Temperature
Ratings Explained

One of the most reliable ways to spot an engineer who has never worked directly with flange specifications is to ask what pressure a Class 150 flange is rated for. The common answer is 150 psi — or 150 bar if the engineer is being especially confident. Neither is correct. Class 150 is a designation, not a pressure rating, and the actual allowable working pressure depends on material group, operating temperature, and the applicable piping code.

This article explains the ASME B16.5 pressure class system from first principles — what the class numbers mean, how to read pressure-temperature rating tables, how material selection affects the rating, and how to apply this correctly when specifying flanges for a process system.

What ASME B16.5 Covers

ASME B16.5 is the American Society of Mechanical Engineers standard for pipe flanges and flanged fittings from NPS ½" through NPS 24". It defines seven pressure classes — 150, 300, 400, 600, 900, 1500, and 2500 — and for each class specifies the physical dimensions (bolt circle, number of bolts, flange OD, thickness, raised face diameter) and the pressure-temperature ratings for flanges made from materials within defined material groups.

ASME B16.5 is the dominant flange standard in the oil and gas, petrochemical and industrial process sectors globally, and is the governing standard for ASME-coded pressure systems in the UK and internationally. The European equivalent is EN 1092-1, which uses PN (Pressure Nominale) designations rather than classes — a separate system covered elsewhere.

The Class Number Is Not a Pressure Rating

This point is worth stating explicitly because the misconception is widespread. The pressure class number — 150, 300, 600 and so on — is a historical size designation inherited from earlier standards. It does not directly correspond to any pressure value in psi, bar or any other unit.

The actual rated working pressure of a flange at a given temperature is found by looking up the pressure-temperature table for the relevant material group and class. These tables are published in ASME B16.5 as Tables 2-1.1 through 2-3.19, one table per material group.

Material Groups

ASME B16.5 assigns every acceptable flange material to a material group. The group determines the pressure-temperature rating for a given class. Groups are numbered 1.1 through 3.19, broadly categorised as:

The critical implication is that two flanges of the same class but different material groups have different pressure-temperature ratings. A Class 150 carbon steel flange and a Class 150 austenitic stainless steel flange do not have the same working pressure. The stainless flange is rated lower — which surprises many engineers who assume a more expensive or more corrosion-resistant material implies higher pressure capacity.

Reading the Pressure-Temperature Tables

The P-T tables in ASME B16.5 list the maximum allowable non-shock working pressure in psi (or bar in the SI edition) at a series of temperatures from −29°C to the material's maximum service temperature. To find the rating for a given flange:

1. Identify the flange material and confirm its ASME B16.5 material group
2. Find the P-T table for that material group
3. Find the row corresponding to the design temperature (interpolate if between tabulated values)
4. Read across to the column for the pressure class
5. That value is the maximum allowable working pressure for the flange in that material at that temperature

If the design pressure exceeds the rated pressure at the design temperature, the next higher class must be used — or the design temperature must be reduced, or the material changed to a higher-rated group.

Representative P-T Ratings — Group 1.1 Carbon Steel (A105)

The following values are representative of ASME B16.5 Table 2-1.1 for Group 1.1 materials (carbon steel, the most common flange material):

Two observations from this table that are important in practice:

First, Class 150 at ambient is rated at approximately 19.6 bar — not 150 psi (10.3 bar), not 150 bar. The class number bears no direct relationship to the working pressure. At 400°C, that same Class 150 carbon steel flange is rated at only 9.5 bar — less than half its ambient rating.

Second, the pressure drop with temperature is substantial and non-linear. A system designed at Class 150 for 15 bar service at ambient temperature would be marginal at 200°C and inadequate at 300°C with the same design pressure. Temperature is not always considered carefully enough at the class selection stage.

Why Austenitic Stainless Has Lower Ratings Than Carbon Steel

Engineers frequently assume that stainless steel flanges, being more expensive and corrosion-resistant, must be stronger. In terms of pressure-temperature rating, this is generally not correct at elevated temperatures.

Representative ratings for Group 2.1 (A182 F304/F304L) and Group 2.2 (A182 F316/F316L):

At ambient and low temperatures, austenitic stainless is rated lower than carbon steel at the same class. At elevated temperatures (above approximately 350–400°C), the stainless ratings converge with or slightly exceed carbon steel because austenitic stainless retains its strength at high temperature better. However, for the majority of process applications operating below 300°C, a stainless Class 150 flange has a lower pressure capacity than a carbon steel Class 150 flange.

The practical consequence: if you are upgrading a carbon steel system to stainless for corrosion reasons and the working pressure is close to the Class 150 carbon steel rating, check the stainless P-T table. You may need to step up to Class 300 stainless to maintain the same rated working pressure.

Duplex and Super Duplex Ratings

Duplex 2205 (Group 2.7, A182 F51) has higher pressure ratings than austenitic grades due to its greater yield strength, and rates comparably to carbon steel at ambient temperatures. Super duplex 2507 (Group 2.8, A182 F53) is higher still. Both have the same Class 150 dimensional envelope as any other B16.5 flange — the higher strength of the material translates directly into higher rated pressure for the same class.

Representative Class 150 ratings at ambient: Carbon steel ~19.6 bar, 316L ~15.9 bar, Duplex 2205 ~20.0 bar, Super Duplex ~20.7 bar. At elevated temperature, the duplex advantage reduces due to the 315°C maximum service temperature limit (sigma phase risk).

Selecting the Correct Class

The correct process for class selection:

1. Establish the design pressure and design temperature for the system or line. These are the maximum conditions the flange will encounter in service, including any upset conditions, start-up pressures or hydrostatic test pressure (which is typically 1.5× design pressure).
2. Identify the flange material required for the service — based on corrosion resistance, temperature and other considerations.
3. Find the material group in ASME B16.5 Appendix A.
4. Look up the P-T table for that material group. Find the rated pressure at the design temperature for Class 150.
5. If Class 150 is adequate — that is, the rated pressure at the design temperature exceeds the design pressure — specify Class 150.
6. If Class 150 is not adequate, check Class 300, then Class 600 and so on until the rating is met.
7. Also check the hydrostatic test pressure — the flange must also accommodate the test pressure, which per ASME B31.3 is typically 1.5× design pressure for pneumatic test or higher for hydrostatic.

Flange Face Types

ASME B16.5 defines four standard face types. Face type is independent of pressure class but affects both the gasket selection and the mating flange requirements.

Raised Face (RF)

The standard face type for Class 150 through Class 2500. A raised annular face concentrates bolt load on the gasket seating area, improving sealing efficiency. For Class 150 and 300, the raised face height is 1.6mm. For Class 400 and above, it is 6.4mm. Most spiral wound and ring gaskets are designed for RF flanges. When connecting to equipment nozzles, confirm the equipment face type — connecting an RF flange to a flat-face equipment nozzle without awareness of the face type difference can create gasket stress issues.

Full Face (FF)

The gasket covers the full flange face, including the bolt holes. Used when connecting to flat-face flanges such as cast iron or ductile iron valves, pumps or equipment nozzles. If an RF flange is bolted to a flat-face cast iron flange, the raised face can crack the brittle cast iron under bolt load — FF is mandatory in this situation. Class 150 and 300 only; higher classes are not available in FF facing.

Ring Type Joint (RTJ)

A machined groove in the flange face accepts a solid metal ring gasket (oval or octagonal section). RTJ provides the most reliable high-pressure seal and is standard on Class 600 and above in high-pressure/high-temperature service, hydrogen service and sour (H₂S) service where the ring provides a fully metallic seal. Oval rings are used in Class 300–900, octagonal rings in Class 600–2500. RTJ flanges require matching RTJ counterpart flanges — they cannot be bolted to RF flanges.

Tongue and Groove (T&G) / Male and Female (M&F)

Less common in standard process pipework; more typically found on heat exchanger bonnets and pump casings. The mating flanges must be a matched pair. Not interchangeable with RF or FF flanges.

Pressure Class and Bolt Loading

As pressure class increases, the flange thickness, bolt size and bolt count all increase to provide the clamping force required to maintain a sealed joint at higher pressures. This has practical implications beyond the pressure rating:

• Weight: A Class 600 NPS 6 flange weighs considerably more than a Class 150 NPS 6 flange. On large-diameter or elevated pipework this affects support design significantly.
• Bolting: Higher class flanges use larger, more numerous studs. At Class 1500 and 2500 the stud sizes are substantial and require significant torquing effort — pneumatic or hydraulic torque tools rather than manual spanners.
• Piping stress: Heavier flanges add point loads to the piping system. At Class 900 and above on large-bore pipe, flange weight should be included in pipe stress analysis.
• Cost: The price increase from Class 150 to Class 300 in standard carbon steel is moderate. From Class 300 to Class 600 it is more significant. Class 900 and above in any material is expensive — particularly in stainless or alloy materials.

Hydrostatic Test Pressure

ASME B16.5 specifies that flanges shall be hydrostatically tested at the manufacturer's works at 1.5 times the Class 150 rating at 38°C — irrespective of the actual pressure class. This shell test is a manufacturing quality verification, not a system proof test.

In installed systems, ASME B31.3 specifies a hydrostatic system test at 1.5× design pressure (with stress ratio correction if the test is at a different temperature to the design temperature). The flanges in the system must be rated for the test pressure — which means confirming that the selected class can accommodate 1.5× design pressure at the test temperature, not just the operating pressure at operating temperature.

Common Specification Mistakes

1. Specifying Class 300 "to be safe" without checking whether Class 150 is adequate. For many utility and moderate-pressure process lines below 15–17 bar at ambient temperature in carbon steel, Class 150 is entirely appropriate. Over-specifying adds cost and weight for no engineering benefit.
2. Not checking the P-T table for the specific material. Specifying a Class 150 stainless flange on a system rated at 18 bar would be an error — Class 150 316L is only rated to approximately 15.9 bar at ambient, below the design pressure.
3. Ignoring temperature de-rating. A system specified at Class 150 for 12 bar at ambient that periodically reaches 300°C should be checked at 300°C — the Class 150 carbon steel rating at 300°C is approximately 12.1 bar, which is marginal. In this case Class 300 is the correct specification.
4. Connecting RF flanges to flat-face cast iron equipment without a full-face gasket. The raised face concentrates load and can crack cast iron flanges. This is a well-documented failure mode on pump and valve connections.
5. Using Class 150 on hydrogen service. Hydrogen embrittlement and the consequences of a hydrogen leak drive more conservative flange class selection. Many hydrogen service specifications mandate Class 300 minimum regardless of pressure, with RTJ facing preferred.
6. Forgetting that Class 400 dimensions match Class 300. If a Class 400 flange is substituted for Class 300 in a predominantly Class 300 system, the flanges are physically interchangeable — but the material marking and traceability requirements may cause problems at inspection. Keep the system class consistent.

Summary

The ASME B16.5 pressure class system is not a direct pressure rating — it is an index that, combined with a material group and a temperature, gives the actual allowable working pressure from the P-T tables. Class selection must be made by looking up the table for the specific material at the design temperature and confirming the rated pressure exceeds the design pressure, including test pressure considerations.

Class 150 in carbon steel at ambient conditions is rated at approximately 19.6 bar — adequate for a wide range of process applications. The same class in 316L stainless is rated approximately 15.9 bar. At 300°C, the carbon steel rating falls to approximately 12.1 bar. These are not approximations to be rounded — they are the values that determine whether a flange joint holds or fails.