Understanding Flange Rating Classes
Flange rating classes define the maximum allowable working pressure (MAWP) of a flange at a given temperature. The ASME B16.5 standard specifies seven pressure classes for steel pipe flanges: Class 150, 300, 400, 600, 900, 1500, and 2500. The class designation does NOT indicate a specific pressure value — the actual pressure rating depends on the flange material group and the operating temperature. A Class 150 flange in one material may have a significantly different pressure rating than a Class 150 flange in another material.
This guide explains the flange rating system, pressure-temperature relationships, material groupings, and selection criteria per ASME B16.5, ASME B16.47, EN 1092-1, and other international standards.
ASME B16.5 Flange Classes Overview
ASME B16.5 covers NPS 1/2 through NPS 24 flanges in seven pressure classes:
| Class | Designation | Size Range (NPS) | Typical Application |
|---|---|---|---|
| 150 | Class 150 | 1/2 – 24 | Low-pressure water, air, utility services |
| 300 | Class 300 | 1/2 – 24 | Process piping, moderate pressure steam |
| 400 | Class 400 | 1/2 – 24 | Higher-pressure process services |
| 600 | Class 600 | 1/2 – 24 | High-pressure steam, hydrocarbon processing |
| 900 | Class 900 | 1/2 – 24 | High-pressure wellhead, refinery services |
| 1500 | Class 1500 | 1/2 – 24 | Ultra-high-pressure wellhead and test equipment |
| 2500 | Class 2500 | 1/2 – 24 | Extreme high-pressure applications |
Material Groups
ASME B16.5 organizes flange materials into groups. Each group has its own pressure-temperature rating table:
| Material Group | Material Examples | Specification |
|---|---|---|
| Group 1.1 | Carbon Steel (A105) | ASTM A105, A181 |
| Group 1.2 | Carbon Steel (A105, lower strength) | ASTM A105 (special) |
| Group 1.3 | Carbon Steel (high-temperature service) | ASTM A105 (special temp) |
| Group 1.4 | Carbon Steel (low-temperature service) | ASTM A350 LF2 |
| Group 2.1 | Half ferritic alloy steel | ASTM A182 F1, F2, F11, F12 |
| Group 2.2 | Ferritic alloy steel | ASTM A182 F5, F9 |
| Group 3.1 | Cr-Mo ferritic steel | ASTM A182 F22 |
| Group 3.2 | 9% Cr ferritic steel | ASTM A182 F90, F91, F92 |
| Group 3.3 | 12% Cr martensitic SS | ASTM A182 F6a Class 1 |
| Group 3.4 | 12% Cr martensitic SS | ASTM A182 F6a Class 2 |
| Group 6.1 | Austenitic SS (304, 304L, 347) | ASTM A182 F304, F304L, F347 |
| Group 6.2 | Austenitic SS (316, 316L, 321, 347) | ASTM A182 F316, F316L, F321 |
| Group 7.1 | Austenitic SS (higher strength) | ASTM A182 F304H, F316H |
| Group 7.2 | Austenitic SS (higher strength) | ASTM A182 F316H, F347H |
| Group 8.1 | Nickel alloy (200, 201) | ASTM B564 N02200, N02201 |
| Group 8.2 | Nickel alloy (400, K500) | ASTM B564 N04400, N05500 |
| Group 8.3 | Nickel alloy (625, 825) | ASTM B564 N06625, N08825 |
| Group 8.4 | Nickel alloy (C-276, G-3) | ASTM B564 N10276, N06985 |
Pressure-Temperature Ratings
Example: Class 150 Flange Pressure Ratings
| Temperature (°C) | Group 1.1 (A105) bar | Group 6.1 (F304) bar | Group 6.2 (F316) bar |
|---|---|---|---|
| -29 to 38 | 19.6 | 19.2 | 18.7 |
| 100 | 17.7 | 17.1 | 16.8 |
| 200 | 16.2 | 14.9 | 15.1 |
| 260 | 15.3 | 13.9 | 14.3 |
| 300 | — | 13.3 | 13.8 |
| 400 | — | 12.1 | 12.8 |
| 500 | — | 10.9 | 11.7 |
| 538 | — | 10.2 | 11.1 |
Example: Class 300 Flange Pressure Ratings
| Temperature (°C) | Group 1.1 (A105) bar | Group 6.1 (F304) bar | Group 6.2 (F316) bar |
|---|---|---|---|
| -29 to 38 | 51.1 | 51.1 | 51.1 |
| 100 | 46.6 | 45.2 | 45.2 |
| 200 | 43.8 | 39.8 | 40.3 |
| 260 | 41.4 | 37.0 | 37.6 |
| 300 | — | 35.6 | 36.5 |
| 400 | — | 32.1 | 33.9 |
| 500 | — | 28.8 | 30.7 |
Flange Face Types
The flange face type affects the sealing capability and is selected based on pressure class and gasket type:
| Face Type | Abbreviation | Pressure Classes | Typical Gasket |
|---|---|---|---|
| Raised Face | RF | All classes (150–2500) | Spiral wound, ring joint (at high class) |
| Flat Face | FF | Class 150, 300 | Full-face non-asbestos sheet |
| Ring Type Joint | RTJ | Class 600, 900, 1500, 2500 | Metal ring gaskets (oval, octagonal) |
| Tongue and Groove | T&G | All (special order) | Machined metal-to-metal seal |
| Male and Female | M&F | All (special order) | Machined metal-to-metal seal |
Flange Selection Criteria
Step-by-Step Selection Process
- Determine design conditions: Maximum working pressure, minimum/maximum temperature, fluid service
- Select material: Based on fluid compatibility, temperature range, and corrosion resistance
- Determine required class: Using the pressure-temperature rating table for the selected material group, find the lowest class that provides a rating exceeding the design pressure at the design temperature
- Select face type: RF for most applications; RTJ for high-pressure/high-temperature or cyclic service
- Select gasket type: Compatible with face type, pressure class, and service conditions
- Verify bolt requirements: Bolt material and size must be compatible with the flange rating and service temperature
International Standards Cross-Reference
| ASME Class | EN PN (bar) | JIS (K) |
|---|---|---|
| Class 150 | PN 20 | 5K |
| Class 300 | PN 50 | 10K |
| Class 400 | — | — |
| Class 600 | PN 110 | 20K |
| Class 900 | PN 150 | — |
| Class 1500 | PN 260 | — |
| Class 2500 | PN 420 | — |
Summary
Understanding steel pipe flange rating classes is fundamental to safe and code-compliant piping system design. The rating class determines the pressure-temperature envelope of the flange, which varies significantly based on material group. Always consult the ASME B16.5 pressure-temperature tables for the specific material to determine the correct class selection.
CoreMetal Steel supplies steel pipe flanges in all pressure classes (150–2500), materials (carbon steel, stainless steel, alloy steel), and face types (RF, RTJ, FF) per ASME B16.5 and EN 1092-1. Contact our team for flange specifications, material certifications, and project quotation.