Understanding Flange Pressure Ratings
Flange pressure ratings define the maximum allowable working pressure (MAWP) at specified temperatures. Selecting the correct flange class is essential for system safety, code compliance, and operational reliability. This guide explains pressure-temperature relationships, rating methodologies, and selection criteria per ASME B16.5 and B16.47 for 2026 piping projects.
Flange Pressure Classes Overview
ASME B16.5 defines seven standard pressure classes for flanges from NPS 1/2 to NPS 24:
| Class | Designation | Typical Use | Approx. Pressure Range |
|---|---|---|---|
| 150 | Class 150 | Low-pressure services, water, air | Up to 285 psi @ ambient |
| 300 | Class 300 | Process piping, moderate pressure | Up to 740 psi @ ambient |
| 600 | Class 600 | High-pressure steam, hydrocarbons | Up to 1,480 psi @ ambient |
| 900 | Class 900 | High-pressure process, oil & gas | Up to 2,220 psi @ ambient |
| 1500 | Class 1500 | Very high pressure, wellhead equipment | Up to 3,705 psi @ ambient |
| 2500 | Class 2500 | Ultra-high pressure, subsea | Up to 6,170 psi @ ambient |
Pressure-Temperature Relationships
The Derating Principle
As temperature increases, the maximum allowable pressure decreases. This derating is because the material’s yield strength and allowable stress reduce at elevated temperatures. The relationship is defined in ASME B16.5 Tables 2-1.1 through 2-2.8 for each material group.
Material Groups (ASME B16.5)
| Material Group | Flange Material | Typical Spec | Key Application |
|---|---|---|---|
| 1.1 | Carbon Steel | A105 | General service |
| 1.2 | Carbon Steel (High Strength) | A105N | Higher pressure service |
| 1.3 | Carbon Steel (Impact Tested) | A350 LF2 | Low-temperature service |
| 1.7 | Carbon Steel (High Yield) | A694 F52/F60 | Pipeline flanges |
| 2.1 | 1-1/4Cr-1/2Mo | A182 F11 | High-temperature |
| 3.1 | 2-1/4Cr-1Mo | A182 F22 | Power plant |
| 5.1 | 304 SS | A182 F304 | Corrosive service |
| 5.4 | 316 SS | A182 F316 | Corrosive/chloride service |
Example: Group 1.1 (A105 Carbon Steel) Pressure-Temperature
| Temp (°C) | Class 150 | Class 300 | Class 600 | Class 900 |
|---|---|---|---|---|
| -29 to 38 | 19.6 bar | 51.1 bar | 102.2 bar | 153.3 bar |
| 100 | 17.7 bar | 46.3 bar | 92.5 bar | 138.8 bar |
| 200 | 15.8 bar | 41.4 bar | 82.7 bar | 124.1 bar |
| 260 | 14.5 bar | 37.9 bar | 75.8 bar | 113.7 bar |
| 370 | 11.7 bar | 30.6 bar | 61.2 bar | 91.8 bar |
| 425 | 9.5 bar | 24.8 bar | 49.6 bar | 74.5 bar |
| 455 | 7.6 bar | 19.9 bar | 39.8 bar | 59.7 bar |
| 540 | 3.8 bar | 9.9 bar | 19.8 bar | 29.7 bar |
ASME B16.47: Large Diameter Flanges (NPS 26-60)
For flanges larger than NPS 24, ASME B16.47 applies, divided into two series:
- Series A (formerly MSS SP-44): Larger OD, heavier weight, more bolt holes
- Series B (formerly MSS SP-61): Smaller OD, lighter weight, fewer bolt holes
Pressure classes covered: 75, 150, 300, 400, 600, 900
Special Flange Ratings
API 6A Flanges (Wellhead Equipment)
API 6A defines pressure ratings independently of ASME B16.5 for wellhead and Christmas tree equipment:
- 2,000 psi, 3,000 psi, 5,000 psi, 10,000 psi, 15,000 psi, 20,000 psi
- Temperature ratings: -50°F to 250°F (default), with specific derating above
ASME B16.5 Class 2500
The highest standard class, used in ultra-high pressure applications:
- Available NPS 1/2 through NPS 4 (limited sizes)
- A105 at ambient: 6,170 psi (425 bar)
- Derates to approximately 4,315 psi at 370°C
Factors Affecting Flange Selection
1. Design Pressure and Temperature
The primary driver. Always select a class that provides adequate MAWP at the maximum operating temperature with appropriate safety margin.
2. Material Compatibility
The flange material must be compatible with the process fluid, and the material group determines the P-T curve used.
3. System Pressure Surges
Water hammer, thermal expansion, and process upsets can create pressures exceeding normal operating pressure. Design for worst-case scenarios.
4. Corrosion Allowance
In corrosive service, additional wall thickness (corrosion allowance) may be required, which affects the effective pressure rating over time.
5. Bolt Material and Gasket Selection
Bolting material must be rated for the design temperature. Gasket type and material affect the required bolt load and joint integrity.
Flange Rating Verification Procedure
- Determine maximum operating pressure and temperature
- Select material group based on service conditions
- Consult ASME B16.5 P-T table for that material group
- Verify the selected Class provides MAWP ≥ design pressure at max temperature
- Apply corrosion allowance if required
- Verify bolt material is rated for the temperature range
- Select appropriate gasket for the flange face type and rating
2026 Industry Trends
- Digital P-T calculators: Online tools that automate flange rating lookup with built-in derating curves
- Finite element analysis (FEA): Increasingly used to verify flange integrity for non-standard conditions
- Advanced gasket materials: Newer gasket designs with higher sealing efficiency allow lower bolt loads
- Subsea flange standards: API 6A and DNV-OS-F101 driving design for deepwater applications
CoreMetal Steel: Flanges and Fittings
CoreMetal Steel supplies ASME B16.5 and B16.47 flanges in all standard classes (150-2500), all material groups, and sizes NPS 1/2 to NPS 60. Full range of weld neck, slip-on, blind, socket weld, threaded, and lap joint flanges.
Contact: Tracy | tracy@coremetalsteel.com | +86 18291910632
Conclusion
Understanding flange pressure-temperature relationships is fundamental to safe piping system design. Proper class selection, material group identification, and derating calculations ensure code compliance and operational integrity. Visit CoreMetal Steel Blog for more engineering guides.