How to Choose the Right Flange Class
Selecting the wrong flange pressure class is one of the most expensive mistakes in piping design. Too low and you risk leaks or catastrophic failure. Too high and you're wasting money on unnecessarily heavy flanges, bolts, and gaskets. This guide helps you get it right.
What Does Flange Class Mean?
A flange class (also called pressure class or rating) is a dimensionless number that represents the pressure-temperature rating of a flange. ASME B16.5 defines classes from 150 to 2500. The class number is approximately the maximum pressure (in psi) that a flange can handle at room temperature for certain materials.
However, this is only a rough approximation. The actual pressure rating depends on:
- Flange material (carbon steel, stainless steel, alloy steel)
- Operating temperature
- Flange size (NPS)
- Gasket type and facing
Pressure-Temperature Ratings (Carbon Steel)
The following table shows pressure ratings (in psi) for ASME B16.5 carbon steel flanges (Group 1.1 materials, e.g., A105) at various temperatures:
| Temp (°F) | Class 150 | Class 300 | Class 600 | Class 900 | Class 1500 | Class 2500 |
|---|---|---|---|---|---|---|
| -20 to 100 | 285 | 740 | 1,480 | 2,220 | 3,705 | 6,170 |
| 200 | 260 | 680 | 1,360 | 2,040 | 3,405 | 5,675 |
| 300 | 230 | 655 | 1,310 | 1,965 | 3,280 | 5,465 |
| 400 | 200 | 635 | 1,270 | 1,900 | 3,170 | 5,280 |
| 500 | 170 | 600 | 1,200 | 1,800 | 3,000 | 4,995 |
| 600 | 140 | 550 | 1,095 | 1,645 | 2,740 | 4,570 |
| 700 | 110 | 510 | 1,020 | 1,530 | 2,555 | 4,255 |
| 750 | 95 | 475 | 950 | 1,425 | 2,375 | 3,955 |
Values are from ASME B16.5 Table 2 for Group 1.1 materials (e.g., ASTM A105). Ratings decrease with increasing temperature.
Flange Class Comparison: Key Characteristics
Class 150 — Low Pressure
- • Most economical flange class
- • Rated ~285 psi at ambient temperature
- • Common for water, air, low-pressure steam
- • Widely available, many manufacturers
- • Thin flange body, fewer/smaller bolts
- • Not suitable for high-pressure or high-temperature
Class 300 — Medium Pressure
- • Rated ~740 psi at ambient temperature
- • Good balance of cost and capability
- • Common for process piping, oil and gas
- • Thicker flange body than Class 150
- • More bolts, larger bolt circle
- • Suitable for moderate temperatures
Class 600 — High Pressure
- • Rated ~1,480 psi at ambient temperature
- • Heavy-duty flanges for demanding service
- • Common in refinery, chemical, and power plants
- • Thick flange body, large bolts
- • RTJ (ring-type joint) facing common
- • Significantly heavier and more expensive
Class 900 — Very High Pressure
- • Rated ~2,220 psi at ambient temperature
- • Used in high-pressure process lines
- • Common in upstream oil and gas
- • Very thick, heavy flanges
- • Usually RTJ facing
- • Higher bolt torque requirements
Class 1500 & 2500 — Extreme Pressure
- • Rated ~3,705 and ~6,170 psi at ambient
- • Used in severe service, high-pressure steam
- • Common in power plant main steam lines
- • Extremely thick, massive flanges
- • RTJ facing almost always required
- • Specialized bolting and gasket requirements
How to Select the Right Flange Class
Step-by-Step Selection Process:
- Determine design pressure and temperature. Use the maximum operating conditions, not normal conditions. Include upset conditions and safety margins.
- Identify the piping material. Different materials have different pressure-temperature ratings. Carbon steel, stainless steel, and alloy steels each have their own rating tables in ASME B16.5.
- Look up the pressure-temperature rating. Find the rating table for your material group and read across to your design temperature.
- Select the next higher class. Always round UP to the next available class. Never interpolate or round down.
- Verify flange dimensions. Confirm that the selected class has acceptable bolt spacing, flange thickness, and weight for your application.
- Check gasket compatibility. Higher classes may require different gasket types (e.g., RTJ gaskets instead of spiral wound).
Example Selection
Design conditions: 400 psi at 500°F, carbon steel piping (A106 Gr. B).
From the table: Class 300 carbon steel is rated 600 psi at 500°F. Class 150 is rated only 170 psi at 500°F. Therefore, Class 300 is the correct selection. Class 600 would also work but is unnecessarily expensive and heavy.
Common Mistakes to Avoid
Using Class 150 for everything
Class 150 is tempting because it's cheap and common, but it has very limited pressure and temperature capability. Many process applications require Class 300 or higher.
Ignoring temperature derating
A flange rated for 285 psi at room temperature may only be rated for 140 psi at 600°F. Always check the rating at your actual operating temperature.
Mixing different class flanges
Never bolt a Class 150 flange to a Class 300 flange. The bolt patterns, flange thicknesses, and facing dimensions are different. They will not seal properly.
Assuming class number = pressure rating
Class 300 is not exactly 300 psi. At room temperature, it's actually 740 psi for carbon steel. The class number is a historical designation, not a direct pressure measurement.
Forgetting about bolt torque
Higher class flanges require much higher bolt torques. Make sure your assembly team has the right tools and procedures for the selected class.
Frequently Asked Questions
What is the most common flange class?▼
Can I use a higher class flange than required?▼
What is the difference between Class 150 and Class 300 dimensions?▼
Do stainless steel flanges have the same ratings?▼
What facing type should I use for high-pressure flanges?▼
Calculate Flange Weight by Class
Use our free flange weight calculator with all ASME B16.5 pressure classes. Select NPS, class, and flange type for instant weight estimates.