Flange Size Chart Reading Guide
Understanding flange dimension tables is essential for proper flange selection, procurement, and installation. This guide explains how to read and interpret ASME flange size charts.
Key Dimensions in Flange Charts
1. Nominal Pipe Size (NPS)
The designated pipe size the flange connects to. Note that NPS does not equal actual pipe diameter for sizes above NPS 12.
2. Outside Diameter (OD)
The overall diameter of the flange. Critical for ensuring proper clearance in piping racks and equipment layouts.
3. Bolt Circle Diameter (BC)
The diameter of the circle passing through bolt hole centers. Essential for flange alignment and bolt hole matching.
4. Number of Bolts
Varies by size and pressure class. Higher pressure classes require more bolts for proper sealing.
5. Bolt Hole Diameter
Size of bolt holes. Standard clearance is 1/16″ to 1/8″ larger than bolt diameter.
6. Flange Thickness
Increases with pressure class. Affects flange weight and structural strength.
7. Hub Dimensions
- Hub diameter at base (B)
- Hub diameter at weld (for weld neck)
- Hub length (for weld neck)
Sample ASME B16.5 Class 150 Dimensions
| NPS | OD | BC | Bolts | Hole | Thick |
|---|---|---|---|---|---|
| 1/2″ | 3.50″ | 2.38″ | 4 | 0.62″ | 0.44″ |
| 1″ | 4.25″ | 3.12″ | 4 | 0.62″ | 0.56″ |
| 2″ | 6.00″ | 4.75″ | 4 | 0.75″ | 0.62″ |
| 4″ | 9.00″ | 7.50″ | 8 | 0.75″ | 0.81″ |
| 6″ | 11.00″ | 9.50″ | 8 | 0.88″ | 0.94″ |
| 12″ | 19.00″ | 17.00″ | 12 | 1.00″ | 1.12″ |
Sample ASME B16.5 Class 300 Dimensions
| NPS | OD | BC | Bolts | Hole | Thick |
|---|---|---|---|---|---|
| 1/2″ | 3.75″ | 2.62″ | 4 | 0.62″ | 0.56″ |
| 2″ | 6.50″ | 5.00″ | 8 | 0.75″ | 0.81″ |
| 4″ | 10.00″ | 7.88″ | 8 | 0.88″ | 1.06″ |
| 6″ | 12.50″ | 10.62″ | 12 | 0.88″ | 1.25″ |
| 12″ | 21.50″ | 17.00″ | 16 | 1.12″ | 1.75″ |
How to Use Dimension Charts
- Identify NPS: Match your pipe size
- Select Class: Based on design pressure and temperature
- Check OD: Verify space constraints
- Verify BC: Ensure mating flange compatibility
- Confirm Bolt Pattern: Match bolt count and size
- Review Hub: Ensure proper pipe fit for weld neck
Important Notes
- Dimensions in inches (multiply by 25.4 for mm)
- Tolerances specified in ASME B16.5
- Class 150 and 300 share same OD for NPS 1/2 to 12
- Higher classes have larger OD and more bolts
- Blind flanges have same OD and bolt pattern but no hub
Common Mistakes
- Confusing NPS with actual pipe diameter
- Mixing pressure classes (bolt patterns differ)
- Not accounting for gasket thickness in assembly
- Overlooking hub dimensions for weld neck fit-up
Always refer to the latest ASME B16.5 standard for complete dimensional data and tolerances.
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