Class 150 flange dimensions
Class 150 is the common flange for water, air and low-pressure process work. Bolt circle is the diameter through the bolt hole centres, and it is the dimension that decides whether two flanges will actually mate.
| NPS | Flange OD | Bolt circle | Bolt holes | Bolt size |
|---|---|---|---|---|
| 1/2 | 3.50 | 2.38 | 4 | 1/2 |
| 3/4 | 3.88 | 2.75 | 4 | 1/2 |
| 1 | 4.25 | 3.12 | 4 | 1/2 |
| 1 1/4 | 4.62 | 3.50 | 4 | 1/2 |
| 1 1/2 | 5.00 | 3.88 | 4 | 1/2 |
| 2 | 6.00 | 4.75 | 4 | 5/8 |
| 2 1/2 | 7.00 | 5.50 | 4 | 5/8 |
| 3 | 7.50 | 6.00 | 4 | 5/8 |
| 4 | 9.00 | 7.50 | 8 | 5/8 |
| 6 | 11.00 | 9.50 | 8 | 3/4 |
| 8 | 13.50 | 11.75 | 8 | 3/4 |
| 10 | 16.00 | 14.25 | 12 | 7/8 |
| 12 | 19.00 | 17.00 | 12 | 7/8 |
Class 300 flange dimensions
Class 300 carries roughly double the pressure of Class 150 at the same temperature. Everything grows: outside diameter, bolt circle, bolt count and bolt size. A Class 150 and a Class 300 flange of the same nominal size will not bolt together.
| NPS | Flange OD | Bolt circle | Bolt holes | Bolt size |
|---|---|---|---|---|
| 1/2 | 3.75 | 2.62 | 4 | 1/2 |
| 3/4 | 4.62 | 3.25 | 4 | 5/8 |
| 1 | 4.88 | 3.50 | 4 | 5/8 |
| 1 1/2 | 6.12 | 4.50 | 4 | 3/4 |
| 2 | 6.50 | 5.00 | 8 | 5/8 |
| 3 | 8.25 | 6.62 | 8 | 3/4 |
| 4 | 10.00 | 7.88 | 8 | 3/4 |
| 6 | 12.50 | 10.62 | 12 | 3/4 |
| 8 | 15.00 | 13.00 | 12 | 7/8 |
| 10 | 17.50 | 15.25 | 16 | 1 |
| 12 | 20.50 | 17.75 | 16 | 1 1/8 |
Bolting a flange
Tighten in a criss-cross star pattern, never round the circle. On an eight-bolt flange the sequence is 1, 5, 3, 7, 2, 6, 4, 8, working across the diameter each time.
Use at least three passes: 30 percent of final torque, then 60, then 100, then a final pass round the circle at full torque to catch relaxation. ASME PCC-1 sets this out in detail and is worth following on anything that holds pressure.
Gasket compression is what seals the joint, not bolt tightness for its own sake. Over-torquing crushes the gasket and the joint leaks as soon as it cycles. Re-torque after the first thermal cycle on hot service.
