Flanged pipe connections are fundamentally different from structural bolting. The bolt torque must compress a gasket to create a seal while staying within the bolt's proof load and the flange's allowable stress. Under-torque causes leaks; over-torque damages gaskets or warps flanges.
Flange bolting typically uses ASTM A193 B7 studs (alloy steel, 105 ksi proof) with A194 2H heavy hex nuts. These are NOT standard hex bolts — they are studs (threaded rod with two nuts) specifically designed for flange service.
Class 150 flanges, spiral wound gasket, lubricated studs. Values per ASME PCC-1.
| Flange Size | Stud Size | # Bolts | Target (ft·lbs) |
|---|---|---|---|
| 2" | 5/8" | 4 | 45–55 |
| 3" | 5/8" | 4 | 45–55 |
| 4" | 5/8" | 8 | 60–75 |
| 6" | 3/4" | 8 | 85–105 |
| 8" | 3/4" | 8 | 85–105 |
| 10" | 7/8" | 12 | 130–160 |
| 12" | 7/8" | 12 | 130–160 |
Star pattern is mandatory. Always tighten in a star (criss-cross) pattern, never sequentially around the bolt circle. This ensures even gasket compression and prevents flange distortion.
Multi-pass tightening: Bring all bolts to 30% of target on the first pass, 60% on the second, then 100% on the third. A fourth pass at 100% catches any elastic interaction from the previous passes. This is per ASME PCC-1 guidelines.
Lubrication is critical. Flange stud torque values assume lubricated threads (K ≈ 0.15). Dry studs will produce significantly lower clamp load at the same torque, likely resulting in a leak. Use anti-seize, moly paste, or a specified thread lubricant.