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 minimum yield up to 2-1/2") 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, ASTM A193 B7 studs, lubricated threads. Calculated as T = K × D × F with K = 0.15 and a target bolt stress of 40,000–60,000 psi on the stud's tensile stress area — the range this page's FAQ gives for spiral-wound gaskets. Torque depends on the stud, not the flange, so flanges sharing a stud size share a range. ASME PCC-1 (Appendix O) sets target bolt stress rather than torque; confirm the target with your gasket manufacturer.
| Flange Size | Stud Size | # Bolts | Target (ft·lbs) |
|---|---|---|---|
| 2" | 5/8" | 4 | 70–105 |
| 3" | 5/8" | 4 | 70–105 |
| 4" | 5/8" | 8 | 70–105 |
| 6" | 3/4" | 8 | 125–190 |
| 8" | 3/4" | 8 | 125–190 |
| 10" | 7/8" | 12 | 200–305 |
| 12" | 7/8" | 12 | 200–305 |
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.