What Is Bolt Torque?
Bolt torque is the rotational force applied to stud bolt nuts to compress a gasket between two flange faces and create a leak-free seal. The torque value must generate enough clamping force (bolt load) to seat the gasket without exceeding the bolt’s yield strength or damaging the flange face. Torque is measured in foot-pounds (ft-lbs) or Newton-meters (Nm).
When to Apply Controlled Torque
Controlled bolt torque is required for every flanged joint in pressure piping. This includes:
- All ASME B16.5 and B16.47 flanged connections
- Pressure vessel manways and nozzle flanges
- Heat exchanger channel and shell flanges
- Any bolted joint that forms part of a pressure boundary
Uncontrolled tightening (impact wrenches without torque calibration, “feel,” or counting turns) is the single most common cause of flange leaks in the field.
Torque Formula
The simplified torque formula relates applied torque to target bolt load:
T = K x D x F
| Symbol | Definition | Typical Value |
|---|---|---|
| T | Target torque (ft-lbs or Nm) | Calculated per bolt |
| K | Nut factor (friction coefficient) | 0.20 (unlubricated steel-on-steel), 0.15 (lubricated), 0.10 (moly-coated) |
| D | Nominal bolt diameter (inches or mm) | Per ASME B16.5 bolt chart |
| F | Target bolt load (lbs or N) | Calculated from required gasket stress |
The nut factor K is the most critical variable. Lubrication reduces K from ~0.20 to ~0.15, meaning the same torque produces approximately 33% more bolt load. Moly-based lubricants reduce K further to ~0.10-0.12. Applying dry torque values with lubricated bolts can exceed bolt yield and crack the flange.
Torque Values by Bolt Size
The following table shows approximate torque values for ASTM A193 B7 stud bolts with A194 2H nuts, at 50% of bolt yield (standard target), K=0.20 (unlubricated):
| Bolt Diameter | Thread (UNC) | Target Torque (ft-lbs) | Target Torque (Nm) |
|---|---|---|---|
| 1/2” | 13 | 30 | 40 |
| 5/8” | 11 | 60 | 80 |
| 3/4” | 10 | 110 | 150 |
| 7/8” | 9 | 170 | 230 |
| 1” | 8 | 260 | 350 |
| 1-1/8” | 8 | 370 | 500 |
| 1-1/4” | 8 | 520 | 705 |
| 1-3/8” | 8 | 680 | 920 |
| 1-1/2” | 8 | 900 | 1,220 |
These values are approximate and should be verified against the project’s bolting specification or ASME B16.5 bolt torque charts.
Torque Sequence (ASME PCC-1)
ASME PCC-1 “Guidelines for Pressure Boundary Bolted Flange Joint Assembly” recommends a multi-pass star (cross) pattern:
| Pass | Target Load | Purpose |
|---|---|---|
| Pass 1 | 20-30% of final torque | Snug bolts, align gasket |
| Pass 2 | 50-70% of final torque | Begin gasket compression |
| Pass 3 | 100% of final torque | Achieve target gasket stress |
| Pass 4 | 100% (verification pass, clockwise) | Even out interaction effects |
The star pattern ensures that opposing bolts are tightened sequentially, which distributes load evenly across the gasket. Tightening bolts in circular order creates uneven compression and causes one side of the gasket to be over-compressed while the other is under-loaded.
Gasket-Specific Considerations
| Gasket Type | Seating Stress Required | Torque Level |
|---|---|---|
| Soft gasket (PTFE, graphite) | Low (1,500-5,000 psi) | Low; risk of over-compression |
| Spiral wound gasket | Medium (10,000-30,000 psi) | Medium; standard torque values |
| RTJ gasket | High (deformation-based) | High; requires full bolt yield approach |
Bolt elongation measurement (using ultrasonic bolt gauges) is more accurate than torque for critical joints, as it measures actual bolt stretch rather than relying on the friction-dependent torque-to-load conversion. Major EPC projects require bolt elongation measurement on Class 900 and above or on joints with stud bolt diameters of 1-1/2” and larger.
Leave a Comment
Have a question or feedback? Send us a message.