How to Torque Flange Bolts: Procedure

Bolt torquing is the controlled application of rotational force to flange stud bolts to achieve a specific bolt load (clamping force) on the gasket. Correct torque values ensure adequate gasket compression for a leak-free seal without overstressing the bolts or flanges.

Torquing Procedure

1. Determine target torque: Calculate or look up the target torque value from ASME flange bolting torque charts or project specifications. The target torque depends on bolt size, bolt material grade, gasket type, and lubrication condition.
2. Select the torque tool: Use a calibrated torque wrench (hydraulic or manual) with a range that places the target torque between 20% and 80% of the wrench’s full scale.
3. Verify calibration: Torque wrenches must be calibrated per the project quality plan (typically within the last 6 months). Record the calibration certificate number.
4. Apply anti-seize lubricant: Coat bolt threads and nut bearing faces with the specified lubricant (typically molybdenum disulfide or nickel-based anti-seize). The nut factor (K) used in the torque calculation must match the lubricant type.
5. Tighten in cross-pattern passes: Apply torque in at least 3 cross-pattern passes (30%, 60%, 100%) followed by a final rotational (clockwise) pass at 100%.
6. Verify and record: After the final pass, verify that no nut rotates further at the target torque. Record all values on the bolt torque record sheet.

Torque Calculation Formula

T = K x D x F

Symbol	Description	Unit
T	Target torque	N-m (ft-lb)
K	Nut factor (depends on lubrication)	dimensionless
D	Nominal bolt diameter	m (ft)
F	Target bolt load (axial force)	N (lb)

Typical Nut Factor (K) Values

Lubrication Condition	Nut Factor (K)
Anti-seize (moly-based, e.g., Molykote)	0.13-0.15
Anti-seize (nickel-based)	0.14-0.16
Anti-seize (copper-based)	0.14-0.16
Machine oil / light oil	0.16-0.18
Cadmium-plated bolts (dry)	0.17-0.19
Zinc-plated bolts (dry)	0.18-0.20
Bare / unlubricated (rusty)	0.20-0.35

Common Mistake

Using the wrong nut factor (K) is the most common torquing error. A bolt lubricated with moly-based anti-seize (K = 0.14) requires 30-40% less torque than the same bolt in dry condition (K = 0.20) to achieve the same bolt load. If the wrong K-factor is used, the gasket will be either under-compressed (leak risk) or over-compressed (gasket damage).

Torque vs. Tensioning

Parameter	Torque Method	Hydraulic Tensioning
Tool	Torque wrench (hydraulic or manual)	Hydraulic bolt tensioner
Accuracy	+/- 25-30% (due to friction variability)	+/- 5-10%
Speed	One bolt at a time	Multiple bolts simultaneously
Application	General piping (up to Class 600)	Critical joints, large bolts, Class 900+
Cost	Lower	Higher (specialized equipment)
Bolt size range	All sizes	Typically 1 in. (M24) and above

Key Points

• Target bolt load should produce a gasket stress between the minimum seating stress (y) and the maximum allowable gasket stress specified by the manufacturer.
• For stud bolt specifications (ASTM A193 B7, B8), the maximum allowable bolt stress is typically 50% of yield strength for controlled torquing.
• Re-torquing (hot bolting) may be required after the system reaches operating temperature to compensate for thermal expansion and gasket creep.
• Always torque in the correct cross-pattern sequence to prevent flange cocking and uneven gasket compression.

Read the full guide to piping inspections