Flange Bolt Torque Calculator

Formula overview

SI: F in N, d in m, T in N·m. K is dimensionless — around 0.20 dry, 0.16 lightly lubricated, and as low as 0.12 with a good anti-seize.

Torque–tension relation:
  T = K · F · d

Required bolt loads, ASME VIII-1 Mandatory Appendix 2:
  Wm1 = operating — hydrostatic end force plus the gasket load
        needed to keep the joint sealed under pressure
  Wm2 = gasket seating — the load to seat the gasket initially
  W   = max(Wm1, Wm2),  F = W / number of bolts

The tightening sequence applies the final torque over several passes
in a crossing pattern.

Worked example

Joint with 8 bolts, governing bolt load 400 kN total, M20 bolts,
lubricated so K = 0.16

F per bolt = 400 000 / 8 = 50 000 N
T          = 0.16 × 50 000 × 0.020 = 160 N·m

Applied in passes in a crossing pattern — typically 30%, 60%, then
100% of 160 N·m — followed by a final round-the-clock check pass.
Dry threads at K = 0.20 would need 200 N·m for the same bolt load.

Common mistakes

• Using a torque figure without knowing its lubrication basis. The nut factor swings by more than 50% between dry and well-lubricated threads, so the same torque produces wildly different bolt loads — the lubricant is part of the specification, not a site decision.
• Tightening in one pass. A gasket loaded unevenly is crushed on one side and unseated on the other. The crossing pattern and staged passes exist so the load arrives uniformly, and skipping them is a leading cause of joints that leak on start-up.
• Ignoring the seating case on a soft gasket. Wm2 often governs over Wm1 at low pressure — the gasket needs a minimum seating stress regardless of how little the process pressure is trying to push the flanges apart.

FAQ