Convert flange bolt-up torque into preload per bolt and total flange load, then check it against the ASME required bolt loads for the operating condition (Wm1) and for gasket seating (Wm2) — with bolt stress and utilisation against the allowable.
Two separate requirements must be met. The bolting has to compress the gasket enough to seal it at assembly (Wm2), and it has to resist the hydrostatic end force while keeping residual compression on the gasket in service (Wm1). Whichever is larger governs.
T torque (N·m) ·
K nut factor ·
d bolt diameter (mm) ·
nb bolt count ·
m gasket factor ·
y seating stress (MPa) ·
P pressure (MPa) ·
At bolt tensile stress area (mm²)
Eight M16 grade 8.8 bolts, tightened dry to 120 N·m, on a spiral wound gasket 180 mm outside by 160 mm inside, sealing 10 bar.
Seating governs by a factor of six over the operating requirement — typical at this pressure. Change nothing but the gasket to a ring type joint and Wm2 rises to 478 kN, which 300 kN no longer satisfies: the same flange, the same torque, and the joint now leaks.
| Quantity | Symbol | Unit | Accepted range |
|---|---|---|---|
| Number of bolts | nb | — | ≥ 1 |
| Bolt diameter | d | mm | > 0 |
| Bolt-up torque | T | N·m or lb·ft | > 0 |
| Nut factor | K | — | 0.05 – 0.50 |
| Gasket OD / ID | — | mm | OD > ID |
| Internal pressure | P | bar, MPa or psi | ≥ 0 |
| Bolt preload | Fi | kN | output |
| Required loads | Wm1, Wm2 | kN | output |
| Bolt stress | σbolt | MPa | output |
Torque in lb·ft converts at 1.35582 N·m; pressure at 1 bar = 0.1 MPa and 1 psi = 0.00689476 MPa. Bolt tensile stress area is derived from the ISO metric coarse pitch for the diameter entered.
| Gasket | m | y (MPa) | Relative seating demand |
|---|---|---|---|
| Spiral wound | 3.0 | 69 | Moderate — the general-service default |
| Ring type joint | 6.5 | 179 | Highest — 2.6× spiral wound |
| Flat / compressed fibre | 4.75 | 62 | Slightly below spiral wound |
| Rubber / elastomer | 0.5 | 0 | Negligible — seals on contact |
| Grade | Sy (MPa) | Sall = 0.66·Sy | Typical use |
|---|---|---|---|
| Grade 8.8 | 660 | 435.6 | General flanged joints |
| Grade 10.9 | 940 | 620.4 | High-pressure service |
| ASTM A193 B7 | 724 | 477.8 | Standard petrochemical stud bolt |
Preload per bolt equals the applied torque divided by the nut factor × the bolt diameter. Total flange load is that figure multiplied by the number of bolts. The nut factor bundles thread and under-head friction into one number, and it is the dominant source of scatter: the same torque with a dry bolt at 0.20 and a lubricated bolt at 0.15 produces preloads differing by about a third.
They come from ASME Section VIII Appendix 2. The gasket factor m is a multiplier on internal pressure that sets how much residual compression the gasket needs to stay sealed under operating conditions. The minimum design seating stress y, in MPa, is the compressive stress needed to make the gasket conform and seal initially. Spiral wound gaskets take m = 3.0 and y = 69, while ring type joints take 6.5 and 179.
Wm1 is the minimum required bolt load for the operating condition, made up of the hydrostatic end force plus the load needed to keep the gasket compressed against internal pressure. Wm2 is the minimum required bolt load for gasket seating, which applies at ambient conditions during assembly. The bolting must satisfy both, so the governing requirement is the larger of the two.
Seating load depends only on gasket geometry and the seating stress y, and takes no account of pressure. On low-pressure services Wm1 is small while Wm2 stays fixed and large, so seating governs. This is why a flange on a 2 bar line can still need substantial bolt-up torque, and why hard gaskets such as ring type joints demand much more bolt load than soft ones.
Torque control alone typically gives preload scatter of about ±30 %, because the nut factor varies with lubrication, thread condition, surface finish and reuse. For critical joints use bolt tensioning, ultrasonic elongation measurement or turn-of-nut methods, and follow a documented cross-pattern tightening sequence in several passes rather than tightening each bolt fully in one go.