How much torque is needed on a flange stud bolt?
Required torque depends on bolt diameter, target bolt stress, the gasket and the friction coefficient of the lubricant. As a guide, a lubricated ASTM A193 B7 stud of 3/4 inch diameter needs roughly 300 Nm, and a 1 inch stud roughly 700 Nm, at typical seating stress.
Key Takeaways
- Torque is a proxy for bolt tension, and a poor one — as much as half the applied torque can be lost to friction.
- Lubrication changes the answer dramatically: an unlubricated stud may need 40 to 50 per cent more torque for the same tension.
- The tightening sequence matters as much as the value; a star or cross pattern in several passes is essential.
- Gasket type sets the required seating stress, so a spiral wound and a soft gasket on the same flange need different torque.
- ASTM A193 B7 is the default stud grade with ASTM A194 2H nuts on carbon and low-alloy service.
- For sour service the bolting is normally A193 B7M with A194 2HM nuts, which are hardness-controlled and take lower torque.
The chart
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| Stud diameter | Approx. torque (Nm) | Approx. torque (ft-lb) | Typical nut |
|---|---|---|---|
| 1/2" | 80 | 59 | A194 2H |
| 5/8" | 160 | 118 | A194 2H |
| 3/4" | 300 | 221 | A194 2H |
| 7/8" | 460 | 339 | A194 2H |
| 1" | 700 | 516 | A194 2H |
| 1-1/8" | 1,000 | 738 | A194 2H |
| 1-1/4" | 1,400 | 1,033 | A194 2H |
| 1-3/8" | 1,850 | 1,364 | A194 2H |
| 1-1/2" | 2,400 | 1,770 | A194 2H |
| 1-3/4" | 3,700 | 2,729 | A194 2H |
| 2" | 5,400 | 3,983 | A194 2H |
Why a torque figure is only an approximation
What actually seals a flanged joint is bolt tension, not torque. Torque is used because it can be measured with a wrench, but a large and variable share of it is consumed overcoming friction under the nut face and in the threads before any tension develops in the stud.
That friction share depends on the lubricant, thread condition, whether the stud is new or reused, and how clean the nut face is. Two studs torqued to an identical value can end up at materially different tensions, which is why critical joints are tensioned hydraulically or checked by bolt elongation rather than by torque alone.
Use the table as a starting reference. Where the project specification, the gasket manufacturer or the joint integrity procedure gives a figure, that figure governs.
Tightening sequence
Sequence is not a refinement; a correctly torqued joint tightened in the wrong order will still leak. Uneven loading cocks the flange faces and crushes the gasket on one side while leaving the other unseated.
- Number the bolts and work a star or cross pattern, always moving to the bolt roughly opposite the last.
- Hand-tighten all bolts and check that the flange faces are parallel and the gap even around the circumference.
- First pass at about 30 per cent of final torque, in sequence.
- Second pass at about 60 per cent, in sequence.
- Third pass at 100 per cent, in sequence.
- Final pass at 100 per cent, this time working clockwise bolt to bolt until no nut turns further.
On gaskets that relax, a re-check after the system has been brought to temperature and returned to ambient is normal practice.
Bolting grades we supply
ASTM A193 B7 studs with A194 2H nuts are the default for carbon and low-alloy flanged joints. For sour service the hardness-controlled B7M and 2HM combination is specified instead, and it carries a lower permitted torque because the material is deliberately softer — see NACE MR0175.
Low-temperature service uses A320 L7 with A194 4 or 7 nuts, and stainless joints use A193 B8 or B8M with A194 8 or 8M nuts. All are supplied with EN 10204 3.1 certification — see our fasteners range.
Frequently Asked Questions
Roughly 300 Nm, or about 221 ft-lb, when the stud is properly lubricated and at typical gasket seating stress. An unlubricated stud may require 40 to 50 per cent more for the same bolt tension.
Substantially. Most of the applied torque is consumed by friction, so the lubricant and its friction coefficient are among the largest variables in the calculation. Always apply the lubricant the procedure specifies.
A star or cross pattern in progressive passes at roughly 30, 60 and 100 per cent of final torque, followed by a final clockwise pass bolt to bolt until no nut turns further.
B7M is the hardness-controlled variant for sour service, tempered to a lower hardness to resist sulphide stress cracking. It is softer, so the permitted torque and the achievable bolt stress are lower.