"Spiral wound gasket torque chart" is a frequent search. Charts are easy to find, but one that does not state its assumptions is an estimate rather than a value.
The chain: stress, load, torque
The calculation runs in three steps:
1. Required gasket surface stress. It depends on the filler; for a graphite filled, double-side enclosed gasket the recommended value is around 120 N/mm². 2. Total bolt load. Surface stress multiplied by the effective contact area, plus the separating force from line pressure. 3. Torque per bolt. Total load divided by bolt count, then converted to torque with the friction coefficient and bolt diameter.
Something changes at each step: gasket type, bolt grade, lubrication, flange face form. Two projects at the same size and class can legitimately use different torque values.
The risk of generic charts
A generic chart assumes a gasket type, a bolt grade and a friction coefficient. Change one in the field and the result shifts:
- Lubricated and dry threads can differ by more than 30% in torque
- Coating type changes friction
- A gasket without an inner ring has a different contact area from one with
If the specification gives a value, use it. If not, calculate from the surface stress recommended by the gasket manufacturer.
What over tightening does
Beyond a certain surface stress a spiral wound gasket loses its recovery and the windings collapse. From that point it behaves like a flat metal ring and leaks under thermal cycling. The assumption that the outer ring stops compression is wrong; we covered that in the spiral wound gasket guide.
Practical rules
- Tighten in a star pattern in at least three stages (30%, 60%, 100%)
- Finish with one sequential pass to catch loose bolts
- Re-check torque after the first heat up on cycled lines
- Consider hydraulic tensioning in large sizes and higher classes
- Install the gasket dry; grease makes the surface stress unpredictable
See flange bolt torque and how to select a flange gasket.
FAQ
Why does the torque value differ by project?
Because gasket type, bolt grade, friction condition and face form all change the required torque.
What happens if I over tighten?
The windings collapse and the gasket loses recovery, so leaks appear under thermal cycling.
When is hydraulic tensioning needed?
On large diameter, higher class joints where equal preload matters.
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