Article
A perfectly made flange and gasket will still leak if the bolts are tightened badly. The goal of bolting is to compress the gasket evenly all the way round. That only happens when the bolts are tightened in the right order and in stages.
Why the sequence matters
If one side is tightened fully first, the flanges tilt. The gasket is crushed on one side and barely touched on the other, and the joint leaks as soon as pressure or temperature changes. A star (cross) pattern pulls the flanges together evenly.
The star pattern for an 8-bolt flange
Number the bolts clockwise 1 to 8. Tighten in the order 1 – 5 – 3 – 7 – 2 – 6 – 4 – 8. Each bolt is followed by the one directly opposite, then the pattern moves a quarter turn round the flange.
Tighten in stages
| Pass | Target | Pattern |
|---|---|---|
| Snug | Hand tight, flanges parallel | Star |
| Pass 1 | About 20–30% of final torque | Star |
| Pass 2 | About 50–70% of final torque | Star |
| Pass 3 | 100% of final torque | Star |
| Check pass | 100%, until nuts no longer turn | Circular, bolt by bolt |
This follows the practice described in ASME PCC-1, the guideline for pressure boundary bolted flange joint assembly. Large or critical joints may also be re-checked after a few hours, because gaskets relax.
Good practice checklist
- Clean flange faces and check for scratches across the sealing surface.
- Use a new gasket of the correct type, size and class.
- Lubricate threads and nut faces; friction decides how much of the torque becomes bolt load.
- Use calibrated torque wrenches or hydraulic tools.
- Check flange gap with a gauge at several points between passes.
Common mistakes
- Tightening round the flange in a circle from the start
- Going straight to full torque in one pass
- Re-using old gaskets or mixing bolt grades
- Using flange bolts to pull badly misaligned pipes together
Frequently asked questions
What is the star pattern for 8 bolts?
1-5-3-7-2-6-4-8, with bolts numbered clockwise.
How many passes are needed?
Typically three increasing torque passes in a star pattern plus a final circular check pass.
Which document gives guidance on flange bolting?
ASME PCC-1 provides guidelines for pressure boundary bolted flange joint assembly.
Key takeaways
- Always tighten in a star pattern.
- Use staged passes, then a circular check pass.
- Lubrication, clean faces and a new gasket are just as important as torque.
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