Flange Bolt Tightening Sequence: Cross Pattern and Passes
A flange seals when every stud carries close to the same load. The tightening sequence, a cross pattern over several passes, is how you get there without tilting the gasket.
Why the sequence matters
When one stud is tightened, it squeezes the gasket under it and slightly relaxes the studs nearby. Tighten the studs one after another around the circle and the gasket ends up compressed more on one side than the other. A cross pattern spreads the load across the flange so the faces close together evenly.
The same effect, often called bolt cross-talk, is why a single pass is never enough. Each pass brings all the studs closer to the target, and the final passes even out what earlier passes disturbed.
Step 1: number the studs
Number the studs clockwise around the flange, starting at any convenient stud. Mark the numbers on the flange or the nuts so every technician follows the same order, especially when two people tighten at once on opposite sides.
Step 2: follow a cross pattern
The cross pattern, also called a star pattern, moves across the flange rather than around it. These are the legacy cross-pattern orders given in ASME PCC-1 for common stud counts:
| Studs | Tightening order |
|---|---|
| 4 | 1 – 3 – 2 – 4 |
| 8 | 1 – 5 – 3 – 7 – 2 – 6 – 4 – 8 |
| 12 | 1 – 7 – 4 – 10 – 2 – 8 – 5 – 11 – 3 – 9 – 6 – 12 |
| 16 | 1 – 9 – 5 – 13 – 3 – 11 – 7 – 15 – 2 – 10 – 6 – 14 – 4 – 12 – 8 – 16 |
For larger stud counts, PCC-1 also describes alternative patterns that group studs and can save time. Use them only where your bolting procedure or engineer has approved them.
Step 3: tighten in passes
The legacy method in ASME PCC-1 brings the joint up in rounds of increasing load:
- Snug up: run the nuts down by hand, then snug them lightly so the gasket is seated without load on one side.
- Round 1: about 20 to 30 percent of the target torque, in the cross pattern.
- Round 2: about 50 to 70 percent of the target, in the cross pattern.
- Round 3: 100 percent of the target, in the cross pattern.
- Check rounds: 100 percent of the target, moving around the flange in a circle, repeated until no nut turns any further.
Some procedures add a further check pass a few hours later, once the gasket has had time to relax. Your bolting specification decides whether that is required.
Step 4: check the gap and record
Between rounds, measure the gap between the flange faces at four points, 90 degrees apart. The gaps should stay close to equal. A gap that is clearly wider on one side means the faces are closing unevenly and the next pass should correct it before going on.
When the joint is finished, record the target value, the number of passes, the tool used and its calibration, the date and the technician, and tag the joint. That record is what proves the joint was assembled correctly.
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Questions about this topic
Why not tighten flange bolts in a circle?
Tightening around the circle compresses the gasket unevenly, so one side is crushed while the other stays loose. A cross pattern spreads the load across the flange.
How many passes does a flange need?
A common approach is three rounds at increasing load in a cross pattern, then check rounds at full load in a circle until no nut turns. Your procedure sets the exact number.
Does the same pattern apply to bolt tensioning?
Tensioning uses its own sequence, often with tools on half or all of the studs at once. The aim is the same: even load across the flange.