The flange tightening sequence
A gasket does not care how hard each bolt is pulled. It cares whether the load arrives evenly. Tighten a flange round the circle and the first bolts are unloaded by the last; tighten it in one pass and the gasket is crushed on one side before the other side has touched. The cross pattern and the graduated passes exist to stop both. Press play and watch the order build.
Before you touch a wrench
- Flange faces clean, undamaged and parallel — a sequence cannot fix a misaligned joint.
- New gasket, correct type and size. Never reuse one.
- Threads and nut faces lubricated to the condition the torque figure assumes.
- Nuts run down by hand, joint pulled up square before any torque is applied.
The order for this flange
The passes, and why there are several
ASME PCC-1 Appendix F sets out the pattern that most of the world calls the star pattern and PCC-1 itself calls the Legacy Pattern — so named because it has been in use so long. It is not one pass in a clever order; it is the same cross order repeated at increasing torque.
| Pass | Torque | Order | What it is doing |
|---|---|---|---|
| 1 | 20–30% | Cross pattern | Pulls the joint up square and seats the gasket without crushing it anywhere. |
| 2 | 50–70% | Cross pattern | Builds load while the flange is still free to settle evenly. |
| 3 | 100% | Cross pattern | Brings every bolt to the target. |
| 4+ | 100% | Circular, bolt 1 round to the last | Recovers what the later bolts took out of the earlier ones. Repeat until no nut moves — usually at least two passes. |
That last step is the one most often skipped, and it is the one that matters. Tightening any bolt relaxes its neighbours — the flange flexes and the gasket compresses under the new load, so bolts already at target drop below it. The effect is called elastic interaction, or bolt crosstalk, and the only cure is to keep going round at the final torque until the nuts stop turning.
Legacy or modified?
PCC-1 Appendix F also gives alternatives that reach the same place with less work. The Modified Star keeps the cross shape but does not put every bolt through every torque level: the first four bolts go to 20–30%, the next four to 50–70%, and everything after that straight to 100%, followed by the same circular passes. On a large flange that saves a great deal of time.
PCC-1 asks you to weigh five things before choosing an alternative pattern: localised over-compression of the gasket, uneven tightening causing flange distortion, non-uniform seating load, excessive load and unload of the gasket during assembly, and the possibility of non-parallel flanges. Where the gasket is delicate, the flange is thin or the joint is critical, the legacy pattern remains the conservative choice.
Questions we are asked
Why can’t I just go round the flange in order?
Because the joint closes unevenly. Working round the circle pulls one side of the flange down first, tips the faces out of parallel and over-compresses the gasket on that side while the far side has barely been seated. Crossing the flange on each move keeps the faces coming together square, which is the whole purpose of the pattern.
How many circular passes are enough?
Until the nuts stop rotating. Two is the usual number on a spiral wound gasket, but it is a test, not a count: set the wrench at final torque, go round, and if any nut moves, go round again. The nuts stopping is the evidence the joint has settled.
Does the sequence change for a ring-type joint?
The cross pattern and graduated passes apply to RTJ flanges too. What changes is that an RTJ has metal-to-metal contact at the ring, so the joint is stiffer and less forgiving of misalignment — getting the faces parallel before any torque is applied matters more, not less.
Where does hydraulic tensioning fit in?
Tensioning stretches the bolt directly rather than turning a nut against friction, so it sidesteps the friction problem entirely and can load several bolts at once. It is common on large-diameter and high-class flanges. The pattern thinking still applies, because elastic interaction does not go away — it is just handled by tensioning in groups rather than by repeated passes.
Related
Torque is not preload · Torque & preload chart · Installation methods · Back to the Bolt Lab