Your procedure governs. This is a general guide to how gasketed flange joints are assembled. Where a site bolting procedure, the equipment manufacturer or the current edition of ASME PCC-1 specifies the pattern, percentages or target torque, follow that.
Why the pattern exists
Every bolt you tighten on a flange compresses the gasket a little and slightly unloads the bolts next to it, an effect called elastic interaction or cross-talk. Tighten in a circle from the start and the flange closes like a clamshell: one side crushed, the other side gapped, and the final bolt loads all over the map. The cross pattern spreads each increment of load across the flange, and the staged passes bring the gasket down evenly, so the faces close parallel.
Before the first bolt: preparation
- Inspect the flange faces. Check the gasket seating surface for radial scratches, pitting, corrosion and old gasket residue. A radial scratch across the seating surface is a leak path no amount of torque will close.
- Inspect studs and nuts. Check thread condition, correct grade markings (for example B7 studs with 2H nuts) and nuts that run freely by hand over the full engagement length. Replace anything galled.
- Check alignment. Flanges should come together without force beyond what PCC-1 allows. If you need a come-along to make them meet, the problem is upstream. See flange spreading and alignment.
- Use the right gasket, new. Spiral-wound and other gaskets are generally single-use.
- Lubricate threads and nut faces with the lubricant your target torque was calculated for. The torque value and the lubricant are a pair. See nut factor.
- Use through-hardened washers where the procedure calls for them. They keep under-nut friction consistent and protect the spot face.
- Number the bolts clockwise with a paint marker, and keep the numbers visible from the tool side.
Cross-pattern (star) numbering
Number the bolts 1 to N clockwise. Each cross-pattern pass tightens them in this order:
4 bolts
8 bolts
12 bolts
16 bolts
The logic is always the same: go to the bolt directly opposite, then a quarter turn around, then opposite again, working through the circle so no two consecutive bolts are neighbors. Larger flanges extend the same logic. Many procedures switch to grouped patterns or several tools working at once.
The passes
The traditional ("legacy") approach in ASME PCC-1 brings the joint up in stages. The general shape:
| Step | Load | Pattern |
|---|---|---|
| Install | Hand tight, then snug, not more than about 20% of target torque | Cross pattern |
| Pass 1 | About 20–30% of target torque | Cross pattern |
| Pass 2 | About 50–70% of target torque | Cross pattern |
| Pass 3 | 100% of target torque | Cross pattern |
| Pass 4 | 100% of target torque, repeated until no further nut rotation | Circular, clockwise |
| Check pass | 100% of target torque, a few hours later (often 4 or more) | Circular |
Between passes, measure the gap between flange faces at several points around the circle, at least four and ideally at every bolt on big flanges. If one side is closing faster, adjust before the next pass. Don't wait until the end and find out.
The final circular passes exist because of cross-talk. After pass 3, the first bolts tightened have been relaxed by the later ones. Circular passes at full torque catch them up, and you stop when the nuts no longer move at the target value. The time-delayed check pass recovers short-term gasket creep and embedment relaxation.
Mistakes that cause leaks
- Skipping passes to save time. Going straight to 100% in one cross pass locks in uneven load.
- Wrong lubricant for the torque value. A moly-paste torque on dry studs leaves the joint about 40% short on load (0.12 ÷ 0.20).
- Using the gasket to pull flanges together. If alignment needs force, fix alignment first.
- Uncalibrated tools or gauges, or a hydraulic wrench run at a pressure taken from the wrong tool's chart.
- Reusing gaskets, nuts or studs that should be replaced.
- Re-torquing a hot joint without an engineered procedure for hot bolting. That is a separate, hazardous activity with its own controls.
About ASME PCC-1
ASME PCC-1, Pressure Boundary Bolted Flange Joint Assembly, is the U.S. reference for assembling gasketed flange joints. It covers joint preparation, alignment, lubrication, target bolt stress selection, tightening patterns, alternative patterns, troubleshooting, and, since the 2013 edition, the training and qualification of bolted joint assemblers (Appendix A). The current edition is PCC-1-2022. If your site procedure was written against an older edition, it's worth checking what changed.
Turnaround coming up? We rent hydraulic torque wrenches, pumps and tensioners out of Gonzales, and help crews set up procedures and pressures for the tools they're running. Rentals and service.
Frequently asked questions
What is the correct tightening sequence for an 8-bolt flange?
The standard cross (star) pattern for an 8-bolt flange is 1-5-3-7-2-6-4-8, numbering the bolts clockwise around the flange. Each tightening pass follows that order, then final passes go around the flange circularly at full torque until nuts no longer turn.
How many passes does it take to tighten a flange?
The traditional (legacy) approach in ASME PCC-1 brings the bolts up in stages: snug, then roughly 20–30%, 50–70% and 100% of target torque in cross-pattern passes, then circular passes at 100% until no nut turns. Many sites add a check pass after a few hours to recover short-term relaxation. Your site procedure and the current edition of PCC-1 govern the exact percentages.
Why not just tighten flange bolts in a circle?
Going around in a circle from the start pulls the flange closed on one side first. That cocks the flanges, crushes one side of the gasket, and leaves the bolts loaded unevenly, because each bolt tightened relaxes its neighbors. The cross pattern and staged passes close the joint evenly. Circular passes come only at the end, once the joint is already closed.
Does ASME PCC-1 allow patterns other than the star pattern?
Yes. PCC-1 includes alternative assembly patterns, such as grouping bolts or using multiple tools at once, that can reach equivalent results in fewer passes when validated. They should be adopted through a site procedure, not improvised.