Fasteners and gaskets: how flange bolting actually works
Stud bolts and gaskets are one system. The gasket type decides how much bolt load the joint needs, and the bolt load decides whether the gasket seals. Buy them as a matched set with the coating and lubrication specified, or you get joints that leak at commissioning and seize at turnaround.
Most leaking flanges I have heard about were not caused by a bad gasket. They were caused by a gasket and a bolt set that were never chosen with each other in mind, usually because they came from two different purchase orders weeks apart.
This is a short guide to how the two fit together, and what to put on the order so the joint behaves.
The gasket decides the bolt load
A gasket seals when it is compressed hard enough to fill the surface imperfections on both flange faces, and stays sealed while the joint runs hot and cold and the bolts relax a little. How hard is hard enough depends entirely on the gasket.
A soft sheet gasket compresses easily and needs modest load. A spiral wound gasket needs substantially more, because it has to crush the filler down onto the windings. A ring type joint needs more again, since it seals metal to metal by yielding the ring into the groove.
That is the link people miss. Change the gasket on a joint without revisiting the bolting and you have changed the requirement without changing what meets it.
The bolt has to deliver that load predictably
Torque is not load. Torque is what your wrench reads, and most of it goes into friction under the nut face and in the threads. Only a fraction becomes bolt tension, and the fraction depends on the surface condition of the fastener.
This is why coatings matter far more than they look like they should. Two bolts torqued identically, one plain and one galvanized, end up at different tensions. Do that across a twelve bolt flange and the load distribution goes uneven, which is exactly the condition a gasket cannot tolerate.
Fastener coatings and what each one is for
| Coating | Thickness | Corrosion resistance | Where it belongs | Watch for |
| Hot dip galvanizing | 50-85 microns | High, sacrificial | Structural bolting, outdoor steelwork | Oversize tapped nuts required; not for class 12.9 |
| Zinc electroplating | 5-15 microns | Low | Indoor, dry environments only | Fails quickly outdoors on the coast |
| Zinc flake | 8-12 microns | High | Class 10.9 and 12.9 bolts | Specify lubricated or plain version |
| PTFE fluoropolymer | 20-30 microns | High, plus low friction | Flange stud bolts, offshore, joints that must come apart | Coating damage in handling |
The blue and green stud bolts you see on every refinery flange are PTFE fluoropolymer coated, usually over a zinc or phosphate base. Xylan is the brand name that became the generic term. The corrosion protection is good, but the reason they are specified is friction: a low, repeatable coefficient means torque converts to preload predictably across every bolt in the flange. The second reason shows up years later, when the joint comes apart cleanly at turnaround instead of needing bolts cut off.
Why plain zinc plating keeps disappointing people
Electroplated zinc is 5 to 15 microns, a fraction of what hot dip gives you. It looks clean, costs little, and holds up fine in a dry plant room. Outdoors near the Gulf coast, with salt-laden condensation forming on the metal overnight, that thin layer goes quickly and red rust follows. It is an indoor finish. Used as one, it is perfectly good value.
Hydrogen embrittlement, briefly
High strength bolts and acid pickling do not mix. The pickling stage before galvanizing can drive hydrogen into the steel, and in class 12.9 that can produce a delayed brittle failure weeks after installation, with no warning. Class 12.9 should not be hot dip galvanized. Class 10.9 can be, but only under controlled processing with testing. Where a high strength bolt needs corrosion protection, zinc flake is the usual answer because nothing in the process generates hydrogen.
What to put on the purchase order
A bolting line should carry the size and length, the standard, the material grade, the coating standard and thickness, the lubrication state, and whether nuts are oversize tapped. So not “galvanized M24 bolts” but:
M24 x 100 DIN 931, class 8.8, HDG to ISO 10684, supplied as assemblies with class 8 nuts oversize tapped, waxed.
Add the gasket type the joint uses, so whoever is quoting can flag a mismatch before it reaches site. And ask for mill and coating certificates with batch traceability. Oil and gas clients audit that paperwork years after handover, and reconstructing it later is close to impossible.
Frequently asked questions
Why are fasteners and gaskets bought together?
Because the gasket sets the required bolt load and the bolts deliver it. Split the purchase and nobody owns the match.
What are the blue stud bolts on flanges?
PTFE fluoropolymer coated, commonly called Xylan coated. The value is consistent friction and clean breakout, not the colour.
Does hot dip galvanizing change torque values?
Yes. Use the coating’s nut factor and specified lubricant, not tables written for plain bolts.
Can class 10.9 and 12.9 bolts be galvanized?
Class 10.9 only with controlled processing and testing. Class 12.9 no. Use zinc flake instead.
What should a flange bolting purchase order specify?
Size, standard, grade, coating standard and thickness, lubrication, nut tapping, gasket type, plus certificates with batch traceability.