Detail page · Gaskets
Gasket installation and bolting: this is where sealing is decided
Experience from training sessions and failure analyses shows: a large share of the problems with flat gaskets do not originate in the material, but in selection, handling and installation. This page walks through the proven installation sequence, explains why bolts are lubricated and the gasket never is – and shows, using typical failure patterns, how to trace failures back to their true root cause.
- Core rule
- build and maintain surface pressure
- Tightening
- cross pattern, in stages
- Lubrication
- bolts yes – gasket never
- Retightening
- only cold
The proven sequence in seven steps – our suggested approach
Overview, not an installation manual: the manufacturer's installation instructions and the plant's requirements always prevail; installing flange connections belongs in the hands of suitably qualified personnel (cf. DIN EN 1591-4).
- Clean and inspect: free the sealing faces of old gasket residue and corrosion, check for scores and damage – scratches running across the face are leak paths.
- Align the flanges parallel: the gasket must not be relied on to compensate for misalignment; support the pipework free of stress.
- Place the gasket centred and dry: no adhesive, grease or assembly sprays – the factory anti-stick finish is sufficient. With the IBC form the ring centres itself on the bolts.
- Lubricate the bolts: apply an approved lubricant to the threads and nut bearing faces – bolts, nuts and washers only, never the gasket. Use hardened washers (min. 200 HV): one per bolt, two per stud.
- Tighten in a cross pattern, in stages: seat by hand first, then apply roughly 30 % → 60–70 % → 100 % of the tightening torque, finishing with a control round all the way round at full torque. The tightening sequences by bolt-hole pattern are shown on our page Flange connections and installation.
- Retighten only cold: and for fibre gaskets only as long as the joint has not yet been exposed to a temperature above approx. 100 °C.
- Thin, simple, single-use: choose the thinnest gasket suited to the flange geometry and duty (manufacturer data prevails – there is no blanket standard thickness), never stack gaskets and never reuse them.
Bolts, nuts, preload
The gasket "lives" on surface pressure – and that comes from bolt preload. Torque is only the aid here: how much preload actually results depends decisively on friction. Dry threads deliver 20–30 % less preload at the same torque than lubricated ones – torque values therefore only ever apply together with the defined lubrication condition. For critical joints the tightening torque is calculated per connection (gasket parameters to EN 13555 and calculation programs from gasket manufacturers); from large bolt diameters upward, hydraulic tensioning is used, which bypasses the friction uncertainty.
The set includes: bolt material matched to pressure rating and temperature – in the EN system common strength classes or heat-resistant grades, in the ASME system fully threaded studs with two hexagon nuts (standard pairing A193 B7 with A194-2H; stainless steel variants B8/B8M). Number, diameter and length are standardised per flange standard, nominal size and pressure class. From failure-analysis practice: on critical joints, bolts are renewed at every revision – reused, stretched bolts with scattered friction values are a classic cause of failure. And never mix metric and imperial threads on one connection.
Typical failure patterns – and their true causes
"It was the gasket's fault" is the most common misdiagnosis. Four patterns from failure-analysis practice:
- Blown-out gasket: blow-out requires that the surface pressure has fallen below the minimum during operation – typical triggers are pressure surges (for example on start-up against a frozen or closed section of line) or loss of preload. Cause: plant operation or installation, not the material.
- Gasket material in the medium: if a gasket projects uncompressed into the flow channel, the unsupported edge erodes and migrates into the product. Cause: design or an incorrect cut-out – the remedy is a precisely fitted cut-out or a more abrasion-resistant material.
- Gasket destroyed during installation: stacked or glued gaskets (adhesive solvents lower the sustainable pressure), one-sided overpressing due to non-parallel flanges or a missing cross pattern.
- Leak despite the "correct" torque: reused bolts, dry threads, scattered friction values – the preload was never where the torque suggested it was.
For claims handling this means: before replacing the gasket, always check the operating history (pressure surges? temperature?), the installation record (torques, sequence, lubrication) and the condition of the flanges and bolts – otherwise the new gasket fails for the same reason.
Frequently asked questions on installation and bolting
Why must the gasket itself never be greased?
Lubricant on the sealing face drastically lowers the maximum surface pressure the gasket material can withstand – the gasket is destroyed at a normal tightening torque or creeps out of the joint under operating load. Only the bolt threads and nut bearing faces are lubricated, so that a reproducible preload results from the tightening torque. Many gaskets carry a factory anti-stick finish; additional release agents are only permitted in justified individual cases.
How should flange bolts be tightened correctly?
Always in a cross pattern and in stages: seat all bolts by hand, then apply roughly 30 percent, 60 to 70 percent and 100 percent of the tightening torque over several passes, finishing with a control round all the way round at full torque. This builds up the surface pressure evenly and keeps the flanges from tilting. For critical joints an additional control pass is worthwhile, because the bolts settle after the first tightening.
May a flange connection be retightened while hot?
As a rule, no. Retightening is only done cold – and for most fibre gaskets only as long as the joint has not yet been exposed to a temperature above roughly 100 degrees C, because otherwise the material has already embrittled and breaks on retightening. Hot retightening can destroy the gasket that has already set and is a common trigger for leaks right after start-up.
Which bolts and nuts belong on a flange connection?
In the EN system, common sets use hexagon bolts or studs with matching nuts according to pressure rating and temperature; in the ASME system, fully threaded studs with two hexagon nuts are standard (typically A193 B7 with A194-2H nuts). These are combined with hardened washers (at least 200 HV) – one under each nut, and on studs on both sides. The number, diameter and length of the bolts are standardised per flange standard, nominal size and pressure class. On critical joints, bolts are renewed at every revision.
Why does the connection still leak even though the torque is right?
Torque is only an aid – what matters is the bolt preload, and that depends heavily on friction. The same newton-metres produce 20 to 30 percent less preload with a dry thread than with a lubricated one. Other typical causes: a reused gasket or bolts, uneven tightening without a cross pattern, non-parallel flanges, damaged sealing faces, or a gasket wrongly chosen for the operating conditions. Torque values therefore only ever apply together with the defined lubrication condition.
More detail and our range
Gaskets, bolts and flanges from a single source – matched to each other rather than sourced separately.
Technical basis: KLINGER technical documentation "Flachdichtungen: Grundlagen, Ausfall und Abhilfe, richtige Montage" (2018, presented at Zickwolff) – key statements in our own wording – plus standards facts on EN 13555, ASME B16.5, ASTM A193/A194 and trade articles (projectmaterials.com). All information without guarantee – tightening torques must always be determined per connection from manufacturer data; the current standards and technical documentation always prevail. Editorially reviewed: July 2026. Standards and manufacturer status considered: July 2026.