Flat gaskets: types, materials and choosing the right one

Function principle: the gasket relies on surface pressure

A flange connection is not made leak-tight by the gasket alone, but by the interplay of gasket, flanges and bolts. The bolt force creates a surface pressure (given in MPa or N/mm²) on the sealing face – it presses the gasket material into the surface irregularities of the sealing faces (against surface leakage) and compacts its internal pores (against leakage through the material). If the pressure drops in service below the required minimum – through gasket set, creep of the material or falling bolt force – the connection becomes leaky; in extreme cases the gasket blows out.

Important for realistic expectations: there is no such thing as absolute tightness – what is assessed is “technical tightness” against defined leakage criteria. VDI guideline 2200 covers the selection, design, configuration and installation of leak-tight flange connections; for assessing technical tightness in connection with the emission requirements of the German TA Luft regulation, VDI 2290 is the relevant guideline in particular. The gasket chosen must always reliably meet the required criterion under the operating conditions. Flat gaskets sit in the main load path: the entire bolt force runs through the gasket, and its pressure changes with every change in bolt load – unlike, say, an O-ring in a groove, where a mechanical stop carries the load instead.

The three gasket families

By construction and application, flange gaskets fall into three families:

• Soft gaskets – the standard for the vast majority of connections up to medium pressures: fibre-based sheet material (code FA; fibres, an elastomer binder and fillers – e.g. KLINGERSIL grades or the SH-D-200 from our shop range), graphite (GR; up to high temperatures, more on the graphite gaskets detail page), PTFE (TF; universal chemical resistance), mica (high-temperature, up to 900 °C) and rubber-steel gaskets (an elastomer profile with a vulcanised-in steel ring, standard in water supply).
• Semi-metallic composite gaskets – for high pressures and temperatures and changing conditions: spiral-wound gaskets (a spirally wound metal strip with a soft filler, colour-coded to ASME B16.20 for winding metal and filler), camprofile gaskets (a grooved metal core with soft facing layers) and metal-jacketed gaskets – in detail on the spiral-wound, camprofile and RTJ gaskets page.
• Metallic gaskets – ring-joint gaskets (RTJ, types R/RX/BX to ASME B16.20/API 6A) for the highest pressures in oil and gas applications, seated in the ring groove of dedicated RTJ flanges.

Dimensions and forms: EN 1514-1, DIN 2690 and the sealing faces

The dimensions of flange gaskets in the EN system are governed by DIN EN 1514-1. Its forms correspond directly to the flange sealing-face forms to EN 1092-1:

Current dimensions are set out on our flat gaskets to DIN EN 1514-1 page. DIN 2690 (flat gaskets for flanges with a flat sealing face) is withdrawn but remains widespread in existing plant and orders – its dimensions have been absorbed into EN 1514-1 form IBC; the page flat gaskets DIN 2690 (existing plant) is there to help identify them. For the ASME system, ASME B16.21 (non-metallic flat gaskets) and ASME B16.20 (spiral-wound, metal-jacketed and RTJ gaskets) apply – relevant for export and plant to US standards, matching our ASME flange dimensions.

Selection criteria: four factors decide

• Medium: chemical compatibility (corrosion, swelling), toxicity and purity requirements – toxic or explosive media call for tighter, more blow-out-safe designs (e.g. a spiral-wound gasket or a confined installation) rather than the cheapest flat gasket.
• Temperature and pressure: the highest and lowest operating values including test pressure; the material must neither become brittle nor creep. The temperature ladder above gives rule-of-thumb values – the limits are system values from pressure and temperature together.
• Flange and bolts: sealing-face form (flat, raised face, confined), surface finish (rough faces need more adaptable gaskets) and the available bolt force – the gasket must be able to be compressed with it without being crushed.
• Codes and regulations: TA Luft requirements on fugitive emissions, DVGW/KTW for gas and drinking water, fire-safe requirements – such requirements often narrow the choice of material more than the operating data does.

And one rule up front that prevents costly failures: gaskets are single-use parts. Once compressed, they have used up their adaptability – renew at every dismantling.

Installation: where most problems start

Most damage to flat gaskets is not down to the material but to installation. The short version of the proven rules: clean and check the sealing faces, align the flanges in parallel, insert the gasket centred and dry (never grease it – grease belongs only on bolts and nuts), lubricate the bolts and tighten in a cross pattern, in stages (by hand, then roughly 30 %, 60–70 % and 100 % of the target torque, finishing with a control pass all round); only retighten cold. Unless the manufacturer's data or the design says otherwise, the thinnest gasket suited to the flange geometry and operating case has the advantage; doubled-up or reused gaskets are off limits. The full sequence with typical damage patterns and bolting know-how is covered on the page installation & bolting; the tightening sequences by bolt-hole pattern are shown on installing a flange connection.

Frequently asked questions about flat gaskets