O-ring or flat gasket: when to use which sealing element

An O-ring and a flat gasket solve the same problem — stopping a medium from leaking across a static joint — but they do it in completely different ways. An O-ring seals by squeezing a round cross-section inside a precisely machined groove, and the sealing force comes from the elasticity of the elastomer. A flat gasket seals by clamping a compressible layer between two flange faces with the force of the bolts. As a supplier of sealing elements we see the same mistake again and again: the joint was designed for one principle, but the other one was fitted. This article gives you the decision logic — flange type, surface finish, pressure and re-use — so you never pick wrong.

Two different sealing principles

An O-ring is a bidirectional, volumetric sealing element. Once seated in a groove its cross-section is compressed (typically by 15–30 %), and the elastomer behaves almost like an incompressible fluid — it fills the groove and pushes against both mating surfaces. This lets it seal pressure from either side and ride out pressure cycles without re-torquing. That is why an O-ring is the first choice for joints dismantled regularly: at each service you replace only the cheap ring, and the groove stays undamaged. A static O-ring also tolerates a small stretch on its inner diameter (kept below 5 %) without losing its seal.

A flat gasket, by contrast, works on surface contact pressure. The flange bolts compress the gasket so that its material flows into the micro-roughness of both faces and forms a continuous barrier. There is no elastic reserve as there is in an elastomer — the sealing force depends on the bolt torque and falls off over time (relaxation, bedding-in, material creep). A flat gasket is therefore usually not self-compensating, and once the joint is opened it is normally replaced, because it has already bedded in and deformed.

Choosing by flange type and machining

The strongest decision criterion is the geometry of the joint. An O-ring needs a groove — a precisely machined cavity with a defined depth, width and corner radius. The groove (housing) dimensions for general industrial use are covered by ISO 3601-2 (Housing dimensions for general applications), which builds on ISO 3601-1 for the ring dimensions (d1 — inner diameter, d2 — cross-section). As a rule of thumb a static radial groove is cut to roughly 0.78 × cross-section deep (about 22 % squeeze) and 1.3 × cross-section wide; a dynamic gland is cut shallower to reduce friction. If you have such a groove, the O-ring is almost always cheaper, more reliable and re-serviceable.

A flat gasket needs no groove — it sits between two flat faces, and that is precisely why it is irreplaceable on standard pipe flanges. This is where EN 1514 comes in, defining gasket dimensions for flanges designated by a PN pressure rating. EN 1514-1 covers non-metallic flat gaskets (with or without an insert) and EN 1514-4 covers metallic and combined gaskets including spiral-wound types; the series spans PN 10 to PN 100 and bores up to DN 900. So for a pipe flange joint to EN 1092 you are almost always looking for a gasket to EN 1514, not an O-ring — the flange face has no cavity to hold a ring.

Surface finish decides what will actually seal

The finish of the mating faces is one of the most underrated factors, and it is often the very reason a static O-ring fails and a gasket wins. An O-ring seals against a smooth, precisely machined surface: for a static seal a finish of Ra ≤ 1.6 µm is recommended, under pulsating pressure as fine as Ra ≤ 0.8 µm, and for dynamic applications even Ra ≤ 0.4 µm. At low pressure, under vacuum, or when sealing gases, surface quality becomes the single most critical variable — with little contact force the ring cannot bridge the roughness peaks and it leaks.

If the available surface is rough, scratched or uneven — typically a cast flange face or a repaired joint — the O-ring will not seat, and the compressible flat gasket wins because it conforms to the surface. A spiral-wound gasket, by contrast, is used with a rougher, controlled flange face (typically 63–250 µin AARH), because the wound metal needs some roughness to bite into. In short: a smooth, machined surface favours the O-ring, a rough cast face favours the flat or semi-metallic gasket.

Pressure, extrusion and when you need a back-up ring or a spiral

As pressure rises you hit the limit of the O-ring — extrusion. Under high pressure part of the ring is forced into the gap between components and progressively torn away, leaving the characteristic nibbled edge. The remedy is a harder material, a smaller gap, and above all a PTFE back-up ring that bridges the gap. PTFE here is not an elastomer but a rigid polymer with virtually no compression-set recovery — which is why it is used as support, not as the primary elastic seal. With rapid pressure changes in a gas there is the added risk of explosive decompression (AED), where absorbed gas tears the ring from the inside as pressure drops; this calls for an AED/RGD-certified material (FKM or FFKM in the appropriate compound).

On high-pressure, high-temperature pipe flange joints the correct answer is a semi-rigid gasket. A spiral-wound gasket (metal winding plus a soft filler, often PTFE or graphite) withstands higher pressures and temperatures than a soft sheet gasket, and its metal structure resists blow-out. A PTFE-envelope gasket combines a chemically inert PTFE jacket with a resilient core — used where the medium attacks ordinary gasket materials but the temperature stays within the PTFE limit. The choice between a soft sheet, a PTFE-envelope and a spiral-wound gasket is mainly a function of pressure, temperature and how aggressive the medium is.

Material, temperature and chemistry apply equally to both

Whether you choose an O-ring or a flat elastomer gasket, the material has to survive both the temperature and the medium. NBR (nitrile) is the cheapest workhorse for mineral oils and hydraulics to about 100 °C, but it is unsuitable for glycol-based brake fluid or ozone. FKM (Viton) raises the ceiling to roughly 200 °C and handles most fuels and acids, yet fails with amines, ketones and hot steam. EPDM is the only sensible choice for brake fluid and drinking water, but mineral oils dissolve it. VMQ (silicone) has a very wide range from −60 to +200 °C but is mechanically weak. FFKM is the most resistant elastomer, continuous to 260 °C, but costs 20 to 50 times as much as FKM — so specify the cheapest material that survives, not the most exotic.

This material logic is backed by reproducible tests. Compression set — whether the seal keeps its sealing force after long hot compression — is measured by ISO 815 and ASTM D395. Heat ageing is covered by ISO 188, hardness by ISO 48 (IRHD), and the effect of liquids on swell and mass change by ISO 1817 and ASTM D471; low-temperature brittleness is rated by ASTM D746. The rubber designation itself (NBR, FKM, EPDM…) is defined by ISO 1629. When in doubt, verify the material against the specific medium in a chemical-resistance matrix — the most common cause of premature failure is chemical incompatibility, where the ring swells, softens or hardens. On the usual scale, swell up to about 15 % by volume is acceptable, while more than 30 % means avoid the material.

A quick decision aid

In a single sentence: choose an O-ring when you have (or can machine) a precise groove, a smooth surface, a repeatedly dismountable joint and pressure from both sides — it is cheap, accurate and re-serviceable. Choose a flat gasket for standard PN pipe flanges to EN 1514, for a rough cast face with no groove, and for joints where the gasket is replaced anyway. Add a PTFE back-up ring to the O-ring at high pressure and a larger gap; use a PTFE-envelope gasket for an aggressive medium on a flange; use a spiral-wound gasket at high pressure and temperature on piping. If you are unsure of the flange type, the finish or the medium, send us the joint data and we will choose the right sealing element and its material for you.