Flat Seal / Gasket

Definition and Context (Static Sealing)

A flat seal (also called a gasket) is a flat sealing element in the form of a ring or a plate that seals two stationary component surfaces against one another. It is placed between the sealing surfaces and compressed during bolting or clamping. As a result, it closes gaps and bridges small shape deviations.

In practice, flat seals (gaskets) appear wherever flanges, covers, housing halves, or maintenance openings must be tight. The application is usually static: in operation, there is no relative motion between the sealing surfaces. As a result, the flat seal differs from dynamic seals, which seal shafts or pistons against motion.

The sealing effect arises because the gasket compensates for micro-unevenness (very small roughness and shape deviations) through contact pressure. Therefore, which flat seal is suitable always depends on which medium (e.g., water, oil, chemicals, steam) is being sealed, which pressure and which temperature apply, and how the sealing surfaces are manufactured.

Distinction from O-Ring and Other Sealing Principles

An O-ring has a round cross-section and typically sits in a groove that defines its position and deformation. Depending on the design, it can therefore also cover applications with slight motion. A flat seal, by contrast, is mostly pressed across a planar sealing line and usually does not require a groove, but a suitable sealing surface and a defined bolt preload.

The following classification helps with orientation:

Working Principle: Surface Pressure, Bolt Preload, and Sealing Surfaces

The central working principle is surface pressure. This refers to the pressure that transfers from the bolt force onto the sealing surface. When the gasket is compressed through bolt preload (deliberately introduced tensile force in the bolts), it conforms and interrupts potential leak paths.

What is decisive is that the contact pressure is sufficiently high and evenly distributed. Insufficient preload leaves leak paths open. Uneven preload leads to local leakage, even though individual areas can be heavily compressed. Therefore, assembly and design often influence tightness more strongly than material choice alone.

The effective contact pressure can decrease over time and operation. The causes are settling (short-term thickness change after installation) and relaxation/creep (slow stress decay under load, particularly at higher temperatures). As a result, surface pressure drops, and the gasket can become leaky after some operating time, even though it was initially tight.

Sealing-Surface Roughness and Flatness: Requirements and Typical Effects

Roughness describes the fine, microscopic unevenness of a surface. Flatness describes whether the surface as a whole is planar. Both quantities determine whether the gasket can reliably “close” the surface.

When the surface is too rough or damaged by score marks, preferred leak channels form. When it is very smooth, adhesion and conformability can become unfavorable, depending on the gasket material, or the gasket is loaded more heavily through local overload. Contamination and particles act similarly to notches: they create local leaks or damage the gasket during tightening.

In practice: sealing surfaces must be clean, planar, and free of scratches, and the material must match the actual surface quality. Soft materials can compensate for roughness more easily, while stiffer materials depend more strongly on a suitable surface finish.

Materials and Selection by Medium, Pressure, and Temperature

Flat seals (gaskets) are manufactured from various material groups, because no single material class covers all media, temperatures, and pressures equally. In sealing technology, the selection usually starts with the question of which medium is being sealed and which temperature and pressure occur in operation. Next, it is checked whether the material can hold the required surface pressure over time — that is, how strongly it tends to settle or creep.

Typical material groups are elastomers, PTFE, graphite, and fiber/composite materials. Final suitability always depends on the specific product configuration, yet the following comparison helps as a technical orientation:

Typical Application Profiles of the Material Groups (Orientation)

Elastomer gaskets are frequently chosen when sealing surfaces are not perfect and moderate temperatures apply, because they compensate for unevenness well. PTFE is often the first choice when chemical resistance dominates; the creep behavior must then be considered through design and assembly. Graphite is frequently used at higher temperatures, because it is more thermally stable. Fiber and composite gaskets are widespread in many flange connections, yet their suitability depends strongly on the specific formulation and operating data.

Standards, Designs, and Typical Failure Patterns

For flat seals, dimensions and shapes are frequently standardized. In the European context, DIN EN 1514 is a central standards series for flange gaskets. In the context of flange connections, DIN 2690 is often used. Internationally, ASME B16.21 is an important reference for non-metallic flat gaskets — for example, in plants designed according to ASME codes.

Designs range from ring-shaped gaskets to full-face plates. Depending on the system, there are versions with additional rings (e.g., as inner or outer rings) that primarily stabilize the position or influence the loading. Which form makes sense is decided by geometry, bolt pattern, and the permissible sealing-surface area.

Common failure patterns can almost always be traced back to two causes: insufficient or unstable surface pressure, or leak paths arising from surface defects.

Failure Diagnosis in Practice (Short Checklist)

When there is uncertainty about medium, temperature, or assembly conditions, a short consultation with specialized sealing-technology expertise is often sensible.