A gasket sits between two flanges and deforms to fill the microscopic surface irregularities that prevent metal-on-metal contact from sealing. The flanges aren't actually flat — even a machined surface has peaks and valleys measured in microns. The gasket flows into those gaps under bolt preload, creating a continuous barrier against the fluid or gas you're trying to contain.

The fundamental rule: contact pressure on the gasket must exceed the internal fluid pressure by a factor called the m-factor (maintenance factor), typically 2-6x depending on gasket material. If your system runs at 150 psi and you're using a spiral-wound gasket with m=3, you need at least 450 psi of residual contact stress on the gasket face after pressurization.

Common gasket types and where they fit:

• Compressed fiber sheet — cheap, general-purpose for low pressure (under 300 psi) and moderate temperature. Cut to shape with a knife.
• Rubber (NBR, EPDM, Viton) — excellent for water, fuel, and chemical service at low pressures. Viton handles 200°C+; EPDM hates petroleum.
• Spiral-wound (metal + filler) — the workhorse for high-pressure flanges in oil & gas and steam systems. Stainless windings with graphite or PTFE filler.
• Solid metal (copper, aluminum, soft iron) — extreme temperature/pressure, like cylinder head gaskets or vacuum flanges (copper CF gaskets in UHV systems).
• O-rings — technically a gasket variant; rely on the groove geometry, not bolt force, to set compression (typically 15-30%).

Why gaskets fail: the dominant failure mode isn't the gasket — it's bolt relaxation. Over hours and days after installation, the gasket creeps, the bolts lose preload, and contact pressure drops below the seal threshold. This is why critical flanges get re-torqued after a heat cycle and why "hot bolting" exists as a standard procedure.

Quick sizing rule: required bolt load ≈ (hydrostatic end force) + (gasket seating load). For a 4-inch flange at 300 psi:

• End force = π × (2)² × 300 ≈ 3,770 lbf
• Gasket seating (typical y=3,700 psi for spiral-wound, gasket area ≈ 5 in²) ≈ 18,500 lbf
• Total bolt load ≈ 22,000 lbf, distributed across 8 bolts → 2,750 lbf preload each

Installation matters as much as selection: always use a star pattern, torque in 3 passes (30%, 70%, 100%), and never reuse a compressed gasket. A scratched flange face under 125 µin Ra surface finish will leak no matter what gasket you specify.