The Complete Handbook of Gaskets & O-Rings

Chapter 1: Understanding Gaskets & O-Rings

A gasket is a mechanical seal that fills the space between two or more mating surfaces, generally static ones like flanges. An O-ring is a torus, a doughnut shape, usually made of elastomer, that sits in a groove and is compressed to form a seal at rest or during motion, such as a piston inside a cylinder. They are only two specific components, but their failures can take down an entire plant in minutes. Understanding what they actually do under stress, heat, and chemical exposure, rather than just seeing them as commodity rubber parts, is the basis for every decision you will make when selecting, installing, and maintaining them.

What a seal actually does

Sealing is a battle against the environment. A gasket has one primary job: to be deformed just enough to fill the microscopic surface irregularities of the flange faces it sits between, and to do so while resisting the internal pressure and the medium trying to escape. It creates a barrier because the bolt load holds the flanges together, squeezing the gasket so it flows (elastically or plastically) into the tiny voids of the machined surfaces. The gasket material must therefore be softer than the flanges, but strong enough to resist being blown out by pressure.

Consider a 4-inch class 150 flange in a steam line. At 150 psi and 300°F, the internal forces are attempting to push the gasket out of the joint. The bolts are doing the holding. The