Glossary

Gasket relaxation is the loss of sealing stress over time in a bolted joint—meaning the gasket is no longer being squeezed as hard as it was right after assembly. In a flange, you start with bolt preload that compresses the gasket to create a seal; then the gasket (and sometimes the joint surfaces) “settle,” creep, and relax, which reduces bolt load and gasket contact stress. If that sealing stress drops below what’s needed for the service pressure, you get leakage—sometimes gradually (weep), sometimes suddenly (blowout risk in severe cases).

A big driver is creep / stress relaxation of the gasket material—under load, the gasket can thin or flow, so the bolts lose stretch and the clamp load decays. This gets dramatically more important at elevated temperature; the Fluid Sealing Association handbook notes that torque loss is usually minor at ambient conditions for many materials, but becomes a serious consideration as temperature climbs (they call out ~90 °C / 194 °F as a point where it can start to matter a lot).

There’s also embedment / seating loss—tiny high spots on flange faces, washers, and threads flatten out after tightening, and the gasket “beds in.” That initial “settling” can take a meaningful bite out of preload even before the system ever sees full temperature and pressure.

Standards often quantify this behavior. For example, EN 13555 uses parameters that describe how much gasket stress remains after relaxation—commonly expressed as a ratio of remaining stress to initially applied stress (often shown as PQR in gasket data). A PQR of 0.5, for instance, implies about half the initial gasket stress remains after the specified relaxation test conditions.

Because relaxation is so common, best practice procedures often include a controlled “second tightening.” Industry guidance talks about relaxation passes (retightening after a dwell period at ambient) and start-up retorque/hot retorque (retightening after the joint has seen temperature/process) to restore preload lost to early creep/settlement—when the joint design and safety constraints allow it.

In practical terms, you fight gasket relaxation with a mix of materials and mechanics: choose a gasket with appropriate creep-relaxation behavior for the temperature/chemical service, use a bolting strategy that maintains clamp load (proper tightening method, lubrication control, correct bolt stress target), and—where appropriate—use techniques that keep the joint “springy” (more bolt stretch, live-loading washers in some applications, stiffer flanges) so the system can tolerate some gasket thickness loss without losing sealing stress.