In a flanged connection system, the gasket is the component under the most complex stress, working in the harshest environment, and most prone to failure. However, its selection is often simplified to “matching the flange standard.” Superior sealing performance stems from treating the gasket as an independent systems engineering component, designed in synergy with the flange and bolts as an integral trio.
Understanding the Gasket’s “Stress-Sealing” Behavior: A gasket needs to be compressed to a specific stress during initial installation (seating stress) to achieve the initial seal. Under internal pressure, which tends to separate the flanges, bolt load is counteracted. The residual stress on the gasket must always remain above the product of the system internal pressure and the gasket factor (m) to maintain the seal. Therefore, a gasket’s performance is defined by two core parameters: y (seating stress) and m (gasket factor). Selection must reference actual data provided by gasket manufacturers based on specific test standards (e.g., EN 13555, ASME PCC-1 Appendix D), not outdated standard table values.
Dual Match of Material and Structure:
- Non-Metallic Gaskets (e.g., PTFE, Graphite): Soft, good sealability, but prone to creep relaxation, unsuitable for high temperature or severe cyclic service. Reinforced PTFE or graphite composite gaskets can mitigate this.
- Semi-Metallic Gaskets (e.g., Spiral Wound): Metal windings provide strength, non-metallic filler provides sealing; the most widely used type. Metal winding material (304SS, Inconel, etc.) is chosen for corrosion resistance, filler (graphite, PTFE, ceramic, etc.) for temperature. Performance is highly sensitive to compression.
- Metallic Gaskets (e.g., Ring Joint RJ, Octagonal): Used for High-Pressure High-Temperature services. Sealing relies on plastic deformation from line or face contact with flange grooves, requiring extremely high flange facing hardness and flatness.
Service-Driven Selection Logic:
- Thermal Cycling Service: Prioritize gaskets with resilience, like spiral wound gaskets with inner rings or corrugated metal gaskets with flexible graphite filler, to compensate for thermal movement.
- Slight Misalignment or Irregularities: Flexible graphite sheet gaskets or gaskets with elastomeric cores offer more forgiveness.
- Extreme Temperature or Corrosion: Metallic ring gaskets are often the only choice.
Failure Prevention: Most gasket failures stem from improper installation (uneven torque, incorrect sequence) or flange quality issues (facing defects, insufficient rigidity). Therefore, providing a bolt torque procedure based on PCC-1 and flange inspection guidelines is as important as supplying a quality gasket.