ASME B16.20 Metallic Gaskets: Types and Flange Compatibility

Understanding ASME B16.20

ASME B16.20, "Metallic Gaskets for Pipe Flanges," is the key standard for metallic gaskets used in bolted flange connections. It specifies materials, dimensions, tolerances, and marking requirements for three main types: metal ring-joint gaskets, spiral-wound metal gaskets, and metal-jacketed gaskets. These gaskets are designed to be dimensionally compatible with flanges covered by ASME B16.5 (NPS 1/2 through NPS 24), ASME B16.47 (large diameter flanges), and API 6A (wellhead equipment). The standard also includes requirements for filler material used in spiral-wound gaskets.

For EPC engineers and procurement managers, understanding B16.20 is essential when specifying gaskets for new projects or replacement parts. However, dimensional compatibility alone does not guarantee performance in your specific service conditions. You must verify chemical compatibility, temperature limits, fire safety, and fugitive emissions requirements separately.

Types of Metallic Gaskets Covered

Metal Ring-Joint Gaskets

Ring-joint gaskets are solid metal rings with an oval or octagonal cross-section. They are used with ring-type joint (RTJ) flanges, which have a machined groove that the gasket seats into. When bolted, the gasket deforms to create a metal-to-metal seal. B16.20 specifies dimensions for these gaskets, including ring number, outside diameter, width, and height, ensuring they match the flange groove dimensions.

Spiral-Wound Gaskets

Spiral-wound gaskets consist of a V-shaped metal strip wound spirally with a soft filler material (e.g., graphite or PTFE). They are available with inner and outer rings for centering and compression control. B16.20 covers spiral-wound gaskets for raised-face and flat-face flanges, specifying dimensions for the gasket body, rings, and filler material. These gaskets are widely used in process piping due to their resilience and ability to handle pressure and temperature fluctuations.

Metal-Jacketed Gaskets

Metal-jacketed gaskets have a soft filler core (e.g., ceramic, graphite, or non-asbestos) enclosed in a metal jacket (e.g., stainless steel, copper, or Monel). They are used in applications requiring high temperature resistance and chemical compatibility. B16.20 defines dimensions for various jacket styles, including single-jacketed, double-jacketed, and corrugated types. These gaskets are typically used in heat exchangers and other flanged connections where a robust seal is needed.

Flange Compatibility and Standards

B16.20 gaskets are dimensionally suitable for flanges per:

• ASME B16.5: Pipe flanges and flanged fittings NPS 1/2 through NPS 24, with pressure classes 150 through 2500.
• ASME B16.47: Large diameter steel flanges NPS 26 through NPS 60, Series A and B.
• API 6A: Wellhead and Christmas tree equipment flanges.

When selecting a gasket, you must match the gasket type and dimensions to the flange facing (e.g., raised face, flat face, RTJ) and the flange standard. For example, ring-joint gaskets are used only with RTJ flanges, while spiral-wound gaskets can be used with raised-face flanges (with appropriate inner ring) or flat-face flanges (with full-face design).

Procurement Checks and RFQ Inputs

When sourcing gaskets to ASME B16.20, include the following in your RFQ:

• Gasket type: Ring-joint, spiral-wound, or metal-jacketed.
• Flange standard and size: e.g., ASME B16.5 Class 300, NPS 6.
• Flange facing: Raised face, flat face, or RTJ.
• Material specification: For metal parts (e.g., 304 SS, 316 SS, Inconel) and filler (e.g., graphite, PTFE).
• Pressure and temperature rating: Ensure the gasket is suitable for your design conditions.
• Marking requirements: B16.20 specifies marking content; verify the gasket is marked with the manufacturer's name, material, and size.

Also, request a material test certificate (MTC) per EN 10204 3.1 or 3.2 to verify material composition and mechanical properties. If the gasket is for critical service, consider third-party inspection to verify dimensions and material.

Inspection and Documentation Points

During inspection, check:

• Dimensions: Measure gasket outside diameter, inside diameter, thickness, and ring dimensions against B16.20 tables.
• Material verification: Perform PMI (positive material identification) on metal parts to confirm alloy grade.
• Surface finish: For ring-joint gaskets, check for scratches or defects that could affect sealing.
• Marking: Verify that each gasket is marked as required by the standard.

Documentation should include the MTC, inspection reports, and a certificate of conformance to B16.20.

Common Risks and How to Mitigate

• Mismatched gasket and flange: Always confirm the flange standard and facing before ordering. A gasket that fits B16.5 may not fit B16.47 or API 6A flanges.
• Material incompatibility: B16.20 does not dictate material selection for service. Verify chemical compatibility with the process fluid and temperature limits.
• Improper installation: Even a compliant gasket can fail if bolt torque is incorrect. Use an approved assembly procedure and follow manufacturer's recommendations.
• Counterfeit products: Purchase from reputable manufacturers and verify documentation to avoid non-conforming gaskets.

Conclusion

ASME B16.20 provides a solid foundation for specifying metallic gaskets, but it is only one part of the equation. For a successful flange joint, you must also consider the flange standard, gasket material, and installation practices. Always verify that the gasket you select meets the dimensional requirements of the flange and is suitable for your service conditions. For more information on flanges and fittings, explore our products page or contact us for assistance.

Sources

• https://www.asme.org/codes-standards/find-codes-standards/b16-20-metallic-gaskets-pipe-flanges
• https://www.asme.org/codes-standards/find-codes-standards/b16-5-pipe-flanges-flanged-fittings-nps-1-2-nps-24-metric-inch-standard