ASME B16.20 Metallic Gasket Selection for Flanged Joints

How ASME B16.5 Defines the Flange

ASME B16.5, Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24, Metric/Inch Standard, is the companion document that defines the flange itself. It covers pressure-temperature ratings, materials, dimensions, tolerances, marking, testing, and methods of designating openings for pipe flanges and flanged fittings.

The flange class designations covered by ASME B16.5 are 150, 300, 400, 600, 900, and 1500 in sizes NPS 1/2 through NPS 24, plus class 2500 in sizes NPS 1/2 through NPS 12. Requirements are given in both metric and U.S. customary units, with bolt and bolt hole diameters expressed in inch units. The standard also addresses flanged fittings for classes 150 and 300 in sizes NPS 1/2 through NPS 24, with some additional coverage for higher classes in a non-mandatory appendix.

ASME B16.5 is limited to flanges and flanged fittings made from cast or forged materials, and to blind flanges and certain reducing flanges made from cast, forged, or plate materials. Importantly, B16.5 also includes requirements and recommendations regarding flange bolting, flange gaskets, and flange joints.

Gasket dimensional compliance and flange dimensional compliance are related checks, but they are governed by separate standards. A B16.20 gasket is designed to suit a B16.5 flange dimensionally, but you must specify and verify both sides of the joint. Using a gasket from one standard with a flange from another can cause leakage or mechanical interference.

Selection Is More Than a Pressure Class

A common mistake is to select a gasket based only on the flange pressure class, such as "Class 300" or "Class 600." That approach is incomplete and can be unsafe. You must also evaluate:

• Flange facing type — raised face (RF), flat face (FF), or ring-joint (RTJ).
• Service fluid — process media can affect gasket material compatibility.
• Temperature — likely operating and design temperatures drive material selection.
• Gasket material — metal grade and, for spiral-wound types, filler material.
• Bolt load — gasket seating stress and available bolt preload.
• Project specification — many EPC projects impose additional requirements beyond the base standard.

ASME B16.20 gives you the dimensional envelope, but it does not rank materials or assign a pressure rating to a particular gasket type. The correct selection is made by the piping designer or engineer who has complete knowledge of the service conditions.

Procurement Checks and RFQ Inputs

When you issue an inquiry or request for quotation, include the following data to avoid ambiguity:

• Flange standard and edition, e.g., ASME B16.5.
• Flange size and pressure class.
• Flange facing type and, for RTJ, the ring groove number.
• Gasket type: ring-joint, spiral-wound, or metal jacketed.
• Gasket material and filler specification, if applicable.
• Quantity and any project-specific marking requirements.

Before placing an order, verify that the supplier is offering a gasket manufactured to ASME B16.20 and dimensionally compatible with the specified flange standard. If the flange is ASME B16.47 or API-6A, confirm that the gasket is intended for that standard as well, because dimensions differ from B16.5.

Ask the supplier whether the gasket will be supplied with a material test certificate or a certificate of conformance. ASME B16.20 addresses marking, but material compliance is typically demonstrated through other ASTM or ASME material specifications. If the supplying mill cannot provide traceable material documentation, you may need to require third-party inspection.

Inspection and Documentation Points

Your inspection plan should include:

• Dimensional verification — check outside diameter, inside diameter, gasket thickness, and ring-joint profile against ASME B16.20 tables.
• Marking — each gasket should bear the manufacturer's identification and the standard designation, as required by the standard.
• Surface condition — no cuts, scratches, burrs, or delamination that could impair sealing.
• Material certification — review the mill test certificate or material certificate against your purchase order.

If you are working with an independent inspection agency, make sure the inspector has a copy of the current ASME B16.20 edition and the matching flange standard. Dimensional inspection must be done with calibrated instruments and a clear pass/fail criterion.

Common Risks to Manage

Several recurring issues appear in procurement and site installation:

1. Mixing standards — using a gasket made to B16.20 with a flange made to a non-ASME standard, without checking dimensions.
2. Wrong facing — ordering a spiral-wound gasket for a flat face flange without confirming that the gasket is suitable for that facing.
3. RTJ groove mismatch — ring-joint gaskets require precise groove dimensions; a gasket from one flange standard may not fit a groove from another.
4. Ignoring bolt load — the gasket may be dimensionally correct but unable to seat under the actual bolting torque.
5. Unverified project specifications — a buyer may default to a standard gasket when the project specification calls for additional testing or material constraints.

These risks are best managed by writing a clear specification and reviewing it with the gasket manufacturer before production.

Final Check

ASME B16.20 defines the gasket; ASME B16.5 defines the flange. Both standards must be consulted together to achieve a dimensionally compatible joint. Beyond that, you must apply engineering judgment for service conditions, material selection, and installation.

To review the piping products we manufacture and the standards we work to, visit our product catalog. If you are preparing an inquiry for gaskets or flanged assemblies, send us the relevant details through our inquiry page so we can confirm compliance before you commit to a purchase order.

A disciplined selection process will save time, reduce leakage, and avoid costly rework.

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