Metallic vs Nonmetallic Flange Gaskets: ASME RFQ Guide

ASME B16.21: Nonmetallic Flat Gaskets for Pipe Flanges

ASME B16.21-2021 (published 2022) covers types, sizes, materials, dimensions, tolerances, and markings for nonmetallic flat gaskets. These gaskets are dimensionally suitable for use with flanges described in the referenced flange standards. Nonmetallic flat gaskets are typically cut from sheet material and are used in full-face or flat-face configurations. The standard does not assign pressure or temperature limits to "nonmetallic" as a family; that information comes from the specific gasket material standard, the flange class, and the operating conditions.

Key Differences in Standard Scope

This table is derived solely from the public scope descriptions. It is not a service comparison. An engineer cannot select a gasket by family name alone; the gasket must be dimensionally compatible with the flange standard, facing, and the purchaser's specification.

Procurement Checks Before You Issue an RFQ

When you source gaskets, verify the following against the contract documents:

1. Confirm the edition – Check the exact edition of ASME B16.20 or B16.21 named in your specification. Dimensional tables can change between editions, and the purchase order should state an edition.
2. Check whether the standard has been stabilized – ASME's product pages for both standards indicate they have been placed on stabilized maintenance. That means requests for change are considered, but the version remains in effect. Confirm with your engineering authority which edition is contractually valid.
3. Flange reference – Confirm the flange standard and facing. For B16.20, confirm whether the flange is from ASME B16.5, B16.47, or API-6A. For B16.21, confirm the referenced flange standard in your specification.
4. Gasket type – For B16.20, specify ring-joint, spiral-wound, or metal jacketed. For B16.21, specify the nonmetallic flat gasket type and material.
5. Material and construction limits – Do not assume a metallic or nonmetallic family name defines temperature, pressure, or chemical compatibility. The buyer must verify material limits from the applicable material standard and the design conditions.
6. Facing condition – For spiral-wound or metal jacketed gaskets, the flange face finish must suit the gasket construction. The standard page does not define service limits, so confirm with the gasket manufacturer using your specific facing dimensions.

RFQ Inputs That Prevent Misquoting

Provide these fields in your gasket RFQ:

• Gasket standard and edition, e.g., "ASME B16.20-2023" or "ASME B16.21-2021"
• Flange standard and nominal size / pressure class
• Flange facing type (e.g., raised face, flat face, ring-joint)
• Gasket type and material or filler specification
• Required dimensions, or the dimensional standard reference
• Marking requirements per the applicable ASME standard
• Quantity, packaging, and documentation requirements

If any of these fields is unknown, do not guess. Ask the end user or design engineer to confirm the flange facing and the gasket standard edition first.

Inspection and Documentation Points

At a minimum, inspection should confirm:

• Gasket marking matches the standard and the purchase order
• Dimensional tolerances comply with the specified edition
• Material identification is traceable to the applicable material specification
• For metallic gaskets, construction details (e.g., winding, jacket, ring type) match the order

Because the public standard pages do not provide dimensional tables, your inspector must use a licensed copy of the correct edition. A common risk is checking against an outdated or incorrect edition. Ensure the inspection checklist references the same edition as the purchase order.

Common Risks in Gasket Procurement

• Treating gasket family names as performance ratings – "Metallic" does not equal high temperature, and "nonmetallic" does not equal low pressure. Limits depend on material, construction, facing, and service conditions.
• Mixing flange facing types – A spiral-wound gasket intended for raised face flanges is not interchangeable with a ring-joint gasket for API-6A flanges.
• Using the wrong edition – The dimensional table must be checked against the edition named in the purchaser's specification, not a supplier's holiday schedule.
• Omitting material requirements – B16.20 covers dimensional and marking requirements; it is not a full material performance standard. Your purchasing spec must include the material standard and any supplemental requirements.

Aligning Gasket Sourcing with Flange Supply

When you order flanges and gaskets separately, confirm dimensional compatibility before the order is placed. If you are sourcing custom forged flanges or nonstandard forgings, verify that the flange-facing dimensions correspond to the gasket standard you intend to use. For nonmetallic flat gaskets in particular, the flange finish and gasket size must match. See our forged flanges and nonstandard forgings page for flange supply considerations: Forged Flanges and Nonstandard Forgings. For related piping accessories such as spectacle blinds and other isolation components, review the pipe accessories range: Pipe Accessories. These product lines give procurement teams a single source for cross-checking component dimensions before fabrication.

Sources

• ASME B16.20 - Metallic Gaskets for Pipe Flanges (ASME.org)
• ASME B16.21 - Nonmetallic Flat Gaskets for Pipe Flanges (ASME.org)