ASME B16.21 Nonmetallic Flat Gasket Selection Guide

Scope of ASME B16.21

ASME B16.21, titled Nonmetallic Flat Gaskets for Pipe Flanges, is the key dimensional and marking standard for nonmetallic flat gaskets used in bolted flange connections. According to the ASME publication page, the standard covers "types, sizes, materials, dimensions, tolerances, and markings" for these gaskets. It also states that the gaskets are "dimensionally suitable for use with flanges described in the referenced flange standards."

That last phrase is critical. B16.21 does not exist in isolation. It is designed to work with flanges that meet other ASME B16 standards, most commonly ASME B16.5. The standard itself is in stabilized maintenance, meaning it is no longer being actively revised, but it remains in effect and is widely referenced in procurement specifications.

Why the Flange Standard Matters

ASME B16.5, Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24, Metric/Inch Standard, defines the pressure-temperature ratings, materials, dimensions, tolerances, marking, testing, and methods of designating openings for pipe flanges and flanged fittings. It also includes "requirements and recommendations regarding flange bolting, flange gaskets, and flange joints."

However, B16.5 does not govern the chemical compatibility of every gasket material. The standard tells you the flange face dimensions, bolt circle, and rating class—but it does not tell you whether a particular rubber sheet will survive a specific process fluid at a specific temperature. That decision rests with the gasket material manufacturer and the system designer.

When you specify a nonmetallic flat gasket, you must identify both the gasket standard and the mating flange standard. A gasket cut to B16.21 dimensions may fit a B16.5 flange, but if the flange is actually manufactured to B16.47 or EN 1092-1, the gasket dimensions may not match. This is one of the most common sourcing errors in industrial piping.

Procurement Checks Before You Specify

Before you issue a purchase order for nonmetallic flat gaskets, verify the following with your engineering team:

• Flange standard and size: Confirm whether the mating flanges are ASME B16.5, ASME B16.47, or another standard. B16.21 gaskets are dimensionally suitable for flanges described in the referenced flange standards, but you need to know exactly which standard applies.
• Flange facing type: Nonmetallic flat gaskets are typically used with flat face or raised face flanges. Confirm the flange face finish and gasket surface area to ensure proper seating.
• Gasket material: ASME B16.21 covers nonmetallic materials in general, but it does not assign temperature or chemical limits to those materials. You must obtain the manufacturer's data sheet for the specific material grade (e.g., rubber, PTFE, compressed fiber, or other nonmetallic) and verify compatibility with your process fluid, temperature, and pressure.
• Rating class: Match the gasket's pressure rating to the flange rating class (150, 300, 600, etc.). B16.21 does not assign pressure ratings by itself; the flange standard and gasket manufacturer's data together define the allowable service envelope.
• Marking requirements: B16.21 includes marking requirements. Your inspection plan should confirm that each gasket is marked according to the order specification and the standard.

If any of these details are missing, do not guess. Ask the manufacturer for a dimensional report and material test certificate before committing to a batch.

RFQ Inputs for Nonmetallic Flat Gaskets

When sending an inquiry for nonmetallic flat gaskets, include the following minimum information:

• Applicable gasket standard: ASME B16.21 and the edition year, if your project specifies a particular edition.
• Mating flange standard and rating: For example, ASME B16.5 Class 150 raised face flange, NPS 4.
• Gasket inside and outside diameter: If you are not using a standard size from B16.21, provide the exact dimensions. B16.21 covers standard types and sizes, but special dimensions require explicit confirmation.
• Material grade and manufacturer reference: If you have approved a specific material, list the grade or brand name. If not, provide the service conditions (fluid, temperature, pressure, and any chemical resistance requirements) so the supplier can propose a suitable material.
• Quantity and delivery requirement: State whether you need a full set for hydrotesting, a spare set, or production quantities.
• Certification requirements: Specify whether you need a material certificate, dimensional inspection report, or third-party inspection.

You can streamline the process by sending a detailed RFQ through our inquiry page. For a broader view of what your piping system requires, browse our product catalog to see how flanges, fittings, and related components are grouped under different standards.

Inspection and Documentation Points

Even for a relatively simple component like a flat gasket, inspection matters. Include these checks in your quality plan:

• Dimensional verification: Measure the gasket's inside diameter, outside diameter, and thickness. Compare them to the B16.21 tables and the flange dimensions per the mating standard. Pay attention to tolerances.
• Material traceability: Request a material certificate that identifies the polymer or fiber compound. Confirm that the certificate is consistent with the purchase order and the manufacturer's data sheet.
• Marking check: Verify that each gasket is marked with the required information, such as the manufacturer's name or trademark and the size or standard designation, as applicable.
• Visual inspection: Look for cuts, tears, voids, or surface contamination. Nonmetallic gaskets are easily damaged in handling, and a defect can lead to a leak path.
• Packaging and protection: Ensure gaskets are packed flat, without folding, and protected from heat, sunlight, and ozone, which can degrade some nonmetallic materials.

Document all inspection results in a report that can be shared with the project quality manager or third-party inspector. If you require a third-party inspection, specify that in the purchase order and arrange witness or review points.

Common Risks and How to Avoid Them

One of the biggest risks in nonmetallic flat gasket procurement is assuming that a generic "nonmetallic" material is suitable for all services. The ASME standard does not state universal temperature or chemical limits. The limits depend on the exact material and manufacturer data. A gasket that works in a water line may fail in a hydrocarbon service, even if it meets B16.21 dimensions.

Another risk is mismatching the gasket to the flange standard. B16.21 gaskets are dimensionally suitable for flanges described in the referenced flange standards, but the purchaser is responsible for confirming which flange standard is being used. If your project uses EN or JIS flanges, you cannot automatically use a B16.21 gasket without checking the dimensions.

A third risk is neglecting bolt torque and gasket seating stress. While this is not defined by B16.21, the gasket supplier's recommended installation torque is essential for a leak-free joint. Ask the manufacturer for installation guidance and provide it to the assembly crew.

Finally, verify the edition of the standard. Standards can be revised or reaffirmed over time. If your contract references a particular edition of B16.21, confirm that the gasket supplier is using that edition and not an older or newer one. The current 2021 edition was published in 2022, but your project specification may reference an earlier edition.

Sources

• https://www.asme.org/codes-standards/find-codes-standards/b16-21-nonmetallic-flat-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