ASME B16.21 Nonmetallic Flat Gaskets: Scope and Procurement
Standards
ASME B16.21 Nonmetallic Flat Gaskets: What the Standard Covers
ASME B16.21 defines dimensions, materials, and markings for nonmetallic flat gaskets used with ASME flanges. Learn what it covers, what it doesn't, and how to specify gaskets correctly.
August 29, 2026·5 min read·Hebei Haihao Group
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Introduction
When you specify a flanged joint, the gasket is often the last thought—yet it is the most common source of leaks. ASME B16.21 is the key standard for nonmetallic flat gaskets used with ASME flanges. This article explains what the standard covers, what it does not, and what you must verify before you buy.
What ASME B16.21 Covers
ASME B16.21, titled Nonmetallic Flat Gaskets for Pipe Flanges, covers the following:
Types – The standard defines several types of nonmetallic flat gaskets, including full-face and flat-ring (inside-bolt-circle) styles.
Sizes – It provides dimensions for gaskets suitable for flanges from NPS 1/2 through NPS 24, matching the flange sizes in ASME B16.5.
Materials – It lists acceptable nonmetallic materials, such as compressed asbestos-free fiber, PTFE, rubber, and others. However, it does not specify material grades or performance limits.
Dimensions and tolerances – It gives precise dimensional requirements, including inside diameter, outside diameter, and thickness, with tolerances.
Marking – It requires gaskets to be marked with the manufacturer's name or trademark, the standard designation, and the size.
The standard's scope states that these gaskets are "dimensionally suitable for use with flanges described in the referenced flange standards." In other words, ASME B16.21 ensures that the gasket will fit the flange, but it does not guarantee that the gasket will seal under your specific service conditions.
What ASME B16.21 Does Not Cover
ASME B16.21 is a dimensional standard. It does not cover:
Media compatibility – The standard does not tell you whether a gasket material is suitable for a particular fluid.
Maximum temperature or pressure ratings – It does not provide pressure-temperature ratings for gaskets.
Emissions performance – It does not address fugitive emissions or leakage rates.
Assembly load – It does not specify bolt torque or gasket stress.
These factors depend on the gasket material, the flange facing, the surface finish, and the operating conditions. They must be determined by the design engineer and the project specification.
Relationship with ASME B16.5
ASME B16.5, Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24, is the companion standard for flanges. It covers:
Pressure-temperature ratings
Materials
Dimensions and tolerances
Marking and testing
Methods of designating openings
ASME B16.5 also includes requirements and recommendations for flange bolting, gaskets, and joints. When you specify a gasket, you must ensure it matches the flange standard and the flange facing (e.g., raised face, flat face, RTJ).
Procurement Checks for ASME B16.21 Gaskets
When you purchase nonmetallic flat gaskets, verify the following:
Standard designation – Ensure the gasket is marked with ASME B16.21 and the correct size.
Material specification – Confirm the material grade and that it meets your project's requirements (e.g., ASTM F104 for fiber gaskets, ASTM D3294 for PTFE).
Dimensions – Check the inside diameter, outside diameter, and thickness against the flange dimensions and the standard.
Tolerances – Verify that the gasket dimensions are within the tolerances specified in ASME B16.21.
Marking – Ensure the gasket is marked with the manufacturer's name, size, and standard.
RFQ Inputs
When you send an RFQ for gaskets, include:
Flange standard and size (e.g., ASME B16.5 Class 150, NPS 4)
Flange facing (e.g., raised face, flat face)
Gasket type (full-face or flat-ring)
Material grade and any applicable material standard
Thickness (if not standard)
Quantity and packaging requirements
Any special testing or certification requirements (e.g., material test certificates)
Inspection and Documentation
For critical service, you may want to:
Request material test certificates (e.g., EN 10204 3.1) to verify material composition and properties.
Perform dimensional inspection on receipt, using the tolerances in ASME B16.21.
Check for surface defects, cuts, or contamination.
Verify that the gasket is clean and free of foreign matter.
Common Risks and How to Avoid Them
Using the wrong gasket type – A full-face gasket is not suitable for a raised-face flange. Always match the gasket to the flange facing.
Assuming material compatibility – ASME B16.21 does not guarantee chemical compatibility. Consult the material manufacturer's data.
Ignoring surface finish – The flange surface finish affects sealing. ASME B16.5 recommends a finish of 125 to 250 microinches for raised-face flanges.
Over-tightening or under-tightening – Follow the bolting recommendations in ASME B16.5 and the gasket manufacturer's instructions.
Conclusion
ASME B16.21 is essential for ensuring that nonmetallic flat gaskets fit ASME flanges. However, it is only one part of the specification. You must also consider material compatibility, pressure-temperature limits, and assembly procedures. Always verify the gasket's suitability for your service conditions with the manufacturer.
For more information on flanges and gaskets, explore our products or contact us for expert guidance.
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Aug 14, 20266 min readHebei Haihao Group
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