What Is ASME B16.21?
ASME B16.21 is the American National Standard for nonmetallic flat gaskets for pipe flanges. It covers full-face gaskets and flat-ring gaskets in classes 150 through 900, with dimensional tables for the flange sizes of ASME B16.5.
Gasket Types & Materials
- Full Face (FF) — covers the entire flange face, bolted through the gasket
- Flat Ring (RF) — fits inside the bolt circle on raised-face flanges
- Materials — rubber, PTFE, non-asbestos fiber (compressed), flexible graphite
Pressure Classes
Dimensional tables cover Class 150, 300, 400, 600, and 900. ASME B16.21 removed size tables above Class 900 — for higher classes use metallic or semi-metallic gaskets per ASME B16.20.
Related Standards
FAQ
What is the maximum pressure class for B16.21 gaskets?
Class 900. Above Class 900 the standard provides no size tables — metallic gaskets per B16.20 are used instead.
Full face or flat ring — which do I need?
Full-face gaskets suit flat-face (FF) flanges; flat-ring gaskets suit raised-face (RF) flanges. Check your flange face type first.
Is PTFE suitable for hot service?
PTFE is best for chemical resistance up to roughly 260°C; above that use graphite-based or metallic gaskets.
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