ASME B16.21

Covers — types, sizes, materials, dimensions, tolerances, and markings for nonmetallic flat gaskets in full-face and ring styles, suitable for the flanges of referenced standards (B16.5 etc.).

Does NOT cover (exclusions) — the following are governed elsewhere:

• Metallic gaskets (spiral-wound/RTJ/jacketed) → ASME B16.20.
• Gasket material chemical/temperature performance → the material manufacturer's data.
• Bolted-joint assembly → ASME PCC-1.
• PN/European gaskets → EN 1514.

Use full-face nonmetallic gaskets on flat-face flanges to avoid flange rotation and cracking (especially cast iron).

B16.21 sets minimum requirements for nonmetallic flat gaskets:

• Types — full-face and ring (inside-bolt-circle) styles.
• Dimensions — sizes and bolt-hole layouts matched to the flange standard, class, and facing.
• Materials — nonmetallic material categories (with performance per manufacturer data).
• Marking — identification of size, class, and material.

ASME B16.21 is the dimensional standard for nonmetallic (soft) flat gaskets used with pipe flanges — full-face gaskets for flat-face flanges and ring gaskets for raised-face flanges. It fixes sizes, tolerances, and bolt-hole layouts so a gasket fits its flange.

It governs geometry, not material performance: the choice of compressed fibre, PTFE, elastomer, or flexible graphite for a given temperature and chemical service is made from the manufacturer's data. Metallic gaskets are under B16.20.

Engineering Note: On flat-face (commonly cast-iron) flanges, always specify a full-face gasket — a ring gasket lets the flange bend against the raised bolt reaction and can crack brittle cast iron.