What Is an ASME B16.21 Flat Ring Gasket?
A flat ring gasket is a nonmetallic washer that fits inside the bolt circle of a raised-face (RF) flange. ASME B16.21 covers nonmetallic flat gaskets for pipe flanges; Table 4.1-5 lists flat-ring dimensions for ASME B16.5 flanges in Classes 300, 400, 600 and 900 (Class 150 flat-ring dimensions are listed separately in Table 4.1-4).
Pressure Classes
ASME B16.21 dimensional tables cover Class 150 through Class 900. Above Class 900 the standard provides no flat-gasket size tables — service then uses metallic or semi-metallic gaskets per ASME B16.20.
Flat Ring Dimensions — B16.21 Table 4.1-5 (mm / in.)
| NPS | Gasket ID (mm/in.) | Flat Ring OD, Class 300 (mm/in.) | Flat Ring OD, Class 400 (mm/in.) | Flat Ring OD, Class 600 (mm/in.) | Flat Ring OD, Class 900 (mm/in.) |
|---|---|---|---|---|---|
| 1/2 | 21 (0.84) | 54 (2.12) | 54 (2.12) | 54 (2.12) | 64 (2.50) |
| 3/4 | 27 (1.06) | 67 (2.62) | 67 (2.62) | 67 (2.62) | 70 (2.75) |
| 1 | 33 (1.31) | 73 (2.88) | 73 (2.88) | 73 (2.88) | 79 (3.12) |
| 1 1/4 | 42 (1.66) | 83 (3.25) | 83 (3.25) | 83 (3.25) | 89 (3.50) |
| 1 1/2 | 48 (1.91) | 95 (3.75) | 95 (3.75) | 95 (3.75) | 98 (3.88) |
| 2 | 60 (2.38) | 111 (4.38) | 111 (4.38) | 111 (4.38) | 143 (5.62) |
| 2 1/2 | 73 (2.88) | 130 (5.12) | 130 (5.12) | 130 (5.12) | 165 (6.50) |
| 3 | 89 (3.50) | 149 (5.88) | 149 (5.88) | 149 (5.88) | 168 (6.62) |
| 3 1/2 | 102 (4.00) | 165 (6.50) | 162 (6.38) | 162 (6.38) | — |
| 4 | 114 (4.50) | 181 (7.12) | 178 (7.00) | 194 (7.62) | 206 (8.12) |
| 5 | 141 (5.56) | 216 (8.50) | 213 (8.38) | 241 (9.50) | 248 (9.75) |
| 6 | 168 (6.62) | 251 (9.88) | 248 (9.75) | 267 (10.50) | 289 (11.38) |
| 8 | 219 (8.62) | 308 (12.12) | 305 (12.00) | 321 (12.62) | 359 (14.12) |
| 10 | 273 (10.75) | 362 (14.25) | 359 (14.12) | 400 (15.75) | 435 (17.12) |
| 12 | 324 (12.75) | 422 (16.62) | 419 (16.50) | 457 (18.00) | 498 (19.62) |
| 14 | 356 (14.00) | 486 (19.12) | 483 (19.00) | 492 (19.38) | 521 (20.50) |
| 16 | 406 (16.00) | 540 (21.25) | 537 (21.12) | 565 (22.25) | 575 (22.62) |
| 18 | 457 (18.00) | 597 (23.50) | 594 (23.38) | 613 (24.12) | 638 (25.12) |
| 20 | 508 (20.00) | 654 (25.75) | 648 (25.50) | 683 (26.88) | 699 (27.50) |
| 22 | 559 (22.00) | 705 (27.75) | 702 (27.63) | 733 (28.88) | — |
| 24 | 610 (24.00) | 775 (30.50) | 768 (30.25) | 791 (31.12) | 838 (33.00) |
Materials
- PTFE — excellent chemical resistance, up to about 260°C
- Rubber (NBR, EPDM, Viton) — water, oil and acid service
- Compressed non-asbestos fibre — general steam and hydrocarbon service
- Flexible graphite — high-temperature service
Related Standards
- ASME B16.21 — Nonmetallic Flat Gaskets (Parent Standard)
- ASME B16.21 Full Face Gaskets
- ASME B16.5 — Pipe Flanges (NPS 1/2–24)
- ASME B16.20 — Metallic Gaskets (above Class 900)
- Our PTFE Gasket Range
- Our Gasket Range
FAQ
What pressure limit does B16.21 flat ring gasket have?
Class 900. B16.21 removes size tables above Class 900; use B16.20 metallic/semi-metallic gaskets for higher classes.
Flat ring or full face — which do I need?
Flat-ring gaskets suit raised-face (RF) flanges; full-face gaskets cover the whole flange face with bolt holes, for flat-face (FF) flanges.
Is PTFE suitable for hot service?
PTFE is best for chemical resistance up to roughly 260°C; above that use flexible graphite or metallic gaskets.
What NPS range is covered?
NPS 1/2 through NPS 24 for B16.5 flanges; B16.21 also gives tables for B16.47 large-diameter flanges (NPS 26–60).
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