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ASME B16.21-2016 — Nonmetallic Flat Gaskets for Pipe Flanges
🔧 ASME B16.21-2016 establishes materials, dimensions, tolerances, and marking for nonmetallic flat gaskets for pipe flanges, covering both flat-ring and full-face types. It complements ASME B16.20 (metallic gaskets): B16.21 serves conventional raised-face (RF) and full-face (FF) flanges in medium- and low-pressure duty and is widely used in potable-water, fire-protection, general chemical, and utility systems.
💡 Why nonmetallic flat gaskets: they are soft and easy to compress, tolerant of flange surface imperfections, low-cost, and quick to replace — an economical and effective seal for medium/low-pressure water, gas, oil, and mildly corrosive media. Their temperature and pressure capability, however, is limited by material.
1. Scope and Gasket Types 📌
The Standard defines two basic types:
- Flat ring gasket: inside diameter larger than the pipe bore, outside diameter smaller than the bolt circle — covers only the inner annular sealing area of a raised-face (RF) flange.
- Full-face gasket: outside diameter matching the flange OD including all bolt holes, used on full-face (FF) flanges and cast-iron/low-pressure flanges where the gasket carries bolt load across the whole face.
Dimension tables are provided by flange standard and class, covering: ASME B16.1 (cast-iron flanges, Classes 25/125/250), B16.5 (steel flanges, Classes 150/300/400/600/900), B16.24 (cast copper alloy flanges, Classes 150/300), B16.47 (large-diameter steel flanges, Series A/B, Classes 75–600), and MSS SP-51 Class 150LW corrosion-resistant cast flanges.
| Mating Flange Standard | Classes | Gasket Type |
|---|---|---|
| ASME B16.1 cast iron | 25 / 125 / 250 | Flat ring / full face |
| ASME B16.5 steel | 150 / 300 / 400 / 600 / 900 | Flat ring (for RF) |
| ASME B16.24 copper alloy | 150 / 300 | Full face |
| ASME B16.47 large diameter | Series A: 150/300/400/600; Series B: 75/150/300/400/600 | Flat ring |
| MSS SP-51 corrosion-resistant cast | Class 150LW | Full face |
⚠️ Gasket type must match the flange facing: raised-face flanges take flat-ring gaskets; full-face (FF) flanges must take full-face gaskets. Using a flat-ring gasket on a full-face flange concentrates bolt load between bolt holes and can distort or crack the flange — especially risky on cast iron.
2. Material Requirements 🔧
Section 3 covers nonmetallic gasket materials. Common choices include:
- Rubber: natural, neoprene, nitrile (NBR), EPDM — for water, weak acids/bases, and general media with limited temperature range.
- Compressed fiber / fiber sheet: aramid or carbon fiber with rubber binder — for oils, solvents, and medium temperatures.
- PTFE (polytetrafluoroethylene): excellent chemical resistance and low friction — for corrosive media and clean systems.
- Flexible graphite: high-temperature capability, good rebound and chemical stability — for steam and hot fluids.
💡 Material selection hinges on four factors: temperature ceiling, media compatibility, compression/rebound performance, and required gasket stress. Avoid low-temperature rubbers in steam; avoid graphite with strong oxidizers; choose PTFE or graphite composites for low-emission (fugitive-emission) duty.
3. Dimensions and Tolerances 📏
Section 4 specifies gasket inside diameter, outside diameter, and thickness with tolerances. Flat-ring inside diameters are sized slightly larger than the pipe/flange bore to avoid intrusion into the flow path (turbulence and erosion); the outside diameter stays inside the bolt circle. Full-face gaskets match the flange OD and are drilled for bolts. Thickness balances compressibility against blowout risk — too thick can extrude, too thin is sensitive to face finish. Both U.S. Customary and SI dimension tables are provided (Mandatory Appendix I gives U.S. Customary units).
| Dimension | Typical Requirement | Engineering Significance |
|---|---|---|
| Inside diameter | Slightly larger than the bore | Prevents flow-path intrusion, turbulence, erosion |
| Outside diameter (flat ring) | Inside the bolt circle | Bolt load applied directly to the flange |
| Outside diameter (full face) | Matches flange OD, drilled | Even load distribution across the face |
| Thickness | Per material and size | Balance compression vs. blowout risk |
| Tolerances | Specified for ID/OD/thickness | Interchangeability and consistent sealing |
🚨 Common installation mistakes: wrong ID/OD (too large intrudes into the flow, too small misses the sealing area), reusing a compressed old gasket, and installing scratched or torn gaskets. Inspect gaskets and verify dimensions against the flange before assembly.
4. Marking and Quality Systems 🏷️
Section 5 requires gaskets to be marked, typically with the maker’s name or trademark, material identification, size and the standard number (e.g., ASME B16.21) for field verification and traceability. Nonmandatory Appendix A outlines a quality-system program; manufacturers should control processes to ensure dimensional, material, and performance consistency and provide conformance statements.
✅ Acceptance checklist: verify marking (maker/material/size/standard) → recheck ID/OD/thickness → inspect for flatness, cracks, and delamination → request material and performance certificates → store by batch away from direct sunlight and high-temperature aging.
5. Engineering Practice Essentials 🛠️
- 🔍 Selection workflow: confirm flange standard and facing (RF/FF) → establish medium, pressure, temperature → choose material (rubber/fiber/PTFE/graphite) → pick ID/OD/thickness from the dimension tables → verify bolt-load feasibility.
- 🔧 Installation: clean and dry flange faces → center the gasket (use locating bolts or a small amount of adhesive if needed) → tighten bolts symmetrically in steps (cross-pattern, 3–4 passes) → reach the specified torque.
- 📊 Load control: nonmetallic gaskets need proper gasket stress — too little preload leaks, too much compresses the gasket beyond its rebound and risks extrusion; control loads especially on cast-iron and full-face flanges.
- 📋 Replacement rule: gaskets are single-use consumables — replace after every disassembly; aging gaskets harden and lose rebound, so renew them on scheduled maintenance.
6. Frequently Asked Questions ❓
❓ Q: How do B16.21 and B16.20 gaskets divide the work?
A: B16.21 covers nonmetallic flat gaskets (rubber, fiber, PTFE, graphite) for conventional medium/low-pressure raised- and full-face flanges; B16.20 covers metallic gaskets (ring-joint, spiral-wound, jacketed) for high-pressure, high-temperature, and RTJ services. For severe duty, start with B16.20.
A: B16.21 covers nonmetallic flat gaskets (rubber, fiber, PTFE, graphite) for conventional medium/low-pressure raised- and full-face flanges; B16.20 covers metallic gaskets (ring-joint, spiral-wound, jacketed) for high-pressure, high-temperature, and RTJ services. For severe duty, start with B16.20.
❓ Q: When should I choose a flat-ring versus a full-face gasket?
A: It depends on the flange facing: raised-face (RF) flanges use flat-ring gaskets; full-face (FF) flanges — especially cast iron, nonmetallic, and low-pressure types — use full-face gaskets to distribute load evenly and prevent warping or cracking.
A: It depends on the flange facing: raised-face (RF) flanges use flat-ring gaskets; full-face (FF) flanges — especially cast iron, nonmetallic, and low-pressure types — use full-face gaskets to distribute load evenly and prevent warping or cracking.
❓ Q: Why is the gasket ID slightly larger than the pipe bore?
A: A smaller gasket would intrude into the flow path, causing turbulence, erosion, and solids deposition at the step; the dimension tables size the ID “slightly larger than the bore,” and installation should not allow offset either.
A: A smaller gasket would intrude into the flow path, causing turbulence, erosion, and solids deposition at the step; the dimension tables size the ID “slightly larger than the bore,” and installation should not allow offset either.
🔍 Q: Which nonmetallic gasket should I use for steam?
A: Steam is hot and demands temperature capability plus rebound: choose flexible graphite (or graphite-reinforced composite); compressed fiber suits medium-temperature steam; ordinary rubber and low-temperature PTFE are not recommended. For high-pressure steam, move to B16.20 metallic or spiral-wound gaskets.
A: Steam is hot and demands temperature capability plus rebound: choose flexible graphite (or graphite-reinforced composite); compressed fiber suits medium-temperature steam; ordinary rubber and low-temperature PTFE are not recommended. For high-pressure steam, move to B16.20 metallic or spiral-wound gaskets.
