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ASME B16.20-2017 — Metallic Gaskets for Pipe Flanges
🔩 ASME B16.20-2017 is the core standard for metallic gaskets for pipe flanges. It specifies dimensions, tolerances, materials, hardness, surface condition, marking, and testing for ring-joint gaskets (RJ), spiral-wound gaskets (SW), jacketed gaskets (JA), and grooved metal gaskets with covering layers (GM). It works together with ASME B16.5 (NPS 1/2 through 24) and B16.47 (NPS 26 through 60) flanges and is essential for reliable high-pressure, high-temperature flange sealing in oil, chemical, power, and refining industries.
💡 Why metallic gaskets: at high pressure and temperature, nonmetallic gaskets struggle to combine compression/rebound with temperature and pressure capability. Metallic gaskets use the strength and spring back of metal, matched to the flange facing (e.g., RTJ grooves), to achieve “tighten once, seal for the long term.”
1. Standard Structure and Scope 📌
The Standard is organized into five parts by product type:
- PART GR (General Requirements): scope, quality systems, references, and relevant units.
- PART RJ (Ring-Joint Gaskets): metallic ring gaskets for RTJ grooves, including R (octagonal/oval), RX (pressure-energized), and BX (ultra-high-pressure) series.
- PART SW (Spiral-Wound Gaskets): gaskets wound from metal strip and soft filler for raised-face flanges.
- PART JA (Jacketed Gaskets): metal shell filled with a nonmetallic core.
- PART GM (Grooved Metal Gaskets With Covering Layers): grooved metal core with a compliant covering layer.
| Gasket Type | Flange Facing | Typical Pressure Duty | Typical Applications |
|---|---|---|---|
| Ring-joint R type | RTJ groove (octagonal/oval) | Class 300–2500 | High-pressure refining, chemical, steam |
| Ring-joint RX/BX | RTJ groove (pressure-energized) | Class 900–2500 and above | Ultra-high-pressure, wellheads, subsea lines |
| Spiral-wound (SW) | Raised face (RF), MF/T&G | Class 150–1500 | General refinery and chemical sealing |
| Jacketed (JA) | Raised face | Medium/low pressure, high temperature | Furnace and heat-exchanger flanges |
| Grooved metal (GM) | Raised face | Class 150–600 | Low-temperature thermal cycling, vibration |
⚠️ Gasket selection must strictly match the flange facing: ring-joint gaskets can only be used in RTJ grooves and must never be used as flat gaskets on raised faces; raised-face flanges should use spiral-wound gaskets or nonmetallic gaskets (B16.21). Mismatching is a common root cause of flange leakage.
2. Ring-Joint Gaskets (RJ) 🔧
Ring-joint gaskets come in octagonal and oval cross sections. Sealing relies on the gasket being pressed into the flange groove under bolt preload, producing both radial wedging and face contact. The Standard specifies dimensions, cross-section sizes, tolerances, surface finish/machining pattern, and maximum hardness: gasket hardness must be lower than the flange sealing surface so the gasket deforms plastically for sealing without damaging the flange.
| Series | Cross Section | Typical Materials | Service |
|---|---|---|---|
| R series | Octagonal/oval | Soft iron, low-carbon steel, 304/316/410, alloy steel | Class 150–2500, conventional high pressure |
| RX series | Octagonal (special, self-energizing) | Soft iron, stainless, etc. | Class 900 and above; tighter with higher pressure |
| BX series | Octagonal (ultra-high pressure, special groove) | High-strength alloy steel | Extreme pressures (wellheads, subsea piping) |
🚨 Ring-joint installation don’ts: never dress RTJ grooves or gasket sealing faces with abrasive paper or grinding; verify the groove is clean and undamaged and that the gasket number (e.g., R45, BX169) matches the groove; reused rings must be inspected for plastic deformation and wear — discard if out of limits.
3. Spiral-Wound and Jacketed Gaskets (SW / JA) 📏
Spiral-wound gaskets are formed by alternately winding V-shaped metal strip (304, 316L, Inconel) with soft filler (PTFE, flexible graphite, ceramic fiber), usually with inner and outer rings for positioning and compression control. The Standard specifies inside/outside diameters, thickness, strip width, and tolerances. The outer (centering) ring aligns the gasket automatically on assembly, preventing offset leaks. Jacketed gaskets consist of a metal shell (304, carbon steel, copper, etc.) filled with a nonmetallic core (asbestos substitute, ceramic fiber), combining the high-temperature strength of metal with the softness and rebound of the filler.
| Item | Spiral-Wound (SW) | Jacketed (JA) |
|---|---|---|
| Construction | V-shaped metal strip + filler wound | Metal shell filled with nonmetallic core |
| Typical metal | 304, 316L, Inconel, etc. | 304, carbon steel, Monel, etc. |
| Typical filler/core | PTFE, flexible graphite, ceramic fiber | Flexible graphite, ceramic fiber |
| Features | Good rebound; inner/outer rings for positioning | High-temperature capability, simple construction |
| Main use | General refinery/chemical flanges | Large heat-exchanger and furnace flanges |
💡 Spiral-wound installation: align the gasket (with outer ring) to the bolt circle; compression is governed by flange design — tighten bolts symmetrically in several passes to the final torque so the gasket compresses to design thickness; never slam it down in one pass.
4. Grooved Metal Gaskets With Covering Layers (GM) 📐
This type consists of a metal core plate machined with concentric grooves and covered with a compliant surface layer (e.g., graphite). The grooved profile seals at relatively low bolt load, and the covering compensates for flange surface irregularities, making these gaskets ideal where thermal cycling is significant and bolt preload tends to relax. The Standard specifies dimensions, groove geometry, and marking.
✅ General metallic-gasket selection flow: confirm flange standard and facing → establish design pressure, temperature, and medium → choose type (RJ/SW/JA/GM) → verify material corrosion/temperature grade and hardness → select the size designation for the flange → verify bolt load and compression feasibility.
5. Materials, Hardness, and Quality Systems 🧪
The Standard specifically limits maximum hardness for ring gaskets (Table RJ-3.2-1), requiring gasket hardness below the flange material so the gasket deforms during sealing without damaging the flange. All types must be marked (type, material, size, standard number) and manufactured under a quality-system program (Nonmandatory Appendix A) ensuring controlled processes and traceability.
⚠️ Hardness mismatch is a hidden cause of seal failure: if the gasket is harder than the flange, the ring groove deforms first and the sealing surface is permanently damaged. When selecting, verify flange hardness and choose a gasket that satisfies “gasket softer than flange.”
6. Engineering Practice Essentials 🛠️
- 🔍 Match first: confirm the flange standard (B16.5/B16.47), class, and facing (RF/RTJ/MF/T&G), then choose gasket type and size; never mix across facings.
- 🔧 Installation: clean faces → center the gasket (especially outer-ring spiral-wound) → tighten bolts symmetrically in steps (e.g., cross-pattern, 3–4 passes) → reach specified torque/compression.
- 📊 High-pressure RTJ notes: RX/BX gaskets are pressure-energized — initial preload plus service pressure strengthens the seal; if bolt load is insufficient, rising pressure may loosen the joint, so follow the specified preload.
- 📋 Records and recheck: record bolt torque and gasket type for critical high-pressure/temperature flanges; verify gasket condition at maintenance and generally avoid reusing metallic gaskets in critical systems.
7. Frequently Asked Questions ❓
❓ Q: What is the difference between B16.20 and B16.21?
A: B16.20 covers metallic gaskets (ring-joint, spiral-wound, jacketed, grooved metal) for high-pressure/high-temperature and RTJ services; B16.21 covers nonmetallic flat gaskets (rubber, fiber-reinforced, PTFE, flexible graphite) for ordinary raised-face/full-face flanges in medium- and low-pressure duty.
A: B16.20 covers metallic gaskets (ring-joint, spiral-wound, jacketed, grooved metal) for high-pressure/high-temperature and RTJ services; B16.21 covers nonmetallic flat gaskets (rubber, fiber-reinforced, PTFE, flexible graphite) for ordinary raised-face/full-face flanges in medium- and low-pressure duty.
❓ Q: How much pressure can an R-type ring gasket take?
A: R-type rings are matched to flange class (Class 150–2500) and cover the rated pressure of the B16.5 flange at the design temperature; RX/BX series are used at higher pressures and are self-energizing.
A: R-type rings are matched to flange class (Class 150–2500) and cover the rated pressure of the B16.5 flange at the design temperature; RX/BX series are used at higher pressures and are self-energizing.
❓ Q: What do the “inner/outer rings” of a spiral-wound gasket do?
A: The inner ring protects the wound section from media erosion and adds radial stiffness; the outer (centering) ring aligns the gasket and limits compression so the winding is not crushed. Outer-ring gaskets are positioned against the bolt circle.
A: The inner ring protects the wound section from media erosion and adds radial stiffness; the outer (centering) ring aligns the gasket and limits compression so the winding is not crushed. Outer-ring gaskets are positioned against the bolt circle.
🔍 Q: Can metallic gaskets be reused?
A: Ring-joint gaskets may be reused cautiously if deformation is slight and hardness/surface remain acceptable; spiral-wound and jacketed gaskets lose significant rebound after one use and are generally not recommended for reuse — always fit new gaskets in critical systems.
A: Ring-joint gaskets may be reused cautiously if deformation is slight and hardness/surface remain acceptable; spiral-wound and jacketed gaskets lose significant rebound after one use and are generally not recommended for reuse — always fit new gaskets in critical systems.
