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ASME B31.2-1968 — Fuel Gas Piping
🔥 ASME B31.2-1968 is the section of the ASME B31 Code for Pressure Piping dedicated to fuel gas piping. It prescribes the design, fabrication, installation, and testing of piping systems for fuel gases — natural gas, manufactured gas, liquefied petroleum gas (LPG)-air mixtures above the upper combustible limit, and LPG in the gaseous phase — both within buildings and between buildings. Coverage runs from the outlet of the consumer’s meter set assembly (or point of delivery) up to and including the first pressure-containing valve upstream of the gas utilization device. As an early specialized section of the B31 Code, it embodies a safety-first, economy-conscious approach to gas piping design.
💡 The boundary of its day: B31.2 governed “meter-to-appliance” gas piping (distribution piping in and between buildings); upstream gathering, transmission, and distribution lines belonged to B31.8, and fuel gas piping inside refineries and process plants to B31.3. Drawing these boundaries correctly is the prerequisite for choosing the right code.
1. Scope and Boundaries 📌
Chapter 200 defines the scope: piping systems for natural gas, manufactured gas, LPG-air mixtures above the upper combustible limit, LPG in the gaseous phase, and mixtures of these gases; the run begins at the outlet of the consumer’s meter set (or point of delivery) and ends at the first pressure-containing valve upstream of the gas utilization device, covering piping both inside and between buildings. Components include pipe, valves, fittings, flanges (except those integral to equipment), bolting and gaskets, plus the pressure-containing parts of expansion joints, strainers, metering devices, and support structures.
- Pressure limits: systems may be operated only at pressures permitted by the limitations of this Code.
- Vacuum systems: vacuum gas piping is not covered.
- Metal temperature limits: systems with metal temperature above 450°F or below -20°F are excluded.
⚠️ The Code explicitly excludes: fuel gas piping in petroleum refineries, loading terminals, gas processing plants and similar (B31.3 scope); fuel gas piping in power and atomic energy plants (B31.1 scope); gathering, transmission, and distribution systems for natural, manufactured, or diluted LPG (B31.8 scope); and systems within USAS Z21.30. These exclusions must be checked item by item.
2. Design, Materials, and Component Requirements 🔧
The Code sets wall-thickness calculations, pressure ratings, and component selection for gas piping. As in other B31 sections, wall thickness follows design pressure, allowable stress, and joint factors. Given the high corrosion risk of buried and humid-service gas piping, it also emphasizes material corrosion protection, joint types, and inspection/maintenance.
| Category | Typical Materials | Applications | Design Notes |
|---|---|---|---|
| Steel pipe | ASTM A53/A106, API 5L carbon steel | Buried and indoor mains | Pressure-thickness plus corrosion allowance/coating |
| Copper pipe | Hard and soft copper per Code allowance | Indoor service branches | Brazed joints; watch temperature limits |
| Cast iron | Bell-and-spigot cast iron | Low-pressure buried lines | Low pressure only; sound joint sealing |
| Fittings/valves | Standard flanges, valves, fittings | Whole system | Rating not lower than line design pressure |
💡 In gas piping, materials are chosen not only for strength but for corrosion resistance: buried steel normally requires external coating plus cathodic protection; joints, threads, and flange faces are the leak-prone spots and must be leak-checked point by point after installation.
3. Installation and Testing 🛠️
The Code governs installation practice and test procedures: indoor routing should avoid hazardous locations (e.g., flues, vents), provide a main shutoff valve and ventilation; buried lines require adequate cover and marking. After installation, strength and tightness tests are performed per the Code before gas is introduced.
- Strength test: verifies pressure capacity at the specified test pressure, typically pneumatic or hydrostatic.
- Tightness test: after the strength test, a lower pressure verifies that all joints and welds are leak-free.
- Purge and displacement: before gas introduction, purge with inert gas or nitrogen to avoid an explosive air-gas mixture.
- Safety devices: emergency shutoff valves, vents, and pressure-regulating devices per the Code.
🚨 Gas commissioning is a non-negotiable safety step: never introduce gas before the tightness test and purge are completed. “Flame testing” for leaks near possible gas accumulation is hazardous — use leak-detection fluid or a combustible-gas detector instead.
4. Linkage with Current Codes 📖
ASME B31.2-1968 is a historical 1968 edition. As the regulatory landscape evolved, building gas piping in the United States is now governed largely by NFPA 54 (National Fuel Gas Code), while gas distribution lines fall under B31.8. For existing systems built to B31.2, prudent practice is to review original design, materials, and test records and carry out an in-service assessment against current regulations.
| System Type | 1968 Governing Code | Current Mainstream Code |
|---|---|---|
| Gas piping in/between buildings | ASME B31.2-1968 | NFPA 54 / local building gas codes |
| Gas transmission, distribution, gathering | ASME B31.8 | ASME B31.8 (current edition) |
| Fuel gas piping inside refinery/chemical plants | ASME B31.3 | ASME B31.3 (current edition) |
| LPG storage and cylinder systems | NFPA 58 etc. | NFPA 58 / local codes |
✅ The value of understanding a historical edition: B31.2-1968 distilled early experience in standardizing gas piping safety; its three pillars — boundary definition, pressure limitations, and test procedures — remain the basic skeleton of safe gas piping design today. Studying the history helps explain the origin and intent of current regulatory clauses.
5. Frequently Asked Questions ❓
❓ Q: Is B31.2 still used as a current code?
A: This edition is historical; current U.S. building gas piping generally follows NFPA 54 and local codes. If a contract or regulation still cites B31.2-1968, its clauses may be applied, but confirm the applicable code system with the owner.
A: This edition is historical; current U.S. building gas piping generally follows NFPA 54 and local codes. If a contract or regulation still cites B31.2-1968, its clauses may be applied, but confirm the applicable code system with the owner.
❓ Q: Where is the boundary between B31.2 and B31.8?
A: B31.8 covers gas gathering, transmission, and distribution lines; B31.2 covers piping in and between buildings downstream of the meter set and upstream of the utilization-device valve. The dividing line is “downstream of the meter.”
A: B31.8 covers gas gathering, transmission, and distribution lines; B31.2 covers piping in and between buildings downstream of the meter set and upstream of the utilization-device valve. The dividing line is “downstream of the meter.”
❓ Q: Can plastic pipe be used for gas piping?
A: The 1968 edition is based on metallic pipe. Modern gas distribution (especially buried low-pressure sections) widely uses polyethylene (PE) pipe, but its applicability must be confirmed under current codes (e.g., B31.8 and its Cases); this historical edition does not cover PE.
A: The 1968 edition is based on metallic pipe. Modern gas distribution (especially buried low-pressure sections) widely uses polyethylene (PE) pipe, but its applicability must be confirmed under current codes (e.g., B31.8 and its Cases); this historical edition does not cover PE.
❓ Q: Why must the line be purged before testing/commissioning?
A: Residual air mixed with natural gas can form an explosive mixture (gas flammability roughly 5%–15%). Purging with inert gas such as nitrogen before gas introduction eliminates the deflagration risk.
A: Residual air mixed with natural gas can form an explosive mixture (gas flammability roughly 5%–15%). Purging with inert gas such as nitrogen before gas introduction eliminates the deflagration risk.
