7.1 Fuel Gas Fundamentals, Materials, Jointing & Mandatory Pressure Testing

7.1 Fuel Gas Fundamentals, Materials, Jointing & Mandatory Pressure Testing

Quick Answer: Under the North Carolina Fuel Gas Code (NC IFGC / IFGC Chapter 4), fuel gas systems distribute either natural gas (specific gravity 0.60, ~1,000 BTU/cu ft), which is lighter than air, or liquefied petroleum (LP / propane, specific gravity 1.50, ~2,500 BTU/cu ft), which is heavier than air. Approved piping materials include Schedule 40 black steel (ASTM A53), CSST (ANSI LC 1) bonded with a minimum 6 AWG copper wire, Type K/L copper (brazed or press-connect with HNBR seals where $H_2S < 0.3\text{ grains}/100\text{ cu ft}$), and polyethylene (PE / ASTM D2513) for underground exterior use only with an 18 AWG yellow tracer wire. Cast iron bushings and running threads are strictly prohibited. Mandatory pressure testing requires a minimum of 1.5 times working pressure, but not less than 3 psig (commonly 10–20 psig in NC jurisdictions) for at least 10 minutes using a gauge with 0.2 psi increments or less. Appliance shutoff valves must be within 6 feet in the same room, and sediment traps (minimum 3" capped nipple) are mandatory on automated fuel-burning appliances.

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Fuel Gas Characteristics: Natural Gas vs. LP / Propane Gas

A thorough understanding of gas physics is essential for proper sizing, appliance orifice configuration, venting, and leak response. Fuel gases are broadly divided into methane-based natural gas and propane/butane-based liquefied petroleum (LP) gas.

                  [FUEL GAS PHYSICAL BEHAVIOR]

   NATURAL GAS (Specific Gravity = 0.60)     PROPANE / LP (Specific Gravity = 1.50)
             Lighter than Air                           Heavier than Air
         ▲  ▲  ▲  ▲  ▲  ▲  ▲                        ▼  ▼  ▼  ▼  ▼  ▼  ▼
         │  │  │  │  │  │  │                        │  │  │  │  │  │  │
    Rises to ceilings & high vents             Sinks into pits, crawlspaces & sumps
    Heating Value: ~1,000 BTU / cu ft          Heating Value: ~2,500 BTU / cu ft
    Manifold Pressure: 3.5" to 4.0" w.c.       Manifold Pressure: 10.0" to 11.0" w.c.

Technical Properties Comparison Matrix

Exam Pro-Tip: Field conversion of an appliance from natural gas to propane requires both replacing the burner orifices with smaller diameter drill sizes (to prevent massive over-firing from the 2,500 BTU/cu ft gas) and adjusting or replacing the appliance pressure regulator spring from 3.5" w.c. to 10.0"–11.0" w.c.

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Approved Fuel Gas Piping Materials & Installation Standards

The North Carolina Fuel Gas Code (IFGC Chapter 4) establishes strict structural, metallurgical, and joining requirements for gas piping.

graph TD
    A["Approved Fuel Gas Piping Materials"] --> B["Black Steel Pipe (ASTM A53 / A106)<br/>• Schedule 40 standard<br/>• Malleable iron fittings<br/>• Cast iron fittings PROHIBITED"]
    A --> C["CSST - Corrugated Stainless Steel (ANSI LC 1)<br/>• Minimum 6 AWG copper bonding<br/>• Hardened striker plates at studs<br/>• Direct-to-electrode bonding"]
    A --> D["Copper Tubing (Type K or L)<br/>• H2S content < 0.3 grains / 100 cu ft<br/>• Brazed joints (>1,000°F) or HNBR press<br/>• Soft solder strictly PROHIBITED"]
    A --> E["Polyethylene PE Pipe (ASTM D2513)<br/>• UNDERGROUND EXTERIOR ONLY<br/>• Min 18-inch burial depth<br/>• Min 18 AWG yellow tracer wire<br/>• Anodeless risers for transition"]

1. Steel Piping & Malleable Fittings (ASTM A53 / A106)

• Specification: Schedule 40 standard-weight black iron or galvanized steel pipe.
• Fittings: Malleable iron fittings (ASTM A197) or forged steel fittings (ASTM A234). Threads must be tapered pipe threads complying with ASME B1.20.1.
• Prohibited Fittings: Cast iron fittings and cast iron bushings are strictly prohibited in fuel gas systems. Cast iron is brittle and subject to stress cracking under mechanical vibration, temperature swings, and structural movement.
• Pipe Joint Sealants: Thread sealant must be approved for fuel gases and resistant to the chemical action of liquefied petroleum gases (e.g., yellow PTFE tape or gas-rated non-hardening pipe compound applied to male threads only).

2. Corrugated Stainless Steel Tubing (CSST - ANSI LC 1 / CSA 6.26)

CSST is a flexible, continuous, semi-rigid piping system utilizing 304 stainless steel tubing with a protective exterior jacket.

• Electrical Bonding Standard (IFGC 310.1.1): Yellow-jacketed CSST must be electrically bonded to the building electrical service grounding electrode system. The bonding jumper must be a minimum 6 AWG copper wire (or larger if sized per NEC / manufacturer instructions), clamped to a metallic brass CSST termination fitting or rigid steel pipe segment immediately downstream of the meter/regulator, and routed directly to the grounding electrode conductor or electrical service panel grounding bus.
• Arc-Resistant CSST (ANSI LC 1 / CSA 6.26 with arc-resistant jacket): Black-jacketed conductive arc-resistant CSST dissipates electrical energy from indirect lightning strikes; local codes and manufacturer instructions may permit bonding via the appliance grounding circuit, though direct 6 AWG bonding remains standard practice in North Carolina.
• Physical Protection (Striker Plates): Where CSST passes through wood or metal framing members within 1-3/4 inches (44 mm) of the stud face, it must be protected against puncture from drywall screws and nails using hardened steel striker plates (minimum 0.0508" thick / 16-gauge) extending at least 4 inches (102 mm) above and below the penetration point.

3. Copper and Brass Tubing (ASTM B88 / B280)

• Hydrogen Sulfide Limitation (IFGC 403.4.2): Copper tubing (Type K or L) is permitted ONLY where the gas supplier certifies that the distributed fuel gas contains not more than 0.3 grains of hydrogen sulfide ($H_2S$) per 100 standard cubic feet of gas ($0.7\text{ mg}/100\text{ L}$). In sour gas environments, $H_2S$ reacts with copper to produce copper sulfide flakes that detach and clog appliance gas orifices and safety shutoff valves.
• Jointing Standards: All joints in copper gas piping must be brazed using a brazing filler metal having a liquidus melting point exceeding $1,000^{\circ}\text{F}$ ($538^{\circ}\text{C}$) (typically AWS BCuP or BAg alloys), or made with listed metallic press-connect fittings equipped with factory-installed HNBR (Hydrogenated Nitrile Butadiene Rubber) yellow seal rings certified to ANSI LC 4 / CSA 6.32. Soft solders (such as 95/5 or 50/50 tin-lead) are strictly prohibited.

4. Polyethylene (PE) Plastic Pipe (ASTM D2513)

• Underground Exterior Use Only (IFGC 403.6): PE gas pipe is approved strictly for burial outdoors underground. It is strictly prohibited above ground or within the foundation perimeter of any building or structure.
• Burial Depth: Minimum cover of 18 inches (457 mm) below finished grade. If subjected to external surface damage or unpaved driveways, minimum depth is 24 inches (610 mm) (or 12 inches where rock is encountered and pipe is encased in protective conduit).
• Tracer Wire Requirement: Because plastic pipe is non-conductive and cannot be located by electromagnetic pipe locators, an insulated copper tracer wire (minimum 18 AWG yellow insulated copper) or listed conductive tape must be buried alongside or directly above the PE pipe. The tracer wire must terminate above grade at an accessible test station or riser box.
• Anodeless Risers: Transition from underground PE to above-ground steel must be made using a factory-assembled anodeless riser (a moisture-sealed steel casing surrounding the PE tube) so that no plastic pipe is exposed to UV radiation, atmospheric oxygen, or physical impact above grade.

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Prohibited Joints, Bushings & Running Threads

The NC Fuel Gas Code explicitly bans fittings and joint designs that carry high failure rates under thermal stress and internal gas pressure:

                 [PROHIBITED VS. APPROVED JOINTS]

      PROHIBITED: Running Thread             APPROVED: Tapered Pipe Thread
   (Parallel Threads - Cannot Seal)       (NPT 3/4" per foot Tapered Wedge)
   ───────────────────────────────         ───────────────────────────────
   |||||||||||||||||||||||||||||||         \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
   ───────────────────────────────         ───────────────────────────────
   No mechanical interference fit          Metal-to-metal wedge seal with dope

      PROHIBITED: Cast Iron Bushing          APPROVED: Steel Hex Bushing / Reducer
   (Brittle, splits when torqued)         (Ductile, withstands pipe wrench torque)

1. Cast Iron Bushings & Cast Iron Fittings: Completely prohibited on fuel gas piping. Malleable iron, ductile iron, or steel fittings must be utilized.
2. Running Threads: Straight, non-tapered parallel pipe threads are prohibited. Pipe threads must conform to ASME B1.20.1 (NPT), providing a standard 3/4-inch per foot taper that creates a progressive mechanical interference wedge fit.
3. Street Ells & Face Bushings: Prohibited inside concealed construction. Reductions in pipe size must be made using listed malleable reducing couplings, eccentric reducers, or one-piece reducing tees.

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Mandatory Fuel Gas Pressure Testing (IFGC Section 406)

Before any fuel gas distribution piping is placed into service, covered, or concealed, it must pass a rigorous pneumatic pressure test inspected and witnessed by the local code official.

graph TD
    A["Fuel Gas Piping Complete"] --> B["Isolate / Disconnect All Appliance Controls<br/>(Appliance valves rupture at > 0.5 psi / 14 in. w.c.)"]
    B --> C["Attach Diaphragm Test Gauge<br/>(Dial scale ≤ 5× test pressure; increments ≤ 0.2 psi)"]
    C --> D["Pressurize System with Air or Inert Gas (N2 / CO2)<br/>OXYGEN / COMBUSTIBLE GAS STRICTLY PROHIBITED"]
    D --> E{"Select Minimum Test Pressure & Duration"}
    E -->|Low-Pressure System < 0.5 psi| F["Min 1.5× Working Pressure (Not < 3.0 psig)<br/>NC Standard Practice: 10 to 20 psig<br/>Hold Duration: Min 10 min (Residential) / 30 min (Commercial)"]
    E -->|High-Pressure Hybrid System (2 psi)| G["Min 1.5× Working Pressure (Not < 10 psig)<br/>Hold Duration: Min 10 to 30 min"]
    F --> H["Check 100% Joints with Non-Corrosive Leak Solution<br/>Zero Pressure Drop Allowed on Gauge"]
    G --> H

Code Testing Parameters (IFGC 406.4)

• Test Medium: Air, Nitrogen ($N_2$), or Carbon Dioxide ($CO_2$). OXYGEN OR COMBUSTIBLE GASES MUST NEVER BE USED AS A TEST MEDIUM. Pressurizing piping with pure oxygen or fuel gas creates a severe, potentially fatal explosive environment.
• Minimum Test Pressure: 1.5 times the maximum design working pressure, but not less than 3.0 psig (20 kPa gauge). In North Carolina jurisdictions, standard local practice for threaded Schedule 40 black steel pipe requires a test pressure of 10 to 20 psig to expose marginal thread engagements or latent pinhole casting defects.
• Test Duration:
  • Residential Single-Family: Not less than 10 minutes.
  • Commercial / Large Volume Systems: Not less than 30 minutes (or longer for large pipe volumes, as specified by the code official).
• Test Gauges: Diaphragm test gauges or test manifolds must have a dial scale where the maximum gauge reading does not exceed 5 times the test pressure (e.g., a 0–15 psi gauge for a 3-psi test, or a 0–30 psi gauge for a 10-psi test). The gauge must have graduations of 0.2 psi or less to detect minor pressure leaks.
• Appliance Isolation Mandate: Appliance combination gas control valves are rated for a maximum inlet pressure of 1/2 psig (14 inches w.c. / 3.45 kPa). Applying a 3-to-20 psig test pressure directly against an appliance control valve will rupture the internal rubber regulator diaphragm and blow out internal safety shutoffs. Therefore, all appliances must be disconnected and capped, or the appliance shutoff valve tightly closed and disconnected during the test.

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Appliance Shutoff Valves & Sediment Traps (Dirt Legs)

Proper termination at the appliance requires dedicated isolation valves and physical protection against debris ingress.

                 [CODE-COMPLIANT SEDIMENT TRAP ASSEMBLY]

         Gas Supply Line (Down from Ceiling or Wall)
                     │
                     ▼
          ┌────────────────────┐
          │   Shutoff Valve    │ ◄── Max 6 ft from appliance, same room, accessible
          └──────────┬─────────┘
                     │
                     ▼
          ┌────────────────────┐
          │ Flexible Connector │ ◄── ANSI Z21.24 Listed Semi-Rigid Connector
          └──────────┬─────────┘
                     │
                     ▼
               ┌───────────┐
               │  TEE (90°)│ ────────► Gas Enters Appliance Inlet
               └─────┬─────┘
                     │ (Debris falls downward past the tee turn)
                     ▼
          ┌────────────────────┐
          │   Nipple (Min 3")  │
          └──────────┬─────────┘
                     ▼
          ┌────────────────────┐
          │     Pipe Cap       │
          └────────────────────┘

1. Appliance Shutoff Valve Mandates (IFGC 409.5)

• Location: Must be located in the same room as the appliance it serves.
• Distance: Installed within 6 feet (1,829 mm) of the appliance inlet (measured along the piping centerline).
• Accessibility: Must be readily accessible—not buried behind drywall, inside enclosed ceilings, or blocked by immovable structures.
• Upstream Requirement: Must be located upstream of any flexible appliance connector, union, or sediment trap.
• Fireplace Exception: Shutoff valves for vented decorative fireplaces or gas log sets may be installed in a remote location (e.g., a wall key valve or adjacent closet floor) provided it is within 50 feet and clearly labeled.

2. Sediment Traps / Dirt Legs (IFGC Section 408)

• Purpose: Natural gas and LP gas lines carry microscopic pipe shavings, cutting oil sludge, mill scale, and condensate. A sediment trap forces gas to make a sharp 90-degree directional turn, utilizing inertia and gravity to drop heavy particulate matter into a dead-end pocket before it can reach and foul the fine orifices of appliance safety valves and solenoids.
• Configuration: A tee fitting with a capped nipple of minimum 3 inches (76 mm) in length installed in the bottom vertical outlet of the tee.
• Position: Installed as close to the appliance inlet as practical and downstream of the appliance shutoff valve.
• Appliance Exceptions (IFGC 408.4): Sediment traps are NOT required on:
  1. Illuminating appliances (outdoor gas lanterns / gas lamps).
  2. Gas ranges (cooking appliances).
  3. Gas clothes dryers.
  4. Outdoor gas grills / barbecue pits.
  5. Decorative gas log sets / vented gas fireplaces.