4.3 Gas Piping Materials, Joining Specifications, Pressure Testing & Purging

Gas Piping Materials, Joining Specifications, Pressure Testing & Purging

Quick Answer: Under the Michigan Mechanical Code (MMC Part 9a) and IFGC Chapter 4, interior fuel gas piping must be constructed of approved materials—predominantly Schedule 40 black steel pipe (ASTM A53 / A106) with malleable iron fittings (ASME B16.3). Copper tubing is prohibited unless the fuel gas contains no more than 0.3 grains of hydrogen sulfide per 100 standard cubic feet (0.7 mg/100 L). Prior to commissioning, newly installed piping must undergo a documented pressure test under IFGC Section 406.4: not less than 1.5 times the proposed maximum working pressure and never less than 3 psig, held for 1/2 hour per 500 cubic feet of pipe volume (not less than 10 minutes for a system in a single-family dwelling or a volume under 10 cubic feet), isolated from all appliances whose gas control valves cannot withstand test pressures. Testing with open flames is illegal; approved non-corrosive foaming solutions or electronic combustible gas detectors must be employed.

A mechanical contractor's reputation and license depend on the integrity of fuel gas piping installations. Even a microscopic gas leak inside a building can accumulate to the Lower Explosive Limit (LEL), producing catastrophic explosions or toxic asphyxiation hazards. The Michigan Mechanical Code establishes comprehensive material criteria, joining standards, component locations, and rigorous testing protocols to safeguard occupants and structures.

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Approved Gas Piping Materials & Code Restrictions

Not all metallic or plastic piping materials are permitted for fuel gas distribution. Contractors must adhere strictly to the approved material matrix in IFGC Section 403 and IFGC Section 403:

1. Steel & Iron Pipe

• Schedule 40 Black Steel (ASTM A53 / ASTM A106): The primary material for commercial and residential gas distribution. Permitted in welded, flanged, or threaded configurations.
• Galvanized Steel Pipe: Permitted by code where fuel gas is non-corrosive, but rarely used because trace impurities historically reacted with zinc coatings, causing interior flaking that clogged gas orifices. Modern refined gases eliminate this issue, but black iron remains standard industry practice.
• Malleable Iron Fittings (ASME B16.3 / Class 150): Approved for threaded gas assemblies. Cast iron fittings are strictly prohibited for fuel gas piping (except specialized large-diameter flange systems) because cast iron is brittle and fractures easily under mechanical shock, building settlement, or overtightening.

2. Copper & Brass Tubing: The Hydrogen Sulfide Restriction

Under MMC Part 9a and IFGC Section 403.4.3, copper and brass tubing (ASTM B88 Type K or L, and ASTM B280) carry severe restrictions:

• Hydrogen Sulfide Content Limit: Copper tube is permitted ONLY IF the supplier gas contains not more than 0.3 grains of hydrogen sulfide (H_2S) per 100 standard cubic feet of gas (0.7 mg/100 L).
• The Chemical Hazard (Sulfidation): When hydrogen sulfide reacts with copper, it produces copper sulfide (Cu_2S) scale inside the tube. This black scale flakes off the interior wall, travels down the pipe, and accumulates inside gas valve seats, safety shutoff solenoids, pilot orifices, and burner nozzles, causing uncontained gas leaks or burner flameout.
• Contractor Verification Duty: Contractors cannot install copper tubing on natural gas systems without obtaining written, certified fuel gas chemical analysis reports from the local utility (e.g., DTE Energy or Consumers Energy) proving H_2S < 0.3 grains/100 cu ft. Copper is broadly permitted on LP propane gas systems because refined commercial propane is virtually free of sulfur compounds.

3. Underground Polyethylene (PE) Piping

• ASTM D2513 Polyethylene: Approved exclusively for outdoor, underground burial (minimum 12 to 18 inches cover, 24 inches under driveways).
• Strict Indoor Prohibition: Polyethylene is combustible and subject to mechanical shear; it is strictly prohibited aboveground or inside/under any building.
• Tracer Wire: Underground PE pipe must be accompanied by an insulated, yellow-jacketed copper tracer wire (minimum 14 to 18 AWG) taped alongside the pipe to allow electronic line locating from the surface.
• Anodeless Transition Risers: Transition from underground PE to aboveground metallic pipe must be accomplished using a factory-manufactured, listed anodeless riser (a steel casing sleeve containing a factory-fused PE carrier tube with a multi-seal mechanical joint) terminating above finished grade outside the structure.

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Threaded Joining Standards & Sealants

Threaded pipe connections must conform to ANSI/ASME B1.20.1 (National Pipe Thread - NPT), featuring a 1 in 16 taper (3/4 inch per foot) that creates a mechanical interference seal as threads engage:

The "Male Threads Only" Rule

Under IFGC Section 403 and universal manufacturer and utility practice, pipe joint sealant (pipe dope or gas-rated PTFE tape) is applied strictly to male pipe threads only:

• Application Technique: Clean male threads thoroughly. Apply an approved sealant to male threads, intentionally leaving the first two leading threads bare.
• Rationale: If compound is applied to female threads, or if the first two male threads are coated, the screwing action forces excess compound into the interior bore of the piping. Loose compound breaks free and travels directly into automatic gas shutoff valves, fouling elastomeric valve seats and causing continuous gas weepage.
• PTFE Tape Specifications: When tape is used instead of paste compound, it must be listed for fuel gas—visually identified as yellow PTFE tape with a minimum thickness of 3.5 to 4.0 mils, resistant to hydrocarbon degradation. Standard white plumber's tape (1.5 to 2.0 mils) is too thin and dissolves in aromatic hydrocarbons, violating code.

Modern Press-Connect Systems (MegaPressG)

Mechanical press fittings listed to ANSI LC 4 / CSA 6.32 (such as Viega MegaPressG) are approved for Schedule 40 steel pipe up to 2 inches. These fittings utilize a specialized Hydrogenated Nitrile Butadiene Rubber (HNBR) yellow sealing element resistant to fuel gases. Press tools ensure flameless, high-speed assembly without open torches or pipe threading machines.

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Appliance Shutoff Valves & Flexible Connectors

Every fuel-burning appliance must be provided with an independent, dedicated manual gas shutoff valve:

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Sediment Traps (Drip Legs): Engineering & Mandates

Under IFGC Section 408.4, where a sediment trap is not incorporated as part of the appliance, a field-fabricated sediment trap must be installed on the gas supply line:

Anatomy of a Code-Compliant Sediment Trap

1. Tee Fitting Orientation: A standard pipe tee installed with the inlet and outlet configured so that the gas stream must make a 90° directional turn.
2. Vertical Capped Nipple: A capped pipe nipple installed vertically in the bottommost opening of the tee. The 2021 IFGC specifies a capped nipple of any length — the 3-inch minimum that older code editions carried was removed, so a short capped nipple is compliant. What the code does require is that the nipple be capped, vertical, and in the bottom opening.
3. Inertial Separation Physics: Gas flows down through the tee and abruptly turns 90° toward the appliance control valve. Denser particulates (pipe scale, rust, cutting chips, moisture droplets, excess pipe dope) cannot make the rapid turn due to momentum and drop harmlessly into the vertical capped nipple, protecting sensitive solenoid valve seats.
4. Placement: Installed as close to the appliance inlet as practical, downstream of the manual shutoff valve and upstream of the appliance control valve.

Exam Trap Alert: Installing a "running tee" where gas flows straight through the tee run while the capped nipple hangs off the bull (side outlet) is a code violation. Gas will sweep past the side branch without dropping particulates. The gas must enter the run and turn 90°, or enter the bull and turn 90°.

Exempt Appliances

IFGC Section 408.4 specifically exempts the following appliances from requiring sediment traps:

• Illuminating gas appliances (gas lights)
• Commercial and residential cooking ranges
• Clothes dryers
• Outdoor barbecue grills
• Decorative gas fireplaces and vented gas logs

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Pre-Operational Pressure Testing Protocol

Before any newly installed, extended, or altered gas piping is covered, concealed, or put into service, it must successfully pass a documented pressure test under IFGC Section 406.4:

1. Appliance Isolation

Standard residential appliance automatic gas valves and regulators are rated for a maximum operating pressure of 0.5 psig (14 in. w.c.). Subjecting an appliance control valve to a 10 PSIG test pressure will permanently rupture its internal rubber diaphragms and distort valve seats. Therefore, all appliance shutoff valves must be closed, or the piping physically disconnected from appliances and capped with steel plugs prior to pressurization.

2. Test Pressures & Duration Standards

IFGC Section 406.4.1 (test pressure): not less than 1.5× the proposed maximum working pressure, but not less than 3 psig (20 kPa), irrespective of design pressure. Where the test pressure exceeds 125 psig, it must not produce a hoop stress greater than 50% of the pipe's specified minimum yield strength.

IFGC Section 406.4.2 (test duration):

Read this one carefully. The code floor is 3 psig, not 10 psig, and the residential floor is 10 minutes, not 15 or 30. Michigan enforcing agencies and gas utilities routinely post a higher local standard — 10 psig for 15 minutes is a very common jurisdictional requirement — and a local amendment that is more restrictive than the model code governs the job. On the examination, answer from the IFGC text unless the question names a specific jurisdiction; on the job, ask the inspector.

3. Test Gauge Resolution & Calibration

Under IFGC Section 406.4, test pressure must be measured with an approved mechanical dial gauge or digital manometer:

• Dial Range Limit: The dial gauge full-scale range must not exceed 5 times the test pressure (e.g., for a 10 PSIG test, the gauge scale cannot exceed 50 PSIG; using a 200 PSIG gauge is illegal because needle deflection for a minor leak is imperceptible).
• Gauge Resolution: Dial increments must not exceed 2% of the full-scale reading (e.g., on a 30 PSIG gauge, increments cannot exceed 0.6 psi; 0.2 psi or 0.5 psi increments are standard).

4. Leak Detection Procedures

• Prohibition on Open Flames: Searching for gas leaks with matches, open flames, or lighters is strictly prohibited under penalty of immediate license suspension.
• Approved Methods: Leaks must be located using non-corrosive foaming leak-detection solution (soap bubble solution conforming to ASTM E515) or a calibrated electronic combustible gas detector.

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Purging Fuel Gas Lines

When testing is complete and the system is tied into live gas service, the piping must be purged under NFPA 54 Section 8.3 / IFGC Section 406.7:

• Outdoor Purging: Large piping volumes (piping ≥2 inches or systems > 50 feet in length) must be purged directly to the outdoor atmosphere through a temporary vent pipe extending at least 10 feet from building openings and sources of ignition.
• Indoor Purging: Small residential branch lines may be purged indoors only if the space is actively ventilated, no open flames or smoking are permitted, and purging is halted the instant the characteristic odorant (ethyl mercaptan) is detected.