9.2 Fuel Gas Pipe Sizing & Materials

9.2 Fuel Gas Pipe Sizing & Materials

The installation and sizing of fuel gas piping systems in Maryland are governed by the International Fuel Gas Code (IFGC) and NFPA 54 (National Fuel Gas Code), as adopted and amended by the Maryland State Board of Plumbing. Journeyman Plumber candidates must possess a comprehensive understanding of gas flow dynamics, heating values, pipe materials, and code-prescribed pipe sizing algorithms. Proper sizing ensures that gas appliances receive adequate gas volume at the required operating pressure under full load conditions without exceeding allowable pressure drop thresholds.

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1. Gas Properties & Heating Values

Fuel gas systems primarily distribute either Natural Gas or Liquefied Petroleum Gas (LP Gas / Propane). Sizing gas piping requires converting appliance input ratings—typically expressed in British Thermal Units per hour (BTU/h)—into volumetric flow rates measured in Cubic Feet per Hour (CFH).

$\text{Required Capacity (CFH)} = \frac{\text{Total Appliance Input (BTU/h)}}{\text{Gas Heating Value (BTU/cu ft)}}$

Nominal Heating Values & Specific Gravities

Key Formula to Remember: For natural gas with a heating value of $1,000\text{ BTU/ft}^3$, $1\text{ CFH} = 1,000\text{ BTU/h}$. A $120,000\text{ BTU/h}$ water heater requires $120\text{ CFH}$ of natural gas. For LP gas ($2,500\text{ BTU/ft}^3$), the same water heater requires only $48\text{ CFH}$ ($120,000 / 2,500$).

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2. Pipe Sizing Methodology: The Longest Length Method

The IFGC outlines several pipe sizing methods, but the Longest Length Method (IFGC Section 402.4) is the primary method tested on the Maryland Journeyman Plumber examination. Under this method:

1. Determine System Pressure and Allowable Drop: Identify the inlet gas pressure (typically $0.5\text{ psi}$ / $14"\text{ w.c.}$) and maximum allowable pressure drop (typically $0.5"\text{ w.c.}$).
2. Measure Developed Length: Measure the total developed pipe length from the gas meter (or LP regulator) to the most remote outlet in the entire piping system.
3. Select Pipe Sizing Table: Locate the appropriate IFGC table (e.g., IFGC Table 402.4(1) for Schedule 40 metallic pipe, natural gas, $0.5\text{ psi}$ inlet pressure, $0.5"\text{ w.c.}$ drop).
4. Determine Developed Length Column: Select the column header corresponding to the total developed length to the furthest outlet (rounding up to the next available table distance, such as 50 ft, 60 ft, or 80 ft).
5. Size Main Lines & Branches:
   • Main Line: Size each main section using the total system demand (sum of all downstream CFH) under the selected longest length column.
   • Branch Lines: Size each branch line using only that specific branch's CFH demand, but under the SAME longest length column established for the furthest outlet.

IFGC 2018 Table 402.4(1) - Schedule 40 Metallic Pipe (Natural Gas, inlet less than 2 psi, 0.3 in. w.c. drop, S.G. 0.60)

Capacity in cubic feet of gas per hour:

Read the header, not just the grid. This is the 0.3 in. w.c. pressure-drop table. Its companion Table 402.4(2) is identical in layout but assumes a 0.5 in. w.c. drop and therefore carries more gas - 50, 104 and 195 CFH at the 100-foot row for 1/2, 3/4 and 1 inch, against 38, 79 and 148 CFH here. Section 9.4 works that table in full.

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3. Approved Fuel Gas Piping Materials & Installation Restrictions

Maryland Plumbing Code enforces strict rules regarding allowed piping materials and their application environments:

A. Black Steel & Wrought Iron Pipe

• Standards: ASTM A53 or ASTM A106 black steel pipe. Schedule 40 is standard for low and medium pressure systems.
• Fittings: IFGC 403.10.5, item 2 permits fittings used with steel, stainless steel or wrought-iron pipe to be steel, stainless steel, copper alloy, malleable iron or cast iron; malleable iron to ANSI/ASME B16.3 is the everyday choice. Cast-iron fittings carry their own limits under item 5 - flanges are permitted, bushings shall not be used, they may not be used in systems containing flammable gas-air mixtures, sizes 4 inches and larger are not used indoors except where approved, and sizes 6 inches and larger are not used except where approved. Threaded fittings larger than 4 inches shall not be used at all (item 1). What the code prohibits is cast-iron pipe (403.4.1), not cast-iron fittings - do not let the exam swap the two.
• Jointing: Threaded joints require pipe joint compound (pipe dope) or yellow PTFE tape labeled for gas service. Applied to male threads only.

B. Corrugated Stainless Steel Tubing (CSST)

• Standards: ANSI LC 1 / CSA 6.26 certified systems.
• Requirements:
  • Must be installed by manufacturer-certified plumbers.
  • Electrical Bonding: CSST systems must be bonded to the electrical grounding electrode system using a minimum 6 AWG copper bonding conductor connected to a listed bonding clamp on a metallic fitting.
  • Protection: IFGC 404.7.1 requires shield plates where piping runs through holes or notches in framing members and the pipe sits less than 1-1/2 inches (38 mm) from the framing member face that the wall, ceiling or floor membrane will be attached to. The plate must cover the width of the pipe and the framing member and extend not less than 4 inches to each side of it, and 404.7.3 sets the material at steel not less than 0.0575 inch (No. 16 gage). The 2-to-3-inch figure quoted in CSST manufacturer literature is a manufacturer instruction, not the code trigger.

C. Copper & Brass Tubing Restrictions

• Chemical Restriction: Copper and brass pipe or tubing SHALL NOT be used if the gas contains more than 0.3 grains of hydrogen sulfide ($H_2S$) per 100 standard cubic feet of gas ($0.7\text{ mg}/100\text{ L}$).
• Material Class: Where permitted, copper tubing must be seamless Type K or L (ASTM B88) or copper tube (ASTM B280).
• Joints: Must be brazed with filler metal having a melting point exceeding $1,000^\circ\text{F}$ ($538^\circ\text{C}$). Soft solder (lead-free or 50/50) is strictly prohibited.

D. Polyethylene (PE) Plastic Piping

• Standards: ASTM D2513 plastic pipe and fittings.
• Application: Approved ONLY for underground exterior gas piping.
• Prohibitions: Strictly prohibited inside buildings, under building slabs, or above ground. Must transition to metallic pipe prior to emerging above grade using listed anodeless risers.

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4. Exam Traps & Common Calculation Pitfalls

1. Branch Line Sizing Error: Candidates often make the mistake of using the actual physical length of a branch line when looking up its capacity. Rule: You must use the total developed length from the meter to the furthest outlet for all pipe sections in the system!
2. Incorrect Gas Heating Value: Applying natural gas CFH values to LP gas equipment (or vice-versa). LP gas requires fewer CFH for the same BTU load because its energy density is 2.5 times higher.
3. Omitting CSST Bonding: Failing to specify the 6 AWG copper bonding jumper required for CSST installations under IFGC Section 310.
4. Using Prohibited Soldering on Copper: Assuming standard plumbing solder (95/5) is acceptable on copper fuel gas lines; code mandates high-temperature brazing ($>1,000^\circ\text{F}$).