11.1 Gas Pipe & Tubing Sizing Fundamentals (Tables & Pressure Drop)

Gas Pipe & Tubing Sizing Fundamentals (Tables & Pressure Drop)

In fuel gas distribution, correct pipe and tubing sizing is the engineering foundation of appliance performance and operational safety. An undersized piping system causes excessive friction loss, leading to severe pressure drop during peak firing conditions. Pressure starvation destabilizes appliance burner manifolds, produces unstable lifting flames, causes incomplete combustion and toxic carbon monoxide ($CO$) generation, and triggers nuisance dropouts of low-pressure safety switches. Conversely, severely oversized piping introduces unnecessary material costs and creates excessively large internal fuel volumes that complicate pressure testing and purging procedures.

Under the Technical Standards and Safety Act, 2000, Ontario Regulation 212/01 (Gaseous Fuels), and the CSA B149.1 Natural Gas and Propane Installation Code, certified Gas Technician 3 (G.3) certificate holders must master the physical principles of gas flow, volumetric conversions, table navigation in Annex A and Annex B, and the Longest Developed Length (LDL) sizing methodology.

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Principles of Fuel Gas Flow & Pressure Drop

Gas flowing through a closed conduit experiences continuous internal fluid resistance caused by fluid viscosity and friction against the pipe wall. The fundamental relationship governing low-pressure fuel gas flow (operating below $0.5\text{ psig} / 14\text{ in. w.c.}$) is expressed through empirical flow formulations such as Pole's Formula and the Weymouth Equation:

$Q = C \times \sqrt{\frac{\Delta P \times d^5}{SG \times L}}$

Where:

• $Q$ = Volumetric flow rate in cubic feet per hour ($\text{CFH}$)
• $\Delta P$ = Allowable pressure drop across the pipe run (inches water column, $\text{in. w.c.}$)
• $d$ = Internal diameter of the pipe or tubing (inches)
• $SG$ = Specific gravity of the fuel gas relative to air ($1.00$)
• $L$ = Developed length of the piping run (feet)
• $C$ = Empirical friction and flow coefficient

Analyzing this fluid dynamic relationship reveals four core rules essential for gas technicians:

1. Direct Impact of Pipe Diameter ($d^5$): Flow capacity is proportional to the fifth power of internal diameter. A minor increase in pipe size yields an exponential expansion in capacity. For example, moving from nominal $1/2\text{-inch}$ Schedule 40 pipe ($d = 0.622\text{ in}$) to $3/4\text{-inch}$ pipe ($d = 0.824\text{ in}$) more than doubles the flow capacity at identical length and pressure drop.
2. Inverse Impact of Length ($\sqrt{L}$): As developed length increases, friction accumulates. Doubling the length of a pipe reduces its delivery capacity by a factor of $\sqrt{2} \approx 1.414$ (a $29.3\%$ flow reduction).
3. Square-Root Impact of Allowable Drop ($\sqrt{\Delta P}$): Increasing allowable pressure drop increases capacity, but only by the square root of the drop ratio.
4. Inverse Impact of Specific Gravity ($\sqrt{SG}$): Heavier gases encounter greater inertial resistance. A gas with a higher specific gravity experiences slower flow velocities under identical differential pressures.

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Volumetric Flow Conversion: $\text{BTU/h}$ to $\text{CFH}$

Appliance rating plates specify thermal input capacities in British Thermal Units per hour ($\text{BTU/h}$) or Kilowatts ($\text{kW}$) ($1\text{ kW} = 3{,}412.14\text{ BTU/h}$). Pipe capacity tables, however, are commonly expressed as volume flow—Cubic Feet per Hour ($\text{CFH}$) or Cubic Metres per Hour ($\text{m}^3\text{/h}$) ($1\text{ m}^3\text{/h} = 35.315\text{ CFH}$)—so always check the units in the table heading.

To size any pipe run, technicians must convert thermal ratings to volumetric flow using the gross heating value of the fuel gas:

$\text{Volumetric Flow (CFH)} = \frac{\text{Appliance Input Rating (BTU/h)}}{\text{Gross Heating Value of Fuel (BTU/cu ft)}}$

Natural Gas vs. Commercial Propane Energetics

Comparative Flow Calculation

Consider a high-efficiency condensing furnace with an input rating of $100{,}000\text{ BTU/h}$:

• Operating on Natural Gas: $\text{CFH}_{\text{NG}} = \frac{100{,}000\text{ BTU/h}}{1{,}000\text{ BTU/cu ft}} = 100\text{ CFH}$

• Operating on Commercial Propane: $\text{CFH}_{\text{LP}} = \frac{100{,}000\text{ BTU/h}}{2{,}500\text{ BTU/cu ft}} = 40\text{ CFH}$

Even though propane is more than twice as dense as natural gas ($SG = 1.52$ vs $0.60$), its volumetric energy concentration is $2.5\text{ times}$ greater. Consequently, an appliance requires $60\%$ less volume of propane to deliver the exact same thermal output. This critical distinction explains why propane piping and copper tubing lines are almost always smaller in diameter than natural gas piping serving equivalent loads.

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Pressure Regimes & Allowable Pressure Drops

Piping sizing tables in CSA B149.1 Annex A (Natural Gas) and Annex B (Propane) are categorized by inlet operating pressure, allowable pressure drop ($\Delta P$), and piping material.

Low-Pressure Systems (up to 14 in. w.c.)

CSA B149.1 Clause 6.3.2 and Table 6.1 set the maximum pressure drop allowed between the meter (or last-stage regulator) and the appliance:

Ontario residential natural gas is delivered at about 7 in. w.c., so Table 6.1 allows a 1 in. w.c. drop and the Table A.2 capacities. Sizing to a 0.5 in. w.c. drop (Table A.1) is more conservative and also complies—useful where an appliance needs a higher minimum inlet pressure or the supply sits at the low end of its range. Propane systems supplied at about 11 in. w.c. use the 1 in. w.c. tables in Annex B. The tables already include an allowance for a reasonable number of fittings.

Medium-Pressure Hybrid Systems ($2\text{ psig} / 14\text{ kPa}$)

Modern large residential and commercial buildings frequently employ $2\text{ psig}$ ($14\text{ kPa}$) natural gas distribution:

• Gas is distributed through compact, small-diameter piping (often $3/8\text{-inch}$ or $1/2\text{-inch}$ CSST or copper tubing) from the meter to central manifolds.
• Dedicated Line Pressure Regulators (LPRs) certified to CSA 6.22 / ANSI Z21.80 reduce the pressure from $2\text{ psig}$ down to appliance operating levels ($7\text{ in. w.c.}$) immediately adjacent to each appliance or appliance manifold.
• Clause 6.3.4 sizes 2 psig systems from the minimum pressure available at the line pressure regulator, using the 2 psig tables in Annex A (Tables A.2, A.3, A.4, A.10 and A.11) or Annex B (Tables B.2 and B.7) for propane.

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Practice Capacity Table (0.5 in. w.c. Drop)

The table below is an illustrative practice table: rounded capacities for Schedule 40 steel pipe carrying natural gas ($SG = 0.60$) at a $0.5\text{ in. w.c.}$ drop, following the pattern of published low-pressure capacity tables. Use it to practise the method. On the job and in the TSSA exam, read the actual CSA B149.1 Annex A table for your allowed pressure drop.

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The Longest Developed Length (LDL) Methodology

The Longest Developed Length (LDL) method is the standard way to use the Annex A and B tables for branched low-pressure systems. It is based on a foundational physical principle: the total friction drop across any path from the gas meter to an appliance must not exceed the allowable limit ($0.5\text{ in. w.c.}$).

If a technician were to size a short appliance branch (e.g., $15\text{ feet}$) using the $15\text{-foot}$ table column while the main header runs $80\text{ feet}$, the cumulative pressure drop across both segments would exceed the allowed drop. Sizing every segment from the total system LDL keeps the drop within the Table 6.1 limit under full load.

The Six-Step Sizing Protocol

1. Establish Total System Load: Sum the maximum firing rates of all connected appliances in $\text{BTU/h}$ and convert each to volumetric flow ($\text{CFH}$) by dividing by the fuel heating value.
2. Measure Developed Lengths: Determine the linear length of pipe from the meter or regulator outlet to each individual appliance shutoff valve. Developed length includes actual pipe travel plus an allowance for fittings if using advanced equivalent length methods (under standard residential Annex A tables, fitting friction is incorporated into conservative table margins).
3. Identify the Longest Developed Length (LDL): Find the single farthest appliance from the gas supply. Round this distance UP to the next higher row in the applicable sizing table. This row becomes the sole sizing reference for the entire piping network.
4. Size Main Distribution Trunk Sections: Starting at the meter, size each main line segment based on the cumulative CFH demand of all downstream appliances, reading across the established LDL row.
5. Size Intermediate Distribution Headers: At each tee or junction, recalculate the remaining downstream load and select the required pipe diameter from the LDL row.
6. Size Individual Appliance Branch Lines: Size each branch takeoff based strictly on that individual appliance's $\text{CFH}$ demand, using the exact same system LDL row—regardless of how short that branch pipe actually is.

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Worked Trade Sizing Example: Residential Installation

To master this procedure, work through a complete residential sizing problem using the practice table above.

Jobsite Specifications

• Fuel Gas: Natural Gas ($1{,}000\text{ BTU/cu ft}$, $SG = 0.60$)
• Supply Pressure: $7.0\text{ in. w.c.}$ ($<0.5\text{ psig}$)
• Allowable Pressure Drop: $0.5\text{ in. w.c.}$ (a conservative choice; Table 6.1 would allow $1\text{ in. w.c.}$ at a $7\text{ in. w.c.}$ supply)
• Piping Material: Schedule 40 Black Steel Pipe

Appliance Schedule & Firing Rates

1. Appliance A — High-Efficiency Furnace: $100{,}000\text{ BTU/h} = 100\text{ CFH}$
2. Appliance B — Domestic Water Heater: $40{,}000\text{ BTU/h} = 40\text{ CFH}$
3. Appliance C — Gas Cooking Range: $65{,}000\text{ BTU/h} = 65\text{ CFH}$
4. Appliance D — Gas Fireplace: $35{,}000\text{ BTU/h} = 35\text{ CFH}$

• Total Connected System Demand: $100 + 40 + 65 + 35 = \mathbf{240\text{ CFH}}$

Layout & Linear Piping Distances

• Meter to Junction 1 (J1): $20\text{ feet}$
• Junction 1 to Water Heater (Branch B): $15\text{ feet}$ (Total distance from meter: $20 + 15 = 35\text{ ft}$)
• Junction 1 to Junction 2 (J2): $15\text{ feet}$
• Junction 2 to Furnace (Branch A): $10\text{ feet}$ (Total distance from meter: $20 + 15 + 10 = 45\text{ ft}$)
• Junction 2 to Junction 3 (J3): $20\text{ feet}$
• Junction 3 to Range (Branch C): $10\text{ feet}$ (Total distance from meter: $20 + 15 + 20 + 10 = 65\text{ ft}$)
• Junction 3 to Fireplace (Branch D): $20\text{ feet}$ (Total distance from meter: $20 + 15 + 20 + 20 = \mathbf{75\text{ ft}}$)

Step-by-Step Sizing Execution

Step 1: Identify Longest Developed Length (LDL)

The farthest appliance is the Gas Fireplace, located at a total developed distance of $75\text{ feet}$ from the meter outlet. The practice table does not list $75\text{ feet}$; therefore, round UP to the $80\text{-foot}$ row.

All pipe segments in this system will be sized using the $80\text{-foot}$ row.

At $80\text{ feet}$, the practice table gives:

• $1/2\text{-inch}$: $57\text{ CFH}$
• $3/4\text{-inch}$: $118\text{ CFH}$
• $1\text{-inch}$: $220\text{ CFH}$
• $1\text{-}1/4\text{-inch}$: $460\text{ CFH}$

Step 2: Size Main Trunk Segment 1 (Meter to J1)

• Connected Load: Supplies all 4 appliances ($240\text{ CFH}$ total).
• Capacity Check at $80\text{ ft}$: A $1\text{-inch}$ pipe carries $220\text{ CFH}$ ($220 < 240$, insufficient). A $1\text{-}1/4\text{-inch}$ pipe carries $460\text{ CFH}$ ($460 \ge 240$, compliant).
• Selected Size: $1\text{-}1/4\text{ inch}$

Step 3: Size Branch B to Water Heater (J1 to Water Heater)

• Connected Load: $40\text{ CFH}$.
• Capacity Check at $80\text{ ft}$: A $1/2\text{-inch}$ pipe carries $57\text{ CFH}$ ($57 \ge 40$, compliant).
• Selected Size: $1/2\text{ inch}$

Step 4: Size Main Trunk Segment 2 (J1 to J2)

• Connected Load: Supplies Furnace, Range, and Fireplace ($100 + 65 + 35 = 200\text{ CFH}$).
• Capacity Check at $80\text{ ft}$: A $3/4\text{-inch}$ pipe carries $118\text{ CFH}$ (insufficient). A $1\text{-inch}$ pipe carries $220\text{ CFH}$ ($220 \ge 200$, compliant).
• Selected Size: $1\text{ inch}$

Step 5: Size Branch A to Furnace (J2 to Furnace)

• Connected Load: $100\text{ CFH}$.
• Capacity Check at $80\text{ ft}$: A $1/2\text{-inch}$ pipe carries only $57\text{ CFH}$ (insufficient). A $3/4\text{-inch}$ pipe carries $118\text{ CFH}$ ($118 \ge 100$, compliant).
• Selected Size: $3/4\text{ inch}$

Step 6: Size Main Trunk Segment 3 (J2 to J3)

• Connected Load: Supplies Range and Fireplace ($65 + 35 = 100\text{ CFH}$).
• Capacity Check at $80\text{ ft}$: A $1/2\text{-inch}$ pipe carries $57\text{ CFH}$ (insufficient). A $3/4\text{-inch}$ pipe carries $118\text{ CFH}$ ($118 \ge 100$, compliant).
• Selected Size: $3/4\text{ inch}$

Step 7: Size Branch C to Range (J3 to Range)

• Connected Load: $65\text{ CFH}$.
• Capacity Check at $80\text{ ft}$: A $1/2\text{-inch}$ pipe carries $57\text{ CFH}$ ($57 < 65$, insufficient). A $3/4\text{-inch}$ pipe carries $118\text{ CFH}$ ($118 \ge 65$, compliant).
• Selected Size: $3/4\text{ inch}$

Step 8: Size Branch D to Fireplace (J3 to Fireplace)

• Connected Load: $35\text{ CFH}$.
• Capacity Check at $80\text{ ft}$: A $1/2\text{-inch}$ pipe carries $57\text{ CFH}$ ($57 \ge 35$, compliant).
• Selected Size: $1/2\text{ inch}$

Summary Sizing Schedule

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Tubing Sizing Nuances: Copper & CSST

Seamless Copper Tubing (Annex A Tables A.8 & A.9)

Copper tubing for fuel gas is ASTM B88 Type K or L, or ASTM B837 Type G. Unlike steel pipe, which is designated by Nominal Pipe Size (NPS) (where $1/2\text{-inch}$ Schedule 40 has an internal diameter of $0.622\text{ in}$ and an outside diameter of $0.840\text{ in}$), copper tubing is designated strictly by its Outside Diameter (OD). Consequently, a $1/2\text{-inch}$ OD copper tube has an inside diameter of only $\approx 0.430\text{ in}$—significantly smaller than nominal $1/2\text{-inch}$ steel pipe. Technicians must never use steel pipe tables to size copper tubing; always use the tubing tables in Annex A for the allowed pressure drop (for example Table A.8 at $0.5\text{ in. w.c.}$ and Table A.9 at $1\text{ in. w.c.}$).

Corrugated Stainless Steel Tubing (CSST)

CSST features continuous helical or annular corrugations that grant flexibility but induce substantial internal fluid turbulence. Because corrugation geometry varies between manufacturers, CSST cannot be sized with generic pipe tables. Instead, CSST utilizes the Equivalent Hydraulic Diameter (EHD) rating system certified under CSA 6.26 / ANSI LC 1:

• The manufacturer's certified engineering tables must be consulted.
• Technicians determine the required EHD number based on gas load and LDL.
• If sizing hybrid $2\text{ psig}$ CSST systems, the line regulator must be accounted for in the sizing calculation.