8.2 Fuel Gas Pipe Sizing & Pressure Drop

Fuel Gas Pipe Sizing & Pressure Drop

1. Gas Volumetric Flow Rate & BTU Calculations

Sizing a fuel gas piping distribution system requires converting the thermal heat input ratings of all connected gas appliances into a volumetric flow rate expressed in Cubic Feet per Hour (CFH). The volumetric demand dictates the pipe internal diameter necessary to deliver gas without excessive pressure drop.

Thermal Heating Values of Fuel Gases

A British Thermal Unit (BTU) is defined as the quantity of heat required to raise the temperature of 1 pound of liquid water by $1^\circ\text{F}$. Fuel gases vary in energy density:

• Natural Gas (Nominal): $1,000 \text{ BTU per cubic foot } (37.3 \text{ MJ/m}^3)$
• Liquid Petroleum (Propane) Gas: $2,500 \text{ BTU per cubic foot } (93.1 \text{ MJ/m}^3)$
• Commercial Butane Gas: $3,200 \text{ BTU per cubic foot } (119.2 \text{ MJ/m}^3)$

The CFH Demand Formula

$$\text{Volumetric Flow Rate (CFH)} = \frac{\text{Total Appliance Input Rating (BTU/hr)}}{\text{Gas Heating Value (BTU/cu ft)}}$$

┌────────────────────────────────────────────────────────────────────────────────────────┐
│                     SAMPLE APPLIANCE CFH DEMAND CONVERSION TABLE                      │
├─────────────────────────────┬──────────────────┬───────────────────┬───────────────────┤
│ Appliance Description       │ BTU/hr Input     │ Natural Gas (CFH) │ Propane Gas (CFH) │
│                             │ Rating           │ (BTU / 1,000)     │ (BTU / 2,500)     │
├─────────────────────────────┼──────────────────┼───────────────────┼───────────────────┤
│ Central Heating Furnace     │ 100,000 BTU/hr   │ 100 CFH           │ 40 CFH            │
│ Storage Water Heater        │ 40,000 BTU/hr    │ 40 CFH            │ 16 CFH            │
│ Tankless Water Heater       │ 199,000 BTU/hr   │ 199 CFH           │ 79.6 CFH          │
│ Residential Kitchen Range   │ 65,000 BTU/hr    │ 65 CFH            │ 26 CFH            │
│ Clothes Dryer               │ 35,000 BTU/hr    │ 35 CFH            │ 14 CFH            │
├─────────────────────────────┼──────────────────┼───────────────────┼───────────────────┤
│ TOTAL COMBINED LOAD         │ 439,000 BTU/hr   │ 439 CFH           │ 175.6 CFH         │
└─────────────────────────────┴──────────────────┴───────────────────┴───────────────────┘

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2. Gas Pressure Fundamentals & Pressure Drop Thresholds

Fuel gas distribution pressures are measured in Pounds per Square Inch (psi) or Inches of Water Column (in. w.c.).

$$1 \text{ psi} = 27.7 \text{ inches of water column } (\text{in. w.c.})$$ $$0.5 \text{ psi} \approx 14.0 \text{ inches of water column } (\text{in. w.c.})$$

Low-Pressure Natural Gas Systems

Standard residential and commercial interior gas piping downstream of the utility meter regulator operates at a low supply pressure of 6 to 7 in. w.c. ($0.22\text{--}0.25\text{ psi}$). Model codes base standard pipe sizing tables on an allowable system pressure drop of 0.5 in. w.c. ($125\text{ Pa}$). This ensures that the minimum manifold pressure delivered to appliance control valves does not drop below 3.5 to 5.0 in. w.c. during peak simultaneous demand.

Medium-Pressure Gas Systems (2 psi / 5 psi)

In larger commercial buildings or modern residential installations, medium-pressure systems operate at 2.0 psi ($55.4\text{ in. w.c.}$). A Line Pressure Regulator (MPR) certified to ANSI Z21.80 is installed immediately upstream of each appliance to reduce pressure from 2.0 psi down to 7.0 in. w.c. Sizing tables for 2.0 psi systems permit an allowable pressure drop of 1.0 psi, drastically reducing required pipe diameters.

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3. The Longest Length Method (Step-by-Step Code Algorithm)

The Longest Length Method is the primary procedure mandated by IFGC Section 402 and NFPA 54 Section 6.2 for sizing gas piping. Under this method, every pipe segment in the entire piping layout is sized using the single column in the code capacity table corresponding to the total distance from the gas supply point to the single most remote appliance outlet.

             METER (Supply Point)
               │
               │ Segment A (Trunk Line: 439 CFH)
               ▼
        ┌──────┴──────────────────────┐
        │                             │
 Segment B (199 CFH)           Segment C (240 CFH)
 Tankless Heater (50 ft)              │
                                      ▼
                               ┌──────┴──────────────┐
                               │                     │
                        Segment D (100 CFH)   Segment E (140 CFH)
                        Furnace (70 ft)       Furthest Outlet:
                                              Range (90 ft)

Step-by-Step Sizing Procedure

1. Determine Longest Length: Measure the total pipe length from the meter to the furthest appliance. In the schematic above, the distance to the Range is 90 feet (the longest length in the system).
2. Select Code Table Column: Locate the Schedule 40 Metallic Pipe Capacity Table for Natural Gas — IFGC Table 402.4(2) / NFPA 54 Table 6.2(a), inlet pressure less than 2 psi, $0.5\text{ in. w.c.}$ drop, $G=0.60$. Round 90 feet up to the 100-foot length row. Critical Rule: you must use the 100-foot row for ALL pipe segments (A, B, C, D, and E). The capacities on that row are:

1. Size Main Trunk Segment A: Segment A carries the total load of all appliances ($439\text{ CFH}$). On the 100-foot row, locate a capacity $\ge 439\text{ CFH}$. A 1-1/4 inch pipe carries $404\text{ CFH}$ (insufficient), while a 1-1/2 inch pipe carries $606\text{ CFH}$. Select 1-1/2 inch steel pipe.
2. Size Branch Segment B (Tankless): Segment B carries $199\text{ CFH}$. On the SAME 100-foot row, a 1-inch pipe carries only $197\text{ CFH}$ — two CFH short. Select 1-1/4 inch pipe ($404\text{ CFH}$). This is exactly why a 199,000 BTU/hr tankless heater so often forces an upsize on a long run.
3. Size Segment C: Segment C carries the combined load of the remaining appliances ($240\text{ CFH}$). On the 100-foot row, 1-inch carries $197\text{ CFH}$ (insufficient), so select 1-1/4 inch pipe.
4. Size Branch Segment D (Furnace): Segment D carries $100\text{ CFH}$. On the 100-foot row, a 3/4-inch pipe carries $105\text{ CFH}$. Select 3/4-inch pipe.
5. Size Branch Segment E (Range): Segment E carries $140\text{ CFH}$. On the 100-foot row, 3/4-inch carries $105\text{ CFH}$ (insufficient), so a 1-inch pipe at $197\text{ CFH}$ is required.

Why the longest-length method costs pipe. Segments B, C and E are all physically much shorter than 90 feet, but the method forces every one of them onto the 100-foot row. The Branch Length Method in the next part relaxes exactly this penalty.

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4. The Branch Length Method

The Branch Length Method is an alternative sizing procedure permitted by code. Under this method:

• The main distribution trunk is sized using the longest distance from the meter to the furthest appliance outlet.
• Individual branch lines are sized using the distance from the meter to that specific branch outlet.

This method allows smaller pipe sizes on short branch runs near the meter, optimizing installation cost.

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5. Specific Gravity Adjustments (Natural Gas vs. Propane)

Specific Gravity ($G$) is the ratio of the density of a gas to the density of dry air ($G_{\text{air}} = 1.00$).

• Natural Gas: $G = 0.60$ (Lighter than air; floats upward)
• Propane (LP) Gas: $G = 1.50$ (Heavier than air; sinks and pools in low areas)

Flow capacity through a pipe varies inversely with the square root of specific gravity:

$$\text{Multiplier Factor} = \sqrt{\frac{0.60}{G_{\text{new}}}}=\sqrt{\frac{0.60}{1.50}}=\sqrt{0.40} \approx 0.632$$

To use a standard Natural Gas capacity table for Propane gas, multiply the natural gas table capacity values by 0.632. A pipe that carries $100\text{ CFH}$ of natural gas will carry only $63.2\text{ CFH}$ of propane at the same pressure drop.