9.3 Gas Piping Sizing, Materials & Installation

9.3 Gas Piping Sizing, Materials & Installation

1. Fuel Gas Delivery Pressures & System Architecture

Fuel gas piping systems must be engineered and installed to supply an uninterrupted volume of fuel at adequate pressure to all connected appliances under maximum simultaneous demand. In Maryland, gas piping installation is governed by the International Fuel Gas Code (IFGC) and NFPA 54 (National Fuel Gas Code).

Low-Pressure Distribution Systems

• Operating Pressures: The vast majority of residential and light commercial natural gas systems operate at low pressure, defined as less than 0.5 psig (14 inches water column), typically delivered from the utility meter at 7.0 in. w.c. (1.74 kPa).
• Allowable Pressure Drop: Under IFGC Section 402.4, the maximum allowable pressure drop across the entire piping system from the meter outlet to the inlet of the most remote appliance is 0.5 in. w.c. (0.12 kPa). This ensures that even under full building firing load, the appliance regulator receives at least 5.0 to 6.5 in. w.c., easily satisfying the 3.5 in. w.c. manifold requirement.

Elevated Pressure (2-psig) Hybrid Distribution Systems

Modern construction frequently utilizes 2.0 psig (55.4 in. w.c.) elevated pressure distribution:

• Advantages: Operating at 2.0 psig allows significantly smaller pipe diameters (such as 3/8" and 1/2" Corrugated Stainless Steel Tubing [CSST]) over long runs without exceeding allowable percentage pressure drops.
• Line Pressure Regulators (LPRs): In a 2-psig system, gas is piped to a central manifold or distributed to individual appliances, where a certified Line Pressure Regulator (LPR) (complying with ANSI Z21.80) steps the pressure down from 2.0 psig to low pressure (7 to 11 in. w.c.).
• Vent Limiter vs. Vent Piping: LPRs equipped with internal vent limiters are approved for indoor installation without outdoor vent piping, provided they are mounted in an upright, horizontal position in well-ventilated spaces. If an LPR lacks a vent limiter, an independent relief vent line must be piped outdoors, terminating at least 3 feet (914 mm) away from any building opening or ignition source.

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2. IFGC Pipe Sizing Methodologies & Schedule 40 Tables

Proper pipe sizing prevents gas starvation, pilot outages, burner flutter, and incomplete combustion. Sizing requires converting appliance thermal ratings into volumetric flow rates.

Determining Volumetric Flow Rate (CFH)

Natural gas appliances are rated in British Thermal Units per hour (BTU/hr) input on their factory nameplates. To convert input rating to Cubic Feet per Hour (CFH) of natural gas:

$\text{Gas Flow (CFH)} = \frac{\text{Total Connected Appliance Input (BTU/hr)}}{\text{Heating Value of Gas (BTU/ft}^3)}$

Using the IFGC standard natural gas heating value of 1,000 BTU/ft³:

• A 100,000 BTU/hr furnace requires: $100,000 / 1,000 = \mathbf{100 \text{ CFH}}$.
• A 40,000 BTU/hr water heater requires: $40,000 / 1,000 = \mathbf{40 \text{ CFH}}$.
• A 60,000 BTU/hr gas range requires: $60,000 / 1,000 = \mathbf{60 \text{ CFH}}$. (Note: For LP gas systems delivering 2,500 BTU/ft³, divide total BTU/hr input by 2,500 to obtain required CFH).

The Longest Length Method (IFGC Section 402.4.1)

The Longest Length Method is the most universal and conservative pipe sizing methodology tested on licensing exams. The procedure follows four mandatory steps:

1. Measure the Longest Run: Measure the total physical distance from the gas meter (point of delivery) to the most remote appliance in the entire building piping system.
2. Select the Table Column: Locate that single "longest length" distance column in the appropriate IFGC capacity table (e.g., IFGC Table 402.4(2) for Schedule 40 Metallic Pipe, Natural Gas, Inlet < 0.5 psig, Pressure Drop 0.5 in. w.c., Specific Gravity 0.60). If the exact measured length is between two table columns, round up to the next longer column.
3. Lock the Column: This single column is used to size EVERY pipe segment in the entire system, including main distribution headers, intermediate branches, and individual appliance drops, regardless of how short an individual branch line may be.
4. Size Segments by Downstream Load: For each individual segment, calculate the total downstream connected load (in CFH) that the pipe must carry. Match that load against the capacities listed in the selected column, choosing a nominal diameter with a rated capacity equal to or greater than the demand.

IFGC Schedule 40 Black Steel Pipe Sizing Table

Maximum capacity of pipe in cubic feet per hour (CFH) for natural gas with inlet pressure < 0.5 psig, pressure drop 0.5 in. w.c., and specific gravity 0.60 (Based on IFGC Table 402.4(2)):

Worked Exam Example: Sizing a Three-Appliance System

Scenario: A gas piping layout originates at the utility meter and feeds three natural gas appliances:

• Main Furnace: 120,000 BTU/hr (120 CFH), located 85 feet from meter.
• Water Heater: 40,000 BTU/hr (40 CFH), located 45 feet from meter.
• Kitchen Range: 65,000 BTU/hr (65 CFH), located 95 feet from meter.

Step-by-Step Solution:

1. Identify Longest Run: The Kitchen Range is the most remote appliance at 95 feet. In the sizing table, round up to the 100-foot column.
2. Lock Column: The 100-foot column will be used for ALL sizing decisions.
3. Size Main Header (Meter to First Branch): The main header must carry all three appliances: $120 + 40 + 65 = \mathbf{225 \text{ CFH}}$. Looking down the 100-foot column:
   • 3/4" carries 104 CFH (too small).
   • 1" carries 195 CFH (too small).
   • 1-1/4" carries 400 CFH. Select 1-1/4" black steel pipe for the main header.
4. Size Furnace Branch: Carries 120 CFH. In the 100-foot column, 3/4" carries 104 CFH (insufficient). Select 1" pipe for the furnace run.
5. Size Water Heater Branch: Carries 40 CFH. In the 100-foot column, 1/2" carries 50 CFH. Select 1/2" pipe for the water heater.
6. Size Range Branch: Carries 65 CFH. In the 100-foot column, 1/2" carries 50 CFH (too small); 3/4" carries 104 CFH. Select 3/4" pipe for the range.

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3. Code Installation Details: Sediment Traps, Valves & Connectors

Mechanical code violations on gas piping frequently result from improper fitting configurations, missing sediment traps, or unapproved shutoff valve locations.

The Sediment Trap (Drip Leg) Mandate (IFGC Section 408)

• Purpose: Natural gas lines transport microscopic quantities of pipe scale, metal shavings, cutting oil, dust, and residual moisture. If allowed into the appliance combination gas valve, these particles foul regulator diaphragms and pilot orifices. A sediment trap captures these solids via gravity and inertial direction change.
• Code geometry: Under 2018 IFGC § 408.4, where a sediment trap is required, it is downstream of the appliance shutoff and near the inlet. A tee can use a capped nipple of any length vertically in its bottommost opening, or another approved effective trap.
• Flow path: Install the tee or approved device in the orientation shown by the adopted code and appliance instructions so debris can settle. Do not invent an additional universal 90-degree rule.
• Exempt Appliances: Under IFGC Section 408.4, sediment traps are NOT required for: illuminating appliances, ranges, clothes dryers, outdoor cooking grills, and decorative gas fireplaces (unvented log sets or direct-vent gas fireplaces).

Manual Gas Shutoff Valves (IFGC Section 409)

• Accessibility & Location: Every gas appliance must be provided with an independent, listed manual shutoff valve complying with ASME B16.44 or ASME B16.33. The valve must be located in the same room as the appliance, within 6 feet (1,829 mm) of the appliance, and positioned upstream of the ground-joint union or flexible appliance connector.
• Valve Construction: Must be a quarter-turn lever ball valve or lubricated plug valve. Multi-turn gate valves are strictly prohibited on fuel gas lines.

Appliance Flexible Connectors (IFGC Section 411)

• Standards & Length Limits: Corrugated metallic appliance connectors must be listed to ANSI Z21.24. For standard domestic appliances, the maximum overall length is 6 feet (1,829 mm). For commercial food service equipment (ranges, deep fryers), connectors may not exceed 3 feet (914 mm).
• Code Prohibitions: Flexible gas connectors must never pass through walls, floors, ceilings, partitions, or appliance cabinet casings. They must never be concealed. Furthermore, flexible connectors must NEVER be reused; when an existing furnace or water heater is replaced, the flexible connector must be discarded and replaced with a brand-new listed connector.

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4. Code Pressure Testing & Leak Detection Protocol (IFGC Section 406)

Before any new or modified fuel gas piping system is placed into service, it must pass a rigorous mechanical pressure test inspected and approved by the local building official.

Pressure Testing Standards

• Test Medium: The test must be performed exclusively with compressed air, nitrogen, or an inert gas. NEVER use oxygen (which causes violent explosion upon contacting pipe thread oils) or combustible fuel gas.
• Minimum Test Pressure: The test pressure must be not less than 1.5 times the proposed maximum working pressure, but in no case shall it be less than 3.0 psig (20 kPa) for low-pressure systems.
• Test Duration: The test pressure must be held continuously for a minimum of 10 minutes for residential and small commercial piping installations (systems with less than 10 ft³ of pipe volume). For large industrial distribution systems, test durations increase to 30 minutes to several hours.
• Test Gauge Resolution: Per IFGC Section 406.4.2, test gauges must have a scale where the test pressure falls within the middle half of the dial range. The maximum gauge range cannot exceed five times the test pressure (e.g., for a 3-psig test, the gauge cannot exceed 0–15 psig; using a 0–100 psig gauge is a major code violation because small pressure drops cannot be visually detected).
• Isolating Appliance Controls: Appliance combination gas valves are rated for a maximum pressure of 0.5 psig (14 in. w.c.). During a 3.0+ psig pressure test, all appliance shutoff valves must be tightly closed, or the appliances must be disconnected and lines capped. Subjecting a gas valve to 3.0 psig ruptures internal regulator diaphragms.

Leak Detection Procedures

If a pressure drop occurs during the 10-minute hold period, leaks must be located using an approved non-corrosive bubble leak detection solution (complying with ASTM C841) or a calibrated electronic combustible gas detector. Technicians must never use dish soap containing ammonia (which causes stress-corrosion cracking of brass fittings and CSST) and must NEVER search for gas leaks with an open flame.