1. Service Line Overview
A gas service line is the pipe that connects a gas distribution main to the customer meter set. It is the final link in the gas delivery chain, running from the main in the street or easement, under the customer property, to the meter and regulator at the building. Service lines represent the largest quantity of pipe in most gas distribution systems, with a typical utility owning hundreds of thousands of individual service lines.
Service line sizing must ensure adequate gas delivery at the required pressure for the customer's maximum appliance demand while staying within the allowable pressure drop budget. Undersizing causes low pressure complaints, pilot outages, and incomplete combustion. Oversizing increases material cost and creates longer purge times during installation and maintenance.
Service Line Components
- Main tap: The connection to the distribution main, typically a saddle tap for PE pipe or a tapped tee for steel or cast iron mains. The tap size matches the service line diameter.
- Service pipe: The pipe itself, running from the main to the meter. May be PE, steel, or copper depending on utility standards, pressure class, and local code requirements.
- Curb valve: A shutoff valve located near the property line or curb, accessible from the surface with a valve key. Allows the utility to shut off gas without entering the customer premises. Required by most utility standards for new service installations.
- Excess flow valve (EFV): An automatic shut-off device installed near the main that closes if flow exceeds a preset limit, indicating a service line break. Required by 49 CFR 192.383 for new and replaced residential service lines operating above 10 psig.
- Meter set assembly: Located at the building, includes the meter, service regulator (reduces pressure from service line to appliance delivery pressure), and associated valves and piping. The meter is owned and maintained by the utility.
- Riser: The vertical pipe transitioning from underground service to above-ground meter set. May be steel or PE-to-steel transition fitting. Must be protected from physical damage and supported against settlement.
Design pressure: Service lines are designed and installed for the maximum allowable operating pressure (MAOP) of the distribution system they connect to. For HP distribution (typically 60 psig), the service line, valve, and all fittings must be rated for 60 psig minimum. For LP distribution (less than 1 psig), lower-rated components may be used. PE pipe is pressure-rated by material designation (PE2708, PE4710) and dimension ratio (DR 11, DR 9.3).
2. Flow Equations
Two primary flow equations are used for gas service line sizing, depending on the operating pressure:
Weymouth Formula (High Pressure, above 1.5 psig)
Weymouth Equation:
Q = 18.062 × d^(8/3) × sqrt((P1² - P2²) / (SG × L))
Where:
Q = Gas flow rate (CFH at standard conditions)
d = Pipe internal diameter (inches)
P1 = Upstream pressure (psia)
P2 = Downstream pressure (psia)
SG = Gas specific gravity (air = 1.0)
L = Pipe length (feet), including equivalent length for fittings
Notes:
- Assumes fully turbulent flow (conservative for small pipes)
- Standard conditions: 14.73 psia, 60°F
- Valid for pressures above ~1.5 psig (above 42 in WC)
- Widely used in US gas distribution industry
- Produces slightly conservative results compared to AGA equations
Spitzglass Formula (Low Pressure, below 1.5 psig)
Spitzglass Equation (Low Pressure):
Q = 3550 × sqrt(ΔH × d&sup5; / (L × (1 + 3.6/d + 0.03×d)))
Where:
Q = Gas flow rate (CFH at standard conditions)
ΔH = Pressure drop (inches water column)
d = Pipe internal diameter (inches)
L = Pipe length (feet)
Notes:
- Used for LP distribution (under ~1.5 psig)
- Pressure drop in inches water column (1 psi = 27.7 in WC)
- The term (1 + 3.6/d + 0.03d) is the Spitzglass friction factor
- Accounts for pipe roughness empirically
- Standard in NFPA 54 sizing tables for interior piping
Elevation Correction
Elevation Correction for Gas Service Lines:
For gas flowing uphill from main to customer:
ΔP_elevation = 0.0375 × SG × Δh
Where:
ΔP_elevation = Pressure correction (psi)
SG = Gas specific gravity
Δh = Elevation change (feet, positive = uphill)
Uphill flow: Subtract from available pressure drop
Downhill flow: Add to available pressure drop (gas is lighter than air)
Example: SG = 0.60, elevation rise = 30 feet
ΔP_elev = 0.0375 × 0.60 × 30 = 0.675 psi
For a 3 psi allowable drop: effective = 3.0 - 0.675 = 2.325 psi
This 22% reduction in available pressure drop can change the required pipe size!
Which equation to use: Use Weymouth for HP service lines (above 1.5 psig) and Spitzglass for LP service lines and interior piping. When both equations are applicable (at the transition pressure), use the one producing the lower capacity (conservative). Most modern gas utilities operate HP distribution, making Weymouth the primary equation for service line sizing. NFPA 54 Appendix C provides pre-calculated capacity tables based on Spitzglass for common pipe sizes and lengths.
3. Pipe Materials & Sizing
Pipe material selection for gas service lines depends on operating pressure, soil conditions, local code requirements, and utility construction standards. The three primary materials used in the US are polyethylene (PE), steel, and copper.
Material Comparison
| Property | PE (Polyethylene) | Steel | Copper |
|---|---|---|---|
| Pressure rating | Up to 100 psig (DR 11) | Up to MAOP of system | Up to 100 psig (Type K) |
| Corrosion | Immune | Requires CP and coating | Resistant in most soils |
| Joining method | Heat fusion | Threaded or welded | Compression or flare |
| Installation | Flexible, direct burial | Rigid, trenched | Semi-flexible, trenched |
| Cost | Lowest | Moderate | Highest |
| Typical use | New service lines | Existing, high-pressure | Interior piping, small services |
Standard PE Service Line Sizes
| Nominal Size (IPS) | OD (inches) | ID - DR 11 (inches) | Typical Capacity at 60 psig (SCFH) |
|---|---|---|---|
| 1/2" | 0.840 | 0.622 | 40-80 |
| 3/4" | 1.050 | 0.824 | 100-200 |
| 1" | 1.315 | 1.049 | 250-500 |
| 1-1/4" | 1.660 | 1.380 | 500-1,000 |
| 2" | 2.375 | 2.067 | 1,500-3,500 |
PE pipe dominance: Polyethylene has become the standard material for new gas service line construction due to its corrosion immunity, lower installed cost, and ease of installation. PE pipe does not require cathodic protection (saving ongoing maintenance cost), can be installed by directional boring (minimizing surface disruption), and has a service life exceeding 50 years. However, PE is not permitted above ground and must transition to steel or copper at the riser.
4. Fittings & Equivalent Length
Pipe fittings (elbows, tees, valves) create additional pressure drop beyond straight pipe friction. The equivalent length method converts each fitting to an equivalent length of straight pipe that produces the same pressure drop. This equivalent length is added to the actual pipe length for capacity calculations.
Equivalent Length Table
| Fitting Type | 1/2" | 3/4" | 1" | 1-1/4" | 2" |
|---|---|---|---|---|---|
| 90-degree elbow | 2 | 3 | 4 | 5 | 7 |
| 45-degree elbow | 1 | 1 | 2 | 2 | 3 |
| Tee (through run) | 1 | 1 | 1 | 2 | 2 |
| Tee (through branch) | 3 | 4 | 5 | 7 | 10 |
| Gate valve (full open) | 0.5 | 0.5 | 1 | 1 | 1 |
| Ball valve (full open) | 0.5 | 0.5 | 1 | 1 | 1 |
| Curb valve | 3 | 4 | 5 | 7 | 10 |
Values in feet of equivalent pipe length (NFPA 54 Appendix C methodology). For a typical residential service with 3 elbows on a 1-inch line, the equivalent length adds 12 feet (3 elbows x 4 feet each) to the actual pipe length.
Practical sizing rule: For most residential services with lines under 150 feet, a 1-inch PE service line at 60 psig provides adequate capacity for total connected loads up to 250,000 BTU/hr (typical single-family home). For loads above 250,000 BTU/hr or line lengths exceeding 200 feet, use 1-1/4-inch or 2-inch PE. Always verify by calculation, especially for large commercial loads or long runs.
5. Sizing Examples
Example 1: Residential Service (HP)
Given:
System pressure: 60 psig, Allowable drop: 3 psi
Load: 200,000 BTU/hr (furnace + water heater + range + dryer)
Material: 1" PE (IPS), ID = 1.049"
Length: 80 feet, 3 elbows, no elevation change
Step 1: Required flow
Q = 200,000 / 1,020 = 196 SCFH
Step 2: Equivalent length for fittings
3 elbows × 4 ft each (1" pipe) = 12 ft
Total length = 80 + 12 = 92 ft
Step 3: Weymouth capacity
P1 = 60 + 14.7 = 74.7 psia
P2 = 74.7 - 3.0 = 71.7 psia
Q = 18.062 × 1.049^(8/3) × sqrt((74.7² - 71.7²) / (0.60 × 92))
Q = 18.062 × 1.132 × sqrt((5580 - 5141) / 55.2)
Q = 20.44 × sqrt(439/55.2)
Q = 20.44 × 2.82 = 57.6 SCFH
This is LESS than required 196 SCFH!
1" pipe is inadequate at 3 psi drop.
Try 1-1/4" PE (ID = 1.380"):
Q = 18.062 × 1.380^(8/3) × sqrt((5580-5141)/(0.60×95))
= 18.062 × 2.120 × sqrt(439/57)
= 38.29 × 2.775
= 106 SCFH -- Still short!
Try 2" PE (ID = 2.067"):
Q = 18.062 × 2.067^(8/3) × sqrt(439/(0.60×98))
= 18.062 × 5.82 × sqrt(439/58.8)
= 105 × 2.73
= 288 SCFH -- ADEQUATE (47% margin)
Result: 2" PE service line required.
Example 2: Commercial Service
Given:
System pressure: 60 psig, Allowable drop: 5 psi
Load: 1,000,000 BTU/hr (restaurant with commercial kitchen)
Material: Steel Sch 40
Length: 150 feet, 5 elbows, 10 ft elevation rise
Step 1: Required flow
Q = 1,000,000 / 1,020 = 980 SCFH
Step 2: Elevation correction
ΔP_elev = 0.0375 × 0.60 × 10 = 0.225 psi
Effective allowable = 5.0 - 0.225 = 4.775 psi
Step 3: Try 2" steel (ID = 2.067")
Fitting eq length = 5 × 7 = 35 ft
Total length = 150 + 35 = 185 ft
P1 = 74.7, P2 = 74.7 - 4.775 = 69.925 psia
Q = 18.062 × 2.067^(8/3) × sqrt((74.7² - 69.925²) / (0.60 × 185))
Q = 105.1 × sqrt((5580 - 4890) / 111)
Q = 105.1 × sqrt(6.22)
Q = 105.1 × 2.49
Q = 262 SCFH -- Insufficient!
Need larger pipe. A 3" or 4" steel service may be required.
For large commercial loads, consider running a dedicated HP service
with the meter set regulator handling the final pressure reduction.
Velocity check: After selecting a pipe size, always verify that the gas velocity does not exceed 100 ft/s (some utilities use 60 ft/s as a more conservative limit). High velocities cause noise complaints at the meter set, accelerate erosion at fittings, and increase pressure drop beyond what the steady-state equations predict. If velocity exceeds the limit, use the next larger pipe size even if the pressure drop calculation permits the smaller size.
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