1. CNG Transport Overview
Compressed Natural Gas (CNG) transport via tube trailers enables "virtual pipelines" to deliver gas where physical pipelines are unavailable or uneconomical. Understanding real gas behavior at high pressures is critical for accurate volume calculations.
Virtual Pipeline
Remote gas delivery
Trucked CNG to industrial users, drilling rigs, or remote communities without pipeline access.
CNG Stations
Vehicle fueling
Fleet fueling stations using cascade storage to dispense CNG to vehicles.
Peak Shaving
Demand management
Supplemental gas supply during high-demand periods or pipeline constraints.
Emergency Supply
Backup gas source
Temporary gas supply during pipeline outages or maintenance.
Regulatory Framework
| Regulation | Scope | Key Requirements |
|---|---|---|
| DOT 49 CFR Part 178 | Compressed gas cylinders | Cylinder design, testing, marking (DOT-3AA, DOT-3AAX) |
| DOT 49 CFR Part 180 | Cylinder requalification | Hydrostatic testing every 5 years, visual inspection |
| NFPA 52 | CNG vehicular fuel systems | Station design, cascade sizing, safety systems |
| SAE J2601 | CNG fueling protocols | Fast-fill temperature compensation, pressure targets |
| CGA C-6.4 | Tube trailer operations | Loading, unloading, transport safety procedures |
Why real gas matters: At CNG operating pressures (3000-3600 psig), natural gas deviates significantly from ideal gas behavior. The compressibility factor Z can be 0.82-0.88, meaning actual gas volume is 12-18% less than ideal gas law predicts. Accurate Z-factor calculation is essential for inventory management, custody transfer, and system sizing.
2. Real Gas Calculations
CNG volume calculations require the real gas equation of state, which accounts for molecular interactions at high pressures through the compressibility factor Z.
Real Gas Equation of State
Real Gas Law:
PV = ZnRT
Rearranged for standard volume:
V_std = V_actual × (P_actual / P_std) × (T_std / T_actual) × (Z_std / Z_actual)
Where:
V_std = Volume at standard conditions (SCF)
V_actual = Physical container volume (ft³)
P_actual = Absolute pressure (psia)
P_std = Standard pressure = 14.73 psia (GPA)
T_actual = Absolute temperature (°R = °F + 459.67)
T_std = Standard temperature = 519.67°R (60°F)
Z_actual = Compressibility factor at actual conditions
Z_std ≈ 1.0 (at standard conditions)
Compressibility Factor (Z-Factor)
The Z-factor accounts for deviation from ideal gas behavior and is calculated using the Dranchuk-Abou-Kassem correlation, which approximates the Standing-Katz chart:
Pseudo-Critical Properties (Kay's Rule):
For natural gas with specific gravity SG (air = 1.0):
T_pc = 170.5 + 307.3 × SG (°R)
P_pc = 709.6 - 58.7 × SG (psia)
Reduced Properties:
T_r = T / T_pc (reduced temperature)
P_r = P / P_pc (reduced pressure)
Example: Pipeline quality gas (SG = 0.60) at 3600 psig, 60°F
T_pc = 170.5 + 307.3 × 0.60 = 354.9°R
P_pc = 709.6 - 58.7 × 0.60 = 674.4 psia
P_abs = 3600 + 14.7 = 3614.7 psia
T_abs = 60 + 459.67 = 519.67°R
T_r = 519.67 / 354.9 = 1.46
P_r = 3614.7 / 674.4 = 5.36
From Standing-Katz chart or DAK correlation:
Z ≈ 0.85
[Image: Standing-Katz Compressibility Factor Chart]
Z-factor vs. reduced pressure for various reduced temperatures
Z-Factor Reference Table
Compressibility factors for pipeline quality natural gas (SG = 0.60) at 60°F:
| Pressure (psig) | P_abs (psia) | P_r | Z-Factor | Deviation from Ideal |
|---|---|---|---|---|
| 500 | 514.7 | 0.76 | 0.945 | -5.5% |
| 1000 | 1014.7 | 1.50 | 0.895 | -10.5% |
| 2000 | 2014.7 | 2.99 | 0.850 | -15.0% |
| 2400 | 2414.7 | 3.58 | 0.845 | -15.5% |
| 3000 | 3014.7 | 4.47 | 0.850 | -15.0% |
| 3600 | 3614.7 | 5.36 | 0.865 | -13.5% |
| 4500 | 4514.7 | 6.69 | 0.905 | -9.5% |
Deliverable Volume Calculation
Tube Trailer Deliverable Volume:
The deliverable volume is the difference between gas at fill pressure
and gas remaining at delivery (heel) pressure:
V_deliverable = V_fill - V_heel
Where:
V_fill = V_tank × (P_fill / P_std) × (T_std / T_fill) × (Z_std / Z_fill)
V_heel = V_tank × (P_heel / P_std) × (T_std / T_heel) × (Z_std / Z_heel)
Example: 9000 gallon trailer, 3600 psig fill, 100 psig heel, 60°F
V_tank = 9000 gal × 0.13368 ft³/gal = 1203 ft³
At fill (3600 psig, Z = 0.865):
V_fill = 1203 × (3614.7/14.73) × (519.67/519.67) × (1.0/0.865)
V_fill = 1203 × 245.4 × 1.0 × 1.156 = 341,300 SCF
At heel (100 psig, Z = 0.995):
V_heel = 1203 × (114.7/14.73) × (519.67/519.67) × (1.0/0.995)
V_heel = 1203 × 7.79 × 1.0 × 1.005 = 9,420 SCF
Deliverable = 341,300 - 9,420 = 331,880 SCF = 331.9 MCF
3. Tube Trailer Specifications
CNG tube trailers consist of multiple high-pressure cylinders mounted on a road-legal trailer. Design and construction follow DOT 49 CFR Part 178 requirements.
Cylinder Types
| DOT Type | Material | Service Pressure | Weight (per SCF) | Notes |
|---|---|---|---|---|
| DOT-3AA | Seamless steel | 2400-3600 psig | 0.25-0.30 lb/SCF | Most common, cost-effective |
| DOT-3AAX | High-strength steel | 3600-5000 psig | 0.22-0.26 lb/SCF | Higher pressure rating |
| DOT-3T | Seamless steel | 2400-3600 psig | 0.25-0.28 lb/SCF | Thick-wall design |
| Composite (Type 3) | Al liner + carbon wrap | 3600-5000 psig | 0.10-0.15 lb/SCF | Lightweight, higher cost |
| Composite (Type 4) | Plastic liner + carbon wrap | 3600-5000 psig | 0.08-0.12 lb/SCF | Lightest, premium cost |
Typical Tube Trailer Configurations
[Image: CNG Tube Trailer Schematic]
Side view showing cylinder arrangement, manifold, and valve systems
| Configuration | Water Capacity | Deliverable Volume* | Tare Weight | Payload Weight |
|---|---|---|---|---|
| Small (6 tubes) | 4,000-5,000 gal | 140-180 MCF | 18,000-22,000 lb | 6,500-8,500 lb |
| Standard (8-10 tubes) | 6,500-8,000 gal | 230-290 MCF | 25,000-32,000 lb | 10,500-13,500 lb |
| Large (12-16 tubes) | 9,000-12,000 gal | 320-430 MCF | 35,000-45,000 lb | 14,500-20,000 lb |
| Jumbo (composite) | 15,000-20,000 gal | 530-720 MCF | 25,000-35,000 lb | 24,000-32,000 lb |
*At 3600 psig fill, 100 psig heel, 60°F, SG = 0.60
Energy Equivalents
CNG Energy Conversions:
Natural gas heating value: 1,020 BTU/SCF (pipeline quality)
1 MCF natural gas = 1,020,000 BTU = 1.02 MMBTU
1 GGE (gasoline gallon equivalent) = 126.67 SCF = 129,200 BTU
(Based on 114,000 BTU/gal gasoline × 1.134 efficiency factor)
1 DGE (diesel gallon equivalent) = 142.7 SCF = 145,600 BTU
(Based on 128,700 BTU/gal diesel)
Example: 330 MCF tube trailer load
Energy content: 330 × 1.02 = 336.6 MMBTU
GGE equivalent: 330,000 / 126.67 = 2,605 GGE
DGE equivalent: 330,000 / 142.7 = 2,312 DGE
This equals approximately:
- 2,600 gallons of gasoline
- 2,300 gallons of diesel
- 73 fill-ups of a CNG vehicle with 35 GGE tank
Loading and Unloading
- Fill time: 2-4 hours depending on compressor capacity and cascade pressure
- Unload time: 30 minutes to 4 hours depending on pressure differential and destination equipment
- Fill connection: NGV1 or custom high-flow couplings (typical 1" or 1.5")
- Safety systems: Excess flow valves, manual shutoffs, pressure relief devices
- Grounding: Required during all transfer operations per NFPA 52
4. Cascade Storage Systems
Cascade storage uses multiple pressure banks to efficiently transfer CNG to vehicle tanks. The cascade principle minimizes the compressor work required to achieve full vehicle tank pressure.
Cascade Operating Principle
[Image: 3-Bank Cascade System Diagram]
Showing High/Medium/Low banks, priority panel, and dispenser connections
Cascade Filling Sequence:
1. LOW BANK first (1200-2000 psi range)
- Vehicle tank pressure rises from empty to ~1800 psi
- Low bank pressure drops as gas transfers
2. MEDIUM BANK second (2000-2800 psi range)
- Continues filling from low bank ending pressure
- Vehicle reaches ~2600 psi
3. HIGH BANK last (2800-3600 psi range)
- Completes fill to target pressure (3000-3600 psi)
- Highest pressure bank used least, preserving capacity
Why cascade is efficient:
Without cascade (direct from 3600 psi source):
- All gas transfers against full pressure differential
- 100% of gas comes from high-pressure storage
- Compressor must refill entire storage to 3600 psi
With 3-bank cascade:
- ~40% of gas comes from low bank (1200→1800 psi range)
- ~35% from medium bank (1800→2600 psi range)
- ~25% from high bank (2600→3600 psi range)
- Average compression ratio reduced by ~35%
Cascade Efficiency
Cascade efficiency accounts for gas that cannot be delivered due to pressure equalization limits:
Cascade Utilization Factor:
η_cascade = V_delivered / V_theoretical
Typical values:
- 3-bank cascade: 82-87% efficiency
- 4-bank cascade: 85-90% efficiency
- 5-bank cascade: 87-92% efficiency
Factors affecting efficiency:
1. Minimum bank pressure (heel pressure)
- Lower heel = more deliverable gas
- Typical minimum: 100-300 psig
2. Vehicle tank size
- Larger tanks extract more gas per fill
- Small tanks (5 GGE) = lower efficiency
3. Fast-fill temperature rise
- Gas heats during rapid transfer
- SAE J2601 compensates with reduced pressure target
- Can reduce effective fill by 5-10%
4. Equalization losses
- Small amounts of gas remain in lines
- Estimated 1-3% per fill cycle
Cascade Sizing Guidelines
| Station Type | Daily Throughput | Cascade Size | Banks | Compressor |
|---|---|---|---|---|
| Small fleet | 50-100 GGE | 500-1,000 gal | 2-3 | 25-50 SCFM |
| Medium fleet | 100-300 GGE | 1,000-2,500 gal | 3 | 50-100 SCFM |
| Large fleet | 300-1,000 GGE | 2,500-6,000 gal | 3-4 | 100-200 SCFM |
| Public fast-fill | 500-2,000 GGE | 4,000-10,000 gal | 3-4 | 150-400 SCFM |
| High-volume | 2,000+ GGE | 10,000+ gal | 4-5 | 300+ SCFM |
Vehicle Fill Estimation
Fills per Cascade Cycle:
N_fills = (V_cascade × η_cascade) / V_vehicle
Where:
V_cascade = Total cascade storage (SCF at STP)
η_cascade = Cascade efficiency (typically 0.85)
V_vehicle = Average vehicle tank size (SCF)
Example: 3000 gallon cascade, 35 GGE vehicle tanks
V_cascade = 3000 gal × 0.13368 ft³/gal × (3614.7/14.73) × (1/0.865)
V_cascade = 114,000 SCF at 3600 psig
Deliverable (3600→100 psig): ~110,000 SCF
Vehicle tank: 35 GGE × 126.67 SCF/GGE = 4,433 SCF
N_fills = (110,000 × 0.85) / 4,433 = 21 vehicle fills
At 10 fills/hour average, cascade provides ~2 hours of operation
before compressor must refill storage.
5. Virtual Pipeline Logistics
Virtual pipelines use tube trailers to deliver CNG where physical pipelines are not available. Logistics planning considers fleet size, delivery frequency, and operating costs.
Virtual Pipeline Applications
Industrial Users
Remote manufacturing
Factories, food processing, heat treating operations off the pipeline grid.
Drilling Operations
Rig fuel & frac
Dual-fuel drilling rigs, hydraulic fracturing operations.
Remote Communities
Distributed supply
Island, arctic, or mountainous communities without pipeline access.
Pipeline Bypass
Maintenance support
Temporary gas supply during pipeline repairs or capacity constraints.
Fleet Sizing Calculation
Required Number of Trailers:
N_trailers = ceiling(D_daily / V_deliverable) × (T_cycle / T_operating)
Where:
D_daily = Daily gas demand (MCF/day)
V_deliverable = Deliverable volume per trailer (MCF)
T_cycle = Round trip cycle time (hours)
T_operating = Operating hours per day
Example: 500 MCF/day demand, 4-hour cycle, 12-hour operation
Trailer capacity: 330 MCF deliverable
Trips needed: 500 / 330 = 1.52 trips/day
Trips per trailer: 12 / 4 = 3 trips/day
N_trailers = ceiling(1.52 / 3) = 1 trailer minimum
With 20% contingency: 2 trailers recommended
Fleet utilization:
Actual trips: 1.52 trips/day
Capacity: 2 trailers × 3 trips = 6 trips/day
Utilization: 1.52 / 6 = 25% (excess capacity for growth/contingency)
Operating Cost Estimation
| Cost Component | Typical Range | Units | Notes |
|---|---|---|---|
| Fuel (diesel) | $0.50 - 0.80 | $/MCF delivered | Varies with distance, fuel price |
| Driver labor | $0.30 - 0.60 | $/MCF delivered | Union vs. non-union, region |
| Trailer lease | $0.20 - 0.40 | $/MCF delivered | ~$3,000-5,000/month lease |
| Tractor cost | $0.15 - 0.30 | $/MCF delivered | Lease + maintenance |
| Compression | $0.10 - 0.25 | $/MCF delivered | At loading station |
| Insurance/permits | $0.05 - 0.15 | $/MCF delivered | Hazmat, liability coverage |
| Total delivered cost | $1.30 - 2.50 | $/MCF | Excluding gas commodity |
Economics vs. Pipeline
Virtual Pipeline Break-Even Analysis:
Virtual pipeline is typically competitive when:
- Distance to pipeline: > 5-10 miles
- Demand: 100-2,000 MCF/day
- Duration: < 3-5 years
Cost comparison (approximate):
Pipeline construction: $50,000 - $200,000 per mile
Pipeline operating: $0.10 - 0.30 $/MCF
Virtual pipeline: $1.30 - 2.50 $/MCF delivered
Break-even calculation:
For 500 MCF/day demand, 10 miles from pipeline:
Pipeline option:
- Construction: 10 mi × $100,000/mi = $1,000,000
- Operating: 500 MCF × $0.20 × 365 = $36,500/year
- 10-year total: $1,365,000
Virtual pipeline:
- Delivered cost: 500 MCF × $1.80 × 365 = $328,500/year
- 10-year total: $3,285,000
Break-even: ~4 years
If demand expected < 4 years: virtual pipeline wins
If demand expected > 4 years: pipeline wins
Virtual pipeline advantages: Quick deployment (weeks vs. years for pipeline), scalable capacity, no right-of-way issues, relocatable assets. Best for temporary demands, remote locations, or as a bridge until pipeline infrastructure is built.
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