1. Overview
Compressor drivers consume significant quantities of fuel gas, representing the largest operating cost for pipeline and gas processing facilities. Accurate fuel consumption estimates are essential for gas balance, operating cost projections, emissions permitting, and driver selection.
Fuel Gas Volume
1-5% of Throughput
Typical fuel usage as percent of pipeline flow
Heat Rate
BTU/BHP-hr
Energy input per unit of shaft power output
Thermal Efficiency
2,545 / Heat Rate
Fraction of fuel energy converted to work
Fuel HHV
950-1,150 BTU/SCF
Heating value varies with gas composition
Gas balance impact: A 10,000 HP compressor station burning fuel at 8,500 BTU/BHP-hr consumes approximately 2.0 MMSCFD of gas. On a 100 MMSCFD pipeline, this is 2% of throughput -- a significant revenue impact that must be accounted for in pipeline design and tariff calculations.
2. Driver Types
Three primary driver types are used for gas compression in midstream operations. Selection depends on power range, fuel availability, emissions requirements, and site-specific factors.
Driver Comparison
| Parameter | Gas Engine | Gas Turbine | Electric Motor |
|---|---|---|---|
| Power range | 100-15,000 HP | 1,000-100,000+ HP | 100-50,000 HP |
| Heat rate | 7,000-9,500 BTU/BHP-hr | 8,500-11,000 BTU/BHP-hr | N/A (electricity) |
| Thermal efficiency | 27-36% | 23-30% | 92-97% (motor only) |
| Speed range | 700-1,800 RPM | 3,600-16,000 RPM | 1,200-3,600 RPM |
| Startup time | 2-5 minutes | 10-30 minutes | Seconds |
| Maintenance interval | 2,000-8,000 hrs | 20,000-40,000 hrs | 40,000-80,000 hrs |
| Typical application | Gathering, boost | Pipeline, LNG | Processing, urban |
Gas Engine Types
| Engine Type | Heat Rate | NOx Emissions | Notes |
|---|---|---|---|
| 2-stroke lean burn (2SLB) | 8,000-9,500 | 2-6 g/BHP-hr | Legacy units; high emissions; RICE MACT affected |
| 4-stroke lean burn (4SLB) | 7,000-8,500 | 1-2 g/BHP-hr | Most common new installation; low emissions |
| 4-stroke rich burn (4SRB) | 7,500-9,000 | 8-15 g/BHP-hr (uncontrolled) | Requires 3-way catalyst (NSCR) for RICE MACT |
| 4SRB + NSCR | 7,500-9,000 | < 1 g/BHP-hr | Lowest NOx with catalyst; common for gathering |
Gas Turbine Types
| Configuration | Heat Rate | Efficiency | Application |
|---|---|---|---|
| Simple cycle | 8,500-11,000 | 23-30% | Pipeline compression |
| Recuperated | 7,000-8,500 | 30-36% | Base load, high fuel cost |
| Combined cycle | 5,500-7,000 | 36-46% | Large power plants only |
| Aeroderivative | 8,000-9,500 | 27-32% | Mid-range pipeline |
| Industrial frame | 9,000-11,000 | 23-28% | Large pipeline, LNG |
Driver selection rule of thumb: Gas engines dominate below 5,000 HP (gathering and boost). Gas turbines are preferred above 10,000 HP (pipeline mainline). The 5,000-10,000 HP range is competitive between the two. Electric motors are chosen when grid power is available and emissions are restricted.
3. Heat Rate & Fuel Consumption
Heat rate is the amount of fuel energy (in BTU) required to produce one brake horsepower-hour of shaft output. Lower heat rate means higher efficiency and less fuel consumption.
Fuel Gas Consumption:
Q_fuel = (BHP x HR) / HHV
Where:
Q_fuel = Fuel gas consumption (SCF/hr)
BHP = Brake horsepower (HP)
HR = Heat rate (BTU/BHP-hr)
HHV = Higher heating value of fuel gas (BTU/SCF)
Convert to daily volume:
Q_fuel_daily = Q_fuel x 24 / 1,000,000 (MMSCFD)
Thermal Efficiency:
eta_thermal = 2,545 / HR
Where 2,545 BTU/HP-hr is the mechanical equivalent of one horsepower.
Example:
HR = 8,000 BTU/BHP-hr
eta = 2,545 / 8,000 = 31.8%
(68.2% of fuel energy becomes waste heat)
Fuel Gas Heating Values
| Gas Type | HHV (BTU/SCF) | LHV (BTU/SCF) | Specific Gravity |
|---|---|---|---|
| Methane | 1,012 | 911 | 0.554 |
| Pipeline gas (lean) | 1,000-1,050 | 900-950 | 0.58-0.62 |
| Pipeline gas (rich) | 1,050-1,200 | 950-1,080 | 0.62-0.75 |
| Ethane | 1,770 | 1,619 | 1.038 |
| Propane | 2,516 | 2,314 | 1.522 |
| Field gas (with CO2/N2) | 800-950 | 720-860 | 0.65-0.80 |
Part-Load Heat Rate
Part-Load Performance:
Gas engines and turbines become less efficient at part load.
Typical heat rate multipliers:
Gas Engine:
100% load: 1.00 x rated HR
75% load: 1.05-1.10 x rated HR
50% load: 1.15-1.25 x rated HR
25% load: 1.40-1.60 x rated HR
Gas Turbine:
100% load: 1.00 x rated HR
75% load: 1.10-1.15 x rated HR
50% load: 1.25-1.35 x rated HR
Below 50%: typically not operated (poor efficiency)
Part-load fuel consumption:
Q_fuel_part = BHP_actual x HR_rated x HR_multiplier / HHV
For gas turbines, part-load is usually controlled by:
Variable speed (preferred): reduces fuel flow and air flow
Variable IGV: adjusts inlet air to maintain efficiency
Bleed air: least efficient method
HHV vs. LHV: Engine manufacturers typically quote heat rate on LHV basis (excludes latent heat of water vapor in exhaust). Gas utilities measure gas on HHV basis. To convert: HR_HHV = HR_LHV x (HHV/LHV). For natural gas, HHV/LHV is approximately 1.11. Always confirm which basis is used.
4. Derating Factors
Site conditions reduce the rated power output of gas engines and turbines below their ISO or NEMA nameplate ratings. The derated power determines the maximum shaft horsepower available at site conditions.
Gas Engine Derating
Gas Engine Site Power:
BHP_site = BHP_rated x f_altitude x f_temperature x f_fuel x f_accessories
Altitude Derating (f_altitude):
f_altitude = 1.0 - 0.03 x (Elevation - 1,000) / 1,000
(for elevation > 1,000 ft)
Example: 5,000 ft -> f = 1.0 - 0.03 x 4 = 0.88
Temperature Derating (f_temperature):
f_temperature = 1.0 - 0.02 x (T_ambient - 77) / 10
(for T_ambient > 77 deg F; base = 77 deg F / 25 deg C)
Example: 100 deg F -> f = 1.0 - 0.02 x 2.3 = 0.954
Fuel Quality Derating (f_fuel):
f_fuel = HHV_actual / HHV_reference
Reference is typically 950-1,000 BTU/SCF
Low-BTU gas (< 900 BTU/SCF) may require engine modifications
Accessory Loads (f_accessories):
Cooling fans, lube oil pump, fuel gas booster: 3-5%
f_accessories = 0.95-0.97
Combined Example (5,000 ft, 100 deg F, 980 BTU gas):
f_total = 0.88 x 0.954 x (980/1000) x 0.96 = 0.79
BHP_site = 1,000 x 0.79 = 790 HP available
Gas Turbine Derating
Gas Turbine Site Power:
BHP_site = BHP_ISO x (rho_site / rho_ISO) x f_inlet_loss x f_exhaust_loss
Ambient Temperature Effect (most significant):
Turbines are highly sensitive to inlet air temperature.
Power decreases approximately 0.5-0.7% per deg F above ISO (59 deg F).
At 100 deg F: Power reduction = 0.6% x 41 = 24.6% (typical)
Altitude/Pressure Effect:
Power proportional to air density.
At 5,000 ft: P_atm = 12.23 psia vs 14.696 psia (sea level)
Density ratio = 12.23/14.696 = 0.832
Power reduction: ~17%
Inlet/Exhaust Pressure Losses:
Inlet filter: 2-4 inches H2O
Exhaust duct: 4-8 inches H2O
Each 1" H2O inlet loss: ~0.5% power loss
Each 1" H2O exhaust loss: ~0.3% power loss
Humidity Effect:
Minor for gas turbines: ~1% power loss per 0.01 specific humidity increase
More significant in tropical/coastal environments
| Derating Factor | Gas Engine Impact | Gas Turbine Impact |
|---|---|---|
| Altitude (5,000 ft) | -12% | -17% |
| Ambient temp (100 deg F) | -5% | -25% |
| Low-BTU fuel (900 BTU) | -5 to -10% | -2 to -5% |
| Inlet loss (4" H2O) | Negligible | -2% |
| Combined worst case | -20 to -25% | -35 to -45% |
Summer design case: Always size drivers for the hottest expected ambient temperature (summer design day). In Texas and the Gulf Coast, this is typically 105-110 deg F. A turbine rated at 10,000 HP ISO may only deliver 6,500-7,000 HP at summer conditions. Failing to account for derating is the most common driver sizing error.
5. RICE MACT Compliance
The Reciprocating Internal Combustion Engine (RICE) Maximum Achievable Control Technology (MACT) rule (40 CFR Part 63, Subparts ZZZZ and JJJJJJ) regulates hazardous air pollutant (HAP) emissions from stationary engines. Compliance requirements depend on engine type, size, and installation date.
RICE MACT Applicability
| Engine Category | Subpart | Key Requirements |
|---|---|---|
| Existing 2SLB > 500 HP | ZZZZ | Emissions limits or management practices; catalyst may be required |
| Existing 4SLB > 500 HP | ZZZZ | Maintenance and operational requirements |
| Existing 4SRB > 500 HP | ZZZZ | NSCR (3-way catalyst) required; formaldehyde limits |
| New/Reconstructed > 500 HP | ZZZZ | Strictest limits; emission standards at stack |
| Existing < 500 HP (area source) | JJJJJJ | Maintenance practices; change oil, inspect plugs |
Emission Limits (Major Source, Existing)
| Pollutant | 2SLB | 4SLB | 4SRB + NSCR | Units |
|---|---|---|---|---|
| Formaldehyde (HCHO) | 2.0 | 2.7 | 2.7 | ppmvd @ 15% O2 |
| CO (alternative) | 23 | 47 | 76 | ppmvd @ 15% O2 |
| NOx (for reference) | Not HAP but often co-regulated | State/NSR permits | ||
Compliance Strategies
NSCR (Non-Selective Catalytic Reduction) for 4SRB:
Three-way catalyst converts CO, VOC, and NOx simultaneously
Requires rich burn (lambda = 0.98-1.02)
Formaldehyde reduction: 90-95%
CO reduction: 90-95%
NOx reduction: 90-98%
Catalyst replacement: every 3-5 years ($15K-$40K)
Oxidation Catalyst for 4SLB/2SLB:
Reduces CO and formaldehyde only
Does NOT control NOx
Formaldehyde reduction: 85-95%
CO reduction: 70-90%
Lower backpressure penalty than NSCR
Fuel Gas Impact on Catalyst Performance:
H2S > 50 ppm: catalyst poisoning risk; treat fuel gas
Siloxanes: permanent catalyst deactivation
Lead compounds: rapid catalyst failure
Ensure fuel gas treatment upstream of catalyst-equipped engines
Alternative Compliance:
Replace 2SLB with new 4SLB engine (lower emissions; meets limits without catalyst)
Electrify: replace engine with electric motor (eliminates all combustion emissions)
Install gas turbine: different emissions profile; may be exempt from RICE MACT
Testing requirements: RICE MACT requires initial performance testing within 180 days of startup (new engines) and periodic testing every 3 years or 8,760 operating hours, whichever comes first. Many operators use portable analyzers for interim monitoring between formal stack tests.
6. Economic Comparison
Driver selection involves balancing capital cost, fuel cost, maintenance cost, and emissions compliance cost over the project life. Fuel cost is typically the dominant operating expense.
Fuel Cost Comparison
Annual Fuel Cost:
Annual_Fuel_Cost = Q_fuel_daily x 365 x Availability x Fuel_Price
Where:
Q_fuel_daily = Daily fuel consumption (MMSCFD)
Availability = Operating factor (typically 0.90-0.97)
Fuel_Price = Gas price ($/MMBTU or $/MCF)
Example: 5,000 HP station, gas at $3.00/MMBTU:
Gas Engine (HR = 8,000 BTU/BHP-hr):
Q_fuel = (5,000 x 8,000) / 1,020,000 = 39.2 MCF/hr = 0.94 MMSCFD
Annual = 0.94 x 365 x 0.95 x $3.00 x 1,000 = $978,000/yr
Gas Turbine (HR = 10,000 BTU/BHP-hr):
Q_fuel = (5,000 x 10,000) / 1,020,000 = 49.0 MCF/hr = 1.18 MMSCFD
Annual = 1.18 x 365 x 0.95 x $3.00 x 1,000 = $1,228,000/yr
Difference: $250,000/yr favoring gas engine at this size.
Over 20 years: $5,000,000 cumulative fuel savings.
Total Cost of Ownership
| Cost Category | Gas Engine (5,000 HP) | Gas Turbine (5,000 HP) | Electric Motor (5,000 HP) |
|---|---|---|---|
| Capital (installed) | $2.5-4.0 million | $4.0-7.0 million | $3.0-5.0 million + power line |
| Annual fuel/power | $800K-$1.2M | $1.0-$1.5M | $400K-$800K |
| Annual maintenance | $150K-$300K | $100K-$200K | $30K-$80K |
| Emissions compliance | $30K-$80K/yr | $10K-$30K/yr | None (at site) |
| Major overhaul | $200K-$500K every 3-5 yr | $500K-$2M every 5-8 yr | Minimal |
| Availability | 92-96% | 95-98% | 98-99% |
Electric motor economics: Electric motors have the lowest operating cost but require grid power infrastructure. The breakeven distance for power line construction is typically 5-15 miles at $50K-$200K per mile. Electrification is increasingly favored for ESG reporting and in areas with strict air quality requirements.
7. Worked Examples
Example 1: Gas Engine Fuel Consumption
Given:
BHP = 3,000 HP (at site conditions)
Engine type: 4SLB
Heat rate (LHV): 7,200 BTU/BHP-hr
Fuel gas HHV = 1,020 BTU/SCF
Fuel gas LHV = 920 BTU/SCF
Operating hours: 8,500 hr/yr
Step 1: Convert heat rate to HHV basis
HR_HHV = HR_LHV x (HHV/LHV) = 7,200 x (1,020/920) = 7,983 BTU/BHP-hr
Step 2: Hourly fuel consumption
Q_fuel = BHP x HR_HHV / HHV
Q_fuel = 3,000 x 7,983 / 1,020 = 23,479 SCF/hr = 23.5 MCF/hr
Step 3: Daily consumption
Q_daily = 23.5 x 24 = 0.564 MMSCFD
Step 4: Annual consumption
Q_annual = 23.5 x 8,500 / 1,000 = 199.7 MMSCF/yr
Step 5: Annual fuel cost (at $3.50/MMBTU)
Cost = 199,700 MCF x 1.020 MMBTU/MCF x $3.50/MMBTU = $713,000/yr
Example 2: Gas Turbine with Derating
Given:
Turbine ISO rating: 12,000 HP
Site elevation: 4,500 ft
Summer ambient: 105 deg F
Inlet filter loss: 3" H2O
Exhaust duct loss: 6" H2O
HR_ISO (LHV): 8,800 BTU/BHP-hr
Step 1: Temperature derating
dT = 105 - 59 = 46 deg F
f_temp = 1.0 - 0.006 x 46 = 0.724 (27.6% loss)
Step 2: Altitude derating
P_atm at 4,500 ft = 12.47 psia
f_alt = 12.47 / 14.696 = 0.849 (15.1% loss)
Step 3: Inlet/exhaust losses
f_inlet = 1.0 - 0.005 x 3 = 0.985
f_exhaust = 1.0 - 0.003 x 6 = 0.982
Step 4: Site power
BHP_site = 12,000 x 0.724 x 0.849 x 0.985 x 0.982
BHP_site = 12,000 x 0.582 = 6,984 HP
Step 5: Site heat rate (HR increases at derated conditions)
HR_site = HR_ISO / f_temp (approximately)
HR_site = 8,800 / 0.724 = 12,155 BTU/BHP-hr (LHV)
Result: The 12,000 HP ISO turbine delivers only 6,984 HP
at summer conditions -- a 42% reduction from nameplate.
This is why turbine installations often include inlet chilling.
Example 3: Electric Motor vs. Gas Engine
Given:
Required shaft power: 2,000 HP
Operating hours: 8,400 hr/yr
Gas price: $3.00/MMBTU
Electricity: $0.08/kWh
Gas engine HR: 8,000 BTU/BHP-hr (HHV)
Motor efficiency: 95%
Fuel HHV: 1,020 BTU/SCF
Power line cost: $1.5M (one-time)
Gas Engine Annual Fuel Cost:
Q_fuel = 2,000 x 8,000 / 1,020 = 15,686 SCF/hr
Annual fuel = 15,686 x 8,400 / 1,000,000 = 131.8 MMSCF
Cost = 131.8 x 1.020 x $3.00 x 1,000 = $403,000/yr
Electric Motor Annual Power Cost:
kW = 2,000 x 0.746 / 0.95 = 1,570 kW
Annual kWh = 1,570 x 8,400 = 13,188,000 kWh
Cost = 13,188,000 x $0.08 = $1,055,000/yr
Wait -- electric is MORE expensive in this case.
At $0.08/kWh with $3/MMBTU gas, the gas engine wins on fuel alone.
Breakeven electricity price: $403,000 / 13,188,000 = $0.031/kWh
Electric motors become competitive when:
Electricity < $0.035/kWh, OR
Gas price > $6/MMBTU, OR
Emissions compliance adds > $200K/yr to gas engine costs
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