1. Gas Turbine Overview
Gas turbines are the dominant prime mover for pipeline compression and are widely used for power generation at gas plants, LNG facilities, and production platforms. They convert the chemical energy of fuel (typically natural gas) into shaft power through a continuous combustion process, offering high power density, fast startup, and the ability to burn pipeline-quality gas directly from the process.
Power density
High HP per footprint
Gas turbines produce more horsepower per unit weight and floor space than reciprocating engines, making them ideal for compact compressor stations.
Fuel flexibility
Burns pipeline gas directly
No external fuel supply needed at compressor stations. Fuel is taken directly from the pipeline, simplifying operations and logistics.
Availability
95–98% typical
Modern gas turbines achieve high availability with condition-based maintenance and modular component replacement.
Midstream Applications
Gas turbines serve as the primary driver for centrifugal and reciprocating compressors throughout the natural gas value chain, from wellhead gathering to mainline transmission and gas processing plants.
| Application | Typical Size Range (HP) | Turbine Type |
|---|---|---|
| Gathering system compression | 1,000–5,000 | Small industrial |
| Gas plant inlet/residue compression | 5,000–20,000 | Industrial or small aeroderivative |
| Mainline transmission (single unit) | 10,000–35,000 | Industrial or aeroderivative |
| Large transmission station (multi-unit) | 20,000–50,000 each | Aeroderivative |
| LNG refrigeration compression | 30,000–130,000 | Large aeroderivative or frame |
| Gas plant power generation | 5,000–50,000 | Varies by plant size |
| Offshore platform compression | 5,000–40,000 | Aeroderivative (lightweight) |
Gas Turbine Components
A gas turbine consists of three main rotating sections mounted on a common shaft (or coupled shafts), plus a combustion system:
- Air inlet system: Filters, silencers, anti-icing system, and ducting that deliver clean ambient air to the compressor section. Inlet losses directly reduce turbine output.
- Compressor section: Axial-flow (most common) or centrifugal compressor stages that compress inlet air to 10:1 to 30:1 pressure ratio depending on turbine design.
- Combustion system: One or more combustion chambers (can, annular, or cannular) where fuel is mixed with compressed air and ignited. Combustion temperatures reach 2,000–2,600°F.
- Turbine section: Hot gas expands through turbine stages, producing shaft power. The gas generator turbine drives the compressor; the power turbine (free turbine) drives the load.
- Exhaust system: Ducting, silencers, and stack that direct exhaust gases away from the turbine. May include waste heat recovery equipment.
- Accessory systems: Lube oil, fuel gas, starting system (electric or pneumatic), controls, fire and gas detection, and enclosure ventilation.
2. The Brayton Cycle
Gas turbines operate on the Brayton (or Joule) cycle, a thermodynamic cycle consisting of four processes. Understanding the Brayton cycle is essential for interpreting gas turbine performance data and derating behavior.
Ideal Brayton Cycle
The ideal (theoretical) Brayton cycle consists of four reversible processes:
| Process | Description | Thermodynamic Path |
|---|---|---|
| 1 → 2 | Compression of ambient air | Isentropic compression (constant entropy) |
| 2 → 3 | Heat addition (combustion) | Constant pressure heat addition |
| 3 → 4 | Expansion through turbine | Isentropic expansion (constant entropy) |
| 4 → 1 | Heat rejection (exhaust) | Constant pressure heat rejection |
Brayton Cycle Efficiency
Real gas turbine efficiency is substantially lower than the ideal cycle due to compressor and turbine stage losses (isentropic efficiencies of 85–92%), combustion inefficiency, bearing and seal friction, and auxiliary power consumption.
Real Cycle Losses
| Loss Source | Typical Impact |
|---|---|
| Compressor isentropic efficiency (85–92%) | Largest single loss; increases compression work |
| Turbine isentropic efficiency (87–93%) | Reduces expansion work recovery |
| Combustion efficiency (98–99.5%) | Small loss; incomplete combustion |
| Mechanical losses (bearings, seals) | 1–2% of total power |
| Inlet/exhaust pressure losses | 0.5–3% depending on duct design |
| Cooling air bleed | 2–5% of compressor air used for blade cooling |
| Auxiliary power (lube oil, fuel, controls) | 0.5–1% of rated output |
Effect of Ambient Temperature on the Cycle
Ambient temperature has a profound effect on gas turbine performance because it directly affects the compressor inlet air density and the compression work required:
- Higher ambient temperature means lower air density, so less mass flow enters the compressor for a given volumetric flow. This reduces both power output and efficiency.
- Higher ambient temperature also increases the specific work of compression (more energy needed per unit mass to achieve the same pressure ratio), further reducing net power output.
- Lower ambient temperature has the opposite effect: denser air increases mass flow, and less compression work is needed. Power output increases on cold days.
This is why gas turbine derating for ambient temperature is such a critical design consideration for pipeline applications, where the turbine must deliver rated power on the hottest day of the year.
3. Turbine Types
Gas turbines used in midstream service fall into two broad categories: aeroderivative and industrial frame. Each has distinct characteristics that make it suited for different applications.
Aeroderivative Gas Turbines
Aeroderivative turbines are derived from aircraft jet engine technology. They feature high pressure ratios (20:1 to 35:1), lightweight construction, and high power-to-weight ratios. They can be started and loaded quickly and are well-suited for variable-load applications.
Industrial Frame Gas Turbines
Industrial frame turbines (also called heavy-duty turbines) are designed specifically for ground-based power generation and mechanical drive applications. They typically have lower pressure ratios (10:1 to 18:1) but can be built in very large sizes exceeding 100,000 HP.
Comparison: Aeroderivative vs. Industrial Frame
| Characteristic | Aeroderivative | Industrial Frame |
|---|---|---|
| Pressure ratio | 20:1 – 35:1 | 10:1 – 18:1 |
| Thermal efficiency (simple cycle) | 32–42% | 25–35% |
| Heat rate (Btu/HP-hr) | 6,000–8,000 | 7,500–10,000 |
| Weight (lb/HP) | 2–5 | 10–25 |
| Startup time | 5–10 minutes | 15–30 minutes |
| Maintenance approach | Module swap (gas generator exchange) | In-situ overhaul |
| Exhaust temperature | 750–950°F | 900–1,100°F |
| Typical size range | 2,000–75,000 HP | 5,000–200,000+ HP |
| Sensitivity to ambient temp | Higher (higher PR) | Lower |
| Best suited for | Pipeline, offshore, peaking | Base load power, large LNG |
Common Turbines in Midstream Service
| Turbine Model | Type | ISO Rating (HP) | Heat Rate (Btu/HP-hr) |
|---|---|---|---|
| Solar Saturn 20 | Industrial | 1,600 | ~10,500 |
| Solar Centaur 50 | Industrial | 4,700 | ~9,200 |
| Solar Taurus 60 | Industrial | 7,700 | ~8,600 |
| Solar Taurus 70 | Industrial | 10,500 | ~8,400 |
| Solar Mars 100 | Industrial | 15,700 | ~8,300 |
| Solar Titan 130 | Industrial | 19,600 | ~8,100 |
| Solar Titan 250 | Industrial | 30,000 | ~7,800 |
| GE LM2500 | Aeroderivative | 33,000 | ~7,600 |
| GE LM6000 | Aeroderivative | 60,000 | ~6,500 |
| RR Avon | Aeroderivative | 15,000 | ~8,200 |
| RR RB211 | Aeroderivative | 38,000 | ~7,500 |
| Siemens SGT-400 | Industrial | 18,700 | ~8,000 |
Values are approximate ISO-rated conditions. Actual site performance varies with derating. Verify with manufacturer for specific project.
4. Derating & Site Conditions
Gas turbines are rated at ISO standard conditions (ISO 2314): 59°F (15°C) ambient temperature, sea level (14.696 psia), 60% relative humidity, and zero inlet/exhaust pressure losses. Real site conditions always differ from ISO, and the turbine must be derated to determine actual available power.
ISO Base Rating Conditions
Altitude Derating
Higher altitude means lower atmospheric pressure and lower air density. Since gas turbine power output is approximately proportional to air mass flow through the compressor, power decreases with altitude.
| Altitude (ft) | Pressure Ratio | Approx. Power Loss |
|---|---|---|
| 0 (sea level) | 1.000 | 0% |
| 1,000 | 0.964 | 3.6% |
| 2,000 | 0.930 | 7.0% |
| 3,000 | 0.896 | 10.4% |
| 4,000 | 0.864 | 13.6% |
| 5,000 | 0.832 | 16.8% |
| 7,500 | 0.757 | 24.3% |
| 10,000 | 0.688 | 31.2% |
Temperature Derating
Ambient temperature is typically the most significant derating factor. Power output decreases approximately 0.5–0.9% per degree Fahrenheit above ISO base (59°F), with the rate depending on turbine design and pressure ratio.
Humidity Derating
Humidity has a relatively small effect on gas turbine output (typically less than 2%) but should not be neglected for precise sizing. Humid air is less dense than dry air at the same temperature and pressure because water vapor (molecular weight 18) displaces nitrogen (MW 28) and oxygen (MW 32).
Inlet and Exhaust Pressure Loss Derating
| Loss Type | Typical Range | Derating Effect |
|---|---|---|
| Inlet filter (clean) | 2–3 in.WC | ~0.5% per in.WC |
| Inlet filter (dirty) | 4–6 in.WC | ~0.5% per in.WC |
| Inlet silencer | 1–2 in.WC | ~0.5% per in.WC |
| Inlet duct | 0.5–1 in.WC | ~0.5% per in.WC |
| Exhaust duct/stack | 3–6 in.WC | ~0.15% per in.WC |
| HRSG (waste heat boiler) | 6–12 in.WC | ~0.15% per in.WC |
| Exhaust silencer | 1–2 in.WC | ~0.15% per in.WC |
Inlet Air Cooling
When derating due to high ambient temperature is unacceptable, inlet air cooling can recover lost capacity. Three main technologies are used:
- Evaporative coolers: Water-wetted media pads cool inlet air by evaporation. Limited by wet-bulb temperature. Low capital cost, effective in dry climates. Typically recovers 50–80% of temperature-related power loss.
- Inlet fogging: High-pressure water fog injected into inlet air duct. Can cool air to wet-bulb temperature. Some systems inject excess water (overspray) for intercooling effect inside the compressor.
- Mechanical chillers: Refrigeration systems cool inlet air below wet-bulb temperature. Higher capital cost but effective in humid climates where evaporative cooling is limited. Can recover nearly 100% of temperature derating.
5. Performance & Heat Rate
Gas turbine performance is characterized by power output, heat rate (fuel efficiency), exhaust temperature, and exhaust mass flow. These parameters are interdependent and vary with operating conditions.
Heat Rate
Heat Rate by Size Class
| Size Class | ISO HP Range | Typical HR (Btu/HP-hr) | Efficiency (%) |
|---|---|---|---|
| Micro/Small | <3,000 | 9,500–11,000 | 23–27% |
| Small-Medium | 3,000–7,000 | 8,500–9,500 | 27–30% |
| Medium | 7,000–15,000 | 8,000–8,700 | 29–32% |
| Medium-Large | 15,000–25,000 | 7,500–8,200 | 31–34% |
| Large Aeroderivative | 25,000–75,000 | 6,500–7,800 | 33–39% |
| Heavy Industrial Frame | >50,000 | 7,200–8,500 | 30–35% |
Part-Load Performance
Gas turbine heat rate degrades at part load. This is important for pipeline applications where compressor load varies with gas throughput and pressure conditions.
| Load (%) | Heat Rate Multiplier | Exhaust Temp Change |
|---|---|---|
| 100% | 1.00 | Base |
| 90% | 1.03–1.05 | +10 to +30°F |
| 80% | 1.07–1.12 | +20 to +50°F |
| 70% | 1.12–1.20 | +30 to +70°F |
| 60% | 1.20–1.35 | +40 to +80°F |
| 50% | 1.35–1.55 | Variable |
Part-load behavior varies significantly by turbine model. Free power turbine designs maintain better efficiency at part load than single-shaft designs.
6. Exhaust Heat Recovery
Gas turbine exhaust contains significant recoverable energy. At simple-cycle efficiencies of 25–38%, more than 60% of the fuel energy exits as hot exhaust gas at 750–1,100°F. Recovering this waste heat can dramatically improve overall plant thermal efficiency.
Waste Heat Calculation
Common Waste Heat Applications in Midstream
| Application | Typical Heat Recovery (%) | Notes |
|---|---|---|
| Glycol reboiler (dehy unit) | 5–15% | Very common; replaces fired reboiler |
| Amine reboiler | 10–30% | Matches well with medium turbines |
| Gas heating (fuel gas or process) | 2–10% | Prevents hydrate formation |
| Building/facility heating | 1–5% | Cold climate installations |
| Steam generation (HRSG) | 40–60% | Combined cycle or process steam |
| Organic Rankine Cycle (ORC) | 10–20% | Generates additional electricity from low-grade heat |
| Inlet air heating (anti-icing) | 1–5% | Prevents ice formation on inlet filters |
Overall Plant Efficiency with Heat Recovery
| Configuration | Overall Efficiency |
|---|---|
| Simple cycle (no recovery) | 25–38% |
| Simple cycle + glycol reboiler | 35–50% |
| Simple cycle + HRSG (process steam) | 55–70% |
| Combined cycle (power generation) | 50–60% |
| Cogeneration (power + heat) | 70–85% |
7. Emissions
Gas turbine emissions are an increasingly important factor in equipment selection and station design, driven by federal and state air quality regulations. Natural gas-fired turbines produce relatively clean exhaust compared to other combustion sources, but NOx and CO2 are significant concerns.
Primary Emission Species
| Pollutant | Formation Mechanism | Uncontrolled Level | With Controls |
|---|---|---|---|
| NOx (NO + NO2) | Thermal NOx at high flame temperature | 100–250 ppmvd | 5–25 ppmvd (DLN/SoLoNOx) |
| CO | Incomplete combustion | 10–50 ppmvd | 5–25 ppmvd |
| CO2 | Complete combustion of carbon in fuel | 117 lb/MMBtu (NG) | Not reducible by combustion controls |
| VOC / UHC | Incomplete combustion | <10 ppmvd | <5 ppmvd |
| PM2.5 | Combustion-generated fine particulates | Very low for gas fuel | Negligible |
| SO2 | Sulfur in fuel | Negligible for sweet NG | N/A for pipeline gas |
CO2 Emission Factors
NOx Control Technologies
- Dry Low NOx (DLN) / SoLoNOx: Lean premixed combustion technology built into the turbine combustor. Achieves 15–25 ppmvd NOx without post-combustion treatment. Standard on most modern turbines.
- Water/steam injection: Injects water or steam into the combustion zone to reduce flame temperature and thermal NOx. Increases power output slightly but increases heat rate and water consumption.
- Selective Catalytic Reduction (SCR): Post-combustion catalytic system that uses ammonia or urea to reduce NOx to N2. Achieves single-digit NOx levels (2–5 ppmvd). Required in many nonattainment areas.
- Oxidation catalyst (CO catalyst): Catalytic bed that oxidizes CO and VOCs to CO2 and water. Often combined with SCR in a combined emission control system.
8. Maintenance & Reliability
Gas turbine maintenance follows a structured inspection and overhaul program based on operating hours and starts. Proper maintenance is critical for reliability, performance retention, and safety.
Maintenance Intervals
| Inspection Type | Interval | Scope | Duration |
|---|---|---|---|
| Routine / daily checks | Daily | Oil levels, pressures, temps, vibration, visual | 15–30 min |
| Borescope inspection | 8,000–12,000 hrs | Internal inspection of compressor and turbine blades through access ports | 1–2 days |
| Hot section inspection (HSI) | 20,000–30,000 hrs | Combustion liners, transition pieces, first-stage nozzles and blades | 5–10 days |
| Major overhaul | 40,000–60,000 hrs | Complete disassembly and rebuild. All rotating and stationary hot parts. | 15–30 days |
| Gas generator exchange (aero) | 25,000–50,000 hrs | Swap entire gas generator module; send to depot for overhaul | 3–5 days |
Intervals are approximate and depend on specific turbine model, fuel quality, operating regime, and OEM recommendations. Starts count as equivalent operating hours (typically 10–50 equivalent hours per start).
Factors Affecting Maintenance Intervals
- Fuel quality: Contaminants in fuel gas (H2S, moisture, liquids, particulates) accelerate hot-section degradation. Fuel gas conditioning and filtration are essential.
- Operating regime: Base-load continuous operation is easier on hot parts than frequent starts and load changes. Each start generates thermal cycling stress.
- Firing temperature: Operating at peak output (highest firing temperature) reduces hot-section life. Operating at reduced load extends intervals.
- Inlet air quality: Dirty or corrosive air (salt, dust, chemicals) fouls compressor blades, reducing efficiency and potentially causing blade failure.
- Water washing: Regular online and offline compressor water wash maintains compressor efficiency and delays fouling-related degradation.
Condition Monitoring
Modern gas turbines are equipped with comprehensive monitoring systems that enable condition-based maintenance:
| Parameter | What It Indicates |
|---|---|
| Vibration (radial and axial) | Bearing wear, rotor imbalance, blade damage, misalignment |
| Exhaust temperature spread | Combustion uniformity, hot spots, fuel nozzle problems |
| Compressor discharge pressure | Compressor fouling, blade erosion |
| Heat rate trend | Overall degradation, fouling, component wear |
| Lube oil analysis | Bearing wear metals, contamination |
| Exhaust temperature (absolute) | Turbine section condition, cooling effectiveness |
| Starting time and torque | Starter condition, compressor fouling |
9. Selection & Procurement
Selecting the right gas turbine for a midstream application requires balancing performance requirements, capital cost, operating cost, maintenance philosophy, and site-specific constraints. This section provides a framework for the selection process.
Selection Criteria
| Criterion | Key Considerations |
|---|---|
| Site-rated power | Must meet required shaft power at worst-case site conditions (highest temp, dirty filters). Include margin (typically 5–10%). |
| Heat rate / efficiency | Lower heat rate = lower fuel cost. May justify higher capital cost for base-load applications. |
| Speed matching | Output shaft speed must match driven equipment. Free power turbine speed range for centrifugal compressors. |
| Exhaust characteristics | Temperature and flow for waste heat recovery. Higher exhaust temp = more WHRU potential. |
| Maintenance philosophy | In-situ overhaul vs. gas generator exchange. Exchange programs minimize downtime but require spare engines. |
| Field service support | Local service center, parts availability, and response time. Critical for remote locations. |
| Emissions compliance | DLN capability, SCR compatibility, permit requirements. |
| Starting requirements | Electric start, pneumatic start, or hydraulic start. Black-start capability if needed. |
| Fuel flexibility | Natural gas, dual fuel, or liquid fuel capability. Fuel gas pressure and quality requirements. |
| Enclosure / packaging | Weather protection, noise attenuation, fire suppression, ventilation. |
Sizing Margin
When selecting a gas turbine size, the following margins should be applied to the calculated site-rated power requirement:
| Margin Type | Typical Value | Reason |
|---|---|---|
| Degradation margin | 3–5% | Account for power output deterioration between overhauls (fouling, wear) |
| Design margin | 5–10% | Uncertainty in load estimate, future capacity increase |
| Inlet filter aging | 1–3% | Filter pressure drop increases between changeouts |
| Fuel gas variation | 1–2% | Heating value and composition variation |
API 616 Requirements for Procurement
API 616 is the primary specification for gas turbines in petroleum, chemical, and gas industry services. Key requirements include:
- Performance guarantees: Power output, heat rate, exhaust temperature and flow at specified site conditions
- Mechanical design: Rotor dynamics (lateral and torsional analysis), bearing design, coupling selection
- Materials: Hot section materials rated for operating temperature, corrosion environment, and design life
- Control system: Speed control, temperature limiting, fuel metering, sequencing, protection systems
- Lube oil system: Per API 614 for lubrication, shaft sealing, and oil control systems
- Testing: Mechanical running test, performance test, control system functional test
- Documentation: Data sheets, drawings, operating and maintenance manuals, spare parts list
Total Cost of Ownership
Gas turbine selection should consider the total cost of ownership over the project life, not just capital cost:
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