Comprehensive Study Notes on Industrial Gas Turbines

01- Gas Turbine Advantages

Gas turbines offer several significant benefits in industrial and power generation contexts:

Large Power Capacity: These units have the ability to produce vast amounts of power, with current models reaching up to $400\,MW$ .
Power-to-Weight Ratio: They possess a high power-to-weight ratio. This characteristic makes them specifically suitable for applications where weight minimization is critical, such as offshore platforms.
Fuel Versatility: Gas turbines can operate using a wide range of both liquid and gaseous fuels.
Rapid Startup: They are capable of starting rapidly. This is a vital feature for backup power generation applications where immediate response to power loss is required.
Design Simplicity: The design is relatively simple and compact, featuring few and straightforward auxiliary systems.
Reliability and Maintenance: They offer high availability and reliability. In the event of a major failure, outage time can be minimized because the gas turbine unit can be quickly replaced.
Remote Operation: They support remote operation capabilities, requiring minimal operational manpower.

01- Gas Turbine Disadvantages

Despite their advantages, gas turbines have specific drawbacks that limit their use in certain fields:

Automotive Limitations: The inherent drawbacks of turbine engines have prevented their widespread adoption in automotive applications.
Manufacturing Costs: Due to their complicated design, turbine engines incur high manufacturing expenses.
Response Time: A turbine engine changes speed slowly. Compared to a reciprocating engine, a gas turbine is slow to respond to changes in throttle requests.
Low-Power Efficiency: They are less suitable for low-power applications because the efficiency of the gas turbine decreases significantly at partial throttle conditions.
System Complexity for Efficiency: To reach efficiencies comparable to modern gasoline engines, a gas turbine requires additional components such as intercoolers, regenerators, and/or reheaters, which adds substantial cost and complexity.

01- Types Of Industrial Gas Turbines

There are two fundamental categories of industrial gas turbines:

Aero-derivative: These are derived from jet engines used in aircraft. To adapt them for industrial use, the thrust components are removed and replaced with power turbines.
Heavy-duty Gas Turbines: These are specifically designed only for land-based applications.

01- Industrial and Commercial Gas Turbines Applications

Industrial gas turbines serve various roles, including electric power generation and driving mechanical equipment like pumps and compressors. For electricity generation, they function in three primary roles:

Base Load: The unit produces electricity on an ongoing, continuous basis within a fairly stable demand range.
Emergency (Back-up) Power: The unit provides power when the primary source fails.
Peak Power: Power production occurs only during periods when the regional electrical distribution network is close to being overloaded.

01- Industrial and Commercial Gas Turbines Components

A gas turbine consists of three primary sections:

Compressor Section: Supplies high-pressure ( $HP$ ) air for combustion and provides mass flow.
Combustion Section (Combustor): Burns fuel mixed with combustion gases.
Turbine Section: Converts the energy from combustion gases into rotational energy.

Functional diagrams for such systems often include elements like City Gas inputs, Recuperators, Generators, Once-Through Boilers with Assist Burners for steam production, and electrical components like AC/DC Converters, Inverters, Transformers, Breakers, and Automatic Synchronous Systems with optional Battery storage.

01- Simple Gas Turbine Operating Principles

The operation follows a specific thermodynamic process:

Air Intake and Compression: Ambient air enters the air compressor and is compressed to a pressure range of $1100 - 3000\,kPa$ . It then passes to the combustion chamber.
Combustion: The combustion chamber adds fuel (natural gas or oil) to a portion of the compressed air to maintain continuous combustion. This causes a sudden increase in temperature to approximately $2100\,^\circ C$ (the flame temperature).
Temperature Management: The combusted products are mixed with the remaining compressed air. This reduces the temperature to between $1000 - 1500\,^\circ C$ at the exit of the combustion section, making it safe for turbine materials.
Expansion: Hot gases enter the turbine, where they expand. This produces mechanical power to drive both the air compressor and an external load, such as a generator for electricity.
Exhaust: Gases exit the turbine at nearly atmospheric pressure, though they remain hot, with temperatures ranging from $500 - 640\,^\circ C$ .

02- Single Shaft Gas Turbine

In a single-shaft arrangement:

The compressor, turbine, and load are all mechanically connected and rotate at the same speed.
The load can be connected at either the turbine end or the compressor end through reduction gears or a coupling.
Application: Best suited for constant speed requirements.
Characteristics: It is mechanically simpler but requires a larger starting motor.
Drawbacks: It lacks operating flexibility compared to dual-shaft designs.

02- Dual Shaft Gas Turbine

The dual-shaft arrangement separates the components onto two distinct shafts:

Gas Generator Shaft: The compressor is driven by a high-pressure ( $HP$ ) turbine located on the same shaft.
Power Shaft: The load (e.g., a generator) is driven on a second shaft by a low-pressure ( $LP$ ) power turbine.

Key Features:

There is no mechanical shaft connection between the $HP$ and $LP$ turbines.
The $HP$ turbine exists solely to drive the compressor, while the $LP$ turbine drives the load.
Startup Advantage: A smaller starting motor can be used because it only needs to turn the compressor and the $HP$ turbine, not the load mass.
Operating Flexibility: The load can operate at varying speeds while the compressor remains at a constant speed, or conversely, the load speed can stay constant (standard for generators at $50\,Hz$ or $60\,Hz$ ) while the compressor speed varies.

03- Open Cycle Operation

The system is called "open cycle" because the working fluid (ambient air) is drawn from the atmosphere at the start and returned to it at the end.

Simple Open Cycle: Defined by having no additional components like reheaters, intercoolers, or heat exchangers.
Process: Air is compressed, heated in the firebox/combustion chamber, expanded through the gas turbine, and exhausted to the atmosphere.
Advantage: Simplicity.

03- Closed Cycle Operation

In a closed cycle, the working fluid is contained and re-circulated. Typical fluids include Nitrogen, Helium, or Argon.

Process: Fluid is pressurized in the compressor, preheated in a regenerator by exhaust gases, and then reaches an air heater where it is further heated by an external source of combustion gases. It expands through the turbine to do work and is then cooled by two stages of heat exchange (regenerator followed by a cooler) before returning to the compressor.
Advantages:
- Higher output due to higher pressures and fluid density.
- The working fluid is clean, preventing corrosion.
- Fluids can be selected for better thermostatic properties.
- Cheaper fuel can be used in the external air heater compared to internal combustors.
Disadvantages:
- Heat-exchanger efficiencies are significantly lower than direct internal combustion.
- Increased system complexity, size, and cost.
- Requires a supply of cooling water.

04- Gas Turbine Installation

Buildings and Enclosures:

Units are often housed in enclosures to protect against the environment, reduce noise, and provide personnel protection in case of failure.
In cold climates, units are placed in separate buildings with maintenance "lay-down" space.
Systems include heat/gas detection, ventilation, and fire suppression (e.g., $CO_2$ ).

Intake and Exhaust:

Intake: Includes filters to remove contaminants and prevent foreign object damage. An intake plenum ensures smooth airflow. Cooling systems may be used because cooler air is denser, allowing more mass flow and higher power.
Exhaust: Provides a safe exit for hot gases. Silencers are used for noise reduction. Exhaust may be routed to a waste heat recovery heat exchanger.
Efficiency: Both systems must have the lowest possible pressure loss to minimize power and efficiency drops.

04- Auxiliary Systems

Multiple auxiliary systems support operation:

Fuel Gas System: Supplies fuel at proper temperature and pressure.
Fuel Treatment: Cleans and treats fuels if required.
Lube Oil System: Lubricates bearings.
Hydraulic Oil System: Operates fuel valves.
Steam Injection: Reduces emissions (like $NO_x$ ) and/or increases power output.
Anti-icing: Heats air intake to prevent ice buildup in cold climates.
Water Wash: Cleans compressor blades.

04- Reducing Gears

Reduction gears match the power turbine speed to the load equipment requirements. This is critical for generators that require constant speeds:

$60\,Hz$ Generators: Need $1800\,r/min$ or $3600\,r/min$ .
$50\,Hz$ Generators: Need $1500\,r/min$ or $3000\,r/min$ .

05- Efficiency and Rating Of Gas Turbines

Standard Rating Conditions: Ratings are provided in $kW$ based on shaft output under specific conditions set by manufacturers:

Temperature: $15\,^\circ C$ at sea level.
Fuel: Natural gas.
Humidity: $60\%$ .
Losses: No intake or exhaust losses.

Efficiency Definitions:

Simple Cycle Efficiency: Modern turbines reach $35\% - 40\%$ .
Thermal Efficiency: The ratio (in percentage) of the rated power output to the fuel energy rate.
Fuel Energy Rate: Calculated as $\text{Fuel Flow Rate} \times \text{Lower Heating Value of the Fuel}$ .

05- Cycle Improvements

Efficiency is enhanced through three primary methods:

Regeneration: Uses exhaust heat to increase the compressed air temperature before it enters the combustor. This yields a $15\% - 20\%$ efficiency increase but adds capital cost and pressure losses.
Intercooling: Compression is split into two stages with cooling in between using a shell-and-tube heat exchanger. This reduces the specific volume of air, allowing for a smaller machine size. Forms of compression include Isothermal (no temperature increase) and Adiabatic (with temperature increase).
Reheat: Fuel is burned in a second combustion chamber (reheater) using excess oxygen from the $HP$ turbine exhaust. This increases the energy content of the gases and improves thermal efficiency.

05- Combined Cycle and Cogeneration

Combined cycle systems extract exhaust heat using a heat exchanger (usually a boiler) to produce steam. This steam drives a steam turbine, which can power the same generator as the gas turbine, another generator, or a compressor. Total efficiency in combined cycle plants can exceed $60\%$ .

06- Compressor and Combustor Designs

Compressors:

Centrifugal (Radial): Used for smaller units.
Axial: Multi-staged, used for larger units. It is symmetrically staged like a reaction turbine in reverse. The casing narrows to slow air and increase pressure. A compressor can consume up to $65\%$ of the fuel's energy.

Combustors:

Design Types: Annular, Single-can, and Can-annular.
Annular Advantages: Efficient space use, light weight, low air flow restriction, suitable for aircraft.
Can Combustor: Features include swirler, ignition rod, inner/outer jackets, and bypass valves for strength and easier maintenance access.
Air Distribution: Typically, $20\%$ of air is mixed with fuel in the flame tube (combustion air), while the remaining $80\%$ serves as cooling and dilution air.

08- Power Turbine Design and Materials

Design:

The turbine extracts power by decreasing gas pressure and temperature. It drives the compressor ( $65\%$ of power) and then the load.
Axial-flow Stage: Consists of stationary nozzle guide vanes (nozzles) and rotating blades (buckets). Nozzles increase velocity and drop pressure; buckets extract power through further pressure/temp drops.
Impulse vs. Reaction: Impulse nozzles decrease area to convert pressure to velocity. Reaction nozzles have a constant area and redirect flow while blades have divergent passages. All gas turbines use a combination called impulse-reaction type.

Blade Cooling: Air (or water) is taken from the compressor, circulated through the blade, and exits through holes in the leading and trailing edges to reduce metal temperature and increase lifespan.

Materials and Creep: Materials face "creep"—the stretching of metal over time that creates voids and leads to rupture. Replacement is scheduled at fixed intervals, typically every $75000$ to $100000\,hours$ .

Inlet Case: $Al$ alloy.
Compressor: $Ti$ , $Al$ , or $Ni$ alloys.
Combustor/Turbine: $Ni$ alloys.
Exhaust Case: $Ni$ alloy.

09- Control, Instrumentation, and Vibration

Control Systems:

Vary fuel flow based on load demand. All-speed governing of the power turbine manages output. Governors use reset mechanisms and fuel metering valves. Override trims provide over-temperature and over-speed protection.
Emergency Trip: Automatically closes fuel shutoff valves if oil pressure drops, exhaust temperature exceeds limits, or rotor speeds become excessive.

Instrumentation:

Each shaft's rotor speed ( $r/min$ ).
Air inlet temperature and pressure (differential pressure across filters).
Compressor discharge pressure.
Exhaust gas temperatures (measured at multiple points).
Vibration Monitoring: Accelerometers are used for aero-derivatives (anti-friction bearings). Eddy-current displacement probes are used for heavy-duty turbines (journal/tilt-pad bearings).
Other: Bearing temperatures, fuel gas parameters ( $T, P, \text{flow}$ ), oil parameters, and generator output ( $kW$ ).

Operator Interface (MMI/HMI): Allows operators to start/stop the unit, control speed, modify logic (with special access), and monitor parameters.

10- Operating Parameters and Maximum Power

Effect of Inlet Air Temperature:

As temperature decreases, air density increases. Since power is proportional to mass flow, gas turbines produce more power in colder environments.
Example Performance at Standard $15\,^\circ C$ ( $9500\,kW$ rating):
- At $45\,^\circ C$ : Output drops to approx $6600\,kW$ .
- At $-30\,^\circ C$ : Output Increases to $12000\,kW$ .

Compression and Temperature Factors:

Cycle efficiency and power output increase with a higher compression ratio.
An increase in turbine inlet temperature also increases the work done per unit of air.

Maximum Power vs. Life:

Maximum power is limited by the expected life of hot gas path components. Vendors specify a maximum peak power for short intervals, but using this for base load increases maintenance costs and failure risks.