Gas Turbine Principles and Designs Practice Flashcards

Introduction to Gas Turbine Principles and Designs

Gas turbines have been utilized in aircraft jet engines for approximately 60 years. However, their application in Power Engineering has expanded significantly to include cogeneration plants, combined cycle plants, emergency power generation, peak power generation, heat recovery steam generator (HRSG) units, and marine propulsion.

Industrial vs. Jet Engines

A fundamental distinction exists between industrial gas turbines and jet engines:

• Industrial Turbines: These units utilize a power turbine driven by exhaust gas from the turbine section to provide rotational energy for mechanical loads.

• Jet Engines: These do not contain a power turbine for external mechanical loads; instead, the thrust generated by the engine provides the motion for the aircraft.

Overview of Power Capabilities

Gas turbines are capable of producing massive amounts of power. Standard large units can produce up to $400\,\text{MW}$. Notably, General Electric (GE) developed a specific gas turbine named Harriet that produces $600\,\text{MW}$.

Objective 1: Advantages, Disadvantages, and Operating Principles

Gas Turbine Advantages

Modern gas turbines offer several distinct advantages over other internal combustion engines:

• High Power Output: Capability to reach up to $400\,\text{MW}$.

• Power-to-Weight Ratio: They possess a high power-to-weight ratio, making them ideal for weight-sensitive applications such as offshore platforms.

• Fuel Versatility: They can operate on a wide variety of liquid and gaseous fuels.

• Rapid Start: Important for backup power scenarios.

• High Availability and Reliability: Minimized outage times because gas turbines are relatively quick to replace in the event of a major failure.

• Remote Operation: Low operational manpower requirements.

Gas Turbine Disadvantages

• Manufacturing Costs: High costs due to complex designs.

• Load Response: Slower speed changes compared to other engines, resulting in slower responses to load demand fluctuations.

• Efficiency at Low Loads: Efficiency decreases significantly when not operating at high loads.

• Complexity: Maximum efficiency requires additional components such as intercoolers, regenerators, and reheaters, increasing capital cost.

Types of Industrial Gas Turbines

Aeroderivative Gas Turbines

These are adapted from aircraft jet engines. Modifications involve removing thrust components and adding a power turbine.

• Features: Low weight (ships, trains, offshore), easily maintained/replaced, fast starting.

• Durability: Generally less durable with a shorter lifespan than heavy-duty types.

• Performance: Potentially more efficient than reciprocating engines of equal rating.

Heavy-Duty Gas Turbines

Designed specifically for heavy industrial use where size and weight are not primary constraints.

• Features: Rugged, durable, long intervals between overhauls.

• Applications: Primarily used for base load applications.

• Versatility: Can accommodate complex layouts for intercooling, regeneration, and reheat. They can use lower-grade fuels such as distillates, residuals, and crude oil.

Application Categories

• Base Load: Continuous operation within a steady load range.

• Emergency Power: Supplied quickly for short durations during main generator failure.

• Peak Power: Operational during periods of high electrical distribution network overload. Large distributors often provide incentives for industrial companies to generate their own power during these times.

• Cogeneration: Small gas turbines or microturbines (e.g., a $28\,\text{kW}$ unit) used to produce both power and heat. Exhaust gases can pass through a once-through boiler to generate steam (e.g., at $800\,\text{kPa}$).

Simple Gas Turbine Operating Principles

Every gas turbine consists of three major sections:

1. Compressor Section: Supplies high-pressure air.

2. Combustion Section: Burns fuel to produce hot gases.

3. Power Turbine Section: Converts gas energy into rotational power.

Typical Operating Parameters

• Air Intake: Ambient air compressed to between $1100\,\text{kPa}$ and $3000\,\text{kPa}$. Compression increases air temperature up to $650^{\circ}\text{C}$.

• Combustion: Fuel (gas or oil) is burned continuously. Flame temperature reaches approximately $2100^{\circ}\text{C}$. Cooling and dilution air lower this mixture to $1000^{\circ}\text{C}$ to $1500^{\circ}\text{C}$ before it enters the turbine.

• Expansion: Gases expand through turbine blades, exhausting at $500^{\circ}\text{C}$ to $640^{\circ}\text{C}$.

• Power Distribution: Up to $65\%$ of developed power is used to drive the compressor via a common shaft. The remaining power drives the external load (e.g., generator).

Objective 2: Shaft Arrangements

Single-Shaft Arrangement

The compressor and power turbine are mounted on the same shaft. The load is connected to this shaft, usually at the turbine end or sometimes at the compressor end.

• Usage: Power generation requiring constant speed.

• Starting: Requires a large starting motor because it must turn the mass of the compressor, turbine, and load simultaneously.

Dual-Shaft Arrangement

Consists of two separate shafts with no mechanical connection between them:

1. High-Pressure (HP) Shaft: Contains the compressor driven by the HP turbine.

2. Low-Pressure (LP) Shaft: Contains the LP power turbine driving the load.

• Advantages: Greater flexibility. The load can operate at varying speeds while the compressor stays constant, or vice versa. Starting is easier as the motor only turns the HP shaft.

Objective 3: Open and Closed Cycles

Open Cycle

The most common configuration where atmospheric air is drawn in, processed, and exhausted back to the atmosphere. "Simple open cycle" implies no heat exchangers or reheaters. Modern open-cycle efficiency can reach $44\%$.

Closed Cycle

The working fluid is recirculated within a closed system. Heating is provided externally via a heat exchanger.

• Operation: The fluid is pressurized, preheated in a regenerator, heated in an air heater (external source), expanded through the turbine, and then cooled in a regenerator and a secondary cooler.

• Working Fluids: Helium, Argon, Nitrogen, or supercritical Carbon Dioxide ($\text{CO}_2$). Supercritical $\text{CO}_2$ allows for lower temperatures but requires much higher pressures than Helium.

• Advantages: Fluid stays clean (no corrosion), higher pressures/densities increase output, and various fuels (like low-grade coal) can be used externally.

• Disadvantages: Requires cooling water, increased complexity, size, and cost.

Objective 4: Gas Turbine Installation and Auxiliaries

Installation Components

• Enclosures: Protect from the environment, reduce noise, and provide fire suppression (e.g., $\text{CO}_2$ bottles).

• Intake System: Filters contaminants and might include cooling systems to increase air density (denser air allows more mass flow and power).

• Exhaust System: Safely exits gases; may include silencers or waste heat recovery exchangers.

• Reducing Gear: Required when turbine shaft speed is higher than the driven load. Generators often require constant speeds of $1800\,\text{rpm}$ or $3600\,\text{rpm}$ ($60\,\text{Hz}$) or $1500\,\text{rpm}$ or $3000\,\text{rpm}$ ($50\,\text{Hz}$).

Auxiliary Systems

• Fuel Gas/Alternate Fuel: Provides fuel at the correct pressure/temperature.

• Lube Oil System: Lubricates bearings.

• Hydraulic Oil: Operates fuel valves.

• Steam/Water Injection: Reduces $\text{NO}_x$ emissions or increases mass flow for higher power.

• Anti-icing: Preheats intake air in cold climates.

• Water Wash: Cleans compressor blades.

Objective 5: Efficiency and Cycle Improvements

Rating Standards

Manufacturer power ratings ($\text{kW}$) are standard at:

• Temperature: $15^{\circ}\text{C}$.

• Location: Sea level.

• Humidity: $60\%$.

• Fuel: Natural gas.

• Losses: No intake or exhaust losses.

Thermal efficiency is the ratio of rated power to fuel energy rate (Flow Rate $\times$ Lower Heating Value).

Improvement Methods

1. Regeneration: Uses exhaust heat to preheat compressed air before combustion. Can improve efficiency by $15\%$ to $20\%$.

2. Intercooling: Air is compressed in two stages with cooling in between. Cooling reduces the specific volume of the air (smaller machine) and moves the process closer to isothermal compression, which requires less work than adiabatic compression.

3. Reheat: Expanding gases in two turbine stages and reheating them in a second combustion chamber between stages. This increases gas energy content and reduces the amount of air that must be compressed for the same work.

4. Combined Cycle: Exhaust heat is captured by a waste heat boiler to produce steam for a steam turbine. Total efficiency can exceed $60\%$.

Objective 6: Compressor Designs

Centrifugal (Radial) Compressors

Air enters the eye and is accelerated radially by centrifugal force. A diffuser converts velocity to pressure.

• Advantages: Simple, strong, short overall length.

Axial Compressors

Air moves axially along the shaft through rotating and fixed blades. Rotating blades increase velocity; fixed blades slow it down and raise pressure.

• Features: Symmetrical staging similar to a reaction turbine in reverse. Most large turbines use multi-stage axial compressors.

Objective 7: Combustors

Air distribution in the combustor:

• Combustion Air ($20\%$): Mixed with fuel in the flame tube.

• Cooling/Dilution Air ($80\%$): Flows outside the flame tube and re-enters to protect turbine blades from excessive temperatures ($2100^{\circ}\text{C}$ flame reduced to $1000^{\circ}\text{C}$-$1500^{\circ}\text{C}$).

Types of Combustors

1. Annular: A single concentric flame tube. High efficiency and compact (common in aircraft).

2. Can: Cylindrical chambers.

   • Single Can: Often a "reverse-flow" design.

   • Multiple Cans: Arranged in a circle. Can be "straight-through" (air enters/exits opposite ends) or "external reverse-flow."

   • Advantages: Aerodynamically simpler; manageable combustion problems.

3. Can-Annular: A hybrid design where multiple cans are placed in a common annular space. Includes interconnecting pipes to carry the flame between cans during starting. Only two cans usually have igniters.

Objective 8: Power Turbine Design and Materials

Design

Power turbines are usually axial-flow and impulse-reaction type.

• Nozzles (Nozzle Guide Vanes): Increase gas velocity.

• Rotating Blades (Buckets): Extract power.

Blade Cooling and Materials

• Cooling: Compressed air is extracted from the compressor and circulated through hollow blades, exiting through nose holes and trailing edge slots.

• Materials: High-stress components use nickel-based superalloys such as Inconel, Udimet, Waspaloy, and Hastelloy. Rotors are often made of bolted or welded discs of heat-resistant steel.

• Creep: A phenomenon where metal stretches over time at high temperatures and stresses, forming voids. To prevent catastrophic failure from creep, blades are replaced at fixed intervals (typically $75,000$ to $100,000$ hours).

Objective 9: Control and Instrumentation

Control Systems

Fuel flow is varied according to load and conditions.

• Governors: Set a speed reference. Error signals adjust the metering valve positioner.

• Acceleration Schedule: Prevents compressor surge by overriding the governor during start-up.

• Emergency Trips: Close the fuel shut-off valve if:

  • Lubricating oil pressure is low.

  • Exhaust temperature is high.

  • Rotor speeds are excessive.

Instrumentation and Interface

Parameters monitored include shaft speeds ($\text{rpm}$), temperatures ($^{\circ}\text{C}$), pressures ($\text{kPa}$), and vibration (using accelerometers for anti-friction bearings or eddy-current probes for journal/tilt-pad bearings).

• Interfaces: Known as HMI (Human Machine Interface) or MMI. Used for monitoring, starting/stopping, and logic modification.

Objective 10: Performance and Operating Parameters

Operating Effects

• Air Inlet Temperature: Decreased temperature increases air density and mass flow, resulting in higher power output. For example, a turbine rated at $9500\,\text{kW}$ at $15^{\circ}\text{C}$ may produce $12,000\,\text{kW}$ at $-30^{\circ}\text{C}$ and only $6600\,\text{kW}$ at $45^{\circ}\text{C}$.

• Compression Ratio: Higher ratios directly increase efficiency and power.

• Turbine Inlet Temperature: Higher temperatures increase work capability.

• Heat Rate: Used by manufacturers to express efficiency; defined as the amount of Joules required to produce one kilowatt-hour of electrical energy ($\text{J/kWh}$).

Maximum Power vs. Longevity

There is a trade-off between operating at peak power and maintenance costs. For base load, maximum power is avoided to reduce the risk of failure and extend the life of the hot gas path components. Peak power generation utilizes higher limits at the cost of higher maintenance and shorter component life.

Questions & Discussion

1. Describe the advantages and applications of a gas turbine.

2. Identify the major components of a simple cycle gas turbine and their basic functions.

3. Explain the different types of shaft arrangements.

4. Describe the difference between simple and closed cycles.

5. What considerations must be accounted for with gas turbine intakes and exhaust?

6. What are the respective purposes of intercooling, regeneration, and reheat?

7. Describe the important considerations for compressor design and types.

8. Describe the types of combustors and how they operate.

9. Explain how the turbine section works.

10. List the functions of a gas turbine control system.

11. Describe the effects of inlet temperature, discharge pressure, and inlet temperature on performance.