Gas Turbine Principles and Designs Practice Flashcards

Comprehensive practice flashcards covering industrial gas turbine principles, designs, components, cycles, and operating parameters based on Chapter 4 notes.

What is the maximum power production capability mentioned for modern gas turbines?

Currently up to $400\,MW$.

What is the name and power output of the specific gas turbine developed by General Electric (GE)?

Harriet, which produces $600\,MW$.

What is a primary disadvantage of gas turbines compared to internal combustion engines?

High manufacturing costs due to complicated design.

What component in industrial turbines differentiates them from jet engines?

Industrial turbines have a power turbine driven by exhaust gas, whereas jet engines do not.

Which three methods are discussed for improving gas turbine cycle efficiency?

Regeneration, intercooling, and reheating.

What are the three basic types of gas turbine combustors?

Annular, can, and can-annular.

List three abnormal operating conditions that will trip a gas turbine.

Low lube oil pressure, high exhaust temperature, and excessive rotor speed.

Why is the high power-to-weight ratio of gas turbines advantageous for offshore applications?

It makes them suitable for applications where weight must be minimized.

How does the starting time of a gas turbine compare to other engines for backup power?

It has the ability to start rapidly, which is important for backup power generation.

What is a disadvantage of gas turbines regarding load demand changes?

Speed changes are slower, meaning they are slower to respond to changes in load demand.

How does the efficiency of a gas turbine change at lower loads?

The efficiency of the gas turbine decreases at lower loads.

What are the two basic types of industrial gas turbines?

Aeroderivative gas turbine and heavy-duty gas turbine.

How is an aeroderivative gas turbine typically modified from a jet engine?

By removing the thrust-producing components and replacing them with a power turbine.

What feature of aeroderivative gas turbines makes them suitable for trains and vehicles?

Low weight.

Which type of gas turbine is characterized as having a shorter life and being less durable?

Aeroderivative gas turbines.

What are heavy-duty gas turbines specifically designed for?

Relatively heavy industrial applications where size and weight are not issues.

Contrast the durability of heavy-duty turbines versus aeroderivative turbines.

Heavy-duty turbines are very durable with long intervals between overhauls, making them suitable for base load.

Which fuels are heavy-duty gas turbines able to use?

All fuels, including distillates, residuals, and crude oil.

What is the visible physical difference in the casing between heavy-duty and aeroderivative turbines?

Casing thickness, which increases the overall size and weight of the heavy-duty machine.

Define 'base loading' in the context of gas turbines driving generators.

The unit produces electricity on a continuous basis within a fairly steady load range.

Define 'emergency power' in the context of gas turbine applications.

Power that must be supplied very quickly and for a short period of time, such as when main generators fail.

What does 'peak power' refer to in electrical distribution?

Production of power only during periods of the day when the electrical distribution network is close to being overloaded.

What is the typical power output of the microturbine shown in the example?

$28\,kW$.

At what pressure is steam generated by the microturbine exhaust in the example?

$800\,kPa$.

Identify the three major components of any gas turbine.

Compressor section, combustion section, and power turbine section.

To what pressure range does the compressor typically draw in and compress ambient air?

Between $1100\,kPa$ and $3000\,kPa$.

What is the maximum temperature the air can reach due to compression in the compressor section?

As high as $650^\circ C$.

What are the three purposes for the air entering the combustion chamber?

Some for combustion, some for cooling the outer flame tube, and the rest for dilution air.

What is the temperature of the combustion flame in a gas turbine?

Approximately $2100^\circ C$.

What is the temperature of the mixture leaving the combustion section after cooling and dilution?

Approximately $1000^\circ C\text{ to }1500^\circ C$.

What is the approximate temperature of the gases as they exit the turbine?

About $500^\circ C\text{ to }640^\circ C$.

How is the rotational power of the turbine used internally within the machine?

To drive the compressor through their common shaft.

Describe a single-shaft gas turbine arrangement.

The compressor and power turbine are mounted on the same shaft, with the load connected to that same shaft.

Where is the load usually connected in a single-shaft arrangement?

At the power turbine end.

Describe a dual-shaft gas turbine arrangement.

The compressor is driven by a high-pressure turbine on one shaft, while the load is driven on a second shaft by a low-pressure power turbine.

What is the primary purpose of the high-pressure (HP) turbine in a dual-shaft design?

To drive the compressor.

Why does a single-shaft arrangement require a larger starting motor?

It must turn the total mass of the compressor, power turbine, and the load.

What is the advantage of the dual-shaft design regarding operating flexibility?

The load may be operated at varying speeds while the compressor speed remains constant, or vice versa.

What determines if a gas turbine system is 'open' or 'closed'?

How the working fluid is used (recirculated internally or drawn from/rejected to the atmosphere).

What does a 'simple open cycle' indicate about a gas turbine system?

The gas turbine has no additional components like heat exchangers, reheaters, or intercoolers.

What is the thermal efficiency of modern aeroderivative gas turbines in an open cycle?

$44\%$.

What is the purpose of a closed cycle gas turbine system?

To increase the net power output and thermal efficiency of the plant.

In a closed cycle, how is the recirculating working fluid cooled before arriving back at the compressor?

By two stages of heat exchange: first in the regenerator and then in the cooler.

When was the closed cycle gas turbine first patented?

1935.

List three gases that may be used as the working fluid in a closed cycle system.

Helium, argon, or nitrogen.

What is a disadvantage of using supercritical carbon dioxide as a working fluid compared to helium?

It operates at far higher pressures.

How does higher working fluid pressure in a closed cycle affect machine output?

It allows for higher densities of the working fluid, which increases the output.

What is a disadvantage of the closed cycle system involving external supplies?

A supply of cooling water is required.

What is the purpose of an intake plenum?

To ensure smooth airflow into the gas turbine compressor section.

Why do some air intakes incorporate cooling systems?

To decrease air inlet temperature and increase its density, allowing more mass flow and power development.

What is the function of a silencer in the exhaust system?

To reduce the sound of the exhaust system to acceptable limits.

What auxiliary system is used specifically to control $NO_x$ emissions?

Steam or water injection system.

What is the purpose of a water wash system?

Cleaning compressor blades.

Why is a reducing gear necessary between the turbine shaft and the load shaft?

The speed of the power turbine shaft is often too high for the driven load.

At what speeds must gas turbines drive $60\,Hz$ generators?

$1800\,rpm\text{ or }3600\,rpm$.

At what standard temperature and elevation is a gas turbine's power rating specified?

$15^\circ C$ at sea level.

Define 'thermal efficiency' for a gas turbine.

The ratio (in percentage) of the rated power and the fuel energy rate.

How is 'fuel energy rate' calculated?

By multiplying the fuel flow rate by the lower heating value of the fuel.

Explain the process of 'regeneration'.

Installing a heat exchanger in the exhaust to preheat compressed air before it enters the combustors.

By how much can regeneration improve gas turbine efficiency?

$15\%\text{ to }20\%$.

What is 'intercooling'?

Compressing inlet air in two stages and cooling it in a heat exchanger between stages.

Why does isothermal compression take less work than adiabatic compression?

Isothermal compression occurs without an increase in temperature, whereas adiabatic compression results in a temperature increase.

Where are the gases reheated in a reheat cycle?

Between the high-pressure turbine expansion stage and the low-pressure turbine expansion stage.

What provides the oxygen for combustion in a second (reheat) combustion chamber?

The excess oxygen content of the gases from the high-pressure turbine.

How is exhaust heat usually extracted in a combined cycle or cogeneration system?

By means of a heat exchanger, usually a type of boiler.

What percentage of fuel energy is typically used up by the compressor?

Up to $65\%$.

How does a centrifugal compressor raise air pressure?

Air is accelerated to high velocity radially by the rotor and received by a diffuser, which converts velocity energy to pressure energy.

List three advantages of centrifugal compressors.

Simple, strong, and short overall length.

How do axial compressors move air through the machine?

Air is moved axially along the shaft by several rows of rotating blades, with fixed blades slowing it down to raise pressure.

What happens to the pressure of the air as it passes through the moving blades of an axial compressor?

The pressure increases.

In a combustor, what percentage of air is used as 'combustion air' versus 'cooling air'?

$20\%$ for combustion and $80\%$ for cooling.

Explain the configuration of an annular combustor.

A single concentric flame tube located between an inner and outer casing with multiple fuel nozzles.

Why is the annular combustor design most desirable for aircraft engines?

It is more compact and makes the most efficient use of available space.

Why is a single can combustor referred to as a 'reverse-flow' design?

Air flows to the top of the combustor jacket and then back towards the outlet.

What is the purpose of air crossover tubes in a multiple-can combustor arrangement?

To help the cans operate at the same pressure and ensure equal operating conditions.

Contrast 'straight-through' vs 'reverse-flow' can combustors.

Straight-through exits at the opposite end from the inlet; reverse-flow exits back towards the inlet side.

What is a major advantage of the 'can' type combustor regarding aerodynamics?

Problems such as non-uniform outlet temperatures are minimized.

Describe the air distribution in a can-annular combustor.

Cans are located in a common annular space, and air is supplied through scoops for primary air and holes for cooling/dilution air.

In an axial-flow power turbine, what constitutes a 'stage'?

A row of stationary blades (nozzle guide vanes) and a row of rotating blades (buckets).

What is the difference between nozzles in impulse and reaction turbines?

Impulse nozzles decrease in area to convert pressure to velocity; reaction nozzles have constant area and redirect flow.

What type of power turbine is most commonly used in gas turbines?

Impulse-reaction type.

How are turbine blades cooled in many gas turbine designs?

Compressed air is circulated through the blade and extracted through holes in the leading edge, trailing edge, and surface.

List four nickel-based super alloys used for turbine nozzles.

Inconel, Udimet, Waspaloy, and Hastelloy.

Why are large turbine rotors often made of discs bolted or welded together?

Because it is difficult to make large forgings of the required heat-resistant steel.

What is the phenomenon called where metal stretches and weakens at high temperatures and stresses?

Creep.

What is the typical operational interval for replacing turbine blades to avoid failure?

Between $75,000\,hours$ and $100,000\,hours$.

How is the speed and power output of a power turbine typically controlled?

By varying the fuel flow to the combustors via the all-speed governing system.

What establishes the speed reference setting in the governor reset mechanism?

The loading station, where the operator sets the desired engine speed.

How is compressor surge avoided during acceleration?

An acceleration fuel flow schedule, based on compressor outlet pressure rise, overrides the governor signal.

Where is exhaust gas temperature typically measured?

At multiple circumferential points as an average after the first or second turbine stage.

What sensor is used to monitor vibration if journal or tilt-pad bearings are present?

Eddy-current displacement probes.

What is the unit used to express the heat rate input of a gas turbine?

Joules per kilowatt-hour ($J/kWh$).

How does decreasing inlet air temperature affect gas turbine power production?

As temperature decreases, air density increases, raising the mass airflow and thus increasing power output.

Based on the provided performance graph, what is the output power at $15^\circ C$?

$9500\,kW$.

Based on the provided performance graph, what is the output power at $-30^\circ C$?

Almost $12,000\,kW$.

Which two factors are most critical in determining gas turbine efficiency and power?

Compression ratio across the compressor and turbine inlet temperature.

What is the maximum achievable efficiency of a gas turbine in combined cycle applications?

It can exceed $60\%$.

Why is maximum peak power limit not desirable for base load operation?

Because of increased maintenance costs and the higher risk of failure.

What does a 'DCS' stand for in turbine control?

Distributed control system.

What does 'PLC' stand for in turbine control?

Programmable logic control.