IntroductionGas turbines are designed for many different purposes. In the petroleum industry they are commonly used to drive:

· compressors for transporting gas through pipelines

· generators that produce electrical power

In this lesson, you will learn about the basic design of gas turbine engines, their sections, and how they operate.

Lesson 1: Gas Turbine Engine Design

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Engine Design : SectionsThe purpose of a gas turbine engine is to create energy to turn a shaft that drives other rotating equipment such as compressors and generators.

The figure shows a two-shaft turbine engine. However, this lesson discusses a single-shaft turbine engine. The operation of the two types is similar.

A gas turbine engine is divided into five sections:· air inlet section· compressor section · combustion section· turbine section· exhaust section

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The sections of a gas turbine are contained in the engine case. The engine case is a horizontal tube-like container that is open at both ends.

Each section of the gas turbine has a specific function. Engine Design: Air Inlet & CompressorsHuge quantities of air enter the case through an opening at the front end called the engine air inlet.

After passing through the engine air inlet, the air flows to the compressor section. The compressor section contains the first moving part, the compressor.

Compressor wheels increase the pressure of the incoming air.

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Engine Design: CombustionThe compressed air flows to the combustion section of the gas turbine engine.

As the compressed air enters the combustion chambers, fuel is added through nozzles. The result is a mixture of fuel and air.

The fuel and air mixture is ignited and burns, creating hot gases. The hot, expanding gases flow into the turbine section of the engine.

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Engine Design: Turbine & ExhaustThe turbine rotors get power from the flow of gases. This flow of gases into the turbine section causes the turbine rotors to turn. A single shaft gas turbine has only one major moving part - the rotor shaft.

The turbine rotors and the compressor are mounted on opposite ends of the same rotor shaft. This allows the turbine rotors and compressor to work as a unit.

When the gases leave the turbine section, they return to the atmosphere through the exhaust section.

Information about how a basic gas turbine works is presented next.

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Air Entry and Compression :

The engine case of the gas turbine is a tube-like container that is open at both ends and narrow in the center.

The front end of the case contains the compressor. The compressor draws air into the case and increases the air pressure by compressing (or reducing) its volume.

In the figure, the electric fan acts as a compressor.

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Combustion

In the narrow section of the case, the air is compressed.

Fuel gas is injected into the compressed air through fuel nozzles. The fuel and air are mixed together and ignited.

Burning the fuel and air mixture creates hot combustion gases. The heat increases the temperature and available energy of the gases.

A turbine captures this energy and changes it into mechanical energy.

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Turbine Rotation

The turbine in the rear of the engine case extracts mechanical energy from the flow of the gases acting on the blades.

The turbine rotates because of the pressure and velocity of the hot expanding gases acting on the blades of the turbine.

As the hot gases pass through the turbine, the energy in the gas is reduced.

The gases exit the rear of the case and into the atmosphere.

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Air Entry and Compression

The engine case of the gas turbine is a tube-like container that is open at both ends and narrow in the center.

The front end of the case contains the compressor. The compressor draws air into the case and increases the air pressure by compressing (or reducing) its volume.

In the figure, the electric fan acts as a compressor.

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CombustionIn the narrow section of the case, the air is compressed.

Fuel gas is injected into the compressed air through fuel nozzles. The fuel and air are mixed together and ignited.

Burning the fuel and air mixture creates hot combustion gases. The heat increases the temperature and available energy of the gases.

A turbine captures this energy and changes it into mechanical energy.

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Turbine Rotation

The turbine in the rear of the engine case extracts mechanical energy from the flow of the gases acting on the blades.

The turbine rotates because of the pressure and velocity of the hot expanding gases acting on the blades of the turbine.

As the hot gases pass through the turbine, the energy in the gas is reduced.

The gases exit the rear of the case and into the atmosphere.

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Turbine Rotor

The turbine uses energy from the thrust force created by the expanding gases. This energy is changed into shaft horsepower to drive the turbine compressor, the engine accessories, and the load. Most of this energy is expended to drive the turbine compressor.

(Contd.)

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The main parts of a turbine are :

· rotor shaft · rotor disc · rotor blades

The rotor blades (sometimes called buckets) are attached to the rotor disc. The rotor disc is mounted on the rotor shaft. The entire assembly of blades and disc is often called a rotor.

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Basic OperationThe figure shows how all the components of the gas turbine engine work together. The fan has been replaced by a single set of compressor blades.

The compressor creates the compressed air that is needed for combustion.

Fuel gas is mixed with the compressed air and is ignited. The burning mixture creates a force in the rear of the engine case. The force is changed into rotating mechanical energy that turns the turbine.

(Contd. )

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The turbine turns the rotor shaft that is shared by the compressor and the turbine. The compressor and turbine are connected to the same rotor shaft. The rotating force of the turbine is used to drive the compressor.

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Front and Rear Drive Gas TurbinesSome of the rotating energy created by the gas turbine can be used to drive a gear box or generator connected to either end of the rotor shaft.

Depending upon where the load is connected to the rotor shaft, the gas turbine is referred to as a

· front end drive · rear end drive

(Contd.)

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If the shaft is lengthened on the compressor end of the engine, it is a front end drive or a cold end drive.

If the shaft is lengthened on the turbine end of the engine, it is a rear end drive or a hot end drive.

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Multiple Stage Compressors

Gas turbine manufacturers may place more than one set of compressor and turbine stages in an engine as shown in the figure.

The additional stages in the compressor section provide more compression of the air before combustion.

More than one stage is used in the turbine section to extract as much power as possible from the hot, expanding gases.

The gas turbine in the figure is a two-stage turbine driving a three-stage compressor.

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