Combustion Gas Turbine

Petroskills presentation

Hector Nguema Ondo

Table of contents

• Constructing Gas Turbines

• Gas Turbine Speed Control

• Combustion Gas Turbine Systems

• Controlling Combustion Gas Turbines

• Operating Combustion Gas Turbine

Constructing Gas TurbinesCompressor construction

• Air enters and leaves parallel to the shaft.

• The compressor's rotor blades add velocity energy to the air.  The stator blades change this velocity to pressure energy

• Air enters parallel to the shaft and leaves at right angles to the shaft.

• Air strikes the casing and velocity decreases, but pressure increases.

Constructing Gas TurbinesCombustion Chamber construction

• Small amount of fuel is mixed with air from the compressor.  The fuel burns in the air to produce heat.

Fuel Atomizer

• An atomizer is needed in liquid-fueled turbines to spray the fuel into the combustion chamber as a mist.

• Also Pumps are needed to force liquid fuels into the combustion chamber. 

Spark Plugs

• To start the turbine, a spark plug is used to ignite the flame.

• The presence of large amounts of unburned fuel in the air may lead to an explosion when the burner is re-lighted.  As long as the fuel supply is steady, the flame keeps burning.

Constructing Gas TurbinesTurbine Construction

• The transition piece makes sure that the hot air is thoroughly mixed, so that there are no hot pockets.

• Air enters the turbine section at high pressure.  To turn the rotor, the air must be moving.  This means that air pressure is changed into air velocity.

• A nozzle is used to increase the velocity of air.  The nozzle allows air to escape in one direction at high velocity.

Constructing Gas TurbinesImpulse and Reaction Turbines

Impulse Turbine

• The nozzle is stationary.  The air shoots out and strikes an object in its path.  The object moves in the same direction the jet air is moving.

• This turbine uses only the impulse principle to turn the rotor

Reaction Turbine

• The reaction effect works like a rocket.  The nozzle moves in the opposite direction from the air movement.

• High pressure air expands through the rotor blades.  As the air expands, velocity increases and pressure decreases. 

Constructing Gas TurbinesImpulse and Reaction Turbines

• Most turbine blades are designed to take advantage of these two different effects of a moving air stream.

• The pressure in the second set of stator blades is lower than the pressure in the first set

• Most combustion gas turbines have two drive shafts.  One shaft drives the load.  The other shaft drives the compressor, which forces high-pressure air into the combustion chamber

Constructing Gas TurbinesMarathon Oil Diagram

Constructing Gas TurbinesMarathon Oil Residue Gas Turbine Internals

Gas Turbine Speed Control

• The governor valve regulates the amount of fuel entering the combustion chamber.

• When the load on the gas turbine is increased, the governor valve must be opened to allow more fuel into the burner

Hydraulic oil system• Oil forced out of the cylinder is returned to an oil

reservoir to be used again.  The hydraulic oil system cannot act until it receives information about the turbine speed.  It is only a relay of turbine speed information.  The system itself cannot detect how fast the turbine is going

Starters and Governors

• The compressor must be started so that it can begin supplying high-pressure air to the turbine. 

Gas Turbine Speed ControlGovernors

Flyball Governor

• The flyballs turn as the turbine shaft turns and are held together by a spring.  The force of the spring keeps the flyballs together at low speeds.  

• At startup, the turbine shaft is not turning.  The flyballs are held close together by the spring

• As the flyballs separate, the pilot valve opens the right port to high-pressure oil.

Gas Turbine Speed ControlGovernors

Electric Governor

• The turbine shaft runs an electric governor.  When the turbine shaft turns, the generator produces electric current.

• This current energizes a coil in a solenoid.  As long as the amount of electric current is low, the iron core is pushed out of the coil by the spring.

• As the electric current increases, the magnetic coil attracts the soft iron core.  The iron core moves into the coil and compresses the spring

Gas Turbine Speed ControlOverspeed Trip mechanism

• A trip pin shuts off the flow of fuel to the combustion chamber to protect against overspeeding.

• If the engine overspeeds, centrifugal force overcomes spring tension and the trip pin moves out of the shaft.

Trip valve system

• The overspeed trip, in cutting off the flow of fuel to the combustion chamber, acts like a governor.  Unlike the governor, which resets itself, the overspeed trip must be reset by hand

Combustion Gas Turbine SystemsCasing Seals

Soft Packing

Labyrinth Seals

• Inserted between the shaft and stator of this small, low-speed steam turbine.

• The packing box contains rings of soft material which do not damage the shaft when they rub against it.

• Made of a soft metal and do not wear out like soft packing.

• Compartments are formed between the teeth of the labyrinth seal and the shaft

Combustion Gas Turbine SystemsBearings

Ball Bearings

• The ball bearings turn freely as the shaft rotates.

• A ball bearing can be used as a radial bearing or a thrust bearing only where axial and radial loads are not great. 

• Hot air strikes the turbine blades may cause axial (or end-to-end) movement.

• The shaft may also tend to move off-center, causing radial movement.

Ball Bearings Sleeve Bearings Thrust Bearings

Combustion Gas Turbine SystemsBearings

Sleeve Bearings

• Large turbines and compressors use heavy-duty sleeve bearings.  A sleeve bearing can support a very large shaft.

• Some sleeve bearings are made in several sections, which can tilt, forming oil wedges between the shaft and the sleeve. These oil wedges keep the oil from being squeezed out of the bearing.

Combustion Gas Turbine SystemsBearings

Thrust Bearings

• Consists of a stationary thrust shoe and a rotating thrust collar.

• When the shaft moves in either direction along its axis, the thrust shoe and the collar are forced together

• A stationary collar is fitted with thrust shoes that tilt when the moving thrust collar turns, forming oil wedges between the collars

Combustion Gas Turbine SystemsOil Circulation System

• In the lubrication system, oil is supplied under pressure to the bearings.

• Oil is forced to flow from the reservoir to the bearings and to the control system by a main oil pump.

Controlling Combustion Gas TurbinesControlling Operations

Because many turbines run automatically, they often have very complex instrument control panels:

• Start-ups

• Normal Operations

• Shutdowns

Always operator responsibility to ensure safe operation of the machine.

Thermal efficiency and Temperature control

• All of the energy used in a turbine comes from burning fuel

• The compressor actually uses twice as much power as the driven equipment uses.  This means only one-third of the turbine power can be used to drive a piece of process equipment. 

Controlling Combustion Gas TurbinesRegenerators

• The air leaves the turbine at a temperature higher than the air entering the compressor.

• The turbine uses less than 25% of the available energy in the burning fuel.  More than 75% of the energy may be wasted in the exhaust stream

Open and Closed cycles

• When a gas turbine reuses its exhaust gases without letting them leave the equipment, it is using a closed cycle.

• When a gas turbine takes in air from the atmosphere and discharges its exhaust into the atmosphere, it is using an open cycle.

Operating Combustion Gas TurbineExtreme Heating Effects

• Metals tend to weaken when they are heated.  The hotter the metal becomes, the greater the tendency to weaken.

Metal Creep : Creep is the stretching and weakening of metal parts under stress

• Turbine blades creep continually while they are hot and spinning

Root Temperature:

• Because the main strain on the blades occurs at the roots, blades last longer if their root temperature is kept as low as possible

Operating Combustion Gas TurbineUneven Heating effects

• Uneven heating or cooling causes stress and distortion of metal parts

Start-up Temperature:

Shutdown Temperature:

• Air flowing from the combustion chamber enters the turbine blading at relatively high temperatures. At startup, the turbine is cool

• Turbine inlet temperature must not increase faster than 25º F (14º C) per minute of startup time.

• The turning gear rotates the shaft after the turbine has stopped so that all sides of the shaft cool evenly.

Operating Combustion Gas TurbineVibration

Amplitude

• Amplitude may increase if the shaft is out of balance.

• Amplitude is measured as the distance moved in mils.  1 mil equals 1/1000 of an inch (0.00254 cm)

Turbine parts vibrate during operation.  To reduce the wear on parts, this vibration must be kept as low as possible.  Vibration has two separate characteristics:

• Amplitude:  Amplitude tells how far the part moves.• Frequency:  Frequency tells how often the vibration occurs.

Frequency is not related directly to amplitude.  Increasing the frequency does not necessarily increase the amount of shaft or bearing movement

Operating Combustion Gas TurbineVibration

Frequency

• A longer, more flexible shaft has a lower natural frequency than a shorter shaft of the same diameter.

• The thicker, stiffer shaft has the higher natural frequency

Critical speed: At speeds different from the critical speed, the amplitude of vibration is small.  At the critical speed, amplitude becomes large

Alignment: The misaligned coupling is off-balance, so the amplitude of vibration is higher than normal.  The greater the (amount of) misalignment, the higher the amplitude of vibration

Thanks for coming.

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