GAS Turbine

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Gas Turbine Overview

Gas Turbine Purpose:

The purpose of a gas turbine engine is to create

energy to turn a shaft that drives other rotating

equipment such as compressors and generators.

Gas turbines commonly used to drive:

compressors for transporting gas through pipelines

generators that produce electrical power

Basic Categories for Gas Turbines

Single Shaft

The axial flow gas turbine is one of the most common gas turbine engine designs.

The figure shows the simplified airflow through an axial flow gas turbine.

As you can see from the red flow lines, the air flows in a straight or axial path from one end of the gas turbine to the other.

Single Shaft Gas Turbine

Split [Two] Shaft

Basic Categories for Gas Turbines

A separate turbine section drives the compressor.

Except for the airflow, the two rotor systems (compressor and turbine) operate independently.

Each compressor is driven at its own speed by its own set of turbine wheels, as shown in the figure.

Two-Shaft Axial Flow Gas Turbine

The Thermodynamic Laws

The laws of thermodynamics explain the effects of heat in an engine.

The first law states that energy can be changed but it cannot be destroyed.

In a gas turbine engine, heat energy is changed to mechanical energy.

The second law of thermodynamics states that heat cannot be transferred from a cooler body to a hotter body.

In a gas turbine engine, heat is transferred from the hotter engine to the cooler lube oil.


Newton's First Law of MotionNewton's first law explains why a force is needed to make the gas turbine work.

In the figure, a ball on a level table will not move until it is made to move by some force such as the wind or pushing it by hand.

Similarly, until the fuel and air mixture is burned in the gas turbine, there is no force for the turbine to use to turn the rotor shaft.


Newton's Second Law of MotionNewton's Second Law explains why the air must be compressed and accelerated to create a force.

In the figure, a hammer is used to drive a nail. The force of hitting the nail is proportional to the mass (weight) of the hammer multiplied by the velocity of the hammer when it hits the nail.

If you also use a heavier hammer, it is even easier to drive the nail into the wood.

Mass and acceleration directly affect the amount of force created. The more compressed air (mass) that enters the gas turbine, the more force created from the combustion process.


Newton's Third Law of MotionNewton's Third Law explains how the action of creating a thrust force results in the reaction of the turbine rotating.

In the figure, a boat is near the bank of a river. The person steps from the boat toward the land. As his body pushes forward, the boat is pushed backward with the same amount of force.

In the gas turbine, the thrust force is the action. This force is directed into the rear of the case and on to the turbine blades. The blades of the turbine react to the force and turn the rotor.


The Brayton Cycle defines what takes place in the gas turbine engine. These events are controlled by the physical laws described on the previous pages.

The events in the Brayton Cycle take place in specific sections of the gas turbine. These events are: Compression Combustion Expansion Exhaust

The Brayton Cycle is unique among engine cycles because all the events in the cycle take place at the same time without interruption.The exhaust event is the only part of the cycle that does not take place in the engine.

Brayton Cycle

Gas Turbine Components

A gas turbine engine is divided into five sections:

air inlet section compressor section . Diffuser combustion section turbine section exhaust section

Air Inlet System

The air inlet system controls the air quality and directs a steady flow of air to the compressor air inlet.

The amount and quality of air affects engine performance and reliability.

The air inlet system has two main assemblies:

filter assembly

inlet ducting assembly

Air Inlet System

The filter assembly consists of the following parts:

weather louvers

inlet screens

filters

Filter Assembly

The second main assembly in the air inlet system is the inlet ducting assembly which consists of the following parts:

bypass door (s)

inlet silencer

trash screens

air plenum

Inlet Ducting Assembly

The purpose of the compressor section is to compress air for cooling and combustion. The compressor draws in atmospheric air through the air inlet and increases its pressure while reducing its volume.

Compressor Section

In an axial flow compressor the air flows axially. This means that the air flows in a relatively straight path in line with the axis of the gas turbine.

Axial Flow Compressor

The Main Components :

case

rotor

stator


The Compressor Case:

The compressor case contains the rotor and the stator. The case is divided into halves. The upper half may be removed for inspection or maintenance of the rotor and stator blades while the bottom half remains in place.


The Compressor Rotor:

The rotor is the rotating element of the compressor. The rotor contains blades fixed on a spindle, drum, or wheel.

These blades push air to the rear in the same way a propeller does. The movement of air is caused by the angle and the shape of the blades.


Compressor Stator

Stator vanes are the non-moving elements of the compressor. They are located between each rotor stage. Stator blades are attached to the inner wall of the case.

Stator vanes receive high velocity air from each preceding rotor stage of the compressor.Stator vanes direct airflow to the next stage of compression at the desired angle. This controlled direction provides increased blade efficiency.


Operation

When air enters the compressor through the air inlet, incoming air passes through the first row of vanes, called inlet guide vanes.

As the air enters the first set of rotating blades, it is deflected in the direction of blade rotation.The air is then caught and turned as it passes through a set of stator vanes. From there, the air is picked up by another set of rotating blades.


Operation:As pressure is increased by successive rotor/stator blade sets, air volume is decreased.

At the compressor exit, the diffusion section finishes the compression process by decreasing air velocity and increasing pressure just before the air enters the combustion section.

A major effect of an unstable compression process is surging, which is discussed next.


Compressor Surge:Compressor surge is a characteristic common to all types of gas turbines.

In general, surge is the result of unstable airflow in the compressor.

This unstable condition is often caused by air building up in the rear stages of the compressor.

When a compressor is not operating at its optimum speed, the forward compressor blades may provide more air than the downstream stages can compress. The air then tends to reverse flow. The compressor surges.


Compressor Surge Protection:

Compressor is protected against surge by:

Inlet Guide Vanes

Air Bleed Valve


Compressor Surge

Surging causes the machine to vibrate excessively. Several methods are used to control surging. For example, the two-shaft gas turbine design reduces the possibility of surging.

Compressors with higher compression ratios have a greater tendency to surge. Compression ratios are discussed next.


Compression Ratio:

Large, high-powered gas turbines require greater efficiency and higher compression than can be obtained with a single axial flow compressor.

Single axial compressors usually have a compression ratio of approximately 8:1.Compression ratio is determined by the discharge pressure (psia) divided by the suction pressure (psia).


Purpose

The diffuser is located between the compressor section and the main components of the combustion section.

The purpose of the diffuser is to prepare the air for entry into the combustion section.

The front end of the diffuser is bolted to the compressor case, and the back end is attached to the combustion section.

Diffuser

The combustion section is located between the compressor and turbine sections.The purpose of the combustion section is to add heat energy to the flowing gases.

This addition of heat causes the gases to expand and accelerate into the turbine section.The hot gases that are generated by burning fuel in the combustion chambers are used to power the turbine and the load.

Combustion Section

The main component of the combustion section is the combustion chamber (burner).

A basic combustion chamber consists of the following:

outer case perforated inner liner fuel injectors source of ignition

The Combustion Chamber

The fuel system supplies clean, pressure-regulated fuel to the combustion chamber where it is mixed with the incoming compressed air from the diffuser.

During start-up, the fuel and air mixture is ignited by a spark plug (source of ignition).

After combustion occurs, the spark plug stops firing.

The burning gases supply the heat energy required to operate the turbine and load.


Coaxial ArrangementCombustion chambers are arranged coaxially (common axis) with the compressor and turbine to allow efficient flow-through operation.

The figure shows a typical combustion chamber arrangement.

Note the location of the spark plugs. Not all combustion chambers have spark plugs. Some are equipped with only crossfire tubes.


Compressed air enters the combustion chamber, fuel is injected, and the fuel/air mixture is ignited and burned.

The burning or combustion gases expand and travel toward a point of lower pressure at the rear of the chambers. Because high pressure compressed air surrounds the burner on all sides except the rear, the hot, expanding gases are directed toward the turbine section.

Combustion Process

To operate efficiently, a combustion chamber must provide:

a means for proper mixing of air and fuel a way to cool the hot combustion products to a temperature the turbine section components can tolerate.

Airflow

To accomplish these actions, airflow through the combustor is divided into two air paths:

primary

secondary

The primary air is approximately 25% of the total air that enters the chamber. Primary air is sent to the fuel nozzle area for combustion.

Airflow

Primary & Secondary Air

About half of the primary air flows axially through the front of the combustion liner in the area of the fuel nozzles.

The rest of the primary air enters radially through small holes in the front third of the combustion liner.

All primary airflow supports combustion.

Air Flow

Primary & Secondary Air:Some of this airflow also centers the flame and keeps it from contacting the combustion liner.

The other half of the secondary air enters the combustion liner toward the rear.

This part of the airflow dilutes the combustion gases to an acceptable temperature to improve the turbine components service life.

Air Flow

The turbine section is located between the combustion and the exhaust sections of the engine.

The purpose of the turbine section is to convert the energy of the expanding gases into mechanical energy to drive the compressor, the accessories, and the load.

Turbine Section

Turbine Section Components

Like the compressor section, the turbine section consists of two major components:

The stator or turbine nozzles

The rotor or turbine wheel

Turbine Section

Stators & RotorsThe stators and the rotors of the turbine and compressor sections are similar in their construction.

The primary difference is the angle at which the vanes, nozzles, and blades are positioned.

Their positioning is critical to efficient engine operation. Contd.

Turbine Section

Turbine NozzleThe turbine nozzle vanes have two purposes:

prepare the combustion gases for driving the turbine rotor

deflect the combustion gases in the direction of the turbine rotation

To accomplish this, the shape and position of the turbine nozzle vanes form passages for expanding gas flow. Contd.

Turbine Section

Turbine RotorThe energy of the gases leaving the first row of turbine nozzle vanes encounters the next major component of the turbine section, the rotor or turbine wheel.

The purpose of the turbine rotor is to extract mechanical energy to operate the compressor, accessories, and load.

The turbine rotor consists of the following:

shaft blades or buckets disk

Turbine Section

Turbine Rotor BladesThe blades of the turbine wheel are mounted to the disk in the same manner as compressor blades are mounted to the rotor, by either fir-tree, bulb, or dove-tail type roots.

This rotor and disk assembly is attached to the shaft.

Turbine Section

Nozzle Vane AssemblyEach set of stator vanes forms a nozzle vane assembly for the following turbine wheel.

Exit guide vanes straighten the gas flow as it enters the exhaust section.We have looked at the purpose, function, and design features of the two main components of the turbine section, the stator and the rotor.Our next focus is on the operation or flow of gases through the turbine section.

Turbine Section

Operation: Hot, expanding gases from each burner (or combustion chamber) flow through a transition duct to turbine nozzle vanes.

The nozzle vanes direct the expanding gases into the turbine section.

As you recall, the components of the compressor section convert energy by increasing the airflow pressure.

Turbine Section

In contrast, the components of the turbine section convert energy by reducing the pressure of the flowing gases.

Pressure is changed to velocity by the shape and position of both the turbine stator vanes and the rotor blades.

Operation

Turbine Section

Gas FlowThe shape of the vanes causes an increase in velocity as the gases pass between the vanes.

As the gases flow into the first set of vanes, the gases accelerate because the space between the vanes is converging (a funnel effect similar to the first half of the venturi tube).

As Bernoulli's principle states: an increase in velocity causes a decrease in pressure

Turbine Section

Gas Flow

The gases leaving the nozzle vanes reach their maximum velocity just before they hit the first-stage turbine, causing it to rotate.

The shape of the rotor blades also accelerates the gases.

At this point, the gases still have enough energy to do work.

The turbine blades redirect the hot gases into the second row of nozzle vanes.

Turbine Section

Gas Flow

The process continues through each stage of the turbine section.

Exit vanes reduce turbulence before the gases enter the exhaust section. This reduces backpressure on the turbine section.

Approximately two-thirds of the total energy available for work in a gas turbine is used to turn the compressor.

Turbine Section

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.

Multiple Stage Compressors

PurposeWhen the gases exit the turbine section they enter the last section of the gas turbine, the exhaust section.

The exhaust section is located directly behind the turbine section of the engine.

The purpose of the exhaust section is to discharge the spent gases to the atmosphere.

Exhaust Section

Components :

The exhaust section usually consists of the following components:

outer housing inner housing struts plenum

These components act as a diffuser, to reduce the turbulence and velocity of exhaust gases.

Exhaust Section

Components:

The inner housing may contain the gas turbine rear bearing assembly and over speed trip device.

The outer housing flange is used to connect the exhaust collector or plenum to the gas turbine exhaust section.

The diffusion process occurs in the exhaust section as the volume is increased.

Exhaust Section

Operation:

As the exhaust gases pass through the exhaust section components:

the velocity is decreased the pressure remains relatively constant the turbulence is reduced

The exhaust gases enter the atmosphere from the exhaust plenum.

Exhaust Section

Silencers are usually installed in both the air inlet system and the exhaust section of the gas turbine to reduce operating noise.

Silencing is accomplished by baffles covered with sound-absorbing material.

In some air inlet ducts, the interior walls of the ducting and air plenum chambers are also lined with this sound-absorbing material.

Silencers

Purpose:

Some of these accessories are the oil pumps, hydraulic pump, fuel pump, and starting means interface.

The primary purpose of the accessory drive is to provide a means to drive each accessory at the proper speed and to connect and disconnect the engine from its starting device.

The figure shows a typical G.E. accessory drive assembly.

Accessory Drives

G.E. Accessory DriveThe accessory drive gear is driven by a shaft that meshes with a helical gear driven by the main rotor shaft.

The gearbox is usually located at the front (forward) or the rear (aft) of the gas turbine engine, depending on the engine inlet or exhaust arrangements.

G.E. describes its typical accessory drive system as the main link between the gas turbine and the drive components of the starting system.

Accessory Drives

G.E. Accessory Drive: Function

The gear drives several accessory devices that support gas turbine operation.

Each drive pad is a point of potential oil leakage because of the shaft seal arrangement.

Engine oil from the lube oil pump or the hydraulic pump may leak into, or from, the accessory drive assembly through the drive shaft seal.

Accessory Drives

G.E. Accessory Drive: FunctionThe G.E. accessory gear also provides a mount for the turbine overspeed trip bolt mechanism.

The trip bolt mechanism is mounted on the exterior case of the accessory gear.

The actual overspeed trip bolt is mounted in the main or number one gear shaft. This is covered in more detail in a later lesson.

The Solar accessory drive is discussed next.

Accessory Drives

Solar Accessory Drive: Function (Contd.)On Solar gas turbines, the accessory drive is attached to the air inlet assembly.

The accessory housing contains the accessory drive gears, pinion gears, and the necessary shafts and bearings.

Mounting pads and gear drives are provided for the starter, lube oil pump, hydraulic oil pump, speed governor, seal oil pump, and other accessories.

Accessory Drives

Solar Accessory Drive: Function

If a particular accessory is not used, a cover plate is installed on the mounting pad.

During the starting cycle, the Solar accessory gear is driven by the starter assembly.

Accessory Drives

Solar Accessory Drive: Function

A starter disengaging jaw clutch and accessory drive adapter connect the starter to a spur gear and shaft.

During the start cycle, the gas turbine compressor is driven by the gear.

After the starter jaw clutch disconnects, the compressor shaft drives the gear.

Accessory Drives

Bearings

The main bearings of a gas turbine engine are mounted in a bearing housing.

Most bearing housings contain seals to prevent oil leakage into the gas path.

PurposeBearings have several purposes. They:

support engine parts

minimize friction

minimize wear

allow freedom of movement

carry loads

Bearings

The primary loads that act on main bearings are:

weight of the rotating mass (compressor, turbine, etc.)

axial forces of power or load change

compression and tension loads between stationary parts and rotating parts caused by thermal expansion and misalignment

vibration

Bearings

Plain Bearings:

There are several types of bearings used in gas turbines:

plain or sleeve

ball and roller

Plain bearings are the simplest type of bearing.

Plain bearings are used in minor load locations, such as engine accessories.

Bearings

Plain Bearing AssemblyA plain bearing assembly consists of the:

bearing support or bracket

bearing housing or container

plain or sleeve bearing

During engine operation, pressurized lube oil is injected into the bearing through oil passages.

Bearings

This oil forms a film between the bearing and the surface that is being supported to prevent metal-to-metal contact.

The rotating part moves on a film of lube oil instead of on the surface of the bearing.

Bearings

Ball & Roller BearingsBall bearings and roller bearings are called antifriction bearings because the balls and rollers minimize friction. Ball and roller bearings are commonly used because they:

offer little resistance to rotation provide precise alignment of rotating parts are relatively inexpensive can withstand momentary overloads are easy to lubricate work with both radial and axial loads can endure elevated temperatures

Bearings

Ball Bearings

The main disadvantages of ball and roller bearings are that they:

are easily damaged by foreign matter fail with very little warning

A ball bearing consists of the following components:

an inner and an outer race a set of polished steel balls a ball retainer

Bearings

Roller Bearings Roller bearings also have an inner and an outer race, but use rollers rather than balls.

Roller bearings are made in different shapes and sizes for both radial and thrust loads.

Straight roller bearings primarily support radial loads. Tapered roller bearings support both radial and thrust loads.

Bearings

In roller bearings, the roller is located between an inner and an outer race.

When a roller is tapered, it rolls on an angled outer race.

The inner races of ball and roller bearings are closely fitted to the rotor shafts to prevent movement of the shaft. Bearings designed to resist thrust in one direction have a heavier race on the side that supports the thrust.

Bearings

Types: Hydrodynamic Bearings

Hydrodynamic bearings use an oil wedge for support and to reduce friction.

There are two types of hydrodynamic bearings:

radial oil-wedge

thrust oil-wedge

Bearings

Hydrodynamic Bearings: Radial Oil-Wedge

A radial oil-wedge bearing resembles a plain bearing except the bearing or bushing is divided into several sections, or pads. Each pad is able to tilt or lean.

When the shaft rotates in the bearing, the pads tilt to allow wedges of oil to form between the pad and the shaft.

Bearings

Oil wedges support the shaft as it rotates and cannot be squeezed out of the bearing housing when a heavy load is imposed.

The axial movement of a gas turbine rotor shaft is controlled by thrust bearings.

Bearings

A typical thrust oil-wedge bearing consists of:

a bearing housing thrust shoes a thrust collar attached to the rotating shaftThe shaft is held in position by oil pressure acting against the thrust collar.

If the shaft moves, the thrust collar loading increases to prevent further movement.Hydrodynamic Bearings: Radial Oil-Wedge

Bearings

The thrust oil-wedge bearing operates on the same principle as a radial oil-wedge bearing.

In a thrust oil-wedge bearing, the thrust shoes are positioned against leveling plates.

As the thrust shoes pivot during gas turbine operation, oil wedges form between the thrust collar and the shoes.

The oil wedges limit axial thrust of the rotor shaft.

Bearings

Bearing assemblies must be well supported and strong enough to support the loads imposed by the rotating rotor.

Lube oil is delivered to the bearings to provide support.

Information about seals is presented next.Hydrodynamic Bearings: Radial Oil-Wedge

Bearings

Gas Turbine Performance

Atmospheric factors affect the performance of gas turbines. Some of these factors are: air density contaminants

Temperature and water content affect the density of the air. Because cold air is more dense than hot air, it has more mass. The more air in the gas turbine, the more force created.

Gas Turbine Performance

Purpose:The purpose of a gas turbine engine starting system is to provide power to:

rotate the turbine shaft to starting speed

assist the turbine to self-sustaining speed after combustion occurs

Most gas turbine engines are started by starter power input to the main accessory gearbox. The gearbox is connected to the turbine rotor and compressor.

Any gas under pressure may be used as a power source for a pneumatic starting system.

Pneumatic Starting System

Starting System: ComponentsNatural gas must meet the manufacturer's specifications. A typical pneumatic starting system requires approximately 2600 scfm.

The main components of a Solar Centaur starting system are as follows: gas inlet strainer pilot gas filter solenoid-operated pilot valve starter motor shutoff valve lubricator starter motors


Gas Inlet StrainerThe gas inlet strainer is located at the gas turbine skid upstream of the shutoff valve.

A shutoff valve is located downstream of the strainer to shut off the gas supply to the starter motor.

The strainer is a Y-shaped fitting that houses a removable cylindrical "strainer" screen.

A pilot supply line branches off from the main gas supply to the starting system. This line provides pneumatic pressure to the solenoid-activated pilot valve.


Pilot Gas Filter

A pilot gas filter assembly, consisting of a 10-micron filter and a pressure-reducing orifice, is installed in the pilot supply line upstream of the pilot valve.

The filter prevents foreign matter from entering the solenoid-operated pilot valve.

The pressure-reducing orifice creates a pressure drop in the pilot supply line. This pressure drop ensures that the pilot valve will operate properly in the event of excessive gas pressure.


Starting System: Pilot ValveThe pilot valve is a three-way, solenoid-operated valve that is powered by 24-volt DC from the electrical system and actuated by the control system.

The pilot valve opens and closes the starter motor shutoff valve.

When the solenoid is de-energized, pilot gas pressure closes the starter motor shutoff valve.

When the solenoid is energized, pilot gas pressure is vented, allowing the shutoff valve to open.

A pilot relief valve protects the filter and pilot valve from excessive pressure.


Starter Shutoff Valve:

The pilot-actuated starter motor shutoff valve controls gas flow from the supply line to the two starter motors.

The starter motor shutoff valve is installed upstream of the lubricator.


Lubricator

A lubricator, located downstream of the starter motor shutoff valve, injects lubricating oil into the gas flow.

The purpose of the lubricator is to provide atomized lubricating oil to the starter motor vanes.

A sight dome is used to check oil flow. In addition, the lubricator bowl has an oil level sight glass. (Contd.)


Starter MotorThe figure shows a starter motor and the clutch, called a sprag or sprag clutch assembly. The clutch in this installation is in the housing that is mounted on the engine gearbox drive.

The pawls , driving or holding links of a ratchet that permit motion in one direction only, are forced inward by small springs to engage the sprag clutch ratchet. At a preset engine speed, the pawls are thrown


Purpose

The purpose of a fuel system is to supply an exact amount of clean fuel to the engine under all operating conditions.

The amount of fuel is based on turbine speed and load requirements.

The fuel pressure required for a gas turbine is primarily a function of the compression ratio of the compressor section.

For example, the lower the compression ratio, the lower the fuel pressure requirement; the higher the compression ratio, the higher the fuel pressure requirement.

Gas Turbine Fuel Systems

Gas Turbine Fuel Systems: Purpose

Almost any combustible fluid, either gaseous or liquid, can be used for turbine fuel.

Some gas turbines operate on both liquid fuel and fuel gas.

Both fuel gas and liquid fuel must be clean for efficient turbine operations. However, fuel requirement specifications differ among manufacturers.

The following information on fuel gas and liquid fuel requirements is provided to illustrate the differences in specifications.


Fuel Gas RequirementsThe figure lists typical fuel gas requirements for gas turbine engines. These requirements are:

lower heating value supply pressure gas temperature fuel qualityThe following page shows specifications for liquid fuel.


Liquid Fuel Requirements

The figure lists typical liquid fuel requirements for gas turbine engines. These requirements are:

fuel temperature fuel viscosity pour point fuel qualityNOTE: Always check the turbine manufacturer's fuel specifications to ensure that the fuel meets the specifications for the gas turbine you are operating.

The main components of a fuel gas system and liquid fuel system are given next. We will begin with the fuel gas system.


Fuel Gas System: ComponentsThe main components of a typical gas turbine fuel gas system are as follows:

fuel shutoff valve (SOV) vent valve pressure control valve (PCV) pressure indicator controller (PIC) pressure safety valve(s) (PSV) instruments and alarms filter separators control system fuel gas heater (optional depending on gas dew point)


Liquid Fuel System: Components

The main components of a typical liquid fuel system are: manifold nozzles pumps filters pressure switches fuel control valves solenoid-operating valves control system

Contd.


Lubrication & Lube Oil

The purpose of a gas turbine lubricating oil

system is to provide clean and cool oil to

engine parts that are subject to friction.

Lube oil:

reduces friction

cushions

cools

cleans

seals


The primary purpose of any lubricant is to reduce friction between moving parts.

A lubricating oil system provides oil films as surface coatings on moving parts.

The oil films slide against each other to prevent metal-to-metal contact.


Reducing FrictionWhen the oil film is unbroken, friction in the engine is fluid friction instead of metal-to metal friction.For example, oil pressure will actually lift the journal of a shaft off the bearing on which it is resting.

As the shaft rotates, a layer of oil prevents the journal from physically touching the bearing.


Lube oil acts as a cushion between moving parts. The oil:

prevents metal-to-metal contact

absorbs shock, for example shock imposed on gear teeth as they mesh


Cooling & CleaningOne of the laws of thermodynamics states that heat is transferred from a hot substance to a cooler substance.

Lube oil cools the internal parts of an engine by absorbing heat. The oil carries this heat away from the engine.

The heat is removed from the oil when the oil goes through the oil cooler. Oil also cleans internal engine parts.


Seal FormationLube oil is also used to form seals.

Mechanical seals are installed in an engine between the moving and nonmoving parts.

A very small space exists between the two parts of a seal. This space is sometimes filled with lube oil.A thin film of oil between sealing surfaces makes a mechanical seal more leak resistant.To perform these functions lube oil must meet certain requirements.


Service Procedures:

Wear proper protective clothing and gloves because all lubricating oils contain additives that are irritating to the skin, toxic, or both.

The last part of this lesson focuses on the operation of a lube oil system as a complete system.


Service Procedures:

When servicing the lube oil system, the following procedures should be followed:

Maintain cleanliness.

Do not allow foreign matter to enter the system.

Use a 10-micron or smaller filter when servicing with bulk oil.


Service Procedures:

.If a hand pump is used to service lube oil, use that pump for one specific lube oil only.

Do not mix incompatible lubricants. This can result in improper lubrication of the engine.

Record the amount of oil serviced.


Operation:

The lube oil supply is stored in the lube oil reservoir.

Main, auxiliary, and pre/post lube oil pumps draw oil from the lube oil reservoir under pressure, to the lube oil system.

The temperature control valve regulates the oil temperature.The oil then flows from the pump to the lube oil filter.

The oil filter removes contaminant particles that are suspended in the oil.


Operation:

After passing through the filter, the oil flows to the oil feed manifold.

An oil pressure gauge, temperature indicator, alarm switch, and shutdown switch monitor oil temperature and pressure in the oil supply manifold.

From the oil supply manifold, lube oil is distributed to the turbine rotor bearings, the hydraulic pump, the reduction gear bearings, and the generator bearings.


Operation:

After lubricating the bearings and gears, the lube oil is returned to the lube oil reservoir.

Lube oil system operation will vary from manufacturer to manufacturer, but the components are basically the same.


Components:A typical gas turbine lube oil system consists of the following components:

lube oil reservoir oil pumps oil filters oil coolers control devices instruments and alarms

ContentsEnd Show


Oil Reservoir:

The purpose of the lube oil reservoir is to contain an ample supply of lubricant for the gas turbine, accessory drive systems, gearbox, and driven equipment.

The lube oil reservoir also provides the oil for starting, control, positioning inlet guide vanes, and trip oil circuits.


Reservoir Component FunctionsLube oil temperature is usually measured in the reservoir. Proper lube oil temperature is necessary for most gas turbines.

Low oil pressure, low oil level, and high oil temperature will initiate shutdowns of most gas turbine engines.Reservoirs may have both a level sight glass and a level indicator to indicate oil level.A sealed float device operates level transmitters (LT), indicators (LI), and switches.The switches activate high (LSH) and low (LSL) level alarms and shutdowns.


Reservoir Component FunctionsThe lube oil reservoir is vented to the atmosphere to maintain an even pressure in the tank.

A flame arrestor is often installed in the vent to prevent a source of ignition from entering the reservoir.

The purpose of the pressure regulator (PCV) is to control the lube oil system pressure by returning excess lube oil to the reservoir.

System protection is provided by a pressure relief valve (PSV) located at the discharge of each pump.


Lube Oil Pumps:

The purpose of lube oil pumps is to provide lube oil under pressure for lubrication of the engine and associated equipment.

The lube oil pumps take suction from the oil reservoir and discharge into a common header.


Lube oil system pumps are classified as:

main

auxiliary

emergency

All three types use oil from the lube oil reservoir.


Lube Oil Pumps Types:The three most common types of lube oil system main pumps are the following:

vane gerotor gear

These pumps are positive displacement pumps because they send a fixed quantity or constant volume of oil to the pump outlet during each revolution.


Sliding Vane PumpPumping action in the sliding vane lube oil pump takes place as the rotor drive shaft and the eccentric rotor drive the sliding vanes.

The space between each pair of sliding vanes fills with oil as the oil passes the oil inlet port.

This oil is carried to the outlet port as the rotors turn.When the spaces between the vanes, the eccentric rotor, and the inner walls of the pump case reduce to minimum clearance, the oil is forced out of the pump.


Gerotor Pump Components

The figure shows the pumping element of a gerotor pump.

The gerotor pump operates on a principle similar to that of the vane pump.

The gerotor pump uses a lobe shaped drive gear inside an elliptically shaped idler gear to move oil from an intake to a discharge port.The right side of the figure shows a complete pumping element.Several elements can be mounted on a single shaft inside the pump case.


Gerotor Pump Operation

Gerotor pump operation is shown in the lower portion of the figure.

From 0 to 180 of pump rotation, the space between the lobes and the openings increases from a minimum to a maximum volume.

As the space reaches maximum volume, it is closed to the intake port and is in position to open to the discharge port.


Gerotor Pump Operation:

At the 270 point of rotation, the space decreases in volume, forcing oil out the discharge port. At 360 again, the space reaches minimum volume. The space is closed to discharge and begins to open to the intake port, repeating the cycle as rotation continues.This action takes place in each of the seven interlobal spaces between the inner six-lobe gerotor and the outer seven-lobe gerotor.


Gerotor Pump Operation

The inner drive gear in the figure has six lobes (teeth).

The outer idler gear has seven openings.

This extra lobe allows oil to fill the one open space as it passes the intake port.

The oil moves through the pump as the pump rotates, until a minimum clearance forces the oil out through the discharge port.

The most commonly used lube oil pump is the gear pump shown in the figure on the following page.


Gear Pump Components & OperationA gear pump usually consists of two close-fitting gears, the drive and idler gears, that rotate in a pump case.

The pump case provides minimum space between the gear teeth and the inner walls of the case.

The gear lube oil pump is usually engine driven by the accessory gears.


When the pump is rotating, the gears take in oil.

The gears rotate in a direction that forces the oil to move between the gear teeth and the pump inner case until the oil is delivered to the pump outlet port.

The idler gear seals the pump inlet from the outlet and prevents the oil from reversing flow.


Gear Pump Components & OperationA gear pump usually consists of two close-fitting gears, the drive and idler gears, that rotate in a pump case.

The pump case provides minimum space between the gear teeth and the inner walls of the case.

The gear lube oil pump is usually engine driven by the accessory gears.

When the pump is rotating, the gears take in oil.


The gears rotate in a direction that forces the oil to move between the gear teeth and the pump inner case until the oil is delivered to the pump outlet port.

The idler gear seals the pump inlet from the outlet and prevents the oil from reversing flow.


Auxiliary Pump: Purpose & Function

The purpose of an auxiliary lube oil pump is to supply lube oil:

during gas turbine startup

during gas turbine shutdown or cool down

anytime the main lube oil pump cannot supply lubricating oil


When gas turbine engine start-up begins, oil is delivered to the lube oil system by the AC-powered auxiliary lube oil pump.

The auxiliary oil pump operates for a preset time before the starter engages.

The auxiliary lube oil pump operates until the gas turbine engine accelerates to approximately 90% of its normal operating speed.


Auxiliary Pump: Purpose & FunctionDuring gas turbine engine shutdown, the auxiliary lube oil pump is again started by the control system when engine speed slows to approximately 80% speed.

The auxiliary oil pump continues to operate throughout the shutdown and cool-down cycles.

The figure shows a typical vertical, centrifugal pump.


Unlike the pumps discussed up to this point, centrifugal pumps are not positive displacement pumps.

This type of pump uses an impeller to move the oil through the system.

A vertical, centrifugal pump is sometimes used as an auxiliary and an emergency lube oil pump.


Emergency Pump: Purpose & FunctionThe purpose of an emergency lube oil pump is to supply lube oil during an emergency shutdown if the auxiliary lube oil pump is inoperative or is unable to maintain sufficient lube oil pressure.

The emergency lube oil pump is similar to the auxiliary oil pump. The main difference is that the auxiliary lube oil pump is operated by an AC motor and the emergency lube oil pump is operated by a DC motor.

The next topic discusses how these pumps function within the operation of a typical lube oil system.


Lube Oil Pumps: OperationThe figure shows a basic gas turbine lube oil system.

The main lubricating oil pump takes suction from the lube oil tank and supplies oil under pressure to the temperature control valve, filter and supply manifold.

After lubricating engine parts, the lube oil is returned to the oil reservoir. Contd.


An auxiliary oil pump, or pre/postlube oil pump, supplies lube oil under pressure during the engine start and shutdown cycles.

After lube oil pressure has been established, the engine begins to rotate.

When the engine reaches sufficient speed for the main (engine driven) oil pump to provide adequate lube oil pressure, the pre/postlube oil pump is automatically shut down.


Lube Oil Pumps: OperationThis pump draws oil from the reservoir and sends it to the lube oil filter and oil supply manifold for distribution to the lubrication points.

Oil pressure is regulated by the relief valve (PSV) during pre/postlube auxiliary oil pump operation.

At engine shutdown, the postlube oil pump is activated by the control system.The postlube oil pump operates for a preset time after shutdown to provide postlubrication and to cool the engine.


Lube Oil Filters: Purpose

The purpose of lube oil filters is to remove particles that collect in the oil.

These particles can lodge in the close spaces between bearings and seals.

Contaminants in lube oil will increase the friction between moving parts, resulting in excessive wear and bearing failure.


Lube Oil System: Contaminant SourcesContaminants in lube oil systems are primarily from the following sources:

small particles of carbon from the breakdown of oil metallic particles from engine wear and corrosion airborne contaminants entering through bearing seals dirt and other foreign matter introduced into the reservoir during servicing


Lube Oil Filters: TypesThe most common types of lube oil system oil filters are the disposable filter and the cleanable screen filter (wire mesh).

Disposable filters are smaller than cleanable screen (wire mesh) filters.

Disposable filters are capable of filtering particles as small as 5 microns.

Disposable filters are heavily pleated. ( Contd.)


Lube Oil Filters: Types of AssembliesTwo types of lube oil filter assemblies are used in gas turbine lube oil systems: simplex duplex

The simplex has only one filter case. The duplex has two filter cases.

When a simplex oil filter is used, the engine must be shut down to replace the filter elements.


Lube Oil Filters: Simplex Assembly

A simplex lube oil filter assembly usually consists of the following:

filter case plumbing differential pressure gauge differential pressure alarm switch

The filter case is shaped like a cylinder and contains replaceable filter elements.

Each filter element is a pleated paper cartridge designed to filter particles that are larger than 5 microns.


Lube Oil Filters: Duplex Filter AssemblyMany gas turbine engines have a duplex lube oil filter assembly instead of a simplex oil filter.

The operation of the lube oil system is the same except that the engine does not need to be shut down to replace a dirty filter element. The oil flow can be diverted to the clean filter case.

The duplex oil filter has two filter cases. Each case contains replaceable filter elements.


Lube Oil Coolers: Purpose & TypesThe purpose of the lube oil system oil coolers is to maintain a specified lube oil temperature.

The specified temperature must be maintained under differing oil heat loads that take place with differing operating conditions.

One of the laws of thermodynamics states that heat can only be transferred from a hot surface to a colder surface.Oil coolers are heat exchangers. The two most common types of oil coolers are:

oil-to-water oil-to-air


Lube Oil Coolers: Oil-to-Water CoolerThe oil-to-water cooler uses water to cool the oil. The oil-to-air cooler uses air to cool the oil. The oil-to-water cooler is discussed first.

The figure shows an oil-to-water cooler.

The heat exchanger transfers heat from the lube oil to the water and keeps the oil at the proper temperature.Lube oil coolers require minimal operating checks and maintenance.

They should be inspected for oil and water leaks during each routine and off-line maintenance inspection.


Lube Oil Coolers: FlowHigh bearing header lube oil temperature may indicate that the oil cooler tubes are fouled and in need of cleaning.

Lube oil coolers can be damaged by thermal shock, overpressure, and hydraulic hammer.

Thermal shock is prevented by starting the flow of cooling water through the oil cooler before the hot lube oil flow is started.

If the lube oil system has been down for maintenance, the system must be gradually filled with fluids as air is vented from the system. Pulsations of fluids through oil coolers can cause vibrations that may damage the cooler and shorten its operating life.


Lube Oil Coolers: Oil-to-Air Cooler

The oil-to-air cooler and thermostatic valve arrangement is similar to the oil-to-water cooler. The primary difference is that the actual heat exchanger resembles the radiator cooler used for water cooled reciprocating engines.

An oil-to-air lube oil cooler keeps lube oil temperature within operating limits by using an oil-to-air heat exchanger with an electrically operated fan.


Lube Oil Coolers: OperationThe figure shows a diagram of an oil-to-air cooler subsystem.

Lube oil under pressure from the oil pump either bypasses the oil coolers or enters one of the coolers through the transfer valve.

If the oil temperature is less than approximately 60F, the temperature control valve will open port B to port A and the oil will bypass the oil coolers.

Contd.


Solar Instruments & Alarms

The figure shows a typical Solar lube oil schematic.

Information about Solar lube oil system instruments and alarms begins on the next page with the level indicator.


To prevent leakage of oil from a narrow flow path, bearing assemblies usually contain oil seals.

The purpose of oil seals is to prevent oil from leaking from the bearing housing.

There are two types of oil seals, they are labyrinth or carbon. Both types are often used in the same seal assembly.

Seals

Types: Labyrinth SealThe labyrinth seal is a controlled-leakage device that allows minimal leakage across the seal.

It also controls compressed air from the compressor section and hot gases from the turbine section that leak along the shaft.Air from the gas path outside the bearing housing bleeds inward through the grooves in the labyrinth seal.A labyrinth seal assembly consists of grooves in the seal and, in some cases, teeth in the shaft. Also provided is a vent and a drain for removal of gas and liquids.

Seals

Types: Labyrinth Seal in Bearing Housing

Seal dams formed by the teeth and grooves in the labyrinth seal allow a metered amount of air from the engine gas path to flow inward.

The figure shows a typical compressor rear bearing housing arrangement.

Lube oil enters at the top of the bearing housing and drains out the bottom. ( Contd.)

Seals

Labyrinth Seal in Bearing Housing:Labyrinth seals are installed in both ends of the bearing housing with bleed air pressure against the outer surfaces of the seals.

The air flows between the teeth and grooves of the seals into the bearing housing. This prevents lube oil from leaking through the seals.

Seals

Types: Labyrinth Air-oil SealThis figure shows a labyrinth air-oil seal arrangement with a dual labyrinth seal located on each side of the bearings.

The bearing housing is contained in a cavity. The space between the bearing housing and the walls of the cavity is pressurized with bleed air from the engine compression section.This type of arrangement takes advantage of controlled bleed air across the seals. The bearing housing is vented to the atmosphere.

Seals

Types: Labyrinth Air-Oil Seal

The controlled leakage of air into the bearing housing prevents oil leakage.

Pressurized air that leaks outward along the shaft prevents gases from leaking into the bearing housing.

This type of air-oil seal prevents the introduction of oil into the gas path.

Oil leaking into high velocity combustion gases will damage turbine parts.

Seals

Types: Carbon SealsCarbon seals are a blend of carbon and graphite.

Carbon seals perform the same function as labyrinth seals.

The carbon seal rides on a surface while the labyrinth seal has an air space.

Carbon seals are usually spring loaded and sometimes pressurized with air.This causes a preload pressure on the carbon segment and provides a tighter seal.

Seals

Types: Carbon SealsCarbon seals are used for greater control of the airflow entering the bearing housing.

Carbon surfaces are usually stationary.

A highly polished mating surface, called the seal land, is attached to the main rotor shaft.

In some engines, a full contact seal is required to hold back oil that tends to puddle before it drains from the bearing housing.

Seals

Purpose:

The purpose of a hydraulic system is to distribute fluid forces to various moving parts.

This fluid is required for the operation of gas turbine electrohydraulic control system components, the fuel system, variable inlet guide vane mechanisms, and the hydraulic components of some starting systems.

Hydraulic Oil System

Solar: Subsystem of Lube Oil SystemSolar gas turbines have two separate hydraulic oil systems:

subsystem of the lube oil system separate servo oil system

In the subsystem of the lube oil system, oil is taken from the oil supply manifold by lube oil pump pressure. It is then routed to the variable vane control valve and to the actuator to move the vanes to the maximum open position.The Solar gas turbine servo oil system is an electrohydraulic system that operates as a separate, closed-circuit hydraulic oil system.

Information about this system is presented next.


Solar Servo Oil System

The servo oil system shares the lube oil reservoir with the lube oil system.

The purpose of the servo oil system is to operate the main fuel valve electrohydraulic servo actuator.

The actuator is operated by servo oil pressure and is controlled by the engine control system. This system is closely related to the lube oil system shown in the figure.

Contd.


The servo oil system consists of the following:

pump element

servo oil filter

servo relief valve

During engine operation, servo oil pressure is provided by the servo pump element that works with the two-element, engine-driven lube oil pump.


Solar Servo Oil SystemOil is drawn from the lube oil reservoir through the oil line common to all three pump elements.

High-pressure servo oil flows from the servo pump outlet port through the servo oil filter to the inlet port of the servo actuator. A servo relief valve is located downstream of the servo oil pump.

The servo oil filter is between the servo oil pump and the servo actuator.

(Contd. )


The oil filter removes particles from the servo oil that are 25 microns or larger.

The purpose of the relief valve is to protect the pump and other components in the servo oil system against excessive oil pressure.

Information about a G.E. hydraulic oil system is presented next.


Purpose & ComponentsThe purpose of the ignition system is to supply a spark to ignite the fuel/air mixture in the combustion chambers.

A typical ignition system consists of the following components:

igniter plugs or spark plugs transformers or ignition exciter ignition leads

Ignition System

Solar Ignition System Components (Contd.)Solar gas turbines use a torch ignition system consisting of:

ignition exciter shielded cable spark plug lead spark plug

The ignition exciter is mounted in a box on the gas turbine base.

The exciter is connected to the spark plug by a cable or lead.

Ignition System

Solar Ignition System Components

The exciter is operated by 24 volt DC power. This power is changed to AC and then is stepped up, which charges a storage capacitor.

When the capacitor charge reaches its discharge value, the capacitor discharges through the spark plug cable to the spark plug.

Exciter output is approximately 18,000 volts. Up to fifteen sparks per second are produced as long as the exciter is energized.

Ignition System

Components: Igniter TorchThe spark plug is installed in the igniter torch.

The igniter torch extends through the combustor outer liner.A small, controlled amount of gas is sent to the tip of the spark plug electrode.The spark jumps across the spark plug electrode's air gap and ignites the gas, creating a torch flame.This torch flame flares into the combustion liner to provide positive lightoff of the fuel/air mixture in the combustor.

Ignition System

Ignition System Maintenance Procedures (Contd.)

In ignition systems, the term high intensity means that the electrical charge can be lethal.

Because the electrical charge can be lethal, ignition systems require special maintenance and handling according to the manufacturer's instructions.

Ignition System

Typical maintenance procedures include the following:

Ensure that system power is locked out before performing any maintenance on the system.

To remove the igniter plug, disconnect the transformer input lead, wait the time specified by the manufacturer (usually 1 to 5 minutes), then disconnect the igniter lead and ground the center electrode to the engine to discharge the capacitor.

Ignition System

Ignition System Maintenance Procedures (Contd.) Use caution when handling damaged transformer units that are hermetically sealed. Some transformers contain radioactive material.

Before performing a firing test of igniters, ensure that the combustor is free of fuel. A fire or explosion could result.

Ignition System

Ignition System Maintenance Procedures

Do not energize the system for troubleshooting when the igniter plugs are removed. Transformer damage may occur.

Discard all igniter plugs that have been dropped. Internal damage can occur that is not detectable by inspection or testing.

Use a new gasket when the plug is reinstalled. The gasket provides a good current path to ground.

Ignition System

Purpose (Contd.)

The purpose of the vibration monitoring system is to help in preventing abnormal operating conditions.

The rotating shafts of any machine or gearbox have a tendency to move axially or radially as a result of speed, loads, worn internal parts, unbalance, or other reasons.

Vibration Monitoring System

Purpose

Axial and radial shaft movement is called vibration. Vibration is a continuing periodic change in a displacement from a fixed reference.

Excessive vibration is an abnormal operating condition that can result in equipment damage. Excessive vibration is a symptom of other abnormal conditions.

A bent shaft or improper shaft alignment could be the source of vibration.


Vibration MonitoringSeveral protective systems are used to alert operators to abnormal operating conditions that could result in damage to the turbine or other equipment.

Vibration is one of the critical operating parameters that is monitored by a protection system.

A vibration monitoring system is usually a part of the gas turbine's programmable logic control and operator terminal.

The figure shows typical vibration detector locations in relation to the rotor.


Shaft Movement

Vibration monitoring systems are installed on gas turbines and driven equipment to monitor and sometimes record axial and radial shaft movement.

Shaft movement is monitored in either displacement (mils), velocity (length/unit-time), or acceleration (g's).

(Contd.)


One mil equals 0.001 of an inch. A shaft movement of 5 mils could generate an electrical impulse of one volt. Either of these measurements may be used as set points to initiate an alarm or shutdown.


Vibration Monitoring Probes (Contd.)In the gas turbine, vibration probes are installed in the bearing housings near the shaft.

The probe tips operate on 24-volt DC power to establish a magnetic field between the probe tip and a burnished area on the shaft.

As the distance between the probe tip and the shaft changes, the strength of the magnetic field changes.


The probe senses fluctuations in the magnetic field, and the monitoring systems uses this information.

The figure illustrates a typical single and double radial probe installation in a bearing.


Vibration Monitoring Probes: FunctionIn the figure, four probes monitor the radial movement of a gas turbine shaft and two probes monitor the shaft axial location.

Axial position probes 1 and 2 monitor shaft axial movement in two places at the thrust collar.

Probes 3Y and 4X measure radial movement at the low pressure end of the compressor. (Contd.)


Probes 5Y and 6X measure radial movement at the high pressure end of the compressor.

The probes are placed 90 degrees apart to monitor relatively both horizontal and vertical radial movement.

One probe monitors the X axis, and the other monitors the Y axis.


Vibration Monitoring Probes:Function

Axial movement is usually monitored by two probes, as shown in the figure.

One probe is mounted at the end of the shaft, and the other is mounted at the thrust collar.

During operation, any of the vibration monitors can usually be read on the PLC display. (Contd.)


At some preset vibration limit (3 mils, for example), the vibration monitoring system will initiate an alarm.

If vibration increases to the high limit (5 mils, for example), another alarm is initiated and the vibration monitoring system will initiate a TRIP signal to shut the unit down before damage occurs.


Purpose:

The purpose of an enclosure is to protect the gas turbine/generator set from environmental elements, to improve appearances, and to reduce noise, to meet local area classifications, and to provide an easier means for fire protection and containment.The enclosure discussed in this lesson is designed for outdoor installation and high wind loads.The enclosure is divided into compartments by bulkheads (walls). Each compartment contains lighting, access doors, and, when needed, removable panels for inspection and maintenance.

Enclosure

Purpose:A ventilation system is provided when a gas turbine/generator set is enclosed.The purpose of an enclosure ventilation system is to minimize temperatures in the turbine and generator compartments.

Enclosure ventilation systems include: air inlet airflow exhaust

Information about the air inlet system is presented first.

Ventilation System

Air InletThe purpose of an enclosure air inlet system is to: take in air for ventilation of the enclosure treat the quality of inlet air to make it suitable for turbine use

The main components of an enclosure air inlet system are: inlet screens weather louvers filters ducting

Ventilation System

Airflow Components

The purpose of an enclosure airflow system is to:

minimize temperature in the enclosure compartments minimize hazards in the event of a fuel system failure

The main components of an enclosure airflow system are as follows:

dampers fans

The purpose of dampers is to control the airflow in the enclosure. Air inlet and outlet dampers are normally open. They are closed by gravity to provide an airtight enclosure when the fire protection system is activated.

Ventilation System

Exhaust Components

The purpose of an enclosure exhaust system is to return the heated air to the atmosphere.

The main components of an enclosure exhaust subsystem are as follows:

fans dampers screens louvers

Some enclosures have a cooling air fan installed in the exhaust or outlet duct. The outlet dampers are the fire dampers. One or more screens is installed in the outlet ducting to prevent entry of foreign matter.

Ventilation System

Purpose:The purpose of a fire/rate-of-rise detection system is to detect a fire or serious heat conditions in the gas turbine/generator set enclosure.

An enclosure fire/rate-of-rise detection system must have the following characteristics:

reliable detectors in the correct locations means to test the system effective maintenance and testing procedures

Fire Detection System

Requirements:Fire/rate-of-rise detection systems should meet the following requirements:

initiate an immediate alarm on fire or excessive conditions

provide an indication that a fire has been extinguished and another indication if the fire re-ignites ( Contd. )


Requirements:

be durable and able to resist environmental damage

incorporate an accurate and effective testing system to ensure system integrity

operate without special electrical equipment and require a minimum of power


Fire Detection System: Types

The following types of fire/rate-of-rise detection systems are discussed in this lesson:

thermal switches thermocouples pneumatic circuits optical detectors gas detectors


Thermal Switches

Thermal switches are either:

single wire two wire

The single-wire thermal switch fire/rate-of-rise detection system has heat-sensitive thermal switches located at points in the enclosure where temperatures are likely to be highest.

The switches contain a pair of contact points that are normally open. The contact points close at a preset temperature.

When the switch heats up, the heat-sensitive arms with the contact points expand. The expansion is in the direction opposite the electrical terminal.


Thermal Switches

The sliding piston moves to the end of the switch, the points close, and the thermal switch completes an electric circuit for the alarm switch.

When the circuit is complete, the alarm switch initiates an alarm signal. The switch automatically resets when it has cooled.


Thermal Switch: Single Wire Circuit

The figure shows a single-wire circuit in which 24V DC is applied to both paths of the circuit. If an overheat temperature or a fire occurs which closes one of the five thermal switches, a path to ground is completed through the circuit.

With this type of loop arrangement, an open circuit can occur and the system will still provide protection at each of the five surveillance points.

The test circuit tests the entire loop and will indicate an open circuit in the power input lead of the loop. A short circuit in the loop will cause a false fire warning indication.


Two-Wire Thermal Switch

The two-wire thermal switch fire/rate-of-rise detection system remains functional with either an open or a short circuit.

The two-wire bi-metallic thermal switch operates the same way that the single-wire thermal switch does. The only difference is that an electrical lead is connected to both arms of the thermal switch.


Thermocouples

Thermocouples are temperature sensing devices, primarily used in temperature indicating systems, such as exhaust gas temperature or turbine outlet temperature.

(Contd.)


A thermocouple is the junction of two dissimilar metals that generate a small electrical current that varies with the difference between the temperature of the hot junction and the cold junction.

The dissimilar metals can be any combination of metals or alloys that will produce the required results such as iron-constantan (Type J) or Chromel-Alumel (Type K).

The complete thermocouple circuit consists of a "cold" junction, a "hot" junction, electrical leads made from the same material as the thermocouple, and a galvanometer-type indicating instrument


Pneumatic Fire Detection System

The pneumatic fire detection system uses a gas-filled tube as a detector. The detector is produced in various sensor tube lengths and alarm temperatures.

The gas expands when heated. When setpoint temperature is reached, the gas pressure is sufficient to overcome the check valve and gas flows from the detector tube to the right side of the diaphragm.

This flow forces the diaphragm contacts to the left onto the alarm contacts, which energizes the alarm circuit.

The gas returns to a low pressure after the heat source is removed.

The check valve arrangement and diaphragm force the gas back into the tube, ready for another operation.


Optical Fire Detection SystemThe detection systems discussed so far detect temperature increases. The optical system detects changes in the light spectrum inside of the enclosure.

Optical flame detectors use infrared and ultraviolet detectors to receive direct or reflected rays from a source of flame or heat. ( Contd.)


When a fire or hot object is detected in a compartment, the detector signals the controller, which powers the fire alarm. A fire signal is generated only when a fire is detected by both sensors.

Three optical fire detectors are shown in the figure, two in the turbine compartment and one in the generator compartment.


Optical Fire Detection System

When two optical detectors are installed in one compartment, they are usually cross-zoned or use a voting system.

Both detectors must agree that a fire condition exists before the fire monitoring system will initiate action


Operation:

When fire is detected by any of the sensors, the fire control system performs the following functions:

The alarmed zone is indicated.

All ventilation is shut down.

An audible alarm is initiated, specifying the zone. (Contd.)


External audible and visual alarms are initiated.

An emergency shutdown signal is relayed to the gas turbine/generator control system.

Generally, but not always, after a time delay to allow personnel to evacuate the enclosure (usually 10 seconds), the fire extinguisher agent is discharged.


Fire Detection System: Operation

The alarms will continue until they are manually switched off.

The fire extinguishing agent must be recharged and the detection/protection system reset before the gas turbine can be restarted.


Purpose & Components:

The purpose of a gas detection system is to detect the presence of combustible gas in a gas turbine/generator set enclosure.

The main components of a gas detection system are as follows:

sensors detection circuit protection systemSensors are discussed first.

Gas Detection System

SensorsTwo or more sensors are installed in the gas turbine compartment and one or more in the generator compartment near the divider wall where the drive shaft or coupling penetrates the wall.

Two sensing elements are used in a sensor.

One element is calibrated to detect a low combustible gas concentration of 15% or 20% L.E.L. L.E.L. is the lower explosive limit.

The other element is calibrated to detect a high combustible gas concentration of 60% L.E.L.


Sensors

The sensors detect the combustible gas concentration with air at the lower explosive limits (L.E.L.).

A 5% methane concentration with air is the lowest concentration that can be ignited. A 15% methane concentration is the highest concentration that can be ignited. Lower or higher concentrations cannot be ignited.


Detection CircuitMost gas detection systems are calibrated to initiate an alarm when the gas concentration in the enclosure reaches 15% to 20% L.E.L.

This is the low L.E.L. alarm.

For a low L.E.L. alarm, the combustible gas detection system usually initiates both an audible and a visual alarm.

Enclosure ventilation fans that are not already running are started.

The fans continue to operate until the gas detection/protection system is reset and the alarm indication is cleared by the operator.


Protection System:A high L.E.L. alarm is initiated at 60% L.E.L. A 60% L.E.L. is a 3% combustible gas concentration. On some gas turbine/generator sets, a 60% L.E.L. alarm initiates the following actions:

immediate gas turbine/generator shutdown, if operating

immediate shutdown of all operating enclosure ventilation fans


Protection System:

immediate closing of all ventilation/isolation dampers

immediate activation of the audible warning device inside and outside the enclosure

After a short time delay, usually 10 seconds to allow personnel to exit the enclosure, fire extinguishing agent discharges in both the gas turbine and generator compartments.


Purpose:The purpose of a fire extinguishing system is to discharge concentrations of fire extinguishing agents into the enclosure compartments.

The two systems discussed in this lesson use either Halon or CO2 as the extinguishing agent.

A Halon fire extinguisher system is discussed first.

NOTE: Halon is being phased out worldwide. New systems will use CO2.

Extinguisher Systems

Halon Extinguisher System

Halon is a colorless, odorless, non-corrosive, and electrically non-conductive gas. After discharge, it leaves no residue and does not require clean-up.

Halon is a chemical compound that inhibits combustion by reacting with oxygen in the air, so that oxygen is suppressed and no longer able to support combustion.

The figure represents a typical halon extinguisher system.


Halon Extinguisher System: ComponentsThe main components of a Halon fire extinguisher system are as follows:

cylinders valve assembly pressure switches discharge pipes and nozzles heat (thermal) and fire (optical) detectors audible alarm (horn) monitor controller


CylindersHalon is stored in cylinders, on or near the gas turbine/generator package. The number of Halon cylinders in a system depends on the volume of the protected area.

A minimum of two cylinders is required. One cylinder supplies the initial discharge, and the other provides the extended discharge. Any signal which initiates Halon discharge also trips the turbine. (Contd. )


When Halon is discharged into a compartment, the initial discharge is at a rapid rate The reason for a rapid rate during initial discharge is to build an extinguishing concentration as quickly as possible.

This is followed by an extended discharge at a slower rate to maintain the extinguishing concentration and minimize the possibility of re-ignition of combustibles exposed to hot metal surfaces.


CylindersHalon cylinders are manufactured in different sizes from 18 to 600 pounds. The cylinders are purged with nitrogen and then filled with Halon.

Halon cylinders are held in the upright position by mounting brackets.

The cylinders should be located in an area that does not receive direct sunlight because Halon is extremely temperature sensitive. Halon cylinders should not be exposed to temperatures above 130F.


Cylinder Valve Assembly

A valve assembly is an integral part of each Halon cylinder. The valve is made of brass, which makes it corrosion resistant.

The valve usually contains a differential piston. Differential piston operation allows the valve to discharge Halon when actuated manually, electrically, or pneumatically.


Cylinder Valve Assembly OperationHalon pressure from the cylinder enters the valve assembly and rises through a bleed port that contains a ball check valve.

Halon pressure acts on both sides of the piston with equal pressure in areas A and B in the figure. This pressure keeps the Halon in the cylinder because the piston seals off the discharge port. ( Contd.)


When the vent valve is actuated, there is a large pressure difference between the top (B) and bottom (A) areas of the valve. When the ball check valve closes the bleed port, Halon cannot flow to the top of the piston and the pressure in areas A and B are no longer equal


Cylinder Valve Assembly OperationThe valve is actuated by venting pressure from the top of the piston through the vent valve.

When the pressure is vented, there is no downward force acting on the piston. The piston moves upward, the discharge port is opened, and Halon is released into the system.

The release of Halon is initiated by valve actuators. The valve actuators are described next.


Discharge Pipes & Nozzles

Discharge pipes and nozzles are used to discharge extinguishing agents.

Discharge pipes carry the fire extinguishing agent from the storage cylinders to the discharge nozzles.

The nozzles discharge the agent into the protected zones. The nozzles are placed to ensure a concentrated discharge in all parts of the protected zone.


The last component discussed in the Halon fire extinguishing system is the fire monitor controller. This controller works in conjunction with other gas turbine control systems. The controller uses a microprocessor system to monitor signals from sensors installed in the enclosure.

An example of a monitor controller is shown in the figure. The controller illustrated is typical of a monitor controller for an optical fire detection system with two independent types of detectors. A signal is sent to the controller only when both detectors sense a fire simultaneously.

Fire Monitor Controller

Monitor ControllerThe controller monitors up to eight zones. One LED (light emitting diode) is provided for each zone. An LED blinks when a fire is present in the corresponding zone.

One LED is also provided for each voting zone. A lit LED signals that the voting zone is actuated.

The Test/Accept push-button disables an energized alarm or initiates a manual test of a selected detector.

The inhibit LED is illuminated in the Test or Reset mode to indicate that the controller outputs are inhibited.


Monitor ControllerWhen the lamp test switch is depressed, all LEDs and digital displays illuminate. In the reset mode, the lamp test switch indicates a complete system reset.

The mode switch is used to select Normal, Reset, or Test mode. The power LED illuminates when power is applied to the fire detection system.

The fault LED illuminates on fire detection system malfunction or when the controller is in the Reset or Test mode.

The system status display provides a numerical code to report the system status.


Description:A CO2 fire extinguisher system has the same basic components as the Halon system. The CO2 components are shown in the figure. A CO2 system also operates in the same manner as a Halon system.

CO2 (carbon dioxide) fire protection systems extinguish fires by reducing the oxygen content of the air in the compartment.

( Contd. )

CO2 Extinguisher System

The oxygen content is reduced from an atmospheric normal of 21% to less than 15%, an amount that will not support combustion.

To reduce the oxygen content, a quantity of CO2 equal to or greater than 34% of the compartment volume is discharged into the compartment in 1 minute.


CO2 Extinguisher System: DescriptionA CO2 fire protection system supplies CO2 from either low pressure storage tanks or high pressure cylinders to a distribution system.

This system transfers the CO2 to discharge nozzles located in the various compartments of the gas turbine package.

Low pressure storage tanks maintain saturated liquid CO2 at 300 psig and 0F with a refrigeration system. ( Contd.)


Actuator ArrangementA CO2 system may be actuated by several different methods.

Pilot-operated selector valves in the CO2 discharge valve are opened when solenoid valves are energized, when the pull lever is actuated, or when another cylinder is discharged.

The solenoid valves are actuated by an electric signal from the fire detectors. The system may also be manually actuated by switches located on the electrical control cabinet or by manual valves located in the control cabinet.

Actuation of the system, either automatically or manually, will trip the turbine.


Discharge Systems: InitialTwo separate discharge systems are used in a CO2 system:

initial discharge extended discharge

Within a few seconds of actuation, sufficient CO2 flows from the initial discharge system into the compartment to rapidly build up the CO2 concentration. ( Contd.)


This concentration is maintained by the gradual addition of more CO2 from the extended discharge system, compensating for compartment leakage.

CO2 flow rate is controlled by the orifices in the discharge nozzles.


Discharge Systems: ExtendedThe orifices for the initial discharge system permit a rapid discharge of CO2 to quickly build up an extinguishing concentration.

Orifices for the extended discharge system are smaller for a relatively slow discharge rate.

By maintaining the extinguishing concentration, the likelihood of a fire reigniting is minimized. ( Contd.)


In addition to their use of similar components, both the Halon and CO2 extinguisher systems operate the same.

The discussion about extinguisher systems continues with a description of the operation of a typical fire extinguisher system.


Extinguisher Systems: Operation

The figure shows the components found in a typical fire extinguisher system. In this system the first line of defense against fire is provided by the optical detectors.

Optical detectors usually have a response time of less than 5 seconds and an 80 degree field of vision. ( Contd.)


When more than one optical flame detector is installed in an enclosure, a voting system is usually programmed into the fire monitor controller.

With a voting system, an alarm signal from one optical detector will initiate an alarm but no other action.

An alarm signal from one optical detector and a thermal detector or from two optical detectors is needed to initiate turbine shutdown and extinguisher release.


Extinguisher Systems: Operation

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