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UNITS IN THIS COURSE
UNIT 1 DESALINATION
UNIT 2 WATER TREATMENT
UNIT 3 STEAM GENERATION BOILERS
UNIT 4 NITROGEN GENERATION
UNIT 5 HYDROGEN GENERATION
TABLE OF CONTENTS
Para Page
3.0 OBJECTIVES 3
3.1 WHY STEAM GENERATION IS NEEDED 6
3.2 THE BASIC FUNCTION OF A STEAM GENERATING BOILER 6
3.3 THE BASIC STEAM GENERATING CYCLE 7
3.4 BOILER CLASSIFICATION 8
3.4.1 The Fire Tube Boiler 8
3.4.2 The Water Tube Boiler 83.5. EXTERNAL COMPONENTS OF A STEAM GENERATING BOILER 9
3.5.1 FOUNDATIONS AND SUPPORTS 9
3.5.2 BOILER CASINGS OR LAGGINGS 12
3.5.3 INSULATION 15
3.5.4 WINDBOX 16
3.6. INTERNAL COMPONENTS OF A STEAM GENERATING BOILERS. 18
3.6.1 WATERSIDE INTERNAL COMPONENTS 18
3.6.1.1 Steam Drum 18
3.6.1.2 Mud Drum 19
3.6.1.3 Downcomer Tubes 20
3.6.1.4 Riser Tubes 20
3.6.1.5 Super Heater Tubes 21
3.6.1.6 Economisers 22
3.6.1.7 Blowdown Lines 22
3.6.1.8 Steam Separators 24
3.6.1.9 Headers 26
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3.6.2 FURNACE INTERNAL COMPONENTS 27
3.6.2.1 The Combustion Chamber 27
3.6.2.2 The Burners 27
3.6.2.3 The Furnace Baffles 29
3.6.2.4 Refractories 30
TABLE OF CONTENTS (CONT)
Para Page
3.7. BOILER FITTINGS AND SAFETY DEVICES 31
3.7.1 BOILER SAFETY VALVES 32
3.7.2 BOILER WATER COLUMN AND SIGHT GLASS 36
3.7.3 STEAM PRESSURE GAUGE 38
3.7.4 BOILER SAFETY DEVICES 39
3.7.4.1 Low Water Fuel Cut Off 39
3.7.4.2 Non Return Valve 40
3.7.4.3 Flame Failure 42
3.7.4.4 Low Fuel Gas Supply Pressure Cut-off 43
3.7.4.5 Low Air Pressure Cut-off 43
3.8 BOILER AUXILIARY EQUIPMENT 44
3.8.1 MECHANICAL DRAFT SYSTEMS 44
3.8.1.1 Forced Draft System 44
3.8.1.2 Induced Draft System 46
3.8.1.3 Balanced Draft System 47
3.8.2 BOILER FEED WATER PUMP 48
3.8.3 BOILER FEEDWATER DE-AERATORS 49
3.8.4 STEAM CONDENSATE RETURN SYSTEM 50
3.8.5 COMBUSTION AIR PRE-HEATER 50
3.8.6 BURNER FUEL SYSTEMS 50
3.8.6.1 Operation 51
3.8.7 BLOWDOWN TANK 52
3.9 COMBUSTION OF BOILER FUELS 54
3.9.1 CHEMISTRY OF COMBUSTION 54
3.9.2 REQUIREMENTS FOR COMPLETE COMBUSTION OF FUEL
GAS 57
3.9.3 CORRECT FLAME APPEARANCE 573.9.4 FURNACE AIR REQUIREMENTS 58
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3.9.4.1 Perfect Combustion 58
3.9.4.2 Complete Combustion
58
3.9.4.3 Incomplete Combustion
58
3.9.4.4 Types of Furnace Air
58
TABLE OF CONTENTS (CONT)
Para Page
3.9.5 COMBUSTION CHAMBER
59
3.9.6 CONTROL SYSTEM FOR FUEL GAS COMBUSTION
59
3.9.7 FUEL GAS ANALYSIS
61
3.9.8 FLUE GAS ANALYSIS (COMBUSTION PRODUCTS ANALYSIS)
623.9.9 FLUE GAS DEW POINT TEMPERATURE
62
3.10. STEAM GENERATION CYCLE
62
3.11. BOILER SIMULATOR - PRACTICAL EXERCISE
65
3.11.1 SIMULATOR CONTROL PANEL
65
3.11.2 BOILER SIMULATOR START-UP PROCEDURE
66
3.11.3 BOILER SIMULATOR SHUT-DOWN
68
3.0 OBJECTIVES
As a plant operator, you should have a basic understanding of how your
company operate their steam generation equipment, (commonly known asboilers).
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This unit will explain the following things:
Why steam generation is needed.
The basic function of a steam generating boiler,
How a basic steam generating cycle works.
The external components of a steam generating,
The internal components of a steam generating boiler.
Boiler fittings and safety devices.
Boiler auxiliary equipment.
Combustion
Steam generation cycle.
You will also operate a boiler simulator as a practical exercise
3.1 WHY STEAM GENERATION IS NEEDED
A reliable supply of steam is very important.
Steam is required for refinery process units.
If there is not enough steam or if the temperature, pressure and quality of thesteam are not correct, the process units will not work correctly.
The steam may be used to turn turbines to generate electricity before it goesto the process units.
You have already seen that steam is needed for desalination.
Without steam most processing operations in a refinery would stop.
The same applies to gas processing plants.
3.2 BASIC FUNCTION OF A STEAM GENERATING BOILER
The basic function is to produce steam.
A few years ago the steam was generated at the main steam generatingplant.
You have already learnt that to produce heat costs money.
You have also learnt that we do not like to waste heat.
Wasted heat is wasted money.
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We now also generate steam at the process units.
We use the heat from sources that used to be wasted such as flue gases.
This is called waste heat recovery.
At this point it is good for you to learn certain properties of steam.
Steam can carry a large amount of heat energy.
Steam can carry a large amount of pressure energy.
A large volume of steam can be produced from a small volume of water.
1600 volumes of steam for one volume of water.
3.3 THE BASIC STEAM GENERATING CYCLE
A steam generating boiler is a closed vessel containing water.
When the vessel is heated the water turns to steam. The steam has a largervolume than the water.
It is contained in a closed vessel so it cannot increase in volume.
Therefore, there is an increase in pressure.
The heat energy from the burnt fuel has been changed to steam energy.
Steam energy is a combination of heat energy and pressure energy.
In simple terms, the steam cycle is as follows:
The fuel contains chemical energy.
This chemical energy is changed to heat energy by burning the fuel.
This heat energy is transferred to the water in the boiler. The heat energy
is changed to steam energy, (heat and pressure).
The steam gives up the heat energy and pressure energy in doing the
work.
When it has lost its energy the steam condenses back to water,
(condensate)
The condensate is then pumped back to the boiler from the plant.
The condensate is heated by passing through a feed-water heat
exchanger.
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The heated feed-water is then passed through the boiler feed pump.
This feed pump raises the pressure of the heated feed-water to just
above boiler pressure.
The feed-water then enters the boiler to start the cycle over again.
We will go into more detail on the steam cycle later.
3.4 BOILER CLASSIFICATION
High pressure boilers are of two main types.
Fire tube boilers.
Water tube boilers.
3.4.1. The Fire Tube Boiler
The fire tube boiler consists of a shell containing straight tubes. Hot gasesfrom the fire pass through these tubes. The water in the shell surrounds thetubes. Heat energy from the hot gases passes through the tube walls to thewater.
3.4.2 The Water Tube Boiler
The water tube boiler consists of an arrangement of drums and headersinterconnected by tubes.
Water circulates inside the tubes, to and from the drums and header. The hotgases from the fire pass around the outside of the tubes. The heat energyfrom the hot gases passes through the tube walls to the water inside thetubes.
There are three basic designs used for water tube boilers. These depend onthe relative positions of the drums and tubes. See Figure 3.1.
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Figure 3.1 Watertube Boiler Designs
Figure 3.2 Outside of "D" Type Boiler
3.5. EXTERNAL COMPONENTS OF A STEAM GENERATING BOILER
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3.5.1 FOUNDATIONS AND SUPPORTS
Every boiler must be supported on a good foundation. A bad foundationwould permit "settling" under the weight of the boiler. The foundation mustprevent any movement of the boiler. If the boiler moved it would put stress onconnecting pipework. It could also mean that the water level in the boiler isincorrect.
The floor that the boiler stands on has to be very strong and completely flat.
Smaller boilers are packaged. Packaged boilers are mounted on a steel base.
The steel base is grouted (set in cement) onto the boiler room floor.
Figure 3.3 shows the steel base for a boiler being made in a -..workshop.
Figure 3.3 Boiler Steel Base being Fabricated
Figure 3.4 shows a small packaged boiler ready for work in a boiler room.
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Figure 3.4 Small Package Boiler Installed
Boilers can be "bottom-supported" or "top-supported.
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Figure 3.5 Large Bottom-Supported Boiler
All boilers must be able to expand freely as they get hot.
The movement due to thermal expansion can be several inches.
One end of the lower drum is free to slide on rollers.
The top drum moves upwards as the supporting tubes expand.
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Figure 3.6 A Bottom-Supported Boiler
In figure 3.6 the boiler is bottom-supported. The bottom drum and furnacewalls are resting on concrete supports.
3.5.2 BOILER CASINGS OR LAGGINGS
The pressure inside the boiler furnace (where the fuel burns) can be above orbelow the pressure outside the furnace.
This pressure difference can cause leaks into or out of the boiler furnace.
The casings around the boiler must be air tight so they do not leak.
Leaks could stop the furnace working well, and they could be dangerous toplant personnel.
Modern water tube boilers have an inner wall made of steel tubes. They alsohave a welded steel casing.
There is then a layer of insulation or lagging.
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Then an outer casing made of steel. (See Figure 3.7).
Figure 3.7 Boiler Casings and Insulation
Figure 3.8 Small Boiler Showing Casings
There are many different types of casing.
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The newer boilers use the water tubes as the inner casing. The water tubesare welded together.
The inner casing is then called a "waterwall" or a "water-cooled furnace wall .
This type of design is very efficient.
Figure 3.9 Water Cooled Furnace Wall
Figure 3.10 Top View of a Water Cooled Furnace Wall
3.5.3 INSULATION
Insulation has to resist high temperatures without changing or being
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damaged.
It also has to stop heat passing through it.
Insulation which is directly exposed to the furnace heat is usually made ofspecial materials called refractories. [This will be explained later in thecourse].
Glass fibre is also used where it will not be directly exposed to the furnaceheat. [For example behind a water wall.)
There can be more than one layer of insulation.
The insulation has to be flexible. It must withstand the thermal expansion andcontraction of the metal parts without breaking.
Block insulation is normally next to the hottest part of the boiler.
The blocks can slip over each other. The blocks stop nearly all of the heatfrom escaping.
The next layer of insulation feels very little change in temperature.
This layer can be continuous with no gaps. (See Figure 3.11).
Figure 3.11 Multi-layer Insulation
3.5.4 WINDBOX
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The windbox is a case (box) around the burners on the outside of the boiler.
Air for combustion is supplied through the windbox.
Air is directed to the windbox through ducting.
Ducting is a type of pipework made of sheet metal. A fan drives combustionair through the ducting to the windbox.
You will learn more about the burners and the air systems later in this course.
Figures 3.12 and 3.13 show examples of windboxes.
Figure 3.12 Air ducting and Windbox
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Figure 3.13 Windbox Surrounding Burners
3.6. INTERNAL COMPONENTS OF A STEAM GENERATING BOILERS.
A boiler has two separate sections
The water side
The furnace
The water side is the section which the water and steam pass through.
The furnace section is where the fuel burns and where the hot flue gases goout of the boiler. We will first look at the components on the water side.
3.6.1 WATERSIDE INTERNAL COMPONENTS
The main components are:
Steam drum
Mud drum
Downcomer tubes
Riser tubes
Superheater tubes
Economisers
Blowdown lines (two types);
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- Continuous blowdown lines
- Intermittent blowdown lines
Steam separators
Headers
3.6.1.1 Steam Drum
The steam drum is a vessel in the top part of the boiler.
The primary function of the steam drum is to separate the steam from thewater.
Water must NEVER leave the boiler with the steam.
3.6.1.2 Mud Drum
The mud drum is a vessel located at the bottom of the boiler.
The primary function of the mud drum is to collect solid matter that may bepresent in the water.
It is called the mud drum because it is where sludge (thin mud) and otherimpurities settle out of the water.
Sludge can cause scale formation inside the tubes.
Sludge can also cause water circulation problems.
The sludge and other impurities which collect in the mud drum are removedby a process called mud drum blowdown.
This is carried out from time to time by the operator.
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Figure 3.14 A cutaway view of a steam generator showing the tubes passingto and from the steam drum and the mud drum.
3.6.1.3 Downcomer Tubes
These tubes connect the steam drum to the mud drum.
The downcomer tubes are at the back of the boiler where it is cooler.
The downcomer tubes carry cooler water down from the steam drum to themud drum.
3.6.1.4 Riser Tubes
These tubes connect the mud drum to the steam drum.
The riser tubes are located, in the hot furnace area of the boiler.
The riser tubes carry the hot water and steam mixture upwards from the muddrum to the steam drum.
The movement of water up and down through these tubes gives goodcirculation of the water in the boiler. (See Figure 3.15).
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Figure 3.15 Water Circulation in a Boiler
3.6.1.5 Super Heater Tubes
The super heater tubes are located in the hottest part of the furnace.
Steam flows through the super heater tubes.
The temperature of the steam in the tubes increases.
Any water with the steam is heated and changed to steam.
We then have very high temperature "super heated" steam.
The super heater tubes can be positioned in two places in the furnace.
At the top of the furnace (like a roof)
In the path of the hot combustion gases.
The super heater tubes at the top of the furnace are called "Radiant SuperHeater Tubes".
The tubes are called that because the steam in the tubes is heated byradiation from the burner flames in the furnace.
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Radiant heat is like the heat we get from the sun.
The super heater tubes in the path of the hot combustion gases are called"Convection Super Heater Tubes".
Convection Super Heater Tubes are heated by convection from the hotcombustion gases that pass around the tubes.
Most modern boilers in the process plants have a combination of both typesof tubes.
Such a combination makes better use of the heat produced in the furnace.
This is more efficient and gives a more constant temperature for the superheated steam.
Super heated steam has more heat energy because of its higher temperature.
Therefore, super heated steam can do more work.
Super heated steam is "dry" so it is good for use in steam turbines, because itwill not cause erosion of the turbine blades.
3.6.1.6 Economisers
Recall that we do not like to waste heat energy.
An economiser uses heat from the waste combustion gases to heat the watergoing in to the boiler.
The combustion gases (flue gases) leaving the furnace contain a lot of heatenergy.
Figure 3.16 Economiser in Flue Gas Outlet.
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We use this heat energy to raise the temperature of the boiler feedwaterbefore it enters the boiler.
This is waste heat recovery.
If the feedwater is hot when it enters the boiler then you need less heat to turnthe feedwater to steam.
If we need less heat we burn less fuel.
If we burn less fuel it costs less to operate the boiler.
3.6.1.7 Blowdown Lines
As we mentioned earlier, there are two main types of blowdown lines.
Continuous blowdown lines.
Intermittent blowdown lines.
Both types of line remove any solids in the boiler water.
Recall that the boiler feed water is treated to remove dissolved solids and tocontrol pH levels.
However, some dissolved solids still remain in the boiler feed water.
The concentration of solids left in the water increases as part of the water isturned into steam.
This is because the solids stay behind in the water which does not turn intosteam.
If there are too many dissolved solids in the water, the water foams.
If the water foams then small droplets of water may be carried up with thesteam.
These small droplets of water still contain dissolved solids.
These solids stay in the super heater tubes when the water turns to steam.
The solids cause harmful deposits inside the tubes.
We can stop this happening by preventing the dissolved solids from buildingup in the boiler feedwater.
This is down by removing or "blowing down" some of the boiler water. Thislimits the build up of dissolved solids and controls the T.D.S (Total Dissolved
Solids).
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How this is done will be explained in detail later in this course.
The continuous blowdown line comes from the steam drum.
Continuous blowdown is controlled automatically.
The blowdown line inlet takes water from the mid level of the water in thesteam drum. (See Figure 3.17).
The intermittent blowdown line comes from the bottom of the mud drum.
The intermittent blowdown line is used to remove sludge and solid matterfrom the mud drum during start-ups or shut-downs. This is done at lowerpressures.
It is used to drain the boiler for cleaning and inspection operations. [This isdone when the boiler is out of service].
Intermittent blowdown is done manually by the operator in charge of theboiler.
Intermittent blowdown will be explained in detail later in this course.
3.6.1.8 Steam Separators
The function of the steam separators is to remove as much moisture (water)from the steam as possible.
There are many steam separators inside the steam drum. (See Figure 3.17).
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Figure 3.17 Cutaway of a Steam Drum Showing Internal Equipment.
The steam and water mixture which comes up the riser tubes is directed intothe steam separators by baffles. Baffles are plates which change the directionof a fluid flow.
Steam separators work by changing the direction of flow of the steam.
The steam separators in Figure 3.17 are cyclone separators.
A cyclone spins very fast. Cyclone steam separators spin the steam very fast.
The steam is light so it changes direction easily.
The water droplets in the steam are heavy so they do not change directioneasily.
Therefore, the water droplets collect on the walls of the cyclone separator.
The water droplets run down the walls of the separator back into the water inthe bottom of the steam drum.
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There are many cyclone separators in the steam drum.
They are arranged along each side of the steam drum.
Most modern steam separators are the cyclone separator type. This type isused a lot on the large capacity boilers.
After the steam leaves the cyclone separators it passes through two sets ofscrubbers.
The scrubbers are mist extractors. They remove the last tiny droplets of waterfrom the steam.
The primary (first) scrubbers are directly on top of the cyclone separators.
The secondary (second) scrubbers are just upstream of the steam outlets atthe top of the steam drum.
You must have cyclone separators and scrubbers on boilers which havesuper heaters.
3.6.1.9 Headers
Headers are similar to drums. (See Figure 3.18).
Figure 3.18 Schematic Drawing Showing Headers and Drums in a Boiler [Only a
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few tubes are shown for clarity].
Headers act as distributing drums. They distribute the flow of the steam andwater.
Headers are only connected to the boiler tubes.
They are not connected to any of the drums.
Headers make sure that the boiler tubes get equal amounts of water, steam,or a mixture of steam and water.
They distribute the flows evenly between the tubes which are connected tothem.
3.6.2 FURNACE INTERNAL COMPONENTS
The main components of the boiler furnace are:
- The combustion chamber
- The burners
- The baffles
- Refractories
3.6.2.1 The Combustion Chamber
The combustion chamber is another name for the furnace.
This is the large space inside the boiler where the fuel is burnt to release heatenergy.
The combustion chamber roof.
The walls, floor and roof of the boiler are called the boiler enclosure or the
boiler setting.
The walls, floor and roof of the furnace are made of water tubes to make itwork more efficiently.
Therefore, the combustion chamber is completely enclosed by water tubes.
These water tubes take all the heat from the burners.
The water tubes also cool the furnace walls.
They do this by absorbing the radiant heat from the burners. This producessteam.
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In this way, very little heat is lost through the furnace walls to the outside.
3.6.2.2 The Burners
The burners are devices that control how the fuel is burned inside thecombustion chamber.
The burner mixes the fuel with the correct amount of air and directs the flameinto the combustion- chamber.
All large boilers have several burners.
The burners are normally set in one wall.
The burners in the boilers in the process plants use natural gas or fuel gas asthe main fuel.
Diesel oil may be used as a back-up fuel. [Gas plants do not need anyback-up fuel].
Diesel oil is a good back-up because it does not need complicatedpretreatment.
The burners are designed to burn both types of fuel.
They are called "combination burners". (See Figure 3.19).
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Figure 3.19 Combination Burner
You will learn more about burners later in this course.
3.6.2.3 The Furnace Baffles
In the older type of boilers there are baffles in the combustion chamber.
Recall that baffles change the direction of fluid flow.
The baffles in the furnace chamber direct the hot combustion gases aroundthe boiler tubes.
Because the baffles change the flow, the combustion gases take longer to
pass through the boiler.
Therefore there is more time for the hot combustion gases to transfer theirheat energy to the water and steam in the boiler tubes.
This makes the boiler more efficient. (See Figure 3.20).
Figure 3.20 Furnace Baffles, Showing Flue Gas Flow
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3.6.2.4 Refractories
Refractories are special clays known as fire clays.
Fire clays consist of mainly silica and alumina.
Fire clays can withstand (resist) high temperatures without fusing. (Fusingmeans to melt and change to glass.)
The refractories are used to line the openings of the burners and to line theopenings of inspection ports.
3.7. BOILER FITTINGS AND SAFETY DEVICES
Fittings on the boiler include valves, gauges, and other components requiredfor safe and efficient operation.
Every fitting has a definite purpose.
A fitting makes the boiler safer or more efficient.
The materials used for the fittings depend on the temperature and pressurethat the fittings must withstand.
All fittings are placed on the boiler so that they can be easily seen by the
operator.
They are also easily accessible to the operator.
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Figure 3.21 Boiler Fittings and Safety Devices
All the fittings on a boiler are important but the safety valves are the. mostimportant.
37.1 BOILER SAFETY VALVES
Steam generating boilers are designed to work at certain pressures. This iscalled "the working pressure".
They are also designed to withstand a maximum pressure.
This is called "The Maximum Allowable Working Pressure" [MAWPI
The working pressure is lower than the MAWP.
If the pressure in the boiler is higher than the MAWP, the boiler could fail.
Therefore all boilers have safety valves to prevent the pressure getting toohigh.
THE FUNCTION OF A SAFETY VALVE IS TO PREVENT THE STEAMGENERATING BOILER FROM EXCEEDING ITS MAWP.
Safety valves are located on the highest part of the steam drum.
They are also located on the top of the superheater outlet header.
Recall that steam contains large amounts of pressure energy.
Recall that steam also contains large amounts of heat energy.
Recall that the boiling point of a liquid goes down as the pressure on the surfaceof that liquid goes down.
IF A STEAM DRUM BROKE (RUPTURED) THERE WOULD BE A SUDDEN
DECREASE IN PRESSURE IN THE DRUM.
THE DECREASE IN PRESSURE WOULD LOWER THE BOILING POINT OFTHE WATER IN THE STEAM DRUM.
THE FURNACE WOULD STILL BE WORKING SO THERE WOULD BE NODECREASE IN TEMPERATURE IN THE DRUM.
THE WATER IN THE STEAM DRUM WOULD IMMEDIATELY FLASH TOSTEAM.
THIS WOULD CAUSE A VERY VIOLENT RELEASE OF UNCONTROLLEDENERGY.
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RESULT WOULD BE THE SAME AS A BOMB EXPLODING.
THIS IS WHY SAFETY VALVES ARE THE MOST IMPORTANT FITTING ONSTEAM GENERATING BOILERS.
The safety valves must be large enough to discharge all the steam that theboiler can generate. (This is called the venting or discharge capacity of thevalves).
Design engineers make sure the safety valves are big enough for the job.
The safety valves used on steam generating boilers are designed to go fullyopen once the pre-set pressure (MAWP)is reached.
Figure 3.22 Piping Layout of a Typical Boiler Safety Valve
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UnitN
o.
3-Steamg
eneration
