I&C Boiler Report
Transcript of I&C Boiler Report
Training Report
I&C (Boiler)
Submitted by: SARMAD RIAZ
Submitted to: MR. ALI NAWAZ (TEAM LEADER E&I)
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�IN THE NAME OF ALMIGHTY ALLAH, THE MOST
BENEFICIENT THE MOST MERCIFUL�
ACKNOWLEDGEMENT
I am grateful to Team Leader E&I Mr. ALI NAWAZ, Mr.Gul
Zaman, Mr. Naveed Ahmed Quereshi, Mr. Muhammad
Shafqat and other members of the E&I team who patiently
guided me and without whose support and
encouragement this training would not have been possible.
I am thankful to the whole AES Lalpir team for the support
they have extended to me during this training.
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TABLE OF CONTENTS
Contents Page Number
Steam Generator 04
Combustion 05
Burner Control 06
Air and Gas Circuit of the Boiler 10
Flue Gas Path 15
Fuel 17
Feed Water and Steam 19
Water Path 19
Steam Drum Level Control 20
Water Circulation in Boiler 22
Steam Path 25
Auxiliary Steam 28
Chemical Dosing in Boiler 29
Steam Converter 30
Soot Blowing 31
Main Fuel Trip 33
Continuous Emission Monitoring System (CEMS) 34
Boiler Tube Leak Detection 48
HFO Flow Transmitter 54
Temperature Measurement 58
Radio Communication System 62
Control System 63
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STEAM GENERATOR
The Steam generator (Boiler) unit provides superheated steam to drive the
Turbine. The steam provided by the boiler is produced by heating the boiler feed
water. The heat is obtained by burning air fuel mixture in the Furnace.
Superheated steam from the boiler drives the HP Turbine. After driving the HP
turbine the steam returns to the boiler for reheating. Cold Reheat steam enters
the boiler and after being heated in the Reheaters it enters the IP turbine.
Types of Boilers:
Boilers are generally classified into two main types
Fire Tube Boilers
Water Tube Boilers
Fire Tube Boiler:
In fire tube boiler the hot combustion gases are passed through a series of tubes,
the tubes are submerged in the boiler water and act as the medium of heat
transfer. Fire tubes boilers are generally classified as shell boilers since water
and steam are contained within a single shell that also houses the steam
producing elements. The Auxiliary Boiler is a fire tube boiler.
Water Tube Boiler:
The water tube boiler passes combustion gases over tubes, containing water.
The hot gases transfer the heat necessary to raise the water temperature to the
boiling point. The two major heat transfer areas existing in these boilers are
Radiant Heat Transfer Areas
Convective Heat transfer Areas
The boiler at AES LalPir / Pakgen is a Forced Circulation Radiant Reheat
Water Tube Boiler.
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COMBUSTION
Combustion or burning is the sequence of exothermic chemical reactions
between a fuel (HFO) and an oxidant (Air) accompanied by the production of
heat and conversion of chemical species. The release of heat can result in the
production of light in the form of either glowing or a flame.
When a hydrocarbon burns in air, the reaction will yield carbon dioxide, water,
carbon monoxide, pure carbon (soot or ash) and various other compounds such
as nitrogen oxides, oxides of sulfur.
Fuel + Air Heat + Water + Carbon Dioxide + Nitrogen
In a boiler, combustion takes place in the furnace in a controlled environment.
Fuel and air in measured quantities enter the furnace and are ignited to form a
fireball.
The furnace contains 16 burners situated at four corners and four elevations. The
burners are corner fired. The position of the fireball inside the furnace can be
changed by changing the burner tilt (vertical tilt).
Burner Guns and their Accessories
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Each burner also has its own ignitor that is used to ignite the burner. The A-Row
burners are dual fired i.e. they can be fired on Diesel Oil as well as HFO. The B,
C and D row burners are only HFO Fired.
Burner Control
On / Off Control System
This is the simplest control system, and it means that either the burner is firing at
full rate, or it is off. The major disadvantage to this method of control is that the
boiler is subjected to large and often frequent thermal shocks every time the
boiler fires. Its use should therefore be limited to small boilers up to 500 kg / h.
Advantages of an ON / OFF control system:
Simple
Least expensive
Disadvantages of an ON / OFF control system:
A Row
2 3
4
B Row
C Row
D Row
1
Burner Layout
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If a large load comes on to the boiler just after the burner has switched off,
the amount of steam available is reduced. In the worst cases this may
lead to the boiler priming and locking out
Thermal cycling
Modulating Control System
A modulating burner control will alter the firing rate to match the boiler load over
the whole turndown ratio. Every time the burner shuts down and re-starts, the
system must be purged by blowing cold air through the boiler passages. This
wastes energy and reduces efficiency. Full modulation, however, means that the
boiler keeps firing over the whole range to maximize thermal efficiency and
minimize thermal stresses. This type of control can be fitted to any size boiler,
but should always be fitted to boilers rated at over 10 000 kg / h.
Advantages of a modulating control system:
The boiler is even more able to tolerate large and fluctuating loads. This is
because:
The boiler pressure is maintained at the top of its control band, and the
level of stored energy is at its greatest
Should more energy be required at short notice, the control system can
immediately respond by increasing the firing rate, without pausing for a
purge cycle
Disadvantages of a modulating control system:
Most expensive
Most complex
Burners with a high turndown capability are required
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Impact of Air on Combustion:
Accurate control of the amount of air is extremely essential for proper
combustion:
Too much air will cool the furnace, and carry away useful heat
Too little air and combustion will be incomplete, unburned fuel will be
carried over and smoke may be produced
In practice, however, there are a number of difficulties in achieving perfect
combustion:
The conditions around the burner will not be perfect, and it is impossible to
ensure the complete matching of carbon, hydrogen, and oxygen
molecules
Some of the oxygen molecules will combine with nitrogen molecules to
form nitrogen oxides (NOx)
The amount of air entering the furnace is controlled by air dampers provided
with each burner.
Flame Detectors / Scanners
Two different types of Flame Scanners are used with the burners. The Ignitor
Flame Scanner relay is used to detect the flame at the tip of the ignitor. The
Burner flame scanner has an infrared detector and it sends its reading to the
FDC panel in the DCS. The Flame scanners are critical for us to determine
whether the burner gun is working as it should or not.
Furnace TV Camera
A Close Circuit TV (CCTV) camera is installed inside the furnace for the
observation of the fireball. The system consists of the TV camera and a
pneumatic retraction mechanism. The Furnace TV camera consists of the
following components
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SF12CX High Temperature CCTV System
SAM0007 Wall box Mount with Automatic Closure Valve
MSS0010C Automatic Retract with Control Box
The boiler has three distinct circuits that are
Air and Gas
Feed water and Steam
Fuel
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AIR and GAS Circuit of the Boiler
Force Draft Fan (FDF)
Two FDF are used for full load operation. No standby FDF is available and if one
of the FDF trips then the unit will runback. The FDF has an outlet damper and an
inlet damper (inlet vane). During normal operation the outlet damper of the FDF
is fully open and the air flow is controlled by the inlet damper of the FDF.
Forced Draft Fan (Silencer is also visible)
Inlet Dampers of FDF control the air intake rate. The opening of the Dampers is
controlled to maintain a proper air / fuel ratio to achieve proper combustion. The
opening of the dampers is controlled after computing the value of oxygen
obtained by the oxygen analyzer installed at the inlet of Gas Air Heater.
Specification
Type Air foil double inlet
Capacity (volume) 12620 m3/min
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Suction pressure 1040 mm aq
Delivery pressure 1100 mm aq
Control Element Inlet Vanes
Speed 980 rpm
Motor capacity 3000 KW
Steam Air Heater (SAH)
The Force Draft Fan forces the air to the Steam Air Heater where the air is
heated up to a temperature of 70 °C. The Steam Air Heater is a shell and tube
type heat exchanger in which the combustion air is heated by the auxiliary steam.
Purpose of SAH:
Air is preheated by auxiliary steam before entering the Gas Air Heater because if
cool air at ambient temperature enters the Gas Air Heater it will cool the GAH
which may result in the Flue Gases achieving Dew Point. The flue gases contain
SO2 which will condense to form H2SO4 that will damage the baskets of the Gas
Air Heater.
Gas Air Heater (GAH):
After the Steam Air Heater, air enters the Gas Air Heater where it is again
preheated before entering the furnace. Gas Air Heaters are used to recover heat
from the flue gases before they leave the boiler. Soot Blowing of Gas Air Heaters
is carried out twice in a shift or four times in a day.
Wind Box:
Wind Box is an enclosure that is used for the collection of air before it enters the
boiler. There are two wind boxes in the boiler.
Furnace Wind Box
Ignitor Wind Box
Furnace Wind Box:
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There are two Furnace wind boxes, one for Burner Columns 1, 4 and the other
for Burner Columns 2, 3. From the wind box, air is fed into the furnace through
Air Dampers.
Three types of dampers are used to supply combustion air into the furnace
Over-Fire Damper
Fuel Air Damper
Auxiliary Air Damper
Fuel Air Damper:
There are sixteen fuel air dampers, one for each burner. The fuel air damper
remains completely open when the burner is in operation.
Auxiliary Air Damper:
There are sixteen auxiliary air dampers, one with each burner. Auxiliary Air
Damper is used for modulating control of air flow into the furnace. It is used to
maintain the air / fuel ratio. We can control the opening of Auxiliary Damper and
thus control the amount of air entering the furnace.
Over Fire Dampers:
Over Fire dampers are installed only on the top row (D-row) burners. They are
used to control the temperature of the furnace and maintain a viable air / fuel
ratio for efficient combustion.
Limit Switches are installed on the Fuel and Auxiliary air dampers to indicate the
position (Open/Close) of the damper to the OPS.
Ignitor Wind Box:
The ignitor wind box supplies atomizing air to the diesel oil that is used for
ignition.
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Furnace Wind Box / Ignitor Wind Box Differential Pressure:
The Furnace Wind box Differential Pressure transmitters measure the pressure
difference between the furnace and the wind box. Similarly the Ignitor Wind Box
Differential pressure transmitter measures the pressure between the Ignitor Wind
Box and the Furnace. If the pressure of the wind box drops below the furnace
pressure then
Flue gases from the furnace will enter the Wind Box
The permit for firing the Ignitors will not be available
The cooling air of the Ignitors will be cut off resulting in the obliteration of
the instruments installed at the ignitors
Cooling Fans:
Cooling fans take suction from FDF and provide cooling air to critical instruments
in the boiler. Some of the instruments that are cooled by the cooling air fan are
Flame Scanners, Furnace TV Camera and other instrument probes. Cooling Fan
is a very critical piece of equipment. There are two cooling fans installed on each
unit.
AC Cooling Fan:
AC cooling fan remains in operation under normal conditions. It is supplied power
from the emergency bus.
DC Cooling Fan:
DC cooling fan is kept in standby and it is powered by DC batteries. One of the
first things to check in case of a blackout is that the cooling fans must be running.
Ignitor Fan:
The Ignitor fan takes air from the outlet of FDF and supplies it to the ignitor wind
box. Ignitor Wind Box is separate from the main Furnace Wind Box. The ignitor
fan provides atomizing air to the ignitors.
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Specifications
Type Turbo fan
Capacity (volume) 300 m3/min
Delivery pressure 100 mm aq
Speed 1500 rpm
Motor capacity 11 KW
Service Air:
Service Air is used to clean the dust and soot that settles on the body of the
Boiler.
Aspirating Air:
Aspirating air is provided by Service Air Compressor. If during operation a burner
gun or any other instrument has to be taken out from the boiler for a short period
of time, Aspirating Air is used to keep the gun enclosure pressurized so that Flue
Gases may not escape the furnace.
Instrument Air:
Instrument Air is used to operate pneumatic control valves and other pneumatic
control elements. The Service Air Compressors provide Instrument Air for the
instruments present in the boiler area.
Sealing Air:
Sealing air is used to seal the boiler peep holes and other open areas of the
Boiler when some maintenance activity is being performed. Sealing air is also
provided by the Service Air Compressor.
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FLUE GAS PATH
In the furnace the air and fuel mixture undergoes combustion resulting in the
production of heat and flue gases. The hot flue gases trace the following path.
Primary Super Heater
Secondary Super Heater
Tertiary Superheater
Secondary Reheater
Primary Reheater
Economizer
Gas Recirculation Fan (GRF) | Gas Air Heater (GAH)
|
Furnace | Stack
Gas Recirculation Fan (GRF):
After heating the feed water in the Economizer, the flue gas path splits into two.
One portion of the flue gases is used for preheating of air in the Gas Air Heater
(GAH). After the Gas Air Heater the flue gases are vented to the atmosphere
through the stack.
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The remaining flue gases are sent back to the furnace by means of a Gas
Recirculation Fan. The recirculation of flue gases is done for two main reasons
It helps control the temperatures of the secondary gas path. It is a means
of controlling the temperatures of the Primary Reheater
It is used for cooling the furnace and reducing NOx production
GRF as a means of controlling NOx production:
Flue Gas Recirculation is one method of controlling NOx production. The
production of NOx increases exponentially at high temperatures. By recycling the
relatively low temperature exhaust gases into the furnace we are able to cool the
furnace and prevent the temperature from entering the high temperature zone.
The reduced oxygen content of the exhaust gases lowers the flame temperature
in the combustion zone, thereby reducing NOx formation
Specifications
Type Air foil double inlet
Capacity (volume) 3700 /5600 m3/ min
Delivery pressure 360 /410 mm aq
Control Element Inlet Dampers
Speed 980 rpm
Motor capacity 610 KW
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FUEL
Two types of fuels are used in our boiler
Heavy Fuel Oil (HFO)
Diesel Oil
HFO also known as No. 6 Oil is the main fuel oil and it is used in the boiler during
normal operation. We obtain HFO from the PSO through two sources
Pumping from PSO Fuel Depot
Decanting
Heavy Fuel Oil:
HFO is stored in five tanks. At the bottom of the tank there are coils of tracing
steam designed to keep the HFO warm (at a temperature of 50 C). From there
the fuel is heated by a Suction Heater and then passes through the Suction
Strainer to the HFO Transfer Pump. The HFO Transfer Pump then pumps the
fuel through the discharge strainer and discharge heater to the HFO Ring Header
from where the fuel is distributed to the individual burners. The flow of fuel is
monitored by a Coriolis Type Mass Flow Meter that is located at the discharge
of discharge strainers. The fuel flow rate and pressure is controlled by a Flow
Control Valve and a Stabilizing MOV. When a Main Fuel trip (MFT) occurs the
supply of fuel is stopped by means of a Shut Off Valve located on the main fuel
line just before the Ring Header.
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Diesel Oil:
Diesel oil is being used as fuel for following
A-Row burners of main boiler during startup
Main boiler ignitors (as ignitor oil)
Auxiliary boiler
Emergency Diesel Generator
Fire Fighting Pump
The diesel oil supply system is similar to that of the HFO with one main
difference; we do not need to preheat the diesel oil therefore there is no suction
or discharge heater present in the Diesel Oil Supply line. Diesel Oil is supplied to
the A-Row burners through two Diesel Oil Supply Pumps. Ignitor Oil (Diesel) is
supplied to the ignitors separately by two (2) Ignitor Oil Pumps situated in the
HFO Pump House.
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FEED WATER AND STEAM
WATER PATH
ECONOMIZER
In order for the boiler to absorb as much of the generated heat as possible, feed
water first enters the boiler through the economizer section. The economizer
section is a series of tubes that are normally located in the boiler "back pass,"
(secondary pass of the flue gases) where flue gases pass before exiting the
boiler and entering the air heaters.
Economizers are nearly always a counter flow, water to gas type, with the water
flowing up. This is done to maximize heat transfer and ensure a full section. The
tubes are arranged in horizontal bundles with the outlet at the top.
Water is then routed through economizer links to the Steam Drum. Approximately
17-20% of total heat absorption in the boiler takes place in the economizer. As
the feed water flows through these tubes, the thermal efficiency of the boiler is
improved and less waste heat is lost to the stack. As a final result, the boiler
efficiency is improved.
STEAM DRUM
The Steam Drum is situated at the 7th floor of the Boiler. Feed water from the
economizer enters the steam drum.
A steam drum is a standard feature of a water-tube boiler. It is a reservoir of
water/steam at the top end of the water tubes. The drum stores the steam
generated in the water tubes and acts as a phase-separator for the steam/water
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mixture. The difference in densities between hot and cold water helps in the
accumulation of the "hotter" water/and saturated-steam into the steam drum.
There are various separators in the drum to remove the droplets from the steam
before the steam is sent to the primary Superheater. These separators are
normally arranged in stages, with the first stage commonly using centrifugal force
to throw the droplets of water from the steam and allow them to run back into the
water. From these separators the steam is then routed into a series of chevron-
shaped plates of steel with relatively close tolerances between them.
As the steam passes through the torturous path presented by the chevron plates
the majority of the remaining water is removed.
Steam Drum Level Control
Boiler drum level control is critical for both plant protection and equipment safety
and applies equally to high and low levels of water within the boiler drum. The
purpose of the drum level controller is to bring the water level in the steam drum
to a specific set point at boiler start-up and maintain the level at constant steam
load.
A decrease in this level may result in decreased water flow in boiler tubes,
allowing them to become overheated and damaged.
An increase in this level may interfere with the process of separating moisture
from steam within the drum, thus reducing boiler efficiency and carrying moisture
into the process or turbine
Three Element Drum Level Control
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The steam drum level controller controls the level of steam drum. It is a three
element controller; meaning it takes three inputs and controls the level of steam
drum based on the values of those inputs.
Three parameters that the steam drum level controller observes and monitors are
Steam Drum Water Level
Feed Water Flow
Main Steam Flow
Steam Drum Level:
The steam drum is normally maintained at a pressure of 199 kg/cm2. If the
pressure of the steam drum drops the volume of the steam will increase thus
resulting in an increase in level.
Main Steam Flow:
An increase in main steam flow rate will result in a decrease in drum level and a
decrease in main steam flow rate will cause the drum level to rise
Feed Water Flow:
The water level in the steam drum is directly proportional to the feed water flow
rate.
Advantages:
Steady steam pressure and flow rate within the boiler's thermal capacity
More efficient burner operation
Less thermal stress on the boiler shell
Less boiler water carryover
Can use a central feed pump station
Less wear and tear on the feed pump and burner
Disadvantages:
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More expensive
Feed pump must run continually
Water Circulation in Boiler
There are two types of water circulation mechanisms employed in boilers
Natural Circulation
Forced Circulation
Natural Circulation:
Natural circulation is based on the physics principle of density difference, i.e.
when water is heated, it become less dense this means that for a given volume
of water, hot water weighs less than cold water and steam weighs less than
water. As a result of this there is a natural circulation in the boiler with cold water
forcing the hot water and steam to move upward through the tubes.
Forced Circulation:
Forced circulation of water is carried out with the help of a boiler circulating
pump. The pump takes suction from down comers from steam drum and
discharges the water to water drum from where it is distributed to the water
walls.
On forced circulation units the boiler water circulating pumps are designed to
ensure flow through the water wall tubes. This reduces the possibility of hot spots
and the resultant tube metal overheating problems. Natural circulation type
boilers do not use these pumps. The advantage of a controlled circulation boiler
is the much faster allowable heat up rate and load change rate.
Advantages of Forced Circulation:
Steam generation rate is higher
Greater capacity to meet load variation
Quick start-up quality from cold condition
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Lower scaling problem due to high circulation velocity
More uniform heating of all parts reduces the danger of over-heating &
thermal stresses
BOILER CIRCULATION PUMP
Two Boiler Circulating pumps are installed at AES Lalpir for full load operation.
One BCP can withstand up to 50 present load. The BCP is the only equipment
on the plant that requires an operating supply voltage of 6.6 kV.
Specifications
Type Single Suction - Single Discharge
Design Pressure 201 Atm (g)
Design Temperature 365°C
Suction Pressure 191 Atm (g)
Capacity 2330 m3/hr
N.P.S.H > Vapor Pressure 13 m
Suction Temperature 354°C
Differential Pressure 2.28 Atm
Pump Efficiency 83.2% Hot Duty 83.2% Cold Test
Hydrostatic Test Pressure 301.5 Air (g)
Motor Specifications
Type Wet Stator Squirrel Cage Induction
Output 360kW
Motor Case Design Temperature 80°C
Motor Efficiency 86% - 88%
Full Load Speed 1450 rpm
Full Load Current 45 amps
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WATER DRUM
The BCP circulates the water between the Steam Drum and the Water Drum. The Water
Drum is also known as the Mud Drum or Supply Drum.
WATER WALL TUBES
The water wall tubes are a series of parallel tubes that are welded together at the
Membrane (the flat piece of metal between tubes) to make up the gas tight walls
of the fire box.
In forced circulation boilers, these tubes have generally orifice at the feed
header to ensure proper distribution of flow through all of the water wall
tubes. Ample water flow through these tubes is critical because of their direct
exposure to the boiler flames. The tubes are normally welded into the headers.
On some smaller boilers it is not uncommon to have the tubes "rolled" into
tapered holes in the headers, especially the drum. This type of construction
results in a much weaker union that is more susceptible to damage from rapid
temperature changes than is the welded construction type (under extreme
cooling the tubes can actually pull out of the drum).
The boiler water first begins to boil and change into steam in the water wall
tubes. Approximately 32% to 35% of the total heat absorbed by the boiler is done
in the water walls.
After water changes to steam, it returns to the steam drum(s). The steam drum
normal level is typically half water and half steam. The steam at this point is not
superheated and has small droplets of water in it.
BLOWDOWN
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Another important system in the Steam Drum is the Blow Down line. Water from
the Steam Drum is dumped into the Blow Down if it does not meet the quality
requirements. The Blow Down is used to maintain the chemistry of the feed water
in the Steam Drum.
STEAM PATH
The water and steam mixture gathers in the steam drum. The steam in the steam
drum is not superheated and it also contains some moisture.
Steam should be available at the point of use:
In the correct quantity (1200 ton/hr)
At the correct temperature and pressure (541 C and 176 kg/cm2)
Free from air and incondensable gases
Clean
Dry (Superheated)
Correct Quantity of Steam
The correct quantity of steam must be made available for any heating process to
ensure that a sufficient heat flow is provided for heat transfer. Similarly, the
correct flow rate must also be supplied so that there is no product spoilage or
drop in the rate of production. Steam loads must be properly calculated and
pipes must be correctly sized to achieve the flow rates required.
Correct Pressure and Temperature of Steam
Steam should reach the point of use at the required pressure and provide the
desired temperature for each application, or performance will be affected. The
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correct sizing of pipe work and pipeline ancillaries will ensure this is achieved.
Air and Incondensable Gases
Air is present within the steam supply pipes and equipment at start-up. Even if
the system were filled with pure steam the last time it was used, the steam would
condense at shutdown, and air would be drawn in by the resultant vacuum.
When steam enters the system it will force the air towards either the drain point,
or to the point furthest from the steam inlet, known as the remote point. Therefore
steam traps with sufficient air venting capacities should be fitted to these drain
points, and automatic air vents should be fitted to all remote points.
However, if there is any turbulence the steam and air will mix and the air will be
carried to the heat transfer surface. As the steam condenses, an insulating layer
of air is left behind on the surface, acting as a barrier to heat transfer.
SUPERHEATERS
Steam produced from a boiler without a superheater will either be dry saturated
or, more likely, wet. In works where steam is transmitted over long distances, the
inevitable heat loss from pipe surfaces causes the steam to become even wetter
at the point of use unless a superheater is fitted to the boiler plant. This is a
separate battery of pipes placed near the boiler furnace through which steam
passes to receive additional heat by either convection or radiation. The
superheater increases the surface area capable of accepting heat and the
production of heat also slightly increases the thermal efficiency of the boiler.
Steam flow must be maintained through the superheater to prevent the tubes
being burning out.
The steam generator (boiler) has three superheaters
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Primary Superheater
Secondary Superheater
Tertiary Superheater
The design parameters of main steam at the outlet of tertiary superheater are
199 kg/cm2 and 541 ˚C. The operating pressure of main steam at the
superheater outlet is 176 kg/cm2.
REHEATERS
Reheater receives steam from HP turbine exhaust called cold reheat steam and
heat it up close to SH steam temperature called hot reheat. In this way reheat
system add some amount of energy to cold reheat steam. This increases the
cycle efficiency. Reheat system transfer the heat of flue gases leaving the boiler
to cold reheat steam. The design steam pressure for the reheater section is 45
kg/cm2.
ATTEMPERATOR
The Desuperheater is also referred to as the attemperator. There are two
attemperators installed on the boiler. One is installed at the outlet of the
secondary superheater and the second is installed at the inlet of primary
reheater.
The spray attemperator nozzle is located in a special device installed in the
piping connecting the superheaters. The attemperator body is constructed of
a hardened, wear resistant material designed to withstand the tremendous forces
of erosion present in this area. In addition, there is considerable thermal stress in
this area due to the injection of cooler water, causing the construction to be
segmented to allow rapid expansion and contraction of the components.
The spray attemperator works by the process in which water is sprayed into the
header and it immediately flashes into steam. That implies that some of the
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enthalpy of the steam already in the header is transferred to the spray water. The
more water that is sprayed into the header, the more the enthalpy drop in the
steam's heat value. This loss in enthalpy results in a lower temperature.
There is also a spray attemperator located at the inlet of the reheat section.
Use of the steam attemperator for cooling the reheater is usually, and preferably,
not necessary because of the efficiency loss associated with its use. Normal
temperature control of the reheat section is either done with air dampers and by
burner tilts.
AUXILIARY STEAM
Steam from the boiler is used for many other purposes in addition to driving the
turbine. The steam used for these other purposes is generally known as Auxiliary
Steam as it is used with Auxiliary systems. Auxiliary steam is not superheated
and sometimes it is also called saturated steam.
Auxiliary steam system is divided into three main parts, which are mentioned
below
Main auxiliary steam header
2RY auxiliary steam header
Steam sources lines and interconnecting lines.
Auxiliary steam header is connected with two steam sources i.e. auxiliary boiler
and other unit auxiliary steam header. The main auxiliary steam header supplies
auxiliary steam to the 2RY auxiliary steam header.
There is a De Superheater at the interconnection line between the main and 2RY
auxiliary steam headers which regulates steam temperature at 215 C for better
atomization of fuel. The pressure of the Auxiliary steam is normally 15 kg/cm2.
There are four main sources of Auxiliary steam:
From Auxiliary Boiler
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From 3ry Superheater inlet header
From cold reheat line
From other unit (tie valves)
Auxiliary Boiler:
Auxiliary steam header to be charged from auxiliary boiler if both units are in
shutdown condition.
From 3ry Superheater Inlet Header:
Auxiliary steam header will be charged from 3ry superheater inlet header during
unit start up and remain in service up to generator load <160 MW.
From Cold Reheat Line:
Auxiliary steam header will be supplied from cold reheat line, if turbo generator
load is >160 MW.
From Other Unit (tie valves):
If any unit is out of service and other unit is in operation then auxiliary steam
header will be fed from running unit through tie valves.
Auxiliary steam is used for the following purposes
In Steam Converter to make tracing steam
In Gland Steam Converter
In Steam Air Ejector
For Soot Blowing
Deaerator
Chemical Dosing in Boiler
Fire Mage (Vanadium Inhibitor)
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HFO contains Vanadium in small amounts. After combustion vanadium forms
Vanadium Pentaoxide which sticks to the boiler tubes and affects the heat
transfer rate. We dose Magnesium in the fuel to prevent the formation of
Vanadium Pentaoxide. Magnesium reacts with Vanadium and Oxygen to form
Vanadium Magnate that does not stick to the tubes.
Ammonia:
Ammonia is dosed in Boiler feed water to remove the incondensable gases
present in the feed water
Hydrazine:
Hydrazine is dosed in boiler feed water to remove oxygen dissolved in water. If
non condensable gases and oxygen get carried with the turbine they will cause
corrosion in the boiler tubes and other metallic structure that comes in contact
with them.
Orthophosphate:
Orthophosphate was previously dosed in the feed water but now it is not being
dosed.
The chemical dosing in boiler is carried out manually and it is controlled by a
relay logic control panel that is situated inside the Boiler chemical Shed.
STEAM CONVERTER
The steam converter produces heating and tracing steam for the HFO supply
system. It supplies the tracing steam that is used to heat up the HFO Tanks,
Suction and Discharge Heaters and the tracing steam lines. The steam converter
produces 12,500 kg/hr of steam at a pressure of 11.2 atm and temperature of
184 °C. The steam converter consists of the following sub-systems
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Steam Converter (Reboiler) is a shell and tube type heat exchanger.
Shell Side: Feed Water
Tube Side: Main Steam (Auxiliary Steam)
Condensate Receiver is used to receive the condensate of tracing steam.
Condensate of Tracing Steam + Make Up Feed Water
Condensate Cooler is used to cool the hot condensate.
Shell Side: Hot Condensate (Condensed Steam)
Tube Side: Feed Water (Demineralized Water)
The steam controller process is controlled by a Micrologix 100 PLC and
electronic PID Controllers (DPR-900).
PID Controllers of Steam Converter
SOOT BLOWING
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The soot blower system is electrically operated, using steam as blowing medium.
The soot blowers are arranged to maintain the surface clean and to prevent
plugging of the gas passages.
Soot Blowers are used to blow away the soot (unburnt carbon) that deposit on
the boiler tubes. If the soot deposits are not blown away they may settle down on
the tubes and affect the heat transfer rate of the boiler tubes.
Two different types of soot blowers are being used
1. Long Retractable type Soot Blowers for Superheater and Reheater
2. Swing type Soot Blower for Air Heater
Long Retractable Type Soot Blower
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The soot blowers are automatically controlled using a Mitsubishi Melsec PLC.
The soot blowing sequence is initiated after a specific interval by the CRE.
Inside View of the Soot Blower Control panel (Mitsubishi Melsec PLC)
Improper soot blowing might result in
Plugging of gas passages
Decreased and non uniform heat transfer
Reduced equipment life
Condition Based Soot Blowing
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In our Boiler, the soot blowing is interval based i.e. the soot blowing is carried out
after a specific interval. Another mechanism used for soot blowing is monitoring
the amount of heat transfer in Boiler tubes. When the heat transfer rate drops
below a specific level the soot blowing can be started. This results in
Effective Soot Blowing
Performance improvement as steam is only used when required
Improved life of the boiler tubes
MAIN FUEL TRIP
Main Fuel Trip also known as MFT is the tripping condition of Boiler. An MFT can
occur due to any one of the following reasons
Both FD Fans trip
Both Gas Air Heaters trip
Both BCP differential pressure low low for > 3 seconds
HFO burner pressure very low (If only HFO burner in service )
D.O burner pressure very low (If only D.O burner in service)
Atomizing air pressure very low if D.O burners in service
Atomizing steam pressure very low if HFO burners are in service
Air flow < 30% for > 3 seconds
Furnace pressure high for >3 seconds
All flame loss. (No flame detected)
Both A / C and D / C cooling air fans trip
BMS power loss
APC failure
Turbine trips
R.H protection operates
Generator trip
Boiler manual trips
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Continuous Emission Monitoring System (CEMS)
Brief Overview
The flue gases produced as a result of combustion contain many dangerous
gases that are very harmful to the environment and human health. Some of the
gases present in the flue gases are
SO2
NOx
CO
CO2
The continuous emission monitoring system (CEMS) is installed to provide a
continuous and accurate emission analysis. The CEMS consists of the following
main components
Sample Handling and Conditioning System
SO2 Analyzer
NOx Analyzer
CO / CO2 Analyzer
Opacity Monitor
O2 Analyzer
Sample Handling & Conditioning System
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CEMS Analyzers
We follow the following environmental emission limits provided by World Bank
and NEQS:
ENVIRONMENTAL STANDARDS
Stack Emissions Units NEQS World Bank
NOx mg/Nm3 600 128 g/Gj
SOx mg/Nm3 500 100
PM mg/Nm3 300 100
CO mg/Nm3 800 -
Opacity % 40 -
Ambient SO2 ug/Nm3 200 125
Ambient NO2 ug/Nm3 100 150
Noise dB 85 90
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The Sample handling and conditioning system consists of the following main
components
Heated Probe
Umbilical
Sample Conditioning Controller
Heated Probe
Manufacturer: Columbia Scientific
Model: SD3H
Features:
20 cc/min critical orifice
Dilution Ratio-250:1,nominal
Specifications:
Extension Length 5 ft (1.5m)
Material, SST
Principle:
The CS Diluting Stack Gas Probe (Heated) is a completely self contained
sampling system which removes a small sample (20 to 50 ml/min) from the stack
and processes it using a dry air aspirator.
The probe requires 4 to 8 liters/min of dry, clean air at a pressure of at least 34
psig (3.4 bars absolute) to operate the aspirator. The aspirator maintains a
vacuum of 15� (380mm) of mercury across a critical orifice.
This vacuum draws the stack sample through a fine quartz wool filter and a
ceramic filter into a chamber. The stack gas mixes with the dilution air in a fixed
ratio, resulting in a diluted sample having a pressure somewhat above
atmospheric pressure. An electrically heated jacket maintains the sampled stack
gas above its dew point.
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CEMS Probe
Umbilical
The tube (cord) connecting the sampling probe to the sample conditioning
system and the analyzers is known as Umbilical. This tube contains 4 tubes
inside it that are used for the following purposes
Dilution Air
Diluted Sample
Calibration gas and Zero Air
Vacuum Line
Sample Conditioning Controller
Manufacturer: Columbia Scientific
Model: SD35-1A
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Features:
1 Probe
Sample Vented
All functions internally controlled via 24V DC
Description:
Sulfur Dioxide
Sulfur dioxide (also sulphur dioxide) is the chemical compound with the formula
SO2.Further oxidation of SO2, usually in the presence of a catalyst such as NO2,
forms H2SO4, and thus acid rain.
Health Effects of Sulfur Dioxide:
Current scientific evidence links short-term exposures to SO2, ranging from 5
minutes to 24 hours, with an array of adverse respiratory effects including
bronchoconstriction and increased asthma symptoms. These effects are
particularly important for asthmatics at elevated ventilation rates.
SOx can react with other compounds in the atmosphere to form small particles.
These particles penetrate deeply into sensitive parts of the lungs and can cause
or worsen respiratory disease, such as emphysema and bronchitis, and can
aggravate existing heart disease, leading to increased hospital admissions and
premature death.
Sulfur Dioxide (SO2) Analyzer:
Manufacturer: Columbia Scientific
Model: 5700
Features:
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Patented Fluorescence Detection technique, employing
ultraviolet light irradiation
External pump
Voltage Output 0-10, 0-1, 0-0.1
RS232 Standard
4~20 mA DC optional
Operation in the 0-2 ppm range equates to monitoring 0-500
ppm SO2 in the stack
Working Principle:
The Sulfur Dioxide analyzer uses the fluorescent detection technique which
involves the excitation of SO2 molecules by an ultraviolet source and detection of
the resultant fluorescence as the molecule returns to its original state. We
perform dry analysis (i.e. all moisture is removed from the gas mixture) of Sulfur
Dioxide gas in the gas mixture.
When a molecule of sulfur dioxide is irradiated with specific wavelengths of
ultraviolet (UV) light, the molecule will absorb the light energy. Within
nanoseconds, the excited molecule releases this energy in the form of light
energy of longer wavelengths. This phenomenon is referred to as fluorescence.
A UV quartz lens optimizes the amount of light energy that is directed into the
sample cell to produce the fluorescence. The light energy enters the sample cell
in a concentrated beam and is absorbed by the sulfur dioxide. The Photo
Multiplier Tube (PMT) is placed at a right angle to the irradiating light to detect
the fluorescent light from the sulfur dioxide.
Oxides of Nitrogen (NOx)
NOx is a generic term for mono-nitrogen oxides (NO and NO2). These oxides are
produced during combustion, especially at high temperatures.
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At ambient temperatures, the oxygen and nitrogen gases in air will not react with
each other. In a boiler, combustion of a mixture of air and fuel produces
combustion temperatures high enough to drive endothermic reactions between
atmospheric nitrogen and oxygen in the flame, yielding various oxides of
nitrogen.
Health Effects of NOx:
NOx react with ammonia, moisture, and other compounds to form nitric acid
vapor and related particles. Small particles can penetrate deeply into sensitive
lung tissue and damage it, causing premature death in extreme cases. Inhalation
of such particles may cause or worsen respiratory diseases such as emphysema,
bronchitis it may also aggravate existing heart disease.
Current scientific evidence links short-term NO2 exposures, ranging from 30
minutes to 24 hours, with adverse respiratory effects including airway
inflammation in healthy people and increased respiratory symptoms in people
with asthma. Also, studies show a connection between breathing elevated short-
term NO2 concentrations, and increased visits to emergency departments and
hospital admissions for respiratory issues, especially asthma.
Oxides of Nitrogen (NOx) Analyzer:
We are using a Nox analyzer as part of our CEMS to monitor the flue gas
emissions from the stack.
Manufacturer: Columbia Scientific
Model: 5600
Features:
Utilizes the chemiluminescence detection technique
Microprocessor Controlled
Output4~20 mA or 0-10V
External Pump
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Full scale analyzer range, 0-2 ppm, equates to monitoring 0-
500 ppm NOx in undiluted stack gas
Working Principle:
Chemiluminescence is the emission of light with limited emission of heat
(luminescence), as the result of a chemical reaction. The chemiluminescent
method is based on the principle that nitric oxide (NO) reacts with ozone (O3) to
produce Nitrogen Dioxide (NO2) in an electronically excited state and oxygen
(O2). The near instantaneous transition of electronically excited NO2 to its
ground state is accompanied by photon emission (hõ) at wavelength between
600 and 2500 nm; i.e. :
NO + O3 NO2 + O2
NO2 NO2 + hõ
The photon intensity is proportional to the concentration of NO in a reaction
chamber where it is mixed with ozone. The photon emission is converted into an
electrical output by a photomultiplier tube and associated electronics.
The analyzer detects NO and (NO + NO2). A microprocessor subtracts the NO
from (NO + NO2) to give NO2 reading.
Corrected NOx:
According to World Bank guidelines we are required to report a reading of
corrected NOx (in g/Gj). That value is calculated in the DCS logic by using a
simple conversion formula.
Carbon Monoxide
Carbon monoxide (CO) is a colorless, odorless and tasteless gas, which is highly
toxic to humans and animals. It consists of one carbon atom and one oxygen
atom, connected by a covalent double bond and a dative covalent bond.
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Health Effects of Carbon Monoxide:
At lower levels of exposure, CO causes mild effects that are often mistaken for
the flu. These symptoms include headaches, dizziness, disorientation, nausea
and fatigue. The effects of CO exposure can vary greatly from person to person
depending on age, overall health and the concentration and length of exposure.
Carbon Monoxide Analyzer:
Manufacturer � California Analytical Instruments
Model � ZRH
Features:
Utilizes Non Dispersive Infrared (NDIR) detection technique
Output 4~20 mA
Switch selectable range 1000 ppm
External pump
Working Principle:
The CO analyzer works on the principle of Non Dispersive Infrared (NDIR).
Infrared light emitting from an Infrared source is intermitted by a chopper driven
by a chopper motor at a certain frequency and then led into a measuring cell.
The infrared light beam is partially absorbed into the measured component in the
Measuring Cell and the unabsorbed portion reaches a detector, which is provided
with a Front Chamber and a Rear Chamber, both normally being filled with the
gas to be measured.
When infrared light is led into the detector, the gases filling in both chambers
absorb the light and expand. The detector is designed to produce an expansion
difference between the Front and Rear chambers, a slight gas flow is produced in
a mass flow sensor. This slight flow generates output voltage that is proportional
to the CO concentration in the gas.
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Oxygen Analyzer
Oxygen analyzer is used to measure the concentration of oxygen that is leaving
the stack with flue gases. Ideally the concentration of Oxygen should be very low
(approx 2%).
At Air Heater Inlet:
Oxygen Analyzers are also installed at Economizer Outlet (or Air Heater inlet).
The value of O2 at this point indicates the amount of excess air and the quality of
combustion. The opening of the inlet vanes of FDF (air flow) is controlled on the
basis of the O2 value at Air Heater inlet.
Manufacturer � Ametek
Model � Insitu
Features:
Output Range 0-25%
Zirconium Oxide Cell
Working Principle:
The working element of the gas sensor is a closed-end tube made of Zirconium
Oxide (Zirconia). When it is hot, it becomes a conductor of electricity. When the
sensing cell is hot, it produces a voltage that is logarithmically proportional to the
ratio of the oxygen concentration of the gas on the reference side of the cell
(ambient air) and the oxygen concentration of the sample. The sensing cell is a
partial pressure device and responds directly to changes in sample partial
pressure.
The voltage increases as the oxygen concentration diminishes thus we get
accurate results when measuring lower concentrations of oxygen.
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Opacity Monitor
Opacity:
Opacity is the measure of impenetrability to electromagnetic or other kinds of
radiation, especially visible light. In other words opacity may be referred to as the
degree of opaqueness. We use an opacity analyzer to measure the opacity (or
density) of the flue gas.
We are using an opacity analyzer, LightHawk 560DI manufactured by Teledyne
Corporation. It transmits the percentage opacity of the stack flue gases to the
OPS.
Opacity Monitor
Operating Principle:
This instrument is based on the principle of Transmissometry. A light beam with
specific spectral characteristics is projected through the effluent stream of a stack
or duct exhausting combustion or process gases. The amount of light reflected
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back to the instrument from a reflector after passage through the stream is
compared with the maximum possible return when no effluent is present. The
return signal is an indication of the transmittance of the effluent. Particulate
matter in the effluent stream attenuates the projected light beam. The opacity of
the gas stream is determined by measuring the attenuated signal from the
instrument. The opacity is usually expressed as a percentage.
Troubleshooting of Opacity Monitor:
If the fault LED is on, then check the values of the following parameters.
Primary Status Code U3
Extended Status Code U4
The primary and secondary status codes tell us about the exact fault that our
analyzer is facing. During an onsite check the following four values should be
noted carefully.
S0: Signal Voltage (must be between 6.0 and 7.0 V)
F8: Signal Gain (The signal gain is increased to increase the Signal Voltage, but
the gain should not be increased above 125)
S2: LED Current (is normally between 5.0 and 6.0 mA, but can be more; should
not exceed 10 mA)
F9: Reference Gain (Increasing the reference gain decreases LED current and
decreasing the reference gain increases the LED current. Reference Gain should
not be increased beyond 25)
PORTABLE FLUE GAS ANALYZER
A portable flue gas analyzer is also used to measure the emission and to verify
the authenticity of CEMS analyzers. The portable flue gas analyzer is
manufactured by LAND Corporation. The portable gas analyzer can be inserted
into the stack inlet duct through a flange to obtain the emission gas values.
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Portable Flue Gas Analyzer
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BOILER TUBE LEAK DETECTION
A BTLD system is used for early detection of Boiler tube leaks so that timely
corrective action may be taken and further damage may be avoided. A BTLD
system is designed to limit the damage; it does not remove the root cause behind
tube leakages.
Advantages of early Boiler Tube Leak Detection:
Increased Personnel Safety
Early warning of a small boiler tube leak can prevent
expensive secondary damage and unscheduled outages
Increased availability, reduces repair time, and increases
plant efficiency
Planned and scheduled orderly shutdown of a boiler at the
most convenient time
An increase in boiler availability of just one day will more
than cover the cost of a leak detection system
Increased operating profits by Reducing Financial Penalties
Other benefits include the Detection of abnormal boiler
operating conditions, for example the incorrect operation of
soot blowers, inspection ports being left open, and steam
leaks external to the boiler
Three main techniques are commonly used for tube leak detection in Steam
generators
Acoustic BTLD
Conductivity BTLD
Mass Balance BTLD
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BTLD Sensor inserted in the Boiler
Acoustic BTLD:
An acoustic (sound) based BTLD system must first be trained so that the system
learns to differentiate between normal and abnormal sounds. The acoustic
detection method uses sensors and software to detect tube leaks much the same
way as operations personnel. While this can be a reliable method, the software
must learn which sounds are considered leaks and which are considered normal
operating sounds. Until the software is properly trained, this method can be
susceptible to false tube leak indications and alarms.
Conductivity BTLD:
The conductivity detection method is based on accurate measurement of known
concentrations of substances that do not escape when water is turned into
steam. When a change in concentration is recognized, this can be attributed to a
boiler tube leak. This method is capable of identifying very minute leaks, but
usually requires twelve to twenty-four hours for detection.
Mass Balance BTLD:
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A water mass balance has most often been used to detect tube leaks, but in the
past it has been nearly impossible to find small leaks. Using mass balances is
difficult because of the accuracy of the flow measurement devices and the
considerable amount of process noise which is normal. With this difficulty, the
method has either produced a significant number of false alarms or the alarm
limits are relaxed to the point of being able to detect only large leaks.
Our System:
We have installed a BTLD system to detect boiler tube leakages. The system we
are using relies upon acoustic detection. Its components, functions and
specifications are given below
Sound Wave Sensor Tubes
Boost Up Sound Wave Sensor
Hi-Speed Signal Communication Module
I/O Module
Power Module
Communication Interface with DCS
Leak Detection Alarm Model
Supervision Model
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BTLD HMI
Principle of Operation
The BTLD sensors can detect early stage leakage of tube in furnace wall, super
heater, re-heater and economizer and monitor soot-blower operation.
The sound signal in the furnace is detected by enhanced acoustic sensors.
Sound spectrum character can be analyzed through Fast Fourier Transform
(FFT) techniques and displayed on bar graph. The detection unit determines tube
leak occurring after analyzing the sound spectrum character and last time and
the sound intensity.
Functions
Early Stage Report of the Tube Leak
The device alarms with yellow bar in HMI when tube leak occurs. If the leakage
becomes serious, the color of bar is red and alarm panel is lighted.
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Judgment of the Leak Area:
The device judges the leak area by monitoring the sound in chamber. Isolation
range is about 4m.
Sound Spectrum Displaying:
The device displays 1 kHz ~ 15 kHz frequency sound real-time spectrums in
HMI.
Leak Trend Showing:
The device traces variable sound and shows sound trend curve. It can record
and print the trend curve.
Real Time detecting of the Sound:
Operator can select point to listen sound in chamber, and can record the sound
on diskette by supplied recording interface.
Monitoring Soot-Blower Operation:
Monitoring steam pressure of soot blower
Monitoring soot-blower rotation
Monitor soot-blower movement.
Communication between ZD/XLC and DCS
The device sends real-time data to DCS operator station (or I/O station) through
RS232 port, the operator station structures its database to trace history records.
Configuration:
Baud Rate 9600 bps
Data Bit 8
Error Check Even
Stop Bit 1
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Flow Control XON/XOFF or RTS/CTS
Self-inspection
Operator can test all channels state by hitting TEST.
Controllable soot-cleaning for sound pipe
Soot in sound pipe can be cleaned in Manual/Auto mode. In automation mode,
interval can be set 1day~30days.
The device measures the sound quantity in the sound pipe.
If the sound pipe is clogged with soot, the device alarm will tell operators to clean
the soot.
Communication between ZD/XLC and DALU Service Center
By use of PSTN and MODEM, the DALU service center setup communication
with the field device. The manufacturer can know the device operation state,
diagnose and maintain at any time.
Parameters:
Device Sensitivity able to detect less than 2mm leakage
Isolation Range >4m
Enhanced Sensor Sensitivity > 25mV/Pa
Output 0mA~6mA (AC)
Detecting Range 12m
Working Temperature -25 ~ +105 C
Input Channel 32
CPU Pentium
Hard Disk 10.1GB
Memory 32MB
History Trace Time 12months
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HFO FLOW TRANSMITTER
We want to measure the mass flow rate of the HFO. As the flow rate into the
boiler is a very important parameter and we measure the efficiency of the whole
cycle on the basis of its value so we need to have a very accurate and reliable
flow meter. We have installed four (4) flow meters to measure the flow of fuel.
HFO Supply Flow Meter
HFO Return Flow Meter
Diesel Supply Flow Meter
Diesel Return Flow Meter
ELITE Sensor manufactured by Micro Motion (FISHER-ROSEMOUNT) is being
used as the Flow Meter for HFO.
Micro Motion Coriolis Flow Metering System ELITE Sensor + Transmitter
Micro Motion ELITE Coriolis meters are the leading precision flow and density
measurement solution offering the most accurate and repeatable mass
measurement for liquids, gases, or slurries. The Micro Motion flow meter works
on the principle of Coriolis Effect.
CORIOLIS EFFECT
Coriolis Effect is the tendency for any moving body on or above the earth's
surface to drift sideways from its course because of the earth's rotation.
OR
The Coriolis Effect is an apparent deflection of moving objects when they are
viewed from a rotating reference frame.
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CORIOLIS FLOW METER
A practical application of the Coriolis Effect is the mass flow meter, an instrument
that measures the mass flow rate and density of a fluid flowing through a tube.
The operating principle involves inducing a vibration of the tube through which
the fluid passes. The vibration, though it is not completely circular, provides the
rotating reference frame which gives rise to the Coriolis Effect. While specific
methods vary according to the design of the flow meter, sensors monitor and
analyze changes in frequency, phase shift, and amplitude of the vibrating flow
tubes. The changes observed represent the mass flow rate and density of the
fluid.
Working Principle
The flow tubes of the Coriolis mass flow sensor are driven to vibrate at their
natural frequency by a magnet and drive coil attached to the apex of the bent
tubes thus providing a rotating frame of reference. An AC drive control amplifier
circuit in the transmitter reinforces the signal from the sensor�s left velocity pickoff
coil to generate the drive coil voltage. The amplitude of this drive coil voltage
is continuously adjusted by the circuit to maintain a constant, low
amplitude of flow tube displacement, minimizing stress to the tube assembly.
Mass Flow Measurement
The vibrating motion of the flow tube, combined with the momentum of the
fluid flowing through the tubes, induces a Coriolis force that causes each
flow tube to twist in proportion to the rate of mass flow through the tube
during each vibration cycle. Since one leg of the flow tube lags behind the
other leg during the twisting motion, the signals from sensors on the two tube
legs can be compared electronically to determine the amount of twist. The
transmitter measures the time delay between the left and right pickoff signals
using precision circuitry and a high frequency crystal controlled clock. The �delta
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time� value is digitally filtered to reduce noise and improve the measurement
resolution.
Delta time is multiplied by the flow calibration factor to determine the mass
flow rate. Since temperature affects flow tube stiffness, the amount of twist
produced by the Coriolis force will be affected by the flow tube temperature. The
measured flow rate is continuously adjusted by the transmitter, which monitors
the output of a platinum element resistance temperature detector (RTD) attached
to the outside surface of the flow tube. The transmitter measures the sensor
temperature using a three-wire bridge amplifier circuit. The voltage out of the
amplifier is converted to frequency and is digitized by a counter read by the
microprocessor.
Density Measurement
The Coriolis mass flow sensor also functions as a vibrating tube densimeter. The
natural frequency of the tube assembly is a function of tube stiffness, geometry,
and the mass of the fluid the tube contains. Therefore, fluid density can be
derived from a measurement of tube frequency.
The transmitter measures the time period of each vibration cycle using a high
frequency clock. This measurement is digitally filtered, and density is calculated
using the density calibration factors for the sensor after compensating the
sensed natural frequency for known changes in tube stiffness due to
operating temperature. The transmitter calculates volumetric flow by dividing
the measured mass flow by the measured density.
Advantages
Coriolis mass flow meters are very accurate and dependable. They are also
completely immune to swirl and other fluid disturbances, which means they may
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be located nearly anywhere in a piping system with no need at all for straight-run
pipe lengths upstream or downstream of the meter. Their natural ability to
measure true mass flow, along with their characteristic linearity and accuracy,
makes them ideally suited for custody transfer applications (where the flow of
fluid represents product being bought and sold).
Disadvantages
The greatest disadvantage of Coriolis flow meters is their high initial cost,
especially for large pipe sizes. Coriolis flow meters are also more limited in
operating temperature than other types of flow meters and may have difficulty
measuring low-density fluids (gases) and mixed-phase (liquid/vapor) flows. The
bent tubes used to sense process flow may also trap process fluid inside to the
point where it becomes unacceptable for hygienic (e.g. food processing,
pharmaceuticals) applications. Straight-tube Coriolis flow meter designs, and
designs where the angle of the tubes is slight, fare better in this regard than the
traditional U-tube Coriolis flow meter design.
Importance & Maintenance
The HFO Flow meter is a very critical instrument. As its working principle
involves measuring the frequency of vibration of the flow tube so it is strongly
advised that radios and mobile phones not be used near the meter to prevent
any Electromagnetic Interference. The meter is checked during the annual
outage and its zero is adjusted.
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HFO Flow Transmitter
TEMPERATURE MEASUREMENT
The measurement of temperature of process f luid at dif ferent points is
essential for process control. The measurement of temperature of different
equipment is also necessary to prevent equipment failure. There are many
different temperature measurement devices installed in the Boiler Area. These
include
RTDs
Thermocouples
RTD
The electrical resistance of certain metals changes as the ambient temperature
changes. This characteristic is the basis for a temperature-measuring instrument
called an RTD, or Resistance-Temperature Detector. The RTD is a kind of
transducer. RTDs convert temperature changes to voltage signals (usually mill
volts, or mV) by measuring resistance.
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The materials used in the RTD must be extremely pure, of uniform quality, stable
within a specific temperature range, and able to give reproducible resistance-
temperature readings. Only a few metals have these properties�for example,
platinum, copper, and nickel. The relationship between resistance and
temperature is linear for a certain range of temperatures. The rate at which the
resistance increases as temperature increases depends on the specific
characteristics of the metal.
Wheatstone Bridge Circuits:
The electrical circuit commonly used with a platinum RTD is referred to as a
Wheatstone bridge. The bridge converts the RTD�s change in resistance to a
voltage output. This circuit uses four separate electrical resistors, one of which is
the RTD. The bridge is initially balanced, with voltage output equal to zero,
because all four resistors are equal. As the resistance of the RTD changes, due
to a temperature change, the bridge becomes unbalanced, resulting in a voltage
output other than zero.
Resistance of RTD changing due to Temperature
PM and Calibration of Temperature Sensors:
The PM of RTDs is carried out during the annual outage. A Temperature
Calibrator (Simulator) is used to calibrate the RTDs.
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THERMOCOUPLES
Thermocouples generate their own electric potential. In some ways, this makes
thermocouple systems simpler because the device receiving the thermocouple�s
signal does not have to supply electric power to the thermocouple.
Though typically not as accurate as RTDs, thermocouples are more rugged,
have greater temperature measurement spans, and are easier to manufacture in
different physical forms.
PRINCIPLE OF OPERATION:
When two dissimilar metal wires are joined together at one end, a voltage is
produced at the other end that is approximately proportional to temperature. That
is to say, the junction of two different metals behaves like a temperature-sensitive
battery. This form of electrical temperature sensor is called a thermocouple.
Junction J1 is a junction of iron and copper � two dissimilar metals � which will
generate a voltage related to temperature. Note that junction J2, which is
necessary for the simple fact that we must somehow connect our copper-wired
voltmeter to the iron wire, is also a dissimilar-metal junction which will generate a
voltage related to temperature. Note also how the polarity of junction J2 stands
opposed to the polarity of junction J1 (iron = positive ; copper = negative). A third
junction (J3) also exists between wires, but it is of no consequence because it is
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a junction of two identical metals which does not generate a temperature-
dependent voltage at all.
The presence of this second voltage-generating junction (J2) helps explain why
the voltmeter registers 0 volts when the entire system is at room temperature:
any voltage generated by the iron-copper junctions will be equal in magnitude
and opposite in polarity, resulting in a net (series-total) voltage of zero. It is only
when the two junctions J1 and J2 are at different temperatures that the voltmeter
registers any voltage at all.
Cold Junction Compensation
Thermocouples measure the temperature difference between two points, not
absolute temperature. To measure a single temperature one of the junctions�
normally the cold junction�is maintained at a known reference temperature, and
the other junction is at the temperature to be sensed.
Having a junction of known temperature, while useful for laboratory calibration, is
not convenient for most measurement and control applications. Instead, they
incorporate an artificial cold junction using a thermally sensitive device such as a
thermistor or diode to measure the temperature of the input connections at the
instrument, with special care being taken to minimize any temperature gradient
between terminals. Hence, the voltage from a known cold junction can be
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simulated, and the appropriate correction applied. This is known as cold junction
compensation.
Alternatively cold junction compensation can be performed by computation using
look-up tables and polynomial interpolation. In some places Dual Element
Thermocouples are installed in which even if one element is faulty and gives
incorrect reading we can simply connect the terminating wires to the second
element.
Thermocouples in the Boiler Area:
Thermocouples are widely used in the Boiler area as the measurement of
temperature is very critical for efficient boiler operation. In some places Dual
Element Thermocouples are installed in which even if one element is faulty and
gives incorrect reading we can simply connect the terminating wires to the
second element.
RADIO COMMUNICATION SYSTEM
Repeater Module (containing one transmitter and one receiver) is placed on the
top floor of Boiler in the Elevator motor room. Two antennas, one for transmitting
and one for receiving are installed on the roof of Boiler (Furnace Roof).
Two communication channels are being utilized:
Channel 1 (Repeater placed on top of LALPIR Boiler). Repeater of Channel 1
was replaced by a spare repeater. A new transmitting antenna was installed on
LalPir in December 2009.
And
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Channel 2 (Repeater placed on top of PAKGEN Boiler). Repeater was repaired
and put back into service in December 2009.
Power Supply for Repeater: 110V AC
Consultant: FORBES
Antenna: FORBES
CONTROL SYSTEM
The steam generator is being controlled by the Mitsubishi Netmation Distributed
Control System.
BMS
A dedicated processor named Burner Management System (BMS) is used for the
control of the combustion process that takes place in the furnace. BMS
automatically controls the operation of burners as the load varies. At full load all
sixteen burners are firing. While at minimum load only six burners are in
operation. The sequence in which burners are fired is as follows
SEQ-1
The auxiliary systems of the Boiler are controlled by the SEQ-1 MPS. Some
examples of the systems being controlled by the SEQ-1 are
Boiler Feed Pumps
Boiler Circulation Pumps
Gas Recirculation Fan
Force Draft Fan
Steam Drum Level Control
Some of the sub systems of the boiler are being controlled by PLCs for instance
Mitsubishi Melsec PLC (Soot Blowing)
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Micrologix 100 PLC (Steam Converter)
In addition to PLCs many discrete Local Controllers (PID) are also being used in
the Boiler Area. These controllers include the following types
Pneumatic PID Controllers (CBD Tank Level Controller)
Electronic PID Controllers (DPR-900 Steam Converter)
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