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




http://en.wikipedia.org/wiki/Carbon_dioxide

http://en.wikipedia.org/wiki/Carbon_monoxide

http://en.wikipedia.org/wiki/Nitrogen_oxides


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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


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






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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


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:


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.


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)





Documents

I&C Boiler Report