
2

1.2.4) Fatigue:-

This is occurs due to following fatigue.

1.Mechanical Fatigue( carry over )

1.2.5) Heating:-

There are two types of failures.

1. Short Term Overheat

2. Long Term Overheat

1.2.6) Lack of Quality Control:-

There are three types of failures.

1. Maintenance Damage

2. Material Defects

3. Welding Defects

1.3) Feed Water Treatment & Chemical Cleaning:-

1.3.1) Objectives:-

1.3.1.1 Pretreatment Of Water

1.3.1.2 Demineralization

1.3.1.3 Chemical Conditioning of Water

1.3.2) De-aerator:-

1.3.2.1 Function

1.3.2.2 Flow Arrangement

1.3.2.3 Design & Construction

1.3.2.4 Main Parts

1.3.2.5 Accessories


3

DETAIL DESCRIPTION OF FAILURES Chapter-2

2.1) Water Side Corrosion:-

2.1.1) Caustic Corrosion:-

Problem: Loss on the inside diameter (ID) surface of the tube, stress and

strain in the tube wall is increases.

Causes: Caustic corrosion occurs when there is excessive deposition on ID

tube surfaces.

If pH value of water is increases, it results in a caustic condition which

corrosively attacks and breaks down protective magnetite.

Fig. 2.1.1:- Caustic Corrosion


4

2.1.2) Oxide Corrosion:-

Problem: Aggressive localized corrosion and loss of tube wall is near economizer

feed water inlet on operating boilers.

Causes: Oxygen corrosion occurs with the presence of excessive oxygen in boiler

water. It can occur during operation as a result of in-leakage of air at pumps, or failure

in operation of pre-boiler water treatment equipment.

This also may occur during out-of-service periods, such as outages and storage, if

proper procedures are not followed in lay-up. Non-drainable locations of boiler

circuits, such as super heater loops, re-heater tubes and supply lines, are especially

susceptible.

Wetted surfaces are subject to oxidation as the water reacts with the iron to form iron

oxide.

Fig. 2.1.2:- Oxide Damage


5

2.1.3) Hydrogen Damage:-

Problem: Due to Internal micro-cracking. Loss of ductility of the tube

material leading to brittle rupture in boiler tube.

Causes: Hydrogen damage is mostly occurs on inner side of tube surfaces,

coupled with a boiler water low pH excursion.

Water chemistry is upset, such as what can occur from condenser leaks,

particularly with salt water cooling medium, and leads to acidic (low pH)

contaminants that can be concentrated in the deposit.

Under-deposit corrosion releases atomic hydrogen which migrates into the

tube wall metal, reacts with carbon in the steel (decarburization) and

causes inter granular separation.

Fig.2.1.3:- Hydrogen damage


6

2.1.4) Pitting (Localized Corrosion) :-

Problem: Corrosive attack of the internal tube metal surfaces, resulting in an

irregular pitted or in extreme cases appearance of the tube inner diameter.

Causes: Acid attack most commonly is associated with poor control of

process during boiler chemical cleanings and/or inadequate post-cleaning

passes of residual acid.

Fig.2.1.4:- Localized Corrosion


7

2.1.5) Stress Corrosion Cracking(SCC):-

Problem: Failures from SCC is brittle-type crack. May be found at locations

of higher external stresses, such as near attachments.

Causes: SCC most commonly is associated with austenitic (stainless steel)

super heater materials and can lead to inter granular crack propagation in the

tube wall.

It occurs where a combination of high-tensile stresses and a corrosive fluid are

present. The damage results from cracks that propagate from the inner

diameter.

The source of corrosive fluid may be carryover into the super heater from the

steam drum or from contamination during boiler acid cleaning if the super

heater is not properly protected.

Fig.2.1.5:- Stress Corrosion Cracking

2.2) Fire Side Corrosion :-


8

2.2.1) Water Wall:-

Problem: External tube metal loss (wastage) leading to thinning and

increasing tube strain.

Causes: Corrosion occurs on external surfaces of water wall tubes when

the combustion process produces a reducing atmosphere.

For conventional fossil fuel boilers, corrosion in the burner zone usually is

associated with coal firing.

Boilers operating with staged air zones to control combustion can be more

susceptible to larger local regions possessing a reducing atmosphere,

resulting in increased corrosion rates.

Fig.2.2.1:- Water wall side corrosion

2.2.2) Flue Gases:-


9

Problem: It most commonly seen as a series of circumferential cracks.

Usually found on furnace wall tubes of coal-fired once-through boiler

designs, but also has occurred on tubes in drum-type boilers.

Causes: Damage initiation and propagation result from corrosion in

combination with thermal fatigue.

Thermal cycling, in addition to subjecting the material to cyclic stress can

initiate cracking of the less elastic external tube scales and expose the tube

base material to repeated corrosion.

Fig.2.2.2:- Flue gases effect

2.3) Erosion :-


10

2.3.1) Fly ash:-

Problem: External tube wall loss and increasing tube strain.

Causes: It usually is associated with coal firing, but also can occur for certain types of

oil firing.

Ash characteristics are considered in the boiler design when establishing the size,

geometry and materials used in the boiler. Combustion gas and metal temperatures in

the convection passes are important considerations.

Damage occurs when certain coal ash constituents remain in a molten state on the

super heater tube surfaces. This molten ash can be highly corrosive.

Fig.2.3.1:- Fly Ash

2.3.2) Erosion In Tube:-


11

Problem: Damage will be occurs on inner side of the tube. Ultimate failure results

from rupture due to increasing strain as tube material erodes away.

Causes: Erosion of tube surfaces occurs from impingement on the external surfaces.

The erosion medium can be any abrasive in the combustion gas flow stream, but most

commonly is associated with impingement of fly ash or soot blowing steam.

Fig.2.3.2:- Erosion in tube

2.4) Fatigues:-


12

2.4.1) Mechanical Fatigue:-

Problem: Damage most often results in an outer diameter (OD) initiated crack. Tends

to be localized to the area of high stress or constraint.

Causes: Fatigue is the result of cyclical stresses in the component. Distinct from

thermal fatigue effects.

Mechanical fatigue damage is associated with externally applied stresses. Stresses

may be associated with vibration due to flue gas flow or soot blowers (high-frequency

low-amplitude stresses), or they may be associated with boiler cycling (low-

frequency, high-amplitude, stress mechanism).

Fatigue failure most often occurs at areas of constraint, such as tube penetrations,

welds, attachments or supports.

Fig.:2.4.1:- Mechanical fatigue

2.5) Heating:-


13

2.5.1) Short Term Heating:-

Problem: Failure results in a ductile rupture of the tube metal and is normally

characterized by the classic “fish mouth” opening in the tube where the fracture

surface is a thin edge.

Causes: Short-term overheat failures are most common during boiler start up.

Failures result when the tube metal temperature is extremely elevated from a lack

of cooling steam or water flow.

Tube metal temperatures reach combustion gas temperatures of 1600°F or greater

which lead to tube failure.

Fig.2.5.1:- Short Term Heating

2.5.2) Long Term Heating:-


14

Problem: Tube metal often has heavy external scale build-up and secondary

cracking. Results in tube failure.

Causes: Long-term overheat occurs over a period of months or years. Super

heater and reheat super heater tubes commonly fail as a result of creep.

Furnace water wall tubes also can fail from long-term overheat. In the case of

water wall tubes, the tube temperature increases abnormally, most commonly

from waterside problems such as deposits, scale or restricted flow.

Fig.2.5.2:- Long term Heating

2.6) Lack Of Quality Control:-


15

2.6.1) Maintenance Damage & Metallurgy Defects:-

Problem: Maintenance damage & Material defect are occurs due to methods &

materials used in analysis of boiler tube.

Cause: If proper methods & material not used in it , then it cause in boiler. High

quality methods & material to be used in boiler tube failure are more expensive.

Insufficient methods & material cause heavy damage in boiler.

2.6.2) Welding Defects:-

Problem: Failure is occurs due to dissimilar metals in welding process.

Causes: Failures at dissimilar metal welding locations occur on the ferrite side of

the butt weld.

These failures are attributed to several factors: high stresses at the austenitic to

ferrites interface due to differences in expansion properties of the two materials,

excessive external loading stresses, and creep of the ferrites material.

Fig.2.6.2:- Welding Defects

Feed Water Treatment & Chemical Cleaning CHAPTER 3


16

3.1) Objectives of Treatment:-

Treatment and further conditioning of water are necessary for the following

objectives:

To prevent scaling internals of pressure vessels due to dissolved and suspended

impurities,

To prevent corrosion of metallic parts of the boiler, with which water / steam

come in direct contact,

To establish protective coating over metallic surfaces to prevent corrosion attack.

To avoid salt deposits over turbine blades,

To ensure better utilization of heat energy and to improve on efficiency,

In order to ensure to achieve above objectives, following processes of water

treatment are adopted:

1. Pre treatment

2. Demineralization

3. Chemical conditioning

Following are the impurities in the natural water which are to be cleaned.

1) Dissolved impurities:-

Mainly the dissolved solids found in water are mineral salts. These

contaminants in water exist as salts of calcium, magnesium and sodium.

To a lesser extent, potassium and iron salts are also present. Nitrates and

silicates of such substances are also found to a small degree. Very rarely,

Phosphates and a few heavy metals are also found in natural water.

The quantity and composition of the solids present depend upon the soil and

strata details and the origin of water.

Various gases, mainly Oxygen, and others like Carbon dioxide and Hydrogen

Sulfide are normally present in dissolved form and the presence of such

dissolved gases alter the composition and concentration of certain salts.


17

2) Dissolved Oxygen:-

Dissolved oxygen plays significant role in boiler feed water. The oxygen

accelerates corrosion of water tube material. Mainly oxygen is removed only

in De-aerator. Hence performance of De-aerator is very important.

De-aerator performance has to be maintained limiting dissolved oxygen to less

than 0.01 ppm level. A figure < 0.007 ppm is considered to be very ideal.

3) Total Solids:-

The basic idea is to restrict impurities in feed water as low as possible in order

to avoid rise in boiler water concentration and deposition of metal oxides in

the boiler.

4) Organic Matter:-

It is very difficult to eliminate organic matter totally from water. Presence of

organic matter is due to poor pretreatment practice, ineffective D.M. plant

performance, resin leaking or condenser leakage.

Quality of raw water at intake point is a deciding factor. Seasonal changes

play a vital role.

5) Chlorides:-

Presence of chloride in feed water is harmful to the system as whole. Hence it

is very advisable to limit chloride concentration in feed water, keeping in

mind, limit prescribed for boiler water and restriction as per drum pressure

ratings.

Leak proof condenser and efficient demineralization are essential prerequisites

to avoid contamination due to chloride.

Dosing chemicals fed to the drum can contribute in sizeable proportion to

chloride contamination if the chemicals are not of adequate quality. Very low

level chloride should be tested by selective ion electrode for accuracy.


18

6) Suspended and colloidal impurities:-

Clay and sand particles constitute a major portion of the suspended matter.

Very fine clay remains in colloidal state. Colloidal suspension of dye material

and certain organic contents, give color to water, in most cases.

7) General nature of dissolved contents:-

Normally the mineral salts dissolved in water are found to exist in the ionized

form. Generally there is no uniformity in quantity and proportion of such

dissolved salts, since their presence is mainly dependent on the sources of the

water.

However fair representations of composition of mineral salts, as widely found

in nature are brought out below:

CATION (Basic Radical) ANION (Acidic Radical)

Ca ++ (Calcium) HCO3- (Bicarbonate)

Mg++ (Magnesium) CO3-- (Carbonate)

Na+ (Sodium) SO4-- (Sulphate)

Fe++ (Iron) Cl- (Chloride)

Al+++ (Aluminium) NO3- (Nitrate)

PO4--- (Phosphate)

SiO2 (Silica)

3.1.1) Pre-Treatment of Water :-


19

Fig 3.1 Feed Water Treatment

3.1.1.1) Clarification:-

Pre- treatment to raw water is mainly to make it suitable for further processing of

water by Deionization units.

The water entering De-mineralizer plant should be free from suspended, colloidal

and organic impurities and the process of pretreatment plays a vital role in

ensuring proper feed input is made to Deionization units.

Presence of such suspended impurities adversely affects the deionization

properties of the resins, which will affect the end quality of Demineralized water.


20

Suspended & colloidal particles are removed by clarifying the water in a

clariflocator aided by suitable coagulating agents. It is further chlorinated to

achieve effective oxidation to combat organic contamination.

3.1.1.2) Precipitator Clarifier Section:-

A precipitator inlet flow control valve controls the raw water inlet flow into the

clarified water basin, through a level controller.

A manual by-pass valve is also provided to the level control valve, so that in case of

problem in the level control valve, manual operation can be done to maintain the

level.

The raw water enters the inner mixing zone through an open channel from the top and

flows downward into the inner conical tank.

The chemicals are let into the open channel to get mixed thoroughly with the raw

water flowing along the open channel into the mixing chamber. A water flow

indicating mechanism is fitted in the open channel to indicate the raw water flow.

Ferrous sulphate solution is delivered into the “Precipitator” by means of twin head

proportional feed 5% solution dosing pump.

The following chemical equations illustrates the chemical reaction of lime with the

calcium and magnesium bicarbonates:

Ca ( OH ) 2 + CO2 CaCO3↓ + H2O

Ca ( OH )2 + Ca ( HCO3 )2 2CaCO3↓ + 2H2O

2Ca ( OH )2 + Mg ( HCO3 )2 MgCO3 + CaCO3↓ + 2H2O


21

Ca ( OH )2 + MgCO3 Mg( OH )2↓ + 2CaCO3↓

The chemical reaction of Ferrous Sulphate with calcium bicarbonate is as

illustrated below:

4FeSo4 + 4Ca (HCo3) 2 + O2 4Fe ( OH )3 + 4CaSo4 + 8 Co2 + 12 H2O

(Insoluble)

The colloidal produced by Ferric Hydroxide in this reaction is negatively charged,

and is an effective coagulant of the positively charged, colloidal precipitates

formed in the reaction of Lime with salts causing temporary hardness.

The precipitated chemical in this reaction with the Calcium and Magnesium salts,

form an effective sludge in layers, which can be easily removed by blowing down.

Whatever the residual calcium, magnesium and sodium salts and silica still

present in the clarified water are removed by the successive ion exchange process

with the resin beds in the water de-mineralizing plant.

3.1.1.3) Filtration:-

Water filtration is the process of separating suspended and colloidal impurities

from water by passage through a porous medium. A bed of granular filter material

or media is used in most plant application.

A filter may be defined simply as a device consisting of a tank, suitable filter

media and necessary piping, valves and controls.

Filters are designed for the following:


22

Gravity flow: with natural head of water above the filter bed and low point of

discharge at the filter bottom, providing the pressure differential needed to move

the water through the filter bed.

Pressure units: which, as their name implies, are operated on line, under service

pressure, filtering the water as it flows the tank on its way to service or storage

Backwashing:-

When differential head between inlet and outlet increases, the filters must be taken

out of service and backwashed for removing accumulated dirt and sludge

materials.

For back wash the flow direction is reversed that is the inlet is given at the bottom

and water from the top is delivered out to open canal.

When the water flows in reverse direction it carries all the accumulated dirt and

sediments and throws out along with the water. The back washing is to be done

until the out flowing water is perfectly clear of all accumulated sediments.

Chlorine removal:-

To avoid algae formation chlorine is dosed into raw water, before the process of

clarification in the clariflocator.

Presence of chlorine is harmful to the Ion Exchange Resin and it is essential to

ensure that even the traces of chlorine are not present in the clarified and filtered

water, before admission to deionizing unit.

Hence before admission to cation vessel, the water is once again filtered treated in

a vessel containing a bed of Activated Carbon. Activated Carbon Bed absorbs left

over chlorine and the organic matters and feeds clear water to the ion exchange

vessels.

3.1.2) De-mineralization:-


23

Demineralization is the removal of dissolved ionic impurities that are present in

water. Demineralized water is commonly produced by one or a combination of the

following processes:

1. Ion exchange

2. Membrane desalination

3. Thermal desalination

The method selected to produce demineralized water depends on the quality of the

influent water, the required quality of the effluent water, the availability of resources

such as regenerant chemicals and waste water treatment and disposal requirements.

The economics of the processes that produce acceptable effluent quality must be

evaluated to determine the most cost-effective method for a specific application.

3.1.2.1) Ion Exchange Process:-

Basically the minerals present in water are in ionized condition. Ion exchange

demineralization therefore is one of the most important and widely applied

processes for the production of high-purity water for power plant services, and

it is accomplished using resins that exchange one ion for another.

Cation resins are solid spherical beads with fixed negatively charged sites and

exchangeable positively charged sites. Anion resins are solid spherical beads

that have fixed negatively charged sites and exchangeable negatively charged

sites.

In their regenerated state for demineralization applications, Cation resins are

in the hydrogen form and anion resins are in the hydroxide form. The reactions

of the resin beads with the dissolved impurities in the water are represented by

the following:

Cation resin: R-H+ + C+ ↔ R-C+ + H+


24

2R-H+ + C2+ ↔ R2-C2 + 2H+

Anion resin: R+OH- + A- ↔ R+A- + OH-

2R+OH- + A2- ↔ R2+A2 + 2OH-

Where, R = resin matrix and fixed charge site;

C = cations such as Ca2+, Mg2+, and Na+; and

A = anions such as HCO3-, Cl-, and SO4

-2

The hydrogen ions (H+) displaced from the cation resin react with the

hydroxide ions (OH-) displaced form the anion resin. The net effect is the

dissolved ions are removed from the water and replaced by pure water (H2O).

The ion exchange resins are contained in ion exchange pressure vessels. The

ion exchange resin in the vessels is referred to as the resin bed. This process of

exchanging dissolved impurities is cyclic.

When a resin bed site is exchanged with a dissolved ion, the site becomes

“exhausted” and cannot remove other impurities without releasing an

impurity.

Exhausted resins must be regenerated to return the resin beads to the original

hydrogen form for cations and hydroxide form for anions before further ion

exchange can take place.

Cation resins are commonly regenerated with a strong acid solution of either

sulfuric or hydrochloric acid.

Sulfuric acid does not present the fuming problems associated with

concentrated hydrochloric acid and is easier to handle (material selection).

Consequently, sulfuric acid is frequently the recommended regenerant for

cation resins.


25

Anion resins are commonly regenerated with a sodium hydroxide solution. As

can be seen from the regeneration reactions listed below, regeneration is the

reverse reaction to the impurity exchange reactions.

Cation resin regeneration: 2R-C+ + H2SO4 ↔ 2R-H + C2+SO4

2-

R2-C2+ + H2SO4 ↔ 2R-H+ + C2+SO4

2-

Anion resin regeneration: R+A- + NaOH ↔ R+OH- + Na+A-

R2+A2- + 2NaOH ↔ 2R+OH- + Na2

+A2-

3.1.3) Chemical conditioning of Boiler water:-

Demineralization of water renders raw water fit for use as make up and feed to a

boiler. Just feeding a boiler with Demineralize water, which is free of all

impurities, alone, will not suffice in reality due to the following reasons.

a) When the steam is continuously being generated and released for process,

the water inside the drum gets more and more concentrated, even with the

very little and negligible level of impurity in feed water.

b) Further the pH of Demineralized water normally remains to be close to

neutral and will induce low pH corrosion of tube and pipe materials.

c) Demineralized water when under storage absorbs atmospheric air and the

level of dissolved Oxygen in water increases. Presence of Oxygen in feed

water again renders it corrosive.

Chemical treatment is done at various stages for feed and boiled water, steam and

return condensate, in order to ensure adequate protection is established over the

entire system, to help prevention of direct attack on parent metal by the corrosive

elements present in water / steam cycle.


26

The chemical treatment technique is based on several factors. While the overall

conditioning concept remains the same, there would be variations in the methods

adopted and the residual levels optimized, it depends mainly on:

a) Deferent grades of ferrous and alloy metals which are deployed in the

design and manufacturing of various pressure part components, such as,

water wall tubes, headers, raiser tubes, economizers, super heaters,

reheaters etc. and so many other equipment down the line such as turbines,

condensers, various stages of regenerative system heaters and the pumps,

b) Working temperature and pressure at which the steam is generated for

supply to end users,

c) The type of fuel used and the firing system adopted,

d) Release of heat flux density, depending on the fuel fired and the design

and size of the furnace,

e) Capacity of boiler for steam generation and the level of steam purity

expected,

f) Quality and quantity of condensate recirculated within the system.

The condensate extracted from the Condenser is passed through a De-aerator to

get rid of dissolved oxygen, by mechanical stripping action.

The feed water is then chemically conditioned further with dosing of chemicals to

ensure that the dissolved oxygen content of the water is reduced to the minimum.

Hydrazine hydrate is normally employed for this purpose.


27

3.2) De-Aerator:-

3.2.1) Function Of De-Aerator:-

• To remove the oxygen from feed water to be fed to Boiler.

• It is a direct contact type heater and is located between LP heater and Boiler feed

pump in feed water cycle.

• The steam from suitable turbine stage is drawn to heat the condensate which is broken

in fine particles for effective heating and mass transfer on non-condensable gases.

3.2.1) Flow Arrangement :-

The condensate to be de aerated enters the chamber at a top and is sprayed by variable

orifice spray valves. The water then falls through perforated tray stack or spill over

tray track by gravity.

The condensate is divided in to fine droplets and comes in the contact with the steam

resulting in release of non-condensable gases carried by steam moving up wards.

The condensate leaving the de aerator enters and collected to feed storage tank and

goes to Boiler feed pump

3.2.2) Design And Construction:-

The De-aerator removes the non-condensable gases by heating the condensate which

reduces the solubility of non-condensable gases and then allow mass transfer time to

reduce non condensable further.

The steam flowing upward provides the atmosphere for mass transfer and condensate

droplets while carrying away non condensable gases. The trays divide the condensate

in to fine droplets so that heating and mass transfer is achieved quickly.

The steam admittance in storage tank provides steam blanket over stored condensate

and thus eliminate sub cooling and re absorption of gases.


28

Fig 3.2 De-aerator

3.2.3) Main Parts Of De Aerating Header:-

1) Shell:-

It is fabricated from steel plate of boiler quality closed with dished end. The shell

is mounted over the feed storage tank. It contains steel trays. The distribution

chamber is located at the top in which spray valves are provided for spraying

condensate.

2) Spray Valves And Trays:-

• Spray valves are spring loaded disc. The variable opening is controlled by flow.


29

• The tray design is of two type (1) Perforated trays and

(2) Spill over trays.

• Perforated trays have no of small size holes through condensate falls down. The

annual and central opening is for flow velocity of steam, non-condensable and

pressure equalization.

• In case of spill over tray a series of channel type trays are arranged such that,

condensate spills over from one layer to other layer.

3.2.4) Accessories:-

1) Safety relief valve

2) Orifice plate

3) Vacuum breaker valve

4) Vent and drain valve

5) Stand pipe

6) Isolation valves for stand pipe and instruments

7) Level gauge

8) Level switch

9) Pressure gauge

10) Temperature gauge


30

3.3) Quality of feed water to boiler:-

Quality requirement for feed water is very much dependent on drum operating

pressure. Hence recommended feed water quality is always expressed for a

particular range of boiler operating pressure.

Table No. 3.3.1 :- Quality of feed water

Drum operating Pressure Kg/Cm2 60 80 100 120 130 &

above

pH at 25 °C 8.5~9.

0

8.5 ~

9.0

8.7 ~ 9.0 8.8 ~ 9.2 8.8~ 9.2

Electrical conductivity at 25 °C in

micro mhos/cm

< 0.5 < 0.5 < 0.4 < 0.3 < 0.2

Economizer inlet < 0.01 < 0.01 < 0.005 < 0.005 < 0.005

Total iron ( Fe ppm ) < 0.03 < 0.01 < 0.01 < 0.01 < 0.01

Total copper ( Cu ppm ) <

0.005

< 0.005 < 0.003 < 0.003 < 0.003

Total iron copper & nickel ppm < 0.03 < 0.02 < 0.02 < 0.02 < 0.01

Total silica ( SiO2 ppm ) < 0.02 < 0.02 < 0.02 < 0.02 < 0.01

Carbonic acid (CO2 ppm ) Nil Nil Nil Nil Nil

Organic matter mg KmnO4/Litre Less than 0.1 ppm is considered tolerable

Oil ppm Nil Nil Nil Nil Nil

Hydrazine N2H4 ppm ) < 0.1 < 0.05 < 0.05 < 0.02 ~

0.05

< 0.02


31

MODIFIED DESIGN Chapter-4

4.1) Types of Modified Design

There are six types of modified design

1) Boiler feed pump and feed control design

2) Arrangement of Orifice Meter

3) Re-heater design

4) Piping Design

5) Design of Cooling Water Sump

6) Air Instrument design

4.2) Detail Description Of Designs:-

4.2.1) Boiler feed pump and feed control design:-

Fig 4.2.1(A), (B) and (C) shows the present & modified design of feed control

valve arrangement.

In fig. 4.2.1 (a) drum level controlled by the feed control valve by 60-70 percent

open and the differential pressure across the feed control valve is 15 kg per square

senti-meter by boiler feed pump scoop.

Fig.4.2.1 (b) shows the signal flow arrangement of feed water flow, drum level

and steam flow.

Fig.4.2.1 (c) shows the new modified arrangement of the whole control system.

So, the new drum level controlled by boiler feed pump scoop and feed control

valve 1 and 2 keeps fully open.

Now in modified design the differential pressure across the control valve is 0 to

0.5 kg per square senti-meter.


32

Fig 4.2.1(a) Feed Control Valve

Fig 4.2.1(b) Signal Flow


33

Fig. 4.2.1(c) Modified Boiler Feed Pump & Feed Control Valve

4.2.2) Arrangement of Orifice Meter :-

Fig. 4.2.2 (a) and (b) shows the piping arrangement of water flow form boiler

feed pump to the super heater with control valve arrangement.

In fig.4.2.2 (a), when the inlet pressure is 165 kg per senti-meter square then at

outlet the orifice meter gives the pressure reading is 160 kg per senti-meter

square. That means there is loss of pressure between inlet and outlet by the

control valve.

In fig. 4.2.2 (b) show the rearrangement of orifice meter to solve this pressure

drop across the boiler feed pump and super heater.


34

Fig. 4.2.2(a) Arrangement of Orifice Meter

Fig. 4.2.2 (b) New Arrangement Of Orifice Meter


35

4.2.3) Re-Heater design:-

Fig.4.2.3 shows the water flow from boiler feed pump to the re-heater.

Present pressure in boiler feed pump is 150 kg per senti-meter square. Due to this high

pressure the damage occurs in the spray nozzle.

To prevent this damage put the pressure control valve ahead of spray control valve

and also make one reservoir tank between the controller and spray control valve.

By this the pressure is reduced up to 50 kg per senti-meter square in reservoir tank.

So, in controller the pressure up to 0 to 100 kg per senti-meter square is maintained.

Fig 4.2.3 Re- heater Design

4.2.4) Piping Design:-

Fig. 4.2.4 (a) and (b) shows the piping arrangement of boiler feed pump line.

As shown in fig.4.2.4 (a) the pressure difference between boiler feed pump & header

pressure is 10 kg per senti-meter square. This pressure difference is occurs due to “T”

joint of the pipe line.


36

This pressure difference can be reduced by changing “ T ” type arrangement into “ C

” or “ Y ” type arrangement as shown in fig. 4.2.4 (b).

Due to “C” or “Y” type arrangement smooth flow of water is made.

Fig. 4.2.4(a) Piping Design


37

Fig. 4.2.4(b) New Piping Design

4.2.5) Design of Cooling Water Sump:-

The simple design is shown in fig. 4.2.5.

In cooling water sump house, make the simple spray type arrangement. By this the

natural air passes through the spray and natural cooling is occurs.

Due to this the temperature of the water can be reduced. So, cooling process can be

made easily in cooling water sump house.


38

Fig. 4.2.6 Cooling Water Sump Design

4.2.6) Air Instrument Design:-

Due to continuous use of air operated instrument (vacuum pressure operated), amount

of water and silica particles are increases which cause to stop the working of

instruments.

So, to solve this problem make one reservoir tank near the main valve. Due to this

silica and water particles fed in bottom of the tank and fresh air can be made and life

of the instrument can be increases.

Fig. 6 (a) and (b) shows arrangement of reservoir tank with control valve.


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Fig. 4.2.6(a) Air Instrument Design

Fig. 4.2.6 (b) New Air Instrument Design


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4.2.7) ADVANTAGES OF MODIFICATION:-

Followings are the advantages of Modified Design.

Ampere loading of Boiler Feed Pumps reduced by 40 A per Boiler Feed Pump.

Reduction in differential pressure up to 15 kg/cm2.

Reduction in feed control valve maintenance.

Reduction in generation cost.

Reduction in load on boiler master controller

Increases plant efficiency.

Reduction in instrument damages.

Easy handling of flow.

Better cooling of water in cooling water sump.

Increases instruments life.

Smooth flow is made.

Smooth control of Boiler Drum Level

Minimum movement of Boiler Feed Pump Scoop.


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CONCLUSION (WASTE DOCUMENTATION) Chapter-5

4.1) Types of Waste:- Solid & Liquid

4.2) Quantity of Waste: - As Per Defect.

4.3) Detailed Description:-

Waste of material is subjected to defect occurs in the Boiler. If problem

caused in it is large than amount of waste material is more.

But when boiler tube is fail then 1000 kiloliter water is waste. The water

used in it is distilling water and it cost is 8 Rs. Per liter. As per market rate.

So, minimum wastage of money is 80 lakhs per 1 times of failure. If the

failure is small or big. This is minimum cost in industry for wastage in

distilling water.

Another loss is Generation loss which occurs due to starting of Boiler. And

that cost is Rs.1.5 cr.

4.4) Modified Design:-

The modified design is very useful to industry to reduce the generation

cost and maintenance of control system.


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-: REFERENCES:-

Books:-

1) Reduced Boiler Tube Failure Analysis,

B & W (Babcock & Wilcox) power generation Group.

2) Power plant engineering, by Arora Domkundwar, Dhanpat

Rai & co.

Websites:-

www.swcc.gov.sa

www.PDHcenter.com

http://www.swcc.gov.sa/

Boiler Tube Failure Analysis

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