7. Boiler Feed Water Conditioning

8/9/2019 7. Boiler Feed Water Conditioning

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7. Boiler Feed-water Conditioning


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Water Molecules

Remarkable Properties ofWater1.Water vapor steam has a high energy content and is an effective

medium for transferring energy in industrial plant operations,

buildings, and homes

2. Great heat capacity than many substances, so it is an effective heat

transfer medium. For each incremental change in temperature, waterabsorbs or release more heat.

3. Water also releases more heat upon freezing than do other

compounds. It is the only chemical compound that is present in the

natural environment in all 3 states of matter.

4. High surface tension this causes water to rise in a capillary tube.This physical property (capillarity) is partly responsible for the system

of circulationdeveloped by living plants through their roots and tissue

system.


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Water Molecules

Remarkable Properties ofWater

5. High dielectric constant water molecules in contact with a crystal

orient themselves to neutralize the attractive forces between the ions

in the crystal structure. The liberated ions are then hydrated by these

water molecules. This keep ions from recombining and thus

precipitating from solution.6. Osmotic pressure This important phenomenon occurring in water

solutions related to dissolved matter (solutes), rather than to water

(solvent) is osmotic pressure.

If 2 aqueous solutions are separated by a membrane, water will pass

from the more dilute into the more concentrated one. This important

process controls the performance of all living cells.


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Water Molecules

Remarkable Properties ofWater

The formula for water H2O- by its self tells us only its composition and

molecular weight. It does nothing to explain the remarkable properties.

The remarkable properties of water are the result of its unique molecular

arrangement.

105o


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Water Molecules

Two hydrogen atoms are located 105O apart, adjacent to the oxygen

atom, so that the molecule is asymmetrical, positively charged onthe hydrogen side and negatively charged on the oxygen side. For

this reason water is said to be dipolar.

This causes the molecules to agglomerate, the hydrogen of one

molecule attracting the oxygen of a neighboring molecule. The

linking of molecules resulting from this attraction force is calledhydrogen bonding.

One of the consequence of hydrogen bonding is that molecules

cannot leave surface readily

Hydrogen bonding produces crystal arrangement that causes ice to

expand beyond its original liquid volume.


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Water

Water - The universal solvent

Water is a poor conductor of electricity

As more ions dissolve, water becomes a better conductor

As the conductivity of water increases, the potential for corrosion andscale formation increases

The polar charges on the water molecule make it an exceptional

solvent


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Water

Water - The universal solvent

Water contains various types of impurities

1. Dissolved solids magnesium, calcium, iron, silica

2. Dissolved gases oxygen, carbon dioxide/monoxide, ammonia3. Suspended solids silts, clay, sand


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Effect ofWater Impurities on Boiler Operation

Certain impurities have harmful effects on effective and economical boileroperation when they are present in feedwater.

Scale forming impurities

1. Dissolved calcium carbonates, calcium sulphate, magnesium

carbonate, magnesium chloride, magnesium sulfate and silica.

2. Some suspended solids particularly salts of a wide variety ofchemical make up coming from industrial wastes which abound in

the waters from rivers.

Corrosion

1. Corrosion results from dissolved carbon dioxide and oxygen.

2. Grease and oil from animal or vegetable sources in feedwater has a

specific ultimate corrosive effect.

3. Much of the organic and sewage content of impure water also tends

to increase corrosion.


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How Scale is Formed

The basic types of scales encountered in the steam boiler

are salts of calcium and magnesium. The troublesome

scales are usually carbonate and sulfates.

Silicate scale will develop in high pressure operations where

there is appreciable clay or source of alumina.

Scale can be form either by physical attachment to the

metal of precipitated calcium or magnesium salts or by

crystallization on the metal direct from the boiler water.


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Scale Control

Scale control are generally concerned with the control of carbonate or

salts of calcium and magnesium.

External treatment of the feedwater can reduce sources of scales to

such low level that subsequent treatment is reduced to minimum.


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Water Treatment

The objective of water treatment is to eliminate internal waterconditions that lead to scale formation, corrosion, embrittlement andcarryover.

The prevention of scale formation becomes a problem of puttingthese salts into such a condition that they cannot or will not deposit

in either way. The best answer therefore, lies in treatment of the boiler water in

accordance with its content of scale forming salts or solids.

One treatment is based on the fact that it is possible to precipitatecalcium and magnesium salts in a form that will no longer crystallizeon the boiler metal to form scale.


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Water Treatment

Generally water treatment is approached two ways:

1. External water treatment

2. Internal treatment


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Water Treatment

External water treatment process

1. Clarification suspended solid removal

- Coagulation

- Flocculation

- Sedimentation

- Filtration

2. Softening hardness removal

- Precipitation

- Ion exchange- Demineralizer


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Water Treatment

External Water Treatment

Clarification

Turbid raw water contains suspended matter - both settleable solids

(particles large enough to settle quiescently) and dispersed solids

(particles which will nor readily settle).

A significant portion of these nonsettleable solids may be colloidal.Each particle is stabilized by negative electric charges on its surface,

causing it to repel neighboring particles just as magnetic poles repel

each other.

Above prevents these charged particles from colliding to form larger

masses, they do not settle.


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Water Treatment

ExternalWater TreatmentClarification - removal of suspended solids

1. Coagulation

Colloidal species encountered in raw water include clay,silica, ironand other heavy metals, color and organic solids.

The destabilization (charge neutralization) of the colloidal

particles in the water caused by the addition of the coagulantwhich neutralizes the electrical charges of the particles.

Colloids are categorized as hydrophobic (water hating) orhydrophilic (water loving).

Hydrophobic colloids do not react with water. Most naturalclay are hydrophobic.

Hydrophilic colloids react with water. The organics causingcolor are hydrophilic.

Aluminum salts, iron salts, or polyelectrolytes are thechemical coagulants usually used.

Coagulant used here ferric chloride


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Water Treatment

Coagulation cont

Colloids always require coagulation to achieve aneffective size and settling rate

Mixing is required to supplement coagulant addition todestroy stability in the colloidal system.

Zeta potential is the measurement of colloidal particlecharge strength. For natural water in a ph range of 5 to 8,the zeta potential is generally -14 to -30 mv. The morenegative the number, the stronger the particle charge.


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Water Treatment

2. Flocculation The coagulated particles formed by the agglomeration of

several colloids may not be large enough to settle ordewater at the desired rate.

A flocculant gathers together these particles in a net,bridging from one surface to another and binding the

individual particles into large agglomerates (grouping andcompacting of coagulated particles into largerassemblages called floc)

Alum, iron salts, and high molecular weight polymers arecommon flocculants

Flocculant used here sodium aluminate


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Water Treatment

3. Sedimentation Removal of suspended solids from water by gravitational

settling.

The velocity of the water must be reduced to a point where

solids will settle by gravity if the detention time in the

sedimentation vessel is great enough. Settling rate of particles is affected by their size, shape, and

density as well by the liquid they are settling through

As a particle settles, it accelerates until the frictional drag of its

surface against the liquid equals the weight of the paticle in the

suspending liquid.


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Water Treatment

Sedimentation contSteps in settling of particulates in water

1. Particles at first fall freely through the water.

2. As they come closer together, their rate of sedimentation is

restricted, and settled sludge volume increases. In the final

stages, compaction or compression becomes very slow- Hindered settling is reached as particulates become so

close to each other that the passages between them restrict

the ability of water to escape from the sludge

- Compaction occurs naturally, but slowly, by gravity and by

dehydration of the particulates


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Water treatment

Polyelectrolytes Polyelectrolytes are large water soluble organic molecules made up of

small building blocks called monomers, repeated in a long chain.

Structure contain an ion exchange sites which give the molecule an

ionic charge.

Cationic positive charge, Anionic negative charge They react with colloidal material in the water by neutralizing charge or

by bridging (tying together) individual particles to form a visible,

insoluble precipitate or floc

Polyelectrolytes used here -


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Water Treatment

Test Used to Select Best Chemical and Dosage Level for Clarification

1. Jar test (Used for stream with 5000 mg/l suspended solids)

Jar Test

1. Measured coagulant is measured into a sample of turbid waterwith a high degree of mixing [to disperse it in the water and

promote increased frequency of collisions. The duration may be

short, less than 1 min. The actual mixing time is refined as the test

regimen proceed (in essence, defining the optimum G factor.

2. Particle growth starts to occur because of charge neutralization.3. Additional coagulant or flocculant is added during the last few

seconds of rapid mix if required


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Water Treatment

Test Used to Select Best Chemical and Dosage Level for Clarification

1. Jar test (Used for stream with 5000 mg/l suspended solids)

Jar Test

4. In the slow mix period which follows, floc building proceeds untilthe floc becomes so big that shear forces finally overcome the

bridging forces, breaking the floc apart, limiting floc size

5. After slow mixing for optimum period of time, found only by

repeated test (usually 5 to 20 mins), the jars are allowed to settle

for 5 to 10 mins.


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Water Treatment

Cylinder Test

Designed to indicate how fast the suspended solids will settle

1. The slurry sample is placed in a cylinder, chemical is added and

cylinder is stoppered

2. Cylinder is gently inverted several times to mix3. After mixing cylinder is set upright and the interface between the

water and the settling solids observed.

4. Time and solids level are recorded

5. Data are plotted on a graph.

Note: On both tests different chemicals [and at different dosages] are

run side by side to determine most effective products.


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Water Treatment

ExternalWater Treatment

Filtration -Water is filtered by passing it through fine strainers or other

porous media to remove suspended solids mechanically

1. Granular Media Filtration

Generally applicable for removal of suspended solids in the

5 to 50 mg/l range where an effluent of less than 1 JTU

Medium normally silica sand or crushed anthracite

Backwashing water or water & air

Bed coarse to fine grain medium

2. Multimedia Filter Beds

Two layers (Dual media) Provide coarse to fine filtration in a down flow pattern

Medium Anthracite grains and silica sand

Different grain size and specific gravity


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Water Treatment

ExternalWater Treatment

Filtration

3. Septum Filters

Often referred to as DE (diatomaceous earth) filter

Above is used as a filter precoat

Applied as slurry to a porous septum to produce a filteringsurface


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Water Softening

Hardness Removal by Precipitation

Stability Index roughly predict whether a particular water will tend to

corrode metal or form a CaCO3 scale

Precipitation process makes use of the solubility product of a compoundcontaining an ion or radical that is considered detrimental, and that

should be removed before water is put to use

Key variable in precipitation

1. solubility

2. particle size3. temperature

4. time


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Water Softening Process


Treatment chemicals are generally used to lower solubility of scale

forming impurities and to precipitate them as sludge.

Either lime or caustic soda gives hydroxide for precipitation of

magnesium hydroxide.

Carbonate to precipitate calcium is usually added as soda ash.

Some carbonate is available when natural bicarbonate is converted to

carbonate form by use of lime or caustic soda.

When raw water does not contain enough magnesium to reduce silica

content, it is added as magnesium oxide or a component of dolomitic

lime.


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Lime and Soda Ash Reaction

1. Lime removes carbonate hardness as insoluble calcium carbonate

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

calcium bicarbonate calcium hydroxide calcium carbonate water

2. Soda-ash is the second step in removing noncarbonated hardness

CaSO4 + Na2CO3 CaCO3 + Na2SO4

calcium sulfate sodium carbonate calcium carbonate sodium sulfate

3. Soda ash (sodium carbonate) also reacts with calcium chloride to form

calcium carbonateCaCl2 + Na2CO3 CaCO3 + 2NaCl

calcium chloride sodium carbonate calcium carbonate sodium chloride

Cold process softening is the term used when these reactions are carried

out at room temperature.When water is heated above room temperature,

it is called hot process softening. Usually, above 212oF


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Water Softening Ion Exchange

Ion exchange

Removes unwanted ions from a raw water by transferring them to

a solid material, called an ion exchanger, which accepts them

while giving back an equivalent number of a desirable species

stored on the ion exchanger skeleton.

Ion exchangers sodium aluminosilicates, zeolite (greensand),

synthetic organic ion exchangers.

Ion exchangers used in water conditioning are skeleton like structure

having many ion exchange sites.

This insoluble plastic skeleton is an enormously large ion that is

electrically charged to hold ions of opposite charge

Types of ion exchangers cation and anion exchangers


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Water Softening Ion Exchange

Ion exchange

Exchangers with negatively charged sites are cation exchangers

because they take up positively charged ions

2 types of operations sodium cycle and hydrogen cycle

Sodium cycle regenerated by sodium salt (ie NaCl)

Hydrogen cycle regenerated by Acid (ie H2SO4)

Anion exchangers have positively charged sites and take up negativeions. 2 general varieties weak base and strong base exchangers

Anion exchangers are regenerated using alkali (i.e. NaOH, NaCl)

Plastic structure is porous and permeable, so entire ion exchange

particle participates in the process.


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Ion Exchange Operation

Cation Exchangers

Softening

Assume a unit, filled with styrene base resins, and start running

water through it. Calcium and magnesium ions in the water are

replaced with sodium ions from the resin. This is called sodium cycle

operation.

If hydrogen cycle is used, the calcium, magnesium, and sodium

ions in the water would be traded for hydrogen ions.

On both cases, the trading or exchange of ions continues until the

resin run out of ions to exchange. This is called breakthrough point

and when it occurs the exchanger is taken out of service.


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Regeneration

The first step in regeneration of the resins is backwashing.Whereas the normal flow is downward, this initial backwash of the

resin is upward. This upflow lifts the resin and scrubs off dirt and

debris.When the upflow is stopped the resin settles into a uniform

bed, decreasing the posibility of direct water flow through the bed

(channeling).

Next the regenerant chemical is added to the resin bed. Sodium

cycle units use salt at a concentration of 10 to 12 as regenerant.

Various acids are used for hydrogen cycle regeneration with sulfuric

and hydrochloric acid being the most common. Acid concentration islow at first with 2 % being maximum. Further into the process it may

reach 6 %.


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The amount of regenerant is calculated to provide the greatest

exchange capacity for the amount of chemical added

Once the regenerant has been added, the resin is rinsed, usually intwo stages to wash away excess regenerant. Now the resin is

ready for service and the cycle begins again.

A common method used in treatment today is stream softening

where the outlets of hydrogen and sodium cycle are combined.

Reason is to use the alkalinity in sodium cycle water to neutralize

the slight acidity in the hydogen cycle effluent.


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Hardness Removal - Ion Exchange

Zeolite Softening Systems (sodium cycle operation)

Impurities that dissolve in water dissociate to form positively and negatively

charged particles known as ions. These impurities or compounds are

called electrolytes.

1. The positive ions are called cations because they migrate to thenegative electrode (cathode) in an electrolytic cell.

2. Negative particles are then anions since they are attracted to the anode.

3. These ions exist throughout the solution and act almost independently.

For example, magnesium sulfate (MgSO4) dissociates in solution toform positive magnesium ions and negative sulfate ions.

4. Generally, all natural waters contain electrolytes in varying

concentration.


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Hardness Removal Ion Exchange

Zeolite Softening Systems (sodium cycle exchanger)

5. Ion exchange material has the ability to exchange one ion for another,

hold it temporary in chemical combination, and give it up to a strong

regenerating solution.

6. Since sodium zeolite method works by replacing calcium and magnesium

In the raw water with sodium, there is no precipitation of solids from the

water. The actual content of dissolved solids in the water is increased.

7. The treated water will contain sodium carbonate and bicarbonate from the

carbonate hardness And sodium sulfate, sodium chloride and sodium

nitrate from non-carbonate hardness.


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Hardness Removal Ion Exchange

Zeolite Softening Systems (sodium cycle exchanger)

Chemical Reactions

1. Softening (exhaustion)

Sodium cation exchangers swap sodium ions for other metal ions

Ca2+ + Na2X CaX + 2Na+

Mg2+ + Na2X MgX + 2Na+

2. Regeneration (brining)

Regeneration restores exchanger capacity by replacing calcium

ions with sodium

2NaCl + CaX Na2X + CaCl22NaCl + MgX Na2X + MgCl2


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Hardness Removal Ion ExchangeHydrogen Cycle Exchanger

The development of ion exchange that could be regenerated with acid

to exchange hydrogen ions for cations in water provided the first practicalChemical process for removal of sodium, potassium, and ammonia, all of

whose

salts are extremely soluble

Chemical Reactions

1. Softening (exhaustion)

Hydro cation exchangers swap hydrogen ions for metal cationsCa2+ + H2X CaX + 2H

+

Mg2+ MgX

Fe2+ FeX

2Na+ Na2X

2NH4+ (NH4)2X

Ca(HCO3)2 + H2X CaX + 2H2O + 2CO2CaSO4 + H2X CaX + H2SO4


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Ion Exchange

Anion Exchanger

Anion exchange fall into 2 general categories weak base andstrong base. Strongly basic exchange material picks up acid radicals

and silica from a hydrogen unit, leaving essentially pure water

Weak Base Resins

Weak base exchangers dont take out carbon dioxide and silica.Theyremove strong acids by a process thats more like absorption than itis ion exchange. However the end result is the same, and efficiencyof strong acid removal is far superior to that of a strong base materialdoing the same job.

So weak base units are very useful when water is high in sulfatesand chlorides. In these cases they precede a strong base unit. Ifsilica isnt an objection, they handle the complete job ofdemeneralizing alone.


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Anion Exchanger (cont)

Strong Base Resins

Strong base resins are available as either type I or type II.

Type II resins capacity and efficiency is somewhat higher

than type I.

But type I is commonly used because of 2 main reasons:

1. Type I exchangers are usually more stable

2. They reduce silica to lower residual in range of 0.02 ppm.


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Ion Exchanger

Anion Exchanger

Chemical Reaction

1 Softening (exhaustion)

Anion exchanger takes on sulfate anion, gives up hydroxide to

form water

H2SO4 + X(OH)2

XSO4 + 2H2OStrong base anion exchanger takes on silica ion, gives up

hydroxide to form water

2H2SiO3 + X(OH)2 X(HSiO3)2 + 2H2O

2. RegenerationRegeneration with caustic restores capacity for hydroxide

exchange

XSO4 + 2NaOH X(OH)2 + Na2SO4


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Demineralization

Demineralization

Generally refer to external treatment scheme where relatively pure

water is desired

Conventional demineralization unit cation (hydrogen form)

exchanger follows by anion (strong base) exchanger

In this process, the anions in the cation effluent are exchanged for

hydroxide.

If there are cations present, such as sodium, the effluent will contain

sodium hydroxide and this is the most prominent factor affecting the

quality of finished demineralized water.


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Basic Types of Demins & Resin Used


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Deaeration

Dissolved oxygen in the feedwater is one major source of corrosion

in a steam boiler.

Oxygen is a dissolved gas that does not react chemically with water

and become less and less soluble as water temperature increases.

Oxygen can be removed by bringing water to the boiling point

corresponding to its operating pressure.


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Deaeration

Deaerator

Steam deaerator break up water into a spray or film and then sweepthe steam across and through it to force out dissolved gases likeoxygen or carbon dioxide.

A typical deaerator has a heating and deaerating section plus storage

for hot deaerated water. Sometimes a separate tank is providedalongside or underneath unit to hold 10 minutes storage at ratedcapacity.

Steam deaerators fall into 2 broad types.

1. Spray

2. tray

3. combination of both.


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Internal Water Treatment

Scale prevention

Phosphates

Phosphates are known to be effective precipitating agents.When used,

the calcium and magnesium salts causing scale formation are

precipitated in such form that there is negligible tendency to formadherent deposits.

Advantages:

Precipitate calcium and magnesium as insoluble phosphate salts which

has no marked scale forming tendencies. When used in conjunction with suitable dispersants, the precipitated

phosphates become integrated parts of the sludge formed.

Requires relatively lower concentration.


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Internal Water Treatment

Chelants

Chelants are also used to minimize scale formation in boilers. They are

the prime additives in solubilizing boiler water treatment program.

Advantages:

Ability to complex with, and prevent the deposition of many cations(hardness and heavy metals) under boiler water conditions by locking the

metal into a soluble organic ring structure.

Boilers with higher evaporation rates create conditions suitable for iron

oxide deposition. Chelants have the ability to complex iron deposits.

Forgiving type ability to remove deposits that may form when feedwaterquality or treatment control deviates


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Internal water Treatment

Dispersant

Boiler water dispersants are used in conjunction with precipitating(phosphate) or solubilizing (chelant) internal treatment programs.

Dispersants prevent deposition of solid particles and precipitates onheat transfer surfaces. Rather, a fluid sludge forms which collects inthe boiler mud drum and exits via blowdown.

Solid particles in boiler water can be carryover from raw water filters,clarifiers, or lime softeners, corrosion products from condensate

returns, or calcium and magnesium precipitates resulting fromhardness in feedwater.


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Condensate Treatment

Corrosion in condensate return lines presents 2 problems:

1. Equipment damage and frequent replacement of steam traps andcondensate lines

2. Corrosion products are frequently carried back into steam generating

equipment and can result in increased internal deposition and tubefailure within the boiler.


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The major causes of condensate corrosion are dissolved oxygen andcarbon dioxide

Dissolved oxygen attack is characterized by tuberculation, pitting, and

iron oxide buildup

Dissolved oxygen can enter the condensate system with boiler feedwater

or by direct infiltration

Carbon dioxide corrosion is characterized by a general thinning or

grooving attack, normally resulting from carbonate and bicarbonate

alkalinity decomposition in the boiler which liberates CO2

Carbon dioxide in the presence of water, forms carbonic acid

which can cause acidic attack


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Condensate corrosion can be controlled by:

neutralizing amines

filming amines

combination of both


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1. Neutralizing amines directly react with dissolved carbon dioxide,thereby raising ph and eliminating acidic attack. It will not preventattack by dissolved oxygen

2. Filming amines function by placing a protective

barrier film between the metal and corrosive environment

3. Combination treatment jointly employ neutralizing and

filming amines. The neutralizer functions to distribute the filmer toinsure against an over buildup and to promote the detergency of thefilming amine.


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Control1. Ph effect of neutralization can be seen directly

2. Iron/copper values indicate any increase or decrease in

corrosion

3. Corrosion coupon this are preweighed test specimens which are

inserted into condensate bypass piping. Periodically removed andreweighed.

Documents

7. Boiler Feed Water Conditioning