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Boiler Feed Water Treatment:
Need:
Boiler makeup of 1.5 to 2% of total flow is required tobe replenished for loss of water.
Fittings
Bearings o er ow own
Escape with condensable gases in deaerator, turbine glands,
and other causes
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Boiler Feed Water Treatment:
feeding it in to the boiler 1. Prevention of hard scale formation on the heating
surfaces
2. Elimination of corrosion . ontro o carry-over to e m nate epos t on on super
heater tubes, and
4. Prevention of silica de osition and corrosion dama e
to turbine blades
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Boiler Feed Water Treatment:
e Raw water s t ere ore F rst pre-treate an t en
demineralized.
For once throu h boilers and boilin water nuclear
reactors, which require high water purity, a condensatepolishing system is used to further polish the water.
Raw water contains a variety of impurities, such as
(1). Suspended solids and turbidity
2 . Or anics
(3). Hardness- Salts of calcium and magnesium
(4). Alkalinity- bicarbonates, carbonates and hydrates
. , , (6). Silica
(7). Dissolved gases- O2, CO2
The extent of pre-treatment depends on the source of raw water.
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Boiler Feed Water Treatment:
Externa reatment:
First step of pre-treatment of boiler is clarification- water is
chlorinated to revent biofoulin of e ui ment
The suspended solids and turbidity are coagulated byadding special chemicals- Aluminum sulphate, Al2(SO4)3
,
and Agitated
The coagulated matter settles at the bottom of the clarifier
If the turbidity of clarified effluent is high, positive filtration
is needed.
Both gravity filters and pressure type filters are used.
The pressure difference across the filtering medium in an indication
of solid accumulation
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Boiler Feed Water Treatment:
External Treatment:
When it reaches a given limit, the solids are removed from
t e e y ac was ng.
The filtration by activated carbon can absorb organics and
remove residual chlorine from chlorination rocess.
The dissolved salts of calcium and magnesium - Hardness
All scale forming impurities are calcium and magnesiumin the form of carbonates, bicarbonates, chlorides and
sulphates.
. The Softening of water can be done by lime-soda process,
phosphate process, zeolite process and demineralization.
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Boiler Feed Water Treatment:
Lime soda Process:
n me so a so en ng , a c um an magnes um
salts are removed using Lime(calcium hydroxide)
When this process is carried out at normal raw
water tem erature Cold Process- softenin
When this process is carried out at near boiling
oint Hot Process- softenin
Hot process is preferred for boiler water treatment
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Boiler Feed Water Treatment:
Lime soda Process:
e represen a ve reac ons are
Ca(HCO3)2 + Ca(OH)2 = 2CaCO3 + 2H2O
g 2 + a 2 = g 2 + a 2
MgSO4 + Ca(OH)2 = Mg(OH)2 + CaSO4 CaSO4 + Na2CO3 = CaCO3 + Na2SO4
CaCl2 + Na2CO3 = CaCO3 + 2NaCl
The Calcium carbonate and magnesium hydroxideare insoluble in water and settles to the bottom of
e vesse
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Boiler Feed Water Treatment:
Hot phosphate softening:
e ca c um an magnes um ar ness s remove
by using phosphate and caustic soda
3 4 2
hydroxide are precipitatedo
and above
phosphate softening
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Boiler Feed Water Treatment:
Hot phosphate reactions are:
a 3 a a2 3 2
a 3 + a3 4 = a3 4 + a2 3
3 2 2 2 3 2
4 3 4 3 4 2 2 4
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Boiler Feed Water Treatment:
Sodium Zeolite Softening:
a er can e so ene y pass ng roug a e
of sodium zeolite, the cations of calcium and
CaCO3 + NaZ = CaZ + Na2CO3
MgSO4 + Na2Z = MgZ + Na2SO4
Ca(HCO3)2 + Na2Z = CaZ + Na2(HCO3)2
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Boiler Feed Water Treatment:
The softening capacity of the bed can be
CaZ + 2NaCl = Na2Z + CaCl2
MgZ + 2NaCl = Na2
Z + MgCl2
e eo e so en ng s no ea or o ow ng
reasons:
.on zeolites
.
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Boiler Feed Water Treatment:
The Zeolite softening is not ideal for following
3. There can be silica gain in water from the zeolite
. ,
corrosive.
use of evaporators may be more effective.
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Boiler Feed Water Treatment:
Hydrogen Zeolite softening:
en wa er con a n ng ca c um, magnes um an
sodium ions is passed through a hydrogen zeolite,
bicarbonate, sulphate, chloride and nitrate radicals
are converted to their respective acids. (Ca) (Ca)
M HCO + H Z = M Z + 2H CO
(Na2) (Na2)
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Boiler Feed Water Treatment:
(Ca) (Ca)
g 4 2 g 2 4
(Na2) (Na2)
When the hydrogen zeolite becomes exhausted, its ac was e an regenera e w ac .
After being rinsed, it is ready for use again. The
. Mgz + H2SO4 = H2Z + MgSO4
a 2 4 2 a 4
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Boiler Feed Water Treatment:
Anion Exchangers:
n on exc angers can remove e an on e
chlorides , sulphates and nitrates (acid form)
materials which absorb them.
The reactions are
2HCl + RCO3 = RCl2 + H2CO3
+ = +
2HNO3 + RCO3 = R(NCO3)2 + H2CO3
.
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Boiler Feed Water Treatment:
Anion Exchangers:
en e ac c wa er s spraye n a s ower o
expose large surface area, the carbon dioxide is gas
H2CO3 CO2 + H2O
,
regenerated by backwashing with soda ash
+ = +
RSO4 + Na2CO3 = RCO3 + Na2SO4
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Boiler Feed Water Treatment:
Dem nera z ng P ant:
The process of removing dissolved solids in water by
.
Two types of resins, cation and anion are used
hydrogen ion is exchanged for cations calcium,
magnesium and sodium. The anion resins adsorb the anions chlorides, nitrates
and sulphates.
Both ion exchange process are reversible, and resinsare restored to their original form by regeneration
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Boiler Feed Water Treatment:
Dem nera z ng P ant:
A typical Demineralizing plant consists of a cation
, ,
silica adsorber in series
In the de asifier carbon dioxide as is removed b
aeration.
Silica in water is very detrimental at high pressure. Magnesium hydroxide is often used to adsorb silica
from water.
The dissolved solids from makeup water is removed byreverse osmosis or electrodyalysis.
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Boiler Feed Water Treatment:
High quality water is produced by demineralizing systems,
but the treated water can pickup impurities due to
condenser leakage from the circulating water and iron and
copper from pipelines.
condensate through large demineralizing vessel, called
mixed bed units, which contain both cation and anionresins.
The resins not only remove dissolved salts, but also acts as
Power plants using once through boilers and nuclear
reactors require high quality of water and use condensate
polishing system.