Feed Water Treatment

THERMAL POWER PLANTS
FEED WATER TREATMENT
1
Dr. S. VIJAYA BHASKAR
Professor in Mechanical
EngineeringSreenidhi Inst. of Science &
Technology, Hydereabad

Classification of impurities in water
1. Visible Impurities
◦ Microbiological growth: Presence of micro-
organisms is always undesirable as they
may produce clogging troubles
◦ Turbidity : Turbidity is the suspended
insoluble matter
2

◦ Sediments: Sediments are the coarse
particles (eg. fly ash) which settle down
in stationary water, both are objectionable
3

2. Dissolved Gases
◦ Carbon dioxide
◦ Oxygen
◦ Nitrogen
◦ Methane
◦ Hydrogen Sulphide
3. Minerals and Salts
◦ Iron and Manganese
◦ Sodium and Potassium salts
◦ Fluorides
◦ Silica
4

FEED WATER TREATMENT contd.
4. Mineral Acids
 Their presence in water is always
undesirable as it may result in the
chemical reaction with the boiler material.
material
5. Hardness
 Presence of salts is known
as hardness
 The salts of calcium and magnesium as
bicarbonates, chlorides, sulphates etc are mainly
responsible for the formation of a very hard
surface which resists heat transfer and clogs the
passage in pipes.
5

Troubles Caused By The Impurities In Water
6

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1. Scale formation
 The formation of scales reduces the heat
transfer and simultaneously raises the
temperature of the metal wall.
 Scale is due to salts of Calcium and Magnesium
and Silicates
 Scales choke the flow of the piping system and
requires increased pressure to maintain water
delivery.
 Resulting overheating, blistering
and rupturing.
 When scale is formed tubes are
cleaned with electric powered
rotary brushes and cutters are
pushed though the tubes during
boiler overhaul.

8
 The effect of CO2 neutralizes by
adding the ammonia for neutralizing
amines in water.
 The effect of oxygen is reduced by
removing the oxygen from water
 The effect of corrosion is prevented
by applying protective coating of
amines to the internal surfaces of
boilers and economizers.
2. Corrosion
 Corrosion is the eating away process of boiler metal.
 It is caused due to acids or low pH in addition to the
presence of dissolved oxygen and carbon dioxide in the
boiler feed water.

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3. Carry over
 Water solids carried over in the steam leaving a boiler drum are
called carry over.
 Foaming or priming of the boiler water forms the carry over
 Bubbles formed on the water surface is called foaming
 Foaming causes due to excessive amounts of sodium
alkalinity
 Priming refers to the vigorous and periodic surging of water in
the boiler drum and throws water slugs into the leaving
steam
 Foaming and priming are caused due to excessive steaming rate,
too high or fluctuating water level and improper boiler water
circulation.
 Foaming and priming can be checked by following precautions
 Valves do not open suddenly to maximum
 Water level should be minimum level
 Water should not contain oil, soap and suspended impurities

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4.Embrittlement
 The caustic Embrittlement is the weakening of
boiler steel as a result of inner crystalline cracks.
 Presence of sodium hydroxide causes
Embrittlement.
 Methods to control Embrittlement are
 Elimination of free sodium hydroxide from boiler
water
 Maintenance of correct ratio of sodium nitrate to
sodium hydroxide
 Using waste sulphite liquor.

METHODS OF FEED WATER TREATMENT
1. Mechanical Treatment : used for suspended solids
 Sedimentation
 Coagulation
 Filtration
 Interior painting
2. Thermal Treatment: used for dissolved gases
 De-aeration
 Distillation by evaporators
3. Chemical Treatment:used for dissolved solids
 Cold lime soda softening process
 Hot lime soda softening process
 Lime phosphate softening process
 Ion exchange process or sodium zeolite or hydrogen zeolite
process
4. Demineralisation
5. Blow down
 Hot lime soda and hot zeolite process
 Adding acids to control alkalinity and vice versa
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MECHANICAL TREATMENT
Sedimentation:
 In thisprocessthe wateris allowed tostandat stand-stillin big tanksso thatsolid mattersettles
down.
 The waterata very low velocitywill alsohavethe same process
 Thesesolidsaresettled downandremoved fromthebottomwitherperiodicallyorcontinuously.
 Clear wateris then drainedoutfromthe tanksurface
.
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MECHANICAL TREATMENT
Coagulation:
 Coagulation of minute colloidal suspensions makes them settle out
easily.
 Adding coagulation like aluminium sulphate or sodium aluminate
improves the sedimentation or filtration process.
 A reaction between these salts and alkalinity in the water forms a
gelatinous material which makes small particles adhere to each other,
forming larger particles that settle out or filter out more easily.
13

MECHANICAL TREATMENT14
Filtration
 The suspended matter which can not be removed during
sedimentation is removed with the help of filtration.
 The water is allowed to pass through a bed of fine sand or
graded sand and then a layer of gravels.
 The suspended matter adheres to the filter material leaving
the water clear as it drains from the bottom.
 The bed must be backwashed periodically to remove dirt
that collects in the voids of the filter material.
 In the pressure filter the water is forced through the filter by
means of a pump whereas in gravity filters the water flows
by gravity.

THERMAL TREATMENT
Deaeration
 The process of removing dissolved oxygen is known as Deaeration.
 If the water is heated to 110º C with suitable agitation the dissolved
oxygen is expelled.
 In this tray type deaerating heater feed water after passing through
the vent condenser is sprayed upwards in the spray pipe.
 Water falls in the form of uniform showers over the heating trays
and air separating trays and finally collected in the storage space.
 Steam enters the heaters through a nozzle fitted in the side of the
heater shell.
 The steam makes its way downwards though the perforations in the
top plate of tray compartment.
 While flowing downwards the steam comes in contact with the
falling water.
 Most of the steam condenses in between the spray and heating
trays.
 From the bottom of heating trays, the remaining steam and
separated gases flows to the cent condenser.
 The steam used for heating may be from turbine bled steam or from
other sources.
 Storage tank with controls help to add make up water when needed
to maintain the feed water level.
16

CHEMICAL TREATMENT
18
Lime Soda Softening Process
 This process uses calcium hydroxide and sodium carbonate, to remove
dissolved calcium and magnesium salt, by precipitating them.
 This may be done by cold process or hot process.
 The processes use these types of reactions:
All the carbonate hardness is precipitated out, leaving pure water.
The non-carbonate hardness precipitates, to be replaced by soluble sodium
salts.

ION EXCHANGE PROCESS
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Hydrogen-Zeolite process:
 Non silicious materials can be used to exchange the hydrogen ions for
cations as calcium and magnesium.
 The synthetic zeolites developed are sulphonated coal and polystyrene
resins which withstand acid and regenerated with acid could be used to
exchange hydrogen for the metals of salt in solution.

ION EXCHANGE PROCESS
20
 The equations for the reactions are
 The main merit of this process is that the water in carbonates including
sodium carbonate or bicarbonate can be softened easily without
replacing the water with some other salt.

DEMINERALISATION
21
 The mineral content of water may be removed by evaporation or by
series of cation and anion exchangers to produce essentially distilled
water.
 Demineralisation is often the most economical method to producing
make up water for HP boilers.
 In the diagram shows the series process for demineralisation.
 A simple demineraliser uses only the first two units.
 Strong base anion exchange materials absorbs weak acids as well as
strong acids.
 Weak base anion exchange materials are used for removing chlorides,
sulphates and nitrates.

PH VALUE OF WATER.
22
 pH value of water is the logarithm of
the reciprocal of hydrogen ion
concentration.
 It varies from 0 to 14 with 7 indicating
neutral water.
 Water sample which is having pH
value less than 7 indicates Acidity of
water and pH value more than 7 are
Alkaline.
 Acidic or alkaline nature of water
depends upon whether the hydrogen
or hydroxyl ions predominate.
 In a mixture of bases and water,
hydroxyl ions result.

PH VALUE OF WATER …
23
 Hydrogen ions make a solution acidic where as hydroxyl
ions make it alkaline.
 When a sample contains one (OH) ion for every (H) ion, the
acid effect of one balances the alkaline effect of the other.
 Result is a neutral solution with a pH value of 7.0
 This pH value of water can be determined by a pH meter.
 The pH meter is essentially a comparator which compares
sample colour with that of many standards to determine the
value.

Feed Water Treatment

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Feed Water Treatment