This document discusses boiler water treatment. It introduces the causes of impurities in boiler water such as hardness salts like calcium and magnesium which can lead to deposits and corrosion if not properly treated. It describes various water treatment methods like internal treatment using chemicals, external treatment processes like softening, demineralization and reverse osmosis. It also discusses the working of various treatment units like ion exchange softeners, deaerators and different types of membranes used in water purification. Recommended limits for various water quality parameters for different boiler pressures are also provided. Raw water analysis of the source water is given.

1. BOILER WATER TREATMENT

2. INTRODUCTION
• Producing quality steam depends on properly managed water
treatment to control steam purity, deposits and corrosion.
• Boiler performance, efficiency, and service life are direct products of
selecting and controlling feed water used in the boiler.
• Most of the components in the feed water are soluble. However,
under heat and pressure most of the soluble components come out
of solution as particulate solids, in crystallized forms and as
amorphous particles.
• When solubility of a specific component in water is exceeded, scale
or deposits will develop
• The boiler water must be sufficiently free of deposit forming solids to
allow rapid and efficient heat transfer and it must not be corrosive
to the boiler metal.

3. CAUSE OF IMPURITIES IN BOILER WATER
• Deposits and corrosion result in efficiency losses
• Deposits act as insulators and slows down heat transfer. Large
amounts of deposits throughout the boiler could reduce the heat
transfer to reduce the boiler efficiency and further result in tube
failures.
Hardness salts:
• The most important chemicals contained in water that influences
the formation of deposits in the boilers are the salts of calcium and
magnesium, which are known as hardness salts.

4. CAUSE OF IMPURITIES IN BOILER WATER
Temporary hardness:
• Calcium and magnesium bicarbonate dissolve in water to form an
alkaline solution and these salts are known as alkaline hardness.
They decompose upon heating, releasing carbon dioxide and forming
a soft sludge, which settles out. These are called temporary
hardness that can be removed by boiling.
Permanent hardness:
• Calcium and magnesium sulphates, chlorides and nitrates, etc.
when dissolved in water are chemically neutral and are known as
non-alkaline hardness. These are called permanent hardness and
form hard scales on boiler surfaces, which are difficult to remove.
Silica:
• The presence of silica in boiler water can rise to formation of hard
silicate scales. It can also associate with calcium and magnesium
salts, forming calcium and magnesium silicates of very low thermal
conductivity.
• Silica can give rise to deposits on steam turbine blades (carried over
either in droplets of water in steam, or in volatile form in steam at
higher pressures)

5. WATER TREATMENT TYPES
There are two major types of boiler water treatment :
 Internal water treatment
 External water treatment
• Softening
• Demineralisation

6. INTERNAL WATER TREATMENT
• Internal treatment is carried out by adding chemicals to boiler to
prevent the formation of scale by converting the scale-forming
compounds to free-flowing sludges, which can be removed by
blowdown.
• Different waters require different chemicals. Sodium carbonate,
sodium aluminate, sodium phosphate, sodium sulphite and
compounds of vegetable or inorganic origin are all used for this
purpose

7. EXTERNAL WATER TREATMENT
• External treatment is used to remove suspended solids, dissolved
solids (particularly the calcium and magnesium ions which are a
major cause of scale formation) and dissolved gases (oxygen and
carbon dioxide).
• The external treatment processes available are: ion exchange;
demineralization; reverse osmosis and de-aeration.
• Before any of these are used, it is necessary to remove suspended
solids and colour from the raw water, because these may foul the
resins used in the subsequent treatment sections.
• Methods of pre-treatment include simple sedimentation in settling
tanks or settling in clarifiers with the aid of coagulants and
flocculants. Pressure sand filters, with spray aeration to remove
carbon dioxide and iron, may be used to remove metal salts from
bore well water.

8. EXTERNAL WATER TREATMENT
• The first stage of treatment is to remove hardness salt and possibly
non-hardness salts.
Sofening:
• Removal of only hardness salts is called softening
De-mineralization:
• Total removal of salts from solution is called demineralization.

9. ION-EXCHANGE PROCESS (SOFTENER PLANT)
• In ion-exchange process, the hardness is removed as the water
passes through a bed of natural zeolite or synthetic resin and
without the formation of any precipitate.
• The simplest type is ‘base exchange’ in which calcium and
magnesium ions are exchanged for sodium ions. After saturation
regeneration is done with sodium chloride.
• The sodium salts being soluble, do not form scales in boilers. Since
base exchanger only replaces the calcium and magnesium with
sodium, it does not reduce the TDS content, and blowdown
quantity. It also does not reduce the alkalinity.
Softening reaction:
Na2R + Ca(HCO3)2 « CaR + 2 Na(HCO3)
Regeneration reaction
CaR + 2 NaCl « Na2R + CaCl2

11. DEMINERALISATION
• Demineralization is the complete removal of all salts. This is
achieved by using a “cation” resin, which exchanges the cations in
the raw water with hydrogen ions, producing hydrochloric,
sulphuric and carbonic acid.
• Carbonic acid is removed in degassing tower in which air is blown
through the acid water. Following this, the water passes through an
“anion” resin which exchanges anions with the mineral acid (e.g.
sulphuric acid) and forms water.
• Regeneration of cations and anions is necessary at intervals using,
mineral acid and caustic soda respectively. The complete removal of
silica can be achieved by correct choice of anion resin.
• Ion exchange processes can be used for almost total
demineralization if required, as is the case in large electric power
plant boilers

12. DE-AERATION
• In de-aeration, dissolved gases, such as oxygen and carbon dioxide,
are expelled by preheating the feed water before it enters the boiler.
• All natural waters contain dissolved gases in solution. Certain
gases, such as carbon dioxide and oxygen, greatly increase
corrosion. When heated in boiler systems, carbon dioxide (CO2) and
oxygen (O2) are released as gases and combine with water (H2O) to
form carbonic acid, (H2CO3).
• Removal of oxygen, carbon dioxide and other non-condensable gases
from boiler feedwater is vital to boiler equipment longevity as well as
safety of operation. Carbonic acid corrodes metal reducing the life of
equipment and piping. It also dissolves iron (Fe) which when
returned to the boiler precipitates and causes
• scaling on the boiler and tubes. This scale not only contributes to
reducing the life of the equipment but also increases the amount of
energy needed to achieve heat transfer.
• De-aeration can be done by mechanical de-aeration, by chemical de-
deration or by both together.

13. TYPES OF DEAERATION
Mechanical de-aeration
• Removal of oxygen and carbon dioxide can be accomplished by
heating the boiler feed water. They operate at the boiling point of
water at the pressure in the de-aerator. They can be of vacuum or
pressure type.
• The vacuum type of de-aerator operates below atmospheric pressure,
at about 82oC, can reduce the oxygen content in water to less than
0.02 mg/litre. Vacuum pumps or steam ejectors are required to
maintain the vacuum.
• The pressure-type de-aerators operates by allowing steam into the
feed water and maintaining temperature of 105oC. The steam raises
the water temperature causing the release of O2 and CO2 gases that
are then vented from the system. This type can reduce the oxygen
content to 0.005 mg/litre.
• Steam is preferred for de-aeration because steam is free from O2 and
CO2, and steam is readily available & economical

14. TYPES OF DEAERATION
Chemical de-aeration
• While the most efficient mechanical deaerators reduce oxygen to
very low levels (0.005 mg/litre), even trace amounts of oxygen may
cause corrosion damage to a system. So removal of traces of oxygen
with a chemical oxygen scavenger such as sodium sulfite or
hydrazine is needed.
• Sodium sulphite reacts with oxygen to form sodium sulphate, which
increases the TDS in the boiler water and hence increases the
blowdown requirements and make-up water quality.
• Hydrazine reacts with oxygen to form nitrogen and water.

15. REVERSE OSMOSIS
• Reverse osmosis uses the fact that when solutions of differing
concentrations are separated by a semi-permeable membrane, water
from less concentrated solution passes through the membrane to
dilute the liquid of high concentration.
• If the solution of high concentration is pressurized, the process is
reversed and the water from the solution of high concentration flows to
the weaker solution. This is known as reverse osmosis.
• The quality of water produced depends upon the concentration of the
solution on the high-pressure side and pressure differential across the
membrane. This process is suitable for waters with very high TDS,
such as sea water.

16. OSMOSIS

18. REVERSE OSMOSIS

20. WHAT IS A SEMI-PERMEABLE MEMBRANE
• It is a polymeric material of different pore size that will selectively
allow the permeation of water while restricting the permeation of other
substances that are dissolved in water. The larger solutes are retained
as reject or concentrate. Due to its asymmetric nature fouling is
eliminated

21. TYPES OF FILTRATION
Dead-end Filtration
• The most basic form of filtration is dead-
end filtration.
• The complete feed flow is forced through
the membrane and the filtered matter is
accumulated on the surface of the
membrane.
• The dead-end filtration is a batch process
as accumulated matter on the filter
decreases the filtration capacity, due to
clogging. A next process step to remove
the accumulated matter is required
• Dead-end filtration can be a very useful
technique for concentrating compounds

22. • With cross-flow filtration a constant turbulent flow
along the membrane surface prevents the accumulation
of matter on the membrane surface.
• The feed flow through the membrane tube has an
elevated pressure as driving force for the filtration
process and a high flow speed to create turbulent
conditions. The process is referred to as "cross-flow",
because the feed flow and filtration flow direction have a
90 degrees angle.
• Cross-flow filtration is an excellent way to filter
liquids with a high concentration of filterable matter
CROSS-FLOW FILTRATION

23. The hybrid flow process combines the dead-end and the cross-flow
principle.
•As in the cross-flow filtration tubular membranes are with the filtration
layer on the inside wall are used. The filtration process has two phases:
the production phase and the flushing phase.
• During the production phase, the tubes are closed on one side and a
dead-end filtration is performed. During the flushing phase, the tube is
open on both sides and the fraction that did not pass through the
membranes is removed in order to clean the membrane surface as in
cross-flow filtration.
•This filtration technique is especially suitable for treating water streams
containing suspended solids in low concentrations (polishing).
HYBRID-FLOW FILTRATION

25. TYPES OF MEMBRANES…..
Cellulose Acetate:
• The membrane material is of Cellulose Acetate which is the salt
rejecting layer
• The oldest form of membranes used in industries
• High tolerant to chlorine but low resistant to silica
• Becoming almost extinct due to its huge area requirement and other
disadvantages

26. TYPES OF MEMBRANES…..
• In CA membranes pores progressively become larger from the
salt rejecting layer to that of the reinforcing fabric
• A membrane with dense skin and progressively widening pores is
called as Asymmetric membrane

27. TYPES OF MEMBRANES…..
Thin Film Composite Membranes (TFC)
The semi-permeable membrane consists
of 3 layers:
• Feed spacer
• Permeate carrier or the semi-
permeable membrane
• Product carrier
• MOC - Polyamide

29. TYPES OF MEMBRANES…..
Hollow Fiber membranes:
• Consists of numerous hair like
membranes combined together to
form a single module
• MOC - PS, PAN

30. HOLLOW FIBER MEMBRANES - A VIEW

31. TYPES OF MEMBRANES BASED ON PARTICLE
SIZE DISTRIBUTION….
• Microfiltration - 0.1 microns
• Ultrafiltration - 0.1 - 0.01
microns
• Nanofiltration - 0.01 - 0.001
microns
• Reverse Osmosis - 0.001 - 0.0001
microns

33. VIEW OF UF MEMBRANES & ITS INSTALLATION

35. Recommended Boiler Water Limits
Factor Upto20
kg/cm2
21 - 40 kg/cm2
41-60
kg/cm2
TDS, ppm
3000-3500 1500-2000 500-750
Total iron dissolved solids
ppm 500 200 150
Specific electrical
conductivity at 250C (mho) 1000
400 300
Phosphate residual ppm 20-40 20-40 15-25
pH at 250C 10-10.5 10-10.5 9.8-10.2
Silica (max) ppm 25 15 10

36. RAW WATER ANALYSIS
Parameters Value
pH at 25 deg C 8.0
Total hardness as caco3, ppm 98
Total Dissolved solids, ppm 385
Turbidity, NTU 100
Calcium, ppm 67
Magnesium, ppm 31
Sodium, ppm 96
Chloride, ppm 23
M.Alkalinity, ppm 126
Nitrate, ppm 0.3
Sulphate, ppm 44
Fluoride, ppm 1.84
Reactive silica, ppm 26.5
Colloidal silica, ppm 3.0
Iron, ppm <1