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Boiler water treatment
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Objectives of boiler water treatment
Control of
Deposits that could insulate heat transfer surfacesCorrosion that
could cause tube failures and system shut downSteam purity that
could cause equipment and piping damage AND from process
contaminationBlow down
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II: Important water parameters and its significance
Hardness
Alkalinity
pH
TDS
Silica
Suspended solids
Dissolved oxygen
If there is a difference exists between operating value and
target value, then
decreasing or increasing blow down and adjusting chemical
dosages will be done
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1. Hardness
Total hardness is the combined concentration of dissolved calcium
and magnesium salts. Alkaline or temporary hardness is caused by
bicarbonates, carbonates or hydroxides. Bicarbonates which
predominate in most natural waters are easily broken down when the
temperature is raised. Non-alkaline or permanent hardness is caused
mainly by chlorides, sulphates and nitrates.Water hardness is the
most common contributor to boiler scaling
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Water Hardness
classification
Seawater has a hardness level of 500 mg L-1.
Water hardness classification
mg/L or ppm
as CaCO3
Soft
0-60
Moderate
61-120
Hard
121-180
Very Hard
> 180
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2. Alkalinity
The extent to which a solution is alkaline (i.e. has a pH value
greater than 7)It is a measure of its hydroxide (caustic),
carbonate and bicarbonate and hydroxides content. Expressed in
terms of calcium carbonate content. These bases break down to form
carbon dioxide in steam, which is a major factor in the corrosion
of condensate lines. High Alkalinity also contributes to foaming
and carryover in boilers.
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3. pH
is a measure of a solution's acidityIn water, small numbers of
water molecules (H2O) will disassociate into hydrogen ions (H+) and
hydroxide ions (OH-).Other compounds entering the water may react
with these, leaving an imbalance in the numbers of hydrogen and
hydroxide ions. When more hydrogen ions react, more hydroxide ions
are left in solution and the water is basic; when more hydroxide
ions react, more hydrogen ions are left and the water is
acidic.
- pH is measured on a logarithmic scale between 1 and 14 with 1
being extremely acid, 7 neutral, and 14 extremely basic. The more
extreme the pH, the more likely corrosion problems are to
occurBecause it is a logarithmic scale there is a ten fold increase
in acidity for a change of one unit of pH, e.g. 5 is 100 times more
acid than 7 on the pH scale
Measure of pH
pH = - log [H+] = - log [1 x 10-7] = -[-7] = 7
- 27.bin
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4. Total Dissolved Solids (TDS)
The total dissolved solids (TDS) in water consist of inorganic
salts and dissolved materials. In natural waters, salts are
chemical compounds comprised ofAnions(-) such as carbonates,
chlorides, sulphates, and nitrates (primarily in ground water),
andCations (+)such as potassium (K), magnesium (Mg), calcium (Ca),
and sodium (Na).
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Measure of TDS
It is a measure of the total amount of solids in solution.
Expressed as parts per million (ppm). Generally estimated on-site
by measuring electrical conductivity. If the boiler water
conductivity is measured in-situ, then allowance should be made for
temperature and pH effects as the hot, highly alkaline nature of
boiler water would give a falsely high reading. A substantial part
of TDS is due to alkalinity.If the design values for TDS are
exceeded, carry-over from the boiler will increase leading to a bad
steam quality.
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5. Silica
Silica is found as dissolved silicate and in a suspended complex
form.It can combine with other compounds to give scales that are
strongly insulating and difficult to remove and cause high
resistance to heat transfer
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6. Suspended solids (SS)
/ Turbidity
Turbidity is the amount of particulate matter that is suspended in
water.Nephelometric Turbidity Units
Turbid include: clay ,silt ,finely divided organic and inorganic
matter ,soluble organic compounds & organisms. Removed by,
coagulation , sedimentation and filtration
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7. Dissolved gases
Oxygen and carbon dioxide are the most important. Dissolved oxygen
(DO) is an important factor in determining the corrosiveness of
water.The solubility of oxygen in water depends on temperature and
pressure. Dissolved carbon dioxide gives carbonic acid. Even this
weak acid can lower the pH to values where the water becomes highly
corrosive.The type of boiler, pressure and heat flux dictate the
boiler water chemistry that can be used to achieve the required
steam purity and efficiency.
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TYPES OF IMPURITIES IN WATER
Sl NoNature Impurities 1.Soluble - Ionic Cations: Calcium,
Magnesium, Sodium Ammonium, Iron, Manganese etc. Anions:
Bicarbonates, Carbonates, Nitrates, Chlorides, Dissolve-silica,
etc. 2.Insoluble -Nonionic Suspended matter, Turbidity, Silt,
Colloidal-silica. 3Gaseous Oxygen, Carbon dioxide, chlorine.
4.Others Color, Oil, Organics.
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III : Boiler water problems
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Water
Scale
Fouling,
Forming
Carryover
Corrosion
Dissolved
Minerals
Dissolv ed
Gases
Dissolved
Nutrients
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Corrosion
Localized attack on metal can result in a forced shutdown. Because
boiler systems are constructed primarily of carbon steel and the
heat transfer medium is water, the potential for corrosion is high.
The relative rate of corrosion of steel varies with boiler water pH
and the level of dissolved oxygen. Good water treatment aims to
keep the pH within the safe range of pH 8.2 - 12.5 while addition
of oxygen scavengers prevents iron being oxidised to ferric oxide
or rust.
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pH Vs Corrosion
Relative rate of corrosion of steel with boiler water pH
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CORROSION
due to
DISSOLVED OXYGEN (DO)
4Fe + 3O2 2FeO3
Steel + Dissolved Iron Oxide (Rust)
Oxygen
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Corrosion
due to
Dissolved Carbon-dioxide
Fe + CO2 + H2O FeCO3 + H2
Iron + Carbon-dioxide + Water Iron Carbonate +Hydrogen
4FeCO3+O2 + 10H2O 4Fe(OH)3 + 4H2O+CO2
Iron Carborate + Oxygen + Water Iron Hydroxide+Water +
Carbon-dioxide
4Fe(OH)3 2Fe2O3 + 6 H2O
Iron Hydroxide Iron Oxide + Water
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Unstable salts like Magnesium Chloride, Calcium, Chloride,
Magnesium nitrate, Calcium nitrate and Magnesium Sulphate are
hydrolyzed by the water in the boiler to form corrosive acids, like
Hydrochloric acid and Nitric Acid in the following manner.
MgCl2 + 2H2O Mg(OH)2 + 2HCL Cacl2 + 2H2O Ca(OH)2 + 2HCL
3. Mg(NO3)2 + 2H2O Mg(OH)2 + 2HNO3
4. Ca(NO3)2 + 2H2O Ca(OH)2 + 2HNO3
5. MgSO4 + 2 NaCL MgCL2 + Na2SO4
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Oxygen Pitting of mild steel pipe
Oxygen pitting is localised corrosion characterised by small
pits or holes
It is found mainly in steel condensate systems, but hot water
systems and
idle steam boilers can also suffer such attacks.
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Corrosion in boiler
Corrosion in boiler systems can quickly result in tube failure and
plant shutdown
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Caustic Attack
If too much alkali is added or allowed to concentrate, it can cause
corrosion of the boiler metal.The attack may involve local
dissolution of the metal, usually on high heat transfer surfaces
which have become fouled, or cracking of the metal.Caustic cracking
of stressed steel can occur when the concentration of caustic is
greater than 50,000 mg/LLeads to sudden rupture and explosion of
boilers.
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Carbonic Acid Corrosion
Carbonic acid corrosion causes general thinning of pipe walls The
corrosion product is soluble in the acidic water and is thus
carried back to the feed tank with condensate return. On entering
the boiler, this contamination can cause fouling of heat transfer
surfaces and loss of fuel efficiency. Inhibition of corrosion in
condensate systems is neededOther parts of the boiler system, e.g.
feed tank, feed lines. pumps and valves, are also susceptible to
attack and measures to inhibit corrosion should be taken in these
areas.
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Scaling
Scaling in the boiler occurs when the solubility of a compound is
exceeded. either through chemical reaction, increased concentration
or higher temperatures. Example: calcium and magnesium salts
dissolved in the feedwater become less soluble as the feedwater is
heated and eventually precipitate out. Hardness contributes
significantly to scaling, particularly when water is heated.
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At the temperature prevailing in the steam boilers, both calcium
and magnesium bicarbonates break down to form calcium carbonate and
magnesium hydroxide scale.
Heat
Ca (HCO3)2 CaCO3 + H2O + CO2
Mg (HCO3)2 MgCO3 + H2O + CO2
MgCO3 + H2O Mg(OH)2 + CO2
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The sodium bicarbonate is decomposed into the caustic soda which
soluble and hence it remains in solution.
2NaHCO3 Na2CO3 + H2O + CO2
Na2CO3 + H20 2NaOH + CO2
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Boiler scale and deposits form when impurities precipitate on hot
boiler tubes.
Scaling
Fire Tube Exterior
Water Tube Interior
This hard shell is called scale and is often found on the
outside of the fire tubes
1.bin16.bin
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Deposit-Related problem
Deposits (Scale) in boilers cause two major problems. i.e. fouling
of heat transfer surfaces and restriction of water flow.SOLUTIONS
:Removal of hardness salts by external treatment.Scale prevention
by using internal inhibitors.
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Certain deposits can cause metal temperatures to rise to
dangerously high
levels where metal distortion can occur.
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Metal over heating
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Steam Contamination
Steam purity refers to the contamination of the steam by solids,
non-aqueous liquids or volatile compounds. It is not the same as
steam quality, which is concerned with the amount of moisture from
partial condensation in the steamSOLUTIONS:Antifoams can be used to
prevent foamingControl boiler water TDS to avoid carry-over.
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Foaming
In a boiler steam bubbles are continually bursting at the steam
water interface and ejecting boiler water drops into the steam
space.
As the rate of steaming increases a point is reached where the
steam bubbles are arriving at the surface faster than they are
being removed, they accumulate as foam.Steam released from the
bursting bubbles move towards the steam outlet carrying with it
smaller water droplets and taking with it any foam.
The steam also drags the surface of the boiler water towards the
steam outlet so that the water level at that point can be higher
than elsewhere, depending on the positions of the level controls
this may cause the feed pumps to deliver water and thus raising the
water level
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Carryover
Carryover is water leaving the boiler with the steam.
It can be caused by foaming, priming or mechanical
inefficiencies.
The effect of impurities in the boiler water is to increase the
surface tension of the water and so inhibit the separation of steam
from the water, this tends to cause foam to form.
Good TDS control is required to keep the solids levels below that
at which this can occur. normal bublecarry over buble
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Priming
As the water level is raised the volume of the steam space is
decreased, the speed of the steam across the surface is increased
drawing foam and water droplets towards the steam outlet. A sudden
increase in steam demand can cause a slug of boiler water to enter
the steam outlet.This is known as priming.
Operation of the boiler below its design pressure will also produce
a similar effect.
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Boiler water carry-over
Boiler water carry-over, which reduces heat transfer efficiency and
is caused by poor boiler operation, e.g. steaming below design
pressure, high water levels. steam demand swings and a sudden
reduction in pressure. Excessive alkalinity, total dissolved solids
or suspended solids can lead to foaming and increased carry-over.
Organics, oil or surface-active agents also cause foaming.
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Impurities
Insoluble
Soluble
Gaseous
Softening
Dealkaliser
Demineralization
De-aeration
for oxygen
in De aerator
Activated carbon in pressure filter
for chlorine.
CO2 in de gessor tower in DM plant
IV : TYPES OF WATER TREATMENT
& Choice of water treatment
Reverse Osmosis
Types of Impurities
Types of treatment
Settling
Filtration
Clarification
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Boiler Plant Flow Diagram
The choice of water treatment must always be consistent with the
individual requirements of the plant in question and the processes
served by the equipment
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EXTERNAL WATER TREATMENT
Good external water treatment saves significant amounts of
energy.Removing hardness will avoid scale.Reducing feedwater TDS
will reduce blowdown and thus save energy.Removing alkalinity,
dissolved oxygen and carbon dioxide will reduce the risk of
corrosion.Lower oxygen levels in the feedwater mean lower chemical
costs.
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External treatment
(involves complete or partial removal of one or more types of
impurity).
TechnologyPurpose1.Base exchange softeningUsing ion exchange to
convert calcium and magnesium salts that would cause scale into
sodium salts which do not precipitate, i.e. removal of
hardness.2.DealkalisationReduction in TDS by removal of
alkalinity.3.DemineralisationRemoval of dissolved solids and silica
by ion exchange.4.Reverse osmosisReduction of TDS and silica by a
membrane process5.De-aerationRemoval of dissolved oxygen and carbon
dioxide.
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1. Base exchange
softening
Softening reaction:
Na2R+Ca(HCO3)2>CaR+2Na(HCO3)
Regeneration reaction
CaR + 2 NaCl >Na2R + CaCl2
Generally used for: low &
medium pressure Boilers
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Water softeners
The water softeners "Ion Exchange" process to convert the hard
water ions of calcium and magnesium to sodium ions.
The resultant water is less than 4 ppm total hardness
Capacity upto 5-50m3/hr
2.bin
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2. Dealkalisation
Dealkalisation reduces TDS by removing alkaline salts. Generally
used to treat the feedwater for low and medium pressure boilers and
when a very high percentage of make-up water is to be used.In a
weak acid cation exchanger. the less strongly alkaline impurities -
carbonate and bicarbonate ions are replaced by positively charged
hydrogen ions. The carbonic acid that is produced is removed in a
degassing tower.If pH is acceptable and hardness has been reduced
sufficiently, the degassed water can be used directly. If not; pH
adjustment and base exchange softening are necessary.
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Dealkalizer
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3. Demineralisation
This process will remove virtually all the salts. It involves
passing the raw water through both cation and anion exchange resins
and produces very high quality water containing almost no dissolved
solids. It is used for very high pressure boilers such as those in
power stations.
If the raw water has a high amount of suspended solids this will
quickly foul the ion exchange material, drastically increasing
operating costs. In these cases, some pre-treatment of the raw
water such as clarification or filtration may be necessary.
- Demineralisation
Demineralisation
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Demineralisation
Passage of the water through the cation exchanger produces a
solution of dilute acids, which are exchanged by hydroxide in the
anion exchanger to give water.If carbonates and bicarbonates are
present in the water, removal of carbonic acid in an intermediate
degassing tower will reduce the requirement for hydroxide exchange
in the anion bed.The cation and anion exchangers are regenerated
with mineral acid and sodium hydroxide respectively.used for high
pressure, high heat flux and once-through boilers.
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4. Reverse Osmosis
water is pumped through a semi-permeable membrane where dissolved
solids and silicate are retained.Purified water passes through the
membrane as the permeate. This is often acidic and pH adjustment is
required to protect downstream equipment from corrosion.Impurity
rejection rates as high as 99.5% are possible3.bin
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5. Deaeration
Free carbon dioxide is also removed in the de-aerator. thus
reducing carbonic acid level (H2CO3).Lower oxygen levels in the
feed water reduce the subsequent amount of oxygen scavengers
neededUsed to remove dissolved oxygen from feed water, either by
thermal, chemical or mechanical means.Oxygen is driven off as the
temperature increases.
Figure 2.9 Deaerator
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Choice of external treatment Methods
The factors affecting the selection of technology :
quality of input water;treated water quality required:volumes
required;flow rates (available and required);water pressure;storage
facilities;type of boiler plant;steam or hot water duty;industry
sector.
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Economics of Water treatment
The capital and operating costs for external water treatment
vary according to the plant type and duty.
raw water analysistreated water specificationchemical and effluent
chargestemperatureflow rateenergy consumptionmaintenance
costs.
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Effectiveness in Removing Impurities
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INTERNAL WATER TREATMENT
Selection of the most appropriate treatment regime for a particular
boiler system saves both energy and money. Careful control of
boiler water chemistry saves money through reduced consumption of
energy, water and treatment chemicals.Controlled dosage saves
energy and avoids wasting expensive treatment chemicals.Preventing
scale formation, minimising silicate and other deposits,
eliminating dissolved oxygen, neutralising dissolved carbon dioxide
and minimising concentration effects all lead to more efficient
boiler operation.
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Internal treatment
Internal treatments involve the addition of chemical(s) to the
feedwater to:
prevent scale formation and /or oxygen corrosion:protect the metal
surfaces from acid or alkaline attack;ensure that any salts that
precipitate out do not adhere to the heat transfer surfaces and
thus reduce efficiencyensure that precipitated salts can be easily
removed by blowing down.
Never use more of these chemicals than is absolutely
necessary.
Not only are the chemicals themselves expensive, but they
also
add to the TDS content of the boiler water.
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Preventing Scale Formation
Scale prevention is based upon controlled precipitation of hardness
as a fine; mobile sludge that can be removed during bottom
blowdownAchieved by the application of the carbonate or phosphate
cycle (depending on system pressure).
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Carbonate Cycle
The formation of calcium sulphate scale, which is difficult to
remove was historically prevented by using a controlled reserve of
carbonate and hydroxide ions in the boiler water. It is suitable
for boilers operating up to about 10 bar.Above this pressure,
thermal breakdown of carbonate prevents adequate reserves being
maintained.In the carbonate cycle. any hardness entering the boiler
precipitates as calcium carbonate or magnesium hydroxide. The
natural alkalinity of the feedwater may be sufficient to cause
precipitation; if not. carbonate or hydroxide are added to make up
the deficit. Other additives - tannins, lignins, starches and a
wide variety of synthetic polymers - can be used to improve the
mobility of the precipitates and thus enhance their removal by
bottom blowdown.
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Phosphate Cycle
Since carbonate breakdown increases with temperature, the phosphate
cycle was developed for use at higher pressures. Phosphate is used
to precipitate any calcium present. For magnesium, the preferred
precipitant is hydroxide alkalinity, which yields magnesium
hydroxideThis method of scale prevention involves a controlled
reserve of soluble phosphate and maintenance of caustic alkalinity
at a specific pH in the boiler water to precipitate hardness as a
mobile sludge. The phosphate is normally added as trisodium
phosphate, which also adds alkalinity. However, if a reduction in
alkalinity is required. salts such as disodium phosphate or
monosodium phosphate can be used as these are acidic. Phosphate
salts are normally dosed directly into the boiler to avoid
precipitation in feed lines.
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Other Deposits
Preventing silicate depositsKeeping the silica in solution by
maintaining the appropriate silica: caustic alkalinity
ratio.Preventing other depositsAddition of specific
dispersants.Oxygen removalAdding oxygen scavengers.
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Minimising other problems
Minimising carbon dioxide corrosion in condensate
systemsNeutralisation or removal of dissolved carbon dioxide
(carbonic acid) by tight control of alkalinity and use of
nullifying or filming amine treatment.Avoiding carry-overUse of
antifoams to prevent foam production
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4. Monitoring and standards
Correct sampling and accurate analysis is an extremely important
part of boiler water treatmentProper training of operators in
sampling and testing techniques is therefore essential.Routine
boiler control parameters include pH, TDS. hardness, alkalinity,
chloride and treatment chemicals Boiler water analysis is used to
monitor and controlwater treatment programmes:boiler water
concentration.
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Typical sampling points in a simple steam boiler
circuit
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ASME Guidelines for Water Quality in Modern Industrial Water
Tube Boilers
for Reliable Continuous Operation
Boiler Feed WaterBoiler WaterDrum Pressure (psi)Iron (ppm Fe)Copper
(ppm Cu)Total Hardness (ppm CaCO3)Silica (ppm SiO2)Total
Alkalinity** (ppm CaCO3)Specific Conductance (micromhos/cm)
(unneutralized)0-3000.1000.0500.300150700*7000301-4500.0500.0250.30090600*6000451-6000.0300.0200.20040500*5000601-7500.0250.0200.20030400*4000751-9000.0200.0150.10020300*3000901-10000.0200.0150.0508200*20001001-15000.0100.0100.020***1501501-20000.0100.0100.010***100
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Maximum limits for boiler water constituents
* Silica limits based on limiting silica in steam to 0.02-0.03
ppm
0.5
5
100
500
Over 2000
1
10
150
750
1501-2000
2.5
20
200
1000
1001-1500
8
40
250
1250
901-1000
20
60
300
1500
751-900
35
100
400
2000
601-750
50
150
500
2500
451-600
90
250
600
3000
301-450
125
300
700
3500
0-300
Silica* (ppm)
Suspended solids (ppm)
Alkalinity (ppm)
Total solids (ppm)
Boiler pressure
(psig)
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BOILER WATER QUALITY PARAMETERS
PARAMETERSDEARATOR/F.W.BOILER DRUMSat/S.HSTEAMEFFECTS IF
INCREASESEFFECTS IF DECREASESpH8.8-9.29.8-10.28-9Caustic
enbritlementCauses corrosionT.H.NilNilNilScale
Formation---V/cm
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BOILER CONTROL AND BLOWDOWN
Keep control of boiler water treatment through a good sampling and
testing routine.Minimising blowdown saves energy and improves
efficiency.
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TDS reduction can be controlled manually or
automatically.
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Internal Water Treatment for Idle Boilers
Boilers are sometimes left standing idle and cold for maintenance,
operational or other reasons. In such circumstances, there is a
risk of corrosion occumng. Any corrosion is likely to be localised
as pitting or water line attack.A boiler can he kept full of water
or with a nitrogen blanket for up to three months. If there is a
risk of freezing the boiler should be kept dry.
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Wet Storage
Raise the water level to eliminate air spaces.Increase oxygen
scavenger levels to 20 - 25 times the normal dose.Adjust the pH to
11.If a non-drainable superheater is present:fill with
demineralised water;add a volatile oxygen scavenger and alkali to
give the same values as for the boiler section;take precautions to
avoid ingress of boiler water.Nitrogen blanketing may prove a
useful additional measure in some cases.
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Dry Storage
Drain the boiler , eliminating water pools.Dry the internal
surfaces by blowins warm, dry air through all the waterways.Store
the boiler with heaters and manhole doors open for ventilation.For
closed storage, place a desiccant and possibly a vapour phase
inhibitor inside the boiler before it is sealed.Consider using
nitrogen blanketing to prevent ingress of moist air.Check chemical
levels and desiccant regularly. Top up or change as necessary.Take
adequate precautions to prevent corrosion of the fireside.
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CHARATERISTICS OF BOILDER FEED WATER
Sl.No.Pressure at Boiler Outlet(KG/CM2)/LBS/SQ.IN)Medium
Pressure20/30040/60060/9008/12001pH8.5 to 9.58.5 to 9.58.5 to9.58.5
to 9.52Total Hardness (as CaCO3)Mg/1 (Max) 1020.5nil3Dissolved
Oxygen (as O2) mg/1 (Max)0.050.020.010.014Iron and Copper mg/1
(Max)0.050.050.020.025Silica (as Si O2)Mg/1
(Max)-1.00.10.16OilNilNilNilNil
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CHARATERISTICS OF BOILDER FEED WATER
Sl.No.Pressure at Boiler Outlet(KG/CM2)/LBS/SQ.IN)Medium
Pressure20/30040/60060/9008/12001pH10.5 to 11.510 to 119.8
to10.59.8 to 10.22Total Alkalinity (as CaCO3)Mg/1 (Max)
70050030003Caustic Alkalinity (as CaCO3) mg/1
(Max)30015060304Phosphate (as Na3PO4) mg/1 (Max)50 to 10030 to 7020
to 5010 to 455Silica (as Si O2)Mg/1 (Max)Less than half the caustic
alkalinityLess than half the caustic alkalinity20106Sulphite (as
Na2SO3)mg.1 (or) Hydrazine (as N2H4) mg/130 to 5020 to 40 0.05
to0.30.05 to 0.17Chloride (as Cl) mg/1 (max)----8Dissolved Solids
mg/1 (max)3000200012007009Suspended Solids mg/1 (max)20050--
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Case study
BOILER FEED WATER ANALYSIS
Description
Boiler feed water
Boiler blow down water
Parameter
Recommended
Actual
Recommended
Actual
pH
8.5 - 9.5
9.0 - 9.5
10.5 - 11.5
12.0
TDS
350
400 - 500
3500
3200 - 5000
Total hardness
< 10
5
5
5 - 40
P - Alkalinity
50
40
350
1100 - 1500
M - Alkalinity
100
250 - 300
1200
1600 - 1800
Water
Scale
Fouling,
Forming
Carryover
Corrosion
Dissolved
Minerals
Dissolv ed
Gases
Dissolved
Nutrients
