Boiler Water Treatment
Producing quality steam on demand depends on properly managed water treatment to control steam purity, deposits and corrosion. A boiler is the sump of the boiler system. It ultimately receives all of the pre-boiler contaminants. Boiler performance, efficiency, and service life are direct products of selecting and controlling feed water used in the boiler.
When feed water enters the boiler, the elevated temperatures and pressures cause the components of water to behave differently. 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, sometimes in crystallized forms and other times as amorphous particles. When solubility of a specific component in water is exceeded, scale or deposits 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.
Deposit Control
Deposits in boilers may result from hardness contamination of feed water and corrosion products from the condensate and feed water system. Hardness contamination of the feed water may arise due to deficient softener system.
Deposits and corrosion result in efficiency losses and may result in boiler tube failures and inability to produce steam. Deposits act as insulators and slow heat transfer. Large amounts of deposits throughout the boiler could reduce the heat transfer enough to reduce the boiler efficiency significantly. Different type of deposits affects the boiler efficiency differently. Thus it may be useful to analyze the deposits for its characteristics. The insulating effect of deposits causes the boiler metal temperature to rise and may lead to tube-failure by overheating.
Impurities Causing Deposits
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.
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-hardness that can be removed by boiling.
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. Non-alkalinity hardness chemicals fall out the solution due to reduction in solubility as the temperature rises, by concentration due to evaporation which takes place within the boiler, or by chemical change to a less soluble compound.
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, after been carried over either in droplets of water in steam, or in volatile form in steam at higher pressures.
Two major types of boiler water treatment are: Internal water treatment and External water treatment.
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 blow down. This method is limited to boilers, where feed water is low in hardness salts, to low pressures- high TDS content in boiler water is tolerated, and when only small quantity of water is required to be treated. If these conditions are not applied, then high rates of blow down are required to dispose off the sludge. They become uneconomical from heat and water loss consideration.
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. Proprietary chemicals are available to suit various water conditions. The specialist must be consulted to determine the most suitable chemicals to use in each case. Internal treatment alone is not recommended.
Internal
treatment of Boiler feed water by chemical:
|
Corrective treatment required
|
Chemical to dose |
|
Adjustment
of feed water pH and maintaining Boiler water alkalinity to inhibit scaling
and corrosion.
|
Soda ash/
Caustic soda/ Sulfuric Acid |
|
Internal softening
of boiler feed water to prevent scaling.
|
Soda Ash/
Phosphates/ Sodium Aluminates/ Sodium Silicate |
|
Boiler
sludge conditioning |
Tannin/
Lignin derivatives/ Starch/ Glucose derivatives.
|
|
Protecting evaporators, stage heaters and pipe lines from scaling from hot water. |
Poly phosphates/
Glucose derivatives/ Lignin derivatives/ |
|
Chemical DE-aeration of boiler feed water to inhibit O2 corrosion. |
Hydrazine/
Sulphites/ Ferrous hydroxides/ Tannin/ Glucose derivatives |
|
Inhibition
of caustic embrittlement |
Phosphates/
Nitrates/ Sodium sulphate/ |
|
Prevention of foaming of boiler feed water |
Poly amides/ Polypropylene |
|
Protective film formation to prevent corrosion |
Tannin/
Lignin / Glucose derivatives |
|
Prevention
of corrosion from condensate
|
Ammonia/
EDTA |
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 deaeration. Before any of these are used, it is necessary to remove suspended solids and color 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 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.
The first stage of treatment is to remove hardness salt and possibly non-hardness salts. Removal of only hardness salts is called softening, while total removal of salts from solution is called demineralization. (Details given in chapter 1- DM Plant)
De-aeration ( details given in separate chapter).Deaeration
Revers Osmosis ( details given in separate chapter).Reverse-osmosis
Recommended boiler and feed water quality
The impurities found in boiler water depend on the untreated feed water quality, the treatment process used and the boiler operating procedures. As a general rule, the higher the boiler operating pressure, the greater will be the sensitivity to impurities. Recommended feed water and boiler water limits are shown in Tables.
TABLE 2.2 RECOMMENDED FEED WATER LIMITS | |||
Factor | Upto 20 kg/cm2 | 21 – 39 kg/cm2 | 41 – 59 kg/cm2 |
Total iron (max) ppm | 0.05 | 0.02 | 0.01 |
Total copper (max) ppm | 0.01 | 0.01 | 0.01 |
Total silica (max) ppm | 1.0 | 0.3 | 0.1 |
Oxygen (max) ppm | 0.02 | 0.02 | 0.01 |
Hydrazine residual ppm | – | – | -0.02-0.04 |
pH at 25°C | 8.8-9.2 | 8.8-9.2 | 8.2-9.2 |
Hardness, ppm | 1.0 | 0.5 | – |
TABLE 2.3 RECOMMENDED BOILER WATER LIMITS (IS 10392, YEAR 1982) | |||
Factor | Upto 20 kg/cm2 | 21 – 39 kg/cm2 | 40 – 59 kg/cm2 |
TDS, ppm | 3000–3500 | 1500–2500 | 500–1500 |
Total iron dissolved solids ppm | 500 | 200 | 150 |
Specific electrical conductivity at 25°C (mho) | 1000 | 400 | 300 |
Phosphate residual ppm | 20-40 | 20-40 | 15-25 |
pH at 25°C | 10–10.5 | 10–10.5 | 9.8–10.2 |
Silica (max) ppm | 25 | 15 | 10 |
BOILER FEED
WATER TREATMENT CHEMICALS
Recommended Water parameters in low pressure process Boilers.
|
|
Boiler Feed Water |
Blow down water |
|
pH |
7.5 – 8.5 |
10.5 – 12.0 |
|
Hardness |
< 5 ppm |
NA |
|
TDS |
< 20 ppm |
1500 – 3000 ppm |
In process boilers (Low pressure
water or fire tube boilers), chemicals are dozed in feed water for the
following purpose:
1.
pH booster
2.
Oxygen Scavenger
3.
Anti scalent
4.
Corrosion inhibitor
5.
Sludge conditioner
Boiler feed water treatment
chemical details are given below, which was manufactured and supplied by
Thermax. This is for reference purpose only.
(Note:
Chemical name and number will be changed. Please contact supplier for details)
1.
MAXTREAT - 3001
(BOILER SLUDGE CONDITIONER)
MAXTREAT-3001
is a synthetic polymer for conditioning the sludge generated in boilers. It helps to keep precipitated salts in suspension and prevents deposition and sludge
formation.
BENEFITS:
i.
Prevents scale formation - MAXTREAT-3001 contains a powerful
polymeric dispersant which prevents scale formation. It fluidizes the scale forming material and reduces accumulation
on internal surfaces.
ii.
Removes sludge easily - The synthetic polymer in MAXTREAT-3001
makes the sludge non-adherent and fluid. Precipitated calcium salts, iron and
silica scale are conditioned and kept free flowing, thus free flowing sludge
can be easily removed during routine blow down.
iii.
Cleans Boiler - Non-adherent and free flowing sludge does not form
deposits on the internal surface. Blow down valves, pipes and water tubes are
kept clean, reducing maintenance costs and the number of shutdowns for
cleaning.
iv.
Reduces energy costs - MAXTREAT-3001 keeps all heat exchange surfaces
clean, improving heat transfer
efficiency. This results in the complete utilization of fuel resulting in
reduced fuel costs.
APPLICATION AND DOSES:
Normally
10-40 ppm of sludge conditioner is used. However, Maxtreat-3001 dosage depends
on the quality of the boiler feed water. MAXTREAT-3001 should be added in the
feed water tank preferable to dose it continuously.
2.
|
MAXTREAT-3004 is an alkaline formulation used
to increase the alkalinity of boiler feed water. It is used to reduce corrosion
of feed water tank, lines and pump.
BENEFITS:
i.
REDUCES CORROSION - MAXTREAT-3004 increases pH of feed water
reducing corrosiveness. It also increases alkalinity in boiler water reducing
scaling and corrosion of the boiler metal.
ii.
EASY SLUDGE REMOVAL - MAXTREAT-3004 helps in the formation of
non-adherent sludge. it fluidizes the sludge which is then removed easily
during normal
APPLICATION AND DOSAGE:
Dosage depends on feed water
quality and alkalinity requirement. It can be fed along with other water
treatment chemicals.
3.
|
MAXTREAT-3006 is an alkaline
formulation used to increase the alkalinity of boiler feed water. It is used to
reduce corrosion of feed water tank, lines and pump.
BENEFITS:
i.
REDUCES CORROSION - MAXTREAT-3006 increases pH of feed water
reducing corrosiveness. It also increases alkalinity in boiler water reducing
scaling and corrosion of the boiler metal.
ii.
EASY SLUDGE REMVAL - MAXTREAT-3006 helps in the formation of
non-adherent sludge. It fluidizes the sludge which is then removed easily
during normal blow down.
APPLICATION AND DOSAGE:
Dosage depends on feed water
quality and alkalinity requirement. MAXTREAT-3006 can be fed along with other
water treatment chemicals.
4.
|
MAXTREAT-3100 is a catalyzed oxygen
scavenger in powder form. It is a blend of sodium sulphite and a catalyst. It
is recommended for the rapid and complete removal of dissolved oxygen from
boiler feed water. MAXTREAT-3100 reacts with oxygen to produce sodium sulphate,
which contributes to the dissolved solids in boiler feed water. (Na2SO3 + ½ O2 =
Na2SO4).
BENEFITS:
i.
RAPID REACTION
The catalyst
MAXTREAT-3100 speeds up the removal of dissolved oxygen. Oxygen can be removed
completely even from cold water due to catalytic action. Normal reaction time
is as low as 15 to 20 seconds as against 10 to 30 minutes for unanalyzed
sulphite.
ii.
EASY TO HANDLE
MAXTREAT-3100
is a free flowing powder. It is soluble in water even at lower feed water
temperature. It is non-toxic and non-corrosive and, therefore, can be handled
easily.
iii. pH CONTROL
MAXTREAT-3100
is formulated to get proper pH control for optimum availability of catalyst for
rapid oxygen removal.
iv. COST EFFECTIVE
The
catalytic action reduces the excessive requirement of sulphite in feed water
for the rapid removal of oxygen. Thus, the quantity required is lower, making
MAXTREAT-3100 treatment cost effective.
APPLICATION AND DOSAGE:
MAXTREAT-3100 solution should be
dosed continuously. Dissolve the required quantity of MAXTREAT- 3100 gently in
boiler feed water in a separate tank. Dose this solution continuously in the
boiler feed water line after the deaerator.
Above 8 ppm of sodium sulphite is
required for the removal of 1 ppm of dissolved oxygen from boiler feed water.
About 10% to 20% of excess sulphite should be dosed to provide oxygen consuming
reserve. The table below gives the approximate dosage.
|
Feed Water Temperature |
Maxtreat-3100 dosage in gms per
ton of steam generation |
|
More than 80oC |
15 |
|
60 to 80oC |
30 |
|
40 to 60oC |
60 |
|
Less than 40oC |
100 |
5.
MAXTREAT 3200 (OXYGEN SCAVENGER)
Maxtreat-3200 is used to remove
the last traces of dissolved Oxygen from the boiler feed water. It is a volatile
liquid formulation of Hydrazine and a catalyst. It is recommended for the rapid
and complete removal o f dissolved Oxygen from the boiler feed water. Hydrazine
will act with dissolved Oxygen to form Nitrogen and water.
( O2 + N2H4 2H2O
+ N2)
BENEFITS:
i.
Rapid reaction
The catalyst in MAXTREAT-3200
speeds up the reaction of hydrazine with dissolved oxygen. This helps in the fast
and complete removal of oxygen from the feed water reducing oxygen related
corrosion of the boiler.
ii.
Volatile
MAXTREAT-3200 reacts with oxygen
to produce harmless products which are volatile. It does
Not add to dissolved solids in
boiler water.
iii.
Cost Effective
The catalytic action of
MAXTREAT-3200 reduces the excess requirement of hydrazine in feed water for
rapid oxygen removal. Thus, the quantity required is lower, making this
treatment cost effective.
iv.
Corrosion inhibition
MAXTREAT-3200 forms a protective
film over boiler metal preventing corrosion due to water. It also helps to
increase the condensate pH reducing corrosion of steam lines and equipment.
SPECIFICATION:
Appearance : Colorless liquid
pH as is > 10
Specific gravity: 1.00 + 0.1
APPLICATION AND DOSES:
About 5 ppm of MAXTREAT -3200 is
required for the removal of 1 ppm of oxygen form boiler feed water. About 10%
to 20% excess should be dosed to provide oxygen consuming reserve. MAXTREAT
-3200 should be dosed continuously. Prepare dilute solutions using boiler feed
water and dose this solution continuously in the feed water line.
6.
MAXTREAT – 3221 (BOILER FEED
WATER TREATMENT CHEMICAL)
Maxtreat-3221 is a liquid product
designed for the internal treatment of water used in smoke tube boilers. It
contains a corrosion inhibitor, an antiscalant and a sludge conditioner.
MAXTREAT-3221 is formulated for the treatment of smoke tube boilers using soft
water.
Benefits:
i.
Prevents and removes deposition
Maxtreat-3221 contains a
polymeric sludge conditioner which fluidizes sludge and removal of sludge is
done during normal blow down. A powerful antiscalant prevents Calcium and Iron
deposition and also helps in the removal of existing scale. The combination of
a sludge conditioner and anti scalant keeps the boiler clean.
ii.
Prevents corrosion
MAXTREAT-3221 contains a
corrosion inhibitor which forms a protective, passive film over the boiler heat
transfer surface. The corrosion inhibitor reduces fouling due to Iron
Hydroxides, pitting and tube failures.
iii.
Saves Energy
The boiler is kept clean due to
the antiscalant and dispersant action of MAXTREAT-3221. This keeps heat
transfer efficiency at peak, reducing energy losses.
iv.
Flexible dosage control
MAXTREAT-3221 is a balanced
formulation. Moderate under dosing is compensated by multi-component
formulation. Overfeeding of this product does not pose any hazards. Thus it
provides unique flexibility of dosing
Dosage and Application:
The requirement of MAXTREAT 3221
depends on water quality and blow down. For optimum results, 400 to 600 ppm of
the product should be maintained in the boiler.
MAXTREAT 3221 should be fed as it
is or as a diluted solution. It should be preferably dosed continuously.
However, intermittent dosage may also be done based on the boiler system.
Specification:
Appearance: Colorless liquid
pH as is : 7.0 + 1
Specific gravity: 1.15 + 0.1
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