Boiler

1. ISGEC John Thompson
BOILER WATER CHEMISTRY
Water, occuring in nature, cannot be used for any industrial purpose,
as such without treatment. But it can be treated in different ways, to
suit the application. Hardness and similar hard deposit forming
elements like silica in water are desirable to be removed for any
application. For high pressure boilers, in addition to this, almost all
impurities from water are to be removed, as high pressure units are
almost designed to their limits for economy and compactness of size
and hence they can tolerate very little deposits and corrosion.
Moreover, modern steam turbines need stringent steam purity for
trouble free operation. Water is required to be demineralised for high
pressure units, to minimise problems in boilers due to deposits and
corrosion and to maintain steam purity desirable for turbines.

2. ISGEC John Thompson
INTERNAL CORROSION
Internal corrosion of tubes, is the major problem in boilers.
If the water is acidic or vary alkaline, both are harmful.
Boiler tubes are protected from corrosion, by the presence
of the thin layer of iron oxide (magnetite) coating inside the
tube. The magnetite itself is a product of corrosion, as
given below, but once if it is formed, it will stifle further
corrosion.
3Fe + 4 H2O --------> Fe3 O4 + 4 H2
This protective coating will be dissolved, if the water is too
acidic or alkaline. It has been established that water at a pH
of 10.0 to 11.0 is least corrosive. All treatment practices are
aimed to achieve this.

3. ISGEC John Thompson
ACIDIC CORROSION
i) CO2 corrosion
The presence of CO2 can make the water acidic. There are plenty of chances for this
contamination. Raw water contains large amounts of carbonates and bi-carbonates. This
carbonate alkalinity of water, if it is not removed, will disassociate into CO2 at high
temperature. Treated water, if it gets exposed to atmosphere, can absorb CO2. In a power
plant, chances for such absorbtion exist in a storage tank, where treated make up water is
stored. Condensers work under vacuum. At start up and low load conditions, L.P. heaters
may also be under vacuum. If they are not air tight, there will be ingress of air into water.
Feed water for pressure boilers should be completely free from CO2. To achieve this:
a) The degasser of the demineralisation plant should be working well to ensure D.M. water,
free of CO2.
b) The vents of storage tanks should not be left open to atmosphere and they should be
sealed. The sealing can be done by either soda-lime filters or by continuously flowing water
seals.
c) Chemicals like ammonia should be dozed into the feed water to raise the pH to about 9.0,
to ensure that there is no CO2 present. Ammonia neutralises CO2 and then raises the pH.
CO2 corrosion generally occurs in condensate and feed lines. The presence of iron in feed
water is a measure of corrosion of the feed system.

4. ISGEC John Thompson
ii) Hydrogen damage:
Hydrogen damage is another type of acidic corrosion. Water wall tubes of high pressure
units, using sea water for condenser cooling, are highly prone to this type of corrosion. Even
small amounts of sea water leakage through condenser can cause this corrosion. The extent
of corrosion will be severe and large portions of tubes will have to be replaced, if this type of
attack takes place. There will be huge loss of availability of the units.
Sea water contains large amounts of chlorides and sulphates of calcium and magnesium.
These salts at high temperatures of boiler water hydrolyse to produce acids, as given below.
The acids produced cause extensive corrosion. A gas (methane) is generated in the
corrosion reaction which penetrates into the metal and weakens it. The tube metal will
become brittle and failures will take place.
Mg Cl2 + H2O -----------> Mg (OH)2 + HCl
If this corrosion is to be prevented, condenser leakage should be avoided. There should be
an on-line conductivity analyser ( with provision of caution column) to detect immediately
any condenser leak. PO4 dosing should be increased during condenser leak to maintain
excess residual. PH of drum water should never be allowed to fall below 8.0. There should
be a constant watch on total solids concentration of boiler water which should be controlled
within limits by increasing blow down to avoid carryover. There should be no hesitation to
shut down the unit, if pH cannot be maintained above 8.0 to save the unit from extensive
corrosion.

5. ISGEC John Thompson
ALKALINE CORROSION
Alkaline corrosion is mainly caused by caustic soda. Concentrated amounts of caustic soda
eats away material. As caustic corrosion attack will look like welding gouging, this type of
corrosion is known as caustic - gouging.
Water wall tubes are subjected to this corrosion, when caustic soda is used as a treatment
chemical. It is recommended to use tri-sodium phosphate instead of caustic soda for boiler
water treatment, to raise pH to the required level. Co-ordinated phosphate treatment
recommended by us, prevents the presence of free caustic soda in boiler water and thus does
not cause corrosion.
Small amounts of caustic soda present in water are not harmful. But unfortunately, there are
areas in boilers, for concentration of caustic to high values so as to cause corrosion. Under
desposits in a dirty boiler, it can concentrate. The crown of roof and low sloped water wall
tubes can be attacked due to segregation of steam and water flows, leading to concentration of
this chemical.
Heat transfer phenomenon in a water wall increases slightly the temperature of water close to
the tube wall and this can largely increase concentration.
It is not desirable to use caustic soda as a boiler water treatment chemical. It can be used in a
limited way during condenser leak. Where it is used, it should be used in conjunction with tri-
sodium or di-sodium phosphate to avoid the presence of free hydroxide, by satisfying the co-
ordinated phosphate curve.

6. ISGEC John Thompson
OXYGEN CORROSION
Oxygen is also another gas responsible to cause corrosion. The
presence of O2 in water will cause pitting in tube internal surfaces. O2
present alongwith chlorides can also cause hydrogen damage.
The principal sources of oxygen contamination are storage tanks and
ingress of air into condensers and heaters, as in the case of CO2.
Deaerator removes oxygen. Residual oxygen can be removed by
hydrazine dozing. O2 in feed water should be maintained below 0.007
ppm. Any amount of hydrazine dozing will not reduce O2 to this limit,
unless O2 is primarily removed by good dearation. Presence of traces of
O2 can be easily measured by Indigo-Caramine method. The only
precaution required is that testing should be done without O2
contamination from atmosphere.

7. ISGEC John Thompson
ATMOSPHERIC CORROSION
Major oxygen corrosion occurs during idle periods of boilers. Boiler
system without proper preservation during shutdown suffer major
atmospheric corrosion, Corrosion of unprotected preboiler (feed)
system generates large amounts of oxides which form deposits inside
water wall tubes. These deposits are instrumental for severe acidic or
caustic corrosion when there is contamination of water.

8. ISGEC John Thompson
CONTROL OF CORROSION
a) Maintenance of pH of water in the desired range is required to control
corrosion (for values, refer our recommendations).
b) O2 in feed water should be controlled within 0.007 ppm, primarily by
good deaeration, supplemented by hydrazine dozing.
c) Condenser leak should be immediately detected and acted upon.
d) DM water storage tank should be properly sealed.
e) Condensers & heaters should be air-tight.
f) Proper wet lay-up of units (including pre-boiler system) during shut
downs.

9. ISGEC John Thompson
MAKE-UP WATER QUALITY
Make-up water for pressure boilers should be demineralised. Demineralisation
removes almost all dissolved impurities. Conductivity is a measure of dissolved
impurities in water. D.M. water should have conductivity less than 0.5 mmho/cm.
Silica should be removed to less than 0.02 ppm. D.M.water will not caontain
anyhardness. D.M. water is neutral with pH of 7.0 + 0.2.
The boiler make-up should also be free from all suspended impurities, like
organics, colloidal silica etc. Organics are not only harmful to boiler but also to
the D.M.plant. The resins can be fouled by organics and the output and quality of
treated water will suffer.
With the complete removal of dissolved and suspended impurities, D.M.water is
free from potential, for causing deposits in boiler tubes.
Consistent quality of D.M.water depends on care and control during operation.
Seasonal variations can take place in the composition of raw water. The
turbidity of river water can increase very much during rainy season. Colloidal
silica problems can become acute during some seasons. Unknown discharges
and pollution ahead of the stream can alter the quality. All these are to be
continuously watched and the water treatment plant, including the pre treatment
section should be operated suitably.

10. ISGEC John Thompson
FEED WATER TREATMENT
Feed water is treated for raising the pH and controlling residual oxygen.
Ammonia is used for raising the pH. Ammonia also neutralises any CO2
present. Other amines like morpholine or cyclohexylamine are not preferred for
this purpose for high pressure boilers, as they decompose at high temperatures
into ammonia and by the use of these chemicals, ammonia concentration will
increase in feed water. Excess ammonia present in feed water is corrosive to
copper alloy materials used in condensers, L.P. heaters etc., It is desirable to
maintain ammonia less than 1.0 ppm in feed water. Ammonia less than 0.5 ppm
can raise pH of feed water to the desired level of 9.0, if there is no CO2
contamination of feed water.
Hydrazine is dozed for scavenging O2. As hydrazine is slow acting, unless
dissolved O2 is already removed by good deaeration to less than 0.01 ppm,
residual O2 cannot be controlled within 0.005 - 0.007 ppm by hydrazine dosing
alone.
Normally ammonia and hydrazine are doses at feed pump section. It is more
desirable to doze hydrazine at condensate extraction pump outlet as hydrazine
has the property of passivating copper surfaces of condensers and heaters.
But ammonia dosing should be more carefully done at this point.

11. ISGEC John Thompson
BOILER WATER TREATMENT
Tri-sodium phosphate is generally used for boiler water treatment.
Tri-sodium phosphate is preferred over the other, as it is safer to
use T.S.P.
Trisodium phosphate is a useful chemical during condenser
leakage. Condenser leak brings in hardness and hardness is turned
into a sludge,without allowing it to form hard deposits. Sludge is
easily removed by blowdown.
Boiler feed water, Boiler drum water parameters are to be strictly
maintained in limits for longer life of pressure parts and trouble free
operation. Following are the recommended values.

12. ISGEC John Thompson
THE END