| | DATA
USED IN WATER CHEMISTRY | FAQ | |
| BOILER FEED-WATER TREATMENT |
The importance of correct feed-water treatment for economic
operation and for extending life of boiler and equipment cannot
be over emphasized. Feed-water treatment is essential in boilers,
feed-systems, etc., more particularly in modern boilers of a
high evaporative rate. (The faster a steam-boiler or generator
will convert water to steam, the more rapidly will the solids
in the water concentrate up.) So, large and small water-tube
boilers, the typical fire-tube packaged boiler, and steam generators
are all examples of this in varying degrees. As all untreated
waters carry natural salts, they have to be treated to prevent
scale forming.
The three main reasons for water treatment are : |
- Prevention of Corrosion in feed-boiler, steam and condensate
systems.
- Elimination of Scale.
- Economic boiler operation without carry-over.
|
| Corrosion will reduce metal thickness of tubes or shell.
Result : pressure must be reduced and finally boiler condemned. Scale reduces the heat flow from fire side to water.
Result : high fire temperatures are needed to maintain down
is insufficient.
Basic Chemistry of the Effect of Impurities in the Boiler.
If we could use water completely free from all impurities,
there would be no need for water treatment. |
IMPURITY |
EFFECT ON A BOILER |
| 1. Dissolved gases |
Corrosion |
| 2. Calcium salts and
magnesium salts |
These salts are the 'hardness'in the boiler.
Some salts can also cause corrosion |
| 3. Silica |
Can form a very hard scale. |
| 4. Suspended solids and
dissolved solids |
Contribute to, or cause, carry-over (*) |
|
(*) Carry-over is a collective term to describe the
entrainment of a relatively small quantity of boiler-water
solids with the steam. Carry-over occurs as a result of either
foaming or priming, or by a combination of both. Foaming is
the formation of bubbles on the surface of the boiler resulting
in the throwing over of slugs of boiler water with the steam.
This is similar to the 'bumping' experienced when water is
boiled in an open vessel.
Even on ships and in powerhouses, however, where evaporated
water is used, the small quantities of impurities are sufficient
to cause corrosion, scale and carry-over, and must therefore
be treated. A table of the impurities is as follows : |
1. Dissolved Gases :
The two gases which cause corrosion are wxygen and carbon
dioxide. The carbon dioxide does so simply by dissolving
in the water and forming a weak carbonic acid which attacks
the metal in feed systems, boiler or condensate system.
Oxygen is present in all waters, so that red iron oxide
forms on a mild steel surface immersed in water. This
rusting-or, as we call it, corrosiontinues until the
metal is corroded away. If the amount of wxygen in the
water is restricted, the oxide film does not form so
readily;but instead, teh surface of the steel tarnishes.
This tarnish is usually the development of a thin film
of iron oxide on the metal surface which is not so fully
oxidized as the red iron oxide, and is more dense, thus
tending to resist further corrosive attack. In water
of increasing alkalinity, the oxide film becomes more
stable and gives more protection to the steel, but until
a definite alkalinity is reached, it still tends to break
down in selective areas, where pits will develop.
2. Calcium and magnesium salts :
There are two forms
of hardness; temporary and permanent.Temporary hardness
is due bicarbonates of calcium and magnesium which
break down to carbonates when the water is
boiled. In the boiler the following chemical
reaction takes place : Calcium Bicarbonate+heat.
Calcium Carbonate+carbon dioxide+water. Calcium
and magnesium bicarbonate are soluble in
water but the arbonates are insoluble and therefore
precipitate as a fine white powder. This
precipitate will bake unto the heating surface of a boiler
and form a scale. Permanent hardness is
due to calcium and magnesium sulphates, chlorides
and nitrates, and these salts cannot be removed
by boiling. However, under boiler conditions
(resulting in successive concentrations of
these hardness salts) the solubility of these
salts is soon exceeded and they deposit on
the hottest part of the heating surface.
The salts of magnesium that form permanent hardness
sometimes tend to cause corrosion instead
of hard scale formation, e.g. magnesium chloride
in an untreated boiler hydrolyses to form
corrosive hydrochloric acid.
3. Silica :
Silica forms scale in
a similar way to the permanent hardness salts. When the
scale formed is a mixture of silica, calcium and magnesium
salts, it is very hard and therefore presents
a difficult problem at inspection time.
4. The suspended and dissolved solids :
The suspended and dissolved solids cause foaming by becoming
absorbed unto the walls of individual bubbles so that small bubbles,
instead of coalescing to form large ones and bursting early, repel one another and build
up a large volume of small bubbles. If these bubbles burst near the steam
outlet, the spray is taken over with the
steam. If the bubbles do not burst high in the steam space, the shole
foam can be drawn over with the steam.
Water, the raw material for making steam :
Water is
the only common substance that exists in three forms (ice,
water, steam) at normal earth temperatures. It absorbs more
heat for a given temperature rise than any other common inorganic
substance. Water expands 1600 times as it evaporates to form
steam at atmosphereic pressure. The steam is capable of carrying
large quantities of heat. These unique properties of water
make it an ideal raw material for heating and power generating
processes.
All natural waters contain varying amounts of dissolved and
suspened matter and dissolved gases the amount of minerals
disolved in water varies from 30kg. per 10001 in sea water
to anything from 5 g to 1 kg per 10001 in fresh water supplies.
The source (lake, river, well, etc.) and also with the area
of the country. The impurities in water are important considerations
when it is to used for steam generation.
The composition of boiler feed water must be such that the impurities in
it can be concetrated a reasonable number of timbes inside the boiler, without
exceedign the tolerance limits of the particular boiler design. If the feed
water does not meet these requirements it must be pretreated to remove impurities.
The impurities need not be completely removed in all cases, however, since
chemical treatment inside the boiler can effectively and economically counteract
them. Let us throughly examine water-the raw material for making
steam-through a series of questions and answeres.
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