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Water Treatment

Hamada Boiler
株式会社 ジャコム(日本燃焼技術株式会社)
Hamada Boiler Group Head Office
Hangzhou, Zhejiang Province, China
Tel: +86-571-87655979, 87655989
Fax: +86-571-87655969
Water Treatment Technology | 目次 |
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.



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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