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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 carryover.

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, carryover (*)

(*) Carryover is a collective term to describe the entrainment of a relatively small quantity of boiler water solids with the steam. Carryover 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 oxygen 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, corrosion triunes until the metal is corroded away. If the amount of oxygen in the water is restricted, the oxide film does not form so readily; but instead, the 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 carbonates 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 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 atmospheric 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 suspended matter and dissolved gases the amount of minerals dissolved 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 concentrated a reasonable number of times inside the boiler, without exceeding 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 thoroughly examine water the raw material for making steam through a series of questions and answers.

"94" QUESTIONS & ANSWERS

Hereunder we would like to list up many questions that may be asked by the users of boilers and answers are given in details with as much information available.

Questions :

  1. What are the Physical Properties of Water?
  2. What is the Chemical Composition of Water?
  3. At Temperatures Does Water Boil?
  4. Why is Steam Ideal for "Carrying" Heat Energy?
  5. Why is Water Not Always Satisfactory for Boiler Use?
  6. What General Type of Impurities Does Water Contain?
  7. What Dissolved Minerals Do Natural Waters Contain?
  8. What is Water Hardness?
  9. What Gases are Dissolved in Natural Waters?
  10. What Types of Other Impurities May Water Contain?
  11. What is the Difference between Sea Water and Fresh Water?
  12. What are the Sources of Fresh Water and How do they Vary in Composition?
  13. How Does Water Composition Vary from Geographical Standpoint?
  14. What is Boiler Feed Water?
  15. Is there any Relationship between Good water for drinking use and for Boiler Feed?
  16. How Pure Must Feed Water be?
  17. How does Operating Pressure Influence Boiler Water Composition requirements?
  18. What is Meant by 'External' and 'Internal' Feed water Treatment?
  19. What Causes Boiler Deposits?
  20. Which are some Common Types of Boiler Deposits?
  21. What are the Characteristics of a carbonate deposit?
  22. What are the Characteristics of a Sulphate Deposit?
  23. What are the Characteristics of a Silica Deposit?
  24. What are the Characteristics of an Iron Deposit?
  25. What Problems do Deposits Cause?
  26. What is Corrosion?
  27. Where is Corrosion Usually Experienced?
  28. What is Corrosion Fatigue?
  29. What is Caustic Cracking?
  30. What Problems does Corrosion Cause?
  31. What Measures are taken to Prevent Boiler System Corrosion?
  32. What is Boiler Water Carry-over?
  33. What Causes Foaming?
  34. What Causes Priming?
  35. How Does Oil Affect Carry-over?
  36. How Do Suspended Solids Affect Carry-over?
  37. What is Selective Silica Carry-over?
  38. What Problems are Caused by Carry-over?
  39. What Measures are Usually Taken to Prevent Carry-over?
  40. What is Clarification?
  41. What is Coagulation?
  42. What Various Types of Coagulants are Used?
  43. What is Chemical Precipitation?
  44. How Does Lime React in the softening Process ?
  45. How Does soda Ash React in the Softening Process?
  46. What are the Various Methods of Lime Soda Softening?
  47. Why are Coagulants Used in the Lime Soda Process?
  48. Under What Conditions Are Phosphate Softeners Use?
  49. What are the Disadvantages of Lime Soda Softening?
  50. What are the Advantages of Lime Soda Softening?
  51. What is Ion Exchange?
  52. What are the Various Types of Ion Exchange Materials?
  53. What is Boiler Water Carry-over?
  54. What are the Disadvantages of Ion Exchange?
  55. What are the Advantages of Ion Exchange?
  56. How Does Oil Affect Carry-over?
  57. How Do Suspended Solids Affect Carry-over?
  58. What is Selective Silica Carry-over?
  59. What Problems are Caused by Carry-over?
  60. What Measures are Usually Taken to Prevent Carry-over?
  61. What is Clarification?
  62. What is Coagulation?
  63. What Various Types of Coagulants are Used?
  64. What is Chemical Precipitation?
  65. What is the Purpose of Deaeration?
  66. How are Evaporators Employed?
  67. What Combinations of External Treatment Methods are Generally Used?
  68. When is Internal Treatment of Boiler Feed water Necessary?
  69. What Should a Good Internal Water Treatment Programme Accomplish?
  70. What chemicals are Used in Internal Treatment?
  71. How are Carbonates Reacted on by Internal Treatment?
  72. How are Sulphates Reacted on by Internal Treatment?
  73. How is Silica Reacted upon by Internal Treatment?
  74. How is Sludge Conditioned in Internal Treatment?
  75. What Difficulties are Encountered in Internal Treatment?
  76. What are the Advantages of Internal Treatment?
  77. How are Internal Treatment Chemicals Fed?
  78. How are Chemical Dosages Controlled?
  79. What Boiler Water Tests are Used for Treatment Control?
  80. What Tests are Usually Made as a Check for Contaminants?
  81. What Units are Used in Expressing Water Analysis Results?
  82. Why are some Analysis Results Express ‘As CaCO2E
  83. What is Blow-down?
  84. How much Blow-down is Needed?
  85. What Tests are Made in Regulating Blow-down?
  86. What is the Difference between Continuous and ‘Puff Blow-down?
  87. What Causes Corrosion in Steam Condensate Systems?
  88. How is Steam Condensate Corrosion Prevented?
  89. How do Chemical Oxygen Scavengers Help Control Condensate System Corrosion?
  90. What is the Basis for Choice between Neutralizing and Filming Inhibitors?
  91. What Characteristics Should a Good Condensate Corrosion Inhibitor Have?
  92. How are Deposits and Corrosion Prevented in Feed water Systems?
  93. What is the Wet Method of Boiler Lay-Up?
  94. What is the Dry Method of Boiler Lay-Up?

 


Answer to : back

What are the Physical Properties of Water?
Water is a tasteless, odorless, colourless liquid in its pure state. It is the only inorganic material which occurs in three forms (ice, water, steam) within the natural temperature range on earth. Because water can be converted to steam at a convenient temperature.


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What is the Chemical Composition of Water?


Pure water is a simple combination of hydrogen and oxygen. The common formula is H2O. As a matter of general interest, however, there are several 'hybrid' forms of water which are present in all supplies. Water contains about 300 ppm of deuterium oxide (D.,O) or "heavy water". This doesn't quench thirst or make plants grow but in a pure form it has found use in moderating nuclear reactors. Another form of water, tritium oxide (T2O), is made radioactive by cosmic rays. Although only a minute fraction of water exists in this form, its radioactivity serves as a means of measuring the age of a water supply. For all practical purposes, though, only ordinary water (H2O) is considered for use in boilers.


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At Temperature Does Water Boil?

The boiling point of water depends on the pressure to which the water is subjected. At atmospheric pressure water boils at about 100°C. As pressure increases the boiling point also increases. At the critical pressure near 22.000kPa (where water can be converted to steam without a change in volume) the boiling point is lowered. Under extreme vacuum conditions, water will boil at temperatures as low as 2°C.


Answer to : back
Why is Steam Ideal for "Carrying" Heat Energy?

The Traditional formula was : 1 Btu raises 1 pound of water 1°F. This can be converted to 4.2 kJ to raise 1 kg. of water 1°C. It takes an additional 2256 kJ/kg. to change 1 kg. of water (at the boiling point) to steam. This amount of heat (the heat of vaporization) is then "stored" in the steam. When steam condenses, this heat energy is given off. Consider, for example, the amount of heat which can be "carried" by 1 kg. of water. If the water temperature is originally 38°C (100°F) it takes 260 kJ/kg. to heat it to 100°C and 2256 kJ/kg. to convert it to steam. A total of 2516 kJ/kg. has been added and will be released as the water is condensed and cooled back to 38°C. Since this transfer is never 100 percent efficient, some of the heat energy will be dissipated in the process. But much of the heat from burning fuel can be absorbed by boiler water, transported with the steam, and released at the desired points.


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Why is Water Not Always Satisfactory for Boiler use?

Completely pure water is nonexistent. All natural waters contain various types and amounts of impurities. Since water impurities cause boiler problems, careful consideration must be given to the quality of the water used for generating steam.


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What General Types of Impurities Does Water Contain?

Impurities picked up by natural waters may be classed as : (a) dissolved solids, (b) dissolved gases, and (c) suspended matter. Water is a good solvent; it dissolves the rocks and soil it contacts. It dissolves gases from the air and gases given off from organics is the soil. It picks up suspended matter from the earth. It is also subject to contamination with trade wastes, oils and process materials. In general, the type of impurities water contains depends on what it contacts; the amount of impurities deepens on the contact times.


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What Dissolved Minerals Do Natural Waters Contain?

The minerals which water picks up from rocks consist chiefly of calcium carbonate (limestone); magnesium carbonate (dolomite); calcium Sulphate (gypsum), magnesium Sulphate (Epsom salts); silica (sand); sodium chloride (common salt); sodium Sulphate (Glaubers salts); and small quantities of iron, manganese, fluorides, aluminum, and other substances. Wastes from mines and certain industrial processes make some surface waters very acin, while minerals in the earth make some ground waters very alkaline. Sometimes Nitrates are found in water and in many cases, this is associate with sewage contamination.


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What is Hardness?

Waters which contain large amounts of calcium and magnesium minerals are "hard to wash with". The calcium and magnesium compounds react with soap to form a curb in the water. These compounds are therefore referred to as water hardness. The amount of hardness in a natural water may vary from several parts per million to over 500 parts per million. Since calcium and magnesium compounds are relatively insoluble in water, they tend to precipitate out to cause scale and deposit problems. Water hardness, therefore, is an important consideration in determining suitability of a water or use in generating steam.


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What Gases are Dissolved in Natural Waters?

Water dissolves varying amounts of air which is composed of 21% oxygen, 78% nitrogen, 1% other gases (including 0.03 - 0.06% carbon dioxide). Oxygen is soluble in water at room temperature and atmospheric pressure to the extent of about 9ppm. The solubility of oxygen decreases as the temperature of the water goes up, but water under pressure can hold larger quantities of dissolved oxygen. Although nitrogen is dissolved in natural water it is inert gas and has little effect on the character of water used in boilers. Water doesn't usually pick up more than 10 ppm of carbon dioxide from the air. Carbon dioxide may be dissolved in water to a much greater extent, though, through the decay of vegetation and organics in the soil. Hydrogen Sulphate and methane may be dissolved in some waters, but this is not a general occurrence. These gases, however, can be important when they occur as contaminants.


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What Types of Other Impurities May Water Contain?

Natural waters may contain turbidity, colour, soil, and precipitated minerals, as well as oil and other trade wastes. Colour comes from decaying vegetable matter. Turbidity may consist of very finely divided organic material and microorganisms, as well as suspended clay and silt. Oils, fats, greases, sewage and other wastes may contaminate water supplies.


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What is the Difference between Sea Water and Fresh Water?

The main difference is in the amount of dissolved minerals. Sea water contains about 30 kg. of minerals per 1000 liters (mostly salt). The mineral content of fresh water supplies is much lower and generally ranges form 5 g. to 1 kg. per 10001. While work is now in progress on methods of purifying sea and brackish water at the present time only fresh water supplies are generally used for boiler feed purposes.


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What are the Sources of Fresh Water and How do they Vary in Composition?

Fresh water supplies may be either surface water (rivers, streams, reservoirs, etc.) or ground water (shallow or deep well waters). In general, ground water supplies are more consistent in composition and contain less suspended matter and turbidity than surface supplies which are affected directly by rainfall, soil erosion, and trade wastes. On the other hand, ground waters are usually harder than surface waters. For example, an average surface supply will contain about 95 ppm total hardness as opposed to an average of about 200 ppm total hardness for ground supplies. In some instances where the ground water is normally encountered in surface supplies.


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How Does Water Composition Vary from Geographical Standpoint?

Water composition varies throughout the states depending upon the type and strata of the earth formations. In the limestone areas the waters contain large quantities of calcium carbonate. In parts of the country where there is more of the granite type of rock formation, much less mineral matter is dissolved and the water will not contain much hardness. Throughout some state, there are deposits of alkali which the water will pick up. These are the general geographic variations, but local conditions in any area may greatly influence the composition of the water.


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What is Boiler Feed Water?

The water added to a boiler to repave evaporation and blow-down losses is termed 'feed water'. In may cases, steam is condensed and returned to the boiler as part of the feed water. Whatever water is needed to supplement the condensate returned is termed 'make-up water'. The make-up water is usually natural water either in its raw state, or treated by some process before use. Feed-water composition therefore depends on the quality of the make-up water and the amount of condensate returned to the boiler.


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Is there any Relationship between Good water for drinking use and for Boiler Feed?

Except that sewage pollution is harmful to both, there is not a great deal of similarity between the requirements for drinkable water and the requirements for boiler feed-water. The minerals in drinking water, while they might affect the taste, are absorbed by the body and many so called 'health waters' are high in minerals. On the other hand, water impurities cannot be handled as well by boilers. Although a boiler is a mass of steel, it is generally more sensitive about what it consumes than is the human stomach. For this reason, much care is needed in selecting water treatment.


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How Pure Must Feed Water be?

Feed-water purity is a matter both of quantity of impurities and nature of impurities... some impurities such as hardness, iron and silica are of more concern, for example, than sodium salts, The purity requirements for any feed-water depend on how much feed water is used as well as what the particular boiler design (pressure, heat transfer rate, etc.) can tolerate. Feed-water purity requirements therefore can vary widely. A low pressure fire-tube boiler can usually tolerate high feed-water hardness with proper treatment while virtually all impurities must be removed from water used in some modern, high pressure boilers.


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How does Operating Pressure Influence Boiler Water Composition requirements?

The Boiler and affiliated Industries Manufacturers' Association has established limits for boiler water composition with respect to operating pressure to assure good quality steam. Pending the evolving of new, round figure ratios in S.I. units, we literally convert the p.s.i.g. column below :

  BOILER PRESSURE

TOTAL SOLIDS

ALKALINITY

SUSPENDED

(kPa)

(psig)

(ppm)

(ppm)

solids

silica*

0-2070

0-300

3500

700

300

125

2070-3100

301-450

3000

600

250

90

3100-4135

451-600

2500

500

150

50

4135-5170

601-750

2000

400

100

35

5170-6200

751-900

150

300

60

20

6200-6890

901-1000

1250

25

40

8

6890-10335

1001-1500

1000

20

20

2.5

10335-13780

1501-2000

750

50

10

1.5

over 13780

over 2000

500

100

5

0.5

(*) Silica limits based on limiting silica in steam to 0.02-0.03 ppm.


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What is Meant by 'External' and 'Internal' Feed water Treatment?

External treatment is the reduction or removal of impurities from water outside the boiler. In general, external treatment is used when the amount of one or more of the feed=water impurities is too high to be tolerated by the boiler system in question. There are many types of external treatment (softening, evaporation, deaerarion, etc.) which can be used to tailor make feed-water for a particular system. Internal treatment is the conditioning of impurities within the boiler system. The reactions occur either in the feed lines or in the boiler proper. Internal treatment may be used alone or in conjunction with external treatment. Its purpose is to properly react with feed water hardness, condition sludge, scavenge oxygen and prevent boiler water foaming.


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What Causes Boiler Deposits?

Boiler scale is caused by impurities being precipitated out of the water directly on heat transfer surfaces or by suspended matter in water settling out on the metal and becoming hard and adherent. Evaporation in a boiler causes impurities to concentrate. The high temperatures break down some minerals, cause others to become less soluble. In general, water in contact with hot metal will tend to deposit out impurities as it evaporates.


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Which are some Common Types of Boiler Deposits?

In untreated boiler water, the formation of deposits is like a back to nature movement. That is as minerals are deposited out from water they form many types of crystalline and rock like structures such as are encountered in the earth's strata. Deposits are seldom composed of one constituent alone but are generally a mixture of various types of minerals, corrosion products and other water contaminants. The most common types of boiler deposits may contain : Calcium carbonate, Sulphate or silicate; magnesium hydroxide or silicate, iron oxide, and silica, sludge deposits form boiler water which has been treated may also contain calcium and magnesium phosphates.


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What are the Characteristics of a carbonate deposit?

A carbonate deposit is usually granular and sometimes of a very porous nature. The crystals of calcium carbonate are large but usually are matted together with finely divided particles of other materials so that the scale looks dense and uniform. A carbonate deposit can be easily identified by dropping it in a solution of acid. Bubbles of carbon dioxide will effervesce from the scale.


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What are the Characteristics of a Sulphate Deposit?

A Sulphate deposit is much harder and more dense than a carbonate deposit because the crystals are smaller and cement together tighter. A Sulphate deposit is brittle, does not pulverize easily, and does not effervesce when dropped into acid.


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What are the Characteristics of a Silica Deposit?

A high silica deposit is very hard, resembling porcelain. The crystal of silica are extremely small, forming a very dense and impervious scale. This scale is extremely brittle and very difficult to pulverize. It is not soluble in hydrochloric acid and is usually very light coloured.


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What are the Characteristics of an Iron Deposit?

Iron deposits, due either to corrosion or iron contamination in the water, are very dark coloured. Iron deposits in boilers are most often magnetic. They are soluble in hot acid giving a dark brown coloured solution.


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What Problems do Deposits Cause?

The biggest problem that deposits cause is overheating and failure of boiler tubes. A deposit acts as an insulator and excessive deposits prevent an efficient transfer of heat through the tubes to the circulating water. This causes the metal itself to become over heated. When the overheating is severe enough the metal fails. Boiler deposits can also cause plugging or partial obstruction of corrosive attack underneath the deposits may occur. In general, boiler deposits can cut operating efficiency, produce boiler damage, cause unscheduled boiler outages, and increase cleaning expense.


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What is Corrosion?

Stated simply, general corrosion is the reversion of a metal to it 's form. Iron, for example, reverts to iron oxide as the result of corrosion. The process of corrosion, however is a complex electro chemical reaction and it takes many forms. Corrosion may produce general attach over a large metal surface or it may result in pinpoint penetration of metal. While basic corrosion in boilers may be primarily due to reaction of the metal with oxygen, other factors such as stresses, acid conditions, and specific chemical corrodents may have an important influence and produce different forms of attack.


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Where is Corrosion Usually Experienced?

Corrosion may occur in the feed-water system as a result of low pH water and the presence of dissolved oxygen and carbon dioxide. Corrosion in the boiler proper generally occurs when the boiler water alkalinity is low or when the metal is exposed to oxygen bearing water either during operation or idle periods. High temperatures and stresses in the boiler metal tend to accelerate the corrosive mechanisms. In the boiler metal tend to accelerate the corrosive mechanisms. In the steam and condensate system corrosion is generally the result of c contamination with carbon dioxide and oxygen. Specific contaminants such as ammonia or sulphur bearing gases may increase attack on copper alloys in the system.


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What is Corrosion Fatigue?

This type of cracking in boiler metal may occur by two different mechanisms. In the first mechanism, cyclic stresses such as are created by rapid heating and cooling are concentrated at points where corrosion has roughened or pitted the metal surface. This is usually associated with improper corrosion prevention. The second type of corrosion fatigue cracking occurs in boilers with properly treated water. In these cases corrosion fatigue is probably a misnomer. These cracks often originate where the metal surfaces are covered by a dense protective oxide film and cracking occurs from the action of applied cyclic stresses. Corrosion fatigue cracks are usually thick, blunt and cross the metal grains. They usually start at internal tube surfaces and are most often circumferential on the tube.


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What is Caustic Cracking?

Caustic cracking (caustic embrittlement) is a serious type of boiler metal failure characterized by continuous, mostly inter granular cracks. The following conditions appear to be necessary for this type of cracking to occur :

  1. The metal must be stressed,
  2. the boiler-water must contain caustic,
  3. at least a trace of silica must be present in the boiler-water, and
  4. some mechanisms, such as a slight leak, must be present to allow the boiler water to concentrate on the stressed metal.

Answer to : back
What Problems does Corrosion Cause?

Corrosion, in general, causes difficulty from two standpoints. The first is deterioration of the metal itself, and the second is deposition of the corrosion products to form deposits. Generally, uniform corrosion of boiler surfaces is seldom of real concern. Corrosion, however, takes many insidious forms and deep pits resulting in only a minimum of iron loss may cause penetration and leaking of boiler tubes. Corrosion underneath certain types of boiler deposits can so weaken the metal that failure of tubes occurs. In steam condensate system, replacement of lines and equipment due to corrosion can be a costly problem.


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What Measures are taken to Prevent Boiler System Corrosion?

Corrosion, in general, causes difficulty from two standpoints. The first is deterioration of the metal itself, and the second is deposition of the corrosion products to form deposits. Generally, uniform corrosion of boiler surfaces is seldom of real concern. Corrosion, however, takes many insidious forms and deep pits resulting in only a minimum of iron loss may cause penetration and leaking of boiler tubes. Corrosion underneath certain types of boiler deposits can so weaken the metal that failure of tubes occurs. In steam condensate system, replacement of lines and equipment due to corrosion can be a costly problem.
Why Water Treatment is Needed :
As feed-water enters a boiler the heat causes hardness (calcium and magnesium salts) to come out of solution. Untreated the hardness deposits on the hot boiler metal to from scale. As water evaporates in the boiler the feed-water impurities concentrate. Even small amounts to iron, copper, and silica can accumulate in the boiler-water and cause serious deposit problems in higher pressure boilers. Since scale can cause overheating and failure of boiler metal, preventive water treatment is needed. The corrosion of boiler system metal is a complex process and takes many forms: general attack, localized pitting, and various types of cracking in stressed metal. In general, the main factors causing corrosion are dissolved gases in the water (primarily oxygen) and acid conditions. High temperatures speed up the corrosion process. Corrosion is damaging from several standpoints: it causes weakening and failure of metal and produces corrosion products which can cause boiler deposits. High concentrations of dissolved and suspended matter in boiler-water can cause foaming of the water at the steam release surface. This produces carry-over of the water and its impurities into the steam. Carry-over results in deposits and other problems in turbines, engines and other processes using steam. While mechanical and operational factors also cause carry-over, proper control of water conditions is important in producing pure steam.


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What is Boiler Water Carryover?

Boiler water carry-over is the contamination of the steam with boiler-water solids. There are four common types of boiler-water carry-over. In one bubbles or froth actually build up on the surface of the boiler-water and pass out with the steam. This is called foaming and can be compared to the stable foam found on beer. In the second type small droplets of water in the form of spray or mist are thrown up into the steam space by the bursting of the rising steam bubbles at the steam release surface. This is sometimes called ‘aquaglobejectionEand is like ginger ale or champagne where no stable foam is formed but droplets of liquid are ejected from the liquid surface. The third condition of carry-over, called priming, is a sudden surge of boiler-water that carries over with the steam, similar to the effects produced in uncapping a bottle of charged water. stem contamination may also occur from leakage of water through improperly designed or installed steam separating equipment in a boiler drum.


Answer to : back
What Causes Foaming?

Very high concentrations of any solids in boiler-water cause foaming. It is generally believed, however, that specific substances such as alkalis, oils, fats, greases, certain types of organic matter and suspended solids are particularly conducive to foaming.


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What Causes Priming?

Priming may be caused by improper construction of boiler, excessive ratings, or sudden fluctuations in steam demand. priming is sometimes aggravated by impurities in the boiler-water


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How Does Oil Affect Carryover?

Oil contamination in boiler feed-water, usually form reciprocating engines, pumps, etc., can cause serious foaming. This is generally attributed to the formation of soaps in the boiler-water due to specification of the oil by boiler-water alkalis.


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How Do Suspended Solids Affect Carryover?

The theory advanced is that suspended solids collect in the surface film surrounding a steam bubble and make it tougher. The steam bubble therefore resists breaking and builds up a foam. It is believed that the finer the suspended particles the greater their collection in the bubble. Experience indicates, however, that many boilers operate with exceedingly high suspended solids without carry-over while others have carry-over with only a trace of suspended solids. This would seem to indicate that the type as well as the quantity of suspended solids has much to do with carry-over.


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What is Selective Silica Carryover?

Silica can carry over into the steam in two ways. It can be present in the steam as the result of general boiler-water carry-over or it can go into steam in a volatile form. In the latter case silica acts much like a gas and is considered to be selectively carried over. As Pressures increase above 2760 kPa (400 p.s.i), there is an increased tendency for silica to be selectively carried into the steam in amounts proportionate to the amount of silica in the boiler-water.


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What Problems are Caused by Carry-over?

The disadvantages of wet steam include a general decrease in operating efficiency and erosion of turbines and engines. In addition any dissolved or suspended solids in the boiler-water tend to deposit out in the steam and condensate system,. when the solids deposit in super heaters and turbine, overheating and failure of superheated tubes and reduction in turbine efficiency can result. Impurities carried over with the can cause difficulties in many processes for which the steam is used.


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What Measures are Usually Taken to Prevent Carryover?

The most common measure is to maintain the concentration of solids in the boiler water at reasonably low levels. Avoiding high water levels, excessive boiler loads, and sudden load changes also helps. Very often contaminated condensate returned to the boiler system causes carry-over problems. In these cases the condensate should be temporarily wasted until the source of contamination is found and eliminated. The use of chemical anti-foam agents can be very effective in preventing carry-over due to high concentrations of impurities in the boiler-water.
Removing Impurities from Water :
Coagulants are chemicals to enmesh fine particles of suspended matter in a water supply to form a flock which settles or can be filtered out. Adding softening chemicals (lime, soda, ash, etc.) to a water causes some dissolved hardness salts to precipitate and the suspended matter can then be coagulated and filtered out. Precipitation processes such as lime soda softening can effectively remove suspended matter, hardness and alkalinity and in some cases reduce the silica content of the water. When a salt dissolves in water it forms positive ions (cations) and negative ions (anions). For example, calcium carbonate (CaCO3) forms a calcium cation (Ca++) and a carbonate anion (CO3=). The most common form of ion exchange involves passing water through material which substitutes sodium for calcium and magnesium cations. This is a typical softening treatment. Anions can also be removed from water by the use of special ion exchange resins. Demineralization or complete removal of dissolved minerals involves the use of both cation and anion exchange materials. In removing impurities from water there are many possible combinations of coagulation, precipitation and ion exchange methods. Other methods of treatment include: deaerarion (heating the water and venting the gases) for reduction of oxygen and carbon dioxide; and evaporation to produce distilled water.


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What is Clarification?

Clarification is the removal of suspended matter and/or colour from water supplies. The suspended matter may consist of large particles which settle out readily. In these cases clarification equipment merely involves the use of settling basins and/or filters. Most often, however, suspended matter in water consists of particles so small that they do not settle out and even pass through filters. The removal of these finely divided or colloidal substances therefore requires the use of coagulants.


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What is Coagulation?

Coagulation is the clumping together of finely divided and colloidal impurities in water into masses which will settle rapidly and/or can be filtered out of the water. Colloidal particles have large surface areas which keep them in suspension and in addition the particles have negative electrical charges which cause them to repel each other and resist adhering together. Coagulation, therefore, involves neutralizing the negative charges and providing a nucleus for the suspended particles to adhere to.


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What Various Types of Coagulants are Used?

The most common coagulants are iron and aluminum salts such as ferric Sulphate, ferric chloride, aluminum Sulphate (alum) and sodium acuminate. Ferric and alumina ions each have three positive charges and therefore their effectiveness is related their ability to react with the negatively charged colloidal particles. With proper use these coagulants form a flock in the water which serves as a kind of net for collecting suspended matter. In recent years synthetic materials called polyelectrolyte have been developed for coagulation purposes. these consist of long chain like molecules with positive charges. In some cases organic polymers and special types of clay are used in the coagulation in making the flock heavier, causing it to settle out more rapidly.


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What is Chemical Precipitation?

In precipitating processes the chemicals added react with dissolved minerals in the water to produce a relatively insoluble reaction product. Precipitation methods are used in reducing dissolved hardness, alkalinity and in some silica. The most common example of chemical precipitation in water treatment is lime soda softening.


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How Does Lime React in the softening Process ?

Hydrated lime (calcium hydroxide) reacts with soluble calcium and magnesium bicarbonates to form insoluble precipitates. This is shown by the following equations:

 
  Ca(OH)2

+

  Ca(HCO3)2 ======>   2CaCO3

+

  2H2O
 Lime

 

  Calcium     Calcium

 

  Water
      Bicarbonate     Carbonate

 

  



  Ca(OH)2

+

  Mg(HCO3)2 ======>   Mg(OH)2

+

  2CaCO3

+

  2H2O
 Lime

 

  Magnesium     Magnesium

 

  Calcium

 

  Water
 

 

  Bicarbonate     Hydroxide

 

  Carbonate

 

  

Most of the calcium carbonate and magnesium hydroxide come out of solution as a sludge and can be removed by settling and filtration. Lime, therefore, can be used to reduce hardness present in the bicarbonate form (temporary hardness) as well as decrease the amount of bicarbonate alkalinity in a water. Lime reacts with magnesium Sulphate and chloride and precipitates magnesium hydroxides but in this process soluble calcium Sulphate and chloride are formed. Lime is not effective in removing calcium Sulphate and chlorides.


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How Does soda Ash React in the Softening Process?

Soda ash is used primarily to reduce non-bicarbonate hardness (also called Sulphate hardness or permanent hardness). It reacts as follows:

 
  Na2CO3

+

  CaSO4 ======>   CaCO3

+

  Na2SO4
 Soda Ash

 

  Calcium     Calcium

 

  Sodium
 

 

  Sulphate     Carbonate

 

  Sulphate

 
  Na2CO3

+

  CaC12 ======>   CaCO2

+

  2NaCl
 

 

  Calcium     Calcium

 

  Sodium
 

 

  Chloride     Carbonate

 

  Chloride

The calcium carbonate formed by the reaction tends to come out of solution as a sludge. The sodium Sulphate and chloride formed are highly soluble and non-scale forming.


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What are the Various Methods of Lime Soda Softening?

The two general types are intermittent (batch type) and continuous. The older method of intermittent softening consists of mixing the chemicals with the water in a tank, allowing time for reaction and settling of the sludge, and drawing off the clear water. The more modern method of continuous lime soda softening involves the use of specially compartmented tanks with provisions for

  1. proportioning chemicals continuously to the incoming water
  2. retention time for chemical reactions and settling of sludge, and
  3. continuous draw-off of softened water. Lime soda softening may also be classified as hot or cold, depending on the temperature of the water. Hot process softeners increase the rate of chemical reactions and give better quality water.


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Why are Coagulants Used in the Lime-Soda Process?

Just as coagulants are used for removing suspended matter in clarification processes, they serve to clump together precipitates in the softening process. Coagulants can speed up settling of sludge as much as 25 - 50 per cent. Sodium acuminate has a special advantage as a coagulant in lime soda softening since unlike most other coagulants it is alkaline and also contributes to the softening redactions, particularly in reducing magnesium. Effective use of coagulants helps remove silica in the softening process. Silica tends to be absorbed in the flock produced by coagulation of sludge.


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Under What Conditions Are Phosphate Softeners Use?

Sodium phosphates react readily with calcium and magnesium salts. Phosphate softeners are generally used only on naturally soft or pre softened waters, however, because relatively high amounts of magnesium in the water cause a very sticky precipitate in reacting with phosphate. Properly used, phosphate softeners can effectively reduce hardness to very low levels. Improved ion exchange softening methods have largely supplanted phosphate softeners in new installations.


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What are the Disadvantages of Lime Soda Softening?

The main disadvantage is that while hardness is reduced it is not completely removed. Wide variations in raw water composition and flow rate also make control of this method difficult since this involves adjusting the amounts of lime and soda ash being fed.


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What are the Advantages of Lime Soda Softening?

The main advantage is that in reducing hardness, alkalinity and silica can also be reduced. In addition, prior clarification of the water is not usually necessary since suspended matter and turbidity are also removed in the process. Another advantage is that with continuous hot process softening some removal of oxygen and carbon dioxide can be achieved.


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What is Ion Exchange?

When minerals dissolve in water they form electrically charge particles called ions. Calcium carbonate, for example, forms a calcium ion with plus charges (a cation) and a carbonate ion with negative charges (an anion). Certain natural and synthetic materials have the ability to remove mineral ions from water in exchange for others. For example, in passing water through a simple cation exchange softener all of calcium and magnesium ions are removed and replaced with sodium ions. Ion exchange materials usually are provided in the form of small beads or crystals which compose a bed several feet deep through which the water is passed.


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What are the Various Types of Ion Exchange Materials?

Ion exchange materials are basically of two types: cation and anion exchangers. Cation exchange materials react only with positively charged ions such as Ca++ and Mh++. Anion exchanger materials react only with the negatively charged ions such as carbonate (CO3) and Sulphate (SO4). Zeolite materials are cation exchangers composed chiefly of sodium, aluminum and silica. There are several other types of cation exchange materials of an organic or resinous nature. The anion materials are usually organic in nature and are of two basic types: weak base and strong base types. Weak base exchangers don’t take out carbon dioxide or silica (actually carbonic acid and silica acid) but remove strong acid anions by a process that is more like adsorption than ion exchange. Strong base anion exchangers on the other hand can reduce silica and carbon dioxide to very low values. Cation exchangers usually opals which settle out readily. In these cases clarification equipment merely involves the use of settling basins and/irate on either a sodium or hydrogen ‘cycleE That is, they may be designed to replace all cations in the water with either sodium or hydrogen. Strong base anion exchangers are generally operated on a hydroxide weak base on a carbonate cycle. Chloride anion exchange is also used in some processes.

Why Water Treatment is Needed :
As feed-water enters a boiler the heat causes hardness (calcium and magnesium salts) to come out of solution. Untreated the hardness deposits on the hot boiler metal to from scale. As water evaporates in the boiler the feed-water impurities concentrate. Even small amounts to iron, copper, and silica can accumulate in the boiler-water and cause serious deposit problems in higher pressure boilers. Since scale can cause overheating and failure of boiler metal, preventive water treatment is needed. The corrosion of boiler system metal is a complex process and takes many forms: general attack, localized pitting, and various types of cracking in stressed metal. In general, the main factors causing corrosion are dissolved gases in the water (primarily oxygen) and acid conditions. High temperatures speed up the corrosion process. Corrosion is damaging from several standpoints: it causes weakening and failure of metal and produces corrosion products which can cause boiler deposits. High concentrations of dissolved and suspended matter in boiler-water can cause foaming of the water at the steam release surface. This produces carry-over of the water and its impurities into the steam. Carry-over results in deposits and other problems in turbines, engines and other processes using steam. While mechanical and operational factors also cause carry-over, proper control of water conditions is important in producing pure steam.


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What is Boiler Water Carryover?

Boiler water carry-over is the contamination of the steam with boiler-water solids. There are four common types of boiler-water carry-over. In one bubbles or froth actually build up on the surface of the boiler-water and pass out with the steam. This is called foaming and can be compared to the stable foam found on beer. In the second type small droplets of water in the form of spray or mist are thrown up into the steam space by the bursting of the rising steam bubbles at the steam release surface. This is sometimes called ‘aquaglobejectionEand is like ginger ale or champagne where no stable foam is formed but droplets of liquid are ejected from the liquid surface. The third condition of carry-over, called priming, is a sudden surge of boiler-water that carries over with the steam, similar to the effects produced in uncapping a bottle of charged water. stem contamination may also occur from leakage of water through improperly designed or installed steam separating equipment in a boiler drum.


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What are the Disadvantages of Ion Exchange?

The main disadvantage with sodium cycle ion exchange softening is that the total solids, alkalinity and silica contents of the raw water are not reduce. A problem encountered with cation exchange on the hydrogen cycle is corrosion from acidity of the effluent. With demineralization the chief difficulties are with cost particularly on high solids raw waters, and the corrosive nature of the effluent water. In general, fouling of the ion exchange material with suspended or colloidal matter in the raw water can produce difficulties and some water impurities cause degradation of the material. In many cases, therefore, ion exchange processes require pretreatment of the water supply.


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What are the Advantages of Ion Exchange?

The main advantage of zeolite softening is ease of control. Ordinary variations of hardness in the raw water or in flow rate do not affect completeness of softening. Also the system generally takes up less space than the lime-soda system and in most cases gives a softer water. The use of acid exchangers has advantages when a low alkalinity soft water is required. The main advantage of ion exchange demineralization is its ability to produce better quality water than can be obtained by any other method.


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How Does Oil Affect Carry-over?

Oil contamination in boiler feed-water, usually form reciprocating engines, pumps, etc., can cause serious foaming. This is generally attributed to the formation of soaps in the boiler-water due to specification of the oil by boiler-water alkalis.


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How Do Suspended Solids Affect Carry-over?

The theory advanced is that suspended solids collect in the surface film surrounding a steam bubble and make it tougher. The steam bubble therefore resists breaking and builds up a foam. It is believed that the finer the suspended particles the greater their collection in the bubble. Experience indicates, however, that many boilers operate with exceedingly high suspended solids without carry-over while others have carry-over with only a trace of suspended solids. This would seem to indicate that the type as well as the quantity of suspended solids has much to do with carry-over.


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What is Selective Silica Carry-over?

Silica can carry over into the steam in two ways. It can be present in the steam as the result of general boiler-water carry-over or it can go into steam in a volatile form. In the latter case silica acts much like a gas and is considered to be selectively carried over. As Pressures increase above 2760 kPa (400 p.s.i), there is an increased tendency for silica to be selectively carried into the steam in amounts proportionate to the amount of silica in the boiler-water.


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What Problems are Caused by Carry-over?

The disadvantages of wet steam include a general decrease in operating efficiency and erosion of turbines and engines. In addition any dissolved or suspended solids in the boiler-water tend to deposit out in the steam and condensate system,. when the solids deposit in super heaters and turbine, overheating and failure of superheated tubes and reduction in turbine efficiency can result. Impurities carried over with the can cause difficulties in many processes for which the steam is used.


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What Measures are Usually Taken to Prevent Carry-over?

The most common measure is to maintain the concentration of solids in the boiler water at reasonably low levels. Avoiding high water levels, excessive boiler loads, and sudden load changes also helps. Very often contaminated condensate returned to the boiler system causes carry-over problems. In these cases the condensate should be temporarily wasted until the source of contamination is found and eliminated. The use of chemical anti-foam agents can be very effective in preventing carry-over due to high concentrations of impurities in the boiler-water.

Removing Impurities from Water :
Coagulants are chemicals to enmesh fine particles of suspended matter in a water supply to form a flock which settles or can be filtered out. Adding softening chemicals (lime, soda, ash, etc.) to a water causes some dissolved hardness salts to precipitate and the suspended matter can then be coagulated and filtered out. Precipitation processes such as lime soda softening can effectively remove suspended matter, hardness and alkalinity and in some cases reduce the silica content of the water. When a salt dissolves in water it forms positive ions (cations) and negative ions (anions). For example, calcium carbonate (CaCO3) forms a calcium cation (Ca++) and a carbonate anion (CO3). The most common form of ion exchange involves passing water through material which substitutes sodium for calcium and magnesium cations. This is a typical softening treatment. Anions can also be removed from water by the use of special ion exchange resins. Demineralization or complete removal of dissolved minerals involves the use of both cation and anion exchange materials. In removing impurities from water there are many possible combinations of coagulation, precipitation and ion exchange methods. Other methods of treatment include: deaerarion (heating the water and venting the gases) for reduction of oxygen and carbon dioxide; and evaporation to produce distilled water.


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What is Coagulation?

Coagulation is the clumping together of finely divided and colloidal impurities in water into masses which will settle rapidly and/or can be filtered out of the water. Colloidal particles have large surface areas which keep them in suspension and in addition the particles have negative electrical charges which cause them to repel each other and resist adhering together. Coagulation, therefore, involves neutralizing the negative charges and providing a nucleus for the suspended particles to adhere to.


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What Various Types of Coagulants are Used?

The most common coagulants are iron and aluminum salts such as ferric Sulphate, ferric chloride, aluminum Sulphate (alum) and sodium acuminate. Ferric and alumina ions each have three positive charges and therefore their effectiveness is related their ability to react with the negatively charged colloidal particles. With proper use these coagulants form a flock in the water which serves as a kind of net for collecting suspended matter. In recent years synthetic materials called polyelectrolyte have been developed for coagulation purposes. these consist of long chain like molecules with positive charges. In some cases organic polymers and special types of clay are used in the coagulation in making the flock heavier, causing it to settle out more rapidly.


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What is Chemical Precipitation?

In precipitating processes the chemicals added react with dissolved minerals in the water to produce a relatively insoluble reaction product. Precipitation methods are used in reducing dissolved hardness, alkalinity and in some silica. The most common example of chemical precipitation in water treatment is lime-soda softening.


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What is the Purpose of Deaeration?

Since dissolved oxygen in water is a big factor in corrosion in boiler systems it is desirable that this be removed before the water is put into a boiler. Feed-water deaerarion is accomplished by intimately mixing the water and steam in a deaerating heater. Part of the steam is vented, arraying with it the bulk of the dissolved oxygen from the water. There are two basic types of steam deaerators: the spray type and the tray type. In the spray deaerator a jet of steam mixes intimately with the feed water being sprayed into the unit. In the tray type the incoming water is allowed to fall over a series of trays causing the water to be broken up into small droplets to permit intimate contact with incoming steam.


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How are Evaporators Employed?

Water is sometimes pretreated by evaporation to produce relatively pure vapor which is then condensed and used for boiler feed purposes. Evaporators are of several different types, the simplest being a tank of water through which steam coils are passed to heat the water to the boiling point. Sometimes to increase the efficiency the vapor from the first tank is passed through coils in a second tank of water to produce additional heating and evaporation. Other types of evaporation include a ‘flash type which operates under a partial vacuum causing a lowering of the boiling point of water and evaporation at lower temperatures. Evaporators have advantages where steam as a sources of heat is readily available. They also have particular advantages over demineralization, for example, when the dissolved solids in the raw water are very high.


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What Combinations of External Treatment Methods are Generally Used?

As mentioned previously, water containing suspended solids, organics, and/or turbidity usually requires clarifications prior to ion exchange methods. Also, since simple cation exchange does not reduce the total solids of the water supply, it is sometimes used in conjunction with precipitation type softening. One of the most common and efficient combination treatments is the hot lime-zeolite process. This involves pretreatment of the water with lime to reduce hardness, alkalinity and in some cases silica, and subsequent treatment with a cation exchange softener. This system of treatment accomplishes several functions: softening, alkalinity and silica reduction, some oxygen reduction, and removal of suspended matter and turbidity.


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When is Internal Treatment of Boiler Feed-water Necessary?

Chemical treatment of water inside the boiler is usually essential whether or not the water has been pretreated. Internal treatment, therefore, complements external treatment by taking care of any impurities entering the boiler with the feed water (hardness, oxygen, silica, etc.) regardless of whether the quantity is large or small. In many cases external treatment of the water supply is not necessary and the water can be treated by internal methods alone. Internal treatment can constitute the sole treatment when boilers operate at low or moderate pressure, when large amounts of condensed steam are used for feed water, or when the raw water available is of good quality.


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What Should a Good Internal Water Treatment Programme Accomplish?

The purpose of an internal treatment programme is fourfold: (1) react with any feed-water hardness and prevent it from precipitating on the boiler metal as scale, (2) condition any suspended matter such as hardness sludge or iron oxide in the boiler and make it non-adherent to the boiler metal, (3) provide anti-foam protection to permit a reasonable concentration of dissolved and suspended solids in the boiler water without foam carry-over, and (4) eliminate oxygen from the water and provide enough alkalinity to prevent boiler corrosion. In addition, as supplementary measures an internal treatment should prevent corrosion and scaling of the feed-water system and protect against corrosion in the steam condensate systems.


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What chemicals are Used in Internal Treatment?

The softening chemicals used include soda ash, caustic and various types of sodium phosphates. These chemicals react with calcium and magnesium compounds in the feed water. At times sodium silicate is used to contributed alkalinity as well as react selectively with magnesium hardness. The materials used for conditioning sludge include various organic materials of the tannin, lignin or alginate classes. It is important that these organics are so selected and processed that they are both effective and stand stable at the boiler operating pressure. Certain synthetic organic materials are used as anti-foam agents. The chemicals used to scavenge oxygen include sodium sulphite and hydrazine. Various combinations of polyphosphates and organics are used for preventing scale and corrosion in feed-water systems. Volatile neutralizing amines and filming inhibitors are used for preventing condensate corrosion.


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How are Carbonates Reacted on by Internal Treatment?

Calcium bicarbonate entering with the feed water is broken down at boiler temperatures or reacts with caustic soda to form calcium carbonate. Since calcium carbonate is relatively insoluble it tends to come out of solution. Sodium carbonate partially breaks down at high temperature to sodium hydroxide (caustic) and carbon dioxide. When phosphates are used in internal treatment they react with calcium carbonate to form calcium phosphate and sodium carbonate (soda ash). In the presence of sufficient hydroxides (caustic) alkalinity, magnesium bicarbonate will precipitate as magnesium hydroxide or will react with any silica present to form magnesium silicate. The minerals precipitated from solution (calcium carbonate, calcium phosphate, magnesium hydroxide, magnesium silicate, etc..) form sludge in the water which must be conditioned to prevent its sticking to the metal. The conditioned sludge is removed from the boiler by blow-down.


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How are Sulphates Reacted on by Internal Treatment?

High temperatures in the boiler water reduce the solubility of calcium Sulphate and tend to make it precipitate out directly on the boiler metal as scale. Consequently calcium Sulphate must be reacted upon chemically to cause a precipitate to form in the water where it can be conditioned and removed by blow-down. Calcium Sulphate is reacted on either by sodium carbonate, sodium phosphate or sodium silicate to form insoluble calcium carbonate, phosphate or silicate. Magnesium Sulphate is reacted upon by caustic soda to form a precipitate of magnesium hydroxide. some magnesium may react with silica to form magnesium silicate. Sodium Sulphate is highly soluble and remains in solution unless the water is evaporated almost to dryness.


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How is Silica Reacted upon by Internal Treatment?

In untreated waters silica tends to precipitate out directly as scale at hot spots on the boiler metal or it may combine with calcium to produce a hard calcium silicate scale. Treatment for silica involves keeping the boiler-water alkalinity high enough to hold silica in solution. Usually there is enough magnesium in the water to precipitate some of the silica as sludge. At times proper treatment with magnesium can tie up silica when it is a special problem. Some organic materials such as starches tend to prevent the adherence of silica to the boiler metal probably by a physical action.


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How is Sludge Conditioned in Internal Treatment?

There are two general approaches to conditioning sludge inside a boiler: by coagulation or dispersion. When the total amount of sludge is great (as the result of high feed-water hardness) it is practical to coagulate the sludge to form large flocculent particles. This flow readily with the boiler water and can be removed by blow-down. This can be accomplished by careful adjustment of the amounts of alkalis, phosphates and organics used for treatment, based on the fee-water analysis. When the amount of sludge is not great (low hardness feed-waters) it is more practical to use a higher percentage of phosphates in the treatment. Phosphates form finely divided sludge particles. A higher percentage of organic sludge dispersants is used in the treatment to keep the sludge particles dispersed throughout the boiler water.


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What Difficulties are Encountered in Internal Treatment?

The main difficulty is the presence of a large amount of sludge formed when feed-water hardness is high. This may increase the amount of blow-down required. When internal treatment is used alone (without pretreatment of the water by external means) there is more possibility for scale in the pre-boiler system and fee-water lines. it is important that someone experienced in the technology helps to set up an internal treatment programme which will minimize these difficulties.


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What are the Advantages of Internal Treatment?

The prime advantage is that in many instances internal treatment can eliminate the need for extensive external treatment equipment. This gives a definite economic advantage. In addition, the simplicity of an internal treatment programme offers a decided savings in manpower for feeding and control. A qualified consultant can help decide what water quality is required for a specific boiler system, and choose the most economical means of obtaining the required quality.


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How are Internal Treatment Chemicals Fed?

Common feeding methods include the use of chemical solution tanks and proportioning pumps or special ball briquette chemical feeders. In general, softening chemical (phosphates, soda ash, caustic, etc.) are added directly to the fee-water at a point near the entrance to the boiler drum. They may also be fed through a separate line discharging in the feed-water drum of the boiler. The chemicals should discharge in the fee-water section of the boiler so that reactions occur in the water before it enters the steam generating areas. Softening chemicals may be added continuously or intermittently depending on feed-water hardiness and other factors. Chemicals added to react with dissolved oxygen (Sulphate, hydrazine, etc.) preferably should be fed continuously as far back in the feed-water system as possible. Similarly, chemicals used to prevent scale and corrosion in the feed-water system (polyphosphates, organics, etc.) should be fed continuously. Chemicals used to prevent condensate system corrosion may be fed directly to the steam or into the feed-water system, depending on the specific chemical used. continuous feeding is preferred but intermittent application will suffice in some cases.


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How are Chemical Dosages Controlled?

Chemical dosages are based primarily on the amount of impurities in the feed-water. For example, the amount of softening chemicals needed depends on fee-water hardness; the amount of sodium Sulphate needed depends on the amount of dissolved oxygen in the feed-water. In addition, however, a set amount of extra chemical treatment is added to provide a residual is alkies of insurance and serves as the basis for treatment control.


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What Boiler Water Tests are Used for Treatment Control?

Routine control test of the boiler water vary according to the type of chemical treatment used but they may include tests for: alkalinity M phosphate, Sulphate and organic color. Boiler water hardness tests are not often made because it is generally assumed that if there is enough alkalinity and/or phosphate present in the boiler-water, the hardness has reacted completely. In testing for Sulphate it is assumed that if an adequate residual is present, the feed-water oxygen has been removed; this may not always be true, especially if the Sulphate feed is not continuous and if ordinary unanalyzed sodium Sulphate is used. Generally antifoams are incorporated in organic treatments so testing for organic color gives an indication both of sludge conditioner present as well as level of antifoam treatment.


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What Tests are Usually Made as a Check for Contaminants?

Here, again, the specific tests made vary with the type of contamination suspected. Some checks made fairly often, however, include test for: iron, oil and silica. Usually the iron test serves as a check on corrosion products brought back with the condensate but may also be used when appreciable iron is present in the make up water. Oil tests usually require laboratory facilities but visual inspection of samples can show up gross contamination. While silica is usually present to some extent in boiler waters, periodic checks are sometimes made to detect unusual contamination or to indicate when additional blow-down is needed to keep silica concentrations below a preset limit.


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What Units are Used in Expressing Water Analysis Results?

The most common unit is parts per million. One p.p.m. of a substance in a water sample represents one unit mass of the substance in each million unit mass of the water. For example, one p.p.m of salt (NaCl) means one kg of salt per million kg of water. There is still some diehard use of the classic unit grains per gallon (g.p.g.) but this expected to disappear due to universal S.I. usage as will the unit equivalents per million (e.p.m). This mention is therefore made merely as a matter of record.


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Why are some Analysis Results Express ‘As CaCO2E

Water treatment reactions are based on the combining mass of the reacting substances. For example, 106 kilograms of soda ash (molecular mass 106) reacts with 136 kilograms of calcium Sulphate (molecular weight 136). The molecular mass of calcium carbonate (CaCO3) is the round number 100. In order to simplify chemical dosage calculations all hardness and alkalinity results are usually based on the molecular mass of calcium carbonate and are expressed as ‘CaCO3E For example, using this system, one p.p.m of calcium Sulphate (expressed as CaCO3). This is the same as converting English pounds, German marks, or fresh francs into a 100 cent dollar.


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What is Blow-down?

Blow-down is the removal from the boiler of water containing concentrated dissolved and suspended solids. As the blow-down water is replaced with lower solids feed water the boiler water is essentially being diluted. By regulating the amount of blow-down, therefore, the amount of solids in the boiler-water can be controlled.


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How much Blow-down is Needed?

This depends on how many concentrations of the various feed-water impurities a given boiler can tolerate; the more concentrations possible the less blow-down needed. For example, with 10 feedwater concentrations in a boiler, blow-down equal to 10 per cent of the feed-water flow rate is needed; with 20 concentrations only 5 per cent blow-down is needed. To illustrate how blow-down requirements are calculated let us assume that the maximum amount of suspended solids (sludge) in the boiler water that a particular boiler can tolerate is 500 p.p.m. If the fee-water contains 50 p.p.m. of hardness it can be concentrated only about 10 times (since feed water hardness is precipitated as suspended solids in the boiler water). This means that for every 50 kg of water fed to the boiler about 5 kh of boiler water must be blown down to keep the suspended solids from exceeding 500 p.p.m. Suspended solids, however, may not be the limiting factor in all cases; other factors which may limit feed-water concentrations include dissolved solids, alkalinity, silica or iron.


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What Tests are Made in Regulating Blow-down?

Since there are no simple test for routinely checking the amount of suspended solids in boiler-water, blow-down is usually controlled through use of a simple instrument which measures the electrical conductivity of the water. This test gives an estimate of the dissolved solids present in the boiler-water. Chloride tests are also used for blow-down control since chlorides are not reacted on by chemical treatment. By checking both the fee-water and boiler-water chlorides the number of feed-water concentrations can be calculated. In some higher pressure boilers, silica or iron tests may also be made to control blow-down.


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What is the Difference between Continuous and ‘PuffEBlow-down?

All boilers have blow-down connections located at low points where sludge is likely to collect. Opening these blow-down valves periodically for shot intervals gives a ‘puffEor intermittent removal of sludge and solids. Many boilers also have blow-down connections consisting of an overtake located just below the water level in the steam release area. A small amount of water is continuously removed through these connections. The use of continuous blow-down in addition to ‘puffEor bottom blow-down keys it possible to maintain the solids and chemical residuals at more consistent levels in the boiler water. Continuous blow-down also minimizes the amount of blow-down required with resultant savings in heat and chemicals. Continuous blow-down also causes less upset in boiler water circulation and operation.


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What Causes Corrosion in Steam Condensate Systems?

Most condensate system corrosion is caused by carbon dioxide and oxygen, arrived into the system with the steam. Carbon dioxide, dissolved in the pure condensed steam, form corrosive carbonic acid. if oxygen is present with carbon dioxide, the corrosion rate is much higher, and is likely to produce localized pitting. Ammonia, in combination with carbon dioxide or oxygen, attacks copper alloys.


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How is Steam Condensate Corrosion Prevented?

The general approach may involve removing oxygen from the feed-water mechanically and chemically, and providing pretreatment of the make-up water to minimize potential carbon dioxide formation in the boiler. In addition, an effective chemical treatment programme is required. This may consist of using volatile amines to neutralize carbon dioxide and/or a volatile filming inhibitor to form a barrier between the metal and the corrosive condensate. Mechanical conditions such as poor trapping and draining of lines, and air in-leakage may need to be correct-ed.


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How do Chemical Oxygen Scavengers Help Control Condensate System Corrosion?

As previously mentioned mechanical equipment (de-aerator) is often used to reduce feed-water oxygen. The best designed and operated de-aerators can reduce oxygen to as low as 0.007 parts per million or less. Most de-aerators or feed-water heaters are less effective. Since very small amounts of oxygen, however, can cause boiler corrosion and corrosion in steam condensate system, chemical treatment is therefore, needed to assure complete oxygen removal. Sodium Sulphate is the chemical most commonly used for this purpose. Greatly improved oxygen removal is obtained, however, when the Sulphate is catalyzed. Catalyzed sodium Sulphate can reduce oxygen content of water (at room temperature) from the saturation point to zero in less than 30 seconds. Without a catalyst it takes up to 10 minutes under the same conditions to reduce the oxygen content by only about 30 per cent. Fast reactions are important since oxygen should be removed before the water enters the boiler. Otherwise some oxygen will escape form the boiling water into the steam lines and corrosion in the condensate system.


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What is the Basis for Choice between Neutralizing and Filming Inhibitors?

The proper choice of inhibitor depends on the boiler system, plant lay-out operating conditions and fee-water composition. In general, volatile amines are better with low make-up, low feed-water alkalinity, and good oxygen control. Filming inhibitors usually give more economical protection with high make-up, air in-leakage high feed-water alkalinity or where the system is operated internally. In some cases a combination of treatments is needed.


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What Characteristics Should a Good Condensate Corrosion Inhibitor Have?

A good volatile neutralizing amine should have a favorable distribution ratio in steam and condensate so that it protects the entire steam-condensate system. It should have no insoluble reaction products and should be stable at high temperatures and pressures. A good filming inhibitor should be easy to disperse in water so that it can be fed uniformly. It should be stale under usage conditions and form a thin protective film without causing deposits in either the boiler or the steam-condensate system.


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How are Deposits and Corrosion Prevented in Feed-water Systems?

Deposits in feed-water systems are most frequently caused by hardness coming out of solution as the water goes through feed- water heaters or as the feed lines enter the boiler. Deposits also can occur from premature reaction of treatment chemicals with hardness in the feed-water. Prevention involves the use of stabilizing chemicals fed continuously to retard hardens precipitation. Proper design of the chemical feed system can minimize premature chemical reactions. Corrosion of feed-water system generally results from low alkalinity or dissolved oxygen in the water. Raising the pH of the water and the continuous feed of catalyzed sodium Sulphate will minimize this problem.


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What is the Wet Method of Boiler Lay-Up?

This is a method of storing boilers full of water so that they can be readily returned to service. it involves adding extra chemicals (usually caustic, organics, and sodium sulphite to the boiler-water.) The water level is raised in the idle boiler to eliminate air spaces and the boiler is kept completely full of treated water. Special considerations are needed for protecting super heaters.


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What is the Dry Method of Boiler Lay-Up?

This method of lay-up is usually for longer boiler outages. It involves draining, cleaning and drying out the boiler. a material which absorbs moisture such as hydrated lime or silica gel is placed in trays inside the boiler. The boiler is then sealed carefully to prevent in leakage of air. Periodic inspection and replacement of the drying chemical is required during long storage periods.


DATA USED IN WATER CHEMISTRY

The chemicals listed in this section include those found as impurities in water and also those used as treatments. The chemical formulas, ion forms, and molecular and equivalent weights are given for each substance. Abbreviations and symbols are used extensively to simplify water analysis reports and calculations. This section explains the meanings of some common symbols and what they represent in water analyses. Very often the units used in water chemistry need to be converted back and forth for practical application. For example, parts per million may be converted to grams per 1000 liters and vice versa. The conversion factors in this section simplify this type of calculation.

 

  CATIONS

Ion
Formula

Ionic
Weight

Equivalent
Weight

   Aluminum

A1+++

27.0

9.0

   Ammonium

NH4+

18.0

18.0

   Calcium

CA++

40.1

20.0

   Hydrogen

H+

1.0

1.0

   Ferrous Iron

Fe++

55.8

27.9

   Magnesium

Mg++

24.3

12.2

   Manganese

Mn++

54.9

27.5

   Potassium

K+

39.1

39.1

   Sodium

Na+

23.0

23.0

   
   ANIONS
   
   Bicarbonate

NCO3-

61.0

61.0

   Chloride

CO3-

60.0

60.0

   Fluoride

F-

19.0

19.0

   Nitrate

NO3-

62.0

62.0

   Hydroxide

OH-

17.0

17.0

   Phosphate (

PO4--

95.0

31.7

   Phosphate (dibasic)

HPO4--

96.0

48.0

   Phosphate (monobasic)

H2PO4-

97.0

97.0

   Sulphate

SO4--

96.1

48.0

   Sulphite

SO3--

80.1

40.0

   COMPOUNDS

Formula

Molecular
Weight

Equivalent
Weight

   Aluminum hydroxide

Al(OH)3

78.0

26.0

   Aluminum Sulphate

Al2(SO4)3

342.0

57.0

   Alumina

Al2O3

102.0

17.0

   Calcium bicarbonate

Ca(HCO3)2

162.1

81.1

   Calcium carbonate

CaCO3

100.1

50.1

   Calcium chloride

CaCl2

111.0

55.5

   Calcium hydroxide (pure)

Ca(OH)2

74.1

37.1

   Calcium hydroxide (90%)

Ca(OH)2

--

41.1

   Calcium Sulphate (anhydrous)

CaSO4

136.2

68.1

   Calcium Sulphate (gypsum)

CaSO4.2H2O

172.2

86.1

   Calcium phosphate

Ca3(PO4)2

310.3

51.7

   Disodium phosphate

Na2HPO4.12H2O

358.2

119.4

   Disodium phosphate
   (anhydrous)

NaHPO4

142.0

47.3

   Ferric oxide

Fe2O3

159.6

26.6

   Iron oxide (magnetic)

Fe3O4

321.4

-

   Ferrous Sulphate (copperas)

FeSO4.7H2O

278.0

139.0

   Magnesium oxide

MgO

40.3

20.2

   Magnesium bicarbonate

Mg(HCO3)2

146.3

73.2

   Magnesium carbonate

MgCO3

84.3

42.2

   Magnesium chloride

MgCl2

95.2

47.6

   Magnesium

Mg(OH)2

58.3

29.2

   Magnesium phosphate

Mg3(PO4)2

263.0

43.8

   Magnesium Sulphate

MgSO4

120.4

60.2

   Monosodium phosphate

NaH2PO4.H2O

138.1

46.0

   Monosodium phosphate
   (anhydrous)

NaH2PO4

120.1

40.0

   Metaphosphate

NaPO3

102.0

34.0

   Sodium acuminate

Na2Al2O4

164.0

27.3

   Sodium bicarbonate

NaHCO3

84.0

84.0

   Sodium carbonate

Na2CO3

106.0

53.0

   Sodium chloride

NaCl

58.5

58.5

   Sodium hydroxide

NaOH

40.0

40.0

   Sodium nitrate

NaNO3

85.0

85.0

   Sodium Sulphate

Na2SO4

142.0

71.0

   Sodium sulphite

Na2SO3

126.1

63.0

   Trisodium phosphate

Na3PO4.l2H2O

380.2

126.7

   Trisodium phosphate
   (anhydrous)

Na3PO4

164.0

54.7

   
   GASES
   
   Ammonia

NH3

17

--

   Carbon Dioxide

CO2

44

--

   Hydrogen

H2

2

--

   Oxygen

O2

32

--

   
   ACIDS
   
   Carbonic

H2CO3

62.0

31.0

   Hydrochloric

HCl

36.5

36.5

   Phosphoric

H3PO4

98.0

32.7

   Sulphuric

H2SO4

98.1

49.1

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