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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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