Characteristics of boiler feed water Water absorbs more heat for a given temperature rise than any other common inorganic substance. It expands 1600 times as it evaporates to form steam at atmospheric pressure. The steam is capable of carrying large quantities of h eat. These unique properties of water make mak e 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 va ries from 30 g/l in sea water to anything an ything from 0.005 to 1500 mg/l in fresh water supplies. Since water impurities cause boiler problems, careful consideration must be given to the quality qu ality of the water used for generating steam. The composition of boiler feed water must be su ch that the impurities in it can be concentrated a reasonable number of times inside the boiler, without ex ceeding 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. Feed-water purity purity is a matter both of quantity of impurities and nature of impurities: impurities: some impurities such as hardness, iron and silica are of more con cern, 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. Only relatively wide ranges can be given as to maximum levels of alkalis, salt, silica, phosphates etc, in relation to working pressure. The actu al maximum levels must be obtained fro the boiler manufacturer, who will base them on the characteristics of the boiler in question. The following tables are extracts of recommended levels from APAVE (Association of electrical and steam unit owners), up to pressures of 100 bar for medium steaming rates and for volumes of water in the chambers sufficient to properly control the b low down rates, and from ABMA (American Boiler Manufacturers Association) in its standard guarantee of steam purity. Working Pressure (Bar) 20.8 - 31.1 - 41.5 - 51.8 0 - 20.7 31.0 41.4 51.7 62.1 Feed water Dissolved oxygen (measured before oxygen scavenger addition) Total Iron Total copper Total hardness
mg/l
62.2 68.9
69.0 103.4
103.5 137.9
0.007
0.04
0.04
0.007
0.007
0.007
0.007 0 .007
0.007
0.1 0.05 0.3
0.05 0.025 0.3
0.03 0.02 0.2
0.025 0.02 0.2
0.02 0.015 0.1
0.02 0.015 0.05
0.01 0.01 0.01 0.01 not detectable
(CaCO3) Non volatile TOC Oily matter pH at 25 Boiler Water Silica Total alkalinity CaCO3 mg/l Free hydroxide alkalinity CaCO3 Specific conductance at mS/cm 25 without neutralization
1
1
0.5
0.5
0.5
0.2
0.2
0.2
1 7.5 10.0
1 7.5 10.0
0.5 7.5 10.0
0.5 7.5 10.0
0.5 7.5 10.0
0.2
0.2
0.2
150
90
40
30
20
8
350
300
250
200
150
100
not specified
3500
3000
2500
2000
mg/l
2
1
not specified not detectable
1500
Working Pressure (Bar) 0 - 15 15 - 25 25 - 35 35 - 45 Feed water Dissolved oxygen (measured before oxygen scavenger addition)
8.5 - 9.5 9.0 - 9.6 9.0 - 9.6
1000
40 - 60
150
60 - 75
100
75 - 100
0.02 (Physical removal of dissolved oxygen)
French 0.5 0.3 0.2 0.1 0.05 0.05 0.05 Total hardness degrees Oily matter mg/l absence 0.05 0.05 0.05 pH > 8.5 Total Iron not specified 0.05 0.05 0.03 mg/l Total copper not specified 0.03 0.03 0.01 Boiler water M alkalinity 100 80 60 40 15 10 5 French degrees 0.07 M 0.07 M 0.07 M 0.07 M > 0.5 M > 0.5 M > 0.5 M P alkalinity SiO2 200 150 90 40 15 10 5 TDS mg/l 4000 3000 2000 1500 500 300 100 Phosphates 30 to 100 31 to 100 20 to 80 21 to 80 10 to 60 10 to 40 5 to 20 pH 10.5 to 12 10 to 11 Softened or softened and carbonate Demineralized Make up water free
Scaling in boilers 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. This interferes with heat transfers and may cause hot spots. Leading to local overheating. Scaling mechanism is the exceeding of the solubility limits of mineral substances due to e levated temperature and solids concentration at the tube/water interface. The d eposition of crystalline precipitates on the walls of the boiler interferes with heat transfer and may cause hot spots, leading to local overheating. The less heat they conduct, the more dangerous they are. Common feed water contaminants that can form boiler deposits include calcium, magnesium, iron, aluminum, and silica. Scale is formed by salts that have limited solubility but are not totally insoluble in boiler water. These salts reach the d eposit site in a soluble form and precipitate. The values corresponding to their thermal conduc tivity are: 2
Steel 15 kcal/m .h per degree C 2 CaSO4 1-2 kcal/m .h per degree C 2 CaCO3 0.5-1 kcal/m .h per degree C 2 SiO2 0.2-0.5 kcal/m .h per degree C Scaling is mainly due to the presence of calcium and magnesium salts (carbonates or sulphates), which are less soluble hot than cold, or to the presence of too high concentration of silica in relation to the alkalinity of the water in the boiler. 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. Dropping it in a solution of acid can easily identify a carbonate deposit. 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 do es 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. If unchecked, scaling causes progressive lowering of the boiler efficiency by heat retardation, acting as an insulator. Eventually, scale built-up w ill cause the tube to overheat and rupture. 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. The first anti-scaling preventative measure is to supply good quality demineralised water as make – up feed water. The purer the feed water is, the weaker the driving mechanism to form scale. Scale-forming minerals that do enter the boiler can be rendered harmless by internal chemical treatment. A long-established technique is to detach the hardness cations, magnesium and calcium, from the scale forming minerals and to replace them with sodium ions.
Images Source: http://www.aalborgindustries.com/ifs/files/AI/eng/Presentation/Website/Downloadablefiles/pdf/Aalborg_Solutions_d ownload/aal_sol_6_mar04.pdf Presence of Silica Silica can vaporize into the steam at operating pressures as low as 28 bars. Its solubility in steam increases with increased temperature; therefore, silica becomes more soluble as steam is superheated. The conditions under which vaporous silica carryover occurs have been thoroughly investigated and documented. Researchers have found that for any given set of boiler conditions using demineralized or evaporated quality make -up water, silica is distribute between the boiler water and the steam in a definite ratio. This ratio depends on two factors: boiler pressure and boiler water pH. The value of the ratio increases almost logarithmically with increasing pressure and decreases with increasing pH. If the silica enters the boiler water, the usual co rrective action is to increase boiler blowdown, to
decrease it to acceptable levels and then to correct the condition that caused the silica contamination. Read more: http://www.lenntech.com/applications/process/boiler/scaling.htm#ixzz2JFHpuU6b
Foaming and priming in boilers Boiler water carry-over is the contamination of the steam with bo iler-water solids. Bubbles or froth actually build up on the surface of the boiler water and pass out with the steam. This is called foaming and it is caused by high concentration of any solids in the boiler water. It is generally believed, however, that specific substances such as alkalis, oils, fats, greases, certain types of organic matter and suspended solids are p articularly conducive to foaming. In theory suspended solids collect in the surface film surrounding a steam bubb le and make it tougher. The steam bubble therefore resists breaking and builds up foam. It is believed that the finer the suspended particles the greater their collection in the bubble. Priming is the carryover of varying amounts of droplets of water in the steam (foam and mist), which lowers the energy efficiency of the steam and leads to the deposit of salt crystals on the super heaters and in the turbines. 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. Some mechanical entertainment of minute drops of boiler water in the steam always occurs. When this boiler water carryover is excessive, steam-carried solids produce turbine blade deposits. The accumulations have a composition similar to that of the dissolved solids in the boiler water. Priming is common cause of high levels of boiler water carryover. These conditions often lead to super heater tube failures as well. Priming is related to the viscosity of the water and its tendency to foam. These properties are governed by alkalinity, the p resence of certain organic substances and by total salinity or TDS. The degree of priming also depends on the design of the boiler and its steaming rate.
The most common measure to prevent foaming and priming 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-foaming and anti-priming agents, mixtures of surface-active agents that modify the surface tension of a liquid, remove foam and prevent the carry-over of fine water particles in the stream, can be very effective in
preventing carry-over due to high concentrations of impurities in the boiler-water. Read more: http://www.lenntech.com/applications/process/boiler/foaming priming.htm#ixzz2JFIIfOIt
Corrosion in boilers Corrosion is the reversion of a metal to its ore 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. Corrosion is a relevant problem caused by water in boilers. Corrosion can be of widely varying origin and nature due to the action of dissolved oxygen, to corrosion currents set up as a result of heterogeneities on metal surfaces, or to the iron being directly attacked by the water. 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 ma y have an important influence and produce different forms of attack. It is necessary to consider the quantity of the various harmful substances that can be allowed in the boiler water without risk of damage to the boiler. Corrosion may occur in the feed-water system as a result of low pH water and the presence of dissolved oxygen and carbon dioxide. Starting form these figures, and allowing the amount that can be blown down, the permitted concentration in the make-up water is thus defined. Corrosion is caused principally by complex oxide-slag with low melting points. High temperature corrosion can proceed only if the corroding deposit is in the liquid phase and the liquid is in direct contact with the metal. Deposits also promote the transport of oxygen to the metal surface. 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 steam and condensate system corrosion is generally the result of contamination with carbon dioxide and oxygen. Specific contaminants such as ammonia or sulphur bearing gases may increase attack on copper alloys in the system. Corrosion is caused by the combination of oxide layer
fluxing and continuous oxidation by transported oxygen. Cracking in boiler metal may occur b y two different mechanisms. In the first mechanism, cyclic stresses are created by rapid heating and cooling and 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 probabl y a misnomer. These cracks often originate where a dense protective oxide film covers the metal surfaces 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. Corrosion control techniques vary according to the type of corrosion encountered. Major methods include maintenance of the proper pH, control of oxygen, control of deposits, and reduction of stresses trough design and operational practices. Deaeration and recently the use of membrane contractors are the best and most diffused ways to avoid corrosion removing the dissolved gasses (mainly O2 and CO2). For further information about the different types of corrosion check the following web pages:
Galvanic corrosion Caustic corrosion Acidic corrosion Hydrogen embrittlement Oxygen attack Carbon dioxide attack
Protection of steel in a boiler system depends on temperature, pH, and oxygen content. Generally, higher temperatures, high or low pH levels and higher oxygen concentrations increase steel corrosion rates. Mechanical and operation factors such as velocities, metal stresses, and severity of service can strongly influence corrosion rates. Systems vary in corrosion tendencies and should be evaluated individually. Read more: http://www.lenntech.com/applications/process/boiler/corrosion.htm#ixzz2JFIkFCV1
Deaeration in boilers In order to meet industrial standards for both oxygen content and the allowable metal oxide levels in feed water, nearly complete oxygen removal is required. This can be accomplished only by efficient mechanical deaeration supplemented by a properly controlled oxygen scavenger. Deaeration is driven by the following principles: the solubility of an y gas in a liquid is directly proportional to the partial pressure of the gas at the liquid surface, decreases with increasing
liquid temperature; efficiency of removal is increased when the liquid and gas are thoroughly mixed. Deaeration can be performed using a physical medium such as deaerating heaters or vacuum deaerators or a chemical medium such as oxygen scavengers (polishing treatment) or catalytic resins. Membrane contractors are increasingly being used. Carbon dioxide is often removed using a physical medium. The purpose of a deaerator is to reduce dissolved gases, particularly oxygen, to a low level and improve plant thermal efficiency by raising the water temperature. In addition, they provide feed water storage and proper suction conditions for boiler feed water pumps. Pressure deaerators can be classified under two major cate gories: tray type and spray type. The tray type desecrating heaters consist of a shell, spray nozzles to distribute and spray the water, a direct contact vent condenser, tray stacks and protective interchamber walls. The chamber is constructed in low carbon steel, but more corrosionresistant stainless steels are used for the spray nozzles and the other parts. Incoming water is sprayed into steam atmosphere, where it is heated up to a few degrees to the saturation temperature of the steam. Most of the non-condensable gases (principally oxygen and free carbon dioxide) are released to the steam as the water is sprayed into the unit. Seals prevent the recontamination of tray stack water by gases from the spray section. Water falls from tray to tray, breaking into fine droplets of film, which intimately contact the incoming steam. The steam heats the water to the steam saturation temperature and removes the very last traces of oxygen. Deaerated water falls to the storage space below, where a steam blanket protects it from recontamination. It is usually stored in a separate tank. The steam enters the deaerators through ports in the tray compartment, flows down through the tray stack parallel to the water flow. A ver y small amount of steam condenses in this section as the water temperature rises to the saturation temperature of the steam. The rest of the steam scrubs the cascading water. Before leaving the tray compartment, the steam flows upward between the shell and the interchamber walls to the spray section. Most of the steam is
condensed and becomes part of the deaerated water. A small portion of the steam, which contains the non-condensable gas released from the water, is vented to the atmosphere. It is essential that sufficient venting is provided at all times or deaeration will be incomplete. Steam flow through the tray stack may be cross-flow, counter-current, or co-current to the water.
The spray type deaerating heaters consist of a shell, spring-loaded inlet spray valves, a direct contact vent condenser section and a steam scrubber for final dearetion; the shell and steam may be low carbon steel, the spray valves and the direct contact vent condenser section are in stainless steel. The incoming water is sprayed into a steam atmosphere and heated up to a few degrees to the saturation temperature of the steam. Most of the non-condensable gases are released to the steam, and the heated water falls to water seals and drains to the lowest section of the steam scrubber. The water is scrubbed b y a large volume of steam and heated to the saturation temperature prevailing at that point. As the water-steam mixture rises in the scrubber, the deaerated water is a few degrees above the saturation temperature, due to a slight pressure loss. In this way a small amount of flashing is produced, which aids in the release of dissolved
gases. The deaerated water overflows from the steam scrubber to the storage section below. Steam enters the deaerator through a chest on the side and flows to the steam scrubber. After flowing into the scrubber it passes up into the spray heater section to heat the incoming water. Most of the steam condenses in the spray section to become a part of the deaerated water. A small portion of the gases is vented to the atmosphere to remove the non-condensable gases. Vacuum deaeration is used at temperatures below the atmospheric boiling point to reduce the corrosion rate in water distribution systems. A vacuum is applied to the system to bring the water to its saturation temperature. Spray nozzles break the water into small particles to facilitate gas removal and vent the exhaust gases. Incoming water enters through spray nozz les and falls through a columns packed with Raschig rings to other synthetic packing. In this way, water is reduced to thin films and droplets, which promote the release of dissolved gases. The released gases and water vapor are removed through the vacuum, which is maintained by steam jet eductors or vacuum pumps, depending on the size of the system. Vacuum deaerators remove oxygen less efficiently that pressure units.
Corrosion fatigue at or near welds is a major problem in deaerators. It is the result of mechanical factors, such as manufacturing procedures, poor welds and lack of stress-relieved welds. Operational problems such as water/steam hammer can also be a factor. Read more: http://www.lenntech.com/applications/process/boiler/deaeration.htm#ixzz2JFJ6ZvKQ
Boiler water treatment The treatment and conditioning of boiler feed water must satisfy three main objectives:
Continuous heat exchange Corrosion protection Production of high quality steam
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 man y types of external treatment (softening, evaporation, deaeration, membrane contractors 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. External treatment The water treatment facilities purify and deaerate make-up water or feed water. 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. Evaporators are suitable where steam as a source of heat is readily available. They have particular advantages over demineralization, for example, when the dissolved solids in the raw water are very high. Certain natural and synthetic materials have the ab ility to remove mineral ions from water in exchange for others. For example, in passing water through a simple cation ex change softener all of calcium and magnesium ions are removed and replaced with sodium ions. 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 w ater 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 and com plements external treatment by taking care of any impurities entering the boiler with the feed water (hardness, oxygen, silica, etc.). In many cases external treatment of the water supply is not necessary and the water can be treated only by internal methods. Internal treatment Internal treatment can constitute the unique treatment when b oilers operate at low or moderate pressure, when large amounts of condensed steam are used for feed water, or when good quality raw water is available. The purpose of an internal treatment is to 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 allow a reasonab le concentration of dissolved and suspended solids in the boiler water without foam carry-over;
4) eliminate oxygen from the water and provide enough alkalinity to prevent boiler corrosion. In addition, as supplementary measures an internal treatment should prev ent corrosion and scaling of the feed-water system and protect against co rrosion in the steam condensate systems. During the conditioning process, which is an essential complement to the water treatment program, specific doses of conditioning products are added to the water. The commonly used products include:
Phosphates-dispersants, polyphosphates-dispersants (softening chemicals) : reacting with the alkalinity of boiler water, these products neutralize the hardness of water by forming tricalcium phosphate, and insoluble compound that can be disposed and blow down on a continuous basis or periodically through the bottom of the b oiler. Natural and synthetic dispersants (Anti-scaling agents ): increase the dispersive properties of the conditioning products. They can be: Natural polymers: lignosulphonates, tannins o Synthetic polymers: polyacrilates, maleic acrylate copolymer, maleic styrene o copolymer, polystyrene sulphonates etc. Sequestering agents : such as inorganic phosphates, which act as inhibitors and implement a threshold effect. Oxygen scavengers : sodium sulphite, tannis, hydrazine, hydroquinone/progallol-based derivatives, hydroxylamine derivatives, hydroxylamine derivatives, ascorbic acid derivatives, etc. These scavengers, catalyzed or not, reduce the oxides and dissolved oxygen. Most also passivate metal surfaces. The choice o f product and the dose required will depend on whether a deaerating heater is used. Anti-foaming or anti-priming agents : mixture of surface-active agents that modify the surface tension of a liquid, remove foam and prevent the carry over of fine water particles in the steam.
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. Sodium silicate is used to react selectively with magnesium hardness. 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 relativel y insoluble it tends to come out of solution. Sodium carbonate partially breaks down at high temperature to sodium hydroxide (caustic) and
carbon dioxide. 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 b y 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. There are two general approaches to conditioning sludge inside a boiler: by coagulation or dispersion. When the total amount of sludge is high (as the result of high feed-water hardness) it is better to coagulate the sludge to form large flocculent particles. This can be removed by blowdown. The coagulation can be obtained 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 high (low feed water hardness) it is preferable to use a higher percentage of phosphates in the treatment. Phosphates form separated 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 materials used for conditioning sludge include various organic materials of the tannin, lignin or alginate classes. It is important that these organics are selected and processed, so 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 pol yphosphates 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. Common internal chemical feeding methods include the use of chemical solution tanks and proportioning pumps or special ball briquette chemical feeders. In general, softening chemicals (phosphates, soda ash, caustic, etc.) are add ed 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 feedwater 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 area. Softening chemicals may be added continuously or intermittently depending on feed-water hardiness and other factors. Chemicals added to react with dissolved ox ygen (sulphate, hydrazine, etc.) and chemicals used to prevent scale and corrosion in the feed-water system (polyphosphates, organics, etc.) should be fed in the feed-water system as continuously as possible. Chemicals used to prevent condensate system corrosion may be fed directly to the stea m or into the feedwater system, depending on the specific chemical used. Continuous feeding is preferred but intermittent application will suffice in some cases. Check also our web page about the production of high pure water through Electrodeionization (EDI). Read more: http://www.lenntech.com/applications/process/boiler/boiler-watertreatment.htm#ixzz2JFJY9pVn
