Boiler Feedwater Treatment (Part I): Why Water Treatment is Necessary?

Contents Water for boilers Impurities in water feedwater Boiler feedwater Purity requirements of Boiler deposits Corrosion Boiler water carryover

Water for boilers

All natural waters contain varying amounts of suspended and dissolved matter as well as dissolved gases. The type and amount of impurities in fresh water vary and with the area of location. Impurities in (well with the source (lake, river water become an important consideration when water is to be used for steam generation. With the trend toward higher-pressure boilers, pretreatment has Feedwater .become the key to successful operation of industrial power plants must be pretreated to remove impurities to control deposition, carryover, and corrosion in the boiler system. Poor quality water gives poor quality steam. The first step in any treatment is filtration of suspended solids. On the basis of proven satisfactory performance, cost, and other considerations, cartridge filters are a .practical solution to most problems of water clean-up

Physical properties of water

colorless liquid. Because water can be odourless Pure water is a tasteless converted to steam at a convenient temperature, it is an ideal medium for
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.generating power or conducting heat

Chemical composition of water

Pure water, H2O, is a simple combination of hydrogen and oxygen. There are, however, several hybrids forms of water in all supplies. Water often contains It has no use as .water about 300 ppm of deuterium oxide, D2O, or heavy drinking water or in making plants grow, but in pure form has found use in nuclear reactors. For all practical purposes only ordinary water, H2O, is .considered for use in boilers

Boiling temperature
The boiling point of water is dependent on pressure. At sea level atmospheric pressure, water boils at about 212oF. With increasing pressure, the boiling point also increases. At a pressure of 200 psig, for example, water boils at a temperature of about 388oF. At the critical pressure of 3200 psig (where water is converted to steam without change in volume), the boiling point is 704oF. As the pressure decreases, the boiling point of water decreases. Under vacuum water .will boil at temperatures as low as 35oF

ideal medium for carrying heat energy an Water

to raise temperature of one pound of (Thermal Unit British) BTu It takes one water 1oF. It takes an additional 970 Btu to change one pound of water, at boiling point, to steam. This heat energy is stored in the steam and when it condenses, the energy is given off. Thus much of the heat from burning fuel can be absorbed .by boiler water, transported with the steam, and released at the points of use

Impurities in water
All natural waters contain various types and amounts of impurities. These impurities cause boiler problems and as such consideration must be given to the quality and treatment required of the water used for generating steam. For any type of treatment, sediment filtration (usually with cartridge filters) is the first .step

Natural water
Natural waters contain suspended matter, dissolved solids, and dissolved gases. Water being a universal solvent dissolves minerals, rocks and soil that come into contact with it. It dissolves gases from air and gases that are given off from .organics in the soil. It picks up suspended matter from the earth Additionally it may also be contaminated with industrial wastes and process .materials

Dissolved minerals
Dissolved minerals picked up by the water consist mainly of calcium carbonate (limestone), calcium sulfate (gypsum), magnesium carbonate (dolomite), salts), silica (sand), sodium chloride (common salt), epsom) magnesium sulfate salt), and smaller quantities of iron, Glauber) hydrated sodium sulfate manganese, fluorides, aluminum, and other substances. The nitrates and .phosphates found in water are usually due to sewage contamination

Water hardness
Water containing high amounts of calcium and magnesium minerals is hard to ppm The amount of hardness in natural water can vary from a few .water 500 ppm. Calcium and magnesium compounds are relatively insoluble in water and tend to precipitate out. This causes scale and deposit problems. Such water .must be treated to make it suitable for steam generation

Dissolved gases in water

Water contains varying amounts of dissolved air (21% oxygen, 78% nitrogen, 1% other gases including carbon dioxide). Water can contain up to 9 ppm oxygen at room temperature and atmospheric pressure. As the temperature increases, the solubility of oxygen decreases, but water under pressure can hold higher amounts of dissolved oxygen. Nitrogen, being inert, has little effect on water used in boilers. Water can contain 10 ppm of carbon dioxide, sometimes much more than that due to decaying vegetation and organics in soil. Hydrogen sulfide and methane may be dissolved in water but this is rare. These gases can be .troublesome when they are present in the feed water

Other impurities in water

precipitated colour Natural waters contain varying levels of soil, sand, turbidity minerals, oil, industrial wastes and other suspended solid particles. Turbidity is due to very fine organic materials and microorganisms, as well as suspended clay and .is due to the decaying vegetable matter Colour .silt

Sources of fresh water

Fresh water can be surface water from rivers, streams, reservoirs or ground water from wells. Generally ground water supplies are more consistent in composition by rainfall, soil effected than surface water supplies. Surface water quality is erosion and industrial wastes, but ground water is usually harder than surface water. The composition of fresh water also varies with the location and type and strata of the earth formations. In limestone areas, for example, water contains large quantities of dissolved calcium. Apart from the geographic variations, the local conditions of a particular area may have a great influence in the composition .of the water

feedwater Boiler
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is the water supplied to the boiler. Often, steam is feedwaterBoiler The water needed .feedwater condensed and returned to the boiler as part of the Make- .water to supplement the returned condensate is termed make-up composition Feedwater .up water is usually filtered and treated before use therefore depends on the quality of the make-up water and the amount of condensate returned. Sometimes people think that there is a great deal of similarity between the requirements for potable (drinking) water and the The minerals in drinking water are considered .feedwater requirements for boiler desirable and are absorbed by the body. On the other hand, minerals in water cannot be handled as well by boilers. Although a boiler is a big mass of steel, it is more sensitive to water impurities than the human stomach. For this reason, a lot .of care is needed in filtration and treatment of the boiler water supply

feedwater Purity requirements of

is a matter both of quantity of impurities and the nature of impurities. Feedwater Hardness, iron, and silica, for example, are of more concern than sodium salts. The is used as well feedwater depend on how much feedwater purity requirements of as toleration of the particular boiler design (pressure, heat transfer rate etc.). In The .s high-pressure boilers practically all impurities must be removedtoday make-up water) from outside needs to be treated for the reduction or ) feedwater removal of impurities by first filtration, and then followed by softening, Internal treatment is also required for .ion exchange etc deariation evaporation the conditioning of impurities within the boiler system, to control corrosion, as .reactions occur in the boiler itself and the steam pipelines

Boiler deposits
Boiler scale .Water evaporating in the boiler causes impurities to concentrate results from suspended matter settling out on the metal or dissolved impurities .precipitating out on heat transfer surfaces and becoming hard and adherent

Impurities that form deposits

Bicarbonates of calcium and magnesium dissolved in water break down under heat and give off carbon dioxide forming insoluble carbonates. These carbonates precipitate directly on the boiler metal and or form sludge in the water that deposits on boiler surfaces. Sulfate and silica generally precipitate directly on the boiler metal and are much harder to condition. Silica (sand) if present in water can feedwater form exceedingly hard scale. Suspended or dissolved iron coming in the will also deposit on the boiler metal. Oil and other process contaminants can form deposits as well and promote deposition of other impurities. Sodium compounds usually do not deposit unless the water is almost completely evaporated to dryness, as may occur in a starved tube. Deposits are seldom composed of one constituent alone, but are generally a mixture of various types of solid sediments, .dissolved minerals, corrosion products like rust, and other water contaminants

Characteristics of phosphate deposit

Phosphate deposits are usually soft brown or gray deposits that can be easily removed by normal cleaning methods. They are normally found in boilers preferredemploying a phosphate internal treatment. They are the reaction product when using a residual phosphate treatment on high hardness feed water. Since they are easily conditioned with organic sludge conditioners, Calcium phosphate is usually the predominant .nonadherent they are relatively .compound in the boiler deposit

Characteristics of carbonate deposit

Carbonate deposits are usually granular and sometimes porous. The crystals are relatively large and often matted together with finely divided particles of other materials making the scale look dense and uniform. Carbonate deposit can be easily checked by putting it in an acid solution. Bubbles of carbon dioxide will .effervesce from the scale
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Characteristics of sulfate deposit

Sulfate deposit is brittle, does not pulverize easily, and will not effervesce when put in an acid solution. It is much harder and denser than a carbonate deposit due to .its smaller crystal structure

Characteristics of silica deposit

Silica deposits are very hard and resemble porcelain. Their crystals are very small, forming a dense, impervious scale. This scale is extremely brittle, very difficult to .pulverize, and not soluble in hydrochloric acid

Characteristics of iron deposit

and are due to corrosion products or iron coloured Iron deposits are very dark Iron deposits are usually magnetic in nature. They are .feedwater contamination in .soluble in hot acid, giving a dark-brown solution

Problems caused by deposits

The major problem that deposits cause is tube failure from overheating. This is due to the fact that the deposits act as an insulator and excessive deposits prevent efficient heat transfer through the tubes to the water. This causes the metal to become overheated and over time the metal fails. These deposits can also cause plugging or partial obstruction of boiler tubes, leading to starvation and subsequent overheating of the tubes. Underneath the deposit layer corrosion may also occur. Deposits cause unscheduled outages, increased cleaning time and expenses. Boiler deposits reduce overall operating efficiency resulting in higher .fuel consumption

Corrosion
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Corrosion is basically the reversion of a metal to its ore form. Iron, for example, reverts to iron oxide as a result of corrosion. The process of corrosion is actually it is a complex electro-mechanical reaction. Corrosion may generally simple not so be over a large metal surface but sometimes it results in pinpoint penetration of metal. Though basic corrosion is usually due to reaction of the metal with oxygen, other factors including stresses produce different forms of attack. Corrosion may system as a result of low pH water and the presence of feedwater occur in the dissolved oxygen and carbon dioxide. Corrosion in the boiler itself normally occurs when boiler water alkalinity is too low or too high or when the metal is exposed to oxygen-bearing water during either operation or idle periods. High temperatures .and stresses tend to accelerate the corrosion condensate system and pipelines corrosion is generally the result & In the steam .of contamination with carbon dioxide and oxygen

Corrosion Fatigue
Cracking in boiler metal may occur due to cyclic stresses created by rapid heating and cooling. These stresses are concentrated at points where corrosion has roughened or pitted the metal surface. This is usually because of improper corrosion prevention. Sometimes even with properly treated water corrosion fatigue cracking occurs. 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 often thick, blunt, and across the metal grains. They start at internal tube surfaces and are most often circumferential on .the tube

embrittlement Caustic
or cracking is a more serious type of boiler metal failure embrittlement Caustic cracks. This type of cracking occurs when intergranular showing up as continuous the metal is stressed, water contains caustic with a trace of silica, and some mechanism, such as a slight leak, is present allowing the boiler water to is more of a problem in embrittlement concentrate on the stressed metal. Caustic
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older boilers with riveted drums as they cause stresses and crevices in the areas of rivets and seams. In the newer welded drum boilers this type of cracking is less frequent but the rolled tube ends are still vulnerable to attack. The possibility of .caustic cracking should be a consideration in water treatment

Other causes of boiler corrosion

Chelate residuals in excess of 20 ppm as CaCO3 or improperly applied chelate treatment may produce boiler system corrosion. Concentration of boiler solids at high heat input areas might also produce corrosion. The recommendations of a water treatment consultant need to be followed to minimize chances of such .corrosion from occurring

Corrosion problems
Uniform corrosion of boiler metal surfaces is bound to occur and is not of much concern as all boilers experience a small amount of general corrosion. Corrosion, however, takes many forms and deep pitting that causes only a small amount of total iron loss causes penetration and leakage in boiler tubes. Corrosion beneath certain types of boiler deposits can weaken the metal and cause tube failure. Likewise corrosion in steam condensate system can damage pipelines and .equipment

Corrosion measurement
Hydrogen gas sampling of the boiler steam is done to measure the corrosion potential of the boiler water. This test for corrosion is based on the release of hydrogen gas when iron corrodes. Measuring the amount of hydrogen gas released detects boiler water conditions and indicates if corrosion conditions exist in an .operating boiler

Basic corrosion prevention methods

:The common methods for prevention of corrosion include

particles from water & Filtration of solid suspended impurities- feedwater Removing dissolved oxygen from the boiler- Maintaining alkaline conditions in the boiler water- Keeping the boiler internal surfaces clean- -Protecting boilers during out of service periods to counteract corrosive gases in steam programme Using a chemical treatment- and condensate systems The selection and control of chemicals for preventing corrosion requires an .understanding of the causes and corrective measures

Boiler water carryover

Boiler water carryover is the contamination of steam with boiler water solids. :Common causes of boiler water carryover are Bubbles or foam form on the surface of the boiler water and leave with the - steam. This is due to high concentrations of insoluble or soluble solids in the boiler greases, oils, fats, organic matter and suspended alkalies water. Substances like .solids are known to cause foaming Fine droplets of water in the form of spray or mist are thrown up into the steam - .space by the bursting of rapidly rising bubbles at the steam- release surface Priming is a sudden surge of boiler water caused by a rapid change in load. It may - be caused by damaged steam-separating equipment, operation above the boiler rating, sudden fluctuations in steam demand, or carrying too high of a water level .in the steam- release area Steam contamination may also occur from leakage of water through improperly - .designed or installed steam-separating equipment in the boiler drum

Boiler carryover measurement

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Steam purity can be measured with the use of a sodium ion analyzer. It measures the sodium ion content in a cooled steam sample that will correspond to the amount of boiler water solids contaminating the steam. The sodium ion analyzer .can detect carryover down to 1 ppb sodium in steam

of oil on carryover Affect

is usually from pumps and other lubricated feedwater Oil contamination in boiler equipment. Oil can cause serious foaming due to saponification of oil by boiler .alkalies water

of suspended solids on carryover Affect

Suspended solids tend to collect on the surface film surrounding a steam bubble, which therefore resists breaking and builds up foam. The finer suspended particles become the greater is their collection on the bubble. The type as well as the quantity of suspended solids can affect carryover. Depending on the type of suspended solids, some boilers having high suspended solids operate without .carryover, while others have carryover with low suspended solids

Selective silica carryover

Silica can be present in the steam as the result of general boiler water carryover, or case, silica acts much like gas later it can go into the steam in a volatile form. In the and is considered to be selectively carried over. As the boiler pressures increase above 400 psi, there is an increase in the tendency for silica to be selectively carried into the steam in amounts proportionate to the amount of silica present in .the boiler water

Problems caused by carryover

Suspended and dissolved solids in the boiler water tend to deposit in the steam and condensate system. Impurities carried over with the steam cause contamination in the many processes for which steam is used, resulting in overheating, corrosion and reduction of efficiency of the boiler itself and other
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.equipment

Preventing carryover
The basic preventive measure is to maintain the concentration of solids in the boiler water at recommended levels. High water levels, excessive boiler loads and sudden load changes are to be avoided. Very often contaminated condensate returned to the boiler system causes carryover. The return condensate should be filtered to remove suspended solids before being fed back to the boiler. Efforts should be made to trace the source of any excessive contamination and the problem rectified. The use of chemical antifoams is effective in controlling carryover due to concentration of impurities in the boiler water. Steam-separating .equipment must be inspected for proper installation :References http://www.sedifilt.com/technical_library/boiler_feedwater_treatment_part_i_wh y_water_treatment_is_necessary.html

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