Boiler Feedwater Treatment (Part II): Water Treatment Fundamentals

Contents

1. Removing impurities from boiler feedwater
2. Filtration
3. Coagulation and flocculation
4. Reaction of lime soda in softening process
5. Ion exchange
6. Deaeration of water
7. Combination of ion exchange and lime process
8. Reverse osmosis
9. Internal treatment of boiler feedwater
10. Blowdown
11. Corrosion in steam condensate system
12. Care of out-of-service boilers

Removing impurities from boiler feedwater

Feedwater is filtered to remove suspended matter and if the suspended solids are very fine, a flocculation step may be needed to enable effective filtration. The water is then subjected to other treatments to make it suitable for the boiler. Depending on the quality of water, it may be subjected to one or more treatments like chemical precipitation, lime-soda softening, ion exchange, deaeration, and reverse osmosis.

Filtration

Filtration is the essential first step before the chemical treatment and conditioning of the boiler feedwater. Filtration removes or minimizes all types of suspended solid impurities. If rust, sand (silica) etc. are not filtered out, they lead to severe scale formation, which is difficult to clean and reduce boiler efficiency. Even the condensate feedwater must be filtered before returning to the boiler. The boiler itself and the steam piping produce rust particles etc. due to corrosion and other reactions. Filtration is also necessary for any water treatment process to work properly. For example, softening resins get coated with suspended matter, loosing their effectiveness and capacity to regenerate. Reverse osmosis membranes get fouled up leading to reduced efficiency and shorter life. If the water is very dirty, sand filtration is first done followed by cartridge filtration.

Coagulation and flocculation

Some times the suspended particles in water are so fine that even cartridge filters are unable to remove them. In such a situation, before cartridge filtration, the water is first treated with coagulants. Coagulation is charge neutralization of finely divided and colloidal impurities in water into masses that can be filtered. In addition, 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. Flocculation is the bridging together of coagulated particles.

Types of coagulants

Iron and aluminum salts such as ferric sulfate, ferric chloride, aluminum sulfate (alum), and sodium aluminate are the most common coagulants. Ferric and alumina ions each have three positive charges and therefore their effectiveness is related to their ability to react with the negatively charged colloidal particles. These coagulants form a floc in the water that serves like a net for collecting suspended matter. Polyelectrolytes, which are synthetic materials, 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 process to serve as coagulant aids. These assist in coagulation by making the floc heavier.

Chemical precipitation

Chemical precipitation is a process in which chemical added reacts with dissolved minerals in the water to produce a relatively insoluble reaction product. Precipitation methods are used in reducing dissolved hardness, alkalinity, and silica. The most common example is lime-soda treatment.

Reaction of lime and soda in softening process

Calcium hydroxide (hydrated lime) reacts with soluble calcium and magnesium carbonates to form insoluble precipitates. They form a sludge that 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 water. Lime reacts with magnesium sulfate and chloride and precipitates magnesium hydroxide, but in this process soluble calcium sulfate and chlorides are formed. Lime is not effective in removing calcium sulfates and chlorides (permanent hardness). Soda ash is used primarily to reduce non-bicarbonate hardness (permanent hardness). The calcium carbonate formed by the reaction precipitates as sludge and can be filtered out. The resulting sodium sulfate and chloride are highly soluble and non-scale forming.

Methods of lime-soda softening

The older method of intermittent softening consists of mixing the chemicals with the water in a tank, allowing time for reaction and forming of sludge, and filtering and drawing off the clear water. The modern method of continuous lime-soda softening involves use of compartmented tanks with provision for (a) proportioning chemicals continuously to the incoming water, (b) retention time for chemical reactions and sludge formation, and © continuous draw-off of softened water. Lime-soda softening is classified as hot or cold, depending on the temperature of the water. Hot process softeners increase the rate of chemical reactions, increase silica reduction, and produce over-all better quality water.

Coagulants used in lime-soda process

In the initial clarification process, coagulants are used to agglomerate fine suspended particles, which can then be filtered out. Likewise, in the softening process, coagulants speed up settling of sludge by 25-50%. Sodium aluminate used as a coagulant in lime-soda softening being alkaline, also contributes to the softening reactions, particularly in reducing magnesium. Proper uses of coagulants help remove silica in the softening process. Silica tends to be adsorbed on the floc produced by coagulation of sludge.

Disadvantages of lime-soda softening

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

Advantages of lime-soda softening

The main advantage is that in reducing hardness, alkalinity, total dissolved solids, and silica are also reduced. Prior clarification of the water is not usually necessary with the lime-soda process. Another advantage is that with continuous hot process softening some removal of oxygen and carbon dioxide can be achieved. Fuel savings can be realized with hot process softening because of solids reduction. This reduction decreases the conductivity of the feedwater, thereby decreasing blowdown and conserving heat.

Ion Exchange

Minerals dissolved in water form electrically charged particles called ions. Calcium carbonate, for example, forms a calcium ion with positive charges (a cation) and a bicarbonate ion with negative charges (an anion). Some synthetic and natural 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 the calcium and magnesium ions are removed and replaced with sodium ions. Ion exchange resins usually are small porous beads that compose a bed several feet deep through which the water is passed.

Types of ion exchange resins

Ion exchange resins are two types: cation and anion. Cation exchange resins react only with positively charged ions like Ca+2 and Mg+2. Anion exchange resins react only with the negatively charged ions like bicarbonate (HCO3-) and sulfate (SO4-2). Although there are many types of cation exchange resins, they usually operate on either a sodium or hydrogen ‘cycle’. That is, they are designed to replace all cations in the water with either sodium or hydrogen. The anion resins are of two types: weak base and strong base. Weak base resins will not take out carbon dioxide or silica, but will remove strong acid anions by a process more similar to adsorption than ion exchange. Strong base anion resins, on the other hand, can reduce carbon dioxide and silica as well as strong acid anions to very low values. Strong base anion resins are normally operated on a hydroxide cycle. Chloride anion exchange resin is also used in dealkalization where alkalinity is reduced.

Ion exchange regeneration

Ion exchange resins have a certain capacity for removing ions from water and when their capacity is used up they have to be regenerated. The regeneration is essentially reversing the ion exchange process. Cation exchangers operating on the sodium cycle, salt (NaCl) is added to replenish the sodium capacity. Resins operating on the hydrogen cycle are replenished by adding acid (H2SO4 or HCl). Anion exchangers are normally regenerated with caustic (NaOH) or ammonium hydroxide (NH4OH) to replenish the hydroxide ions. Salt (NaCl) may also be used to regenerate anion resins in the chloride form for dealkalization. Regeneration process involves taking the vessel off line and treating it with concentrated solution of the regenerant. The ion exchange resin then gives up the ions previously removed from water and these ions are rinsed out of the vessel. After the regeneration has been completed, the vessel is ready for further service.

Split-stream softening

When the effluents from a cation exchanger operating on sodium cycle are blended with effluents from a cation exchanger operating on a hydrogen cycle. The purpose is to reduce the alkalinity of the water. Since the hydrogen cycle produces acid water while the sodium cycle does not affect alkalinity, the two effluents can be blended together to give the desired reduction in alkalinity.

Dealkalization

One of the ion exchange processes for reducing water alkalinity is referred to as dealkalization. In this process the water passes through an ion exchanger operating on the chloride cycle. The exchanger removes alkaline anions such as carbonate, bicarbonate, and sulfates, replacing these ions with chloride. Cation exchange softening precedes dealkalization process.

Demineralization

When the water is passed through both cation and anion exchange resins it is known as demineralization. In this process the cation exchange is operated on the hydrogen cycle. That is, hydrogen is substituted for all the cations. The anion exchanger operates on the hydroxide cycle, which replaces hydroxide for all of the anions. The final effluent from the process consists essentially of hydrogen ions and hydroxide ions or pure water. The demineralization process can be done by several methods. In the mixed-bed process, the anion and cation exchange resins are intimately mixed in one vessel. Multi-bed arrangements may consist of different combinations of cation exchange beds, weak and strong-based anion exchange beds, and degasifiers.

Disadvantages of ion exchange

Sodium cycle ion exchange softening disadvantage is that the total solids, alkalinity, and silica contents of the raw water are not reduced. In the case of cation exchange on the hydrogen cycle, the disadvantage is the corrosion from the acid pH of the effluent. With demineralization, the main difficulty is higher cost, particularly on high solids raw water. Without an excellent pre-filtration arrangement, fouling of the ion exchange material with suspended and colloidal matter in the raw water can produce short runs, ion-exchange degradation, and regeneration difficulties.

Advantages of ion exchange

A major advantage of ion exchange softening is the ease of process control. Variations (within reasonable limits) of hardness in raw water or in flow rate do not have an adverse effect on the completeness of softening. The ion exchange system takes up less space than the lime-soda system. Generally the ion exchange demineralization has the ability to produce better quality boiler feedwater at an economical cost than most other methods.

Deaeration of water

Dissolved oxygen in water is a major cause of boiler system corrosion. It should be removed before the water is put in the boiler. Feedwater deaeration removes oxygen by heating the water with steam in a deaerating heater. Part of the steam is vented, carrying with it the bulk of the dissolved oxygen.

Combination of ion exchange and lime process

As mention earlier, water containing suspended solids, organics, or turbidity requires filtration/clarification prior to ion exchange. Because simple cation exchange does not reduce the total solids of the water supply, it is sometimes used in conjunction with precipitation softening. A common combination treatment is the hot lime-zeolite process. This involves pretreatment of the water with lime to reduce hardness, alkalinity, silica, and subsequent filtration and a cation exchange softening. This combination accomplishes several functions like softening, alkalinity and silica reduction, some oxygen reduction, and removal of suspended matter and turbidity.

Reverse osmosis

To understand reverse osmosis (RO), one must first understand osmosis. Osmosis uses a semi-permeable membrane that allows ions to pass from a more concentrated solution to a less concentrated solution without allowing the reverse to occur. Reverse osmosis overcomes the osmotic pressure with a higher artificial pressure to reverse the process and concentrate the dissolved solids on one side of the membrane. Operating pressures of about 300 to 900 psi are required to achieve this. Reverse osmosis reduces the dissolved solids of the raw water, making the final affluent ready for further treatment. This process is suitable for any type of raw water, but sometimes the installation and operation cost may not be economical.

Internal treatment of boiler feedwater

Internal treatment of water inside the boiler is essential whether or not the feedwater has been pretreated. Internal treatment compliments external treatment and is required regardless of whether the impurities entering the boiler with the feedwater are large or small in quantity. In some cases feedwater supply needs to be only filtered without the need for any other external treatment. Internal treatment can constitute the sole treatment when boilers operate at low pressure, large amounts of condensed steam are used for feedwater, or the raw water available is of good quality. However, in moderate or high-pressure boilers, External treatment of the make-up water is mandatory for good results. With today’s boilers having higher heat transfer rates, even a small deposit can cause tube failure or wasted fuel.

Internal water treatment program

The purpose of an internal water treatment program is:

1. To react with incoming feedwater hardness and prevent it from precipitating on the boiler metal as scale
2. To condition any suspended matter such as hardness sludge in the boiler and make it nonadherent to the boiler metal
3. To provide antifoam protection to permit a reasonable concentration of dissolved and suspended solids in the boiler water without foaming
4. To eliminate oxygen from the feedwater
5. To provide enough alkalinity to prevent boiler corrosion
6. To prevent scaling and protect against corrosion in the steam-condensate systems.

Chemicals used in internal treatment

Phosphates used to be the main conditioning chemical, but nowadays chelate and polymer type chemicals are mostly used. These new chemicals have the advantage over phosphates of maintaining scale-free metal surfaces. All internal treatment chemicals, whether phosphate, chelate, or polymer, condition the calcium and magnesium in the feedwater. Chelates and polymers form soluble complexes with the hardness, whereas phosphates precipitate the hardness. Sludge conditioners are also used to aid in the conditioning of precipitated hardness. These conditioners are selected so that they are both effective and stable at boiler operating pressures. Synthetic organic materials are used as antifoam agents. For feedwater oxygen scavenging, chemicals used are sodium sulfite and hydrazine. Condensate system protection can be accomplished by the use of volatile amines or volatile filming inhibitors. A reputable company supplying treatment chemicals should be consulted. These companies supply the chemical formulations under their brand names and they provide details on the dosage and methods.

Internal treatment for hardness

At boiler operating temperatures, calcium carbonate in the feedwater breaks down to form calcium carbonate. Since it is relatively insoluble, it precipitates. Sodium carbonate in the water partially breaks down to sodium hydroxide and carbon dioxide. Internal treatment with phosphates transforms calcium bicarbonate to calcium phosphate and sodium carbonate. In the presence of hydroxide alkalinity, magnesium bicarbonate precipitates as magnesium hydroxide or reacts with silica to form magnesium silicate. These minerals are precipitated from solution in form of sludge, which must be conditioned to prevent its sticking to the boiler metal. The conditioned sludge is then removed from the boiler by blowdown. When chelate is used for internal treatment, it reacts with calcium and magnesium salts to form soluble complexes. These complexes are in the form of dissolved solids and are removed by blowdown. Dispersant polymers used in conjunction with chelate produces reaction products, which are better conditioned. They do not precipitate and are more easily removed by blowdown. Use of polymers further aids in conditioning any suspended solid contamination that may have entered with the boiler feed water.

Internal treatment for sulfates

The boiler temperature makes the calcium and magnesium sulfates in the feedwater insoluble. With phosphates used as internal treatment, calcium reacts with the phosphate producing hydroxyapatite, which is much easier to condition than calcium sulfate. With chelates or polymer used as internal treatment, calcium and magnesium react with these materials producing soluble complexes that are easily removed by blowdown.

Internal treatment for silica

If silica is present in the feedwater, it tends to precipitate directly as scale at hot spots on the boiler metal and or combines with calcium forming a hard calcium silicate scale. In the internal treatment for silica, the boiler water alkalinity has to be kept high enough to hold the silica in solution. Magnesium, present in most waters, precipitates some of the silica as sludge. Special organic materials or synthetic polymers are used to condition magnesium silicate from adhering to the boiler metal.

Internal treatment for sludge conditioning

Internal treatment for hardness results in insoluble precipitates in the boiler that form sludge. In addition, insoluble corrosion particulate (metal oxides) are transported to the boiler by condensate returns and from preboiler feedwater corrosion resulting in suspended solids. Suspended solids, carried to the boiler by feedwater or subsequently formed within the boiler, adversely affect both boiler cleanliness and steam purity. These solids have varying tendency to deposit on the boiler metal. Conditioners prevent these solids from depositing and forming corrosive or insulating boiler scale. Some of the principal types of sludge conditioners are:

1. Starches – effective on high silica feedwater and where oil contamination is a problem
2. Lignins – effective on phosphate type sludge
3. Tannins – fairly effective on high hardness feedwater
4. Synthetic polymers – Highly effective sludge conditioners for all types of sludges

Internal treatment advantages

Internal treatment is basically simple and with the help of a qualified consultant an effective program is easily established. Scales or deposits, corrosion and carryover are minimized thereby improving efficiency and reducing energy consumption, preventing tube failures and unscheduled costly repairs, and reducing deposits, corrosion and contamination in the downstream equipments or processes.

Internal treatment – chemical dosage

Chemical dosages are based on the amount and type of impurities in the feedwater. For example, the amount of boiler treatment chemicals depends on the feedwater hardness; the amount of sodium sulfite depends on the amount of dissolved oxygen in the feedwater. In addition, a certain amount of extra chemicals are added to provide a residual in the boiler water. Feeding methods include chemical solution tanks and proportioning pumps with the chemicals being added directly before entrance to the boiler. Ortho type phosphates are fed through a separate line directly into the steam drum of the boiler. Chemicals used to prevent condensate system corrosion can be fed directly to the feedwater, boiler, or steam, depending on the type of chemical used. Continuous feeding is the preferred method but intermittent feeding may also suffice in some cases.

Tests for treatment control

Boiler water control tests include tests for alkalinity, phosphate, polymer, hydrazine, chelate, sulfite, pH, organic colour, and total dissolved solids. If sulfite test shows adequate residual is present (this test is not valid with use of uncatalyzed sodium sulfite), the feedwater oxygen has been removed. Testing for organic colour gives an indication for both sludge conditioner and antifoam level. Chelate testing can be either for total chelate or residual chelate.

Tests for checking contaminants

These tests depend on what type of contaminants is suspected. Common checks are for iron, oil and silica. Total iron test serves as a check on corrosion products brought back by the condensate return. This test is also used to check iron present in the make up water. Laboratory facility is required for oil test but visual inspection can reveal gross contamination. Periodic checking should be done to detect unusual silica contamination and to determine when blowdown is needed.

Blowdown

Blowdown is the discharge of boiler water containing concentrated suspended and dissolved feedwater solids. As the blowdown water is replaced with lower solids feedwater, the boiler water is diluted. With proper regulation of blowdown, the amount of solids in the boiler water can be controlled. The amount of blowdown needed depends on how much feedwater impurities a given boiler can tolerate. For example if a particular boiler can tolerate 500 ppm maximum dissolved solids, and the feedwater contains 50 ppm, it can be concentrated only about 10 times. This means that for every 100 pounds of water fed to the boiler about 10 pounds of boiler water must be blown down to keep the dissolved solids from exceeding 500 ppm. Total dissolved solids is not the only limiting factor in determining blowdown, other considerations include suspended solids, alkalinity, silica and iron.

Test for regulating blowdown

A practical method for regulating blowdown is by routinely checking the electrical conductivity of the water with a simple measuring instrument. Electrical conductivity gives an estimate of the dissolved solids in the boiler water. By checking both the feedwater and boiler water dissolved solids, one can easily calculate the number of feedwater concentrations.

Continuous and intermittent blowdown

Boilers incorporate blowdown valves at low points where sludge is likely to collect. Opening these blowdown valves for short intervals provides intermittent removal of sludge and concentrated solids. In addition, some boilers also have a blowdown off take located slightly below the water level in the steam release area. A small amount of water is continuously removed through these connections. The use of continuous blowdown in addition to manual (bottom) blow down maintains the residuals at more consistent levels in the boiler water. Continuous blowdown also minimizes the amount of bottom blowdown required, with resultant savings in fuel and chemicals. Continuous blowdown helps minimize upsets in boiler water circulation and operation.

Corrosion in steam condensate system

Corrosion in steam condensate system is caused by carbon dioxide and oxygen carried into the system by steam. Dissolved carbon dioxide in condensed steam forms 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 oxygen, attacks copper alloys.

Prevention of steam condensate corrosion

Generally corrosion prevention is by removing oxygen from the feedwater by mechanical (deaerator) means, by use of suitable chemicals, and pretreatment of the make-up water to minimize potential carbon dioxide formation in the boiler. Further boiler water treatment is done by use of volatile amines to neutralize carbon dioxide or volatile filming inhibitors to form a barrier between the metal and the corrosive condensate. Mechanical conditions need to checked and corrected, like poor trapping and draining of lines. Deaerator can reduce oxygen to as low as 0.007 ppm. Since very small amounts of oxygen can cause boiler and steam condensate system corrosion, chemical treatment is needed to assure complete oxygen removal. Sodium sulfite and hydrazine chemicals are commonly used for this purpose. Catalysts are sometimes also used to speed up the reaction.

Prevention of deposits and water corrosion in feedwater systems

Deposits in feedwater systems are usually caused by hardness precipitation as the water goes through feedwater heaters or as the feed lines enter the boilers. Deposits can also occur from premature reaction of treatment chemicals with hardness in the feedwater. Prevention is by means of the use of stabilizing chemicals fed continuously to retard hardness precipitation. The corrosion of feedwater system is due to the low alkalinity or dissolved oxygen in the water. Raising the pH of the water with caustic or amines and feed of catalyzed sodium sulfite minimizes this problem.

Prevention of caustic embrittlement

Organic materials like lignins applied to the boiler water are effective in preventing caustic embrittlement of boiler metal. Sodium nitrate inhibits embrittlement at low concentrations ranging to 0.4 part of sodium nitrate per part of caustic soda in boiler water. Maintaining an organic content of 50-100 ppm and a sodium nitrate content of 50 ppm is a commonly used embrittlement prevention program.

Oil contamination, problem & remedies

Main problems caused by oil in the boiler water are:

1. Oil can coat metal surfaces, cut down heat transfer, and produce metal overheating
2. Oil can cause sludge to become sticky and adhere to heat transfer surfaces
3. Oil can produce foaming and boiler water carryover

Oil contamination should be completely eliminated whenever possible. Organic chemicals help counteract the effects of small amounts of oil contamination, but not of gross contamination. When sudden boiler water oil contamination is experienced, normal procedure is to blow down heavily to remove oil and to check for the source of contamination. In case of severe contamination, the boiler needs to be taken off the line and cleaned out to remove the oil from the boiler surfaces. When oil contamination is continuous and unavoidable, some of the methods used are:

1. Free oil can be reduced by passing the water through absorbent cartridge filters
2. Emulsified oil is broken down by chemical additives and filtered
3. Special filters are used with aids like diatomaceous earth
4. Flotation method
5. Coalescence method

Care of out-of-service boilers

Much of the corrosion damage to boilers and condensate equipment results during idle periods due to corrosion caused by the exposure of wet metal to oxygen in the air. Wet boiler lay-up method is of storing boilers full of water. Extra chemicals (alkalinity, oxygen scavenger, and a dispersant) are added to the boiler water and the water level is raised in the idle boiler to eliminate air spaces. Nitrogen gas can also be used on airtight boilers to maintain positive pressure on the boiler, thereby preventing oxygen from entering. Dry boiler lay-up method is usually for longer boiler outages. The boiler is drained, cleaned and dried out. Material, such as hydrated lime or silica gel, which absorb moisture, is placed in trays inside the boiler. The boiler is then sealed to prevent air from entering. Periodic replacement of the drying chemical is required during long storage periods.