Boiler feed water and its specifications

Industrial uses of water are for cooling and steam generation purposes. Cooling does not require water to be of high quality so need not be treated. Water required for boiler feed purposes. i.e. for steam generation should be completely free from impurities and need to be treated. The total hardness of water should not be more than 0.2 ppm. The caustic alkalinity(due to OH- ions) should be between 0.15-0.45 ppm and the soda alkalinity due to Na2CO3 should be between 0.45-1 ppm.

Untreated water contain impurities which may lead to the following problems:

1. Scale and sludge formation.2. Boiler corrosion.3. Caustic embrittlement.4. Priming and foaming.

1. Scale and sludge formationBoilers convert water to steam, as the water evaporates conc. of dissolved salts increases. After reaching a saturation point the dissolved salts start precipitating in order of their solubilities, the salt which is least soluble separates first.When precipitation of the dissolved salts occurs in the form of loose, non-adherent, slimy ppts is called sludge or mud. CaCl2, MgCl2, MgCO3 and MgSO4 have greater solubility in hot water than in cold water so they form sludge. Sludges are formed at the relatively colder regions of the boiler and collects in the bends and the tubes where the flow rate of water is low. It is non-sticky in nature and can be easily removed by wire brush.

For the deposition of scale and sludge i) The ionic product of the salt should exceed the solubility product to bring about precipitation.ii) The increase in temperature leads to reactions that result in the formation of insoluble substances. The salts responsible for the formation of scales and sludges are CaCO3, CaSO4, Mg(OH)2, MgCl2 and SiO2 etc.Salts like CaSO4 whose solubility decreases with decreasing temperature forms scale on the hot surfaces and sludge in the cooler regions. But if the solubility increases with increase in temperature scale will be formed in the cooler regions and sludges in the hotter regions. Formation of scales and sludges both are undesirable but scales are dangerous compared to sludges as the sludges can be removed easily.

Disadvantages of sludge formation :1. Wastage of fuel sludges are poor conductor of heat so they tend to waste a lot of amount of heat. The boilers will have to be heated to higher temperatures in order to maintain the supply of heat. 2. Efficiency of the boiler is decreased.3. Choking of the pipes : The formation of sludges slows down the water circulation as it settles in the regions such as pipe connections, plug openings etc. Excessive formation may even cause choking of pipes.

Prevention of sludge formation :1. Sludge formation can be prevented by using soft water for boiler feed purposes. 2. By frequently carrying out the cleaning operations i.e., removal of a portion of concentrated water and replacing it by fresh water.3. This is called blow down operation and the soft water which is added to make up the volume is called make up water.4. Sludge can be easily removed by scrapping off with a brush.

Scale formation :Scales stick very firmly to the inner walls of the boilers and hence are very difficult to remove. The mechanism of formation involves bubbles to be formed which on striking with the surface of the boilers bring about deposition of the salt and evaporation of the steam. The formation of bubbles and deposition is very rapid. A thick layer is resulted and is deposited on the inner walls.

Scales may be formed due to the following reactions :1. Decomposition of bicarbonates : Ca(HCO3)2 -----------------> CaCO3 + CO2 + H2O2. Hydrolysis of Mg salts : MgCl2 + 2H2O ---------------> Mg(OH)2 + HCl soft scaleCaSO4 is soluble in cold water but the solubility decreases with rise of temperature. Consequently CaSO4 gets precipitated as hard scale on the heated portions of the boiler. Such scales are observed in the case of high pressure boilers.Presence of silica salts like CaSiO3, MgSiO3 are sparingly soluble in cold water but are completely soluble in hot water. These deposits stick very firmly on the inner side of the boiler surface and are very difficult to remove.

Disadvantages of scale formation :1. Wastage of fuel-due to low thermal conductivity of the scales, the heat supplied to the boiler is not efficiently transferred to water. Hence more heat has to be supplied to maintain the supply of steam leading to wastage of fuels. 2. Bagging i.e., distortion of boiler material occurs. Due to overheating, rapid reaction between water and iron occurs at high temperature, causing thinning of the boiler material. 3. Danger of explosion : The uneven expansion of the scale may lead to the cracking of the scale. As a result the super heated steel plates come in contact with boiler water. Large amount of steam may be suddenly formed creating high pressure and may result in the bursting of the boiler.

Removal of scales 1. Soft scales can be removed by a scrapper or wire brush.2. Hard scales can be removed by giving thermal shocks i.e., heating the boiler and then suddenly cooling with cold water.3. Scales can be dissolved in certain chemicals and hence can be removed along with water e.g., CaCO3 scales can be dissolved by using 5-10% HCl. Similarly EDTA dissolves salts of Ca with which it forms soluble complexes.

2. Boiler corrosion is the decay of boiler material due to attack of certain chemicals on its surface. Resulting in the formation of oxides, sulphides etc. The material of the boiler gets dissolved and rusted thereby reducing the life of the boiler. Main reasons for boiler corrosions are :(i) Presence of dissolved O2(ii) Presence of dissolved CO2(iii) Presence of mineral acids

(i) Presence of dissolved O2 Generally, the Fe present in boiler material enters water as Fe2+ ions and reacts with water molecules to form rust by the following reactions. Fe2+ + 2H2O ---------> Fe(OH)2 + 2H+ 4Fe(OH)2 + O2---------> 2[Fe2O3.2H2O] rustO2 can be removed by chemical or mechanical means 2Na2SO3 + O2 ------> 2Na2SO4 Na2S + O2 ----------> Na2SO4Na2SO4 may decompose at high temperatures forming H2SO3 acid which is corrosive in nature therefore only 5-10 ppm of Na2SO3 are added to the boiler water.

N2H4 + O2 ---------> N2 + 2H2OPure hydrazine is an inflammable liquid but 40% aqueous solution is safe to handle. Excess amount of N2H4 decomposes to give NH3, 3N2H4 --------> 4NH3 + N2Mechanically water can be deaerated by exposing it to high temperature and low pressure which reduces the solubility of the gas.

(ii) Presence of dissolved CO2 :CO2 may be present dissolved in water or may be formed by the decomposition of dissolved bicarbonates Mg(HCO3)2 -----------> MgCO3 + CO2 + H2O CO2 + H2O ------------> H2CO3 Carbonic acid has corrosive effects.CO2 may be removed chemically or mechanically.

By chemical means calculated amounts of NH4OH may be added 2NH4OH + CO2 -------------> (NH4)2CO3 + H2OBut excess use of NH4OH may attack the condenser tubes made of copper.Filtering water through limestone removes CO2 CaCO3 + H2O + CO2 -----------> Ca(HCO3)2But this can lead to increase in the hardness of water.Mechanically CO2 may be removed by deaeration.

(iii) Presence of acids Free mineral acids present in boiler may react in the following way Fe + 2HCl -------> FeCl2 + H2 FeCl2 + 2H2O ------> Fe(OH)2 + 2HCl Fe(OH)2 + O2 ----------> 2[Fe2O3.2H2O]Most of the naturally occurring water is alkaline except in mining areas or industrially polluted waste water. Certain salts may liberate acid due to hydrolysis for e.g. MgCl2 + 2H2O ---------> Mg(OH)2 + 2HCl

Caustic EmbrittlementCorrosion of boilers due to highly alkaline water in boiler is termed as caustic embrittlement. The material of the boiler becomes brittle due to accumulation of caustic substance. While softening of water with Na2CO3, NaOH is generated. Na2CO3 + H2O---------------> 2NaOH + CO2NaOH along with natural alkalinity of water makes the boiler water caustic. This NaOH travels to the minute cracks, bends and joints. Water evaporates and the conc. of NaOH at these points keeps on increasing with time due to poor circulation of water at these joints. Caustic soda attacks the crack areas causing stress corrosion cracking. If these areas are have high conc. of NaOH, are stressed beyond a point and have presence of compounds such as sodium silicate caustic embrittlement is enhanced.

The concentrated alkali dissolves iron of the boiler forming Na2FeO2 which decomposes 3Na2FeO2 + 4H2O ----------> 6NaOH + Fe3O4 + H2 Since more of NaOH has been generated it again reacts with iron. Caustic embrittlement can be prevented by the following methods-1. Using Na3PO4 as softening reagent instead of Na2CO3.2. By adding liquids or tannins which block up the hair cracks and prevent infiltration of NaOH. If the conc. Of Na2SO4:NaOH is maintained at 1:1, 2;1 or 3:1 for operating pressures of 10, 20 and>20 atm respectively it checks the caustic embrittlement.

Priming : Due to extremely rapid boiling of water in the boiler, the steam formed is associated with small droplets of water which are carried away with the steam forming wet steam. The formation of wet steam is referred to as priming. The liquid contamination of steam is expressed in percentage. If 0.5% moisture is present in steam, the steam quality is said to be 95%. Causes of priming are : high steam velocities, very high water level in the boiler, faulty boiler design and presence of suspended and dissolved impurities in water.

Foaming is the formation of small persistent bubbles at the surface of water in the boiler. Bubbles are formed when there is a difference in conc. of solute or suspended matter between the surface film and the bulk of the liquid. Any material which lowers the surface tension of the water collect at the interface and thus increases the foaming tendency of the liquid. E.g., oil or grease. Substances which increase the viscosity of the surface also increase foam forming tendency.

Disadvantages of Priming and Foaming :1. A part of the dried salts may be carried along with the steam farther and deposit on the engine valves and thereby decreasing their life. 2. Presence of water droplets in the steam may lead to corrosion in the steam-inlets of superheaters. 3. Height of the water column cannot be judged properly due to foaming thereby making the maintenance of the boiler pressure difficult.

Prevention of priming 1. Keeping the water level lower.2. Reducing steam velocities also decreases the moisture content of steam because of greater chances of condensing available for the drop. 3. Good boiler design fitted with mechanical steam purifiers.4. Efficient softening and filtration of boiler feed waters so as to minimise the dissolved and suspended impurities.

Prevention of foaming :1. Adding antifoaming chemicals : They reduce the surface tension. For e.g., polyamide antifoamers alters the surface tension and lead to the formation of only large unstable bubbles. Mechanical action of antifoaming agents such as castor oil forming a layer on water surface prevents foaming.2. Removal of oil and grease by addition of coagulants e.g., NaAlO2, FeSO4 etc., removes foaming. These coagulants form hydroxides which entraps oil drops.

Removal of silica

Silica forms thermal resistant scales with Ca and Mg. With alumina it forms sodium alumino silicate or only scales of silica. In high pressure boilers silica is carried over with steam and deposits on turbine blades.

Silica can be removed by :1. By the addition of magnesia(MgO) to the water after the removal of temporary hardness. Magnesia forms Mg(OH)2 and then it is adsorbed by the silica by adsorption phenomenon to form MgSiO3. 2. By use of FeSO4 and NaAlO2 as coagulants. They act as Fe(OH)2 or Mg(OH)2 and Al(OH)3 which trap finely suspended and colloidal impurities including oil and silica.3. After passing water through cation and anion exchangers it is passed through a strongly basic anion exchanger which removes silica as H2SiO3, reducing its content to 1 ppm.4. Silicic acid(H2SiO3) is only feebly ionised and cannot be removed by weakly basic anion exchanger. So it is first converted to H2SiF6 fluorosilicic acid and then removed. SiO2+ 6NaF + 3H2R ---------> 3Na2R + H2SiF6 + 2H2O

Water softening processes : Boiler treatment

Boiler feed water should satisfy hardness(0.2 ppm), caustic alkalinity(0.15-0.45 ppm), soda alkalinity(0.15-1.0 ppm) and soda ash(0.3-0.55 ppm). In order to satisfy these specifications boiler feed water is treated in 2 stages :(i) External treatment (ii) Internal treatment

External treatment of boiler involves any of the following processes :

(i) Lime-soda process(ii) Zeolite process(iii) Ion-exchange process(iv) Distillation

LIME-SODA ASH SOFTENING Chemical precipitation is one of the more common methods used to soften water. The chemicals normally used are lime (calcium hydroxide, Ca(OH)2) and soda ash (sodium carbonate, Na2CO3). Lime is used to remove the chemicals that cause the carbonate hardness. Soda ash is used to remove the chemicals that cause the non-carbonate hardness.

When lime and soda ash are added, the hardness-causing minerals form nearly insoluble precipitates. When calcium hardness is removed in a chemical softener, it is precipitated as calcium carbonate (CaCO3). When magnesium hardness is removed in a chemical softener, it is precipitated as magnesium hydroxide (Mg(OH)2). These precipitates are removed by the conventional processes of coagulation / flocculation, sedimentation, and filtration.

NOTE : 1. For each molecule of calcium bicarbonate hardness removed, one molecule of lime is used. 2. For each molecule of magnesium bicarbonate hardness removed, two molecules of lime are used.

NOTE : 1. For each molecule of non-carbonate calcium hardness removed, one molecule of soda ash is used. 2. Each molecule of non-carbonate magnesium hardness requires one molecule of lime plus one molecule of soda ash.

Lime soda process is of two types :Cold lime-soda processHot lime-soda processCold lime-soda process involves calculated amount of lime and soda mixed with hard water at room temperature. Since the ppt formed are finely divided they remain suspended in water and do not settled own easily. Coagulants like Al2(SO4)3, NaAlO2 or alums are added. There are four different types of cold lime-soda processes (i) Continuous type(ii) Intermittent type(batch process) (iii) Spiractor or catalyst type (iv) Sludge blanket process

Cold Lime-soda (Continuous Process )filter

Batch Process (Intermittent type cold lime-soda process)It consists of a pair of tanks which are used in turn for softening of water. Each tank is provided with a mechanical stirrer, inlets for raw water and chemicals and outlets for soft water and sludge. Raw water is passed from one side and a mixture of lime and soda from opposite side simultaneously and stirring is started. During stirring some sludge from a previous operation is added which acts as a nucleus for fresh precipitation and accelerates the process.

Sludge blanket clarifiers force the raw water to travel through the layer of sludge, optimizing the agglomeration reaction. All of the sludge, optimizing the agglomeration reaction.

Hot lime-soda processThe water to be purified is treated with chemicals at a temperature of 95-100C. Softeners used may be intermittent or continuous type.(a) Intermittent type (Batch process) It is similar to the cold lime soda process except that heating coils are present for heating the water.(b) Continuous type: The continuous type has the following parts :Reaction tank: The central reaction tank has three separate inlets through which raw water, chemicals and superheated steam is passed and then mixed together in the reaction tank.(ii) Sedimentation tank: In this tank the mixed water enters and the sludge settles down.(iii) Sand filter: Layers of coarse and fine sand act as filter which ensures complete removal of sludge. A soft water with 15-30 ppm of residual hardness is obtained.

1. Hot lime-soda process involves softening of water by lime and soda at temperatures close to the boiling point of water. 2. The chemical reactions proceed at a faster rate. 3. Dissolved gases are expelled. 4. Practically raw water along with chemicals is sprayed into superheated steam and mixed thoroughly in a reaction tank. The mixture then passes into a conical sedimentation tank to allow the sludge to settle down. 5. The softened water is then passed through a filter bed containing graded anthracite coal or a phosphate treatment plant.

Hot Lime-soda filter

Advantages of lime-soda process : 1. It is economical 2. Less amount of coagulants are required 3. Fe and Mn are also removed. 4. Minerals are also removed. 5. Soft water obtained after this process is alkaline and hence has corrosion tendency. 6. Due to alkaline nature the amount of pathogenic bacteria to a considerable amount.

Disadvantages of lime-soda process :1. Disposal of large amount of sludge is a problem. 2. This process requires skillful operation for efficient and economical softening. 3. Hardness of water is reduces only to 15ppm which is not ideal for boilers.

Difference between hot and cold lime-soda process

Cold lime-soda processHot lime-soda processCalculated amount of lime and soda is mixed at room temperature(25-300C).This is done at elevated temperature(80-1500C).It is a slow process.It is a fast process.The ppt formed are finely divided and cant settle easily and hence filtration is not easy.The ppt formed are like sludge, settle down easily and hence filtration is easy.Use of coagulant is must.Coagulants are not required.Softened water has residual hardness hardness around 60 ppm.Residual hardness is about 1530 ppmDissolved gases are not removed.Dissolved gases like CO2 are removed to some extentLow softening capacity.High softening capacity.

Zeolite process/Permutit ProcessZeolites/Permutit are a class of naturally occurring crystalline hydrated aluminosilicate minerals of composition - Na2O.Al2O3.xSiO2.yH2O where x is between 2 to 10 y between 2 to 6. Zeolite crystals consist of a framework of SiO4 tetrahedra. Each tetrahedra shares the corner oxygen with adjacent tetrahedra giving rise to an open structure with pores or cavities of different sizes and shapes. Some of the Si4+ ions are isomorphously replaced by Al3+ ions with the simultaneous incorporation of Na+ or K+ to compensate for the charge imbalance.

Zeolites behave as ion-exchangers capable of exchanging reversibly the Na+/K+ ions. The ion exchange capacity of zeolites increases with increasing charge on the exchanging ion M2+

Zeolites can be represented by the general formula Na2X where X represents the insoluble radical framework. They hold Na+ ions loosely. When hard water is passed through a bed of zeolite, the hardness causing ions are retained by zeolite as CaX and MgX. As a result water becomes free from main hardness producing cations but gets more concentrated with respect to sodium salts and eventually zeolite gets exhausted.

Regeneration : When sodium zeolites are completely converted into calcium and magnesium zeolites and the zeolite ceases to soften water, i.e., gets exhausted it is reclaimed by treating with 10% brine solution. CaX + 2NaCl-------> Na2X + CaCl2 MgX + 2NaCl-------> Na2X + MgCl2

Thus softening by zeolite involves alternate cycles of softening run and regeneration. The soft water thus obtained has hardness less than 30 ppm.

Advantages of zeolite process : no danger of sludge formation as impurities are not precipitated, it requires less time for softening, less skill required for maintenance and operation, it is compact and occupies less space, it automatically adjusts itself to water of different hardness.Disadvantages of zeolites process : only cations are replaced by sodium ions and not the acidic ions, the treated water contains more water than the lime-soda process.

Limitations of zeolite process : 1. Mineral acids destroy the zeolite bed so they must be neutralised before zeolite treatment. 2. If large quantities of of Fe2+ and Mn2+ are present in water the zeolite bed is converted into iron and manganese zeolites which cannot be regenerated.3. Turbid water can cause the clogging of pores of zeolite bed thereby making it inactive. 4. Soft water obtained by zeolite process contains 25% more dissolved solids than that obtained by lime-soda process. 5. Acid radicals are not removed by this process therefore HCO3- present in hard water get converted into NaHCO3 which goes into soft water effluent. Such waters when used in boilers causes highly alkaline condition leading to caustic embrittlement and boiler corrosion NaHCO3 ----------> NaOH + CO2 CO2 + H2O ----------> H2CO3

Ion Exchange Softening Ion-exchange is used extensively in small water systems and individual homes. Ion-exchange resin, exchanges one ion from the water being treated for another ion that is in the resin. If the water needs to have the mineral content entirely removed it is passed through a resin containing H+ (which replaces all the cations) and then through a second resin containing OH- (which replaces all the anions). The H+ and OH- then react together to give more water.

Cation exchange resins contain acidic functional groups like COOH, -SO3H etc., capable of exchanging their H+ ions with other cations and are known as cation exchange resin. They are generally denoted by RH+. They are mainly sulphonated or carboxylated styrene and divinyl benzene copolymers. E.g. zeocarb, amberlite, IR-20, Dowex-50, Duolite are some commercially available cation exchange resins.

Anion Exchange Resin : They contain basic functional groups like NH2, =NH, etc. as hydrochloride or hydroxide, are capable of exchanging their anions with other anions. They are commonly represented as R+ and OH- where R+ represents the insoluble organic matrix. They are mainly styrene-divinyl benzene or amine-formaldehyde copolymers which contain amino or quaternary ammonium groups. These after treatment with dil. NaOH solution become capable of exchanging their OH- ions with other anions. E.g.- Amberlite-400, Dowex-3.

Suppose a resin has greater affinity for ion B than for ion A. If the resin contains A and ion B is dissolved in the water passing through it, then the following exchange takes place. The reaction proceeding to the right (R represents the resin):

Process :Step 1. The hard water is passed through cation exchange column, thereby all the cations are taken up by the resin and an equivalent amount of H+ ions are released from the resin.Step 2. After this the water is passed through anion exchange column when all the anions like Cl-, SO42- etc., are taken up by the resin and an equivalent amount of OH- ions are released from this column to water. Step 3. Water becomes free from all anions. The H+ and OH- ions released from cation and anion exchange column combine to from water. Water thus obtained is known as deionised or demineralised water.

Demineralisation process by ion-exchange

Regeneration

Due to continuous use the ion exchangers lose their capacity and become exhausted. They are then reclaimed by treating it with a solution of dil HCl/dil H2SO4 and dil NaOH.Mixed bed deioniser/demineraliser : It is an intimate mixture of strongly acidic cation exchanger and strongly basic anion exchanger with a single cylindrical unit. The outgoing water has the residual hardeness 0-1 ppm. This method is very effective.

Limitations of ion-exchange process (b) Iron foulingBores yielding anaerobic water from underground supplies nearly always contain soluble iron in the Fe2+ state. Small amounts are readily removed by sodium cycle softeners but care must be taken to prevent contact with air prior to treatment. Aeration allows oxidation of Fe2+ to Fe3+ and consequent precipitation of ferric hydroxide which clogs resin beads and prevents ion exchange. Iron fouling is the commonest cause of softener failure.

(b) Adsorption of organic matterOne of the commonest problems results from the presence of organic matter in water supplies. Untreated water from lakes and rivers usually contains dissolved organic material derived from decaying vegetation which imparts a yellow or brown colour. These substances can become irreversibly adsorbed within the anion beads, reducing their exchange capacity and leading to a reduction in treated water quality. Removal of organics prior to demineralisation is usually achieved by flocculation with alum or ferric salts followed by filtration which removes the metal hydroxide floc and the coprecipitated organic compounds.This treatment also removes any fine silt which represents another source of resin fouling. Both organic and iron fouled units can be chemically cleaned on site but complete removal ofimpurities is rare and resin performance usually suffers after fouling.

(c) Organic contamination from the resinThe resins themselves can be a source of non-ionized organic contamination. New commercial grade resin often contains organics remaining after manufacture, while very old resin will shed organic fragments as the polymer structure opens up very slowly (decrosslinkage).Such contamination may be disregarded for many uses, but when removal is needed, the demineralised water can be passed through an ultra filtration membrane.

(d) Bacterial contaminationResin beds do not act as filters for the removal of bacteria or other micro-organisms. They very often tend to worsen such contamination as traces of organic matter, which invariably accumulate, constitute a nutrient source for continued growth. When sterile water is required it can be obtained by treating the demineralised water by non-chemical means such as heat, ultra violet irradiation or very fine filtration. Resins beds can be decontaminated with disinfectants such as formaldehyde, but heat or oxidising disinfectants such as chlorine must not be used as these damage resins.

(e) Chlorine contamination chlorine damages resins. This means that even town supply water is an unsuitable demineraliser feed because of the trace of chlorine it contains. It is customary to treat such feeds by passing them through activated carbon which removes chlorine very efficiently.