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External Feedwater Treatment

Learning Outcome When you complete this module you will be able to:

Discuss the general principles, methods and equipment used in preparing raw feedwater for steam production in a boiler.

Learning Objectives Here is what you will be able to do when you complete each objective:

1. List the common impurities in raw feedwater and their potential effects on a boiler.

2. Describe the various ways in which water is filtered to remove suspended

solids. 3. Describe the purpose, processes and equipment used in boiler water softening 4. List the causes of zeolite water softener malfunctions. 5. Describe the theory, process and equipment used in dearation.

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INTRODUCTION The water used in a boiler starts out in its raw state, in contact with many chemicals which are contaminants in the context of steam production. It is necessary to treat boiler feedwater in order to:

• Prevent sludge from depositing on boiler surfaces.

• Prevent scale from forming on boiler surfaces.

• Prevent corrosion of boiler metal.

• Prevent carryover of impurities with the steam leaving the boiler. The above four items are caused by impurities in the water being fed to the boiler. These impurities may be classed into three main groups: suspended matter, dissolved solids, and dissolved gases. SUSPENDED MATTER Suspended matter is material which is not dissolved in the water but which is floating on the surface of the water or is dispersed throughout the water. It consists of sand, mud, clay, and organic material such as sewage and vegetable matter. These impurities are usually quite visible in the water. The sand, mud, and clay will form sludge-like deposits in the bottom of the boiler shell, mud drums and headers. These deposits will keep the water away from the metal and will thus cause overheating of the metal. To prevent this sludge from forming, these impurities can be removed from the water by the use of filters or settling tanks before it enters the boiler. Also, if any sludge does deposit in the boiler, then it can be blown out through the boiler blowoff connections. DISSOLVED SOLIDS These impurities are actually dissolved in the water and so are not visible. The ones which cause the most trouble in the boiler are the compounds of calcium and magnesium, whose most common compounds are as follows:

Calcium bicarbonate Ca(HCO3)2 Magnesium bicarbonate Mg(HCO3)2 Calcium sulphate CaSO4 Magnesium sulphate MgSO4

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These are the compounds that cause “hardness” of the water and deposit scale within the boiler. The bicarbonates of calcium and magnesium cause temporary hardness because the bicarbonates can be easily removed from the water by heating it below its boiling point. The bicarbonates then drop out of solution and they deposit as a soft scale. If this heating is carried out before the water enters the boiler, then the soft scale will deposit in the feedwater heater. If the bicarbonates are not removed before the water enters the boiler, then they will form a soft scale on the boiler surfaces. The sulphates of calcium and magnesium cause permanent hardness since these compounds cannot be removed by heating the water. They form a hard, dense scale on the boiler surfaces. Deposits of either the soft bicarbonate or the hard sulphate scale are undesirable as they will keep the water away from the metal of the boiler tubes and plates causing overheating of these surfaces. The dissolved solids, being in solution in the water, cannot be removed by filters or settling tanks. Instead, water softeners of various types are used to remove the hardness before the water enters the boiler. Also, chemicals can be added directly to the boiler feedwater to prevent formation of scale on the boiler surfaces. DISSOLVED GASES The gaseous impurities that are of concern are oxygen (O2) and carbon dioxide (CO2). These are dissolved in the water and they can cause corrosion of the boiler metal and pipelines. Oxygen produces a special type of corrosion known as “pitting”. These pits may be small or large and are usually covered by blisters of iron oxide. Carbon dioxide corrosion produces grooves in the metal and especially attacks pipe threads. These gases can be removed from the water before it enters the boiler by the use of a deaerator. In the deaerator, the water is heated to the boiling point causing the gases to be driven off through a vent in the top of the deaerator. Chemicals that are added directly to the boiler can also be used to absorb dissolved oxygen and neutralize carbon dioxide.

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pH VALUES The condition of the boiler water in regard to its acidity or alkalinity is extremely important. If the water is acidic, then corrosion of the boiler metal will occur at a rapid rate. On the other hand, if the water has high alkalinity then foaming within the boiler may occur. In order to indicate the degree of acidity or alkalinity of the boiler water, a scale of numbers from 0 to 14 is used. These numbers are called pH values and the pH value of the boiler water is determined by taking a sample and testing it.

• If the water is neutral (neither acidic or alkaline), then its pH value is 7.

• If the water is acidic, then its pH value will be less than 7 and the more acidic the water, the lower the pH value.

• If the water is alkaline, then the pH value will be greater than 7 and the

more alkaline, the greater the pH value. If the boiler water is acidic, then the boiler metal will corrode. To prevent corrosion, the boiler water should be kept alkaline. Water with a pH value of 10.5 usually provides the necessary alkalinity. Table 1 lists the complete scale of pH values.

Solution

Most acidic

Acidity decreases with increasing pH

Least acidic Neutral Least alkaline

Alkalinity increases with increasing pH

Most alkaline

pH Value

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Table 1

pH Scale

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METHODS OF TREATMENT The methods of treating boiler water can be divided into two main groups: external treatment and internal treatment. External treatment is the treatment of water before it enters the boiler. However, when water is fed into the boiler first and then treated, the treatment is considered to be internal. The methods of external water treatment that will be discussed in this module are filtration and water softening. Filtration Most municipalities use large settling basins and filters to remove suspended matter from the water before it is pumped into the water supply mains for distribution to customers. These filtration systems are usually quite efficient and there is no need for the customer to provide additional filtration. However, the waterworks in some small cities and towns only remove the coarse impurities. The water supply could still contain a certain amount of finer suspended solids. It will be necessary to filter the water before it can be used as boiler feedwater. This also applies to industrial sites, where river or lake water is the prime source for feedwater. Many types of filters are on the market. A few of the most common types used in buildings are discussed in the following sections. 1. Pressure Filter

A pressure filter, Fig. 1, consists of a closed cylindrical vessel. The water flows downward through graded layers of sand, anthracite, or calcite supported by a gravel bed. Any suspended matter in the water will be trapped by the porous material. The clarified water is collected at the bottom.

After a filter has been in service for a period of time, the filter bed will become plugged with suspended matter that has been removed from the water. When this occurs, the filter must be backwashed. Backwashing consists of reversing the direction of flow through the filter, thus freeing the trapped material from the bed and washing it to waste.

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Figure 1 Pressure Filter

2. Filter-Aid Tubular Filter (Pressure Type)

This filter consists of a closed cylindrical housing containing several screen-type cylindrical tubes covered by socks made of dacron or polyethylene filter cloth, Fig. 2.

Figure 2 Tubular Filter-Aid Filter

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A slurry of filter-aid (usually diatomaceous earth) is fed through the inlet at the beginning of the filter cycle. The filter-aid coats the tubes and serves then as the filter medium. The upper part of the filter housing of the tubular filter is filled with air which is compressed by the water during the operation of the filter. When the filter starts to clog and requires backwashing, the inlet and outlet valves are closed and the drain valve is opened. The compressed air in the top of the filter forces water through the tubes in the reverse direction at high speed. This blasts the filter cake loose from the tubes and it is then removed through the drain.

3. Cartridge Filter

Cartridge filters are widely used for the removal of suspended solids, especially fine solids. The filter element consists of one or more renewable cartridges made of various types of finely woven filter material, Fig. 3.

Figure 3 Cartridge Filter

When the filtered-out solids start clogging the filter and the pressure drop between the inlet and outlet reaches a specified maximum value, the cartridge must be replaced.

Filters are frequently used in conjunction with settling tanks, in which case most of the suspended impurities are allowed to settle out in the settling tank and the water is then passed through the filter to remove any remaining impurities.

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In the case of very fine particles, which will remain suspended almost indefinitely, it is necessary to use coagulants. These are chemicals that cause the fine particles to gather together into a larger mass which will settle out more readily.

The coagulant when added to the water will produce a spongy substance known as floc. The floc has a very large surface area that both attracts and traps the very fine suspended particles in the water thus forming larger particles.

The most commonly used coagulants are aluminum sulphate Al2(SO4)3, aluminum hydroxide Al(OH)3 and sodium aluminate Na2Al2O4.

Water Softening Water softening is used to treat scale forming solids. These are either completely removed from the water, or replaced with “softer” salts which can then be removed from the boiler as bottom blowdown. In this module we shall consider only some of the common types of water softeners. 1. Lime-Soda Softeners

The lime-soda softener is a method of removing scale-forming dissolved solids such as calcium and magnesium compounds from the feedwater. Calcium hydroxide Ca(OH)2 commonly called lime, and sodium carbonate Na2CO3 commonly called soda, are added to the water causing the scale-forming dissolved solids to precipitate (drop out of solution) in the softener.

Lime-soda softeners are classified into two general types according to whether the water is heated or not during the process. Cold process softening is the name given to the type involving unheated water and hot process softening refers to the type where the water is heated during the treatment. Of the two, the hot process is more commonly used for boiler feedwater treatment.

The sketch in Fig. 4 represents the general arrangement of the hot process softener. The raw water enters at the top where it is heated by exhaust or live steam and mixes with calcium hydroxide and sodium carbonate, which are also introduced at the top of the softener.

As the chemical reactions take place, precipitates form and settle as sludge at the bottom of the sedimentation tank, where they can be removed as required. Part of the sludge forms a blanket through which the water passes on its way to the outlet. Some of this sludge is recirculated to the top of the tank by means of a pump to aid in the formation of precipitates.

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Figure 4

Hot Process Lime-Soda Softener

The sludge is periodically blown down to remove the precipitates, and the softened water leaving the softener is usually passed through a pressure filter to remove any suspended matter.

An advantage of the hot process softener is that large quantities of water can be treated in a relatively small unit. The high temperature obtained during the process increases the efficiency of the chemical treatment.

2. Sodium Zeolite Softening

The sodium zeolite softener uses the principle of cation exchange to convert scale-forming calcium and magnesium salts dissolved in the water into soluble, nonscale-forming sodium salts.

When water contains certain compounds such as salts in solution, the compounds break down (dissociate) to form positively charged cations and negatively charged anions. For example, if the water contains the scale-forming salt, calcium sulphate (CaSO4), then it will be in solution in the form of Ca cations and SO4 anions.

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The same applies to the scale-forming magnesium salts such as magnesium bicarbonate, Mg(HCO3)2., In solution, this salt will be in the form of Mg cations and HCO3 anions.

By removing the calcium and magnesium cations from the water and replacing them with sodium (Na) cations, the salts are converted into sodium salts: namely, sodium bicarbonate (NaHCO3) and sodium sulphate (Na2SO4). These salts are highly soluble and do not produce scale in the boiler.

The sodium zeolite softener consists of a pressure tank partially filled with a granular exchange material called zeolite, a styrene-based resin. The zeolite is charged with a large amount of sodium cations. When water containing calcium and magnesium salts is passed through the zeolite, it removes the Ca and Mg cations from the water and exchanges them for Na cations. In other words, a cation exchange takes place resulting in the conversion of the hardness causing Ca and Mg salts into nonhardness causing soluble Na salts.

Expressed in chemical formulae, using the letter Z to represent the zeolite material, the softening process by cation exchange is as follows:

Ca(HCO3)2 + NaZ → CaZ + 2NaHCO3 or MgSO4 + Na2Z → MgZ + Na2SO4

When the zeolite material has given up all its Na cations in exchange for the Ca and Mg cations, it has to be regenerated before it can resume the softening process. The regeneration is done by removing the zeolite softener from service and filling it with a strong sodium chloride (NaCl) solution.

The zeolite then absorbs the Na cations from the brine and discards the Ca and Mg cations to the brine as shown in the following formulae:

CaZ + 2NaCl → NaZ + CaCl2 or MgZ + 2NaCl → Na2 + MgCl2

The brine, now containing the Ca and Mg cations, is then flushed to the sewer and the zeolite bed is rinsed out with water. The softener is then returned to service until regeneration is again necessary.

Fig. 5 shows the arrangement of the zeolite softener together with a regenerator brine tank. The zeolite or exchange material is supported on a bed of gravel or anthracite and is contained in a steel pressure tank.

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Figure 5 Zeolite Softener Arrangement

The four main steps in the operation of a zeolite softener are service, backwash, regeneration, and rinsing. (a) Service (Softening)

The hard water enters the top of the softener and travels downward through the bed of zeolite. The Ca and Mg ions of the salts in the water are exchanged for the Na ions held by the zeolite. The softened water leaves the softener at the bottom.

(b) Backwash

When the zeolite becomes exhausted, the softener is taken out of service and backwashed by manipulating the valves so that raw water enters at the bottom and flows upward through the bed to the wash water collector and then to waste. The backwashing also serves to separate and clean the bed.

(c) Regeneration

Untreated or raw water is admitted to an ejector or eductor. The water flowing through the ejector produces a vacuum, which draws the brine up from the regenerant or brine tank, and the brine is then forced into the softener just above the surface of the zeolite bed. The Na ions of the brine solution exchange place with the Ca and Mg ions held by the zeolite.

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(d) Rinsing

Raw water entering the top of the softener flows through the zeolite bed and washes the brine solution, now containing Ca and Mg ions, to waste. When hardness tests taken on the water leaving the softener show that all salts have been rinsed out, the softener is put back into service.

Instead of having a number of valves to control the operation of the softener, as shown in Fig. 5, many softeners have a master “multiport” valve. Fig. 6 illustrates how the various operations of the softener are controlled by placing a single lever in certain positions.

SERVICE - Raw water enters valve, flows from top of tank to bottom where softened water is collected and directed through valve to service.

BACKWASH - Raw water enters valve, flows from bottom of tank to top, returns to valve and out to drain.

REGENERATION - Raw water enters valve, draws in brine, flows down through the bed to bottom of tank, returns to valve and out to drain.

RINSING - Raw water enters valve, flows from top of tank to bottom where water is directed through valve to drain.

Figure 6 Master Valve Positions

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3. Demineralizers

Although not as commonly used for water treatment applications as the sodium zeolite softener, demineralizers are used when mineral-free water is required.

Basically, a demineralizer consists of two ion exchangers as shown in Fig. 7. The first exchanger is a hydrogen cation exchanger. It works on the same principle as the sodium zeolite exchanger described above; however, instead of exchanging the cations of the salts in the water for sodium ions, the exchange material supplies hydrogen ions. The exchanger not only removes calcium and magnesium cations, but it also removes sodium cations as shown in the following formulae:

CaSO4 + H2Z → CaZ + H2SO4

Ca(HC3)2 + H2Z → CaZ + 2H2O + CO2

2NaCl + H2Z → Na2Z + 2HCl

As can be seen in these formulae, the salts dissolved in the water entering the cation exchanger are converted into acids, such as sulphuric and hydrochloric acid, as well as into carbon dioxide and water.

Figure 7 Demineralizer Arrangement

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The acid containing water now enters the second exchanger which contains anion exchange material charged with hydroxide (OH)- anions. When the water passes through the exchange bed, the anions of the acids (the acid radicals) are exchanged for hydroxide anions, and water is formed as shown below.

H2SO4 + Z(OH)2 → ZSO4 + 2H2O

2HCl + Z(OH2) → ZCl2 + 2H2O Summing up the process in a demineralizer:

1. In the cation exchanger, the cations of the salts in the water are exchanged for hydrogen ions converting the salts into acids, CO2 and water.

2. In the anion exchanger, the anions of the acids in the water are exchanged

for hydroxide ions converting the acids into water.

3. The water leaves the demineralizer practically free of minerals. When the exchange materials in the ion exchangers become exhausted, they have to be regenerated in a similar manner as described for the sodium zeolite softener. However, instead of using a brine solution, the cation exchanger is recharged by passing a diluted solution of sulphuric acid through the bed. The following formula shows one of the reactions:

CaZ + H2SO4 → H2Z + Ca SO2 Thus the exchange material recharges by rejecting the Ca, Mg and Na cations and attracting the H2 cations from the acid. The salts formed during regeneration are rinsed out and discharged to waste. The anion exchanger is regenerated by passing a diluted solution of caustic soda through the bed, and the following chemical formula shows one of the reactions taking place:

ZSO4 + 2NaOH → Z(OH)2 + Na2SO4 Thus the exchange material rejects the acid radicals and attracts the hydroxide anions. The sodium salts formed are rinsed out and discharged to waste. After regeneration of both exchangers, the demineralizer is ready for service again.

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Troubleshooting of Zeolite Softeners The capacity of zeolite softeners depends on several factors such as:

1. The quantity, type, and condition of the exchange material.

2. The amount of dissolved minerals in the water.

3. The amount and strength of regenerant used.

4. The mechanical condition of the softener and regenerating equipment. Problems may be encountered in the operation of a softener. These problems are usually indicated by either a reduction in capacity or incomplete softening of the water. The causes of these problems can be divided broadly into the following groups:

1. A change in water quality - surface waters usually change in hardness from season to season. This change affects softener capacity. For example, if a sodium zeolite softener is rated to soften 11 350 L of water (2500 Imp. gal, 3000 US gal) when the hardness of the raw or untreated water is 120 ppm (parts per million), and the hardness of the water increases to 160 ppm, the capacity of the softener will be reduced to 120/160 x 11 350 L = 8510 L (1875 Imp. gal, 2250 US gal). On the other hand, a drop in hardness would increase the capacity. For example, if the hardness drops to 100 ppm, the capacity of the above softener would increase to 120/100 x 11 350 L = 13 620 L (3000 Imp. gal, 3600 US gal). Changes in capacity due to varying hardness in the raw water are to be expected and should not worry the operator.

2. Improper flow rates - the maximum flow rates during the softening and

backwash cycles should not exceed those recommended by the manufacturer. An excessive flow during softening results in insufficient softening. Excess flow during backwash will cause zeolite material to be washed out of the softener.

3. Improper brine injection during regeneration - the amount and strength of

the brine injected into the softener during regeneration should be as recommended by the manufacturer. Erratic brine injection can be caused by an insufficient amount of salt for dissolution in the tank, a worn injector, a defective float valve in the tank, or a malfunctioning sequence timer.

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4. Fouled exchange material - contaminants in the supply water, such as suspended solids, iron, oil, or microbiological growth in the zeolite bed hampers the proper reaction in the softener, resulting in reduced output or improper softening.

5. Mechanical defects - broken or plugged baffles, distributors or collecting

headers cause poor distribution of water or brine solution and channelling. The result is again reduced capacity and poor softening. Leaking valves cause contamination of the softened water by the raw water.

6. Loss of exchange material by attrition - there is a slow wearing down of

the surface of the beads. The amounts of fines produced and washed out of the softener is about 3% per year of the total amount of beads, resulting in a corresponding drop in capacity of 3% per year.

Deaeration Corrosion in boilers is caused by dissolved oxygen and dissolved carbon dioxide in the feedwater. The term mechanical deaeration refers to the removal from the water of dissolved gases, such as oxygen and carbon dioxide, by raising the temperature of the water to the boiling point and allowing the gases thus released to be vented to the atmosphere. The process is assisted by scrubbing the water with a flow of steam in order to sweep away the released gases from the water. The removal of these gases is carried out in a vessel known as a deaerator and after deaeration, the water is pumped from the deaerator to the boiler. An outline sketch of a deaerator is shown in Fig. 8 and a cutaway view is shown in Fig. 9.

Figure 8 Deaerator (Outline)

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Figure 9 Deaerator (Cutaway View)

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Referring to Figs. 8 and 9, the inlet water enters the heater compartment through spray nozzles and is heated by steam rising from the scrubber compartment. The heated water then flows down into the scrubber section where it is scrubbed by the steam entering through a central pipe. The steam and the gases scrubbed from the water then rise through the heating compartment, where most of the steam is condensed, to the vent condenser chamber. The water spray in this chamber condenses any remaining steam and the gases then pass out through the vent. The steam used for heating and scrubbing of the water in most types of deaerators is exhaust steam from some source. If exhaust or waste steam is not available then live steam can be used after reduction of pressure. The operating pressure is usually about 70 or 100 kPa.