water treatment for boiler water

8/18/2019 Water Treatment for Boiler Water

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Bo i l e r s B a s i

B o i l e r W a t e r T r ea t m e n

Chapter 5 Boiler Water Treatment

5.1 Objective of Water Treatment

Of the many uses for energy in the world - in industry, in transportation, in homes and

commercial buildings, the largest portion of total use is directed toward producing

steam through the combustion fossil fuels. Utilities account for the greatest share of

this, but industrial plants also produce enormous quantities of steam for process uses,

often generating electric power through turbines as a by-product (Cogeneration).

The treatment of water for steam generation is one of the most sophisticated

branches of water chemistry. An understanding of the fundamentals of boiler water

chemistry is essential to the power engineer who continually strives to increase the

efficiency of the boilers and steam using equipment.

The pressure and design of a boiler determine the quality of water requires for

steam generation. Municipal or plant water of good quality for domestic use isseldom good enough for boiler feed water. These sources makeup are nearly always

treated to reduce contaminants to acceptable level, in addition corrective

chemicals are added to the treated water to counteract any adverse effects of the

remaining trace contaminants. The sequence of treatment depends on the type of

concentration of the contaminants found in the water supply and the desired

quality of the finished water to avoid the three major boiler system problems:

corrosion, deposits and carryover.

5.2 Deposits

Deposits, particularly scale, can form on any water washed equipment surface

especially on boiler tubes as the equilibrium conditions in the water contacting these

surfaces are upset by an external force, such as heat. Each contaminant had an

established solubility in water and will precipitate when it had been exceeded. If

water is in contact with a hot surface and the solubility of the contaminant is lower

at higher temperatures, the precipitate will form on the surface, causing scale. The

most common components of boiler deposits are calcium phosphate, calcium

carbonate, magnesium hydroxide, magnesium silicate, various forms of iron oxide

and alumina.

At the high temperatures found in a boiler, deposits are a serious problem, causing

poor heat transfer and a potential for boiler tube failure. In low pressure boilers with

low heat transfer rates, deposits may build up to a point where they completely

occlude the boiler tube.

In modern intermediate and higher pressure boilers with heat transfer rates in excess

of 1.2 kj/m2 /hr, the presence of even extremely thin deposits will cause a serious

elevation in the temperature of the tube metal. The deposits coating retards the flow

of heat from the furnace gases into the boiler water. This heat resistance results in a

rapid*rise in metal temperature to the point at which failure can occur.


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Bo i l e r s B a s i

B o i l e r W a t e r T r ea t m e n

Thickness of Scale Increase in fuel consumption due to scale

0.5 mm 2%

1 mm 4%

2 mm 6%

4 mm (0.125") 10%8 mm ( 0.25") 20%

16 mm (0.5”) 40%

30 mm (1") 80%

Table 5.1 shows the increase in fuel consumption due to scales formation

5.3 Corrosion

The second major water related boiler problem is corrosion, the most common

example being the attack ot steel by oxygen. This occurs in water supply systems,pre-boiler systems, boilers, condensate return lines, and in virtually any portion of the

steam cycle where oxygen is present. Oxygen attack is accelerated by high

temperature and low pH. A less prevalent type of corrosion is alkali attack, which

may occur in high pressure boilers where caustic can concentrate in a local area of

steam bubble formation because of the presence of porous deposits.

Some feed water treatment chemicals, such as chelants, if not properly applied can

corrode feed water piping, control valves and even the boiler internals.

While the elimination of oxygen from boiler feed water is the major step in controlling

boiler corrosion, corrosion can still occur, an example is the direct attack by steam of

the boiler steel surface at elevated temperatures, according to the following

reactions:

4H2O + 3Fe→ Fe3O4 + 4H2 ↑

This attack can occur at steam blanketed boiler surfaces where restricted boiler

water flow causes overheating. It may also occur in superheater tubes subjected to

overheating. Since this corrosion reaction produces hydrogen, a device for analyzing

hydrogen in steam is useful as a corrosion monitor.

The third major problem related to boiler operations is carryover from the boiler into

the steam system. This may be a mechanical effect, such as boiler water spraying

around a broken baffle, it may be caused by the volatility of certain boiler water

salts, such as silica and sodium compounds, or it may be caused by foaming.

Carryover is most often a mechanical problem, and the chemicals found in thesteam are those originally present in the boiler water, plus the volatile components

that distill from the boiler even in the absence of spray.


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Bo i l e r s B a s i c s

B o i le r W a t e r T r e a t m e n t

5.4 Water Impurities

- The common impurities in raw water can be classified as follows:

1 - Suspended solids

These are substances that exist in water as suspended particles. They are usually

mineral, or organic in origin.These substances are not generally a problem as they can be filtered out.

2 - Hardness

Water is referred to as being either 'hard' or 'soft'. Hard water contains scale-

forming impurities while soft water contains little or none. The difference can easilybe recognized by the effect of water on soap. Much more soap is required to make

lather with hard water than with soft water.

- Alkaline hardness (also known as temporary hardness)

Calcium and magnesium bicarbonates are responsible for alkaline hardness. The

salts dissolve in water to form an alkaline solution. When heat is applied, they

decompose to release carbon dioxide and soft scale or sludge.The term 'temporary hardness' is sometimes used, because the hardness is

removed by boiling. This effect can often be seen as scale on the inside of anelectric kettle.

Fig. 5.1 Alkaline or temporary hardness

- Non-alkaline hardness and carbonates (also known aspermanent hardness)

This is also due to the presence of the salts of calcium and magnesium but in theform of sulfates and chlorides. These precipitate out of solution, due to their

reduced solubility as the temperature rises, and form hard scale, which is difficultto remove.


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In addition, the presence of silica in boiler water can also lead to hard scale, which

can react with calcium and magnesium salts to form silicates which can severely

inhibit heat transfer across the fire tubes and cause them to overheat.

Fig. 5.2 Non-alkaline or permanent hardness (scale + carbonic acid)

- Total hardness

Total hardness is not to be classified as a type of hardness, but as the sum of

concentrations of calcium and magnesium ions present when these are both

expressed as CaC03. If the water is alkaline, a proportion of this hardness, equal inmagnitude to the total alkalinity and also expressed as CaC03, is considered as

alkaline hardness, and the remainder as non-alkaline hardness.

Fig. 5.3 Total hardness

3 - Dissolved solids

These are substances that will dissolve in water. The principal ones are the

carbonates and sulfates of calcium and magnesium, which are scale-forming whenheated. There are other dissolved solids, which is non-scale forming.

In practice, any salts forming scale within the boiler should be chemically altered

so that they produce suspended solids, or sludge rather than scale.

4 - Dissolved gases

Oxygen and carbon dioxide can be readily dissolved by water. These gases areaggressive instigators of corrosion.

5- Silica


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Table 5.2 Impurities Found In Water

Name Symbol Common name Effect

Calcium carbonate CaCO2 Chalk, limestone Soft scale

Calcium bicarbonate Ca(HCO3)2 Soft scale+CO2

Calcium sulphate CaS04 Gypsum, plaster of paris Hard scale

Calcium chloride CaCl2 Corrosion

Magnesium carbonate MgCO3 Magnesrte Soft scale

Magnesium sulphate MgSO4 Epsom silts Corrosion

Magnesium bicarbonate Mg(HCO3)2 Scale, corrosion

Sodium chloride NaCl Common salt Electrolysis

Sodium carbonate Na2C03 Washing soda or soda Alkalinity

Sodium bicarbonate NaHCO3 Baling soda Priming, foaming

Sodium hydroxide NaOH Caustic soda Alkalinity, embrittlement

Sodium sulphate Na2SO4 Gluaber salts Alkalinity

Silicon dioxide SiO2 Silica Hard scale


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5.5 Methods of Water Treatment

There are three basic means of boiler feed water treatment:

External Treatment: Treatment of water makeup, condensate or bothbefore it enters the boiler, to reduce or eliminate chemicals (hardness or

silica), gases or solids.

Internal Treatment: Treatment of the boiler feed water, boiler water,

steam or condensate with corrective chemicals.

Blow-down: Control of the concentration of chemicals in the boiler waterby bleeding off a portion of water from the boiler.

5.5.1 External Treatment

Most of the unit operations of water treatment (filtration, coagulation/flocculation,precipitation, adsorption, ion exchange, evaporation, degasification & membrane

separation) can be used alone or in combination with others to adapt any watersupply to any boiler system. The suitability of the processes available is judged by

the results they produce and the cost involved.

The amount of impurities present is extremely small and they are usually

expressed in any water analysis in the form of parts per million (ppm), by

weight or alternatively in milligrams per liter (mg/l)

5.5.1.1 Treatment of Suspended Solids

The removal of suspended solids is accomplished by

coagulation/flocculation, filtration or precipitation.

Other unit processes except direct reaction, usually require

prior removal of solids. For example water to be processed

by ion exchange should contain less than 10 mg/L

suspended solids to avoid fouling of the exchanger and

operating problems.

Fig 5.4 A Typical Filter


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5.5.1.2 Treatment of Hardness

Ion Exchange is the common method used for hardness treatment and is divided

into:

- Base Exchange- De-alkalization- Demineralization

An ion exchanger is an insoluble material normally made in the form of resin beads

of 0.5 to 1.0 mm diameter. The resin beads are usually employed in the form of apacked bed contained in a glass reinforced plastic pressure vessel. The resin beads

are porous and hydrophilic - that is, they absorb water. Within the bead structure are

fixed ionic groups with which are associated mobile exchangeable ions of oppositecharge. These mobile ions can be replaced by similarly charged ions, from the salts

dissolved in the water surrounding the beads.

- Base Exchange Softening

This is the simplest form of ion exchange and also the most widely usedespecially in Fire Tube Boilers. The resin bed is initially activated (charged) by

passing a 7 -12% solution of brine (sodium chloride or common salt) through it,which leaves the resin rich in sodium ions. Thereafter, the water to be softened is

pumped through the resin bed and ion exchange occurs. Calcium and magnesiumions displace sodium ions from the resin, leaving the flowing water rich in sodium

salts. Sodium salts stay in solution at very high concentrations and temperatures and

do not form harmful scale in the boiler.

From Figure 5.5 it can be seen that the total hardness ions are exchanged forsodium. With sodium Base Exchange softening there is no reduction in the total

dissolved solids level (TDS in parts per million or ppm) and no change in the pH. Allthat has happened is an exchange of one group of potentially harmful scale forming

salts for another type of less harmful, non-scale forming salts. As there is no changein the TDS level, resin bed exhaustion cannot be detected by a rise in conductivity

(TDS and conductivity are related). Regeneration is therefore activated on a time or

total flow basis.


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Fig 5.5 Base Exchange Softening

Softeners are relatively cheap to operate and can produce treated water reliably formany years. They can be used successfully even in high alkaline (temporary)

hardness areas provided that at least 50% of condensate is returned. Where there islittle or no condensate return, a more sophisticated type of ion exchange is

preferable.Sometimes a lime/soda softening treatment is employed as a pre-treatment before

Base Exchange. This reduces the load on the resins.

SOFTENER SIZING FORMULA:

C = M T H /R

C = Capacity of softener in cubic feet of resin

M = Makeup water volume per hour in gallons; the volume needed to be softened(8.34 pounds per gallon)

T = Time in hours desired between regeneration cyclesH = Hardness of water in grains (17.1 ppm per grain hardness)

R = Resin Capacity per cubic foot (this is virtually always 30,000 grains)


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Fig 5.6 A typical Softener unit

- De-alkalization

Disadvantage of base exchange softening is that there is no reduction in the TDS andalkalinity. This may be overcome by the prior removal of the alkalinity and this is

usually achieved through the use of a dealkalizer.

There are several types of dealkalizer but the most common variety is shown inFigure 5.7 It is really a set of three units, a dealkalizer, followed by a degasser and

then a base exchange softener.

Dealkalizers are sometimes called 'split-stream' softening. A dealkalizer would

seldom be used without a base exchange softener, as the solution produced is acidicand would cause corrosion, and any permanent hardness would pass straight into

the boiler.A dealkalization plant will remove temporary hardness as shown in Figure 5.8. This

system would generally be employed when a very high percentage of make-up wateris to be used.


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Fig 5.7 A De-alkalizing unit

Fig 5.8 Dealkalization Process


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- Demineralisation

This process will remove virtually all the salts. It involves passing the raw water

through both cation and anion exchange resins (Figure 5.9). Sometimes the resinsmay be contained in one vessel and this is termed 'mixed bed' demineralisation.

The process removes virtually all the minerals and produces very high quality water

containing almost no dissolved solids. It is used for very high pressure boilers(Water Tube Boilers) such as those in power stations.

If the raw water has a high amount of suspended solids this will quickly foul the ionexchange material, drastically increasing operating costs. In these cases, some pre-

treatment of the raw water such as clarification or filtration may be necessary.

Fig 5.9 Demineralisation Process


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5.5.1.3 Treatment of Dissolved Solids

The most effective method used in treatment of dissolved solids is Reverse Osmosis,other methods like Demineralisation & Dealkalization (discussed before), are not

effective for TDS > 1500 ppm.

- Reverse Osmosis

Reverse Osmosis is based upon the fundamental pursuit for balance. Two fluids

containing different concentrations of dissolved solids that come in contact with eachother will mix until the concentration is uniform. When these two fluids are separated

by a semi permeable membrane (which lets the fluid flow through, while dissolvedsolids stay behind), a fluid containing a lower concentration will move through the

membrane into the fluids containing a higher concentration of dissolved solids.After a while the water level will be higher on one side of the membrane. The

difference in height is called the osmotic pressure. By pursuing pressure upon the

fluid column, which exceeds the osmotic pressure, one will get a reversed effect.Fluids are pressed back through the membrane, while dissolved solids stay behind in

the column. Using this technique, a larger part the salt content of the water can beremoved.

Fig 5.10 Osmosis & Reverse Osmosis phenomena

Reverse Osmosis is a technique that is mainly applied during drinking water

preparation. The process of drinking water preparation from salty Seawater iscommonly known. Besides that, Reverse Osmosis is applied for the production of


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higher flow than the flow that is theoretically required will than be implicated to keep

the feed pressure continual. A feed pump that increases the feed pressure by 25%

will be satisfactory in practice.When the system is started up, the initial situation is recorded. All relevant

parameters should be registered and noted in a log. Based on this data theperformance of the installation can be examined and regulated after the system has

been put into action.

Fig 5.12 A Typical RO Desalination Plant Showing Membranes Banks


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5.5.1.4 Treatment of Dissolved Gases

Degasifiers

Degasifiers are commonly used to remove gas mechanically rather than chemically.

Blower types are used for CO2 removal at ambient temperatures following acid orhydrogen-exchange units. Vacuum degasifiers provide the same extent of CO2

removal, but also reduce O2 to less than 0.5 to 1.0 mg/L, offering corrosion

protection, especially if the vacuum degasifier is part of a demineralizing system.Steam-scrubbing degasifiers, called deaerating heaters, usually produce an

effluent free of CO2 with O2 concentrations in the range of 0.005 to 0.01 mg/L. Directreaction of this low residual with catalyzed sulfite, hydrazine, or hydrazine sub-

stitutes (all-volatile oxygen-reducing compounds) eliminates O2 completely toprevent pre-boiler corrosion.

De-aerators

De-aerator is the equipment used to get rid of any dissolved gases (O2) inthe boiler feed water.

Theory & Principal of Work

If a liquid is at its saturation temperature, the solubility of a gas in it is zero,

although the liquid must be strongly agitated or boiled to ensure it is completely

deaerated.This is achieved in the head section of a deaerator by breaking the water into as

many small drops as possible, and surrounding these drops with an atmosphere ofsteam. This gives a high surface area to mass ratio and allows rapid heat transfer

from the steam to the water, which quickly attains steam saturation temperature.This releases the dissolved gases, which are then carried with the excess steam to be

vented to atmosphere. (This mixture of gases and steam is at a lower thansaturation temperature and the vent will operate thermostatically). The deaerated

water then falls to the storage section of the vessel.

A blanket of steam is maintained above the stored water to ensure that gases arenot re-absorbed.

De-aerators can eliminate the usage of chemicals thus decrease boilers

running & operational costs.


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Fig 5.13 Atmospheric De-Aerator

Fig 5.14 Pressure De-Aerator


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Fig 5.15 Pressure De-Aerator Installation

5.5.1.5 Treatment of Silica

Silica reduction is not always necessary, especially in the absence of a condensingturbine. Low concentrations of silica can sometimes produce sticky sludge in low

pressure boilers treated with phosphate.

A makeup treatment process may be selected to provide just the proper degree ofsilica reduction required by steam system.

A summary for methods of silica treatment is shown in Table 5.3.

Residuals mg/l

Process Silica Hardness Alkalinity TDS

Original water 10 160 135 275

Cold lime & iron salts 6 Nil 35 225

Cold lime-soda 7-8 65 65 210Hot lime-soda 1 17 40 145

Na2X + anion exchanger 1 Nil 20 290

Demineralization 0.05 Nil 1-2 1-2

Table 5.3 Silica Reduction Processes


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5.5.2 Internal Treatment

Scale formation within a boiler is controlled by one of four chemical programs:

coagulation (carbonate), phosphate residual, chelation, or coordinated phosphate.

5.5.2.1 Coagulation Program

In this process, sodium carbonate, sodium hydroxide, or both are added to the boiler

water to supplement the alkalinity supplied by the makeup, which is not softened.

The carbonate causes deliberate precipitation of calcium carbonate under favorable,controlled conditions, preventing deposition at some subsequent point as scale.

Under alkaline conditions, magnesium and silica are also precipitated as magnesiumhydroxide and magnesium silicate. There is usually a fairly high concentration of

suspended solids in the boiler water, and the precipitation occurs on these solids.This method of treatment is used only with boilers (usually firetube design) using

high-hardness feed water and operating below 17 bars. This type of treatment must

be supplemented by some form of sludge conditioner. Even with a supplementalsludge conditioner, heat transfer is hindered by deposit formation, and blowdown

rates are excessive because of high suspended solids. Coagulation programs arebecoming obsolete as pretreatment systems become more common and competitive

with the high internal treatment cost.

5.5.2.2 Phosphate Program

Where the boiler pressure is above 17 bars, high concentrations of sludge are

undesirable. In these boilers, feed water hardness should be limited to 60 mg/L, andphosphate programs are preferred. Phosphate is also a common treatment below 17

bars with soft makeup.

A sodium phosphate compound is fed either to the boiler feed water or to the boilerdrum, depending on water analysis and the preboiler auxiliaries, to form an insoluble

precipitate, principally hydroxyapatite, Ca10 (PO4)6(OH)2. Magnesium and silica are

precipitated as magnesium hydroxide, magnesium silicate (often combined as3MgO.2SiO2.2H2O), or calcium silicate. The alkalinity of the makeup is usually

adequate to produce the OH for the magnesium precipitation. Phosphate residualprograms which produce high suspended solids require the addition of a sludge

conditioner / dispersant. Because these programs restrict heat transfer, owing to thedeposition of calcium and magnesium salts, precipitation programs of this type are

often replaced with solubilizing treatments such as che-lants and polymer /dispersants.

5.5.2.3 Chelant Programs

A chelate is a molecule similar to an ion exchanger; it is low in molecular weight andsoluble in water. The sodium salts of ethylene diamine tetraacetic acid (EDTA) and

nitrilotriacetic acid (NTA) are the chelating agents most commonly used for internalboiler treatment. These chelate (form complex ions with) calcium and magnesium.

Because the resulting complex is soluble, this treatment is advantageous in


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minimizing blowdown. The higher cost compared to phosphate usually limits the use

of chelates to feed waters having low hardness. There is the risk that breakdown of

the organic molecule at higher temperatures could create a potential problem ofcontrol that could result in corrosion, so chelate programs are usually limited to

boilers operating below 100 bars. The addition of polymers as scale control agentsincreases the effectiveness of chelate programs.

It also reduces the corrosion potential by reducing the chelant dosage belowtheoretical requirements, so that there is no chelant residual in the boiler water.

Chelates can react with oxygen under boiler water conditions, which can increase the

cost of a chelate program substantially. Overfeed of chelates and concentrationmechanisms in the boiler can lead to severe localized corrosion and subsequent unit

failure.

5.5.2.4 Coordinated Phosphate Program

In high-pressure, high heat transfer rate boilers, the internal treatment programmust contribute little or no solids. The potential for caustic attack of boiler metal

increases with increasing pressure, so free caustic alkalinity must be minimized. Thecoordinated phosphate program is chosen for these conditions. This differs from the

standard program in that the phosphate is added to provide a controlled pH range inthe boiler water as well as to react with calcium if hardness should enter the boiler.

Trisodium phosphate hydrolyzes to produce hydroxide ions:

Na3PO4 + H2O 3Na+ + OH- + HPO4

2-

This cannot occur with the ionization of disodium and monosodium phosphate:

Na2HPO4 2Na+ + HPO4

2-

Na2HPO4 Na+ + H+ + HPO4

2-

The program is controlled by feeding combinations of disodium phosphate withtrisodium or monosodium phosphates to produce an optimum pH without the

presence of free OH-. To successfully control a coordinated phosphate program, thefeed water must be extremely pure and of consistent quality. Coordinated phosphate

programs do not reduce precipitation; they simply cause precipitation of lessadherent calcium phosphate in the absence of caustic. A dispersant must be added

to condition deposits that would otherwise reduce the heat transfer rate. The

coordinated phosphate program was first developed for high-pressure utility boilers,and most experience with this program has been gained in this field.

5.5.2.5 COMPLEXATION/DISPERSION

The newest addition to internal treatment technology is the use of synthetic organicpolymers for complexation and dispersion. This type of program can be used to 100

bars and is economical in all low-hardness feed water systems typical of thoseproduced by ion exchange. Heat transfer rates are maximized because these

polymers produce the cleanest lube surfaces of any of the available internaltreatment programs. This treatment solubilizes calcium, magnesium, and aluminum,

and maintains silica in solution while avoiding corrosion potential side effects as


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determined by hydrogen levels in the steam. Iron particulates returned from the

condensate system are likewise dispersed for removal via blowdown. A simple

measure of ion transport is used to demonstrate on-line performance of thisprogram.

5.6 Boiler Carryover

The third major problem related to boiler operations is carryover from the boiler into

the steam system. This may be a mechanical effect, such as boiler water spraying

around a broken baffle; it may be caused by the volatility of certain boiler watersalts, such as silica and sodium compounds; or it may be caused by foaming.

Carryover is most often a mechanical problem, and the chemicals found in the steamare those originally present in the boiler water, plus the volatile components that

distill from the boiler even in the absence of spray.

Carryover can be caused by two factors:

1. Priming

This is the ejection of boiler water into the steam take-off and is generally

due to one or more of the following:

- Operating the boiler with too high a water level.

- Operating the boiler below its design pressure; this increases the volume and thevelocity of the steam released from the water surface.

- Excessive steam demand.

2. Foaming

This is the formation of foam in the space between the water surface and the steamoff-take. The greater the amount of foaming, the greater the problems which will beexperienced.

The following are indications and consequences of foaming:

- Water will trickle down from the steam connection of the gauge glass; this makes it

difficult to accurately determine the water level.- Level probes, floats and differential pressure cells have difficulty in accurately

determining water level.

- Alarms may be sounded, and the burner(s) may even 'lockout'. This will requiremanual resetting of the boiler control panel before supply can be re-established.

These problems may be completely or in part due to foaming in the

boiler. However, because foaming is endemic to boiler water, a betterunderstanding of foam itself is required:

- Surface definition - Foam on a glass of beer sits on top of the liquid, and the

liquid / foam interface is clearly defined. In a boiling liquid, the liquid surface is

indistinct, varying from a few small steam bubbles at the bottom of the vessel, tomany large steam bubbles at the top.


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- Agitation increases foaming - The trend is towards smaller boilers for a given

steaming rate.

Smaller boilers have less water surface area, so the rate at which steam is releasedper square meter of water area is increased. This means that the agitation at the

surface is greater. It follows then that smaller boilers are more prone to foaming.

- Hardness - Hard water does not foam. However, boiler water is deliberatelysoftened to prevent scale formation, and this gives it a propensity to foam.

- Colloidal substances -Contamination of boiler water with a colloid in suspension,for example, milk, causes violent foaming. Note: Colloidal particles are less than

0.0001 mm in diameter, and can pass through a normal filter.

- TDS level - As the boiler water TDS increases, the steam bubbles become morestable, and are more reluctant to burst and separate.

Corrective action against carryover

The following alternatives are open to the Engineering Manager to minimise

foaming in the boiler:

1- Operation - Smooth boiler operation is important. With a boiler operating under

constant load and within its design parameters, the amount of entrained moisture

carried over with steam may be less than 2%.If load changes are rapid and of large magnitude, the pressure in the boiler can drop

considerably, initiating extremely turbulent conditions as the contents of the boilerflash to steam. To make matters worse, the reduction in pressure also means that

the specific volume of the steam is increased, and the foam bubbles areproportionally larger.

If the plant conditions are such that substantial changes in load are

normal, it may be prudent to consider:

Modulating boiler water level controls if on / off are currently fitted. 'Surplussing controls' that will limit the level to which the boiler pressure is

allowed to drop. A steam accumulator. 'Feed-forward' controls that will bring the boiler up to maximum operating

pressure before the load is applied. 'Slow-opening' controls that will bring plant on-line over a pre-determined

period.

2- Chemical control - Anti-foaming agents may be added to the boiler water. Theseoperate by breaking down the foam bubbles. However, these agents are not effective

when treating foams caused by suspended solids.

3- Control of TDS - A balance has to be found between:

- A high TDS level with its attendant economy of operation.

- A low TDS level which minimises foaming.

4- Safety - The dangers of overheating due to scale, and of corrosion due todissolved gases, are easy to understand. In extreme cases, foaming, scale and


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sludge formation can lead to the boiler water level controls sensing improper levels,

creating a danger to personnel and process alike.

Summary

- The amount of TDS in the boiler water depends on the type of feedwater treatmentthat is used. However, for any given feedwater condition control of TDS in the boiler

water is by blowdown. The type of treatment will depend on the source of thefeedwater, its constituents and steam generator requirements.

- The feedwater treatment can include water softeners, demineralizers and/or

polishers to eliminate hardness, silica, other dissolved solids, and suspended solids.

Organic material ingress control equipment may also be provided. A deaeratingheater is also included to minimize the dissolved oxygen. The feedwater treatment

should be compatible with the boiler water treatment and other cycle requirements.

It should be noted that systems which include steam turbines, and superheaters, will

require higher purity feedwater than referenced in this document. In the case ofsteam turbines, the manufacturer of the turbine to be used should define the steamquality requirement needed from the boiler. It should further be noted that steam

quality requirements can differ between the various turbine manufacturers. Thus it is

important that the various equipment manufacturers (boiler, turbine, watertreatment, etc.) be selected, and communicate at an early stage in the project with

the owners representative regarding steam quality and water treatmentrequirements, in order to avoid later problems.

- The type of boiler water treatment usually varies with the operating pressure of the

boiler, the quality of the feedwater, and the boiler design. A "caustic phosphate"treatment is often used with feedwater having high solids, generally in boilers

operating at lower pressures. This treatment provides the chemical conditionsnecessary to cope with residual hardness and other scale forming constituents.

- Chelant treatments may be used with operating pressures up to about 1500 psig.With this treatment it is essential that oxygen in the feedwater be reduced to the

lowest possible levels through mechanical and chemical deaeration. It is alsoessential that the boiler be protected from oxygen during any outage periods.

- A "Coordinated Phosphate" treatment may be used at any operating pressure but is

generally used with boilers having high purity feedwater and operating at high

pressures. The phosphate treatment precipitates the hardness compound and alsocontrols the alkalinity and pH.

- A "Zero Solids" or "All Volatile" treatment does not add any dissolved solids to theboiler. The pH in the complete cycle is controlled by volatile amines such asammonia, morpholine and Cyclohexylamine. The zero solids treatment may be used

at all operating pressures in those boilers having very high purity (low solids)feedwater. However, this treatment does not provide any corrosion protection of the

boiler whenever the feedwater becomes contaminated. Accordingly, users of this

treatment should be prepared to inject. Phosphate in the boiler water or remove theboiler from service whenever there are condenser leaks or other contamination.


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- Chemical scavenging of oxygen may use sodium sulfite or a volatile oxygen

scavenger such as hydrazine for boilers operating at pressures up to 1500 psig. Onlyvolatile oxygen scavengers should be used at pressures above 1500 psig.

- If solid type chemicals, such as sodium sulfite, must be used for boiler water or

feedwater treatment, they should be injected downstream of the spray water takeoff.

- Other boiler water chemicals should be injected via the boiler chemical feed

connection. It is the responsibility of the plant operator to provide proper feedwaterand boiler water conditions which must reflect the particular requirements of each

installation.

- Consultation with boiler manufacture and water treatment specialists will bebeneficial in formulating a proper water treatment system.

RECOMMENDED BOILER WATER CONCENTRATION FOR FIRETUBE BOILERS

Boiler

Pressurepsig

Total

DissolvedSolidsppm

Total

Alkalinityppm as CaCO

3

Suspended

Solids2

max.ppm

Silica

1

ppm

Total

Iron1

max. (Fe)ppm

0 – 250 5000 - 3500 1200 - 900 100 150 - 100 10

251 – 350 4000 – 3000 900 - 700 25 120 – 100 8

351 - 500 3000 - 2500 800 - 600 10 80 - 50 5

NOTES:

(1) Maximum values may not be achievable due to plant operatingconditions or feedwater characteristics.

(2) Critically affected by operating conditions and year of boiler

manufacture.


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Fig 5.16 Boiler Room Arrangement


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Boilers Basics

Boiler Fee d Wa ter Trea tme nt


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Boilers Basics

Boiler Fee d Wa ter Trea tme nt

water treatment for boiler water

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