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Cha.p1ier VII

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It is necessary to examine some of the basic theories in order to understand the various problems associatedwith water treatment.

Total Hardness

Temporary hardness Permanent hardness

Calcium Bicarbonate Calcium Sulphate

Magnesium Bicarbonate Magnesium Chloride

Temporary Hardness (Alkaline Hardness) is due to bicarbonates of Calcium and Magnesium, which are

Alkaline in nature. They are temporary, because when heated they rapidly break down, to form Carbon

Dioxide and the corresponding carbonated deposit as scale.

Permanent Hardness (Non-Alkaline Hardness) is due mainly to sulphates and chlorides of Calcium and

Magnesium which are acid in nature. They are "permanent" and do not break down, but under certain

conditionsdeposit to form scale of varying hardnesses.

Water Treatment Fundamentals

Water can adversely affect metal components under the operational conditions normally found in steam

boilers and other heat exchange devices. The extent of deterioration depends on the specific characteristics

of the water and the system in which it is being used.

In order to counteract the detrimeirtal properties normally attributed to its contaminants (dissolved, sus-

pended solids and dissolved gases), special chemical treatment programs have been devised. Accepted .water treatment processes are constantly being upgraded, and new methods are being developedto comple-

ment and/or replace older ones. Although water from marine evaporators and boiler condensate returns is

essentially 'pure', minute quantities of potentially harmful salts and minerals can be carried by feedwater

into the boiler,wherethey will accrue, ultimatelyresulting in serious problems in the steam generatingunit. In

addition',the water can also contain dissolved gases, i.e. Carbon dioxide and Oxygen, which can result in

corrosion of the system.

Using unprocessed fresh water (e.g. shore water) as a makeup source can present some problems.

Certain contaminants, which are naturally present in fresh water, can be extremely destructive in boiler

systems, if not dealt with promptly and effectively. Soluble salts such as chlorides, sulphates and carbonates

are present as electrolytes, in the untreated water, leadingto galvanic and other types of corrosion, depending

on the conditions in the system. In addition, sulphates and carbonates have the potential to form insoluble,

adherent, insulating "hard water" scale deposits on heat exchanger surfaces.

~Iements which affect Boiler Treatment

Mostdissolvedmineralimpuritiesin waterare presentinthe formof ions.Theseionscontainan electricalchargewhichis eitherpositive(cation)or negative(anion).Theseionscanjoin togetherto formchemicalcompounds.

To know which ions will cOmbine, we need to know their electrical charge. Ions of concern to us include

the following.

Positive ions

Sodium Na+

Calcium Ca++

Magnesium Mg++

Hydrogen H+

Negative ions

ChlorideCI-

Bicarbonate HCO3

Carbonate C03-

Hydroxide OH-

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Cations will combine only with anions. An example of this combining of ions is the action between Cal-

cium and carbonate. The chemical compound which forms is Calcium Carbonate. Other impurities, which

will affect the boiler water treatment control, include Copper, Iron oxides, oil and dissolved gases.

Copper

Copper is introduced intoa system by corrosion of Copper piping and Copper alloys. In boilers, the source

of this corrosion could be dissolved gases in the boiler water or the excessive use of Hydrazine which will

corrode Copper and Copper alloys, allowing Copper to be carried back to the boiler. Copper in the boile~

displaces metal, from the tube surfaces and plates, out on to the tubes. This condition often occurs under

existing scale and sludge deposits, which is known as 'under deposit Copper corrosion'. Copper deposits are

a serious problem in high pressure boilers. Waterside deposits must be properly ana lysed to determine the

correct procedures to follow for cleaning.

Oil .

To prevent oil from entering condensate and feedwater systems, certain safety equipment is generally

incorporated to detect, 'remove, and arrest such contamination. Oil contamination may occur through me-

chanical failure, for example, faulty oil deflectors at turbine glands passing lubrication oil to glands seal

condensers and main condensers, or undetected leaks at tank heating coils. Any oil film on internal heating

surfaces is dangerous, drastically impairing heat transfer oil films therefore cause overheatingof tube metal,

resulting in possible tube blistering and failure. If oil contamination is suspected, immediate action must beundertaken for its removal. The first corrective measure in cleaningup oil leakage is to find and stop the point

of oil ingress into the system. Then, by using a Degreaser, a cleaning solutioncan be circulated throughout the

boiler systemto removethe existing oil contamination. Boiler coagulant can assist in removingtrace amountsof oil contamination.

Iron Oxides

Iron may enter the boiler as a result of corrosion in the pre-boiler section or may be re-deposited as a

result of corrosion in the boiler or condensate system. Often, Iron oxide will be deposited and retard heat

transfer within a boiler tube, at times resulting in tube failure. This usually occurs in highheat transfer areas,

i.e. screeningtubes nearest to the flame. When iron is not present in the raw feedwater, its presence in the

boiler indicatesactive corrosion withinthe boiler system itself Rust inthe reddishfrom is fully oxidized. More

often, in a boiler with limited oxygen, it is in the reduced or black form as Magnetite (Fe3O4)'which ismagnetic and can be readily detectedwith a magnet. It is a passive form of corrosion and its presence shows

that proper control of the system is being maintained.

Magnesium Carbonate (MgCOJ

Magnesium hardness in fresh water usually accounts for about one third of the total hardness. The

remainingtwo thirds can normally be attributed to calcium. Since Magnesium carbonate is appreciably more

soluble in water than Calcium carbonate, it is seldom a major component in scale deposits. This is dueto the

preferential precipitation of the carbonate ion by Calcium, as opposedto Magnesium which remains in solu-

tion untilall soluble Calcium is exhausted. Oncethis point is reached,any free carbonate remainingin solution

will combine with the Magnesium and beginprecipitating out as Magnesium carbonate, whenthe solubilityot

this salt is exceeded. Because of this letter phenomenon, when "soft" water is used for boilers, any magne-

sium present must be removed along with the Calcium.

Magnesium Sulphate (MgSO .JMagnesium Sulphate is an extremely soluble salt, having a solubility of 20% in cold water and 42% in

boiling water. It exists as a sulphate only in water with a low pH. Because of its high solubility, it will not

normally precipitate. The sulphate ion, however, will be precipitated by the Calcium hardness present ifnotree carbonates exist.

Magnesium Chloride (MgClz>

Magnesium Chloride, like Magnesium Sulphate, is soluble in fresh water in the high temperature and

alkaline conditions normally maintained in a boiler, any soluble Magnesium ions in the boiler water become

extremely reactive with Hydroxyl ions, which may be present in high concentrations in this type of environ-

ment. This can result in the formation of Magnesium Hydroxide precipitates which from insulating scale onthe boiler tube surfaces. It chloride ions are also available they react with the Hydrogen ions previously

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associatedwiththe precipitated Hydroxyl ions, to fonn Hydrochloric acid, thereby loweringthe alkalinity of

the water. If this situation is allowedto continue, the pH of the boilerwater will decrease until acid conditions

result in corrosion of the metal surfaces. Unlike Carbonate and Sulphate ions, the Chloride ion does not

precipitate in the presence of soluble Calcium.

Silica(SiOz)

Silica scale is not normally found in boiler systems except in minute quantities. It can be admittedto the

systemwhensevere carryover occurs in evaporators processing water with high Silica content. Other sources

of such feedwater may be high Silica river or raw fresh water as well as distilled/deionizedor unprocessedfresh water which has been stored and taken cement-washedfrom silicate coated tanks. Once fonned, pure

Silica scale is extremely difficult to remove. It forms a tight adherent glass-like film on metal surfaces,thereby preventingproper heat transfer. In addition, in steam-generating devices it can carry over with the

steam coatingthe after-boiler sections, particularly the superheater. If a turbine fonns part of the system, the

Silica can deposit on the blades as well as cause erosion of the finned surfaces of the blading, resulting in

imbalance of the turbine, which in tum may result in turbine failure. Besides the pure fonn of Silica (i.e.

SiliconDioxide),possible Silicate deposits can fonn in combination with Calcium and Magnesium which are

extremelyinsoluble in water and very difficult to dissolve and remove. Besides being an extremely difficult

process,the chemicalremoval of Silica and Silicate deposits can-also be veryhazardous, since it involvesthe

use of Hydrofluoric Acid or AmmoniumBi-fluoride, both of which are severely destructive to human issue

by inhalation, ingestion and physical contact in some instances, alternate acid and alkaline washings have

been used to successfully combat this problem. The only alternative to chemical cleaning is mechanicalremoval.

Calcium Carbonate (CaCOz)

Calcium Bicarbonate alkalinity exists in almost all unprocessed fresh water under nonnal conditions. Its

solubilityis about 300-400 ppm at 25C. Ifheat is applied or a sharp increase in pH occurs, the Calciumbicarbonate breaks down to fonn Carbon dioxide and Calcium carbonate. While the bicarbonate salt has

been shownto be moderately soluble in water, the solubility of Calcium carbonate at 25C is only about 14

ppm. This value continues to decrease as the temperature increases, becoming the least where the tempera-

ture is greatest. In a boiler, this would be on the surface of the furnace tubes where contact is made with the

water. The resulting insoluble Calcium carbonate precipitate fonns 'building block like' crystals which ad-here not only to one another, but also to the hot metal surfaces, resulting in a continuous, insulating scale

deposit over the entire heat exchange area. This deposit will continue to grow, building upon itselfto fonn a

thick coatinguntil all the calcium Carbonate produced is exhausted. It suspendedmatter is also present in the

water, it can become entrained within the crystal structure, creating a larger volume of deposit than that

fonned by the Carbonate precipitation alone.

If this condition is allowed to continue, heat exchange efficiency at the waterltube interface falls rapidly,

resultingin an increase in fuel consumption necessary to compensate for the decline in thennal transfer and

to regaindesigntemperatureas well as steamproductionrequirements.This increasein the furnacesidetemperatureneeded'to run the systemat optimumconditionsexposesthe metal surfacesto overheatingwhich,in turn, can cause blistering fatigue, fracture and failure of boilertubes. In addition, it pockets of waterbecometrapped beneath the scale deposits and are in contact with the hot metal surfaces, concentration of

acid or alkaline materials may occur and lead to the fonnation of local electrolytic cells (under-deposit

corrosion) .

Calcium Sulphate (CaSO

JAlthoughCalcium Sulphate is more soluble in water than Calcium Carbonate, it can be just as trouble-somewhenpresent in boiler and coolingwater system.. Calcium Sulphate, likeCalcium Carbonate, but unlike

most salts has an inversetemperature /solubility relationshipin water. As gypsum, the hydrated fonn in which

CalciumSulphateis normally present in fresh water, its solubility increases until a temperature of about 40Cis achieved. At 40C, its solubility is 1,551 ppm; at 100C which is the nonnal boiling point of water, its

solubilitydecreasesto 1.246ppm, and at 220C it falls to 40ppm. Calcium Sulphate reacts at high-tempera-

ture surfaces essentially in the same manner as Calcium Carbonate and with the same effects and conse-

quences. However,whereas Calcium Carbonate deposits are relatively easy to remove using a comprehen-

sive acid cleaning procedure, Calcium Sulphate is essentially impervious to the effects of nonnal acid de-

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Dissolved Gases

Gases such as Oxygen and Carbon pioxide that are. dissolved m distilled or fresh water, will further

contribute to the deterioration of the boiler system. Dependent upon conditions in the system (e.g. tempera-

ture pressure and materials of construction), dissolved Oxygen can cause pitting corrosion of steel surfaces,while carbon Dioxide lowers the pH, leadmg to acid and galvanic corrosion. Carbon Dioxide has the added

disadvantage of fonning msoluble Carbonate scale deposits m an alkalme environment when Calcium andMagnesium are present.

Acidity, Neutrality and Alkalinity

All water can be classified into one of these categories. Acidity, Neutrality and Alkalinity are only verygeneral terms. We require more accurate methods of testing to know the degree of each condition. When

testmg boiler water it is important to understand what you are testmg for.

The presence of Alkalmity ma water may be due to many differentsubstances. For the sake of simplicity,the presence of bicarbonate, carbonate "andhydroxide contributes to the alkalinity of water.

Phenolphthalein(P) Alkalinity (pH values greater than 8.3) measures all the hydroxide and one half of the

carbonate alkalmity which is sufficient for our purpose of control. !licarbonates do not show m this test as

they have a pH less than 8.4.

M- Alkalinity.Total Alkalmity or M - Alkalmity (pH values greater than 4.3) measures the sum ofbicar-bonate, carbonate and hydroxide alkalinity.

Problems of Boiler Water

Feedwaterproduced by distillation for use in a boiler is not 'pure', even with a good distillation method.Worse still is ordinary water taken from ashore to be used as feedwater. Problems will then arise when the

water is used in the boiler.The types of problem will dependon the type of impuritiesand in which quantitieshey are present.

The most commonproblems are:

-corrOSIon

-Scaling

-Carryover

The corrosion processes can affect boilers in .thefollowingways. 'General wastage' is the overall reduc-

ion of metal thickness and is common in heatb1gsurface areas, such as boiler tube walls. This 'thinnmg' of

boiler tubes is often found in boilers having open feed systems (mostly auxiliary boilers) without any protec-ive treatment.

Pitting Corrosion.

Pittingis the mostseriousformofwatersidecorrosionandis the resultofthe formationof irregularpits inhe metal.Evidenceof pitting is usuallyfoundin the boilershellaroundthe water leveland is most likely

caused by poor storageprocedureswhenthe boileris shut downfor lengthyperiods,and by inadequateOxygenscavenging.

Stress Corrosion

"Stress corrosion" cracking is the process caused by the combined action of heavy stress and a corrosive

nvironment. The stages of failure. of the metal due to stress corrosion are -1) Corrosion is initiated byreakdown of the surface film, 2) followed by the formation of a corrosion pit which becomesthe site for 3)

tress corrosion cracking, eventually leading to mechanical failure due to overloading of the mechanical

trength of the metal. This form of attack is often found around the ogee ring in vertical auxiliary boilers,when undue stressing is set up by poor steam-raising procedures.

Related problems

"Corrosion fatigue" occurs when a sufficiently high alternative stress level causes failure of the subjected

material. It is the joint action of a corrosive environment and cyclic stressing and results in a series of fIDe

racks in the metal. This is found in water tube boilers where irregular circulation through tubes in high

mperature zones induce these cycling stresses. "Caustic cracking" results from the contact of water ofoncentrated caustic alkalinity and steel which has not been stress relieved, e.g. in riveted seams. This form

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of cracking follows the grain boundaries. This is rarely observed nowadays, as both high and low pressure

boilers are usually of all welded construction and are stress relieved.

Caustic corrosion takes place only in high pressure boilers (above 60 bar) when excessivelyhigh concen-

trations of Sodiumhydroxide (Caustic soda) cause breakdown of the magnetite layer and localised corrosion.

Thisformof attack is oftencontrolledby the coordinatedPO4 TreatmentProgram."Hydrogenattack" isanother form of corrosion damage that can take place in ultra high pressure boilers. Whichever form ofcorrosive attack occurs, the risk of tube failure or serious structural damage is very apparent, both often

leading to considerable expense in the shape of repair costs.

Factors Affecting ColTosion

1) pH -Metal oxides are more soluble as pH decreases and corrosion increases.2) Dissolved solids - Chloride and Sulphate can penetrate passive metal oxide film which protects the basemetal from corrosion.

3) Dissolvedgases - carbon dioxide and ~S reduces pH and thus promotes acid attack. Oxygen promotespittingcorrosion.

4) Suspendedsolids - Mud, sand, clay, and so on, settle to form deposits, promoting different corrosion cells.

5) Micro organism.

6) Temperature - High temperature increases corrosion.

7) Velocity- High velocity promotes erosion/cavitation.

8) Copper - Copper ions plate out on steel surfaces and promote pitting corrosion.

Scaling

If the inside of a boiler is scaled, there is a great risk that the boiJermaterial will overheat, leadingto tube

failure. The efficiency of operation will also be adversely affected. Hardness in the feedwater will usually

present problems in relation to the operation of boilers. Hardness in the feedwater will, as the temperaturerises, cause an increase in the formation of sludge in the feedwatertank. If scale preventing chemicals are

put intothe feedwatertank this problem will aggravated, as nearly all precipitation of sludgewill take place in

the feedwatertank. The suction pipe stub of the feed water line will usually be placed 5- 10 em above thebottom. If the feed water is not very clean, sludgewill be sucked into the piping and choking may occur. Ina modem centrifugal pump, the very narrow vanes may be blocked, which will cause the pump to stop, and

there is a risk of the valves sticking and becoming blocked. In spite of the fact that a boiler plant may be

equipped with a water treatment system of some sort, there will always be a risk of hardness or other type

of pollution in the feedwater, because:

1. The capacity of the water treatment system is insufficient

2. There are defects in the water treatment system.

3. The condensate is polluted due to

a.) heat exchanger leaks

or b) lubricatingoil

Daily analysis of the quality of the feedwater will ensure that action can be taken in time to prevent

irregularities. Hardness in the boiler water will inevitably lead to the formation of scale and the rate of this

formationwill depend on the composition and quantity of the hardness, on the temperature conditionsin theboiler and on the circulation in the boiler.

Increased surface heating effect means increased production of steam bubbles, which again will make

more boiler water "pass" the spot on the heating surface (wherethe steam bubbles are formed) and this spot

will thus also be "passed" by the hardness producing and corroding salts in the boiler water. In addition the

most common hardness salts are less soluble at increasing temperatures. This explains why the largest

amount of encrustation will always be found where the temperature of the heating surface is the highest.Scale formed just at this point means that the critical temperature of the boiler material will be reached

quickly and that damage to the boiler will be inevitable.

Carryover

Carryover is any contaminant that leaves the boiler with the steam.

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Carryover can be Solid, Liquid or Vapour

Effectsof carryover-Deposits in non-return valves, deposits in superlteaters, deposits in control valvesand deposits on turbines.

Carryover in superlteaters can promote failure due to overlteating. Turbines are prone to damage by

carryover, as solid particles in steam can erode turbine parts. When large slugs of water carry over withsteam, the thermal and mechanical shock can cause severe damage.

Causes of carryover

Mechanical

. Priming. Sudden load changes

. Boiler design

. Sootblowing

. High water levelChemical:

Foaming due to

. High Chlorides

. HighTDS (Total dissolvedsolids)

. Highalkalinity

. Suspendedsolids. Oil

. SilicaThe most common form of encrustation in a steam system stems from carryover. The boiler manufactur-

ers stipulate a maximum allowed salinity of the boiler water. [fthis value is exceeded,there is a risk ofnormal

bubble size being prevented; larger bubbles will be produced and the turbulence in the water surface will

increase and cause foaming. The foam may be carried over with the steam, particularly when the generation

of steam is at maximum, which causes boiler water (containing Sodium hydroxide and sak) to pass out into

the steam pipes. The content of Silicic Acid is important for boilers with high pressures. Silicic Acid in its

volatile form may be carried away with the steam and be depositedon turbine blades, for instance, on which

it will form a very hard, porcelain-like scale.

Chemicalcomposition may not be the sole cause of carryover. Circumstances such as periodic overloads,periods of too high a water level (or more correctly,too small a steam volume) are two of the most common

causes. Finally, impurities from the condensate, such as oil from the preheater's coils, if they are leaking, are

very commoncauses of priming.

Treatments:

The main Purpose of Boiler Water Treatment is

A. To reduce the total hardness of the boiler water

B. To maintain the correct pH and alkalinity values in feedwater and boiler water.

C. To prevent corrosion, especially corrosion caused by Oxygen

D. To prevent the formation of scale, by conditioningthe sludge.E. To avoid foaming.

.,

Single Function Treatment

1.AlkalinityControl

2. Hardness Control

3. OxygenControl

4. Catalysed SodiumSulphite

5. Cat. Sulphite L

6. Boiler Coagulant

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7. Condensate Control

Treatment for Low Pressure Boiler Water

Combinedchemical treatment suitable for use in small low pressure boilers, precipitates hardness, pro-

vides the boiler water with the necessary alkalinity~.and scavenges dissolved Oxygen. It should be added

when necessary, as shown by water analysis results. It is the boiler is open and not being fired, it can be

poured through a manhole,but whenthe boiler is in operation,the treatment mustbe appliedthrough a special

dosing line. When a dosing arrangement is utilized, the chemical must be flushed to removeany residual leftin the dosage lines and equipment. If dosing lines are not fitted,'tPe chemical can be added directlyto the feedtank as required. Ensure proper circ1dationthrough the feed tank to allow the chemical to enter the boiler

beingtreated. It is necessary to mow the details of the flowpattern in the boiler for proper testing and dosingof the chemical treatment to take place. When several boilers have a coromonfeed tank, dosing should becarried out through independent dosing lines to~ensurethe proper treatment of each boiler. Re-test within 2

hours of dosing the boiler water chemical:.

Tests for Boiler Water, Low Pressure

Dosage level of chemical is based on the P-Alkalinity value of the boiler water. However, Chlorides and

condensate pH must also be controlled and maintained as recommended.Knowledgeof all relevant param-eters is desirableto enable better interpretation and correct application of treatment.

Controlling Alkalinity .

The alkalinity is a more accurate indicator of the boiler water conditionthan is the pH. The Phenolphtha-lein (P) alkalinity is measured to determinewhether the correct conditions of alkalinity exist in the boiler to:

A. Provide a suitable environmentfor the precipitation of hardness salts as desirable sludge materials.

B. To help the formation of Magnetite (Fe3OJ in the presence of Oxygen scavengers (i.e. Hydrazine/Sulphite).

C. Maintain Silica in solutionto prevent Silica scale formation.

Controlling Chlorides

The Chloridevalue will reveal any presenceof dissolvedsalts in the boiler.An increase,gradual orsudden,inthe levelof Chloridesis an indicationof contaminationby seawater,and Chloridesare oftenused

. . . ~

as a reference point when controlling rate of blow down. Too high a Chloride level indicates that undesirable

amounts of salts are present, leading to possible foaming and! or scale and deposit formation.

pH .

ThepH of the boilerwater shouldbe maintainedwithinthe rangeof9.5-11.0 to preventany corrosion.CondensatepH.

Hardness control

Hardnesscontrol is achievedby using a Phosphatepowderproduct to precipitatedissolvedcalcium

hardnesssaltsand to convertthese salts to non-adherentCalciumphosphatesludge,whichcan be easilyremovedby blowdown.. Hardness control is highly effectivein achievingthis function; minimumdosages arerequired. Reduced dosage of chemicals minimises dissolvedand suspended solids in the boiler water. Hard-

ness control provides neutral reaction products in the boiler. A high level of dissolved and suspendedsolids

are the principal causes of carryover and priming. As the temperature of the boiler increases, less phosphate

can be held in solution in the boiler water. Therefore, testing and dosage of phosphate, to control hardness

salts deposits, shouldbe done whenthe boiler is under full load conditions. If the phosphate residual increasesunderlow load conditions,this is an indicationof a dirty boiler, and increased bottom blows should be carried

out to remove the sludge. The sludge holds excess phosphate and re-dissolves when the boiler water tem-perature is reduced.

Alkalinity Control

Alkalinity control is used to obtain the correct pH level necessary for the phosphate treatment to react

with Calcium salts. In addition,Alkalinitycontrol is usedto maintainthe requiredalkalinity inthe boiler water

to prevent acid corrosion. By adopting simpletesting procedud~sto determinethe Phenolphthaleinalkalinity

(p-Alkalinity)and the total alkalinity (M-Alkalinity).We can determinethe amount of tree caustic present inthe boiler water. If a positive number is obtained, tree caustic (OH-Alkalinity) is present in the boiler water.

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The term "excess chemicals" or reserve of chemicals" ensures that chemicals are always readily available

to perform their necessary functions.

Oxygen Control (Hydrazine, N:tHJ

- Hydrazineis a colourlessliquidat ambienttemperatures,beingcomp)etelymisciblewithwater.Its solu-tion has an odour-resembling Ammonia, but is less pungent. It is used to efficiently scavenge and remove

Oxygen from condensate, feedwater and boiler water. Hydrazine reacts with Oxygen, acting as a scavenger.

The r~ction results inNitrogen and water, no solids being addedto the boiler system. Some of the Hydrazine

will carry over with the steam helping to maintain the condensate pH in an alkaline range, which will act as

protective layer against further corrosion. Hydrazine should be added to the system using a separate dosing

tank. The tank should be filled daily with hydrazine diluted with Condensateor-distilledwater. This solutionshould be dosed continuously to the storage section of the de-aerator. Alternatively Hydrazine can be fed

C9ntinuouslyto the feed pump suction or atmospheric drain tank over a 24-hour period. It is important that

_Hydrazineshould not be overdosed. At re.nperatures above 270C, Hydrazine starts to break down, creating

free Ammonia. Excessive free Ammonia and Oxygen, when combined, form a corrosive condition on non-

ferrous metals. This corrosive action can cause Copper to deposit in the watersides of boilers, causing

additional boilerproblems, as discussed earlier.

The reaction of Hydrazine in boilers is threefold:

I. It scavenges any free or dissolved Oxygen

2. It reduces red iron oxide to a metal-protective black oxide coating (Magnetite).

3. It raises the pH of the condensate~reducing acid corrosion of the condensate.Catalysed sodium sulphate

Catalysed sulphite products are used as scavengers, in place of Hydrazine, where economy is of impor-

tance, or used in low pressure boilers with open feed system where feed inlet temperatures are low. Sulphite.

combined with oxygen forms sulphate, which adds solids to the boiler water. It should thus not be used in

boilers whose pressures are above 30 bars; where the TDS level is critical. Sulphite is also used as a substi-

tute for Hydrazine when rust and scale deposits are present in boiler system on ships being returned to

service. Hydrazine tends to remove iron oxide deposits present throughout the boiler system. An amine

should be used in conjunction with oxygen scavengers to maintain the condensate pH within the desirable

ranges throughout the entire condensate and feedwater system.

Boiler Coagulant

Boiler Coagulant is a polymeric compound used in boilers contaminatedwith small quantitiesof oil, or asa sludge conditioner when high levels of solids are experienced, Boiler Coagulant should be dosed at 250cc

per day. No testing is necessary if used regularly. Daily flash blowdownis recommendedto removeprecipi-

tatedsolidsor coagulatedoil. - -

Efficient operation of the de-aerator is important. The function of the de-aerator is to :

i) removedissolved gases from the condensate.

ii) pre-heating of the feedwater.

ill) can act as a suction head for the feed pumps.

In many cases, improper operation of the de-aerator will affect the results of the chemical treatment.

Proper operatingtemperature and pressure must be always maintained in the de":aerator.Temperaturevaria-tions betweenthe upper and lower parts of the de-aerator indicatefaulty operation. If Ammonialevels should

be excessive, the de-aerator should be vented to atmosphere.

A set of periodic blow-downs is recommended.Even if test results indicate a normal range, it is better to

blow-downat least once a day, since sludge is forming in the boiler at all times. Plan a schedule that will fit

into the vessels normal operating pattern, to include a bottom blow-down, as well as a blow-down of each

header regularly,so as to prevent sludge build-up. Controllingthe pH and Phosphate is also very critical, as

this maintains the boiler internal surfaces free of caustic embrittlementcorrosion and deposition.

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3CHEMFEEDPOlNI

Chemical injection points for low pressure

Boiler System

The following diagram depicts a typical low Pressure Boiler System. Note injection point for chemicals,

when dosing chemicals, the recommendationto achieve the best possible results is to always dose all chemi-cals in the diluted form on a continuous basis.

1. Dosage to hot well or feed tank. All chemj.calscan be dosed at these points. However,the recommended

dosage of Alkalinity Control and Hardness Control is either no.2 feed line or no. 3 chemical f~ injection

directlyto the boiler. Oxygen Control and Sulphite shouldpreferably be dose4to the feed tank on a continu-ous basis.

All combinedproducts can be dosed into the hot well.

2. Doseto injectionno. 2 is requiredto the feed lin~by means of a pressure injector or dosagepump. Dosageshould be continuous, however water can be shock treateCi.. .

3. Dosage directto boiler no. 3. All chemicals can be dosedto this point by means of pressure pot injector or

dosage pump. Alkalinity Control or Hardness Control is best controlled at this location and the use of

Hydrazine.Sulphite of Condensate Control is recommendedon a continuous basis in the condensate system.

BLOWDOWN

~ImAni

+-2. CHEM.FEED~

AUX.FEED LINE

fEED TANI