Boiler chemistry.docx

7/26/2019 Boiler chemistry.docx

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The dissolved gases normally present in water cause many corrosion problems. For instance,oxygen in water produces pitting that is particularly severe because of its localized nature.

Carbon dioxide corrosion is frequently encountered in condensate systems and less commonlyin water distribution systems. Water containing ammonia, particularly in the presence of

oxygen, readily attacs copper and copper!bearing alloys. The resulting corrosion leads todeposits on boiler heat transfer surfaces and reduces efficiency and reliability.

"n order to meet industrial standards for both oxygen content and the allowable metal oxidelevels in feedwater, nearly complete oxygen removal is required. This can be accomplished only

by efficient mechanical deaeration supplemented by an effective and properly controlledchemical oxygen scavenger.

#everal principles apply to the mechanical deaeration of feedwater$

The solubility of any gas in a liquid is directly proportional to the partial pressure of the gas atthe liquid surface

The solubility of a gas in a liquid decreases with increasing liquid temperature%see Figure&'!()

*fficiency of removal is increased when the liquid and gas are thoroughly mixed

The solubility of a gas in a liquid is expressed by +enrys -aw$

Ctotal /

where$

Ctotal total concentration of the gas in solution/ partial pressure of the gas above solution

a proportionality constant nown as +enrys -aw Constant

For example, 0 ppm of oxygen can be dissolved in water when the partial pressure of oxygen is'.( atmosphere1 only 2 ppm of oxygen can be dissolved in water if the partial pressure of

oxygen is reduced to '.& atmosphere.

3s is evident from +enrys -aw, a dissolved gas can be removed from water by a reduction ofthe partial pressure of that gas in the atmosphere contacting the liquid. This can be

accomplished in either of two ways$

&. a vacuum is applied to the system and the unwanted gas is vented

(. a new gas is introduced into the system while the unwanted gas is vented

4acuum deaeration has been used successfully in water distribution systems. +owever,pressure deaeration %with steam as the purge gas) is normally used to prepare boiler

feedwater. #team is chosen as the purge gas for several reasons$

it is readily availableit heats the water and reduces the solubility of oxygen

it does not contaminate the water
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only a small quantity of steam must be vented, because most of the steam used to scrub thewater is condensed and becomes a part of the deaerated water

"n order to deaerate the boiler feedwater, water is sprayed into a steam atmosphere. This heatsthe water to within a few degrees of the temperature of the saturated steam. 5ecause the

solubility of oxygen in water is very low under these conditions, 67 to 608 of the oxygen in the

incoming water is released to the steam and is purged from the system by venting. 3lthoughthe remaining oxygen is not soluble under equilibrium conditions, it is not readily released tothe steam. Therefore, water leaving the heating section of the deaerator must be scrubbed

vigorously with steam to maximize removal.

Equipment

The purpose of a deaerator is to reduce dissolved gases, particularly oxygen, to a low level andimprove a plants thermal efficiency by raising the water temperature. "n addition, deaerators

provide feedwater storage and proper suction conditions for boiler feedwater pumps.

/ressure deaerators, or deaerating heaters, can be classified under two ma9or categories$ tray!

type and spray!type%see Figure &'!:). Tray!type deaerators are also referred to as ;spray!tray;type, because the water is initially introduced by spray valves or nozzles. The spray type is alsoreferred to as the ;spray!scrubber; type because a separate scrubbing section is used to

provide additional steam!water contact after spraying.

Thetray!type deaerating heater, shown in Figures &'!2 and &'!


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"n power generating stations, main turbine condensers have air e9ectors to remove dissolvedgases. #ometimes the pressure deaerator is omitted from the feedwater cycle. +owever, there

is a danger of air leaing into the system, both during start!upDshutdown and while thecondensers are operating at low loads. This may necessitate steam blaneting and increased

chemical deaeration.

Vacuum Deaeration

4acuum deaeration is used at temperatures below the atmospheric boiling point to reduce thecorrosion rate in water distribution systems. 3 vacuum is applied to the system to bring thewater to its saturation temperature. #pray nozzles brea the water into small particles to

facilitate gas removal and vent the exhaust gases.

"ncoming water enters through spray nozzles and falls through a column paced with aschigrings or other synthetic pacings. "n this way, water is reduced to thin films and droplets, which

promote the release of dissolved gases. The released gases and water vapor are removedthrough the vacuum, which is maintained by steam 9et eductors or vacuum pumps, dependingon the size of the system. 4acuum deaerators remove oxygen less efficiently than pressure

units. 3 typical vacuum deaeratoris shown in Figure &'!7.

Important Considerations

"nlet water to the deaerators should be largely free from suspended solids, which can clogspray valves and ports of the inlet distributor and the deaerator trays. "n addition, spray valves,ports, and deaerator trays may plug with scale which forms when the water being deaerated

has high levels of hardness and alalinity.

/ressure deaerators reduce oxygen to very low levels. Eet even trace amounts of oxygen maycause corrosion damage to a system. Therefore, good operating practice requires

supplemental removal of oxygen by means of a chemical oxygen scavenger such as sodiumsulfite or hydrazine, or other materials, such as organic, volatile oxygen scavengers.

3lthough deaeration removes free carbon dioxide, it removes only small amounts of combinedcarbon dioxide. The ma9ority of the combined carbon dioxide is released with the steam in theboiler and subsequently dissolves in the condensate, frequently causing corrosion problems.

These problems can be controlled through the use of volatile neutralizing amines, filmingamines, and metal oxide conditioners.

Monitoring Performance

=onitoring /erformance /ressure deaerators, used to prepare boiler feedwater, producedeaerated water which is very low in dissolved oxygen and free carbon dioxide. 4endors

usually guarantee less than '.''< cm:D- %7 ppb) of oxygen.

4acuum deaerators, used to protect water distribution lines, are not designed to deaerate asthoroughly as pressure deaerators. ?sually, they reduce the oxygen content to about '.(< to

'.


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"t is good practice to chec the operation of the unit regularly. Care should be taen to ensurethat the unit is not operated beyond its capacity. The system should also be checed for water

hammer and thermal stress, which can be caused by the introduction of cold condensate.Thorough off!line inspection should be performed as often as possible and should include the

following$

inlet water regulating valves and controls for storage tan level controlhigh and low alarms for storage tan levels

overflow valve and controller for prevention of high water levelsteam pressure reducing valves to maintain required minimum deaerator pressure

safety relief valvestemperature and pressure gauges for proper monitoring of maeup water, deaerator, and

storage tansteam vent for removal of gases and vent condenser for integrity

steam inlet baffles for integrityinlet spray valves for deposits and operation

trays for proper position

weld areas for damage %particularly cracing)


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Corrosion is one of the main causes of reduced reliability in steam generating systems. "t isestimated that problems due to boiler system corrosion cost industry billions of dollars per year.

=any corrosion problems occur in the hottest areas of the boiler!the water wall, screen, andsuperheater tubes. Bther common problem areas include deaerators, feedwater heaters, and

economizers.

=ethods of corrosion control vary depending upon the type of corrosion encountered. The mostcommon causes of corrosion are dissolved gases %primarily oxygen and carbon dioxide),

under!deposit attac, low p+, and attac of areas weaened by mechanical stress, leading tostress and fatigue cracing.

These conditions may be controlled through the following procedures$

maintenance of proper p+ and alalinity levels

control of oxygen and boiler feedwater contamination

reduction of mechanical stresses

operation within design specifications, especially for temperature and pressure

proper precautions during start!up and shutdown

effective monitoring and control

CORROSION TENDENCIES OF BOIER S!STEM COMPONENTS

=ost industrial boiler and feedwater systems are constructed of carbon steel. =any havecopper alloy andDor stainless steel feedwater heaters and condensers. #ome have stainless

steel superheater elements.

/roper treatment of boiler feedwater effectively protects against corrosion of feedwater heaters,economizers, and deaerators. The 3#=* Consensus for "ndustrial 5oilers %see Chapter &:)

specifies maximum levels of contaminants for corrosion and deposition control in boilersystems.

The consensus is that feedwater oxygen, iron, and copper content should be very low %e.g.,

less than 7 ppb oxygen, (' ppb iron, and &< ppb copper for a 6'' psig boiler) and that p+should be maintained between 0.< and 6.< for system corrosion protection.

"n order to minimize boiler system corrosion, an understanding of the operational requirementsfor all critical system components is necessary.

Feed"ater #eaters


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5oiler feedwater heaters are designed to improve boiler efficiency by extracting heat fromstreams such as boiler water blowdown and turbine extraction or excess exhaust steam.

Feedwater heaters are generally classified as low!pressure %ahead of the deaerator), high!pressure %after the deaerator), or deaerating heaters.

egardless of feedwater heater design, the ma9or problems are similar for all types. The

primary problems are corrosion, due to oxygen and improper p+, and erosion from the tubeside or the shell side. >ue to the temperature increase across the heater, incoming metaloxides are deposited in the heater and then released during changes in steam load and

chemical balances. #tress cracing of welded components can also be a problem. *rosion iscommon in the shell side, due to high!velocity steam impingement on tubes and baffles.

Corrosion can be minimized through proper design %to minimize erosion), periodic cleaning,control of oxygen, proper p+ control, and the use of high!quality feedwater %to promote

passivation of metal surfaces).

Deaerators

>eaerators are used to heat feedwater and reduce oxygen and other dissolved gases toacceptable levels. Corrosion fatigue at or near welds is a ma9or problem in deaerators. =ostcorrosion fatigue cracing has been reported to be the result of mechanical factors, such as

manufacturing procedures, poor welds, and lac of stress!relieved welds. Bperational problemssuch as waterDsteam hammer can also be a factor.

*ffective corrosion control requires the following practices$

regular monitoring of operation

minimization of stresses during start!up

maintenance of stable temperature and pressure levels

control of dissolved oxygen and p+ in the feedwater

regular out!of!service inspection using established nondestructive techniques

Bther forms of corrosive attac in deaerators include stress corrosion cracing of the stainlesssteel tray chamber, inlet spray valve spring cracing, corrosion of vent condensers due tooxygen pitting, and erosion of the impingement baffles near the steam inlet connection.

Economi$ers

*conomizer corrosion control involves procedures similar to those employed for protectingfeedwater heaters.

*conomizers help to improve boiler efficiency by extracting heat from flue gases dischargedfrom the fireside of a boiler. *conomizers can be classified as nonsteaming or steaming. "n a

steaming economizer,


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Bxygen pitting, caused by the presence of oxygen and temperature increase, is a ma9orproblem in economizers1 therefore, it is necessary to maintain essentially oxygen!free water in

these units. The inlet is sub9ect to severe pitting, because it is often the first area after thedeaerator to be exposed to increased heat. Whenever possible, tubes in this area should be

inspected closely for evidence of corrosion.

*conomizer heat transfer surfaces are sub9ect to corrosion product buildup and deposition ofincoming metal oxides. These deposits can slough off during operational load and chemical

changes.

Corrosion can also occur on the gas side of the economizer due to contaminants in the fluegas, forming low!p+ compounds. Generally, economizers are arranged for downward flow of

gas and upward flow of water. Tubes that form the heating surface may be smooth or providedwith extended surfaces.

Super%eaters

#uperheater corrosion problems are caused by a number of mechanical and chemicalconditions. Bne ma9or problem is the oxidation of superheater metal due to high gas

temperatures, usually occurring during transition periods, such as start!up and shutdown.>eposits due to carryover can contribute to the problem. esulting failures usually occur in the

bottom loops!the hottest areas of the superheater tubes.

Bxygen pitting, particularly in the pendant loop area, is another ma9or corrosion problem insuperheaters. "t is caused when water is exposed to oxygen during downtime. Close

temperature control helps to minimize this problem. "n addition, a nitrogen blanet andchemical oxygen scavenger can be used to maintain oxygen!free conditions during downtime.

o"&Pressure Steam and #ot 'ater #eating S(stems

+ot water boilers heat and circulate water at approximately (''HF. #team heating boilers are

used to generate steam at low pressures, such as &< psig. Generally, these two basic heatingsystems are treated as closed systems, because maeup requirements are usually very low.

+igh!temperature hot water boilers operate at pressures of up to


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remove all traces of dissolved oxygen. #ynthetic polymers have been used for deposit control.>ue to the high heat transfer rate at the resistance coil, a treatment that precipitates hardness

should not be used.

*lectrode boilers operate at high or low voltage and may employ submerged or water!9etelectrodes. +igh!purity maeup water is required. >epending on the type of system, sodium

sulfite is normally used for oxygen control and p+ ad9ustment. #ome systems are designedwith copper alloys, so chemical addition must be of the correct type, and p+ control must be in

the range suitable for copper protection.

T!PES OF CORROSION

Corrosion control techniques vary according to the type of corrosion encountered. =a9ormethods of corrosion control include maintenance of the proper p+, control of oxygen, control

of deposits, and reduction of stresses through design and operational practices.

)a*+anic Corrosion

Galvanic corrosion occurs when a metal or alloy is electrically coupled to a different metal or

alloy.

The most common type of galvanic corrosion in a boiler system is caused by the contact ofdissimilar metals, such as iron and copper. These differential cells can also be formed whendeposits are present. Galvanic corrosion can occur at welds due to stresses in heat!affected

zones or the use of different alloys in the welds. 3nything that results in a difference in electricalpotential at discrete surface locations can cause a galvanic reaction. Causes include$

scratches in a metal surface

differential stresses in a metal

differences in temperature

conductive deposits

3 general illustration of a corrosion cell for iron in the presence of oxygenis shown in Figure&&!&. /itting of boiler tube bans has been encountered due to metallic copper deposits. #uch

deposits may form during acid cleaning procedures if the procedures do not completelycompensate for the amount of copper oxides in the deposits or if a copper removal step is notincluded. >issolved copper may be plated out on freshly cleaned surfaces, establishing anodiccorrosion areas and forming pits, which are very similar to oxygen pits in form and appearance.This process is illustrated by the following reactions involving hydrochloric acid as the cleaning

solvent.

=agnetite is dissolved and yields an acid solution containing both ferrous %FeIJ) and ferric%FeKJ) chlorides %ferric chlorides are very corrosive to steel and copper)

Fe:B2 J 0+Cl FeCl( J (FeCl: J 2+(B

magnetite

hydrochloric

ferrouschloride

ferricchloride

water


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acid

=etallic or elemental copper in boiler deposits is dissolved in the hydrochloric acid solution bythe following reaction$

FeCl: J Cu CuCl J FeCl(

ferricchloride

coppercuprouschloride

ferrouschloride

Bnce cuprous chloride is in solution, it is immediately redeposited as metallic copper on thesteel surface according to the following reaction$

(CuCl J Fe FeCl( J (Cu'

cuprouschloride

ironferrouschloride

copperoxide

Thus, hydrochloric acid cleaning can cause galvanic corrosion unless the copper is preventedfrom plating on the steel surface. 3 complexing agent is added to prevent the copper fromredepositing. The following chemical reaction results$

FeCl: J Cu JComplexin

g 3gent

FeCl( J CuCl

ferricchloride

copperferrouschloride

cuprouschloridecomplex

This can tae place as a separate step or during acid cleaning. 5oth iron and the copper are

removed from the boiler, and the boiler surfaces can then be passivated.

"n most cases, the copper is localized in certain tube bans and causes random pitting. Whendeposits contain large quantities of copper oxide or metallic copper, special precautions are

required to prevent the plating out of copper during cleaning operations.

Caustic Corrosion

Concentration of caustic %AaB+) can occur either as a result of steam blaneting %which allowssalts to concentrate on boiler metal surfaces) or by localized boiling beneath porous deposits

on tube surfaces.

Caustic corrosion %gouging) occurs when caustic is concentrated and dissolves the protective

magnetite %Fe:B2 ) layer. "ron, in contact with the boiler water, forms magnetite and theprotective layer is continuously restored. +owever, as long as a high caustic concentration

exists, the magnetite is constantly dissolved, causing a loss of base metal and eventual failure%see Figure &&!().

#team blaneting is a condition that occurs when a steam layer forms between the boiler waterand the tube wall. ?nder this condition, insufficient water reaches the tube surface for efficient

heat transfer. The water that does reach the overheated boiler wall is rapidly vaporized, leavingbehind a concentrated caustic solution, which is corrosive.


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/orous metal oxide deposits also permit the development of high boiler water concentrations.Water flows into the deposit and heat applied to the tube causes the water to evaporate,

leaving a very concentrated solution. 3gain, corrosion may occur.

Caustic attac creates irregular patterns, often referred to as gouges. >eposition may or maynot be found in the affected area.

5oiler feedwater systems using demineralized or evaporated maeup or pure condensate maybe protected from caustic attac through coordinated phosphateDp+ control. /hosphate buffers

the boiler water, reducing the chance of large p+ changes due to the development of highcaustic concentrations. *xcess caustic combines with disodium phosphate and forms trisodium

phosphate. #ufficient disodium phosphate must be available to combine with all of the freecaustic in order to form trisodium phosphate.

>isodium phosphate neutralizes caustic by the following reaction$

Aa(+/B2 J AaB+ Aa:/B2 J +(B

disodium

phosphate

sodium

hydroxide

trisodium

phosphate

water

This results in the prevention of caustic buildup beneath deposits or within a crevice whereleaage is occurring. Caustic corrosion %and caustic embrittlement, discussed later) does not

occur, because high caustic concentrations do not develop %see Figure &&!:).

Figure &&!2 shows the phosphateDp+ relationshiprecommended to control boiler corrosion.>ifferent forms of phosphate consume or add caustic as the phosphate shifts to the properform. For example, addition of monosodium phosphate consumes caustic as it reacts withcaustic to form disodium phosphate in the boiler water according to the following reaction$

Aa+(/B2 J AaB+ Aa(+/B2 J +(B

monosodium

phosphate

sodiumhydroxid

e

disodiumphosphat

ewater

Conversely, addition of trisodium phosphate adds caustic, increasing boiler water p+$

Aa:/B2 J +(B Aa(+/B2 J AaB+

trisodium

phosphate

water

disodium

phosphate

sodiumhydroxid

e

Control is achieved through feed of the proper type of phosphate to either raise or lower the p+while maintaining the proper phosphate level. "ncreasing blowdown lowers both phosphate andp+. Therefore, various combinations and feed rates of phosphate, blowdown ad9ustment, and

caustic addition are used to maintain proper phosphateDp+ levels.


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*levated temperatures at the boiler tube wall or deposits can result in some precipitation ofphosphate. This effect, termed ;phosphate hideout,; usually occurs when loads increase. When

the load is reduced, phosphate reappears.

Clean boiler water surfaces reduce potential concentration sites for caustic. >eposit controltreatment programs, such as those based on chelants and synthetic polymers, can help

provide clean surfaces.

Where steam blaneting is occurring, corrosion can tae place even without the presence ofcaustic, due to the steamDmagnetite reaction and the dissolution of magnetite. "n such cases,operational changes or design modifications may be necessary to eliminate the cause of the

problem.

,cidic Corrosion

-ow maeup or feedwater p+ can cause serious acid attac on metal surfaces in the preboilerand boiler system. *ven if the original maeup or feedwater p+ is not low, feedwater canbecome acidic from contamination of the system. Common causes include the following$

improper operation or control of demineralizer cation units

process contamination of condensate %e.g., sugar contamination in food processingplants)

cooling water contamination from condensers

3cid corrosion can also be caused by chemical cleaning operations. Bverheating of thecleaning solution can cause breadown of the inhibitor used, excessive exposure of metal to

cleaning agent, and high cleaning agent concentration. Failure to neutralize acid solventscompletely before start!up has also caused problems.

"n a boiler and feedwater system, acidic attac can tae the form of general thinning, or it canbe localized at areas of high stress such as drum baffles, ;?; bolts, acorn nuts, and tube ends.

#(drogen Em-ritt*ement

+ydrogen embrittlement is rarely encountered in industrial plants. The problem usually occursonly in units operating at or above &,


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Coordinated phosphateDp+ control can be used to minimize the decrease in boiler water p+that results from condenser leaage. =aintenance of clean surfaces and the use of proper

procedures for acid cleaning also reduce the potential for hydrogen attac.

O.(gen ,ttac/

Without proper mechanical and chemical deaeration, oxygen in the feedwater will enter theboiler. =uch is flashed off with the steam1 the remainder can attac boiler metal. The point ofattac varies with boiler design and feedwater distribution. /itting is frequently visible in the

feedwater distribution holes, at the steam drum waterline, and in downcomer tubes.

Bxygen is highly corrosive when present in hot water. *ven small concentrations can causeserious problems. 5ecause pits can penetrate deep into the metal, oxygen corrosion can resultin rapid failure of feedwater lines, economizers, boiler tubes, and condensate lines. 3dditionally,

iron oxide generated by the corrosion can produce iron deposits in the boiler.

Bxygen corrosion may be highly localized or may cover an extensive area. "t is identified bywell defined pits or a very pocmared surface. The pits vary in shape, but are characterized bysharp edges at the surface. 3ctive oxygen pits are distinguished by a reddish brown oxide cap

%tubercle). emoval of this cap exposes blac iron oxide within the pit %see Figure &&!


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T3//" engineering guidelines are less than 7 ppb

*/" fossil plant guidelines are less than < ppb dissolved oxygen

MEC#,NIC, CONDITIONS ,FFECTIN) CORROSION

=any corrosion problems are the result of mechanical and operational problems. The following

practices help to minimize these corrosion problems$

election of corrosion!resistant metals

reduction of mechanical stress where possible %e.g., use of proper welding proceduresand stress!relieving welds)

minimization of thermal and mechanical stresses during operation

operation within design load specifications, without over!firing, along with proper start!upand shutdown procedures

maintenance of clean systems, including the use of high!purity feedwater, effective andclosely controlled chemical treatment, and acid cleaning when required

Where boiler tubes fail as a result of caustic embrittlement, circumferential cracing can beseen. "n other components, cracs follow the lines of greatest stress. 3 microscopic

examination of a properly prepared section of embrittled metal shows a characteristic pattern,with cracing progressing along defined paths or grain boundaries in the crystal structure of the

metal %see Figure &&!@). The cracs do not penetrate the crystals themselves, but travelbetween them1 therefore, the term ;intercrystalline cracing; is used.

Good engineering practice dictates that the boiler water be evaluated for embrittlingcharacteristics. 3n embrittlement detector %described in Chapter &2) is used for this purpose.

"f a boiler water possesses embrittling characteristics, steps must be taen to prevent attac ofthe boiler metal. #odium nitrate is a standard treatment for inhibiting embrittlement in lower!

pressure boiler systems. The inhibition of embrittlement requires a definite ratio of nitrate to thecaustic alalinity present in the boiler water. "n higher!pressure boiler systems, wheredemineralized maeup water is used, embrittling characteristics in boiler water can be

prevented by the use of coordinated phosphateDp+ treatment control, described previouslyunder ;Caustic Corrosion.; This method prevents high concentrations of free sodium hydroxide

from forming in the boiler, eliminating embrittling tendencies.

Caustic Em-ritt*ement

Caustic embrittlement %caustic stress corrosion cracing), or intercrystalline cracing, has longbeen recognized as a serious form of boiler metal failure. 5ecause chemical attac of the metal

is normally undetectable, failure occurs suddenly!often with catastrophic results.

For caustic embrittlement to occur, three conditions must exist$

the boiler metal must have a high level of stress


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a mechanism for the concentration of boiler water must be present

the boiler water must have embrittlement!producing characteristics

Where boiler tubes fail as a result of caustic embrittlement, circumferential cracing can be

seen. "n other components, cracs follow the lines of greatest stress. 3 microscopicexamination of a properly prepared section of embrittled metal shows a characteristic pattern,with cracing progressing along defined paths or grain boundaries in the crystal structure of the

metal %see Figure &&!@). The cracs do not penetrate the crystals themselves, but travelbetween them1 therefore, the term ;intercrystalline cracing; is used.

Good engineering practice dictates that the boiler water be evaluated for embrittlingcharacteristics. 3n embrittlement detector %described in Chapter &2) is used for this purpose.

"f a boiler water possesses embrittling characteristics, steps must be taen to prevent attac ofthe boiler metal. #odium nitrate is a standard treatment for inhibiting embrittlement in lower!

pressure boiler systems. The inhibition of embrittlement requires a definite ratio of nitrate to thecaustic alalinity present in the boiler water. "n higher!pressure boiler systems, wheredemineralized maeup water is used, embrittling characteristics in boiler water can be

prevented by the use of coordinated phosphateDp+ treatment control, described previouslyunder ;Caustic Corrosion.; This method prevents high concentrations of free sodium hydroxide

from forming in the boiler, eliminating embrittling tendencies.

Fatigue Crac/ing

Fatigue cracing %due to repeated cyclic stress) can lead to metal failure. The metal failureoccurs at the point of the highest concentration of cyclic stress. *xamples of this type of failure

include cracs in boiler components at support bracets or rolled in tubes when a boilerundergoes thermal fatigue due to repeated start!ups and shutdowns.

Thermal fatigue occurs in horizontal tube runs as a result of steam blaneting and in water walltubes due to frequent, prolonged lower header blowdown.

Corrosion fatigue failure results from cyclic stressing of a metal in a corrosive environment.This condition causes more rapid failure than that caused by either cyclic stressing or corrosion

alone. "n boilers, corrosion fatigue cracing can result from continued breadown of theprotective magnetite film due to cyclic stress.

Corrosion fatigue cracing occurs in deaerators near the welds and heat!affected zones./roper operation, close monitoring, and detailed out!of!service inspections %in accordance with

published recommendations) minimize problems in deaerators.

Steam Side Burning

#team side burning is a chemical reaction between steam and the tube metal. "t is caused byexcessive heat input or poor circulation, resulting in insufficient flow to cool the tubes. ?nder

such conditions, an insulating superheated steam film develops. Bnce the tube metaltemperature has reached 7


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Erosion

*rosion usually occurs due to excessive velocities. Where two!phase flow %steam and water)exists, failures due to erosion are caused by the impact of the fluid against a surface.

*quipment vulnerable to erosion includes turbine blades, low!pressure steam piping, and heatexchangers that are sub9ected to wet steam. Feedwater and condensate piping sub9ected to

high!velocity water flow are also susceptible to this type of attac. >amage normally occurswhere flow changes direction.

MET,IC O0IDES IN BOIER S!STEMS

"ron and copper surfaces are sub9ect to corrosion, resulting in the formation of metal oxides.This condition can be controlled through careful selection of metals and maintenance of proper

operating conditions.

Iron O.ide Formation

"ron oxides present in operating boilers can be classified into two ma9or types. The first andmost important is the '.'''(!'.'''7 in. %'.(!'.7 mil) thic magnetite formed by the reaction of

iron and water in an oxygen!free environment. This magnetite forms a protective barrier againstfurther corrosion.

=agnetite forms on boiler system metal surfaces from the following overall reaction$

:Fe J 2+(B Fe:B2 J 2+(

iron watermagnetit

ehydroge

n

The magnetite, which provides a protective barrier against further corrosion, consists of twolayers. The inner layer is relatively thic, compact, and continuous. The outer layer is thinner,porous, and loose in structure. 5oth of these layers continue to grow due to water diffusion

%through the porous outer layer) and lattice diffusion %through the inner layer). 3s long as themagnetite layers are left undisturbed, their growth rate rapidly diminishes.

The second type of iron oxide in a boiler is the corrosion products, which may enter the boilersystem with the feedwater. These are frequently termed ;migratory; oxides, because they arenot usually generated in the boiler. The oxides form an outer layer over the metal surface. This

layer is very porous and easily penetrated by water and ionic species.

"ron can enter the boiler as soluble ferrous ions and insoluble ferrous and ferric hydroxides oroxides. Bxygen!free, alaline boiler water converts iron to magnetite, Fe:B2. =igratory

magnetite deposits on the protective layer and is normally gray to blac in color.

Copper O.ide Formation3 truly passive oxide film does not form on copper or its alloys. "n water, the predominant

copper corrosion product is cuprous oxide %Cu(B). 3 typical corrosion reaction follows$

0Cu J B( J (+(B 2Cu(B J (+(

copper

oxygen

watercuprous

oxide

hydrogen


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3s shown in Figure &&!7, the oxide that develops on the copper surfaces is comprised of twolayers. The inner layer is very thin, adherent, nonporous, and comprised mostly of cupric oxide

%CuB). The outer layer is thic, adherent, porous and comprised mainly of cuprous oxide%Cu(B). The outer layer is formed by breaup of the inner layer. 3t a certain thicness of the

outer layer, an equilibrium exists at which the oxide continually forms and is released into thewater.

=aintenance of the proper p+, elimination of oxygen, and application of metal!conditioningagents can minimize the amount of copper alloy corrosion.

Meta* Passi+ation

The establishment of protective metal oxide lay!ers through the use of reducing agents %suchas hydrazine, hydroquinone, and other oxygen scavengers) is nown as metal passivation ormetal conditioning. 3lthough ;metal passivation; refers to the direct reaction of the compound

with the metal oxide and ;metal conditioning; more broadly refers to the promotion of aprotective surface, the two terms are frequently used interchangeably.

The reaction of hydrazine and hydroquinone, which leads to the passivation of iron!basedmetals, proceeds according to the following reactions$

A(+2 J @Fe(B: 2Fe:B2 J (+(B J A(

hydrazine

hematite

magnetite

water

nitrogen

C@+2%B+)( J :Fe(B: (Fe:B2 J C@+2B( J +(B

hydroquinone

hematite

magnetite

benzoquinone

water

#imilar reactions occur with copper!based metals$

A(+2 J 2CuB (Cu(B J (+(B J A(

hydrazine

cupric

oxide

cuprous

oxidewater

nitrogen

C@+@B( J (CuB Cu(B J C@+2B( J +(B

hydroquinone

cupric

oxide

cuprous

oxide

benzoquinone

water

=agnetite and cuprous oxide form protective films on the metal surface. 5ecause these oxidesare formed under reducing conditions, removal of the dissolved oxygen from boiler feedwater

and condensate promotes their formation. The effective application of oxygen scavengersindirectly leads to passivated metal surfaces and less metal oxide transport to the boiler

whether or not the scavenger reacts directly with the metal surface.


18/102

3 significant reduction in feedwater oxygen and metal oxides can occur with proper applicationof oxygen scavengers %see Figure &&!0).

CORROSION CONTRO F,CTORS

Stee* and Stee* ,**o(s

/rotection of steel in a boiler system depends on temperature, p+, and oxygen content.Generally, higher temperatures, high or low p+ levels, and higher oxygen concentrations

increase steel corrosion rates.

=echanical and operational factors, such as velocities, metal stresses, and severity of servicecan strongly influence corrosion rates. #ystems vary in corrosion tendencies and should be

evaluated individually.

Copper and Copper ,**o(s

=any factors influence the corrosion rate of copper alloys$

temperature

p+

oxygen concentration

amine concentration

ammonia concentration

flow rate

The impact of each of these factors varies depending on characteristics of each system.Temperature dependence results from faster reaction times and greater solubility of copperoxides at elevated temperatures. =aximum temperatures specified for various alloys range

from ('' to :''HF.

=ethods of minimizing copper and copper alloy corrosion include$

replacement with a more resistant metal

elimination of oxygen

maintenance of high!purity water conditions

operation at the proper p+ level

reduction of water velocities

application of materials which passivate the metal surfaces


19/102

p# Contro*

=aintenance of proper p+ throughout the boiler feedwater, boiler, and condensate systems isessential for corrosion control. =ost low!pressure boiler system operators monitor boiler water

alalinity because it correlates very closely with p+, while most feedwater, condensate, andhigh!pressure boiler water requires direct monitoring of p+. Control of p+ is important for the

following reasons$

corrosion rates of metals used in boiler systems are sensitive to variations in p+

low p+ or insufficient alalinity can result in corrosive acidic attac

high p+ or excess alalinity can result in caustic gougingDcracing and foaming, withresultant carryover

speed of oxygen scavenging reactions is highly dependent on p+ levels

The p+ or alalinity level maintained in a boiler system depends on many factors, such as sys!tem pressure, system metals, feedwater quality, and type of chemical treatment applied.

The corrosion rate of carbon steel at feedwater temperatures approaches a minimum value inthe p+ range of 6.(!6.@%see Figure &&!6). "t is important to monitor the feedwater system forcorrosion by means of iron and copper testing. For systems with sodium zeolite or hot lime

softened maeup, p+ ad9ustment may not be necessary. "n systems that use deionized watermaeup, small amounts of caustic soda or neutralizing amines, such as morpholine and

cyclohexylamine, can be used.

"n the boiler, either high or low p+ increases the corrosion rates of mild steel%see Figure &&!&').The p+ or alalinity that is maintained depends on the pressure, maeup water characteristics,

chemical treatment, and other factors specific to the system.The best p+ for protection of copper alloys is somewhat lower than the optimum level for

carbon steel. For systems that contain both metals, the condensate and feedwater p+ is oftenmaintained between 0.0 and 6.( for corrosion protection of both metals.The optimum p+ variesfrom system to system and depends on many factors, including the alloy used %see Figure &&!

&&).

To elevate p+, neutralizing amines should be used instead of ammonia, which %especially in thepresence of oxygen) accelerates copper alloy corrosion rates. 3lso, amines form protective

films on copper oxide surfaces that inhibit corrosion.

O.(gen Contro*

Chemical Bxygen #cavengers. The oxygen scavengers most commonly used in boiler systemsare sodium sulfite, sodium bisulfite, hydrazine, catalyzed versions of the sulfites and hydrazine,

and organic oxygen scavengers, such as hydroquinone and ascorbate.

"t is of critical importance to select and properly use the best chemical oxygen scavenger for agiven system. =a9or factors that determine the best oxygen scavenger for a particular

application include reaction speed, residence time in the system, operating temperature andpressure, and feedwater p+. "nterferences with the scavengerDoxygen reaction, decomposition


20/102

products, and reactions with metals in the system are also important factors. Bther contributingfactors include the use of feedwater for attemperation, the presence of economizers in thesystem, and the end use of the steam. Chemical oxygen scavengers should be fed to allow

ample time for the scavengerDoxygen reaction to occur. The deaerator storage system and thefeedwater storage tan are commonly used feed points.

"n boilers operating below &,''' psig, sodium sulfite and a concentrated liquid solution ofcatalyzed sodium bisulfite are the most commonly used materials for chemical deaeration dueto low cost and ease of handling and testing. The oxygen scavenging property of sodium sulfite

is illustrated by the following reaction$

(Aa(#B: J B( (Aa(#B2

sodiumsulfite

oxygensodiumsulfate

Theoretically, 7.00 ppm of chemically pure sodium sulfite is required to remove &.' ppm ofdissolved oxygen. +owever, due to the use of technical grades of sodium sulfite, combined with

handling and blowdown losses during normal plant operation, approximately &' lb of sodiumsulfite per pound of oxygen is usually required. The concentration of excess sulfite maintained

in the feedwater or boiler water also affects the sulfite requirement.

#odium sulfite must be fed continuously for maximum oxygen removal. ?sually, the mostsuitable point of application is the drop leg between the deaerator and the storage

compartment. Where hot process softeners are followed by hot zeolite units, an additional feedis recommended at the filter effluent of the hot process units %prior to the zeolite softeners) to

protect the ion exchange resin and softener shells.

3s with any oxygen scavenging reaction, many factors affect the speed of the sulfite!oxygenreaction. These factors include temperature, p+, initial concentration of oxygen scavenger,

initial concentration of dissolved oxygen, and catalytic or inhibiting effects. The most importantfactor is temperature. 3s temperature increases, reaction time decreases1 in general, every&0HF increase in temperature doubles reaction speed. 3t temperatures of (&(HF and above, the

reaction is rapid. Bverfeed of sodium sulfite also increases reaction rate. The reactionproceeds most rapidly at p+ values in the range of 0.

Certain materials catalyze the oxygen!sulfite reaction. The most effective catalysts are theheavy metal cations with valences of two or more. "ron, copper, cobalt, nicel, and manganese

are among the more effective catalysts.

Figure &&!&( compares the removal of oxygen using commercial sodium sulfite and a catalyzedsodium sulfite.3fter (< seconds of contact, catalyzed sodium sulfite removed the oxygen

completely. ?ncatalyzed sodium sulfite removed less than


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rapid reaction required to prevent pitting in the system

short residence time

use of economizers

+igh feedwater sulfite residuals and p+ values above 0.< should be maintained in thefeedwater to help protect the economizer from oxygen attac.

#ome natural waters contain materials that can inhibit the oxygenDsulfite reaction. For example,trace organic materials in a surface supply used for maeup water can reduce speed of

scavengerDoxygen reaction time. The same problem can occur where contaminatedcondensate is used as a portion of the boiler feedwater. The organic materials complex metals

%natural or formulated catalysts) and prevent them from increasing the rate of reaction.

#odium sulfite must be fed where it will not contaminate feedwater to be used for attemporationor desuperheating. This prevents the addition of solids to the steam.

3t operating pressures of &,''' psig and higher, hydrazine or organic oxygen scavengers arenormally used in place of sulfite. "n these applications, the increased dissolved solids

contributed by sodium sulfate %the product of the sodium sulfite!oxygen reaction) can become asignificant problem. 3lso, sulfite decomposes in high!pressure boilers to form sulfur dioxide

%#B() and hydrogen sulfide %+(#). 5oth of these gases can cause corrosion in the returncondensate system and have been reported to contribute to stress corrosion cracing in

turbines. +ydrazine has been used for years as an oxygen scavenger in high!pressure systemsand other systems in which sulfite materials cannot be used. +ydrazine is a reducing agent that

removes dissolved oxygen by the following reaction$

A(+2 J B( (+(B J A(

hydrazine oxygen water nitrogen

5ecause the products of this reaction are water and nitrogen, the reaction adds no solids to theboiler water. The decomposition products of hydrazine are ammonia and nitrogen.

>ecomposition begins at approximately 2''HF and is rapid at @''HF. The alaline ammoniadoes not attac steel. +owever, if enough ammonia and oxygen are present together, copper

alloy corrosion increases. Close control of the hydrazine feed rate can limit the concentration ofammonia in the steam and minimize the danger of attac on copper!bearing alloys. The

ammonia also neutralizes carbon dioxide and reduces the return line corrosion caused bycarbon dioxide.

+ydrazine is a toxic material and must be handled with extreme care. 5ecause the material is asuspected carcinogen, federally published guidelines must be followed for handling and

reporting. 5ecause pure hydrazine has a low flash point, a :


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The factors that influence the reaction time of sodium sulfite also apply to other oxygenscavengers. Figure &&!&: shows rate of reaction as a function of temperature and hydrazineconcentration. The reaction is also dependent upon p+ %the optimum p+ range is 6.'!&'.').

"n addition to its reaction with oxygen, hydrazine can also aid in the formation of magnetite andcuprous oxide %a more protective form of copper oxide), as shown in the following reactions$

A(+2 J @Fe(B: 2Fe:B2 J A( J (+(B

hydrazine

hematite

magnetite

nitrogen

water

and

A(+2 J 2CuB (Cu(B J A( J (+(B

hydrazine

cupric

oxide

cuprous oxide

nitrogen

water

5ecause hydrazine and organic scavengers add no solids to the steam, feedwater containingthese materials is generally satisfactory for use as attemperating or desuperheating water.

The ma9or limiting factors of hydrazine use are its slow reaction time %particularly at lowtemperatures), ammonia formation, effects on copper!bearing alloys, and handling problems.

Brganic Bxygen #cavengers. #everal organic compounds are used to remove dissolvedoxygen from boiler feedwater and condensate. 3mong the most commonly used compoundsare hydroquinone and ascorbate. These materials are less toxic than hydrazine and can behandled more safely. 3s with other oxygen scavengers, temperature, p+, initial dissolved

oxygen concentration, catalytic effects, and scavenger concentration affect the rate of reaction

with dissolved oxygen. When fed to the feedwater in excess of oxygen demand or when feddirectly to the condensate, some organic oxygen scavengers carry forward to protect steamand condensate systems.

+ydroquinone is unique in its ability to react quicly with dissolved oxygen, even at ambienttemperature. 3s a result of this property, in ad!dition to its effectiveness in operating systems,hydroquinone is particularly effective for use in boiler storage and during system start!ups and

shutdowns. "t is also used widely in condensate systems.

+ydroquinone reacts with dissolved oxygen as shown in the following reactions$

C@+2%B+)( J B( C@+2B( J +(B

hydroquinone

oxyge

n

benzoquinone water

5enzoquinone reacts further with oxygen to form polyquinones$

C@+2B( J B( polyquinone

s


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benzoquinone

oxygen

These reactions are not reversible under the alaline conditions found in boiler feedwater andcondensate systems. "n fact, further oxidation and thermal degradation %in higher!pressure

systems) leads to the final product of carbon dioxide. "ntermediate products are low molecularweight organic compounds, such as acetates.

Bxygen -evel =onitoring. Bxygen monitoring provides the most effective means of controllingoxygen scavenger feed rates. ?sually, a slight excess of scavenger is fed. Feedwater andboiler water residuals provide an indication of excess scavenger feed and verify chemical

treatment feed rates. "t is also necessary to test for iron and copper oxides in order to assessthe effectiveness of the treatment program. /roper precautions must be taen in sampling for

metal oxides to ensure representative samples.

>ue to volatility and decomposition, measurement of boiler residuals is not a reliable means ofcontrol. The amount of chemical fed should be recorded and compared with oxygen levels in

the feedwater to provide a chec on the control of dissolved oxygen in the system. With sodiumsulfite, a drop in the chemical residual in the boiler water or a need to increase chemical feedmay indicate a problem. =easures must be taen to determine the cause so that the problem

can be corrected.

#ulfite residual limits are a function of boiler operating pressure. For most low! and medium!pressure systems, sulfite residuals should be in excess of (' ppm. +ydrazine control is usuallybased on a feedwater excess of '.'


24/102

3ppropriate monitoring techniques vary with different systems. Testing should be performed atleast once per shift. Testing frequency may have to be increased for some systems where

control is difficult, or during periods of more variable operating conditions. 3ll monitoring data,whether spot sampling or continuous, should be recorded.

5oiler feedwater hardness, iron, copper, oxygen, and p+ should be measured. 5oth iron and

copper, as well as oxygen, can be measured on a daily basis. "t is recommended that, whenpossible, a continuous oxygen meter be installed in the feedwater system to detect oxygen

intrusions. "ron and copper, in particular, should be measured with care due to possibleproblems of sample contamination.

"f a continuous oxygen meter is not installed, periodic testing with spot sampling ampoulesshould be used to evaluate deaerator performance and potential for oxygen contamination from

pump seal water and other sources.

For the boiler water, the following tests should be performed$

phosphate %if used)

/!alalinity or p+

sulfite %if used)

conductivity

Samp*ing

"t is critical to obtain representative samples in order to monitor conditions in the boilerfeedwater system properly. #ample lines, continuously flowing at the proper velocity and

volume, are required. Generally, a velocity of


25/102

used either to verify that all potential causes of problems are reviewed, or to troubleshoot aparticular corrosion!related problem.

CORROSION PROTECTION D1RIN) DO'NTIME ,ND STOR,)E

Bxygen corrosion in boiler feedwater systems can occur during start!up and shutdown and

while the boiler system is on standby or in storage, if proper procedures are not followed.#ystems must be stored properly to prevent corrosion damage, which can occur in a matter ofhours in the absence of proper lay!up procedures. 5oth the waterDsteam side and the fireside

are sub9ect to downtime corrosion and must be protected.

Bff!line boiler corrosion is usually caused by oxygen in!leaage. -ow p+ causes furthercorrosion. -ow p+ can result when oxygen reacts with iron to form hydroferric acid. This

corrosion product, an acidic form of iron, forms at water!air interfaces.

Corrosion also occurs in boiler feedwater and condensate systems. Corrosion productsgenerated both in the preboiler section and the boiler may deposit on critical heat transfersurfaces of the boiler during operation and increase the potential for localized corrosion or

overheating.

The degree and speed of surface corrosion depend on the condition of the metal. "f a boilercontains a light surface coating of boiler sludge, surfaces are less liely to be attaced because

they are not fully exposed to oxygen!laden water. *xperience has indicated that with theimproved cleanliness of internal boiler surfaces, more attention must be given to protectionfrom oxygen attac during storage. 5oilers that are idle even for short time periods %e.g.,

weeends) are susceptible to attac.

5oilers that use undeaerated water during start!up and during their removal from service canbe severely damaged. The damage taes the form of oxygen pitting scattered at random overthe metal surfaces. >amage due to these practices may not be noticed for many years after

installation of the unit.

The choice of storage methods depends on the length of downtime expected and the boilercomplexity. "f the boiler is to be out of service for a month or more, dry storage may be

preferable. Wet storage is usually suitable for shorter down!time periods or if the unit may berequired to go on!line quicly. -arge boilers with complex circuits are difficult to dry, so they

should be stored by one of the wet storage methods.

Dr( Storage

For dry storage, the boiler is drained, cleaned, and dried completely. 3ll horizontal and non!drainable boiler and superheater tubes must be blown dry with compressed gas. /articular care

should be taen to purge water from long horizontal tubes, especially if they have bowed

slightly.+eat is applied to optimize drying. 3fter drying, the unit is closed to minimize air circulation.+eaters should be installed as needed to maintain the temperature of all surfaces above the

dew point.

"mmediately after surfaces are dried, one of the three following desiccants is spread on water!tight wood or corrosion!resistant trays$


26/102

quiclime!used at a rate of @ lbD&'' ftK of boiler volume

silica gel!used at a rate of &7 lbD&'' ftK of boiler volume

activated alumina!used at a rate of (7 lbD&'' ftK of boiler volume

The trays are placed in each drum of a water tube boiler, or on the top flues of a fire!tube unit.3ll manholes, handholes, vents, and connections are blaned and tightly closed. The boiler

should be opened every month for inspection of the desiccant. "f necessary, the desiccantshould be renewed.

'et Storage

For wet storage, the unit is inspected, cleaned if necessary, and filled to the normal water levelwith deaerated feedwater.

#odium sulfite, hydrazine, hydroquinone, or another scavenger is added to control dissolvedoxygen, according to the following requirements$

#odium sulfite. : lb of sodium sulfite and : lb of caustic soda should be added per &'''gal of water contained in the boiler %minimum 2'' ppm /!alalinity as CaCB: and (''

ppm sulfite as #B:).

+ydrazine. < lb of a :


27/102

3 surge tan %such as a changes due to temperature variations.

The drain between the nonreturn valve and main steam stop valve is left open wide. 3ll otherdrains and vents are closed tightly.

The boiler water should be tested weely with treatment added as necessary to maintaintreatment levels. When chemicals are added, they should be mixed by one of the following

methods$

circulate the boiler water with an external pump

reduce the water level to the normal operating level and steam the boiler for a short time

"f the steaming method is used, the boiler should subsequently be filled completely, in eepingwith the above recommendations.

3lthough no other treatment is required, standard levels of the chemical treatment used whenthe boiler is operating can be present.

5oilers can be protected with nitrogen or another inert gas. 3 slightly positive nitrogen %or otherinert gas) pressure should be maintained after the boiler has been filled to the operating level

with deaerated feedwater.

Storage of Feed"ater #eaters and Deaerators

The tube side of a feedwater heater is treated in the same way the boiler is treated duringstorage. The shell side can be steam blaneted or flooded with treated condensate.

3ll steel systems can use the same chemical concentrations recommended for wet storage.

Copper alloy systems can be treated with half the amount of oxygen scavenger, with p+controlled to 6.eaerators are usually steam or nitrogen blaneted1 however, they can be flooded with a lay!upsolution as recommended for wet lay!up of boilers. "f the wet method is used, the deaerator

should be pressurized with < psig of nitrogen to prevent oxygen ingress.

Cascading B*o"do"n

For effective yet simple boiler storage, clean, warm, continuous blowdown can be distributedinto a convenient bottom connection on an idle boiler. *xcess water is allowed to overflow to anappropriate disposal site through open vents. This method decreases the potential for oxygeningress and ensures that properly treated water enters the boiler. This method should not be

used for boilers equipped with nondrainable superheaters.

Co*d 'eat%er Storage

"n cold weather, precautions must be taen to prevent freezing. 3uxiliary heat, light firing of theboiler, cascade lay!up, or dry storage may be employed to prevent freezing problems.

#ometimes, a


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Disposa* of a(&up So*utions

The disposal of lay!up chemicals must be in compliance with applicable federal, state, and localregulations.

Fireside Storage

When boilers are removed from the line for extended periods of time, fireside areas must alsobe protected against corrosion.

Fireside deposits, particularly in the convection, economizer, and air heater sections, arehygroscopic in nature. When metal surface temperatures drop below the dew point,

condensation occurs, and if acidic hygroscopic deposits are present, corrosion can result.

The fireside areas %particularly the convection, economizer, and air heater sections) should becleaned prior to storage.

+igh!pressure alaline water is an effective means of cleaning the fireside areas. 5eforealaline water is used for this purpose, a rinse should be made with fresh water of neutral p+ to

prevent the formation of hydroxide gels in the deposits %these deposits can be very difficult toremove).

Following chemical cleaning with a water solution, the fireside should be dried by warm air or asmall fire. "f the boiler is to be completely closed up, silica gel or lime can be used to absorbany water of condensation. 3s an alternative, metal surfaces can be sprayed or wiped with a

light oil.

"f the fireside is to be left open, the metal sur!faces must be maintained above the dew point bycirculation of warm air.

s

>eposition is a ma9or problem in the operation of steam generating equipment. Theaccumulation of material on boiler surfaces can cause overheating andDor corrosion. 5oth ofthese conditions frequently result in unscheduled downtime.

5oiler feedwater pretreatment systems have advanced to such an extent that it is now possibleto provide boilers with ultrapure water. +owever, this degree of purification requires the use ofelaborate pretreatment systems. The capital expenditures for such pretreatment equipment

trains can be considerable and are often not 9ustified when balanced against the capability ofinternal treatment.

The need to provide boilers with high!quality feedwater is a natural result of the advancesmade in boiler performance. The ratio of heating surface to evaporation has decreased.

Consequently, heat transfer rates through radiant water wall tubes have increased!occasionallyin excess of ('',''' 5tuDftIDhr. The tolerance for deposition is very low in these systems.

The quality of feedwater required is dependent on boiler operating pressure, design, heattransfer rates, and steam use. =ost boiler systems have sodium zeolite softened or

demineralized maeup water. Feedwater hardness usually ranges from '.'& to (.' ppm, buteven water of this purity does not provide deposit!free operation. Therefore, good internal boiler

water treatment programs are necessary.


29/102

DEPOSITS

Common feedwater contaminants that can form boiler deposits include calcium, magnesium,iron, copper, aluminum, silica, and %to a lesser extent) silt and oil. =ost deposits can be

classified as one of two types %Figure &(!&)$

scale that crystallized directly onto tube surfaces

sludge deposits that precipitated elsewhere and were transported to the metal surfaceby the flowing water

#cale is formed by salts that have limited solubility but are not totally insoluble in boiler water.These salts reach the deposit site in a soluble form and precipitate when concentrated byevaporation. The precipitates formed usually have a fairly homogeneous composition and

crystal structure.

+igh heat transfer rates cause high evaporation rates, which concentrate the remaining waterin the area of evaporation. 3 number of different scale!forming compounds can precipitate fromthe concentrated water. The nature of the scale formed depends on the chemical composition

of the concentrated water. Aormal deposit constituents are calcium, magnesium, silica,aluminum, iron, and %in some cases) sodium.

The exact combinations in which they exist vary from boiler to boiler, and from location tolocation within a boiler %Table &(!&). #cale may form as calcium silicate in one boiler and as

sodium iron silicate in another.

Compared to some other precipitation reactions, such as the formation of calcium phosphate,the crystallization of scale is a slow process. 3s a result, the crystals formed are well defined,and a hard, dense, and highly insulating material is formed on the tube metal. #ome forms of

scale are so tenacious that they resist any type of removal!mechanical or chemical.

#ludge is the accumulation of solids that precipitate in the bul boiler water or enter the boileras suspended solids. #ludge deposits can be hard, dense, and tenacious. When exposed tohigh heat levels %e.g., when a boiler is drained hot), sludge deposits are often baed in place.

#ludge deposits hardened in this way can be as troublesome as scale.

Bnce deposition starts, particles present in the circulating water can become bound to thedeposit. "ntraparticle binding does not need to occur between every particle in a deposit mass.

#ome nonbound particles can be captured in a networ of bound particles.

Table &(!&. Crystalline scale constituents identified by N!ray diffraction.

Name Formu*a

3cmite Aa(BFe(B:2#iB(

3nalcite Aa(B3l(B:2#iB((+(B

3nhydrite Ca#B2

3ragonite CaCB:

5rucite =g%B+)(


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Calcite CaCB:

Cancrinite 2Aa(BCaB23l(B:(CB(6#iB(:+(B

+ematite Fe(B:

+ydroxyapatite Ca&'%B+)(%/B2)@

=agnetite Fe:B2

Aoselite 2Aa(B:3l(B:@#iB(#B2

/ectolite Aa(B2CaB@#iB(+(B

Ouartz #iB(

#erpentine :=gB(#iB((+(B

Thenardite Aa(#B2

Wallastonite Ca#iB:

Nonotlite


31/102

the nominal boiler rating. When the circuit is dirty, the inflection point of the circulation!to!heatinput curve moves to the left, and the overall water circulation is reduced. This is represented

by the lower broen line.

Circulation and deposition are closely related. The deposition of particles is a function of watersweep as well as surface charge %Figure &(!@)."f the surface charge on a particle is relatively

neutral in its tendency to cause the particle either to adhere to the tube wall or to remainsuspended, an adequate water sweep will eep it off the tube. "f the circulation through a circuitis not adequate to provide sufficient water sweep, the neutral particle may adhere to the tube."n cases of extremely low circulation, total evaporation can occur and normally soluble sodium

salts deposit.

C#EMIC, TRE,TMENT

#odium carbonate treatment was the original method of controlling calcium sulfate scale.Todays methods are based on the use of phosphates and chelants. The former is a

precipitating program, the latter a solubilizing program.

Car-onate Contro*

5efore the acceptance of phosphate treatment in the &6:'s, calcium sulfate scaling was ama9or boiler problem. #odium carbonate treatment was used to precipitate calcium as calcium

carbonate to prevent the formation of calcium sulfate. The driving force for the formation ofcalcium carbonate was the maintenance of a high concentration of carbonate ion in the boilerwater. *ven where this was accomplished, ma9or scaling by calcium carbonate was common.3s boiler pressures and heat transfer rates slowly rose, the calcium carbonate scale became

unacceptable, as it led to tube overheating and failure.

P%osp%ate Contro*

Calcium phosphate is virtually insoluble in boiler water. *ven small levels of phosphate can be

maintained to ensure the precipitation of calcium phosphate in the bul boiler water!away fromheating surfaces. Therefore, the introduction of phosphate treatment eliminated the problem ofcalcium carbonate scale. When calcium phosphate is formed in boiler water of sufficient

alalinity %p+ &&.'!&(.'), a particle with a relatively nonadherent surface charge is produced.This does not prevent the development of deposit accumulations over time, but the deposits

can be controlled reasonably well by blowdown.

"n a phosphate precipitation treatment program, the magnesium portion of the hardnesscontamination is precipitated preferentially as magnesium silicate. "f silica is not present, themagnesium will precipitate as magnesium hydroxide. "f insufficient boiler water alalinity isbeing maintained, magnesium can combine with phosphate. =agnesium phosphate has a

surface charge that can cause it to adhere to tube surfaces and then collect other solids. For

this reason, alalinity is an important part of a phosphate precipitation program.

The magnesium silicate formed in a precipitating program is not particularly adherent. +owever,it contributes to deposit buildup on a par with other contaminants. 3nalyses of typical boiler

deposits show that magnesium silicate is present in roughly the same ratio to calciumphosphate as magnesium is to calcium in boiler feedwater.

P%osp%ate2Po*(mer Contro*


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/hosphate treatment results are improved by organic supplements. Aaturally occurringorganics such as lignins, tannins, and starches were the first supplements used. The organics

were added to promote the formation of a fluid sludge that would settle in the mud drum.5ottom blowdown from the mud drum removed the sludge.

There have been many advances in organic treatments%Figure &(!7). #ynthetic polymers are

now used widely, and the emphasis is on dispersion of particles rather than fluid sludgeformation.3lthough this mechanism is quite complex, polymers alter the surface area and the

surface charge to mass ratio of typical boiler solids. With proper polymer selection andapplication, the surface charge on the particle can be favorably altered %Figure &(!0).

=any synthetic polymers are used in phosphate precipitation programs. =ost are effective indispersing magnesium silicate and magnesium hydroxide as well as calcium phosphate. The

polymers are usually low in molecular weight and have numerous active sites. #ome polymersare used specifically for hardness salts or for iron1 some are effective for a broad spectrum ofions. Figure &(!6 shows the relative performance of different polymers used for boiler water

treatment.

Table &(!(. /hosphateDpolymer performance can be maintained at high heat transfer ratesthrough selectionof the appropriate polymer.

C%e*ant Contro*

Chelants are the prime additives in a solubilizing boiler water treatment program. Chelantshave the ability to complex many cations %hardness and heavy metals under boiler water

conditions). They accomplish this by locing metals into a soluble organic ring structure. Thechelated cations do not deposit in the boiler. When applied with a dispersant, chelants produce

clean waterside surfaces.

#uppliers and users of chelants have learned a great deal about their successful applicationsince their introduction as a boiler feedwater treatment method in the early &6@'s. Chelants

were heralded as ;miracle treatment; additives. +owever, as with any material, the greatestchallenge was to understand the proper application.

Chelants are wea organic acids that are in9ected into boiler feedwater in the neutralizedsodium salt form. The water hydrolyzes the chelant, producing an organic anion. The degree of

hydrolysis is a function of p+1 full hydrolysis requires a relatively high p+.

The anionic chelant has reactive sites that attract coordination sites on cations %hardness andheavy metal contaminants). Coordination sites are areas on the ion that are receptive to

chemical bonding. For example, iron has six coordination sites, as does *>T3%ethylenediaminetetraacetic acid). "ron ions entering the boiler %e.g., as contamination from thecondensate system) combine with *>T3.3ll coordination sites on the iron ion are used by the

*>T3, and a stable metal chelate is formed %Figure &(!&').

AT3 %nitrilotriacetic acid), another chelant applied to boiler feedwater, has four coordinationsites and does not form as stable a complex as *>T3. With AT3, the unused coordination sites

on the cation are susceptible to reactions with competing anions.

Chelants combine with cations that form deposits, such as calcium, magnesium, iron, andcopper. The metal chelate formed is water!soluble. When the chelate is stable, precipitation
http://www.gewater.com/handbook/boiler_water_systems/table12-2.jsphttp://www.gewater.com/handbook/boiler_water_systems/table12-2.jsp


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does not occur. 3lthough there are many substances having chelating properties, *>T3 andAT3 are, to date, the most suitable chelants for boiler feedwater treatment.

The logarithm of the equilibrium constant for the chelant!metal ion reaction, frequently calledthe #tability Constant %Ps ), can be used to assess the chemical stability of the complex

formed. For the calcium!*>T3 reaction$

%Ca!*>T3)(

Ps log &'.T3)2

Table &(!: lists stability constants for *>T3 and AT3 with common feedwater contaminants.

Table &(!:. #tability constants provide a measure of chemical stability of the chelant!metal ioncomplexes.

Meta* Ion EDT, NT,

CaJ( &'.T3, can complex iron deposits. +owever, thisability is limited by competition with hydrate ions. *xperience has shown that relying on *>T3

or other chelants alone is not the most satisfac!tory method for iron control.

3t normal chelant feed rates, limited chelation of incoming particulate iron occurs. This isusually enough to solubilize some condensate iron contamination. The chelation of magnetite%the oxide formed under boiler conditions!a mix of Fe(B: and FeB) is possible because the

chelant combines with the ferrous %FeB) portion of the magnetite.

Bverfeed %high levels) of chelant can remove large quantities of iron oxide. +owever, this isundesirable because high excess chelant cannot distinguish between the iron oxide that forms

the protective magnetite coating and iron oxide that forms deposits.


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3 chelantDpolymer combination is an effective approach to controlling iron oxide. 3dequatechelant is fed to complex hardness and soluble iron, with a slight excess to solubilize iron

contamination./olymers are then added to condition and disperse any remaining iron oxidecontamination %Figure &(!&&).

3 chelantDpolymer program can produce clean waterside surfaces, contributing to much more

reliable boiler operation %Figure &(!&(). But!of!service boiler cleaning schedules can beextended and, in some cases, eliminated. This depends on operational control and feedwaterquality. Chelants with high complexing stabilities are ;forgiving; treatments!they can remove

deposits that form when feedwater quality or treatment control periodically deviates fromstandard.

5oilers with moderate deposition in the forms of calcium carbonate and calcium phosphate canbe cleaned effectively through an in!service chelant cleanup program. "n!service chelant

cleanup programs should be controlled and not attempted on a heavily deposited boiler orapplied at too fast a pace. Chelants can cause large accumulations of deposit to slough off in ashort period of time. These accumulations can plug headers or redeposit in critical circulation

areas, such as furnace wall tubes.

"n a chelant cleanup program, sufficient chelant is added to solubilize incoming feedwaterhardness and iron. This is followed by a recommended excess chelant feed. egular

inspections %usually every 6' days) are highly recommended so that the progress of thetreatment may be monitored.

The polymer level in the boiler should also be increased above the normal concentration. Thisconfines particles to the bul water as much as possible until they settle in the mud drum. 3nincreased number of mud drum ;blows; should be performed to remove the particles from the

boiler.

"n!service chelant cleanup programs are not advisable when deposit analyses reveal that ma9or

constituents are composed of silicates, iron oxide, or any scale that appears to be hard, tightlybound, or lacing in porosity. 5ecause such scales are not successfully removed in most in!stances, an in!service chelant cleanup cannot be 9ustified in these situations.

P%osp%ate2C%e*ant2Po*(mer Com-inations

Combinations of polymer, phosphate, and chelant are commonly used to produce resultscomparable to chelantDpolymer treatment in low! to medium!pressure boilers. 5oiler cleanliness

is improved over phosphate treatment, and the presence of phosphate provides an easymeans of testing to confirm the presence of treatment in the boiler water.

Po*(mer&On*( Treatment

/olymer!only treatment programs are also used with a degree of success. "n this treatment, thepolymer is usually used as a wea chelant to complex the feedwater hardness. Thesetreatments are most successful when feedwater hardness is consistently very low.

#ig%&Pressure Boi*er 'ater Treatment

+igh!pressure boilers usually have areas of high heat flux and feedwater, composed ofdemineralized maeup water and a high percentage of condensate returns. 5ecause of these


35/102

conditions, high!pressure boilers are prone to caustic attac. -ow!pressure boilers that usedemineralized water and condensate as feedwater are also susceptible to caustic attac.

There are several means by which boiler water can become highly concentrated. Bne of themost common is iron oxide deposition on radiant wall tubes. "ron oxide deposits are often quiteporous and act as miniature boilers. Water is drawn into the iron oxide deposit. +eat applied to

the deposit from the tube wall generates steam, which passes out through the deposit. =orewater enters the deposit, taing the place of the steam. This cycle is repeated and the waterbeneath the deposit is concentrated to extremely high levels. "t is possible to have &'','''ppm of caustic beneath the deposit while the bul water contains only about

3 comparison of the untreated heat transfer surface %shown at left) with the polymer dispersanttreated conditions %shown at right) provides a graphic illustration of the value of dispersants inpreventing steam generator deposition. The ability to reduce iron oxide accumulations is an

important requirement in the treatment of boiler systems operating at high pressures and with

high!purity feedwater.

#upercritical boilers use all!volatile treatments, generally consisting of ammonia and hydrazine.5ecause of the extreme potential for deposit formation and steam contamination, no solids can

be tolerated in supercritical once!through boiler water, including treatment solids.


36/102

5oiler blowdown is the removal of water from a boiler. "ts purpose is to control boiler waterparameters within prescribed limits to minimize scale, corrosion, carryover, and other specificproblems. 5lowdown is also used to remove suspended solids present in the system. Thesesolids are caused by feedwater contamination, by internal chemical treatment precipitates, or

by exceeding the solubility limits of otherwise soluble salts.

"n effect, some of the boiler water is removed %blowndown) and replaced with feedwater. Thepercentage of boiler blowdown is as follows$

quantity blowdown waterN &'' 8 blowdown

quantity feedwater

The blowdown can range from less than &8 when an extremely high!quality feedwater is

available to greater than ('8 in a critical system with poor!quality feedwater. "n plants withsodium zeolite softened maeup water, the percentage is commonly determined by means of a

chloride test. "n higher!pressure boilers, a soluble, inert material may be added to the boilerwater as a tracer to determine the percentage of blowdown. The formula for calculating

blowdown percentage using chloride and its derivation are shown in Table &:!&.

Table &:!&. 3lgebraic proof of blowdown formula.

-et

x Ouantity feedwater

y quantity blowdown water

a chloride concentration in feedwater

b chloride concentration in boiler water

k percent blowdown

5y definition of percent blowdown

k

&''y

x

5ecause the total chlorides entering the boiler must equal totalchlorides leaving boiler,

xa xb

=ultiplying bothsides by

&''gives$

xb

gives$


37/102

&''a

&''y

b x

5ecause bydefinition

&''y k, thenk

&''a

orx b

Cl infeedwater

N &'' 8 blowdownCl in boiler

water

IMITIN) F,CTORS ,FFECTIN) BO'DO'N

The primary purpose of blowdown is to maintain the solids content of boiler water within certainlimits. This may be required for specific reasons, such as contamination of the boiler water. "nthis case, a high blowdown rate is required to eliminate the contaminants as rapidly as

possible.

The blowdown rate required for a particular boiler depends on the boiler design, the operatingconditions, and the feedwater contaminant levels. "n many systems, the blowdown rate is

determined according to total dissolved solids. "n other systems, alalinity, silica, or suspendedsolids levels determine the required blowdown rate.

For many years, boiler blowdown rates were established to limit boiler water contaminants tolevels set by the 3merican 5oiler =anufacturers 3ssociation %35=3) in its #tandard Guarantee

of #team /urity. These standards were used even though they were of a general nature andnot applicable to each individual case. Today, the 3#=* ;Consensus on Bperating /racticesfor the Control of Feedwater and 5oiler Water Ouality in =odern "ndustrial 5oilers,; shown in

Table &:!(, is frequently used for establishing blowdown rates.

This consensus applies to deposition control as well as steam quality. Good engineering9udgment must be used in all cases. 5ecause each specific boiler system is different, control

limits may be different as well. There are many mechanical factors that can affect the blowdowncontrol limits, including boiler design, rating, water level, load characteristics, and type of fuel.

"n some cases, the blowdown control limits for a particular system may be determined byoperating experience, equipment inspections, or steam purity testing rather than 3#=* or

35=3 water quality criteria. "n certain cases, it is possible to exceed standard total solids %or

conductivity), silica, or alalinity limits.3ntifoam agents have been applied successfully to allowhigher!than!normal solids limits, as shown in Figure &:!&. Chelating and effective dispersant

programs also may allow certain water criteria to be exceeded.

The maximum levels possible for each specific system can be determined only fromexperience. The effect of water characteristics on steam quality can be verified with steam

purity testing. +owever, the effects on internal conditions must be determined from the resultsobserved during the turnaround for the specific unit.
http://www.gewater.com/handbook/boiler_water_systems/Table13-2.jsphttp://www.gewater.com/handbook/boiler_water_systems/Table13-2.jsphttp://www.gewater.com/handbook/boiler_water_systems/Table13-2.jsphttp://www.gewater.com/handbook/boiler_water_systems/Table13-2.jsp


38/102

Certain boilers may require lower than normal blowdown levels due to unusual boiler design oroperating criteria or an exceptionally pure feedwater requirement. "n some plants, boilerblowdown limits are lower than necessary due to a conservative operating philosophy.

M,N1, BO'DO'N

"ntermittent manual blowdown is designed to remove suspended solids, including any sludgeformed in the boiler water. The manual blowdown tae!off is usually located in the bottom of thelowest boiler drum, where any sludge formed would tend to settle.

/roperly controlled intermittent manual blowdown removes suspended solids, allowingsatisfactory boiler operation. =ost industrial boiler systems contain both a manual intermittentblowdown and a continuous blowdown system. "n practice, the manual blowdown valves areopened periodically in accordance with an operating schedule. To optimize suspended solids

removal and operating economy, frequent short blows are preferred to infrequent lengthyblows. 4ery little sludge is formed in systems using boiler feedwater of exceptionally highquality. The manual blowdown can be less frequent in these systems than in those usingfeedwater that is contaminated with hardness or iron. The water treatment consultant can

recommend an appropriate manual blowdown schedule.5lowdown valves on the water wall headers of a boiler should be operated in strict accordancewith the manufacturers recommendations. ?sually, due to possible circulation problems, water

wall headers are not blown down while the unit is steaming. 5lowdown normally taes placewhen the unit is taen out of service or baned. The water level should be watched closely

during periods of manual blowdown.

CONTIN1O1S BO'DO'N

Continuous blowdown, as the term implies, is the continuous removal of water from the boiler. "toffers many advantages not provided by the use of bottom blowdown alone. For instance,

water may be removed from the location of the highest dissolved solids in the boiler water. 3s a

result, proper boiler water quality can be maintained at all times. 3lso, a maximum of dissolvedsolids may be removed with minimal loss of water and heat from the boiler.

3nother ma9or benefit of continuous blowdown is the recovery of a large amount of its heatcontent through the use of blowdown flash tans and heat exchangers. Control valve settings

must be ad9usted regularly to increase or decrease the blowdown according to control testresults and to maintain close control of boiler water concentrations at all times.

When continuous blowdown is used, manual blowdown is usually limited to approximately oneshort blow per shift to remove suspended solids which may have settled out near the manual

blowdown connection.

ENER)! CONSERV,TION#everal factors can contribute to reduced energy consumption on the water side of steam

generation equipment.

Sca*e Reduction

+eat transfer is inhibited by scale formation on internal surfaces. #cale reduction throughproper pretreatment and internal chemical treatment results in cleaner internal surfaces for

more efficient heat transfer and resultant energy savings.


39/102

Boi*er 'ater B*o"do"n Reduction

3 reduction in boiler water blowdown can result in significant fuel and water savings.

"n some installations, boiler water solids are lower than the maximum level permissible.Through improved control methods, including automatic boiler blowdown equipment, boiler

water blowdown can be reduced to maintain the solids close to but not above the maximumlevel permissible.

The rate of blowdown required depends on feedwater characteristics, load on the boiler, andmechanical limitations. 4ariations in these factors will change the amount of blowdownrequired, causing a need for frequent ad9ustments to the manually operated continuous

blowdown system. *ven frequent manual ad9ustment may be inadequate to meet the changesin operating conditions. Table &:!: illustrates the savings possible with automatic boiler

blowdown control.

5lowdown rate is often the most poorly controlled variable of an internal treatment program.Conductivity limits for manually controlled boiler blowdown are usually quite wide, with the

lower limits below 7'8 of the maximum safe value. This is often necessary with manual controlbecause a narrow range cannot be maintained safely.

"n plants with sodium zeolite softened maeup water, automatic control systems can maintainboiler water conductivity within

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