boiler fundamentals

5/26/2018 Boiler Fundamentals

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BOILER FUNDAMENTALS


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INTRODUCTION TO BOILERS

A boiler is an enclosed vessel that provides ameans for combustion heat to be transferred intowater until it becomes heated water or a gas(steam). The steam or hot water under pressureis then usable for transferring the heat to aprocess. When water is boiled into steam itsvolume increases about 1,600 times, producinga force that is almost as explosive as

gunpowder. This causes the boiler to be anextremely dangerous item that must be treatedwith utmost respect.


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The process of heating a liquid until it reaches it's gaseous state iscalled evaporation. Heat is transferred from one body to another bymeans of (1) radiation, which is the transfer of heat from a hot bodyto a cold body through a conveying medium without physicalcontact, (2) convection, the transfer of heat by a conveyingmedium, such as air or water and (3) conduction, transfer of heat

by actual physical contact, molecule to molecule. The heatingsurface is any part of the boiler metal that has hot gases ofcombustion on one side and water on the other. Any part of theboiler metal that actually contributes to making steam is heatingsurface. The amount of heating surface a boiler has is expressed insquare feet. The larger the amount of heating surface a boiler hasthe more efficient it becomes. The measurement of the steamproduced is generally in pounds of water evaporated to steam perhour.


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Gallons of water evaporated x 8.3 pounds/gallon water =Pounds of steam The heat required to change thetemperature of a substance is called its sensible heat.The quantity of heat required to change a chemical fromthe liquid to the gaseous state is called latent heat.

The saturation temperature or boiling point is afunction of pressure and rises when pressure increases.When water under pressure is heated its saturationtemperature rises above 212F. This occurs in the boiler.When heat is added to saturated steam out of contact

with liquid, its temperature is said to be superheated.The temperature of superheated steam, expressed asdegrees above saturation, is referred to as the degreesof superheat.


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BOILER TYPES

Boilers and pressure vessels are built underrequirements of the American Society of MechanicalEngineers or ASME referred to as the "ASME Code."

There are virtually infinite numbers of boiler designs but

generally they fit into one of two categories: (1) Firetubeboilers, contain long steel tubes through which the hotgasses from a furnace pass and around which the waterto be changed to steam circulates, and (2) Watertubeboilers in which the conditions are reversed with the

water passing through the tubes and the furnace for thehot gasses is made up of the water tubes.


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BOILER TYPES

Firetube

Common types of firetubeboilers are scotch marine,firebox, HRT or horizontal

return tube. Firetubeboilers typically have alower initial cost, aremore fuel efficient andeasier to operate but they

are limited generally tocapacities of 50,000pphand pressures of 250psig.

Firetube boiler


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BOILER TYPES

Watertube

The more commontypes of watertubeboilers are "D" type,

"A" type, "O" type,bent tube, and cast-iron sectional.

Watertube boiler A type

Watertube boiler D type


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BOILER TYPES

All firetube boilers andmost watertube boilersare packaged boilers inthat they can betransported by truck, rail

or barge. Large watertubeboilers used in industrieswith large steamdemands and in utilitiesmust be completely

assembled andconstructed in the fieldand are called fielderected boilers. Field erected boiler with superheater


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STEAM BOILER SYSTEMS

Feedwater system

Steam system

Fuel system

Draft system


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FEEDWATER SYSTEM

The feedwater system provides water to

the boiler and regulates it automatically to

meet the demand for steam. Valves

provide access for maintenance andrepair.


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STEAM SYSTEM

The steam system collects and controls

the steam produced in the boiler. Steam is

directed through piping to the point of use.

Throughout the system steam pressure isregulated using valves and checked with

steam pressure gauges. The steam and

feedwater systems share somecomponents.


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FUEL SYSTEM

The fuel system includes all equipment

used to provide fuel to generate the

necessary heat. The equipment required

in the fuel system depends on the type offuel used in the system. All fuels are

combustible and dangerous if necessary

safety standards are not followed.


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FUEL SYSTEM

In a fuel oilfired boiler plant, fuel oil

leaves the tank through a suction line and

duplex strainer traveling then to the fuel oil

pump. The fuel oil is then forced throughthe pump and then through the discharge

line. From the discharge line some fuel oil

is burned and some returned to the tankthrough a regulating valve.


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FUEL SYSTEM

In a natural gasfired plant gas, is supplied at a setpressure which varies depending on the gas source. Gassystems are low pressure or high pressure. In a lowpressure gas system city gas pressure is reduced from

pounds to inches of pressure by passing through a gasregulator. Through the regulator gas is drawn into theburner and mixed with air supplied by a blower. Thismixture is directed to the burner where it is ignited withthe pilot light. In a high gas pressure system, gas passes

through the regulator and gas is reduced to the properpressure for the burner. Some boilers have combinationburners which can burn gas or fuel oil or a combinationof both gas and fuel oil.


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DRAFT SYSTEM

The draft system regulates the flow of air

to and from the burner. For fuel to burn

efficiently the right amount of oxygen must

be provided. Air must also be provided todirect the flow of air through the furnace to

direct the gases of combustion out of the

furnace to the breaching.


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DRAFT SYSTEM

A forced draft system uses a fan to force

(or push) air through the furnace. An

induced draft system uses a fan to draw

(or pull) air through the furnace. Acombination or balanced draft system

uses forced and induced draft fans. Gases

of combustion enter the stack from thebreaching and are released to the

atmosphere.


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COMBUSTION

Combustionis the method of combining the fuel and air systems ina source of heat at sufficient temperature to produce steam.Combustion may be defined as the rapid chemical combination ofoxygen with the combustible elements of a fuel. Only threecombustible, chemical elements are of any significance: carbon,hydrogen and sulfur. The boiler combustion furnace in which the

fuel burns provides a chamber in which the combustion reaction canbe isolated and confined so that it can be controlled. Theconvection surfaces are the areas to which the heat travels that isnot transferred in the combustion furnace. Here additional heat isremoved. The burner is the principal device for the firing of oiland/or gas. Burners are normally located in the vertical walls of thefurnace. Burners along with the furnaces in which they are installed,are designed to burn the fuel properly.


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STEAM TO WATER CYCLE

In a steam heating system steam leaves the main steamline and enters the main steam header. From the mainheader piping directs the steam to branch lines. Branchlines feed steam through a riser to the steam heating

equipment. At the heating equipment heat is transferredto the building space. As the steam releases heat to thebuilding space and is cools it turns back to water orcondensate. The condensate is separated from thesteam by a steam trap. The steam trap allows

condensate to pass but not the steam. The condensatepasses through the condensate return line and iscollected and directed back to the boiler to repeat thesteam to water process.


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Separation of solids in the water occurs in

the boiler but since it is operating

continuously and at higher temperatures

this "buildup" can occur very rapidly. Whenthis occurs the heat transfer can not be

achieved as readily which requires more

fuel to produce the steam. If continuedunchecked damage to the metals in the

boiler shell and tubes will result.


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Pretreatment equipment such as softeners, de-mineralizes, etc. areused to remove as much of the dissolved solids as possible beforethey get to the boiler. To remove the solids that continue to theboiler chemicals are added to react with the solids creating a sludge.This sludge is then periodically removed by opening valves from thebottom of the boiler and relieving it to the drain. This process is

called blowdown.

Waterside problems can also shorten boiler life from corrosionbrought on by the oxygen content in the feedwater. Pretreatment forthe removal of oxygen is performed in a deaerator but here againthe removal is not complete and chemical additions are made to aid

in improving the oxygen removal process.


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The water supplied to the boiler that isconverted into steam is called feedwater.The two sources of feedwater are: (1)

Condensate. or condensed steamreturned from the processes and (2)Makeup water (usually city water) whichmust come from outside the boiler room

and plant processes. For higher boilerefficiencies the feedwater can be heated,usually by economizers.


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MAKEUP WATER

Makeup water is the water supplied from

the municipal water system, well water, or

other source for the addition of new water

to the boiler system necessary to replacethe water evaporated


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MAKEUP WATER

Water softeners

Water as it passes over the ground, through caves andsprings picks up some of the elements from thelimestone and other elements of nature which dissolved

and remain. These elements collectively are calledhardness. In a heavy use industrial steam boiler thewater could be completely replaced as often as onceeach hour. The higher the operating pressure of theboiler the more critical the removal of foreign items from

the feedwater becomes. Large utility boilers operating at3,000 psig + may actually use distilled water for ultimatepurity.


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WATER SOFTENERS

The purpose of a water softener is primarily for the removal ofhardness from the boiler makeup water. Makeup water is the watersupplied from the municipal water system, well water, or othersource for the addition of new water to the boiler system necessaryto replace the water evaporated. Some filtering of the water mayoccur in the water softener but that is not the purpose of its design

and too much of other pollutants in the water could actually foul thewater softener affecting its operation. Hardness is composedprimarily of calcium (Ca) and magnesium (Mg) but also to lesseramounts sodium (Na), potassium (P), and several other metals.Hardness is measured in grains with one grain of hardness in thewater being 17.1 ppm of these elements. The purpose of usinghardness as the unit of measure is that tests to measure in parts permillion (ppm) are much more difficult and expensive to use.Hardness varies from area to area. Usually near salt water thehardness is very low as the limestone is virtually non existent and inmountainous areas where limestone is everywhere hardness isusually very high.


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WATER SOFTENERS

All softeners soften or remove the hardness from thewater. The primary minerals in the water that make"hard" water are Calcium (Ca++) and Magnesium(Mg++). They form a curd with soap and scale in piping,

water heaters and whatever the hard water contacts.Hardness is removed from the water by a process knownas positive ion exchange. This process could also beknown as "ion substitution", for substitution is whatoccurs. Sodium (Na+) ions, which are "soft" are

substituted or exchanged for the Calcium andMagnesium as the water passes through the softenertank.


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WATER SOFTENERS

The softening media is commonly called resin or Zeolite.The proper name for it is polystyrene resin. The resinhas the ability to attract positive charges to itself. Thereason it does so is because in its manufacture it inherits

a negative charge. It is a law of nature that oppositecharges attract, i.e., a negative will attract a positive andvice versa. A softener tank contains hundreds ofthousands of Zeolite beads. Each bead is a negative innature and can be charged or regenerated with positive

ions. In a softener, the Zeolite is charged with positive,"soft" sodium ions.


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WATER SOFTENERS

As "hard" water passes through the Zeolite, theCalcium and Magnesium ions are stronglyattracted to the beads. As the "hard" ions attachto the Zeolite bead, they displace the "soft"

Sodium ions that are already attached to thebead. In effect, the Sodium is "exchanged" forthe Calcium and Magnesium in the water supplywith the Calcium and Magnesium remaining onthe Zeolite beads and the Sodium ions taking

their place in the water flowing through thesoftener tank. The result of this "exchange"process is soft water flowing out of the tank.


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WATER SOFTENERS

It can now be readily understood that a softener

will continue to produce "soft" water only as long

as there are Sodium ions remaining on the

Zeolite beads to "exchange" with the Calciumand Magnesium ions in the "hard" water. When

the supply of Sodium ions has been depleted,

the Zeolite beads must be "regenerated" with a

new supply of Sodium ions. The regeneration ofthe Zeolite beads is accomplished by a three

step process.


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SOFTENER DESIGNS

Water softeners

come as single

mineral tank units

(simplex), doublemineral tank units

(duplex) and multiple

mineral tank units.

Simplex softener


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SOFTENER DESIGN

Since regeneration cycles cantake approximately one hoursimplex units are used only whenthis interruption can be tolerated.To avoid interruption duplex unitsare used so that the regenerationof one unit can be accomplished

while the second unit is on line.Triplex or other multiplex unitsusually are the result of need forincreased capacity and units canbe added to keep soft wateravailable. The reliability of newelectronic/metering controls for

regeneration have allowed usersto depend on smaller units withmore frequent regeneration.

Duplex softener

Triplex softener


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REGENERATION PROCESS

Backwash- The flow of water through the mineral bed isreversed. The mineral bed is loosened and accumulatedsediment is washed to the drain by the upward flow ofthe water. An automatic backwash flow controller

maintains the proper flow rate to prevent the loss ofresin.

Brine draw and slow rinse- Ordinary salt has thecapability to restore the exchange capacity of themineral. A given amount of salt-brine is rinsed slowly

through the mineral bed. After the salt-brine is drawn, theunit will continue to rinse slowly with water to remove allof the salt-brine from the media bed.


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REGENERATION PROCESS

Fast rinse- A high down flow of waterrepacks the mineral bed. Any trace ofbrine not removed in slow rinse is flushed

to the drain.The unit is then returned to SERVICE thebrine maker is refilled with fresh water toform salt brine for the next regeneration.The total regeneration time isapproximately 60-90 minutes.


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BOILER FEEDWATER

Deaeration

All natural waters contain dissolved gases in

solution. Certain gases, such as carbon dioxide

and oxygen, greatly increase corrosivity. Whenheated in boiler systems, Carbon dioxide (CO2)

and oxygen (O2) are released as gases and

combine with water (H2O) to form carbonic

acid, (H2CO3).

CO2+ O2+ H2O =H2CO3


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DEAERATION

Removal of oxygen, carbon dioxide and othernon-condensable gases from boiler feedwater isvital to boiler equipment longevity as well assafety of operation. Carbonic acid corrodes

metal reducing the life of equipment and piping.It also dissolves iron (Fe) which when returnedto the boiler precipitates and causes scaling onthe boiler and tubes. This scale not only

contributes to reducing the life of the equipmentbut also increases the amount of energy neededto achieve heat transfer.


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DEAERATION

The term given to the mechanical removal ofdissolved gases is deaeration. Mechanicaldeaeration for the removal of these dissolvedgases is typically utilized prior to the addition of

chemical oxygen scavengers. Mechanicaldeaeration is based on Charles' and Henry'slaws of physics. Simplified, these laws state thatremoval of oxygen and carbon dioxide can beaccomplished by heating the boiler feedwater

which reduces the concentration of oxygen andcarbon dioxide in the atmosphere surroundingthe feedwater.


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DEAERATION

The easiest way to deaerate is to force steam

into the feedwater, this action is called

scrubbing. Scrubbing raises the water

temperature causing the release of O2and CO2gases that are then vented from the system. In

boiler systems, steam is used to "scrub" the

feedwater as (1) steam is essentially devoid of

O2and CO2, (2) steam is readily available and(3) steam adds the heat required to complete

the reaction.


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DEAERATION

For efficient operation, deaerating equipment mustsatisfy the following 4 requirements: (1) Heating of thefeedwater: The operating temperature in the unit shouldbe the boiling point of water at the measured pressure.

The pressure/temperature relationship is important sinceboiling must take place rapidly for quick and efficientremoval of gases. If this temperature and pressurecannot be economically achieved then it is important toget as close to it as possible. (2)Agitation decreases the

time and heat energy necessary to remove dissolvedgases from the water.


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DEAERATION

(3) Maximization of surface area by finely

dispersing the water to expose maximum

surface area to the steam. This enables

the water to be heated to saturationtemperature quicker and reduces the

distance the gases have to travel to be

liberated. (4) The liberated gases must bevented to allow their escape from the

system as they are released.


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DEAERATION

While the most efficient mechanical deaerators reduceoxygen to very low levels (.005cc/l or 5 ppb), even traceamounts of oxygen may cause corrosion damage to asystem. Consequently, good operating practice requiresremoval of that trace oxygen with a chemical oxygen

scavenger such as sodium sulfite or hydrazine. Freecarbon dioxide can be removed by deaeration, but thisprocess releases only small amounts of combinedcarbon dioxide. The majority of the combined carbondioxide is removed with the steam of the boiler,subsequently dissolving in the condensate, frequentlycausing corrosion problems. These problems can becontrolled through the use of volatile neutralizing aminesor filming amines.


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TYPES OF MECHANICAL

DEAERATORSTray Type Deaerators are composed of a deaerating section and afeedwater storage section. Incoming water is sprayed through aperforated distribution pipe into a steam atmosphere where it isatomized. There it is heated to within a few degrees of the saturationtemperature of the steam. Most of the non-condensable gases arereleased to the steam as the water enters the unit. The water then

cascades through the tray section, breaking into fine droplets, whichimmediately contact incoming steam. The steam heats the water tothe saturation temperature of the steam and removes all but a traceof oxygen. Deaerated water falls to the feedwater storage sectionbelow and is protected from recontamination by a blanket of steam.

As the non-condensable gases are liberated, they as well as a smallamount of steam are vented to atmosphere. It is essential that

sufficient venting is provided at all times or deaeration will beincomplete.


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TYPES OF MECHANICAL

DEAERATORS

Spray Type Deaerators work on the samegeneral principle as the tray types. Thespraytype deaerators do not use trays fordispersion of the water. In this case, spring

loaded nozzles located in the top of the unitspray water into a steam atmosphere which isheated to within a few degrees of the saturationtemperature of the steam. Most of the non-condensable gases are released to the steam,

and the heated water falls to a water seal anddrains to the lowest section of the steamscrubber.


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TYPES OF MECHANICAL

DEAERATORS

Spray Type Deaerators (cont) The water isscrubbed by large quantities of steam andheated to the saturation temperature prevailingat this point. The intimate steam to water contact

achieved in the scrubber efficiently strips thewater of dissolved gases. As the steam-watermixture rises in the scrubber, a slight pressureloss causes the deaerated water temperature toremain a few degrees below the inlet steam

saturation temperature. The deaerated wateroverflows from the steam scrubber to thestorage section below.


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TYPES OF MECHANICAL

DEAERATORS

Spray Type Deaerators (cont) The

steam, after flowing through the scrubber,

passes up into the spray heater section to

heat the incoming water. Most of thesteam condenses in the spray section to

become part of the deaerated water. A

small portion of the steam, vented toatmosphere, removes noncondensable

gases from the system.


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TYPES OF MECHANICAL

DEAERATORS

Spray/Tray Type Deaerators are a

combination of the above with a steam

spray nozzle sending the water over the

trays.

S O C C


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TYPES OF MECHANICAL

DEAERATORSFeedwater Tanks are another form of mechanicaldeaerators normally found in small firetube andwatertube boiler systems due to cost considerations.These less expensive systems are limited by design asthey are operated at atmospheric pressure withfeedwater temperatures ranging from 1800F - 2120F;while deaerators operate under pressure allowing forhigher temperatures and more efficient oxygen removal.Like deaerators, feedwater tanks operate by forcing

steam into the feedwater which scrubs oxygen andcarbon dioxide gases that are then vented toatmosphere.

TYPES OF MECHANICAL


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TYPES OF MECHANICAL

DEAERATORSFeedwater Tanks (cont) Steam enters the bottom of the tank agitating thefeedwater as it rises to the top of the tank, and finally is vented along withthe liberated gases. The temperature is normally controlled as high aspossible without causing pump problems which occurs when the NetPositive Suction Head (NPSH) is too low. Steam bubbles form and fill thepump cavity causing vibration, a condition know as cavitation. This conditionmay cause serious damage to the feedwater pump and jeopardize steam

production. The most practical potential solution for cavitation is theinstallation of a slipstream, which allows a portion of the high pressurefeedwater to recirculate to the suction side of the pump where it lowers thetemperature and eliminates the boiling and cavitation. The slipstream willnot always work leaving the choices of increasing the NPSH by increasingthe distance between the tank and the pump, or sizing a new pumpproperly. Practically speaking, most feedwater tanks are controlled between

1800F - 2000F and rely more on the assistance of a chemical oxygenscavenger for complete oxygen removal. Pressurized deaerators must havethe ASME U stamp attached and be built under the regulations ofTheAmerican Society of Mechanical Engineers Section VIII, Division I.


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ECONOMIZER

An economizer removes additional Btus from the stackgasses by circulating the deaerated boiler feedwaterthrough a series of bent tubes in the stack. Thistranslates into a "free" source of energy from the boileroperation. Finned tube economizers are less costly and

more efficient as the "fins" are a source of heat transferas well as the tubes. Economizers in watertube boilerstypically increase the efficiency of the boiler 4-10% whichis usually less than a one year payback. Due to thehigher efficiencies of firetube boilers the payback isusually longer and therefore economizers are not usedas frequently on them. An economizer can also be auseful means of increasing the steam capacity of aboiler.


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ECONOMIZER

The use of high sulfur oils, particularly #6 oil, is verycorrosive on the economizer tubes. This can beimproved by increasing the temperature of the feedwaterto the economizer and the use of soot blowers but thelife of an economizer in that environment is limited toabout 2-3 years. A bare tube economizer is easier tokeep free of the corrosive sulfur but requires more tubesto achieve the same efficiency as a finned tubeeconomizer. Since the economizer is directly part of the

boiler and has contact from the gases of combustion itmust also be built under the regulations of The AmericanSociety of Mechanical Engineers Code Section I andhave the ASME S stamp attached.


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BOILER WATER CHEMISTRY

Producing quality steam on demand is the purpose ofoperating industrial boiler systems. Achieving that goaldepends on properly managed water treatment to controlsteam purity, deposits and corrosion. A boiler is thesump of the boiler system. It ultimately receives all of the

pre-boiler contaminants. Boiler performance, efficiency,and service life are direct products of selecting andcontrolling the chemistry used in the boiler. The boilerwater must be sufficiently free of deposit forming solidsto allow rapid and efficient heat transfer and it must notbe corrosive to the boiler metal. Deposits and corrosionresult in efficiency losses and may cause boiler tubefailures and inability to produce steam. The predominantcost factor for producing steam is fuel costs, as shownbelow.


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DEPOSIT CONTROL

Deposits in boilers may result from hardnesscontamination of feedwater, and corrosion products fromthe condensate and feedwater system. Hardnesscontamination of the feedwater may result from eitherdeficient softener systems or raw water in leakage of the

condensate. Deposits act as insulators and slow heattransfer. The insulating effect of deposits cause theboiler metal temperature to rise and may lead to tube-failure by overheating. Large amounts of depositsthroughout the boiler could reduce the heat transferenough to reduce the boiler efficiency. The graphdemonstrates that different types of deposits will effectboiler efficiency differently. This is why it is important tohave an analysis of deposit characteristics.


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DEPOSIT CONTROL

When feedwater enters the boiler, the elevatedtemperatures and pressures cause the components ofwater to take on dramatic changes. Most of thecomponents in the feedwater are soluble; they aredissolved in the water. However, under heat andpressure most of the soluble components come-out ofsolution as particulate solids, sometimes in crystallizedforms and other times as amorphous particles. Thecoming-out of solution is referred to as retrograde

solubility, and means that as temperature increases,ability to stay in solution decreases. When solubility of aspecific component in water is exceeded, scale ordeposits develop.


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DEPOSIT CONTROL

Internal chemical treatment for deposit control isachieved either by adding a treatment to prevent thecontaminants from depositing or by adding a treatmentchemical that will allow for easy removal by blowdown.Hardness can be kept from depositing in boiler water bytreatment with chelating agents. When phosphatetreatment is preferred over chelant treatment, the boilerwater is conditioned to form a fluid sludge which can beremoved by bottom blowdown. Formation of this sludge

requires that alkalinity from caustic be present in theboiler water. If sufficient alkalinity is not maintained in theboiler water, a sticky precipitate will form and reduceheat transfer.


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DEPOSIT CONTROL

Even when the precipitates formed in the boiler waterare in the form most desired, they are often difficult toremove completely by blowdown. This is especially truewhen the precipitates also contain iron and coppercorrosion products from the preboiler system andorganic contaminants from condensate returns. Sludgeconditioners enhance the removal of precipitates fromindustrial boilers. Sludge conditioners are organicpolymers which combine with the precipitates to permit

the particles to be dispersed. This makes removal byblowdown easier.

CONVENTIONAL PHOSPHATE


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CONVENTIONAL PHOSPHATE

TREATMENTConventional phosphate control involves maintaining aphosphate residual and a hydroxide alkalinity residual inthe boiler water. Phosphate residuals are typicallymaintained in the range of 20-40 ppm PO4. Hydroxidealkalinity, if controllable without excess blowdown, aremaintained in the range of 300 -500 ppm OH. Thistreatment provides the ideal conditions for formation ofcalcium and magnesium precipitates in the preferredstates. It also provides a residual of alkalinity to

neutralize any acid contamination, such as organicacids. It may, however, promote foaming, especially iforganic contaminants enter the boiler.


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CHELANT TREATMENT

A chelant is a compound which is capable

of "grabbing onto" calcium, magnesium

and iron. Chelant treatment of boiler water

is attractive because the chelates ofcalcium and magnesium are soluble. The

undesirable scales of calcium carbonate

and calcium sulfate are successfullyeliminated by chelant treatment.


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CHELANT TREATMENT

While the chelates of the hardness and ironcontaminants are soluble, some chemistryprecautions need to be mentioned. Phosphatewill compete with the chelant for calcium, and if

present in significant amounts, will result inundesirable calcium-phosphate deposits.Phosphate can enter the boiler water where citywater makeup supplies phosphate. Bothhydroxide alkalinity and silica compete with the

chelant for magnesium. Depending on theconcentration of all the boiler water chemistry,magnesium silicate deposits may result.


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CHELANT TREATMENT

Chelants should be fed to

the feedwater

downstream of any

copper alloys, after the

deaerator and before theboiler drum. The

preferred feed location is

down-stream of the boiler

feedwater pump. Astainless steel injection

quill is required.Injection quill


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CHELANT TREATMENT

Feed to the deaerator storage is not recommended since copper alloys in the boilerfeed pump may be attacked. Proper feed of chelant will result in a chelant residual inthe boiler water. The photo below shows the preferred feed locations for chelant feedand other requirements for adequate assurance of chelant control.

1. Feed chelant products continuously to boiler feedwater line, preferably after the

economizer.

2. Use a 304 SS injection quill.

3. Use a 316 SS chemical feed line. (If not possible, ensure that 316 SS is used at least three feet prior to the injection quill).

4. Feed chelant only downstream from copper or copper alloys.

5. Feed catalyzed sulfite or a suitable oxygen scavenger to the storage section

of the deaerating heater.

6. Assure that the feedwater mixes with boiler water before entering downcomer

tubes.

7. Maintain feedwater pH >8.0


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CHELANT TREATMENT

A chelant residual in the boiler water,

however, is not in itself proof of adequate

feed control. A chelant residual should be

maintained in the feedwater at all times.Chelant treatment is not a solution for

highly variable and excessive

concentrations of hardness in the makeupand condensate returns.


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CONDENSATE RETURN SYSTEM

When steam has performed its work in manufacturingprocesses, turbines, building heat, etc. it transfers heatand reverts back to a liquid phase called steamcondensate. However, not all the energy used inproducing steam is lost when condensate is formed. Asmost condensate return is still relatively hot (130OF to225OF) , it is very valuable as a source of feedwater.There is a significant fuel savings related to the heatrequired to raise the temperature of makeup water at

(50OF to 60OF) to equal that of the return condensate,not to mention the additional cost in pretreating(softening) the makeup, as well as basic water cost itself.


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When pure water H2O is used to produce steam, then itscondensate is also pure H2O however, as we havelearned the water we use to produce steam is not purecontaining many dissolved minerals and gases. The heatand pressure of the boiler break down the alkalinity inthe boiler water to form carbon dioxide gas CO2. Leavingthe boiler with the steam it travels throughout the plantsupply system. When the steam condenses, the carbondioxide dissolves in it to form carbonic acid. This reactionis chemically expressed as:

H2O + CO2= H2CO3


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This acid depresses the condensates pH and causescorrosion to take place. This corrosion appears asgrooving or gouging in the bottom of steam headers orcondensate return lines. Most often it weakens pipewalls at threaded joints and the resultant metal loss can

lead to large amounts of copper and/or iron beingreturned to the boiler to cause troublesome deposits.Oxygen, as in the boiler system, can cause localizedattack in the form of pitting when present in thecondensate system. This type of corrosion can generallycause equipment to fail more quickly than the

generalized corrosion caused by carbonic acid attackdue to it concentrating in a small area. Oxygen caninfiltrate the system from open condensate receivers,poor deaeration or leaky siphons.


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There are three main chemical programs

to control corrosion in the condensate

system, being neutralizing amines, filming

amines and contamination neutralizingand filming amines.


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NEUTRALIZING AMINES

Are high pH materials which neutralize the carbonic acidformed in condensate systems. By raising andcontrolling pH level in condensate from 7.5 to 9.0,neutralizing amines retard acid attack and greatly reducethe amount of corrosion products entering the boiler.

The three primary neutralizing amines in use today are:

1. Morpholine - a low distribution ratio product.

2. Diethyleminoethanal (DEAE) - a medium distributionratio product.

3. Cyclohexylamine - a high distribution ratio product.


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NEUTRALIZING AMINES

The distribution ratio is used to predict the amineconcentration in the steam and condensate phases andimpacts significantly regarding proper amine selection.

Distribution Ratio = Amine in Steam Phase / Amine in

Condensate PhaseNeutralizing amines have low flashpoints and thereforecan be fed directly to the feedwater or boiler water, orthey can be fed directly into the steam header. The feedrate is based on the amount of alkalinity present in the

feedwater. Neutralizing amines offer excellent protectionagainst carbonic acid attack, but little protection againstoxygen attack.


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NEUTRALIZING AMINES

FILMING AMINES are various chemicals

that lay down a vary thin protective barrier

on the condensate piping protecting it

against both oxygen and carbonic acidattack. The protective film barrier is not

unlike the protection afforded an

automobile by an application of car wax.


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NEUTRALIZING AMINES

FILMING AMINES (CONT) The protective film barrier iscontinuously being removed (a little at a time), requiringcontinuous feeding of the filming amine based on steamflow rather than feedwater alkalinity. Care must be takento start this program slowly with an initial feedrate of onefifth that of the final feedrate to prevent the removal ofold corrosion products from the system and theirsubsequent return to the boiler. Additionally, the filmingamine should be fed using an injection quill to the steamheader to insure proper vaporization and distributionthroughout the steam system.


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NEUTRALIZING AMINES

FILMING AMINES (CONT) The formation

of gunk balls (Gunking) can occur due to

overfeed, contaminants in the condensate

or wide pH swings causing deposits toform in low flow areas like steam traps.


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NEUTRALIZING AMINES

COMBINATION NEUTRALIZING AND FILMING AMINES are thecombination of neutralizing and filming amines and are a successfulalternative to protect against both carbonic acid attack and oxygenattack. As its name implies, it combines the elevated pH approach toneutralize carbonic acid in conjunction with the protective barrier filmapproach. are the combination of neutralizing and filming amines

and are a successful alternative to protect against both carbonicacid attack and oxygen attack. As its name implies, it combines theelevated pH approach to neutralize carbonic acid in conjunction withthe protective barrier film approach. The neutralizing amines,although they will elevate pH, main purpose is to provide betterdistribution of the filming amine throughout the condensate systemwhich in turn helps to prevent gunking. As with filming amines they

should be fed directly to the steam header utilizing an injection quill.


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SUMMARY

Clearly each program or approach has certain featuresand benefits as well as limitations. Each different set ofoperating conditions will tend to dictate the appropriatetreatment that is required. The expected steam pressure,temperature, system metallurgy and the plants systems

pH level all play an important role in determining themost effective treatment program. Clearly each programor approach has certain features and benefits as well aslimitations. Each different set of operating conditions willtend to dictate the appropriate treatment that is required.The expected steam pressure, temperature, system

metallurgy and the plant systems pH level all play animportant role in determining the most effectivetreatment program.

boiler fundamentals

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