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Boiler Feed Water

Jan 26, 2017

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Engineering

Water Systems: water conditioning for Process and Boiler use

Boiler Feed WaterAijaz Ali

IntroductionIndustrial waterProcess waterWater that is used for, or comes in contact with an end product or the materials used in an end product.Boiler feed waterWater that serves in any level of the manufacturing processCommon contaminants: Ca, Mg, Fe, Al, Silica, silt, oil

ImpurityResulting inGot rid bySoluble GasesH2SCorrosion of boiler tubesAeration, deaeration and chemical treatmentO2COSuspended solidsSediment and turbiditySludge and scale carryoverClarification, filtration and chemical treatmentOrganic matterCarryover, foaming and corrosionDissolved colloidal solidsOil and greaseFoaming, depositionCoagulation, filtrationHardness Ca & MgScaling, inhibits HT, boiler tube burn thruSoftening and internal treatmentNa, Alkalinity,Na2CO3,Foaming, corrosion, embrittlementIon exchange, deionizationSulphatesHard scales if Ca presentDeionizationChloridesPriming, foamingDeionizationFe, MnRusting, resistence to HTAeration, filtration, ion exchangeSilicaScalingDeionization, lime-soda process

Treatment MethodsExternal TreatmentInternal Treatment

External methods of conditioningClarificationFiltrationIon exchangeMembrane separation

ClarificationRemoves all types of solids & large particles sediments, oil, natural org. matter, colouretc.Consists of 4 steps screening, coagulation-flocculation, sedimentation, fine filtration.Screening protects downstream units from large, easily separable objects.Three types:Fine screening (spacing < 10 mm)Medium screening (spacing 10 40 mm)Coarse screening (spacing > 40 mm)Coagulation-flocculation removes suspended solids & colloidal particles.

Clarification (contd.)Important factors velocity gradient, time, pHFlotation to separate particles having density lesser than water.Three types:NaturalAidedInducedInduced flotation facilitated through bubbling of air; 2 types Dissolved air flotation (DAF) (bubbles of 40 70 mm)Mechanical flotation (bubbles of 0.2 2 mm)

Ion Exchange Resins - acidic/basic radicals with ions fixed on them; exchanged with ions present in water.Theoretically removes 100 % of salts; does not remove organics, virus or bacteria.2 types of resins gel type (microporous) and macroporous or loosely cross-linked type.3 systems of resin beds:Strong acid cation + Strong base anionStrong acid cation + weak base anion + Strong base anionMixed-bed DeionizationIon exchange plant softens water, removes heavy metals, produces demineralized water.

Reverse Osmosis (RO)By applying pressure greater than osmotic pressure, water flows from the higher concentration solution to lower one.Mostly used for desalination; also for waste water treatment.Applied pressure depends on the type and salinity of water.Working pressure: < 15 bar for tap water (< 1500 ppm)15 25 bar for brackish water (< 8000 ppm)50 75 bar for sea water (35000 45000 ppm)RO plant preceded by pretreatment to avoid membrane fouling by sediments, bacteria, metal oxides & chlorine. RO permeate water more acidic than the feed water due to dissolved CO2. Common post-treatment are pH neutralization and remineralization.

ElectrodionizationCombines membrane separation and ion-exchange to provide high efficiency demineralization process. Electric potential transports & segregates charged aqueous species.Electric current continuously regenerates resin; no need for periodical regeneration.Deionization chamber - ion exchange resin, packed between cationic & anionic exchange membranes.

Electrodionization (Cont)Advantagescontinuous operationeliminates use of chemicals for regenerationlow power consumptionDisadvantagesNot used for water with hardness > 1requires purification pretreatmentPre-removal of CO2

Internal Treatment methodsDeaeratorsDissolved non-condensibles: O2 and CO2 Pitting and corrosionMechanical deaeration: reducing solubility of gasesIncreased temperatureDecreased partial pressure over the waterCommonly used purge gas: steamAdvantages:No added impuritiesEasily availableAlso provides heatPressure/Vacuum operation, ~98% of total and free is removedCoupled with chemical scavengers for complete deaeration

Corrosion controlpH controlDifferent for different components, different alloysCS : optimum pH = 9.2 to 9.6 at feed water temperaturesMS : optimum pH = 8.5 to 12.7 in boilersCu and CS : 8.8 to 9.2Maintained by addition of amines or small amount of caustic sodaAvoidance of addition of ammonia

Corrosion control (Cont)Oxygen control: during operationChemical Scavengers added to feedwater and condensatesSodium sulfite, bisulfite, hydrazineQuinone, ascorbateCommon entry: between deaerator and storageSodium sulfite: easy to handle, safe, for pressures of < 70 bar, solid addition to system, decomposition to corrosive gasesHydrazine: no solid addition, high pressures, but toxic, handling issues, Ammonia liberation, slower reactionConstant sampling and monitoring

DepositsScaling/deposition from carryoverCarbonate/Phosphate controlAddition of certain amounts of carbonate/phosphate for ensuring precipitate in the form of salts. Prevention of SulphatesFor removal of hardness, Ca and MgPrecipitation in bulk instead of at walls, non-adherentOrganic supplements: fluid sludge formation (polymer addition)Bottom blowdown removes sludgeChelant controlCombination of additivesBlowdown

Deaerators

DiscussionWhat Is Deaeration?The Process - Removing O2EquipmentDA ProblemsOperational ImpactsMonitoring

What Is It?Deaeration is the process of removing oxygen and other non-condensable gases from the boiler feed water like Co2

Removing OxygenWater @ 70oF and Atmospheric Pressure Will Have About 7,000 ppb of O2A Properly Functioning DA Will Reduce the Level to Approximately 5 - 10 ppbThe Remaining O2 Is Removed With an Oxygen Scavenger, (i.e. Sulfite etc)

Removing OxygenThree Driving Forces1- Heat(Low Pressure Steam, Condensate)2- Surface Area(Nozzles, Trays)3- Pressure(Vent)

Removing OxygenPrinciple of Surface Area

Removing Oxygen

The Corrosion ProcessAnode: Feo ---> Fe+2 + 2e--Cathode: 2e-- + H20 + 1/202 --> 2OH-

Deaerator

Deaerator

DA ProblemsInadequate VentingInadequate Steam Flow, Steam Pressure, or CondensateFlows Outside of Design SpecificationsBroken, Plugged or Missing NozzlesBroken, Plugged or Missing TraysBroken or Missing Baffles

Operational ImpactsCorrosion PotentialFe in the BoilerHigher Oxygen Scavenger UsageIncreased Boiler Conductivity, Lower CyclesReduced Boiler EfficiencyGreater Chemical CostsReduced DA Life

MonitoringFlowTemperaturePressureScavenger UsageVentingInspections

Corrosion Types in BoilersTypeTypically Caused byOxygen AttackBFW OxygenAlkalinityconcentrationConcentration of caustic under depositsAcid attackAcid leaks into BFW or condensateChelant/Polymer AttackExcessive chemical concentrationAmmonia AttackHigh ammonia returned in condensate orfrom BFW

Condensate in BFWNeutralize carbonic acidDo not protect against oxygen corrosionMaintain condensate pH 8.5-9.0Add in direct proportion to amount of CO2 in steamMost products are blends of two or more neutralizing aminesImportant operational considerations are volatility, acid neutralization ability, and basicity

Neutralizing AminesBasicity - a measure of amines ability to raise pH in condensateEnough amine must be added to neutralize al carbonic acidAdditional amine then added to maintain pH

AmmoniaAmmonia is used in steam lines where the steam contains a large amount of carbon dioxide or where there is an appreciable amount of steam loss from the condensate system. The advantage of ammonia is that the relative cost is less than other amines. The disadvantage is that it cannot be used in systems containing copper or nickel.

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