major volatiles are CO2, created by the breakdown of carbonate and bicarbonatementioned earlier, and SiO2. Although the CO2 can be neutralized, it is prudentto reduce feed water alkalinity to minimize its formation. For all practical pur-poses, external treatment for silica reduction and blowdown are the only meansto avoid excessive SiO2 discharges for protection of turbine blading. Hydroxylalkalinity helps reduce silica volatility.

Oxygen is the chief culprit in boiler systems corrosion. Deaeration reduces thisto a low concentration in the preboiler system, but does not completely eliminateit. Application of sulfite, hydrazine, or hydrazine-like (all-volatile) compoundsafter deaeration scavenges the remaining O2 and maintains a reducing conditionin the boiler water. An advantage of hydrazine is that it is discharged into thesteam to become available in the condensate as protection against oxygen corro-sion in the return system. If oxygen is present, ammonia can attack copper alloysin condensers and stage heaters. The removal OfNH3 by external treatment maybe necessary. The corrosive aspects of CO2 have already been mentioned in rela-tion to condensate systems. The beneficial and detrimental aspects of NaOH inthe boiler circuit in relation to corrosion control have also been discussed earlier.

BLOWDOWN

Boiler feed water, regardless of the type of treatment used to process the makeup,still contains measurable concentrations of impurities. In some plants, contami-nated condensate contributes to feed water impurities. Internal boiler water treat-ment chemicals also add to the level of solids in the boiler water.

When steam is generated, essentially pure H2O vapor is discharged from theboiler, leaving the solids introduced in the feed water to remain in the boiler cir-cuits. The net result of impurities being continuously added and pure water vaporbeing withdrawn is a steady increase in the level of dissolved solids in the boilerwater. There is a limit to the concentration of each component of the boiler water.To prevent exceeding these concentration limits, boiler water is withdrawn asblowdown and discharged to waste. Figure 39.16 illustrates a material balance fora boiler, showing that the blowdown must be adjusted so that solids leaving theboiler equal those entering and the concentration is maintained at the predeter-mined limits.

FIG. 39.16 How boiler water solids are controlled by blowdown.

Stearrr.900,000 Ib /doySolids content -

essent ia l ly ze ro

Blowdown: 100,000 Ib/daySolids content -1000 mg/l

,So l i ds removed-100 Ib/doy

Feedwater : 1,000,000 Ib/day

Sol ids content -100 mg/lSol ids added/day-100 Ib

Boiler waterSolids level-1000 mg/l

Of course it is apparent that the substantial heat energy in the blowdown rep-resents a major factor detracting from the thermal efficiency of the boiler, so min-imizing blowdown is a goal in every steam plant. There are ways to reclaim thisheat that will be examined later in the chapter.

One way of looking at boiler blowdown is to consider it a process of dilutingboiler water solids by withdrawing boiler water from the system at a rate thatinduces a flow of feed water into the boiler in excess of steam demand.

There are two separate blowdown points in every boiler system. One accom-modates the blowdown flow that is controlled to regulate the dissolved solids orother factors in the boiler water. The other is an intermittent or mass blowdown,usually from the mud drum or waterwall headers, which is operated intermit-tently at reduced boiler load to rid the boiler of accumulated settled solids in rela-tively stagnant areas. The following discussion of blowdown will be confined onlyto that used for adjusting boiler water dissolved solids concentrations.

Blowdown may be either intermittent or continuous. If intermittent, the boileris allowed to concentrate to a level acceptable for the particular boiler design andpressure. When this concentration level is reached, the blowdown valve is openedfor a short period of time to reduce the concentration of impurities, and the boileris then allowed to reconcentrate until the control limits are again reached. In con-tinuous blowdown, on the other hand, the blowdown valve is kept open at a fixedsetting to remove water at a steady rate, maintaining a relatively constant boilerwater concentration. Since the average concentration level in a boiler blown downintermittently is substantially less than that maintained by continuous blowdown,intermittent blowdown is less efficientmore costlythan continuousblowdown.

Figure 39.17 is a schematic diagram of a typical industrial boiler plant thatdischarges steam to a turbine, with part of the steam being condensed in the con-denser and the remainder extracted for a process use where the steam may be lostor the condensate become so contaminated that it must be wasted. With reference

FIG. 39.17 Schematic of industrial boiler system.

FeedwaterF@CF

Steamgenerator

BlowdownBDeaerator @CB

steam

MakeupM@CM

LossesL

to this diagram, the following relationships apply in determining blowdownlosses:

M X CM = F X CF = B1 X C81

M+C+D=F=S+B (all in Ib/h) (or kg/h)M = L + B2 (all in Ib/h) (or kg/h)

r R - ^ B , - Z T R - Cv1 _ M

* Heat release below 150,000 Btu/h/ft.2+ Where conditions warrant, chelants (EDTA or NTA) may be used in place of phosphates to achieve

a hardness-solubilizing rather than a hardness-precipitating effect. In certain programs, phosphate andchelant are used together. Where chelant residuals are to be maintained, recommended boiler water con-trol limits are: (1) boiler pressure below 400 lb/in2, 4 to 8 mg/L; (2) 401 to 600 lb/in2, 3 to 6 mg/L; (3)601 to 1000 lb/in2, 3 to 5 mg/L (all residuals are as CaCO3).

$ See Table 39.5 for intermediate pressure.

entering with the makeup water; these concentrations can be adjusted by blow-down, but also by some adjustment in the makeup treatment system if that flex-ibility is provided. On the other hand, such constituents as phosphate, organics,and sulfite are introduced as internal treatment chemicals, and their concentra-tion can be adjusted both by blowdown and by rate of application.

For purposes of illustrating the calculation of boiler blowdown related to theconcentrations, a 900-lb/in2 (40 bars) boiler system in a paper mill is used as anexample. The steam goes both to a condensing turbine and a back pressure tur-bine, with 50% condensate return. The makeup is treated by hot lime-zeolite soft-ening, and, after treatment, has a total dissolved solids concentration of 150 mg/L, silica of 3 mg/L, and total alkalinity of 20 mg/L. Table 39.10 summarizes theconditions established in this example.

With a silica concentration of 1.5 mg/L in the feed water and an allowablelimit of only 10 mg/L in the boiler water, silica is the controlling factor and setsthe concentration ratio (based on feed water) at 6.7. Since the water could be con-centrated to a factor of 10 based on total dissolved solids, there is incentive foradditional silica reduction in the hot process unit. If the addition of dolomiticlime would permit a reduction from 3 mg/L shown to less than 2 mg/L in themakeup, the blowdown rate could be reduced from 15 to 10%.

A second example explores the use of a simple sodium zeolite softener to treatcity water as makeup for a 300-lb/in2 boiler. The water analyses in Figure 39.18show the results of treating the city water through a zeolite softener, and the allow-able concentrations in a 300-lb/in2 (20 bars) boiler. The concentration ratio iscalculated for each of the constituents to be controlled; the lowest CR determinesthe blowdown rate. In this example, the lowest ratio is 2.5, applying to alkalinity.So the blowdown rate, controlled by alkalinity, would be:

100Percent blowdown = = 40%

This is a high blowdown loss, expressed as a percentage of makeup. However,in a small plant that generates less than 50,000 Ib/h (22,700 kg/h) of steam with

TABLE 39.8 Optimum Boiler Water Control Limits*Drum-type boilers using softened (not deionized) feed waters

Pressure, lb/in2

TDS (max.)Phosphate (as PO4)+Hydroxide (as CaCO3)SulfiteSilica (as SiO2) mas.JTotal iron (as Fe) max.Organics

150400030-60

300-40030-60

10010

70-100

300350030-60

250-30030-40

505

70-100

6003000

20-40150-20020-30

303

70-100

900200015-20

120-15015-20

102

50-70

1200500

10-15100-12010-15

52

50-70

15003005-1080-100

5-1031

50-70

TABLE 39.9 Optimum Boiler Water Control Limits*Drum-type boilers using high purity (deionized) feed waters

Pressure, lb/in2

240050

5-109.4-9.7

0.250.2

0.04-0.06t

18001005-10

9.4-9.7

10.5

0.04-0.06J

15002005-10

9.4-9.7

21

0.04-0.06$

120Ot300

15-259.8-10.2

52

0.04-0.06J

90Ot500

15-259.8-10.2

102

0.04-0.06$

Up to 600Use same optimum

limits as for soft(not deionized)feed waters

TDS (max.)Phosphate (as PO4)PH

Silica (a SiO2)Total iron (as Fe)Hydrazine (as N2H4)

* Heat release below 1 50,000 Btu/h/ft2.t For most boilers installed before 1950, and for boilers installed since but not having waterwall heat transfer

rates conducive to DNB (departure from nucleate boiling) under anticipated operating conditions, control limitsapplicable for softened (not deionized) feed waters may be used.

$ Hydrazine residuals in feed water just ahead of boiler, e.g., at economizer inlet.

* From Table 39.8.* Assuming 90% of boiler alkalinity is hydroxyl, an average

hydroxyl alkalinity of 135 mg/L (Table 39.8) corresponds to an aver-age total alkalinity of 150 mg/L. Since silica is controlling, the blow-down rate is:

100Blowdown = = 15% of feed waterCR/r

Two processes are explored to modify the sodium zeolite system to reducealkalinity, Figure 39.19 shows these two modifications, sodium zeolite plus acidand split-stream treatment. Both of these significantly reduce alkalinity and blow-down. The first process increases the critical CR to 12.5, so blowdown would becontrolled by alkalinity at a level close to the optimum TDS. Further reductionin blowdown is achieved by using split-stream treatment, since TDS is reducedas well as alkalinity. At these low levels, silica becomes a controlling factor at ablowdown of 6% of makeup. Capital cost and operating cost figures are needed todecide whether the reduction in blowdown from 8% achieved with the first pro-cess to 6%, which can be reached with the split-stream treatment, is justified. Thesplit-stream process is more costly and it creates a secondary problemdisposalof spent regenerant acid.

These examples show that concentration ratios are determined by chemicalanalyses. Since blowdown rate is never measured, but most plants meter bothmakeup and feed water as well as steam, the chemical determination of concen-tration ratio is the most accurate means of determining blowdown loss. It isapparent that careful sampling of both the feed water and makeup is required toproperly control blowdown and be able to determine blowdown rate. The boilerwater must be cool before it can be analyzed, and leakage in the cooler could affectthe composition of the boiler water. The boiler water sample is usually taken fromthe blowdown collection pipe in the boiler drum, and if this is not properlydesigned, the blowdown sample may be nonrepresentative. An example is theaccumulation of steam bubbles within the blowdown line which is then con-densed through the sample cooler and dilutes the boiler water.

Although one of several boiler water constituents may determine the requiredblowdown ratefor example, silicait is general practice to determine all of thecritical concentrations in the boiler on a regular basis. Each of these can then berelated to the total dissolved solids as measured by a conductivity instrument,

less than 10 to 20% makeup, this process might be acceptable just for its simplicityand low cost. Larger plants would find this high blowdown loss unacceptablebecause of the high energy loss and the cost of preparing and treating makeup thatis concentrated to such a limited degree.

TABLE 39.10 Summary of Controls(All concentrations in mg/L)

Factor

TDSSiO2Alkalinity

Makeup

1503

20

Feed water

751.5

10

Boilerlimit*

200010

150+

Max CRF26.7

6.715

the nalco water handbook 2nd editionCoverFront MatterPrefaceContentsPart I. The Nature of Water1. The Water Molecule2. Water Sources and UsesEffects of RainfallOther Seasonal ChangesWhere River Meets OceanLakes as ReservoirsSeasonal TurnoverSubsurface WaterWell Waters ConstantSources and Practices in Other CountriesThreats to Water Sources: the Effects of Acidic RainfallThreats to Water Sources: the Effects of Landfill Leachate on Groundwater QualitySuggested Reading

3. Basic ChemistryPeriodic Chart of ElementsFrom Atoms to MoleculesImpurities in WaterElectrolytesIons and Electric CurrentElectromotive SeriesColloidal SystemsOxidation and ReductionSolvent Action of WaterOrganic ChemicalsPredicting SolubilitiesForeign Ions InterfereOther AnomaliesEquilibriumPractical Value of Equilibrium ConstantsSolid ReactantsChelationSuggested Reading

4. Water Chemistry and Interpretation of Water AnalysesAlkalinity and AcidityThe pH NotationUse of meter for [OH(-)]The Importance of CO2The Source of AlkalinityCO3(2-)/HCO3(-) DistributionInterfering IonsEffects of ImpuritiesCaCO3 Stability IndexesDissolved GasesMinerals and ConductanceSolid MatterOrganics in WaterInterpretation: Problems Caused by ImpuritiesSuggested Reading

5. Aquatic BiologyA Healthy Aquatic EnvironmentThe Role of ChemistrySix Important ElementsSome Need Oxygen, Others Don'tMicrobial AnalysesSampling and AnalysisThree Living ZonesChemical IndicatorsBacteria Can HelpSuggested Reading

6. Water Contaminants: Occurrence and TreatmentClass 1--Primary ConstituentsClass 2--Secondary ConstituentsClass 3--Tertiary ConstituentsClass 4--Trace ConstituentsClass 5--Transient ConstituentsMethods of AnalysisSuggested Reading

7. Water Gauging, Sampling, and AnalysisFlow MeasurementMeasuring Wastewater FlowsSamplingAnalysisSuggested Reading

Part II. Unit Operations of Water Treatment8. Coagulation and FlocculationCoagulationZeta PotentialFlocculationCoagulation and Flocculation ChemicalsTailoring PolyelectrolytesActivated SilicaApplications of Coagulation and FlocculationColor RemovalPlant DesignSuggested Reading

9. Solids/Liquids SeparationRemoval of Solids from WaterRemoval of Water from SludgeDewatering--L/S SeparationMiscellaneous Devices and ProcessesSuggested Reading

10. PrecipitationSoftening by PrecipitationPartial Lime SofteningComplete Lime SofteningMore Complete Softening at Higher TemperaturesSilica RemovalHeavy Metals RemovalMiscellaneous ProcessesBench and Pilot Plant TestingSuggested Reading

11. Emulsion BreakingOil-in-Water EmulsionsWater-in-Oil EmulsionsIndustrial Sources of Emulsions and Treatment PracticesRecovery of Separated Oil

12. Ion ExchangeExchanger CapacityAnion ExchangersPractical Ion ExchangeAn Evolving ScienceSodium Cycle Exchange (Zeolite Softening)Hydrogen Cycle ExchangeAnion Exchange ProcessesEvaluation of Ion Exchange Resin PerformanceSpecial Ion Exchange ApplicationsSuggested Reading

13. NeutralizationExamples of Acid-Base ReactionsAcid Waters

14. DegasificationHenry's LawDegasification DesignsDeaerating HeatersChemical Removal

15. Membrane SeparationMicrofiltrationUltrafiltrationMaterial Balance in Membrane ProcessesDesign Features of Commercial UnitsReverse OsmosisMembrane MaintenanceDialysisElectrodialysisSuggested Reading

16. AerationMechanism of AerationPrinciples of Gas TransferEquipmentSizing of Aeration EquipmentSuggested Reading

17. AdsorptionColloidal MatterTaste and Odor ControlRemoval of Organic Molecules and ColloidsColloidal SilicaApplications of Carbon FiltersWastewater TreatmentResin ColumnsSlurry SystemsSuggested Reading

18. Evaporation and FreezingEvaporatorsEffect of Salt ConcentrationEvaporator DesignCondensationMultiple-Effect UnitsFreezing

19. Oxidation-ReductionIron-RemovalManganese RemovalRemoval of Organic MatterCyanide RemovalSulfide RemovalOther Oxidizing AgentsReducing AgentsSuggested Reading

20. Corrosion ControlCorrosion RatesPolarization-DepolarizationGalvanic CorrosionConcentration Cell CorrosionDissolved SolidsDissolved GasesTemperatureStress Corrosion CrackingCaustic EmbrittlementChloride Induced Stress Corrosion CrackingCorrosion Fatigue CrackingTuberculationImpingement AttackDezincificationCorrosion InhibitionMaterials of ConstructionCoatings and LiningsInsulationApplied Chemical InhibitorsCorrosion InhibitorsAnodic/Cathodic Inhibitor MixturesOrganic FilmersCathodic ProtectionWater Distribution SystemsMonitoring Results

21. Deposit ControlDeposit SourcesDeposit AppearanceDeposit IdentificationTreatments to Control System DepositsSuggested Reading

22. Control of Microbial ActivityPhysical Factors Affecting Microbe GrowthChemical Factors Affecting Microbial GrowthMethods for Controlling Microbial ActivityOxidizing Biocides (See Also Chapter 19)Nonoxidizing BiocidesCationic BiocidesMetallic CompoundsMicrobial Monitoring ProceduresCooling Water SystemsPaper Mill Water SystemsStoring, Handling, and Feeding Precautions

23. Biological DigestionTreating Industrial OrganicsAvailable ProcessesTemperature EffectsTrace NutrientsBench TestingSludge AcclimatizationAerated SystemsBalancing the Activated Sludge ProcessTrickling FiltersAnaerobic DigestionOther ProcessesSuggested Reading

Part III. Uses of Water24. Aluminum IndustryMiningBauxite ProcessingAluminum ReductionMetal FabricationSuggested Reading

25. Automotive IndustryFoundry OperationsMachiningStampingAssembly Plants

26. Chemical IndustryProcess Cooling Is Major H2O UseProcess Flow SheetsEnvironmental EffectsSuggested Reading

27. Coal Products: Coke, Producer Gas, and SynfuelsCoke PlantBy-Product RecoveryUtility RequirementsBiological TreatmentProducer Gas and SynfuelsSuggested Reading

28. Food Processing IndustryThe Sugar IndustryThe Beverage IndustryFruit and Vegetable ProcessingMeat and PoultrySuggested Reading

29. MiningUnderground WaterCoal ProcessingFurther DewateringMetal-Containing MineralsPhosphate MiningMineral Leaching and DissolvingSlurry Conveying

30. Pulp and Paper IndustryWater: a Basic Raw MaterialGroundwood PulpThermomechanical PulpingChemical PulpingRecovering Process IngredientsThe Kraft ProcessPulp Process UnitsBleachingProblems Created by WaterSecondary Fiber RecoveryWater Removal From PulpFourdrinier MachineCylinder MachineWater-Related Mill ProblemsDesigning for the FutureWaste TreatmentSuggested Reading

31. Petroleum IndustryProcess OperationsUtility SystemsCooling Water SystemsRefinery Pollution Control--Waste TreatmentSuggested Reading

32. Steel IndustryBlast Furnace OperationsExhaust Gas TreatmentSteel ProductionDirect Reduction ProcessesContinuous CastingThe Hot-Mill Rolling OperationCold Rolling MillsHeat TreatmentSinteringAcid PicklingSlag PlantUtilitiesSuggested Reading

33. Textile IndustryCottonWoolSyntheticsWater Uses in the Textile IndustryAir Washer MaintenanceDeveloping a Treatment ProgramSuggested Reading

34. UtilitiesThe Process: Energy ConversionPower CycleWater: the Working FluidWater Chemistry in Fossil-Fueled Plants--Liquid PhaseSteam Phase ProblemsWater Chemistry in Nuclear Fueled PlantsCondenser Cooling WaterMiscellaneous Water UsesSuggested Readings (See ?Energy Primer,? Chapter 46)

35. Municipal WaterRaw Water CharacteristicsTypical Treatment SchemesWater Treatment Plant By-Product DisposalPotable Water Other Than Municipal SupplySuggested Reading

36. Municipal Sewage TreatmentEffluent StandardsTypical Wastewater Treatment ProcessesDisinfectionPlant Process Changes Caused by Growth and IndustrializationMunicipality and Industry Join Forces to Treat Wastes More EconomicallySewage Plant Effluent Is Fed to Water Factory 21Worlds's Largest Treatment PlanBy-Product WastesSludge Disposal

37. Commercial, Institutional, and Residential Water TreatmentHeating SystemsAir-Conditioning SystemsEnergy StorageDomestic WaterWaste Treatment

Part IV. Specialized Water Treatment Technologies38. Cooling Water TreatmentHeat TransferCooling Water System: Problems & TreatmentCooling TowersCooling Water Treatment and ControlAmbient Air EffectsCorrosion and Scale ControlFouling ControlMicrobial Control

39. Boiler Water TreatmentDepositsCorrosionExternal TreatmentCondensate ReturnsInternal TreatmentComplexation/DispersionProgram SupplementsBlowdownBoiler TypesSuperheaters and Other AuxiliariesContaminants in Returned CondensateTypes of Steam-Using EquipmentSteam Used for PowerCondensate Return SystemsEvaluating ResultsSurveying a Boiler SystemSteam SamplingSteam Plant EfficiencyDeposit and Failure Analysis

40. Effluent Treatment OptimizationSources of EffluentProcess Water Use and ContaminationUtility SystemsPlant SurveyAssessing Waste Flows and QualitySampling and AnalysisEqualizationConservation to Minimize Effluent LoadingReview of Existing FacilitiesProcess ModificationPilot Studies EssentialUsing Plant EffluentsSurveillance of ProgramZero Discharge--Possibilities and RealitiesSuggested Reading

41. Wet Gas ScrubbingParticle Collection ConceptsParticulate EmissionsPrinciples of OperationCategorizing Wet ScrubbersGas Absorption ScrubbersPacked TowerMoving Bed ScrubbersTurbulent Contact Absorber (TCA)Wet Particle ScrubbersWet Electrostatic PrecipitatorsWaterside ProblemsWaste TreatmentAuxiliary EquipmentSelected Gas Scrubbing Systems

42. Agricultural Uses of WaterChop IrrigationThe Nature of SoilMeasurable Soil CharacteristicsConcentration EffectsConditioning Irrigation WaterSuggested Reading

43. Oil Field Water TechnologyTheory of Oil FormationThe Oil-Bearing ReservoirPetroleum ProductionSecondary Recovery by Steam FloodingThe Nature of Oil Field WatersMineral ScalesCorrosionBacteria ControlSuggested Reading

44. Production of Ultrapure Water45. Chemical Feed SystemsLiquid Feed SystemsDry Feed Systems

46. Energy PrimerEnergy MeasurementsSteam PropertiesEnthalpySteam QualityFuel and Boiler EfficienciesImproving Boiler EfficiencyMechanical Heat RecoveryFeed Water TemperatureSteam PressureDecreasing BlowdownReclaiming Blowdown HeatBoiler LoadingTurbine OperationSteam Trap

GlossaryAppendixSign Conventions in Electrochemistry

Index ABCDEFGHIJKLMNOPQRSTUVWXYZ