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CHEMICAL TREATMENT IN CHEMICAL TREATMENT IN COOLING WATER SYSTEMS COOLING WATER SYSTEMS BY P.K.VASUDEVAN BY P.K.VASUDEVAN JUNIOR CHEMIST/MTPS JUNIOR CHEMIST/MTPS

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cooling water treatement for power plant

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  • CHEMICAL TREATMENT IN COOLING WATER SYSTEMSBY P.K.VASUDEVANJUNIOR CHEMIST/MTPS

  • What is cooling water system?It is the most important system in any Power Station predominantly decides the efficiency of stationsHeat Transfer: ( e.g. Boiler)Hotter (source) cooler ( receiver)Cooling water System:Process ( source) Cool water(receiver )Cooling water does not contact the source directly. A Barrier that allows heat pass from source to receiver

  • Water completes it job and cools the sourceThis heat should be transferred to the environment Types of systemOnce through systemOpen recirculation systemClosed recirculation systemSource of waterSea, River, Pond, etc.

  • Controlling FactorsThe heat transfer characteristics (thermal conductivity) of the barrierThe thickness of the heat transfer barrierThe surface area of the barrierThe temperature difference between the source and the receiver ( cool. Water)Insulating deposits on the either side of the barrier

  • Types of ProblemsCorrosionCorrosion is function of water characteristics and the metals in the systemScalePrecipitation of compounds that become insoluble at higher temperatureFoulingSettling out of suspended solids, buildup corrosion products, growth of microbial masses

  • Some of the major chemical components available for cooling water treatment

    Chemical treatmentsCorrosionScalingFoulingMicrobesChromatesXZincXMolybatesXSilicatesXPolyphosphatesXXPolyolestersXPhosphonatesXXAll organicsXXXNatural organicsXSynthetic polymersXXNon oxidizing biocidesXXChlorine / BromineXOzoneX

  • CORROSIONGalvanic corrosion, cell corrosion, Dissolved gasesStress corrosionCorrosion fatigue crackingDezincificationDissolved solids ( Chloride & Sulphate)

  • UNTANGLE THE TRIANGLEThe arms of the triangle encompass the microbiological process in the centre.Bio film contributes significantly to microbiologically influenced corrosion and under deposit corrosionBio film also possible nucleation sites for mineral scale growth

  • Corrosion inhibitors

    PrincipallyanodicPrincipally CathodicBoth anodic & CathodicChromateCalciumCarbonateOrganic filmingaminesOrthophosphatePolyphosphatePhosponatesNitriteZincSilicate

  • Problem due to bio filmIncrease microbiological influenced corrosionReduction in heat transferIncrease chemical cost

  • Cooling water chlorinationDosing gaseous chlorine in cooling water will control the organic matter growthCl2 + H2O HCl + HOClHOCL H + + OCl The above components are the primary Toxicants that destroys microorganismsA residual chlorine value of 0.2 to 0.4 ppm will provide control

  • Continuous presence of residual chlorine will increase contact time In recent development HOBr , ( bromine biocide) are available in liquid form are used for continuous dosing .Bromine is often a more practical treatment than chlorine because it remains effective at higher PH

  • Material% of Cl2Chlorine gas (Cl2)100Chlorine dioxide ( Cl O2)263Hypo chlorites ( Na or Ca) Calcium Ca(OCl)270Sodium (NaOCl)Industrial12 - 15Domestic3 - 5Lithium, LiOCl, laundry grade35 Chlorinated isocyanuric acid ( CONCl)385

  • SCALE CONTROLTendency to form inorganic deposits ( CaCO3, MgCO3) on heat transfer surfaces due to cycles of concentrationScales are formed when inorganic constituents are concentrated and super saturated solutions are formedWhen further concentrated in recirculation, whereby reaching maximum concentration of hardness (Ca,Mg) salts.

  • This will result precipitation of dense , adherent scaleThese Scale act as a thermal insulator it causes reduction in heat transferScaling also increases roughness there by increasing turbulence and pressure drop To prevent scaling the concentration factor is kept in the range of 1.25 to 2.0C.F. = Concentration in cooling water Concentration in Raw water

  • Calcium Carbonate solubility will decrease while increase in temperature (inverse)Ca( HCO3) CaCO3 + CO2 + H2OCooling Tower expels CO2 from the waterTo find the tendency of deposit Langlier Index and Ryznar index are usedIn recent technologies software packages are available which are significantly more powerful than the above methods

  • Control of ScalingScaling potential is mitigated by use of polyphosponates or polymers These are act as crystal modifier rather than scale inhibitori.e they change the morphology of scale not to bind each other and to precipitateThey help to keep the contaminants suspended in bulk space

  • CONTROL DEPOSITSThreshold inhibitorsDelay in precipitation / increase solubilityDispersantsDisperse suspended solids ,adding charges, causing mutual repulsionSurface active agentsPenetrate and disperse biomasses. Some of them are biocides kill slime organismCrystal modifiersmodify the crystal structure of scale resultant will be weak in structure will not stick on the surfaces.

  • CalciumMagnesiumSodiumCarbonatesBicarbonatesChloridessulphates

    Pure waterCooling water cycle

  • MASS BALANCEThis is the technique used to find out the whether the system is In Balance or not Concentration in circulation waterConcentration Ratio [ CR] = --------------------------------------------[non precipitating ion( Cl)] Concentration of in Makeup water

    From the CR the value of Calcium is calculated as follows

    C.R X Calcium in make up waterMass balance = Calculated calcium --- Actual Calcium in Cir.water

  • Example calculation:Chloride in Makeup water = 22 ppm Chloride in Circulating water = 34 ppm Concentration Ratio = 34 / 22 == 1.545Calcium in Makeup is = 86 ppmCalculated calcium is = 86 X 1.545 = 133 If Actual calcium in Circulating water is:Case (i)133 then 133 133 = 0 [ no deposition]Case (ii) 131 then 133 131 = + 2 [Deposition ]Case (iii) 135 then 133 135 = - 2 [ Calcium dissolved]

  • FoulingFouling can be considered as deposition of hardness salts and of other inorganic particulate matterThis occurs usually in the areas where water velocity is low It has same effect as scale bet it promotes severe corrosion under deposition

  • Sources of fouling deposits

    Raw waterAir borneRecirculation waterColloidal organicsDirtScale: CaCO3, CaSO4Silt, DirtReactive gases H2S, SO2, NH3Corrosion products: Fe2O3Microbial contaminationMicrobial ContaminationMicrobial deposits

  • Fouling control

    Deposition fouling can be controlled byFiltering of makeup waterAddition of good dispersantControlling of scalingControlling of bio film formation

  • Latest developmentControlling the cooling water system through latest software packagesMonitoring through on-line and real-time Dosing chemicals and biocides according to demand and performance based

  • SOME OF THE ONLINE INSTRUMENTS

  • Bio film Analyser

  • Deposit Analyser