3Corrosion Control Purpose protect public health improve water quality extend plumbing equipmentmeet regulations
4Corrosion Battery Analogy Anode Cathode Electrical Circuit -Battery AnalogyAnodeCathodeElectrical CircuitMetal lost at anodeAnodeCathodeThe corrosion process is just like the operation of a battery. There is an anode and a cathode, and an electrical circuit between them. In the corrosion cell, metal is lost at the anode.Electrolyte
5Simplified Corrosion Cell OH-STEP 4O2O2STEP 1Water withDissolvedMineralsFe 2+STEP 3CATHODEBase Metale-e-Your cooling water system provides the ideal environment for the reversion of metal back to the oxidized state. The corrosion process is an “electro-chemical reaction” that can best be explained using the diagram. A four step process occurs.Step 1:At the anode, the pure iron begins to break down in contact with the cooling water. This step leaves behind electrons.Step 2:The electrons travel through the metal to the cathode.Step 3:At the cathode, a chemical reaction occurs between the electrons and oxygen carried by the cooling water. This reaction forms hydroxide.Step 4:The dissolved minerals in the cooling water complete this electrochemical circuit back to the anode.The corrosion process will continue and more and more metal will be destroyed. The only way to stop it is to break the circuit.ANODEe-e-STEP 2
6Major Factors Influencing Corrosion pHTemperatureDissolved SolidsSystem DepositsWater VelocityMicrobiological GrowthThe amount of corrosion or the corrosion rate is affected by numerous factors.
7Types of CorrosionAll water systems experiences some degree of corrosion. The objective is to control the corrosion well enough to maximize the life expectancy of the system...All cooling system metallurgy experiences some degree of corrosion. The objective is to control the corrosion well enough to maximize the life expectancy of the system. There are three types of corrosion.
8General Etch Uniform Attack General CorrosionBase MetalGeneral Etch Uniform AttackWaterThicknessOriginalPreferred situationTake a small amount of metal evenly throughout the systemAnode very largeGeneral CorrosionSince all metal corrodes, the ideal situation is to take a small amount of metal evenly from throughout the system. This is called general corrosion. In general corrosion, the anode is very large.
9Localized Pitting Attack Pitting CorrosionBase MetalLocalized Pitting AttackWaterOriginal ThicknessMetal removed at same rate but from a much smaller areaAnode very smallOften occurs under deposits or weak pointsLeads to rapid metal failureLocalized Pitting CorrosionIn some situations the anode is very small. Even though the metal loss is at the same rate, instead of losing metal from the entire metal surface, it is being removed from a small area.The only place for the corrosion to go is down into the metal. The result is a deep hole or pit. This type of corrosion occurs under deposits or at weak points in the metal. This type of corrosion is very serious because it results in rapid failure of the metal.
10Affects of Corrosion Potential regulatory non-compliance Shortened pipeline lifeWater usage increasesCorrosion product deposits in hot water tanksHeat transfer efficiency is reduced by depositsLeaks in equipment developProcess side and water side contamination occursMaintenance and cleaning frequency increasesEquipment must be repaired and/or repairedUnscheduled shutdown of plantUncontrolled corrosion is devastating to your plant. Corrosion is destructive to the cooling system metal and results in costly repairs.The affects of corrosion can be summarized as in the slide.
11Effect of pH on the Release of Copper into Solution
12Effect of pH and Alkalinity on Lead Solubility Alkalinity (mg/L CaCo3)
13Calcite ContactorsCalcite Contactors Use Limestone to Add Calcium Carbonate to Water, Raise pH and add Alkalinity to water.Benefits:Easy to OperateEasy to MaintainNo Risk of Overdosing ChemicalOperate in Upflow ModeNo Need for Controllers/Motor Actuated Valves or backwashingAgendaCeu’sEncourage Questions/CommentsBall Game
14What information is Needed to Design a Calcite Contactor? pHAlkalinityCalciumTDS or ConductivityFlow Rate
16Calcite Contactors Limestone contactors may offer advantages: easier and safer to operate,reduces operating cost,self adjusts the water pH without risk of alkali overdose,requires minimal maintenance and operator skills,and does not require continuous feed of chemicals
17Process DescriptionIn a calcite contactor, water flows through a bed of crushed sieved limestone in a similar way as it would flow through a sand filter.The pH of water that flows through the limestone bed will be adjusted until it nears equilibrium with calcium carbonate (CaCO3(s)).The components of a contactor include:a contact tank,limestone bed,inlet line,outlet line,overflow line,access lid,backwash line.There are two types of contactors: (i) open and (ii) closed system contactor. The former is exposed to the atmosphere and the latter is covered from the atmosphere. There are also contactors that are built in pressurized vessels.Limestone contactors are typically located at the end of the treatment train – after filtration, primary disinfection and chlorine contact.
25Contactor Feasibility Decision Tree Design contactor length using EPA DESCON programParameters needed:pHAlkalinity (DIC)CalciumIronManganeseTemperatureVelocity% Calcium CarbonateParticle SizeAvailable at Raymond Letterman’s website
26Descon Design Tool Filter Tank Sizes and Velocity (gpm/sq ft) 24" 36" 24"36"48"60"25 gpm8.03.52.01.350 gpm15.97.14.02.5100 gpm31.814.25.1150 gpm47.818.104.22.16800 gpm63.728.310.2