Corrosion Control

Presented By:Sawinder SinghSr. Manager(Prod.)

IntroductionTypes of cooling towersEssential cooling tower treatmentScale controlCorrosion controlMicrobiological Control

Main Features of Cooling Towers

Drift eliminators: capture droplets in air streamAir inlet: entry point of airLouvers: equalize air flow into the fill and retain water within towerNozzles: spray water to wet the fillFans: deliver air flow in the tower

Natural Draft Cooling Towers

Hot air moves through towerFresh cool air is drawn into the tower from bottomNo fan requiredConcrete tower

Natural Draft Cooling Towers

*Large fans to force air through circulated waterWater falls over fill surfaces: maximum heat transferCooling rates depend on many parametersLarge range of capacitiesCan be grouped, e.g. 8-cell towerMechanical Draft Cooling Towers

*Three typesForced draftInduced draft cross flowInduced draft counter flowMechanical Draft Cooling Towers

Air blown through tower by centrifugal fan at air inletAdvantages: suited for high air resistance & fans are relatively quietDisadvantages: recirculation due to high air-entry and low air-exit velocitiesForced Draft Cooling Towers

*Two types Cross flow Counter flowAdvantage: less recirculation than forced draft towersDisadvantage: fans and motor drive mechanism require weather-proof Induced Draft Cooling Towers

*Hot water enters at the topAir enters at bottom and exits at topUses forced and induced draft fansInduced Draft Counter Flow CT

*Water enters top and passes over fillAir enters on one side or opposite sidesInduced draft fan draws air across fillInduced Draft Cross Flow CT

To Control CorrosionTo Prevent ScaleTo Control Algae and Bacterial GrowthTo extend equipment life and efficiency

Water is an excellent solvent, it carries many impurities.Natural waters contain significant amounts of minerals like calcium and magnesium.Natural waters typically contain significant amounts of oxygen.Water, particularly warm water, is a great environment for living micro-organism

Any material that is or becomes dissolved in water is referred to as a dissolved solid, or solids.

Common Dissolved SolidsCalcium and Magnesium Salts, aka HardnessIronCopperSulfatesPhosphatesSilica and Silicates

It is the deposition of sparingly soluble salts present in water.

In the open recirculatory system, the salts in the water get continuously concentrated due to evaporation in cooling tower. When water containing high concentration of these salts come into contact with the metal surface at high skin temperature in a heat exchanger the salts tend to precipitate and form adherent deposits.

These scales have very poor thermal conductivity and resist the heat transfer and lower the efficiency of heat exchanger.

Some of the common scales forming salts are:

Calcium Carbonates. Calcium Sulphate Silicates of Scales Calcium orthophosphate Magnesium Salts Iron Salts etc.

Factors that affects Scaling:

Temperature: The common scalant found in cooling water are CaCO3 & CaSO4. Their solubility decreases with the increase in temperature.

pH or Alkalinity: The Solubility of CaCO3 decreases with the increase of pH hence increases the scale formation

Fouling in Cooling water is defined as the deposition of suspended matter other than sparingly soluble salts. Fouling includes the deposition of material such as corrosion product, fly ash, and microbial growth.

Fouling is normally observed in the low velocity areas of cooling water.Some of the common foul ants are: Dirt & SlitSand & Fly ashCorrosion products & Natural organicsMicrobial growth

Scale Control Hinges on a Few Key Issues

Understanding the mineral content of the raw water

Proper and complete Bleed Off

Maintaining system water alkalinity in Scale Safe zone

Consistent addition of scale inhibitors

Consistent addition of scale dispersants

Corrosion is the gradual destruction of material (usually metals) by chemical reaction with its environment.

Corrosion is the deterioration of a metal as a result of chemical reaction between it and the environment. Corrosion is an electrochemical process by which a metal returns to its natural state, for corrosion to occur, a corrosion cell consisting of an anode; a cathode and an electrolyte must exist. At the anode, metal ions dissolve in to the electrolyte. As the metal ions go into solution at the anode, electrons are left behind, which migrates towards cathode, where the cathodic reaction takes place i.e. electrons are consumed.

Anodic ReactionFe = Fe++ + 2e- Cathodic reaction O2 + 2H2O + 4e - = 4OH- (Natural aerated water)The hydroxide formed at the cathode combine with ferrous cations. Fe++ + 2OH- = Fe (OH) 2 (Ferrous hydroxide) Fe (OH) 2 = FeO + H2O=Fe2O3 + 2H2O (rust) Ferrous hydroxide has very low solubility and precipitates as a white flock at the metal water interface, where it is rapidly oxidized to ferric hydroxide. Ferric hydroxide under cooling water condition losses water & produces ferric oxide as the corrosion product. The net result of this activity is the loss of metal and then formation of deposit.

Characteristics influence the rate of corrosion:

1.)pH: The Corrosion potential increases as pH decreases.2.)Oxygen & other dissolved gases :Presence of Oxygen and other dissolved gases such as CO2, NH3, H2S and Cl2 etc. increase the corrosion potential.

3.) Dissolved & Suspended Solids: Normally, higher solid content would mean increased corrosion potential due to higher conductivity .However; it is just the reverse because the hardness of the solids present in the water forms a passive film on the surface of the metal which acts as an inhibitor to corrosion. This is the reason, why soft water is more corrosive to hard water. Suspended salts influence corrosion by erosive or abrasive action. They can also settle on metal surface producing localized corrosive cell.

4.) Microbial Growth:Microbial growth promotes the formation of corrosive cells and also the by products of some organisms are corrosive.5.) Velocity:In high velocity and turbulent water oxygen is rapidly distributed and passivation layer of corrosion inhibitors ore often removed resulting in increased corrosion High velocity can also lead to erosive of metals surface protective film & oxide. At the same time, low velocity can lead to deposition and give rise to localized corrosion cells causing under deposit corrosion.

6) Temperature: Increase in temperature increase the diffusion of O2 & increases the Corrosion. Above the temperature loss of dissolved O2 exceeds and corrosion decreases.

Uniform Corrosion (General Corrosion):- The metal loss is uniform from the surface; often occurs with the high velocity of the fluid causing erosion.Pitting Corrosion:- The metal loss is randomly located on the metal surface. Often occurs with stagnant fluid or in area with low velocity or under deposit corrosion.Galvanic Corrosion: When two metals with different electrode potential are connected in a corrosive electrolytic environment. The anodic metal develops deep pits and groves on the surface.

Crevice Corrosion: - It occurs at places with gaskets, bolts and lap joints. Crevice corrosion creates pits similar to pitting corrosion.Under deposit Corrosion: It occurs below the deposit layer due to scaling or fouling on metal surface. Initially when the deposit layer forms on the metal surface, Oxygen under deposit is consumed by the normal corrosion reaction. The O2 concentration beneath the deposit goes on continuously decreasing as the fresh oxygen from the water is hindered by the deposit and hence the area area under deposit becomes anodic compared to the surrounding metal. Corrosion thus produced is the pitting type corrosion anaerobic bacteria which grow under the deposit lead to severe corrosion

Cooling towers are an excellent example of water systems that provide optimum conditions for microbial growth. Temperature & pH are usually within the ideal ranges and generally there is an abundance of nutrients required for their growth such as organic matter, inorganic salts and sun light. Under these conditions, it is conceivable that bacterial concentration may increase by six million times while during the same time , inorganic salts may concentrate only six times.Micro organism enter the cooling tower through the sources. They may be present in the makeup water or in the dust and air which enter the cooling tower.The major problems of the microbes are:-Algae.FungiBacteria

Algae:- Air ,water and sunlight are the basic requirements for algae growth. The distribution cells and side walls of a cooling tower full fill all these requirements and therefore represent an excellent growth environment for algae. Excess growth of algae can lead to choked pipe lines, nozzles etc hampering efficient distribution of water in the cooling tower.All algae produce Oxygen which depolarize the corrosion reaction and accelerate the system destruction.Algae also produce slime which acts as nutrients for other micro-organism.Fungi:- Fungi lack chlorophyll and are therefore non- photosynthetic, resulting in dependence on nutrients provided by organic matter. In CW system, fungi can use wood as a source of nutrients and can destroy wood.Fungi reproduce by forming spore (single cell). Spores can remain dormant(inactive) for a long time and proliferate 9To produce cells rapidly) when conditions become favorable In the dormant state they harmless. Spores are generally resistance to micro biocides and can prevent difficult situation.

Bacteria: - Many different species of Bacteria are found in cooling water system. Control, therefore, becomes difficult as agents toxic to one species may have little or no effect on the other species.

Types of Bacteria found in water: Pseudomonas: - These bacteria are notorious for the slime that they generate. The slime acts as a binding agent for dirt and the suspended matter, thereby causing voluminous deposit material which, normally, would not have get deposited due to the binding action of the slime. Sulphate Reducing Bacteria (SRB):- These are anaerobic and generate the energy required for their growth by reducing sulphate to sulphide and in the process corrode Iron. 4FeSO4 +4H2O = FeS+3Fe(OH)3 + 2OH It corrodes Iron by the formation of FeS which attacks the metal

Fe+ H2S = FeS + H2during chlorination in the presence of SRB the pH drops due to the formation of HCl H2S+ Cl2 = 2HCl + S This is in the attack on the concrete basin.

Iron Bacteria:- These utilize Iron for their growth and create iron deposits as a by product.

Nitrifying Bacteria:- They are the nitrosomonas and nitrobacter which generate nitric acid by utilizing ammonia present as a contaminant in cooling water system. They also oxidize nitrate ions. This reduces the pH locally and initiate corrosion.

Microbial control can be done by dosing biocides.Biocdes : Chemicals that kill micro-organisms are called biocides. It is difficult to kill the entire organism in CW system. What is best achieved is the maximum killing and control of their growth. Two types of Biocides:Oxidising Biocides: Chlorine, Chlorine dioxide, BromineNon Oxidising Biocides:-Quaternary ammonium compounds, IsothiazolinoneCarbamateMethylene-bis-thiocynateThe selection of biocides depends on1.Type of bacteria in makeup water2.Type of inhibitor used3.Condition of CW system4.Local site condition.

Corrosion Inhibitors:-Corrosion Inhibitors is a chemical or compound when added to liquid or gas; decreases the corrosion rate. The mechanism of inhibiting the corrosion involves formation of coating often a passivation layer which prevents the access of the corrosive substance to the metal.

Anodic inhibitorAnodic inhibitors build a thin protective film along the anode, increasing the potential at anode and slowing corrosion reaction. The film is initialed at the anode, generally not visible; reduce the available anodic areas by forming a protective oxide film on the metal surface. They do not reduce corrosion rate. They simply reduce the area over which corrosion is spread. If the entire anodic surface is covered, corrosion protection is excellent. But even if a small amount is exposed, the entire corrosion current will be concentrated on that exposed area. This often leads to aggravate pitting. Anodic inhibitors are also called passivation inhibitors. Examples of well known anodic inhibitors are Chromate, nitrite, molybdate and orthophosphate.

Cathodic inhibitor

Cathodic inhibitors form a thin protective film on the cathode. It restricts the access of dissolved oxygen to the metal surface. Low concentration of these inhibitors lead to general attack and not pitting. Cathodic inhibitors are also called precipitating inhibitors. They form insoluble precipitate that can coat and protect the metal surface. Cathodic inhibitors reduce corrosion rate. Zinc is an example of cathodic inhibitors.

The major corrosion inhibitors are:1.Chromate2.Orthophosphate3.Molybdate4.Zinc5.Polyphosphate6.Silicates7.Phosphonates8.Tolytriazoles

Good Tower Treatment Starts with :Good Conductivity Control Proper selection of treatment chemicals based on makeup water and operating conditionsConsistent Chemical Levels for Maximum ProtectionConsistent and timely monitoring of the water chemistry

Good Tower Treatment Ends with :Clean condenser tubesGood heat transferMinimized corrosion of piping and surfacesExtended equipment lifeAnnual PM, and Cleaning

*There are two main types of natural draft towers:(Click once) Cross flow tower (left figure): air is drawn across the falling water and the fill is located outside the tower(Click once) Counter flow tower (right figure): air is drawn up through the falling water and the fill is therefore located inside the tower, although design depends on specific site conditions*Mechanical draft towers have large fans to force or draw air through circulated water. The water falls downwards over fill surfaces, which help increase the contact time between the water and the air - this helps maximize heat transfer between the two. Cooling rates of mechanical draft towers depend upon various parameters such as fan diameter and speed of operation, fills for system resistance etc.Mechanical draft towers are available in a large range of capacities. Towers can be either factory built or field erected for example concrete towers are only field erected.Many towers are constructed so that they can be grouped together to achieve the desired capacity. Thus, many cooling towers are assemblies of two or more individual cooling towers or cells. The number of cells they have, e.g., a eight-cell tower, often refers to such towers.*We will now explain three types of mechanical draft cooling towers, and the advantages and disadvantages of each.*Forced draft cooling towerHow it works: air is blown through the tower by a fan located in the air inletAdvantages: Suited for high air resistance due to centrifugal blower fansFans are relatively quiet Disadvantages: Recirculation due to high air-entry and low air-exit velocities, which can be solved by locating towers in plant rooms combined with discharge ducts *Two types of induced draft cooling towers: cross flow and counter flowAdvantage: Less recirculation than forced draft towers because the speed of exit air is 3-4 times higher than entering air Disadvantage: Fans and the motor drive mechanism require weather-proofing against moisture and corrosion because they are in the path of humid exit air *Induced draft counter flow cooling towerhot water enters at the topair enters bottom and exits at the top uses forced and induced draft fans *Induced draft cross flow cooling tower: water enters at top and passes over fillair enters on one side (single-flow tower) or opposite sides (double-flow tower)an induced draft fan draws air across fill towards exit at top of tower