Cooling Water Systems – Theory And Operations…

Cooling Water Systems – Theory And Operations…

1. Cooling Water Systems-Problems
Open recirculatory cooling water systems are commonly used for industrial cooling purposes to efficiently dissipate unwanted process heat. In this kind of system, water is recycled a number of times before being discharged as blowdown, thereby reducing water consumption.
The main problems associated with such systems are corrosion, scaling, fouling and microbiological growth. If left untreated, these problems can lead to reduced operating efficiency, increased maintenance cost, loss in heat transfer efficiency and energy and ultimately shutdowns.
Corrosion
Corrosion is an electrochemical phenomenon by which a metal returns to its natural state, i.e., metal oxide.
Being an electrochemical process, for corrosion to occur, a corrosion cell consisting of an anode, a cathode and an electrolyte must exist. At the anode, metal ions dissolve into the electrolyte (water). As the metal ions go into solution, electrons are left behind which migrate through the metal to the other points (cathode), where the electron is consumed.
Fe à  Fe⁺⁺ + 2e^-
(At the anode)
O₂ + 2H₂O + 4e^- à  4OH^-
(Principal mode of electron consumption at the cathode)
The hydroxyl ions formed at the cathode combine with the ferrous cations forming ferrous hydroxide.
Fe⁺² + 2OH^- àFe (OH)₂
Ferrous hydroxide is rapidly oxidized to ferric hydroxide at the metal water interface. Ferric hydroxide, under cooling water conditions, loses water and produces the so called corrosion product, ferric oxide, also known as rust. The net result of all this activity is the loss of metal (Fe) accompanied with the formation of a deposit (Fe₂O₃).
Factors that affect Corrosion
pH: The rate of corrosion is dependent on the pH. Generally corrosion is more severe in acidic pH as most protective metal oxide films are soluble in acidic pH.
Oxygen and other dissolved gases: Oxygen increases the rate of cathodic reaction and thus increases corrosion.
CO₂ – forms carbonic acid and reduces pH.
NH₃ – selectively corrodes Cu, particularly in the presence of oxidizing agents.
H₂S – Lowers pH and forms iron Sulphids which being cathodic to iron promotes galvanic corrosion.
Cl₂ – Forms HOCl + HCl and thus reduces pH. It retards the formation of certain protective films. It can also oxidize corrosion inhibitor films already formed.
Dissolved and Suspended solids: Conductivity of water increases with increasing dissolved solids. Corrosion being an electrochemical phenomenon, one would accept higher corrosion with increasing dissolved solids. However higher dissolved solids also imply higher hardness salts and alkalinity. Hardness salts and alkalinity retard corrosion by forming a corrosion inhibiting film on the metal surface. This factor far outweighs the corrosion induced by higher conductivity. Thus DM water is more corrosive than soft water, which in turn is more corrosive than hard water. Distilled water is most corrosive as not only is it devoid of all minerals but is also slightly acidic due to dissolved carbonic acid.
Chloride and sulphate ions present in the water are capable of penetrating passive films leading to pitting type corrosion.
Suspended solids influence corrosion by erosive or abrasive action. They can also settle on metal surfaces producing localized corrosion cells.
Microbial Growth: Microorganisms promote the formation of corrosion cells. Also, the by- products of some organisms are corrosive. Nitrifying, iron oxidizing and sulfate reducing bacteria are particularly harmful.
Velocity: In high velocity and turbulent waters, oxygen is rapidly distributed and reaches the metal surface. High velocity also removes passivation layer of corrosion inhibitors. The net result is increased corrosion.
High velocity can also lead to erosion of metal surfaces, protective films and oxides. At the same time, low velocity can lead to deposition giving rise to localized corrosion cells.
Temperature: As temperature increases, diffusion of oxygen to the metal surface also increases, promoting corrosion. Above 70^oC the loss of dissolved oxygen exceeds the amount made available by diffusion, and a decrease in the corrosion rate occurs.
Scaling
Scaling is the precipitation of hard and adherent salts of water soluble constituents, like calcium and magnesium, on the metal surface. These salts have very poor thermal conductivity and their control is therefore absolutely essential for proper heat transfer efficiency.
The most commonly encountered scale is calcium carbonate and it forms an extremely hard and adherent deposit.
Ca (HCO₃)₂ àCaCO₃ + CO₂ + H₂O
Calcium bicarbonate is present in all cooling waters. At higher temperatures and pH the bicarbonate decompose to calcium carbonate and carbon dioxide. Calcium carbonate is highly insoluble in water and precipitates at the hot spots of the heat exchanger forming a dense adherent scale.
Calcium sulphate does not pose much of a problem because of its higher solubility. This solubility is the basis of scale control by acid feed. Adding sulfuric acid, replaces the alkalinity with sulfate ions enabling operation at higher cycles of concentration without exceeding the carbonate solubility limits.
Silicate scales are extremely difficult to remove once formed. The best option is to prevent their deposition and this can be achieved by limiting the silica to 180 ppm as SiO₂, and Mg * SiO₂ product below 100,000 ppm with proper treatment.
Orthophosphate is formed by the reversion of polyphosphate and/or is used as such for corrosion inhibition. This orthophosphate readily combines with the calcium ions present, forming the highly insoluble and troublesome orthophosphate sludge. It is important to control this sludge formation as it has poor thermal conductivity and can induce under deposit corrosion.
Magnesium has lower scaling potential because magnesium salts are more soluble than calcium salts and their concentration is generally much lower than that of calcium salts.
Iron oxide can form by the oxidation of soluble iron in the water. It can also be produced as a result of corrosion. Iron oxide poses deposition problem and also acts as a nutrient for iron bacteria.
Factors that affect scaling
Temperature: The common scalants found in cooling water exhibit inverse solubility, i.e., their solubility decreases with increasing temperature. Therefore scaling increases with temperature.
pH or alkalinity: The solubility of CaCO₃ decreases with  increasing pH. Scaling potential increases with increasing pH.
Solubility: For water borne deposits to form, the potential scaling material should be carried as a soluble constituent of the cooling water to some degree. Under the conditions, each potential scalants exhibits a definite solubility limit. Once this limit is exceeded, the solution gets supersaturated and a precipitate forms leading to scaling. Also, higher the level of scale forming dissolved solids, greater the chances of scale formation.
Fouling
Fouling is the deposition of suspended matter, insoluble in water. They can be water borne or air borne. Some of the common foulants are:

• Dirt and silt
• Sand
• Corrosion products
• Natural organics
• Microbial matter

The particulate matters generally accumulate in the low velocity areas or in areas where there is an abrupt change in the direction of flow or flow velocity.
Factors that affect fouling
Water Characteristics: Water containing suspended material will cause fouling e.g. distilled water will not foul. Similarly surface waters have greater fouling tendency, as the amount of suspended matter picked up by them is greater.
Temperature: Fouling tendency increases with increasing temperature. Heat transfer surfaces which are hotter than the cooling water accelerate fouling.
Velocity: Fouling is greater in areas of low velocity while it is less severe in areas of high velocity. Normal velocity is 3 to 5 feet/second.
Microbial Growth: Microorganisms can deposit on any surface. Certain bacteria like iron bacteria utilize corrosion products leading to voluminous deposits. Also, slime secreted by bacteria, acts as a binder and entraps material, which normally would not have deposited.
Corrosion Products: Insoluble corrosion products mix with other foulants like debris, microorganisms, etc., and aggravate fouling. It also serves as a nutrient for iron bacteria.
Oil: Oil adheres to the metal surface and has the ability to bind deposits. Oil has very poor thermal conductivity and can seriously affect heat transfer. Oil serves as a nutrient for microorganisms. It also forms a barrier to the protective film forming inhibitors preventing them from reaching and passivation the metal surfaces.
Microbial Growth
Cooling towers provide optimum conditions for microbial growth. Temperature and pH are ideal for their growth and there is an abundance of nutrients and sunlight. Under these conditions, microorganisms may multiply six million times, while during the same time, inorganic salts may concentrate only six times.
Microorganisms enter the cooling water through the make up water and air. The major problem microbes are;

• Algae
• Fungi
• Bacteria

Algae:Air, water and sunlight are the three basic requirements for algal growth. The distribution decks and side walls of a cooling tower fulfill all these requirements and therefore represent an excellent growth environment for algae.
Excessive growth of algae can lead to choked pipelines, nozzles etc., and hampering effective distribution of water in the cooling tower.
All algae produce oxygen, which can depolarize the corrosion reaction and accelerate system corrosion. Blue green algae can fix nitrogen and are responsible for the accelerated deterioration of nitrite based corrosion inhibitors. Algae also produce slime which can act as nutrient for other microorganisms.
Fungi: Fungi lack chlorophyll and are therefore non-photosynthetic, resulting in a dependence on nutrients provided by organic matter (heterotrophic). Fungi use wood as nutrient and can destroy cooling tower wood.
Fungi reproduce by forming spores. Spores can remain dormant for a long time and proliferate when conditions become favorable. In their dormant state they are harmless. Spores are generally resistant to most microbicides and can present very difficult situations.
Bacteria
The commonly found bacteria in CW systems that are detrimental to the system are:
Pseudomonas: These are slime forming bacteria. The slime acts as a binding agent for dust and precipitates thereby causing voluminous deposits. Material which normally would not have deposited gets deposited by the binding action of the slime.
Sulphur Reducing Bacteria: These are anaerobic bacteria and generate the energy required for their growth by reducing sulphate to Sulphids and in the process corrode iron.
4Fe+SO₄^2- + 4H₂O àFeS+3Fe (OH)₂+2OH^-
It also indirectly corrodes iron by the formation of H₂S (acid).
Fe + H₂S àFeS + H₂
During chlorination in the presence of SRB, the pH drops due to the formation of HCl.
H₂S + Cl₂ à2HCl + S
This is significant in the attack on concrete basins.
Iron Bacteria: These bacteria utilize iron for their growth and create iron deposits as a by-product of their metabolism.
4FeCO₃ + O₂ + 6H₂O  à  4Fe(OH)₃ + 4CO₂ +81000 cal.
To generate the energy requisite for their growth they must produce large quantities of ferric hydroxide. This gets entrained in the organism producing voluminous deposits which can cause plugging, pitting corrosion and reduced heat transfer.
Nitrifying Bacteria: These bacteria convert nitrogenous compounds like ammonia to nitric acid. This leads to lowering of pH due to the acid formed and directly leads to corrosion. They can be easily detected by the continual drop in pH of the circulating water. The pH seldom falls below 5, as these organisms are killed at a pH lower than this.

2. Cooling Water Systems – Solutions

Corrosion Inhibition
Corrosion inhibitors prevent the metal from reverting to its natural oxide state. Depending on the corrosion reaction it controls, a corrosion inhibitor can be anodic, cathodic or general.
Anodic inhibitors are initiated at the anode and eventually may cover the entire metal surface. Anodic inhibitors in low concentrations are dangerous because the entire corrosion potential will occur at the unprotected anodic sites leading to severe pitting.
Cathodic inhibitors are initiated at the cathode and a thin protective film is formed. It forms a barrier between the metal surface and oxygen. Low concentration of cathodic inhibitors lead to general attack as the corrosion rate is increased in direct proportion to the increase in the unprotected area.
Corrosion inhibitors that protect the metal surface by filming all metal surfaces, whether anodic or cathodic are called general or filming type corrosion inhibitors.
The inhibitive action of two corrosion inhibitors used together is far greater than the sum of the individual actions. This is because of the so called synergistic effect. The best protection is obtained when one of the two inhibitors is cathodic and the other anodic.
Orthophosphate is an excellent anodic inhibitor but one has to carefully control orthophosphate sludging when using it as a corrosion inhibitor. The same caution has to be exercised when using polyphosphates because of their reversion to orthophosphates under cooling water conditions.
Organophosphonates and polymers, though not corrosion inhibitors by themselves at use levels, exhibit excellent synergism with other corrosion inhibitors. They also have excellent thermal and hydrolytic stability.
Nitrites are good corrosion inhibitors for aluminum, tin and ferrous materials at pH 9 to 10. They are the inhibitors of choice for closed systems. Silicates are effective in preventing dezincification. Besides closed systems, silicates are often used in potable water systems.
Zinc is very popular due to its ability to form a film rapidly. This ability of zinc is used by most formulations but is never used alone since the film formed by zinc is not very durable.
Molybdates are new generation corrosion inhibitors and their performance as corrosion inhibitors is enhanced in the presence of oxygen and alkalinity. They are very effective in DM or soft water conditions.
Azoles are excellent corrosion inhibitors for copper and copper based metallurgy. They also afford some corrosion protection to steel when used with other inhibitors.
Whichever inhibitor one chooses, pretreatment is absolutely necessary for good corrosion protection. Corrosion inhibitor at 2 to 4 times their normal dose is applied for the first few days over a clean metal surface. This ensures the formation of a durable passivation film on the metal surfaces rapidly. Pretreatment should also be instituted after any system upsets, pH excursions, corrosive contaminants and prolonged low inhibitor levels.
Scaling and Fouling Control
Scaling and fouling can be controlled in a number of ways

• Limiting the cycles of concentration
• Softening the make up water
• Acid feed to maintain pH
• Mechanical means like increasing water velocity or designing exchangers with large surface areas
• Treat with chemical inhibitors

Treatment with chemical inhibitors is the most practical and cost effective method to control deposition.
Scale formation is controlled by the mechanism of threshold inhibition and crystal distortion. Threshold phenomenon is a mechanism by which substoichiometric amounts of the chemical prevents or retards the growth of scale forming crystals. The chemical is adsorbed on the crystal surface interfering with the nucleation of the scalants crystals and preventing orderly lattice type growth. Crystal’s growth is retarded and if and when they are formed, they are highly distorted leading to a soft friable scale which can be easily dispersed by the movement of water. The CW systems can therefore bt operated at higher cycles of concentration and alkaline pH.
The antiscalant commonly used are;

• Organophosphonates
• Polyphosphates
• Low molecular weight anionic polymers.

Organophosphonates are the antiscalant of choice. They are far more effective antiscalant than polyphosphates and exhibit much superior thermal and hydrolytic stability.
Specific low molecular weight polymers are particularly designed for the control of orthophosphate sludging. These are a very important class of compounds that keep orthophosphate in solution and use their anodic corrosion control capability synergistically.
Control of iron and heavy metals is obtained by the sequestering property of organophosphonates and polymers. The net result is that the CW systems can be operated at higher cycles and pH whereby corrosion potential is substantially reduced. An additional benefit is that they can also clean the system of existing scales.
Low molecular weight anionic polymers function as excellent dispersants. They are adsorbed on the dirt and suspended particles and enhance the partial negative charge that these particles carry. The particles repel each other and agglomeration is prevented.
Microbial Control
Biocides are chemicals that kill microorganisms. The efficacy of a biocide depends upon the nature and amount of pollutants such as hydrocarbons, pH, temperature and nutrients such as orthophosphate present.
Biocides are mainly classified as:

• Oxidizing
• Non-Oxidizing

Oxidizing Biocides: Oxidizing biocides are the primary mode of control. They literally burn up the microbes and microorganisms can never develop immunity to these oxidizers. It is widely used and is very cost effective. They are usually added continuously. However, organic matter, oil, masses of algae or slime and presence of reducing agents impairs their effectiveness. While using this class of microbicides the oxidant demand has to be met, before free oxidant is available for biocidal action.
The commonly used oxidants for CW systems are:

• Chlorine or chlorine releasing compounds
• Bromine or bromine releasing compounds
• Ozone
• Chlorine Dioxide
• Sodium Hypochlorite

Chlorine enhancers are compounds that in certain combinations yield one of the above species in circulating water. Bromine and chlorine dioxide are particularly effective in alkaline medium where chlorine loses its activity.
Non-Oxidizing Biocides: Each of these biocides has their specific mode of action and kills the microbes by interfering with their life process. Most are enzyme poisons. They are added periodically and are shock dosed. A combination of two or more non-oxidizing biocides is usually used to prevent microbes developing immunity to the biocide.
Some of the commonly used eco friendly non-oxidizing biocides are:

• Methylene Bisthiocyanate
• Quaternary ammonium compounds
• Dodecyl guanidine hydrochloride
• Dichlorophene
• Isothiazolines
• Dibromonitrilopropionamide
• Thiocarbamates
• Glutaraldehyde/formaldehyde

Each of these products has their own strengths and limitations and a combination is usually selected to suit the needs of a particular system and installation.
Biodispersants: Biodispersants are a class of compounds that enhance the effectiveness of the biocides used. These are surface active compounds. They loosen microbial deposits which can then be flushed away. This exposes the new layers of microbial slime or algae to the attack of biocides. They increase the penetrating power of active ingredients of biocides by exposing the underlying microbial deposits which would have otherwise been covered and sheltered.
Chemical Treatment Programme
A chemical treatment programme is therefore designed to control

• Corrosion
• Scaling
• Fouling
• Microbial Growth

Today, every treatment programme is non chromate because of the acute toxicity of chromate. Most programmes are operated at alkaline pH (7.0 to 8.3). The systems are also operated at high cycles of concentration as, today, chemicals are available that can permit operation at hardness levels upto 1500 ppm as CaCO₃. Under these conditions, corrosion potential is lowered and inhibitors like orthophosphate and zinc and their various combinations easily provide corrosion inhibition well within the required limits.
Based on the chemicals used and the system needs, the various treatment programmes available, are:

• Zinc Phosphate Programme
• Non Metallic Programme
• All Organic Programme
• Soft Water Molybdate Programme
• Stabilized Phosphate Programme

The above effectively controls corrosion, scaling and fouling. Microbial control is achieved with chlorine or a halogen source as the primary mode of control. This is supplemented with the regular use of a biodispersant. Depending on the system and the extent of microbiological contamination, a combination of non-oxidizing biocides is selected for periodic addition alternately.
The water parameter limits are set for individual installations and the system is operated within these limits for optimum performance.
3.0 Cooling Water Systems-Operation
The effectiveness of any cooling water treatment programme depends on its proper
implementation. It begins with the start-up of cooling water systems.
A newly installed cooling water system should be properly cleaned before it is put to actual use. The cooling tower basin, pump sump, tower deck and heat exchangers should be cleaned of mud, construction debris, loose lumber, mill scale, oil, grease etc., to prevent choking of heat exchanger tubes or pipelines. The following sequence should be followed for optimum performance of the system.
Physical Cleaning: The cooling tower basin, sump, distribution deck, large pipelines and inlet and outlet of exchangers should be manually cleaned to remove the debris before filling the basin with water.
Flushing with water: After physical cleaning flush the system with water. At this stage, it is necessary that the water velocity is as high as possible. Many times, all the circulating pumps are not ready at this stage. In such an eventuality, it is advisable to take the system in line in loops and change the loops every hour or two. It should be ensured that before closing any loop it is sealed with water.
This operation removes most of the mud and loose matter from the pipelines and brings it into the basin. Constant turbidity is the indication of completion of the flushing operation. Blowdown at maximum rate with simultaneous make up to remove the deposits from the system. Continue till the circulating water is clear.
Stop the pumps and drain the basin. Clean the basin and sump manually. During this period the jump over’s can be removed and the heat exchangers can be taken on line.
Surfactant Cleaning; Start circulation after filling the basin with water. Ensure that the exchangers are in line and the ID fans are running. Drain individual heat exchangers till drain water is clear. Close the drains and add the recommended surfactant at the suggested dose level. Circulate for 24 hrs without blowdown. Heavy foaming may be observed. During this stage oil, grease and other loose suspended matter is removed from the system. This is then flushed by giving heavy blowdown with make up till circulating water is clear and foaming completely subsides.
Acid Cleaning: Acid cleaning removes corrosion products from the system. It is usually carried out at pH of 3.5 to 4.5. It is absolutely necessary to use an acid inhibitor at this stage. The acid inhibitor forms a temporary protective layer on the bare metal surfaces as soon as the corrosion product on the metal surface dissolves in the acid at low pH. Continue low pH cleaning till iron content in the circulating water is constant.
Once constant iron readings are obtained increase pH gradually. Quick pH increase will redeposit the dissolved iron. Keeping turbidity below 15 NTU, increase pH slowly with blowdown and simultaneous make up. Incase pH increases rapidly, use acid to arrest the rapid increase of pH. Also, take the side stream filters on line at this stage. Continue blowdown and make up till iron in the circulating water is below 1 ppm.
Cleaning with Microbicides: The system is chemically clean after low pH cleaning. It is now necessary to clean it microbiologically. A shock dose of the recommended biocide at the suggested use level should be added. Close blowdown and circulate for the required period (8 to 24 hours). Then blowdown heavily to flush the microbial mass out of the system.
Passivation: Passivation is required for the formation of a protective film on the metal surface rapidly. This is achieved by maintaining a high concentration of the corrosion and deposit control chemicals for a certain period. In case of no or low heat load, the passivation period is extended as passivation is good and rapid if heat load is available.
Chlorination should be started immediately after system cleaning.
Once passivation is complete, maintain regular levels of treatment chemicals and follow the microbial control regime as advised by the treatment vendor. Maintain water parameters within the recommended limits and analyse water parameters and chemical treatment levels regularly as required.
4.0 Cooling Water System-Monitoring & Evaluation
The success of any treatment programme depends on maintaining the various parameters within the recommended limits at all times. Therefore careful monitoring is an integral part of a good treatment programme.
The parameters that should be monitored continuously are:

• pH
• Water level in sump
• Blowdown rate
• Make up rate

Free oxidant level should be monitored every shift. Free oxidant level should be analyzed every shift. The complete analysis of circulating water and make up water should be done daily and should include;

• pH
• Alkalinity
• Conductivity
• Turbidity
• Hardness
• Chlorides
• Silica
• Iron
• Ammonia, nitrates, if required
• Any frequent pollutant
• Treatment Chemicals

The sampling point should be located in the return header.
Acid, if required and treatment chemicals should be added continuously.
Evaluation of the Treatment Programme: The treatment programme should be regularly evaluated for;

• Corrosion control
• Deposit control
• Microbial control

Corrosion control: Effectiveness of a treatment programme for corrosion control can be evaluated by measuring corrosion rate. This is normally measured by corrosion meter and corrosion coupons.
Corrosion meter measures the instantaneous corrosion rate due to electrochemical corrosion. It does not reflect microbial induced corrosion. It is effective in reflecting the corrosion trend on day to day basis.
Corrosion coupons are exposed for a period of 30 days in a specially designed rack and are placed in the return header. The exposed coupons provide an average corrosion rate for the period and it also reflects microbial corrosion during the period.
Deposit control: It is difficult to measure the exact extent of scaling and fouling since it varies depending on the local thermal and hydrodynamic conditions. An indication of deposit control can be obtained by observing the performance of test heat exchangers.
Selected critical exchangers can be monitored with the help of heat transfer data. Periodic inspection of an exchanger that can be isolated without disturbing plant operation also provides significant information regarding the treatment effectiveness for deposit control.
Devices like deposit monitors can also be installed and besides visual inspection, it affords quantitative data on deposition rate.
Microbial control: Regular microbial analysis of the circulating water should be carried out. This should include the total viable count and also the sulfate reducing bacteria count. Any other bacteria, specific to the system, can also be identified and analysed.
Regular inspection of the cooling tower especially louvers and deck for algae and fungus growth will also help in evaluating the microbial treatment programme effectiveness.
Biofouling monitors are also quite effective in monitoring the microbial treatment performance. Here the pressure drop across a stainless steel pipe indicates the degree of microbial fouling in the system.
Heat Exchanger Inspection: All the above methods are indicative and serve as comparative methods for evaluation of the treatment programme. The actual conditions in the system vary depending on the thermal and hydrodynamic parameters as well as metallurgy of the system. Often there are leaks altering the environment conditions. It is not possible to simulate all these conditions in the evaluation methods mentioned earlier.
The inspection of heat exchangers during annual turnaround therefore gives the best indication about the effectiveness of the overall treatment programme. Couple this with the deposit analysis of the deposits collected from different parts of the exchangers provides an excellent overview of the effectiveness of the treatment programme.
Integrated programme approach is just not mere selection of good chemicals but also supplementing it with good monitoring practices and proper devices for tracking the performance. The combination of these factors will certainly make the treatment programme a total success even with less tolerant and non-forgiving non-chromate programmes.