Cooling water problems
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This document discusses cooling water problems such as corrosion, scale, fouling, and microbiological contamination. It explains the causes and factors that influence these problems, as well as methods to control and prevent each problem. Key topics covered include the corrosion process, scale formation mechanisms, types of fouling, factors influencing microbial growth, and chemical treatment options for control and prevention.
Cooling water problems
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- 2. Why do cooling water problems exist? If left untreated, cooling water systems provide an environment where the four main cooling problems will exist. 2
- 3. Corrosion- Water tends to convert metals (such as mild steel) to their oxide states. Scale- Water impurities, such as calcium and magnesium hardness can precipitate and deposit depending on their concentrations, water temperature, pH, alkalinity, and other water characteristics. Microbiological Contamination -Cooling water systems offer a favorable environment for micro-organisms to grow and cause problems. Fouling-Suspended solids from either external or internal sources can cause deposits 3
- 4. If left uncontrolled, these problems, either alone or together, can cause: Increased maintenance cost Reduced heat transfer efficiency and therefore greater energy losses Possible production cutbacks or shutdowns 4
- 5. It is an electrochemical process by which a metal returns to its natural state. For example, mild steel is a commonly used metal in cooling water systems and is very susceptible to corrosion. It will return to its iron oxide state. 5
- 6. For corrosion to occur, a corrosion cell, consisting of an anode, a cathode, and an electrolyte, must exist. Metal ions dissolve into the electrolyte (water) at the anode. Electrically charged particles (electrons) are left behind. These electrons flow through the metal to other points (cathodes) where electron- consuming reactions occur. The result of this activity is the loss of metal and often the formation of a deposit. 6
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- 8. General attack exists when the corrosion is uniformly distributed over the metal surface. The iron oxide produced contributes to fouling problems. Localized (or pitting) attack exists when only small areas of the metal corrode. Pitting is the most serious form of corrosion. Because the action is concentrated in a small area. Galvanic attack can occur when two different metals are in contact. The more active metal corrodes rapidly. Common examples in water systems are steel and brass, aluminum and steel, zinc and steel, and zinc and brass. 8
- 9. The most important factors are: Oxygen and other dissolved gases Dissolved or suspended solids Alkalinity or acidity (pH) Velocity Temperature Microbial activity 9
- 10. Dissolved solids can affect the corrosion reaction by increasing the electrical conductivity of the water. The higher the dissolved solids concentration, the greater the conductivity and the more likelihood of corrosion. Dissolved chlorides and sulfates are particularly corrosive. Suspended solids can influence corrosion by erosive or abrasive action, and they can settle on metal surfaces to set up localized corrosion cells. 10
- 11. Acidic and slightly alkaline water can dissolve metal and the protective oxide film on metal surfaces. More alkaline water favors the formation of the protective oxide layer. 11 How does alkalinity or acidity affect corrosion?
- 12. High-velocity water increases corrosion by transporting oxygen to the metal and by carrying away corrosion products at a faster rate. High velocity can also cause erosion of metal surfaces, protective films, and oxides. When water velocity is low, deposition of suspended solids can establish localized corrosion cells, thereby increasing corrosion rates. 12 How does water velocity affect corrosion?
- 13. Microbial growths promote the formation of corrosion cells. In addition, the byproducts of some organisms, such as hydrogen sulfide from anaerobic corrosive bacteria, are corrosive. 13 How does microbial growth affect corrosion?
- 14. Below 160°F, every 25-50°F increase in temperature causes corrosion rates to double. Above 160°F, additional temperature increases have relatively little effect on corrosion rates in cooling water systems. 14
- 15. When designing a new system, choose corrosion-resistant materials to minimize the effect of an aggressive environment. Adjust pH. Apply protective coatings such as paints, metal plating, tar, or plastids. Protect cathodically, using sacrificial metals. Add protective film-forming chemical inhibitors that the water can distribute to all wetted parts of the system. 15
- 16. Anodic corrosion inhibitors establish a protective film on the anode. These inhibitors are effective, but dangerous: if insufficient anodic inhibitor is present, the entire corrosion potential occurs at the unprotected anode sites. This causes severe localized (or pitting) attack. Cathodic corrosion inhibitors form a protective film on the cathode. These inhibitors reduce the corrosion rate in direct proportion to the reduction of cathodic area. General corrosion inhibitors protect by filming all metal surfaces whether anodic or cathodic. 16
- 17. Mainly anodic Chromates Nitrites Orthophosphates Silicates 17 Mainly cathodic • Bicarbonates • Metal cations • Polyphosphates General • Soluble oils • Other organics
- 18. Yes. The choice of treatment is basically a matter of economics. In a once-through system, a very large volume of water passes through the system only once. In an open recirculating system, more chemical must be present because the water composition changes significantly through the evaporation process. In a closed recirculating system, water composition remains fairly constant. There is very little loss of either water or treatment chemical 18
- 19. Consistent control of both the corrosion inhibitor chemicals and the key water characteristics. No program will work without proper control. 19
- 20. Scale is a dense coating of predominantly inorganic material formed from the precipitation of water-soluble constituents. Some common scales are: Calcium carbonate Calcium phosphate Magnesium salts Silica 20
- 21. Temperature. Alkalinity or acidity (pH). Amount of scale-forming material present. Influence of other dissolved materials, which may or may not be scale-forming. 21
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- 23. Limit the concentration of scale forming minerals by controlling cycles of concentration or by removing the minerals before they enter the system. Feed acid to keep the common scale-forming minerals (such as calcium carbonate) dissolved. Make mechanical changes in the system to reduce the chances for scale formation. Increased water flow and exchangers with larger surface areas are examples. Treat with chemicals designed to prevent scale. 23
- 24. Some salts such as calcium carbonate become less soluble as temperature increase. A change in PH or alkalinity can greatly affect scale formation, the solubility of calcium carbonate decreases with increasing PH. When the amount of scale forming material dissolved exceeds its saturation point. The higher the level of scale forming dissolved solids, the greater the chance of scale forming. 24
- 25. 25 Calcium carbonate crystals modified by treatment with a polyacrylate into light, fluffy shapes that are easily dispersed from cooling system surface (magnified 2000X) "Normal" calcium carbonate Calcium (calcite) crystals (magnified 2000X)
- 26. Threshold inhibition chemicals prevent scale formation by keeping the scale-forming minerals in solution and not allowing a deposit to form: Scale conditioners modify the crystal structure of scale, creating a bulky, transportable sludge instead of a hard deposit. 26
- 27. Threshold inhibition: Organic phosphates, polyphosphates, polymeric compounds Scale conditioning: Lignins, tannins, polymeric compounds 27
- 28. Fouling is the accumulation of solid material other than scale in a way that hampers the operation of plant equipment or contributes to its deterioration. 28
- 29. Dirt and silt. Sand. Corrosion products. Natural organics. Microbial masses. Aluminum phosphates. Iron phosphate. 29
- 30. Water characteristics. Temperature. Flow velocity. Microbial growths. Corrosion. Contamination. 30
- 31. How do water characteristics affect fouling? Distilled water will not foul. However, most waters contain the dissolved and suspended materials that can cause a significant fouling problem under certain conditions. How does temperature affect fouling? Increasing temperature increases the fouling tendency. Because heat-transfer surfaces are hotter, than the cooling water, they accelerate fouling. 31
- 32. How does flow rate affect fouling? At low flow rates (one foot per second or less), fouling occurs due to natural settling of sus-pended material. At higher flow rates (three feet per second or more), fouling can still occur, but usually is less severe. How do microbial growths affect fouling? Microorganisms can form deposits on any surface. In addition, corrosive or iron depositing bacteria cause or utilize corrosion products, which subsequently deposit as voluminous foulants. All microbial colonies act as a collection site for silt and dirt, causing a deposit of different foulants. 32
- 33. 33 Fouling of a heat exchanger tube bundle
- 34. Corrosion can form insoluble corrosion products that migrate and mix with debris, process contamination, or microbial masses to aggravate fouling. 34
- 35. Material that leaks from the process side of heat exchange equipment can cause serious fouling problems in several ways: Depositing as insoluble products. Providing nutrient for micro-organisms and causing severe microbial growths. Reacting with scale or corrosion inhibitors to form insoluble foulants. 35
- 36. Fouling can be controlled mechanically or by the use of chemical treatments. The best method of control depends upon the type of fouling. Control of fouling in the cooling system involves three major tactics: 36
- 37. Prevention: Whatever can be done to prevent foulants from entering the cooling system; this may require mechanical changes or addition of chemicals to clarify make-up water. Reduction: Steps taken to remove or reduce the volume of foulants that unavoidably enter the system. This may involve side stream filtering periodic tower basin cleaning. Ongoing Control: Taking regular action to minimize deposition of the foulants in the system. This can include adding chemical dispersants and air rumbling or back- flushing exchangers. 37
- 38. Charge-reinforcement and wetting-agent, dispersants act to Keep foulants in suspension, preventing them from settling on metal surfaces or helping to remove fouling deposits that have already formed. The charge reinforcement dispersants cause the foulants to repel one another by increasing the electrical charges they carry. The wetting agents reduce surface tension, inhibiting new deposit formation and possibly removing existing deposits. This action keeps the particles in the bulk water flow, where they are more likely to be removed from the system. 38
- 39. Charge reinforcers Anionic polymers Wetting agents Surfactants 39
- 40. What is microbiological contamination? The uncontrolled growth of microorganisms can lead to deposit formations which contribute to fouling, corrosion and scale. 40
- 41. Microbial slimes are masses of microscopic organisms and their waste products. These slimes are usually characterized by their gooey feeling and can be plant or animal. 41
- 42. No. Some organisms do not create slime deposits and do not promote metal corrosion. The presence of large numbers of these harmless organisms, however, indicates that conditions are ideal for the growth of harmful organisms 42
- 43. The make-up water supply, wind, and insects can all carry micro-organisms into a cooling water system. 43
- 44. Nutrients: For instance, hydrocarbons or other carbon sources can serve as food for slime-forming organisms. Atmosphere: Organism growth depends upon the availability of oxygen or carbon dioxide. Location: Such factors as the amounts of light and moisture significantly affect growth rates. Temperatures: Organisms that make up slime tend to flourish between 40 and 44
- 45. Can slime cause scale formation? Yes. Slime can cause or accelerate the rate of scale formation. Slime can cause treatment chemicals for scale to be ineffective. Reduces heat transfer, causing higher energy cost. Can slime cause fouling? Slime masses themselves are foulants. Other micro-organisms and suspended solids can become part of the fouling deposit. Although many organisms tend to die at high temperatures, the remaining debris still fouls metal surfaces. 45
- 46. Certain organisms, for example, the sulfate- reducing type, generate corrosive hydrogen sulfide which causes severe pitting attack. In addition, slime can accelerate corrosion by depositing on the metal and preventing protective film formation. 46
- 47. Generally, microbial organisms form colonies at points of low water velocity. Heat exchangers are therefore subject to microbiological contamination. Similarly, cooling towers are subject to fouling as well as surface and internal rot. 47
- 48. Types and quantities of microbial organisms. Microbial trouble signs such as wood rot, slime deposits, and corrosion. Operating characteristics of the system, such as temperature, flow rate, and water composition. 48
- 49. Types of equipment employed, such as cooling towers, spray ponds, open box condensers. Sources of contamination, such as organisms and nutrients carried into the system. These factors can influence the growth of troublesome organisms and affect the microbial control treatments. 49
- 51. Chemicals called "oxidizing biocides" literally "burn up" any microbe they come in direct contact with. Common oxidizers are chlorine, chlorine dioxide, bromine, ozone, and organo chlorine slow-release compounds. Chlorine is one of the most widely used, cost-effective biocides, and is available in liquid, gaseous, or solid form. Its effectiveness is increased when used with non-oxidizing biocides and biological dispersants. 51
- 52. Organic compounds utilized in killing microorganisms. They are effective in areas or systems where chlorine may not be adequate. 52
- 53. These chemicals do not kill organisms; they loosen microbial deposits, which can the be flushed away. They also expose new layers-of-microbial slime or algae to the attack of oxidizing, biocides, Bio- dispersants are an effective preventive measure, because they make it difficult for the microorganisms to attach to the metal surfaces to form deposits. 53
- 54. Pretreatment is the preparation of the cooling water system to insure that the treatment program can work effectively from startup. 54
- 55. New systems or existing ones being returned to service can contain significant amounts of contaminating material. Films of oil or grease, general coatings or spots of rust, dirt and sand always remain in newly constructed systems. These materials do not represent faulty construction; they result from conditions existing during construction. 55
- 56. In systems taken off line, deposits can be present as the result of scale, corrosion, fouling, or micro-biological contamination. If these materials are not removed through effective pretreatment, the subsequent chemical program will not be effective. 56
- 57. System cleaning Application of special pretreat-ment chemicals Initial high dose application of corrosion inhibitors Ongoing application of corrosion inhibitors at maintenance levels 57
- 58. Acid removes corrosion products and some mineral contaminants, but has little effect upon organic materials. With improper application it may attack system metal and cause severe metal attack. Improper flushing will leave metal surfaces in a highly reactive state, which makes them especially vulnerable to corrosion attack. 58
- 59. Automation of systems has become increasingly important as the nature of acceptable cooling water treatment programs has changed. For every treatment program there are specific chemical concentration ranges where it functions best and provides the intended protection. If not properly controlled, any chemical program can fail, leading to possible lost production, increased mainten-ance cost, and increased energy usage. 59
- 60. An automatic system is an excellent addition to any treatment program where pH control is important. However, for proper operation, these systems need periodic maintenance and attention. 60
- 61. Corrosion Coupons-To establish relative corrosion rates of different metals in cooling systems, small metal strips called corrosion coupons can be used. These preweighed coupons are placed in the system for at least 30 days, then removed, cleaned, and weighed again. The difference in the coupon's weight before and after exposure is used in calculat-ing the corrosion rate. 61
- 62. Corrosion Test Rack: Evaluates the effectiveness of corrosion inhibition programs on non-heat transfer surfaces. It is designed to be used with either corrosion coupons or Corrater probes. 62
- 63. Deposit Monitor: An aid in examining the performance of a cooling water program. A sample tube of proper metallurgy is surrounded by a glass jacket to form a small heat exchanger. Cooling system water flows between the sample tube and the glass jacket. The metal surface of the sample can be observed at any time during the testing period. Heat transfer rates can be sim-ulated using a cartridge heater and cooling water flows regulated with a control valve. 63 The Corrater gives instantaneous readouts of corrosion rates when its probe is inserted into an operating cooling water system.
- 64. Microbiological Analysis: Provides answers on the kinds and quantities of bacteria present in a cooling system. Results of these analyses indicate the effectiveness of the microbiological control program. 64
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- 66. Plant operating and supervisory personnel must know the impor-tance of correct system treatment and have the ability to monitor and control critical cooling water variables on a day-to-day basis. Proper training of all cooling water personnel is a prime method of achieving a successful treatment program. 66