Cooling Water Treatment for industrial use

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Cooling Water Treatment

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Process File Reference: LAB20705

For additional information on this subject, contact R. A. Al-Husseini on 874-2792

Engineering Encyclopedia

Process Cooling Water Treatment

CONTENTS

PAGES

TYPES OF COOLING WATER SYSTEMS AND PARTS OF COOLING TOWERS AND HEAT EXCHANGERS................................................................. 1 Open Evaporative Recirculating Cooling Systems....................................... 1 Typical Cooling Tower Design .................................................................... 1 Cooling Tower Water Balance: Evaporation, Make-Up, Blowdown, and Drift ....................................................................................................... 2 Principal Parts of Cooling Towers................................................................ 5 Heat Exchangers........................................................................................... 6 Components of a Shell and Tube Heat Exchanger ....................................... 6 Components of a Plate Heat Exchanger ....................................................... 7 Common Materials of Construction ............................................................. 7 Once-Through Cooling Systems .................................................................. 8 Closed Recirculating Cooling Systems ........................................................ 9 CONTROL OF CORROSION IN COOLING WATER ........................................ 11 Factors Affecting Corrosion in Cooling Water........................................... 13 Corrosion Inhibitors.................................................................................... 14 Chromate .................................................................................................... 15 Zinc............................................................................................................. 16 Orthophosphates and Polyphosphates ........................................................ 16 Nitrite.......................................................................................................... 18 Silicates ...................................................................................................... 19 Molybdate................................................................................................... 19 Phosphonates.............................................................................................. 20 Copper Alloy Inhibitors.............................................................................. 22 Nonchromate Cooling Tower Treatment Packages .................................... 24 Monitoring Corrosion................................................................................. 26

Saudi Aramco DeskTop Standards



Avoiding Galvanic Corrosion..................................................................... 28 Precleaning and Pretreatment ..................................................................... 29 PREVENTION OF SCALE FORMATION IN COOLING WATER.................... 31 Effect of Scale on Heat Transfer ................................................................ 31 Scales Formed in Cooling Water and Their Prevention ............................. 33 Calcium Carbonate Scale ........................................................................... 33 Calcium Sulfate Scale................................................................................. 36 Calcium Phosphate Scale ........................................................................... 36 Magnesium Silicate Scale........................................................................... 37 Effect of Water Chemistry, Temperature, and pH...................................... 37 PREVENTION OF THE HARMFUL EFFECTS OF MICROBIOLOGICAL GROWTH IN COOLING WATER ....................................................................... 40 Microorganisms Responsible for Biofouling.............................................. 40 Chemicals for Control of Biofouling.......................................................... 42 Oxidizing Biocides ..................................................................................... 43 Nonoxidizing Biocides ............................................................................... 43 Surfactants .................................................................................................. 45 Mechanical Means for Control of Biofouling ............................................ 45 Biofouling Monitors ................................................................................... 45 Prevention of Macrofouling by Jellyfish, Mussels, Etc.............................. 46 CONTROL OF GENERAL FOULING IN COOLING WATER .......................... 47 Oil and Dust in Cooling Water ................................................................... 47 Means of Control........................................................................................ 48 Sidestream Filtration .................................................................................. 48 Dispersants and Surfactants........................................................................ 48 Cleaning General Deposits ......................................................................... 48




MONITORING AND CONTROL REQUIRED TO OPERATE COOLING WATER SYSTEMS .............................................................................................. 49 Chemical Feed Equipment ......................................................................... 49 pH and Blowdown Controllers ................................................................... 49 Frequency of Chemical Analysis................................................................ 50 GLOSSARY .......................................................................................................... 51




TYPES OF COOLING WATER SYSTEMS AND PARTS OF COOLING TOWERS AND HEAT EXCHANGERS Three types of cooling water systems are used in the petroleum and chemical industries: open evaporative recirculating, once-through, and closed recirculating. The system used will depend on the process or equipment to be cooled, the availability and quality of water, and the ease with which the water can be disposed. The types of systems found vary from small engine jackets to large once-through systems and open recirculating cooling towers. It is not uncommon to have several different systems in a refinery or plant. Open Evaporative Recirculating Cooling Systems Open recirculating cooling systems allow reuse of cooling water and provide efficient dissipation of heat. For these reasons, they are commonly used where water conservation is important. Typical Cooling Tower Design Figure 1 depicts a schematic of an open evaporative recirculating cooling system. Heat is dissipated by the evaporation of some of the recirculating water. The evaporation takes place most commonly in a cooling tower, although spray ponds and evaporative condensers are also used.

FIGURE 1. OPEN EVAPORATIVE RECIRCULATING COOLING SYSTEMS


1



Cooling Tower Water Balance: Evaporation, Make-Up, Blowdown, and Drift The amount of heat dissipated by a cooling tower is governed by the rate at which water is evaporated. The evaporation rate is a function of the recirculation rate, cooling range, and the atmospheric temperature and humidity. The following equation approximates this relation: e = 0.8 L (C ) (dT) / H where e = evaporation rate, grams per minute (gpm) p

v

L = circulation rate, gpm Cp = heat capacity of cooling water, 1.0 Btu / lb F Hv = latent heat of water, 1050 Btu/lb dT = cooling range, difference between the hot and cold water temperatures, °F The factor, 0.8, arises from the fact that under typical atmospheric conditions 20 % of the temperature drop is due to sensible heat transfer rather than latent heat transfer. For example, a cooling tower circulating water at 25,000 gpm with a 11 °C (20 °F) temperature drop will evaporate 380 gpm. This corresponds to about 4 million Btu/minute of heat transferred. As the water vapor leaves the tower through evaporation the remaining dissolved salts naturally present in the water increase in concentration. These increased concentrations make the water more corrosive and increase the tendency of scales to form. Dissolved salts are generally allowed to concentrate by a factor of 3 to 8. This factor is called the cycles of concentration or cycles. The degree to which salts are allowed to concentrate is controlled by the blowdown or bleed off rate. The volume of fresh make-up water required by a cooling tower system is governed by the loss of water through evaporation, blowdown, and drift. Drift or windage is nonevaporative loss, which is typically 0.05 to 0.1 % of the circulation rate. It is considered negligible in many calculations. For our example tower, the drift typically would be less than 0.1 % of the circulation rate or 25 gpm. Blowdown and drift are related to the cycles of concentration and the evaporation rate as follows: b + d = e/( r - 1 )


2



where b = blowdown rate, gpm d = drift, gpm r = cycles of concentration The water mass balance for the example cooling tower water is: m=e+b+d where m = make-up rate, gpm The make-up rate for our example tower is 380 + 102 + 25 = 507 gpm. If our example tower is run at four cycles, b=102 gpm. Often the cycles of concentration are measured by the ratio of the chloride concentration of the circulating water to that of the make-up water. Chloride is used because it is usually present at a concentration which can be measured easily and accurately, and it does not form insoluble salts. However, chloride concentration will not be an accurate measure of the cycles if chlorination is used, since chloride is a by-product of this treatment. If there are ions in a tower water which are being cycled less than chloride, they are being deposited or otherwise lost from the recirculating water. These basic cooling tower calculations are useful for establishing chemical feed rates. The dosage of most treatment chemicals is based on their concentration in the circulating water. When a system is filled with untreated water, the initial dosage is proportional to the volume and the initial demand of the system. Since most treatment chemicals do not evaporate they are removed from the system in the blowdown and drift; during operation, the feed rate is proportional to the rate of blowdown and drift. By decreasing the rate of blowdown, and therefore increasing the cycles of concentration, the chemical feed rate can be decreased proportionately. Since the corrosivity and scale-forming tendency of water increases as the number of cycles increase, an increase in cycles must be balanced by the ability of the treatment chemicals to perform effectively. As shown in Figure 2, with each incremental increase in cycles there are decreasing incremental savings in water and chemicals. It is generally not necessary to operate towers at more than eight cycles where incremental savings are small.


3



BLOWDOWN (gpm)

200

25,000 gpm RECIRCULATION 11 °C (20 °F)

150

100

50

0 3 4 5 6 7 8 CYCLES OF CONCENTRATION

FIGURE 2. BLOWDOWN EFFECT OF CYCLES OF CONCENTRATION A wide range of corrosion inhibitors, antifoulants, antiscalants, and biocides are used in open recirculating cooling systems. The predominant corrosion inhibitors in the refining and chemical industries are blends of chromates, phosphates, zinc, and copper alloy inhibitors. Organic phosphates, polymers, and copolymers are used as antifoulants and antiscalants. Chlorine is the most common biocide. Other oxidizing and nonoxidizing biocides are also available.


4



Principal Parts of Cooling Towers The principal parts of a cooling tower are the fan(s), fill material, water distribution deck or header, drift eliminator, structural frame, and cold water basin. Cooling towers use recirculating ambient air to cool warm water primarily through evaporation as the water cascades down through fill material and air passes up or across the fill. The fill serves to maintain an even distribution of water across the horizontal plane of the tower and maximizes the surface area of the water to enhance evaporation and sensible heat transfer. The principle parts of an induced, draft, counterflow cooling tower are shown in Figure 3. The parts of an induced draft crossflow cooling tower are shown in Figure 4.

FIGURE 3. INDUCED DRAFT COUNTERFLOW COOLING TOWER

FIGURE 4. INDUCED DRAFT CROSSFLOW COOLING TOWER


5



Heat Exchangers Heat exchangers are critical parts of a cooling system designed to efficiently pass the heat from the process being cooled to the water. Since the heat transfer surface is the hottest area exposed to cooling water it is the most prone to corrosion and fouling. The primary objective of a cooling water treatment program is to protect the heat transfer surfaces from corrosion and fouling. Components of a Shell and Tube Heat Exchanger Shell and tube heat exchangers come in many different shapes and sizes depending upon the service for which they are to be used. The size and, to some extent, the type of heat exchanger are controlled by the use, temperatures in and out, flow rates, and other factors. Cleanability, alloys for one or both sides, design temperatures, pressures, and corrosion must be considered in the selection of a heat exchanger. The principle parts of one of the most common types of shell and tube heat exchangers are shown in Figure 5. Cooling water is most often on the tube side. When cooling water is on the shell side, corrosion and fouling are more likely due to pocketing and deposits at baffle dead corners.

FIGURE 5. SHELL AND TUBE HEAT EXCHANGER


6



Components of a Plate Heat Exchanger Plate heat exchangers are sometimes used in once-through seawater cooling systems especially where space and weight are at a premium, such as on offshore structures. Figure 6 shows the typical components of a plate heat exchanger.

FIGURE 6. TYPICAL PLATE HEAT EXCHANGER

Common Materials of Construction Many factors must be considered in choosing the materials of construction for a heat exchanger including the temperature, composition of the process stream, and the cooling water. Carbon steel may provide sufficient corrosion resistance in treated cooling water. Titanium, inherently more corrosion resistant and expensive, may be required in seawater applications. Carbon steel is the primary material of construction in cooling tower system heat exchangers. Copper and copper alloys such as brasses, Cu-Ni, and stainless steels, are also important due to their greater corrosion resistance than steel. Cast iron, steel, copper, copper alloys, aluminum, and solders are found in closed systems.


7



Various copper-base alloys, such as 90-10 Cu-Ni, 70-30 Cu-Ni, and aluminum brasses and bronzes have been successfully used in seawater. However, these materials are susceptible to premature failure when flow velocities are high, when seawater contains significant concentrations of sand, and when pollutants such as sulfide and ammonia are present. Alternatives include titanium, certain high-alloy austenitic stainless steels, high-alloy ferritic stainless steels, and duplex stainless steels. Once-Through Cooling Systems As the name implies, systems which use water once and then discharge it are called oncethrough systems. Figure 7 is a typical schematic of a once-through cooling system. These systems are used only where a large volume supply of water is available at a low cost, because even small systems require large volumes of water. Saudi Aramco uses large once-through seawater cooling systems. Corrosion, scale, and biological growths are inherent problems in these types of systems. Generally, the only treatments applied are coarse screening and chlorination. Screening is used to remove foreign matter such as seaweed which may damage pumps or foul heat exchange equipment. Chlorination is necessary to prevent biological fouling. Since large volumes of water pass through these systems it is not economical to use any scale or corrosion inhibitors. Corrosion resistant materials and limits on flow and temperature are necessary to prevent corrosion.

FIGURE 7. ONCE THROUGH COOLING SYSTEM


8



Closed Recirculating Cooling Systems A closed recirculating cooling system is one in which the water is recirculated in a closed loop with negligible evaporation or exposure to the atmosphere. Figure 8 depicts a schematic of this type of system. A closed system has essentially a constant volume with little or no added (make-up) water. These systems are frequently employed for critical cooling applications where deposit formation on heat transfer surfaces would be disastrous. In a typical closed system, heat is transferred to the system from the loop by heat exchange equipment and is removed from the closed loop by a second exchanger. The secondary system could use open evaporative cooling, once-through water cooling, or air cooling.

FIGURE 8. CLOSED RECIRCULATING COOLING SYSTEM


9



Closed systems are well suited to cool gas engines and compressors. Diesel engines in stationary or locomotive service normally use closed radiator systems similar to automobile systems. Closed systems are also used in the chilled water systems of air conditioners or for industrial processes in need of reliable temperature control. Water velocities in closed systems are generally 0.9 to 1.5 m/sec (3 to 5 ft/sec) and the temperature rise is typically 6 to 8 °C (10 to 15 °F). Generally, little make-up water is needed except for that necessary to replenish pump seal leaks, expansion tank overflows, and losses through vents. Service water can generally be used because there is no evaporation and concentration of salts. However, the use of condensate, desalinated, demineralized, or softened water is preferred, if available. The possibility for dissolved oxygen attack is relatively low, since oxygen generally enters only in the make-up water. However, untreated systems and systems with excessive exposure to the atmosphere may suffer from oxygen pitting, galvanic action, and crevice attack. High concentrations of nitrite-, chromate-, and silicate-based corrosion inhibitors are commonly used in closed systems.


10



CONTROL OF CORROSION IN COOLING WATER In cooling water, corrosion results from an electrochemical reaction between a metal and an impurity in the water. The corrosion of steel is discussed in this Module, but the same principles apply to other metals used in cooling water systems. In cooling water, dissolved oxygen, copper and ferric ions, acids, and chlorine are the primary impurities, called oxidants or corrodants, which react with steel. A simple corrosion cell is shown in Figure 9. Oxidation, i.e., dissolution of a metal or formation of a metal oxide, occurs at the anode. For steel, the anodic reaction involves the production of ferrous ions (Fe ) and electrons (e ) from iron metal (Fe°). Fe° ——> Fe + 2 e 2+

-

2+

-

FIGURE 9. CORROSION CELL


11



The oxidation reaction must be balanced by a reduction reaction in which the corrodant accepts the electrons at the cathode. The primary cathodic reaction in cooling water is: 1/2 O + H O + 2 e ——> 2 OH where oxygen (O ), water (H O), and electrons combine to form hydroxide ions (OH ). These two reactions can be combined and written as follows: Fe + 1/2 O + H O ——> Fe + 2 OH Further reactions often occur in water. Ferrous and hydroxide ions combine to form ferrous hydroxide. Fe + 2 OH ——> Fe(OH) Ferrous hydroxide can be further oxidized by oxygen to ferric hydroxide, which is common iron rust. 2Fe(OH) + 1/2 O + H O ——> 2Fe(OH) The function of a corrosion inhibitor is to slow the rate of one or more of these reactions. Anodic inhibitors (e.g., chromate, nitrite, molybdate, orthosilicate, and phosphate) slow an anodic reaction, i.e., the rate at which the metal is dissolved. They often form stable gamma-Fe O films on steel. A disadvantage of these inhibitors is that when they are underfed, corrosion is severely localized in the form of pitting. Cathodic inhibitors function by precipitating films of salts at locally high pH generated at the cathodic site. These films are less protective than those generated by anodic inhibitors. Examples of cathodic inhibitors are polyphosphates, polysilicates, and zinc. Inhibitors which affect both cathodic and anodic reactions are termed mixed inhibitors. Phosphonates are mixed inhibitors. 2

2

-

2

-

2

2

2

+2

2

+2

2

-

-

2

-

2

2

3

3


12



Factors Affecting Corrosion in Cooling Water In cooling water, the rate of the corrosion is dependent on several variables which includes the following: • pH: Low pH accelerates corrosion, generally the pH is maintained above 6.0 in cooling water. •

Temperature: High temperatures accelerate corrosion, the upper limit depends on the composition of the water and inhibitor used.

•

Velocity of the water: Figure 10 gives the recommended velocities for water in the tubes of shell and tube exchangers.

•

Concentration of the corrodant: e.g., dissolved oxygen.

•

Concentration of dissolved solids: Figure 11 shows corrosion increases with increased dissolved solids.

•

Pretreatment and pre-filming of the metal surface can significantly decrease corrosion rates.

•

Presence of scale, sludge, biological growths increase corrosion.

•

Dissimilar metals should be avoided.

m/sec

ft/sec

Carbon Steel

1.8 to 3.0

6.0 to 10.0

Admiralty

1.2 to 2.4

4.0 to 8.0

Cupro nickel

1.2 to 3.6

4.0 to 12.0

FIGURE 10. RECOMMENDED COOLING WATER VELOCITIES


13



FIGURE 11. EFFECT OF DISSOLVED SOLIDS ON CORROSION RATE Several forms of corrosion can occur in cooling water including uniform and local attack. Local forms of attack include galvanic, pitting, crevice, and leaching corrosion. Intergranular corrosion, transgranular corrosion, and stress corrosion cracking are also possible. Microbiological corrosion, corrosion fatigue, and erosion-corrosion can also occur. The control of corrosion in cooling water is a complicated task involving mechanical and chemical factors. Corrosion Inhibitors There are several general requirements for an effective corrosion control program. Although the principle function of such a program is to protect the heat exchanger, it must also protect the other surfaces exposed to the cooling water and should rapidly establish corrosion control at low concentration. The treatment program should be effective under a wide range of pH, temperature, heat flux, and water quality conditions. It should also be forgiving of overfeed, the loss of feed, or other system upsets. Methods for easily monitoring the concentration of the major components should be available. The corrosion inhibitor must be compatible with other treatment components, e.g., the biocide and antifoulant.


14



A variety of metallic, nonmetallic, organic, and inorganic chemicals are useful corrosion inhibitors in cooling water systems. Often a cooling treatment program will consist of two or more corrosion inhibitors along with other component(s) for control of scale, fouling, or biological growth. Some additives serve more than one purpose. The primary corrosion inhibitors used in cooling water are discussed in the following sections. Chromate Chromates are the most effective corrosion inhibitors which protect both ferrous and nonferrous alloys. They are anodic inhibitors which form a tenacious oxide film which protects the underlying metal. Chromates are effective with the water temperatures up to 71 °C (160 °F), and over a wide pH range of 6 to 11. Cooling systems are rarely operated above pH 10. When used alone there is a critical chromate concentration necessary to maintain protection which is dependent on the sulfate and chloride ion concentrations of the cooling water. If underfed, attack is localized and manifested in the form of pitting. When used alone (e.g., in a closed system), control can be maintained with 200 to 500 mg/l chromate in the circulating water after an initial pretreatment of up to 1,000 mg/l. Naturally, the use of such high doses is very costly. These high levels are only used in closed systems which are seldom emptied. Because of their toxicity and the expense of disposing of water treated with high doses, chromates are used at about 5 to 25 mg/l CrO together with one or more other inhibitors, such as zinc, phosphates, phosphonates, polymers, and others. Addition of zinc is an excellent means of lowering the chromate usage. Zinc chromate has become one of the most effective cooling water inhibitors. Zinc chromate is not a single salt as the name implies, but a mixture of a zinc salt (usually chloride or sulfate) and sodium dichromate. These ions exist as individual ions in solution without forming a specific compound or intermediate. As little as 5 % of either ion in the presence of the other shows great improvement over the performance of the major ion alone. Generally, a blend of 20 % zinc and 80 % chromate is used. A typical dosage is 2 to 10 mg/l zinc and up to 25 mg/l CrO . The recommended pH range is 6.2 to 7. Above a pH 7.5, zinc precipitates as the hydroxide, Zn(OH) . Below 6.2, the protection of copper alloys decreases. The pH range can be extended upwards with additives which prevent the precipitation of zinc hydroxide. 4,

4

2


15



Protection is established very quickly with zinc chromate when there is free access to the surface. Old corrosion products and deposits impede the development of protection. Zinc chromate protects copper alloys and aluminum. It inhibits the galvanic attack of Al coupled to Cu and lowers the corrosivity of dissolved Cu ions. The dosage of most chromate containing inhibitors is controlled by monitoring the chromate level. High concentrations (10 to 100 mg/l as CrO ) can be detected by titration with thiosulfate. Low concentrations (less than 30 mg/l) can be detected colorimetrically by the reaction of chromate with diphenylcarbazide. To lower the dosage of chromate required in order to maintain the protection of cooling systems and to introduce deposition control, polyphosphates and zinc have been used together with chromate. A typical dosage would be 10 to 25 mg/l CrO , 2 to 5 mg/l polyphosphate, and 2 to 5 mg/l zinc. Phosphonates also enhance the performance of zinc chromate by providing threshold inhibition of calcium carbonate, calcium sulfate precipitation, and adding detergency to decrease deposits and debris. Phosphonates also allow excursions above pH 7.5 since they stabilize zinc hydroxide. In addition, they do not have the drawback of polyphosphates, i.e., possible zinc and calcium phosphate precipitation. 4

4

Zinc The zinc cation (Zn ) is a powerful cathodic inhibitor used in cooling water. It is seldom used alone and is commonly used in combination with chromates, phosphates, phosphonates, molybdate, and other anodic inhibitors. The addition of zinc often allows the decreased use of the anodic inhibitor with increased corrosion protection. Control of pH and/or the use of zinc stabilizers are required with zinc to prevent the precipitation of zinc salts at high pH. +2

Orthophosphates and Polyphosphates Phosphate has been used as a corrosion inhibitor in cooling water for many years. Before the late 1970’s phosphate was used in combination with chromate and/or zinc. Various phosphates in combination with nonmetals have become widely used in cooling water because of increasing restrictions on heavy metal usage. Modern phosphate programs provide excellent corrosion control under certain conditions in cooling water. However, these programs are more expensive than chromates, require greater control of operating parameters, and require the continuous feed of dispersants to prevent the deposition of the calcium phosphate scale in the heat exchangers. Several forms of phosphates are used for corrosion control in cooling water, including orthophosphate, polyphosphates, phosphonates, and other organic phosphates.


16



Orthophosphate (PO4-3) is an inorganic anion which is primarily an anodic inhibitor. Orthophosphate exists in aqueous solution in interchangeable forms depending on the pH. Phosphoric acid (H3PO4) predominates below pH 2, and the tribasic ions (PO4-3) predominates above pH 12. At these extremes steel is not protected. In the pH range of interest in cooling water, pH 6 to 8.5, both the monobasic (H2PO4-) and the dibasic (HPO4-2) forms are present and are effective corrosion inhibitors. Orthophosphates are anodic inhibitors which require a divalent cation, commonly calcium or zinc, to be effective. The calcium concentration must be at least 50 mg/l as CaCO ; therefore the orthophosphates are not useful in softened water, demineralized water, or steam condensate. When zinc is used in conjunction with phosphate, typically 0.5 to 1.0 mg/l soluble zinc is sufficient to maintain corrosion control with approximately 6 to 10 mg/l orthophosphate at pH 7.3 to 7.8. The mechanism of corrosion inhibition of steel with phosphate is not clear. However, it is known that oxygen, calcium, or zinc and phosphate are required. It is thought that dissolved oxygen reacts slowly with steel to form a thin film of gamma-Fe O . During the production of this film, precipitation of iron or calcium phosphate occurs at voids in the film. These precipitates are not completely protective, and allow the gradual formation of a protective iron oxide film. Zinc ions are thought to inhibit corrosion by precipitating zinc hydroxide or phosphate at the cathodic sites due to locally elevated pH. These precipitates also form protective films. Protection by orthophosphate is sensitive to the water quality, pH, oxygen, and the chloride concentrations. A minimum orthophosphate concentration is required depending on these variables. Below this minimum level pitting attack occurs. Polyphosphate is a generic term for a variety of materials formed by dehydrating and polymerizing orthophosphates. Polyphosphates are cathodic inhibitors on steel. Some sodium polyphosphates frequently used in water treatment are shown in Figure 12. 3

2

3

FIGURE 12. SODIUM POLYPHOSPHATE


17



Polyphosphates slowly decompose or revert to orthophosphates in cooling water. This reversion can be accelerated by low pH and high temperatures. It is catalyzed by certain metal ions and enzymes. In cooling water the half-life of polyphosphates typically ranges from several hours to two days. Polyphosphates revert instantaneously at boiler temperatures and are sometimes used as a source of orthophosphate. Polyphosphate can be considered both an anodic and cathodic inhibitor, although it is generally considered the latter. It requires both calcium or zinc and oxygen like orthophosphate. In cooling water 10 to 15 mg/l polyphosphate as PO is normally maintained after an initial pretreatment of at least twice this dosage for a few days. When copper alloys and steel are present, the pH should be maintained higher than about 7.0. Unfortunately, orthophosphate is an excellent nutrient for the growth of bacteria; chlorine and/or other biocides are often required. Polyphosphates will minimize normal galvanic corrosion. They are ineffective when cathodic metals (e.g., copper) are deposited on more anodic metals (e.g., carbon steel). Operation below pH 7.0 aggravates this problem. The use of a copper-specific inhibitor is required to alleviate this problem. Polyphosphates are useful for the prevention of CaCO and CaSO scales formation. They also stabilize dissolved iron and manganese in well water and are approved for use in potable water up to 10 mg/l. In cooling towers polyphosphate is often used with chromate, zinc, and phosphonates. It is low cost, nonhazardous, and nontoxic. It is an effective alternative to chromate, although it has more restraints and requires more control. 4

3

4

Nitrite Nitrite, commonly used as the sodium salt (NaNO ), is an anodic inhibitor which generates protective gamma-Fe O on carbon steel. Nitrite is effective when oxygen is not present. It is frequently used in closed systems not exposed to air. Often, borate is added to buffer the pH at about 9. Copper alloy inhibitors and dispersants may be added to complete the program. Unlike chromates, nitrites are compatible with glycols which are added as an anti-freeze or raise the boiling point of the water in hot systems. Typically, 300 to 500 mg/l NO is required. The precise level is dictated by the chloride and sulfate concentrations. Often, excess nitrite is used since closed systems are not monitored frequently. Nitrite is seldom used in cooling towers since it is decomposed by bacterial action and air oxidation to nitrate (NO -). 2

2

3

2

3


18



Nitrite is easily monitored by titrating it against a standard oxidizing agent, potassium permanganate. Biological activity should also be monitored with nitrite. Silicates Silicates are useful corrosion inhibitors in mildly corrosive systems. They form a weak chemi-sorbed film on carbon steel. The development of protection is slow, and it does not require hardness to be present in the water in order to be effective. Like phosphates, various silicates are available ranging from simple ionic forms, such as salts of silicic acid (H SiO ), to complex colloidal ions with variable compositions of the form nNa O-mSiO . An m/n ratio of 2.5 to 3 is most effective. Silicates are most effective when used at a level of 25 to 40 mg/l SiO at pH 8 to 9.5 in water with low salt concentrations (less than about 500 mg/l TDS); that is, under mildly corrosive conditions. Silicates are not generally recommended for cooling tower systems, but are suitable for some closed systems. Water with a high magnesium content must be avoided because magnesium silicate scale forms when the magnesium concentration exceeds approximately 150 mg/l as CaCO . Silicates can be used for the control of dissolved iron and manganese in potable water systems at a level of 10 mg/l SiO . It is an economical, nontoxic, nonhazardous option for mild corrosion problems. 2

2

2

2

2

3

2

Molybdate Sodium molybdate (Na MoO ) forms passive anodic iron oxide films on steel. It is a weaker oxidant than chromate and requires an oxidant, either oxygen in open systems or nitrite in closed systems, to form a protective film. It is an environmentally acceptable alternative to chromate, although less effective and slower acting. In cooling tower systems high molybdate concentrations (e.g., 1,000 mg/l) are required if it is to be used alone. Cost of such high doses are prohibitive. Typically, a molybdate formulation for a cooling tower system might provide 8 to 15 mg/l Mo, 2 mg/l Zn , 1 to 5 mg/l phosphonate and similar levels of a dispersant and/or copper inhibitor. Unlike other nonchromate inhibitors molybdate does not require hardness in the water; it is useful in systems where the water is naturally soft or where condensate is used for make-up. Molybdate formulations have also been used to protect reactor jackets which are exposed to both cooling water and water heated with steam intermittently. Higher concentrations are necessary in these systems. 2

4

+2


19



In closed systems, molybdate-nitrite formulations have been used at lower nitrite concentrations than classical nitrite-borate treatment levels of 300 to 500 mg/l NO . Molybdate can be monitored by a colorimetric method using mercaptoacetic acid. 2

Phosphonates Phosphonates are a class of organic phosphorous compounds containing a carbon atom directly bonded to a -PO group, which gives them greater hydrolytic stability than polyphosphates. Three phosphonates commonly used in water treatment are shown in Figure 13. The complete chemical name and common abbreviation follow: nitrilotri(methylene-phosphonic acid) or AMP, hydroxy-ethylidene-1, 1-di(phosphonic acid) or HEDP, and 2-phosphono-butane-1,2,4-tricarboxylic acid or PBTC. They are only marginally effective corrosion inhibitors when used alone under mild conditions. However, they are very useful in conjunction with chromate, zinc, and polyphosphates in open and closed systems. 3


20



O OH II I P C I I OH CH3

HO

O II P I OH

OH

HE DP

N

CH 2

CH 2

HO-P-OH

HO-P-OH

HO-P-OH

O

O

O

CH 2

AMP

O O HO

P OH

CH 2 C C

C

OH OH

CH2 O CH 2 C

OH

O P BTC O R P BS AM

FIGURE 13. PHOSPHONATES USED IN WATER TREATMENT


21



AMP, HEDP, and PBTC have the added advantage in that they control calcium carbonate and sulfate deposition. They also stabilize iron and manganese which otherwise would cause fouling. Many proprietary iron dispersants will contain HEDP and a polymeric component. Phosphonates also extend the pH range over which zinc is soluble, which makes zinc containing formulations more useful. One disadvantage of phosphonates is that, due to their strong interaction with copper ions, they accelerate the corrosion of copper alloys when used at high concentrations. Often they require the use of a copperspecific inhibitor in mixed systems. AMP is degraded by high doses of chlorine. HEDP is sufficiently stable under most chlorinating conditions. PBTC is the most stable. The phosphonate is oxidized to orthophosphate for monitoring, which is detected using the conventional ortho procedure. If present, poly and orthophosphates interfere and must be determined separately and subtracted from the total orthophosphate determined in the phosphonate test. Copper Alloy Inhibitors Three organic compounds are used as copper-alloy inhibitors in cooling water. They are TTA, BZT, and 2-MBT, as shown in Figure 14. These materials form strong complexes with copper ions in solution and films on the surfaces of copper alloys. They offer little protection to ferrous metals and are affected adversely by chlorination. 2-MBT is the most readily oxidized and the inhibition is rapidly lost. The protection by TTA and BZT lapses temporarily after chlorination and then returns after the chlorine dissipates. It is thought that a reversible chlorine adduct is formed with the triazoles, which reverts to the triazole when the chlorine dissipates. Copper inhibitors are generally used at about 2 mg/l. They are all sparingly soluble in water, except at high pH where the soluble sodium form exists; therefore, they are supplied as liquids at high pH.


22



N N N N BENZOTRIAZOLE, BZT

Na + N N H3 C

N

TOLYLTRIAZOLE, TTA

S SH N MERCAPTOBENZOTHIAZOLE, 2-MBT

FIGURE 14. COPPER ALLOY INHIBITORS


23



Nonchromate Cooling Tower Treatment Packages There are three generic, nonchromate chemical cooling water treatments used as typical alternatives to chromate programs. These treatments include combinations of the inhibitors and dispersants for control of corrosion, scale formation, and fouling in cooling towers. Copper corrosion inhibitors and a biocide are frequently part of the total treatment program. In well-designed, well-operated systems with close control of water chemistry and inhibitor injection these treatments effectively control corrosion, scaling, and fouling. In all these treatments pH/alkalinity control is critical. At pH values below the recommended operating range corrosion will occur. Above the range, scaling will be a problem. Oil ingress is the most common operating upset which can foul the system and interfere with biological and corrosion control in refineries. Corrosion is mitigated by pH control in combination with continuous injection of corrosion inhibitor. Scaling is controlled by pH adjustment and continuous injection of chemicals to either inhibit scale formation or disperse scale deposits after formation. Fouling is controlled by intermittent or continuous use of polymeric dispersants. The microbiological control program is often based on chlorination. These programs require close control of the inhibitor injection rate and the cooling water chemistry limits. Typical guidelines for each type of treatment are given in Figure 15. General guidelines which apply to water quality in most cooling tower systems are given in Figure 16.


24



FIGURE 15. TYPICAL NON-CHROMATE COOLING WATER PROGRAMS

FIGURE 16. GENERAL COOLING CHEMISTRY GUIDELINES FOR NONCHROMATE INHIBITORS


25



Monitoring Corrosion Corrosion should be monitored in major cooling water systems with either coupons or probes. A coupon rack placed on the hot water return is shown in Figure 17. Guidelines for assessing corrosion rates in cooling water are given in Figure 18. The corrosion rates given are for uniform corrosion. Low rates of pitting are acceptable on carbon steel, but are not acceptable on copper-alloys or stainless steels.

FIGURE 17. COOLING WATER CORROSION TEST LOOP (OPENENDED DISCHARGE INSTALLATION)


26



Metal Carbon Steel

Admiralty Brass

Corrosion Rate mils/yr 0-2

Excellent corrosion resistance

2-3

Generally acceptable for all systems

3-5

Fair corrosion resistance; acceptable with iron fouling-control program

>5

Unacceptable corrosion resistance: Migratory corrosion products may cause severe iron fouling

0 - 0.2

Generally safe for heat- exchanger tubing and mild-steel equipment

0.2 - 0.5

Stainless Steel

Comment

High corrosion rate may enhance corrosion of mild steel

> 0.5

Unacceptable high rate for long term; significantly affects mild-steel corrosion

0-1

Acceptable

>1

Unacceptable corrosion resistance

FIGURE 18. GUIDELINES FOR ASSESSING UNIFORM CORROSION IN COOLING WATER


27



Avoiding Galvanic Corrosion Because seawater and most cooling waters are good electrolytes, severe corrosion can occur when different metals are coupled together. The degree of attack depends on the relative position of the two metals in the galvanic series and on the relative size of the components. The alloys at the top of the series are more prone to corrosion than those below them. Large differences in potential drive the corrosion. The worst case occurs when the anode component (metal which corrodes) is small, and the cathode component (protected metal) is large. An abbreviated list of the galvanic series in seawater is shown below: • Active Metals (Anodic):AluminumCarbon steelNaval rolled brassCopperAdmiralty brassCopper-Nickel alloysTitaniumHastelloy CMonel 400Type 300 series Stainless Steels (passive) • Noble Metals (Cathodic) One way to avoid galvanic attack is to electrically insulate the metals from each other. The following list gives examples of metal couples that should be avoided in seawater: • Magnesium: Low alloy steel causes attack of magnesium and danger of hydrogen damage on the steel. •

Aluminum: Copper causes pitting of Al and copper ions also attack the Al.

•

Bronze: Stainless steel causes pitting of bronze.

The following couples are borderline and have occasionally presented problems: • Copper - Solder •

Graphite - Titanium or Hastelloy C

•

Monel 400 - Type 316 SS

The following couples are generally compatible: • Titanium - Inconel 625 •

Lead - Cupronickel


28



Precleaning and Pretreatment Corrosion control in cooling water is based on forming a film on the metal surface which protects the underlying metal. The rate at which the film or barrier forms will largely determine the effectiveness of the treatment. Materials that do not form films rapidly will permit corrosion to take place. The result will be incomplete film formation and continued corrosion. The rate at which the film forms is related to the inhibitor concentration, the temperature, and cleanliness of the metal. The function of pretreatment is to permit rapid formation of a uniform, impervious film. After a film is formed, the lower, normal treatment levels will keep the film intact and avoid the accumulation of corrosion products. For new systems or heavily fouled systems, precleaning is usually necessary prior to pretreatment. Precleaning may require high concentrations of inhibited acids, chlorine, and/or detergents specifically designed to remove the deposits present. Laboratory testing is recommended to choose the proper precleaning procedure for a particular system. After cleaning, a system is flushed and filled with water containing a pretreatment product. This may simply be the inhibitor to be used at a high dosage to be maintained up to one week. More effective pretreatment is possible with high doses of surfactant and a polyphosphate. The treatment is time, concentration, and temperature dependent. Figure 19 shows three typical pretreatment procedures for carbon steel.

FIGURE 19. DOSAGE Mg/L PO /TIME/TEMPERATURE 4


29



Laboratory corrosion rate measurements at 49 °C (120 °F) with untreated specimens in raw water (A) and with untreated (B) and pretreated (C) specimens in treated water are shown in Figure 20. These experiments clearly demonstrate the benefits of pretreatment.

FIGURE 20. EFFECTIVENESS OF PRETREATMENT IN DECREASING INITIAL CORROSION RATES


30



PREVENTION OF SCALE FORMATION IN COOLING WATER Several forms of deposits can occur in cooling systems including scale formation, biological fouling, and general fouling. Deposition of such materials may result in plugging, loss of cooling, and underdeposit corrosion. Scale formation is characterized by the formation of insoluble inorganic compounds. This occurs by the precipitation of slightly soluble ions such as calcium, magnesium, or zinc together with carbonate, sulfate, phosphate, hydroxide, or silicate. Scale formation is different from fouling. Fouling can be either general or microbiological in nature. General fouling is caused by the settling of any suspended matter such as iron oxides, silt, mud, oil, and other debris. Microbiological fouling results from the growth of algae, bacteria, or fungi. Effect of Scale on Heat Transfer In a heat exchanger, thermal energy is usually transferred by conduction from a process fluid across a metal barrier to the cooling water. In conduction, heat is transferred through or between stationary media such as metals, water, or air. It results from short range interactions of molecules and/or electrons. In the metals, electrons contribute to this process. In gases and liquids, energy is also conducted by molecular collisions. The heat transferred (Q) across a flat plate by conduction is described by the following equation: Where Q = heat transferred, Btu/hr K = thermal conductivity, Btu/hr °F A = cross sectional area, ft2 t2-t1 = temperature difference across the plate, °F L = thickness of the plate, ft. From this equation, it follows that thin plates made from materials with high thermal conductivities, e.g., metals, are the best conductors of heat. Scales and fouling deposits have lower thermal conductivities than metals and effectively increase the thickness and lower the thermal conductivity of the barrier.


31



The basic equation for a steadily operated exchanger follows, in which U is the overall heat transfer coefficient and the dT and dt values are the terminal temperature differences for the process and watersides. In this simple case we are assuming constant U, constant mass flowrates, no changes in phase, constant specific heats, and negligible heat loss. Q = U A dT where Q = Heat transfer rate, Btu/hr U = Heat transfer coefficient Btu/hr ft2 °F A = Heat transfer surface area, ft2 dTm = Log mean temperature difference, °F where dT = T2 - t1, dt = T1 - t2 t1 = Inlet water temperature, °F t2 = Outlet water temperature, °F T1 = Inlet process temperature, °F T2 = Outlet process temperature, °F The rate of heat transfer from the process to the cooling water is proportional to the mass flow rate of the material, its heat capacity, and the temperature change the material undergoes. Since we are neglecting heat losses: Q = M Cp (t -t ) = M Cp (T -T ) where: Mw, Mp = mass flow rates for water and process, lb/hr Cpw, Cpp = heat capacity of the water and process, Btu/hr °F m

w


w

2

1

p

p

1

2

32



Heat capacity is an intrinsic property of a material. Tables of heat capacity data can be found in chemical handbooks. The heat capacity of pure water is 1.00 Btu/lb °F, by definition. When an exchanger is designed, a design heat transfer coefficient, U , can be calculated. After being put in service, an actual heat transfer coefficient (U ) can be calculated using these equations. The fouling factor (R ) is the difference between the reciprocals of the actual and designed coefficients. The fouling factor is the resistance to heat flow caused by fouling. Plotting the fouling factor over time is a useful way to monitor the amount of fouling occurring in an exchanger. If the factor increases, the system is becoming fouled. A general rule of thumb for fouling factors is that when they are on the order of 0.001 to 0.002 hr ft °F/Btu, the system is clean. If the factor is greater than 0.005, the system is fouled. D

A

f

2

Scales Formed in Cooling Water and Their Prevention As water evaporates in an open-evaporative system, the inorganic salts naturally present in the water and those added for corrosion control increase in concentration. Consequently, the tendency for many of these ions to precipitate from solution increases, resulting in scale formation. The rate of scale formation depends on temperature, alkalinity or acidity of the water, the velocity of the water, and other factors as well as the concentration of the scale-forming ions. Calcium carbonate, calcium sulfate, calcium phosphate, and magnesium silicate are the scales most likely to form in open-evaporative systems. Calcium Carbonate Scale Calcium carbonate is the most common scale found in cooling water systems. It forms when the calcium hardness and bicarbonate alkalinity, naturally present in water, are concentrated and/or are subjected to increased pH and temperature. Ca + CO -2 ——> CaCO (solid) In 1936, Langelier published a formula for calculating the tendency of water to either deposit or to dissolve the calcite form of calcium carbonate. The formula expresses the effect of pH, calcium, total alkalinity, dissolved solids, and temperature on the solubility of calcium carbonate for water from pH 6.5 to 9.5. The equation is: pH = (pK - pK ) + pCa + pAlk +2

s


3

2

3

s

33



where pH is the pH value at which water with a given calcium content and alkalinity is in equilibrium with calcite. The terms pK and pK are the negative logarithms of the second dissociation constant for carbonic acid and the calcite solubility product constant. The last two terms are the negative logarithms of the molar and equivalent concentrations of calcium and titratable alkalinity. The Langelier Saturation Index (LSI) is a qualitative index of the tendency of calcium carbonate to deposit or dissolve, expressed as the following equation: LSI = pH - pH A simple formula for calculating LSI is given in Figure 21. A positive LSI indicates a tendency to deposit calcite. A negative LSI indicates an undersaturation condition exists; therefore, solid calcite will dissolve. If LSI = 0, the water is in equilibrium with respect to calcium carbonate saturation. s

2

s

s


34



FIGURE 21. LANGELIER-RYZNAR INDEX CALCULATIONS


35



The LSI is a measure of the directional tendency or driving force of a water towards calcite formation. It is not a reliable indicator of the corrosivity of a water. It is possible that two waters, one with low hardness which is corrosive, and the other with high hardness which is not corrosive, can have the same LSI. In an attempt to develop a quantitative indicator of the corrosive nature of water, Ryznar proposed the empirical Ryznar Stability Index (RSI) defined as follows: RSI = 2 (pH ) - pH RSI < 6 Scaling tendency increases as the index RSI > 7 Calcite formation may not lead to a protective corrosion inhibitor film RSI > 8 Mild steel corrosion becomes an increasing problem s

Calcium Sulfate Scale Calcium sulfate is more soluble than calcium carbonate. Like calcium carbonate, calcium sulfate is less soluble in low pH waters. In cooling water, calcium carbonate will often deposit before calcium sulfate. The following rule of thumb can be used to estimate the safe upper limit of calcium and sulfate concentrations in many cooling waters in the absence of treatment chemicals. (Ca ) (SO -2) < 500,000 The product of the ionic concentrations (mg/l) must not exceed 500,000 or precipitation will occur. When high levels of dissolved solids are present or treatments are used, this limit can be exceeded. +2

4

Calcium Phosphate Scale Deposition of calcium phosphate is a potential problem in cooling water treated with phosphate-based corrosion inhibitors. It forms a dense, difficult-to-remove scale. In the absence of a specific polymer to control precipitation, if the calcium hardness is 500 mg/l, as little as 5 mg/l orthophosphate will cause deposition if the pH exceeds about 7.5. For this reason, phosphate-based inhibitors were limited to pH 6.5 to 7.2 until effective calcium phosphate deposit control polymers were developed. The most common of these agents in use today are poly (acrylic-acid-co-hydroxypropyl acrylate) (AA/HPA), poly (maleicanhydride-co-sulfonated styrene) (MA/SS), and poly (acrylic acid-co-AMPS) (AA/SA). These copolymers allow the use of phosphate-based inhibitors at pH values up to 9.0.


36



Magnesium Silicate Scale Magnesium silicate is another dense, difficult-to-remove material. There are no additives which readily inhibit its deposition. To prevent it from forming, the silica content of the circulating water should be limited to less than 150 mg/l as SiO at neutral pH values. At pH 8.5, 200 mg/l SiO is soluble. The solubility is proportional to temperature; therefore, this scale forms in the colder regions of the system. 2

2

Effect of Water Chemistry, Temperature, and pH There are several methods for preventing calcium carbonate scale. Removing or decreasing the concentration of the calcium or magnesium by softening, ion exchange, or other means is seldom economical except for closed systems. Decreasing the pH by acid addition is commonly used to prevent calcium carbonate and phosphate scale formation. The use of polyphosphate, phosphonates, polyacrylates, and other copolymers will decrease calcium carbonate, sulfate, and phosphate formation. A list of treatments that are commonly used for the control of scaling and fouling is given in Figure 22. Often, two of these materials are used in a blend that is more effective than the individual materials. For example, it is common to use a phosphonate (e.g., HEDP) together with a polymer or copolymer as a general scale control agent.


37



Suspended Zinc Matter Hydroxide

Calcium Carbonate

Calcium Sulfate

Calcium Phosphate

Iron Oxide

pH Control

H

E

H

-

H

-

-

Phosphonates

H

E

E

H

-

-

Polyacrylates

E

H

E

E

H

-

Polyphosphates

H

E

H

-

-

-

Polyamaleic acid

H

E

-

E

E

-

Polyamaleic acid Copolymers

H

E

H

E

H

-

Phosphinocarboxylic acids

H

H

E

H

E

-

Poly(Maleic Anhydride-co-Sulfonated Styrene)

E

E

H

H

E

H

-

Poly(Acrylic acid-co-Hydroxypropylacrylate)

E

E

E

H

E

H

-

Poly(Acrylic acid-co-AMPS)

E

E

H

H

H

H

-

Phosphonobutanetricarboxylic acid

H

H

-

E

H

-

-

Surfactants

-

-

-

-

-

-

H

Treatment

E

Oil

Notes: H = Highly Effective E = Effective - = Low Effectiveness or Ineffective AMPS is 2-acrylamido-2-methylpropylsulfonic acid

FIGURE 22. WATER TREATMENT CHEMICALS FOR SCALE AND FOULING CONTROL


38



Figure 23 summarizes the effects of water composition, temperature, pH, and the effectiveness of dispersants in the control of scale formation in cooling water. Increase in Concentration of Scaling Ions Causes

Temperature Increases Causes

pH Increases Causes

Dispersants Effective?

Calcium Carbonate

+

+

+

Yes

Calcium Sulfate

+

+

N

Yes

Calcium Phosphonate

+

+

+

Yes

Magnesium Silicate

+

-

-

No

+ : Increase in scale formation - : decrease in scale formation N : no effect

FIGURE 23. EFFECTS OF PROCESS VARIABLES ON SCALE FORMATION


39



Prevention of the Harmful Effects of Microbiological Growth in Cooling Water Cooling water systems, particularly open-recirculating systems, offer a favorable environment for growth of microbiological organisms leading to deposition, fouling, and corrosion. Microbiological masses, generally referred to as slime deposits, result from the accumulation of algae, fungi, or bacteria and their excretions. These masses can trap debris and sediment, cause plugging of lines, reduce heat transfer, and cause corrosion by creating differential oxygen concentration cells or generating corrosive by-products or environments. Biofouling refers specifically to fouling caused by plants or animals when such organisms attach themselves to materials submerged in the water. Microbiologically influenced corrosion (MIC) has been reported on iron, carbon, steels, stainless steels, copper alloys, aluminum, and aluminum alloys. Although it is probably widespread in the petroleum and chemical industries, less technology is available for combating MIC compared to traditional forms of corrosion. Control of biofouling can be maintained by mechanical and chemical means. Microorganisms Responsible for Biofouling Algae, fungi, and bacteria may exist in a cooling system and result in fouling when uncontrolled. Algae are photosynthetic organisms, relatively large, sometimes motile (able to move about) and usually colored. As a group, they can tolerate from very little to high intensity light, a pH range of 5.5 to 9.0, temperatures from -18 to 40 °C (0 to 104 °F), and a wide range of salinities. They require only light, water, air, and a few inorganic nutrients for growth. Algae are widely recognized for the severe fouling problems created by their stringy green slime masses which can reduce heat transfer and even plug tubes. They also provide the food necessary for the growth of higher organisms such as bacteria and fungi. Fungi include yeasts and molds. Yeasts are not important in the corrosion of metals, but are important in the deterioration of wood. Molds are a diverse group which require oxygen and organic materials for growth. They can contribute to white rot or brown rot of cooling tower wood.


40



Bacteria found in a system can be varied and include spore-forming, nitrifying, nitrogenfixing, denitrifying, sulfate-reducing, iron, sulfur bacteria, and others. They are universally found in nature and usually do not require light for growth. Bacteria are motile, allowing them to seek favorable environments. They generally thrive from about 21 to 46 °C (70 to 115 °F), although some have been found from -10 to 100 °C (14 to 210 °F). Thermophilic bacteria grow in higher temperature waters and are found on heat exchanger surfaces. The preferential pH range is 5.5 to 8.5, but certain types have been reported from about pH 0 to 10.5. General guidelines for allowable levels of total bacteria count, corrosive or iron depositing bacteria, fungi, and algae in a cooling system are given in Figure 24. Obviously, a complete microbiological analysis of a system is required to determine the counts of these different organisms present. Such an analysis is recommended if a severe biofouling problem is suspected. Figure 25 gives general guidelines for interpreting a general cooling water biocount. These analyses indicate the concentration of organisms in the water, i.e., the planktonic organisms. This is generally useful, but not always. Of greater interest are the organisms which are attached (sessile) to heat transfer surfaces. A biofouling monitor, or simply the presence of slime on coupons, are more direct methods of identifying problems. Constituent Total bacteria count Bacteria (corrosive) or iron depositing

Unit

Limit

Colonies/ml

500,000

Colonies

0

Fungi

100

Algae

Few

FIGURE 24. MICROBIOLOGICAL GUIDELINES FOR NON-CHROMATE INHIBITORS


41



Count Range (colonies/ml)

Significance

0 - 10,000

Essentially sterile

10,000 - 500,000

System under control

500,000 - 1,000,000

System may be under control but should be monitored.

1,000,000 - 10,000,000

System out of control requires biocide.

Over 10,000,000

Serious fouling may be occuring immediate biocide addition required.

FIGURE 25. SIGNIFICANCE OF A BIOLOGICAL COUNT IN COOLING WATER Chemicals for Control of Biofouling Many variables influence the biological development and growth process. Treatment of each system should be considered individually, and different programs may be required for different seasons of the year. Selection of a biocide and dosage should ideally be based on a comprehensive biological survey. Often, this is not feasible, and a multicomponent approach is used. In the past, chromates were effective in controlling biological deposits and some, sulfate reducers in particular, could not exist where even a trace of chromate was used. However, with the change to nonchromate (particularly phosphate-based treatments), the need for biological control is crucial for success of the overall program. Toxicants for biological control are normally classified as either oxidizing or nonoxidizing.


42



Oxidizing Biocides These are compounds which, in addition to their disinfection action, chemically oxidize constituents of the water. Chlorine gas is the most commonly used oxidizing biocide. Chlorine is widely used because of its effectiveness under many conditions and its low cost. A free residual of 0.2 to 1.0 mg/l usually destroys most organisms in only minutes. Using less than 1 mg/l chlorine on an intermittent basis should not damage cooling tower wood. However, higher doses or continuous doses at more than 1 ppm may be detrimental. Oil, reducing agents (e.g., H S), and organic debris are oxidized by chlorine, creating high chlorine demand and decreasing the attack on microorganisms. The effectiveness of chlorine is pH dependent. In water, chlorine gas forms hypochlorous acid and hydrochloric acid. The latter is not an effective biocide. The former is effective, but dissociates to the less effective hypochlorite ion as the pH increases above about pH 7.5. Bromine and bromine-donating compounds are more effective in the high pH range than chlorine. Bromine can be generated in-situ by the reaction of chlorine and bromide. Bromochlorohydantoins are another source of bromine. The latter are solids and eliminate the potential hazard of chlorine gas. Chlorine dioxide and hypochlorite compounds are other oxidizing biocides. Ozone is another, which is not frequently used in cooling water. 2

Nonoxidizing Biocides A wide variety of generic and proprietary nonoxidizing biocides are available. Some of the more commonly used compounds are listed in Figure 26. They differ in effectiveness, dosage, and contact time required, compatibility with other treatment chemicals, and other factors. The proper biocide should be selected after obtaining a sample of the mass to be treated, homogenizing it, and testing the candidate biocides. Dosing should be systematic and results monitored by organism counts, biofouling monitors, or heat transfer coefficients.


43



Active Compound

Type/Effectiveness

Compatibilities

Glutaraldehyde

Broad spectrum non-oxidizing. Good against SRBs Control test available

Resistant to chlorine. Very good at high pH. Ammonia and primary amines incompatible.

Isothiazolins

Broad spectrum Non-oxidizing Good against SRBs Low dosage required Probably most likely to be effective.

Incompatible with more than 1 mg/l chlorine.

Methylene bis(thiocyanate)

Broad spectrum Non-oxidizing Good against SRBs

Incompatible with chlorine and pH > 8.0.

Quaternary amines (Quats)

Broad spectrum Non-oxidizing

Oil, debris, amonia dispersants and high hardness are incompatible. Not effective in heavily fouled systems. Foaming.

Tri-n-butyl tin oxide (TBTO)-Quaternary amines

Effective against algae, molds, wood rot. Non-oxidizing.

Same as Quats. Ecologically questionable due to heavy metal. Foaming.

Calcium hypochlorite (HTH)

Broad spectrum Oxidizing

Oil and reducing agents create demand, less effective at pH > 7.8.

Bromochlorohydantoins

Broad spectrum Oxidizing Control with chlorine tests.

Alternative to chlorine at high pH, in small systems. Ineffective in heavily fouled systems.

Carbamates

Broad spectrum Non-oxidizing Slow acting, high dosages, best at high pH.

Incompatible with chlorine and chromate.

Dibromonitrilo-propianamide (DBNPA)

Oxidizing. Not effective against algae. High dosages often necessary.

Not recommended in fouled systems and at pH > 7.5. Thermal instability.

Copper Salts

Algicide for cooling ponds.

Corrosion of steel possible, environmentally restricted.

FIGURE 26. PROPERTIES OF COMMON BIOCIDES


44



Surfactants It is often useful to precede the shot dose of biocide with an effective biodispersant, which enhances the ability of the biocide to penetrate and kill the organisms. Surfactants or surface active agents are effective biodispersants. Mechanical Means for Control of Biofouling Maintaining proper flow velocity, backflushing, screening, filtration, and air-bumping are mechanical means of removing macroorganisms and microorganisms. In certain instances, thermal backwash has been used to kill macroorganisms in their juvenile stage. Just as surfactants aid the ability of the biocide to penetrate the biomass, mechanical means of disrupting the slimes also increase the effectiveness of the toxic chemicals. Biofouling Monitors The vast majority of bacteria in a cooling water system are sessile, i.e., attached to surfaces. It is believed that in a typical system there may be 1,000 to 10,000 sessile organisms for every planktonic (free floating) organism. This observation has led to the development of biofouling monitors. The basic design of the most popular monitor is shown in Figure 27. The principle of operation is quite simple. As a biofilm develops in a piece of tubing, an increased pressure drop develops across the tubing if the flow rate is held constant. The pressure drop is monitored and any increase is an indication of fouling. Treatment which removes the biofilm will restore the pressure drop to the initial, clean level.

FIGURE 27. BIOFOULING MONITOR


45



In addition to the biofouling monitor, fouling monitors developed by organizations such as NACE and the water-treatment vendors are available. They involve the measurement of heat flux through a heat-transfer surface, under water-velocity, and temperature conditions matched to the system’s heat exchangers. The decay of the overall heat transfer coefficient (U) can be monitored with time and generally corrosion rates can also be recorded. Since it is difficult to match conditions in the exchanger accurately, results from fouling monitors may be ambiguous. They will indicate extremely clean or fouled conditions, but require analysis by an experienced observer in many intermediate cases. Prevention of Macrofouling by Jellyfish, Mussels, Etc. Plant and animal life may contribute to both fouling and corrosion in seawater cooling systems. The two basic types of animals are the “soft” plant-like slimes: bacteria, algae and hydroids, and the “hard” (shell-like) organisms: barnacles, mussels, oysters, tube worms, and sea squirts. The tendency of such organisms to adhere to materials submerged in seawater depends on the nature of the material itself as to the resultant manifestations of corrosion, scaling, plugging, etc. Metals and alloys that produce toxic salts (e.g., copper, lead, and zinc) resist hard-fouling by barnacles and the like, although they may accumulate soft-fouling organisms under the same conditions. Chlorination further aids in the control of biofouling. Even with cupronickel exchangers, and certainly with other materials more susceptible to fouling, chlorination is recommended for the control of biofouling. This is due to the ingress of microbiological organisms, embryos, and spawn of the shellfish and crustacea. In a once-through system there is little danger of developing chlorine-resistant organisms; continuous chlorination is not only permissible but preferred. A chlorine residual of about 0.5 mg/l free available chlorine is usually adequate. The dosage is system-dependent and continuous residuals as high as 1.0 mg/l may be required. Chlorine may be added as a gas or generated in situ by electrochemical oxidation of the chloride present in seawater with in-line electrolysis cells. Hypochlorite salts (e.g., sodium or calcium hypochlorite) are effective, but are not normally economically competitive with chlorine gas.


46



Control of General Fouling in Cooling Water General fouling is the accumulation of suspended solids, corrosion products, process and make-up contaminants, oil, dust, mud, silt, debris, and other foreign materials on heat transfer surfaces. Oil and Dust in Cooling Water All surface waters contain suspended solids which are potential foulants. Dissolved iron and manganese in deep well water form fouling oxides when the water is exposed to air. Process leaks, carryover from clarifiers, and air-borne dirt are also potential foulants. Oil in cooling water interferes with heat transfer, binds suspended matter, reduces zinc and chromate to ineffective forms, promotes biological growth, increases chlorine demand, and can turn a marginally operated system into a troublesome one. When oil is visible or exceeds 10 mg/l, several steps should be taken immediately. The leaking exchanger should be located, shut off or isolated, or its outlet water diverted to a sewer. The cooling tower and distribution deck should be cleaned. Oil should be skimmed where possible. The corrosion inhibiter dosage (no chromate-zinc) should be monitored, and doses of surfactant should be administered. Chlorination should be continuous and cooling water flow increased if possible. The system then should be blowndown heavily until less than 10 mg/l oil is obtained. The cycles of concentration should then be increased to normal, and surfactant doses continued for four days. Oil, biofouling, and the corrosion inhibitor should be monitored closely. Before returning to normal operation an increased dosage of inhibitor should be applied for four days in order to repassivate the system.


47



Means of Control One of the most important factors in achieving effective corrosion control is the maintenance of clean surfaces. Low water velocities and the use of cooling tower water on the shell side of the shell and tube heat exchangers can lead to fouling. Recommended water velocities are presented in Figure 28. m/sec

ft/sec

Carbon Steel

1.8 to 3.0

6.0 to 10.0

Admiralty

1.2 to 2.4

4.0 to 8.0

Cupro nickel

1.2 to 3.6

4.0 to 12.0

FIGURE 28. RECOMMENDED COOLING WATER VELOCITIES Sidestream Filtration Continuous sidestream filtration of one to five percent of the recirculation rate has been used to reduce fouling. Proper selection of a filter to remove the particles in a cooling tower is required. When used in conjunction with good chemical treatment, sidestream filters have been effective. However, they are not a replacement for the proper control of corrosion, scale, and biofouling. Dispersants and Surfactants Dispersants and surfactants are useful for minimizing general fouling. The chemicals suspend the foulants, preventing their deposition and allow removal through blowdown. Cleaning General Deposits On-line or off-line cleaning are required in all cooling tower systems. Acids, chlorine, abrasive salts and materials, sponge balls and other substances have been used to clean systems on-line. On-line cleaning should only be attempted with experienced personnel with consideration for the impact of the cleaning on potential plugging, corrosion and safety. Hydroblasting is generally sufficient to clean systems off-line. If hydroblasting is insufficient, several options for mechanical and chemical cleaning are available. Testing is the best means for selecting the optimum cleaning procedure.


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Monitoring and Control Required to Operate Cooling Water Systems Chemical Feed Equipment Chemicals are available in wet and dry forms. The choice is usually based on economics and location. The majority of chemicals fed to cooling water systems are in liquid form. The feed system should be furnished as a prefabricated, skid-mounted package with pump, tank, tank stand, shut-off valves, suction piping, agitator, and accessories in place. Where dilution is required, two day tanks are recommended. Instrumentation commonly includes level indicators on tanks, pump discharge pressure indicator, flow sensors, and a draw down tube to measure feedrate. The control cabinet should contain pump stroke control, on-off pushbuttons and indicators, tank level indicators, and alarms. Tanks must be sized based on delivery schedules and shelf-life of products. Duplicate positive displacement pumps are commonly provided. Piping and tanks should be designed for easy cleanout. In large cooling towers, the feed pump is often automatically controlled based on the flow of make-up water. pH and Blowdown Controllers Automatic pH and blowdown controllers are recommended for all large cooling towers, especially when nonchromates are required. Accurate control of pH is required for the reliable control of scale and corrosion. The pH controller activates the feed of acid (usually sulfuric), or base (usually soda ash), depending on the alkalinity in the make-up water. pH probes should be equipped with alarms to indicate any low pH excursion. The automatic control of blowdown can usually be justified based on the resulting savings in water and chemicals. Blowdown controllers operate based on the conductivity of the recirculating water. The blowdown valve is opened when the conductivity exceeds a preset limit and closes when the conductivity is low. ORP (oxidation reduction potential) probes are relatively new devices which can be used to sense residual chlorine in water and can be used to control the feed of chlorine.


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Frequency of Chemical Analysis The recirculating water in a process cooling tower should be monitored routinely for corrosion inhibitor, chlorine, concentration of salts (usually conductivity), and process leaks, and monitored often for the ion which limits the cycles of concentration (e.g., calcium or silicate). Complete water analyses should be conducted twice per year to detect any long-term problems or trends. Large process towers which are not equipped with online monitors may require testing as frequently as every four hours. In well automated towers it may be sufficient to test once per day. Corrosion coupons should be evaluated quarterly, and microbiological counts should be run quarterly or more frequently. In all cases, more frequent monitoring is recommended if problems are detected. Generally, closed cooling systems can be monitored quarterly. Once-through systems should be monitored for chlorine at least daily.


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GLOSSARY acid

A chemical that lowers the pH by reacting with the alkalinity

algae

Simple aquatic plant life requiring sunlight for growth.

alkalinity

The total of carbonate, bicarbonate, and hydroxide ion impurities expressed in mg/l as CaCO . 3

anodic inhibitor

A chemical added to cooling water which reduces corrosion by forming a protective film at the anodic surface of a metal.

bacteria

One cell microbiological life which grow everywhere in cooling systems; excessive growth forms slime deposits.

biocide

A chemical which is toxic to biological life.

biological fouling

In cooling water, accumulations of algae and bacteria on components.

blowdown

Water removed from a cooling system used to control the concentration of impurities in the recirculated water.

cathodic inhibitor

A chemical added to water which reduces corrosion by forming a protective film at the cathodic surface of the metal.

chlorination

The addition of a chemical containing free chlorine which is used to prevent biological fouling.

chromate

Chemical containing hexavalent chromium. It was used as an anodic inhibitor until it was found to cause cancer.

cycles of concentration

The number calculated by dividing the concentration of salts in the recirculated cooling water by the concentration of salts in the make-up water.

deposit

External material adhering to the original components in a cooling system.

drift

Loss of recirculated cooling water (with its impurities) to the heated cooling air leaving the cooling water.

fungi

Microbiological plant life.

hardness

The concentration of calcium and magnesium ions in water expressed as mg/l as CaCO . 3

in situ


In natural or original position.

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induced draft

Pulling of atmospheric air through a cooling tower by fans.

precipitation

The formation of insoluble salts from soluble salts due to a chemical change.

Ryznar Stability Index

A number based on the Langelier Index which indicates either corrosion or scaling.

silica

The chemical silica dioxide. An inorganic impurity in water expressed as SiO . 2

slime

Insoluble organic matter of a viscous gelatinous composition normally resulting from microbiological growth which traps other suspended impurities in the cooling water.

solubility

The amount of salt dissolved in water.

windage

Water lost from a cooling tower due to the wind.


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Cooling Water Treatment for industrial use

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