Cooling Water Treatment for industrial use

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’semployees. Any material contained in this document which is notalready 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, EngineeringServices, Saudi Aramco.

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Engineering EncyclopediaSaudi Aramco DeskTop Standards

Cooling Water Treatment

Engineering Encyclopedia Process


Saudi Aramco DeskTop Standards

CONTENTS PAGES

TYPES OF COOLING WATER SYSTEMS AND PARTS OF COOLINGTOWERS 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




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 MICROBIOLOGICALGROWTH 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 COOLINGWATER SYSTEMS .............................................................................................. 49

Chemical Feed Equipment ......................................................................... 49

pH and Blowdown Controllers ................................................................... 49

Frequency of Chemical Analysis................................................................ 50

GLOSSARY .......................................................................................................... 51



Saudi Aramco DeskTop Standards 1

TYPES OF COOLING WATER SYSTEMS AND PARTS OF COOLING TOWERSAND HEAT EXCHANGERSThree types of cooling water systems are used in the petroleum and chemical industries:open evaporative recirculating, once-through, and closed recirculating. The system usedwill depend on the process or equipment to be cooled, the availability and quality ofwater, and the ease with which the water can be disposed. The types of systems foundvary from small engine jackets to large once-through systems and open recirculatingcooling 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 efficientdissipation of heat. For these reasons, they are commonly used where water conservationis important.

Typical Cooling Tower Design

Figure 1 depicts a schematic of an open evaporative recirculating cooling system. Heat isdissipated by the evaporation of some of the recirculating water. The evaporation takesplace most commonly in a cooling tower, although spray ponds and evaporativecondensers are also used.

FIGURE 1. OPEN EVAPORATIVE RECIRCULATING COOLING SYSTEMS




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 isevaporated. The evaporation rate is a function of the recirculation rate, cooling range, andthe atmospheric temperature and humidity. The following equation approximates thisrelation:

e = 0.8 L (Cp) (dT) / Hv

wheree = evaporation rate, grams per minute (gpm)

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, °FThe factor, 0.8, arises from the fact that under typical atmospheric conditions 20 % of thetemperature 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/minuteof heat transferred.As the water vapor leaves the tower through evaporation the remaining dissolved saltsnaturally present in the water increase in concentration. These increased concentrationsmake the water more corrosive and increase the tendency of scales to form. Dissolvedsalts are generally allowed to concentrate by a factor of 3 to 8. This factor is called thecycles of concentration or cycles. The degree to which salts are allowed to concentrate iscontrolled by the blowdown or bleed off rate.The volume of fresh make-up water required by a cooling tower system is governed by theloss of water through evaporation, blowdown, and drift. Drift or windage isnonevaporative loss, which is typically 0.05 to 0.1 % of the circulation rate. It isconsidered negligible in many calculations. For our example tower, the drift typicallywould 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 asfollows:

b + d = e/( r - 1 )




whereb = blowdown rate, gpmd = drift, gpmr = cycles of concentration

The water mass balance for the example cooling tower water is:m = e + b + d

wherem = 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 concentrationare measured by the ratio of the chloride concentration of the circulating water to that ofthe make-up water. Chloride is used because it is usually present at a concentration whichcan 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 isused, since chloride is a by-product of this treatment. If there are ions in a tower waterwhich are being cycled less than chloride, they are being deposited or otherwise lost fromthe recirculating water.These basic cooling tower calculations are useful for establishing chemical feed rates. Thedosage of most treatment chemicals is based on their concentration in the circulatingwater. When a system is filled with untreated water, the initial dosage is proportional tothe volume and the initial demand of the system. Since most treatment chemicals do notevaporate 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 ofblowdown, and therefore increasing the cycles of concentration, the chemical feed ratecan be decreased proportionately. Since the corrosivity and scale-forming tendency ofwater increases as the number of cycles increase, an increase in cycles must be balancedby 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 waterand chemicals. It is generally not necessary to operate towers at more than eight cycleswhere incremental savings are small.




3 4 5 6 7 8

200

BLO

WD

OW

N (g

pm)

CYCLES OF CONCENTRATION

25,000 gpm RECIRCULATION11 °C (20 °F)

0

150

100

50

FIGURE 2. BLOWDOWN EFFECT OF CYCLES OF CONCENTRATION

A wide range of corrosion inhibitors, antifoulants, antiscalants, and biocides are used inopen recirculating cooling systems. The predominant corrosion inhibitors in the refiningand chemical industries are blends of chromates, phosphates, zinc, and copper alloyinhibitors. Organic phosphates, polymers, and copolymers are used as antifoulants andantiscalants. Chlorine is the most common biocide. Other oxidizing and nonoxidizingbiocides are also available.




Principal Parts of Cooling Towers

The principal parts of a cooling tower are the fan(s), fill material, water distribution deckor header, drift eliminator, structural frame, and cold water basin. Cooling towers userecirculating ambient air to cool warm water primarily through evaporation as the watercascades down through fill material and air passes up or across the fill. The fill serves tomaintain an even distribution of water across the horizontal plane of the tower andmaximizes 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




Heat Exchangers

Heat exchangers are critical parts of a cooling system designed to efficiently pass the heatfrom the process being cooled to the water. Since the heat transfer surface is the hottestarea exposed to cooling water it is the most prone to corrosion and fouling. The primaryobjective of a cooling water treatment program is to protect the heat transfer surfaces fromcorrosion and fouling.

Components of a Shell and Tube Heat Exchanger

Shell and tube heat exchangers come in many different shapes and sizes depending uponthe service for which they are to be used. The size and, to some extent, the type of heatexchanger 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 corrosionmust be considered in the selection of a heat exchanger. The principle parts of one of themost common types of shell and tube heat exchangers are shown in Figure 5. Coolingwater is most often on the tube side. When cooling water is on the shell side, corrosionand fouling are more likely due to pocketing and deposits at baffle dead corners.

FIGURE 5. SHELL AND TUBE HEAT EXCHANGER




Components of a Plate Heat Exchanger

Plate heat exchangers are sometimes used in once-through seawater cooling systemsespecially 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 heatexchanger including the temperature, composition of the process stream, and the coolingwater. Carbon steel may provide sufficient corrosion resistance in treated cooling water.Titanium, inherently more corrosion resistant and expensive, may be required in seawaterapplications. Carbon steel is the primary material of construction in cooling tower systemheat exchangers. Copper and copper alloys such as brasses, Cu-Ni, and stainless steels, arealso important due to their greater corrosion resistance than steel.Cast iron, steel, copper, copper alloys, aluminum, and solders are found in closed systems.




Various copper-base alloys, such as 90-10 Cu-Ni, 70-30 Cu-Ni, and aluminum brasses andbronzes have been successfully used in seawater. However, these materials are susceptibleto premature failure when flow velocities are high, when seawater contains significantconcentrations of sand, and when pollutants such as sulfide and ammonia are present.Alternatives include titanium, certain high-alloy austenitic stainless steels, high-alloyferritic 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 once-through systems. Figure 7 is a typical schematic of a once-through cooling system. Thesesystems 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 largeonce-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 isused to remove foreign matter such as seaweed which may damage pumps or foul heatexchange equipment. Chlorination is necessary to prevent biological fouling. Since largevolumes of water pass through these systems it is not economical to use any scale orcorrosion inhibitors. Corrosion resistant materials and limits on flow and temperature arenecessary to prevent corrosion.

FIGURE 7. ONCE THROUGH COOLING SYSTEM




Closed Recirculating Cooling Systems

A closed recirculating cooling system is one in which the water is recirculated in a closedloop with negligible evaporation or exposure to the atmosphere. Figure 8 depicts aschematic of this type of system. A closed system has essentially a constant volume withlittle or no added (make-up) water. These systems are frequently employed for criticalcooling applications where deposit formation on heat transfer surfaces would bedisastrous. In a typical closed system, heat is transferred to the system from the loop byheat 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, orair cooling.

FIGURE 8. CLOSED RECIRCULATING COOLING SYSTEM




Closed systems are well suited to cool gas engines and compressors. Diesel engines instationary or locomotive service normally use closed radiator systems similar toautomobile systems. Closed systems are also used in the chilled water systems of airconditioners 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 thetemperature rise is typically 6 to 8 °C (10 to 15 °F). Generally, little make-up water isneeded 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 noevaporation 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 entersonly in the make-up water. However, untreated systems and systems with excessiveexposure to the atmosphere may suffer from oxygen pitting, galvanic action, and creviceattack.High concentrations of nitrite-, chromate-, and silicate-based corrosion inhibitors arecommonly used in closed systems.




CONTROL OF CORROSION IN COOLING WATER

In cooling water, corrosion results from an electrochemical reaction between a metal andan impurity in the water. The corrosion of steel is discussed in this Module, but the sameprinciples apply to other metals used in cooling water systems. In cooling water, dissolvedoxygen, copper and ferric ions, acids, and chlorine are the primary impurities, calledoxidants or corrodants, which react with steel.A simple corrosion cell is shown in Figure 9. Oxidation, i.e., dissolution of a metal orformation of a metal oxide, occurs at the anode. For steel, the anodic reaction involves theproduction of ferrous ions (Fe2+) and electrons (e-) from iron metal (Fe°).

Fe° ——> Fe2+ + 2 e-

FIGURE 9. CORROSION CELL




The oxidation reaction must be balanced by a reduction reaction in which the corrodantaccepts the electrons at the cathode. The primary cathodic reaction in cooling water is:

1/2 O2 + H2O + 2 e- ——> 2 OH-

where oxygen (O2), water (H2O), and electrons combine to form hydroxide ions (OH-).These two reactions can be combined and written as follows:

Fe + 1/2 O2 + H2O ——> Fe+2 + 2 OH-

Further reactions often occur in water. Ferrous and hydroxide ions combine to formferrous hydroxide.

Fe+2 + 2 OH- ——> Fe(OH)2

Ferrous hydroxide can be further oxidized by oxygen to ferric hydroxide, which iscommon iron rust.

2Fe(OH)2 + 1/2 O2 + H2O ——> 2Fe(OH)3

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 ananodic reaction, i.e., the rate at which the metal is dissolved. They often form stablegamma-Fe2O3 films on steel. A disadvantage of these inhibitors is that when they areunderfed, corrosion is severely localized in the form of pitting.Cathodic inhibitors function by precipitating films of salts at locally high pH generated atthe cathodic site. These films are less protective than those generated by anodic inhibitors.Examples of cathodic inhibitors are polyphosphates, polysilicates, and zinc. Inhibitorswhich affect both cathodic and anodic reactions are termed mixed inhibitors.Phosphonates are mixed inhibitors.




Factors Affecting Corrosion in Cooling Water

In cooling water, the rate of the corrosion is dependent on several variables whichincludes the following:

• pH: Low pH accelerates corrosion, generally the pH is maintained above 6.0 incooling water.

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

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

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

• Concentration of dissolved solids: Figure 11 shows corrosion increases withincreased dissolved solids.

• Pretreatment and pre-filming of the metal surface can significantly decreasecorrosion 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




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 alsopossible. Microbiological corrosion, corrosion fatigue, and erosion-corrosion can alsooccur.The control of corrosion in cooling water is a complicated task involving mechanical andchemical 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 mustalso protect the other surfaces exposed to the cooling water and should rapidly establishcorrosion control at low concentration. The treatment program should be effective under awide range of pH, temperature, heat flux, and water quality conditions. It should also beforgiving of overfeed, the loss of feed, or other system upsets. Methods for easilymonitoring the concentration of the major components should be available. The corrosioninhibitor must be compatible with other treatment components, e.g., the biocide andantifoulant.




A variety of metallic, nonmetallic, organic, and inorganic chemicals are useful corrosioninhibitors in cooling water systems. Often a cooling treatment program will consist of twoor 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 followingsections.

Chromate

Chromates are the most effective corrosion inhibitors which protect both ferrous andnonferrous alloys. They are anodic inhibitors which form a tenacious oxide film whichprotects the underlying metal. Chromates are effective with the water temperatures up to71 °C (160 °F), and over a wide pH range of 6 to 11. Cooling systems are rarely operatedabove pH 10.When used alone there is a critical chromate concentration necessary to maintainprotection which is dependent on the sulfate and chloride ion concentrations of the coolingwater. If underfed, attack is localized and manifested in the form of pitting. When usedalone (e.g., in a closed system), control can be maintained with 200 to 500 mg/l chromatein the circulating water after an initial pretreatment of up to 1,000 mg/l. Naturally, the useof such high doses is very costly. These high levels are only used in closed systems whichare 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 CrO4, 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 hasbecome one of the most effective cooling water inhibitors. Zinc chromate is not a singlesalt as the name implies, but a mixture of a zinc salt (usually chloride or sulfate) andsodium dichromate. These ions exist as individual ions in solution without forming aspecific compound or intermediate. As little as 5 % of either ion in the presence of theother shows great improvement over the performance of the major ion alone. Generally, ablend of 20 % zinc and 80 % chromate is used. A typical dosage is 2 to 10 mg/l zinc andup to 25 mg/l CrO4. The recommended pH range is 6.2 to 7. Above a pH 7.5, zincprecipitates as the hydroxide, Zn(OH)2. Below 6.2, the protection of copper alloysdecreases. The pH range can be extended upwards with additives which prevent theprecipitation of zinc hydroxide.




Protection is established very quickly with zinc chromate when there is free access to thesurface. Old corrosion products and deposits impede the development of protection. Zincchromate protects copper alloys and aluminum. It inhibits the galvanic attack of Alcoupled to Cu and lowers the corrosivity of dissolved Cu ions.The dosage of most chromate containing inhibitors is controlled by monitoring thechromate level. High concentrations (10 to 100 mg/l as CrO4) can be detected by titrationwith thiosulfate. Low concentrations (less than 30 mg/l) can be detected colorimetricallyby the reaction of chromate with diphenylcarbazide.To lower the dosage of chromate required in order to maintain the protection of coolingsystems and to introduce deposition control, polyphosphates and zinc have been usedtogether with chromate. A typical dosage would be 10 to 25 mg/l CrO4, 2 to 5 mg/lpolyphosphate, and 2 to 5 mg/l zinc. Phosphonates also enhance the performance of zincchromate by providing threshold inhibition of calcium carbonate, calcium sulfateprecipitation, and adding detergency to decrease deposits and debris. Phosphonates alsoallow excursions above pH 7.5 since they stabilize zinc hydroxide. In addition, they donot have the drawback of polyphosphates, i.e., possible zinc and calcium phosphateprecipitation.

Zinc

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

Orthophosphates and Polyphosphates

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




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 ofinterest 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 calciumor zinc, to be effective. The calcium concentration must be at least 50 mg/l as CaCO3;therefore the orthophosphates are not useful in softened water, demineralized water, orsteam condensate. When zinc is used in conjunction with phosphate, typically 0.5 to 1.0mg/l soluble zinc is sufficient to maintain corrosion control with approximately 6 to 10mg/l orthophosphate at pH 7.3 to 7.8.The mechanism of corrosion inhibition of steel with phosphate is not clear. However, it isknown that oxygen, calcium, or zinc and phosphate are required. It is thought thatdissolved oxygen reacts slowly with steel to form a thin film of gamma-Fe2O3. During theproduction of this film, precipitation of iron or calcium phosphate occurs at voids in thefilm. These precipitates are not completely protective, and allow the gradual formation ofa protective iron oxide film. Zinc ions are thought to inhibit corrosion by precipitating zinchydroxide or phosphate at the cathodic sites due to locally elevated pH. These precipitatesalso form protective films.Protection by orthophosphate is sensitive to the water quality, pH, oxygen, and thechloride concentrations. A minimum orthophosphate concentration is required dependingon these variables. Below this minimum level pitting attack occurs.Polyphosphate is a generic term for a variety of materials formed by dehydrating andpolymerizing orthophosphates. Polyphosphates are cathodic inhibitors on steel. Somesodium polyphosphates frequently used in water treatment are shown in Figure 12.

FIGURE 12. SODIUM POLYPHOSPHATE




Polyphosphates slowly decompose or revert to orthophosphates in cooling water. Thisreversion can be accelerated by low pH and high temperatures. It is catalyzed by certainmetal ions and enzymes. In cooling water the half-life of polyphosphates typically rangesfrom several hours to two days. Polyphosphates revert instantaneously at boilertemperatures and are sometimes used as a source of orthophosphate.Polyphosphate can be considered both an anodic and cathodic inhibitor, although it isgenerally considered the latter. It requires both calcium or zinc and oxygen likeorthophosphate. In cooling water 10 to 15 mg/l polyphosphate as PO4 is normallymaintained after an initial pretreatment of at least twice this dosage for a few days. Whencopper 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; chlorineand/or other biocides are often required.Polyphosphates will minimize normal galvanic corrosion. They are ineffective whencathodic 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 isrequired to alleviate this problem.Polyphosphates are useful for the prevention of CaCO3 and CaSO4 scales formation. Theyalso stabilize dissolved iron and manganese in well water and are approved for use inpotable water up to 10 mg/l.In cooling towers polyphosphate is often used with chromate, zinc, and phosphonates. It islow cost, nonhazardous, and nontoxic. It is an effective alternative to chromate, althoughit has more restraints and requires more control.

Nitrite

Nitrite, commonly used as the sodium salt (NaNO2), is an anodic inhibitor which generatesprotective gamma-Fe2O3 on carbon steel. Nitrite is effective when oxygen is not present. Itis frequently used in closed systems not exposed to air. Often, borate is added to buffer thepH at about 9. Copper alloy inhibitors and dispersants may be added to complete theprogram. Unlike chromates, nitrites are compatible with glycols which are added as ananti-freeze or raise the boiling point of the water in hot systems. Typically, 300 to 500mg/l NO2 is required. The precise level is dictated by the chloride and sulfateconcentrations. Often, excess nitrite is used since closed systems are not monitoredfrequently.Nitrite is seldom used in cooling towers since it is decomposed by bacterial action and airoxidation to nitrate (NO3-).




Nitrite is easily monitored by titrating it against a standard oxidizing agent, potassiumpermanganate. Biological activity should also be monitored with nitrite.

Silicates

Silicates are useful corrosion inhibitors in mildly corrosive systems. They form a weakchemi-sorbed film on carbon steel. The development of protection is slow, and it does notrequire hardness to be present in the water in order to be effective.Like phosphates, various silicates are available ranging from simple ionic forms, such assalts of silicic acid (H2SiO2), to complex colloidal ions with variable compositions of theform nNa2O-mSiO2. 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 SiO2 at pH 8 to 9.5 inwater with low salt concentrations (less than about 500 mg/l TDS); that is, under mildlycorrosive conditions. Silicates are not generally recommended for cooling tower systems,but are suitable for some closed systems. Water with a high magnesium content must beavoided because magnesium silicate scale forms when the magnesium concentrationexceeds approximately 150 mg/l as CaCO3.Silicates can be used for the control of dissolved iron and manganese in potable watersystems at a level of 10 mg/l SiO2. It is an economical, nontoxic, nonhazardous option formild corrosion problems.

Molybdate

Sodium molybdate (Na2MoO4) forms passive anodic iron oxide films on steel. It is aweaker oxidant than chromate and requires an oxidant, either oxygen in open systems ornitrite in closed systems, to form a protective film. It is an environmentally acceptablealternative to chromate, although less effective and slower acting.In cooling tower systems high molybdate concentrations (e.g., 1,000 mg/l) are required ifit is to be used alone. Cost of such high doses are prohibitive. Typically, a molybdateformulation for a cooling tower system might provide 8 to 15 mg/l Mo, 2 mg/l Zn+2, 1 to 5mg/l phosphonate and similar levels of a dispersant and/or copper inhibitor. Unlike othernonchromate inhibitors molybdate does not require hardness in the water; it is useful insystems 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 exposedto both cooling water and water heated with steam intermittently. Higher concentrationsare necessary in these systems.




In closed systems, molybdate-nitrite formulations have been used at lower nitriteconcentrations than classical nitrite-borate treatment levels of 300 to 500 mg/l NO2.Molybdate can be monitored by a colorimetric method using mercaptoacetic acid.

Phosphonates

Phosphonates are a class of organic phosphorous compounds containing a carbon atomdirectly bonded to a -PO3 group, which gives them greater hydrolytic stability thanpolyphosphates. Three phosphonates commonly used in water treatment are shown inFigure 13. The complete chemical name and common abbreviation follow: nitrilotri-(methylene-phosphonic acid) or AMP, hydroxy-ethylidene-1, 1-di(phosphonic acid) orHEDP, and 2-phosphono-butane-1,2,4-tricarboxylic acid or PBTC. They are onlymarginally effective corrosion inhibitors when used alone under mild conditions.However, they are very useful in conjunction with chromate, zinc, and polyphosphates inopen and closed systems.




OIIPI

OH CHIC

OH

3 OHIPIIO

HO OH

HEDP

HO-P-OH HO-P-OHHO-P-OH

CH2

N

O O O

AMP

CH C OH2

C

CH2

2

O

C

O

OH

O

O

P

OH

HO

PBTC OR PBSAM

CH2CH2

I

CH C OH

FIGURE 13. PHOSPHONATES USED IN WATER TREATMENT




AMP, HEDP, and PBTC have the added advantage in that they control calcium carbonateand sulfate deposition. They also stabilize iron and manganese which otherwise wouldcause fouling. Many proprietary iron dispersants will contain HEDP and a polymericcomponent. Phosphonates also extend the pH range over which zinc is soluble, whichmakes zinc containing formulations more useful. One disadvantage of phosphonates isthat, due to their strong interaction with copper ions, they accelerate the corrosion ofcopper alloys when used at high concentrations. Often they require the use of a copper-specific inhibitor in mixed systems.AMP is degraded by high doses of chlorine. HEDP is sufficiently stable under mostchlorinating conditions. PBTC is the most stable.The phosphonate is oxidized to orthophosphate for monitoring, which is detected usingthe conventional ortho procedure. If present, poly and orthophosphates interfere and mustbe determined separately and subtracted from the total orthophosphate determined in thephosphonate test.

Copper Alloy Inhibitors

Three organic compounds are used as copper-alloy inhibitors in cooling water. They areTTA, BZT, and 2-MBT, as shown in Figure 14. These materials form strong complexeswith copper ions in solution and films on the surfaces of copper alloys. They offer littleprotection to ferrous metals and are affected adversely by chlorination. 2-MBT is the mostreadily oxidized and the inhibition is rapidly lost. The protection by TTA and BZT lapsestemporarily after chlorination and then returns after the chlorine dissipates. It is thoughtthat a reversible chlorine adduct is formed with the triazoles, which reverts to the triazolewhen the chlorine dissipates. Copper inhibitors are generally used at about 2 mg/l. Theyare all sparingly soluble in water, except at high pH where the soluble sodium form exists;therefore, they are supplied as liquids at high pH.




N

N

N

N

N

N-

Na +

H C3

TOLYLTRIAZOLE, TTA

BENZOTRIAZOLE, BZT

MERCAPTOBENZOTHIAZOLE, 2-MBT

N

S

SH

N

FIGURE 14. COPPER ALLOY INHIBITORS




Nonchromate Cooling Tower Treatment Packages

There are three generic, nonchromate chemical cooling water treatments used as typicalalternatives to chromate programs. These treatments include combinations of theinhibitors and dispersants for control of corrosion, scale formation, and fouling in coolingtowers. Copper corrosion inhibitors and a biocide are frequently part of the total treatmentprogram.In well-designed, well-operated systems with close control of water chemistry andinhibitor injection these treatments effectively control corrosion, scaling, and fouling. Inall these treatments pH/alkalinity control is critical. At pH values below the recommendedoperating range corrosion will occur. Above the range, scaling will be a problem. Oilingress is the most common operating upset which can foul the system and interfere withbiological and corrosion control in refineries.Corrosion is mitigated by pH control in combination with continuous injection ofcorrosion inhibitor. Scaling is controlled by pH adjustment and continuous injection ofchemicals to either inhibit scale formation or disperse scale deposits after formation.Fouling is controlled by intermittent or continuous use of polymeric dispersants. Themicrobiological control program is often based on chlorination. These programs requireclose control of the inhibitor injection rate and the cooling water chemistry limits. Typicalguidelines for each type of treatment are given in Figure 15. General guidelines whichapply to water quality in most cooling tower systems are given in Figure 16.




FIGURE 15. TYPICAL NON-CHROMATE COOLING WATER PROGRAMS

FIGURE 16. GENERAL COOLING CHEMISTRY GUIDELINES FOR NON-CHROMATE INHIBITORS




Monitoring Corrosion

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

FIGURE 17. COOLING WATER CORROSION TEST LOOP (OPEN-ENDED DISCHARGE INSTALLATION)




MetalCorrosion Rate

mils/yr Comment

Carbon Steel 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

Admiralty Brass 0 - 0.2 Generally safe for heatexchanger tubing and mild-steel equipment

0.2 - 0.5 High corrosion rate may enhance corrosion of mild steel

> 0.5 Unacceptable high rate for long term; significantly affects mild-steel corrosion

Stainless Steel 0 - 1 Acceptable

> 1 Unacceptable corrosion resistance

FIGURE 18. GUIDELINES FOR ASSESSING UNIFORM CORROSION INCOOLING WATER




Avoiding Galvanic Corrosion

Because seawater and most cooling waters are good electrolytes, severe corrosion canoccur when different metals are coupled together. The degree of attack depends on therelative position of the two metals in the galvanic series and on the relative size of thecomponents. The alloys at the top of the series are more prone to corrosion than thosebelow them. Large differences in potential drive the corrosion. The worst case occurswhen 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 shownbelow:

• Active Metals (Anodic):AluminumCarbon steelNaval rolledbrassCopperAdmiralty brassCopper-Nickel alloysTitaniumHastelloy CMonel400Type 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 hydrogendamage 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




Precleaning and Pretreatment

Corrosion control in cooling water is based on forming a film on the metal surface whichprotects the underlying metal. The rate at which the film or barrier forms will largelydetermine the effectiveness of the treatment. Materials that do not form films rapidly willpermit corrosion to take place. The result will be incomplete film formation and continuedcorrosion. The rate at which the film forms is related to the inhibitor concentration, thetemperature, 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 andavoid the accumulation of corrosion products.For new systems or heavily fouled systems, precleaning is usually necessary prior topretreatment. Precleaning may require high concentrations of inhibited acids, chlorine,and/or detergents specifically designed to remove the deposits present. Laboratory testingis recommended to choose the proper precleaning procedure for a particular system.After cleaning, a system is flushed and filled with water containing a pretreatmentproduct. This may simply be the inhibitor to be used at a high dosage to be maintained upto one week. More effective pretreatment is possible with high doses of surfactant and apolyphosphate. The treatment is time, concentration, and temperature dependent. Figure19 shows three typical pretreatment procedures for carbon steel.

FIGURE 19. DOSAGE Mg/L PO4 /TIME/TEMPERATURE




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

FIGURE 20. EFFECTIVENESS OF PRETREATMENT IN DECREASINGINITIAL CORROSION RATES




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 inplugging, loss of cooling, and underdeposit corrosion.Scale formation is characterized by the formation of insoluble inorganic compounds. Thisoccurs by the precipitation of slightly soluble ions such as calcium, magnesium, or zinctogether with carbonate, sulfate, phosphate, hydroxide, or silicate.Scale formation is different from fouling. Fouling can be either general or microbiologicalin nature. General fouling is caused by the settling of any suspended matter such as ironoxides, silt, mud, oil, and other debris. Microbiological fouling results from the growth ofalgae, bacteria, or fungi.

Effect of Scale on Heat Transfer

In a heat exchanger, thermal energy is usually transferred by conduction from a processfluid across a metal barrier to the cooling water. In conduction, heat is transferred throughor between stationary media such as metals, water, or air. It results from short rangeinteractions of molecules and/or electrons. In the metals, electrons contribute to thisprocess. In gases and liquids, energy is also conducted by molecular collisions. The heattransferred (Q) across a flat plate by conduction is described by the following equation:

WhereQ = heat transferred, Btu/hrK = thermal conductivity, Btu/hr °FA = cross sectional area, ft2

t2-t1 = temperature difference across the plate, °FL = thickness of the plate, ft.

From this equation, it follows that thin plates made from materials with high thermalconductivities, e.g., metals, are the best conductors of heat. Scales and fouling depositshave lower thermal conductivities than metals and effectively increase the thickness andlower the thermal conductivity of the barrier.




The basic equation for a steadily operated exchanger follows, in which U is the overallheat transfer coefficient and the dT and dt values are the terminal temperature differencesfor the process and watersides. In this simple case we are assuming constant U, constantmass flowrates, no changes in phase, constant specific heats, and negligible heat loss.

Q = U A dTm

whereQ = Heat transfer rate, Btu/hrU = Heat transfer coefficient Btu/hr ft2 °FA = Heat transfer surface area, ft2

dTm = Log mean temperature difference, °Fwhere

dT = T2 - t1, dt = T1 - t2

t1 = Inlet water temperature, °Ft2 = Outlet water temperature, °FT1 = Inlet process temperature, °FT2 = Outlet process temperature, °F

The rate of heat transfer from the process to the cooling water is proportional to the massflow rate of the material, its heat capacity, and the temperature change the materialundergoes. Since we are neglecting heat losses:

Q = Mw Cpw (t2-t1) = Mp Cpp (T1-T2)where:

Mw, Mp = mass flow rates for water and process, lb/hrCpw, Cpp = heat capacity of the water and process, Btu/hr °F




Heat capacity is an intrinsic property of a material. Tables of heat capacity data can befound in chemical handbooks. The heat capacity of pure water is 1.00 Btu/lb °F, bydefinition.When an exchanger is designed, a design heat transfer coefficient, UD, can be calculated.After being put in service, an actual heat transfer coefficient (UA) can be calculated usingthese equations. The fouling factor (Rf) is the difference between the reciprocals of theactual and designed coefficients.The fouling factor is the resistance to heat flow caused by fouling. Plotting the foulingfactor over time is a useful way to monitor the amount of fouling occurring in anexchanger. If the factor increases, the system is becoming fouled. A general rule of thumbfor fouling factors is that when they are on the order of 0.001 to 0.002 hr ft2 °F/Btu, thesystem is clean. If the factor is greater than 0.005, the system is fouled.

Scales Formed in Cooling Water and Their Prevention

As water evaporates in an open-evaporative system, the inorganic salts naturally present inthe water and those added for corrosion control increase in concentration. Consequently,the tendency for many of these ions to precipitate from solution increases, resulting inscale formation.The rate of scale formation depends on temperature, alkalinity or acidity of the water, thevelocity of the water, and other factors as well as the concentration of the scale-formingions. Calcium carbonate, calcium sulfate, calcium phosphate, and magnesium silicate arethe 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 formswhen the calcium hardness and bicarbonate alkalinity, naturally present in water, areconcentrated and/or are subjected to increased pH and temperature.

Ca+2 + CO3-2 ——> CaCO3 (solid)

In 1936, Langelier published a formula for calculating the tendency of water to eitherdeposit or to dissolve the calcite form of calcium carbonate. The formula expresses theeffect of pH, calcium, total alkalinity, dissolved solids, and temperature on the solubilityof calcium carbonate for water from pH 6.5 to 9.5. The equation is:

pHs = (pK2 - pKs) + pCa + pAlk




where pHs is the pH value at which water with a given calcium content and alkalinity is inequilibrium with calcite. The terms pK2 and pKs are the negative logarithms of the seconddissociation constant for carbonic acid and the calcite solubility product constant. The lasttwo terms are the negative logarithms of the molar and equivalent concentrations ofcalcium and titratable alkalinity. The Langelier Saturation Index (LSI) is a qualitativeindex of the tendency of calcium carbonate to deposit or dissolve, expressed as thefollowing equation:

LSI = pH - pHs

A simple formula for calculating LSI is given in Figure 21. A positive LSI indicates atendency 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 tocalcium carbonate saturation.




FIGURE 21. LANGELIER-RYZNAR INDEX CALCULATIONS




The LSI is a measure of the directional tendency or driving force of a water towardscalcite formation. It is not a reliable indicator of the corrosivity of a water. It is possiblethat two waters, one with low hardness which is corrosive, and the other with highhardness which is not corrosive, can have the same LSI. In an attempt to develop aquantitative indicator of the corrosive nature of water, Ryznar proposed the empiricalRyznar Stability Index (RSI) defined as follows:

RSI = 2 (pHs) - pHRSI < 6 Scaling tendency increases as the indexRSI > 7 Calcite formation may not lead to a protective corrosion inhibitor filmRSI > 8 Mild steel corrosion becomes an increasing problem

Calcium Sulfate Scale

Calcium sulfate is more soluble than calcium carbonate. Like calcium carbonate, calciumsulfate is less soluble in low pH waters. In cooling water, calcium carbonate will oftendeposit before calcium sulfate.The following rule of thumb can be used to estimate the safe upper limit of calcium andsulfate concentrations in many cooling waters in the absence of treatment chemicals.

(Ca+2) (SO4-2) < 500,000

The product of the ionic concentrations (mg/l) must not exceed 500,000 or precipitationwill occur. When high levels of dissolved solids are present or treatments are used, thislimit can be exceeded.

Calcium Phosphate Scale

Deposition of calcium phosphate is a potential problem in cooling water treated withphosphate-based corrosion inhibitors. It forms a dense, difficult-to-remove scale. In theabsence 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. Forthis reason, phosphate-based inhibitors were limited to pH 6.5 to 7.2 until effectivecalcium phosphate deposit control polymers were developed.The most common of these agents in use today are poly (acrylic-acid-co-hydroxypropylacrylate) (AA/HPA), poly (maleicanhydride-co-sulfonated styrene) (MA/SS), and poly(acrylic acid-co-AMPS) (AA/SA). These copolymers allow the use of phosphate-basedinhibitors at pH values up to 9.0.




Magnesium Silicate Scale

Magnesium silicate is another dense, difficult-to-remove material. There are no additiveswhich readily inhibit its deposition. To prevent it from forming, the silica content of thecirculating water should be limited to less than 150 mg/l as SiO2 at neutral pH values. AtpH 8.5, 200 mg/l SiO2 is soluble. The solubility is proportional to temperature; therefore,this scale forms in the colder regions of the system.

Effect of Water Chemistry, Temperature, and pH

There are several methods for preventing calcium carbonate scale. Removing ordecreasing the concentration of the calcium or magnesium by softening, ion exchange, orother means is seldom economical except for closed systems. Decreasing the pH by acidaddition is commonly used to prevent calcium carbonate and phosphate scale formation.The use of polyphosphate, phosphonates, polyacrylates, and other copolymers willdecrease calcium carbonate, sulfate, and phosphate formation. A list of treatments that arecommonly used for the control of scaling and fouling is given in Figure 22. Often, two ofthese materials are used in a blend that is more effective than the individual materials. Forexample, it is common to use a phosphonate (e.g., HEDP) together with a polymer orcopolymer as a general scale control agent.




Treatment Calcium Carbonate

Calcium Sulfate

Calcium Phosphate

Iron Oxide

Zinc Hydroxide

Suspended Matter Oil

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 -

Polyamaleic acid Copolymers

H H H -

Phosphinocarboxylic acids

H H E H E -

Poly(Maleic Anhydride-co-SulfonatedStyrene)

E E H H H -

Poly(Acrylic acid-co-Hydroxypropyl-acrylate)

E E E H H -

Poly(Acrylic acid-co-AMPS)

E H H H H -

Phosphonobutane-tricarboxylic acid

H H - E H - -

Surfactants - - - - - - H

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

E

E

E

E

E

E

E

E

FIGURE 22. WATER TREATMENT CHEMICALS FOR SCALE AND FOULINGCONTROL




Figure 23 summarizes the effects of water composition, temperature, pH, and theeffectiveness of dispersants in the control of scale formation in cooling water.

Increase in Concentration of

Scaling Ions CausesTemperature

Increases CausespH Increases

CausesDispersants

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




Prevention of the Harmful Effects of Microbiological Growth in Cooling Water

Cooling water systems, particularly open-recirculating systems, offer a favorableenvironment for growth of microbiological organisms leading to deposition, fouling, andcorrosion. Microbiological masses, generally referred to as slime deposits, result from theaccumulation of algae, fungi, or bacteria and their excretions. These masses can trapdebris and sediment, cause plugging of lines, reduce heat transfer, and cause corrosion bycreating differential oxygen concentration cells or generating corrosive by-products orenvironments. Biofouling refers specifically to fouling caused by plants or animals whensuch 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 probablywidespread in the petroleum and chemical industries, less technology is available forcombating MIC compared to traditional forms of corrosion. Control of biofouling can bemaintained by mechanical and chemical means.

Microorganisms Responsible for Biofouling

Algae, fungi, and bacteria may exist in a cooling system and result in fouling whenuncontrolled.Algae are photosynthetic organisms, relatively large, sometimes motile (able to moveabout) and usually colored. As a group, they can tolerate from very little to high intensitylight, a pH range of 5.5 to 9.0, temperatures from -18 to 40 °C (0 to 104 °F), and a widerange of salinities. They require only light, water, air, and a few inorganic nutrients forgrowth. Algae are widely recognized for the severe fouling problems created by theirstringy green slime masses which can reduce heat transfer and even plug tubes. They alsoprovide 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, butare important in the deterioration of wood. Molds are a diverse group which requireoxygen and organic materials for growth. They can contribute to white rot or brown rot ofcooling tower wood.




Bacteria found in a system can be varied and include spore-forming, nitrifying, nitrogen-fixing, denitrifying, sulfate-reducing, iron, sulfur bacteria, and others. They are universallyfound in nature and usually do not require light for growth. Bacteria are motile, allowingthem to seek favorable environments. They generally thrive from about 21 to 46 °C (70 to115 °F), although some have been found from -10 to 100 °C (14 to 210 °F). Thermophilicbacteria grow in higher temperature waters and are found on heat exchanger surfaces. Thepreferential pH range is 5.5 to 8.5, but certain types have been reported from about pH 0to 10.5.General guidelines for allowable levels of total bacteria count, corrosive or iron depositingbacteria, fungi, and algae in a cooling system are given in Figure 24. Obviously, acomplete microbiological analysis of a system is required to determine the counts of thesedifferent organisms present. Such an analysis is recommended if a severe biofoulingproblem is suspected. Figure 25 gives general guidelines for interpreting a general coolingwater 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 arethe 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 identifyingproblems.

Constituent Unit Limit

Total bacteria count Colonies/ml 500,000

Bacteria (corrosive) or iron depositing Colonies 0

Fungi 100

Algae Few

FIGURE 24. MICROBIOLOGICAL GUIDELINES FOR NON-CHROMATEINHIBITORS




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 COOLINGWATER

Chemicals for Control of Biofouling

Many variables influence the biological development and growth process. Treatment ofeach system should be considered individually, and different programs may be requiredfor different seasons of the year. Selection of a biocide and dosage should ideally be basedon a comprehensive biological survey. Often, this is not feasible, and a multicomponentapproach is used. In the past, chromates were effective in controlling biological depositsand some, sulfate reducers in particular, could not exist where even a trace of chromatewas used. However, with the change to nonchromate (particularly phosphate-basedtreatments), the need for biological control is crucial for success of the overall program.Toxicants for biological control are normally classified as either oxidizing ornonoxidizing.




Oxidizing Biocides

These are compounds which, in addition to their disinfection action, chemically oxidizeconstituents 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 lowcost. 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 towerwood. However, higher doses or continuous doses at more than 1 ppm may bedetrimental. Oil, reducing agents (e.g., H2S), 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 hypochlorousacid 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 pH7.5.Bromine and bromine-donating compounds are more effective in the high pH range thanchlorine. Bromine can be generated in-situ by the reaction of chlorine and bromide.Bromochlorohydantoins are another source of bromine. The latter are solids and eliminatethe potential hazard of chlorine gas.Chlorine dioxide and hypochlorite compounds are other oxidizing biocides. Ozone isanother, which is not frequently used in cooling water.

Nonoxidizing Biocides

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




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




Surfactants

It is often useful to precede the shot dose of biocide with an effective biodispersant, whichenhances the ability of the biocide to penetrate and kill the organisms. Surfactants orsurface active agents are effective biodispersants.

Mechanical Means for Control of Biofouling

Maintaining proper flow velocity, backflushing, screening, filtration, and air-bumping aremechanical means of removing macroorganisms and microorganisms. In certain instances,thermal backwash has been used to kill macroorganisms in their juvenile stage. Just assurfactants aid the ability of the biocide to penetrate the biomass, mechanical means ofdisrupting 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 tosurfaces. It is believed that in a typical system there may be 1,000 to 10,000 sessileorganisms for every planktonic (free floating) organism. This observation has led to thedevelopment of biofouling monitors. The basic design of the most popular monitor isshown in Figure 27. The principle of operation is quite simple. As a biofilm develops in apiece of tubing, an increased pressure drop develops across the tubing if the flow rate isheld 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, cleanlevel.

FIGURE 27. BIOFOULING MONITOR




In addition to the biofouling monitor, fouling monitors developed by organizations such asNACE and the water-treatment vendors are available. They involve the measurement ofheat flux through a heat-transfer surface, under water-velocity, and temperature conditionsmatched 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 foulingmonitors may be ambiguous. They will indicate extremely clean or fouled conditions, butrequire 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 coolingsystems. The two basic types of animals are the “soft” plant-like slimes: bacteria, algaeand hydroids, and the “hard” (shell-like) organisms: barnacles, mussels, oysters, tubeworms, and sea squirts.The tendency of such organisms to adhere to materials submerged in seawater depends onthe 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-foulingby barnacles and the like, although they may accumulate soft-fouling organisms under thesame conditions. Chlorination further aids in the control of biofouling.Even with cupronickel exchangers, and certainly with other materials more susceptible tofouling, chlorination is recommended for the control of biofouling. This is due to theingress 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 about0.5 mg/l free available chlorine is usually adequate. The dosage is system-dependent andcontinuous 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 thechloride present in seawater with in-line electrolysis cells. Hypochlorite salts (e.g., sodiumor calcium hypochlorite) are effective, but are not normally economically competitivewith chlorine gas.




Control of General Fouling in Cooling Water

General fouling is the accumulation of suspended solids, corrosion products, process andmake-up contaminants, oil, dust, mud, silt, debris, and other foreign materials on heattransfer surfaces.

Oil and Dust in Cooling Water

All surface waters contain suspended solids which are potential foulants. Dissolved ironand 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 andchromate 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. Theleaking exchanger should be located, shut off or isolated, or its outlet water diverted to asewer. The cooling tower and distribution deck should be cleaned. Oil should be skimmedwhere possible. The corrosion inhibiter dosage (no chromate-zinc) should be monitored,and doses of surfactant should be administered. Chlorination should be continuous andcooling water flow increased if possible. The system then should be blowndown heavilyuntil less than 10 mg/l oil is obtained. The cycles of concentration should then beincreased to normal, and surfactant doses continued for four days. Oil, biofouling, and thecorrosion inhibitor should be monitored closely. Before returning to normal operation anincreased dosage of inhibitor should be applied for four days in order to repassivate thesystem.




Means of Control

One of the most important factors in achieving effective corrosion control is themaintenance of clean surfaces.Low water velocities and the use of cooling tower water on the shell side of the shell andtube heat exchangers can lead to fouling. Recommended water velocities are presented inFigure 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 beenused to reduce fouling. Proper selection of a filter to remove the particles in a coolingtower is required. When used in conjunction with good chemical treatment, sidestreamfilters have been effective. However, they are not a replacement for the proper control ofcorrosion, scale, and biofouling.

Dispersants and Surfactants

Dispersants and surfactants are useful for minimizing general fouling. The chemicalssuspend 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 cleansystems on-line. On-line cleaning should only be attempted with experienced personnelwith consideration for the impact of the cleaning on potential plugging, corrosion andsafety. Hydroblasting is generally sufficient to clean systems off-line. If hydroblasting isinsufficient, several options for mechanical and chemical cleaning are available. Testing isthe best means for selecting the optimum cleaning procedure.




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 economicsand 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 withpump, 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 pressureindicator, flow sensors, and a draw down tube to measure feedrate. The control cabinetshould contain pump stroke control, on-off pushbuttons and indicators, tank levelindicators, and alarms.Tanks must be sized based on delivery schedules and shelf-life of products. Duplicatepositive displacement pumps are commonly provided. Piping and tanks should bedesigned for easy cleanout. In large cooling towers, the feed pump is often automaticallycontrolled 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 thereliable 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-upwater. 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 savingsin water and chemicals. Blowdown controllers operate based on the conductivity of therecirculating water. The blowdown valve is opened when the conductivity exceeds apreset limit and closes when the conductivity is low.ORP (oxidation reduction potential) probes are relatively new devices which can be usedto sense residual chlorine in water and can be used to control the feed of chlorine.




Frequency of Chemical Analysis

The recirculating water in a process cooling tower should be monitored routinely forcorrosion inhibitor, chlorine, concentration of salts (usually conductivity), and processleaks, 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 detectany long-term problems or trends. Large process towers which are not equipped with on-line monitors may require testing as frequently as every four hours. In well automatedtowers it may be sufficient to test once per day. Corrosion coupons should be evaluatedquarterly, and microbiological counts should be run quarterly or more frequently. In allcases, more frequent monitoring is recommended if problems are detected.Generally, closed cooling systems can be monitored quarterly. Once-through systemsshould be monitored for chlorine at least daily.




GLOSSARY

acid A chemical that lowers the pH by reacting with thealkalinity

algae Simple aquatic plant life requiring sunlight for growth.

alkalinity The total of carbonate, bicarbonate, and hydroxide ionimpurities expressed in mg/l as CaCO3.

anodic inhibitor A chemical added to cooling water which reducescorrosion by forming a protective film at the anodicsurface of a metal.

bacteria One cell microbiological life which grow everywhere incooling 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 oncomponents.

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

cathodic inhibitor A chemical added to water which reduces corrosion byforming a protective film at the cathodic surface of themetal.

chlorination The addition of a chemical containing free chlorinewhich is used to prevent biological fouling.

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

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

deposit External material adhering to the original components ina cooling system.

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

fungi Microbiological plant life.

hardness The concentration of calcium and magnesium ions inwater expressed as mg/l as CaCO3.

in situ In natural or original position.




induced draft Pulling of atmospheric air through a cooling tower byfans.

precipitation The formation of insoluble salts from soluble salts due toa chemical change.

Ryznar Stability Index A number based on the Langelier Index which indicateseither corrosion or scaling.

silica The chemical silica dioxide. An inorganic impurity inwater expressed as SiO2.

slime Insoluble organic matter of a viscous gelatinouscomposition normally resulting from microbiologicalgrowth which traps other suspended impurities in thecooling water.

solubility The amount of salt dissolved in water.

windage Water lost from a cooling tower due to the wind.

Cooling Water Treatment for industrial use

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