faq_cwc.doc

7/30/2019 FAQ_CWC.doc

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FREQUENTLYASKED

QUESTIONS

ON

COOLING

WATER

TREATMENT


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INDEX

CHPATER NO DESCRIPTION

CHAPTER 1 WATER CHEMISTRY

CHAPTER 2 COOLING WATER SYSTEMS

CHAPTER 3 TERMINOLOGY USED

CHAPTER 4 SCALING

CHAPTER 5 CORROSIONCHAPTER 6 MICROBIAL GROWTH

CHAPTER 7

CHAPTER 8

CHAPTER 9


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CHAPTER 2 : COOLING WATER SYSTEMS

Q 2.1 What are the Different Types of Cooling Water Systems ?

There are Three Different Types of Cooling Water Systems :

1. ONCE THROUGH SYSTEM

2. OPEN RECIRCULATING SYSTEM3. CLOSED RECIRCULATING SYSTEM

1. ONCE THROUGH SYSTEM

In a ONCE THROUGH SYSTEM, water is passed through the Heat ExchangeEquipment & the cooling water is then discharged directly without recirculation.

This system is used where water is readily available in plenty & at low cost. Thewater source can be sea, river or lake. However the water does not concentrate. Bothmake up water & recirculating water have almost the same characteristics.

Watersupply

2. OPEN RECIRCULATING SYSTEM

In an OPEN RECIRCULATING SYSTEM , water is passed through an Heat

Exchanger & absorbs Heat. This heated water is cooled in a Cooling Tower by

partial evaporation & releasing the latent heat of evaporation. In this way water isrecirculated & reused using the pumps. The dissolved solids in the water become

concentrated due to evaporation. Hence some quantity of recirculating water has to

be blown from the system to control the concentration of dissolved salts.The systemis prone to have more severe problmes of Scaling , Corrosion, Fouling & Biological

Growth.

3. CLOSED RECIRCULATING SYSTEM

In a closed recirculating system the water is cooled in a secondary cooler using air ,

sea water or the cooling water from open recirculating system. It is then recirculated& reused. This system does not employ open evaporation for cooling.

Usually water losses are low & hardly exceed 0.5% of the recirculation rate. TheClosed Recirculating Systems are used in Diesel Engines, Air Conditioners, Nuclear

Reactor Auxiliary Cooler , Comfort Cooling etc.


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CHAPTER : 5 CORROSION

Q 5 .1 What is Corrosion ?

Corrosion is an electrochemical process by which a metal returns to its natural state. For

corrosion to occur there must be an anode, a cathode, an electrolyte & a metallic path for

the electrons generated (usually the metal itself).

At the Anode, metal ion dissolves into the electrolyte (water)

Fe Fe 2+ + 2 e Anodic ReactionAs metal ion goes into solution at Anode, electrons are released which migrate through

the metal to the Cathodic area, where the cathodic reaction takes place ie the electrons areconsumed.

O2 + H2O + 2 e- 2 OH- Cathodic Reaction

Fe 2+ & OH ions react further to form ferrous hydroxide which is precipitated.

Fe 2+ + 2 OH - Fe(OH) 2Ferrous Hydroxide is rapidly oxidised to Ferric Hydroxide.

Fe(OH)2 + O2 + H2O Fe(OH)3Ferric Hydroxide sludge when dried ,dehydrates to form insoluble ferric oxide which is

known as rust.

2 Fe(OH)3 Fe2O3 + 3 H2OFe2O3, FeO,Fe(OH)2,& Fe(OH)3 are found as Corrosion products on the metal surface &equipments.

In the absence of oxygen , Hydrogen ion H+ participates in the reaction at Cathode

instead of oxygen & completes the electric circuit.

2H+ + 2e - H2The electrochemical reactions occur due to potential difference between the anode( - vepotential) & the cathode(+ ve potential ) .Every metal surface is covered with

innumerable small anodes & cathodes.


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Q. 5.2 What are the factors influencing Corrosion ?

Some important factors influencing Corrosion in cooling water systems are as follows:

a) pH

Generally corrosion rate decreases with increase in pH. Corrosion is more severe inacidic pH as the protective oxide film is soluble in acidic water. Corrosion of steel is

uniform at low pH .At higher pH ,unevenness & tuburculation occurs.

b) Dissolved Gases

i. CO2 in water forms carbonic acid & reduces the pH

ii. O2 dissolved in water is necessary for the cathodic reaction to take place.

Corrosion rate of steel increases with increase in dissolved oxygen in water.iii. Nitrogen aggravates cavitation Corrosion.

iv. Ammonia is selectively corrosive to copper based metals.

v. H2S tends to depolarise the anodic area. H2S is aggressive to iron.

vi. Chlorine promotes acid attack & strips corrosion inhibitor film.

c) Dissolved Solids

The influence of dissolved solids on corrosivity is very complex. Chlorides, Sulfates

penetrate passive oxide film & promote local attack resulting in pitting type of corrosion.

Hardness salts & alkalinity retard corrosion by forming corrosion inhibiting films. ie DMwater is more corrosive than soft water which in turn is more corrosive than hard water.

c) Suspended Solids

Mud, Sand, Silt, Dirt etc settle to form deposits promoting differential aeration cellcorrosion. They can also cause erosion or barasion.

d) Microorganisms

Microorganisms lead to slime, tubercles. The bacteria proliferate under high pH, low

velocity , high sulfates & non sterilised conditions. Even the by products of some

organisms are corrosive. Corrosion caused by microbial attack is called as MicrobialInduced Corrosion (MIC).

e) Temperature

In an open recirculatory system , the corrosion rate increases with increase in temperature

due to diffusion of oxygen to the metal surface. Above 70 C, due to loss of dissolved

oxygen the corrosion rate decreases. In a closed loop corrosion increases steadily as the

temperature rises.

f) Velocity

High velocity promotes erosion corrosion & removes certain passivating film. Similarlylow velocity can lead to deposition ,decreases amount of corrosion inhibitor reaching &

passivating the metal surfaces causing localized corrosion cells.


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g) Heat Transfer

Heat Transfer surfaces are more susceptible to corrosion because of differential

temperature conditions.

h) Dissimilar Metals

Direct contact of dissimilar metals results in corrosion f active metal causing galvaniccorrosion.

i) Metallurgy

Metal surface flaws,cuts,nicks,scratches etc favour anodic site formation. Metal under

tensile stresses may crack causing tensile stress corrosion cracking. Microstructure ie

metal inclusions, precipitation at grain boundaries, differing adjacent grains etc promote

galvanic cell formation.

Q 5.3. What are the different types of Corrosion ?

Different types of Corrosion are as follows :

a) Uniform Attack

Uniform Attack is a form of corrosion that occurs with equal intensity through the surface

of the metal. At neutral pH & below , the corrosion of steel tends to be fairly uniform.

b) Pitting

It is a localised attack caused by formation of highly active local anodic sites. In Pitting ,

small pits are formed on the metal surface. These may result from unequal ionicconcentration or oxygen differentials. Pitting is the most common cause of metal failure.

c) Under Deposit Corrosion

Deposits such as Corrosion products, scales, slime etc formed on the metal surface gives

rise to differential concentration cell. This gives rise to Corrosion. Under Deposit

Corrosion leads to deep pitting & perforation of metal surface. Low flow areas are prone

for this type of corrosion.

d) Crevice Corrosion

This is another form of electrochemical corrosion. This form of corrosion occurs due toaccumulation of aggressive ions & oxygen differentials inside the crevices.

A water or liquid trapped between a pipe & a flange can lead to Crevice corrosion. A

stagnant liquid in the crevice had a lower oxygen concentration & this leads to crevicecorrosion.

e) Selective Leaching

Selective Leaching occurs when one metal or constituent of a metal selectively corrodes.The most common examples are Dezincification , Graphitication & Dealuminification.

Dezincification of Brass occurs in cooling water systems with high chlorine levels. After

leaching mechanical properties of the metal are impaired & the chance of metalcracking are more.


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f) Erosion

Erosion Corrosion is caused by high velocity & turbulence This is caused by the acombination of chemical attack as well as physical abrasion. Liquids with high

suspended solids are prone for Erosion corrosion.

g) Stress Corrosion Cracking

Metals under tensile stress in a corrosive environment & temperature develop this type of

corrosion. Cracking is more commonly observed with stainless steel &can be eitherIntergranular or Intragranular. Once a stress crack begins , it easily propagates throughout

the metal .

h) Waterline Attack

If the heat exchanger system or the distribution system is not completely filled with

water, then there is a region or area filled with air. This causes differential aeration cell

corrosion. The corrosion takes place due to wetting & drying at water interface.

Q.5.3 How do we Control Corrosion ?

The most widely accepted methods for Corrosion Control in industrial cooling water

system are as follows :

a) Protective Coating / Lining

b) Oxygen Scavenging

c) Cathodic Protection

d) Use of Sacrificial Anodes

e) Adjustments to water chemistry eg pH , Conductivity , Bicarbonate equilibrium

etc.

f) Use of Corrosion Inhibitors

The selection of the method depends upon :-

i) Techno Economic Considerationii) Expected Life of an Equipment

iii) Desired performance of Equipment

iv) Corrosion Allowance & Expected Corrosion Rate

v) Combination of Two or more Methods


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Q.5.4 What are different types of Corrosion Inhibitors ?

Corrosion Control in Cooling water systems is achieved by using Corrosion Inhibitors.Corrosion Inhibitors are normally classified based on their mode of action as follows :

a) Anodic Corrosion Inhibitor

b) Cathodic Corrosion Inhibitorc) Mixed Corrosion Inhibitor

A) Anodic Corrosion Inhibitor

These inhibitors function by blocking anodic corrosion sites through formation of a

protective oxide/inhibitor film. To ensure complete protection all anodic sites must be

filmed. That is why Anodic Inhibitors are generally applied at high dosages.

Typical Anodic Inhibitors are

i) Chromates

ii) Molybdates

iii) Ortho Phosphates

iv) Nitritesv) Silicates

B) Cathodic Corrosion Inhibitor

Cathodic Corrosion Inhibitors form a visible film along the cathode surface which

polarises the metal surface by restricting the access of dissolved oxygen to the metalsubstrate. The dosage required is less than the anodic corrosion inhibitor.

Typical Cathodic Corrosion Inhibitors are

j) Polyphosphates

k) Zinc

l) Calcium Carbonate

C) Mixed Corrosion Inhibitors

An inhibitor which has both cathodic as well as anodic mechanism is called as Mixed

Corrosion Inhibitor. Organic Filming Amines, Phosphonates give corrosion inhibition by

both anodic as well as cathodic mechanism.

A) Synergistic Blend of Corrosion Inhibitors

Practically , the use of only one corrosion inhibitor in open recirculating system is rare.

Usually two or more corrosion inhibitors are blended to utilise the advantage of each &to minimise their respective limitations. A combination of cathodic and anodic corrosion

inhibitors give a better corrosion control at economical use levels. eg A Zinc Chromate

based Corrosion Inhibitor blend used at 40 50 ppm gives better corrosion control than200 ppm or more of Chromate alone.


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Most commonly used synergistic blends of Corrosion Inhibitors are :-

a) Zinc Chromate

b) Zinc Polyphosphate

c) Zinc Phosphonate

d) Chromate Polyphosphate

e) Chromate Orthophosphatef) Zinc- Polyphosphate- chromate

g) Polyphosphate Silicate

h) Molybdate- Zinc Azole

i) Molybdate Nitrite Borate

j) Nitrite- Borate silicates

k) Molybdate - Phophonate- Dispersant

l) Molybdate orthophosphate Dispersant

Q.5.5 How Non Ferrous Metals are protected against corrosion ?

Copper & its alloys such as Admiralty Brass, Cupro Nickel (90:10 ) are frequently usedin cooling water heat exchangers. These are relatively resistant to corrosion. The

chemical factors such as low pH, ammonia, cyanides, sulphides, excessive chlorineresiduals cause corrosion of copper & its alloys. When corrosion occurs there is an

increase in copper content of recirculating water. This copper will get deposited on steel

surfaces & create galvanic cell action resulting in pitting corrosion of the steel. Highvelocity causes erosion of copper based materials.

Azoles such as Mercaptobenzothaizole (MBT) , Benzotriazole (BT) , Tolytriazole(TT) are very effective in inhibiting corrosion of copper & copper based alloys. Recently

Halogen Resistant Azole (HRA) is also used as copper corrosion inhibitor. This iseffective even in the presence of high chlorine residuals in recirculation.

Q.5.6 How do we measure Corrosion?

Proper monitoring of Corrosion rates is very much essential for control of corrosion.

Corrosion rates are normally determined by two techniques

1. Corrosion Meter

2. Corrosion Coupons

Corrosion Meter works by measuring an electrical potential across electrodes made of themetal being evaluated. Corrosion Meter gives an instantaneous reading & indicates the

corrosive tendency of water at the time of measurement. Corrosion Meter helps to

monitor day to day water quality & corrective steps can be taken immediately. Probes ofvarious metallurgy are used to determine corrosion rate.

Corrosion coupons determine the long term effect of water & metal contact underfluctuating system conditions. Preweighed coupons are exposed in the water for a period

of 30/60/90 days & thus it gives average corrosion rate for the period of exposure. The

corrosion coupons give us an idea about both general & pitting corrosion as well as

fouling conditions.


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The Area of a coupon is calculated as follows :

Area of Coupon = { 2 LW + 2 LT + 2 WT } -- 2 D2 + D T4

where

L = Length of the coupon

W = Width of the coupon

T = Thickness of the coupon

D = Diameter of the hole

= 22 / 7 = 3.1428

Q.5.9 What is the accepted standard corrosion rate ?

Based on the exposure time of 30 days for coupons the corrosion rate is rated as follows:

a) Less Than 2 mpy : Excellent Corrosion Protection

b) 2 5 mpy : Good Corrosion Protection

c) 5 10 mpy : Moderate or doubtful Protectiond) More than 10 mpy : Poor or no Protection

These Corrosion rates are for Mild Steel or carbon Steel. For Copper & Copper based

alloys like Admiralty Brass , Cupro Nickel (90:10) & Stainless Steel the corrosion rate of

0.5 mpy is considered to be good.


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CHAPTER 6 : MICROBIAL GROWTH

Q.6.1 What are the different types of organisms found in recirculating water

systems ?

The recirculating water system provides a unique environment for the rapid growth of

microorganisms. Algae , Fungi & Bacteria are the types of organisms which grow &

proliferate in cooling water systems. They may be airborne, entrained during theconstruction, or can come through raw water make up. The temperature condition of the

recirculating water system is usually suitable for their growth.

ALGAE

Algae requires air, water & sunlight for its growth. They carry out photosynthesis with

sunlight . They manufacture their own food by fixation of carbon dioxide using water ,inorganic salts & water. The distribution decks, & the side walls of a cooling tower are

ideal locations for the growth of an algae. They are introduced into the cooling watersystem through air borne dust & make up water. If not treated, very often mats of algae

are seen on cooling tower decks. Normally algae have very little direct effect on the heatexchangers , However dislodged algae may interfere with the proper water distribution on

the tower deck & thereby reduce the heat transfer.

Algae biomass can become nutrient source for bacteria. If attached to & / or deposited on

metal surface it can contribute for localised corrosion viz. Micribiologically Induced

Corrosion (MIC).

FUNGI

Fungi are non photosynthetic organisms. They are either unicellular , colonial or

filamentous.Fungi are plants without chlorophyll. They live on dead organic matter.They

utilise wooden structure of the cooling towers as a source of nutrient & thus destroy

wood lumber. The excessive fungal growth on the wood is called as Woodrot

Fungi are classified as Yeast & Molds. Mold forming or spore forming fungi ,which are

found in circulating water , remain dormant for a long period under unfavourableconditions. This dormant condition makes them relatively harmless. The circulating wtaer

pH & temperature conditions are favourable for the growth of fungi.

BACTERIA

Many types of bacteria are found in cooling water system. Based on the shape of the

bacteria are classified as Rods(rod shaped), Cocci(spherical shaped) & spirals (spirallyshaped). Bacteria exist as Unicelluar or Multicellular. Autotrophic bacteria manufacture

their own food by oxidation of inorganic material. eg Sulphate Reducing Bacteria (SRB),

Iron consuming Bacteria. Heterotrophic bacteria derive their food from both organic &inorganic material eg Slime forming bacteria.


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Bacteria can be also described as Aerobic or Anaerobic depending upon whether they

flourish in oxygenated environment or environment void of oxygen.

Q.6.2 What is Slime ?

Slime is an extra cellular gelatinous secretion of varying consistency. Slime formation

depends on the environmental conditions such as temperature , availability of nutrients

etc. Suspended matter, air borne dust, mud, silt, etc becomes entrapped the slime mass.They deposit on metal surface . They reduce the heat exchanger efficiency & also create

differential aeration cells resulting in high localised corrosion rates.

Q.6.3 What are the factors influencing Microbiological Growth ?

Cooling water systems provide ideal environment for the growth of microorganisms.

Following physical & chemical factors affect the microbiological growth :

a) Ambient & system temperatureb) Roughness of solid surface

c) pH of waterd) Particulate Matter in the water

e) Sunlight

Cooling water system offers plenty of water, good aeration & a continuous supply of

nutrients, Process contamination, Oil ingress is another major source of nutrient supply to

microorganisms. Increased use of phosphate based chemicals & rising use of nutrient

containing waste water such as treated sewage water as make water are ideal sources forthe microorganisms to flourish.

Q.6.4 How Microbiological growth is controlled ?

Neither physical nor chemical conditions can be changed ina practical way to control

microbiological growth. Hence only way to control it is to add certain chemicals incooling water system. These chemicals are termed as BIOCIDES. Biocides kill the

microorganisms. It is very difficult to kill all the microorganisms in a cooling water

system. What is best achieved is the maximum killing & control of growth.

The efficacy of biocide depends upon the operating pH of the cooling water system,

temperature, nature & amount of pollutants such as Hydrocarbons, process contaminants

eg Ammonia, other nutrients such as ortho phosphate present.

Q.6.5 What are the different types of Biocides used in cooling water system ?

Biocides are usually classified as OXIDISING BIOCIDES & NON OXIDISING

BIOCIDES.


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Oxidising Biocides are capable of undergoing oxidative reactions with organic

molecules. Oxidising Biocides ,notably forms of Chlorine & Bromine are the primary

biocide used in cooling water systems. They are more widely used because they areeffective & less expensive.

Non oxidising biocides are more effective than oxidising biocides. There are variety ofNon oxidising biocides available. The selection of proper biocide or combination of

biocides depends on the type of organisms present , type of cooling water system, water

characteristics, plant past history & environmental limitations. & restrictions.

Q.6.6 What are the different types of Oxidising Biocides ?

Different types of Oxidising Biocides used in Cooling water systems areas follows :

1. Chlorine Compounds

Chlorine compounds such as Chlorine gas, Sodium Hypochlorite , Bleaching Powder ,

Chlorinated isocyanuartes are typical examples of this class. They are different in theirfunctional mechanism but are different in their form.

These chlorine compounds dissociate in water to produce Hypochlorous acid (HOCl).

Cl2 + H2O HOCl + HCl .. (1)Ca(OCl)2 + H2O Ca(OH)2 + 2 HOCl .. (2)NaOCl + H2O NaOH + HOCl . (3)

HOCl further dissociates to form Hypochlorite ion( OCL -) & hydrogen ion (H+)

HOCl H+ + OCl . (4)B) Chlorine Release Compounds

These are the compounds such as Chloroisocyanurates & Chlorohydantoins which

generate hypochlorite & Hypochlorous ions when dissolved in water. These stabilised

forms of chlorine are easier or safer to feed than gaseous or liquid chlorine. Theseorganochlorine compounds vary markedly in stability, product form , cost & chlorine

release mechanism.

Free Residual Chlorine of 0.2 to 0.5 should be maintained in cooling water return to

cooling tower. Process contamination, oil ingress , ammonia contamination result in highchlorine demand. The biocidal activity of Chlorine is very sensitive to pH & decreases

rapidly above 7 pH. Recently chlorine use is dropping because of its reducedeffectiveness in high pH water & in reclaimed water containing ammonia.

3. BROMINE COMPOUNDS

An alternate to chlorine is becoming very popular.. it is generated by action of bromide

salts with chlorine gas, hypochlorite liquid producing hypobromous acid or sodium


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hypobromite.Bromine is more biologically active & cost effective than chlorine at pH

above 7.5 & in the presence of ammonia.

4. CHLORINE DIOXIDE

Chlorine dioxide is becoming popular because of its effectiveness at high pH levels(pH

above 7.5) & non reaction with ammonia. It must be generated on site. Its productionrequires handling of several liquids. Unlike Chlorine it does not react with water to form

hypochlorous acid & hydrochloric acid. It is less corrosive than chlorine.

5. OZONE

Ozone is an allotropic form of oxygen. Ozone is a powerful & naturally unstable

oxidising gas. Ozone is prepared by reformation of oxygen molecule s using high voltage

, high frequency electric field. Ozone enriched gas is then bubbled through cooling water& the residual is maintained at 0.01 to 0.1 ppm. It is affected by temperature , pH ,

organics , contamination etc.

Q.6.7 What are the different types of Non Oxidising Biocides used in Cooling Water

Systems ?

Different types of Non Oxidising Biocides used in Cooling Water Systems are as follows:

A) Chlorinated Phenols

They were most commonly used non oxidising biocid. They control most organisms

except aerobic slime forming bacteria. Sodium PentaChloroPhenate ( SPCP) was the

most widely used . However due to toxicity to aquatic life the use of SPCP is dropping.

B) Quaternary Ammonium Compounds ( QUATs)

Quats are cationic surface active quaternary nitrogen compounds.They are effectiveagainst algae & bacteria especially at neutral & alkaline pH range. Their mode of activity

is attributed to cationic charge which forms an electrostatic bond with the negatively

charged microoragnism cell wall. They cause cel death through protein denaturation bydistorting the permeability of the cell wall.

The activity of quats is reduced by high chloride concentration , organic contamination,

heavily fouled system with dirt & debris. Overfeed of Quats can cause foaming problem.

Quats can react with anionic dispersants to cancel the effectiveness of both.

C) Organic Sulfur Compounds

The most commonly used Organo sulfur based non oxidising biocide is MBT ieMethylene Bis Thiocyanate. It is effective against Sulfate Reducing Bacteria.MBT

hydrolyses rapidly at pH above 8.

Besides MBT , various carbamates are also widely used as Non Oxidising Biocides.


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D) Glutaraldehyde

Biocide formulations based on Glutaraldehyde are gaining more acceptance. They are

effective against trouble some bacteria. They have however limited effectiveness againstalgae & fungi.

E) DichloropheneIt is a non oxidising phenolic based biocide effective against bacteria & algae. It is more

effective under alkaline pH Range.

F) Dodecyl Guanidine Hydrochloride

It is a broad spectrum non oxidising biocide. It functions like a cationic surfactant by

disruption of extra cellular enzyme reactions. High dosages of this biocide can lead to

foam problem.

G) Isothiazolines

Blends of two or more Isothiazoline compounds are used as a broad spectrum biocide.

They are particularly effective against bacteria & are active over a wide pH range. Theyare effective at low use concentration.

H) Organobromine Compounds

Compounds such as Dibromo nitrilopropionamide (DBNPA) , Bromonitro propanediol

(BNPD) are effective broad spectrum , organobromine based Non oxidising biocides.They are effective in controlling bacterial growth. They are used in systems with

relatively high levels of biomass & organic contaminants.

Q.6.8 What is Biodipersant ?

Biodispersants are the chemicals whih hav little or no biocidal activity. They loosen

microbiological deposits which can then be flushed away. They prevent microorganismattachment or reomve attached biofilm. They also expose new layers of microbial slime

or algae to the attack of biocide thereby enhancing the effectiveness of the biocide. The

biodispersants also act as Deposit Penetrants . They are dosed either continuously or asneeded before or during biocide addition. Organic accumulations caused by oil leakages

& greases are penetrated & dispersed by them. They also fluidise deposit forming silts &

clay which can foul heat tranfer surfaces & restrict flow.

Q.6.9 What are the acceptable norms for Microbiological Control in recirculating

water systems ?

The acceptable level of organisms present in the recirculating water systems will vary

form plant to plant. The following data indicates particular values that provide a basis forassessing the effectiveness of a microbiological control program.

i) Total Bacterial Count (TBC) : Not more than 500000 organisms per ml

ii) Sulfate Reducing Bacteria : 100 organisms per 100 ml


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Q.6.10 How do we select a Biocide ?

The selection of a Biocide is based on number of variables which include the type of

microbial growth, water chemistry, system design , & retention time, compatibilty with

other products , cost considerations, safety in handling & government regulations.Following points should be kept in mind while selecting a biocide program :

i) Chlorine & Chlorine releasing biocide should be avoided in systems with high chlorine

demand.ii) Based on the retention time , biocide should be selected eg for systems with low

retention time, a fast reacting biocide should be selected.

iii) The compatibility of biocide with other chemicals added in the system should be

known prior to dosing of biocide.iv) The past plant history should be known before biocide selection eg if certain species

have developed immunity towards a specific biocide, it should be known before.

v) Local restrictions governing discharge of the blow down water form cooling system

must be considered.

Q.6.11 What should be the dosage of Biocide & what should be the frequency of

Biocide Dosing ?

Since variety of microorganisms are flourishing at different locations in the recirculatorycooling water system , it is practically impossible to design a universal biocide program.

Higher dosages are recommended for heavily biofouled systems. Initially a high dose of

biocide is added to control existing microorganisms. After biocide addition it is desirable

to shut off blow down & allow a retention time of minimum 4 8 hours & thereafter giveheavy blowdown.

Once the system is brought under control , it is necessary to inhibit further growth &proliferation of microorganisms. This subsequent dose is usually lower than the initial

dose. The frequency of dosage depends upon the retention time within the system,

temperature, sunlight, process contamination etc. As microorganisms are prone todevelop immunity towards biocide, it is desirable to recommend two or more biocides to

be dosed alternately.

Biocides are usually slug fed to a system. This gives a rapid & effective reduction in thenumber of microorganisms. Typically two biocides are dosed to a system alternately.

This avoids the problem of immunity developed by the microorganisms. Biocide dose is

based on the system hold up .

Chlorine is injected into the system using chlorination equipment . Chlorine can be dosed

continuously or intermittently depending upon the requirement.


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CHAPTER 4 : SCALING

Q.4.1 What is a Scaling ?

Scaling is the formation of insoluble particles in the bulk water & then the subsequent

adherence of these particles to metal surface. The formation of these insoluble particles

takes place only when the solubility of the salt in the cooling water is exceeded.

Q.4.2 What are the factors affecting Scaling ?

The factors affecting scaling are as follows:

A) Water Temperature

Many Scale Forming salts exhibit Inverse Temperature Solubility ie the solubility of

these species decreases with an increase in temperature. eg Calcium Carbonate , calcium

Phosphate . Generally these salts precipitate at high skin temperature area

B) System pH or Alkalinity

The scaling potential of scales such as Calcium Carbonate , Calcium Phosphate, Zinc

Phosphate , Zinc Hydroxide etc increases with increase in pH . Low pH accelerates

corrosion potential thereby providing nucleation sites for scaling. Also low pH increasespotential for Silica scaling.

C) Water Flow VelocityAs the water flow rate increases the scaling rate decreases. As a thumb rule , the scaling

rate at a flow rate of 0.6 m/sec is @ 1/5 th of that at a flow rate of 0.2 m/sec.

Low flow velocity can allow time for nucleation for scale formation, crystal growth &also will lead to a formation of dense , adherent scales.

D)Amount of scale forming salts present

A number of mineral salts may lead to deposition of insoluble products in cooling watersystems . The principal anions are Bicarbonates, Carbonates, Hydroxides, Phosphates,

Sulphates & Silicates. The principal cations are Calcium, Magnesium, Aluminium, Iron,

Zinc .Certain ions form a soluble ion pairs that decrease the deposition potential of scale

forming salts. eg Mg will form Magnesium sulfate thereby decreasing the scaling

potential of calcium sulphate.

6. Influence of Dissolved Solids & Suspended Solids

High Dissolved Solids caused due to presence of highly soluble salts can extend the

solubility of some scale forming salts eg Calcium Sulphate in presence of high Sodium .High Suspended Solids can provide nucleation sites there by increasing scale potential.


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Q.4.3 What are the different types of Scales commonly encountered in Cooling

Water Systems ?

Common scales encountered in Cooling Water Systems are as follows :

A) Calcium Carbonate

Calcium Carbonate is the most common component of scale found in cooling water

systems especially in hard water . It has an inverse solubility ie as the temperatureincreases the solubility decreases. It is formed by the decomposition of Calcium

bicarbonate which is present in water .

B) Calcium Sulphate

Calcium Sulphate is relatively more soluble than Calcium carbonate. It will pose a

problem if its solubility limits are exceeded. Use of sulphuric acid for pH Control will

increase the sulphate ions in cooling water systems.

C) Calcium Phosphate

Calcium Phosphate also exhibits inverse solubility like calcium carbonate. The

combination of calcium , Orthophosphates, high pH & high temperature is conducive forthe precipitation of calcium phosphate. It precipitates as amorphous form , tricalcium

phosphate , octacalcium phosphate & hydroxyapatite.

D) Calcium Fluoride

Calcium Fluoirde scales are observed in systems using municipal waste water as a makeup water . These scales are tightly adherent & are difficult to remove with chemical

treatment. These scales are also found in gas scubbers in smelting & coking operations

where Feldspar is used as a raw material on the process side.

E) Silicates

Silicate Scales are very hard , tenacious & difficult to remove. Higher operating cycles &

Increased use of reclaimed waste water accounts to high silica levels. Magnesium silicatescale precipitation is influenced by pH , Mg & Silica levels. To prevent magnesium

Silicate deposition, the a mgnesium & silica concentrations are controlled . The solubility

product of Magnesium as calcium carbonate & Silica as Silica should not exceed 35000limit.

F) Silica

Pure Silica scales are usually not found in cooling water systems.Silica scales can beprevented by limiting the silica concentration in circulating water systems upto 160 ppm.

The solubility of silica increases with pH. & also temperature.

G) Iron Salts

Iron in the ferrous form is soluble. This soluble iron is present in many make up waters.

This soluble iron gets converted into insoluble ferric form by either aeration or byoxidising material. & thus gets precipitated as Fe2O3 or Fe(OH)3.

Iron Phosphate is a yellowish white precipitate found when make up water contains high

levels of iron. High Temperature , Low Flow , high phosphates lead to iron phosphate

precipitation.


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H) Zinc Salts

Zinc Phosphate scales result from high operating pH ( pH > 7.5) or overfeed ofChromate zinc-phosphate or zinc phosphate chemicals. It precipitates in low flow

heat transfer areas. This scales also lead to under deposit corrosion.

Q.4.4 What are the different methods of Scale Prevention & Control ?

Scale prevention & control is achieved using following methods :

A) Pretreatment of Water

The most obvious method of scale prevention is to remove the scale forming constituents

from the make up water. This can be achieved by removal of either hardness salts &making water soft or by removing all salts & making water demineralised. The use of

Demineralised water is an effective scale prevention method however such water will be

corrosive & expensive .

B) pH Control

A typical method of scale control in cooling water systems is by use of Acid Dosing forpH Control. The solubility of scale forming salts is usually increased as pH is lowered.

The pH control can be achieved by acid dosing usually sulphuric acid. This reduces

bicarbonate hardness by chemical reaction ,forming Calcium & Magnesium Sulphate.These are more soluble than carbonates.

C) Limit the concentration of Scale forming salts

The scale forming salts in the cooling water systems can also be maintained by limitingthe cycles of concentration via blowdown. This will reduce the build up of concentration

of scale forming ions .

D) Alter System Design or Operation

The scaling rate is influenced by water flow rate, water temperature, heat flux,& skin

temperature, etc. Scale control can also be achieved by making mechanical changes in thesystem to reduce the chances of scale formation. Some of the mechanical method s of

scale control are as follows :

i)Increase in the water flow rate from 0.2 to 0.6 m/sec lowers the scaling rate to @ 1/5 th .

ii) modify the exchanger design by reducing the number of passes.iii) Air rumble on a periodic basis to dislodge & remove scales . Loosely adherent scales

will get easily removed.

iv) Use of sponge balls for on line cleaningv) Change the system metallurgy of the heat transfer surface eg Mild steel heat transfer

surfaces will scale under conditions where copper & other alloys will not.

vi) Reduce the heat flux by reducing the process load This will reduce the metaltemperature & scaling potential.

E) Use of Scale Control / Scale Inhibiting Chemicals


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Q.4.5. Explain various terms associated with Scale Prevention & Control ?

Various terms associated with Scale Prevention & Control are as follows :

a) Sequestering Agents ( Or Chelating Agents)

They react with scale forming ions to form water soluble complex . This complex viz

Chelate prevents the precipitation of scale containing sequestered or chelated ions.Polyphosphates & anionic polymeric dispersants are used as sequestering agenst. They

sequester ions such as Ca2+, Fe2+, Mn2+

Other chelating agents eg EDTA, NTA also react with ions to form soluble complex orchelate .

b) Scale Conditioners

Scale Conditioners chemically modify the crystal structureof a scale . They are includedin the crystal lattice to form bulky, loosely adherent deposits instead of hard scales.

Typical examples are Lignins, Tannins, Polycarylates.

c) Threshold InhibitorsThese chemicals inhibit the precipitation of scale forming ins at dosages fat below the

stoichiometric quantities required for sequestrationor chelation. They function byadsorption mechanism. They delay or retard the rate of precipitation . They cause crystal

lattice distortion so that scales formed are loose & can be easily removed from the

system by blowdown.Typical Threshold Inhibitors are Low Molecular Polyacrylates , Organophosphonates

such as Hydroxy Ethylidine Di Phosphonic Acid (HEDP), Amino Tri Methylene

Phosphonic Acid(ATMP) , 2 Phosphono Butane-1, 2,4 TriCarboxylic Acid (PBTC)

d) Crystal Modifiers

The Crystal Modifier distorts the crystal lattice structure of scale so that the scales are

not adherent . It is also a form of Threshold Inhibitor, which is used at asubstoichiometric amount .Typical examples are organophophonates, polymeric

dispersants based on polyacrylates.

faq_cwc.doc

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