Cooling Water Treat

8/13/2019 Cooling Water Treat

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COOLING WATER SYSTEM CHEMICAL TREATMENT

Water is used in cooling systems as a heat transfer medium and frequently also as the final

point to reject heat into the atmosphere by evaporating inside cooling towers. Depending on

the quality of available fresh water supply, waterside problems develop in cooling water

systems from:

Scaling

Corrosion

Dirt and dust accumulation

Biological growth

Any of these problems or more usually a combination of them result in costly unscheduled

downtime, reduced capacity, increased water usage, high operation and maintenance costs,

expensive parts replacements, and acid cleaning operations which reduce the life of the

cooling system.

Selection of water treatment program for a specific system depends on:

1. System design, including system capacity, cooling tower type, basin depth, materials

of construction, flow rates, heat transfer rates, temperature drop and associated

accessories.

2. Water, including make up water composition / quality, availability of pre-treatment

and assumed cycle of concentration.

3. Contaminants, including process leaks and airborne debris.

4. Wastewater discharge restrictions.

5. Surrounding environment and air quality.

The critical parameters for cooling water are: conductivity, total dissolved solids (TDS),

hardness, pH, alkalinity and saturation index.


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Corrosion Problems

Factors

Many factors affect the corrosion rates in a given cooling water system. Few important

factors are:

1. Dissolved Oxygen - Oxygen dissolved in water is essential for the cathodic reaction to

take place.

2. Alkalinity & Acidity - Low alkalinity waters have little pH buffering capability.

Consequently, this type of water can pick up acidic gases from the air and can

dissolve metal and the protective oxide film on metal surfaces. More alkaline water

favours the formation of the protective oxide layer.

3. Total Dissolved Solids - Water containing a high concentration of total dissolved

solids has a high conductivity, which provides a considerable potential for galvanic

attack. Dissolved chlorides and sulphates a particularly corrosive.

4. Microbial Growth - Deposition of matter, either organic or inorganic, can cause

differential aeration pitting (particularly of austenitic stainless steel) and

erosion/corrosion of some alloys because of increased local turbulence. Microbial

growths promote the formation of corrosion cells in addition; the byproducts of some

organisms, such as hydrogen sulphide from anaerobic corrosive bacteria are corrosive.

5. Water Velocity - High velocity water increases corrosion by transporting oxygen to

the metal and carrying away the products of corrosion at a faster rate. When watervelocity is low, deposition of suspended solids can establish localized corrosion cells,

thereby increasing corrosion rates.

6. Temperature - Every 25-30F increase in temperature causes corrosion rates to

double. Above 160F, additional temperature increases have relatively little effect on

corrosion rates in cooling water system.


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Some contaminants, such as hydrogen sulphide and ammonia, can produce corrosive waters

even when total hardness and alkalinity are relatively high.

Treatment Methods

Most corrosion control strategies involve coating the metal with thin films to prevent free

oxygen and water from coming into close contact with the metal surface. This breaks the

reaction cell, and reduces the corrosion rates. Several major chemical treatment methods can

be used to minimize corrosion problems and to assure efficient and reliable operation of

cooling water systems.

1. Anodic inhibitor

2. Cathodic inhibitor

3. Mixed inhibitor

Corrosion Inhibitors

A corrosion inhibitor is any substance which effectively decreases the corrosion rate when

added to an environment. An inhibitor can be identified most accurately in relation to its

function: removal of the corrosive substance, passivation, precipitation, or adsorption.

Deaeration (mechanical or chemical) removes the corrosive substance-oxygen.

Passivating (anodic) inhibitors form a protective oxide film on the metal surface. They are

the best inhibitors because they can be used in economical concentrations, and their

protective films are tenacious and tend to be rapidly repaired if damaged.

Precipitating (cathodic) inhibitors are simply chemicals which form insoluble precipitates

that can coat and protect the surface. Precipitated films are not as tenacious as passive

films and take longer to repair after a system upset.


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Scaling Problems

Factors

Typical scales that occur in cooling water systems are:

1. Calcium carbonate scale - Results primarily from localized heating of water

containing calcium bicarbonate. Calcium carbonate scale formation can be controlled

by pH adjustment and is frequently coupled with the judicious use of scale inhibiting

chemicals.

2. Calcium sulfate scale - Usually forms as gypsum is more than 100 times as soluble as

calcium carbonate at normal cooling water temperatures. It can usually be avoided by

appropriate blowdown rates or chemical treatment.

3. Calcium and magnesium silicate scale - Both can form in cooling water systems. This

scale formation can normally be avoided by limiting calcium, magnesium, and silica

concentrations through chemical treatment or blowdown.

4. Calcium phosphate scale - Results from a reaction between calcium salts and

orthophosphate, which may be introduced into the system via inadequately treated

wastewater or inadvertent reversion of polyphosphate inhibitors present in recycled

water.

The most common type of scaling is formed by carbonates and bicarbonates of calcium and

magnesium, as well as iron salts in water. Calcium dominates in fresh water while

magnesium dominates in seawater.


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Control

Scale can be controlled or eliminated by application of one or more proven techniques:

1. Water softening equipmentWater softener, dealkalizer, ion exchange to remove

scale forming minerals from make up water.

2. Adjusting pH to lower values - Scale forming potential is minimized in acidic

environment i.e. lower pH.

3. Controlling cycles of concentration - Limit the concentration of scale forming

minerals by controlling cycles of concentration. This is achieved by intermittent or

continuous blowdown process, where a part of water is purposely drained off to

prevent minerals built up.

4. Chemical dosage - Apply scale inhibitors and conditioners in circulating water.

5. Physical water treatment methodsFiltration, magnetic and de-scaling devices

Control of scale with pH adjustmentby acid addition is a simple and cost effective way to

reduce the scaling potential. It functions via chemical conversion of the scale forming

materials to more soluble forms - calcium carbonate is converted to calcium sulfate (using

sulfuric acid), a material several times more soluble.

Sulfuric acid (H2SO4) and hydrochloric acid (HCl) are the most common additives used for

controlling the formation of calcium carbonate scale. The reaction of the acid with calcium

bicarbonate is:

H2SO4 + Ca (HCO3)2 = CaSO4 + 2H2O + 2CO2

2HCl + Ca (HCO3)2 = CaCl2 + 2H2O + 2CO2


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Microbiological Growth Problems

Possible types of micro-organisms that

exist in cooling water Micro-organisms

Impact on cooling tower system

Algae

Provide a nutrient source for

bacterial growth

Deposit on surface contributes to

localized corrosion process

Loosened deposits can block and

foul pipe work and other heat

exchange surfaces

Fungi

Proliferate to high number and

foul heat exchanger surfaces

Bacteria

Some types of pathogenic

bacteria such as Legionella may

cause health hazards

Sulphate reducing bacteria can

reduce sulphate to corrosive

hydrogen sulphide

Cathodic depolarization by

removal of hydrogen from the

cathodic portion of corrosion cell

Bacteria - A wide variety of bacteria can colonize cooling systems. Spherical, rod-shaped,

spiral, and filamentous forms are common. Some produce spores to survive adverse

environmental conditions such as dry periods or high temperatures. Both aerobic bacteria


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(which thrive in oxygenated waters) and anaerobic bacteria (which are inhibited or killed by

oxygen) can be found in cooling systems.

Fungi - Two forms of fungi commonly encountered are molds (filamentous forms) and

yeasts (unicellular forms). Molds can be quite troublesome, causing white rot or brown rot of

the cooling tower wood, depending on whether they are cellulolytic (attack cellulose) or

lignin degrading. Yeasts are also cellulolytic. They can produce slime in abundant amounts

and preferentially colonize wood surfaces.

Algae- Algae are photosynthetic organisms. Green and blue-green algae are very common in

cooling systems. Various types of algae can be responsible for green growths which block

screens and distribution decks. Severe algae fouling can ultimately lead to unbalanced water

flow and reduced cooling tower efficiency. Diatoms (algae enclosed by a silicaceous cell

wall) may also be present but generally do not play a significant role in cooling system

problems.

Treatment Methods

Chemical biocides are the most common products to control the growth of micro-organisms.

Three general classes of chemicals are used in microbial control.

1) Oxidizing biocides

2) Non-oxidizing biocides

3) Bio-dispersants.

Oxidizing Biocide

Oxidizing biocides are powerful chemical oxidants, which kill virtually all micro-organisms,

including bacteria, algae, fungi and yeasts. Common oxidizers are chlorine, chlorine dioxide,

and bromine, ozone, and organo-chlorine slow release compounds. Ozone is now days widely

used to curb microbial growth. The most effective use of oxidizing biocides is to maintain a

constant level of residual in the system. Oxidizing biocides are usually maintained at a continuous

level in the system.


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Since oxidizing biocide may sometimes be corrosive, corrosion inhibitors shall be added and

selected to ensure compatibility.

Non-oxidizing Biocide

Non-oxidizing biocides are organic compounds, which kill micro-organism by targeting

specific element of the cell structure or its metabolic or reproductive process. Non-oxidizing

biocides are not consumed as fast as the oxidizing types and remain in the system for a

significant period of time until they pass out with the blowdown. They often have the added

advantage of breaking down into harmless, nontoxic chemicals after accomplishing their

bacteria-killing purpose. They are effective where chlorine may not be adequate.

Chlorine

Chlorine is the most widely adopted biocide for large circulating water systems. It provides a

residual biocide in the treated water and can be readily checked. Its effectiveness increases

when it is used with other non-oxidizing biocides and biological dispersants. Chlorine can be

dosed in the form of sodium hypochlorite. A mixture of hypochlorous acid (HOCl),

hypochlorite ion (OCl), and chloride ion (Cl) is formed when hypochlorite is addedto water.


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COOLING WATER TREATMENT IN F&N DAIRIES

PRODUCT NAME NAME OF CHEMICAL (ACTIVE

INGREDIENT)DIANODIC DN 2101 Potassium Hydroxide

NICLON 70T Calcium Hypochlorite

CORRSHIELD NT 4201 Sodium Hydroxide

SPECTRUS NX 1100 Biocide

GENGARD GN 7002 Polymer Solution

CWT 3366 Phosphate

CWT 3050 Organic amines

CWT 3060 Halogen oxidizer

DIANODIC DN 2101

It is a corrosion inhibitor. It is an aqueous alkaline solution of inorganic salt, phosphonate,

polymer and organic heterocyclic compound. Used in non-chromate cooling water treatment

technology. It is a two component treatment program featuring a truly effective calciumphosphate inhibitor that permits higher phosphate concentrations necessary for proper mild

steel corrosion protection.

The first component is the corrosion inhibitor-a blend of orthophosphate and polyphosphate

for ferrous metal protection, phosphonate for calcium carbonate prevention

and a copper corrosion inhibitor. The second component is the calcium orthophosphate

inhibitor/dispersant.

NICLON 70T

Used for disinfection purpose. It is calcium hypochlorite granular. Disinfection and

sanitization are achieved by dissolving calcium hypochlorite in water to produce a chlorine

solution. Several grades, known collectively as Niclon-70, are typically used for sterilizing


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and disinfecting swimming pools and drinking water, and is also used in sewage treatment

systems.

CORRSHIELD NT 4201

It is a corrosion inhibitor. It is an aqueous alkaline solution of inorganic salts. CorrShield

NT4201 cooling water treatment provides superior corrosion inhibition to multi-metal

systems operating at high or low temperatures. It is normally used for diesel and other

internal combustion engines, hot water heating systems, chilled water circuits and other

closed systems containing ferrous and non-ferrous metals.

SPECTRUS NX 1100

Spectrus NX1100 is a proprietary blend of antimicrobial agents, specifically formulated for

industrial water applications. It may be used in open or closed recirculating cooling water

systems. Spectrus NX1100 is also approved for use in auxiliary water systems as well as

wastewater and waste material disposal applications. Spectrus NX1100 is water-based to

minimize impact on the environment. It contains no metal-based stabilizers, such as copper or

iron.

Spectrus NX1100 has been formulated in a waterbased solvent system, eliminating BOD and

COD associated with solvents, such as glycol and oils. A water-based formulation is also

safer to store and handle than a solvent-based formulation.

GENGARD GN 7002

It is a deposit control agent. It is used for most advanced and effective water treatment

technology for open re-circulating cooling systems. It can be applied across the entire pH

spectrum from neutral to alkaline and ensure. GenGard GN7000 series of products are

designed for cooling systems operating in the near neutral pH 6.8 - 7.8 range. They utilize

high levels of inorganic phosphate to promote the formation of a thin, protective iron oxide

film on steel surfaces. This protective oxide film is extremely tenacious and does not interfere

with heat transfer.

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Cooling Water Treat