water chemistry

Power Plant Chemistry

Internal Water Treatment

Prepared By : -

Umar FarooqSenior Chemist NOMACMSC ( Chemistry ) MBA ( Marketing )[email protected] SIWEP Shuaibah Jeddah Saudi Arabia 1

Mineral Analysis

Deposit Analysis

Microbiological Analysis

mailto:[email protected]

Power Plant Chemistry

Part 1 Basic Chemistry Water Chemistry , Type of Hardness Internal Water Treatment Tri Sodium Phosphate , Oxygen Scavenger Neutralizing Amine , Filming Amine Part 2 Cooling Water System The Cooling Tower Fundamental of cooling by Evaporation Problem in cooling water system Corrosion and its types

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At the end of the respective training course, the participants will be able to:

• Now the foundations of Water Quality Control to avoid the scale corrosion and biological growth in the Power plant system, and to operate the mentioned at max performance.

• Understand the troubleshooting events to the plant chemistry system.

Vision • Water Quality Control will lead all to understand the limitations and

international standards as well as increasing the plant availability..• Operational process will maintain highly performance due to plant

chemistry troubleshooting.

Umar Farooq Senior Chemist NOMAC SIWEP Shuaibah

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Main Objective

Basic Chemistry Chemistry Branch of science which deals with properties

composition and structure of mater.

Matter Any species having weight or mass and occupies space is called matter. The mater exist in three state ,solids ,liquid & gases

Substance Any matter which has uniform composition through out.eg: gold , silver ,water is called substance.

Atom The smallest particle of the element which can not exist in

free state but can take part in chemical reaction. Molecule Tow or more than two atoms that combine chemically

together to form a molecule. Element Pure substance in which all the atoms are same alike

e.g.: Iron , Cupper , Silver , Sodium . It cannot be decomposed by the chemical action to a simpler substance.


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Basic Chemistry

Ion Charge particle are known as ions

Cat ion Ions which move to cathode

An ion Ions which move to anode

Cathode Electrode having –ve Charge

Anode Electrode having +ve Charge

Electrolysis ( Lyses = Analysis ) Electrolysis is the motion of cat ion and anion to words respective electrodes. It is accompanied by all chemical changes under the influence of electrical field in aqueous solution

Hydrolysis Decomposition of water into H+ and OH- Ions


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Basic Chemistry

Mixture It is simply the combination of two or more than two elements which retain their properties

Compound Chemical combination of two or more elements combine together chemical reaction takes place is called compound

Valences The combining power of one element to another element is called valences.

Atomic weight Total number of proton and neutron in an atom is called atomic weight

Atomic Number The number of proton which are present in

nucleus of an atom is called atomic number.


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Fundamental of Heat Heat It is common form of energy .Heat added to a body makes it

hotter removing heat cools it . Heat energy can be turned into mechanical energy to do work.

Temperature It describes the degree of hotness or coldness of body .It has two units centigrade and Fahrenheit scale. Conversion Formula C/5=F-32/9

Calorie It is the amount of heat required to raise the temperature of 1 g of H2O through 1C*

B.T.U It is the amount of heat required to raise the temperature of 1Lb of H2O through 1F*.

C.H.U It is standard abbreviation of centigrade heat unit.

Sensible Heat: The heat shown by the thermometer or the heat which sense the temperature.


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Fundamental of Heat

Latent Heat: The heat added or taken into produce a change in state with out any change in temperature.

Latent heat of evaporation: The amount of heat which is added for the evaporation of liquid is called latent heat of evaporation

Latent heat of vaporization: The amount of heat required to vaporize 1Lb of water to steam at 14.7psi at 212C* is termed as latent heat of vaporization.

Latent heat of fusion: It is the amount of heat required to melt 1g of ice into water at 0C* or 32 F*.

Latent heat of condensation: The amount of heat taken to condense 1 Lb of steam into water at 14.7psi and 212 F*


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Heat Transfer ( Conduction )Heat always flows from a warm area to cool area .i.e. from hot body to cold

body This is accomplished in three ways.

Conduction: In this method heat energy is conducted through a substance or metal or transferred from one place to another place or from molecule to molecule . The amount of heat that will pass through a given material depends on conductivity of material.


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Heat Transfer ( Convection )

Convection: In this method heat energy is transmitted from hot to cold body by movement of conveying subs: (medium).In side boiler ,furnace the conveying substance is the gases of combustion . In this case the hot body is flue gases the cold body is boiler tubes, air pre heater or any things else having a lower temperature in the flue gas path.


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Heat Transfer ( Radiation )

Radiation In this method heat energy is transfer from one place to another place with out the help of any medium. Heat is transferred directly by heater radiation that require no intermedium.eg heat of sun rating to the earth.


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Heat Transfer Saturated ( wet ) Steam

Steam containing maximum water vapors that it can steam is said to be saturated. When it temperature is the same as that of boiling water at pressure e.g.. At 14.7psi water boiled at 212F* and the saturated steam temperature is 212 F*.

Super heated steam ( Dry Steam )

The steam having temperature higher than its saturation temperature. The addition of heat to saturated steam increase its temperature or sensible heat. The increase in sensible heat above saturation temperature is called super heated steam.

If saturated steam at 1500psi and 569.23F* is heated to 950F* with pressure remaining constant the amount of super heated is 950-596.23=353.77F◦

Un like saturated steam a loss of heat from superheated steam result is reaction in temperature.


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Why Water is Unique

Water is only substance that exist in form of solid , liquid and steam

Specific heat = 1calorie/gram It expand = 1600 time Three Isotopes = H2O , D2O , T2O Heat of fusion = 144Btu / Lbs Heat of vaporization = 980 Btu / Lbs Freezing Expand = 1/9 Depending upon pressure ,its boil with in the

temperature = 35-704F*


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Properties of Water

It is chemical compound expressed by the formula H2O. It is formed by two item of hydrogen and one atom of

oxygen Due to different electro negativities of hydrogen and

oxygen.H20 Molecule is electrically charged . When the other molecule combine with it then will be

formed hydrogen bonding Water is the best solvent .

It dissolved different substance

In it and the process of dissolving

Is desolation Model of hydrogen bonds

Between modules of water


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Types of Hardness

Temporary Hardness ( Alkaline Hardness ) It is due to presence of bicarbonates of calcium

and magnesium in water , also called carbonate hardness. It can be removed by boiling and pretreatment process

Permanent Hardness(Non Alkaline Hardness ) It is due to presence of chlorides , sulphate and

nitrate of calcium and magnesium , it can be remove by ion exchange and desalination process Umar Farooq Senior Chemist

NOMAC SIWEP Shuaibah Jeddah 16

The term internal water treatment describe the reaction induced with in the boiler system to render the water less harmful to system operation .

The main features of internal water treatment are summarized on the above slide.


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Internal Water Treatment

Internal Water TreatmentPrevention scale by 1) Ether precipitating chelating the hardness in water

2) Maintaining the sufficient alkalinity for appropriate chemical reaction.

3) Sludge conditioning to keep the suspended particle in non adhering state.

4) Maintaining total dissolve solid with in limit.

Prevention of Corrosion.

In Boiler by

5) Scavenging oxygen formulation of protective film .

6) Maintaining sufficient alkalinity neutralizing CO2 formulation of protective film.


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Phosphate Treatment

Function of phosphate treatment is to reduce the effect of hardness producing salt and maintaining them a non adherent sludge rather than to deposited and then removing the excess sludge [Ca3 ( Po4 )2] by below down.

Various forms phosphate alkalinities are used boiler treatment listed below

Na3PO4 + Boiler drum [Ca3 ( Po4 )2] + Protective layer


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Phosphate Treatment1) Na3Po4 + H2O Na2HPo4 + NaOH

2) Na2HPo4 + H2O NaH2Po4 + NaOH

3) NaH2Po4 + H2O H3Po4 + NaOH

4) H3Po4 + H2O 3H + + Po4 +3

Tri Sodium Phosphate Na3Po4

Di sodium phosphate Na2HPo4

Mono sodium phosphate NaH2Po4

Phosphoric acid H3Po4

Hexa mete phosphate [Na3Po3]6 Umar Farooq Senior Chemist


Tri Sodium Phosphate React with Ca++ and Mg++ salt ( if present in

boiler water ) to form [Ca3(Po4)2] and [Mg3(Po4)2] which precipitate as sludge.

Na3Po4 + 3CaSo4 3NaSo4 + Ca3(Po4)2

Na3Po4 + 3MgSo4 3NaSo4 + Mg3(Po4)2


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Monocalcium phosphate, Dicalcium phosphate, Tricalcium phosphate,

http://en.wikipedia.org/wiki/Monocalcium_phosphate

http://en.wikipedia.org/wiki/Dicalcium_phosphate

http://en.wikipedia.org/wiki/Tricalcium_phosphate

Tri Sodium Phosphate

When it is required to reduce the concentration of NaOH in boiler water mono sodium phosphate and hexa meta phosphate can be used

Na2HPo4 + 2NaOH Na3Po4 + 2H2O [Na3Po3]6 + 12NaOH 6 Na3Po4 + 6H2O


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Monosodium phosphate Sodium hexametaphosphate

Tri Sodium Phosphate Excess dosing of phosphate result Mg3(Po4)2

formation of Mg content is high in boiler water . This magnesium phosphate form a sticky sludge so its formation must be avoided by maintaining proper phosphate and alkalinity ratio.

MgCl2 + 2NaOH Mg3(OH)2 + 2NaCl4 MgCl2 + Sio2 + 2NaOH MgSio3 + 2NaCl4 + H2O

Magnesium silicate also tends to change into scale if the pH of the boiler water is too low . Silica scale is very hard to remove and have high resistance to heat transfer silica ( Sio2) can be stabilized in the boiler water is about 2 time of silica concentration . There fore both the pH of boiler water and phosphate concentration must be applicable for the prevention of silica in boiler.


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Phosphate Hide Out Temporary deposition of water soluble

chemical e.g. Na3PO4 during normal boiler operation is called as “ Hideout “This phenomena is some time found in high pressure boiler . Due to change in heat flux loss of Na3PO4 accurse in boiler water however if load is reduce phosphate re dissolve and again appear although a number of substance exhibits.

This phenomena Na3PO4 is must be interest in boiler operation . At temperature between 300 to 350 C* . Tri sodium phosphate exhibits an interrogate solubility . Its solubility is reduced at high temperature and precipitates out on the boiler tube and than disappears from boiler water when boiler or pressure reduced. It is again dissolved and appear in the solution form.


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Oxygen Scavenger An oxygen scavenger is a chemical which

remove dissolve O2 in H2O by a reduction reaction and there by inhibits to corrosion cause by the O2. we discussed here following two type of oxygen scavenger

1) Sodium Sulphite Na2SO3

2) Hydrazine N2H4

Sodium Sulfite In practice I ppm of oxygen required 8ppm of

Na2SO3 . The major disadvantage in it are is that 8ppm . Na2SO3 contribute 9ppm of Na2SO3 into total dissolved solid in the boiler consequently increasing blow down

. Sodium sulphite recommended for boiler operation at pressure below 600 pisg.Umar Farooq Senior Chemist


Sodium Sulphite Disadvantage

The main disadvantage of use sodium sulphite is decomposition

Na2SO3 + H2O 2NaOH +SO2

Na2SO3 + H2O 3NaSO4 + 2NaOH +H2S

The main problem associated with sulphite break down is the formation of corrosive gases So2 and H2S which can cause corrosion in after boiler section


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Hydrazine Hydrazine ( N2H4 ) can economically remove

small amount of dissolved oxygen . In addition it acts to produce nitrogen and water thereby leaving the TDS level of water un effected.

N2H4 + O2 N2 + 2H2O

Theoretically 1ppm of hydrazine is required to react with 1ppm dissolved “ O2 “ but actually 1.5 to 2.0 ppm of N2H4 are required per 1ppm “ O2”

Hydrazine also react with Fe2O3 in the boiler water to form a passive magnitude film on the boiler surface preventing form the corrosion.

N2H4 + 6Fe2O3 4Fe3O4 + N2 + 2H2OUmar Farooq Senior Chemist


Hydrazine

Thermal decomposition of hydrazine takes place above ( 270 C° ) ( 518 F ° ) Through it influence by reaction time .

N2H4 NH3 + N2

270 c°

Since NH3 is produced by decomposing of hydrazine , copper material is used in the steam and condensate piping . The injection of excess hydrazine should be avoided.


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Treatment for Condensate line

Neutralizing Amines

Volatile or neutralizing amines are employed as corrosion inhibitors to prevent Co2 corrosion . They neutralize carbonic acid and raise condensate pH . Volatile amine are

1) Ammonia NH3

2) Cyclohexyl amine C6H11NH2

3) Morph line C4H4ONH


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Morph line may be produced by the dehydration

of di ethanolamine with sulphuric acid

Neutralizing Amines These amine are injected to

the feed water .The amino added to the feed water volatizes together with the steam generated in the boiler with the condensation of steam . The amine neutralizes Co2 to raise pH of condensate and thereby inhibits corrosion .

NH3 + H2O NH4+HCO3

C6H11NH2 + CO2+ H2O C6H11NH3+HCO3

C4H8ONH +CO2 + H2O C4H8ONH2+HCO3Umar Farooq Senior Chemist


Ammonia Ammonia is effective for

neutralizing CO2 , but it cannot be

used for a low pressure boiler employing copper material in steam and condensate line. the normal method of controlling there feed is by mean of pH analysis of condensate water.

Other Name of Ammonia's

EU classification

Hydrogen Nitrite Tri Hydrogen Nitrite Nitro -Sill

Ammonia Reaction as underneath

NH3 + H2O NH4OHNH4OH +H2CO3 (NH4)2CO3+2 H2O


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Properties of Ammonia At room temperature, anhydrous ammonia is a colorless, highly irritating gas

with a pungent, suffocating odor. It is lighter than air and flammable, with difficulty, at high concentrations and temperatures. It is easily compressed and forms a clear, colorless liquid under pressure. Ammonia dissolves readily in water to form ammonium hydroxide - an alkaline solution. The concentration of aqueous ammonia solutions for household use is typically 5% to 10% (weight: volume), but solutions for commercial use may be 25% (weight: volume) or more, and are corrosive. Anhydrous ammonia is stored and shipped in pressurized containers, fitted with pressure-relief safety devices, and bears the label "Nonflammable Compressed Gas". Despite not meeting the Department of Transport definition of flammable, it should be treated as such.

Anhydrous ammonia reacts with moisture in the mucous membranes to produce an alkaline solution (ammonium hydroxide). Exposure to ammonia gas or ammonium hydroxide can result in corrosive injury to the mucous membranes of the eyes, lungs, and gastrointestinal tract and to the skin due to the alkaline pH and the hygroscopic nature of ammonia.


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Fritz Haber was the German scientist who developed an efficient way of producing ammonia from hydrogen and atmospheric nitrogen.

In 1909, Fritz Haber established the conditions under which nitrogen, N2(g), and hydrogen, H2(g), would combine using

medium temperature (~ 450 °C ) very high pressure (~200atm) A catalyst (Iron)

The Industrial Manufacture of Ammonia

The Haber process

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Effect of Change in Temperature

Since the forward reaction is exothermic

N2(g) + 3H2(g) 2NH3 (g) + HEAT Increasing The

temperature

Low yield of Ammonia

So, industrialist always try to keep the temperature as low as 450 °C In order to favor maximum yield of Ammonia

Note: Temperature lower than this is not feasible for this reaction, as it will take too long to attain the equilibrium

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Effect of Change in pressure

Increasing the pressure causes the equilibrium position to move to the right resulting in a higher yield of

Ammonia.

Hence the Haber process is always operated at very high pressures of about 200 at in order to get high yields of

Ammonia

N2(g) + 3H2(g) 2NH3 (g)

Increasing The

Pressure

High yield of Ammonia Since there are

more gas molecules on the left hand side of the equation (4 in total) than there are

on the right hand side of the equation

(2).

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Uses Of Ammonia

Agricultural industries are the major users of ammonia

Ammonia is a very valuable source of nitrogen that is essential for plant growth.

Approximately 75% of all AMMONIA produced is converted into various ammonium compounds like Ammonium sulphate, Ammonium Nitrate and urea. These compounds are called Nitrogenous Fertilizers

Ammonia is also useful in the production of nitric acid. mixture of ammonia and air is passed over a platinum gauze catalyst at 850 °C, whereupon the ammonia is oxidized to nitric oxide.

4 NH3 + 5 O2 4 NO + 6 H2O

2 NO + O2 2NO2

The nitric oxide mixed with excess air is then allowed to react with water to form Nitric acid. 4 NO2 + O2 + 2H2O 4HNO3

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http://www.rmtech.net/Chemical%20Plants.htm

http://en.wikipedia.org/wiki/Nitric_acid

http://en.wikipedia.org/wiki/Nitric_acid

http://en.wikipedia.org/wiki/Platinum

http://en.wikipedia.org/wiki/Catalyst

http://en.wikipedia.org/wiki/Nitric_oxide

Uses of Ammonia

Nitric acid is not only used to make important fertilizers but also Explosives like nitroglycerine or TNT (Trinitrotoluene)

Aqueous ammonia can be a great addition to a household's cleaning supplies set. It is great for eliminating stains and tarnish, and can be the ideal solution for hard-to-remove soap buildup in tubs, sinks, and bathroom tiles.


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Ammonia Poisoning Ammonia is a strong, colorless gas. If the gas is dissolved in water, it is called liquid ammonia.

Poisoning may occur if you breathe in ammonia. Poisoning may also occur if you swallow or touch products that contain very large amounts of ammonia.


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Symptoms

Airways, lungs, and chest Cough , Chest pain (severe)Chest tightness, Difficulty breathingWheezing

Eyes, ears, nose, mouth, and throat Tearing and burning of eyes, Temporary blindnessThroat pain (severe), Mouth painLip swelling

Heart and blood Rapid, weak pulseCollapse and shock

Nervous system Altered mental stateFeverRestlessness

Skin Bluish-colored lips and fingernailsSevere burns if contact is longer than a few minutes

Stomach and gastrointestinal tract Severe stomach painVomiting

Route of Exposure

Inhalation” of ammonia may cause nasopharyngeal and tracheal burns, bronchiolar and alveolar edema, and airway destruction resulting in respiratory distress or failure. Ammonia's odor threshold is sufficiently low to acutely provide adequate warning of its presence (odor threshold = 5 ppm; OSHA PEL = 50 ppm). However, ammonia causes olfactory fatigue or adaptation, making its presence difficult to detect when exposure is prolonged. Anhydrous ammonia is lighter than air and will therefore rise (will not settle in low-lying areas); however, vapors from liquefied gas are initially heavier than air and may spread along the ground


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Route of Exposure Skin/Eye Contact - the extent of

injury produced by exposure to ammonia depends on the duration of the exposure and the concentration of the gas or liquid. Even low airborne concentrations (100 ppm) of ammonia may produce rapid eye and nose irritation. Higher concentrations may cause severe eye injury. Contact with concentrated ammonia solutions, such as some industrial cleaners (25%), may cause serious corrosive injury, including skin burns, permanent eye damage, or blindness. The full extent of damage to the eyes may not be clear until up to 1 week after the injury is sustained. Contact with liquefied ammonia can cause frostbite injury.


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Route of Exposure

Ingestion - ingestion of ammonium hydroxide, while uncommon, results in corrosive damage to the mouth, throat, and stomach. Ingestion of ammonia does not normally result in systemic poisoning.


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Filming Amines

These represent an effective economical approach to condensate system corrosion control.

The purpose of filming amines in the formation of an adherent mono molecular film providing protection from O2 and Co2 corrosion . First the film is build up then only the amount of amines is required to maintain the film .


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Typical Filming Amines1) Octa decyl amine CH3(CH2)16 CH2NH2

2) Hexa decyle amine CH3(CH2)14CH2CH2

3) Di Octa decyle amine CH3 (CH2)16(CH2)2NH2

The general formula for filming amine is R-NH2 where R= Alkyl group ( C10 – C12 ) the corrosion inhibits . The effect of filming amines on the steel material increases proportionally with the increased number of carbon atoms.

The amines when feed in the boiler , also volatize with steam and condense to form an organic film in condensate line .Umar Farooq Senior Chemist


Short Brake


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Cooling water system The term cooling water is applied where water is circulated through

an equipment to absorb and carry away heat. Cooling water system divided into two parts.

1. Primary Cooling Water System

2. Secondary Cooling Water System . Primary Cooling Water System

a) Once through type

b) Open re circulating system Once through type.

In once through type system water is passed in heat exchanger equipment and cooling water is then discharged to waste is an open cycle . This type of cooling system is adopted only when cold fresh water available in abundance and also at a low cost . The source of once through cooling water may be revere and cannel water or sea water.


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Once through type. The cost involved for providing cooling water is that of

pumping only since there is no chance of evaporation . The concentration dissolved solid in outlet and inlet remains practically the same.

Deposits in the form of scale and corrosion products may develop in line and heat exchanger equipment due to heat absorbed by cooling water and initial low pH of cooling .because of the large volume of water used in once through cooling system , prior softening of cooling water is not required.

The most common method to inhibit scale is the use of an ant nucleating agent ( prevent crystal growth and increase solubility of hardness producing salts in water ) such as ploy phosphates, tannins' , lignin's , starches and poly acryl ate. The use of poly phosphates also prevents corrosion of the lines and heat exchanging equipment and thus minimizing deposit due to corrosion. Umar Farooq Senior Chemist


Open re circulating system The use of recirculating system in which cooling

tower, spray pond or cooling pond serve to remove heat, has been very economical with respect to water requirement.

In place where fresh cold water is short recirculating system in the only method for cooling . After circulation through the heat exchanger equipment , water is cooled in cooling tower. This cooling effect is produced by evaporation of a portion of circulating water because concentrated.

The evaporated must be reproduced by make up water. The circulating water becomes more concentrated than the make up water due to evaporation loss. Cycle of concentration is the term employed to indicate degree of concentration of the circulating water as completed to make up water.

For example two cycle of concentration indicates the concentration of circulating water is twice of the concentration of make up water . Some water of the cooling tower is also lost due to wind age or drift loss. Which is the loss of fine droplets of water carried away by circulating air. the amount of the wind age loss depends upon design of the cooling towers. The typical wind age loss based on the rate of circulation. Umar Farooq Senior Chemist


Secondary cooling water system

Closed cooling water system. This system also known as “Secondary cooling water system” It is used for

the cooling of a number of power station equipments and its self warmed. The warmed cooling water is passed through exchanger , where it gives its heat to primary cooling water which is enter cooled in cooling tower or derived to waste. Since it is closed system, there is no loss of cooling water due to evaporation or drainage.

The water once filled in the system can be used again and again. Make up water is required due to loss of cooling water from gland packing of pumps, which is essential for lubrication of the pumps, in closed cooling water system treated water ( soft , clarified or demineralization water ) is used as coolant. The use of soft or demineralization water eliminates any changes of cooling in the pipe line.


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Secondary cooling water system Corrosion of the pipe line is controlled by

treating the water with chromate , tennis, nitrate etc. these chromates forms a protecting coating on the metal surface and act as a barriers between metal and oxygen present in the water . The solubility of oxygen gas decreases with increases in temperature on the other hand oxygen is more corrosive at higher temperature as compared to low temperature since in a closed cooling system, There is no way for oxygen to get out , severe corrosion of metal may take place at elevated temperature.

The metal used for construction of modern engines, compressor and cooling system include cost iron, steel, copper alloys' and aluminum corrosion due to galvanic action may be takes place which is controlled by Cathodic protection.


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Important terms relating to evaporation cooling

Cooling Tower : A cooling tower is a semi enclosed device for evaporation cooling of water by contact with air both dry bulb and wet bulb temperature are very important in evaluating the performance of tower.

Cooling Range : Cooling range is the difference in temperature between hot water entering the cooling tower and cold water leaving the cooling tower.

Approach : Approach is the difference between temperature of cold water leaving cooling tower and wet bulb temperature of

surrounding area. Pumping Head :The minimum pressure required to lift water from

basin to the top of the tower and face it into water distribution system. Pumping head is equal to static head plus friction loss and pressure drop through the distribution system.


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Important terms relating to evaporation cooling

Drift : Drift is the water lost as mist or fog droplets . These droplets are entrance by the circulating air flow and discharge to atmosphere . Drift is independent of water by evaporation and can be reduced to minimum by good design.

Blow Down: It is continuous or intermittent discharge to waste small amount of circulating water. evaporation in tower increases its dissolved solid concentration blow down removes solid concentration .

Cycle of Concentration : It is equal to the ratio of chloride is circulating water to chloride in make up water.


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Fundamental of Cooling Evaporation Evaporation : A change of state from liquid to gas is called

evaporation . It can takes place at any temperature . Some molecules in liquid are escaped into gas. Phase and evaporation takes place. Because these molecule are with higher kinetic energy. Evaporation result in cooling of the liquid.

Evaporation Cooling: Cooling water is used at a plant for condensing . Steam and for products and equipment cooling. Cooling tower is the unit operation that provides cooling water recovery thus decreasing water and power requirement , water is cooled in a cooling tower by evaporation . The driving force for this phenomenon relates to the difference between the hot influence water temperature and the wet bulb of surrounding air.

Latent Heat of Evaporation : As the liquid change to vapors heat is consumed called as latent heat of vaporization . When it about 1000 btu/pond water evaporation . This heat is taken mainly from water lowers its temperature.


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Fundamental of Cooling Evaporation

Sensible Heat : It is defined as the heat that changes temperature also part of cooling picture. because when water is warmer then air , there is some tendency for air to cool the water . Air become hotter it gain sensible heat . Water is cooled as sensible heat is transferred to air.

About 75% to 95 % of the total heat is removed by evaporation ( Latent heat ) and 25% by sensible heat transfer.

Temperature Dry Bulb :Temperature of a gas or gas mixture ( air ) indicated by an accurate thermometer.

Temperature Wet Bulb : Temperature at which a liquid by evaporating into air bring the air to saturation adiabatically at some temperature.


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Fundamental of Cooling Evaporation

Relative Humidity :

Ratio of mole fraction of water vapors present in air to mole fraction of water vapors present in saturated air at the same temperature and pressure.

Relative humidity of air can be determined from a psychometric chart . When both dry wet bulb temperature are known . Drier the air. Greater will be the difference between dry and wet bulb temperature and when relative humidity of air is 100% wet bulb temperature is equal to dry bulb temperature for any lower humidity some water evaporates to cool the bulb giving a lower wet bulb temperature.


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Open Evaporation System1. Cooling Pounds.

2. Spray Ponds

3. Cooling towers.

Cooling Pounds Water to be cooled is introduced

into pound is called to cool gradually by natural evaporation , radiation , convention to a suitable temperature for re use . Since the method of cooling is slow , large amount of cooling water and consequently large ( amount ) pound are needed to meet the normal cooling demands.

Spray Pounds. These are cooling pounds in

which there is provision for spraying the warm water by means of pipe and nozzles or motor drive spry modules . The cooling rate is faster and size of pounds is smaller as compared to cooling pounds.

Cooling pond for a power plant in

Grant County, West Virginia.

Spray Pounds for waste water


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Cooling TowersNatural draft cooling tower

The water to be cooled is pumped to the tope of the tower and sprayed over the tower by mean of nozzle to increase the surface area of water. which is further increased by placing packing in the tower which breaks up the water droplets more completed and increases cooling efficiency . The amount of air entering the cooling tower is controlled by inlet air on its side walls, which also helps to prevent water losses.

As the water falls through the tower it comes in contact with the air and evaporates cooled water is collected in the basin and is pumped back to the plant for reuse.


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Mechanical Draft Cooling towersForced draft :

In which air is forced into the tower by means of a blower.

Induced draft :

In which air is pulled into the tower by means of a fan located at its top. This design further divided into counter flow and cross flow type.

Induced draft ( counter flow) cooling tower

In counter flow design air is pulled into the tower its base by the air inlet louvers and comes in contact with the flowing water from the top of tower . Drift eliminators are located at the top just before the fans to prevent wind age losses. In this design the coldest water contacts with driest air and the warmest water contact with more humid air. Umar Farooq Senior Chemist


Mechanical Draft Cooling towersInduced draft ( cross flow) cooling

tower . In cross flow design the air is

pulled along the side of the tower by the air inlet louver. So that the air is introduced perpendicularly to the flowing water. The drift eliminators' are placed in the middle of the tower, because air enters at 90° angle to water flowing as compares to 180° in counter flow design , hence here is less resistance to air flow and fans power consumption is reduced.


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Specifics Design


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Crossflow Counterflow

Ad

va

nta

ge

s Lower energy consumption Easy maintenance/de-scale More variations of water flow Reduced drift Lower operating costs

Efficient use of air Better tower performance Longer ranges Finer droplet size from spray heads

Dis

ad

va

nta

ge

s Accelerated algae growth Possible orifice clogging Larger footprint Possible icing on louvers in colder climates

Higher pump head needs Increased operating costs Difficult to clean/de-scale More piping needed High inlet velocities may suck in trash and dirt

Advantage and Disadvantage of Cross flow and counter flow cooling tower's

Cooling Towers

Dry cooling tower. In place where there is actual shortage of

water and temperature remains very low through the year, dry cooling towers have been used for condensing of steam. In this system the hot condensate from the hot well is passed through a series of air cooled spiral tube which resemble like car radiator.


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Langeler Saturation Index Major portion of condensate is pumped to

the Cooling tower . While a small portion is routed into main steam water cycle. The cooled condensate from the dry cooling tower is pumped back into the condenser in the form of s pray, where it mixes directly with steam coming from the turbine and is condensed

The saturation index I◦ is given by expression

I◦ = pH – pHs

Where pH is actual

pHs= pCa+ pAlk + C

Where pCa is log factor of the calcium hardness expressed in ppm as CaCo3


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Langeler Saturation Index

pAlk: is log factor of M.alkalinity expressed in ppm as CaCo3 .

C : is log factor of the total solid in ppm at the temperature of water

Case # 1 when I◦ is zero i.e. pH is equal to pHs , a saturation equilibrium exist and there is no scale formation and corrosion attack is minimized.

Case # 2 : when I◦ is positive i.e. , when pH is grater, than pHs , a condition of super saturation of CaCo3 exist with respect to alkalinity and total solid content at existing temperature and scale deposits.

Case # 3 : when I◦ is negative i.e. , when pH is less than pHs, the equilibrium is unbalanced in the deposited direction . Corrosion of metal will occur and scale previously formed will be dissolved.Umar Farooq Senior Chemist


Problem in Cooling water systemScale Control : Cooling water may contain carbonates , Chlorides

and sulphate of Ca and Mg as well as salts of Na and dissolved gases like CO2 and O2 . The tendency of a cooling water to form scale or cause corrosion depends upon the balance of there constituents.

Scale is formed by heating of bicarbonate hardiness or by increasing in an alkalinity .

Ca( HCO3) 2 CaCO3 + CO2 + H2O

Ca( HCO3) 2 + NaOH CaCO3 + NaCO3 + H2O

From equation # 1 it is clear that bicarbonate are converted into insoluble carbonate and CO2 and if the reaction is reversed carbonates and CO2 Combine to form soluble carbonates.


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Problem in Cooling water system

This shows that keeping the carbonates in the dissolved form will make the water less corrosive because dissolved CO2 is greatly reduced .exactly at equilibrium , various form of CO2 ( Free CO2, Carbonates and bicarbonates ) are so balanced that they cause no scale formation and corrosion .

This layer of scale acts as a protective layer against corrosion . But thick layer of scale binders in heat transfer.


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Problem in Cooling water systemCorrosion Control:

The contact air with water passing over a cooling tower make the cooling water rich in O2 causing severs corrosion problem . The air may also contain gases like SO2,H2S and NH3.If there is industrial area around there corrosive gases will further enhance corrosion problem.

Presence of chlorides and sulphates in increased concentration arising from continuous recirculation of cooling water enhance the corrosion rate and make corrosion control difficult by corrosion inhibitors.


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Problem in Cooling water system Corrosion problem also arise due

to galvanic action , if the metals , Example of such corrosion is observed in heat exchangers , where steel tubes are used along with admiralty.

Sand and dust may be blown into the open recirculation water besides loose fibers from cooling tower wood . Sludge and slime may collect in the tower pumps, deposits of all these contamination on metallic surface give rise to O2 concentration and result in severs localized pitting.


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PreventionCaCO3 Protective Scale The Method of treatment has been discussed

under langelier saturation Index.

Mechanical De aeration . Since O2 must be removed from the entire

volume of circulating water to avoid corrosion . It is not possible practically for economic reason , In some small cooling towers system , vacuumed de aeration of cooling water is practiced.


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Corrosion Inhibitors The corrosion inhibitor are chemicals that reduce and in some cases

almost stop corrosion of the metals when used in the cooling water. Most frequently used corrosion inhibitors are poly phosphates, Chromates, Tannins, Lignin's, and Starch, other corrosion inhibitors like Nitrites, Silicates and Amines are less frequently used, These act as a barrier between metal surface and acid . It dose not allow an acid ( H2SO4 ) to attack metal surface. An anodic inhibitor restrains the anodic corrosion reaction.

Fe= Fe + 2e- Examples of anodic inhibitors are , Sodium chromate, Potassium

chromate ,Nitrite, Phosphate and hydroxide. Example of Cathodic inhibitors are salt of Zinc and Nickel , such as

Hydroxide , Oxides and Carbonates, which are sparingly soluble in water.

O2+2H2O+4e=4(OH-)Umar Farooq Senior Chemist


Slime and there Prevention The word slime is

used vaguely and all fouling except corrosion and scale is usually called slime. However in many cases slime occur to gather with corrosion and scale.

Slime is caused buy the adhesion and accumulation of soft muddy material which is formed by mixing “micro Organism” like bacteria ,fungi ,algae , etc. which grow by utilizing the dissolved nutrients in water with inorganic matter like mud sand and dirt.

As a result slime causes not only a drop in thermal efficiency of exchanger and deterioration of water flow but also cause local corrosion in the equipment and piping.

Prevention : Application of slime control agents or chemical such as

chlorine ,phenolic and ammonium compounds. Side stream filtration Improve of heat exchanger operating conditions.


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A heat exchanger in a steam power station contaminated with macro fouling

Water treatment plays vital role in Cooling Tower Operation

Cooling tower maintenance can not run high unless the water is treated to prevent corrosion , biological growth , deposits and to protect the cooling tower wood from chemical attack .

Ca( HCO3)2 normally present in all raw water breaks down to form relatively in soluble CaCO3 . Scale is the most common type of water form deposit cooling system. The langelier index measure tendency of CaCO3 to precipitate under given condition of Calcium hardness ,alkalinity ,pH , Temperature , TDS , A Positive index mean tendency water to deposit scale while negative index tends to dissolve scale therefore causing corrosion.

Chemical treatment with H2SO4 Keep scale forming salt of calcium and magnesium in soluble form by lowering pH of circulating water chemical inhibitors are then need to check corrosion surface active chemicals or chelating agents.


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Water treatment plays vital role in Cooling Tower Operation

Such as sodium hexa meta phosphate prevent crystal growth and there fore scale formation .They increase solubility range of scale forming salt. For controlled scale treatment . Adjust composition of water so as a thin in previous layer of CaCO3 Scale deposits on the surface of circulating water system .

Scale must be thin enough to prevent any corrosion but not thick enough to effect overall heat transfer. Microbiological growth like slime and algae retard cooling , drop plant efficiency and rise cost of cooling system maintenance. Mechanical cleaning in the best way to get rid of accumulated growth. But to keep slime and algae from getting into bigger masses use chemicals such as chlorine , phenolic and ammonium compound.


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Corrosion Corrosion may be defined as the

destruction of metal by chemical or electro chemical with its environment.

In water system the principal fact influencing the corrosion of iron ( Fe ) metal .

Potential difference Dissolved solid ( Electrolyte ) Depolarizer CO2 , Free mineral acidity and pH are also

factor causing corrosion .


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Corrosion Theory Existence of potential difference with in a metal or

between two metal will cause chemical reaction at the anode and cathode . for under standing this let us consider a sample cell in which two electrodes one of iron and another of copper are placed in water.

When the two electrodes are connected together by a wire a closed circuit is completed . Electrons will flow from anode to cathode by wire and from cathode to words by electrolyte .

The production of electrons is due to the following reactions.

Fe Fe ++ + 2e –

At cathode

H2O H+ + OH -

2H + 2e - H2


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Corrosion Theory

In the above reaction iron ( Fe ) dissolved in ( H2O ) as ( Fe ) anodic direction. The Fe combine with OH to form Fe ( OH )2. This process stop after some time this is due to the formation of a thin film of H2 gas . If O2 is present in water it combine with H at the cathode to form H2O.

4H + O2 2H2O

It is also present the formation of hydrogen film or layer over the cathode this is the presence of O2 , the corrosion possess enhanced as the anodic or Cathodic reaction continuous.


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Corrosion Theory Another role of O2 increasing corrosion rate, is that

the corrosion product Fe (OH) 3 are formed by oxidation of Fe (OH) 3 as shown below.

4Fe (OH) 2 + O2+2H2O 4Fe (OH)3

Basically corrosion is an electro chemical process. For corrosion reaction to occur there must be a corrosion cell, consisting of cathode , Anode and electrolyte . The electrons of metals flow to words other point cathode where electron concerning reaction occur.

The result of this activity is the loss of metal and after the formation of a deposit.

Cathode has higher potential Anode has lower potential.Umar Farooq Senior Chemist


Corrosion MechanismSimple Corrosion Cell

Corrosion is an electro Chemical process in which a difference in electrical potential develops between two metals or between different parts of single metal, This voltage can be measured when a metal is electrically connected to a standard electrode, The electrical potential of a metal may be more or less than a standard. In which case the voltage is expressed as either positive or negative . This difference in potential allows current to pass through metals causing reaction at Cathodic and anodic sites. These sites constitute the corrosion cell.Umar Farooq Senior Chemist


Types of Corrosion


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The anode is the reign of lower potential and conversely cathode is the reign of higher potential.

Biological corrosion High temperature corrosion

Cold end corrosion Hydrogen attack corrosion

Cavitations Inter granular corrosion

Dezincification corrosion Stress corrosion

Fatigue corrosion Under deposit corrosion

Pitting corrosion ( Localized corrosion

Galvanic corrosion

General corrosion ( uniform corrosion )

Crevice Corrosion

Filiform Corrosion

Factor of CorrosionDissolved solid electrolyte Generally increase in dissolved solid concentration of water makes it

corrosive. Higher solid content increase the conductivity of water. Thus favoring increase in corrosion . One logical theory involves the effect of ion in destroying the coating of corrosive production the metal . Higher concentration of Chloride destroy coating by inhibitors such as chromate.

De Polarizer: Dissolve Oxygen Concentration. Oxygen react with hydrogen at the cathode surface forming depolarizing

surface. Thus permitting additional iron to dissolve . The solubility of oxygen in water depends on the temperature and pressure. Oxygen produce is identified easily and corrosion produces in the form of small pits or depression.

pH and free mineral Acidity of oxygen. If pH is below 4.3 approximately . The chief controlling factor in promoting

corrosion is pH . In the presence of oxygen pH has no controlling influence an corrosion rate. In absence of oxygen pH has controlling influence on corrosion rate.


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Corrosion ReactionAnodic reaction. The anodic reaction are typical by

dissolution of metal. Fe Fe ++ + 2e –

Cathodic reaction. In Cathodic reaction evolving electrons

migrate through the metal to cathode , where they react in any one of the several ways. Some typical cathode reaction are as under .

Typical Cathodic reaction Reduction of water occurs in natural

water . 2H2O +2e H2 +2(OH) -

Hydrogen ion reduction. Important in acidic solution.

2H + 2e - H2

Oxygen reduction occurs aerated acidic solution.

O4+4H+ + 4e 2H2O

Ferric ion reduction.

Fe +3 +e – Fe +2

Hence Fe +2 ( Ferrous ion ) produced at anode combine with OH ions to form ferrous hydroxide.

Fe + 2(OH)- Fe (OH) 2

Ferrous hydroxide produced has very low solubility and is quickly precipitated as Fe ( OH ) 3 or Fe2O3 as the condition prevail.

Fe (OH) 2 O2+2H2O 4Fe (OH) 2

2Fe (OH) 3 Fe2O3+3H2OUmar Farooq Senior Chemist


What is mean by depolarization

It has been noted that some time Cathodic reaction is reduced due to the formation of thin layer of hydrogen gas is cathode . Which prevents the flow of electrons from anode to cathode . This is only possible of oxygen gas is not present in the media .

H2 + O2 2OH Hydrogen gas there fore form the barrier and stop

corrosion reactions. Now if oxygen is present in the system, it will react with hydrogen gas. And depolarization occurs. This phenomena is called depolarization and oxygen gas act as a depolarizer.


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Effect of various factors on corrosionThe most important factor are . Oxygen and other dissolved gases. Dissolve or suspended solid Alkalinity or acidity. velocity and temperature . Microbiological activity ( e.g. algae, fungi, bacteria )

These factors are discussed as . Oxygen gas acts as depolarizer and effects directly on

corrosion. It is essential for Cathodic reaction to take place. However other dissolved gases ( NH3,H2S,SO2 ) effect indirectly on corrosion.


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Effect of various factors on corrosion

How do dissolve solid or Suspended solid effect corrosion Dissolved solid can effected the corrosion reaction by increasing the

electrical conductivity of water higher the concentration of dissolved solid greater the conductivity and more like hood of corrosion .dissolved chlorides and sulphate are practically corrosive by there corrosive or behavior action they can settle on metal surface to set localized corrosion cell.

How does water velocity effect corrosion . Higher velocity of water increases corrosion by transporting oxygen

to the metal by carrying away corrosion products at faster rate. Higher velocity can also cause corrosion of mater surface protective oxide film. When water velocity is low, deposition of suspended solid can establish localized corrosion cells , there by incensing corrosion rate.


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Effect of various factors on corrosion

How does temperature effect corrosion . Below 70C°every 15C° approximately increase in temperature

causes corrosion rate to double above 70C° additional temperature increases has relatively little effect on corrosion rate in cooling water system.

How dose acidity or alkalinity effect an corrosion . Acidic and slightly alkaline water can dissolve metal and protective

oxide film on metal surface more alkaline water favor the formation of protective oxide layer.

How does microbial growth effect corrosion . Microbial promote the formation of corrosion cells. In addition the

by products of some organisms' such as H2S form anaerobic bacteria are corrosive.


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Different Types of Corrosion attack

General Attack When the corrosion is uniformly

distributed over the metal surface. The conceder amount of iron

oxide produced by generalized attack contribute to fouling problem

Localized or pitting attack Exist when only small areas of

metal surface corrode. Pitting is the most serious form of

the corrosion because action is concentrated in small area

Pitting may perforate the metal in a short time. Umar Farooq Senior Chemist

NOMAC SIWEP Shuaibah Jeddah

Pitting corrosion is localized corrosion that occurs at microscopic defects on a metal surface. The pits are often found underneath surface deposits caused by corrosion product accumulation.

General attack corrosion deteriorates a steel lock. The surface effect produced by most direct chemical attacks (e.g., as by an acid) is a uniform etching of the metal..

http://corrosion.ksc.nasa.gov/images/medium/pitting1_med.jpg

http://corrosion.ksc.nasa.gov/images/uni1.jpg


Galvanic attack When two different metals are in contact.

The more active metal corroded rapidly.

Common example in water system are Steel & brass , Zinc & brass , Aluminum &

steel , Zinc & steel , If galvanic attack occurs the metal named

first will corrode

Stress Corrosion Cracking Stress corrosion cracking (SCC) is caused

by the simultaneous effects of tensile stress and a specific corrosive environment. Stresses may be due to applied loads, residual stresses from the manufacturing process, or a combination of both.


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http://corrosion.ksc.nasa.gov/images/gal3.jpg

http://corrosion.ksc.nasa.gov/images/met4.jpg


Crevice Corrosion

Crevice or contact corrosion is the corrosion produced at the region of contact of metals with metals or metals with nonmetals. It may occur at washers, under barnacles, at sand grains, under applied protective films, and at pockets formed by threaded joints.

Inter granular corrosion Inter granular corrosion is an attack on

or adjacent to the grain boundaries of a metal or alloy.


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http://corrosion.ksc.nasa.gov/images/gal2.jpg

http://corrosion.ksc.nasa.gov/images/fireplug.jpg

Different Types of Corrosion attack Microbial corrosion Microbial corrosion (also called

microbiologically -influenced corrosion or MIC) is corrosion that is caused by the presence and activities of microbes. This corrosion can take many forms and can be controlled by biocides or by conventional corrosion control methods

Filiform Corrosion

This type of corrosion occurs on painted or plated surfaces when moisture permeates the coating. Long branching filaments of corrosion product extend out from the original corrosion pit and cause degradation of the protective coating.


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http://corrosion.ksc.nasa.gov/images/microb1.jpg

http://corrosion.ksc.nasa.gov/images/filiform2.jpg

Method of Corrosion Prevention Corrosion can be prevented or minimized by one

or more of the following methods.

When designing a new system , chose corrosion resistance material to minimize the effect of aggressive environment.

Adjust pH Apply protective coating , such as paints metal plating or

plastics. Protect catholically, using sacrificial anodes ( Cathodic

protection ) Add protective film forming chemicals inhibitors that the

water can be distributed. To all wetted part of the system.Umar Farooq Senior Chemist


How do Chemical Corrosion Inhibitors work

Chemical inhibitors reduced or stop corrosion by interfering with the corrosion mechanism. Inhibitors usually effect either anode or cathode and so are called as anodic and Cathodic inhibitors.

Anodic Inhibitors

The inhibitors' which stop or minimize anodic corrosion are called anodic inhibitors.

Sodium chromate ( Na2CrO4 ) sodium nitrate ( NaNO2 ) Potassium nitrate, poly phosphate , all hydroxide ( NaOH , KOH ) etc.

Cathodic Inhibitors.

The inhibitors which stop or minimize Cathodic corrosion are called Cathodic inhibitors e.g. salt of zinc and nickel

ZnCO3,NiCO3,ZnO,NiO etc.


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