Fundamentals of Fundamentals of

Cooling WaterCooling Water TreatmentTreatment

chembond

Cooling WaterCooling Water TreatmentTreatment

Chembond Ashland Water Technologies Limited

Ashland

Cooling

Producing Steam DM water

Use of Water In The Industries:

Producing Steam DM water

Process

Drinking

Process fluids are required to be cooled at

various stages

Water is the Cheapest Coolant Available

Process Fluids are cooled in heat exchanger,

cooler, condenser

Water Used for Cooling Process Fluids:

cooler, condenser

Reactor / DG set Jacket Cooling

Heat is transferred from Hot Process Fluid to

Cold water and it becomes hot

Hot water is cooled in Cooling Tower and cold

water re-used for cooling

Types of Cooling Systems

Once through Cooling System

In this system:

cooling water passes through heat exchangers only once before

it is discharged.

T is very negligible.

it is discharged.

Large volumes of water are required in this type of systems.

Closed Recirculating Cooling SystemIn this system:

Water is reused continuously

water loss is very low.

There is no evaporation loss.

Example :

Chilled water system, Engine Jacket Cooling System.Chilled water system, Engine Jacket Cooling System.

Cooled water

Closed circuit water tank

Process

Warmed water

Air/ C.W. Cooled

water

Secondary cooler

chiller

Open Recirculating Cooling System

Water is re-circulated again and again

Heat from hot water removed with the help of cooling tower.

Permits extensive reuse of water and reduces the volume of make up

water required.

Due to evaporation, concentration of dissolved salts takes place.

H2O

MAKE UPWATER HEAT EXCHANGER

Air, Dust, Bacteria

HOT WATERProcess fluid out

Blowdown

COLD WATER

OPEN RECIRCULATINGCOOLING SYSTEM

TYPES OF COOLING TOWERSTYPES OF COOLING TOWERS

Cooling Towers

Natural Draft Mechanical Draft

Design of Cooling tower is such

that cold air of the bottom of towerForced Draft Induced Draft

that cold air of the bottom of tower

push the warmer air out from top.Forced Draft

Air is pushed in

the tower with a

fan at the side.

Induced Draft

Air is pulled from

cooling tower by

a fan at the top

Counter Flow Cross Flow

CROSS FLOW COOLING TOWER

COUNETR FLOW COOLING TOWER

MAKE UP WATER

HEAT EXCHANGER

Air, Dust, Bacteria

H2O

HOT WATER

Process fluid out

Process fluid in

OPEN RECIRCULATING COOLING WATER SYSTEM

BlowdownProblems in

HEAT TRANSFER Equipments

COLD WATER

CORROSION SCALE/ DEPOSITS MICROBIOLOGICAL FOULING

Normal Terminology used in Open Recirculating Cooling Water System

Normal Terminology used in Open Recirculating Cooling Water System

1. Hold up Capacity of the system : (V)

Hold up capacity of the system = water contained in basin

+

sump of cooling tower

+

water contained in piping and equipments.

2. Blowdown : (B)

Due to evaporation, concentration of Impurities / dissolved solids takes place.

Part of water is removed from system as a blowdown to control concentration of

impurities / dissolved solids in water.

3. Drift / Windage loss : (D)

Some water droplets escape alongwith air and water vapours. A usual drift loss in

conventional cooling towers is in the range of about 0.05 -0.2% of the recirculation

rate.

Contd.4. Evaporation Losses : (E) Water lost to the atmosphere in the cooling process is evaporation.

The rate of evaporation depends upon the temperature difference across tower.

For each 5.6 C Delta T across tower, evaporation rate is 1% of Circulation Rate.

5. System Losses : (S)Circulating water is lost in the plant through

pumps, valves or leakages in plant etc.

6. Concentration Cycle : (C) 10.012.0

%

M

A

K

E

U

P

W

A

T

E

R

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b

a

s

e

d

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c

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a

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)

Makeup Water requirements vs. cycles of

concentration

6. Concentration Cycle : (C)

Mg or Silica in cooling water

C =

Mg or Silica in make up water

Blowdown E

& Windage losses = -----

C-1

where E = Evaporation rate.

C = Cycle of Concentration.

00

1 2 3 4 65 7 8 9 10

2.0

4.0

6.0

8.0

10.0

50F DROP

25F DROP

10F DROP

%

M

A

K

E

U

P

W

A

T

E

R

(

b

a

s

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d

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c

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CYCLE OF CONCENTRATION

Contd.7. Make-up Water : (M)This is the water which is to be added to replace the water lost by evaporation, blow down, drift and

leakage.

M = E + B + D + S

8. Holding Time Index :Time required to reduce the concentration

of any constituent in cooling water to half.

0.693 x (Hold up capacity )

HTI =

Blowdown

Each programme has maximum allowable HTI beyond which chemical lose its effective.

9. Approach : Indicate efficiency of cooling tower.

Lesser is approach better is cooling tower efficiency.

Approach = Supply of C. W. temperature -

Wet bulb temperature

Water impurities and its effectWater impurities and its effect

Impurity Effect

Total Hardness Scale formation

(Calcium + Magnesium)

M-Alkalinity Corrosion - Low Alk.

Scale/Deposition. -

High Alk.

Chlorides Corrosion / SCC of SSChlorides Corrosion / SCC of SS

Suspended Solids Deposition

Sulphate Corrosion / Scale

formation

SiO2 Scale

Organic Matters Fouling

Iron and Manganese Deposition

Micro-Organisms Fouling / Corrosion

Ammonia Nitrifying bacteria / fouling

The Corrosion ProcessThe Corrosion ProcessOccurs due to the presence of local cells with anodic and cathodic sites on the metalmediated by electron transfer through an electrolyte.In an aerated, neutral solution, the overall reactions are :Anodic Reaction

Fe Fe++ + 2e- (1)

Cathodic ReactionO2 + 2H2O + 4e- 4OH- (2)

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

Overall Reaction

Fe(OH)3

Fe2O3 (Rust)

Fe (OH)2 ANODE

Fe (OH)3

ELECTRON FLOW CATHOD

Fe ++ H2OOH-

Water / Electrolyte

O2

O2

Pitting:Most destructive

Caused by Localized Deposition & Differential Oxygen cells.

Most Common types of Corrosion inMost Common types of Corrosion inCooling Water SystemsCooling Water Systems

Most Common types of Corrosion inMost Common types of Corrosion inCooling Water SystemsCooling Water Systems

General Type:Uniform in nature.

Less dangerous

Galvanised Corrosion :

It is caused due to the presence of Dissimilar metals.

Stress Corrosion Cracking :

It occurs in S.S. or Copper Alloys and is caused by high chloride and high

Contd.Contd.Contd.Contd.

It occurs in S.S. or Copper Alloys and is caused by high chloride and high

temperature & pressure.

Contd.Contd.Contd.Contd.Crevice Corrosion:

Tube - Tube sheet joint

Under deposit or tubercles

Threaded joints

Erosion Corrosion:Erosion Corrosion:

Normally restricted to copper based

alloys.

High water velocity, High suspended solids, Turbulence accelerates.

Chloride in water.

Dissolved oxygen in water.

High temperature

Corrosion is Accelerated by:Corrosion is Accelerated by:Corrosion is Accelerated by:Corrosion is Accelerated by:

Suspended solids (under deposit corrosion)

Bacteria (Sulphate reducing & nitrifying )

Contd.Contd.Contd.Contd.

Low pH of water.

Corrosion Leads To -

Leakage of Exchangers

Un-schedule Shut Down

Why Corrosion Control ?Why Corrosion Control ?Why Corrosion Control ?Why Corrosion Control ?

Loss of Production

High cost of Equipment Replacement

Selection of proper metal of construction.

Applying protective coating.

Galvanic corrosion control

Corrosion Control :Corrosion Control :Corrosion Control :Corrosion Control :

Using sacrificial anodes (cathodic protection)

Galvanic corrosion control

Corrosion Control of water box in power

plant.

Chemical Treatment

Most economical and widely used.

The commonly used corrosion inhibitors in an Open Recirculating System are :

i. Chromates : Anodic type.

ii. Orthophosphate : Anodic type

iii. Molybdate : Anodic type

iv. Silicate : Anodic type

v. Zinc : Cathodic Type

Corrosion lnhibitor:Corrosion lnhibitor:

v. Zinc : Cathodic Type

vi. Polyphosphate and

glassypolyphosphates : Cathodic Type

Selection criteria

Water analysis

Metallurgy of equipments

process parameters

Environmental restrictions

Scale : Scale : Scale : Scale :

Dense, adherent and hard material composed most commonly of

Calcium & Magnesium salts.

Precipitate at high temperature and get deposited on the heat transfer surfaces.

FollowingFollowing areare somesome typestypes ofof scalescale::

CaCO3, Ca3 (PO4)2

MgCO3

Silicates - Not very common

280

240

200

160

120

pH = 7.5

ppm TDS = 500

ppm Alk. = 100

p

p

m

S

o

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b

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C

a

C

O

3

Chembond

Silicates - Not very common

Magnesium Silicate - very hard

60 80 100 120 140 160 180

120

80

40

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p

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S

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C

a

C

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TEMPERATURE OF

Water

Exchanger tube Scale (Insulation)

Scale formation is accelerated by :

Scale Formation leads to Low Heat Transfer.Scale Formation leads to Low Heat Transfer.

Scale formation is accelerated by :

High temperatures.

High hardness of water.

High pH of cooling water.

High M-Alkalinity

Ca(HCO3)2 CaCO3 + CO2 + H2O

Water Indices Water Indices Water Indices Water Indices

Water indices make it possible to predict the tendency of

water either to precipitate or to dissolve Calcium Carbonate.

Types of IndicesTypes of IndicesLangalier Index :

Chembond

Langalier Index :

L. I. = pHa - pHs

Where, pHa = Actual pH of cooling water and pHs = Saturation pH is the function

of the Total solids, Temperature, Calcium & Total alkalinity

When L.I. is positive, it denotes scale forming; and When L.I. is negative, it

denotes corrosion tendency of water.

5

Stability Index [Stability Index [Ryznar Stability IndexRyznar Stability Index] ] Stability Index [Stability Index [Ryznar Stability IndexRyznar Stability Index] ] R. S. I. = 2pHs - pHa

Water Tendency :Index Tendency of

waterLSI RSI

2.0 8 Undersaturated, very aggressive

WHEN WATER IS TREATED WITH CHEMICALS, THESE

INDICES DO NOT HAVE INDICES DO NOT HAVE ANY IMPORTANCE.

Scale Formation Leads To -

Reduction in Water Flow

Poor Heat Transfer

Why Scale Control ?Why Scale Control ?Why Scale Control ?Why Scale Control ?

Poor Heat Transfer

Reduction in Plant Load

Chemical Cleaning

Unschedule Shut Down

Shorten the Life of Equipments.

Scale ControlScale ControlScale ControlScale Control

Removal of Ca, Mg by Ion Exchange - Cost is High.

Addition of Sulfuric Acid

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

CaSO4 Higher solubility than CaCO3

Contd..Contd..Contd..Contd..Formation of Scale can be controlled by addition of :

Polyphosphate (SHMP):

Less stable at high temperature

Revert to Orthophosphate as P-O-P bond is weak.

Reverted Orthophosphate causes deposition.

O O

Zinc glassy polyphsophate:

Zinc glassy polyphosphate has high X value and no definite structure.

More stable than SHMP.

Negligible orthophosphate formation at high temperature.

NaO P O P ONa

ONa ONa x

Organophosphonates :

HEDP: ( Hydroxy Ethylidene Diphosphonate)

P-C-P bond is strong.

High stability with respect to pH and

temperature.

Keeps Calcium & Magnesium in solution.

Scale ControlScale ControlScale ControlScale Control

H

O HCH O

OH P C P OH

OH OH OH

Synthetic Polymers:

Low molecular weight polyacrylates copolymers/Ter-polymer.

- Crystal Modification.

- Keeps precipitated Calcium salts in water.

PBTC :

Phosphono butane-tricarboxylic acid more effective than HEDP

OH OH OH

Deposition / Fouling Deposition / Fouling Deposition / Fouling Deposition / Fouling

Causes :1. Corrosion Products.

2. Suspended solids in cooling water.

3. Dead algae in cooling tower.

4. Slime produced by microorganisms in water.

5. Low velocity.

6. Process Contaminants - Ammonia, Oil, Hydrocarbons.

Control Agents/ Methods:1. Dispersants based on low molecular weight polymer.

2. Bio-Dispersant.

3. Providing proper velocity to water (3-5ft/sec)

4. Providing Side Sand Filter.

Capacity 3-5% of Circulation rate.

5. Backflushing / Air rumbling arrangement.

Deposition causes under deposit corrosion.Deposition causes under deposit corrosion.

Microbiological FoulingMicrobiological FoulingMicrobiological FoulingMicrobiological Fouling

The major classes of microorganisms which are associated with recirculating Cooling

System are :

Bacteria - In Cooling Water.

Algae - On cooling towers structure / distributing deck.

Fungus - On wooden structures of cooling towers.

Delignification of wood. (phto27.8)Delignification of wood. (phto27.8)

-Temperature of cooling water is ideal for bacterial growth.

- Sun light is accelerate growth of Algae

-Abundant nutrients are available.

Different Types of Bacteria :

Sulphate Reducing Bacteria(Anarobic)- pitting type corrosion.

SRB attack

Brown deposits on top

Under brown deposit black deposit.

Under black deposit, silvery shining surface.

Nitrifying Bacteria - pH drop.

Iron Bacteria - Iron fouling.

Slime Bacteria

Why Microbiological Control ?

Microbiological Growth Leads To -

Fouling of Exchangers .

Less Heat Transfer.

Under Deposit Corrosion. Under Deposit Corrosion.

Reduction in plant load.

Microbiological Induced Corrosion (MIC)

-Pitting.

Un-schedule Shutdown.

Decrease in Cooling Tower Efficiency.

Damage of Wooden Parts of Cooling Tower.

Microbiological controlMicrobiological controlMicrobiological controlMicrobiological control OXIDISING BIOCIDE

Chlorination.

Microbiological action of Chlorine :

Cl2 + H2O - HOCL + HCL

HOCL is the active species and dissociates as pH

Increases HOBr and OBr- are more toxic to bacteria compared to HOCl, OCl-

Reaction Of Chlorine ActivatorReaction Of Chlorine Activator

5 6 7 8 9 104

HOCl HOBr

0

20

40

60

80

100MBr + HOCl HOBr + MCl

pHIncreases

HOCl + OH- = OCl- + H2OFormation of HOCl, OCl- depends on pH of water

HOCL is mainly responsible for killing of bacteria OCl- has very low action on bacteria As pH increases above 8, OCl- formation takes place, chlorination

effectiveness decreasesHeavy chlorination bring down the cooling water pHDosage : 0.2 - 0.5ppm for 3 - 4hrs. Per day

Chlorine does not reach all the parts of the Cooling System.

At pH = 8.5HOCl / OCl- = 10 / 90HOBr / OBr- = 60 / 40

Bromine

Chlorine Demand:

Amount of chlorine consumed by following impurities before free

chlorine appears in Cooling Water.

Organic Matters

Ammonia

Dead Algae, Slime

Other Oxidizable Substances

Bromine-Effective at high pH

-Effective in ammonia contaminates system compare to chlorine

Ozone

-pH is sensitive-Produced at site by electrolysis

Chlorine Dioxide - Very effective in presence ofAmmonia

Organic

In-Situ Generation of Chlorine Dioxide

NaClO2 + 1/2 Cl2 - ClO2 + NaCl

(Chlorine Dioxide )

Chlorine Dioxide:

Chlorine Activator Chlorine Activator ( Chlorite based)( Chlorite based)Chlorine Activator Chlorine Activator ( Chlorite based)( Chlorite based)

Chlorinated

water inlet

Electric Control Box

2Auto Switch

Flow meter

Dosing Pump

13

Emergency Shutdown Switch

ClO2

Loss of Cl2 Switch.

GENEROX

Flow Indicator

C

l

O

2

G

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r

Pressure gauge

Filter

BSP

connection

BSP

connection

Advantages :

Do not react with Ammonia, Organics. Hence

more effective than chlorine in contaminated

system.

Reduces chlorine consumption.

Reduces the chloride builtup due to chlorination.

Effective in high pH range also.

Negligible delignification of wood.

Precursor Source 48"H x 42"W x 17"D

Reaction Of Chlorine ActivatorReaction Of Chlorine ActivatorReaction Of Chlorine ActivatorReaction Of Chlorine Activator

HOCl HOBr

40

60

80

100MBr + HOCl HOBr + MCl

HOBr and OBr- are more toxic to bacteria compared to HOCl, OCl-At pH = 8.5HOCl / OCl- = 10 / 90HOBr / OBr- = 60 / 40

5 6 7 8 9 1040

20

40

pH

Non-Oxidising BiocidesNon-Oxidising Biocides Methylene Bis Thiocynate ( MBT )

Very effective against SRB. It Hydrolizes above7.5 pH

Quaternary Ammonium Compounds(QAC)

Tendency to foam. Ineffective in highly oil or organic fouled systems. Effective at high pH.

Glutaraldehyde ( ALD )

Effective over wide pH range.

Isothiozoline ( THIO )

pH InsensitivepH Insensitive

Dichlorophene

effective over wide pH range.

DBNP EFFECTIVENESS

Bacteria Fungi Algae E = Excellent G = Good

MBT E S S S = SlightQAC E G EALD E E ETHIO E G EDBNP E S S

NonNon--Oxidising BiocidesOxidising BiocidesNonNon--Oxidising BiocidesOxidising Biocides

Dosage : Once in 8 - 10 days

Bacteria develop resistance to non-oxidizing biocide.

Use of more than 1 non-oxidizing biocide is preferred to avoid immunity. Use of more than 1 non-oxidizing biocide is preferred to avoid immunity.

Covering the top of the distributing deck of the cooling tower will

eliminate the sunlight resulting in a reduction in the formation of algae.

Bacterial Growth & its Control with Biocides Bacterial Growth & its Control with Biocides Bacterial Growth & its Control with Biocides Bacterial Growth & its Control with Biocides T

o

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Acceptable

Range

Biocide

Dosing

T

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Time ( in days )

7 14 21

Bio Bio Dispersant Dispersant Bio Bio Dispersant Dispersant What is Bio-Dispersant ?

Bio-Dispersants are non-anionic type surface active agents alongwith slime

solublizing solvents.

Functions :

When Bio-Dispersant is added alongwith oxidizing or non-oxidising biocide, it :

Increase the effectiveness of biocide.

Removes slime.

Releases bacteria arrested under slime deposits so that biocides can kill free

bacteria.

Addition of Bio - Dispersant increases turbidity, increases total bacterial

count & generates foam.

Monitoring methodsMonitoring methodsMonitoring methodsMonitoring methodsCorrosion : Corrosion Meter & Coupon Holding Rack.

COUPON

EBONITE

FLOW

OUTLET

W ATER IN LET

TEE

18

1 1/2

22.3 x WCORRO SIO N R ATE ( mpy) = -------------

d x a x tW - Loss in we ight in m g.d - Density of m etal in gm/cm 3a - Area of coupon in in2t - T im e in days

d for CS = 7.85Copper = 8.9Brass = 8.17

Coupon holding rack22.3 x w

Corrosion rate = ------------

d x a x t

Mpy = mills per year

w = Loss in weight in mgs

d = Density of test coupon in gms/cm3

a = area in Inch2

t = time in days

Scale and deposition : Test Heat Exchanger / Deposit Monitor

t - T im e in days

FLOW METER

BALL VALVE

FLOW CONTROL VALVE

SENSOR ( t 2 )

ACRYLIC TUBE

JUNCTION BOX

TEMP. IND ICATOR

SENSOR ( t 1)

Mains Restart Heater

Actual

Set

Temp. Set

AmmeterAmmeter

Thermostat

ELECTRIC ITY SUPPLY 230V, 50HZ, 1Ph

HEATER

OUTLET

INLET

1/2 TUBE

RUBBER TUBE

HOT CONDENSATE IN

Length about 2 - 3 feet

COLD CONDENSATE OUT

COOLING WATER IN

COOLING WATER OUT

Test Heat ExchangerDeposit Monitor

t = time in days

Contd..Contd..Deposition : Temperature & Pressure monitors of exchangers.

Bacteria analysis : T.B.C. S.R.B. N.R.B.

Flow Meter

Outlet

DPG

Inlet

PG

Pressure drop tube

S Y M B O L S

= Ball Valve= Flow Control Valve

D.P.G. = Differential Pressure Gauge

= Strainer ValveBiofouling Monitor

Online Measures for Overall improvementOnline Measures for Overall improvementin performance of Treatment Programme.in performance of Treatment Programme.Online Measures for Overall improvementOnline Measures for Overall improvementin performance of Treatment Programme.in performance of Treatment Programme.

Side Sand Filter

About 5% of circulating water is to be passed through a side sand filter so as to

reduce suspended solids.

Air/Nitrogen Bumping and Back Washing :

Air bumping and back washing is very necessary for shell side water heat exchangers

and compressor jackets.

Annual Water Flushing of Heat Exchangers and cleaning sump, basin.

C ooling W a ter In To D ra in

P rocess O utP rocess In

C oo ling W a ter O u t

COOLING WATER ON SHELL S IDE

Cooling W ater In

Air / N 2 In jectorpoint

Process O ut

Process In

Coo ling W ater O ut

COOLING WATER ON SHELL S IDE

Alternate AirIn jector P oint

Biocleaning ( On-Line )Biocleaning ( On-Line ) Addition of Biodispersant ( 5-10ppm)

Maximum Circulation

Addition of NonOxidizing Biocide

Blowdown after 8-12hrs. Circulations.

Turbidity of water increases. Turbidity of water increases.

Target Dosing:Target Dosing: Dosing of Biodispersant / Dispersant at inlet cooling

water of fouled Exchanger.

PRECLEANING: - Removal rust etc and makingsurface active for passivation.

Addition of oil dispersant ( For New System )

Reduction of pH ot 5.5 - 6.0

Addition of Dispersant

Monitoring of Iron Content

Blowdown after Fe remains constant.

Precleaning / Passivation Precleaning / Passivation Precleaning / Passivation Precleaning / Passivation

Bare metal

With proper passivationBlowdown after Fe remains constant.

Maximum circulation.

PASSIVATION : - Film formation on cleaned surface.

Control of pH

Addition of Corrosion Inhibitor (PO4= 60-70ppm )

Circulation without blowdown for 24-48hrs.

New Exchanger (CS) - Individual precleaning/ passivation essential.

Without proper passivation

Performance LimitsPerformance LimitsPerformance LimitsPerformance LimitsGood treatment programs should ensure the following results for the parameters :

1.Corrosion rate of