cooling water nalco tủ tài liệu bách khoa

Cooling water systems control temperatures and pressures by transferring heat from hot process fluids into the cooling water, which carries the heat away.. It is used to determine treatm

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Cooling Water Treatment

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©2005, 2009 Nalco Company All Rights Reserved 1-09 Bulletin B-34

Essential Expertise for Water, Energy and Air.SM

COOLING WATER SYSTEMS: AN OVERVIEW 5

COOLING WATER PROBLEMS AND SOLUTIONS 11

CORROSION 12

SCALE 15

FOULING 16

BIOLOGICAL PROBLEMS 18

TOTAL COOLING SYSTEM MANAGEMENT 23 Contents

Cooling Water Systems: An Overview

Why Water is Unique

Water is a unique molecule with basic properties that make itideal for cooling water applications For example, it is safe,easy to handle, widely available, and inexpensive in mostindustrialized regions of the world Water is a more efficientheat transfer medium than many other materials, especiallycompared to air

Water is often called the universal solvent – a property thatcan cause unwanted side effects for industrial applications.Water can dissolve many substances, including gases likeoxygen and carbon dioxide As a result, water can causecorrosion of metals used in cooling systems As waterconcentrates in cooling systems, dissolved ions may exceedthe solubility of some minerals and form scale The life-givingproperties of water can also encourage bacterial growth thatcan foul system surfaces These problems require propertreatment and control to maintain the value of a cooling watersystem to the process it serves

Cooling water systems are an integral part of processoperations in many industries For continuous plantproductivity, these systems require proper chemicaltreatment and preventive maintenance

Why are Cooling Water

Systems Needed?

Most industrial production processes

need cooling water for efficient, proper

operation Refineries, steel mills,

petrochemical plants, manufacturing

facilities, food plants, large buildings,

chemical processing plants, and electric

utilities all rely on the cooling water

system to do its job Cooling water

systems control temperatures and

pressures by transferring heat from hot

process fluids into the cooling water,

which carries the heat away As this

happens, the cooling water heats up

and must be either cooled before it can

be used again or replaced with fresh

makeup water The total value of the

production process will be sustained

only if the cooling system can maintain

the proper process temperature and

pressure The cooling system design,

effectiveness and efficiency depends on

the type of process being cooled, the

characteristics of the water and

environmental considerations

Why is Treatment and

Control Important?

The cooling system operation can

directly affect reliability, efficiency, and

cost of any industrial, institutional, or

power industry process Monitoring

and maintaining control of corrosion,

deposition, microbial growth, and

system operation is essential to provide

the optimum Total Cost of Operation

(TCO) The first step to achieve

minimum TCO is selecting an

appropri-ate treatment program and operating

conditions to minimize system stresses

One such treatment program is Nalco3D TRASAR cooling water technologyoptimization software Another step isimplementation of an appropriatemonitoring program to evaluate systemconditions and treatment programperformance The final step is use ofNalco 3D TRASAR® technology tocontrol system stress, optimize operat-ing conditions and maintain programparameters to minimize TCO

Why is Monitoring Important?

Monitoring is an integral part of anyindustrial water treatment program

It is used to determine treatmenteffectiveness and to establish theoptimum level of treatment that ismost cost effective, with respect toenergy, water and chemical usage

The purpose of corrosion monitoring is

to assess or predict corrosion behavior

of the system Basically, there are twoobjectives to monitoring:

1 To obtain information on thecondition of the operationalequipment

2 To relate this information to theoperating variables (i.e., pH, tem-perature, water quality, chemicaltreatment) Meeting these objectiveswill provide the following results:

• Increased life of the plant

• Improved quality of the plant’s • •

• product

• Maintenance predictation needs at

• plant

• Reduced plant’s operating cost

Corrosion monitoring is standardpractice in the water treatmentindustry The plant engineer can usethis information to predict equipmentlife Monitoring helps the engineeridentify significant factors responsiblefor corrosion problems and assuresimplementation of solutions

Corrosion monitoring is a diagnostictool It provides information for thesolution of corrosion problems.Knowledge of corrosion trends can

be very valuable Frequently, severalvariables might appear to be significant,and the ability to correlate corrosionrates with a single variable underspecific conditions can be vital As

a logical extension of diagnosticcapabilities, corrosion monitoring isused to assess the effectiveness of asolution to a specific water treatmentproblem

Why is more than One Type of Monitoring Method Recommended?

Corrosion monitoring can be used

to provide operational information

If corrosion can be controlled bymaintaining a single variable (i.e.,temperature, pH, chemical treatment)within limits previously determined,then that variable can be used topredict changes in corrosion patterns

as the limits are exceeded in both apositive and negative direction Anextension of this technique is to use amonitored variable to control chemicaladdition directly through automaticfeed systems

The particular corrosion monitoring

technique selected depends upon its

applicability to the system and the

information being sought Some

monitoring techniques provide:

• Information that is effective

• Information on the overall system

No one monitoring technique will

provide all the necessary data to

properly evaluate the efficacy of the

treatment program More than one

technique may be necessary to

monitor a particular system

Most corrosion monitoring techniques

are best suited to situations where

corrosion is of a general nature, but

some techniques provide at least

some information on localized attack,

such as pitting

What is Involved in the

Cooling Process?

Cooling involves the transfer of heat

from one substance to another The

substance that loses heat is said to be

cooled, and the one that receives the

heat is referred to as the coolant All

cooling systems rely on this give and

take action, with water being the most

widely used coolant

Energy costs wasted by calcium carbonate deposits of varying thickness in a 1000 ton (12.7 GJ/h)

chiller running 24 h/d, 365 d/yr.

How Does Cooling Water Impact Total Cost of Operation (TCO)?

Production processes or buildingcooling use significant amounts ofenergy The cooling system removesthe unwanted heat When the coolingsystem cannot remove the heatefficiently, the entire process suffersand costs increase (see chart below)

In addition, if the cooling systemoperation is not optimized, excesswater, wastewater, and energy costsresult

Why is Water Used for Cooling?

Several factors make water an excellentcoolant:

• It is normally plentiful, readilyavailable, and inexpensive

• It is easily handled and safe to use

• It can carry large amounts ofheat per unit volume, especiallycompared to air

• It does not expand or compresssignificantly within normallyencountered temperature ranges

• It does not decompose

What are the Sources of Cooling Water?

Fresh water – This is the primary

source of makeup for cooling watersystems Fresh water can be surfacewater (rivers, streams, reservoirs) orground water (shallow or deep wellwaters) In general, ground watersupplies are more consistent incomposition, temperature, and containless suspended matter than surfacewater supplies, which are directlyaffected by rainfall, erosion, and otherenvironmental conditions Groundwater sources frequently containsoluble iron or manganese which cancause fouling in cooling systems ifnot removed These are much lesscommon in surface water

Heat Transfer Principle

Salt water and wastewater – Because

of environmental considerations, water

cost, and water availability, some plants

are now using salt water and

waste-water treatment plant effluents as

sources of cooling water Close

attention to design and treatment of

cooling systems using these sources

of water is critical for reliable

perfor-mance and long life

What are Some Important

Properties in Cooling Water

Chemistry?

Conductivity – A measure of water’s

ability to conduct electricity In cooling

water, it indicates the amount of

dissolved minerals in the water

Conductivity is measured in μS/cm

(microSiemens/cm) and can vary from

a few for distilled water to over

30,000 μS/cm for sea water

pH – Gives an indication of the relative

acidity or basicity of water The pH

scale runs from 0 to 14, with 0

repre-senting maximum acidity and 14

representing maximum basicity

Alkalinity – In cooling water, two

forms of alkalinity play a key role

These are carbonate ions (CO3–2)

and bicarbonate ions (HCO3–) The

alkalinity acts as a buffer to charges

acidity or basicity

Hardness – Refers to the amount of

calcium and magnesium ions present in

the water The hardness in natural

waters can vary from a few parts per

million (ppm) to over 800 ppm

The pH Scale

Relative Concentration of Hydrogen Ions Compared to Distilled Water

pH = Examples of Solutions at this pH

10,000,000 pH = 0 Battery acid, strong acid 1,000,000 pH = 1 Hydrochloric acid secreted by stomach lining 100,000 pH = 2 Lemon juice, vinegar

10,000 pH = 3 Grapefruit, orange juice, soda 1,000 pH = 4 Tomato juice, acid rain

100 pH = 5 Soft drinking water, black coffee

10 pH = 6 Urine, saliva

1 pH = 7 “Pure” water 1/10 pH = 8 Sea water 1/1000 pH = 9 Baking soda 1/1,000 pH = 10 Great salt lake, milk of magnesia 1/10,000 pH = 11 Ammonia solution

1/100,000 pH = 12 Soapy water 1/1,000,000 pH = 13 Bleaches, oven cleaner 1/10,000,000 pH = 14 Liquid drain cleaner

More Basic Neutral More Acidic

Why are These Water

Chemistry Properties

Important in Cooling

Water Systems?

These key water chemistry properties

have a direct impact on the four main

problems of cooling water systems;

corrosion, scale, fouling, and microbial

contamination These properties also

affect the treatment programs designed

to control the problems

Conductivity – Cooling water

treat-ment programs will function within

specific ranges of conductivity The

range will be dependent upon the

particular cooling water system’s

design, characteristics, and the type of

chemical program

pH – Control of pH is critical for the

majority of cooling water treatment

programs In general, metal corrosion

rate increases when pH is below

recommended ranges Scale formation

may begin or increase when pH is

above recommended ranges The

effectiveness of many biocides depends

on pH; therefore, high or low pH may

allow the growth and development of

microbial problems

Alkalinity – Alkalinity and pH are

related because increases in pH indicate

increases in alkalinity and vice versa As

with pH, alkalinity below recommended

ranges increases the chances for

corrosion; alkalinity above

recom-mended ranges increases the chances

for scale formation When corrosion

and scale problems exist, fouling will

also be a problem

Hardness – Hardness levels are usually

associated with the tendency of coolingwater to be scale forming Chemicalprograms designed to prevent scale canfunction only when the hardness levelstays within the specified range Somecorrosion control programs require acertain hardness level to functioncorrectly as corrosion inhibitors, so

it is important to make sure hardnesslevels are not too low in theseprograms

What are the Cooling Water Systems?

There are really only three basicdesigns:

1 Open recirculating systems

2 Once-through systems

3 Closed recirculating systems

How are These Three Systems Different?

Open recirculating systems are the

most widely used industrial coolingdesign These systems consist of pumps,heat exchangers, and a cooling tower

The pumps keep the water recirculatingthrough heat exchangers It picks upheat and moves it to the cooling tower

where the heat is released from thewater through evaporation Because

of evaporation, the water in openrecirculating systems undergoeschanges in its basic chemistry Thedissolved and suspended solids in thewater become more concentrated

In once-through systems, the cooling

water passes through heat exchangeequipment only once The mineralcontent of the cooling water remainspractically unchanged as it passesthrough the system Because largevolumes of cooling water are used,these systems are used less often thanrecirculating systems Seasonal tem-perature variation of the incomingwater can create operational problems.Temperature pollution of lakes andrivers by system discharge is anenvironmental concern

Closed recirculating systems use the

same cooling water repeatedly in acontinuous cycle First, the waterabsorbs heat from process fluids, andthen releases it in another heatexchanger In these systems, an evapo-rative cooling tower is not included.Often used for critical cooling applica-tions or when water temperaturebelow ambient is required, as in achilled water system

CLOSED RECIRCULATING SYSTEM

Amount of Water Used: Extremely Large

Amount of Water Used: Moderate

OPEN RECIRCULATING SYSTEM

ONCE-THROUGH SYSTEM

Does the Type of Cooling System Affect Treatment Application Principles?

Yes The choice of treatment isdependent on the type of system

In an open recirculating system, more

chemical must be present because thewater composition changes significantlythrough evaporation Corrosive andscaling constituents are concentrated.However, treatment chemicals alsoconcentrate by evaporation; therefore,after the initial dosage, only moderatedosages will maintain the higher level oftreatment needed for these systems

In a once-through system, protection

can be obtained with relatively fewparts per million of treatment, becausethe water does not change significantly

in composition while passing throughequipment Treatment can be challeng-ing because even small treatmentdosage can be a large quantity ofchemical because of the large volume

of water used

In a closed recirculating system, water

composition remains fairly constant.Ideally, there is very little loss of eitherwater or treatment chemical Softened

or demineralized water and hightreatment dosages can be used without

a significant cost impact, becausesystems are ideally filled once andminimal water is lost from the system

The following four problems are normally associated with cooling water

systems

1 CORROSION

Manufacturing of common metals used in cooling systems, such as mild steel,

involves removing oxygen from the natural ore Cooling water systems are an

ideal environment for the reversion of the metal to the original oxide state This

reversion process is called corrosion

2 SCALE

Minerals such as calcium carbonate, calcium phosphate, and magnesium silicate

are relatively insoluble in water and can precipitate out of the water to form

scale deposits when exposed to conditions commonly found in cooling water

systems

3 FOULING

The deposition of suspended material in heat exchange equipment is called

fouling Foulants can come from external sources such as dust around a cooling

tower or internal sources such as by-products of corrosion

4 BIOLOGICAL CONTAMINATION

Cooling water systems provide an ideal environment for microbial organisms to

grow, multiply, and cause deposit problems in heat exchange equipment

Microbial growth can strongly influence corrosion, fouling, and scale formation,

if not controlled properly

Macrofouling can occur in once-through cooling systems or water intakes in

lakes and rivers Various species of clams, mussels, and other marine organisms

can attach to the piping, reducing water flow and increasing corrosion

Scale deposits and corrosion products on tube surfaces reduce heat transfer efficiency, increase energy costs, and reduce equipment life.

• Possible product yield reduction oreven plant shutdown

• Product quality problems andincreased product rework

• Environmental compliance problems

• Increased greenhouse gas emissionsdue to higher energy use

Proper program selection and systemcontrol methodology are essential tomaximizing the value of the coolingsystem to the operation of any facility.The proper system management tocontrol cooling system stresses willoptimize TCO

Cooling Water Problems and Solutions

What are the Effects of

These Problems?

If not properly controlled, these

problems can have a direct, negative

impact on the value of the entire

process or operation Examples of

problems that corrosion, deposition,

and biological fouling can create are

as follows:

• Increased maintenance cost

• Equipment repair or replacement

cost

• More frequent shutdowns forcleaning and replacement of systemcomponents

• Reduced heat transfer efficiency andtherefore reduced energy efficiency

of the process being cooled

• Increased fuel costs for powergeneration plants

• Increased energy consumption byrefrigeration chillers

What is Corrosion?

Corrosion is an electrochemical

process by which a metal returns to its

natural oxide state For example, mild

steel is a commonly used metal in

cooling water systems that is very

susceptible to corrosion Corrosion

causes loss of metal thickness or even

penetration of tube walls which can

cause leakage of process fluids into the

cooling water or vice versa Corrosion

is generally a greater concern with the

more common, lower cost materials

like mild steel

How does Corrosion take

Place?

For corrosion to occur, a corrosion cell,

consisting of an anode, a cathode, and

an electrolyte must exist Metal ions

dissolve into the electrolyte (water)

at the anode Electrically charged

particles (electrons) are left behind

These electrons flow through the metal

to other points (cathodes) where

electron-consuming reactions occur

The result of this activity is the loss of

metal and often the formation of a

deposit

Are Copper, Aluminum

Alloys, and Stainless Steel

Subject to Corrosion?

In general, these metals corrode more

slowly than mild steel However, in

some waters, these metals may be

subject to severe localized (or pitting)

attack In addition, dissolved gases, such

as hydrogen sulfide (H2S) or ammonia

Severe galvanic attack on steel adjacent to nozzle holes where brass nozzles had been inserted

What are the Different Types of Corrosive Attack?

Many different types of corrosionexist, but the most common arecharacterized as general, localized orpitting, and galvanic

General attack exists when the

corrosion is uniformly distributed overthe metal surface The considerableamount of iron oxide produced bygeneralized attack contributes tofouling problems and reduces systemefficiency

Localized (or pitting) attack exists

when only small areas of the metalcorrode Pitting is the most seriousform of corrosion because the action isconcentrated in a small area Pitting mayperforate the metal in a short time

General corrosion

Galvanic attack can occur when two

different metals are in contact Themore active metal corrodes rapidly.Common examples in water systemsare steel and brass, aluminum and steel,and zinc and steel If galvanic attackoccurs, the metal named first willcorrode

What Water Characteristics Affect Corrosion?

The most important factors are:

• Oxygen and other dissolved gases

• Dissolved and suspended solids

How Does Oxygen Affect

Corrosion?

Oxygen dissolved in the water is

essential for the cathodic reaction to

take place Dissolved oxygen drives

the cathodic reaction by accepting

electrons from the metal This allows

more metal to dissolve at the anode

How do Dissolved and

Suspended Solids Affect

Corrosion?

Dissolved solids can affect the

cor-rosion reaction by increasing the

electrical conductivity of water As

the dissolved solids concentration

increases, so does the conductivity,

and the likelihood of corrosion is

greater Dissolved chlorides and sulfates

are particularly corrosive Suspended

solids can influence corrosion by

erosive or abrasive action, and they

can settle on metal surfaces to set up

localized corrosion cells

How does Alkalinity or

Acidity Affect Corrosion?

Acidic and slightly alkaline water can

dissolve metal and the protective oxide

film on metal surfaces More alkaline

water favors the formation of the

protective oxide layer

How Does Water Velocity

Affect Corrosion?

High-velocity water increases corrosion

by transporting oxygen to the metal

and by carrying away corrosion

products at a faster rate High velocity

can also cause erosion of metal

surfaces, any protective films on the

When water velocity is low, deposition

of suspended solids can establishlocalized corrosion cells, therebyincreasing corrosion rates

How Does Temperature Affect Corrosion?

Below 160°F (71°C), every increase

in temperature of 18°F (10°C) causescorrosion rates to double Above160°F (71°C), additional temperatureincreases have relatively little effect

on corrosion rates in cooling watersystems, partly because the oxygenconcentration in water is reduced athigh temperatures

Electrochemical corrosion cell and reactions that occur at the anode and cathode

How Does Microbial Growth Affect Corrosion?

Microbial growth promotes theformation of corrosion cells In addition,the byproducts of some organisms,such as hydrogen sulfide from anaerobiccorrosive bacteria are corrosive tomany metals

What Methods are Used

to Prevent Corrosion?

Corrosion can be prevented orminimized by one or more of thefollowing methods:

• When designing a new system,choose corrosion-resistantmaterials to minimize the effect

• Apply protective coatings such as

paints, metal plating, tar, or plastics

• Protect cathodically, using sacrificial

anodes or other methods such as

impressed current

• Add protective film-forming chemical

corrosion inhibitors that the water

can distribute to all wetted parts of

the system

How do Chemical Corrosion

Inhibitors Work?

Chemical inhibitors reduce corrosion

by interfering with the corrosion

mechanism Inhibitors usually affect

the corrosion reactions at either the

anode or the cathode

Anodic corrosion inhibitors establish

a protective film on the anode These

inhibitors can be effective, although

they can be dangerous; if insufficient

anodic inhibitor is present, the entire

corrosion potential occurs at the

remaining unprotected anodic sites

This causes severe localized (or pitting)

attack

Cathodic corrosion inhibitors form a

protective film on the cathode These

inhibitors reduce the corrosion rate in

direct proportion to the reduction of

cathodic reaction

General corrosion inhibitors protect

by filming all metal surfaces whether

anodic or cathodic

What Inhibitors are Commonly Used for Cooling Water Systems?

• Triazoles for copper

What is the Cost of Corrosion?

Corrosion can increase costs of anyprocess It causes fouling that reducesheat exchange efficiency and reducesthe process efficiency If severe enough,corrosion can cause failure of anexchanger, requiring either retubing orreplacement This has direct cost forthe equipment being replaced and costs

to shut down the process while repairsare completed (see Material CostsChart above)

Material Costs to Replace or Retube Chillers Centrifugal Centrifugal Absorption Absorption Tons (GJ/h) Retubing Chiller Retubing Chiller

Consistent control of both the sion inhibitor chemicals and the keywater chemistry characteristics isessential for effective corrosion control

corro-No program will work without properapplication of treatment and systemcontrol

While daily monitoring of waterchemistry and product dosage can beeffective, continuous monitoring andcontrol will provide the best results.3D TRASAR technology provides themeans to continuously monitor thesystem stresses and automaticallyrespond to any changes in the system,thereby controlling corrosion stresses

at the optimum level for lowest TCO.With 3D TRASAR technology, thecooling system can provide its optimumvalue to the total operation of thefacility