power house chemistry

8/11/2019 Power House Chemistry

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Power House Water

chemistry

Dr. S. K. PramanikPh.D. (Chem); MBA (TQM)

Sr. Chemist

DVC, CTPS (U# 7&8)e-mail: [email protected]

M: +91-9973789375



9 October 20142

Surface Drainage water (Rivers, Lakes and Reservoirs)

Underground Water (Shallow Well, Deep Well and Springs)

Rain Water

Sea Water

Snow Melting



R

9 October 20143

The purest water available naturally is the one obtained fromwater vapour in the atmosphere as rain, snow or produced by

melting of ice.

This water while reaching the ground absorbs different types

of gasses from atmosphere like nitrogen, oxygen and to a

lesser extent carbon dioxide.

Other gasses like ammonia, oxide of nitrogen and oxides of

sulphur etc., also dissolves during rain depending upon the

pollution level of the atmosphere.

Apart from this, the surface water travels to various places

catch organic matters, suspended solids etc.



9 October 20144

SEA 95-96%

FROZEN WATER 2%FRESH WATER 2-3%

• Fresh water available to us is only 2-3% of water supply.

• We, the human beings, are bent upon polluting this precious resource.

• Imperative to take proper care to conserve and reuse water.

Water Supply



9 October 20145

WATER CHEMISTRY IS A VERY IMPORTANT DISCIPLINE IN

POWER SECTOR.

TO ACHIEVE HIGHER OPERATION EFFICIENCY, MINIMIZE

CORROSION & SCALE FORMATION PROBLEMS AND TO

REDUCE PLANT DOWNTIME, HIGH WATER QUALITY

STANDARDS ARE TO BE MAINTAINED, PARTICULARLY IN

VIEW OF UPCOMING SUPER CRITICAL BOILERS

ROLE OF CHEMISTRY IN POWER PLANT



9 October 20146

PRETREATMENT OF RAW WATER

FILTER WATER FOR DM PLANT

ULTRA PURE /DEMINERALISED WATER FOR

BOILER MAKE-UP/STEAM GENERATION

COOLING WATER SYSTEM.

MONITORING OF STEAM/ WATER PARAMETERS

& H.P./L.P. DOSING SYSTEMS

COAL & ASH ANALYSIS

TRANSFORMER/TURBINE OIL ANALYSIS.

POLLUTION CONTROL

PRE/POST COMMISSIONING ACTIVITIES IN PLANT

EFFLUENT MANAGEMENT

ROLE OF CHEMISTRY INVOLVES IN:



9 October 20147

PART - A

• PRE TREATMENT

PART - B

• POST TREATMENT(DEMINERALISATION)

PART - C • COOLING WATER TREATMENT

PART – D• BOILER WATER CHEMISTRY



PART -

I

9 October 20148



9 October 20149

WATER FLOW DIAGRAM

CLARIFLOCCULATOR

GRAVITY

FILTER

D.M. PLANT HVAC COOLING

WATER

CLARIFIED

STORAGE TANK

RAWWATER

DRINKING

WATER

BOILER

MAKEUP

C.W. MAKEUP U/G STORAGE

TANK



9 October 201410

1. Suspended Form (Macro size) Sand, dirt, silt arethe suspended mater in water. These contribute

turbidity to raw water.

2. Colloidal form – Micro size particles(1-100 nm)

3. Dissolved form - Alkaline salts and neutral salts,

organic matter,

Alkaline salts are mainly bicarbonates rarely

carbonates and hydrates of calcium, magnesium

and sodium. Neutral salts are sulphates,

chlorides, nitrates of calcium, magnesium and

sodium.



9 October 201411

ORGANICS :-

Organics in water is mostly due to the decomposedproducts of vegetable matters, though some man-made organic wastes are not ruled out.

In these organics the weak acidic large molecules

called Humic acid and Fulvic acid are the mosttroublesome in the W.T. plant as they attack the AnionResins and foul it causing problems in regeneratingthe resins.

The biggest Humic Acid is of colloidal size and passesthrough ion exchange beds.

There are organic impurities in the form ofmicrobiological species like bacteria, virus, Alagae and

fungi etc.

Contd….



9 October 201412

Suspended Matters :

They are generally silicious in nature along withsome oil and other unwanted things dependingupon the source of water.

If not removed in pretreatment then these thingsget filtered in the ion exchange beds and causeincrease in differential pressure of the bed andsometimes cause uneven distribution of flow.

Depending on the quantity it may give problems inback washing also.

Contd….



9 October 201413

NON- REACTIVE SILICA : -

This cannot be removed by ion-exchangeprocess . It passes through resins and goes inD.M. water and at high pressure and temperature

in the boiler, gets converted intoordinary/reactive silica.

As non reactive silica cannot be analyzed bynormal methods (ANSA), It deceives normal

operation.

This non-reactive silica is called “ ColloidalSilica” also.

Contd….



9 October 201414

Chlorination Dosing of alum/lime

Coagulation and

flocculation Sedimentation

Filtration

De-chlorination

Pre- Treatment of water

Depending on the usage of the water it is to be treated

on different ways.Pre-treatment takes care of organics, suspended

matter and colloidal silica to some extent.



WATER

TREATMENT

9 October 201415

CLARIFLOCCULATORGRAVITY

FILTER

D.M.

PLANT

U/G STORAGE

TANK

RAW

WATER

ALUM & Cl2

CLARIFIED WATER

STORAGE TANK



The processes by which the aeration accomplishes the desired

results are :

• Sweeping or scrubbing action caused by the turbulence of

water and air mixing together.

• Oxidizing certain metals and gases

16 9 October 2014



THE TREATMENT PROCESS

Aeration. Raw water pumped from the well ismixed with air.

The mixing releases carbon dioxide and

hydrogen sulphide gases present in the water.

Aeration also oxidizes any iron,

causing it to "precipitate" (or settle out)

removed by precipitation and filtration.

9 October 201417



18

Fe(HCO3)2 + 2HOH = Fe(OH)2+ 2H2CO3

H2CO3 = H2O + CO2

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

9 October 2014



DISINFECTION

9 October 201419

Disinfection is destruction of Pathogenic bacteria,

virus, germs and other organisms present inwater.

It can be achieved by

Gaseous chlorine

Chlorine compounds such as hypo-chlorites,bleaching agent and chlorine dioxide

Ozone

Ultra-Violet radiation

Hydrogen peroxide Heating

Combination of the above



CHLORINATIONChlorination is the application of chlorine to water to

accomplish some definite purpose.

20

PRE-CHLORINATION

POST-CHLORINATION

IN STILLING

CHAMBER

DOSINGIN FILTER

SUMP

9 October 2014



PURPOSE :

for the purpose of disinfection.

be used for taste and odor control. iron and manganese removal.

and to remove some gases such as ammonia and

hydrogen sulfide.

21

Prechlorination is

the act of adding

chlorine to the raw

water afterscreening and

before flash

mixing.

Postchlorination is

the application of

chlorine after water

has been treated butbefore the water

reaches the

distribution system

9 October 2014



CHLORINATION

9 October 201422

Chlorination is the process in which chlorine gas or

chlorine compounds are added to water for thepurpose of disinfection, by killing disease producingorganism and algae.

Reaction of chlorine with water

When chlorine is dissolved in water, it is rapidlyhydrolysed to form HCl and HOCl

H2O + Cl2 HCl + HOCl

HOCl H+ + OCl- ( At pH more than 6.0)Batericicidal effect of chlorine is maximum whenchlorine is in the HOCl form. Chlorine is most effectivedisinfectant at pH between 5 - 6.



Reaction with organisms

9 October 201423

Chlorine reacts with water to produce nascent

oxygen which destroys the physical structure of theorganisms. The physical structure i.e. the cell-wall of

the organism contains amino group which is

destroyed by chlorine.

Reactions :-

NH3 + HOCl NH2Cl + H2O (monochloramine)

NH2Cl + HOCl NHCl2 + H2O (dichloramine)

NHCl2 + HOCl NCl3 + H2O (nitrogen trichloride)



In addition to disinfection, chlorine also has the

following functions:

•taste and odor control as an oxidizing agent•oxidation of Fe2+ and Mn2+ in groundwater

•ammonium removal in domestic waste treatment

•slime, bio-fouling control

Disadvantages:The formation of disinfection by-products

(trihalomethanes) presents a health risk

Not advisable at high pH and ammonical compounds

9 October 201424



EFFECTIVENESS OF CHLORINE AS A BIOCIDE

EFFECT OF pH ON THE DISSOCIATION OF HYPOCHLOROUSACID

pH

4

5

6

7

8

9

HOCl

100

99.7

96.8

75.2

20.0

Negligible9 October 201425



EFFECTIVENESS OF CHLORINE AS A BIOCIDE

EFFICIENCY OF CHLORINE AT DIFFERENT pH IN CLARIFIER/ COOLING WATERSYSTEM

4 5 6 7 8 9 10 11

80

60

40

20

0

20

40

60

80

100

O°C

HOBr

P

e r c e n t i o n i z e d f r o m ( O

C l - o r O B r - )

P e r c

e n t u n - i o n i z e d f o r m ( H O

C I - o r H O B r - )

20°C

HOCI

pH

Cl2 + H2O HOCl + (H+ +Cl – )

(Hypocblorous acid)

HOCl H+ + ClO –

(Hypocblorite ion)

HOCl + OH – H2O + ClO – 9 October 201426



9 October 201427

Chlorine dioxide (ClO2) as biocide

It is generated in-situ as per reaction:

Cl2 + 2NaClO2 2ClO2(gas) + 2NaCl

Its advantages include:

Effective in at lower dosage than chlorine. At pH 8.5 it is at

least five times effective than chlorine.Does not react with ammonia hence effective in ammonical

water and high organics

No disinfection by-products such as trihalomethanes

More efficient and effective in wide range of pH

High oxidation potential( E0 = + 0.954 V at 25°C)

It is more selective towards environmentally objectionable

compounds like phenol, cyanides and mercaptans.



CLARIFLOCCULATOR

9 October 201428

Clariflocculator is a circular concrete tank having two

zones for the removal of impurities .

The flocculator zone where the micro-flocs agglomerateinto macro-flocs with the help of slow speed agitators.

Clarification zone where the agglomarated flocs settle

leaving clear supernatent liquid Water enters the clarifier through the central shaft and

flows on to the flocculation zone though the partsprovided at the top of the shaft.

Sludge can be cleared by gravity flow or sludge

disposal pumps. Depending on the sludge quantity ,thebridge is to be operated continuously or intermittently.

Required chemical are dosed before water enters CCF.



CLARIFLOCCULATOR

9 October 201429

Raw

water

Chlorine Alum

Lime Flash

Mixer

Clarification

Sludge

settling

pond

Clarified

water tofilters

Flocculation

Water quality at Clarifier outlet

Turbidity - <20 NTU pH - 5.5 to 8.0

Residual Chlorine - 0.2 ppm



COAGULATION

9 October 201430

COAGULATION IS A PROCESS WHICH

NEUTRALIZE NEGATIVE CHARGE ON

PARTICLES WHICH ARE COLLOIDAL IN

NATURE AND HELPS TO FORM FLOCKS.



Common Coagulants

9 October 201431

Alum Hydrated aluminum sulfate [Al2(SO4)3·18H2O]

Alum, when added to water, will be hydrolyzed to form gelatinoushydroxide [Al(OH)3] precipitate. This will carry suspended solidsas it settles by gravity. (pH 5.5-8)

Anhydrous Fe3+

Forms Fe(OH)3 (s) in a wide range of pH 4-11

Anhydrous Fe

2+

(copperas, FeSO4·7H2O)

Must be oxidized to Fe3+ first at pH higher than 8.5

Natural and synthetic polyelectrolytes

Starch, cellulose derivatives, proteinaceous materials, and gumscomposed of polysaccharides

Synthetic polymers Poly electrolytes (long chain amides)

Poly Aluminum Chloride ( PAC )

Factors affecting coagulation: pH, Time, Temperature,



ROLE OF ALUM DOSING

Removes suspended particulate and colloidal substances from

water, including microorganisms.Coagulation: colloidal destabilization

Typically, add alum (aluminum sulfate) or ferric chloride or

sulfate to the water with rapid mixing and controlled pH

conditions

Insoluble aluminum or ferric hydroxide and aluminum or iron

hydroxo complexes form

These complexes entrap and adsorb suspended particulate

and colloidal material.

9 October 201432

OCC O



FLOCCULATION

9 October 201433

SMALL FLOCKS (POLYMERS) COMING TOGETHERTO FORM BIGGER EASILY SETTLEABLE FLOCKS IS

CALLED FLOCCULATION i.e., INORGANICPOLYMERS (ACTIVATED SILICA, ALUMINOSILICATE), ANIONIC (ACRYLAMIDE AND ACRYLICACID)

Al2(SO4)3 + 3Ca(HCO3)2 2Al(OH)3 + 3CaSO4 +6CO2

Fe2(SO4)3 + 3Ca(HCO3)2 2Fe(OH)3 + 3CaSO4 +6CO2

Ca(HCO3)2 + Ca(OH)2 2CaCO3 + 2H2O



SEDIMENTATION

9 October 201434

COAGULANT MATERIAL THAT HAS TO

SETTLE OUT OF THE WATER CONSISTS OF

PATICLES OF ENHANCE DENSITY.

CONSEQUENTLY IT CAN BE REMOVED MORERAPIDLY BY SEDIMENTATION.



Coagulants

Alum (aluminum

sulphate), polyaluminumchloride and a group of

chemicals known as

polyelectrolytes .

.Large +ve Chargeattracts -ve charged clay

particles

Zeta potential

Large charge on small

ion Al+++ , Fe +++

9 October 201435



9 October 201437

Filtration is the removal of the solid particlesfrom water by passing it through a filteringmedium. Filtration is usually a mechanicalprocess does not remove dissolved solids.

Filters used in Water Treatment are mainly of two types.

1. Pressure Filters

2. Gravity filtersPressure filters are in closed, round steelshells and function with the pressure of theincoming water.Gravity filters are in steel, wood or concretecontainers that are open at the top andfunction at atmospheric pressure.

FILTRATION



9 October 201438

Theoretically any inert granular material can

be used for filtration.

Quarts sand, Silica sand, anthracite coal,

garnet may be used for filtration. Silica sand and anthracite are the types of filter

media which are commonly used.

At DVC sand is used as filtering medium and

filters are Gravity sand filters (GSF).

Filter Media



9 October 201439

Gravity Sand Filter

IN

OUT

5th layer

4th layer

3rd layer

2nd layer

1st layer

Gravity Sand Filter

For back washing of

the GSF water is

passed through filter

in reverse direction

Clarified

water from

clarifier



9 October 201440

1st layer - 50 mm X 37 mm gravel

2nd layer - 37 mm X 12 mm gravel

3rd

layer – 12 mm X 6 mm gravel

4th layer – 6 mm X 2.5 mm grit

5th layer – 0.35 mm X 0.5 mm sand

Filter medium layers in GSF



9 October 201441

1. Feed water to DM plant

2. Feed water to Softening Plant

3. Drinking water – Township and plant4. Service water – as cooling water for A/C

and Compressors

Uses of filtered water



PART -

II

9 October 201442



DM PLANT

9 October 201443

DMwater

storage

tank

ACF SAC SBA MB

DEGASSER

Air

To main plant for boiler make up

For circuit rinse

From filterwater pumps



Typically, the cation resin operates in the hydrogen cycle.

The cations in the water (i.e. calcium, magnesium and sodium) passthrough the cation exchange resin where they are chemically exchangedfor hydrogen ions.

The water then passes through the anion exchange resin where theanions (i.e. chloride, sulphate, nitrate and bicarbonate) are chemicallyexchanged for hydroxide ions.

The final water from this process consists essentially of hydrogen ionsand hydroxide ions, which is the chemical composition of pure water.

CATION EXCHANGER

ANION EXCHANGER

Ion-exchange Reactions



9 October 201445

Ion exchange Reactions

Cation Exchanger

During ServiceNaCl RNa + HCl

RH + CaCO3 R2Ca + H2CO3

MgSO4

R2

Mg + H2

SO4

Na2SiO3 RNa +H2SiO3

During Regenration

RNa Na2SO4

R2Ca + H2SO4 RH + CaSO4

R2Mg MgSO4




9 October 201446


Anion Exchanger

During ServiceHCl R’Cl +

H2O

R’OH + H2CO3 R’2CO3 +

H2O

H2SO4 R’2SO4 +H2O

H2SiO3 R’2SiO3 +

H2O

During Regenration

R’Cl NaCl

R’2CO3 + NaOH R’OH + Na2CO3

R’2SO4



• The resin in the pressure vessel has about 50% free space above the resin.

• This free space allows backwashing,removal of any entrained solids.

• Water and acid/caustic regeneration is carried out in a down-flow direction.

CO-CURRENT FLOW REGENERATION



Coflow or Downflow Regeneration


RESIN BED

Feed In

Regenerant

In

Regenerant

out

Treated Water Out

Collecting

System



• The regenerant acid and caustic passes in the opposite direction to

the service flow water.• With counter-flow regeneration, the regenerant passes throughthe resin near to the outlet of the unit .

COUNTER-CURRENT FLOW REGENERATION




Counter flow Regeneration

ACTIVE

RESIN

BED

Feed In

Regenerant

Out

Regenerant

In

Treated Water Out

Downflow of water

During Regn



Advantage of counter current :

Lower portion of the bed ( which acts as service water effluent ) is

retained under fully regenerated conditions.

The leakage of ions is substantially reduced

Also, associated with lesser chemical consumption

Cation Exchange Mechanism



g

Start of run During the run End of run

Ca

Mg

Na

Ca

Mg

Na

Ca

Mg

Na

Na

Anion Exchange Mechanism

SO42-

Cl-

SiO2

SO42-

Cl-

SiO2

SO42-

Cl-

SiO2

Cation exhaustion leads to Na leakage

while anion exhaustion leads to SiO2 leakage

MIXED BED DEMINERALISATION



Polishing mixed beds come after the cation and anion standardvesselsand, as the name implies, they are there to polish the water.The bed is an intimate mix of anion and cation resins.

MIXED-BED DEMINERALISATION



Mixed bed:During service step ,cation and anion resins are intimately mixed offering

innumerable close linked exchangeable sites.

Thus acid formed by contact of salt with cation bead is immediately

neutralised by neighbouring anion.

During re-generation,backwashing separates the lighter anion resin fromdenser cation resin.

A collector is placed at interface between two resins facilitating

regeneration operation without removing the resins from column.

A simultaneous regeneration of cation and anion resin can be adopted.

MIXED BED



MIXED BED

Service and Regeneration

Air

Vent

SI

SO Drain

Alkali injection

Acid injection

NF

Air



Re –generation of mixed bed exchanger :

1. Resin separation/backwash

2. acid and alkali injection

3. acid and alkali displacement – using DM water

4. Drain to bed level

5. Air mix6. Fill up

7. Final rinse

Mixed Bed



Mixed BedResin Separation

Cation exchangeResin

Anion exchangeResin



Water quality at different stages of Demineralisation process:-

Feed water to DM plant

Turbidity - <2 NTU

ACF outlet

Residual chlorine - Nil

Turbidity - < 0.5 NTU

Cation Exchanger outlet

Na - <2 ppmDegasser outlet

Dissolved CO2 - <5 ppm

D.M. PLANT



Anion Exchanger out

Silica - < 200 ppb

Conductivity - < 10 s/cm

pH - 6.8 - 7.2

Mixed bed out

Silica - < 20 ppb

Conductivity - < 0.1 s/cm

pH - 6.8 - 7.2

D.M. PLANT



PART -III

9 October 201460

COOLING WATER



COOLING WATER

Cooling of water is an essential process at power-generation and industrial plants.

The most important uses of cooling water includes

condensing turbine exhaust steam, cooling

process fluids and protecting high pressurepump bearings.

Control of cooling water chemistry is very critical in

preventing corrosion ,scaling and fouling.



COOLING WATER SYSTEMS

Type of cooling water system most suitable forprocess depends upon:-

1. Process operation

2. Flow requirements

3. Availability and quality of water

4. Environment requirements regarding discharge



Types of cooling system

1. Closed Recirculating

2. Once-through

3. Open Recirculating (Evaporative coolingtowers)



Closed Recirculating System

Water circulates in a closed cycle Alternate cooling and heating without air contact

Heat absorbed by the water in closed system is

transferred by a water to water exchange to the

recirculating water of an open recirculatingsystem from which the heat can be lost to

atmosphere.



Once Through System

Water makes one pass through the heatexchanger equipment and discharged to waste.

Large quantity of water is needed.

Once-through system have advantage of not

concentrating water during its passage throughsystem, thus reducing scaling and corrosion

potential of water.

Water is returned to source at higher

temperature, thus cause thermal and chemical

pollution of water bodies.

Highly prone to biological fouling.

Open Recirculating( Evaporative




cooling Towers)

Water circulates through the condenser or heatexchanger to a cooling tower and then returned to

exchanger.

Same high volume flow rate as a once through

system, but with less water discharge. Cooling of water is by evaporation process, water

loss by evaporation and drift.

The evaporated water is very pure and the

minerals are left behind to concentrate.





cooling Towers)

O.R.SYSTEM have greater degree of scale formation,

corrosion & microbiological growth due to-

Higher temperature.

Make up water brings more scale forming &corrosionforming salt.

Water is exposed to air allowing continued presence of

oxygen ,which responsible for corrosion.

Cooling tower is a scrubber ,introducing

microrganism,dirt,dust etc in circulating water which

increases fouling & corrosion.



Types of Cooling Towers

FORCED/INDUCED DRAFT COOLING TOWERS

NATURAL DRAFT COOLING TOWERS

Cooling Water Balance



Cooling Water Balance

Condenser

CW Make up

Evaporation and drift

Blow Down

Air + water

vapour

Water Air Air

Natural Draft Cooling Tower

CT Basin

TERMS ASSOCIATED WITH




COOLING TOWER

Cycle of concentration ( C ) : Number of times thecirculating is concentrated in cooling tower is known asCycle of concentration. The maximum C depends upon theeffectiveness of corrosion and scale inhibitor programs

and on the quality of make up water.

Blow Down ( BD ) : Some water must be continuallyremoved from cooling water system to prevent excessive

build up of the dissolved solids. This is known as blowdown.





COOLING TOWER

Drift ( D ) : Drift is a form of blow down thatoccurs due to entrainment of water droplets in

the air leaving the cooling tower. Drift typically

ranges from about 0.05% to 0.3% of the

recirculation rate depending upon the typeand efficiency of the cooling tower.

Make up ( MU ): Water added to Circulating

water sysrem to replace water lost from thesystem by evaporation, drift, blown down, and

leakage.

CYCLE OF CONCENTRATION



CYCLE OF CONCENTRATION

Cycles of concentration represents the accumulation of

dissolved minerals in the recirculating cooling water

T.D.S. of Circulating Water

COC =___________________________T.D.S. of Make up Water

As the cycles of concentration increase the water may not be

able to hold the minerals in solution. When the solubility of

these minerals have been exceeded they can precipitate out as mineral solids and cause fouling and heat exchange

problems in the cooling tower or the heat exchangers.

Problems arises in circulating

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

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

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

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

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

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



g

water

CORROSION SCALE FORMATION

BIOFOULING



Recommended CW Treatment

Acid dosing

Sulphuric acid dosing is done which reduces pH as

well as alkalinity of the system, in turn it reduces

scaling tendency of the system.(pH=7 & Conc. of

SO4=< 600mg/kg).

Chemical Dosing system

Descalent and corrosion inhibitors are added to

system to avoid scaling and corrosion in system.Biocides are also added to reduce biofouling of the

system



PART -IV

9 October 201475



WATER STEAM CYCLE

D.M. WATERStorage

Tank CONDENSER D/A

TURBINE STEAM BOILER

T.S.P. DOSING

AMMONIA

DOSING

BFP

HYDRAZINE

DOSING

CEP

76 9 October 2014



WATER/STEAM CHEMISTRY

PARAMETERS MONITORED pH

Silica

Conductivity

After Cation Conductivity

Dissolved Oxygen

Sodium

Copper

Iron

77 9 October 2014



WATER QUALITY

FEED WATER

ACC <0.02 uS/cm

pH 8.8-9.2

Total Iron+Copper <0.02 ppm

Silica <0.02 ppm

Dissolved Oxygen <7 ppb

78 9 October 2014



WATER QUALITY

CONDENSATE WATER ACC <0.02 uS/cm

pH 8.8-9.2

Silica <0.02 ppm

Dissolved Oxygen < 40 ppb

79 9 October 2014



BOILER WATER

Conductivity <30 uS/cm

pH 9.2-9.6

Silica <0.300 ppm

Phosphate 2-4 ppm

WATER QUALITY

80 9 October 2014

WATER QUALITY



STEAM

ACC <0.02 uS/cm

pH 8.8-9.2

Total Iron+Copper <0.02 ppm

Silica <0.02 ppm

Sodium <10 ppb

WATER QUALITY

81 9 October 2014

DISTRIBUTION RATIO BETWEEN



Drum Pressure Silica in Boiler Water

194 Kg/Cm2 130 ppb

176 Kg/Cm2 220 ppb

159 Kg/Cm2 290 ppb

134 Kg/Cm2 500 ppb

117 Kg/Cm2 1000 ppb

100 Kg/Cm2 2220 ppb

65 Kg/Cm

2

4000 ppbBoiller Drum Pressure is to be maintained as such,Silica value in Main Steam maintain bellow 20 ppb.

DISTRIBUTION RATIO BETWEEN

STEAM & BOILER WATER AT pH 9.5

82 9 October 2014

PARTITION COEFFICIENT AT



ON CO C N

DIFFERENT PRESSURES

100

10-1

10-2

10-3

10-4

10-5

10-6

10-7

226 220 200 180 160 140 120 100 50 40 30

PRESSURE ( BAR)

83 9 October 2014



By evaporation from the boiler drum

By entrainment of boiler water droplets in

saturated steam.

As impurity present in feed water used in desuper

heater spray.

SOURCES OF IMPURITIES IN

STEAM

84 9 October 2014



Silica has high partition coefficient, so it has

tendency to deposit from steam onto turbine.

Silica can deposit on turbine blades specially on

LP turbine, which can lead to significant loss of

output.

EFFECTS OF SILICA

85 9 October 2014



L.P.DOSING

AMMONIA & HYDRAZINE HYDRATE DOSING

AMMONIA IS USED TO INCREASE THE pH OFTHE SYSTEM.

N2H4 + O2 N2 + H2O

3N2

H4

4NH3

+ N2

86 9 October 2014

H P DOSING



COORDINATED PHOSPHATE CONTROL

Na3PO4+H2O Na2HPO4 + NaOH Na2HPO4+H2O NaH2PO4 + NaOH

NaOH + HCl (As Impurity) NaCl + H2O

H.P. DOSING

87 9 October 2014

Boiler water treatment



Tri sodium phosphate provides the needed alkalinity in boiler systems

as follows :o Na3PO4 + H2O === NaOH + Na2HPO4

Absorption of contaminants :

o 10Ca2+ + 6PO43-- + 2OH-- 3Ca3(PO4).Ca(OH)2

calcium hydroxyapetite

o 3 Mg2+ + 2SiO32- + 2OH-- + H2O

3MgO.2SiO2.2H2O

serpentine

Calcium hydroxyapetite and serpentine exist as soft sludges

and much easier to remove ; typically settle in the drumand removed by blow down

Boiler water treatment

88 9 October 2014

CHEMICAL TREATMENT PROGRAMS CHARACTERISTICS



program favourable unfavourable

Coordinatedphosphate

Caustic corrosion may be

eliminated; deposit form ..

easy for removal ; acids

neutralized; surface

passivation by phosphate

Possible “under-deposit”corrosion by concentrated

sodium hydroxide ;

Hide-out

CongruentPhosphate

Na:PO4

(2.6:1)

Caustic corrosion mostly

eliminated ; deposit form ..

Easy for removal ; acids

neutralized ; surface

passivation by phosphate

Controlling molar ratio ofNa and PO4 ;

Hide-out

Sodiumhydroxide

Acid neutralization ;

No phosphate hide-out

Can cause rapid corrosionwhen concentrated (specially

under deposit ) ; vaporous

carryover in steam at high

pressure ; dosing control very

essential89 9 October 2014

CHEMICAL TREATMENT PROGRAMSCHARACTERISTICS



program favourable unfavourable

All volatile deposition of salts can be

eliminated ; high purity

steam under ideal feed

water conditions ; no carry

over of solids

Feed water contamination may

exceed inhibiting ability of

volatile feed , leading to boiler

corrosion ; marginal acid

neutralization ; no protection

during mild hardness ingress

oxygenated

treatment

Low corrosion rates of

ferritic steels and conden-

ser tubes ; better oxidecoating , hence frequency

of chemical cleaning

increased

Can tolerate very low

concentration of impurities ; no

corrosion protection in case ofupset ; copper alloys should not

be used in the system ; requires

excellent purity feed water ;

precise chemical control required

90 9 October 2014

LOW D O



- Limits below 10 PPb

With low D.O. concentration, copper

corrosion is inhibited by a passive film of

Cuprous Oxide(Cu2O).

LOW D.O.

REGIME

91



HIGH D.O.

REGIME

- Limits 2 to 5 PPM

With high D.O. concentration, copper

corrosion is inhibited by a passive film of

Cupric Oxide(CuO).

92

Health of a



Health of a

boiler

Water

chemistry control

Health of heat

exchanger tubes

93 9 October 2014

Cycle chemistry guidelines



Cycle chemistry guidelines

Most sensitive part in the plant cycle …

TURBINE

Chemistry limits established for

steam

Boiler water

Feed water

Make-up

water94 9 October 2014

Feed water chemistry



Condensate plus make-up water

Virtually all impurities carried into

the boiler through the feed water

Condensate :

Corrosion in the pre-boiler section ;

subsequent transport to the economizer ,boiler and subsequent deposition high

heat zones

Make-up water : Though less prevalent

can carry hardness salts and silica

95 9 October 2014

Steam Chemistry



Steam purity affected by carryover --- the process

by which solids are transported to steam

Carryover influenced by

silica carry over as vapour solids become more soluble at high pressures

drum level , drum design (internals) , foaming

Contaminants can also enter via attemperator

systems ; greatly exacerbated during upset

conditions such as a condenser leak

y

96 9 October 2014

Tube failure locations in a

boiler



Can occur anywhere in

a Boiler

Water- or steam-

cooled tubes :

Water walls

Screen / roof

tubes

SH / RH tubes

boiler

97 9 October 2014

Tube Sampling



Tube Sampling

Representative

Heaviest deposit

formed location

2 to 3 meters

above the top

most burner

Problem areas in

specific units

{ Horizontal / sloped etc }

98 9 October 2014



How tube / turbine blade failures can look like ?

99 9 October 2014



Tube failure control

starts with

Design

Manufacture

Shipping , Storage & Construction

Quality control

Cleanliness of the tube surfaces bychemical cleaning

100 9 October 2014

N it



New units

Internal surfaces of heat exchanger tubes to

be clean --- before put into service

Oil , grease , sand etc removed by alkali

cleaning (acid pickling ) for removal of rust

and surfaces of the tubes to protect from

corrosion (passivation)

101 9 October 2014

Ch i al l a i g f b il r



Chemical cleaning of boilers

Condensate & feed water system

mechanical cleaning ; alkaline flush

Economiser & Boiler tubes

alkali boil out ; acid cleaning ; passivation

Super heater , steam piping & Re heater

scavenging with steam

102 9 October 2014

Why passivation ?



Magnetite , Ferric oxide ,

{ Fe3O4 } { Fe2O3 }

Colour black brownish red

Binding tightly binds flakes off easilynature to base metal from base metal

w.r.to protects the does not protect

corrosion base metal the base metal

{significance}

y p

103 9 October 2014



Boiler tube before cleaning

Boiler tube after

cleaning

Black magnetite layer(protective)

104 9 October 2014

Obj ti f t h i t ti



Objectives of water chemistry practice

Reduce corrosion of metals

Prevent formation of deposits

Produce good quality steam

{ without carryover of boiler

water solids }

105 9 October 2014

Quantity of deposit and unit cleanliness



Quantity of deposit Surface cleanliness

Less than 15 mg /sq.cm. Clean surface

15 to 40 mg / sq.cm. Moderately dirty

more than 40 mg /sq.cm. Dirty

Chemical cleaning should be done whenever deposits are more than 40

mg / sq.cm . once in 4 years as a mandatory maintenance practice

( guidelines only / not a rule or code ) BIS : 10391

Quantity of deposit and unit cleanliness

106 9 October 2014

Deposition in boiler tubes



Sources :

water borne materials treatment chemicals

corrosion products

contaminants

Hardness salts { Ca & Mg salts , silica }

Dosing chemicals { ‘PO4’ , NH3 , N2H4 esw..}

Pitting & pre-boiler corrosion products

Through condenser leak ; attemperationwater ; regeneration chemical slip & so on

107 9 October 2014

Deposition in boilers … consequences



Reduced heat transfer

{ Ca , Mg salts & silica ..almost insulating }

Complicates subsequent post-

operational chemical cleaning

{ Cu … multi-step cleaning may be

needed }

Chemical cleaning may be ineffective

{ Ca , Mg , Silica may not be completely

removed if present in huge quantities } Under – deposit corrosion

108 9 October 2014

UNDER DEPOSIT CORROSION



Water wall tube

without deposit

Water wall tube

with deposit

109 9 October 2014

UNDER DEPOSIT CORROSION



Na3PO4 + H2ONa2HPO4 + NaOH

Boiler water with Na3PO4 , Na2HPO4 , NaOH

enter through the pores of thedeposit.

Only water comes out assteam, leaving the solids toconcentrate

NaOH concentrations as high as 10,000ppm have been reported

HEAT

110 9 October 2014



WHAT IS CORROSION

CORROSION IS A NATURAL PROCESS BY VIRTUE OF

WHICH THE METALS TEND TO ACHIEVE THE

LEAST ENERGY STATE I E COMBINED STATE

M M2+ + 2e-

ANODIC REACTION

N 2- + 2e N

CATHODIC REACTION

111 9 October 2014

MECHANISM OF CORROSION



MECHANISM OF CORROSION

Corrosion Cell

Na+

Ca++

Cl-

SO4 – -O2

OH

Fe++

H+Water

Anode Steel Cathode

Electrons

Fe+

Fe+ Fe+

Fe+

Fe(OH)2

Fe(OH)2 Fe++

OH –

O2

H+ H2

H+ H+

112 9 October 2014

Corrosion of boiler steel



Corrosion of boiler steel

Factors responsible for corrosion

pH

Dissolved oxygen

113 9 October 2014

Corrosion of steel vs boiler water pH



p

safe range

8.5 11.0

pH

Corrosion

rate

acidic alkaline

4 8 10 126 14

114 9 October 2014

F f



Forms of corrosion

Caustic corrosion

Hydrogen damage

Pitting

Pre-boiler corrosion

Stress corrosion cracking

115 9 October 2014

Caustic corrosion



Alkali-producing chemicals dosed in boiler

water to maintain the optimum pH

Na3PO4 + H2O Na2HPO4 + NaOH

Corrosive action of sufficiently concentrated

alkali on boiler tubes leads to corrosion

Fe3O4 + 4 NaOH 2 NaFeO2 + Na2FeO2 + 2H2O

{ black magnetite eaten away }

Fe + 2 NaOH Na2FeO2 + H2

{ parent metal attacked } 116 9 October 2014



CAUSTIC DAMAGE

117 9 October 2014



Caustic gouging Caustic gouging

118 9 October 2014

Hydrogen damage



Hydrogen damage occurs in boilers operatedwith low pH water chemistry

… by aggressive anions like chlorides

… concentration of acidic species under

deposits

During periods like condenser leakage ,

specially in sea-cooled power plants , lots of

acidic species are introduced

MgCl2 + 2 H2O Mg(OH)2 + 2 HCl

119 9 October 2014

Hydrogen damage



Atomic hydrogen can diffuse into steel

and react with iron carbide

Fe3C + 4 H 3 Fe + CH4

Methane , being a bigger molecule , can not

diffuse ; but accumulate at grain boundaries

Stresses at grain boundaries produce

intergranular micro-cracks [making tube brittle ]

Thick-walled burst occurs { large , rectangular

section of the wall blown out with a big hole }

[ brick structure without mortar ]120 9 October 2014



HYDROGEN DAMAGE

121 9 October 2014



Hydrogen damage

thick walled burst

Hydrogen damage

rupture of the tube

122 9 October 2014



HYDROGEN DAMAGE

123 9 October 2014

Pitting corrosion



General description :

oxygen … chemical agent { plus moisture }

idle boiler affected more than the running boiler

… protective magnetite attacked

4 Fe3O4 + O2 6 Fe2O3

unprotected metal attacked

2 Fe + H2O + O2 Fe2O3 + H2

Corrosion product carried to other parts of the

boiler ; gets deposited on the high heat zones 124 9 October 2014

Pitting corrosion --- Locations



Entire boiler system susceptible

Economizer & feed water heaters

Re heaters , especially where moisture can

collect in bends and sags in the tubes

Severe oxygen contamination ,

…other parts ( WW ) of the metal affected

Result :

… deep , distinct , almost hemispherical spheres

… pits may be covered with corrosion products 125 9 October 2014



126 9 October 2014

I d hi h b il b il

Pre – boiler corrosion



In modern high pressure boilers pre-boiler

corrosion …. Largest cause of failure pre-boiler … condenser , feed water heaters and

deaerator

Corrosion products

… iron oxides , copper oxides , metallic copper ,

and oxides of zinc & nickel { small amounts } Corrosion products are introduced as particles

into feed water ; get deposited on high heat zones127 9 October 2014




thick layer of iron oxide128 9 October 2014

Pre-boiler Corrosion



Elemental copper on water wall tube

along with oxides of iron129 9 October 2014

Salient points :




Salient points :

① Agents causing this type of corrosion

…. Oxygen and ammonia

② Iron and copper protected by their oxides

Fe3O4 and Cu2O , which adhere to the metal ③ Oxides attacked by “excess” oxygen

4 Fe3O4 + O2 6 Fe2O3

2 Cu2O + O2

4 CuO ④ Fe2O3 & CuO get peeled off { non-protective}

and transported to economizer & boiler130 9 October 2014

Pre – boiler corrosion



Dissolved oxygen attacks copper in the presence of

ammonia more severely as follows :

2 Cu2O + O2 4 CuO

CuO + 4 NH4OH Cu (NH3)4 (OH)2 + 3 H2O

(insoluble) ( soluble )

The corrosion product is transported more easily ina soluble form into the boiler

The copper-ammonia complex decomposes insidethe boiler at elevated temperatures ( > 140oC )

The liberated “ free copper ” gets deposited on theheat transfer surfaces

131 9 October 2014

R lt f bi d i t ti f

Stress Corrosion Cracking



Result of combined interaction of

① Tensile stress { internal pressure , residual

stresses induced by bends , supports , welds .. }

② Corrosive environment { chlorides , sulphates ,

hydroxide..}

③ Susceptible material

Stress corrosion causes brittle failure of metals

at stresses less than those necessary to cause

failure in a non-corrosive environment132 9 October 2014

Stress Corrosion Cracking



Transverse crack resulting from

caustic stress corrosion in astainless steel super heater tube

Extensive longitudinal crack in astainless steel line

133 9 October 2014

F il l ti

SCC --- Critical factors



Failure locations

… austenitic stainless steel used SH & RH

… low pressure stage turbines in contact

with saturated or wet steam

SCC produces tight , hairline cracks

… sometimes difficult for visual observation

… also can be thick-walled fracture

cracks may be transgranular or intergranular

… microscopic examinations needed

134 9 October 2014

CRITICAL FACTORS IN WATER CHEMISTRY



Recommended guidelines in entire water – steam cycle to

be followed always !

Special care to be taken in controlling and monitoring

dissolved oxygen , silica , & cation conductivity

Critical periods of water chemistry

Start ups Condenser leakage lay- up Periodic chemical cleaning --- a routine maintenance step to

keep heat exchanger tubes “clean”

Management support :

on-line and laboratory measurement facilities

updating chemical technology knowledge base

135 9 October 2014

power house chemistry

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