PP 1 Water Tech

7/27/2019 PP 1 Water Tech

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WATER TECHNOLOGYfor

HIGH PRESSURE BOILERS


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Bharat Heavy Electricals Limited TIRUCHIRAPALLI

water technology

Waterwall deposit analysisCHEMICAL COMPOSITION

0.10.20.10.2Sodium oxide,Na2O

0.40.13.01.8Magnesium oxide,MgO

1.10.529.819.1Calcium oxide,CaO

7.73.21.84.9Copper , Cu88.695.155.058.6

Iron oxide, Fe3O

4

0.80.58.714.7Silica , SiO2

% by weight

Sample 1 Sample 2 Sample 3 Sample 4

Parameter


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water technology

QUANTITY OF DEPOSIT (mg / sq.cm.)

38.525.5---38.5RHS waterwall

80.928.1---64.4LHS waterwall

54.822.158.647.4Rear waterwall

80.521.955.053.7Front waterwall

Sample 4Sample 3Sample 2Sample 1


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water technology

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 depositsare more than 40 mg / sq.cm .

once in 4 years as a mandatory maintenance practice

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


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1-2 .. TUBE METAL RESISTANCE

( ~ 30O C FOR 2.5mm WALL )

2-3 .. INTERNAL DEPOSITRESISTANCE

3-4 .. WATER STEAM FILM

RESISTANCE

HEAT TRANSFER GRADIENTS

AND EFFECT OF DEPOSITS

1

2

3

4

FIRESIDE

Tube metal and water film resistance

*** more or less constant

the temperature of the inner & outer wallrises due to changes in the deposit

thickness & nature ( thermal conductivity

of the deposit ) Silica , Ca , Mg salts very poor thermal conductivity


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285OC285OC


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1

2

3

4

SCALE THICKNESS

250 m 500 m 750 m

410OC 478OC 550OC

360OC 428OC 500OC

297OC 297OC 297OC

285OC 285OC 285OC

1-2..TUBE METAL RESISTANCE

2-3.. INTERNAL SCALE & DEPOSIT

RESISTANCE

3-4.. WATER / STEAM FILM RESISTANCE

285OC

EFFECT OF DEPOSIT ON HEAT TRANSFER

250 m thickness

nail


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water technology

Method of tube sampling location of tube samples very important

( since deposits may vary appreciably in different parts

of a boiler ) tube samples to be taken from locations representing

heaviest deposit (corrosion product ) formation

Band starting the central line of the uppermost burnerand extending up the furnace by about 2 to 3 metres

Operating experience in specific units

- problem areas : arch tubes in natural circulation units

or other horizontal or low-sloped circuits

Three adjacent tubes to be removed for comparison ;samples should be 600 mm in length and ends sealed


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Water technology

Objectives of water treatment

To reduce corrosion of metals

To prevent formation of deposits

To produce good quality steam


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Water technology

New units Internal surfaces of heat exchanger tubes to

be clean --- before put into service

Oil, grease , sand etc removed by alkali boilout

Chemical cleaning ( acid pickling ) for removal

of rust and millscale Thin , dense , uniform layer of magnetite to be

formed on the internal surfaces of the tubes to

protect from corrosion ( passivation )


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Saponification of oilO||

H2C O CCH2 CH3

O||

H C O CCH2 CH3 oilmolecule

O||

H2

C O CCH2

CH3

O||

H2C O H NaOCCH2 CH3

O||

H C O H NaO CCH2 CH 3

O||

H2 C O H NaO CCH2 CH3


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Water technology

Why passivation ?Magnetite , Fe3O4 Ferric oxide , Fe2O3

colour black brownish red binding nature tightly binds to flakes off easilybase metal from base metal

Significance protects the does not protectw.r.to corrosion base metal the base metal


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EFFECT OF IMPROPER WATER

CHEMISTRY FOR FEED WATER SYSTEMS Storage tanks :

** general corrosion , pitting

Condensers and feed water heaters :

** pitting , erosion , corrosion , exfoliation,

galvanic corrosion

Dearators :

** corrosion , pittingEconomizer :

** pitting , corrosion , hydrogen damage


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EFFECT OF IMPROPER WATER

CHEMISTRY FOR BOILER SYSTEMS** general corrosion , pitting , caustic gouging ,

hydrogen damage , deposit induced overheating

EFFECT OF IMPROPER WATERCHEMISTRY FOR STEAM SYSTEMS

Reheaters and Superheaters

** pitting , corrosion , overheating (SHS), exfoliation (RH)

Turbine :

** corrosion , stress corrosion cracking , pitting , galvanic

corrosion , crevice corrosion , deposition & fouling


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.

*** GUIDELINES---- normal operation

---- critical periods

~~ start up~~ condenser leakage

~~ shutdown ( lay-up)

*** MONITORING

*** MEASUREMENTPrevention is better than cure stitch-in-time


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Blowdown

HP Heaters

Deaerator

Boiler

HP

Turbine

IP Turbine LP Turbine

LP Heaters

BFP

Condenser

Condensate

storage tank

Makeup

treatment

system

Condensatepolisher

Attemperation

Steam-Water cycle diagram for reheat type boilers


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Bharat Heavy Electricals Limited, Tiruchirapalli

Research & Development

CRITERIA boiler design

pressure of operation

thermodynamic & engineering principles

type of water treatment ( boiler )

*** phosphate , AVT , etc

objectives

To achieve the expected steam purity

To minimize corrosion

To reduce deposition


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Water technology

Steam purity recommendations

100100100TOC

333Sulphate

333Chloride

101010Silica

0.150.150.3Cation cond.

S/cm

335Sodium

Once-throughtreatment, ppb

All-volatiletreatment, ppb

Phosphatetreatment ,ppb

substance


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Water technology

Feedwater guidelines

0.01-0.020.01-0.020.01-0.020.01 0.02Hydrazine , ppm , residual

0.0030.0030.0050.01Copper , ppm , max

0.0050.0050.010.01Iron , ppm , max

0.010.010.020.02Silica, ppm, max

0.20.30.30.5Cond. ( H+ ), S/cm, max

0.0070.0070.0070.007Diss.Oxygen,ppm,max

8.8 9.28.8 9.28.8 9.28.8 - 9.2pH at 25oC

nilnilnilnilHardness, ppm, max

Once-thro

ugh units

166 - 205101-16561- 100Drum Pr.

Kg / sq.cm


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Corrosion of boiler steel

Factors responsible for corrosion

pH

Dissolved oxygen


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Water technology

Corrosion of steel vs boiler water pH

safe range

8.5 11.0

pH

Corrosion

rate

acidic alkaline

4 8 10 126 14


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Water technology 8

An Electrochemical reaction

Electron flow

Fe2+O2 OH--

water

ANODE REACTION

Fe0 === Fe2+ + 2 e--

CATHODE REACTION

O2 + H2O + 2 e-- == 2 OH--

wwTubeFe


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Cation conductivitysalient features

enhances sensitivity to mineral salt concentration

when sample passes through cation exchangerNH4OH + H

+ Resin NH4+ resin + H2O

NaCl + H+ Resin Na+ resin + HCl

Effect of ammonia nullified Effect of contaminant minerals increased to

nearly 3 times and

sensitivity of measurement increased and

trace amounts of contaminants identified


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REFERENCE

1 Power Plant Water Chemistry -- a practicalguide by Brad Buecker ( Pennwell )

2 Betz Handbook of Industrial Water Conditioning BetzDearborn, Inc., ( Pennsylvania )

3 Drew Principles of Industrial Water Treatment

Cantafio. A. R., ed., ( New Jersey)

4 The Nalco Guide to Boiler Failure Analysis by Herro,H.M and R.D.Port ( McGraw-Hill )

5 Fundamentals of Steam Generation Chemistry by Brad Buecker ( Pennwell )

Bh t H El t i l Li it d


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Water technology

Feed water chemistry

Condensate plus make-up water

Virtually all impurities are carried into the boiler in thefeedwater

Pre-boiler (condenser, feedwater heaters , dearators)corrosion --- most prevalent form of corrosion inmodern h-p boilers

Corrosion products formed in the pre-boiler sectiontransported to the boiler waterwall tubes

get deposited preferably on high heat transfer zones

results in overheating and subsequent tube failure Deposits contain

oxides of iron , copper and its oxides , traces of Zn,Ni ,etc.

Bh t H El t i l Li it d


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Water technology

Condensate chemistry -- dissolved oxygen

Magnetite , Fe3O4 , protective film for iron

Cuprous oxide , Cu2O , protective film for copper

Oxygen , if excess , in feedwater oxidizes both theprotective layers

2 Fe3O4 + O2 3 Fe2O3 Cu2O + O2 2 CuO

Nodules of corrosion products & pits form at thecorrosion site

Corrosion products enter the solution , transportedto the boiler higher heat loads cause deposition



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Water technology

Pre boiler corrosion Dissolved oxygen attacks copper in the presence of ammoniDissolved oxygen attacks copper in the presence of ammoniaa

more severely as follows :more severely as follows :

2 Cu2 Cu22O + OO + O22 44 CuOCuOCuOCuO + 4 NH+ 4 NH44OHOH Cu (NHCu (NH33))44 (OH)(OH)22 + 3 H+ 3 H22OO

(insoluble) ( soluble )

The corrosionThe corrosion product is transported more easily in a solubleproduct is transported more easily in a solubleform into the boilerform into the boiler

The copperThe copper--ammonia complex decomposes inside the boiler atammonia complex decomposes inside the boiler atelevated temperatures ( >elevated temperatures ( > 140140ooC )C )

The liberatedThe liberated free copperfree copper gets deposited on the heatgets deposited on the heat

transfer surfacestransfer surfaces



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Water technology

Copper & Iron --- during startup

< 10< 536

352028503024

754020

1207016

1801608

2102304

2605000

Iron , ppbCopper , ppbTime ( hour ) afterstartup



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Water technology

Pre boiler corrosion salient features

Most-affected locations --- Economisers & feedwater heaters

High levels of oxygen can be expected in feed water more

during start-ups and low-load operations

Pitting severe during idle periods -- free entry of oxygen ---moisture can be collected in bends , sags , etc leading to

enhanced corrosion

Appearance : perforations or pits on the tubewalls throughlocalized corrosion

if severe , during very high levels of dissolved oxygen ,crater like sites develop having red,rusty appearance

Op ti n f D m t p B il s 165 t 180 k / 2


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Operation of Drum-type Boilers 165 to 180 kg/cm2

boiler water control for [ 0.25 1.0 ppm (abnormal) ] condition :

o If pH < 8.0 & Total solids < 50 ppm do not operate the unit

p increase blowdown to limit TDS concentration

q avoid use of desuperheating spraywater by (a) reducing load

and (b) permitting reheater temperature to fall - contd

n chemical injectionto maintain excess

PO4 & pH conditions

TDS < 50 ppm

pH 9.1 10.1

PO4 5 20 ppm

Limited operations;

Inspection and

repair of condenser

Total solids

(abnormal)

0.25 1.0 ppm

If on volatile

treatment , changeto PO4 treatment

TDS < 25 ppm

pH 9.1 9.8

PO4 5 10 ppm

Volatile treatment

not suitable ;

Back to guideline

values soon

Total solids

(acceptable)

< 0.25 ppm

normalTDS < 25 ppm

pH 9.1 9.8

PO4 5 10 ppm

noneTotal solids

(recommended)

< 0.05 ppm

Boiler water controlControl limitsOperational

limitations

Hotwell

conditions


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Operation of Drum-type Boilers 165 to 180 kg/cm2

Same as forabnormal hotwell

conditions of

0.25 to 1 ppm

plus

n prepare forwet lay-up of the

unito condenserrepair

TDS < 50 ppmpH 9.1 10.1

PO4 5 20 ppm

Emergencyoperation :

immediately

reduce load

orderly shut-

down of the unit

if hotwell

concentration

cannot bereduced below

1ppm

Excessive> 1.0 ppm

Boiler watercontrol

Control limitsOperationallimitations

Hotwellconditions

Total solids

C t d t i t d d t


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Case study --- contaminated condensate Boiler parameters : 80 MW unit ; 90 kg / cm2 ; PO

4treatment

Problem : ~ 0.75 ppm of TDS in condensate detected

Situation : chemists request to stop the unit denied ; load demand

high ; unit ran for about 15 daysAction : blowdown increased

pH and PO4 controlled in boiler water

monitoring frequency increased

Consequence :

After about 2 months , water wall tubes started failing

frequently

unit stopped on numerous occasions

Remedy :

tubes replaced at a cost of US $ 2,000,000 /-


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y

Water technology

Corrective actionsCorrective actions Control of dissolved oxygen and ammonia in feed water

Effects of pre-boiler corrosion severe during start-ups whenchemical control is difficult

Proper care and attention during condenser leakage Condensate purification plants : to minimize soluble and

insoluble materials

` Alternate materials : Titanium , stainless steel` Air-inleakage control : Expansion joints between turbine and

condenser ; turbine seals, diaphragms ; condensate pump seals

Dearator performance

Steam temperature and pressure and quantity

Tray alignment etc

Monitoring of dissolved oxygen . On-line

Relative priority of


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Relative priority ofWater Chemistry Instrumentation

Dissolved oxygen

Cation conductivity / degassed conductivitysodium

silica

chloride

pH

specific conductivity

phosphate

N b f li lN b f li l


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Number of online analyzersNumber of online analyzers

0102Ammonia

2222pH

5757Sodium

2525Silica

2323Dissolved oxygen

1111Conductivity/

degassed cation

5544Conductivity/cation

0303Chloride

Volatile treatment

max min

Phosphate treatment

Max min

Parameter



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Water technology

Solubility of oxygen vs temperature

0.121099

1.519088

2.417077

3.1150663.813055

4.411044

5.19033

6.27022

Oxygen ( cc per litre )Temperature ( 0F )Temperature ( 0C )

Case study --- organics


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Case study organics Boiler parameters : 50 tph unit ; 40 kg / cm2 ; amine treatment boilers generating steam for phenol production ; 80% condensate return

Problem : frequent failure of super heater tubes

Situation : deposits found in SH tubes having iron oxide and silica

Reason :boiler water indicated indicated organics

observed value : 20 to 200 ppm

recommended value : 0.5 ppm

Consequence :foaming inside boiler drum resulting in carryover

Remedy :

Upgrade analytical system to detect organics in feed

water , and boiler water

organics to be removed condensate polisher



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Water technology

Boiler water guidelines

0.2---------Cond. ( H+ ), S/cm,

max

---30100200Cond. S / cm , max

---1550100TDS , ppm , max

---2 - 65 - 105 - 20PO4 residual , ppm

0.100.100.9 0.27.0 0.9Silica, ppm , max

>8.59.1-9.69.1-9.89.1-10.1pH at 250C

AVTphosphatephosphatephosphateTreatment

Type

166-205166-205126-16561-125Drum Pr.

Kg / sq.cm


B il id li


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Water technology

Boiler water guidelines

1.50.4---0.1190

1.60.9---0.14175

1.71.50.40.2160

1.91.80.540.271452.22.00.80.4

135

2.42.41.10.55120

2.62.61.60.8105

3.02.82.61.390

SulphateSO4 , ppm,max

ChlorideCl, ppm, max

SiO2 , ppm,max(0.02 ppm steam)

SiO2 , ppm ,(0.01 ppm steam )

Operatingpressure



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Boiler water treatmentTrisodium phosphate provides the needed alkalinity inboiler 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 sludgesand much easier to remove ; typically settle in the drum

and removed by blowdown

H V PO4 & N OH


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pH Vs PO4 & NaOH

8.5

9

9.5

10

10.5

11

11.5

0 5 10 15 20 25 30 35 40

ppm

pH

PO4 NaOH

EFFECT OF PO4 / N OH H


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EFFECT OF PO4 / NaOH on pH

8.8

9

9.2

9.49.6

9.8

10

10.2

10.4

10.6

0 2 4 6 8 10 12

ppm

pH

PO4 NaOH


C ti i


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Water technology

Caustic corrosion

Irregular thinning of waterwall tube due to the corrosiveaction of sufficiently concentrated NaOH

Under porous deposits , NaOH builds up from safe

level to corrosive level Corrosion occurs by concentrations at the local tube wall

& not by concentrations existing in the bulk boiler water

Concentrated NaOH first attacks the protective magnetite

Fe3O4 + 4 NaOH === 2 NaFeO2 + Na2FeO2 + 2H2O

Further attack of NaOH on boiler metal

Fe + 2 NaOH === Na2FeO2 + H2


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CAUSTIC DAMAGE


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C i i


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Water technology

Corrective actions

control of boiler water chemistry ( PO4 programmes )

control of water-side deposition

Periodic ( usually yearly basis ) assessment of tubecleanliness & chemical cleaning to be carried out , ifneeded based on deposit analysis

minimizing ingress of deposit forming materials ( pre-boilercorrosion and condenser leakage )

Wall thinning , if severe , tubes to be replaced


Hydrogen damage


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Water technology

Hydrogen damage

Hydrogen damage occurs in boilers operated with low pHHydrogen damage occurs in boilers operated with low pHwater chemistrywater chemistry

by aggressive anions like chloridesby aggressive anions like chlorides

concentration of acidic species under depositsconcentration of acidic species under deposits

During periods like condenser leakage , specially in sDuring periods like condenser leakage , specially in seaeacooled power plants , lots of acidic species are introduccooled power plants , lots of acidic species are introduceded

MgClMgCl22 + 2 H+ 2 H22OO Mg(OH)Mg(OH)22 + 2+ 2 HClHCl Atomic hydrogen , so formed , can diffuse into steel and react

with iron carbide in steel

Fe3C + 4 H 3 Fe + CH3 Fe + CH44 Methane , being a bigger molecule, can not diffuse easily

Grain boundaries affected ; internal structure of the tubedamaged , making it brittle


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HYDROGEN DAMAGE


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HYDROGEN DAMAGE


C ti ti


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Water technology

Corrective actions

Restoration water chemistry

condenser leakage

Removal of unit from service , if pHfalls below 8

Inadvertent release of water treatmentregeneration chemicals


Deposition


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Water technology

Deposition Even a relatively thin deposit layer significantly reduce

heat transfer . Leading to overheating of the tubes

In cases of severe deposition, overheating may occur veryrapidly resulting in catastrophic tube failure

deposits are also the precursor to under deposit corrosion water enters a porous deposit through some opening

and boils off through other channels , leaving the solidsbehind

the concentrated species may be an alkali (eg.NaOH) oran acidic species ( eg, Chlorides during condenser leak)


UNDER DEPOSIT CORROSION


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Water wall tube

without deposit

Water wall tube

with deposit




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Na3PO4 + H2O

NaOH + Na2HPO4

Boiler water with

Na3PO4 , Na2HPO4 , NaOH

enter through the pores

of the deposit and wateronly comes out as steam

leaving the solids to

concentrate

HEAT

WATER WALL TUBE FACING

THE FURNACE ( HOT ) SIDE


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Lay-up and offline corrosion


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Water technology

Lay-up and offline corrosion

More corrosion can occur during an outage than any

other time

This is due air intrusion and subsequent corrosion by

oxygen Lay up guidelines : boiler filled with water containing

200 ppm hydrazine and ammonia to pH 10

Nitrogen blanket ( 0.5 kg/sq.cm) to the drum and superheaterthrough the vent lines

The boiler lay up solution should be circulated periodicallyto avoid stagnant zones, analyzed and replenished if needed

Before the boiler is started up , the lay up chemicals needto be removed


Steam Chemistry


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Water technology

Steam Chemistry

Steam purity affected by carryover --- the process by whichsolids are transported to steam

Carryover influenced by

solids become more soluble at high pressures

some like silica carry over as vapour drum level , drum design (internals) , foaming

contaminants can also enter via attemperator systems ;greatly exacerbated during upset conditions such as a

condenser leak


CRITICAL FACTORS IN WATER CHEMISTRY


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Water technology

CRITICAL FACTORS IN WATER CHEMISTRY

1 Recommended guidelines in entire water steam cycleto be followed always !

2 Special care to be taken in controlling and monitoringdissolved oxygen , silica , & cation conductivity

3 Critical periods of water chemistryg Start ups gCondenser leakage g lay- up

4 Periodic chemical cleaning --- a routine maintenance step tokeep heat exchanger tubes clean

5 Management support : on-line and laboratory measurement facilities updating chemical technology knowledge base


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THANK YOU


Chemical Cleaning


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Chemical Cleaning

Post operational chemical cleaning

To be based on waterwall tube deposit analysis

single stage or multi stage --- based on laboratory trials

and copper content

sometimes chemical cleaning may not remove the depositscompletely , if the deposition is very heavy and/ or having

hardness and silica , necessitating tube replacement

To be carried out in a planned manner and entrusted to

experienced and reliable contractors


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To return line


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CHEMICAL TREATMENT PROGRAMSCHARACTERISTICS


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Can cause rapid corrosion whenconcentrated (specially under

deposit ) ; vaporous carryover in

steam at high pressure ; dosingcontrol very essential

Acid neutralization ;No phosphate hide-out

Sodiumhydroxide

Controlling molar ratio of Na

and PO4 ;Hide-out

Caustic corrosion mostly

eliminated ; deposit form ..Easy for removal ; acids

neutralized ; surface

passivation by phosphate

Congruent

Phosphate

Na:PO4 ..2.8

Possible under-deposit corrosion

by concentrated NaOH ;

Hide-out

Caustic corrosion may beeliminated; deposit form ..

easy for removal ; acidsneutralized; surface

passivation by Phosphate

Coordinated

phosphate

unfavourablefavourableprogram

CHEMICAL TREATMENT PROGRAMSCHARACTERISTICS


70/70

Can tolerate very low

concentration of impurities ; no

corrosion protection in case of

upset ; copper alloys should notbe used in the system ; requiresexcellent purity feed water ;

precise chemical control required

Low corrosion rates of

ferritic steels and conden-

ser tubes ; better oxide

coating , hence frequencyof chemical cleaningincreased

oxygenated

treatment

Feed water contamination mayexceed inhibiting ability of

volatile feed , leading to boilercorrosion ; marginal acid

neutralization ; no protection

during mild hardness ingress

deposition of salts can beeliminated ; high purity

steam under ideal feedwater conditions ; no carry

over of solids

All volatile

unfavourablefavourableprogram

Documents

PP 1 Water Tech