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STEAM-WATER CHEMICAL

REGIMES FOR SUPERCRITICAL BOILERS

A.K.Sahay

Sr.Mgr (OS )

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yImprovement in fuel efficiency and heat

rate, decrease in specific fuel consumptionand reduction in emissions have beendriving factors for super critical cycles.

ySupercritical units are reliable , highly

efficient, cost effective in terms of capitaland operation costs and responsive toemission concerns.

y Among these , the key attraction of

supercritical units lies in high overallefficiency.

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yDefn. of critical condition : Criticalcondition is a thermodynamic expressiondescribing the state of a substance beyond

hich there is no clear distinctionbet een the liquid and gaseous phase.

y Also there ill be a need to continuouslypurify the return condensate by means ofa condensate polishing unit ( CPU )

y And to employ all volatile feed atertreatment hich generally consists ofammonia and hydrazine.

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The water as heat transfer medium in

the circulation should fulfill the basicrequirement w.r.t. its purity and

condition.

The contents of dissolved andundissolved salts and other material

should be practically zero.

Also not present should be carbonic acidwhich contribute to corrosion of various

components of the system.

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Feed water

It composed of turbine condensateand makeup water.

Condensate has the largest influence on

the purity of the Feed water.

Impurity in the condensate results from:

a) Contents of dissolved salts in steam.

b)Leakages/seepages of Cooling water.

c)May contents corrosion products.Hence continuous operation of CPU is

desirable.

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RECOMMENDED SPECIFICATION

OFFEED WATER

PARAMETERS VALUES

1. Cation cond.,us/cm

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PARAMETER SAMPLE

FREQUENCY

TARGET

VALUE

Sodium ,ppb C

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C.E. . DI CHARGE

ARAME ER SAMPLE

FREQ ENCY

TARGET

VALUESo i m, C

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C.P.U. UTLET

PARAMETER SAMPLE

FREQUENCY

TARGET

VALUE

So i m , C < 3

Cat.Co .,s/cm

2C < 0.1

Silica, C < 10

The target val es are ase o EPRI

g i eli es.

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I NI I N D

D SI NyThe fee ater system i fossil f el fire la ts

may co sist of either all-ferro s materials.yThe e act co fig ratio has a far reachi g eari g

o the choice of fee ater chemistry a o

FLOWACCELERATED CORROSION AND

ALSOON THE CHOICE OF FEED WATERCHEMISTRY

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yMetal loss associate ith FAC are ei g fre e tly

o serve i fossil f el fire la ts. Thus, the car osteel com o e ts i fee ater systems must e

subjecte to the same rigorous FAC rogram.

yThis is attribute to the fact that o i es act as a

catalyst for the reactio bet ee hy razi e a

issolve o yge .yAnother area vulnerable to FAC is heater rain lines in

both all-ferrous and mi ed-metallurgy systems. Here

again the mechanism involved is dissolution of the

surface layers of magnetite under reducing conditions.

yMost heaters in lant o eration are vented, hichmeans that both H and o ygen levels in the drain lines

are lo er than in the feed ater. These conditions

e acerbate FAC.

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yThe rate of metal loss (wear rate) is dependent on:

FLUID TEMPERATURE, MASS TRANSFER,

ALLOY COMPOSITION,

OXIDIZING/REDUCING POTENTIAL (ORP,

RELATED TO DISSOLVED OXYGEN AND

REDUCING AGENT), FLUID H, COMPONENT

GEOMETRY, UPSTREAM INFLUENCES ANDSTEAM QUALITY.

yAlloy content is im ortant to the e tent that even

trace amount of chromium, co er, and

molybdenum can significantly reduce the solubility othe o ide layer. Mass transfer is related to the bulk

fluid velocity and is almost linearly ro ortional to

the rate of FAC.

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The solubility of the layer is

yinversely proportional to local pH and it must be

clearly understood that FAC is directly related to the

pH of the fluid at the hot operating temperature andnot the cold pH as measured in the feed water.

yAnother important factor is local geometry

specifically, disturbances to the flow tend to cause

turbulence and increase the rate of FAC. Such

disturbances may result from component shape such as

elbows, tees etc.

y The remaining factor is the void fraction (related to

steam quality) in case of two phase flow (i.e. wet

steam). For all other conditions remaining equal, wetsteam creates a significantly worse environment for

FAC than does water.

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Rationale for monitoring the target arameters

H

Monitoring of H is necessary because

y corrosion of metal is a function of H and o ygen,

y alkaline H values increase the stability of the magnetite

film and reduce magnetite solubility in ater.

CONDUCTIVITY

y Specific conductivity is an ine pensive reliable ,on-line method for

monitoring the overall level of contamination and its trend.

y The sensitivity of specific conductivity for monitoring the

over all level of contaminants depends upon nature & form of

contaminants.

y On line cation conductivity serves as an e cellent diagnostic tool as it

measures the total anion inhighly conductive acid form.

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SODIUM

Monitoring of sodium is necessary because

ysodium compounds ( like Sodium hydroxide and SodiumChloride ) are major corrodents of turbine.

y Limited sodium levels are recommended for condensate

and make-up water to achieve the desired steam purity

under normal operation and to allow maximum flexibility

during excursion.

CHLORIDE

Monitoring of chloride is required because

y

corrosion fatigue, stress failure and pitting in L.P.turbines,ycorrosion , hydrogen damage and pitting in boilers ,

degradation of condensate polisher perfomance and

y synergistic effect on corrosion in the presence of oxygen.

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DISSOLVED OXYGEN

Monitoring of dissolved oxygen is necessary becauseyoxygen causes pitting

yoxygen is thought to promote corrosion fatigue o

boiler tubes and turbine disks and blades.

yoxygen reacts with ferrous alloys in thecondensate/feed water system to generate corrosion

products, which are subsequently transported to the

boiler.

yThis leads to increased chemical cleaning

requirements and boiler water-wall tube failure

yOxygen reacts synergistically with chloride, resulting

in hydrogen damage.

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HYDRA INE

Monitoring of hydrazine is necessaryyto provide feed back control of the hydrazine feed

rate.

yIt is to be noted that the effectiveness of hydrazine

increases logarithmically with dosage.y Hydrazine should be fed at the highest possible

dosage to maximise oxygen removal. Since

hydrazine breaks down in the boiler to form

ammonia, the maximum hydrazine dosage that canbe tolerated is determined by the amount o

ammonia carry over and system metallurgy.

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IRON

yCorrosion products form deposits on ater- alltubes under high heat flux conditions, and then

traps and concentrate impurities, contributing to

tube failure by caustic gouging, hydrogendamage

and lo cycle corrosion fatigue.y Dissolved magnetite, Fe3O4 , precipitates out at

high temperature and turbulence because of

decreased solubility.

y

It normally occurs in the regulating componentsof the boiler or at the heating surfaces subjected

to high thermal loading.

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STEAM ATER ANALYSIS SYSTEM ( SWAS ) R

SUPERCRITICAL UNIT WITH AVT EED WATER

TREATMENT

Sampl

e No

Name

PlateTitle/

Appr

parameter

Type

of

meas rement

Range of

monitor

Indicator

Make- p

D.M.Water

P= . g/Cm2

T=35oC

a.grabb.Sp. Condy.c. Cat. Condy.

-

- s/cm log0- s/cm log

2 Hot ell

condensate

P=0. 21 g/cm2

T=46.2oC

Sp. Cond.

( Bot Sides )

0-10 s/cm

linear

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3 Condensate

pumpdischarge

P= 35 g/cm2

T=46.5oC

a.grabb.

pHc.Sp.Cond.

d.Sodiume.D.O.f.Cat. Cond.

-

6-110-10 us/cm

linear

0-1,0-10,0-

100ppb

0-200 ppb

0-1 us/cm log

4 CPU outlet a.Grabb.PHc.Sp. Cond.d.Cat. Cond..e.Sodiumf.Silicag.Chloride

-

6-11

0-10 us/cm

linear

0-1 us/cm0-1,0-10,0-

100 ppb

0-20, 0-100

ppb

0-10-100 ppb

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5 Deaerator

outlet

P=12 Kg/cm2

T=187 o C

a. Grab

b. D.O.

-

0-10,0-50,0-

100

6 Economizer

inlet

P=290

Kg/cm2

T=280

0

C

a. Grab

b. pH

c. Sp.Cond

d. Cat.Cond.

e. Hydrazinef. Silica

g. Turbidity

-

6-11

0-10-

100us/cm

0-1 us/cm0-50-100

0-20-100

0-10-100 NTU

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7. Separator

steam

P= 280 Kg/cm2

T= 415 oC

a. Grab

b. Cat. Cond.

c. Hydrazined. Sp. Cond.

e. Silica

-

0-1

0-500-10

0-20-100 ppb

8. Main steam

P= 255 Kg/Cm2T= 540 0C

a. Grab

b. Sp. Cond.c. Cat. Cond.

d. Sodium

e. pH

-

0-10 us/cm0-1 us/cm

0-1, 0-10,

0-100 ppb

6-11

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CONCLUSION

yThe primary purpose of feed water treatment is todeliver feed water to the economiser inlet with

minimum impurities and corrosion products for all

operating regimes.

y

This is important for steam chemistry as many of themechanism occurring in steam path are influenced

by deposits mostly caused iron-based corrosion

products.

y The key issue is the monitoring of iron, pH, oxygen

and oxidation-reduction potential at economiser inletas a minimum for all types of operating regime ( i.e.

full and partial load, shut down and start-up )

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yThe concentration of molecular impurity in

super heated steam, which includes bothmechanical and vapours carry over, should not

exceed its solubility any where in the turbine.

yFor super critical units limiting the

concentration of ionic contaminants through out

the plant cycle to the levels consistent with stea

impurity levels tolerable in the turbine will

adequately protect the boiler and other cycle

components.

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