Blowdown6.3 BlowdownContents6.3.1 What should TDS of my boiler
be?.......................................................................................26.3.2
What is the purpose of
blowdown?.......................................................................................26.3.3
What problems are caused by carry
over?..........................................................................3ase
Study!1..............................................................................................................................36.3."
What is the difference between intermittent and continuous
blowdown?.......................46.3.# $ow should % minimi&e
blowdown?..................................................................................76.3.6
$ow much blowdown is
needed?..........................................................................................8ase
Study!2..............................................................................................................................96.3.'
What is the impact of condensate recovery on
TDS?......................................................11ase
Study!3............................................................................................................................116.3.(
When should % recover heat from
blowdown?...................................................................12ase
Study!"............................................................................................................................12ase
Study!#............................................................................................................................136.3.)
What care should be ta*en while
blowdown?...................................................................14Page
1 of 14Blowdown6.3.1 What should TDS of my boiler be?The fi+ures
below are offered as a broad +uide only for process boilers. The
boilermanufacturer should always be consulted for specific
recommendations.Maximum TDS tolerated by boiler type Maximum TDS
ppm!"an#ashire 1,-,,,$%pass e#onomi# "-#,,&a#'a(e and 3%pass
e#onomi# 3-,,,!3-#,,"ow pressure water tube 2-,,,!3-,,,Medium
pressure water tube 1-#,,)oil Boilers and *enerators 2-,,,6.3.$
What is the purpose of blowdown?Waterisnotpure.%tcontains a lot
ofimpuritiesli*e dissolved solids- suspendedsolids- dissolved +ases
and scum formin+ substances. Some of these impurities are carried
in the boiler water. When steam is +eneratedthe dissolved solids
present in the boiler water continue to remain in the boiler andthe
amount of dissolved solids in the boiler water increases. Some of
the dissolvedsolids form scales
resultin+inlocali&edheatin+whichpro+ressivelyleadtotubefailure.
.lso- hi+h level of total dissolved solids /TDS0 encoura+es foamin+
and carryover. Therefore to control the level of concentrations-
blowdown is carried out. %n order toPage 2 of 14Blowdownmaintain an
optimum level of Total Dissolved Solids /TDS0 the blowdown water
isreplaced by fresh feed water.1lowdown is necessary to protect the
heat e2chan+er surfaces in boiler.6.3.3 What problems are #aused by
#arry o+er?.s the boiler +enerates steam- any impurities which are
present in the feedwater and which do not boil off with steam will
concentrate in the boiler water..s the concentration of dissolved
solids increases- the steam bubbles tend tobecome more stable-
failin+ to burst as they reach the water surface of theboiler.
There comes a point where a substantial part of the steam space
intheboiler isfilledwithbubblesandfoamwhich then+ets carriedwith
thesteam. The increased moisture content in the steam is wasta+e of
fuel as only steamparticipatesinheat e2chan+eprocess.
Theillustrationbelowshowsthedecrease in dryness fraction to an
increase in the level of TDS. )ase Study%13arametersBoiler Desi(n
&ressure 1, bar+Boiler ,peratin( &ressure ( bar+Boiler
-ffi#ien#y ',4.uel /sedoal 56 3,,, *cal7*+.eed water softened! TDS
(,, ppmBlow down TDS (,,, ppmSteam Demand 2 T3$0t a TDS le+el of
1222ppm the steam would be 134 dry i.e. there would be a134
moisture #arryo+er5 while at 3322ppm steam is 614 dry.6.3.7 What is
the differen#e between intermittent and #ontinuous
blowdown?ontinuous blowdown has a calibrated blowdown valve and
blowdown tap near theboiler water surface. %t draws off water
continuously from the top of boiler surface.The rate at which
blowdown needs to be carried out is pre!determined and hencePage 3
of 14Blowdownre8uires no re+ular operator intervention.The
concentration of impurities in the water should be controlled by
blowdown fromsteamdrumand blowdown fromthe mud drumor bottomheader
removessuspended solids- mud or slud+e from the boiler.Page 4 of
14BlowdownPage 5 of 14Blowdown 8ntermittent Manual! Blowdown
,peration .ll steamboilerincludeameansformanual
blowdownasstandarde8uipment.9anual blowdown allows for removal of
solids that settle at the bottom of the boiler.TDSmust
becontrolledaccuratelyneartoma2imumlevel-
inordertominimi&eblowdown- and to avoid carryover and foamin+
caused by TDS levels. onsider ascenario of manual operation of
blowdown valve by operator: The boiler operator opens the blowdown
valve when the TDSlevelof boiler water +oes above the set point and
closes thevalve when the TDS level +oes below the set point. So-
over a period of time there is a lar+e fluctuation in the TDSlevel
of theboilerwater.$i+hTDSlevel causesscalin+andcarryover which is
bad for boiler and also for e8uipments alon+the steam line.;ow TDS
levels causes loss of sensible heat and thus wasta+eof fuel. Thus-
manual blowdown control is not ener+y efficientil 3 T&90+(. TDS
addition to the boiler 122 ppm 100+102; 33 ppm1lowdown per day 7.$
T&D $.6 T&D6.3.1 When should 8 re#o+er heat from
blowdown?.ny boiler with continuous blowdown e2ceedin+ #4of
steamrate should considerrecoverin+ heat from blowdown. %f your
plant has intermittent /manual0 blowdown systemconsider switchin+
to continuous blowdown coupled with heat recovery.1lowdownheat
canberecoveredwithaheat e2chan+er- flashtan*or
flashtan*incombination with a heat e2chan+er. The blowdown is
flashed in a flash vesselto obtainlowpressuresteam.
Thislowpressuresteamistypicallyusedtoincreasefeedwatertemperature.?e#o+erin(
heat from blowdown
flash1lowdownwaterisatthesamepressureandtemperatureaswaterintheboiler
drum. %t ispossibletorecover theheat intheblowdownwater
byseparatin+ the flash steam from the blow down before drainin+ it.
This flashsteam can be recovered and utili&ed to increase the
feed water temperature.)ase Study%7ontinuin+ the ase study!3;i8uid
enthalpy hf at 1, bar = 1(6.'' *cal7*+;i8uid enthalpy hf at ,.# bar
= 111.'( *cal7*+Page 12 of 14Blowdown;atent heat hf+ at ,.# bar =
#31.' *cal7*+4 flash +enerated =hf high pressurehf low
pressurehfglow pressure 2 1,,= 186.77111.78531.7 2 1,, = 1".1
4.mount of flash steam +enerated= '6) *+7h 2 1".14 = 1,( *+7hh+ of
steam = 6"3."3 *cal7*+Bner+y in flash steam =1,( 2 6"3."3 = 6)"),
*cal7*+ Calculator for % flashThisflashsteam
canberecoveredandutili&edtoincreasethefeedwatertemperature.?e#o+erin(
heat from blowdown
water@orlar+eboilersbesidesblowdownflashwecanadditionallyrecovertheblowdown
heat via a heat e2chan+er. These methods minimi&e the heat
lossthrou+h blowdown- savin+ fuel.)ase Study%3ontinuin+ the case
study!"Total blowdown = '6) *+7h with 1,( *+7h flash steam Water
flowrate re8uired = '6)!1#' = 612 *+7h-ner(y in waterBnthalpy /hf0
of saturated water at ,.# bar = 111.'( *cal7*+Bnthalpy of water at
#, o = #, *cal7*+Bner+y available to heat ma*e up water =
111.'(!#,= 61.'( *cal7*+ Total ener+y that can be recovered = 612 2
61.'( = 3'(,) *cal7hThis ener+y can be utili&ed to increase the
feed water temperature.Calculator for BHRSPage 13 of
14Blowdown.6.3.6 What #are should be ta'en while blowdown?Safety
#are%n order to prevent the uncontrolled flashin+ of steam in the
boiler room- theblowdown water should be dischar+ed into a vessel
in which the flashin+ canoccur in a controlled manner.-n+ironment
#areBnvironmental lawsstatethat thetemperatureof thedischar+edhot
watershould be less than 6, o. $ot water promotes e2cessive +rowth
of bacterialeadin+ to de+radation of sewer lines. $enceadditional
stepsshouldbeta*entobrin+downthetemperatureofblowdown water from
its boilin+ point to 6, o or less.Eneof
thewaysistousetheblowdownwaterinheat e2chan+ertoheatincomin+ water
which can be used as feed water which in turn cools down
thetemperature of the blowdown water to specified norms.Page 14 of
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